GEOLOGY OF EAGLE MOUNTAINS AND VICINITY, TRANS-PECOS TEXAS OF EAGLE MOUNTAINS AND VICINITY, GEOLOGY TEXAS TRANS-PECOS GEOLOGY OF EAGLE MOUNTAINS AND VICINITY, TRANS-PECOS TEXAS A DISSERTATION Presented to the Faculty of the Graduate School of The University of Texas in Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY By Q OO James R« Underwood, Jr , BoSo, M A Texas Austin, August 1962 . . in a completely homogeneous material /with the greatest principal . stresses ori­ented to northeast-southwest,/ overthrusting the southwest to the north- and overthrusting east are equally probable. When confronted by the observation that the northeastward overthrusting is dominant or predominant, we remark that actual deformation is obviously controlled by local inhomogeneities or bound­ ary conditions; example, by pre-existing for faults, joints, and bedding planes, by changes in composition from shale to limestone, by notable strata in one thinning of direction, by newly laid sedimentary material a on steeply sloping, strong, metamorphic base­ment. Once deformation has started it goes on creating its own inhomogeneities <> Here is a problem in integrationo We have a variable stress tensor as a derivative function of three-dimensional coordinates of geological space, the values varying from point to point. The number of points is in­ finite, and the integration to predict the resulting deformation of the rock is humanly impossible. Fortunately nature does this integration, and our problem is the much sim­pler one of deriving a fairly reasonable stress history from a known pattern of deformation o This is difficult enough. In a complexly de­formed area such as the Indio Mountains, it is hard to be sure what the pattern really is. We at least can be thankful that we are work­ ing in the Chihuahuan desert rather than a Guatamalan rain forest 0•• DeFord, 1958b, p. 73~74« GEOLOGY OF EAGLE MOUNTAINS AND VICINITY, TRANS-PECOS TEXAS James R« Underwood, Jr. ABSTRACT Devil The Eagle Mountains and adjacent highlands. Ridge to the northwest and the Indio Mountains to the south, rise steeply above flanking intermontane lowlands in southeastern Hudspeth County, Texas, to an elevation almost 7,500 feet above sea level. The Precambrian Carrizo Mountain Formation, about SjOOO feet of steeply dipping metamorphosed sedimentary rocks, out on the northeast flank of the Eagles, Rock crops rang­ ing in age from Ordovician through Pennsylvanian has been eroded, but the basal siliciclastic Powwow Conglomerate and the overlying Hueco Limestone record the transgression of the Permian sea over the Diablo platform during the Wolfcamp Epoch. Erosion has removed younger Permian rocks. The Mexican halting transgression of the Cretaceous sea over the Diablo platform is recorded about feet of by 7,000 alternating siliciclastic and carbonate rock of the Comanche and Gulf series that were deposited on the erosional surface of Permian rock along the western margin of the platform and the adjacent Chihuahua trough. The Comanche Series consists from oldest to the of, youngest, Yucca Formation, Bluff iv Benevides Forma- Formation* Cox Sandstone* Finlay Limestone* tion, Espy Limestone, Eagle Mountains Sandstone, and Buda Limestone. The conformably overlying Chispa Summit Forma­ tion the Gulf Series. represents Some of the major structural elements of the Eagle Moun tains and vicinity were created during the Laramide orogeny when sediments that had been deposited in the Chihuahua folded and overthrust northeast- were trough asymmetrically ward . blanket of Widespread mid-Tertiary vulcanism spread a flow and pyroclastic rock over Trans-Pecos Texas. In the Mountains and volcanic Eagle rhyolitic trachytic flow rock, breccia, and flow breccia overlie centroclinally dipping Cretaceous strata. The high, interior part of the Eagles is a stock of the Peak crescent-shaped composed Eagle Syenite. and with Rhyolitic trachytic fine-grained tuff alternating welded tuff and trachyte cover part of the tilted Cretaceous rock of the southern Indio Mountains. Cretaceous rock in the northern Indio Mountains and in Devil Ridge has been intruded by rhyolite. Regional uplift that followed the vulcanism was accom­ panied by block faulting that created the mountains, or horsts, and the intermontane basins, or grabens. Subsequent erosion has partly filled the grabens to form bolsons and has reduced the mountains to their present form. The newly is a series established Rio Grande drainage system forming the bolson fill. of stream terraces as it incises CONTENTS TEXT Page Introduction. 1 . ©«. ®. ©.» ©.*.*» ©«. © . Location* . ® ©. .»«.©».. » ® ® ©«. © ........©©©*© Physiographic setting. 5 ...*. ©«... ©. © © ® Accessibility. 9 ©© Previous work and work in © © 18 . progress © . © . ... . Origin and scope of project. . © © © © 24 © Acknowledgments. 25 Field and laboratory procedure 29 ......... - . ©«....«.. © ®. © .. ©. Cartography. 31 Horizontal and vertical control. 31 ........ Terms and abbreviations. 35 ©•«.©..«©».. ............. Stratigraphy. Prec ambrian rocks. 39 . .©. ©. ... ©«. ... Carrizo Mountain Formation. ........ 39 Feldspathic metaquartzite (p£Cq) 41 ... Meta-arkose (pCCma). 41 ......... Mixed unit 1 (pCCmI) 42 Mixed unit 2 ©• •©...©• ......... (pCCm2) 43 Amphibolite (pCCa) 44 Quartz tourmaline veins. 45 *©..... -x--;(-pre-Permian Paleozoic rocks. 45 ©..©.©.©. Permian rocks. 47 . . ....... Hueco Limestone 47 .. ... .. . •. ©©©© rocks. 57 .©...©.©«».© -x-i;-Pre-Aptian Mesozoic rocks 58 Cretaceous rocks © © .* * 57 Yucca Formation 62 . ©.*© ©© © © ©® .*©» ®«© © ®« © ©©© Thickness, lithology, and fossils. 62 © * Correlation and 74 .. ..©©©© © age. . Origin 7*7 .©..©©©.© © © . ... BluffFormation » © » ©. « © © 85 . o oo©© • Thickness, lithology, and fossils. © 85 Correlation and . © 98 * *. ©. ©» age. Origin 101 ..*.*.. ® o ©e ©a ©a o Cox Sandstone 106 . ..«. ®*« » © ®*© .© Thickness, lithology, and fossils. 106 .. Correlation and 113 age. Origin 11 6 *»e. * © aa•®»a ... © ‘''-''•Stratigraphic units marked with asterisks do not out crop in the area. map vii Stratigraphy--Continned Cretaceous rocks--Continued Finlay Limestone Thickness, lithology, Correlation and age. Origin Benevides Formation Thickness, lithology, Correlation and age Origin Espy Limestone Thickness, lithology, Correlation and age. Origin Eagle Mountains Sandstone Thickness, lithology, Correlation and age Origin Buda Limestone Thickness, lithology, Correlation and age. Origin Summit Formation Page 123 and fossils. 123 .. 129 ......... 131 . 133 and fossils. 133 ...... .. 138 . 140 144 and fossils. 144 ...... . . 154 157 159 and fossils. 159 ......... .. 162 • 163 168 and fossils. 168 . . 169 .• ....... 171 174 ........ .......... Chispa and fossils. . . Thickness, lithology, 174 Correlation and age. ......... 178 ....... Origin 180 ¦>'-*Post-Turonian rocks. 183 •. • pre-Duchesnean Cenozoic rocks 183 ..... Intrusive rocks of Devil Ridge area 185 .... Quartz latite porphyry Rhyolite Lamprophyre. Age and correlation. 185 185 187 188 ........ Eruptive rocks of Eagle Lower rhyolite Trachyte porphyry. Upper rhyolite .. ......... Mountains 190 ..... 191 202 210 ... .....• . Lower rhyolite sills 216 ......... Eagle Peak Syenite 220 .......... Diabase dikes 226 Late rhyolite dikes. 230 ......... rocks of Indio Mountains 231 ..... Eruptive » .. «.« Hogeye Tuff. 233 ... .«» . Pantera 244 ........... Trachyte units marked with asterisks do not out crop in the area. map Page Stratigraphy—Centinued Cenozoic rocks—Continued Eruptive rocks of Indio Mountains--Continued Tuff (Ttu) and trachyte (Ttr). 252 • dikes and sills 260 ••••• © « Rhyolite Older gravel..©.*®.©. »»©•*»« 26/). BolSOn fill o 266 a ©o a e«aao» oa«» a Terrace gravel. © 268 •••••«••»••• © Alluvium. © 2?0 «© © *.*»«©»©»».© © Windblown sand. 274 ©© .«.« © ©.©•>© © © Tectonicsetting. » © © « ®•« © « ©*» » ® © © »* 27p Structure 280 .««»..».©» ©»®©©» ©©©©©» Structural features of Devil Ridge area© © © © © 280 Thrust faults of Devil Ridge area » © © 280 . © Devil Ridge fault. .•>...©*. © © 280 Red Hills fault. • ©©©©©©© ©©• Minor thrust faults. 284 ©©©©©»©©© © Strike-slip faults of Devil Ridge area. © 287 Folds of Devil Ridge area • 289 Normal faults of Devil Ridge area . © . © © 295 Structural features of Eagle Mountains © © 298 . ©. Thrust faults of Eagle Mountains. © © . © © 298 Devil Ridge(?) fault 298 .©©«©©©©© ©© Carpenter fault. 301 ...©.© ©. © Spar Valley fault. 302 ..«©.« © ©» © Minor thrust faults. 302 . •© © ©•©.• faults of Mountains • © . Strike-slip Eagle 304 .» ©©«© Rhyolite fault » © » © © • 304 fault. Wind Canyon 305 Eagle Spring fault , © 306 ..©..»© © Folds of Mountains. 307 Eagle ©©«.©©©.. Normal faults of Eagle Mountains. 309 . ©© ©© Structural features of Indio Mountains 311 Thrust faults of Indio Mountains. • 314 . ©©© © •.« © fault Willoughby 314 Squaw fault. 315 .» «««« «© ©* ««© Bennett fault. 316 * .« ®«» «© ®»«» Borrega fault. . . • . 317 •.. ©. © ©. Red Mountain fault • • 317 Minor thrust faults. 318 Strike-slip faults of Indio Mountains 319 . «* Folds of Indio Mountains. © • • ••••• © 319 Normal faults of Indio Mountains. e . * 327 Indio fault. 327 .« «« ©©•».«.»» ®© Bramblett Ridge area • » • 328 ***** .> ** •* Lost Valley syncline • . 0 • » 328 M3,37g1ri3.1f3.UltS» * 32C * ®«o oo*ao a Page Structure--Continued • Bo1sons#*• ©•.• o•*. *. » « • « »* © 330 Tectonicanalysl.•* «• « © » • © 332 s•« •«© © • .©©..©©©©©©e Geologic history 339*»•»©. Precambrian time • ••••••••••«•••• 339 Pre-Permian Paleozoic time ©•©©©©©©©©© 342 Permian time 343 .©..©©.©e.©©..©©©. Early Mesozoic time* 344 ©..*©»©.«©©©©© Cretaceous Period. 345 ©©.»*©.©©©©©©©« Comanche Epoch• 34p • ©.«»©« © © ©««© © Gulf Epoch••»©»©«»©»»©••©«© 347 .© Laramide orogeny .......©«©©..© 348 Cenozoic Era 348 ©..©.©©© Vulc3.nl sm 349 •«•••••••••©•©©•© Intrusion 350 © ••© ©.©©© ©• Folding 3pl •*© ©©. ©» Block faulting© 35^ ©•.»••«« «»••» © Evolution of drainage 354 .....•.©©©. Terrace development 358 Economic 361 ... ©.©99©.©. geology. • » ••• Water* ••«• .••« ••• «• © © ©361 Surface water 361 ...• © Ground water 363 © .©. Aquifers 363 Wells© 363 •••.. ©•»• © © ©« Springs • 364 © . .. Quality 36? © Recharge and movement. 367 ..©..©.© Recommendations for . • . development. 369 Soil 370 © «. silver Lead, zinc, copper, . ©.©«©..© © © 374 .» Fluorspar© 3/6 »©*«»• © *« ®.* © © ©.• •© © Barite .•©*«.©«© ©© ©©© ©« ©«379 Petroleum..»•«©•...».©. 380 Uranlum 382 ©©©©©. **•«««.*•»©©.©©.©*©©« Coal©.©.*©*........©.©©©©© 384 Quartz 387 © ©*. ©•. .. ©*. «.»« © ©©© © Sand and gravel. 387 » © © •.•..*• .« © «©© Dimension stone. 388 • ... •••••• Limestone 389 Rhyolite 390 ..... ...... Tuff or pumicite 390 . Perlite © 391 Batguano. 392 • ©. ©.«..»•. ©». ©. . ** Silicified wood. 392 ........ . aa* Fossils©.. . •. « «... • * • «. © 393 stones Semi-precious 393 Page Appendix 394 Geography 395 Climate 395 • . Regional 395 Local 395 Classification 400 Vegetation 400 Herbs and shrubs ........... 401 Grasses 405 Trees 405 * Wildlife. . 408 Mammals. 408 . Reptiles, insects, and spiders 411 .... Birds. . » . •** •412 .... • ... . Culture 413 ........ Economy 415 Measured sections 419 MS 1. Yucca Formation 420 MS 2. and tuff - - Trachyte (Ttr) (Ttu), Garren Group ........... 438 MS 3* Trachyte (Ttr) and tuff (Ttu), - Garren Group ........... 441 ~ MS 4* Tuff (Ttuj, Pantera Trachyte, . . and Hogeye Tuff, Garren Group. 445 “ MS 5» Benevides Formation and Finlay Lime5t0ne............. 448 - MS 6. Buda Limestone, Eagle Mountains ., Sandstone, and Espy Limestone. 455 - ....... MS 7* Powwow Conglomerate, 464 MS 8. -Benevides Formation. ......• 468 “ MS 9* Bluff Formation and Yucca For­ •.. C. O0 mation »»©»«..420 - MS 10. Cox Sandstone. .......... 493 MS 11. Espy Limestone 501 -.. .. •.. a•. MS 11a.-Espy Limestone 508 MS 12a Buda Limestone and Eagle .. - Mountains Sandstone. ....... 511 - MS 13. Trachyte porphyry. ...... 516 ®. MS Hueco Limestone. 518 - • 14* «.••. « ®• MS 15. Lower rhyolite 522 -• ••.. •.•. • MS 16. Finlay Limestone and Cox - oandstone. 52 6 • « »« «*. »«»•• References • .......® • 540 Vita. ............a®#*.*..®... 559 TABLES Table Pa£e 1, Location of triangulation stations. Eagle Mountains and vicinity ....... 34 2. Index to of rocks. petrographic descriptions 46 .. 3. Petrography of representative samples of Hueco Limestone. 54 .. «... .... . ... .. of of 4* Petrography representative samples Powwow Conglomerate. 56 .............. 5. Petrography of representative samples of the Yucca Formation. 82 .............. 6. Petrography of representative samples of the Bluff Formation. 104 7. Petrography of representative samples of Cox Sandstone. 120 ................. 8. Petrography of representative samples of Finlay Limestone 132 . of of 9. Petrography representative samples the Benevides Formation. 143 10. Petrography of representative samples of the Limestone Espy 158 11. Petrography of representative samples of the Eagle Mountains Sandstone. 166 ......... 12. Petrography of representative samples of the Buda Limestone 173 ............... 13* Petrography of representative samples of the Chispa Summit Formation. 181 .......... of 14* Petrography representative samples of intrusive rocks, Devil Ridge area. 189 ....... of 15* Petrography representative samples of lower rhyolite, Eagle Mountains. 197 ......e • 16. Petrography of representative samples of Table Page •. trachyte porphyry. Eagle Mountains 207 of of 17* Petrography samples upper rhyolite. Eagle Mountains. 215 ». ». «... « .. .... . 18. of from sills and dikes Petrography samples of lower rhyolite and late rhyolite. Eagle Mountains 219 19. Petrography of representative samples of Peak Mountains. ...... Eagle Syenite, Eagle 223 20. Petrography of representative samples of diabase dikes, Eagle Mountains • 228 • 21. Petrography of representative samples of member of the the trachyte (Thtr) Hogeye Tuff, Indio Mountains. 238 ..... 22. Petrography of representative samples of the upper tuff member (Thtu) of the Hogeye Indio Mountains. Tuff, 243 • 23• Petrography of representative samples of Pantera Trachyte, Indio Mountains. 247 ....... 24* Petrography of representative samples of tuff (Ttu), Indio Mountains, 257 .......... 25» Petrography of representative samples of trachyte (Ttr), Indio Mountains. 259 ........ 26. Petrography of representative samples of intrusive rock, northern Indio Mountains 263 .... 27* Estimated thickness, feet, of formations in Devil Ridge area. 282 ...... . • .....„ 28. Water analysis, PPM. 368 • 29* Monthly and annual average temperatures, in at Van Horn, Culberson County, Texas, 397 ,• °F, 30, Monthly and annual total precipitation, in inches, at Van Horn, Culberson County, Texas 399 . * 31m U. S. Geological Survey air photographs. Table Page GSLU of the Eagle Mountains and series, vicinity 418 • .............. ILLUSTRATIONS Figures Figure Page 1. MAP: Physiographic features, western Trans- Pecos Texas and adjacent Chihuahua, Mexico 4 « 2. MAP: Previous work and work in progress, western Trans-Pecos Texas and adjacent Chihuahua, Mexico. 17 . • 0 ......... 3. CHART: Stratigraphic section of Cretaceous rock in Mountains and . . Eagle vicinity 63 4. CHART: Correlation of Cretaceous formations, Trans-Pecos Texas and adjacent Chihuahua 64 5. MAP: The relation of the west edge of Diablo platform to the Chihuahua trough and the to the progressive advance of the sea north during the Early Cretaceous Epoch. 128 . MAP: Mesozoic 6. Late paleogeographic features, Trans-Pecos Texas and northern Mexico. 277 .« Photographs Photograph Page 1. Southward view of Yucca Mesa 66 •. • 2. Northwestward view of Love Hogback and Devil Ridge 8? 3. Eastward view of Indio Pass. 11l 4. Westward view of MS 8, Benevides Formation 135 . .......... .. . Photograph Page Northwestward view $. along Wyche Ridge 150 6. North-northwest view of Eagle Bluff. . • . 211 • •. 7. South-southeastward view of Pantera Trachyte and Hogeye Tuff. 245 *• • 8. Southward view of Flat Top 253 •« • 9. North-northwest view along west flank of Indio Mountains 26$ ... 10. Southeastward view from Love Hogback 290 ...... Plates Plate Page 1. MAP; Geologic map of Eagle Mountains and vicinity, Trans-Pecos Texas (Underwood, 1962). pocket ..... 2. MAP: Structure contour map of top of Pre­ cambrian in Eagle Mountains and vicinity pocket INTRODUCTION Eagle Mountains and vicinity* the map area (plo l), includes Devil Ridge* Eagle Mountains* and Indio Mountains.® This includess superb geological workshop (l) well-exposed rocks ranging in age from Precambrian to Recent* including a small inlier of metamorphosed Precambrian sedimentary rocks* small inliers of lower Permian limestone* approxi­ mately 7,000 feet of marine Cretaceous rocks with varied composition and complex facies* an early Tertiary sequence of flow and and intrusive fine-grained pyroclastic rocks, rocks in the form of dikes and sills and a small stock? (2) structural created late Creta­ complicated relationships by and and ceous to early Tertiary folding thrust faulting late Tertiary block faulting* and (3) a recently developed, through-flowing drainage system* the Rio Grande* that has created conspicuous alluvial fans and terraces® The Eagle Mountains together with the Indio Mountains and Devil Ridge* like other ranges in the region* form a horst that is flanked by grabenso The highlands and the partly filled intermontane basins constitute bolsonso The Eagle Mountains are a topographically high mass of Tertiary volcanic and intrusive rock overlying marine lime- and sandstone of stone* shale* quarts Cretaceous age a In small areas on the north and northeast flanks* Permian lime­ stone and Precambrian metasedimentary rocks are exposed® 1 Devil Ridge is a northwesterly trending ridge that ex­ tends from the northwest flank of the Eagle Mountains almost to Sierra Blanca, Texas. In Devil Ridge, Lower Cretaceous and shale are thrust over Cre­ limestone, sandstone, Upper taceous sandy limestone and shale. Small early Tertiary dikes and sills have penetrated the Cretaceous rocks. The Indio Mountains, which extend southward from the Eagle Mountains to the Rio Grande, are composed largely of marine strata of Cretaceous age. Tertiary volcanic rocks cover a large area in the southern part of the mountains; in intrusive rocks are volumetrically minor and occur only the northern Indies. The area is near the northwest end of the Laramide map Chihuahua tectonic belt The (DeFord, 1956b, p. ?2, 74)• Eagle Mountains are part of the easternmost range of the belt, which begins more than 100 miles to the southeast near Ojinaga, Chihuahua, and terminates at Sierra Blanca, Texas. North-to northwest-trending thrust faults are promi­ nent in Devil Ridge and in the Indio Mountains. The south­ westward and westward movement of the overthrust block along some of the faults in the Indio Mountains is opposite to the northeast movement of the overthrust blocks along the faults in the Devil Late and Ridge area. Tertiary normal faulting erosion subsequent are responsible for the present topography. LOCATION The Mountains make the skyline southwest of Van Eagle the Culberson seat located at the inter- Horn, Texas, County section of U. S. Highways 80 and 90, 120 miles east-southeast of El Paso, Texas. Although these mountains are in Hudspeth 20 miles viewed from Van County almost away, their height Horn is impressive. Eagle Peak, the highest point, is about feet above level. sea 7,500 The 600-square-mile map area (pi. l) is in the south­ eastern corner of Hudspeth County. Its boundaries ares north —Southern Pacific tracks; east--long 104°56T W.; south--Rio and west--Red Draw Grande; Light (Quitman Arroyo on Eagle Mountain Sheet, Texas, 1897). The map area lies between lat 30°40* N. and 310 10 , N. and between long 104°56’ W. and 105°22 f W.j although it is entirely within Hudspeth it includes of the County, parts Chispa, Eagle Mountain, and Sierra Blanca 30* quadrangles. The area extends map northwestward to Sierra Blanca, the county seat of Hudspeth County, which is also the junction of the Southern Pacific and Texas and Pacific railroads. Although the map area is now in Hudspeth County, it was in El Paso County from the time that county was created in 1850 until Hudspeth County was organized in 1917. Culberson County was separated from El Paso County in 1911. Figure 1.-Physiographic features, western Trans-Pecos Texas and adjacent Chihuahua, Mexico, PHYSIOGRAPHIC SETTING The summit of Eagle Peak affords an unexcelled view of vast of Trans-Pecos Texas and northern Chihuahua. a part To the north* Guadalupe Peak* the highest point in Texas with an elevation above sea level of 8*752 feet* and El Cap­ itan* only 500 feet lower, are both remnants of the mighty Permian Capitan barrier reef that bordered the Delaware basin. These lofty limestone masses are a majestic back- for the Diablo which stretches to the ground Plateau, away north and northwest at an elevation of s*ooo-6,000 feet* and Hueco bolson to the west from Salt Basin to the separates east. Sierra Diablo, a north-trending range along the east­ ern margin of the Diablo Plateau, overlooks Salt Basin to the east and contains the most nearly complete rock record of any physiographic feature in the region. In these moun­ tains, rocks ranging in age from Precambrian through Creta­ excepting those of Cambrian, Triassic, and Jurassic ceous, age, are well exposed. Salt Basin is a remarkably smooth-floored depression more than 100 miles long, averaging 20 miles wide* into which water drains from all directions. This closed basin is a long* narrow north-trending half graben* bordered on the west by Sierra Diablo and on the east by the Delaware and Apache mountains, which compose the distant northeast skyline. The Delawares are an elongate north-northwest trending fault block in which Permian sandstone and shale of the Delaware basin facies have been uplifted and exposed. In contrast, the Apache Mountains are an above-ground northwest- trending part of the ancient Capitan barrier reef. The sharp notch in the northeast skyline is Seven Heart Gap, a pass between the Delaware Mountains and the Apache Mountains. Much nearer and just north of Van Horn are the Beach and Baylor mountains, relatively small fault blocks composed largely of rocks of Precambrian, Ordovician, and Permian age. Just west of Van Horn and bordering Eagle Flat on the north­ east are the Carrizo Mountains in which metamorphosed sedi­ mentary rocks of Precambrian age predominate. Bass Canyon, marked the well-defined in the of the by depression profile southern Carrizo Mountains, is the pass through which the old stage road runs. The eastern skyline is formed by the lofty Davis Moun­ tains, a vast Tertiary volcanic field of alkalic igneous • rocks. The sharp, triangular summit of Mt Livermore, the jagged silhouette of Sawtooth, and the rugged north-facing prominence of Gomez Peak are all distinctive features, even at a distance of 50-60 miles from Eagle Peak. In the foreground, dividing the southern end of Salt Basin into Michigan Flat on the east and Lobo Flat on the west, are the Wylie Mountains, a horst block composed largely of Permian and Cretaceous rocks. Just south of the main block of the Wylies, the Three Sisters (three small buttes monzonite of volcanic rock), Canning Ridge quartz intrusion, and Chispa Peak (composed of volcanic rock) are prominent features along the east margin of Lobo Flat. Just beyond Green River to the east and southeast are the Van Horn Mountains in which High Lonesome, elevation 5,622 feet above sea level, is the high volcanic rock mass at the north end. High on the distant southeastern skyline, a displaying distinctive angular silhouette, is Sierra Vieja, or the Rim Rock Mountains, composed of a sequence of Tertiary flow and pyroclastic rocks. Still farther south and east and plainly visible on clear days are the Chinati Mountains near Ruidosa, Texas, some 75 miles away. The view southward into Chihuahua is no less impressive. In this region the Rio Grande, in its path toward the Gulf of Mexico, wanders alternately through broad open valleys and deep, steep-walled canyons cut in rugged highlands. South of the Indio Mountains, beyond where the Rio Grande cuts the in the Sierra Sierra through range, are, order, Pilares, de los Fresnos, Sierra de Ventana, and Sierra Grande. This which extends a hundred miles south-southeastward to range, the vicinity of Ojinaga, Chihuahua, is composed largely of folded Cretaceous rocks. Far to the south, west of the southern Sierra Pilares and Sierra de Ventana, is Bolson del Cuervo. Just west, in succession, are the geologically unexplored northwest- trending Sierra de la Puerta del Alambre and the high Sierra de los Pinos, which compose the distant southwest skyline. In the near foreground and extending north to the Rio Grande is the Sierra de la Cieneguilla, a low of overturned range Cretaceous rocks. The Quitman Mountains make the peculiarly uneven skyline to the west and northwest. Folded and thrust faulted Cretaceous rocks the southern of the compose part mountains, and Tertiary intrusive and flow rocks make up most of the northern Quitmans. The low point of the skyline is Quitman Gap, the canyon that separates the southern from the northern Quitmans. The old stage road went through this pass and on west about 9 miles to Fort Quitman, near the Rio Grande. Just north of the Quitmans and near the western margin of the Diablo Plateau is easily the most distinctive feature of this vivid panorama. Sierra Blanca, "White Mountain," a laccolith with convex slopes sweeping gracefully to a summit 7,000 feet above sea level, its light-colored intrusive igne ous rock brilliantly reflecting the sunlight, is a truly magnificent landmark. The Malone and Finlay mountains, low ranges farther west and northwest on the western of the Diablo Pla­ margin folded and teau, contain gently rocks, largely of Permian Cretaceous The Malone Mountains contain the out­ age. only in Texas of marine strata of Jurassic crop age. Largely the result of block faulting, the valley flats flanking the highlands of the map area are: east, Green Valley; north, northeast, and northwest. Eagle Flat; and west and southwest. Red Light Valley. The map area is divided into three principal physio­ graphic units. The boundary between the Eagle Mountains and the Indio Mountains is the east-northeast-trending topo­ graphic low just south of Eagle Bluff that roughly coincides with the fault contact between the Tertiary volcanic rock of the Eagle Mountains and the Cretaceous strata of the Indio Mountains (pi. l). The Indio Mountains are subdivided along the east-trending Indio Pass road into the northern Indio Mountains and the southern Indio Mountains. To the northwest, the boundary between the Devil Ridge area and the Eagle Mountains is the steep cliffs of lower west rhyolite and overlying trachyte porphyry along the flank of the Eagles. In structural and stratigraphic dis­ cussions, however, that Cretaceous rock near the mouth of Horse Canyon and that in the vicinity of Black Butte are con sidered to be in the Devil Ridge area. ACCESSIBILITY Four factors control accessibility: (l) attitude of the ranchers toward work on their property, (2) type of field vehicle, (3) weather, and (4) location and condition of roads. The ranchers in the Eagle Mountains and vicinity, with­ out exception, were sympathetic toward the project. Three ranchers allowed use of small cabins or adobes at no range and information charge, all willingly supplied concerning roads, fences, springs, wells, tanks, names of geographic features, and local history. In return it was only neces­ sary to respect their wishes concerning closed or locked gates and to take time for a visit, always pleasant and usu­ ally enlightening, whenever and wherever we happened to meet. Effective work in the demands 4-wheel drive ve­ area a less is a serious and hicle; anything handicap actually dangerous. Field vehicles used during the three field sea- were a sons a jeep l/4-ton 4-wheel drive pick-up truck, standard and a military l/4-ton jeep, 6-cylinder l/4-ton pick-up truck. Weather is an important factor in accessibility of the area. Following a dry season, most roads are in good order, and those maintained by the county will be put in good order shortly after each severe rain. Most ranchers maintain their ranch roads in reasonably good order, but immediately following severe rains many roads are washed out and the little-used ones remain so for months. several Heavy rains in the middle and late summer and early fall wreak havoc with the roads* Those across low-lying areas are passable on the interstream divides but washed out at arroyo crossings* Those in higher country may simply be non-existent * On rare occasions high water at the mouth of Green River, near Scott Crossing, at Hot Wells, and at the T. 0* Red Draw block Crossing on Light may traffic for a day or two * There are five main access routes to the Eagle Moun­ tains and vicinity* The southernmost part of the area, the Indio Mountains, may be reached by driving 22.5 miles south from Van Horn on Uo So Highway 90 to a point about a mile northwest of Chispa siding on the Southern Pacific Railroad, where a well-traveled unpaved county road leads west over Chispa Summit, a low pass between the Sierra Vieja Mountains on the south and the Van Horn Mountains on the north. From U, S. 90 it is approximately IB miles to the river road and another 4®5 miles upriver to the new Adobe headquarters of Moody Bennett, just north of the Rio Grande and some 5-6 miles southeast of the mouth of Green River* An unimproved road leads northwest from the new Adobe of Bennett to the headquarters upriver old Adobe headquar­ ters and the Green River both near the mouth of Green farm, River, where the road turns north and extends the length of the Indio Mountains* Access to the eastern part of the area and the northern Indio is road U. S. Mountains by an unpaved county leaving 80 east of the Western motor hotel Highway directly Lodge and leading southwest from Van Horn 8 miles to Scott Cross­ ing on the Southern Pacific Railroad. Just south of the Southern Pacific tracks the road forks, the left fork lead­ ing into the Neal ranch and the right fork leading southwest alongside the tracks. The right fork leads to a second fork 0.6 mile beyond; the road to the right continues northwest along the railroad tracks and the road to the left leads al­ most due south to Mica Mine and the Green River road. At a point opposite a large windmill and concrete tank to the west and 4*B miles south of the Southern Pacific tracks, Mica Mine road continues straight ahead, but Green River road leads west a few hundred yards, then turns south and runs along the east bank of Green River. Just past the junction of Green River and Mica Mine roads, at the point where Green River road turns south, a dim road leads southwest past the old Taylor place, then west to the eastern margin of the mountains. Farther south another useful route leads west from the Green River road to the Indio Mountains. This east- west road, which leaves Green River road about a mile south of Double Wells and 11 miles south of Scott if in Crossing, good order allows easy access to the Indio Mountains, and it connects with the main north-south road through the Indio Mountains about a mile south of Indio headquarters. This east-west road is referred to as the Indio Pass road. From the junction about a mile south of Double Wells, the Green River road continues in the river bed and on the south, to the old Adobe the Rio Grande. near banks, headquarters Mountains reached south The Eagle proper are by turning off U. S. Highway 80 about 11 miles west of Van Horn (road to north leads to Allamoore) and crossing the Southern Pacific tracks at Hot Wells. An improved gravel road leads southeast from here some 9-10 miles to the Eagle Mountains ranch house and the general vicinity of the old fluorspar mines . The road to the Speck Ranch provides access to the northwest and west flanks of the Eagle Mountains and the southeast part of the Devil Ridge area. It leads south from U. S. Highway 80 about 9»5 miles east of Sierra Blanca, crosses the Southern Pacific tracks 1.5 miles south of the highway and continues south-southeast through the western part of the Espy Ranch to the Speck Ranch, and thence to the Speck ranch house at the southeast end of Love Hogback. Ranch Road 1111, leading south from Sierra Blanca, is now paved out to 5-Mile Point. Here the right fork leads southwest through Quitman Gap and the left fork leads south­ east to the Hayter, Guerra, and Bramblett ranches and to Indian Hot Springs. The road roughly parallels Red Light Draw, the main drainage between the Eagle Mountains on the east and the Quitman Mountains on the west, southeast to a point near the junction between Red Light Draw and the Rio Grande. This is also the road to Indian Hot Springs, that is reached by turning southwest off Red Light Draw road miles southeast of Red windmills. approximately 6 Light In addition to the five main roads to the moun­ access there four routes the all but tains, are across mountains, one of which requires a 4-wheel drive vehicle. This one is the route from U. S. Highway 80 southeast to the Speck ranch house, thence by one of several roads southwest over to the Red Draw-Indian Hot Light Springs road, joining it at a point near Red Light windmills, about 13 miles southeast of 5­ Mile Point. The most spectacular route across the mountains is from Hot Wells Mountain ranch to the Eagle house, up Wind Canyon and over the divide into Broad Canyon, and on to the Guerra ranch headquarters. This route is impassable, even with a 4-wheel drive vehicle, unless the roads in the canyons have been worked since the last severe rains. A third route leads across the northern Indio Moun­ tains from the large mill at the head of Green River past the old Taylor place, west to Oxford Ridge and the Oxford ranch then down Oxford Draw and southeast on the house, gravel-capped terraces to the intersection with Red Light Draw road near the Guerra school and farm headquarters. The southernmost route over the from Green mountains, River to the Rio Grande, is by way of the Indio Pass road which begins one mile south of Double Wells ranch house and leads over Indio Pass to the main road running north almost the length of the Indio Mountains. From this junction, the route leads north past Indio headquarters and on to a high knoll some two miles farther north, where the road forks. The road to the left (west) and through a fence leads to the western of the mountains forks chosen at margin if right are each intersection after leaving the main road. From the on a west boundary the road drops down to gravel-capped ter­ through another then into the race, passes fence, drops to the south. in the bed of the and arroyo Driving arroyo following the southern (left) bank will lead to the river- road at a point approximately half way between Bramblett headquarters and the Guerra farm and school. There is a reasonably good dry-weather unimproved road along the southern side of the Southern Pacific tracks from Scott Crossing to Sierra Blanca. Numerous ranch roads lead south to the mountains from this road that runs along the northern boundary of the area. Explanation of Figure 2 Area Year mapping completed 1* Devil Ridge, Eagle Mountains, Underwood, 1960 Indio Mountains 2. Northern Rim Rock Country Braithwaite, Frantzen, Bridges, and Dasch, 1957 3« Northern Sierra Pilares Clutterbuck and Ferrell, 1957 4. Central Sierra Pilares Harwell and Vest, 1958 del (Sierra Porvenir) 5. Central Sierra Pilares Atwill, Campbell, and (Borrachera Anticline) Daugherty, 1958 6. Southern Sierra Pilares Hamilton and 1960 Yeager, 7. Sierra de los Fresnos Allen and 1956 Nichols, 8. Northern Sierra de Ventana Dill and Spiegelberg, 1960 9. Northern Rim, Triple Hill, Brunson and 1953 Wade, Cox Mountain 10. Malone Mountains Albritton, 1935 11. Northern Quitman Mountains Huffington, 1941 12. Southern Quitman Mountains 1940 (measured Scott, section); Jones, 1962 Sierra de las 13. Cieneguillas Powell, 1959; Reaser, 1962 Van Horn Mountains 14• Twiss, 1958 15• Porvenir Bilbrey, Colton, Ferguson, McKinney, Miller, and Schulenberg, 1956 16. Pinto Canyon Amsbury, 1957 17. Southern Rim Rock Country Buongiorno, Carlisle, (Candelaria) Duchin, Mankin, McGrew, Moran, Peterson, Sewell, and Smith, 1954 18. Cuchillo Parado King and Adkins* 1946 Placer de and 19• Guadalupe King Adkins* 1946 20 Sierra de los Fierros Ramirez and o Acevedo* 1957 21. Cajoncito area Bell and Milton* 1962 Figure 2.-Previous work and work in progress western Trans-Pecos , Texas and Mexico. adjacent Chihuahua, PREVIOUS WORK AND WORK IN PROGRESS In 1535* Cabeza de Vaca and his three companions, the first Europeans to enter Trans-Pecos Texas, travelled on foot the east bank of the Rio Grande in the course of up their incredible six-year trek from the Texas Gulf Coast westward to the Pacific coast (DeFord, 1958a, p* 1-2), More than 350 when the still-remote re- years later, gion was barely free of rampaging Apaches and only recently spanned by rails, the first detailed geological studies were made by W* H. von Streeruwitz, geologist of the Trans-Pecos district for the third Texas Geological Survey, the Dumble Survey. Von Streeruwitz travelled widely throughout the region from late 1888 to late 1893. The results of his work (1888, p. 31-44; 1890, p, 219-234; 1891, p. 669-7'38; 1892, contained in the p. 381-389; 1893, p« 141-175) are largely reports of the Texas Geological Survey. The Cretaceous rocks of the region were studied in 1890 by J, A. Taff, a member of von Streeruwitz f s party. Taff f s informative report (1891* 714-738) includes three measured p. sections on the north and east flanks of the Eagle Mountains* one at the old Phinney ranch, one at Carpenter Spring, and one at Eagle Spring. The section between the Eagle Mountains and Finlay station was described in a general way by E, T. Bumble (1895* p-375-388) 0 T. W. Stanton (1905* p» 23-33) recorded a few observations, based on field work in 1897 and the the west flank of the 1899jconcerning rocks on Eagle Mountains. other R. T. Hill (1900) briefly described* among areas* the physiographic features* drainage* climatic features, and population distribution of Trans-Pecos Texas. Near the turn of the century* the U. S. Geological Sur­ vey prepared four topographic quadrangle maps of the region* three of which include a part of the Eagle Mountains and vicinity? (l) Sierra Blanca Quadrangle was surveyed in 1891 and issued in was map 1895 j (2) Chispa Quadrangle surveyed in 1892 and map issued in 1897* but this sheet was re-issued in and Mountains in 1938; (3) Eagle Quadrangle was surveyed 1896 and map issued in 1897* The fourth, the Van Horn Quad- in issued in does not rangle* surveyed 1904-1905* map 1906* include part of the map area. The scale of these any maps is The contour interval is either feet or 100 1?125*000. 50 feet. G. B. Richardson made a reconnaissance north of the Texas Railroad and later and Pacific (1904) geologically mapped the El Paso Quadrangle (1909) to the northwest and the Van Horn Quadrangle (1914) to the northeast of the Eagle Mountains. In B. F. Hill and J. A. 1904 Udden published a general­ ized geological map (scale 1?300*000) of a part of Trans- and Pecos Texas including the Eagle Mountains vicinity. This map was based on reconnaissance field work by W. B. Phillips, B. F. J. A. Udden. Hill, and J. A. Udden, C. L. Baker, and E. Bose (1916) described in a general way the stratigraphy and igneous petrography, historical and economic geology of Trans-Pecos Texas. The accompanying geological map, scale 1:1,500,000, differenti­ ates Upper and Lower Cretaceous rocks within the map area and records the of Carboniferous rocks. Precambrian presence metasedimentary and intrusive rocks, now known to be present, were not shown. In two months of rapid and skillful reconnaissance in 1922, C. L. Baker mapped the region in Trans-Pecos Texas ex­ tending from Finlay station on the northwest to the Chinati Mountains to the southeast. His report (1927) set forth for the first time the of thrust faults in presence major the the region and described in a general way stratigraphy, and of the Baker did not structure, igneous geology region. distinguish between the formations now known as Bluff and Finlay nor between those now known as Yucca and Cox; i.e., he mapped the Yucca and Cox as Cox and the Bluff and Finlay as Finlay. Subsequent papers by Baker (1928, 1930, 1935) elaborate on the structure of the Trans-Pecos region. Four graduate students from Harvard University worked on doctoral problems in the Sierra Blanca region in the middle and late 1930*ss C. C. Albritton, Jr. (1938) in the Malone Mountains! R. M. Huffington (1943) in the northern Quitman Mountains; J. F. Smith, Jr. (1940) in Devil Ridge; and J. D. Boon, Jr. in the Sierra Blanca region. P. B. and J. B. the Sierra King Knight (1944) mapped Diablo P. B. and H. C. Fountain region. King (1944) mapped the southern Guadalupe Mountains and P. B. King, R. E. King, and J. B. Knight (1945) mapped the Hueco Mountains. A later map (King, 1949) includes much of the Devil Ridge area. W. S. Strain and J. S. Luckett (1944) topographically and geologically mapped parts of five sections in the gen­ eral vicinity of the Black Diamond mine in the Indio Moun­ tains. The unpublished map, prepared at the request of shows locations William Rossman, Fredericksburg, Texas, the of the numerous mineral prospect pits. In other work prior to the 1950-1960 decade J. F. Smith, Jr. the G. L. (1941) investigated Eagle Spring area, Evans (1946) and W. E. Dennis (1946a, 1946b) reported on various min eral deposits of the Eagle Mountains, and Elliott Gillerman (1946a, 1946b, and 1948) reported on the fluorspar deposits in the Eagle Mountains. Gillerman f s (1953) excellent final report on these deposits describes the stratigraphy, struc­ ture, and economic geology of the central part of the Eagle Mountain s, E. P. and J. A. Wood Chapman, Jr., (1951)* mining engi- New neers and geologists of Albuquerque, Mexico, prepared a well structure section topographic and geologic map as as a of the area of the Mountains-Part of Rocky Ridge Eagle their map was taken from Gillerman (1946a)® P. T-Flawn (King and Flawn, 1953> p. 45-50) mapped the Precambrian and adjacent rocks in the Eagle Mountains in 1951. That of the area north of 31° N. latitude is part map shown on the quadrangle map; Van Horn Texas, United States; Mexico scale contour interval Chihuahua, (1958), 1;250,000; 200 feet. R. K. DeFord (1958a) described the Tertiary stratigraphy of the Rim Rock and included a detailed historical country summary of exploration in the region. A comprehensive study of the foraminiferal genus Orbitolina has been made R. C. He by Douglas (i960-meas­ ured the Bluff section at Yucca Mesa and nearby localities. Graduate students of The University of Texas who have worked in the map area are; D. L« Bostwick (1953) and J. N. Smith in the northern Indio Mountains; Edwin Allday (1953) and G. B. Adams (1953) in the southern Indio Mountains; D. R. Frantsen (1958) and Philip Braithwaite (1958) in the southern Indio Mountains. Other graduate students of The University of Texas who have worked in adjoining or near-by areas are; Hugh Hay-Roe in the C. Twiss (1957, 1958) Wylie Mountains, Page (1959a, 1959b) in the Van Horn Mountains, A. D. Ferrell (1958) and D. B. Clutterbuck (1958) in the northern Sierra Pilares, L. W. Bridges II (1958) and £• J. Dasch (1959) in the north­ ern Rim Rock area, and J. D. Powell (l96l) in the Sierra de las Cieneguilla area. Graduate students still active in the general area are D. F. Reaser, The University of Texas, in the Sierra de las Cieneguilla area; B. R. Jones, Texas A & M College, in the southern Quitman Mountains; and A. P. Milton and J. J. Bell, The in the University of Texas, area immediately upriver from the southern Quitman Mountains. A number of graduate students of Texas Technological College, Lubbock, Texas, under the direction of J. P. Brand, have worked in the map area and surrounding regions Priddy (1956) made sedimentary analyses of the Cox Sandstone from Cox and the Kent Mount the Finlay Mountains, Mountain, area; a ina (i960) used quartz-grain photometer making petro­ fabric analysis of the Cox Sandstone from measured sections in the Finlay Mountains, Cox Mountain, the southern Quitman Mountains, and the Indio Mountains. Lithostratigraphic and biostratigraphic analysis of the zone of Orbitolina (Bluff Formation) in Trans-Pecos Texas has been the aim of a research project supervised by Brand. Based on measured and sampled sections in the southern Quit- man Mountains, Devil Ridge, and Indio Mountains, and in the Pinto Solitario Canyon, Shafter, and areas, graduate stu­ dents have completed or nearly completed the following proj­ ects; Thomerson (1961) studied the microfauna (exclusive of Orbitolina); Barnette (1961) reported on the carbonate pe- Sutcliffe made a statistical trography; (1961) analysis of the heavy minerals; and Toney is studying the genus Orbit­ olina. William D. Miller, candidate for the Ph.D. degree at of is the University Missouri, Columbia, Missouri, investi­ the and of the Cox Sandstone gating stratigraphy petrography of Trans-Pecos Texas. He has, or will, measure sections in Devil Ridge and in the Eagle and Indio mountains. Two U. S. Geological Survey professional papers are being prepared on areas to the north, and together they over­ lap the map area as far south as lat 31° N. One by J. F. Smith, Jr. and C. C. Albritton, Jr. deals with the Sierra Blanca region; the other by P. B. King is on the Sierra Diablo region. ORIGIN AND SCOPE OF PROJECT The University of Texas Bureau of Economic Geology has had in for progress some years a mineral survey and geologic mapping program in Trans-Pecos Texas. south Following earlier work and east, the program cen­ tered in the Van Horn area. Hay-Roe (1957, 1956) investigated the Wylie Mountains east of Van Horn, and Twiss (1959a, the Van Horn Mountains south of Van Horn 1959b) investigated and west-southwest of the Wylie Mountains. Lying immediately west of the Van Horn Mountains, the Eagle Mountains and its ancillary ranges were the next logi­ cal objective, and I undertook the study of this geologically intriguing area in the summer of 1957* Field work continued during the summer of 195 S and from mid-April to mid-July and early September to late October of 1959* Later, one field trip and several brief collecting trips were made to the area. The primary objectives were to investigate the mineral resources of the area and to produce a geologic map with accompanying text. Arrangements were also made to use this study as a doctoral problem under the supervision of Ronald K. DeFord. ACKNOWLEDGMENTS I would like to express my appreciation to those who gave their assistance in this project. They ares Professor Ronald K. of The DeFord, Department Geology, of as University Texas, Supervising Professor, gave gener­ ously of his time and effort in the field and in editing the manuscript, was an unfailing source of information concern­ ing Trans-Pecos Texas and northern Mexico, and was a constant inspiration to achieve a high level of proficiency in the science of geology. The late Professor John T. Lonsdale, The University of Texas Bureau of Economic Geology, and Professor William R. Muehlberger, Department of Geology, The University of Texas, served as members of the thesis committee and aided me in the field on several occasions. Dr. Lonsdale me invalu gave able the rocks the in thin aid in examining igneous of area section. Dr. Muehlberger offered much helpful criticism con cerning structural interpretation. Dr. Peter T. Flawn, Director of The University of Texas Bureau of Economic Geology, served on the thesis committee in place of the late Dr. John T. Lonsdale. Professor Sylvain J. Pirson, Department of Petroleum Engineering, The University of Texas, served on the thesis committee. The University of Texas Bureau of Economic Geology as sponsor of the project, provided field assistants, field thin and and expenses, vehicles, sections, air photographs to No. gave permission use Geologic Quadrangle Map 24 as plate 1 of this report. Messrs. J. C. Yeager, W. R. Cleaves, S« C. Hamilton, Jr., and D. L. Kirksey were exceptionally capable and will­ ing field assistants. Dr. Page C. Twiss, Department of Geology, Kansas State University, and Professor William S. Strain, Department of me Geology, Texas Western College, guided through their near thesis in the several by areas, joined me Eagle Mountains on occasions, and engaged in stimulating conversation about our mutual geologic problems. Research Dr. Hugh Hay-Roe, Jersey Production Corpora- Dr. J. Dan Continental Oil tion, Tulsa, Oklahoma, Powell, Company, Ponca City, Oklahoma, and Messrs. Donald F* Reaser, Philip Braithwaite, Dan R. Frantzen, Harry A. Vest, E. Robert Atwill IV, George M. Harwell, Jr., Franklin W. Daugherty, Richard M. Campbell, John C. Yeager, Samuel C. Hamilton, Jr., E. Julius Dasch, Luther W. Bridges 11, Don B. Clutterbuck, Alton D. Ferrell, and Bill R. Jones, all either currently or at one time University of Texas graduate stu­ dents and workers in adjoining or near-by areas, freely their shared the knowledge gained by work in respective areas. Professors S, E. Clabaugh and R. L. Folk, Department of Geology, The University of Texas, aided me in examining thin sections of igneous and sedimentary rocks, respectively. Professor K. P. Young, Department of Geology, The Uni­ versity of Texas, identified the invertebrate fossils. Professor John A. Wilson, Department of Geology, The University of Texas, identified the vertebrate fossils and accompanied me to the field for a instructive and very successful search for bones. moderately Mr. James W. Macon, Cartographer, The University of Texas Bureau of Economic Geology, directed the preparation of No. 1 of this and aided Quadrangle Map 24? plate report, greatly in air photograph interpretation. Dr. Derek V. Ager, Imperial College of Science and Technology, University of London, identified the rhycho­ . nelloid brachiopods Dr. C. S. Ross, Peabody Museum, Yale University, Mr. Garner Wilde, Humble Oil and Refining Company, Midland, Texas, and Mr. Donald A. Myers, United States Geological Survey, Denver, Colorado, identified the Permian fusulinids. Dr. Bob F. Perkins, Shell Development Company, Houston, identified the corals sent to him for examination and Texas, and in provided expenses for participated a collecting trip to the Eagle Mountains. Mr, Sam Bishop, Humble Oil and Refining Company, Mid­ land, Texas, visited me in the field and piloted me on a photographic flight over the Eagle Mountains and vicinity* Mr, Philip W* Beckley, consulting geologist, was a help ful source of Information concerning the structure and Cre­ taceous stratigraphy of the Eagle Mountains and vicinity as well as the surrounding region. The following owners, lessors, and foremen of ranches their and much graciously permitted entry on land provided helpful information and pleasant companionship during the three field seasons? Mr. and Mrs. Moody Bennett, Mr. and Mrs. Tommy Bell, Mr. Buddy Neal, Mr. "Blackie" Woods, the late Mr. Bob Mr. W. R. Mr. and Mrs. Dod, McAfee, Bill Wyche, Mr. and Mrs. John D. Bramblett, Mr. and Mrs. Mann Bramblett, Mr. Mr. Mr. W. L. Mrs. Bill Patton, Henry Willbanks, Berry, R. H. and the late Mr. R. H. Mr. Espy "Judge" Espy, Manning Fowlkes, Mr. and Mrs. Jack Hayter, Mr. Mr. Robert Guerra, Mr. and Mrs. J. R. Love, Mr. and Mrs. Bill Mr. and Mrs. Nick Mr. Mrs. Melbarth, Rose, Murray Fashkin, Charles M. Speck and the late Mr. Speck. Standard Oil Company of Texas, Sunray-Mid-Continent Oil Corporation, National Science Foundation, and The University of Texas provided fellowships during the investigation. Most importantly, this project could not have been com­ pleted without the encouragement and assistance of wife, my Margaret Ann; my father, Mr. James R. Underwood; and my mother, the late Mrs. James R. Underwood. Whatever measure of success is represented by this effort is largely the re­ sult of their esteem for and faith in the value of scholar­ . ship FIELD AND LABORATORY PROCEDURE The modus operandi in mapping the Eagle Mountains and vicinity was to re-examine and map the areas reported on by earlier workers, then to extend the mapping into intervening areas hitherto unmapped at such large scale. The reports by Smith (1941), Gillerman (1953), Allday (1953), Adams (1953), and Bostwick (1953) were invaluable. The mapping was done on matte surface contact vertical air photographs, 9 by 9 inches, scale 1523,600, flown in 1950 under U. S. Geological Survey Series LU contract no. Aero Service I-GS-11846 by the Corporation, Philadelphia, Pennsylvania. The photographs are designated GS-LU series. Earlier vertical, stereo-paired air photographs of the Eagle Mountains and vicinity are available. They were flown for the U. S. Geological Survey in 1948 and are designated Series GS-ER. Two series of air were flown the Rio photographs along Grande for the Commodity Stabilization Service of the U. S. Department of Agriculture. Series DTP was flown in February 1955, and Series DWP was flown in April 1960. The photo- cover a narrow graphs only strip along the Rio Grande. measured Stratigraphic sections of dipping strata were with Jacobs staff and Brunton sections of horizontal compass; strata were measured with hand level, Jacobs staff, and 6­ foot steel tape. Sections were designated MS 1, MS 2, and so on . Sixteen sections were measured totalling nearly 18,000 feet, and 926 rock samples were taken from the measured sections. This of gives an average a sample every 19 feet. Miscellaneous rock samples total 551> and collections were made from 355 fossiliferous localities including collections from measured sections and other localities. Fifty thin-sections of pre-Cenozoic sedimentary rocks and 89 thin-sections of Tertiary volcanic and intrusive rocks were examined. The thin-sections were prepared by ¥. Ho von Huene; they and the samples from which they were cut, are on deposit at The University of Texas Bureau of Economic Geology* CARTOGRAPHY Cartographers of The University of Texas Bureau of Economic Geology, under the direction of J. W. Macon, pre­ pared the geologic map and accompanying cross-sections (pi* of the radial . l) Following completion plot, compilation was done at the final map scale of a radial 1:48,000 using plotter connected to a percent-reducing pantograph designed and built by the cartographers of the Bureau. HORIZONTAL AND VERTICAL CONTROL work in Early geodetic the area was done by von in Streeruwitz who, in 1889 preparation for topographic mapping, laid out a 6,400-meter base line along the north side of the Southern Pacific tracks beginning six miles west of the old station of Torbert (von Streeruwitz, 1890, p. Ixxxi)o The location of the monuments marking the east and west ends of this base line, known as East Base and West Base, are shown by King (1949)* In 1890 the U. S. Geological Survey established an astronomical pier at Sierra Blanca (Woodward, 1890, 12), p. laid out a 24,277-foot base line between the pier and Arispe station to the east-southeast along the Texas and Pacific Railroad, and established triangulation stations Taff, Mesa, and Yucca (Woodward, 1890, p. 71-78) that are shown by King The stations (1949)• triangulation were not described; was unable to recover triangulation station shown "Yucca,” by King (1949) to be on Yucca Mesa. In the summer of the same year, the U. S. Geological Survey set two markers ostensibly on the 105th meridian, but their location is error slightly in (Woodward, 1890, p. 71­ Woodward described these markers their 78)o and placement (p. 78)s The meridian stones were set June 16, 1890, by Mr. A. Po Davis with the assistance of Prof. W. von Streeru­witz and Mr. J. A. Taff, of the Texas Geological Sur­and Col. J. R. vey, Marmion, county surveyor of Pre­ sidio County. The stones are about one mile apart in a true north and south line, the northerly one being in latitude of . . . 30°59f51•56lf. They are limestone, dressed to a square cross section at their upper ends, and few inches project a above ground. Von Streeruwitz (1891, p. xcii) described the location of the markers more specifically: . . one near the intersection of the old stage road . to El Paso with the Southern Pacific Railway on the north side of the railway, and one about one mile south of the first one ... Both of these markers were in place in 1961, but they are blocks of rhyolite, not limestone. Today, horizontal and vertical control is based on 48 bench marks and 17 triangulation stations, all within or just outside the area. map Eagle, the first U. S. Coast and Geodetic Survey tri­ angulation station in the map area, was set in 1909-It was destroyed and Eagle 2 set in 1934, the year in which the eleven other U. S. Coast and Geodetic Survey triangulation stations in the area were established. In 1957, the bronze Eagle 2 marker disc was lying loose on the ground and, as requested by the Coast and Geodetic Survey, I returned it to them mail in So far I know this station has not by 1958. as been re-set. The five U. S. stations Geological Survey triangulation were set in 1950. In 1917 a first-order level line, no. 238, was run by the U. S. Coast and Geodetic Survey along the Southern Pacific tracks, and in 1957 it was releveled and additional bench marks set. Level lines were also run by the U. S. Coast and Geodetic Survey and the U. S. Geological Survey in 1950 in the northwestern part of the area. One Coast and Geodetic Survey level line extends from Sierra Blanca along the county road to Indian Hot Springs. their Triangulation stations are listed, together with date of and coordinates in table 1. setting, agency, map Table 1.--Location of triangulation stations. Eagle Mountains and vicinity Triangulation Stations Map Coordinates DEVIL USCGS 1934 F .5 -5.2 JUDGE do 1934 K.3 8.8 — *EAGLE 2 do 1934 L 8 12.2 - o — DALE do 1934 P.8 7.2 GREEN do 1934 T.7 16.0 — BOYKIN do 1934 U. 7 11.5 _ ROCKY USGS 1950 F .8 — 8.3 — NEST do 1950 G 0 10o0 o POINT do 11.2 — 1950 Go? EAGLE SPRING do 1950 H ,2 12.2 - TANK do 1950 H 2 13.1 o- disc had not been re--set as of June 1959. TERMS AND ABBREVIATIONS No new terms have been proposed herein; an explanation of the use of some common ones as well as two uncommon ones follows: The terms "calciclastic" and "siliciclastic" were pro­ posed by Braunstein (1961, p. 2017) for, respectively, "clastic carbonate rocks” and "clastic non-carbonate rocks (which are almost exclusively silicon-bearing either as forms of quartz or as /otheiy7 silicates) ," I have also used the terms "carbonate” to refer to rocks composed of limestone of any origin. The term "high-energy environment" has been used to de­ scribe beach or other shallow-water environment in which the sediments were reworked by current and wave action The term a "low-energy environment" refers to an environment of any depth where wave and current action were negligible• Where the term refers to two "contemporaneous" separate bodies of rock, it means that their time of deposition over- This is in contrast to the term that lapped, "synchronous" implies simultaneous deposition« The term "paleontologically correlative" is used herein to refer to separate bodies of rock that have faunas similar enough so that one may infer that the deposition of the rock was contemporaneous, if not synchronous<> The definitions of the first three of the following terms are those of Young (Harwell, 1959? p» 89)s - aff* related to species of fossil which have affinity with but are not identical with the species named* cfo compare it with the species named; it is - similar but not the same* - cfr* compare it with the species named; it may be the same but it is too poorly preserved for accurate identification* - n. sp. new species* - sp. species (singular)* - spp* species (plural)* The term "MS" refers to measured sections that are tab­ ulated in the appendix and located on plate 1„ The term less "microphenocryst" designates phenocrysts than 1 mm in diameter; the term "microporphyry" refers to a rock with 20 percent or more microphenocrysts. If the rock contains less than 20 percent microphenocrysts, it is termed "microporphyritic <*" Data in the tables of petrography are based on thin- section examination. The right column of the tables is estimated percentage composition; the left is, in orders (l) rock name, (2) thin-section number, (3) location, (4) median grain size, (5) sorting, (6) roundness, and (7) brief description* Grain size distribution of some samples is bimodal; for these, the median grain size of both modes is the within mode* rock listed as is sorting each Igneous terms and names are those of Williams* Turner* and Gilbert terms (1955)* Sedimentary rock and names are largely those of Folk (1956, 1959, 1961). Some thin sections that are described in the tables of petrography are designated according to the strati­ graphic position of the sample within a measured section, for example, MS 2.11. Other thin sections are from miscel­ laneous rock samples collected other than within measured sections. An example of the nomenclature used in desig­ the thin nating these samples and corresponding sections is; DU~KbI~3OO ”D” stands for Devil Ridge? ”E” was used for Eagle Mountains % "I” for Indio Mountains« ”U” signifies collector, Underwoodo ~Kbl,t indicates sample came from Bluff Formation of Cretaceous age o means is rock ”300” sample 300th sample, not counting those from measured sections, collected since of beginning projecto The following abbreviations appear in the tables of petrography 2 - foram foram(s) Md median - Mnft index of refraction - tr trace® - STRATIGRAPHY in the Mountains and Rocks exposed Eagle vicinity range in from Precambrian to Recent. Igneous intrusive and age extrusive rocks as well as basin fill, terrace gravel, allu­ vium, and wind-blown sand constitute the rocks of Tertiary and Quaternary Pre-Cenozoic rocks exposed in the age. map area are of Precambrian, Permian, and Cretaceous age, but the subsurface Pre-Cenozoic stratigraphic record is probably more nearly complete. Lacking well data in the map area, we must of the rocks there gain our knowledge not exposed from regional surface and subsurface geology. PRECAMBRIAN ROCKS King and Flawn (1953) and Flawn (1956, 1962) have de­ scribed the stratigraphy and structure of Precambrian rocks in the Van Horn region. The best of these base- exposures ment rocks in the region are in the foothills of the south­ ern Sierra Diablo and in the Carrizo Mountains. Carrizo Mountain Formation The Carrizo Mountain Formation includes the oldest ex­ posed rock in the and is best about miles region exposed 5 northeast of the Mountains in the Eagle Carrizo Mountains, where the formation consists of more than 19,000 feet of quartz-feldspathic metasedimentary rock with interbedded 39 intrusive argillite, some carbonate rock, and metamorphosed, rhyolite and diorite (King, 1953 > P* 126; Flawn, 1956, p. 33) The Carrizo Mountain Formation is also exposed along the western margin of the Wylie Mountains about 16 miles east, in the northeastern and northwestern parts of the Van Horn Mountains about 8 miles east-southeast, and on the northeast flank of the Eagle Mountains (K-14) • The small exposure of basement rocks in the Eagle Moun­ tains is the southwesternmost exposure on the Van Horn up- Flawn in lift; it was mapped by (1953? P» 45-50, pi. 7) 1950 on a scale of 1,000 feet to the inch using air photographs. Because I could add little to Flawn f s of the Precambrian map rocks of the Eagle Mountains and because his map units are clearly defined in the field and in his report, that part of plate 1 that shows Precambrian rocks is reproduced from his Precambrian map. The following description of the rocks is abstracted from Flawn. In the Eagle Mountains the Carrizo Mountain Formation is of altered rocks about feet a body sedimentary 5?000 thick that includes meta-arkose, metaquartzite, schist, phyllite, and limestone. The metamorphic grade is low, com­ parable to the greenschist facies of Eskola. Retrogressive cataclastic metamorphism superimposed on an earlier progres­ sive metamorphism failed to obliterate many of the sedimen­ tary structures. Flawn mapped five unnamed rock units in the Carrizo Mountain Formation exposed in the Eagle Moun­ tains, and they are, beginning with the lowest and oldest? (l) feldspathic metaquartzite, p€Cq; (2.) meta-arkose, p€Cmaj mixed interbedded (3) unit 1, metaquartzite, phyllite, and sericite schist, p€cml; (4) mixed unit 2, interbedded slate, and intruded phyllite, limestone, p€Cm2. Amphibolite, p€Ca, these units. Foliation is commonly parallel to bedding. Strike N. E. and SE. averages 70° dip, 60°-65° Feldspathic metaquartzite (p€Cq) .--The feldspathic meta- quartzite is the major Precambrian rock unit; it is about 3,200-3,400 feet thick and constitutes 60 percent of the ex­ posed Precambrian section. This fine-grained, hard, brown metaquartzite with interbedded phyllite layers forms rough, It is micaceous and blocky ledges. commonly (sericitic), has a striped appearance owing to dark thin bands of a black metallic mineral. Quartz constitutes 70-80 percent of the rocks, and microcline, albite, and sericite are the other major con­ stituents. Accessory minerals are or magnetite, ilmenite, apatite, zircon, sphene, leucoxene, and rutile. Texture ranges from granoblastic to lepidoblastic. Bedding is well­ sericite preserved; flakes are parallel to bedding. Gross- bedding is also common. Meta-arkose --A of fine- o (p€Cma) 50-foot (maximum) wedge biotite-rich grained, massive, brown, meta-arkose conformably overlies the metaquartzite. Interbedded with the meta­ arkose are thin beds of quartzite and phyllite and thin An sill the layers of amphibolite. amphibolite separates meta-arkose from the overlying metamorphic unit* This meta­ morphosed arkose is composed of poorly sorted grains of quartz (40 percent) and twinned albite (40 percent) in a matrix of quartz, albite, and non-oriented biotite flakes (15 percent). Other minerals present are epidote, chlorite, magnetite or ilmenite, and apatite. An equigranular texture has been partially developed through recrystallization; a relict clastic texture is still visible. Mixed Unit 1 unit 1 consists of alter­ (p€Cml).--Mixed nating 2-5 foot beds of fine-grained, brown metaquartzite, brown-to -gray phyllite, and fine-grained mica schist. The fine-grained schistose metaquartzite contains 50-85 percent and less than micro- quartz, 5-40 percent albite, 5 percent these minerals form mosaic in dine. Grains of a part; they are also surrounded by masses of sericite, chlorite, biotite, and finer crushed quarts. Accessory minerals are sphene, rutile, magnetite or ilmenite, apatite, and tourmaline. This rock has compound texture, for superimposed by recrys­ tallization on the original clastic texture are granoblastic and textures. has re­ lepidoblastic Incipient mylonisation sulted in a mortar texture. The is a the are phyllite microlayered rock; layers defined by grain size and micaceous minerals. This rock of smeared, partly bleached biotite (30 percent), chlorite has a relict (penninite, 20 percent), and quartz (30 percent) porphyroblastic texture with a superimposed diapthoritic or retrograde texture. Elongate quarts augen of relatively coarse grain contain helicitic garnets almost completely altered to chlorite. Minor minerals are almost completely sericitized fibrous and tour- feldspar, epidote, sericite, maline . This mixed unit is incompetent and chevron folds are common, especially in the phyllitic units along the Rhyolite fault. Because of the folding, thickness is difficult to estimate, but 600 feet is a reasonable minimum. The unit is underlain the sill cited earlier and uncon­ by amphibolite fonably overlain by the Hueco Limestone. Mixed Unit 2 (p€Cm2).--Dark slate, dark phyllite, and black limestone compose mixed unit 2, which is separated from the unit just described by the Rhyolite fault. Colluvium from the massive, overlying Hueco Limestone covers much of this unit. The aphanitic, gray-to-black limestone with siliceous laminae occurs with the dark slate and phyllite in beds less than 5 feet thick. The limestone is actually 55“75 percent carbonate and 10-30 percent in a quartz fine-grained matrix. Occurring between the grains and perhaps comprising as much as 5 percent of the rock is a fine-grained black, opaque, probably carbonaceous. Partly bleached, sooty material, rock. oriented biotite flakes comprise 1-10 percent of the Sericite (4 percent) occurs in one sample; other minerals leucoxene. are are magnetite or ilmenite, and Micro-layers defined by quartz or carbon content of the layers or by both . sill-like Amphibolite (p€Ca).--Two major amphibolite masses and a number of smaller amphibolite bodies occur in the Carrizo Mountain Formation of the Eagle Mountains; only the two larger masses were mapped. The larger of the two is 150 feet thick and characteristically massive, although schistose outcrops are present. The amphibolite is green- black with grain size averaging 0.01-1 mm with local coarsen ings. Small prisms of hornblende are visible to the naked . eye The amphibolite is 35-60 percent non-oriented or poorly- oriented and hornblende and 0-10 blue-green grass-green per­ cent red-brown biotite in an oligoclase-andesine matrix com­ prising 10-45 percent of the rock. The plagioclase has partly altered to sericite or to sericite and epidote. Chlorite ranges from 3-10 percent. Minor minerals are epi­ dote, magnetite or ilmenite, apatite, sphene, and rutile. The rock has a compound texture, and the original texture has obliterated hypidiomorphic granular not been by metamorphism, which partly reduced the large grains of horn- needles blende to masses of small and partly recrystallized the plagioclase. Progressive and retrograde metamorphism cannot be distinguished; the amphibolite probably underwent only the later cataclastic metamorphism. Quartz-tourmaline veins .--Broken and crushed lenses and veins of white quartz containing black tourmaline and ilmenite occur throughout the area. Their shattered nature may repre­ sent a pre-or syn-metamorphic period of hydrothermal activ­ ity. PRE-PERMIAN PALEOZOIC ROCKS In the Eagle Mountains, the Carrizo Mountain Formation of Precambrian age is overlain by rocks of Permian age. Dur­ ing much of the Paleozoic Era, however, pre-Permian Paleozoic rocks probably covered the map area. The twofold evidence for this statement is; (l) rocks ranging in age from Ordo­ vician through Pennsylvanian are exposed in the Franklin, Hueco, Sierra Diablo, Beach, and Baylor mountains (Nelson, 1940; King and Knight, 1944; King, 1949; King, 1953) and (2) rocks of these same systems have been Identified from place to place in the subsurface northwest, northeast, and southeast units marked with asterisks do not out crop in the area. map Index to of rocks Table 2. petrographic descriptions Formation Table Page Hueco Limestone 3 Powwow Conglomerate 4 Yucca Formation 5 Bluff Formation 6 Cox Sandstone 7 Finlay Limestone 8 Benevides Formation 9 Espy Limestone 10 Eagle Mountains Sandstone 11 Buda Limestone 12 Chispa Summit Formation 13 Intrusive rocks. Devil Ridge area 14 Lower Mountains rhyolite, Eagle 15 Eagle Mountains 16 Trachyte porphyry, Upper rhyolite, Eagle Mountains 17 Lower rhyolite and late rhyolite dikes and sills. Eagle Mountains IB Mountains Eagle Peak Syenite, Eagle 19 Diabase dikes. Eagle Mountains 20 Trachyte member (Thtr) of the Hogeye Tuff, Indio Mountains 21 Upper tuff member (Thtu) of the Hogeye Indio Mountains 22 Tuff, Pantera Trachyte, Indio Mountains 23 Tuff (Ttu), Indio Mountains 24 Indio Mountains Trachyte (Ttr), 25 Intrusive rocks, northern Indie-Mountain s 26 and of the Eagle Mountains (Nelson, 1940; King Knight, 1945; Snider, 1955; Smith, 1956). Cambrian rocks seem to be ab­ sent . Basal Permian rocks in the Hueco Mountains rest on rocks that range in age from Pennsylvanian to Ordovician (King and Knight, 1945), and in the Sierra Diablo, basal Permian rocks rest on rocks of all the older systems as well as on rocks of Precambrian age (King, 1953, P* 98). On the surface in the Carrizo, Wylie, Van Horn, and Eagle Mountains oldest Permian rocks overlie the Carrizo Mountain Formation with marked angular unconformity. In the subsurface in the southwestern part of the map area or a little farther west, pre-Permian Paleozoic formations may intervene between the basement rock and basal Permian rock. PERMIAN ROCKS Hueco Limestone Erosion of Tertiary and Cretaceous rocks has exposed Permian rocks in relatively small outcrops along the north flank of the Eagles; specifically, (l) in a small outcrop (E-8) northwest of Partition Tank; (2) east of Eagle Spring rounded which of in low, hills, structurally are part a faulted and eroded anticline; (3) in the lower half of Espy Ridge; (4) in the scarp that borders the Precambrian outcrop south and southeast; and (5) in the low, rounded hills south of the Rhyolite fault and just west of the main road from Hot Wells to the Eagle Mountain ranch.. The Hueco Limestone is subdivided into dom­ an upper, inantly carbonate member (Ph) and a basal terrigenous member, Powwor the Conglomerate (Php). Although workers in near-by areas (Hay-Roe, 1957, 1958; Twiss, 1959a, 1959b) considered the Powwow as a formation, I have treated it as a basal mem­ ber the Hueco Limestone because its small in of of outcrop the Eagle Mountains. About 1,060 feet of the upper limestone member (Ph) was measured at MS 14 (K.9~14•8); the base is not exposed and the section crosses a gently dipping thrust fault. Gillen­ man (1953, 14) measured a little more than feet of p. 1,000 Permian rock in the low hills northeast of Eagle Spring and in the area south of the Rhyolite fault. The Powwow Conglom­ at has erate, not everywhere present the base of the Hueco, a minimum thickness of nearly 170 feet at the easternmost Permian exposure (MS ?), where it is faulted at the base and covered at the top. Twiss (1959a, 1959b) reported a combined thickness of about 650 feet of Hueco Limestone and Powwow Conglomerate in the southern Carriso and the northern Van Horn mountains. Hay-Roe (1957) a thickness of reported 1,200 feet of Hueco Limestone in the subsurface on the east- flank of the Wylie Mountains. In the northern Quitman Mountains as well as in the Malone and Finlay mountains, Permian rock no older than Leonard has been recognized (Huffington, 1943, P* 994-995; Albritton, 1936, p. 1756; Albritton and Smith, 1949, p. 1869); it is probably underlain in the subsurface by rock of Wolfcamp age* The Hueco Limestone, with the Powwow Conglomerate at the base, is widely exposed to the north in the Sierra Diablo and southern foothills, where it overlies all older rock with angular unconformity (King, 1953, P* 99)• The thick-bedded Hueco Limestone is clearly visible from place to place just north of U. S. Highway SO between Van Horn and Sierra Blanca. an In the Eagle Mountains, angular unconformity sepa­ rates Permian rocks from the underlying Precambrian rocks; the nature of the upper contact, however, is not everywhere certain*. Yucca rests disconformably on the Hueco at MS 14 and near Eagle Spring, but along the northeast face of Espy Ridge, limestone of the Bluff Formation either rests discon­ formably on the Hueco or is faulted against it so that the Yucca has been eliminated. The how­ pre-Cretaceous surface, ever, may have been so uneven that the Yucca was deposited only in topographically low areas that flanked hills of Permian rock. The Hueco is a medium to medium dark thin- gray gray, bedded, compact, very finely crystalline limestone, which gives off a fetid odor when fractured* There are irregular patches and nodules of brown-weathering chert in the upper half; veins of calcite are common in the lower half* The basal Powwow Conglomerate (Php) is horizontally and At MS it is at least 168 feet vertically heterogeneous* 7 thick and consists ofs light to dark gray, generally finely crystalline, compact limestone; light gray, fine-to coarse- grained quarts sandstone; and light gray calcirudite and calcarenite. Eight feet of pale red siltstone near the base contain subrounded to subangular calcareous nodules. The Powwow at MS 7 is correlative only with the upper part of the Powwow of the Van Horn, Carrizo and Wylie mountains. The lower part of the Powwow, composed of poorly consolidated and poorly sorted angular fragments of the underlying PreCam­ brian rock ranging up to boulder size, is best exposed in small gullies (K.6-13*7) southwest of the Rincon tank and in the reaches (K.2-13.8) of the gully that drains north- upper northeast into Corner tank. In places the Powwow is missing; for example, in the hillside (K.3“14»5) south of the old mines in the Precambrian where the Hueco rests area, directly on the Precambrian rock. In the Eagle Mountains and in the surrounding region, the is Powwow distinguished bys (l) a sharp, angular, uncon­ formable contact with the underlying rock, (2) an upward de­ crease in grain size, (3) a conformable contact with the and (4) wide variation in thick- overlying Hueco Limestone* ness, as it thins and thickens over hills and valleys cut in the underlying rock. About 1,000 feet southeast of MS 7, low on the north­ east flank of the Permian there are beds of lime- outcrop, which resemble stone and chert-pebble conglomerate, parts of the Yearwood Formation of the Van Horn Mountains (Twiss, 196l). These strata, however, may be part of the Powwow Conglomerate lower than that measured at MS 7* Fusulinids collected from the base of the northeast flank (H.4”13‘0) of the low Permian hills northeast of Eagle Spring are not silicified. Ross (1959) identified them as Schwagerina Iaxissima Dunbar and Skinner and Pseudo sch- correlated the wagerina uddeni (Beede and Kniker) and the lower of the Hueco fusulinid-bearing beds part Limestone of the Hueco Mountains. Other fusulinids were collected from the round Permian hills south of the low, Rhyolite fault from beds that are skeptically correlated with non-fusulinid-bearing beds at MS 14 approximately feet above the base of measured section. Wilde (1959) re­ ported s .•. The species present in the samples represent but their Wolfcampian types, general affinities pre­sent somewhat of an enigma. Most of them fall into a which is intermediate between Triticites and group Pseudoschwagerina. To be more specific, they are related to forms such as Pseudofusulina? closely moranensis Thompson and Pseudoschwagerina convexa Triticites be • .* o Thompson s s. also appears to pres­ ent in the samples* This fauna is found in the Wiley and Hueco Mountains and is well developed in New Mexico* It appears to be­long to the upper part of the Wolfcampian. The age of the Permian limestone of the Eagle Mountains is thus almost certainly Wolfcamp* and the formation is cor­ relative, totally or in part* with the Hueco Limestone of the Hueco Mountains* Finding no fossils in the Powwow* I had to infer its Its and the age* stratigraphic position* four species of ostracods and the one species of foramini­ fera reported from near the top of the Powwow in the Wylie Mountains (Hay-Roe* 1957), indicate that the age is Late Pennsylvanian or early Permian* Theoretically there could be about 2*ooo feet of Permian rock exposed in the Eagles if, in fact* the two largest exposures are of totally dif­ ferent Because the fusulinid-bearing beds near Eagle age. Spring* which are correlative with the lower part of the Hueco Limestone of the Hueco Mountains* are low in the sec­ tion because those MS and fusulinid-bearing beds near 14, which are upper Wolfcamp in age* are relatively high in the the rocks in the two sections are in section* probably large part contemporaneous * In addition to fusulinids I collected the following * fossils from rocks of Permian age in the Eagle Mountains? Marginifera sp»* Pictyoclostus sp** and silicified Composlta sp., Euomphalous sp., and echinoid spines and plates. Baker (1927, 10) collected the following fossils from the out- p. crop of Permian rock northeast of Eagle Spring: Derbya sp., Productus sp., Bellerophon sp., Omphalotrochus sp *, Pugnax and sp., Composlta sp., Schwagerina sp., Fusulina elongata. I processed about 30 samples of the Hueco for conodonts, but I found none. The large, coarse, poorly sorted, locally derived rock making up the lowest Powwow suggests an envoronment of was so deposition in which considerable transporting energy suddenly dissipated that little remained for sorting, abrad­ ing, and cleaning the deposits. That some of the Powwow, at was derived from material reworked from the least, regolith which developed on the pre-Permian erosional surface is indi cated by the micrite fragments with septarian fractures that compose the calclithite of unit 8, MS 7 (tbl. 4, p» 56). These are probably caliche balls (Folk, 1962). The upper of the Powwow was deposited in continental-fluvial part a or littoral environment as the sea encroached the conti­ upon nental shelf. There is a gradual transition to the very finely crystalline limestone member of the Hueco, which was probably deposited in a warm, quiet, neritic sea. The posi­ tion of the Diablo platform and the inferred environment (160-180 feet deep) of fusulinids (Dunbar, 1957, p. 753), suggest that the sea was shallow. Table 3. —Petrography of representative samples of Hueco Limestone Fossiliferous MICRITE; U-14«l; micrite 69 MS 14, Unit 1, Eagle Mts; sparry calcite 5 Micrite (Md 0.003mm) with ostracods tr scattered irregular patches algae tr (Md 0.02mm) sparry calcite; calcite spheres 2 calcite spheres up to 1.0mm, some fossil debris 4 with scalloped rims; spheres are pellets tr largely sparry calcite, but may feldspar tr have micrite border; some are pelecypods tr entirely micrite; feldspar frag­ ments angular and less than 0.05mm; fossil debris is sparry calcite; fractures sparry calcite-filled. DISMICRITE; U-14.11; micrite 50 MS 14, Unit 6, Eagle Mts; sparry calcite 43 Irregular patches and grains of ostracods tr sparry calcite intermixed with fossil debris 3 micrite (highly burrowed?); large calcite spheres 2 untwinned fragments sparry calcite opaque minerals 2 (Md 0.5mm) may actually be dolomite; calcite spheres (Md 0.1­ 0.2mm); some with scalloped rims, largely sparry calcite; rock is highly fractured, fractures sparry calcite filled; widely disseminated black, metallic iron oxide. Foraminifera! BI0MIGRITE; U-14.19; micrite 20 MS 14, Unit 7, Eagle Mts; sparry calcite 15 Poorly washed micrite-sparry calcite echinoids tr matrix with abundant organic forarn 65 bodies, probably largely forarn but opaque minerals tr perhaps partially algae; some may be clasts; organic bodies vary widely in shape and size (Md 0.15mrn); gradation between micrite and sparry calcite grain size; sparry calcite ranges up to 0.3mm; fills fractures. Table 3. —Continued BIOMICRITE; U-14.27; micrite 63 MS 14, Unit 8, Eagle Mts; sparry calcite 2 Micrite matrix encloses abundant ostracods 2 unidentified fossil debris pellets 3 (Md 0.065mra); some burrowed zones; foram tr pellets range up to 0.13mm fossil debris 30 (Md 0,05mm); fractures filled with opaque minerals tr sparry calcite. Pelecypod BIOSPARITE; U-14.39; sparry calcite 49 MS 14, Unit 9, Eagle Mts; micrite 35 Matrix is intimate mixture of echinoids tr sparry calcite (Md 0.0185mm) pelecypods 10 microsparry calcite, and micrite; algae tr pellets (Md 0.065mm). foram 1 ostracods tr pellets 5 Table 4*—Petrography of representative samples of Powwow Conglomerate Dolomitized INTRAMICRITE; U-7.4* micrite 30 MS 7, Unit 4, Eagle Mts; sparry calcite 3 Dolomitized micrite matrix; dolomite 35 dolomite rhombs Md 0,07mm; round. intraclasts 25 poorly sorted micrite intraclasts quartz 2 (Md 0.65); fracture-filling feldspar tr sparry calcite; cavity-filling opaque minerals 1 opal; angular quartz and feldspar opal 4 fragments (Md 0.045mm) in intraclasts plus micrite and dolomite; borders of some intraclasts show dolomite replace­ ment. Granular very course sandstone; micrite tr calcitic and dolomitic supemature sparry calcite 14 CALCLITHITE; U-7.18; dolomite 5 MS 7, Unit 8, Eagle Mts; quartz 1 Micrite intraclasts (caliche balls?) intraclasts 77 have sparry calcite-filled septarian opaque minerals 3 fractures; cement is sparry calcite, replacement dolomite with a little micrite; scattered angular quartz grains up to 0.8mm (Md 0.05mm); they are in cement as well as intraclasts; many intraclasts stained with orange iron oxide; some dog-tooth sparry calcite cement; some intraclasts dolomitized along borders; dolomite rhombs, Md 0.0015mm. **Post-Hueco Rocks Permian rocks younger than Wolfcamp have not been iden­ tified in the map area, but presumably a thick Permian sec­ tion was removed by pre-Cretaceous erosion. It is probable that rocks of Leonard age are still present in the subsur­ face in the southwest part of the map area (King and Flawn, 1953, pi. 19). Twiss referred about In the Van Horn Mountains, (1959a) 240 feet of unfossiliferous sandstone and limestone to the Leonard, based on lithologic correlation with similar but fossiliferous rocks of known Leonard age in the Wylie Moun­ tains, where Hay-Roe (1957) reported about 1,900 feet of limestone, marl, and dolomite of Leonard and Guadalupe age. In the Malone Mountains to the northwest, the Briggs Formation of Leonard consists of "a minimum of 630 feet age of and with interbedded anhydrite gypsum /fossiliferoujsy7 limestone,” which was probably deposited by "recurrent par­ tial evaporation of lagoonal waters cut off from /th£J open sea" Ho fossils known (Albritton, 1936, p. 1733> 1757). are from the upper 500 feet of this formation; this part may be younger than Leonard (p. 1756). An estimated 200 feet of black limestone that has been correlated with the of Briggs *-*Stratigraphic units marked with asterisks do not crop out in the area. map the Malone Mountains out in the northern Quitman Moun­ crops tains (Huffington, 1943, P* 994-995)* The thickness of the shelf limestone deposited in the map area during Leonard and Guadalupe time and since eroded, was probably about the same as that deposited and still in place behind the reefs that are exposed to the north in the Sierra Diablo and Guadalupe Mountains. This thickness is about 4,000 feet (King, 1948, pi. 7). It is doubtful that Ochoa rocks ever existed in the area. map **PRE-APTIAN MESOZOIC ROCKS Marine Triassic is not known in Trans-Pecos Texas, but continental of the Dockum of Late Triassic deposits Group out in western Reeves County about 100 miles east-age crop northeast of the Eagle Mountains, According to McKee and showed the others (1959, pi. 9) regional extrapolation that depositional margin of the Dockum Group passed through the northern part of the map area. Other than this extrapola­ tion of the contour representing zero thickness, there is no evidence that rock of Triassic ever existed in the age map area. Marine rocks of Early or Middle Jurassic have not age been identified in Trans-Pecos Texas or Chihuahua, but the *-»-Stratigraphic units marked with asterisks do not crop out in the map area. age of the conglomerate, sandstone, shale, siltstone, and limestone of the Malone Formation in the Malone and northern Quitman Mountains is Late Jurassic. The Malone is separated from underlying Permian of Leonard age by an angular uncon­ formity. Neocomian rock in the Malone and northern Quitman moun­ tains rest conformably on the rocks of Upper Jurassic (Kim­ meridgian) age (Albritton, 1938, p. 1754, 1765; Huffington, 997). No rock of Neocomian is known to be ex­ 1943, p. age posed in the map area, but the thick, basal Cretaceous unit, the Yucca Formation, is largely unfossiliferous. The age of the uppermost part of the exposed Yucca is Late Aptian; the age of the lowest part of the exposed Yucca may be Neocomian. it is certain Although reasonably that no Jurassic exists the northeast flank of the Jurassic and along Eagles, Neocomian rock and a thicker and more nearly complete Paleo­ zoic section may lie at considerable depth in the subsurface in the southwest part of the map area or a little farther west . CRETACEOUS ROCKS Marine Cretaceous which about feet rocks, average 7,000 in thickness and range in age from Late Aptian to perhaps as young as Late Turonian, were deposited along the eastern margin of the Chihuahua trough; they constitute most of the pre-Cenozolc rocks exposed in the Eagle Mountains and vicin­ ity. The Comanche and Gulf series have been recognized, and together they comprise nine lithologically distinct forma­ tions. An unconformity separates the Comanche and Gulf series in many places in Trans-Pecos Texas and elsewhere, but it was not observed in the area. The series bound- map ary, which coincides with the Chispa Summit Buda contact, - is generally covered, however. Within the Cretaceous section, I saw evidence only for two short periods of erosion; one at the Cox-Finlay contact and the other within the sandstone member of the Benevides Formation. All other formation contacts within the Creta­ ceous are conformable. The lower boundaries of the Creta­ ceous are placed not at the base of the first bed composed of rock typical of the formation but at that stratigraphic level above which little of the of rock that type composes the underlying formation is present. The Cretaceous section shows cyclic lithologic al­ ternation of carbonate and siliciclastic rock. Within the rocks of the Comanche Series four such cycles are repre­ sented, each of which consists of siliciclastic rock over­ lain carbonate rock. These by lithologic couplets reflect which Cretaceous cyclic deposition, was widespread during time; each unit of a couplet was mapped as a formation. The boundaries of the couplets are distinct but the siliciclastic-carbonate boundary within a couplet is grada­ tional and somewhat indistinct; e.g., the boundary between the Yucca and the Bluff formations and that between the Cox and the Finlay formations. The sharp boundary between a carbonate formation and an overlying siliciclastic formation indicates a rapid regression of the sea wherein the base of the siliciclastic rock was a time horizon. There be an may each siliciclastic if the unconformity within unit; so, hiatus should increase shoreward (Grabau, 1913, P» 736-73#). A transitional or gradational boundary between a silici clastic unit and an overlying carbonate unit may represent the ever-slower erosion of a rapidly uplifted source area wherein the base of the carbonate unit, representing a slowly transgressive sea, would be discordant with the time surface (Braithwaite, 195#, P» 12). The significance of the widespread cyclic sedimentation is not certain. The cyclic deposits may be a record of a series of relatively slight transgressive and regressive movements of the sea that accompanied eustatic change in sea level. These fluctuations in sea level could also have orig inated through regional elevation and subsidence of the plat form. Periodic local tectonic activity and subsequent ero­ sion in source could have the the area produced alternating rock types with little accompanying change in sea level. Climate, drainage patterns, longshore currents, and the chemical state of the itself other fac­ physical and sea are tors that could have affected deposition. The over-all up­ ward increase of carbonate in the Comanche section indicates increasing stability of conditions through early Cretaceous time . In the of the advance of the Cretaceous onto course sea the platform and foreland, the Comanche formations overlapped each other and overstepped older formations; thus the basal siliciclastic or marginal facies is younger to the north and east. The of the basal siliciclastic facies of the Cre­ age taceous in the and is System Eagle Mountains vicinity Aptian; at the Cornudas Mountains about 50 miles north of Sierra Blanca, the is Late Albian. age The discussion of the formations of the Cretaceous Sys­ tem follows this format: (l) occurrence and thickness, (2) lithology, including fauna, (3) age and correlation, and (4) origin. Yucca Formation The Yucca Formation includes the oldest Cretaceous rock exposed in the map area; its outcrops are the most wide­ spread, and its red-brown color clearly makes it the most distinctive unit. map lithology,t and fossils p* 725) Thickness, lithology fossilsTaff (1891, named the Yucca Formation for its excellent the on exposures Figure 3 CHIHUAHUA FORMATIONS ADJACENT 4. CRETACEOUS AND Figure OF TEXAS PECOS CORRELATION TRANS­ north face of Yucca Mesa (C-l, 2) at the northwest end of Devil about feet of the formation is Ridge; 1,200 exposed there. The Yucca is sparsely exposed to the southeast along the northeast face of Front Ridge but is well exposed along the northeast face of Love Hogback (G-6, 7) where the Yucca is protected by the thick-bedded limestone of the conform- ably overlying Bluff Formation. Northeast of Devil Ridge a isolated of Yucca lies Par- small outcrop (E-8) just west of tition tank. The Yucca is widely exposed south and east of the Speck (formerly Love) ranch house, where it has been southwest intensely faulted, and it crops out along the flank of Back Ridge, where it has been thrust over the Cox and Finlay formations. Red Hills (H-5> 6) are composed en- which also out in small erosional rem­ tirely of Yucca, crops nants to the southeast. Smith (1940, p. 605) measured 1,567 feet of Yucca at Red Hills, but because his traverse crossed so many small faults the figure is questionable. Thin beds of and red to gray, brown, aphanocrystalline very finely crystalline limestone and sandy limestone and fine-grained quartz sandstone, equally colorful shale and quartz siltstone, and rather drab limestone-pebble conglom­ erate the Yucca at Yucca Mesa. Limestone, either compose as an orthochemical precipitate, as the gravel fraction of a conglomerate, or as the clasts of calclithites, constitutes more than half of the Yucca at Yucca Mesa and predominates Photograph 1. Southward view of Yucca Mesa from 8.5-I*9; thick-bedded, gray limestone of Bluff Formation overlies red, brown, and orange conglom erate, shale, and limestone of Yucca Formation; offset of strata that resulted from normal fault­ing clearly visible; trace of Devil Ridge thrust fault hidden beneath alluvium, near foreground. in the lower the at half of formation. Sandstone, places indistinctly cross-bedded and generally spotted with iron oxide, and shale are more abundant in the upper part than in the lower of the formation. Pisolites to an inch in part up diameter (Md 10 mm) occur about 600 feet above the base in dark limestone. gray Although the Yucca is largely unfossiliferous, there is a hash of pelecypods and high-spired gastropods in a 6-foot zone about 100 feet below the top. Area Ostrea sp., sp., and caprinids that were reported from the Yucca by Adkins (1933, P* 296) are probably the fossils reported by Taff (1891, p. 726) from the "second caprotina horizon," a zone which both Smith and I have included in the overlying Bluff. Taff did report the occurrence of Exogyra resembling Exogyra texana Roemer about feet below the 30 "Caprotina horizons." Smith (1940, p. 628) recorded ostracods at about this same stratigraphic position at his measured section 3» The insignificant quantity of chert pebbles and almost total lack of quartz pebbles in the Yucca at Yucca outcrops Mesa, Front Ridge, and Love Hogback as contrasted to the relative abundance of this material in the Yucca in the Red Hills and Back Ridge, led Smith (1940, p. 606-609) to sug­ gest the existence of a source area to the southwest as well as another to the north and northeast. As envisioned by Smith, the southwestern of the source chert and quartz pebbles in the Yucca of Red Hills and Back Ridge was a peninsula extending northwestward from the main­ land so as to create an embayment 24 miles or more wide and open to the sea on the northwest. The geometry of the embay­ ment and its location were based on the distribution of the known outcrops of the Yucca after palinspastic adjustment. The most straightforward explanation for the distribu­ tion of the chert and quartz pebbles is that the Yucca ex­ posed in Red Hills and Back Ridge is older than that to the north and east and is simply not exposed there. Smith discounted this (1940, p. 607) possibility: All the Yucca formation in the Red Hills cannot be older than that to the north because the contact upper of the Yucca is about a half mile east of the Red Hills and not many feet higher stratigraphically than that in the Red Hills . The closest Yucca-Bluff contact to the Red Hills is more than a mile to the east; in between are alluvial-covered areas that could mask faults of considerable displacement. Furthermore, in the Indio Mountains where several thousand feet of Yucca are exposed but highly faulted, the lower part of the Yucca abounds in and chert pebble quartz conglomerate; the upper part differs from the Yucca at Yucca Mesa mainly in that there is less limestone in the Indio outcrops. Huffington (1943> p. 1013-1016) measured more than 5,600 feet of Yucca at Quitman Gap, only 5-6 miles southwest of Yucca Mesa. His section showed that the first limestone conglomerate is 1,052 feet below the top of the Yucca and that there is a series of quartz-pebble conglomerate beds 2,429-2,846 feet below the top of the Yucca. There is thus to believe the beds of good reason that quartz-pebble and chert-pebble conglomerate exposed in Back Ridge and Red Hills are stratigraphically lower than the Yucca rocks exposed at Yucca Mesa and along Front Ridge and Love Hogback. I see no need to postulate a southwestern source for any part of the Yucca exposed in the map area. Furthermore, Smith T s (1940, fig. 5, p. 607) palin­ spastic map of part of Trans-Pecos Texas, which includes the present site of the Eagle Mountains and vicinity and the northern part of the Sierra Pilares, shows these areas to be part of the foreland adjacent to Sierra Blanca Bay. Smith, presumably, did not know of the thick section of quartz and chert pebble conglomerate, conglomeratic sandstone, sand- and shale which the Yucca of the Indio Moun­ stone, compose tains as well as the strata correlative with the Yucca in the ranges in northeast Chihuahua. The Etholen Conglomerate, composed of chert pebbles and cobbles and limestone pebbles, cobbles, and was boulders, for its named by Taff (1691, p. 723-724) exposures at Etho­ len which are about miles west of Sierra Blanca. Knobs, 4 In agreement with Baker (1927, p. IB), I suggest that some of the limestone and chert conglomerate exposed along the east of the ranch house be correlative with scarp Speck may the Etholen. The Etholen was considered by Taff, as well as Baker and Adkins to lie at the (1927, p. 15) (1933, p-285, 292), base of the Cretaceous in the Sierra Blanca region; Buffing- considered the Etholen to ton (1943, p. 992, 1008), however, be the uppermost part of his Washita Group. that in the Etholen I agree with Buffington vicinity of Knobs the Etholen overlies beds of his Washita Group, but I that these beds in and that and suggest are Gulfian age they the Etholen are in thrust contact. Bazzard (i960; West Texas Geological Society, 1950, p. 14) found Permian limestone interbedded with Etholen Con- on the westernmost of the Etholen Knobs. This glomerate limestone yielded Waagenoceras sp., an ammonoid of Guadalupe Be also found Parafusulina a common of the age. sp., genus Leonard, in the matrix as well as in the clasts of the con­ glomerate; in addition, the matrix yielded Dictyoclostus sp. with the spines intact. These fossils were not reworked. Because he found Permian fossils in the matrix as well as in limestone interbeds of the conglomerate, Bazzard considered the of the Etholen to be Permian and the formations to age be in thrust contact with the underlying Gulfian sandstone, which, he suggested, is correlative with the sand below zone at tbe "zone of Collopoceras" of Adkins (1933, p* 437) Chi Summit. spa Yucca the Mountains are few. It Exposures of in Eagle crops out in the vicinity of Eagle Spring and just south of the Precambrian outcrop (K-13, 14) where the Yucca discon­ formably overlies the Permian. There are no other exposures The Yucca beds in both areas contain more siliceous conglomerate and less limestone than the Yucca at Yucca Mesa, and the formation is much thinner. Gillerman (1953, p* 15) estimated the thickness of the Yucca to near Eagle Spring be 330 feet; Smith (1941, pi. 1) and I have shown about twice that much, because we included a colorful sequence of and con- sandstone, shale, sandy limestone, limestone-pebble glomerate that Gillerman considered to be a basal part of the Bluff. The Yucca-Hueco contact is at the surface at only two localities in the map area; near Eagle Spring (H-11, 12) and at the easternmost outcrop of Hueco Limestone (K, L-14). At the easternmost exposure the contact is covered, but the basal Cretaceous rock is a medium gray, conglomeratic, coarse-to very fine-grained, poorly sorted calclithite; the gravel fraction is composed of rounded granules and peb­ bles largely of limestone but also of white vein quartz and black chert. The Yucca-Hueco contact here as well as at Eagle Spring is disconformable. From the Rhyolite fault north to the end of Espy Ridge, rocks of the Bluff Formation rest on Hueco Limestone. The somewhat anomalous absence of the Yucca may reflect nondepo­ sition over a high,or faulting. Because the contact between the two formations is so even and because the Yucca is pres­ ent between the Bluff and Hueco formations immediately south of the Rhyolite fault, I believe that the presence of a fault between the Bluff and Hueco formations north of the Rhyolite fault is likely. This fault is not shown on plate 1, how­ ever . The Indio Mountains are composed largely of the Yucca Formation. Stratigraphic and structural relationships are complicated, because the Yucca has been intimately involved in folding, thrust faulting, and normal faulting. Unfortu­ nately, nowhere in the Indio Mountains is there a complete exposure of the Yucca Formation, although more of it is ex­ posed there than in other of the area. The any part map conformable and transitional contact with the overlying Bluff is visible in many parts of the area, but the lower part of the formation is everywhere truncated by a fault. Although several thousand feet of Yucca is probably exposed in the Indies, the maximum thickness that has been measured there is the 2,021 feet that was recorded at MS 9 (5.7-14-7) . There, as throughout the Indies, the Yucca may be grossly subdivided into two unitss a lower unit composed of sandstone, a conglomeratic sandstone, and conglomerate, and an upper unit composed of sandstone, siltstone, shale, and limestone. The prevailing red-brown color is still the most distinctive characteristic. The sandstone is largely thin-bedded, fine-to medium- grained, moderately to well sorted grains of quartz cemented by calcite; intergranular iron oxide is abundant. The gravel of the Yucca is largely rounded to subrounded granules and pebbles of gray limestone, pale red and black chert, and white and pink quartz. The shale and siltstone of the upper part of the Yucca characteristically contain rounded calca­ reous nodules which weather green and accumulate on slopes. The first bed of limestone at MS 9 is slightly more than 200 feet below the next higher one, the base of which was chosen as the Yucca-Bluff contact; this second limestone is 13 feet thick, sandy and very finely crystalline, and contains Exogyra quitmanensis Cragin, colonial corals and other fos­ sils. Because the Yucca-Bluff boundary probably is not everywhere at the horizon and because the zone of transi same tion from uppermost Yucca to the lowermost Bluff is very fos­ siliferous, I list the fossils that I collected from this zone in the Indio Mountains and do not assign them to the Yucca or to the Bluff. These fossils ares Actinastrea whitneyi (Wells), Actinastrea n A, Act inastrea n. sp« B, Actinastrea n * . sp. sp. C, Montastrea whitneyi (Wells), Montastrea n. sp., Complexastrea? glenrosensis Wells, Isastrea whitneyi Wells, Microsolena texana Wells, Helioporidae sp., (new genus, n. sp.; may be related to Polytnemacis? hancockensis Wells), Lamellaerhynchia indi Ager, Cyprimeria sp*, Protocardia sp., Unio ~ Cardium Lima Arctica 0strea sp sp., sp., sp., sp., Trigonia mearnsi Stoyanow, Trigonia stolleyi Hill, Trlgonia sp., Pecten (Neithea) sp., Homomya sp®, Anatina sp., Crassaltelites? sp., Corbula sp., Aptyxiella sp., Exogyra Nerinoides sp., Exogyra quitmanensis Cragin, Toucasia sp., roemeri M® Whitney, Nerinoides sp., Nerinea sp®, Natica sp«, Tylostoma sp., Turritella sp., noded gastropod, nautiloid, Acanthohoplites? sp., Turrilites sp., silicified wood. Correlation and age the northern Sierra Pilares, age.—ln Ferrell collected the fossils from (1958* P* 27) following beds that are probably laterally continuous with the upper part of the Yucca-Bluff transition zones Parahoplites cfr. ?, comalensis (Scott), Exogyra cfr. E* quitmanensis Cragin, and Trigonia stolleyl Hill. The Yucca of Smith, Huffington, and Underwood is equivalent to the red sandy shale, impure limestone, and limestone pebble conglomerate of the "Las Vigas" of the Malone Mountains (Albritton, 1938* p® 1754)® The Yucca is not exposed at Triple Hill; the Yucca and per­ haps the Bluff have been overstepped by the Cox somewhere south of Cox Mountain, where no Yucca is present and the Cox rock which either be Permian rests on may or Campagrande (a Bluff equivalent )o The Yucca is in the Van Horn Mountains area present at the south end where Twiss feet only (1959a) reported 250 of unfossiliferous, siliceous, and friable sandstone and lenses of quartz-and chert-pebble conglomerate. The Yucca is unrecorded farther out in or west of east; it wedges just the Van Horn Mountains. At Pinto Canyon about 50 miles southeast of Eagle Peak, the limestone-cobble conglomerate and red limestone, sand­ stone, and shale of the Yucca Formation rest on cherty, dolomitic siltstone of Permian age® The maximum thickness of the Yucca is more than 650 feet (Amsbury, 1958)® In northeast between the latitude of Pinto Chihuahua, Canyon and that of the Indio Mountains, the thickness of rocks beneath beds to the Bluff of the equivalent Formation Eagle Mountains and vicinity ranges up to 4*519 feet (base These rocks resemble the Yucca of the Indio unexposed)• Mountains. In the southern Quitman Mountains near the Rio Grande Scott (1940, pi. 55) recorded about 5*300 feet of rock be­ neath beds that he identified as ”Glen Rose.” Because of the suspected presence of unrecognized structure, there has long been doubt about the true thickness of Scott’s measured section, but Jones (1962) has reported that Scott 1 s pub­ lished section is accurate to the base of the "Glen Rose." up Correlating the section in the southern Quitmans with a sim­ ilar section near Cuchillo Parade, Chihuahua, described by Burrows Scott the name "Las to about (1909), applied Vigas" 2,300 feet of gray to white coarse-grained sandstone which is overlain by interbedded red to brown sandstone and red shale; he applied the name "Cuchillo" to the overlying sec­ tion of about 3,000 feet of interbedded limestone, shale, and sandstone. Beds of conglomerate are conspicuously absent, although there are a few beds of conglomeratic sandstone (Reaser, 1962), In his Cuchillo in the southern Quitmans, Scott recog­ nized three ammonite zones! (l) zone of Dufrenoya at the base, (2) zone of Sonneratia about midway of the formation, (3) zone of Douvilleiceras near the top. He considered the top of the Dufrenoya zone to be the Aptian-Albian boundary; within this same zone he also collected Acanthohoplites. Two fragmental and badly worn specimens of Acanthohop­ lites ? from the Yucca-Bluff transition zone in the Indio Mountains suggest that the age of the zone of transition is probably Late Aptian-Early Albian. Assuming that Dufrenoya and Acanthohoplites occupy roughly the same zone, the col­ lection of Acanthohoplites? and Exogyra quitmanensis from the transition zone in the Indio Mountains means that there the zone of Dufrenoya is only about 500 feet below the zone of Qrbitolina * In the southern Quitman Mountains the zone of Dufrenoya is 1,350 feet below the zone of Exogyra quit­ manensis and about 2,050 feet below the zone of Qrbitolina. In the Sierra Pilares, Harwell (1958, p. 15-19) and Yeager (i960, p. 28-35) collected Dufrenoya and Exogyra quitmanensis from the same zone. Yeager found Douvilleiceras only about 90 feet above Dufrenoya, whereas in the southern Quitmans these zones are separated by about 2,280 feet of rock. Because the Indio and Sierra Pilares sections are closer to the platform than the section in the southern Quit- it is reasonable to assume that the is mans, convergence evidence of different rates of subsidence and deposition in the two of erosion of both areas or subsequent or (Harwell, 1959, P-18). The age of the Yucca in Devil Ridge and in the Eagle Mountains must rest mainly on the well-documented f,Glen Rose” or Early Albian age of the overlying Bluff Formation. By inference, the of the part of the Yucca is Late age upper Aptian-Early Albian. OriginOrigin.--The source of the Yucca Formation, especially of the siliciclastic and calciclastic conglomerate, is an that interesting problem was considered by Yeager (i960, p. 56-66) . He pointed out that pre-Cretaceous erosion in the Sierra Diablo and Carrizo Mountains not only removed a cover of the late Paleozoic rocks but also exposed Van Horn, Hazel, and Allamoore formations and perhaps the Carrizo Mountain Formation as well. These local possible source rocks con­ tain all of the varied material that the Yucca composes Formation and stratigraphically equivalent units in the general region. also the Davis Moun- Yeager (i960, p. 58) postulated tains area, the Marathon uplift, and a conjectured island in the Neocomian sea as possible sources for material of the Yucca and equivalent rock. DeFord (Yeager, 1960, p. the erosional of 63) suggested that pre-Cretaceous surface the Wichita of R. T. Hill Texas, Paleoplain (1901, p. 363­ 367), may have been covered with terrigenous material that was eroded other the late Paleozoic from, among sources, mountains of the Ouachita-Marathon trend. Some of this material must have been swept into the Chihuahua trough the during Early Cretaceous Epoch. In the Indio Mountains much of the sandstone of the Yucca may be classified as siliceous, bimodal, immature to submature, chert-bearing subgraywacke. Common and vein quartz predominate, and chert composes as much as 25 per­ cent of the rock. Because it has more than 5 percent of metamorphic-quartz and metamorphic-rock fragments in the terrigenous fraction, it may be classified as subgraywacke rather than orthoquartzite. The iron oxide that coats most of the grains predates the authigenic quartz overgrowths. Accessory minerals are euhedral apatite and rounded to sub- rounded zircon and tourmaline. and The metamorphic quartz metamorphic-rock fragments indicate that metamorphic rocks of Precambrian age in the Van Horn contributed to the sediments of the Yucca. region That the sedimentary cover of the Precambrian basement was being stripped off and its debris added to the sediment is indicated the of chert the forma­ by abundance throughout tion. Anomalous size-roundness relations as well as the bimodality of the grain size indicate multiple sources of the sediment. The subrounded to rounded tourmaline and zircon suggest that these minerals have undergone more than one cycle of sedimentation. The red-brown hue of the Yucca, easily its most dis­ tinctive characteristic, stems not from the color of the larger constituent particles but from the color of red iron oxide that coats the grains and stains the clay. The origin of redbeds is still controversial; as is true with so natural evidence indicates many phenomena, that there is no one set of conditions or series of steps under and by which they form. At least it is now certain that redbeds do not necessarily originate in an arid environ­ ment . The coating of the grains of the Yucca Formation by- iron oxide must have occurred after deposition; abrasion during transport and attendant rounding would surely have removed coating inherited from the source area. Where the grains are coated with iron oxide, it always separates the grain from the authigenic quartz overgrowths. Because the quartz overgrowths show no evidence of having been reworked, the iron oxide as well as the siliceous must be overgrowths post-depositional. The terrestrial origin of many redbeds is indicated by their abundant plant remains, mud cracks, raindrop impres­ sions, freshwater mollusks, and footprints of terrestrial vertebrates (Pettijohn, 1949, P» 173)-Of the above, the Yucca contains only a small amount of silicified wood. Al- some of the Yucca be because of the though may terrestrial, absence of any positive evidence for terrestrial origin, because of well-developed bedding, and because of the pres­ ence of beds of marine limestone, the Yucca is interpreted as a near-shore marine deposit. Some of the minute particles of iron oxide may have been inherited from such red source rocks as the Van Horn and Hazel formations and the rocks of the Dockum Group. The regolith that developed on the pre-Cretaceous erosional surface was no doubt reworked by the sea as it advanced onto the platform, and much of the reworked material must have been incorporated in the Yucca. The material that formed the Yucca actually may have been deposited in a re­ but because ducing marine environment, deposition was so all the ferric iron was not reduced. Preserva rapid perhaps tion of the iron oxide in the ferric oxidation state would have been enhanced by the lack of decaying organic remains. The Yucca, however, may have been deposited in an oxidizing marine environment. It seems likely that much of the red iron oxide may have been inherited from a regolith developed in a warm alternately wet and dry climate, in which abundant ferric oxide was formed and much of the organic matter elim­ inated. In summary, the Yucca is a transgressive, largely near- shore deposit that was laid down during the advance of the Cretaceous sea north and east onto the Diablo platform. The and advance was sufficiently rapid that prolonged sorting rounding of the sediments along the shore could not occur. The varied composition reflects the fluctuation of the advancing sea as well, perhaps, as mild fluctuating tectonic activity in the source area. Longshore currents as well as waves distributed and slightly reworked the material. The depositional environment was not conducive to animal life; water it was probably an oxidizing environment, and the must often have been choked with terrigenous debris. of the Yucca of Table 5 Petrography representative samples 0 Formation siliceous bimodal Fine sandstone; supermature chert-bearing SUBGRAYWAGKE; IU-Ky-71; Summit of Red Mountain (V.5~14®7); well 0.4mm, 0.15mm, well sorted, sorted; subround; Quartz largely common; 75$ quartz grains have overgrowths; large quartz grains chert both round and angular; grains some calcite round to subangular; iron oxide coats almost all cement; grains. siliceous Fine to medium sandstone; and calcitic submature chert-bearing SUBGRAYWACKE; IU-Ky-72; Red Just west of Borrega fault, Mountain (V.5-14®8); 0.25mm, moderately to sorted; subangular subround; Common quartz dominant; overgrowths on 80$ of quartz grains; large quartz from angular to round; grains range to some chert grains round subround; calcite cement; almost all grains coated by iron oxide that preceded authigenic quartz; because of parallel stress zones, several grains quartz resemble albite-twinned plagioclase. Clayey fine and very fine sandstone; weakly bimodal immature chert-bearing SUBGRAYACKE; IU-Tb-80; that Beneath rhyolite, along ridge leads southeast from Flat Top, about road 2,000 feet south of point where cuts through ridge (V.3-16»2); 0.13mm, 0.065mm, well sorted, moderately sorted; subangular; Dominantly common quartz; elongate quartz common; no overgrowths; clay is kaolin; larger mode angular-round; silt largely chert grains subangular; angular; large round. tourmaline and zircon detrital quartz chert metamorphic quartz metamorphic fragments authigenic calcite clay rock quartz minerals opaque apatite tourmaline sericite detrital quartz chert authigenic quartz metamorphic quartz metamorphic rock fragments clay calcite minerals opaque apatite tourmaline zircon plagioclase detrital quartz chert metamorphic quartz metamorphic rock fragments calcite clay minerals opaque tourmaline zircon plagioclase 65 7 3 6/ 5 2 2 10 tr tr tr 64 8 3 5 5 2 3 10 tr tr tr tr 68 8 4 2 4 10 4 tr tr tr Table 5 0 —Continued Clayey fine sandstone; chloritic detrital immature chert-bearing SUBGRAYWACKE; quartz 60 IU-Ky-84# chert 12 Just beneath summit, Evans Peak (Vo7“-15»9)# authigenic quartz 1 0.13mm, 0o06mm, well sorted, moderately metamorphic quartz 4 sorted; subangular; metamorphic rock Largely common quartz; 15$ quartz grains fragments 3 elongate; overgrowths on 20% quartz clay (including grains; graded bedding; laminations and chlorite) 15 irregular zones outlined by iron oxide- calcite 5 stained clay; matrix combination of plagioclase tr unidentified clay, chlorite and calcite; opaque minerals tr large grains angular to subround; open apatite tr packing. zircon tr muscovite tr Granular coarse sandstone; detrital siliceous bimodal submature chert- quartz 49 bearing SUBGRAYWACKE; U-9ol6; chert 25 MS 9, Unit 4, Indio Mts; 2 0 7nm, authigenic quartz 5 0.65inm, poorly sorted, poorly sorted; metamorphic quartz 10 subangular; metamorphic rock Granules of chert; sand, common and vein fragments 5 quartz; chert is subround to subangular; volcanic rock 70$ quartz grains have overgrowths; clay fragments 1 occurs as iron oxide-stained grains and calcite 1 interstitially; quartz grains angular opaque minerals 3 to round. clay 1 muscovite tr zircon tr Slightly granular coarse sandstone; detrital siliceous and calcitic submature chert- quartz 62 bearing SUBGRAYWACKE; U-9.102; chert 15 MS 9, Unit 23# Indio Mts; 0o52mm, poorly authigenic quartz 5 sorted; angular -subangular; metamorphic quartz 4 Gravel fraction subangular metamorphic metamorphic rock quartz; sand grains largely common fragments 2 and vein quartz and chert; 90$ quartz calcite 7 grains have overgrowths; calcite cement opaque minerals 2 is microsparry calcite and some micrite fragments 3 appears dolomitic. Table 5® “-“Continued Granular medium-coarse sandstone $ detrital calcitic bimodal mature chert-and quartz 18 quartz-bearing CALCLITHITE; U-9ol27; chert 15 MS 9, Unit 27, Indio Mts; 3*5mm, 0o5mm, micrite moderately sorted, well sorted; angular- fragments 50 subround; metamorphic quartz 3 Largely micritic granules (caliche metamorphic rock balls?); chert and metamorphic quartz fragments tr granules also present; micrite granules sparry calcite 12 varied in grain size and in allochemical opaque minerals 2 constituents; no fossils in micrite plagioclase tr granules; sand-size quartz and chert round to angular; quartz is common and vein; cement as well as micrite granules, dolomitic e Medium sandstone; siliceous detrital mature chert-bearing SUBGRAYWACKE; quartz 71 U-9cl87; chert 10 MS 9, Unit 51, Indio Mts; authigenic quartz 4 0.3mm, well sorted; subangular; metamorphic quartz 5 Common quartz, round to subangular; metamorphic rock chert angular to subround; some fragments 3 authigenic chert; 85% quartz clay 2 grains have overgrowths; round opaque minerals 5 tourmaline and zircon 0 calcite tr zircon tr tourmaline tr apatite tr blotite tr Bluff Formation Bluff Mesa, on the east flank of the northern Quitman Mountains about three-fourths of a mile northwest of 5 Mile Point, is the source of the name "Bluff beds" used by Taff (1891, p. 726, 727). In describing the "Bluff beds," how- Taff referred to section measured at Yucca a ever, Mesa, where he applied the name only to 104-106 feet of sandstone and limestone between the "second and third Caprotina hori­ zons." Smith the (1940, p. 609-611, 621-623) applied name "Bluff formation" to the beds of sandstone and Orbitolina­ bearing limestone that are bounded stratigraphically by the colorful limestone and shale of the conformably underlying Yucca Formation and by the light-colored sandstone of the conformably overlying Cox. Because the amended Bluff is an it easily recognized useful stratigraphic unit I have used throughout the Eagle Mountains and vicinity. s The U. Se Geological Survey, despite Taff f many years of priority, applied the name "Bluff" in 1936 to a Jurassic member in southern Utah. Gillerman (1953, p» 16) therefore used the name "Bluff Mesa" instead of Bluff in his on report the fluorspar deposits of the Eagle Mountains. and fossils addition the Thickness,Thickness lithology,lithology» fossils.--In to , at Yucca the Bluff forms most of the three exposure Mesa, which make Front and large outcrops up Ridge, it composes most of Love Hogback. In the Pagoda Hill area where it lies in thrust contact on the Yucca, the Bluff forms prominent of rise the ridges. Small west-facing hogbacks Bluff above alluvial flats south and southeast of Speck Ridge; there are a few low-lying ridges of Bluff southeast of Grayton Lake® On Devil Ridge about two miles southeast of Yucca Mesa, Smith (1940, p. 622-623? 627-628) measured 1,080 feet of Bluff, some 300 feet less than he recorded at Yucca Mesa. A northwest thickening of the Bluff is thus indicated, but this evidence should be tempered by the knowledge that Smith’s measured section 2 at Yucca Mesa crossed numerous normal faults with throws of the order of tens of feet. That this thickening may exist, however, is indicated by an apparent thinning of the Cox to the northwest. The upper part of the Bluff thus thicken at the of the may expense lower part of the Cox. The Bluff in the Devil Ridge area is dominantly thin- to thick-bedded, gray, sandy, very finely crystalline lime­ stone with interbeds of light-colored, Cox-like, fine-to and sandstone coarse-grained, calcareous siliceous quartz and some sandy shale® Oolitic limestone occurs about 600 feet above the base (Smith, 1940, p. 610) and about 400-700 feet from the top (Barnette, 1961, p. 89-“90). On the northeast flank of Yucca Bluff Mesa, the basal unit is a dark gray, sandy limestone with abundant oyster Photograph 2. Northwestward view of Love Hogback and Devil colorful sandstone Ridge from H.5-B*s; and shale of Yucca Formation in foreground; Love Hogback, left center, capped by thick-bedded lime stone of the Bluff Formation; northeastward move­ ment along Devil Ridge thrust fault at the base of Love Hogback brought Yucca into juxtaposition with Chispa Summit Formation, largely covered in valley, center; Speck ranch house at base of hog­ back, left center; Devil Ridge, upper center; northern Quitman Mountains on skyline, upper left; Sierra Blanca on skyline, upper right. fragments. It has a rough weathered surface, and its thick­ ness is not constant along strike, ranging from 4 to 30 feet. The lower of the Bluff is thicker bedded and more re- part sistant than the upper part. The most diagnostic fossil of the Bluff is the large foraminifer, Orbitolina d’Orbigny, which is found about 80 feet above the base and at the intervals throughout upper 600 feet of the formation (Barnette, 1961, p. 88-89)® in his Douglass (i960, p. 11, 17, pl« 6,7), comprehensive of this measured and collected from a section survey genus, roughly parallel to Smith’s section at Yucca Mesa. Douglass identified the Foraminifera in the lower zone as Orbitolina texana (Roemer) and those in the upper zone he assigned to Orbitolina minuta Douglass, n. sp. According to Douglass (1960, p. 38)s . . Generally, specimens of 0. minuta are smaller and more conical than those of 0, texana, although individ­ . ual specimens cannot be distinguished on size and shape alone. Stratigraphically the two species occur at differ­ent horizons. 0. minuta occurs in the upper part of the Glen Rose limestone whereas 0. texana is con­ .•. fined to the lower part of the Glen Rose o I shall henceforth refer to this fossil only as Orbitolina for I have made at sp., no attempt specific identification. At Yucca Mesa, the Orbitolina minuta zone also contains Porocystis sp., Protocardia sp., gastropods, and echinoids; reef fossils such as Exogyra sp., rudistids, and caprinids occur in abundance at horizons throughout the section, along and echinoids with a variety of other pelecypods, gastropods, (Smith, 1940, p. 620-623). In addition to Orbitolina minuta, the microfauna of this upper zone comprises ostracods, abundant miliolids associated with Textularia and Rotaliidae, and <> charaphytes (Thomerson, 1961, p. 23, 30, 35) In 1898 in the Devil Ridge area, Stanton (1947, p • 10, from the Bluff Forma­ 80, 81, pi. 58) collected, presumably the of a new species, Nerinea aquilina Stanton. tion, cotypes He described the type locality as follows: about 13 miles southeast of the town of Sierra ... Blanca, Hudspeth County, Tex., a little south of the old and at the end of a prominent lime- stage road, stone ridge trending northwest from Eagle Mountain. The position is low in the Trinity group. Stanton’s type locality is probably the northwest end of Love Hogback. On the northeast flank of the Eagle Mountains, the Bluff is less easily distinguished from the underlying Yucca and the overlying Cox, because there the Bluff contains more sandstone and shale than in the Devil Ridge area. This is to be expected because the Bluff in the Devil Ridge area, with the exception of the small outcrops just south of the railroad, is part of a thrust sheet that moved northeast out of the The Bluff the northeast flank trough. exposed along of the Eagles, however, is beneath the thrust fault and is presumably a nearer-shore facies than most of that in the Devil area. More shale and sandstone well a as as Ridge thinner section would be expected. At limestone of the Bluff an east- Eagle Spring, caps facing scarp just west and northwest of the Eagle Spring ranch house. Gillerman (1953, p. 17) measured only about 250 feet of Bluff there; Smith (1941, p. 72) estimated the thickness to be about 200 feet. Contrary to Gillerman, I have considered as Yucca the light colored sandstone, red and brown shale, and conglomerate in the immediate vicinity of the adit at the Eagle Spring fluorspar prospect, and I have placed the Yucca-Bluff boundary at the base of the thick Orbitolina-bearing limestone that caps the ridge above and west of the adit. Because my Bluff-Cox boundary is also stratigraphically higher than Gillerman T s, the thicknesses of Bluff are comparable. About midway in the Bluff section there I collected Arctica roemeri (Cragin), Tylostoma sp., and Anatina and near the base Trigonia sp., sp. Limestone of the Bluff Formation is sparsely exposed low on the northwest flank of Lone Hill, and an anomalously thin section is exposed in Espy Ridge where the Bluff over­ lies the Permian with little, if discordance. This any, may be the result of faulting, or it may be that the Bluff is merely thinner there than to the northwest or to the south. The outcrop along the ridge maintains such a constant thick­ ness that the of a fault there seems presence likely. In the of shaft vicinity 3 (K-13) Gillerman (1953> P* 16) estimated the thickness of the Bluff to be almost 1,000 feet. Considering the part of his Bluff Mesa to be upper I show a much thinner Bluff. The feet of Cox, upper 385 Gillerman f s Bluff Mesa beds lack Orbitolina, which elsewhere throughout the map area is abundant in the uppermost lime­ stone of the formation. Gillerman found Porocystis globularis (Giebel) in these beds. It might, there- upper fore, be argued that these beds are, in fact, "of Glen Rose age" and are correlative with those beds elsewhere that con­ tain Orbitolina. From the beds of the upper part of his Bluff Mesa in the vicinity of shaft 3 (lower Cox of Underwood) Gillerman texana collected Exogyra Roemer, Monopleura sp., Trigonia sp., Astarte cf. A. roemeri (Cragin). In the lower part he collected Orbitolina sp., Exogyra quitmanensis Cragin, and Porocystis globularis (Giebel) o South of the Rhyolite fault, the Bluff overlies the Yucca Formation. There Bluff Formation is my roughly equiv­ alent to Gillerman T s Bluff but the 89 feet of Mesa, upper shale and sandstone of the Bluff Mesa at his measured sec­ tion 2 belongs in the Cox rather than in the Bluff. From limestone beds in the lower part of the Bluff the I collected Anatina along Rhyolite fault, Arctica? sp., sp., Exogyra quitmanensis Cragin, and Tylostoma sp. Piso­ litic limestone occurs in the Bluff on Espy Ridge. Near Eagle Spring and in places southeast along the trace of the Devil Ridge fault, rough-weathering limestone beds of the Bluff overlie the Chispa Summit Formation. This relationship may be seen in Coal Mine Arroyo (H, J-ll) just upstream from the old coal mine, around the flanks of TC Peak and east-southeast (K. 6-12.9) of Carpenter Spring. Several relatively small, isolated outcrops of Bluff occur in the Eagle Mountains? in the Black Hill (M-9); area in between Cottonwood and Broad Rocky Ridge (M-ll); canyons (N-11, 12) ; and just east of Cottonwood Canyon on the east and west flanks of Eagle Bluff (M, N-12, 13). In the Black Hill more than 1,500 feet of thin-to thick-bedded area, fossiliferous limestone with interbedded shale are overlain disconformably by the well-bedded tuff and tuffaceous breccia of the lower rhyolite. Because of the abundance of such fos­ sils as Caprinula cf. crassifibra (Roemer) and Toucasia C_. cf. T. texana (Roemer) the lower part resembles reef- limestone zones in the or even Finlay Espy (Gillerman, 1953> p. 17). The upper part of the outcrop, however, is char­ acterized by beds containing Orbitolina in abundance. The Rocky Ridge outcrop of limestone, shale, and sand- in feet stone, plan view roughly 3,500 by 1,500 feet, is highly fractured and has been intruded by a diabase dike. The limestone contains rudistids and caprinids as well as Orbitolina much of the limestone has been sp.; crushed, fractured, and recemented. The sandstone and shale are brittle and highly sheared. This outcrop rests on the flow rock and pyroclastic rock of the upper rhyolite. Three similar outcrops of Bluff on the south flank of the Eagles between Broad and Cottonwood canyons also rest on the rhyolite. In contrast to the outcrop at Rocky upper Ridge, these exotic blocks are largely limestone. The larg­ est two are triangular in plan view; the largest has a long dimension of 4,500 feet, a maximum width of 3,500 feet, and is 200-300 feet thick. Where the contact is exposed the limestone and the underlying volcanic rock do not interpene­ trate and the limestone has been discolored and only faintly baked. On the other hand, Gillerman (1953, 50) reported P* silicic material in fractures at the base of the Rocky Ridge block; he suggested that these may have been stringers and were apophyses of rhyolite that silicified and recrystal­ lized sometime after emplacement. The to Gillerman Rocky Ridge block, according (1953, p. 49, pi. 17), dips beneath the volcanic rock to the north­ west; yet the only evidence that the blocks between Cotton­ wood and Broad canyons ever were covered is a small outcrop of rhyolite within or on the largest limestone block. upper be the orbe It may resting on limestone, it may underlying rock cropping out where the limestone has been removed by erosion. Several smaller blocks (M, N-12, 13) crop out on the east and west flanks of Eagle Bluff; the largest is about 100 feet thick. While it is not unreasonable to think of these smaller blocks as xenoliths, it is difficult to as­ cribe that origin to the larger blocks to the west. The mechanism by which the blocks of the Bluff Forma­ tion were is unknown. It is that there emplaced possible such relief the overthrust sheet of Cretaceous was on strata, even as late as the time of outpouring of the upper rhyolite, that the Bluff, under the influence of gravity and perhaps movement related to the subsidence that accom­ triggered by volcanic slid panied the outburst, downslope onto fairly cool rocks of the upper rhyolite. Succeeding eruption of lava, partly or wholly covered some or perhaps all of these blocks. It is also possible that the upwelling volcanic rock penetrated the Cretaceous rock as sills so thick that the molten material simply raised large blocks and carried or ’’floated” them some distance and subjected them to stresses of varied orientation so that they were sheared, fractured, and brecciated. In the Indio the Bluff out widely in Mountains, crops the area north of Oxford Springs where it has been highly folded. Probably the most distinctive outcrop of the Bluff in the map area is Bramblett Ridge (Q, P-12, 13) that ex­ tends more than two miles and a half northwest from the ranch house at Oxford Springs. In this ridge the strata range in attitude from near-vertical to slightly overturned. To the south there are three linear exposures, roughly parallel, east of the Indio fault. West of the fault the Bluff is highly faulted, in places folded, and incompletely exposed. The southernmost exposure is in the Lost Valley syncline (W, X-15, 16). There is a small, isolated outcrop of Bluff west of Escondido well. (V-17) just MS 9 includes 796 feet of Bluff. The lowest unit is a bed of medium 25-foot light gray, very finely crystalline thick-bedded and fossiliferous limestone. Near the base this bed contains Exogyra quitmanensis Cragin; next above, a zone of rudistids and caprinids; and above that a zone of colonial corals. Near the top are Exogyra rudistids, sp., and caprinids. Oolitic limestone is characteristic of the lower part of the Bluff throughout the map area. At MS 9 it ranges from about 130 feet to 285 feet above the base. The oolites in diameter from to and in range 0.15 mm 1.9 mm shape from discoidal to oblate spheroidal; they are enclosed in a matrix of sandy and calcite. silty sparry In the Indio Mountains, I found Orbitolina near the base of the Bluff only at the foot of the limestone ridge (T.5-14*8) which leads southeast from the vicinity of the Indio Adams 23) collected Orbit- headquarters. (1953* P* olina from the base of the Bluff in the Lost Valley area, and Barnette (1961, p. 93) identified a few specimens of Orbitolina in thin-sections of samples collected near the base of Bluff just east of the Indio fault along the Indio Pass road. At MS 9 the first Orbitolina zone is in the upper lime­ stone unit of the Bluff about 730 feet above the base in medium thin-bedded nod- light gray, very finely crystalline, ular limestone. The first zone bearing abundant Orbitolina is 740 feet above the base; Orbitolina ranges up to the base of the Cox with varied abundance. locally Also from the uppermost limestone unit of MS 9, I col­ lected Anatina Porocystis globularis (Giebel), Tapes sp., sp . , Trigonia sp . , Pecten (Neithea) sp., Cyprimeria sp., Tylostoma sp., Hemiaster sp., Enallaster sp., and Holectypus sp. In the Indies most of the microfauna of the Bluff is confined to the upper part; the microfauna comprises charo­ phytes, ostracods, and foraminifers, among which are milio­ lids (Thomerson, 1961, p. 22, 35)­ of the Bluff In the uppermost part (P.B-15-7) just north of the east end of Oxford Ridge I collected Orbitolina Trigonia and Loriolia The part of the sp., sp., sp. upper Bluff also characteristically yields large, cardid clams, and bulbous gastropods. Between the lower and limestone sections of the upper Bluff, beds of thin-bedded, light gray, very finely crystal­ line fossiliferous limestone alternate with beds of thin- well bedded, gray, fine-to medium-grained, moderately to sorted and subangular to subrounded quartz sandstone. Cal­ careous cement binds the grains; much of the sandstone re­ sembles that of the Cox. The Bluff in the Lost is feet Valley area only 446 Orbitolina thick; the uppermost limestone containing the zone is 140 feet thick and the alternating limestone and sandstone and sandy limestone of the lower part is 306 feet thick (Braithwaite, 1958, p. 18-19). At the base is a gray, 50-foot rudistid-reef limestone. Adams (1953 , P» 23) collected H emiaster sp, and Ostrea sp. from the basal unit and Tylostoma sp., Trigonia sp., and Homomya sp. from the upper Orbitolina zone in the Lost Valley area. In the northern Sierra feet of Orbitolina- Pilares, 225 bearing limestone lies beneath the Cox; most of the lime­ stone of the lower part of the Bluff north of the river disappears to the south and in the northern Sierra Pilares has been sandstone and shale replaced by (Ferrell, 1958, p. 19). Where the lateral changes of facies are so abrupt, it is difficult to select formation boundaries and to correlate the different sections. The Bluff is a unit containing con­ siderable carbonate rock that a in the ter­ represents pause recorded in the Yucca and Cox formations. rigenous deposition The base of the Bluff Formation is that stratigraphic level above which a significant quantity of carbonate rocks occur. Because of the lithologic variation at the upper Yucca-lower Bluff interval from place to place, the measured thickness of the Bluff varied. In practice, I used the method or arbitrary cut-offs in choosing the base of the Bluff; the base is not everywhere at the same stratigraphic level (Braithwaite, 1958, p. 25-2?)» Correlation and age p. 1002) esti­ _a£_e .--Huffington (1943, mated that 1,500 feet of Bluff are present in the northern Quitman Mountains. At Bluff Mesa the formation exhibits the gross lithologic sequence so characteristic of the Bluff throughout the areas massive, gray, sandy limestone at the base overlain successively by oolitic limestone, sandstone, and the thin-bedded Orbitolina-bearing limestone. At Quitman Gap, however, the Yucca and Bluff grade imper­ ceptably one into the other, i.e., a transitional zone has been substituted for the sharp formational contact at Yucca . Mesa (Huffington, 1943, p* 1001) Here again, an arbitrary cut-off must be selected; Huffington chose the lowermost fossiliferous bed as the basal unit of the Bluff. Bluff of Bluff is The Yucca and mesas probably roughly equivalent to the "Glen Rose" of the Malone Mountains, to the 630 feet of siltstone and impure limestone of the Cam­ pagrande that underlies the Cox in the Finlay Mountains, and to the Yearwood Formation of the Van Horn Mountains and the Kent Quadrangle. At Triple Hill, only a few feet of medium gray, thick- bedded limestone of the Campagrande, or Bluff, is exposed beneath the Cox and above the granite porphyry laccolith (Brunson, 1954, p. 18). P. B. King (Brunson, 17-18) reported 20 feet of p. Campagrande at Cox Mountain. Smith (1940, p. 601), Wade (1954, 28), and Brunson (1954, 18), however, did not P* P» recognize Campagrande, or Bluff, there and reported that Cox rests on Permian. King (i960) wrote concerning the presence of Campa­ grande, i.e., Bluff or Yearwood, at Sierra Prietas Your suggestion that the basal limestone at Sierra Prieta might be Finlay is worth some thought, and might be worth mentioning as an alternative interpre­tation in my report. At the moment this does not seem as likely as a correlation with the basal limestone ("Campagrande" or "Yearwood"?) of Cox Mountain. This is because the is principally Finlay preserved only about 5 miles southwest and west of Sierra and Prieta, most of the limestone is from the Finlay at these localities. Only a few beds less than 5 feet thick remain, and the rest of the unit has changed to marl, sandy shale, and sandstone. is quite different; gone In the Van Horn Mountains area, 600 feet of Bluff is exposed on the west flank of the Colquitt Syncline; the Bluff grades northward into the Yearwood Formation, which feet in thickness. The Yearwood is com- averages only 225 of of and posed alternating beds conglomerate, sandstone, shale, which is overlain by massive, unfossiliferous, reddish-brown to limestone pinkish-gray (Twiss, 1959a). Although the Yearwood is absent in the Wylie Mountains, where Cox rests directly on Permian, it is 150 feet thick at the type section in the Kent Quadrangle (Brand and DeFord, The of the Yearwood in the 1958, p. 376) . ab sence Wylie Mountains suggests the presence of a high there during the deposition of the Yearwood elsewhere. The Bluff at Pinto Canyon is widely exposed and con­ sists of 200 feet of thick-bedded limestone overlain by 450 feet of inter-bedded nodular, Orbitolina-bearing limestone and light-colored, cross-bedded sandstone. In northeast Chihuahua, Orbitolina-bearing beds of gray limestone are laterally continuous with the upper Orbitolina­ bearing limestone beds of the Bluff Formation of the Eagle Mountains and vicinity. In the southern Sierra Pilares and northern Sierra de Ventana, Orbitolina sp. occurs as high as 989 and 407 feet above the base of the Cox Sandstone (Yeager, respectively 1960, p. 38; Spiegelberg, 1961, p. 21). Southwest, and farther out in the trough, Jones (1962) has reported that beds roughly equivalent to the Bluff For­ mation of the Eagle Mountains and vicinity are thicker. A black shale zone containing Exogyra quitmanensis Cragin and Douvilleiceras is about 800 feet thick. This is overlain sp. by about feet of massive reef-lime stone, with inter­ 1,000 beds of Cox-like sandstone containing Orbitolina sp. Jones has reported that Orbitolina occurs locally in beds equiva­ lent to the lower 200-300 feet of the Cox. As the Bluff Formation of the Eagle Mountains and vicin­ ity contains Orbitolina sp., it is paleontologically correl­ ative with the Glen Hose Limestone of central Texas. The age of the Bluff is Early Albian. Origin.--SuchOrigin.rocks as intrasparite, intramicrosparite, intrasparrud.it e, odsparite, and fine-grained sandstone are typical of the lower part of the Bluff, whereas biomicrite is typical of the upper part. Relatively strong currents prevailed during deposition of the lower part of the Bluff; the micrite of the upper part must have been deposited in a such a or offshore low-energy environment, as lagoon quiet, water. The small quantity of quartz silt in the lower part of the Bluff have been transported to the site of may deposition by wind. The Cox-like sandstone of the Bluff is a fine-grained, Common siliceous, mature, chert-bearing subgraywacke. quartz composes most of the rock, but metamorphic quartz and of metamorphic-rock fragments constitute from 5-10 percent the terrigenous fraction. The chert indicates a sedimentary rock source; the metamorphic quartz and the metamorphic-rock fragments suggest a metamorphic-rock source, although these constituents could have been derived from sedimentary rocks as well. Kaolinite is more abundant upward in the Bluff section in the Indio Mountains and in Devil Ridge (Sutcliffe, 1961, vi). This could indicate increasingly severe conditions p. of chemical weathering in the source area, namely, increas­ or warmer climate or both; it could also indi­ ing humidity cate increasing availability for weathering in the source area of sodium-and potassium-rich rocks. Celestite, garnet, goethite, rutile, tourmaline, and zircon are the dominant heavy minerals in the Bluff Forma­ tion in Devil Ridge and the Indio Mountains; others are apatite, chlorite, and rutile. The well rounded to sub- rounded stable heavy minerals indicate that they were de­ rived from the reworking of previous sediments. The pres­ ence of garnet, in addition to the metamorphic quartz and that the metamorphic-rock fragments, suggests some of the a source. sediments were derived from metamorphic-rock Sections of the Bluff Formation at Shatter, in the Soli- Pinto the Indio at Devil tario, in Canyon, in Mountains, Ridge, and in the southern Quitman Mountains are correlated on the basis of statistically significant heavy mineral zones. Be­ cause the Bluff section at Shatter and in the Solitario and in the southern Quitman Mountains to contain rock appear younger than the sections nearer the margin of the Diablo platform, the Bluff Formation may well have been deposited during a rapid the Comanche sea transgression of (Sutcliffe, 1961, p. 43-44)• The Bluff whose varied Formation, lithology represents equally varied conditions in the source area and in the en­ vironments of deposition, was laid down during the general transgression of the Cretaceous sea onto the platform, a trans­ gression that was interrupted by regressive movements or per­ haps by sudden and short-lived tectonic uplifts in the source area. Reefs were locally abundant from time to time during Bluff deposition. As the sea moved farther onto the platform, the Bluff overlapped the Yucca Formation to the north and east . The Yearwood and Campagrande formations, probably equiv­ alent only to the upper part of the Bluff Formation, may rep­ resent a lagoonal or near-shore deposit correlative with the very finely crystalline limestone of the Bluff, which was de­ water. posited in quieter and perhaps deeper Table 6,—Petrography of representative samples of the Bluff Formation Fine sandstones siliceous mature detrital chert-bearing SUBGRAYWACKE; DU-Kbl-38; quartz 75 Yucca Mesa, Cox-like sandstone chert 10 beneath upper zone of Orbitolina; authigenic quartz 6 well 0.15mm, sorted; subangular; metamorphic quartz 5 rock Largely common quartz; 85$ quartz grains metamorphic have overgrowths; detrital grain of fragments 1 sparry calcite (0.9mm) composed of calcite 2 chert subround to minerals 1 abundant calcispheres; opaque tr angular. zircon muscovite tr tr clay Sandy and silty intraclastic microsparry BIOMICROSPARITE; U-9.192; calcite 28 MS 9, Unit 53, Indio Mts; pelecypods 20 and Angular sand silt, foram. blue-green algae(?) 25 micrite fragments of blue-green algae(?) 5 micrite intracuasts foram 2 and pelecypods, all in and zircon tr sandy silty microsparry calcite matrix (Md 0.007mm) sand tourmaline tr and silt fragments are quartz. muscovite tr metamorphic quartz, plagioclase, sand and silt 20 chert, muscovite, metamorphic rock fragments; plagioclase is fresh; a few euhedral zircon crystals. Sandy and silty fossiliferous sparry calcite 36 00SPARITE; U-9.202; o61.ites 60 MS 9, Unit 55, Indio Mts; foram tr Oolites of varied shape ranging pelecypods 1 up to 1,9mm (Md 0.5mm) in a echinoids 2 calcite matrix 1 sparry (Md 1,6mm) quartz that contains some angular to subangular plagioclase tr chert quartz, echinoid fragments, tr and other material in addition tr foram, metamorphic quartz to the abundant and varied material zircon tr incorporated as a nucleus within algae tr ostracods tr oblites, e.g. foram, pelecypod fragments, quartz, chert, metamorphic quartz, echinoid fragments, plagioclase, ostracods, algae; composite oblites are common as are oblites enclosing oolites; 99$ oolites are concentric; aside of from nucleus, oblites composed calcite and micrite. sparry Table 6,—Continued Fine sandstone; siliceous mature detrital chert-bearing SUBGRAYWAGKE; U-9.226; quartz 74 MS 9, Unit 60, Indio Mts; chert 8 0.20mm, well sorted; subangular; authigenic quartz 7 Common quartz; 80% quartz grains metamorphic quartz 4 have overgrowths; chert angular metamorphic rock to subangular; some clay in fragments 2 detrital grains; principal cement is clay 2 authigenic quartz, but iron carbonate opaque minerals tr cement (ankerite?) also present. zircon tr ankerite? 3 tourmaline tr muscovite tr Foraminiferal BIOMICRITE; U-9.238; micrite 66 MS 9, Unit 65, Indio Mts; foram 30 Abundant Orbitolina sp. in micrite oysters 1 matrix which also contains other sparry calcite 3 forams; a trace of angular quartz echinoids tr silt (Md 0.03mm), oyster and echinoid quartz tr fragments, and scattered irregular patches of sparry calcite (fossil debris), Cox Sandstone The Cox Sandstone, widespread in Trans-Pecos Texas, is an excellent stratigraphic marker because of its lithologic and color contrast with the limestone of both the under- gray lying Bluff Formation and the overlying Finlay Limestone. Richardson (1904, 47) named this unit the T,Cox for- p. mation,” no doubt taking the name from Cox Mountain (Taber­ nacle Mountain), about 16 miles northeast of Sierra Blanca. He did not, however, specify a type section or locality. the Because Richardson f s description is applicable to Cox that is exposed near the ranch headquarters near the south end and on the north flank of the Finlay Mountains, Brunson (1954, p. 3, 20) chose this site as the type section of the Cox Sandstone. Of the 603-foot Cox section of massive sand­ stone and interbedded limestone, marl, and shale, only 280 feet is sandstone (Brunson, 1954, P» 21). Although sandstone is the characteristic material of the Cox, at the type sec­ tion as well as at other localities in the region, substan­ of material other than sandstone occur in tial quantities the Cox. Thickness , lithology . , fossils.--In the Devil and fossils Ridge area, relatively small outcrops of Cox are on the northeast flanks of Sand Mountain and Texan Mountain. The most ex­ tensive outcrop of Cox, however, is on Devil Ridge; the Cox forms the major and the highest part of this 7*5 mile-long low southeast hogback. The Cox is poorly exposed in a ridge of Grayton; it is well exposed but folded and faulted in Little Hill (F, G-9, 10). The formation is also exposed in the eroded crest of the overturned anticline in Back Ridge and in the general area east and southeast of the Speck ranch house. Two small hills of Cox Sandstone rise above the and alluvium just west of Love Hogback. gravel The Cox thickens rapidly southeast along strike: near Yucca Mesa, Smith (1940, p. 612) measured only 557 feet; his section crossed only minor faults. About 6.5 miles south­ east, at MS 16, the thickness is 1,737 feet. Because the Bluff thickens to the northwest, the lower part of the Cox northwestward into the of the Bluff. At may grade upper part Yucca Mesa some 260 feet of thin-bedded limestone, sandstone, and shale overlie the zone of nodu­ thin-bedded, light gray, lar, Orbitolina-bearing limestone so characteristic of the Bluff elsewhere in the area. These beds uppermost uppermost of Bluff at Yucca Mesa may well be equivalent to a lower the Cox to the southeast. part of well Throughout the Eagle Mountains and vicinity, as as in other parts of Trans-Pecos Texas, a gray, nodular, fos­ siliferous limestone in the upper part of the Cox separates the unfossiliferous upper and lower sections of sandstone; shale occurs near the base. In the Devil Ridge the sandstone of the Cox is area, various shades of gray, brown, orange, and pink, and is characterized, as it is throughout the map area, by speckles reddish iron oxide 2 of intergranular, orange, averaging mm or less in diameter. Fine-to medium-grained quartz composes the bulk of the formation, although it does contain beds of limestone well as shale. The sandstone at Sand Mountain as and at Flat Mesa is coarser and less indurated than that on Devil The Cox contains a few Ridge (Smith, 1940, p. 6ll). local lenses of limestone-, quartzr-, and chert-pebble conglom­ erat e. The formation is thin bedded as well as cross bedded; oscillation ripple marks are also preserved. On his plate 1, Smith cross-bed and data so to (1940) plotted ripple-mark as indicate the direction of the of beds. These data top every­ where confirm the attitude of the beds that one would deduce from superposition. Throughout the lower 200 feet, sandstone and limestone are interbedded; covered zones are very likely shale. Begin­ ning about 905 feet above the base, however, beds of dark nodular limestone contain abundant Actaeonella dolium gray Roemer as well as Nerinea sp., Tylostoma sp., Gryphaea sp., and Exogyra sp. I found Pecten (Neithea) irregularis Bose about 1,050 feet above the base, Exogyra texana Roemer and caprinids about 1,080 feet above the base, and Gryphaea mucronata Gabb at about 1,255 feet above the base, all in thin-bedded, nodular limestone. Smith (1940, p. 612) collected Engonoceras sp. from a thin, limestone bed about 650 feet above the base of sandy the Cox some two miles northwest of MS 16. From about the same horizon he also collected Actaeonella texana (Roemer), presumably the same as Actaeonella dolium Roemer. The Cox is widely exposed in the Eagle Mountains. From just west of Eagle Spring the Cox outcrop extends in a broken but broad band along the northeast flank of the mountains. The Cox is also in Lone prominently exposed Hill; it makes up the upper part of Espy Ridge and most of the ridge that the and flanks Spar Valley on north is widely exposed south of the mouth of The beds Spar Valley. dip generally southwest and have been intruded by rhyolite sills, most of which are too small to show on plate 1. A complete section of Cox is not exposed in the Eagles, but well over 1,000 feet crop out in the Spar Valley area. Gillerman (1953# P» 21-22) measured two partial sections in the Spar Valley area, both around 700 feet thick. As in the Devil the Cox Ridge area, is largely a well-indurated, fine- to medium-grained sandstone but with more conglomeratic lenses scattered than throughout in Devil Ridge. Ripple marks, burrows and trails, and cross beds are common fea­ tures. In in the places Eagle Mountains as throughout the sandstone of the Cox is desert varnished. map area, A few beds of gray, nodular limestone in the vicinity texana Protocardia of shaft 3 yielded Exogyra Roemer, sp., I Ostrea sp., and Nerinea sp. (Gillerman, 1953> p. 22). collected Tylostoma sp. and Anatina? sp. from this locality. In the Eagle Mountains and elsewhere the Cox contains silici­ fied wood. Throughout the map area platy, angular fragments of the sandstone in the colluvium of the Cox and mask predominate less resistant rock. This gives the erroneous impression that the Cox is 100 percent sandstone, whereas shale and limestone interbeds are common. Small xenoliths(?) of white, highly brecciated and re- cemented Cox sandstone are exposed southeast of East mill, where out within the The they crop upper rhyolite. largest of these blocks has a maximum dimension of about 500 feet; most are much smaller. In the Indio Mountains, the Cox is present in the folded area north and northeast of Oxford Springs,in the principal ridges of the mountains south of Squaw Peak where thrust faults have resulted in two roughly parallel en echelon ex­ posures. The Cox is also exposed along the east margin of the mountains, and in the Lost Valley area. MS 10 about mile southeast of a (S.B-15*4)* Squaw has feet of Cox. Spring, 1,267 Although generally light colored, in detail the color of the Cox from white ranges Photograph 3* Eastward view of Indio Pass from T thin-to-thick beds of 6-15»2; conglomerate, shale of of Yucca For- b sandstone, and upper part mation just east of Indio fault; darker rock toward of section is thick-bedded limestone top of Bluff Formation; skyline is Cox Sandstone; picture taken in vicinity of Stop No. 6 (DeFord, 1958b). through different shades of gray, orange, and brown. Colors of weathered surfaces are equally varied. Sandstone of the Cox is largely fine-to medium-grained quartz cemented by calcite but mostly by silica in the form of overgrowths on quartz grains. The übiquitous spots of intergranular iron oxide are Beds of common throughout the section. conglomerate are rare, but the section has several conglomeratic sandstone lenses with a gravel fraction composed of rounded quartz, chert, and limestone granules and pebbles. Such features as cross beds, ripple marks, and penecontemporaneous slump folds are present. The 9-foot, medium light gray, very finely crystalline limestone about 1,015 feet above the base contains the follow ing fossils: Exogyra texana Roemer, Gryphaea washitaensis Hill, Toucasia sp., Artica? sp., caprinid, Exogyra cf. E. texana Roemer. Actaeonella was collected near the north sp. end of Red Mountain from Cox that has been overridden by a thrust sheet of Yucca that moved east. There are other thin beds of limestone in the Cox, but they are largely covered. Near the base of the Cox, also largely covered in all but a few localities, is a red, brown, and green shale. The contact in six in Cox-Finlay widely separated areas Trans-Pecos Texas (including the Indio Mountains) was re- Wade and Brunson ported by (1954, P* 41-48) (1954, p. 28-29, 33-36) to be disconformable• Wade f s figure 12 and Brunson 1 s figure 14 are the same photograph of the undulating Cox- Finlay contact near the mouth of Snake Canyon (T, U, V-16). The total relief of the uneven surface pictured probably is not greater than two inches. At MS 5, there is a 1-inch caliche zone at the Cox-Finlay contact, but I did not exca­ vate the contact sufficiently to determine whether it is undulatory. Braithwaite (1958, p. 30) reported 1,592 feet of Cox in the Lost in the southern Indies. section Valley area The is largely sandstone with some conglomerate plus a total of 79 feet of limestone and 5 feet of shale. The Cox on the west limb of the Lost Valley syncline may be traced southward the river into the northern Sierra Pilares where Fer- across rell (1958, p. 28) reported 1,673 feet. Correlation and age the northwest in the northern age.--To Quitman Mountains, a complete section of Cox is not exposed. A 704-foot section of Cox that out about 2.5 miles crops northwest of 5 Mile Point is noteworthy, however, for its relatively abundant limestone and sandy shale beds are in contrast to the sandstone in the Cox that out crops just south of Yucca Mesa (Buffington, 1943, p. 1003). At Triple Hill, Actaeonella sp. occurs in a limestone and shale about 212 feet from the of 600-foot sequence top a section of Cox (Wade, 1954, p. 24). The Cox at Cox Mountain is 461 feet thick and is largely sandstone and quartz-, is chert-, and limestone-pebble and conglomerate. The Cox not present in the Cornudas Mountains (Clabaugh, 1941, p. 8); at Sierra Prieta about 95 feet of sandstone between the under lying Permian Hueco Limestone and the overlying Duck Creek Formation are of disputed age. The upper part is probably to the Fredericksburg; the lower paleontologically equivalent part may be correlative with the Bluff or Campagrande of the believes that the feet of sandstone region. DeFord (1962) 95 are correlative with the Finlay. section Twiss (1959a) reported an incomplete 1,215-foot of Cox in the Van Horn Mountains. In the Wylie Mountains, where the Cox rests on Permian limestone, Hay-Roe (1957) re­ ported thicknesses ranging from 130 feet at the north to 530 feet at the south end of the mountains. The fossiliferous limestone in the widely occurring, Cox is missing in the Kent Quadrangle where the 120-foot sec­ tion of Cox consists of sandstone and conglomerate (Brand and DeFord, 1958, p. 376-378) Farther east in the Ft. c described Stockton area, Adkins (1927, p. 31-33) a 140-foot section of ferruginous varicolored medium-to coarse-grained sandstone with at the base. This section is conglomerate overlain by limestone that has fossils indicative of the Fredericksburg. In the absence of diagnostic fossils, the basal Cretaceous unit in the Ft. Stockton area well be may assumed to be correlative with all or part of the Cox in the Kent Quadrangle. The Cox Sandstone did not, however, cover all Trans- Pecos Texas Limestone that is (fig. 4). paleontologically correlative with the Edwards Limestone of central Texas overlies rocks of Permian in the Del Norte Mountains age and in the Glass Mountains (McAnulty, 1955# p. 539) (King, 1937, p. 14). recorded feet of Amsbury (1958) 450 sandstone, shale, and nodular limestone in the Cox in the Pinto Canyon area about 65 miles south-southeast of Eagle Peak, where there are Exogyra texana and Actaeonella dolium in limestone in the of the formation. upper part In the ranges of northeast Chihuahua, the Cox contains progressively more limestone and shale, and Orbitolina is present in the lower part of the formation. In the Sierra de the Ventana, Spiegelberg (1961, p. 21) reported presence of feet of Cox lime­ 2,179 (60 percent sandstone, 35 percent stone, 5 percent shale) with Orbitolina as high as 400 feet above the base. Thickness of the beds in the southern Quitmans which are stratigraphically equivalent to the Cox is difficult to determine because the beds are faulted; the thickness, how- is in of feet ever, probably excess 2,000 (Mount, 1960, p* 63-67)* This section, which is composed of subequal amounts of limestone and sandstone in addition to minor quantities of siltstone and shale, contains Orbitolina sp., Actaeonella dolium and Exogyra texana (Jones, 1962)* t The upper part of the Cox is paleontologically correla­ tive with the Fredericksburg Group of central Texas; the lower be correlative with the part may upper Trinity (fig. 3). The age of the Cox is Albian. OriginOrigin.--The Cox Sandstone records the transgression of the Cretaceous sea over the Diablo platform as it is the youngest Cretaceous formation that rests on Paleozoic and rocks. rocks older Although pre-Cretaceous exposed locally on the Diablo platform may have contributed sediments to the lower of the material that the part Cox, composes upper part of the formation in the Eagle Mountains and vicinity must have from distance come some away. The transition from thick shelf or marginal geosynclinal deposits featuring subequal amounts of limestone and sand­ stone to the thin, largely sandstone deposits of the platform is strikingly shown by the series of Cox sections from the southern Quitmans east-northeast to Ft. Stockton, including the Indio, Van Horn, Wylie, and Kent sections described earlier. The same transition is recorded between the mar­ ginal section of the Sierra de Ventana and the platform sec­ tion of Pinto Canyon. A rough outline of the west boundary of the Diablo platform is a line connecting the sections marginal to the Devil and Indio Moun­ platform, i.e.. Ridge, Eagle Mountains, tains. In Chihuahua, the boundary falls between the Sierra Pilares and the Sierra Vieja; farther south the boundary lies between the Sierra de Vantana and Pinto Canyon. A palinspastic correction must be made, however, to ac­ count for the northeast and east movement along thrust faults Laramide All the with the ex- during the orogeny. sections, ception of those in the Eagle Mountains and at Pinto Canyon, are on thrust blocks that moved some distance northeast and east. The depositional boundary of the Cox probably ran roughly east-west through a point north of the present site of Sierra Prieta but south of the present site of the Cor­ nudas Mountains (figs. 1,5)» The relatively large quantity of limestone and shale in the Cox in the northern Quitman Mountains and in Triple Hill suggest the presence of an embayment in this area during Cox time. The gypsum in the Cox in the Finlay Mountains (Brunson, also that condi­ 1954* p. 74) suggests temporary evaporative tions may have developed behind a reef or bar. The cleanness and high degree of sorting and rounding of the of the Cox indicate that the material quartz grains of the Cox was deposited in a high energy environment, e 4g., beach, bar, or other shore or near-shore environment. Much of the material was derived from older sedimentary rocks, the Cox well for heavy minerals in are characteristically rounded and much of the roundness of the quartz grains was a probably inherited. It is entirely possible that large of the that the Cox has experi­ part quartz grains compose enced several cycles of erosion and deposition. Angular and rounded quartz grains of the same size and chert grains more angular than quartz grains of equal size indicate that multiple source rocks contributed to the sediments. The Cox offers some evidence of volcanic activity dur- Albian. Twiss and I ing Early-Middle (1959a) recognized only a trace of plagioclase and no orthoclase in the Cox; Hay-Roe trace of volcanic-rock (1958, p. 52) reported only a frag­ ments. Braithwaite (1958, p. 31-33), however, cited the occurrence of euhedral crystals of plagioclase feldspar in the Cox, and Harwell (1959, p. 21-22) reported that in Sierra del Porvenir the Cox contains chlorite volcanic-rock cement, fragments, and volcanic minerals. It is thus likely that near-by volcanic activity contributed some sediment to the Cox . Because about 3 percent of the terrigenous grains of the Cox consists of metamorphic-rock fragments and meta­ morphic quartz, metamorphic rocks probably contributed sedi­ ments to the Cox. The presence of chert in all samples indi­ cates that chert-bearing limestone made a part of the up terrain being eroded. The high degree of frosting and pitting of the quartz grains of the Cox suggest that much of the Cox could have originally been wind-blown material that accumulated on beaches and was later reworked by waves and currents (Priddy, 1956, p. 36). Table 7.—Petrography of representative samples of Cox Sandstone Fine to medium sandstone; detrital siliceous chert- 83 supermature quartz chert bearing ORTHOQUARTZITE; DU-Kcx-34| 5 8 DEVIL triangulation station authigenic quartz (F.4-5*2), Devil Ridge; metamorphic quartz 1 rock 0.2mm, well sorted; well rounded; metamorphic Largely common quartz; 80$ quartz fragments 3 grains have overgrowths; slight opaque minerals tr crenulation some tr along grain apatite zircon tr boundaries; chert angular to round; round tourmaline and zircon; leucoxene tourmaline tr forms fraction of opaque minerals. Medium sandstone; siliceous super- detrital mature ORTHOQUARTZITE; DU-Kcx-41; quartz 83 Just below JUDGE triangulation chert 3 station (K.3-8.8); 0.27nun, well authigenic quartz 10 1 sorted; well rounded; metamorphic quartz Common quartz predominates; 90$ metamorphic rock chert subround quartz have overgrowths; fragments 3 to round; slight crenulation along some opaque minerals tr grain boundaries. apatite tr zircon tr Fine sandstone; siliceous and detrital calcitic supermature chert-quartz 64 bearing ORTHOQUARTZITE; IU-Kcx-63; chert 10 Red Mountain thrust fault Along some authigenic quartz 6 (U.7-14*9), Indio Mts; 0.18mm, metamorphic quartz 2 well sorted; subround; metamorphic rock Quartz is largely common; 75$ quartz fragments 2 grains have overgrowths; calcitio plagioclase tr cement introduced along abundant micrite 2 fractures; chert subangular to subround; sparry calcite 13 opaque tourmaline and zircon round. minerals 1 zircon tr tourmaline tr Table 7*—Continued Course sandstone: siliceous and detrital calcitic submature chert-bearing quartz 64 SUBGRAYWACKE; IU-Ky-65; chert 15 Along Red Mountain thrust fault authigenic quartz 7 (V.1-15.1), Indio Mts; 0 o 7mm metamorphic quartz 5 moderately sorted., subround; metamorphic rock Common quartz dominates; 80$ fragments 3 quartz grains have overgrowths; sparry calcite 5 chert subangular to angular; some opaque minerals 1 cement may be iron carbonate apatite tr (ankerite?), zircon tr plagioclase tr Sandy siltstone; calcitic bimodal detrital immature ORTHOQUARTZITE; IU-Tr-345; quartz 63 Immediately above last unit of chert 10 Bluff Formation at MS 9 (S.7-14.7); plagioclase 1 0.05mm, 0.10mm, well sorted, well sparry calcite 15 sorted; subangular; micrite 10 Common quartz dominant; micritic opaque minerals 1 zones may be filled burrows; zircon metamorphic quartz tr well rounded. muscovite tr zircon tr Sandy and silty pelletiferous micrite 75 foraminiferal MICRITE; U-10.13 sparry calcite 8 MS 10, Unit 4, Indio Mts; foram 10 Micrite matrix grades to microsparry pellets 4 calcite, then to sparry calcite; quartz 2 abundant miliolid foram filled by chert 1 sparry calcite; sand and silt angular. plagioclase tr bimodal (Md 0.10mm, 0,05mm). metamorphic quartz tr pelecypods tr ostracods tr Table 7*—Continued Fine sandstones siliceous and detrital calcitic submature chert-bearing quartz 77 SUBORAYWACKE; U-10.42; chert 5 MS 10, Unit 13, Indio Mts; 0.18mm, moderately sorted; subangular; authigenic quartz metamorphic quartz 7 4 Quartz largely common; 65$ quartz metamorphic rock grains have overgrowths; chert round to angular; plagioclase highly fragments sparry calcite 2 2 altered; tourmaline round. clay 2 opaque minerals 1 plagioclase tr muscovite tr apatite tr zircon tr tourmaline tr Fine to medium sandstone: siliceous detrital supermature ORTHOQUARTZITE; U-10.61B; MS 10, Unit 18, Indio Mts; 0.25mm, quartz chert 86 2 very well sorted; well rounded; authigenic quartz 10 Quartz largely common; 90$ quartz metamorphic quartz 2 grains have overgrowths; chert metamorphic rock subangular to round; small euhedral fragments tr zircon; larger round and angular clay tr zircon. opaque minerals tr muscovite tr zircon tr apatite tr Coarse sandstones siliceous detrital submature chert-bearing ORTHOQUARTZITE; quartz 83 U-10.69; chert 5 MS 10, Unit 20, Indio Mts; 0.5mm, authigenic quartz 8 moderately sorted; well rounded; metamorphic quartz 2 Common quartz dominates, but metamorphic rock considerable vein quartz as well; fragments tr 85$ quartz grains have overgrowths; volcanic rock chert subangular; large sedimentary fragments tr rock fragment (2.5mm) is very fine feldspar tr sandstone, a submature clay 1 orthoquartzite. opaque minerals 1 zircon tr apatite tr Finlay Limestone beds of limestone that form the outer The massive, gray rim of about 20 miles northwest of the Finlay Mountains, Sierra Blanca, Texas, were named the Finlay Formation by Richardson (1904, p. 47). He specified no type section or but Brunson chose the ex- locality, (1954> P* 37) 205-foot Flat the north rim of the mountains posure in Canyon on as the type section of the Finlay. Thickness and fossils .--In the northwest Thickness,, lithology, part of the Devil Ridge area, the Finlay Limestone crops out at Texan Mountain and at Sand Mountain and in three southeast trending, roughly parallel exposures: the first is the low cuesta that leads southeast from Grayton Lake; the second is the on the of Devil and the exposure dip slope Ridge proper; third is the outcrop along Back Ridge where Yucca is in thrust contact with the underlying Finlay. This same Yucca- at small Finlay relationship may be seen a outcrop (G— 5) of Finlay on the north flank of Red Hills. In the Black Butte and where the Speck Ridge area, Finlay was intimately involved in complicated structural movements, it is the most conspicuous formation exposed in the near-vertical ridges and plunging folds. Nowhere in the Devil Ridge area is a complete section The feet in MS is medium of Finlay exposed. 588 16 a gray, pale yellowish brown weathering, thin-to thick-bedded, very finely crystalline, nodular limestone with a few thin beds of shale, siltstone, and very fine-grained quartz sandstone base. of near the There is a 5-foot zone brown-weathering, irregular, rounded chert nodules about 530 feet above the base of the formation. Near the north end of Devil Ridge at the point (D.3-2.5) where the old road crosses the lower of the southwest- part dipping Finlay, thin stringers of calcareous, cross-laminated, This pale yellowish brown-weathering siltstone crop out. siltstone is noteworthy in that it bears well-developed flute casts. The Finlay forms a series of resistant ledges separated by less resistant and characteristically covered beds; it weathers to a rough surface owing mainly to the dissolving action of meteoric water. A correlation between the of type solution feature and slope on the weathered surface of the Finlay was described by Smith and Albritton (1941> 61-78). p. Their plate 1, figure 3 was taken at the northwesternmost outcrop (D.4-2.2) of Finlay on Devil Ridge where solution furrows are unusually well developed. The almost flat-lying Finlay surfaces exposed south of Grayton Lake show extensive development of solution pits. The most diagnostic fossil of the Finlay is the fora­ minifer Dictyoconus walnutensis (Carsey), which at MS 16 occurs in a zone about 17 feet thick beginning about 355 feet above the base of the Finlay. Smith (1940, p. 615) reported the occurrence of D. walnutensis approximately 220 feet above the base of the formation along a line of section a short distance northwest of MS 16. Exogyra texana Roemer occurs throughout the Finlay. At or near MS 16, I found Engonoceras sp. about 50 feet above the base and Oxytropidoceras cf. 0, chihuahuaensis Adkins about 470 feet above the base of the Finlay. The lower 100 feet of the Finlay at MS 16 is typified by thin beds of hash A oyster (probably Exogyra sp.)« 9-foot zone of rudistids and about 400 feet above the caprinids begins base; a second zone, approximately 5 feet thick, of these about feet above the base. reef-building organisms begins 560 At JUDGE triangulation station (K.3-8.8) the underlying Toucasia aff. T. texana 10 feet be- Finlay yielded (Roemer); low is a zone of Dictyoconus walnutensis (Carsey). I also collected Tylostoma Lunatia and Nerinea just sp., sp., sp. below the Dictyoconus zone. Smith (1940, p. 615) reported Haplostiche texana (Conrad) in the Finlay, but I found this fossil only in the Eagle Mountains Sandstone. The lithology, fauna, and weathering habit of the Finlay in the Eagle Mountains is much like that in Devil Ridge. No­ where is a complete section exposed. In the Eagles, the is in the Strike and the Finlay exposed Lucky prospect Spar rock. Valley areas. It is everywhere faulted against younger Gillerman (1953, p. 24) reported the occurrence of corals in the zone of Toucasla texana (Roemer) 105 feet above the base of his measured section 4 in the Lucky Strike area. I collected Requienia sp., presumably from this zone, from the Finlay south of the mouth of Spar Valley. Giller- also the man (1953, P« 23) reported presence locally of oolitic limestone in the interval above the tTToucasia-reef beds.” Because it is repeated by a thrust fault, the Finlay out in two en echelon in the Indio Mountains crops exposures south of Peak. There the resistant limestone forms Squaw west-facing hogbacks and ridges; the west wall of Snake Canyon is a dip slope of Finlay. The formation is also well-exposed in the Lost Valley area; there as along the westernmost of the two outcrops to the north, there are com­ plete sections of the Finlay. At MS 5 (S. 8-15-8) I measured 401 feet of Finlay; Adams (1953# pi. 1# p. 31) measured 440 feet of Finlay 3*5 miles to the south near the mouth of Snake Canyon. The Finlay is much thicker in the Lost Valley area where Braithwaite in the northern Sierra (1958# p» 36) reported 797 feet; Pilares about two miles south of the Rio Grande, Ferrell measured feet of limestone (1958, p. 29) 709 equivalent to the to structural how- Finlay. According my interpretation, the sections south of Snake Canyon on blocks which ever, are were thrust northeast. These sections, therefore, are from farther out in the trough and greater thicknesses are to be expected. At MS 5 the Finlay is a medium gray to light brownish gray, thin-to thick-bedded, very finely crystalline lime­ stone that weathers to a pale and very orange grayish orange, rough surface. The lower 8.5 feet of the Finlay is a yellow- an ish gray siltstone; 8-foot grayish orange, fine-grained sandstone begins about 45 feet above the base. The Finlay contains largely unidentified microfossils and abundant macrofossils. As in areas to the north and northwest, the most diagnostic fossil is Pictyoconus walnut­ ensis (Carsey), which occurs in a 45-foot zone beginning about 223 feet above the base of the Finlay. I collected Engoneceras sp. from zones 53-125 feet and 193-224 feet above the base, and Pervlnquieria sp. from a zone 315-355 feet above the base of the Finlay. Reef fossils characteristic of the feet are upper 300 of the Finlay. At MS 5* I collected Toucasia texana Eoradiolltes (Roemer), Hequienia sp., Monopleura sp., sp., texana radiolitids, rudistids, caprinids, Exogyra Roemer, Adams collected and Exogyra sp. (1953, p. 34, 35) Alec­ tryonia carinata Lamarck* from this same zone in Lost Val- the ley. Braithwaite (1958, p. 39) reported occurrence in the feet the Lost upper 150 of Finlay in Valley, of Figure 5•-The relation of the west edge of Diablo platform to the Chihuahua trough and the progressive advance of the sea to the north during the Early Cretaceous Epoch. Shaded line, edge of the platform; a-a 1 , limit of Neocomian deposits; b-b', limit of Cox Sandstone, prohahly approximate position of shoreline, c-c', limit of Finlay Limestone and strata correlative (King, 1935j Kellum, 193&; Brunson, 195^-j Braithwaite, 1958) • Eoradiolites davidsoni (Hill), Toucasia patagiata (C. A. White), and Toucasia aff. . texana (Rosier). Lunatia which is common in the Glen Rose of cen­ sp., tral Texas, occurs in the lower half of the Finlay through­ out the Eagle Mountains and vicinity. Ranging throughout the Finlay are a wide variety of less distinctive pelecy­ and echinoids. pods, gastropods, Correlation and age is a widely recognized age.--The Finlay formation in Trans-Pecos Texas; it thins and contains more marl and sand northward and eastward from the Eagle Moun­ tains and vicinity. Twiss (1959a) reported 175 feet of Finlay in the Van Horn Mountains; Hay-Roe (1958, p. 54) esti mated 110-170 feet in the Wylie Mountains; and farther east in the Kent Brand and DeFord quadrangle. (1958, p. 374, 378) feet from the Cox dis- reported 40 of Finlay separated by a conformity. Huffington (1943, p. 1004) reported more than 100 feet of Finlay at Texan Mountain (top not exposed), where the zone of Dictyoconus walnutensis (Carsey) is within 50 feet of the base of the formation. Wade and Brunson re­ (1954, P* 50-53) (1954, p. 37-47) ported 256 feet of Finlay at Triple Hill, just north of Sierra and feet not Blanca, they reported 85 (top exposed) at Cox Mountain 16 miles northeast of Sierra Blanca. Accord ing to Clabaugh (1941, p. 8) and Adkins (1933, p. 354) the Finlay is absent in the Cornudas Mountains to the north; Adkins (1933, p. 347) also reported the absence of Finlay at Sierra Prieta to the northeast. King (i960) cited the occurrence of Finlay about 5 miles southwest of Sierra Prieta but DeFord r s suggestion that the basal questioned limestone at Sierra Prieta instead of is Finlay Campa­ grande* Contrasted with the Finlay in the map area and that in the of northeast Chihuahua, the Finlay at Pinto Can- ranges yon is a nearer-shore infraneritic deposit* It is about 300 feet thick and is composed of sand and marl at the base overlain by thick-bedded aphanitic limestone locally con­ taining abundant rudistids and zones of nodular chert (Amsbury, 1958)* In the ranges of northeast Chihuahua, the thickness of the Finlay stratigraphic interval averages about 650 feet; the upper part of the formation is characterized by chert nodules and silicified fossils. The Finlay thickens south­ west into the Chihuahua trough; in the Cieneguilla area, the formation is more than 700 feet thick and contains Dictyoconus walnutensis (Carsey) about 100 feet from the top (Reaser, 1962) & In the southern the consists Quitman Mountains, Finlay of 700-750 feet of massive reef limestone, limestone, sandy limestone, sandstone, and shale; the facies relationships along the length of the mountains are complex (Jones, 1962) Conditions of deposition must have been more varied there than those that prevailed during deposition of the Finlay to the north and east. The age of the Finlay Limestone is Middle Albian, and correlative the it is paleontologically with Fredericksburg Group of central Texas, i.e., the Walnut, Comanche Peak, and Edwards formations. Because the upper part of the Cox may lower of the of be synchronous with the part Fredericksburg central Texas, the Finlay of Trans-Pecos may be synchronous with all but the lowest part of the Fredericksburg Group of central Texas* neritic Origin .--The Finlay was deposited in a environ­ ment and in quieter and perhaps deeper water than the next older carbonate unit, the Bluff Formation. The vague dis- conformity at the base represent a brief interim of sub- may aerial erosion of the Cox surface prior to inundation; the thin sandstone beds near the base probably reflect a short retreat of the advancing sea* The warm, clear, marine water to reef bank was favorable rudistid-caprinid or development as well as to the support of a varied marine fauna. The fine sand silt that angular, very quartz and constitute a small part of several samples of the Finlay were probably swept by weak currents into the zone of micrite deposition. of Finlay Limestone Table 8«-Petrography of representative samples Very fine sandy and silty echinoid BIOMICRITE; U~5o4| MS 5, Unit 2, Indio Mts; Subangular, poorly sorted micrite quartz (Md 0.75nim)> intraclasts armored by sand and silt; and foram and fragments of pelecypods echinoids in micrite matrix with of scattered irregular patches sparry calcite; micrite and sparry calcite gradational in size; iron oxide along numerous styloliteso Fossiliferous MICRITE; U-5»12j MS Unit 7, Indio Mts; 5, Micrite matrix, perhaps slightly burrowed, containing scattered angular quartz silt, echinoid and pelecypod one micrite a fragments, few foram; calcite intraclast 2.75nmi and sparry several ellipsoidal masses sparry long; calcite, 0o5nim long® Foraminiferal BIOSPARRUDITE; U-5e22; MS 5, Unit 12, Indio Mts; Abundant tests to Dictvoconus walnutensis up 2.5mm (Md i 5mm) in poorly washed e calcite matrix also containing sparry echinoid and pelecypod fragments, scattered angular quartz pellets and filled with fractures iron oxide stain along silt; abundant sparry calcite; numerous stylolites. Pelecypod BIOSPARRUDITE; U-5°42; MS Unit IB, Indio Mts; 5, Abundant pelecypod fragments (probably assorted shapes and rudistid) of sizes to 5o0mm (Md 1.5mm) in sparry up calcite matrix that ranges widely in grain size; some appears dolomitic; many fossil fragments outlined by micrite calcite within and without with sparry of equal size. micrite 56 calcite 5 sparry quartz 10 intraclasts 2 echinoids 10 pelecypods B foram 5 pellets 3 tr calcispheres ostracods tr minerals 1 opaque micrite 90 2 calcite sparry 1 intraclasts tr pellets quartz 1 pelecypods 4 echinoids 2 tr foram minerals tr opaque calcite 30 sparry micrite 24 foram 35 echinoids 3 1 pelecypods pellets 5 quartz 1 tr plagioclase minerals 1 opaque pelecypods 60 calcite 30 sparry micrite 10 Benevides Formation nonresistant The Benevides Formation, a thin, largely of siltstone and sandstone, is one of several use- sequence ful siliciclastic units in the map area that separate the rather similar carbonate units of the Comanche Series. Amsbury (1958) proposed the name "Benevides" in the Pinto area for a 125-foot section of dark shale over- Canyon gray lain by a yellow-brown skeletal calcarenite. In the Van Horn Twiss similar Mountains, (1959a) recognized a sequence, that the is sandstone instead of except upper part quartz calcarenite. He justified his use of the term in the Van Horns on the basis of similarity of thickness, lithology, and fauna to the Benevides of Pinto Canyon. For the same I have extended the Benevides the Moun­ reasons, into Eagle tains and vicinity. Thickness, lithology, and fossils .--In the Thickness , lithology. map area, the Benevides characteristically has two members (undiffer­ entiated on plate l)s a lower, nonresistant and sparsely exposed siltstone and an upper, ledge-forming sandstone. The largest outcrops (D-6) of the Benevides in the Devil Ridge area are just south of Grayton Lake where gray­ ish pink, pinkish gray, and pale red fine-grained, calcareous and somewhat argillaceous and friable quartz sandstone is poorly exposed in the flat between ridges of Finlay and Espy limestone. South-southeast of Grayton, sandstone that crops out from place to place in the drainage channels that lead west into Triple tanks may also be Benevides. The poorly exposed very pale orange and pinkish gray, fine-to medium-grained, calcareous and somewhat argilla­ well-indurated sandstone that out near ceous, quartz crops the Love ranch house is shown on plate 1 as Benevides. The identification is based solely on superposition and lith­ ology; no fossils were recovered from these outcrops. In the Eagle Mountains, interbedded black, gray, and siltstone and limestone of the Bene­ yellow black, nodular vides are exposed in a small fault slice west of Espy Ridge. bravoense and 0. I collected Oxytropidoceras (Bose) genicu­ latum Conrad there, and Gillerman (1953* p. 25) collected the texana followings Exogyra plexa Cragin, Exogyra Roemer, Gryphaea navia Hall, Gryphaea corrugata Say, Pecten (Neithea) cf. P. subalpina (Bose), Pecten (Neithea) sp., Sphaera? sp., Cerithium? cf« £. bosquense Shumard, Tylostoma sp., Oxytropido ceras belknapi (Marcou), Oxytropido ceras aff. 0. trinitense cf. 0* autocarinatum (Gabb), Oxytropidoceras (Shumard), Oxytropidoceras sp., and a brachiopod(?). At MS 8 on the east flank of the Eagles in the valley bordered on the southwest feet of Bene­ by Wyche Ridge, 56 vides was measured. Olive gray and black sandy quartz silt­ stone is interbedded with sandy, very finely crystalline nod­ ular limestone. This is overlain by 28 feet of fine-to Photograph 4» Westward view of MS 8, Benevides Formation, from M.5-14»B; man standing on lower, nonresistant, interbedded gray shale and nodular limestone of Benevides Formation; above is fine- to sandstone medium-grained 28 feet thick, over­lain by sill of yellow-brown lower rhyolite at least 25 feet thick: Espy Limestone of Wyche Ridge on skyline, upper right. calcareous sandstone. Near the coarse-grained quartz top calcareous cement is replaced by siliceous and iron oxide the sandstone be "case hardened” and is cement; appears to stained iron oxide. The sandstone has been in- widely by truded by a sill of lower rhyolite 25 feet thick, above which more of the sandstone of the upper member is sparsely exposed. Fossils collected from this locality ares Craglnites sp., Pecten (Neithea) sp., Pervinquieria sp., and Pervinquieria? A short distance northwest of MS 8 sp. the sandstone member of the Benevides is poorly exposed at intervals in the bed and bank of one of the main drainage channels. Probably the most typical of the Benevides exposure occurs in the Indio Mountains in an outcrop extending south- southeast from the Bennett thrust fault. The lower, non­ resistant shale member forms an alluvium-covered strike val­ ley, whereas the upper very pale orange to pinkish gray mem­ ber forms a resistant ledge. At MS 5> the Benevides is 121 feet thick; only the sandstone member, 33 feet thick, is ex­ posed. This upper part is fine-to medium-grained, calcare­ ous, quartz sandstone; it is thin to thick bedded and cross bedded. The lower of this member is character- part upper ized by abundant, roughly spherical, calcite-cemented, fine- sandstone nodules to 12 cm across (Md grained quartz up 5 cm) . At the mouth of Snake Canyon, 6-10 feet below the top of the sandstone member, there is a slightly uneven surface that has been bored, probably by crabs (Perkins, i960), and the cross beds of the underlying unit have been truncated and channeled. Primary bedding above and that below the disconformity or diastem are concordant. There are three small outcrops in the Lost Valley area, where the formation is largely covered by terrace gravel, alluvium, and volcanic rock. On the east flank of the syn­ cline there are from bottom to top, 34 feet of shale with interbedded nodular limestone and 23 feet of calcareous sand­ stone with sandy shale interbeds (Braithwaite, 1958, p. 89)® The upper contact with the overlying Espy Limestone is con­ formable, but Braithwaite reported a disconformable contact at the base, because ’’small sedimentary dikes extend into the irregular surface of the Finlay limestone.” The upper part of the Benevides is largely covered at Lost Valley; the ,Tgray shale” and ’’light colored limestone” that Adams (1953, p. 38) cited as the uppermost units of the Benevides (i.e., his ’’Kiamichi”) are actually the basal units of the These basal units are in Espy. clearly exposed posi­ tion stratigraphically and topographically above the sand­ stone member of the Benevides in Snake Canyon at the point (U. 6-16.6) near Palmas well (abandoned) where the road crosses the sandstone member of the formation. Correlation and age .--In the northern Sierra Pilares, Ferrell (1958, 31, 33) reported that the Benevides inter- p. val attains a thickness of 729 feet of which 354 feet is a massive caprinid-reef limestone. This is a local development, because the reef is replaced at the south end of Ferrell’s map area by thin-bedded nodular limestone. Farther south, the Benevides interval in thickness from 311 feet ranges (Campbell, 1959, p* 28) to 637 feet (Yeager, 1960, p. 41) and is largely shale and limestone. In the Sierra de Ventana, the Benevides stratigraphic interval is 500 feet thick and of limestone and limestone composed sandy (Spiegelberg, 1961, p. 25). The platform section in Pinto Canyon, about 10 miles southeast of the marginal section of the Sierra de Ventana, is 125 feet thick and is shale and calcarenite. only In the Kent feet of interbedded sand- Quadrangle, 30 stone and shale at the base of the Levinson Member of the Boracho Formation contains fossils correlative with those in the Kiamichi and Duck Creek formations of north Texas and this section (Brand DeFord, 1958, p. 380); represents the eastward thinning of the platform facies of the Bene­ vides . Northward thinning of the Benevides is represented by the 56 feet of sandstone and interbedded in limestone, part fossiliferous, which crops out on Cox Mountain and is cor­ relative with the Benevides (Brunson, 1954, p. 95-97). Still further thinning of the Benevides is reflected at Sierra Prieta, about 30 miles northeast of Sierra Blanca, by the 26 feet of thin-bedded and cross-bedded sandstone that underlies beds correlative with the Duck Creek Forma- In the tion of north central Texas (Adkins, 1933, P» 354)* Cornudas Mountains, about 45 miles north of Sierra Blanca, no rocks correlative with the Benevides have been reported; the oldest Cretaceous rocks are paleontologically correla­ tive with the Washita Group of central Texas and rest on rocks of Permian age (Adkins, 1933, p. 354-355; Clabaugh, 1941, P-8). Baslnward thickening has been reported by Jones (1962) in the southern Quitman Mountains where the Benevides strati­ graphic interval is composed of about 300-400 feet of black and dark shale interbedded with nodular limestone. green Jones has collected Oxytropidoceras sp. from this section. The Benevides is not exposed in the northern Quitman Moun­ tains (Buffington, 1943, P* 1004); neither is it exposed in the Malone the mountains Albrit­ nor Finlay (Albritton, 1938; ton and Ham, 1941) Between Triple Hill and Flat Mesa, • about 5 miles north-northwest of Sierra Blanca, the Benevides stratigraphic interval consists of 108 feet of marl with interbedded sandstone and limestone overlain by 31 feet of cross-bedded sandstone with lenses of sandy limestone (Brunson, 1954, P* 83-85)• These exposures are the same as those cited as Kiamichi by Adkins (1933, p. 352 , 353 ) • The Benevides is paleontologically correlative with the Kiamichi of north central Texas and possibly with the lower part of the Duck Creek Formation as well. The age is Upper Albian • Origin.--TheOriginBenevides thins to a wedge edge north and east of the Eagle Mountains and vicinity and thickens greatly south and southwest where deposition occurred farther out in the trough. This formation represents a relatively brief but abrupt break in carbonate sedimentation. The disconformity in the upper part of the Benevides might be a regressive- surface of erosion which would a transgressive represent greater section of missing rock shoreward or north and east. The siliciclastic upper part of the Benevides would corre­ spond to the hypothetical intercalated shore formation of Grabau (1913, p. 736-738)* The local caprinid patch reefs in the formation in northeast Chihuahua probably grew along the margin of the platform but were not sufficiently contin­ uous to trap terrigenous material as it was brought in from the east. The varied thicknesses of the Benevides reported south and southwest of the area Indicate that local map irregularities existed in the floor of the basin. The fine-grained quartz sandstone of the upper unit varies from a calcitic mature fossiliferous dolomitic ortho­ quartzite near the bottom to a siliceous and slightly The lower calcitic mature orthoquartzite near the top. part of this unit is almost a sandy and silty dolomitic sparite. Common is the dominant constituent of the rock quartz and was probably derived largely from a sedimentary source rock. Rounded and angular quartz and chert grains of the same size as well as rounded grains of zircon and tourmaline indicate multiple source rocks, some of which were quartz­ itic sandstone. The grains of chert also suggest that chert bearing limestone in the source area was also a contributor to the material of the Benevides. The sandy shale and sandy limestone of the lower part of the Benevides (see MS 8) was deposited in a low-energy, neritic environment during a slight regression of the advanc ing Comanche sea. This regression continued until the pres­ ent area of the Eagle Mountains and vicinity was near the old the sandstone of shoreline, during which fine-grained Benevides The sand- the upper part of the was deposited. stone was deposited in an environment with sufficient energy the was not suf­ to remove clay-sized particles, yet energy ficient to alter the sand grains to a high degree of round­ ness. The dolomitic calcite cement indicates that the sea water had slightly higher-than-normal salinity. Deposition have occurred in a in which water circulation was may bay restricted. That part of the sandstone above the slight dis conformity was deposited as the sea once again moved north and east. This brought the present area of the Eagle Moun­ tains and vicinity into a low-energy environment in which the shale and limestone of the lower part of the Espy were depos­ ited. Table of the Benevides 9.—Petrography of representative samples Formation Fine sandstones Calcitic mature detrital fossiliferous dolomitic quartz 50 ORTHOQUARTZITE; U-5.43; chert 3 MS 5, Unit 20, Indio Mts; metamorphic quartz 2 0.20mm, well sorted; angular; metamorphic rock Quartz largely common; a few fragments tr grains up to 1,0mm; large grains pelecypods 1 both angular and round; small foram tr grains angular; sparry calcite ostracods tr ranges from 0.010mm up to 0,3mm; sparry calcite 33 much of it dolomitized, with dolomite dolomite 10 rhombs up to 0.08mm; tourmaline and chlorite tr zircon round; rock almost a sandy opaque minerals 1 and silty sparite. tourmaline tr zircon tr Fine sandstones Siliceous and detrital slightly calcitic mature quartz 81 ORTHOQUARTZITE; U-5«45; chert 1 MS 5, Unit 21, Indio Mts; authigenic quartz 8 0.20mm, well sorted; subangular; metamorphic Largely common quartz; Q0% quartz quartz 3 grains have overgrowths; chert angular metamorphic rock to round; some chert authigenic; fragments tr sparry calcite slightly dolomitized oyster 1 (Md 0.004mm); zircon round. echinoids 1 clay 1 zircon tr calcite 3 dolomite 1 opaque minerals tr Espy Limestone For- Huffington (1943, p. 1005) proposed the name "Espy mation" for "the Washita beds mapped by Smith (1940, pi. l) east of Love Hogback and the southeastern part of Devil Ridge ." Because of the distinctive, brown-weathering sandstone (the Eagle Mountains Sandstone) in the upper part of Huffing- ton's Espy and Smith’s Washita, the section is easily divis­ ible into three units, and the division is advantageous in structural mapping. I propose, therefore, to restrict the term "Espy Lime­ stone" to that body of rock between the Benevides Formation and the Eagle Mountains Sandstone. The type section is that rock exposed at MS 11 and MS 11a (pi. l). Thickness, lithology,lithology , and fossils .--The Espy crops out in a series of low, northeast-facing hogbacks east of Devil Ridge and Love Hogback. Northeast of the Speck ranch house, Smith (1940, p* 628-629) measured 6?6 feet of beds equiva­ lent to the Espy Limestone (restricted). MS 11 (G.O-8.8), also in this general area, records 2,193 feet of Espy; but this measurement includes inferred thicknesses of alluvium- covered flats between the hogbacks, whereas Smith’s measure­ ment does not. At MS 11, the thickness of the strata exposed in the low ridges of limestone is 736 feet. The estimated total thickness of Espy Limestone of 2,193 feet is probably too because the measured section traverse crosses great, a number of faults. On the other hand, the base of the Espy and therefore the measurement does not in- is not exposed clude all the formation. in Devil is The Espy Ridge largely medium light gray to brownish gray, very finely crystalline, thin-bedded, fossiliferous limestone that weathers to pale very orange. It is interbedded with less resistant marl. Iron oxide pseudomorphs after pyrite are common. The lowermost part of the Espy is black shale interbedded with thin beds of nodular limestone and is exposed in only one place (J.4-9*l) where colluvium has slipped downward on a steep slope. Fossils in the Espy exposed in the Devil Ridge area are largely restricted to five relatively thin zones, as follows? 5. Upper 100 feet Toucasia caprinids, - sp., Nerinea Nerinoides sp., Mortonlceras sp., sp., Enallaster sp., Pedinopsis sp. feet Lima wacoensis 4* 1,825-1,900 Roemer, above the base Feeten (Neithea) george­ townensis Pervin- Kniker, quieria sp., Tetragramma Enallaster Pedi­ sp., sp., nopsis sp. 3. 1,510-1,660 feet Prohysteroceras sp., above the base Holectypus sp. 2. 430-530 feet Pecten (Neithea) texanus above the base (Roemer), Pecten (Neithea) georgetownensis Kniker, Cyprimeria sp., Gryphaea washitaensis Hill, wlntoni Mortoniceras cf. M. (Adkins), Mortoniceras sp., Drakeoceras cf. drakei JD. Young, Drakeoceras cf. JD. gabrielensis Young, Drakeoceras Para­ sp., cymatoceras texanum cf. (Shumard), Pervinquieria P. trinodosa (Bose) Eutre­ , phoceras Enallaster sp., sp., Epiaster elegans, Holaster simplex Shumard. 1. Lower 50 feet Gryphaea washitaensis Hill, N erino ides sp., Engono­ ceras Mortoniceras sp., sp., Pervinquieria sp. MS 11a supplements MS 11. Although there is some over­ lap, the rock exposed at MS 11a is largely covered at MS 11. at MS 11a extends from The section exposed approximately feet above the base of the and is 1,660 to 1,830 Espy clearly more more more homogeneous, fossiliferous, yellow-weathering, and more nodular than rock immediately above and below it. collected, among others, the following fossils from MS 11a: wacoensis Haplostiche texana (Conrad), Kingena (Roemer), Gryphaea washitaensis Hill, Pecten (Neithea) texanus (Roemer), Pecten (Neithea) georgetownensis Kniker, Protocardia texana (Conrad), Mortoniceras cf. M. wintoni (Adkins), Mortoniceras Plesioturrilites sp., Plesioturrilites brazoensis (Roemer), sp., Paracymatoceras sp., Enallaster sp., Holectypus sp., and Pedinopsis sp. Probably the most unusual occurrence of the Espy is in the tight, synclinal fold in the limestone at the base of Black Butte (K.O-9*0); the Espy also crops out (J-8, 9) in the Indian Springs area and on the steep cliffs that form the west flank of the Eagles. Because of the similarity in lithology and the presence of Kingena wacoensis (Roemer), the Espy exposed in the immedi­ ate vicinity of Indian Springs is roughly correlative with that at MS 11a. The folded limestone beneath exposed sharply Black Butte was identified as Espy on the basis of the occur­ rence there of Gryphaea washitaensis Hill, Pecten (Nelthea) georgetownensis Kniker, Nerinoides sp., Nerinea sp., Enallaster Pedinopsis sp. and Hemiaster The occurrence sp., sp. of Kingena wacoensis (Roemer), Gryphaea aff. G. graysonana Stanton, Pecten (Neithea) georgetownensis Kniker, and Pedinopsis sp. in the gently dipping limestone beds that out in the cliffs that form the west flank of the crop Eagles identifies them as Espy. Although not in his map area, Gillerman (1953, P« 30~ 31) collected fossils from the uppermost Espy (his reef limestone member of the Grayson Formation) in the low ridges on the northeast flank of Devil Ridge; he did not specify the exact location. In addition to several species of pelecypods and gastropods, a varied collection of corals was made there, among which J. W. Wells recognized 15 genera and 9 new species (Gillerman, 1953, P* 30). Poorly exposed limestone beds (B. 9-2.3) north of Yucca Mesa are shown on plate 1 as Espy on the basis of lithology and stratigraphic position. I found no diagnostic fossils there. In the Carpenter Spring area and in Wyche Ridge the Espy has been repeated by the Carpenter fault; in Wyche Ridge, the Espy has been intruded by sills of lower rhyolite as well as by dikes of a dark rock. In the Moun green Eagle tains the thickness of the Espy stratigraphic interval is about 95G feet, the lower 800 feet of which is interbedded gray shale and black limestone, the lower half of which the Strike yielded, in Carpenter Canyon and near Lucky prospect Kingena sp., ostrea (Arctostrea) carinata (Lamarck), Gryphaea washitaensi s Hill, Pecten (Neithea) subalpinus Pecten cf. (Bose), (Neithea) texanus (Roemer), Pervinquieria JP. kiliani (Lasswitz), Pervinquieria cf. P. trinodosa (Bose), cf. brazoense Pervinquieria sp., Eopachydiscus E. (Shumard) and many others (Gillerman, 1953* p« 25-26). The succeeding 90 feet of nodular, thin-bedded, black limestone (Gillerman’s "Carpenter Limestone Member of the is fossiliferous in Can- Grayson Formation”) very Carpenter yon where Kingena sp., Exogyra arietina Roemer, Exogyra drakei Cragin and others were collected (Gillerman, 1953 , P* 24-30). This is overlain by about 60 feet of massive, gray, rough-weathering limestone (Gillerman’s "reef-lime stone member of the Grayson Formation") which is characterized by abundant caprinids and rudistids. the northwest of the road Espy exposed along end spur to the Lucky Strike prospect yielded Beudanticeras sp., Eopachy­ discus sp., Mortoniceras sp., Pervinquieria aff. P. equi­ distans (Cragin), Pervinquieria equidistans (Cragin), and Cymatoceras sp. In the Wyche Ridge area, the thin-bedded limestone and marl of the lower 100 feet of the Espy yielded Pervinquieria? Pervinquieria equidistans (Cragin), Eopachydiscus sp., brazoense and Mortoniceras (Shumard), Eopachydiscus sp. sp., One isolated, linear outcrop of limestone low on the Photograph 5* Northwestward view along Wyche Limestone in- Ridge from M.9-14»8; gray Espy truded by sills of lower rhyolite that weather orange; largest Yucca trees in center are more than 6 feet high. southern flank of the Eagles beneath Eagle Bluff is mapped as Espy because it yielded Pedinopsis sp. At the mouth of Broad Canyon the road crosses an out- of limestone. Without diagnostic fossils, it re- crop gray sembles the Espy Limestone lithologically. Because well- bedded and interbedded volcanic and sedimentary rock underlie this block of limestone and because the attitude of these beds differs from that of the beds of the lime- limestone, stone block may be a xenolith. Its origin may, however, be similar to that of the erratic blocks of the Bluff Formation. It is also possible that this block of Espy is in normal or slightly faulted position stratigraphically above the Finlay that out to west. crops immediately the In the Indio Mountains, the upper strata of the Espy form the major part of Willoughby Ridge, in which all forma­ tions are overturned; the high ridge east of Green Peak is made up largely of the Espy, the upper part of which is char­ acteristically thicker bedded and more resistant than the lower also out t in the Lost part. Espy crops (l) Valley (2) in a small window (V.3-17-2) in the thrust area, sheet, and (3) in a small outcrop (5.4-17-1) just west of Green River. The color of the in the Indio Mountains Espy ranges from medium dark to brownish and the rock gray light gray weathers very pale orange and yellowish brown. It is very finely crystalline and thin to thick bedded; karren are com mon on weathered surfaces. The Espy is 1,094 feet thick at MS 6, however the line of section crosses a fault of unknown magnitude. The shale and interbedded nodular limestone and marl of the lower 50-60 feet of the Espy are very fossiliferous, especially in the Lost Valley area. I collected the follow­ ing fossils from this zone: Kingena wacoensis (Roemer), Pecten (Neithea) texanus (Roemer), Gryphaea washitaensis Hill, Trigonia stolleyi Hill, Pervinquieria equidistans (Cragin), Pervinquieria aff. P. equidistans (Cragin), Pervinquieria sp., Pervinquieria? sp., Craginites? sp., Eopachydiscus sp., Eopachydiscus? sp., Idiohamites sp., Idiohamites? sp., Goodhallites aff. G. aguilerae (Bose), Pervinquieria cf. P. equidistans (Cragin), Mortoniceras Mortoniceras? Beudanticeras Elobiceras sp., sp., sp., sp., Hemiaster cf. H. elegans Shumard, Hemiaster Holaster sp., simplex Shumard, Epiaster sp. Braithwaite fossils this (1958, p. 47-48) reported from same zone: Prohysteroceras sp., Eopachydiscus brazoense (Shumard), Mortoniceras aff. shumardi (Marcou), and Pedi­ nopsis aff. _P. symmetrica (Cragin), At MS the more resistant 6 (T. 4-16.3), thick-bedded, limestone beds from about 460 to 720 feet above the base yielded the following fossils: Pecten (Neithea) georgetownensis Kniker, Pecten (Neithea) roemeri Hill, Phola­ domaya cf. P. sancti-sabae Roemer, Gryphaea washitaensis Hill, Turritella Nerinea Salenia sp., sp., sp., Tylostoma sp., Alepes Drakeoceras drakei Drakeoceras Cyma­ sp., Young, sp., toceras sp., Pervinquieria equidistans (Cragin), Holectypus cf. H. planatus Roemer, Holectypus sp., Enaliaster sp., Tetragramma sp., Pedinopsis sp., and Hemlaster sp. Large rudistids occur from about 735 to 770 feet above the base of the formation. An 194-foot fossiliferous zone extends from about upper 900 feet above the base to the top of the formation; about the feet characterized rudistids and upper 95 are by caprinids as well as by Gryphaea washitaensis Hill, Pecten (Neithea) georgetownensis Kniker, Pect en (Neithea) sp,, Pholadomaya sancti-sabae Tylostoma Turritella and Roemer, sp., sp., Enallaster sp. The Isolated outcrop of Espy Limestone just west of the road leading north from the Rio Grande to Lost Valley yielded these fossils? Pervinquieria equidistans (Cragin), Pervin­ quieria cf. JP. equidistans (Cragin), Mortoniceras sp. , Epiaster cf. E. elegans, Epiaster sp., Enallaster sp., and Tetragramma sp. The identification of the limestone outcrop (V.B-17.0) immediately east of Campo Bonito as Espy is questionable, because the lithology is not definitive and I found only the following fossils there s Exogyra sp., N erlnoide s sp., gastropods, Hemiaster sp., and Enallaster sp. North of the water in this limestone there are brown- gap outcrop, weathering nodules of chert which resemble silicified rudi­ stids and caprinids. Chert nodules are common in the upper part of the Finlay in the ranges in northeast Chihuahua this (Atwill, 1960, fig. 9); thus, outcrop may well be earlier workers as Finlay it was mapped by (Allday, 1953? pi. 1; Adams, 1953? pi* l). Its proximity and general lith­ ologic resemblance to the outcrops of Espy to the north, how­ ever, led me to map the limestone as Espy. Correlation and ageage.--In the northern Quitman Mountains, the "Espy Formation" (unrestricted) of Huffington (1943? p. 1005-1006, 1011) is unfossiliferous and more than 1,287 feet thick, and it consists from the base up of 680 feet of inter- bedded sandstone and sandy limestone, 340 feet of platy, and and feet white, arenaceous argillaceous limestone, 266 of fine-grained greenish sandstone. The lack of fossils, the the relatively greater quantity of sandstone, and gen­ eral lack of resemblance to Smith 1 s "Espy" of the Devil Ridge lead me to question Huffington’s correlation of the two units. Huffington (1943, plo l) mapped the strata that underlie the Etholen at Etholen Knobs as Espy, but they could well be correlated with the Chispa Summit Formation® If in of the northern is fact Huffington’s "Espy" Quitmans correlative with the Espy of Devil Ridge, a marked change in facies occurs along strike. The Espy has been completely removed by erosion in the Malone Mountains, in the Finlay Mountains, at Cox Mountain, and at Triple Hill, although correlative beds do crop out in northwest of Sierra Blanca* places At Sierra Prieta, Adkins (1933 P» 354) identified the "Duck Creek," "Fort Worth," "Denton?" and "Weno?" which con­ sist of and limestone. The first three have sandstone, marl, a combined thickness of more than 92 feet. The Cretaceous section in the Cornudas Mountains is not well known. Richardson (1904, p. 49) reported 112 feet of limestone shale, sandy shale, and sandy there; Arick measured a section in the Cornudas, the location of which is not known, and recognized rocks equivalent to the Denton, Fort Worth and Duck Creek? (Adkins, 1933, P* 355)* The lower 28 feet of the 35-foot "Duck Creek?" is fine sandstone and lime­ stone-pebble and chert-pebble conglomerate. The Espy also thins eastward over the platform; in the Van Horn Mountains the Espy, called Loma Plata Limestone, is 685 feet thick (Twiss, 1959a). Farther east at Kent, the correlative stratigraphic equivalent consists of about the 110 feet of the Levinson Member and all the 260-foot upper San Martine Member of the Boracho Formation (Brand and DeFord, 1958, p. 374, 379-385). In the Wylie Mountains, Hay-Roe (1956, p. 55) estimated the maximum thickness of the Boracho to be 230 feet; the interval is Espy stratigraphic probably slightly less. A of the thicker section at Kent and in the Van comparison Horn Mountains with the estimate of Hay-Roe suggests that the been Wylie Mountains may have a local high during deposition of the Espy. The type section of the Loma Plata Limestone in the Pinto Canyon area, 720 feet thick (Amsbury, 1958), occupies about the same position relative to the Diablo platform as the Van Horn section to the north. From the Pinto Canyon area the correlative strata thickens westward into the Chi­ huahua trough. The Loma Plata Limestone in the Sierra de Ventana is 944 feet thick (Spiegelberg, 1961, p. 29-30); the thickness from to feet northward in the Sierra ranges 1,004 1,785 Pilares (Campbell, 1959, p. 30-32; Ferrell, 1958, p. 19)* In the sections just cited, the lower strata of lime­ stone are characteristically thin bedded and non-resistant; the upper limestone strata are thick bedded, massive, and resistant . habit reversed the This outcrop is in Cieneguilla area, however, where the lower 300 feet of thick-bedded limestone forms a prominent ridge and the upper 250 feet forms a slope (Reaser, 1962). This same outcrop habit is expressed in the southern Quitman Mountains where the correlative section, feet contains an invertebrate fauna similar to 1,120 thick, that of the Espy in the Eagle Mountains and vicinity. The age of the Espy is Late Albian to early Cenomanian; it is paleontologically correlative with the Georgetown Lime­ stone of central Texas and is approximately paleontologically correlative with part of the Duck Creek, as well as the Fort Worth, Denton, Weno, Pawpaw, and Main Street formations of northeast Texas. Origin.--TheOriginEspy is largely a biomicrite that contains zones of abundant micro-and macrofossils. The macrofossils have been described elsewhere; the microfossils consist of ostracods and foraminifera plus calcispheres• Fragments of echinoids and oysters are common. The thick relatively homogeneous, very finely crystal­ line (micritic) fossiliferous limestone of the Espy repre­ sents a rather long period of deposition in quiet, warm water along the margin of a stable platform or shelf, i.e., a low­ neritic environment. The abundance of rudistids and energy caprinids at different stratigraphic levels indicate that reefs developed from time to time. The intrasparrudites (tbl. 10, thin-section no. U-6.30) were developed in the rel­ atively higher-energy reef environment. Limestone Table 10.—-Petrography of representative samples of Espy micrite Foraminiferal BIOMICRITE; U-6.1; 73 calcite 1 MS Unit 1, Indio Mts; sparry 6, tr Micrite matrix (Md 0.003mm); oysters foram 15 abundant foram, calcispheres(Md 0.04mm), 2 ostracods echinoid and oyster fragments; echinoids 4 scattered irregular patches sparry 1 fractures filled with opaque minerals calcite; faint indication of calispheres 4 sparry calcite; burrows. micrite 95 Calcispheric MICRITE; U-6.6; MS Unit 3, Indio Mts; calcispheres 4 6, foram tr micrite matrix Homogeneous echinoids 1 (Md 0.004mm); many calcispheres in minerals tr have mineral (orange opaque opaque reflected light) in center. micrite 84 Fossiliferous MICRITE; U-6.23; calcite MS 6, Unit 7, Indio Mts; sparry 5 echinoids Micrite matrix (Md 0.002mm); sparry 3 calcite fills burrows and micro- pelecypods 3 ostracods tr iron oxide in fractures; (orange 1 reflected light) concentrated in gastropods tests and shells foram 2 calcite of sparry well as in opaque minerals 2 of fossils as many calcite that fills burrows. sparry calcite 41 INTRASPARRUDITE; U-6.30; sparry MS Unit Indio Mts; pelecypods 8 6, 10, reef-like rock; intraclasts 50 Heterogeneous echinoids 1 large pelecypod fragments plus abundant smaller fragments pelecypods and echinoids; intraclasts largely micrite, grade into sparry calcite but many inward from margin; many are elongate and most are round; intraclasts range up to 2.5mm (Md 0.65mm) 0 Pelecypod BI0MICR0SPARITE; U-6.41* microsparry MS Unit Indio Mts; calcite 40 6, 14, micrite Matrix grades from microsparry calcite to 32 calcite 3 micrite; abundant pelecypod fragments; sparry sparry calcite fills fractures; pelecypods 25 abundant pellets (Md 0.075mm); some microsparry calcite may actually be dolomite. Mountains Sandstone Eagle and calcareous The platy, orange-brown-weathering, Eagle Mountains Sandstone is in color, lithologic, and faunal contrast to the and conformably underlying overlying Comanche limestone. gray Gillerman (1953, p. 27) divided his "Grayson formation" Mountains into three members in the Eagle on "lithological and faunal bases." From oldest to youngest they are the Limestone Member," the "reef-limestone "Carpenter member," and the "Eagle Mountains Sandstone Member." He described the follows youngest as (1953, p. 31)? The Eagle Mountains Sandstone member of the Grayson formation, named from the excellent exposures on the north side of the Eagle Mountains, is also a distinct and persistent member and is found throughout the mapped area adjacent to the reef-limestone member except where rhyolite intervenes. About 70 feet of interbedded brown sandstones (some of which are quarzitic), brown shales, siltstones, and sandy limestones make up the member* The characteristic brown color of the beds contrasts strongly with the dominant blue-gray color of the other formations of late Comanche Exogyra cartledgi age. /"sic? Bose is characteristic of the Eagle Mountains member and is abundant in the beds immediately overly-the reef-limestone member. Other fossils identi­ ing fied are Protocardla and Trigonia? sp., sp. Gillerman (1953, p. 27) designated the type section of the three members as Carpenter Canyon, about a mile above Carpen­ ter wells. Thickness, lithology and fossils .--I consider the Eagle t Mountains Sandstone as a formation. It comprises that sequence of orange-and brown-weathering interbedded sandy limestone, calcareous and shale that rests on the lime- sandstone, gray stone of the Espy and that is overlain by the nodular, gray limestone of the Buda. In the Van Horn Mountains Twiss (1959a; 1959b, p. 39) described 29 feet of interbedded lime­ stone and shale that becomes sandy toward the Eagle top, as Mountains Sandstone. In the Devil Ridge the Eagle Mountains Sandstone area, crops out in the low northwest-trending hills east of Devil Ridge, Love Hogback, and the Speck ranch house. At MS 12 there are feet of Mountains (G.7-8.7) 130 Eagle Sandstone, much of it covered. Of the part exposed nearly 80 percent is sandstone with brown-weathering, very fine-grained quartz calcareous and iron oxide cement. The rock is characteristi­ cally thin bedded as well as laminated and cross laminated. Platy, angular fragments of the sandstone litter the surface and are apt to be desert varnished. Exogyra cartledgel Bose in great abundance and Gryphaea graysonana Stanton occur about 10 feet above the base of the formation in a 1-foot oyster hash® Numerous small unidenti­ fied clams averaging about 7.5 mm in maximum dimension occur at intervals through the formation. Weathered surfaces of the lowermost sandstone unit show small characteristically depressions, averaging 13-15 mm wide and 8-10 mm deep, that indistinct outline. have an heart-shaped Perkins (i960) the burrows of suggested that these depressions represent clams in soft sediment. In the Eagle Mountains, the Eagle Mountains Sandstone crops out southeast of Eagle Spring in a ridge that runs parallel to and just west of the northwest-trending part of the Lone Hill fault. Farther southeast, the Eagle Mountains Sandstone is exposed in the Carpenter Springs area and along Wyche Ridge; in both areas it is repeated by a high angle thrust fault. South in the Indio Mountains, the Eagle Mountains Sand­ stone is sparsely exposed in four areas: (l) in an over­ turned attitude along the west flank of Willoughby Ridge, (2) east-northeast of Green Peak where it out low on crops the east flank of the highest ridge of the southern Indio (3) in the folded inlier west of Escondido Mountains, highly well, and (4) just north of the Rio Grande in the Lost Valley area. At MS 6 in the southern Indio Mountains, the Eagle Moun­ tains Sandstone is ?8 feet thick. Nineteen percent of the section is covered, but of the exposed part 92 percent is orange-and brown-weathering sandstone or sandy limestone similar to that at MS 12. Within about 17 feet of the base of the formation at MS 6, the large, agglutinated foraminifer Haplostiche texana (Conrad) occurs with Exogyra cartledgei Bose. of brown sandstone north of the Rio At an outcrop just Grande and east of the road that leads north to immediately Bose and the Lost Valley area, Exogyra cartledgei Gryphaea graysonana Stanton have been collected; Metengonoceras sp. was also found in the Eagle Mountains Sandstone in the south­ ern Indies (Braithwaite, 1959* p. 50). Correlation and age.--Across the Rio Grande in the age northern Sierra Pilares, the Eagle Mountains Sandstone is and shale with thin beds composed of 106 feet of gray orange of sandstone and sandy limestone scattered throughout (Fer­ rell, 1958, p. 36-40). Fossils are abundant in the section; Plesioturrilites brazoensis (Roemer) occurs near the base; Exogyra arietina Roemer, 24 feet above the base; Haplostlche texana (Conrad), 54 feet above the base; Gryphaea graysonana Stanton and Exogyra cartledgei Bose, 10 feet from the top. In addition to Haplostiche texana, Ferrell also identified 14 other of foraminifers• genera The distinctive Eagle Mountains Sandstone is unusually helpful in geological mapping in most of the ranges to the south in Chihuahua. In the southern Sierra Pilares Yeager (i960, p. 46-47) reported 41 feet of Eagle Mountains Forma­ tion, composed of shale rather than sandstone and limestone. The formation thins along what was probably depositional strike over reefs of Espy that extended upward into the Eagle Mountains sea. Southeast, in the Pinto Canyon area, the Mountains Formation thickens to 87 feet and is composed Eagle of limestone interbedded "light gray, nodular, argillaceous marl” with light gray to light yellow-gray (Amsbury, 1958). In the Cieneguilla area, west-southwest of the Indio the formation is feet thick and is Mountains, 300 composed largely of shale with sandstone interbeds (Reaser, 1962). Jones (1962) has reported that the correlative strata in the southern Quitman Mountains comprise 285 feet of brown sandstone, brown-weathering limestone, and shale. Jones has identified Haplostiche texana (Conrad), Exogyra cartledgei Bose, and nerinids, as well as rudistids and caprinids, which occur in a 5-6-foot orange-weathering black limestone within 50 feet of the base of the unit. The Eagle Mountains Sandstone may be exposed in the northern Quitman Mountains but it is not readily identifi­ ,T Tt able in the measured section of Buffington’s Espy(1943, sediments p. 1011). Contemporaneous were probably widely deposited north, northwest, and northeast of Sierra Blanca, but subsequent erosion has removed them. The Eagle Mountains Sandstone is paleontologically cor­ relative with the Del Rio Clay of southwest Texas but proof of between the Del Rio and lateral continuity Clay the Eagle Mountains is The of the Moun- Sandstone lacking. age Eagle tains Sandstone is early Cenomanian. Origin .--The Eagle Mountains Sandstone changes upward to from a biosparite an orthoquartzite and then to a sub­ graywacke. The terrigenous material is angular and well sorted and consists of quartz silt and very fine-grained quartz sand. Glauconite, cellophane, muscovite, zircon, and tourmaline are minerals. Foraminifers, ostracods accessory and of and echinoids are common. fragments oysters Common quartz constitutes most of the terrigenous mate­ rial; the major source rock was probably quartz sandstone. Braithwaite (1958, p. 49) found small quantities of biotite, plagioclase, and igneous rock fragments in the Eagle Mountains Sandstone and suggested that these constituents had a distant volcanic source. I found only a trace of plagioclase in the formation in the map area. Deposition was in an environment with sufficiently high energy to remove the micrite matrix and sort the terrigenous material but without sufficient to round the sand and energy silt grains® Shells of invertebrate animals may have been broken in the environment of deposition; they could, however, have been broken elsewhere and transported by relatively weak currents• Following deposition of the Espy Limestone in a low­ neritic the area was into the lit- energy zone, map brought toral zone as a result of (l) a flood of terrigenous debris seaward and concomitant of the regression shoreline, (2) a broad of the land area with concomitant regression of upwarp the sea, and (3) an eustatic drop in sea level, and (4) some combination of these as well as other factors, such as change in and currents. The Moun­ climate, drainage patterns, Eagle tains Sandstone represents only a brief pause in the general northward advance of the Comanche sea. Table 11,—Petrography of representative samples of Eagle Mountains Sandstone Glauconitic, silty, very fine sparry calcite 37 sandy echinoid BIOSPARITE; U-6,45; quartz 40 MS 6, Unit 15, Indio Mts; metamorphic quartz tr Sparry calcite and microsparry calcite chert tr matrix with angular,well-sorted glauconite 3 very fine quartz sand and quartz intraclasts 4 silt; largely common quartz; glauconite foram tr subround and partially altered to iron pelecypods 4 oxide; zircon round; intraclasts are echinoids 8 micrite; may be alightly dolomitic. ostracods 2 zircon tr opaque minerals 2 Glauconitic, silty, very fine sandy sparry calcite 30 BIOSPARITE; U-6.47A; quartz 40 MS 6, Unit 15, Indio Mts; metamorphic quartz 1 Matrix is cloudy to clear sparry chert 1 calcite; quartz sand and silt glauconite 2 angular; largely common quartz; echinoids 4 glauconite (Md 0,075mm) angular pelecypods 15 to round; zircon subround to angular; ostracods 4 microfractures filled with sparry foram tr calcite; sparry calcite is zircon tr poikiloblastic, opaque minerals 3 tourmaline tr brachiopods tr Silty very fine sandstone; calcitic detrital mature ORTHOQUARTZITE; U-6.51; quartz 60 MS 6, Unit 18, Indio Mts; 0,065mm, chert tr very well sorted; angular; metamorphic quartz 1 Quartz largely common; all grains cellophane tr angular; many are elongate and glauconite tr subparallel; laminations caused by opaque minerals 3 thin zones micrite; burrowed?; zircon tr glauconite (Md 0.036mm) angular; muscovite tr zircon round; micropoikiloblastic. sparry calcite 30 pelecypods tr echinoids tr micrite 6 Table 11.—Continued Silty, very fine sandstone; detrital calcitic mature SUBGRAYWACKE; quartz 45 U-6.56; chert 2 MS 6, Unit 21, Indio Mts; 0.065nan, metamorphic quartz 6 very well sorted; angular; metamorphic rock Largely common quartz; many fragments tr grains elongate and subparallel; cellophane tr rock finely laminated; laminae plagioclase tr outlined by concentration of opaque minerals 25 opaque minerals; one plagioclase pelecypods tr grain observed-angular and fresh. ostracods tr muscovite tr zircon tr sparry calcite 22 Buda Limestone The top of the Buda Limestone coincides with the top of the Comanche Series in the area. The nodular lime- map gray, stone of the where it is exposed along with the under- Buda, and Summit lying Eagle Mountains Sandstone overlying Chispa shale, stands out in marked contrast to those orange-and brown-weathering siliclastic units. Lithologically, the Buda and Espy limestones are not easily distinguished. In the Devil Ridge area, the Buda crops out in the hills northeast of Devil Ridge, Love Hogback, and the Speck ranch house. In the Eagle Mountains, the Buda crops out in the Carpenter Spring area and along Wyche Ridge; it has been the The Buda repeated in both areas by Carpenter fault. out in the Indio Mountains in an overturned position crops along Willoughby Ridge and in two small outcrops (T-16, 17) just east of the high, north-trending ridge in the mountains. Thickness , lithology ,and fossilsfossils.--The Buda is 239 t t feet thick at MS 12 (G.7-8.7) in the Devil Ridge area, 225 feet in Carpenter Canyon (Gillerman, 1953* p. 31), and 218 feet thick at MS 6 (T. 4-16.3) in the Indio Mountains, where the part is truncated by a thrust fault. The Buda upper the Mountains and conformably overlies Eagle Sandstone, is, in turn, conformably overlain by the Chispa Summit Formation. In a few places where the contact of the Buda and Chispa evidence of is Summit is exposed, an unconformity lackingo The Buda is a characteristically light brownish gray, very finely crystalline, nodular, thin-bedded limestone that weathers very pale orange-pale yellowish brown. Microscopic fora shell fragments, largely gastropods and pelecypods, and minifers occur in varied abundance throughout the Buda; macrofossils are sparse in the Buda in the Indio Mountains, although the lower 46 feet of MS 6 yielded Pecten (Neithea) Alectryonia cf. A. carinata Turritella sp., Lamarck, sp., and Hemiaster sp. At or in the vicinity of MS 12 in the Devil the lower of the Buda contains abun- Ridge area, part dant macrofossils, including Cyprimeria sp., Pholadomya Pecten Anchura Tur­ sp., (Neithea) sp., Trigonia sp., sp., ritella sp., Tylostoma sp., Budaiceras sp., and Enallaster sp. From Carpenter Canyon, Gillerman (1953» P» 3l) reported Lima shumardi Shattuck, Pholadomya shattucki Bose, Isocardia Exogyra Turritella Nerinea Tylostoma cf« sp. , sp., sp., sp», To harrisi Whitney, and Enallaster texanus (Roemer)• In addition he cited the occurrence of a persistent bed of Tur­ ritella and Nerinea at the base of the formation immediately above the Mountains Sandstone. I did not find these Eagle fossils in this position elsewhere in the area, but immedi­ ately northeast of the Carpenter fault the uppermost Buda contains abundant turritellid gastropods. south Correlation and age --From the Buda just of the Rio Grande in the northern Sierra Pilares, Ferrell (1958, p. 41-42) collected the following cephalopods: Plesioturrilites brazoensis (Roemer), Budaiceras mexicanum Bose, Budaiceras re­ cfr. B. curvata (Lasswitz), and Stoliczkaia sp. He also 15 of Foraminifera from the Buda at this loca­ ported species tion . Eastward the Buda does not thin, relatively, as much as of the older Cretaceous formations. It is 135 feet many thick in the Van Horn Mountains (Twiss, 1959a) and slightly more than 160 feet thick in the Kent where it is Quadrangle, bounded disconformities (Brand and p. by DeFord, 1958, 374, sandstone in the lower of the Buda 385)* There, part may represent the Eagle Mountains Sandstone. In the Pinto Canyon area, the Buda is only 5 feet thick, but the top is not exposed. The rock is typically light gray, nodular limestone, and the formation is overlain by a conglomerate of Tertiary age (Amsbury, 1958). Incomplete sections of the Buda are exposed throughout most of the Sierra Pilares, but westward in the Cieneguilla area and in the southern Quitman Mountains the Buda is a gray, nodular and is feet thick limestone, 200-300 (Reaser, 1962| Jones, 1962). Like the Eagle Mountains Sandstone the Buda in the northern Quitman Mountains is not readily identifiable within Buffington’s (1943, p. 1011) "Espy v Buda outcrops are not , present in the Malone and Finlay mountains, at Triple Hill, Flat Mesa, Cox Mountain, Sierra Prieta, or the Cornudas Mountains; the Buda may crop out, however, from place to place between these well-known localities. No doubt it was present in all this area at one time, but in most places it has been removed by erosion. It does occur at Cerro de El where it consists of about 80 feet of near Muleros, Paso, limestone (Adkins, 1933* p. 368). Buda is the only central Texas name that has been used in the map area: the formation can be traced from central Texas into western Trans-Pecos Texas and very likely was rock once a continuous body of (Brand and DeFord, 1958, p. of the Buda is its in 385). The age Cenomanian; top Trans- Pecos Texas, northern Chihuahua, and in central Texas coin­ cides with the top of the Comanche Series. Origin12 shows the of Origin.--Table petrographic summary of Buda. foraminiferal representative samples the It is a biomicrite which also contains fragments of echinoids, oysters, ostracods, as well as calcispheres. The Buda also contains up to 1 percent of angular quartz and feldspar grains, which range in size from silt to fine sand. Some of the feldspar is authigenic. Pellets and burrowed zones are other common features of this formation. The very finely crystalline limestone (micrite) of the Buda originated in a low-energy neritic or lagoonal environment where lime mud and very little terrigenous mate­ rial were deposited. Gentle currents probably swept the silt and fine sand as well as the fossil fragments into the area of deposition. The relatively consistent lithology and thickness of the Buda over a large area indicate that the area was tectonically stable and that uniform conditions pre­ vailed . Table 12.—Petrography of representative samples of Buda Limestone Foraminiferal and calcispheric micrite 88 MICRITE; U-6.59; sparry calcite 5 MS 6, Unit 23, Indio Mts; quartz tr Micrite (Md 0.003mm); calcispheres feldspar tr average 0.03-0. OAmm; pellets echinoids 1 average 0.025mm; fractures filled pelecypods 1 with sparry calcite; stylolite ostracods tr crosses section; iron oxide­ calcispheres 3 stained; quartz and feldspar angular foram 2 (Md 0.036mm); burrowed? zones. brachiopods tr pellets tr opaque minerals tr Fossiliferous MICRITE; U-6.69, micrite 80 MS 6, Unit 27, Indio Mts; sparry calcite 6 Micrite (Md 0.002-0.003mm); sparry feldspar 1 calcite fills fractures and foram 3 composes bodies with oval cross echinoids 3 section up to 0.75mm; a trace of pelecypods 3 angular feldspar up to 0.2mm, some ostracods 2 of which is euhedral and very calcispheres 2 likely authigenic; rock highly opaque minerals tr fractured. Summit Formation Chispa The Chispa Summit Formation includes the youngest Cre­ taceous rock exposed in the Eagle Mountains and vicinity. The base of the Chispa Summit coincides with the base of the Adkins the for Gulf Series. (1933, p. 437) proposed name the approximately 800 feet of yellow and brown interbedded, flaggy limestone, marl, and clay that crop out near Chispa Summit about 20 miles south-southeast of Eagle Peak. Thickness,Thickness lithology, and fossilsfo ssils.--In the Devil Ridge t area, the Chispa Summit Formation is sparsely exposed in small outcrops northwest of Lucky well and along the north­ east flank of Love Hogback where it is in thrust contact with the Yucca Formation; in the valley east of the Speck (for­ merly Love) ranch house, Smith (1940, p. 628-629) measured feet of Summit. His did not 1,420 Chispa section, tiowever, extend south to the thrust contact with the Yucca; in all, about feet of Summit be there. 1,600 Chispa may present Much of it is covered by gravel and alluvium, and it has been intruded by dikes and sills of mid-Tertiary igneous rock. East of the Speck ranch house, the Chispa Summit is thin-bedded and calcareous, and grades upward from gray and black fissile shale and flaggy limestone at the base through interbedded shale and sandy to inter- poorly exposed shale, bedded fine-to coarse-grained brown sandstone and gray and olive shale and sandy shale in the upper part. subdivisions Adkins (1950) recognized the three general described above, but he believed the section was repeated by faulting and accorded it a thickness of only 600 feet. Small line but faults may cross the of section, although incompe­ tent near the Devil Ridge fault, the Chispa Summit Formation is little disturbed. Hazzard stated that the (i960), commenting from memory, Gulf Series at the Speck ranch is more than 2,000 feet thick and that it contains a zone of desmoceratids near the base, a zone of Pseudaspidoceras and Fagesia spp. about midway, a zone of Ostrea soleniscus Meek and gastropods in the upper third, and a zone of Coilopoceras sp. in a shale at the top. Smith’s (1940, p. 628-629) measured section 5, east of the ranch Inoceramus at intervals Speck house, yielded sp. in the lower 250 feet and Ostrea sp. in a 40-foot zone about 365 feet from the top. In the same area, about 375 feet above the base of the formation I collected the following fossils from a black, nodular limestone that weathers very pale orange: Ino ceramus sp., Plesiovascoceras and sp., Pseudaspidoceras sp. The brown-weathering sandstone about feet the 850 above base yielded Gyrodes? sp. In the Eagle Mountains, the Chispa Summit Formation out the and crops in Eagle Spring and Carpenter Spring areas in the valley south of Wyche Ridge, where it is largely covered by gravel and alluvium. The Chispa Summit Formation in the Eagle Mountains is similar except near Eagle Spring, where rather more than 1,400 feet is exposed (Smith, 1941, p. 74) including coal seams in black shale. The Chispa Summit is well exposed in Carpenter Canyon, measured feet of where Taff (1891, p. 734-735) 1,120 brown, fissile, calcareous shale and sandy shale, calcareous fissile shale and shale flaggy sandstone, and black, sandy containing Inoceramus sp. Gillerman (1953, p* 33) collected ostrea Coilopoceras sp., Romaniceras? sp., soleniscus Meek, and Inoceramus sp. near Carpenter Spring. I found fish teeth and fish scales in the shale within a few feet of Carpenter Spring. The of Chispa Summit in the Indio largest exposure Mountains is along the base of the southern half of Willoughby Ridge, and in the valley northeast of the ridge where much of the formation is covered by gravel and allu­ vium. One small, isolated outcrop (Q.4-15*5) is just west of the north end of Willoughby Ridge in a window in the overlying thrust sheet of the Yucca Formation; another (T.l-17*5) is short distance west of Green River and just a south of the road to Indio Pass. The Chispa Summit in this general area is thin-bedded and consists of: calcareous, fissile, yellow to black shale with a bituminous odor; olive to brown flaggy calcareous and to olive with discoidal sandstone; gray shale large septarian concretions. Gypsum permeates much of the shale. There are also distinctive sandstone cylinders of unknown origin, to 6 inches in diameter and perpendicular to up bedding, in the Chispa Summit. In the Chispa Summit outcrop just east of Willoughby Ridge, I found Inoceramus sp. and Collignoniceras aff. chispaense Adkins; north-northeast (P B-l6.0) near the point o where the road leading west to Oxford crosses the main stream channel, I found Calycoceras sp. float and as Romaniceras sp. in place at an undentermined height above the base of the formation. The widespread gravel and allu­ vium that cover much of the formation in this valley pre­ clude measuring the thickness of the strata, but more than 1,000 feet is probably present. Where several hundred feet of highly deformed, yellow- brown gypsiferous shale and marl are exposed (T.l-17.5) just south of the Indio pass road, I collected Inoceramus sp., Eucalycoceras underwood! Powell, Vascoceras sp., and Man­ telliceras sp. Adams (1953, 46) collected Prionocyclus P* sp. and Collignoniceras sp. from the same locality. Powell (1961, p. 11), who mistakenly cited the loca­ tion of this outcrop as in the Eagle Mountains instead of the Indio that the Mountains, suggested fauna is probably correlative with zone 5 of Adkins (1931? 37) at Chispa P* Summit. If the of the Summit Formation in so, age Chispa this small outcrop is late Turonian (Twiss, 1959, 49-50). p. Correlation and .--The estimated thickness of the age shale, limestone flags, and clay that compose the Chispa Summit Formation in the northern Sierra Vieja is about 2,800­ 3,000 feet (Braithwaite, 1958, p. 60-64; Miller, 1956, p. 23~ 56). Southeastward in the Pinto Canyon area, no Cretaceous rocks younger than Buda are exposed (Amsbury, 1958)» Up­ river, however, and opposite the Sierra de Vantana and Sierra Pilares, rocks of Gulf age are preserved in down-faulted blocks west of the Sierra Vieja rim. Because the rocks are characteristically folded and poorly exposed, no complete sections have been measured. Resting conformably on the Buda Limestone in the Ciene­ south the Powell’s guilla area, and west of Indies, Ojinaga Formation (Powell, 1961, 1962; Reaser, 1962) is about 2,000 feet thick. It consists of about 1,400 feet of shale and thin limestone beds overlain by about 600 feet of shale and thin limestone beds and about 600 feet of interbedded sand­ stone and shale. The lower in part ranges age from late Early Cenomanian through Early Turonian (Powell, 1961, p. iii) . Within a radius of about 7 miles north and west of Sierra showed Blanca, King (1949) scattered outcrops of ’’Upper Cretaceous rocks” including the ’’Eagle Ford Formation” and rocks in places. As mentioned earlier, perhaps younger part or all of Buffington’s (1943, P« 1011) measured section of ’’Espy” in the northern Quitman Mountains may well be Summit. Chispa The Chispa Summit, once probably widespread over the region north of the map area, has been largely removed by erosion . The Buda-Chispa Summit contact, which is also the Comanche-Gulf Series boundary, seems to be conformable in the Eagle Mountains and vicinity; it is, however, poorly exposed. This contact is reported to be unconformable in areas many (Adkins, 1933, p. 401; Twiss, 1959a; Ferrell, 1958, P» 42; Nichols, 1958, p. 34)• Adkins (1933, P» 423) mentioned an undulating Chispa Summit-Buda contact in the Eagle Mountains, but I have not seen it. Powell (1961, p. 23) concluded? it seems safe to that there is no disconform­ . . . say at of the Buda e ity the top in the Cieneguilla area The same is probably true in the Chispa Summit area* Some doubt is also cast on the "disconformity" farther south and east of the Chispa Summit area. In my opinion if the disconformity exists in the southern part of Trans-Pecos Texas and in Chihuahua, the corresponding lacuna is rarely large* The ammonites from just east of the Speck ranch house indicate that the of the formation in the Devil age Ridge area ranges from Late Cenomanian to perhaps as young as Late Turonian (Adkins, 1933, p* 437). The Chispa Summit exposed the east flank of the Indio Mountains also be as along may as Late Turonian. young The Chispa Summit is paleontologically correlative with the Eagle Ford of central Texas, the Ojinaga of Burrows (1909), the of Powell (1961), and the Boquillas in Ojinaga the Big Bend area. Its age is Late Cenomanian and Turonian. It is possible that the upper part of the Chispa Summit in the Sierra be Vieja region may as young as Campanian (Twiss, 1959b, p. 50). Origin .--In the map area the sandy limestone, limestone, and siltstone of the Summit sandy shale, sandstone, Chispa are the latest record of Cretaceous siliciclastic deposition. The sediment was deposited in a changeable neritic sea. The some organic material suggests that was deposited along the shore in stagnant lagoons. Table 13,--Petrography of representative samples of Chispa Summit Formation bearing foraminiferal? sparry calcite 80 - Cellophane cellophane 10 BIOSPARITE) Du-Kcs-503) foram 6 Northeast of Speck ranch house, tr about 10 feet above base of quartz tr formation (G.9-8.5)S plagioclase 4 Sparry calcite ranges 0.025mm calcispheres minerals tr to 0.75mm) encloses calcispheres opaque up to 0,04112111 spherical and of calcite ellipsoidal masses sparry (foram?) resembling calcispheres to 0.26mm) some are composite, range up some are enclosed within others) a few angular silt grains quartz and plagioclase) zones lamination caused by of cellophane. Foraminiferal BIOMICROSPARITE) microsparry calcite 86 Du-Kcs-509) calcite 3 Northeast Speck ranch house$ sparry 1 southwest flank gravel-capped calcispheres foram hill (G.5-8.5h 6 tr Microsparry calcite (Md 0.006mm) pelecypods tr with irregular patches of sparry quartz tr calcite up to 0.1mm) calcispheres feldspar of sparry calcite pellets 3 up to O.OAmm) spheres minerals 1 (foram?)up to 0.55mm (Md 0.2mm))boundary opaque between matrix and foram or calcispheres not sharp| trace of angular feldspar and scattered grains quartz silt? of opaque minerals (hematite). detrital fine Silty very sandstone) immature chert-bearing ARKOSE) quartz 37 chert 3 Du-Kcs-514| East-northeast Speck ranch anorthoclase 36 south bank of first tributary plagioclase 4 house, illite 20 from east to Rattlesnake Draw tr (H.1-8.3)| 0.065mm, very well glauconite tr sorted) angular) cellophane zircon tr Quartz common) subpaiallel alignment muscovite tr of detrital grains) glauconite and zircon minerals tr cellophane average 0.05mm) opaque round and angular) feldspar fresh. Table 13.—Continued Fine to medium sandstone; detrital calcitic bimodal mature chert quartz 20 and oyster bearing ARKOSE; chert 20 Du-Kcs-518; metamorphic quartz 5 Summit of small hill of metamorphic rock Chispa Summit Formation, fragments 3 due east of Speck ranch house volcanic rock (H.2-8.3); 0.32mm, 0.16mm, fragments 1 well sorted, well sorted; microsparry angular; calcite 4 Quartz largely common; chert grains plagioclase 16 angular; large quartz grains alkalic angular and round; feldspar fresh feldspar 10 and weathered; a few round clay pelecypods 5 balls; oyster fragments up to sparry calcite 15 5mm. opaque minerals 1 biotite tr glauconite tr **Post-Turonian Pre-Duchesnean Rocks The youngest Cretaceous rocks of the Gulf Series exposed in the map area are possibly as old as Late Turonian, but has removed much of the Gulf Series. the erosion Presumably Gulf Series could have been as thick in the area as it map was in the northern Rim Rock country, where there is a com­ bined thickness of about s>ooo feet of the Chispa Summit and the San Carlos formations (Frantzen, 1958, p. 38). The age of variegated shale in the upper part of the San Carlos be as late as Maestricthian, and the rock may youngest of the Gulf Series in the map area prior to erosion may have been as It seems likely that Late Cretaceous rock young. than Late Turonian be in the area younger may preserved map in the subsurface in intermontane beneath a blanket of areas volcanic rock and overlying basin fill. In the Big Bend Park (Wilson jet _al., 1955) Paleocene and Eocene rocks have been deformed by Laramide orogeny. CENOZOIC ROCKS The most remarkable feature of the Tertiary eruptive rocks of the map area is the difference between the eruptive rocks of the Eagle Mountains and those of the Indio Mountains. **Stratigraphic units marked with asterisks do not crop out in the area. map Although both sequences are sodium-rich rhyolitic and tra­ surface chytic rocks, the homotaxis, thickness, expression, and color of the extrusive rocks of the two areas texture, are quite different. In the Indio Mountains there is an alternation of relatively homogeneous fine-grained tuff and microporphyritic welded tuff or trachyte, whereas in the Eagle Mountains the extrusive igneous rocks are volcanic two breccia, flow breccia, and rhyolite flows. The sequences extrusive rock cannot be chrono­ of igneous directly compared are not in contact at the surface. logically because they Because vulcanism throughout Trans-Pecos Texas was roughly contemporaneous, the inference is that igneous activity in the Eagle Mountains was contemporaneous with that in the Indio Mountains. I did not a conglomerate at the base of the Ter­ see tiary volcanic rock sequence in the Eagle Mountains and vicin ity such as in the Rim Rock country (DeFord, p. occurs 1958a, 14). Admittedly its distribution would be patchy and irreg­ ular, but it should be present in old topographic lows be­ neath the basal extrusive rock. The in- light colored, fine-grained, microporphyritic trusive rhyolite of the Devil Ridge area resembles that of the northern Indio Mountains, and both are similar to the rhyolite of the Eagle Mountains. Basin fill of Tertiary and Quaternary age and terrace gravel, alluvium, and windblown sand almost encircle the of the these sediments and rocks complete highlands map area; the stratigraphic record of the Eagle Mountains and vicinity. Intrusive Rocks of Devil Ridge Area Quartz porphyry sill from 40-50 thick latite porphyry.--A feet in the Cox Sandstone on the northeast flank of Texan Moun­ tain extends southeastward several hundred feet into the map area (A-l). The rock is a medium light gray, quartz latite euhedral of porphyry, distinguished by crystals pinkish gray sanidine to in and up 1.5 cm diameter crystals of very light gray oligocase up to 3 mm in diameter in a ground mass of feldspar microlites and quartz (ingerson, 1952, p. 191)• Great numbers of sanidine crystals have weathered out of the matrix. The quartz latite porphyry is the youngest of the of intrusive rocks of the northern Quitman Moun­ sequence tains (Buffington, 1943# p. 1037, pi. l); it is seemingly unrelated to the igneous rocks of the Eagle and Indio Moun­ tains. Rhyolite The sill-like intrusive rock in the Espy . Limestone north of Wiggleton tank (E-5, 6) is white and very light gray, compact, fine-grained microporphyritic rhyolite. A sample from the southwesternmost outcrop contains alkalic feldspar spherulites up to 0,5 mm, a few embayed quartz alkalic phenocrysts and feldspar phenocrysts averaging 1,0 mm, and microphenocrysts of euhedral partly resorbed alkalic feldspar laths (Md 0.1 mm) in a cryptocrystalline matrix of alkalic feldspar and quartz. The rock shows pronounced con­ choidal fracture. In the area east of the Speck ranch house, dikes and sills of rhyolite have intruded the Yucca, Espy, Eagle Moun­ formations. tains, Buda, and Chispa Summit Sills predominate, and have an thickness of feet. that average 30-40 Magma formed the dikes probably intruded along faults; the largest dike is about 100 feet wide and is in of exposed a series light-colored outcrops (H-8) aligned slightly north of east in the valley just north of Pagoda Hill. The rock is very pale orange with abundant light brown concentrations of iron oxide less than 0.1 mm in diameter derived from the oxidation of iron-bearing minerals. Quartz and alkalic feldspar phenocrysts up to 2.5 rim are enclosed in a fine-grained cryptocrystalline matrix of quartz and alkalic shows and feldspar, which rudimentary granophyric spherulitic texture. A rhyolite sill about 50 feet thick crops out (G-9) within the Espy Limestone east-northeast of the Speck ranch house. The rock is greenish gray with abundant clear quartz and chemically altered biotite 0.7 mm phenocrysts averaging in maximum dimension. Pinkish gray phenocrysts of albite­ oligoclase, of them altered to yellow and red-brown many iron about 1.5 mm in maximum diameter. The oxide, average matrix is a of and alkalic feld­ fine-grained mosaic quartz A of iron in spar (Smith, 1940, p. 619) • patina oxide, places up to 4 mm thick, is characteristic of the rhyolite of this area. that a small Beckley (1961) reported northeast-trending dike crops out just northwest of the midpoint of Love Hogback. The easternmost outcrop of intrusive igneous rock in the Devil Ridge area is more than two miles from the steep cliffs the flank of lower rhyolite on west of the Eagle Mountains, but the rhyolite of the two areas probably stemmed from the same chamber. The intrusion of igneous rocks in the Devil magma Ridge area was probably contemporaneous with extrusion or in­ trusion of rhyolite in the Eagles. Lamprophyrethe color and texture vary from Lamprophyre.--Although place to place, the composition of the igneous rocks of the Devil Ridge area is relatively uniform; the small dikes of ,T 1’ dark-colored trap rock reported by Smith (1940, p. 619) two are the exception. These lamprophyre dikes crop out in localities: (l) along the road (H.3-B*6) leading to the windmill and tank in Goat Arroyo upstream from the Speck ranch house, and (2) along the west bank of Rattlesnake Draw the Summit and (G.9-8.5) near Buda-Chispa contact, several hundred feet farther upstream (G.B-8.6) where the same rock has intruded the Buda, either along the fault that runs just west of the draw or along a fracture related to the fault. The rock is various shades of olive it weathers gray; readily; it is fine-grained and contains equidimensional phenocrysts of calcite (Md 0.5 mm), which probably originated through the alteration of olivine. The rock also contains much secondary iron oxide. With the exception of faint local baking, the intrusive rocks have had little effect on the rock. country Age and correlation evidence permits dating the correlation.--Direct rhyolitic intrusive rock of the Devil Ridge area only as than Chispa Summit and older than basin fill. The younger rhyolite resembles that of the Eagle Mountains; probably in­ trusion in the Devil Ridge area was contemporaneous with some phase of extrusion or intrusion of rhyolite in the Eagle Moun tain s. of intrusive Table 14—Petrography of representative samples e rocks. Devil Ridge area DU-Trd-10; phenocrysts Porphyritic RHYOLITE; 8 Sill (H.2-8.7) east of Speck quartz ranch house along Goat Arroyo alkalic feldspar 3 matrix 78 north of large outcrop of rhyolite; Alkalic feldspar and round and caleispheres 2 to chert and embayed quartz phenocrysts up (Md 1.0mm) in chalcedony 4 2.5mm minerals 5 opaque cryptocrystalline matrix of quartz some and alkalic feldspar; feldspar phenocrysts totally altered to calcite and chert; have most quartz phenocrysts poorly developed coronas; indistinct granophyric and cavities spherulitic texture; filled by quartz, chert, iron oxide„ Microporphyritic RHYOLITE; DU-Ti-24; microphenocrysts Summit, largest exposure of quartz 6 intrusive rock (H.3-8*7) east of alkalic feldspar 9 ranch house and just northeast matrix 75 Speck chert 2 of base of Pagoda Hill; 8 Round, embayed quartz and severely opaque minerals fractured alkalic feldspar micro­phenocrysts up to 2.6mm (Md 0.75nim) in a cryptocrystalline matrix of alkalic feldspars and quartz; quartz phenocrysts slightly resorbed; chert fills cavities and replaces feldspars; rudimentary granophyric texture; indistinct spherulites. Spherulitic, porphyritic RHYOLITE; phenocryst tr quartz DU-Tr-487; half a mile Sill (E.5-5*8) about alkalic feldspar 3 matrix - west northwest of Wiggleton tank; 97 Anhedral alkalic feldspars and a opaque minerals tr to 2mm few quartz phenocrysts up in cryptocrystalline matrix of quartz and alkalic feldspars; abundant euhedral, partly resorbed alkalic feldspar laths (Md Oelmm); spherulites up to 0®5mni (Md 0.2mm); intersertal texture. Eruptive Rocks of Eagle Mountains The eruptive rocks of the Eagle Mountains compose the interior, highest part of the mountains. Plate 1 and Giller- man*s map (1953* pi. l) show the rock units but with a same slightly different distribution of eruptive rocks. Plate 1 probably is oversimplified. The texture of each of the three - major units from oldest to youngest, lower rhyolite, tra­ - chyte porphyry, and upper rhyolite is both horizontally and vertically variable. With detailed work, each of these units shown might be subdivided into a of sequence intertonguing flow and pyroclastic rocks. The intrusive rocks include rhyolite, microgranite, quartz syenite, basalt, and diabase. The small stock is com­ posed of the Eagle Peak Syenite; it, in particular, deserves additional study. Aside from the basalt that occurs within the rocks shown on plate 1 as diabase dikes, the only basalt (M. 7-14.4) that is known in the Eagle Mountains is exposed on the crest of north-northwest s concrete-and-rock Wyche Ridge of ¥yche T dam, where a small outcrop of dark gray, fine-grained, compact basalt rests on lower rhyolite. Whether the rock is intrusive or extrusive was not determined. Because of its small area, this outcrop is not shown on plate 1. Although the age of the eruptive rocks of the Eagle Moun­ tains may be inferred from regional geology to be Oligocene in age (DeFord, 1958a), no well-documented vertebrate fos­ sils have been recovered from there* Baker (1927, p. 35) reported that % The rhyolitic volcanics in the Rim Rock country have a total maximum thickness of feet or more ... 4>ooo but more than half the total is tuff. Tuff-breccias hundreds of feet in thickness form the base of the rhyolitic series in Eagle Mountain. Pyroclastics are not nearly so prevalent in the igneous succession younger than the rhyolitic. Bones of land tortoises are found in the rhyolitic tuffs. This statement has long been interpreted (Plummer, 1933, p. to 802; Buffington, 1943, P» 1032; Gillerman, 1953, p« 34) mean that Baker collected the tortoise bones in the Eagle Mountains and not in the Rim Rock country. Baker (1962) wrote i recently I think the statement that I found land tortoise bones scutes from the (probably mainly carapaces) in rhyolitic tuffs in the Eagle Mtns. is correct ,.. Lower rhyolite .--The ’’lower rhyolite” is a sequence of sedimentary rock, extrusive and intrusive rhyolite, volcanic breccia, flow breccia, and tuff that ranges in thickness from several hundred to more than a thousand feet. This se­ quence was deposited on an uneven surface of Cretaceous rock on which differences in elevation were as much as 500 feet. Formations of Comanche and Gulf are overlain unconform­ age the lower ably by rhyolite. The lower is rhyolite widely exposed in the Spar Valley and the Carpenter Spring area, where it is difficult to dis- between the that was extruded as lava and the tinguish part part that was intruded in sills. From the Rhyolite fault north the lower is more extensive and thicker than rhyolite to the southeast, and steep-walled canyons with high gradi­ ents, such as Black Rock, Horse, and Goat canyons, are in­ cised in it. East of Broad Canyon along the south flank of the mountains the lower rhyolite does not crop out; it may be covered or it may not be present. Along the west flank of the mountains and in the Black the vicinity of Butte, lower rhyolite is exposed in precipitous cliffs and probably reaches a thickness of at least 800 feet, although much of it is covered by colluvium. Beneath the dark caprock of trachyte porphyry, the very light gray lower rhyolite at Black Butte consists primarily of two kinds of rock; rhyolite with microphenocrysts of quartz and tuffaceous, fine-to medium-grained, poorly sorted, quartz sandstone. Near the large solution caves in the lower rhyolite on the west flank of the Eagles just above Indian Spring, a vol­ canic breccia with a pale yellowish green matrix and with red volcanic-rock 1 grayish fragments (Md in.) grades upward to a volcanic breccia with a white matrix and with smaller fragments that consist of volcanic rock, chert, and quartzite (Md 3 mm). Just beneath the overlying trachyte porphyry there is a of and pale red purple rhyolite with microphenocrysts quartz with pronounced flow structure. This rock weathers to frag­ ments that resemble silicified wood. At MS 13, there is a lower the tra­ 6-foot zone between the rhyolite and overlying that consists of blocks of both chyte porphyry angular types of rock. To an observer at some distance, however, the con­ tact is and distinct. sharp On the high ridge of lower rhyolite that overlooks the head of Horse Canyon from the south, there are several poorly exposed outcrops of seemingly intrusive, dark greenish gray pitchstone that strikes N. 80° E. and dips 40° SE. The out- are about 6 feet wide and feet Farther crops 10-15 long. west, on the south slope of the west-southwest-trending can­ yon just north of the one in which there is a dam, there is a circular about 80-100 feet in diameter of a breccia outcrop of and black pitchstone; it also contains blocks gray, green, of moderate red rhyolite that range up to 2 feet in diameter. The outcrop appears to be a lava remnant; it might, however, be the remains of a small vent. At MS 15 on the northeast flank of Panther Peak, more than 1,000 feet of the lower rhyolite is exposed. At the base a distinctive 34-foot outcrop of thin-bedded lithic tuff composed of grains of volcanic rock dips 14° N. The remainder of the section is largely volcanic breccia, and Much of the volcanic spherulitic rhyolite, rhyolite. breccia is composed of red volcanic-rock fragments embedded in a green, aphanitic matrix. Beginning about 417 feet above the base of MS 15, there is an 8-foct zone of spheru­ litic rhyolite in which white spherulites of alkalic feld­ 2.0 enclosed in red spar (Md mm) are a pale purple aphanitic matrix of quartz and alkalic feldspar. The uppermost layer of rhyolite at MS 15 is grayish pink, is slightly spheru­ litic, and has flow structure similar to that of the upper part of the lower rhyolite above Indian Spring. At TC Peak less than a mile north of Panther Peak, much of the lower rhyolite is a thick-bedded, volcanic breccia that contains angular and round fragments of fine-grained quartzite, spherulitic rhyolite, and limestone up to 5 feet in diameter (Gillerman, 1953, p* 35; Smith, 1941, p. 74)* The volcanic breccia forms the resistant cap of the peak as well as about half of the section below the Beneath cap. the volcanic breccia there is a light greenish gray sandy tuff. On the southwest flank of the mountains, about 300 feet of rock that is probably near the base of the lower rhyolite is well exposed in a draw near where it is crossed (L.9-9»4) by the pasture road that leads southwest from the Hayter ranch house. Downstream from the road crossing there is an outcrop of Orbitolina-bearing Bluff limestone that strikes N. 15° W. and dips 25° NE.; the limestone underlies the The two formations concord- lower rhyolite. are seemingly ant. The outcrop of lower rhyolite consists of a variegated sequence of sandy and lithic tuff, volcanic breccia, and rhyolite. The breccia consists of quartzite, limestone, and volcanic rock fragments that range up to 6 feet in diameter. The rock of this section is and it generally light-colored, from thin-bedded to thick-bedded and massive. ranges Typical of the rocks that compose the lower part of the lower rhyolite on the west and southwest flanks of the Eagles is the light greenish gray tuff or volcanic breccis that contains irregular fragments of a grayish green serpentine- like mineral. Much of the rock that the lower of the composes part lower rhyolite is well stratified and probably represents lacustrine or fluvial deposits into which volcanic debris has been incorporated in varying amounts. Gillerman (1953> p. 35) was particularly interested in the rocks of the lower rhyolite in the Spar Valley area be­ cause there they contain fissure veins of fluorspar. He wrote ; the lower part of the rhyolitic series consists of flows of porphyritic rhyolite containing quartz and sericitized orthoclase phenocrysts in a fine-grained groundmass. Small granophyric intergrowths of quartz and orthoclase can be observed in some specimens. Flow banding is and flow breccias containing ... common, fragments of rhyolite and sedimentary rocks are pres­ent. The of the series ... is more upper part andesitic in composition and is a buff-colored to yellowish-gray weathered lava composed of phenocrysts of albitized plagioclase and a little orthoclase in a groundmass of quartz and feldspar. The dark min­ erals . . . have been oxidized. Rock fragments are numerous. A possible vent near the northwest end of Wyche Ridge lies east of the Eagle Mountains ranch house and south of the at the entrance to the Eagle Mountains cattle-guard ranch. This vent is at the summit of a ridge that leads a few degrees east of south from the outcrop of Espy Limestone at the sharp bend in the stream channel near the mouth of Wind Canyon. The lower part of the ridge is composed of hard, compact, trachytic, fine-grained tuff whose color, be­ tween and is controlled grayish orange very pale orange, by oxide. The of the hill is intergranular iron upper part characterized by an assortment of rocks such as silicified spherulitic rhyolite, volcanic breccia, silicified tuff, basalt, and silicified limestone. Although it is not easily seen on the ground, on air photographs one sees a roughly circular seemingly vertical contact between rocks of the and lower of the hill. This upper parts locality merits further investigation. Table 15•—-Petrography of representative samples of lower rhyolite. Eagle Mountains Lithie TUFF; MS 15.2; volcanic rock MS 15, Unit 1; fragments 73 Volcanic rock fragments ("n” less glass matrix 20 than balsam) composed of fine- quartz 4 grained quartz? and feldspar?; alkalic unidentified green, alteration? feldspar 1 mineral in volcanic rock plagioclase tr fragments; quartz angular to sedimentary rock subangular, ranges up to 0 o 8mm fragments tr (Md 0.4mm); feldspar angular; opaque minerals 2 glass partly devitrified and tridymite tr stained with orange iron oxide; tridymite in cavities« Microporphyritic RHYOLITE; MS 15®9s matrix MS 15, Unit 3; alkalic Microphenocrysts of quartz and highly feldspar and vacuolized and corroded zoned quartz 92 alkalic feldspar, up to 2.5mm microphenocrysts (Md 0,25mm) in a fine-grained quartz 4 seriate matrix of quartz and alkalic alkalic feldspar ("n11 less than feldspar 2 balsam); poorly developed volcanic rock spherulites of alkalic feldspar up fragments tr to 0.2mm (Md 0«lmm); one volcanic opaque minerals 1 rock fragment 5®0mm in diameter; chert 1 chert and tridymite in vesicles. tridymite tr Microporphyritic RHYOLITE; MS 15®12; matrix MS 15, Unit 4| alkalic Microphenocrysts of quartz. feldspar and plagioclase and zoned alkalic quartz 93 feldspar up to 2.5mm (Md 0 o 2mm) in microphenocrysts a seriate fine-grained matrix of quartz 3 quartz and alkalic feldspar alkalic ( f,n f* less than balsam); alkalic feldspar 2 feldspar vacuolized; plagioclase plagioclase 1 corroded; a few indistinct opaque minerals 1 spherulites (Md 0.1mm) of alkalic zircon tr feldspar; vague granophyric texture„ Table 15,--Continued Microporphyritic Spherulitic RHYOLITE | MS 5*16; MS Unit 5; 15, seriate matrix of Fine-grained and alkalic feldspar quartz ("n" less than balsam) encloses subhedral and anhedral microphenocrysts of quartz, and zoned alkalic vacuoLized and plagioclase upfeldspar, matrix has (Md 0.2mm); to 3.0mm and felsophyric, granophyric, spherulites spherulitic texture; largely alkalic feldspar (Md 0,2ram) o Microporphyritic RHYOLITE; MS 15.191 MS Unit 15, 6; of Anhedral microphenocrysts vacuolized alkalic quartz and to 2.0mm (Md 0.3mm) feldspar up in a fine-grained poikilitic matrix of quartz and alkalic feldspar ("n" less than balsam); large of in irregular patches quartz matrix optical continuity; indistinctly granophyric and spherulitic• Porphyritic, Spherulitic RHYOLITE; MS 15.23; MS 15, Unit 7; Enhedral to anhedral of and phenocrysts quartz sericitized alkalic vacuolized, feldspar up to 2.5mm (Md 1.0mm) in a fine-grained spherulitic to matrix; spherulites range up 3.0mm (Md 1.0mm); red iron oxide stains matrix around periphery of of some spherulites; nucleus alkalic spherulites and mostly intricate intergrowths feldspar; of alkalic feldspar and quartz compose spherulites. matrix *i quartz and alkalic feldspar 89 s microphenocryst quartz 2 1 plagioclase alkalic feldspar 7 minerals 1 opaque • tr zircon tr sericite „ chert i,tr microphenocryst s quartz 7 alkalic feldspar 3 /“•» /N matrix 90 minerals tr opaque phenocrysts quartz 2 alkalic feldspar 3 matrix quartz and alkalic feldspar 95 minerals tr opaque zircon tr Table 15.-“Continued Spherulitic and Microporphyritic RHYOLITE; MS 15.24 microphenocrysts MS 15, Unit 7; quartz 3 Fine-grained spherulitic matrix alkalic of quartz and alkalic feldspar feldspar 7 enclose euhedral to anhedral matrix microphenocrysts of quartz and alkalic highly vacuolized, armored feldspar and alkalic feldspar up to lo3mm quartz 89 (Md 0«25mm); spherulites range up opaque minerals 1 to 1.0mm (Md 0.6mm); matrix slightly granophyric in places 0 Microporphyritic RHYOLITE; MS 15.28 microphenocrysts MS 15, Unit 8; quartz 10 Fine-grained matrix of alkalic alkalic feldspar and quartz encloses feldspar 6 euhedral to anhedral microphenocrysts plagioclase 3 of quartz, alkalic feldspar, and matrix plagioclase up to 0„8mm (Md 0o4mm); quartz and alkalic feldspars vacuolized; a few feldspar 81 spherulites about 0.4nim diameter; opaque minerals tr slight granophyric texture; abundant apatite tr veinlets filled with mineral with straw-colored birefringence; vague outlines of cognate xenoliths?. Lithic TUFF; MS 15.33; volcanic rock MS 15, Unit 10; fragments 73 Fine-grained matrix of partly quartz 4 devitrified glass encloses angular alkalic rock and mineral fragments up to feldspar 2 2.4mm (Md 0c4mm); volcanic rock plagioclase 1 fragments fine-grained and some are chert 4 spherulitic; volcanic rock fragments sedimentary rock contain pale green mineral that has fragments 1 greenish yellow birefringence with glass matrix 15 ,,n* 1 greater than balsam, in irregular opaque minerals tr patches and stringers; irregular cavities in most volcanic rock fragments. Table 15,--Continued Spherulitic RHYOLITE; MS 15*3$; MS 15, Unit 11| Iron oxide-stained matrix is fine­ phenocrysts quartz alkalic 1 grained (0.015-0.03mm) spherulitic and granophyric mosaic of quartz and alkalic feldspar with spherulites up to 2.2mm (Md limn); few micro­phenocrysts of quartz and alkalic feldspar (maximum 0.9mm); part of matrix is microcrystalline quartz. feldspar matrix quartz and alkalic feldspar opaque minerals tr 98 1 Spherulitic RHYOLITE; MS 15*42; MS 15, Unit 11; Fine-grained seriate and spherulitic mosaic of quartz and alkalic feldspar (Md 0.0075mm) with irregular patches and stringers of quartz and some alkalic feldspar up to 0.9mm; (Md 0.15mm); spherulites indistinct and range quartz alkalic feldspar opaque minerals plagioclase 35 64 1 tr up to 0.5mm. RHYOLITE; MS 15*46; MS 15, Unit 12; Fine to medium-grained seriate mosaic of quartz and quartz alkalic feldspar opaque minerals 54 45 1 alkalic feldspar; fine-grained (Md 0.0075mm) matrix has Mn" less than balsam and is micrographic in places; medium-grained quartz alkalic feldspar (Md 0.15mm) in irregular patches and stringers. and RHYOLITE; MS 15*48; MS 15, Unit 13; Fine to medium-grained seriate mosaic of quartz and feldspar up to 1mm; micrographic and granophyric; coarser grains in irregular patches and stringers; flow structure evident in places; larger feldspar laths highly vacuolized. quartz alkalic feldspar opaque minerals zircon 59 40 1 tr Table 15.~~Conti.nued Spherulitic RHYOLITE; MS 15°52; quartz 47 MS 15, Unit 13; alkalic Fine-to medium-grained seriate. feldspar 50 spherulitic mosaic of quartz and microcrystalline alkalic feldspar; small (maximum quartz 3 0,3mm) spherulites abundant; opaque minerals tr some are quartz, some are alkalic apatite tr feldspar, and some are composite; olivine tr much feldspar vacuolized; micrographic. Rhyolitic VOLCANIC BRECCIA; EU-Tlr-86; igneous rock Near base large caves (J.6-9o3) west fragments 40 flank, Eagle Mts; sedimentary rock Igneous rock fragments and fragments 2 sedimentary rock fragments about quartz 10 10mm in diameter plus euhedral to alkalic anhedral quartz and alkalic feldspar feldspar 5 (Md 0.20mm) in a fine-grained matrix matrix (T,nn less than balsam) of alkalic alkalic feldspar and quartz; feldspar vacuolized; feldspar and matrix slightly spherulitic« quartz 41 calcite 2 opaque minerals tr Rhyolitic Welded Lithic TUFF;EU-Tlr-87; igneous rock Below caves and southwest of steeply fragments 40 dipping limestone beds, west flank. quartz 2 Eagle Mts; alkalic Irregularly shaped igneous rock fragments feldspar 3 up to ?mm (Md l 0 25mm) plus subhedral matrix quartz and alkalic feldspar (Md 0.8mm) glass shards 55 in a partly devitrified matrix of glass shards ("n” less than balsam) ;»»nn fragments greater than balsam; igneous rock fragments patchily altered to unidentified green mineral. Trachyte porphyry belt of por­ porphyry.--The outcrop "trachyte phyry" along the east flank of the Eagles is narrow, and this rock is missing or hidden by younger rock along the south flank of the mountains. It thickens to the west, how- the lower at MS as does rhyolite; 13 (J.6-9*4) 743 ever, feet of trachyte porphyry was measured. Black the covers The trachyte porphyry caps Butte, area on the west flank of the mountains known as the Roof Garden (J-9), forms the upper part of Black Peak (J-10), and con­ stitutes several small outliers in the Eagle Spring area. Beginning at the mouth of Snowline Canyon, the trachyte por­ phyry outcrop extends along the southwest flank of the moun­ tains as far north as Frenchman Canyon. The lower rhyolite and the upper rhyolite are in con­ tact near Eagle Rock and also just north of Frenchman Canyon Whether this absence indicates that the trachyte porphyry was eroded or whether it means that it never existed in that area is uncertain* In places to the east the lower rhyolite and trachyte porphyry grade into one another. The gradation suggests con the lack tinuous eruption. Perhaps of trachyte porphyry be­ tween the upper rhyolite and the lower rhyolite means that it was never there. On the other hand, at Black Butte there is an obvious discordance of dip between the lower rhyolite and overlying trachyte porphyry. It is, therefore, likely that the trachyte porphyry on the west was never continuous with that on the east; indeed, they may well not be synchro­ nous or even contemporaneous. Along the west flank of the Eagles, there is a distinct change in color and weathering habit at the lower rhyolite- The rounded trachyte porphyry boundary. light-colored, weathered surface of the lower is in contrast rhyolite sharp with the dark-colored angular blocks of the trachyte por­ . phyry Near the head of Spar Valley the lower rhyolite grades upward into the trachyte porphyry; along the east flank of the mountains south of the Eagle Mountains ranch house, however, the contact between the two is distinct. There, the color lower of the uppermost part of the rhyolite is between gray­ ish pink and pale red; the rock is characterized by pro­ nounced flow structure and microphenocrysts of quartz and feldsparo This is in strong contrast with the hard, compact grayish red trachyte porphyry with euhedral to anhedral, very pale orange phenocrysts of alkalic feldspar.* Just south of Wind Canyon, the trachyte porphyry has either intruded or it intertongues with, the lower rhyolite. East along the ridge that forms the south wall of Wind Can­ yon, beginning just south of the Eagle Mountains ranch, the of west-or rock is sequence southwest-dipping upper rhyo­ lite, trachyte porphyry, lower rhyolite, and again trachyte porphyry. The same sequence holds for the ridge parallel to and just south of this one. The outcrops of trachyte por­ phyry within the lower rhyolite are small and are not shown on plate 1. The color of the trachyte porphyry in this area is varied; it is light gray, medium light gray, light brownish gray, pale yellowish brown, or grayish red. Compositionally 'and texturally the rock is less varied; it consists of euhedral to anhedral phenocrysts of white alkalic feldspar ranging up to 9 mm (Md 4 mm) in an aphanitic, darker matrix. The rock is hard and and it forms rounded hills and compact, relatively smooth slopes. The trachyte porphyry near the upper reaches of French­ man Canyon as well as that just north of the booster mill on south side of the from to the canyon, ranges light gray medium gray. Composition and texture are similar to that of the north south of Wind trachyte porphyry and Canyon. Just behind Hayter T s house and southeast along the southwest flank of the mountains and at Eagle Spring, Black Peak, Roof Garden and Black Butte, the trachyte porphyry is grayish red, or pale red, or pale brown and the feldspar phenocrysts are smaller (Md 3 mm) and less distinct. The rock is hard and compact, and it weathers to angular blocks. The trachyte porphyry at MS 13 is typical of that in the western part of the map area. Euhedral and subhedral of anorthoclase that to 15 mm (Md 2.0 phenocrysts range up mm) are enclosed in a fine-grained, granophyric matrix of alkalic feldspar and interstitial quartz. Zircon and apatite minerals. The rock is brittle and are accessory intensely fractured and highly weathered. The red-brown color is derived from the finely dis­ seminated dust of and black metallic iron orange, red, oxide, that together with larger fragments of these same materials compose 25-30 percent of the rock. The shape of some of the grains of opaque minerals suggests that they were originally mafic minerals that have been altered to secondary minerals of iron oxide. interstitial much 10­ Although quartz may compose as as 15 percent of the rock at MS elsewhere is less 13, quartz abundant The rock over-all (Gillerman, 1953, P» 36). is a trachyte porphyry, but locally it may be a quartz trachyte or a porphyry rhyolite porphyry. Gillerman (1953, P« 36-37) suggested? .. . Although most of the trachyte porphyry is be­lieved to be extrusive the outliers in the Eagle o.. Spring area and some of the trachyte porphyry within the should be in upper /sic» lower/ rhyolitic series the Spar Valley area may be intrusive. The evidence is not conclusive, but in the Eagle Spring the area, outliers of trachyte porphyry seem to occur as sills within the lower rhyolitic series and the Cretaceous sedimentary rocks, or between the two units. The tex­ture of much of the trachyte porphyry and the absence of and vesicular structure is further evi­ flow banding dence of the intrusive character of some of the rocko Gillerman and I have mapped as trachyte porphyry, rock that may well have originated as two (or more) separate and not necessarily synchronous or even contemporaneous flows. Table 16.—Petrography of representative samples of trachyte porphyry, Eagle Mountains Porphyritic QUARTZ TRACHYTE; MS 13.3; phenocrysts MS 13, Unit 1; alkalic feldspar 12 Euhedral phenocrysts of sericitized opaque minerals 3 and vacuolized alkalic feldspar up matrix to 4.25mm (Md 2.0mm) and metallic quartz and opaque minerals (black and red in alkalic reflected light and of crystal shape feldspar 70 that suggests they may have originally zircon tr been mafic silicate minerals) in a apatite tr fine-grained matrix (,! n u less than balsam) opaque minerals 15 of alkalic feldspar and quartz with abundant flecks black metallic opaque mineral (Md 0.02mm). Porphyritic QUARTZ TRACHYTE; MS 13.10; phenocrysts MS 13, Unit 4; alkalic feldspar 8 Euhedral to subhedral phenocrysts of matrix intensely fractured, sericitized and quartz and otherwise altered alkalic feldspar alkalic up to 2.5mm (Md 1.8mm) in a fine- feldspar 67 grained matrix (,fn n less than balsam) opaque minerals 25 of quartz and alkalic feldspar and apatite tr opaque minerals (black and red in zircon tr reflected light); widespread red and brown iron oxide stain as well as alteration to iron oxide. QUARTZ TRACHYTE PORPHYRY; MS 13.13; phenocrysts MS 13, Unit 4; alkalic Fine-grained, iron oxide-stained feldspar 15 matrix (n n IT less than balsam) of opaque minerals 2 alkalic feldspar, quartz, and calcite? 3 black and red opaque minerals encloses matrix intensely fractured irregularly zoned alkalic phenocrysts of alkalic feldspar up feldspar to 5,1mm (Md 2.0mm); subhedral and quartz 60 phenocrysts of black and red opaque zircon tr minerals (probably altered pyrabole), apatite tr and subhedral to anhedral calcite? opaque minerals 20 (probably altered pyrabole); alkalic feldspar phenocrysts partly carbonatized. Table 16Continued QUARTZ TRACHYTE PORPHYRY; MS 13*15; phenocrysts MS 13, Unit 4; alkalic Fine-grained iron oxide-stained matrix ("n" less than balsam) of feldspar opaque minerals 18 2 alkalic feldspar, quartz, and black calcite 1 and red opaque minerals encloses pyrabole tr euhedral to anhedral intensely matrix fractured, carbonatized, irregularly alkalic zoned phenocrysts of alkalic feldspar and feldspar up to 6<>75mm (Md 2,0mm) quartz 59 plus a few phenocrysts of highly opaque minerals 20 altered pyrabole, and calcite (probably carbonatized pyrabole), and opaque minerals. Microporphyritic QUARTZ TRACHYTE; microphenocryst s MS 13.18; MS 13, Unit 4; alkalic Fine-grained iron oxide-stained feldspar 12 orthophyric matrix (’’n” less than opaque minerals tr balsam) of alkalic feldspar, quartz, matrix and black and red opaque minerals alkalic encloses euhedral to anhedral. feldspar and zoned, fractured micro- quartz 68 phenocrysts of alklic feldspar up opaque minerals 20 to 3mm (Md 0.5mm) plus a few opaque zircon tr phenocrysts; slightly glomero­ apatite tr porphyritic. Microporphyritic QUARTZ TRACHYTE; microphenocrysts MS 13.23 alkalic MS 13, Unit 4; feldspar 15 Euhedral to anhedral, intensely matrix fractured irregularly zoned ,partly alkalic sercitized microphenocrysts of alkalic feldspar up to 2.5mm feldspar quartz and 68 (Md 0,75mm) in fine-grained iron opaque minerals 20 oxide-stained orthophyric and apatite tr felsophyric matrix ( M n M less than muscovite tr balsam) of alkalic feldspar, quartz, zircon tr and black and red opaque minerals? flightly glomeroporphyritic, Table 16,—Continued QUARTZ TRACHYTE PORPHYRY; MS 13.31; phenocrysts MS 13, Unit 41 alkalic Fine-grained (Md 0 o 008mm) iron oxide- feldspar 30 stained matrix (tt n ff less than balsam) matrix of alkalic feldspar, quartz, and alkalic opaque minerals; encloses euhedral to feldspar and anhedral phenocrysts of alkalic quartz 50 feldspar up to 15mm (Md 1.75mm); opaque minerals 20 some phenocrysts seem to have core zircon tr of sodic plagioclase and armor of muscovite tr alkalic fedlspar; somewhat apatite tr glomeroporphyritic; abundant fine-grained (Md OoOlmm) opaque minerals that were originally mafic minerals? QUARTZ TRACHYTE PORPHYRY; EU-T(?)-19; phenocrysts Roof Garden, small hill (J«4-9®6) alkalic to north overlooking Horse Canyon; feldspar 26 Fine-grained matrix ( flnM less than plagioclase 4 balsam) of alkalic feldspar, quartz, opaque minerals 2 and brown and black opaque minerals; matrix encloses euhedral to anhedral phenocrysts alkalic of alkalic feldspar, plagioclase feldspar and (oligoclase-andesine) armored with quartz 48 alkalic feldspar, and opaque minerals opaque minerals 20 that are probably altered pyrabole; zircon tr phenocrysts range up to 4mm (Md l*2mm); spatite tr feldspar microphenocrysts intensely calcite tr fractured; slightly glomeroporphyritic. Upper rhyolite .--The precipitous, bare, light-colored slopes of Eagle Bluff rise nearly 2,000 feet above the basin fill on the east flank of the Eagle Mountains. Eagle Bluff and much of the higher parts of the Eagles are composed of a variety of rhyolitic volcanic rocks shown on plate 1 as "upper rhyolite." This formation includes rhyolite, volcanic breccia, and flow breccia. Its roughly circular outcrop surrounds the stock of Eagle Peak Syenite. At Eagle Bluff, the upper rhyo lite is 1,500-2,000 feet thick; it thins northward and west­ ward . a Gillerman (1953* p* 36, pi. l) mapped conglomerate at the base of his "upper rhyolitic series" between Siphon and Wind canyons. Because outcrops of this basal conglomerate are small and because it is genetically related to the upper rhyolite, I have included the conglomerate within the upper rhyolite• The conglomerate consists of boulders of trachyte por­ phyry in a rhyolite matrix (Gillerman, 1953, p. 36). Just south of the Marine ranch house and just from a upstream small of dark limestone that be outcrop gray may Buda, there is a pale red purple rhyolitic flow breccia with angular fragments several inches in diameter. This rock grades up to a rhyolite of the same color and composition that has structure. pronounced flow Photograph 6. North-northwest view of Eagle Bluff from P.O-12.5! narrow, elongate, dike-like shape of upper rhyolite clearly visible; terrace gravel (Qgl) in foreground; dark rock left and right center is lower part of upper rhyolite (Tur), Eagle Mountains; light-colored rock, left center, is volcanic breccia of the upper part of the upper rhyolite (Tur); high point, upper center, is about 2 miles distant at an elevation of about 6,400 feet; elevation of gravel-covered terrace, fore­ground, is about 5,200 feet. The upper rhyolite, excluding for the moment the small outcrops of the basal conglomerate, consists of two types of rocks a rhyolite and an overlying volcanic breccia. The volcanic breccia is best exposed at Eagle Bluff on the east flank of the mountains; the less conspicuous underlying the central rhyolite is widely exposed in part of the Eagles, The rhyolite is compact and aphanitic with micropheno­ crysts of quartz and light-colored feldspar in a much darker aphanitic groundmass. The color ranges from light gray to medium dark gray to dark greenish gray to pale blue. The rock is brittle and intensely fractured; it weathers to smooth-surfaced, angular blocks. In Cottonwood Canyon, this lower part of the upper brownish medium and flow rhyolite is gray to gray shows structure marked by elongate silica-filled vesicles. Its habit in contrast angular, blocky weathering is to the smooth, weathered surface of the overlying volcanic breccia of the of the formation. In this lower unit upper part there is a change in dip toward the mouth of Cottonwood S® Canyon, where apparent dip of the rhyolite is about 20° be the result of subsidence or It may during following erup­ it be the result tion; or may of post-volcanism folding. At Rocky Ridge, the lower part of the upper rhyolite caps the highest point and is a medium light gray, compact, brittle, spherulitic rhyolite. The spherulites are white feldspar and range up to 5 mm (Md 1.25 mm). The rock also contains microphenocrysts of quartz and feldspar. Gillerman (1953, p. 37) described the lower part of the upper rhyolite adjacent to the syenite stocks . . . the rock is more calcic . . . trending toward andesite, and has been recrystallized into a dark-gray, slightly coarser-grained rock, Quartz and large oligo­in clase phenocrysts, some albitized, are set a ground-of and Clusters of of mass quartz oligoclase. granules recognizable secondary aegerite augite are present, as well as augite and pigeonite. Magnetite is common. The rock is extensively albitized and was evidently baked in soda-rich solutions. Farther from the away stock of Eagle Peak syenite the albitization is ... localized fractures. along The volcanic breccia that the of composes upper part the upper rhyolite forms the upper and steeper part of Eagle Bluff as well as much of the high ridges and peaks in the vicinity of High mill (M-12). At Eagle Bluff erosion has carved this rock into smooth-surfaced spires and pinnacles with broad, rounded silhouettes. The aphanitic matrix ranges from white to light gray to very pale orange and con­ tains microphenocrysts of quartz as well as angular fragments of volcanic rock, quartzite, and limestone that from range sand size to several inches in diameter. Near the top of Eagle Bluff there is a zone of undetermined thickness of very pale orange tuffaceous rhyolite that is almost free of breccia. It is characterized intense by liesegang banding. The lowest part of the volcanic breccia as well as the at underlying compact gray rhyolite are exposed the mouth of Cottonwood Canyon. The breccia has poorly developed bedding and is poorly sorted. Angular fragments of sandstone very and Yucca-like conglomerate range up to 2 feet in diameter (Md 1 in.). Looking north-northeast at the highest and steepest part of Eagle Bluff or looking at Eagle Bluff on air photo­ graphs, one is struck with its dike-like form. Rock near the base of this highest part of the bluff, however, has pro­ nounced horizontal flow structure. Just west of Cottonwood Canyon and between two of the large anomalous limestone blocks of the Bluff Formation, the upper rhyolite has the gross outline of a volcanic cone that has been breached to the south. The shape is especially sug­ gestive of a cone when viewed from the south or from the air, but I found no evidence on the ground to indicate the pres­ ence of such feature. any Within the upper rhyolite on the flanks of Eagle Bluff, there are outcrops that have been identified as Bluff Forma­ tion, Cox Sandstone, and Espy Limestone. Neither the iden­ tity of these rocks nor their mode of emplacement within the is certain. The smaller be upper rhyolite blocks may xeno­ liths . . Table 1? —Petrography of samples of upper rhyolite. Eagle 0 Mountains s Microporphyritic RHYOLITE; EU-Tur?--8; microphenocryst Summit of Snowline Ridge (M.6-LU3) quartz 7 just above adit at Rocky Ridge; alkalic feldspar 3 Euhedral to anhedral microphenocrysts of sedimentary rock 1 quartz and alkalic feldspar plus fragments to tr sedimentary rock fragment up clay? 4 8mm (Md 0.3mm) enclosed in a fine matrix 9 less devitrified grained matrix (’’n” than balsam) glass of devitrified glass with relict plus quartz and shards up to 0.1mm; feldspar highly alkalic calcite also either carbonatized; feldspar 69 minerals tr replaced matrix or filled opaque interstices. caleite 20 zircon tr TRACHYTE; EU-?-9i alkalic feldspar 45 Below adit at Rocky Ridge (M.7-11.3) igneous rock near floor of Snowline Canyon; fragments 5 Closely packed alkalic feldspar laths apatite 5 minerals (Md 0.15mm), many carbonatized, opaque 40 intimately associated with brown 5 and calcite black opaque minerals (Md 0.06) that may be altered mafic minerals; abundant to apatite in thin laths up 0.32mm (Md 0.02mm). Lower rhyolite sills .--On the north and east flank of the mountains near the contact of the lower rhyolite volcanic rock sequence and the Cretaceous and Permian sedimentary rocks, sills of orange-weathering lower rhyolite have in­ truded the Cox Sandstone in Lone Hill (H, J-12), in Espy Ridge (J, K-13)> and in the ridge just north of Spar Valley Because the Cox (M-14,15)« slopes on are characteristically mantled with colluvium, the sills are not well exposed. Even from a distance, however, discontinuous bands of the orange-weathering rhyolite are in reasonably sharp contrast and sandstone the with the gray brown-weathering of Cox. Not all these sills were mapped, and the width of some shown on the map is necessarily exaggerated. The sills of lower that intrude well-exposed rhyolite the gray Espy Limestone along Wyche Ridge are in strong color contrast to the country rock. Also well exposed is a 25-foot sill of lower rhyolite that intrudes the Benevides Formation at the locality of MS 8 (M.6-14®3)* The sills in thickness from a few feet to several range hundreds of feet; they show well-developed liesegang bands. Sills and irregular bodies of the lower rhyolite have also intruded the Hueco and Yucca formations in the Eagle and the Summit Formation Spring area Chispa in the Carpenter Spring area; in the vicinity of the Spar Valley fluorspar workings and in the area of the Lucky Strike prospect, the rocks Cretaceous lower rhyolite and of age are intimately associated. The ridge of Finlay Limestone south-southwest of Black Butte near JUDGE triangulation station is cut by dikes of lower rhyolite 2-3 feet wide. These small dikes are not shown on plate 1. Clearly all these intrusive bodies of lower rhyolite must have been emplaced at about the time the lower rhyolite lava was extruded. Perhaps the localization of the intru­ sive rhyolite along the north and east flank of the Eagles is an indication that the magma gained access to the surface in this more vents area through one or or fissures. Although no vents were identified in the the Peak map area, Eagle Syenite stock may well be emplaced within a major vent. It is pos­ sible that the older magma (pre-Eagle Peak Syenite), gained access to the surface along faults that were created by Lara­ mide tectonism, for example, the Carpenter fault or the Devil 0 Ridge fault Although locally variable, the rhyolite is character­ istically light colored (white, gray, very pale orange), fine grained, and in places is microporphyritic and spheru­ litic. Patinas of iron oxide are common on weathered sur­ of iron oxide scattered faces; irregular patches throughout also give the rock a speckled appearance on a fresh surface. Some of the rhyolite is distinctly laminated. It weathers readily but remains relatively competent• In the of Cox that northeast-facing scarp Sandstone overlooks the west flank of the Precambrian a 20­ outcrop, 30 foot sill (K.B-13.6) that crops out for several hundred feet is composed of altered and zoned laths of white feld­ spar up to 1 cm long in a light olive gray, fine-grained matrix. Although shown on plate 1 as a sill of lower rhyo­ this rock is different from the lower lite, distinctly rhyo­ lite that the other sills. composes from sills and dikes of lower Table 18.—Petrography of samples rhyolite and late rhyolite. Eagle Mountains Microporphyritic RHYOLITE; microphenocrysts EU-Trd-6; quartz 6 Dike along Eagle Spring fault, altered feldspar about 1,000 feet east northeast (chert, calcite. - sericite) 4 of Eagle Spring; Euhedral and subhedral embayed matrix phenocrysts of quartz along with quartz 70 chert 10 altered phenocrysts of feldspar sericite 10 (now chert, sericite, calcite) minerals tr up to 0.9mm (Md 0.5mm) in a fine-opaque highly vacuolized grained matrix of and anhedral equigranular quartz chert and sericite; quartz have phenocrysts coronas, average width 0«025mmo Porphyritic RHYOLITE; Eu-T?-39jl Small dikes near JUDGE triangulation phenocrysts altered feldspar 7 station (Ko3-8®8); alkalic feldspar tr Phenocrysts of altered feldspar (?), matrix now chert, calcite, and sericite, up quartz 35 to 2.?mm (Md 1.0mm) in a fine-grained altered feldspar 54 matrix of equigranular quartz calcite 1 (Md 0.1mm) and altered feldspar with “n" sericite 2 lower than balsam; distinct lath-shape zircon tr of some clay particles indicates they opaque minerals 1 may be altered feldspar; quartz up to 0.2mm occurs in irregular zones® Spherulitic RHYOLITE; EU-Trd?-43; spherulites 5 Small intrusive body (Ho5~12®6), (alkalic feldspar hill of Permian rock east north-and quartz) - matrix east of Eagle Spring; and alkalic quartz and clay Spherulites of quartz in a fine-(sericite ?) 79 feldspar? (Md 0®25mm) chert 15 grained equigranular matrix of quartz, chert, and an unidentified clay? zircon tr minerals 1 mineral (sericite?); euhedral opaque hexagonal opaque crystals altered to orange iron oxide; a few altered phenocrysts now chert, up to 2.25ram; quartz phenocryst 1 (0.4mm) with corona® Eagle Peak Syenite.--Gillerman (1953, 38) proposed Syenite p. the name "Eagle Peak Syenite" for the compact rock of a small stock that makes up the interior and highest part of Mountains. Small of the the Eagle apophyses Eagle Peak out east of the stock in a small near Syenite crop canyon East mill (M-13) and just north of the Marine ranch house (1-13), where the syenite caps a nearly-circular, low, rounded hill. North of the main stock, small intrusive bodies of the syenite crop out within the lower rhyolite, the in the trachyte porphyry, and upper rhyolite the general of the reaches of Frenchman vicinity upper Canyon (K, L-11, . 12) Vertical contact between the Eagle Peak Syenite and the upper rhyolite is well established at the east end of the crescent-shaped stock, but in places toward the west end the syenite appears to be concordant with and underlain by extrusive upper rhyolite (Gillerman, 1953, p. 38). The syenite may have intruded as a sill between the extruded layers of upper rhyolite, or the syenite may simply have ex­ truded as lava also. The weathered color and habit of the Eagle Peak Syenite and the upper rhyolite are so similar that locating their contact with accuracy requires careful searching. Al- any though I made several traverses across the stock, I am not sure whether the western part is extrusive or intrusive. This body of rock merits additional investigation, both in the field and in the laboratory. The west of Middle Mountain is in syenite (K, L-9) just vertical contact with the lower rhyolite through most of the but the two extensions of the outcrop, eastward syenite (pi. 1, section C-C f ) look like sills in the lower rhyolite. The color of most of the Eagle Peak Syenite is pale brown-pale yellowish brown; at the east end of the stock near the contact with the upper rhyolite, it is medium dark gray. Samples from this area invariably have a discolored zone extending as much as half an inch inward from the weathered surface. Weathered surfaces are various subdued shades of brown and orange. Erosion has carved V-shaped valleys and ridges with relatively steep, unbroken slopes in the syenite, but the effects of erosion on the upper rhyolite and the Eagle Peak Syenite are not different enough to dis­ tinguish these formations. The syenite is composed of phenocrysts of subhedral alkalic feldspar and minor amounts of anhedral iron olivine and subhedral plagioclase in a fine-grained orthophyric matrix of quartz, alkalic feldspar, and an unidentified mafic mineral. and zircon are the minerals. Apatite accessory Much of the olivine has been altered to iron oxide; viz., hematite, limonite, or magnetite. The few grains of plagio­ clase are commonly armored with alkalic feldspar; the phenocrysts of alkalic feldspar are commonly armored or patchily zoned with alkalic feldspar of lower index of re­ fraction* The rock has a granophyric texture. The Eagle Peak Syenite is a microgranite locally, where quartz constitutes as much as 15-20 percent of the rock. The terms "porphyritic" and "porphyry" are also applicable be­ cause phenocrysts of alkalic feldspar are up to 6 mm long and constitute as much as 25 percent of the rock. Table 19,-—Petrography of representative samples of Eagle Peak Syenite, Eagle Mountains Porphyritic MICROGRANITE; phenocrysts EU-Tep-81; alkalic Summit, Eagle Peak; feldspar 10 Hypidiomorphic granular with matrix euhedral and subhedral phenocrysts of quartz 15 alkalic feldspar up to 3•75mm (Md 2.0mm) alkalic in orthophyric matrix of laths of feldspar 64 alkalic feldspar (Md 0 o 15mm) and apatite tr interstitial quartz; alkalic feldspar zircon tr vacuolized and irregularly zoned 0 opaque minerals 11 Olivine, Quartz SYENITE PORPHYRY; phenocrysts EU-Tep-176; iron olivine 2 Outcrop (M,8-13.3) in small canyon alkalic about 1,000 feet west of East mill; feldspar 25 Phenocrysts of irregularly zoned matrix alkalic feldspar up to 2.5mm (Md 2mm) quartz 8 and altered iron olivine up to l„lmm alkalic (Md 0.25mm) in an orthophyric matrix feldspar 52 of iron olivine, alkalic feldspar and iron olivine 7 interstitial quartz (Md Oolmm); apatite 2 alkalic feldspar phenocrysts are zircon tr cryptoperthitic and most are slightly opaque minerals 4 resorbed along their borders; olivine altered to iron oxide. Porphyritic Quartz SYENITEs EU-Tep-265; phenocrysts Along ridge (M.0-13.2); leading west - alkalic southwest from Marine ranch house to feldspar 17 high peak; plagioclase 1 Subhedral alkalic feldspar and iron olivine 2 plagioclase up to 3®5mm (Md 1.2mm) matrix and severely altered iron olivine up quartz 10 to 1,1mm (Md 0,5nim) in a fine-grained plagioclase orthophyric matrix of quartz, alkalic and alkalic feldspar, plagioclase and iron olivine feldspar 59 with rudimentary granophyric texture; iron olivine tr some alkalic feldspar phenocrysts are apatite tr cryptoperthitic; many are patchily zoned opaque minerals 11 and armored. calcite tr chert tr Table 19,--'Continued Microporphyritic Quartz SYENITE microphenocrysts EU-Tep-379l plagioclase 4 About midway to crest of hill alkalic (K.7-10.0) of dark colored rock feldspar 10 north of mouth of Black Rock iron olivine 1 Canyon; matrix Subhedral microphenocrysts of quartz 10 plagioclase and alkalic feldspar alkalic up to 3.75mm (Md 0,5mm) and feldspar 58 anhedral microphenocrysts of iron plagioclase 5 olivine almost toally altered to apatite tr iron oxide, in a fine-grained zircon tr orthophyric matrix (Md 0,1mm); calcite tr plagioclase microphenocrysts opaque minerals 12 armored and patchily replaced by alkalic feldspar; microphenocrysts of alkalic feldspar armored and patchily zoned; some microphenocrysts cryptoperthitic ® Quartz SYENITE MICROPORPHYRY % microphenocrysts EU-Tep-393; alkalic Along ridge (L.4-12,2) about 4,000 feldspar 25 feet due north of Eagle Peak; augite 6 Subhedral microphenocrysts of alkalic iron olivine 2 feldspar up to 3.0mm (Md 0«6mm) and plagioclase tr anhedral altered microphenocrysts matrix of augite and iron olivine up to alkalic 1,4mm (Md 0,75mm) in a fine-grained feldspar 52 orthophyric matrix (Md 0 o lmm) of quartz 8 alkalic feldspar and quartz; about mafic mineral 5 5$ irregular masses of an unknown zircon tr mafic mineral (Md 0,05mm); olivine apatite tr more altered than augite; alkalic opaque minerals 2 feldspar microphenocrysts irregularly and patchily zoned and armored; rudimentary granophyric texture. Table 19.—-Continued Olivine SYENITE MICROPORPHYRY; micropheno cryst s EU-T?-433; plagioclase 5 Outcrop (L.0-11.2) in upper reaches alkalic of Frenchman Canyon, west -north- feldspar 20 west of Cherry tank; iron olivine 5 Subhedral microphenocrysts of matrix alkalic feldspar up to 2,0mm (Md quartz 3 0.5mm) and euhedral to anhedral alkalic microphenocrysts of iron olivine feldspar 49 up to 2.25mm (Md 0.08ram) in a mafic mineral 15 fine-grained felsophyric matrix of apatite tr quartz, laths of alkalic feldspar, opaque minerals 3 and an unknown mafic mineral (Md 0.05mm); alkalic feldspar microphenocrysts patchily zoned, resorbed along borders, and severely fractured; subtrachytic texture. Porphyritic MICROGRANITE;EU-Tep-443; phenocrysts High hill of dark rock (K.6-11.2) plagioclase 4 north of upper reaches of French­ alkalic man Canyon; feldspar 11 Subhedral microphenocrysts of alkalic iron olivine tr feldspar and plagioclase up to 3*0mm matrix (Md 0.9mm) in a fine-grained alkalic orthophyric matrix (Md 0.1mm) of feldspar 40 alkalic feldspar, quartz, and an quartz 20 unknown mafic mineral; alkalic mafic mineral 10 feldspar and plagioclase apatite tr microphenocrysts armored; alkalic opaque minerals 15 feldspar microphenocrysts irregularly and patchily zoned; some microphenocrysts are cryptoperthitic, some have resorbed borders; granophyric texture. Diabase dikes .--In several areas within the Eagle Moun­ tains near-vertical or vertical dikes composed of a dark greenish gray, fine-grained rock are in sharp contrast to These light-colored sedimentary and igneous country rock. dark rocks do not have everywhere the same composition or but be classified as diabase. texture, they may generally On south flank of Lone Hill the the (J.2-12.3), diabase has intruded along the Lone Hill fault and is exposed for several hundred feet in an outcrop about 100 feet wide. The of this dike is outer part a dark gray basalt porphyry com­ posed largely of laths of andesine and labradorite in a cryptocrystalline matrix. The rock has a subtrachytic tex­ ture. The inner part is a coarser-grained, greenish gray quartz diabase of similar composition but with a granophyric texture. At Rocky Ridge (M-ll) a dike of greenish gray and light brownish diabase cuts the Bluff Formation as well as gray the upper rhyolite. The dike is only about 4 feet wide but crops out for a distance of over 3,000 feet. At the mouth of Snowline Canyon (M.B-10.4), little a more than a mile west-southwest of Rocky Ridge, the road dike crosses a coarser-grained quartz diabase more than 50 feet wide that has intruded the lower rhyolite and trachyte porphyry (Gillerman, 1953, p» 39). What is presumably the same rock has also intruded the lower rhyolite and the Cox Sandstone few hundred feet to the southwest. The diabase a that the there, however, is finer-grained than near road; faults. the dike has been offset by northwest-trending a south- At about the midpoint of Wyche Ridge (M, N-14) black has in- west trending dike of greenish basalt porphyry truded the Mountains, and Buda formations. Be- Espy, Eagle cause the dike is aligned with near-by normal faults, it may have intruded along a parallel fracture. fine- A dike of altered light gray and greenish gray, the rocks of intrusive breccia also cuts to coarse-grained about 4,000 feet northwest of the dike of basalt Wyche Ridge The is too small to show just described. outcrop prophyry on plate 1. of diabase Table 20.—Petrography of representative samples dikes. Eagle Mountains Intrusive BRECCIA; EU-T?~212; quartz 5 Outcrop (M.7-14.3) in small chlorite a gully southwest flank ¥yche calcite 30 Ridge; epidote 2 Enigmatic, highly altered compact mosaic of several igenous rock apatite matrix tr 55 fragments up to 3«75mm in a matrix of quartz, alkalic feldspar, chlorite, euhedral epidote, calcite, and an unidentified nearly isotropic mineral (fluorite?) that in euhedral form is hexagonal* Intrusive BRECCIA; EU-T7-213; limestone Saddle (M.7-14.3) in Wyche Ridge at fragments 35 head of small gully from which igneous rock EU-T?-212 was collected; fragments About angular fragments of limestone (diabase) 10 (sparry calcite) and diabase up to matrix 52 3.0mm (Md 0.35mm) in a cryptocrystalline sericite 1 matrix of Mn Tf higher than balsam. opaque minerals 2 BASALT MICROPORPHYRY; EU-T7-251; microphenocrysts Outcrop (M.9-14. 8) on north side plagioclase 40 of gully, northeast flank of matrix Wyche Ridge; quartz 3 Euhedral and subhedral microphenocrysts plagioclase 22 of andesine-labradorite up to 4.4mm calcite 15 (Md 0.5mm) in a fine-grained matrix of chlorite 8 plagioclase and quartz; plagioclase opaque minerals 12 has altered to calcite and chlorite, which also fill cavities; subtrachytic texture. BASALT MICROPORPHYRY; EU-Td-30?A; microphenocrysts Dike (J.2-12.4) along Lone Hill plagioclase 35 fault, south flank Lone Hill; matrix 55 Euhedral microphenocrysts of andesine opaque minerals 10 and labradorite up to 1.75mm (Md 0.25mm) in a cryptocrystalline matrix (Md 0.005mm) with "n" higher than balsam; matrix shows slight pinpoint birefringence; interserial and subtrachytic texture. Table 20.—Continued Quartz DIABASE; EU-Td-307B; microphenocrysts Dike (J.2-12.4) along Lone Hill plagioclase 67 fault, south flank Lone Hill; matrix Euhedral and subhedral microphenociysts quartz 5 of andesine -labradorite up to 2.0mm chlorite 12 (Md 0.5mm) in a fine-grained matrix of calcite 7 quartz and calcic plagioclase; sericite 3 plagioclase altered to sericite and apatite 1 calcite; irregular patches of opaque minerals 5 chlorite abundant; granophyric texture. DIABASE; EU-Tdd-332; phenocrysts Dike (M.6-11.2) on north flank calcite 3 Rocky Ridge block; matrix Subhedral phenocrysts of feldspar. plagioclase 50 completely altered to calcite and quartz 3 chlorite up to 1.8mm (Md 1.0mm) chlorite 8 in a fine-grained matrix of subhedral calcite 25 andesine-labradorite laths up to apatite tr 1.25mm (Md 0.5mm) and subequant opaque minerals 11 quartz, chlorite and calcite; plagioclase laths altered to calcite and chlorite and resorbed along borders; subtrachytic texture. Late dikes the Wind Canyon fault off­ rhyolite dikes.--Because sets the Eagle Peak Syenite, the faulting must be younger than the emplacement of the syenite. Presumably at the same movement also took place the other east-west time, along faults, such as the Eagle Spring, Lone Hill, and Rhyolite faults. Perhaps, therefore, the rhyolite that intruded along the and Lone Hill faults as well as similar rock Eagle Spring north of Eagle Spring and Siphon Canyon are products of the latest igneous activity in the area. Yet it seems probable that the east-west faults have had two periods of movement, the first during the Laramide tectonism and the second after the intrusion of the syenite. If so, the emplacement of the rhyolite along the east-west faults could have happened at any time during the period of igneous activity. The relation of an outcrop of trachyte porphyry to the rhyolite that intruded along the Lone Hill fault south- southeast of Eagle Spring is unclear. If the trachyte por­ phyry overlies the rhyolite and the fault along which it is intruded, the intrusion as well as the faulting is older than the trachyte porphyry. The southernmost of the two long, late rhyolite dikes north of Siphon Canyon cuts the trachyte porphyry. If this intrusion is contemporaneous with that at Eagle Spring, all the dikes are younger than the trachyte porphyry. Hand specimens, however, from the dikes in the two localities are not strikingly similar, although both rocks are light-colored fine-grained rhyolite. Nevertheless the general east-west trend of both sets of dikes suggests that they be con- may . temporaneous The rocks mapped as late rhyolite sills closely resemble those shown as lower rhyolite sills. There is no more varia­ tion in color, texture, and weathering habit between the rocks of these two groups than there is between samples of the same from different localities. group The color of the rhyolite ranges from white to very pale orange, and the rock is fine-grained and compact. Micro­ phenocrysts of alkalic feldspar are common; no spherulites detected. Northwest of Eagle Spring, grayish olive associated the the pitchstone is with rhyolite along Eagle Spring fault. Eruptive Rocks of Indio Mountains The northern Indio Mountains lack extrusive igneous rocks; the southern Indies lack intrusive igneous rocks. The Cretaceous rocks of the northern Indios have been intruded dikes and sills by of Tertiary rhyolite. The Cre­ taceous rocks in the southern Indio Mountains were once partly, or more covered probably, totally by Tertiary welded tuff, tuff, rhyolite, basalt, sandstone, and conglomerate. The intrusive igneous rocks of the northern Indios have dikes sills the been mapped as rhyolite and under symbol "Tr” because the rocks are not lithologically distinctive, and they are difficult to correlate with igneous rocks of nearby areas. They do not resemble the extrusive igneous of southern Indio but more like the rock the Mountains, are rhyolite of the Eagle Mountains in color, texture and compo­ sition , I have extended the Garren Group of Hay-Roe (1957? 1956) and Twiss (1959a, 1959b) westward into the southern Indio Mountains becauses (l) the upper part of the group can be traced on the surface from the Van Horn Mountains west- area ward across Green River into the Indio Mountains, and (2) there is marked the Pantera lithologic similarity between Trachyte in the Wylie, Van Horn, and southern Indio Moun­ tains* In the Van Horn Mountains Twiss (1959a) corre­ area, lated the Hogeye Tuff of the Garren Group with the Chambers Tuff of the in the northern Rim Rock Vieja Group country. The lava flows, nuee ardentes, and ash falls that de­ posited the volcanic material in the southern Indio Moun­ tains probably extended some distance westward. The extru­ sive section igneous rock reported by Reaser (1962) in the north of the Rio is Cieneguilla area Grande remarkably like that in the southern Indio Mountains. Rocks of the Garren Group crop out in three areas in the southern Indio Mountains; (l) along and just west of the Green River; (2) in vicinity of Flat Top (U-15); and (3) in Lost Valley (W, X-15). The group is a sequence of alternating non-resistant tuff and ledge-forming welded tuff or quartz trachyte. A tuff forms the basal unit. The Pantera Trachyte is the middle resistant unit; the three units below the Pantera compose the Hogeye Tuff, and the two units above are mapped simply as tuff (Ttu) and trachyte The best of the Garren in the (Ttr). exposures Group are vicinity of MS 4 (U.4-17.0). Hogeye Tuff the southern Indio Mountains the Hog­ Tuff.--In eye Tuff is composed of three unnamed members? a lower tuff and (Thtl); a middle trachyte (Thtr); an upper tuff (Thtu). The lower tuff member (Thtl) is present only in the vicinity of MS 4, where it overlies the Yucca Formation. Although the lower tuff is covered at MS 4> its thickness there is about feet. A short distance north of the 32 near­ by road that leads generally southeast to Green River, a few feet of the lower tuff are exposed. There it is a white, highly calcareous tuff with fragments of quartz and volcanic rock, and a very light gray, poorly sorted, limestone breccia. The breccia is composed of angular and rounded fragments of finely crystalline limestone that to two inches very range up in diameter (Md 0.5 in.). The tuff as well as the breccia are brittle and exhibit conchoidal fracture. A few feet of the lower tuff may be present from place to place in the Flat Top area and in Lost Valley; if the tuff is covered. so, The member (Thtr) of the Hogeye is well exposed trachyte at MS 4, where it is about 170 feet thick and forms a prom- E. and extends inent ledge. The trachyte dips 17° about 4,500 feet along strike. About a mile and a quarter north of MS 4 there are two small isolated outcrops (T. 6-17.3) of the trachyte member. The trachyte member also composes the roughly circular outcrop (X-15) of igneous rock at Lost Val- it is more than feet thick at the ley; 250 large outcrop (X.­ 15.3) just north of the mouth of Eagle Draw. At Lost Valley the igneous rock rests on tilted strata of the Cox, Finlay, Benevides, and Espy formations. To the north in the vicinity of Flat Top, immediately northwest of the large stock tank is west of the north end of the the (U.5-15»2) that mountain, member overlies the Yucca Formation. It is well ex­ trachyte the draw leads west from the tank. A smaller posed along that outcrop (U.3-14«3) lies just north of the point at which this draw joins a larger one. A still smaller outcrop overlies south-dipping beds of the Yucca Formation about 3,500 feet south of the largest outcrop of the trachyte member and about 3,000 feet west of the road. The characteristic color of the trachyte is pale red, but in places parts of it are also pinkish gray, grayish rock orange pink, light brown, and moderate brown. The is compact, and displays conchoidal fracture. In places, elongate, subparallel generally weathered amygdules and rock fragments give the rock some degree of flow structure. The trachyte consists of crystals and crystal fragments of anorthoclase and some quartz (Md 0,3-0,4 mm) and frag­ ments of and rock that to 7 mm sedimentary igneous range up in diameter enclosed in a red-brown iron oxide-stained matrix that in places is fine grained and in other places is partly devitrified glass and glass shards that or not welded. are are In most places, sufficient quartz is present as fragments or as an interstitial component of the matrix to classify the rock as a quartz trachyte or a rhyolite. Much of the rock welded is a quartz trachytic tuff in which elongate, poorly devitrified glass shards are clearly bent around the crystals and rock fragments. Tridymite-filled vesicles are common, and some of the rock is permeated with thin, elongate amygdules of tridymite. Other secondary minerals are microcrystalline quartz and cal­ cite . Accessory minerals are apatite, zircon, and a black, metallic opaque mineral, probably magnetite. At many outcrops, the typical red trachyte is underlain feet of dark rock by 2-3 a gray, compact with a glassy luster. It is a welded or vitric tuff with both crystal and lithic ( ,f ,f fragments in a matrix nless than 1,537) of glass shards that average 0.25 mm in length. Some of the lithic fragments to 15 mm or more in length. This rock is range up probably the chilled base of a flow. Because of the of this presence rock at the base of the igneous rock section at Lost Valley, this Lonsdale (1958) discounted the possibility of outcrop vent. representing a At Lost Valley just east of the mouth of Eagle Draw, a black basalt out fine-grained, compact, greenish crops (X.­ 15»4) on the surface of a low ridge. I could not deter- upper mine whether it is extrusive intrusive rock. an oran About a mile west of Green River, the upper member (Thtu) of the Hogeye Tuff is poorly exposed for almost three miles in a narrow, north-trending outcrop that is protected by the At MS overlying, resistant, east-dipping Pantera Trachyte. 4, near the south end of this outcrop, this tuff member is about 25 feet thick. It is not present at Lost Valley. To the north in several scattered, poorly exposed outcrops west and northwest of Flat it is thinner than Top, considerably at MS 4* There, the color of the exposed tuff is light gray, and The light greenish gray, yellowish gray. grains range in size from silt to fine-grained sand, and the rock is char­ acteristically friable. Some of it is finely laminated, whereas other massive. About 160 feet above the parts are base of the upper tuff member at MS 4> is a yellowish gray tuff that is friable beneath a firm weathered the rock crust; is composed of spheres of calcite that range in size up to mm and that are enclosed in a matrix. 0.5 (Md 0.15 mm) glass The upper part of the upper member of the Hogeye Tuff is well exposed at the extreme north end of the outcrop belt about a mile west of Green River and half a mile south of the Indio Pass road. There, 50-75 feet of alternating ledge- and less resistant vitric and crystal tuff rest on forming the Chiispa Summit Formation. The color of the tuff ranges from medium to to light gray grayish orange pink pale pink; the rock is fine-grained and friable. The upper reistant tuff bed contains spherical calcareous concretions up to 2 inches in diameter; a lower zone of yellowish gray tuff contains ,fcylinders" that average 1 inch in diameter and 2 inches in length and are perpendicular to bedding. The upper member of the Hogeye Tuff has yielded fossil vertebrate teeth of and bones of animals including Mesohippus oreodont tooth. to Wilson the fauna indi­ an According (1962) cates a Chadronian or late Eocene-early Oligocene age for the tuff. The fauna as well as the tuff are correlative with those at the Ash Springs locality of Bridges (1958, p. 35““ 36). of the Table 21.--Petrography of representative samples trachyte member (Thtr) of the Hogeye Tuff, Indio Mountains Quartz Trachytic Devitrifled quartz 1 Welded MS 4.11 alkalic TUFF; MS U, Unit 2; feldspar 4 Volcanic rock fragments up to volcanic rock 8.5mm plus euhedral to anhedral fragments 8 and alkalic matrix crystals of quartz feldspar up to 2.0mm (Md 0.25mm) in alkalic a fine-grained iron oxide-stained feldspar, quartz 9 less of devitrified matrix^n1* than balsam) ( r,n devitrifled glass shards nless glass shards 86 than balsam) up to 1mm (Md 0.4mm) opaque minerals 1 zircon tr plus quartz and alkalic feldspar; glass shards show spherulitic apatite tr crossed cross under nicols; microeutaxitic * Microporphyritic QUARTZ TRACHYTE; microphenocrysts MS 4.4; quartz 1 MS Unit alkalic 4, 2; Fine-grained iron oxide-stained feldspar 1 matrix ( Mntf less than balsam) of matrix and quartz, alkalic feldspar, quartz, alkalic subparallel stringers of brown feldspar. glass encloses few microphenocrysts of brown dust 93 and alkalic to 1 quartz feldspar up tridymite 2.0mm (Md 0«35mm) plus several sedimentary rock rock 1 fragments (Md 2mm); secondary fragments rock calcite is vesicle filling and metamorphic product of alteration of feldspar. fragments 1 minerals tr opaque calcite 2 zircon tr Table 21.—Continued Microporphyritic QUARTZ TRACHYTE 5 microphenocrysts MS 4.7a; alkalic MS 4, Unit 2; feldspar 3 Fine-grained matrix ("n” less than quartz 1 balsam) with elongate iron oxide- matrix. stained stringers;encloses finegrained 91 euhedral to anhedral calcite 2 micronhenocrysts of quartz and sedimentary rock alkalic feldspar up to l0 .lmm fragments 1 (Md 0o3mm) plus rock fragments; tridymite 2 tridymite fills vesicles as does chert tr chert, calcite; calcite also zircon tr alteration product; microeutaxltico volcanic rock fragments tr Slightly Devitrifled Vitric TUFF; alkalic MS 4.9* feldspar 4 MS 4, Unit 2; quartz tr Crystal and crystal fragments of glass matrix 84 alkalic feldspar up to 1.75mm tridymite 10 (Md ®2mm) in slightly devitrified opaque minerals 1 iron oxide-stained glass matrix calcite 1 ( ,! n n less than balsam) in which zircon tr tridymite fills vesicles and chert tr pore spaces. Slightly Devitrified Welded Vitric alkalic TUFF; MS 4.10; feldspar 6 MS 4, Unit 2; volcanic rock Crystals and crystal fragments of fragments 3 alkalic feldspar up to 2.25mm quartz tr (Md 0o3mm) plus fine-grained glass matrix 91 volcanic rock fragments up to 4025mm opaque minerals tr in iron oxide-stained slightly devitrified calcite tr n glass matrix ( ftn less than balsam) with interstitial alkalic feldspar plus a little quartz and calcite. Table 21Continued Slightly Dsvitrifled Welded alkalic Vitric TUFF; MS 4oil; feldspar 15 MS U, Unit 2; quartz 5 Crystals and crystal fragments igneous rock of alkalic feldspar and some fragments 3 quartz up to lo5mm (Md 0©i+mm) glass and plus igneous rock fragments in cryptocrystalline an iron oxide-stained glassy and matrix 77 cryptocrystalline matrix (ttnft less opaque minerals tr than balsam)| quartz and alkalic apatite tr feldspar fill elongate vesicles• tridymite tr sericite tr Slightly Devitrified Vitric TUFFs anorthoclase 4 IU-Tr-54; cryptocrystalline Iron oxide-stained matrix of and glass matrix 87 cryptocrystalline material (’’n11 volcanic rock less than balsam) and equidimensional fragments 3 and elongate particles of glass opal 1 (Md 0,3mm) encloses corroded tridymite 5 crystals and crystal fragments of apatite tr anorthoclase (Md 0,4mm) plus volcanic opaque minerals tr rock fragments; opal, tridymite fill zircon tr cavities; microeutaxitico Microporphyritic TRACHYTE; IU-Tr-58; microphenocrysts Outcrop (U,3-15*0) south and west of anorthoclase 2 road that connects Trap tank and matrix Purple Sage mine; cryptocrystalline 88 Iron oxide-stained cryptocrystalline sedimentary rock spherulitic matrix encloses euhedral fragments 3 to anhedral microphenocrysts of calcite 3 anorthoclase (Md 0 o 3mm) plus tridymite 4 sedimentary rock fragments up to 7mm quartz tr long; cavities filled with tridymite. opaque minerals tr opal, calcite, quartz; microeutaxitic; zircon tr shard ghosts. apatite tr opal tr Table 21.—Continued Microporphyritic TRACHYTE; IU-Tr-66; about 3,500 Outcrop feet south of Trap tank and about 2,500 feet west of road; Iron oxide-stained cryptocrystalline less than and glassy matrix ( nnn balsam); glass in equidimensional and elongate fragments and imparts microeutaxitic texture; encloses euhedral to anhedral microphenocrysts some of anorthoclase, corroded, up to 2mm (Md 0.5mm); tridymite and ealcite in vesicles® Microporphyritic TRACHYTE; IU-Tr-69; Outcrop (X.0-15.5) rests on Finlay west flank Lost Valley Limestone, synclnne east of Eagle Draw; Iron oxide-stained cryptocrystalline matrix ("n” less than balsam) encloses of sanidine to microphenocrysts up and sandstone and (Md 0ol5mm) 2.5mm limestone fragments up to 6 o25mm; tridymite and calcite fill pore spaces® Tuffaceous LIMESTONE; IU-T?~52; of north Outcrop (U.5-15.2) west end Flat Tops Arcuate and branched subparallel glass shards in a matrix of sparry sanidine calcite; angular fragments of scattered throughout; opaque minerals placered parallel to fine laminations. Vitric TUFF; IU-Tb-55; downstream from Outcrop (U.5-15.1) west of north end Flat Top; large tank Iron oxide-stained glassy matrix ,f Tf ( n less than balsam) very slightly encloses a few igneous devitrified, rock fragments plus crystals and crystal fragments of anorthoclase and albite tol075mm (Md 0.4mm) up 0 microphenocrysts anorthoclase matrix cryptocrystalline and glassy matrix tridymite calcite minerals opaque apatite microphenocrysts sanidine sedimentary rock fragments cryptocrystalline matrix tridymite calcite minerals opaque clinopyroxene apatite chert sanidine calcite glass shards minerals opaque anorthoclase albite glass matrix igneous rock fragments minerals opaque zircon 3 86 10 1 tr tr 2 12 77 5 2 2 tr tr tr 1 58 40 1 3 2 92 2 1 tr Table 21*-“Continued Welded TUFF; IU-T-56; anorthoclase 4 Outcrop (Uo5~15*l) downstream sedimentary rock from large tank west of north fragments 2 end Flat Top; glass shards 40 Matrix ( ,Jn" less than balsam) cryptocrystalline of glass shards and matrix 51 cryptocrystalline material chert 1 encloses crystals and crystal apatite tr fragments of anorthoclase up to zircon tr 0.8mm (Md 0.3mm); a few chert­ biotite tr fragments and elongate sedimentary­ plagioclase tr rock fragments (sandstone) up to opaque minerals 2 5mm. Slightly Devitrified Welded TUFF; sanidine 2 IU-Tr-68; glass shards Outcrop (X.0-15*5) rests on and matrix 96 Finlay Limestone, west flank Lost volcanic rock Valley syncline east of Eagle fragments 2 Draw; calcite tr Crystals and crystal fragments sanidine up to lo2ram (Md 0.1mm) plus round volcanic rock fragments (Md 0.1mm) in a glassy matrix ( TfnM less than balsam) containing abundant poorly devitrified glass shards up to 1.25mm long (Md 0.25mm). of the Table 22.—Petrography of representative samples upper Indio tuff member (Thtu) of the Hogeye Tuff, Mountains Vitric TUFF; MS 4d3; anorthoclass 1 MS 4, Unit 3; glass shard Crystal fragments anorthoclase (Md 0.06mm) enclosed in a matrix of arcuate matrix opaque minerals 94 and branched glass shards (leucoxene) 5 (Md 0.25mm) and irregular but zircon tr roughly equidimensional grains of apatite tr leucoxene? (Md 0.07mm); microeutaxitic; laminated® shards 45 Calcareous Vitric TUFF; IU-Tr-22; glass Lowest resistant ledge (T.2-17.5) calcite 54 of tuff about half a mile south anorthoclase 1 minerals tr of Indio Pass road and almost a opaque mile west of Green River; Arcuate and branched glass shards and to 0.6mm (Md 0.4mm) glass up spheres up to 0«2mm (Md 0«15mm) cemented by sparry calcite, of which are in irregular patches scattered angular optical continuity; grains of anorthoclase® anorthoclase 45 Partly Devitrified Crystal TUFF; quartz 5 IU-Tt-24; partly devitri- Just beneath Pantera Trachyte half a mile south fied glass matrix 50 about minerals tr (T.2-17.5) of Indio Pass road and almost a opaque mile west of Green River; Angular fragments anorthoclass plus a little quartz up to 0.6mm (Md 0®08mm) in matrix (nnft less than balsam) of a partly devitrified glass® Tuffaceous LIMESTONE; IU-Tt-62; calcite 77 about 1,800 feet glass 3 Outcrop (U.7-15o2) south of Trap tank and west of road opal 20 that lies west of Flat Top; anorthoclase tr and branched minerals tr Calcite-replaced arcuate opaque and shards (Md 0.2mm) spheres (Md 0.3mm) covered with thin crust of opal; all cemented a few scattered by calcite| angular grains anorthoclase (Md 0.1mm); iron oxide- several irregular pieces stained glass (Md 0.2mm). Pantera Trachyte Trachyte is a resistant, Trachyte.—The Pantera characteristically pale red or grayish red, ledge-forming rock. From the north end of the outcrop belt about a mile west of Green River, a west-facing hogback capped by Pantera extends south almost three miles. In the Flat vicinity of Top there are several low-lying, resistant ledges of the Pantera Trachyte. The Pantera is 45 feet thick at MS 4, including 6 feet of welded crystal tuff at the base that ranges in color from black to light gray. At High Lonesome in the Van Horn Moun­ about seven miles to the the Pantera is tains, northeast, about 450 feet thick (Twiss, 1959a); the westward thinning indicates a source to the east. In the Wylie Mountains farther the Pantera is the most widespread extrusive east, rock and thickens westward to a maximum of perhaps 350 feet (Hay-Roe, 1957). The main of the Pantera is a rock that dis part compact plays subconchoidal to conchoidal fracture. Rock fragments and crystals and crystal fragments of anorthoclase and some and plagioclase that to 3*o mm in diameter quartz range up (Md 0.2 mm) are enclosed in a matrix of partly devitrified glass and glass shards that are stained with red iron oxide. The rock has pronounced microeutaxitic texture, and may be classified as a welded trachytic crystal tuff. The flow structure is not evident in hand specimen. Tridymite is a Photograph 7. South-southeastward view from T.O­ -red-brown Pantera Trachyte, right center, and white and gray Hogeye Tuff, center, overlie shale of Forma­ yellow gypsiferous Chispa Summit tion, lower center; tuff was gently warped prior to eruption of Pantera Trachyte; low hills, upper left, are east of Green River in Van Horn Moun­ tains area; barely visible on distant skyline, center and left is the center, Sierra Vieja. common mineral that filled or lined cavities. secondary Other accessory minerals are zircon, apatite, and a black metallic mineral, probably magnetite. The pale red to grayish red color of the upper part of the Pantera is in contrast with the varied shades of strong as well the black color of the lower of the gray as part formation. Microscopically, texture and composition are almost identical, except that the glass shard matrix of the lower part is not stained by red iron oxide. The lower part is less compact. In places, the top of the lower part of the Pantera is black and has a vitreous luster. too, is It, a welded trachytic crystal tuff, but some samples show vague perlitic fractures. Where this black welded tuff contains abundant crystal fragments, it is granular and crumbly; where it has few fragments and is largely glass shards, it is compact and weathers to a smooth surface. In a few black well-bedded places, gray or tuff, probably welded, feet thick lies above the pale red to grayish red welded tuff of the upper part of the Pantera. Northwest of Flat Top, a north-trending fence crosses a light greenish gray friable tuff several feet thick that seemingly lies within the upper part of the Pantera there. Table of Pantera 23•—Petrography of representative samples Trachyte, Indio Mountains Welded Crystal Trachyte TUFF; anorthoclase 20 MS 4.16; plagioclase 6 MS 4, Unit 4; glassy matrix 70 Crystals and crystal fragments of opaque minerals 1 anorthoclase (zoned) and volcanic rock plagioclase (armored) up to 3.25mm fragments 1 (Md 0.3mm) plus volcanic rock zircon tr fragments in a red-brown iron oxide- apatite tr stained welded glass shard matrix opal 2 (Tt n n less than balsam); incipient devitrification in places; eutaxitic. Welded Crystal Trachyte TUFF; MS 4»18; anorthoclase 23 MS 4, Unit 4; quartz 2 Crystals and crystal fragments of glass matrix 72 anorthoclase and quartz up to 5*0mm calcite 1 (Md 0.3mm) in a red-brown iron oxide- opaque minerals 1 stained matrix (n n u less than balsam) tridymite 1 of glass and glass shards; tridymite in sodic plagioclase tr some cavities; incipient devitrification; apatite tr eutaxitic. zircon tr Welded Crystal Trachyte TUFF; MS 4»19j anorthoclase 15 MS 4, Unit 4; plagioclase tr Red-brown iron oxide-stained matrix quartz tr ( ,TnM less than balsam) of glass and glass matrix 84 glass shards encloses crystals and opaque minerals 1 crystal fragments of anorthoclase up zircon tr to 2.0mm (Md 0.3mm) plus a few crystals apatite tr of quartz, plagioclase, and a volcanic volcanic rock rock fragment; eutaxitic. fragments tr Crystal Trachyte TUFF; MS 4®20; anorthoclase 20 MS 4, Unit 4; plagioclase tr Irregularly zoned crystals and crystal glass matrix 79 fragments of anorthoclase up to 3*0mm opaque minerals 1 (Md 0.3mm) in iron oxide-stained apatite tr matrix ("n" less than balsam) of glass zircon tr and glass shards; slightly microeutaxitic. Table 23.“"Continued Welded Crystal Trachyte TUFF; MS 4*21; MS Unit 5; 4, red-brown iron Microeutaxitic Tl Tt less oxide-stained matrix ( n than balsam) of glass and glass shards encloses crystals and of anorthoclase crystal fragments and plagioclase up to 3*Omm (Md 0.3mm); tridymite and opal fill or line cavities; microeutaxitic * Welded Crystal Trachyte TUFF; MS 4*23; MS Unit 4, 5; of Crystals and crystal fragments anorthoclase and a little plagioclase up to 2.5mm (Md 0.2mm) iron oxide- enclosed in a brown stained matrix ("n" less than balsam) of glass and glass shards, both partly calcite devitrified; tridymite and microeutaxitic. occur in cavities; Welded Crystal Rhyolite TUFF; MS 4*24; MS Unit 5; 4, and corroded Elongate crystals and crystal fragments of anorthoclase up to 2.5mm (Md 0.5mm) a in red-brown n iron oxide-stained matrix ( n,! less than balsam) of glass and glass both partly divitrified; shards, microeutaxitic. Welded Crystal Rhyolite TUFF; MS 4*25; MS 4, Unit 5; Crystals crystal fragments of and 8mm in anorthoclase up to l 0 (Md 0.2mm) a red-brown iron oxide-stained, partly devitrified matrix ("n" less than balsam) of glass and glass shards; fills cavities and tridymite lines and occurs as discrete roughly equidimensional microeutaxitic. (Md 0.08mm); masses anorthoclase plagioclase glass matrix tridymite opal minerals opaque zircon chert apatite anorthoclase plagioclase glass matrix tridymite calcite minerals opaque zircon apatite anorthoclase glass matrix tridymite opal minerals opaque apatite volcanic rock fragments anorthoclase glass matrix tridymite minerals opaque apatite igneous rock fragments plagioclase 9 1 83 4 2 1 tr tr tr 8 tr 84 5 2 1 tr tr 6 83 10 1 tr tr tr 10 69 20 1 tr tr tr Table 23«—Continued Welded Crystal Trachyte TUFF; anorthoclase 15 IU-T-491 plagioclase tr Outcrop (U.2-15®3) "west of road glass matrix 84 and just north of draw that runs opaque minerals 1 north of Flat Top; zircon tr Crystals and crystal fragments of apatite tr anorthoclase and plagioclase up to volcanic rock 6mm (Md 0.4mni) pins igneous rock fragments tr fragment in a brown iron oxide- stained matrix (”nn less than balsam) of glass and glass shards. both partly devitrified; microeutaxitic« Vitric Trachyte TUFF; IU-Tb-67; anorthoclase 2 Outcrop (U®8-15.4) on east flank of quartz tr hill about 4,000 feet southeast of plagioclase tr Trap tank and west of road; glass matrix 96 A few crystals and crystal fragments volcanic rock of anorthoclase, quartz, and fragments 2 plagioclase up to 0®9rom (Md 0 2mm) opal tr plus round volcanic rock fragments apatite tr up to 2mm in a matrix ( ftn" less than zircon tr balsam) of glass and glass shards opaque minerals tr with incipient devitrification® Welded Crystal Trachyte TUFF; anorthoclase 15 IU-Tg-88; plagioclase tr Outcrop (U#7-15«3) immediately west clinozoisite? 1 of road and about 2,000 feet southeast quartz tr of Trap tank® glass matrix 81 Crystals and crystal fragments of volcanic rock anorthoclase up to 8®5mm (Md 0®65mm) fragments 2 plus a few volcanic rock fragments opaque minerals 1 enclosed in a matrix ( H n M less than opal tr balsam) of glass and glass shards; apatite tr devitrified; scattered grains of clinozoisite? occur in cavities and as inclusions in anorthoclase; microeutaxitic• Table 23.—Continued Welded Crystal Trachyte TUFF; IU-Tg-89; Outcrop (U.7-15o3) immediately west of road and about 2,000 feet southeast of Trap tank (sample just above IU-Tg-88); Brown matrix (!,nu less than balsam) both of glass and glass shards, showing incipient devitrification, enclose crystals and crystal fragments of anorthoclase up to few (Md 0.5mm) plus a volcanic rock fragments. 2.25mm Welded Crystal Trachyte TUFF; IU-Tt-25; West facing scarp (T2-17<>4) 0 about half a mile south of Indio mile west a Pass road and almost of Green River; iron oxide-stained matrix Red-brown ("n" less than balsam) of glass shards encloses crystals and glass and crystal fragments of anorthoclase, plagioclase and quartz up to 2.9mm (Md 0.35mm) plus volcanic fragments; opal and rock calcite tridymite occur in cavities; is alteration product; microeutaxitic. Welded Crystal Trachyte TUFF; iu-t-48; west of road and Outcrop (U.2-15o3) northnorth of draw that just runs of Flat Top (underlies IU-T-49)| of Crystals and crystal fragments a anorthoclase, oligoclase, and little quartz up to 3*0mm (Md 0o5mm) in red-brown iron oxide-stained a matrix ( tTnn less than balsam)of glass several vesicles and glass shards; up to 5mm filled with fibrous, radiating, cryptocrystalline material; microeutaxitic« anorthoclase plagioclase glass matrix volcanic rock fragments minerals opaque apatite zircon anorthoclase plagioclase quartz glass matrix tridymite volcanic rock fragments minerals opaque calcite apatite anorthoclase oligoclase quartz glass matrix minerals opaque opal apatite zircon 8 tr 91 tr 1 tr tr 8 tr tr 85 4 2 1 tr tr 7 2 1 89 1 tr tr tr Table 23.—Continued Crystal Trachyte TUFF; IU-Tr-6l; anorthoclase 10 Outcrop (U0 7”15a2) about 2,000 plagioclase tr feet south of Trap tank and about quartz tr 1,500 feet west of road? glass matrix 82 Red-brown iron oxide-stained matrix volcanic rock ( T,nu less than balsam) of glass and fragments 2 glass shards encloses crystals and calcite 1 crystal fragments of anorthoclase, opaque minerals tr quartz, and plagioclase up to 2 e 5mm opal 5 (Md ,18mm) plus volcanic rock apatite tr fragments; opal lines cavities; matrix zircon tr shows incipient devitrification® Tuff (Ttu) and trachyte (Ttr) tuff unit (Ttr).--The uppermost (Ttu) of the rock of the southern Indio Igneous sequence Mountains overlies the Pantera Trachyte and at MS 4 is about 170 feet thick* Included within the tuff (Ttu) is a zone of olivine basalt* The tuff (Ttu) is overlain by a resistant (Ttr), which is the igneous rock of the trachyte youngest southern Indies. At MS 3 (V.6-17*2), about 2 miles south of MS 4j a partial section of the tuff is about 525 feet thick, and the thickness of an incomplete section of the tuff at MS 2 at Flat Top to the west is about 305 feet. The tuff thickens south and west because there a tongue of the out is of the lowest part trachyte wedges and re- a placed by tuff (pi. l) At MS 4 only about the lower 18 feet of the tuff are exposed; it is yellowish gray to light olive gray, friable, and weathered surfaces are smooth and rounded. It is a crystal tuff composed of crystals and crystal fragments of anorthoclase and plagioclase, ranging in size to 2.2 mm up (Md 003 mm), set in a matrix of partly devitrified glass shards. The rock contains a trace of zircon, about 1 percent of apatite and 2-3 percent of magnetite?. About feet north of MS the feet of 2,000 4, upper 40-50 the tuff is exposed; its color is distinct, ranging from grayish orange pink (10R8/2) to grayish orange pink (SYR7/2) to Itis to moderate orange pink (lOR6/4) pale pink. Photograph 8, Southward view of Flat Top from is lower T. 6-15.2; upper part trachyte (Ttr); half is tuff (Ttu); low-lying outcrop, left center, also trachyte (Ttr); limestone of Bluff Formation, right center; northern Sierra Pilares on distant skyline, right center; Indio fault just out of picture to left. fine grained, thin to thick bedded to massive, and moder­ ately hard. tuff that The calcareous grayish orange pink composes the lower part of the outcrop yielded several specimens of fossil snails. According to Taylor (1961): • Two specimens have moderately well preserved sur­face and nuclear whorls In the rela­ ., ... sculpture tively large, flat, nuclear whorls, the coarse retrac­tive growth lines, and general size and shape the fos­sils well with shells of Humboldtiana which now agree lives in the region. From the similarity in these morphologic features and from the geographic occurrence there is no reasonable doubt that the fossils are Humboldtiana, but without knowing the locality, one could not identify them surely. • . . This is the first well-established Tertiary record of the genus, and in any case the oldest known occurrence. These snails do not provide precise environmental information. Some species can live in rather dry places, others are restricted to humid or forested areas• At MS 3, within a partly covered zone that extends from 231 feet to 400 feet above the base of the section, compact, dark gray basalt and a scoriaceous, brittle, grayish red purple basalt crop out. This same zone underlies the road northwest of Flat Top and is well exposed in a small draw (U.2-l$o3) east of the road where a "baked zone” of light red tuff underlies the basalt. At this much crystal exposure of the rock is a flow breccia composed of angular blocks of vesicular basalt. This zone of dark gray olivine basalt, along with the brightly colored baked zone, crops out from place to place along the west face of Flat Top. The zone is best exposed just north of the water gap at the south end of Flat Top. There the thickness ranges from 10 to 40 feet, and the basalt is underlain by a baked zone. The top of the basalt holds a constant stratigraphic position along strike; there­ fore the change in thickness along strike is the result of the irregular surface over which the basalt flowed. The yellowish and light gray, fine-grained, friable tuff of the lower part of the section at MS 2 and MS 3 re­ sembles that at MS 4° The upper zone of light-colored tuff that the snails near MS is not at MS yeilded 4 exposed 3* At MS 2 at Flat Top, rock of the part of the section upper is a lithic or crystal tuff and much less homogeneous than at MS 4» In of the rock is a tuff breccia. fact, part over Quartz trachyte (Ttr) is present much of the area that extends about a mile west of Green River and about 3 miles south of the Indio Pass road. The rock is ex- poorly posed, however, because of a cover of gravel (QTg)« To the west the trachyte caps Flat Top, where it is 313 feet thick and well exposed. Two small outliers of the trachyte lie imme­ diately north of the mountain. The section of at trachyte MS 3 at the south end of the outcrop belt of igneous rock west of Green River is 512 feet thick. At MS 3j the rock is a microporphyritic quartz trachyte with euhedral to of anorthoclase anhedral microphenocrysts and a little quartz that in size to 3.0 mm (Md 6.0 range up mm) set in an aphanitic matris (n" less than 1.537). Rock are abundant in some Welded tuff zones. fragments composes part of the unit; much of the rock is eutaxitic, Tridymite and calcite have been deposited in many of the elongate vesicles that give rise to the eutaxitic texture. A black metallic mineral, probably magnetite, is a common accessory mineral. Color of the rock is light brown, pale red purple, pale red, and varied shades of gray. The trachyte is compact and shows conchoidal fracture. It, as well as the underly­ tuff and the Pantera be traced ing (Ttu) Trachyte (Tp), can east across Green River into the Van Horn Mountains area these rock units a sound correlation (Twiss, 1959a); provide of the between the two Garren Group ranges. The trachyte (Ttr) is more than 500 feet thick in the Indio Mountains about a mile west of Green River, whereas at High Lonesome to the northeast in the Van Horn Mountains it is only a little more than 300 feet thick. These dimensions, however, probably reflect not original thickness of the flow or sequence of flows but differential erosion after emplace­ ment . Table 24•—Petrography of representative samples of tuff (Ttu), Indio Mountains Devitrifled Vitric TUFF; MS 2.11; anorthoclase 3 MS 2, Unit 6; quartz 1 Cryptocrystalline matrix ( ,!n n less tridymite 5 than balsam) has faint shard ghosts volcanic rock and encloses crystal fragments of fragments tr anorthoclase and quartz up to cryptocrystalline 1,25mm (Md 0,5mm) plus a volcanic matrix 91 rock fragment l 0 25mm in diameter; opaque minerals tr tridymite in cavities. zircon tr Olivine BASALT; MS 3.5; labradorite 45 MS 3, Unit 2; alkalic Alkalic feldspar interstitial feldspar 20 between labradorite laths (Md 0.3mm) olivine 15 and angular grains of olivine (O^lmm); opaque minerals 20 several grains of plagioclase about apatite tr 2.5-3.Omm long; abundant biotite tr equidimensional grains of opaque zircon tr minerals. Crystal Trachyte TUFF; MS 4.26; anorthoclase 20 MS 4, Unit 6; plagioclase 5 Crystals and crystal fragments of glass matrix 71 anorthoclase and plagioclase up to apatite 1 2,2mm (Md 0.4mm) in a matrix (,,n M zircon tr less than balsam) of glass and glass calcite 1 shards (Md 0.1-0.2mm); feldspar opaque minerals 2 grains show extensive exsolution? and most are zoned or armored; matrix shows incipient devitrification; apatite as inclusions and in matrix. Crystal Trachyte TUFF; MS 4°27; anorthoclase 20 MS 4, Unit 6; plagioclase 5 Crystals and crystal fragments of glass matrix 70 anorthoclase and plagioclase up volcanic rock to 1,2mm (Md 0 o 25mm) plus volcanic fragments 2 rock fragments in a matrix (M nn less opaque minerals 1 than balsam) of glass and glass calcite 1 shards (0,2-0,3mm long); matrix zircon tr partly devitrified; apatite occurs in apatite 1 matrix and as inclusions. Table 24®—Continued Olivine MICROGABBRO; IU~Tb~47; Dark outcrop (Uo2“15*3) colored west of road and just north of north fork of draw north of Flat Top; Grains of olivine (Md 0.2mm), alteration along borders and fractures, plus grains of clinopyroxene (Md 0.025mm) and labradorite up laths to 0«7mm (Md 0.008mm) with interstitial and plagioclase. probably oligoclase, abundant black chlorite; metallic opaque minerals; calcite fills cavity. Olivine BASALT; IU-Tb-74; Outcrop (V.2-16.1) southwest flank of ridge that extends southeast from Flat Top, immediately northwest of point where road and draw cut through ridge; Grains of olivine (Md 0.3mm), altered along fractures and borders, plus laths of labradorite up to 0.75mm and (Md 0.25mm) grains of clinopyroxene ina T,fl matrix nthan balsam) of plagioclase more sodic than labradorite, plus chlorite; abundant black metallic opaque grains. (Md 0.025mm) (greater Vitric Quartz Trachyte TUFF; IU-Tr-75; Outcrop (V.2-16.1), just beneath IU-Tr-74; Crystals and crystal fragments of anorthoclase, quartz, and plagioclase up to OcAjmi (Md O.lram) plus a few volcanic rock fragments in a red-brown iron oxide-stained matrix ( ,Tnn less than and greater than balsam) of glass and glass both shards, partly devitrified. labradorite olivine chlorite plagioclase matrix clinopyroxene minerals opaque calcite labradorite olivine chlorite clinopyroxene plagioclase matrix minerals opaque apatite anorthoclase plagioclase quartz glass matrix opaque minerals volcanic rock fragments chert 30 10 1 20 12 25 2 35 2 1 25 17 20 tr 5 tr 5 90 tr tr tr Table 25•—Petrography of representative samples of trachyte (Ttr), Indio Mountains Vitric Quartz Trachyte TUFF; MS 2,22; anorthoclase 3 MS 2, Unit 7; quartz 2 Crystals and crystal fragments of glass matrix 80 anorthoclase and quartz up to 1.7mm tridymite 10 (Md 1mm) in a partly devitrified calcite 1 glass matrix (,r n n less than balsam); opaque minerals 4 vesicles lined or filled with tridymite, calcite; eutaxitic texture results from elongate, parallel vesicles* Microporphyritic TRACHYTE; MS 2.25; anorthoclase 10 MS 2, Unit 7| quartz 1 Microphenocrysts of anorthoclase cryptocrystalline (2 percent) and quartz (1 percent) matrix 68 up to 1.5mm (Md 0.6mm) plus one volcanic rock volcanic rock fragments 12.8mm long. framents 15 in a cryptocrystalline matrix ( nnM opaque minerals tr less than balsam); elongate vesicles zircon tr filled with anorthoclase and tridymite tridymite 6 create flow structure. Microporphyritic RHYOLITE; MS 2.29; sanidine 3 MS 2, Unit 7; quartz 2 Microphenocrysts of sanidine and quartz matrix of quartz in a fine-grained matrix (M n w less than and alkalic balsam) of alkalic feldspar and quartz; feldspar 94 a few grains of a highly altered mafic mafic mineral tr mineral. opaque minerals tr calcite 1 Rhyolite dikes and sillssills.--Intrusive rocks in the Indio Mountains are concentrated in the northern Indies in the numerous near- vicinity of Oxford Springs (pi* l), where vertical dikes of rhyolite range in thickness from ten to feet and trend from east-northeast to due several hundred east* The rhyolite has also intruded as sills, but they are less numerous than dikes. The majority of the intrusive bodies are aligned roughly parallel to the east-northeast faults. These intrusive rocks cut the Yucca, Bluff, and Cox formations; contact metamorphism is negligible. Northeast of Oxford Springs near the axis of Oxford syn- a sill or small stock of out cline, rhyolite crops (P-15) within the Bluff Formation. It is arcuate in plan, about 3,000 feet long and 700 feet wide. The rock ranges from red to pale pink-pale purple grayish orange pink; it is fine- grained, hard, compact, and exhibits conchoidal fracture. It contains microphenocrysts of alkalic feldspar in a micro- crystalline matrix of quartz, alkalic feldspar, and glass. The parallel alignment of quartz and feldspar lenses o l-0,2 e mm long constitutes microeutaxitic texture. Southwest toward Oxford Springs, a rhyolite dike (P, Q­ U) has intruded the Bluff and Yucca formations. The intru­ sive rock in color from to ranges very light gray grayish orange and is similar to the rock in the igneous mass to the The dike northeast. rock, however, has relatively more quartz and biotite and exhibits a rudimentary granophyric texture., West and southwest of Oxford Springs, dikes of light- colored rhyolite are abundant. Typical of these is the dike crossed by the road that runs north past Norte well to Ox­ ford Draw. At the first road crossing (U.9-13*2) the dike is 10-12 feet wide, strikes N. 65° E., and dips 74° SE. It is light gray, hard and compact, but it is not sufficiently resistant to extend more than several inches above the sur­ face . The northwesternmost outcrop of intrusive rhyolite, about 1.6 miles north of Oxford Draw (P-12), is dike strik­ a ing almost due east and dipping 72° S. that cuts steeply dipping beds of the Yucca Formation. The color of the rhyo­ lite is between pale red purple and pale pink; the rock is compact and has pronounced conchoidal fracture. Micropheno­ of and sericitized alkalic enclosed crysts quartz feldspar are the in a fine-grained matrix of the same composition. At west end of the dike, columnar jointing is well developed. The long axes of the columns are near-horizontal; that is, they are aligned perpendicularly to the cooling surfaces of the dike. Bostwick (1953* P« 53, pi. 1) reported, but did not de­ scribe three dikes in the northern Indies; (l) in the steeply dipping Bluff Formation of Bramblett Ridge about 1.3 miles northwest of the Oxford ranch house (P.B-13.4), (2) in the Chispa Summit about 0,1 mile southwest of Medicina well (Q­ and in the Bluff Formation about mile south of -16), (3) 0,3 His 1 shows this dike to be Squaw Spring (S-15)» plate within the Yucca; on page $3 he reported it to be in the Bluff Formation, The average length of these dikes, esti­ mated from his plate 1, is about a hundred yards® Of the dikes mentioned by Bostwick, I examined only the one in the Bluff Formation northwest of Oxford Springs; it is actually a sill and is a yellow green, severely weathered lamprophyric rock. About 300 feet south of the southwestern most dike of the northern Indio Mountains (Q-12) a near-vertical dike of hard, compact, basalt porphyry has in­ truded the Yucca Formation. It is feet wide and 3-4 crops out for a distance of about 30 feet in an east-northeast direction. Because I found no other rocks of these types and be­ cause their not shown on exposures are very small, they are plate 1 of this report. It seems likely that the rhyolite in the northern Indio Mountains intruded along what may have been extension frac­ tures created during the Laramide orogeny. If the frac­ so, tures existed at the outset of the volcanic activity in the and the rock intruded along them be related to the area, may earliest igneous rock in the Eagle Mountains, the lower rhyo lite, which there is both extrusive and intrusive. Table of 26«—Petrography of representative samples intrusive rock, northern Indio Mountains RHYOLITE; IU-Tr-133; alkalic in Outcrop (Q 5“13»3) feldspar 72 e tributary to Oxford Draw; quartz 25 chalcedonic Fine-grained matrix of quartz quartz 2 and turbid euhedral alkalic calcite 1 chert tr feldspar laths and microlites; poorly developed granophyric opaque minerals tr texture; spherulites; subparallel amygdules calcite, are chert and radial chalcedonic quartz. Microporphyritic RHYOLITE; microphenocrysts IU-Tr-142; quartz 1 Outcrop (P.9-12.7) along draw alkalic about half a mile east of feldspar 4 Red tank; matrix Microphenocrysts of anhedral quartz 28 quartz and subhedral sericitized alkalic alkalic feldspar (Md 0.6mm) in a cryptocrystalline feldspar 60 matrix of quartz and alkalic feldspar. sericite 5 minerals 2 opaque RHYOLITE; IU-Tr-316; microphenocrysts Outcrop (Q.3-14*1) crossed by road alkalic about 2,000 feet east of Oxford ranch feldspar 2 matrix house; Holocrystalline fine-grained mosaic quartz 25 of anhedral and euhedral alkalic alkalic quartz and biotite feldspar laths (Md 0,15mm), feldspar 72 flakes (Md 0,8mm); subhedral alkalic biotite flakes 1 feldspar microphenocrysts; amygdule is calcite tr calcite and chalcedonic quartz; poorly chalcedonic quartz tr developed granophyric texture« opaque minerals tr RHYOLITE; IU-Tr-317; phenocrysts Outcrop (P,7-15*3) alkalic in Oxford northeast flank 1 syncline, large feldspar igneous mass; matrix of Cryptocrystalline mosaic quartz quartz 25 and alkalic feldspar with a few alkalic alkalic to 0 2mm and anhedral o feldspar laths up feldspar 74 quartz less than 0,1mm; indistinct biotite tr microeutaxitic texture caused minerals tr by opaque zones of subparallel elongate relatively coarse quartz and feldspar (Md 0,15mm), Older Gravel In places in the southern Indio Mountains a poorly sorted, well-indurated gravel overlies the volcanic rock. Along the Indio fault east of Flat Top and on south, there are great mounds of gravel. These hills are remnants of a body of gravel that was emplaced during movement along the Indio fault. Southeast of Flat Top and near the mouth of Snake Can- the hills and the is yon (U, V-15,16) are rounded, gravel poorly exposed; much of it has been removed by erosion. On south, in the vicinity of Campo Benito, steep-walled draws incise the gravel and expose it to view. The gravel is com­ posed largely of round boulders of Tertiary volcanic rock with few boulders of Cretaceous rock. The dip is s°-6°, S. E. 55° is correlative with the Very likely this gravel poorly and Tarantula Gravel and sorted, pebbly cobbly (DeFord Bridges, 1959), which is similarly related to faults in the Rim Rock country. The roundness of the gravel as well as its bedding is derived in places enigmatic. Presumably it was locally and distance of made transport was short. It is, however, of soft rock. up largely relatively volcanic Photograph 9-North-northwest view along west flank Indio Mountains of from X,4-14-5; sand­stone and conglomerate of Yucca Formation, fore ground; Red Mountain composed of red-brown sand stone of Yucca, right center; terrace gravel overlies bolson which fill, in some beds dip east toward Indio Mountains; Eagle Mountains on skyline, upper left. Bolson Fill Fluvial, lacustrine, and aeolian deposits, also the re­ sult of erosion in to the difference in elevation response almost created by late-Tertiary normal faulting, completely surround the Eagle Mountains and their subsidiary ranges. the "older described and of the Probably gravel" just some oldest part of the "bolson fill” are contemporaneous and intergradational, although in general, the two formations are related to two different episodes of faulting. These which filled the deposits, troughs (grabens) normal created by late Tertiary faulting, range widely in grain size and composition. They are generally coarse near the margins of the intermontane basins and fine toward the center. The composition of the coarse material is directly related to the rock that was available for erosion in the nearby mountains; much of the fine material came down the ancestral Rio Grande that by the process of flooding and spilling over, poured fine material into many of the bolsons of Trans-Pecos Texas and northern Chihuahua (DeFord, 1962). Thickness of the bolson deposits can be estimated from the known depths of deep water wells bottomed in the fill. At Hot Wells, this depth is 1,000 feet; at Sierra Blanca, about 900 feet; at the old station of Torbert (F-11, 12) on the Southern Pacific, about 1,100 feet; at the head of Green River, about 1,100 feet; at Red Light mills, about 500 feet. A locality (T.B-12.3) along Arroyo Escudo produced a of vertebrate remains from the of the variety youngest part bolson fill beneath the capping terrace gravel. The fauna Ist Equus sp. Nannippus ? sp. Lepus sp• Urocyon sp. Geomys bursarius Camelops sp. Platygonus sp. texanus Glyptotherium Testudo (large) sp. Proboscidean (mastodont) to Wilson this fauna According (1962), definitely indi­ cates an early Pleistocene (pre-Illinoian) age for the bolson in which they were found. The deposits be as deposits may old as Nebraskan or as as Kansan. young Strain (1959, p. 375-377) has discovered a Blancan mammalian fauna in the fill of the Hueco bolson, near the town of McNary, about 25 miles west of Sierra Blance. Two zones are more fossiliferous than the rest of the section. he is uncertain of the of the lower he Although age zone, has dated the zone as early Kansan or older. Based on upper bones found in a gravelly sand beneath a layer of surface caliche near El Paso, Strain concluded? that the gravelly sand near El Paso was deposited between and . .. at some time early Kansan medial Illinoisan; that the Rio Grande, in the Hueco Bolson, began to en­ trench itself in the older basin fill after late Kansan and before medial Illinoian time. probably If, in fact, both Equus sp, and Nannippus sp. were pres­ ent in the bolson fill along Arroyo Escudo, a firm correla­ tion could be made with the zone of Strain 1 s section in upper the Hueco bolson. Because the identification of Nannippus from Escudo is the correlation sp. along Arroyo questionable, is only tentative. Terrace Gravel There are three principal formations of terrace gravel along Green River, the Rio Grande and Red Light Draw. These gravel formations, mapped from highest and oldest to lowest and youngest as Qgl, Qg2, and Qg3 on plate 1, represent suc­ levels the Rio Grande cessively lower base of drainage sys­ tem. They are not identified by lithology but by relative position above the present drainage system. Within a mile of the Rio Grande, the upper surfaces of the terrace gravels were at the following elevations above the adjacent flood­ 20 and plain? Qg3, feet; Qg2, 75 feet; Qgl, 150 feet. Like the bolson terrace in fill, the gravels range widely grain size and composition. They are a loose to well-indurated mixture of particles ranging in size from the finest silt to boulders. Because their composition depends on the type of rock available in the mountains, composition may vary more widely geographically within one terrace gravel than it does stratigraphically within one gravel or between successive terrace gravels. Near the Rio Grande, the broad, prominent gravel ter­ race that catches the is the second terrace gravel, or eye The terrace is Qg2. highest gravel, Qgl, patchily preserved the near the margins of the bolsons only. Along arroyos, terrace the lowest gravel, Qg3, is also sparsely preserved. The time during which the broad, prominent, second ter­ race was being laid down was a time when much gravel (Qg2) material was being eroded from the mountains and spread out over the low-lying bolson surface, both as terrace gravel but also in the form of the large alluvial fans that are so of prominent at the mouths Spar Valley and Carpenter, Goat, Horse, Frenchman, Broad, and Cottonwood canyons. After a change in climate, during an uplift in the area, or after a drop in base level of the Rio Grande, the Qg2 was eroded. The erosion was followed another gravel by conditions the low- period of relatively stable during which est terrace gravel, Qg3> was deposited. Much of its material was eroded from the mountain, but much of it probably came from erosion of the Qg2 gravel. It was merely eroded near the head of the alluvial fans and deposited widely across the foot of the fans. The same is true today; drainage channels are being cut deeper in the mountains and at the head of the fans; the eroded material is being deposited downslope in the gullies and draws and on the generally flat surfaces where anastomos­ ing drainage channels regularly capture one another. Because of their ever-changing courses, these streams spread sand and the surface that gravel widely over will eventually be­ come the gravel cap of the next lower terrace. The correlation of the terrace gravels of the Rio Grande drainage system with those of the Salt Basin drainage system is based primarily on the belief that the same conditions that produced the great alluvial fans on the southwest flanks of the mountains, produced those on the northeast flanks as well. The careful delineation and correlation of the terrace gravels of Trans-Pecos Texas is a challenging problem for the future. The rather cursory treatment herein is only a first step toward an understanding of their development and geologic significance. Alluvium The floodplains and stream beds of the present streams other than the Rio Grande were mapped as alluvium, Qal 1. The floodplain of the Rio Grande is cut some 10-12 feet below the floodplain of many of the tributary streams, and this lower level along the Rio Grande is shown as Qal 2 on plate 1. Of course, the stream bed of the Rio Grande and those of the tributaries are substantially at grade. Widespread undifferentiated alluvium covers much of the lower parts of the Devil Ridge area and the lower areas along the northeast and east flanks of the Eagle Mountains. Taken as a the material that the rock whole, composes mapped as Qal 1, Qal 2, and Qal ranges in size from the fin­ est silt in the valley flats and the large floodplains to the large boulders in stream channels within the mountains. The range in composition is just as great and includes the many types of rocks that crop out in the map area. Baker (1927, p. 40) reported ,Tdeep, straight, and narrow cracks” trending north in the alluvium northeast of the old he that these cracks Taylor place (P.3-17.7); suggested ”may have formed during the great earthquake in northeastern Sonora in 1887.” to similar set of cracks sud- According Espy (1957) a denly appeared in the alluvium about two and a half miles south-southeast of Hot Wells in about 1925* He recalled that he had ridden over his the week and had pasture preceding seen nothing of them, and he also recalled that at or very near the same time, a set of cracks appeared in the alluvium west and northwest of Red Light mills. He could remember no earth tremors during this period. What are presumably the locations of these same sets of cracks are clearly preserved today by relatively dense lines of the old cracks that are nothing more now vegetation along than shallow ditches. The main crack south-southeast of Hot Wells is about 2,500 feet long and trends northeast, then north. Espy re­ called that when he first saw it, in places it was 8-10 feet wide and deeper than he could see. Its location, as well as that of three or four smaller cracks that branch off at right angles and lead southeast or east, can be seen on GSLU air number photograph series, photograph 4“124* The set of fractures west and northwest of Red Light mills are much more complex; three roughly parallel main fractures, two of which are about two miles long, trend north-northwest and are parallel to Red Light Draw. The principal fractures are connected cross-fractures. Toward the north by minor, end, the main cracks together with the minor ones take on a polygonal outline. These fractures clearly shown on are GSLU air photograph series, photograph number 4-161. The cracks near Green River mentioned by Baker (1927, p. 40) do not appear on the air photographs of the GSLU series which were taken in 1950. In late May 1959, however, a north-trending fracture about 2,400 feet long and in places 4-5 feet wide and 8-10 feet deep, appeared in the alluvium north-northeast of the old Taylor place at or near where Baker the The main fracture reported cracks. effectively blocked travel on the road that leads north-northeast from the old Taylor place to the main Green River road. Only two weeks earlier I had driven along this road without dif­ ficulty . The main fracture had several branches, each about 1,200 feet long, that were perpendicular to the main fracture. At both ends of the main fracture it gradually narrowed until it was a mere hairline in the alluvium. Parts of the main frac­ ture held water left over from a rain several days earlier. There was no evidence that the erosive power of running water had created this break in the alluvium. About a mile and a half north, near the drainage divide between Green Valley and Eagle Flat, a smaller set of cracks in the alluvium lies west of the sheep loading pens on the Neal ranch. The main fracture trends northeast then north? it has several smaller branches. Fractures similar to those in the area have been re­ map ported in Salt Basin by Pratt (1958). Are these cracks the result of tectonism, erosion, des­ or controlled of the bolson fill? iccation, gravity shifting They do not seem to be the result of erosion or tectonism. The vague polygonal pattern developed in places indicates that they may be giant desiccation cracks or ,fmud cracks,” Windblown Sand Light brown windblown sand has accumulated north and east of Grayton Lake, East of Grayton its areal extent is well marked by a stand of tall yucca. There the sand is composed largely of angular to rounded, frosted, very fine- to medium-grained quartz as well as feldspar, rock fragments, and of caliche. grains The sand north of Grayton contains a much higher per­ centage of quartz and the sand grains are distinctly finer- grained than those to the east. The sand consists of very fine-grained, subangular to subrounded, frosted quartz grains, and little else. At both accumulations the surface of the sand is only there is difference of foot slightly irregular; a only a or less between adjacent high and low points. TECTONIC SETTING Regional geology focuses attention on two pre-Cretaceous tectonic events of the region, including the Eagle Mountains and vicinity; 1. During late Precambrian time the map area was undetermined and part of a geosyncline of size, shape, orien­ tation, which lay within the Texas craton (Flawn, 1962, p. Deformation of the sediments created 20). geosynclinal folds and faults, which even now are prominently displayed a short distance north of the Eagle Mountains. 2. The area was of the foreland of the map part Marathon Late Paleozoic that ended geosyncline. orogeny early in the Permian Period, deformed the geosynclinal sedi­ ments, uplifted and tilted the foreland area, created the Van Horn uplift as well as the larger Diablo platform, and erected the present structural framework of Texas, indeed of the central United States (King, 1942). The Diablo platform was a positive area throughout the remainder of the Permian Period and was probably exposed the Triassic the Jurassic and the throughout Period, Period, Neocomian Age (King, 1942). During most of Aptian time it was all above water and most of it was still exposed in early Albian time. The Coahuila platform, which was a separate and larger positive area south of the older Diablo platform during the 275 Middle and Late Jurassic and the Early Cretaceous epochs (fig. 6), is often referred to as the Coahuila peninsula. It was a peninsula only so long as it was exposed, which the Late Jurassic the and was during Epoch, Neocomian Age, the early part of the Aptian Age (Kellum, 1944; DeFord, 1962). The Diablo platform and the Coahuila platform, (fig. 6), had related though somewhat different histories as the Cretaceous sea advanced northward. The Coahuila platform was inundated in late Aptian time, the Diablo platform by mid-Albian (Kellum, 1936, 1944; DeFord, 1962). Viewed today, the Indio Mountains, Eagle Mountains, and Devil Ridge are part of a mountain range that begins just west of Ojinaga near La Mula, Chihuahua, and extends north” west about 150 miles to Sierra Blanca. This is the range easternmost and largest in a belt of folded ranges that from a Cenozoic cover in east-central Chihuahua emerge just east of the Rio Conchos and extend north-northwestward to the international boundary. These ranges roughly parallel the course of the Rio Grande, but two of them cross the river into Texas. In Texas the eastern includes the Indio Mountains, the Eagle Mountains, and Devil Ridge; the west­ ern, the Quitman Mountains. these two characterized Structurally, ranges are by (l) northwest-trending thrust faults, along most of which the Mesozoic Trans-Pecos Texas and northern Mexico. Figure 6.-Late paleogeographic features, overthrust block moved northeast, and (2) northwest-trending folds, many of which are asymmetrical to the northeast and some of which are overturned to the northeast® referred to this as the Baker (1935, P» 156) region Others "Mexican overthrust province." (Albritton, 1936, p. 1748, 1901; Smith, 1940, p. 599, 629; Buffington, 1943, p. 988) have considered the ranges in Texas to be part of a northwest extension of the Sierra Madre Oriental. Follow­ ing DeFord (1958b, p. 72, 74) I refer to these northwest- in northern Chihuahua and Hudspeth County, trending ranges as the Chihuahua tectonic belt. The sediments that Texas, were deformed to create the folds of the Chihuahua tectonic belt were deposited in the Chihuahua trough (Atwill, 1960, p. 22)• The Chihuahua tectonic belt was formed contemporane­ ously with the Sierra Madre Oriental by less intense defor­ mation of the sediments of the Chihuahua trough. Although folds and thrust faults are prominent in the of the Chihuahua tectonic belt, the outlines of ranges gross the ranges near and within the map area were produced by late-Tertiary normal faults that created a series of debris- filled grabens (bolsons) and eroded horsts (mountain masses). Thus, the map area lies within and near the margins of two superimposed structural provinces, namely, near the east margin of the Basin-and-Range province and near the northeast margin of the Chihuahua tectonic belt. That part of Texas immediately north of the Eagle Moun­ tains is characterized by flat-lying strata cut by normal the strata in the area and in the faults, whereas map ranges and southwest intensely folded and are southeast, south, thrust faulted. The nearly east-west zone of demarcation between these two physiographic and structural provinces, which roughly follows the Texas and Pacific Railroad between Sierra Blanca and Van Horn, was thought by Hill (1902, p. 173) to be the locus of part of a transcontinental zone of faulting that Ransome (1915, p. 294) later named "the Texas lineament. ,f Albritton and Smith (1957, p« 501-518), who reviewed the evidence for the Texas lineament in the light of recent work, concluded (p. 515): After more than half a the idea of the century, Texas lineament remains in the class of plausible hypothesis. This is so in spite of the fact that much information about Cordilleran structure which has lately come to light actually tends to increase its probability ... . . no fault zone is established until it is mapped; . and there seems to be no concensus as to where, pre­cisely, the Texas lineament should be drawn .. • Assigning a type locality to the lineament would give the name something to stand for, without dis­ couraging fruitful speculation as to possible exten­sions. We that the the Texas propose type locality of lineament be the segment of about 55 miles in length which runs along the corridor of Eagle Flat in ... Culberson Texas «.• Hudspeth and counties, STRUCTURE The structure of the Eagle Mountains and vicinity is dominated by thrust faults, strike-slip faults, and folds that formed in to severe compression in Late Cre­ response - taceous Early Tertiary time. Late Tertiary normal faults gave the mountains their gross outline; subsequent erosion has altered them to their present form. STRUCTURAL FEATURES OF DEVIL RIDGE AREA In the Devil Ridge area, large blocks have been over- thrust to the northeast an unknown distance (section A-A f , Later normal the relaxa­ pi. l). faulting that accompanied tion of compression has created structural relationships that but most of Devil are complex decipherable throughout Ridge. The area immediately east and southeast of the Speck ranch house, however, has been so severely broken by faults and contorted by folds that only the major structural features have been delineated (section B-B T , pi. l) . Thrust Faults of Devil Ridge Area Devil Ridge fault movement along fault.--Northeast overthrust Devil fault sufficient to Yucca and the Ridge was bring the r Chispa Summit formations into juxtaposition (section A-A , pi. l), that is, the oldest Cretaceous strata now overlie the youngest Cretaceous strata. Stratigraphic separation is 280 more than 8,000 feet. Throughout most of its extent, the trace of the northwest trending Devil Ridge fault is hidden beneath the alluvium just northeast of Front Ridge, but the trace is exposed from place to place along the northeast face of Love Hogback as well as along the north-facing scarp east of the Speck ranch house. The relatively incompetent rocks of the Chispa Summit overturned somewhat far feet from are and crumpled as as 75 the fault (Smith, 1940, p. 630), but they have not been dis­ torted as much as one might expect from their close prox­ to movement. imity so much I did not see the fault but Smith plane, (1940> p. 630) cited an exposure ”on Love Hogback 2000 to 2800 feet north­ ,T west of the Judge Love Ranch /now Speck ranch/ where • the fault dips 54° SW. As about feet of Yucca is above the fault 1,000 exposed in the overriding block and about 1,500 feet of Chispa Summit lies beneath the fault in the overridden block, the strati­ graphic separation should be roughly 8,000 feet. This esti­ mate is based on the thicknesses given in table 27. Assuming that the fault plane becomes less steep at depth, average angle of dip along the plane might be 25°. A stratigraphic separation of about 8,200 feet and a dip angle of 25° would entail about 19,000 feet of movement fault. along the Table 27 .-"“Estimated thickness, feet, of formations in Devil Ridge area Formation Chispa Summit 1,500 Buda Limestone 240 Eagle Mountains Sandstone 130 Limestone Espy 1,800 Benavides Formation 100 Finlay Limestone 550 Cox Sandstone 1,700 Bluff Formation 1,200 Yucca Formation 1,000 Total 8,220 The Devil Ridge thrust fault may be inferred to extend into the northern Quitman Mountains (fig. l), where the trace lies beneath the alluvium about a mile and a half southwest of Texan Mountain (A.2-1.8) The stratigraphic • throw decreases to the northwest* East of the Speck ranch house, the fault trace is largely covered; the in exposure Coal Mine Arroyo near Eagle Spring is the first exposure of the trace east of the mouth of Horse Canyon, Red Hills fault .--The Red Hills fault parallels the Devil Ridge fault, and for the most part the trace lies be­ neath the alluvium between Devil Ridge and Back Ridge. The Devil Red Hills thrust faults divide the Ridge and major part of the Devil Ridge area into three tectonic elements: (l) the foreland block to the northeast, (2) the Devil Ridge thrust block, and (3) the Back Ridge thrust block to the southwest. Movement along the Red Hills fault placed the Yucca Formation opposite the Finlay Limestone. This relationship is well exposed in Back Ridge where, in places, the trace of the fault The the is exposed. dip of fault plane, as meas­ ured on Back Ridge just south of the water gap (F.3-3.6) is SW. feet of the 24° If about 450 Finlay are exposed beneath fault plane and about 1,500 feet of Yucca are exposed above the fault plane and the thicknesses in table 27 are correct, the is feet. stratigraphic separation about 4,800 If an average angle of dip of 15° is assumed, movement along the fault was about 19,000 feet. Although the Yucca is in contact with the Finlay in most places where the trace is exposed, near the southeast end of Back Ridge the fault sheared an overturned anticline and placed the Yucca opposite the Cox Sandstone, The over­ turned anticline has also been cut out by the Red Hills fault northwest of the water in Back just gap Ridge as well as between Back Ridge and Red Hills to the southeast. West-northwest of Yucca Mesa, Huffington (1943, pi. 1) showed the inferred location of the trace of the Red Hills fault in the northern Quitman Mountains., The trace is cov­ ered, however, from a point just north of the water gap in Back Ridge to the inferred termination of the fault in the northern Quitmans. Southeast of Back Ridge, the trace of the Red Hills fault may be identified from place to place. On the north of Red Hills Yucca overlies southeast of this flank Finlay; point the location of the trace is uncertain. Minor thrust faults .--Coincident with the . . major over- thrust movement in the Devil area in to a max- Ridge response were imum horizontal compressive stress regime, there also structural the relatively minor adjustments throughout area* The stratigraphic separation of all these minor thrust faults is probably less than 500 feet; of most of them less than feet. 50 Minor faults with varied but generally low dip have dis­ placed the Finlay Limestone on the dip slope of Devil Ridge opposite Back Ridge. The "back thrust 11 mentioned by Smith (1940, p. 632) is at the upper end (F*B-4*B) of MS 16; it seems to be the southeast extension of the minor thrust fault that extends a short distance northwest. This thrust fault generally dips southwest, but at the location of the "back thrust" of it is overturned. Just beneath the Smith, southwest limb of the small syncline (F.B-4*9) the fault dips northeast. At Sand Mountain the Finlay is repeated by a small has a thrust fault (8.2-2.9), which stratigraphic separa­ tion of about 250 feet. Movement along a low-angle thrust account for the thin section of Cox fault may anomalously (D.l-2.1) at the northwest end of Devil Ridge just south of Yucca Mesa. There is a thrust fault in the Finlay that overlies the Cox on the northwesternmost block (D.3-2.1) of Devil Ridge. The the prominent steep-sided, southeast-trending ridge on southwest flank is an anticline that has been sheared so that the northeast limb overlies the southwest limb along part of the crest. This block of Finlay to be in seems thrust contact with the underlying Finlay. The distance between the of the Yucca at the north- top west end of Love Hogback and the base of the Cox that forms the small hills (G.2-6.3) to the west is not sufficient to accommodate a normal thickness of Bluff. A branch of the Devil Ridge thrust probably lies between the hills and the hogback. Another thrust fault probably extends eastward from a point (F.B-6.8) near McAdoo tank to a point (G.O-8,3) south of Triple tanks, thence southeastward between low, east- facing ridges of Espy Limestone. This fault also accounts for the offset of strata northwest and southeast of the point at which the fault from beneath the Devil emerges thrust the two faults Ridge plate* Actually, may merge. tanks Because it repeats the section, the McAdoo-Triple fault also helps to account for the slightly thick section of Espy Limestone recorded at MS 11 (G.O-8,8). This some­ what thick section of Espy may also be attributed to other thrust faults, for several have cut the Espy strata in the foreland block. When the water level in the westernmost tank of Triple tanks is sufficiently low, beds of Espy Lime­ stone in normal southwest-dipping position may be seen over­ lying limestone strata of the Espy that change attitude from N. 37° W. 17° SW. to N. 85° W. 20° NE. over a lateral dis­ tance of 30-40 feet. that the thrust Smith (1940, p. 632) reported small fault (H.3-7.7) in the Bluff Formation at the southeast end of Love Hogback has a stratigraphic separation of 25 feet and a net slip of 100 feet. I did not determine the net slip or stratigraphic separation, but they are small. The outcrops in the area (H, J-8, 9) that lie east and south of the Speck ranch house and west of Horse Canyon (j.O-10.1) are composed of thrust-faulted, folded, and nor­ mally faulted beds of the Yucca, Bluff, Cox, Finlay, and Espy formations. The nature of the Yucca made incompetent it yield readily to severe compression. It broke in a series of low thrust B-B shingle-like angle faults (section T , pi. l), but the movement was not great along any of them. probably That the strata were folded prior to thrusting is indicated the thrust fault contact of the Yucca and overlying Bluff by in the hill (H.B-8.2) just north of the north end of Speck the limestone of the Bluff is sharp Ridge. There, gray in contrast to the red siltstone and sandstone of the Yucca; the attitude of the two formations, which are in their nor- is in contrast mal stratigraphic positions, equally strong (pi. 1). The vertical and near-vertical beds of Finlay Limestone (j.3_9.6) that extend northwest from beneath the cover of volcanic rock on the west flank of the Eagles are in thrust- contact with limestone strata of the Bluff Formation. fault At the surface only the upper, Orbitolina-bearing beds of Bluff are present beneath the fault. Although the Bluff at the southeast end of the ridges is parallel with the Finlay, the Bluff and Finlay diverge northwestward. The Bluff under lies the valley to the north and reappears to form the high, the mouth of rugged, northeast-facing scarp just west of Horse Canyon. Devil Ridge Area Strike-Slip Faults of An inferred strike-slip fault that trends almost due east between Yucca Mesa and Sand Mountain accounts for the westward displacement of the Cox and Finlay formations in Sand Mountain as compared to the same formations on the southeast. The inferred movement along the fault was left lateral, but because of the distance separating the outcrops, it is not feasible to estimate the amount of movement. It re- is possible, however, that the displacement could have sulted from the folding of the beds southwest of Grayton Lake into a very gently plunging, southwest-trending syncline« North of Love Hogback about 1,400 feet of right-lateral movement of beds along a northwest-trending strike-slip fault (G.2-8.2) has offset strata of the foreland block. A left-lateral, northeast-trending strike-slip fault (j.7-B*7) is inferred to account for the offset in folds north and west of Black Butte. This fault is only in the overridden thus its inferred trace extends from be- block; neath the thrust sheet. South of the water gap, Speck Ridge is offset by a left-lateral strike-slip fault along which there was later vertical or near-vertical movement, north side down after the volcanic rocks were in place. The strike separation of the beds is almost 300 feet. The four strike-slip faults just cited represent pref­ erential movement along one of two potential shear fractures usually oriented at an angle of about 30° on either side of the direction of the greatest prinicpal stresses (Moody and Hill, 1956, p. 1209-1210), which in this area during the Laramide orogeny was approximately northeast-southwest„ fault near Yucca Mesa is oriented about The strike-slip 30° from the direction of the greatest principal stress; of the other three, is at a greater angle and the other two one are at a lesser angle than 30°. This variation from the average angle may be ascribed to the heterogeneity and iso­ tropy of the rocks, or to non-elastic deformation of the rocks, or to still other factors (Moody and Hill, 1956, p. 1213-1214) . Folds of Devil Ridge Area The most prominent folds in the Devil Ridge are area along the line of structure section B-B f plate 1. Begin­ , ning at the southwest end, beds of the Yucca and Bluff for­ mations dip about 30° northeast. Because the thick-bedded limestone of the the west limb of Finlay that composes Speck southwest and is overlain the Cox Ridge dips by Sandstone, the section there is overturned to the northeast. I infer, therefore, that immediately northeast of the first outcrop along the line of section B-B’ there is a syncline followed by an anticline overturned to the northeast. Speck Ridge, then, is made up of the faulted limbs of two folds: an overturned anticline to the southwest and a f syncline to the northeast (section B-B pi. l) e , Along the line of section the limestone beds of the Photograph 10. Southeastward view from G.9-7.8; thick-bedded limestone of Bluff Formation capping Love Hogback, foreground; Yucca Formation, left center; Speck Ridge composed of Finlay Limestone, right center, with southeast plunging anticline of Finlay Limestone to left; Black Butte, upper center and west flank of Eagle Mountains, upper left, where dark trachyte porphyry overlies light-colored lower rhyolite; northern Sierra Pilares, distant skyline, right center. southwest half of Speck Ridge dip 70°-82° SW. and are part of the overturned limb of the anticline described earlier. The thick limestone beds of the northeast half of Speck Ridge dip 50°-75° NE. and are separated from the beds of the southwest half of the ridge by a surface along which there thrust later normal The was earlier faulting and faulting. beds of limestone that the northeast half of the compose were now ridge dragged upward during the thrust faulting and form the steeply dipping southwest limb of a syncline. Northeast of Speck Ridge along the line of section the topography is dominated by two prominent, southeast-plunging anticlines; the outcrops are composed largely of Finlay Lime stone and rise several hundred feet above the surrounding terrain. The southwestern anticline to is asymmetrical the the to the southwest. The northeast; other, asymmetrical folds have an average plunge of about 20°; their steep limbs are vertical or near vertical. Between these two southeastward-plunging anticlines, a sharp syncline in Espy Limestone plunges southeastward and is asymmetrical to the northeast. Of special interest is the sharp symmetrical syncline exposed on the northwest flank of Black Butte where the con­ verging limbs of massive limestone of the make an Espy angle of about 100°. Presumably this syncline is the same as the one to the northwest between Speck Ridge and the anticline immediately to the northeast. South of the water gap that cuts Speck Ridge, the beds of limestone of the northeast steeply northeast-dipping half of the ridge continue southeastward until they are cov­ ered by alluvium. A strike-slip fault offsets them to the east about midway of the ridge. The limestone beds of the Finlay that make up the southwestern half of the ridge dip steeply southwest or are vertical as far southeast as the strike-slip fault. Immediately beyond that point, the attitude of the thick beds of Finlay Limestone as well as the beds of Cox Sandstone abruptly changes. Along the ridge on which JUDGE triangulation station is located the strata dip 30° ESE. I have interpreted this change in attitude to be the result of increased movement to the southeast along the nor­ mal fault that traverses the length of Speck Ridge. Imme­ diately south of the fault that transects the ridge, the crest of the overturned anticline has been moved down still limb the against the steeply-dipping of syncline to the northeast. In the area that lies between Horse Canyon (J.O-10.1) and the Speck ranch house, the compression created not only the imbricate thrust faults in the thrust sheet but also numerous small folds. Most of the fold axes were perpendicu the stress. lar to greatest principal At the northeast end of the thrust block of Bluff Lime- is of Hill stone that immediately south Pagoda (H.6-8.5), the gently dipping beds of limestone bend sharply upward; there is 55° W. At the southwest end of this small dip thrust block the attitude of the gently dipping beds abruptly steepens to about 65° SW. The dip of the beds roughly paral­ lels the of the thrust fault and the net effect is a dip monoclinal fold in the overthrust block of limestone of the Bluff Formation. There are two small southeast-plunging folds, asymmetrical to the southwest, in the nearby Bluff Formation; an anticline in the overthrust sheet and a syn­ cline in the overridden block. Although the Yucca that is exposed east of the Speck ranch house is little folded, the Yucca in the Red Hills has into small which been compressed numerous folds, many of are asymmetrical. The folds in strata of the Yucca and the Bluff in small outcrops southeast of Red Hills also reflect the of response the rocks to the compressive stress regime of the Laramide orogeny. Asymmetry of the folds in Red Hills as well as that in the small outcrops to the southeast is both northeast and southwest. Northwest, along the dip slope of Devil Ridge opposite Back Ridge, the Finlay is gently folded. Along the northeast flank of Back Ridge smaller but closer and overturned folds accompanied the creation of the relatively larger overturned Back Ridge anticline. The northeast, overturned limb of the limb this anticline dips 55°-65° SW.; southwest dips 30°-50° SW. In the Finlay just north of the water gap, a small is overturned to the south­ doubly-plunging syncline we st. At least one large fold and possibly several smaller folds lie between Back Ridge and Devil Ridge. As shown in section A-A T , plate 1, a syncline overturned to the north­ east the two separates ridges. In the foreland block in the vicinity of the Love ranch house, there are small folds (8.3-2.6) in the Benevides and the Finlay. Those in the Benevides are poorly exposed. Just north of McAdoo tank, there are a myriad of small the folds (F.4-6.7) in outcrops of the Espy, Eagle Mountains, and Buda formations. The attendant thrust faulting and sub­ sequent normal faulting and erosion have created a complex outcrop pattern. There are a few small folds in the Espy elsewhere in the foreland block. The change in strike of strata in the vicinity of Gray- ton Lake suggests that the foreland block might be located on the southwest flank of a northwest-plunging anticline. No evidence of the northeast flank of such a fold can be found to the north, however (King, 1949); I have concluded this is homocline. in that structurally area a The change strike west of Grayton may be the result of gentle, southwest plunging folds. Normal Faults of Devil Ridge Area Throughout the Devil Ridge area the dominant trend of normal faults is northeast; the secondary trend is northwest. a few of the normal faults has movement been Only along tens feet. The greater than a few of faults are largely near vertical or vertical; many of the minor faults, especially the of foreland dif­ those in low-lying ridges the block, are ficult to see on the ground and were mapped by their expres­ air sion on photographs. At Yucca several faults with throws of 100-300 Mesa, feet have displaced the Yucca and Bluff formations. Faults with slightly smaller throws have displaced the same forma­ tions at Love Hogback. At least two faults believed to have been reverse are the locus of much later movement during the period of normal faulting. The first is the fault that extends the length of Speck Ridge. Initial movement along the fault thrust the southwest block to the northeast which resulted in beds of the underlying block being dragged upward and beds of the overlying block being overturned. This zone of weakness per­ sisted through time, and during a later stress regime a nor­ mal fault developed, along which the southwest block moved down relative to the northeast block (section B-B f pi. l). , The roughly east-west fault that truncates the large anticline (J.O-8.9) in which Finlay is folded asymmetrically to the southwest, may be a normal fault or it may be a strike-slip fault related to the Eagle Spring fault. If it is a normal fault, minimum movement along it must have been about 1,700 feet to account for the missing Cox Sandstone. If it is a strike-slip fault, strike separation would prob- about ably be 5,000 feet. Volcanic rocks in Black Butte and in the mountains to the east-northeast appear to have been displaced vertically by a fault, which had offset the southeastern half of Speck Ridge during an earlier period of strike-slip movement. The outpouring of the lava occurred long after the strike-slip movement. Probably most of the mountain blocks in Trans-Pecos Texas are bounded by normal faults. The resulting basins or grabens have been filled with debris eroded from the moun­ tains or horsts. Although no direct evidence of such bound- faults exists in the Devil Ridge water wells ary area, along the Southern Pacific tracks 900 to 1,000 feet deep and still in basin fill indicate that a depression at least a thousand feet deep exists roughly parallel to the mountain front. agree with Smith (1939, p. 116-119) that this depression is the result of block faulting and not erosion or folding. There is a similar basin on the southwest flank of the Devil Ridge area. The well at Red Light (H.9-4*7) is reported to be about 500 feet deep (Espy, 1961); a well at the old Babb place just a mile and a half west-southwest of Red Light and a well at the Patterson and Elliot place about four miles northwest of Red both than feet Light are more 650 deep (Bramblett, 1961). The Devil Ridge area may be bounded on the southwest by normal faults with stratigraphic separa­ tions approaching 1,000 feet. There and throughout the map area, the boundary faults are roughly parallel to the fold axes . Relatively recent movement along faults just east of Red Light has cut the alluvial fill, but erosion has kept with the A fault a foot two or pace faulting. scarp high exists in a few places; elsewhere there is no difference in elevation across the fault. An anomalous north-northwest-trending cut in the allu­ vium (G.2-8.9) southeast of Triple tanks and southwest of Little Hill, seems to be the surface reflection of an ancient fault hidden beneath the alluvium; or it may repre­ sent a recent fault along which erosion has created a cut about 2,000 feet long, 15-20 feet wide, and up to 5 feet deep. STRUCTURAL FEATURES OF EAGLE MOUNTAINS beds Lime- Steeply-dipping, southeast-striking of Espy stone beneath the northeast of (J.7-9.2) disappear rhyolite Black Butte. There is little doubt that the complex and diverse structure of the Cretaceous rocks of the Devil Ridge extends east and southeast beneath the blanket of vol- area canic rock of the Eagle Mountains. Because of the general lack of stratification within the volcanic rocks, they tend to mask the effects of post-volcanism normal faults. Thrust Faults of Eagle Mountains Devil Ridge(?) faultfault.--What is probably a southeast ex­ tension of the Devil Ridge thrust fault is exposed south- southwest of Eagle Spring on the flanks of TC Peak and In Coal Mine Arroyo. Although the fault trace is covered by alluvium, steeply dipping Orbitollna-bearing limestone beds of the Bluff Formation overlie steeply dipping beds of sand­ stone and shale of the Chispa Summit Formation. Generally, the fault strikes northwest and dips gently southwest; locally, according to Smith (1941, p* 77) it dips gently northeast. In Coal Mine Arroyo (H.B-11.5) beds of the Bluff Forma­ tion strike N. 51° W. and dip 88° NE. The coal in the seam underlying Chispa Summit strikes N. 75° W. and dips 82° NE.; the along arroyo, however, dip in the overlying Bluff and underlying Chispa Summit may be as low as 50° (Baker, 1927, the and erratic in rocks above p, 22). Despite steep dips and below the fault, the sinuous trace in the vicinity of TC Peak indicates that the fault plane has a gentle dip. I do not think it feasible to estimate the stratigraphic the movement the because of the separation and along fault, uncertain stratigraphic positions of the beds of Bluff and Chispa Summit on the two sides of the fault. Stratigraphic separation, however, is less than that in the Devil Ridge area where the Yucca overlies the Chispa Summit along the Devil Ridge fault. West of Eagle Spring, between Horse Canyon and Goat Canyon, isolated blocks of limestone of the Bluff Formation are of the overthrust block. The dry water probably part well (H. 5-10.4) at the mouth of Horse Canyon, drilled during the summer of 1959, encountered the Chispa Summit Formation the cover of just beneath gravel. There are two small klippen of limestone of the Bluff Formation on the east flank of TC Peak. Southeast of TC Peak the only evidence of the Devil Ridge fault is a block of limestone, questionably identified as Bluff, about 2,000 feet south-southeast of this Carpenter Spring; limestone may be part of the Devil Ridge thrust block. Because the block is so highly fractured, its attitude is uncertain; it appears to strike N. 55° E., and dip 60° SE. That it is in contact Summit not be- with the Chispa was definitely established, of the of volcanic rock and colluvium. The cover cause Summit Formation does in Chispa crop out, however, gullies from the limestone block. Neverthe­ immediately downslope less, the block could be a xenolith within the volcanic rock of the lower rhyolite. Five miles southwest, in the valley south of Wyche Ridge and east of Eagle Bluff, the Chispa Summit Formation crops out from place to place, but only one outcrop is suf­ ficiently large to show on plate 1. Probably the Chispa Sum­ mit is in thrust fault contact with the Yucca Formation along the southern margin of the valley, although I did not see these formations in contact there. That the fault be an may extension of the Devil Ridge thrust fault is also merely a hypothesis, albeit a reasonable one. The fault is in proper alignment to be a southeast extension of the Devil Ridge fault and the stratigraphic separation is about the same. Because the of the lowermost volcanic rocks of the age Eagle Mountains that overlie the Devil Ridge thrust is prob­ ably middle Tertiary, the thrust faulting appears to belong - to the Late Cretaceous Early Tertiary Laramide orogeny. Gillerman (1953, p. 41) wrote that the Devil Ridge thrust fault earliest fault was the recognizable in the area. This may be true, but there is no certainty that some of the faults in the Permian rocks not be older. let most may of these faults cut Cretaceous rocks as well, and the rest are in alignment with faults that do. .--The Carpenter fault high angle, northwest-trending thrust fault parallels the northeast flank of the Carpenter Eagle Mountains, and it repeats the upper part of the Cre­ taceous section. Stratigraphic separation varies along the strike of the fault from about 600-900 feet. Along the Carpenter fault north of the Rhyolite fault, the Espy is in juxtaposition with either the Chispa Summit or the Buda, and where the Carpenter fault is exposed south of the Rhyolite the Espy is opposite the Buda Limestone. The fault fault, may be traced for more than 7 miles; it is offset from place northeast-faults. to place by or east-trending At an expos­ ure on the northeast flank of the ridge northeast of Carpenter the the fault is the fault trace is Spring, dip of 63° SW.; also exposed a short distance to the north in Carpenter Can­ . yon Gillerman (1953, p. 45) stated: The fault is a normal fault in the vicinity of and NE. To the Spar Valley dips 63°-Bs° north, however, the of the fault is reversed is 60° dip and SW.; hence, in this area it is a reverse fault. to This explanation perhaps unusual is sufficiently cause us to search for a simpler one. Because neither Giller- man nor I found evidence that the Carpenter fault cut the I the the volcanic rock, suggest that faulting preceded vulcanism; in other words, this is a Laramide fault. Except for minor reversals, the Carpenter fault may dip southwest its if it should be classed as a throughout length; so, high-angle thrust fault or a high-angle reverse fault. faultfault.—The trace of the fault Spar Valley Spar Valley is exposed at the upper end of Spar Valley; the fault is in­ ferred to run almost the length of the valley. The fault plane was penetrated by four diamond drill holes, and it is exposed in drift in shaft 1 (Gillerman, a There is little doubt that the fault 1953, p. 43)• dips generally 20°-30° SW. and that it has cut out some of the beds. It is a question, however, whether the fault was originally a low-angle normal fault that was later tilted or whether it was originally a low-angle thrust fault that originated after tilting of the strata and their intrusion by rhyolite. Gillerman (1953 p* 43-44) favored the latter > with hypothesis and correlated the post-volcanic compression that cited by Baker (1935a, P* 150). It is not certain, how­ ever, that the Spar Valley faulting is later than the intru­ sion of magma or the extrusion of lava. Perhaps both the fault and the fault results of Spar Valley Carpenter are Laramide compression. Minor thrust faultsfaults.--A low-angle, southwest-dipping thrust fault has displaced the Hueco Limestone to the north­ east along the base of Espy Ridge. The trace of the thrust fault disappears to the north beneath the alluvium and ter­ race At its southeast this fault terminates gravel. end, fault On against a that is either normal or strike-slip. the prominent nose of Permian limestone near the southeast end of the thrust fault, a slice of Cox-like sandstone, about 10 feet thick, is exposed along the sole of the fault for a short distance. The limestone just above the sand­ stone strikes N. 77° W. and dips 36° SW.; just below the the limestone N. E. and sandstone, strikes 44° dips 9° NW. The sandstone must be part of the overlying Bluff; its pres­ ence along the sole of the thrust fault together with the angle of the fault suggests that the fault formed near the top of the Hueco (Flawn, 1953 , p* 49)• At MS 14 (L.2-14.7), a low-angle thrust fault has dis­ placed the Hueco Limestone an unknown, but probably short, distance. The thrust faults and a fold in the limestone of Per­ mian age probably date from the Laramide orogeny, because the (l) there was no episode of severe compression in gen­ eral area after the deposition of the Hueco Limestone and prior to the Laramide orogeny, and (2) the thrust faults and features the fold are approximately parallel to similar in the Cretaceous rock. In the southwest of Carpenter Spring area, Espy Ridge, the Limestone has been two Espy displaced by minor, parallel. thrust faults. northwest-trending In the large hill (M.7-15*3) just north of Wyche Ridge, the Cox Sandstone has been folded and thrust east-northeast over Finlay Limestone. Along the trace of the fault, drag the Cox to ENE. steepened the dip of 55° Strike-Slip Faults of Eagle Mountains the Gillerman (1953> P* 46-47) interpreted nearly east- west faults that transect the Eagle Mountains to be younger rocks two of the than the eruptive because them displaced extrusive rocks and one of them displaced part of the syenite stock. These and similar faults be left-lateral strike- may faults that the in slip originated during Laramide orogeny to stresses east- response greatest principal aligned northeast and west-southwest. Later '’normal” movement along these faults in to an entirely different stress response regime offset early Tertiary eruptive rocks. Rhyolite fault .—Near the east end of the Rhyolite fault the dip is 77° SE. In the vicinity of shafts 2 and 3, which were sunk along this mineralized fault, the dip is 60° SE, (Gillerman, 1953* P* 47)­ The strike separation of the Eagle Peak Syenite along the fault is about 1,000 feet; the strike separation of the Cretaceous formations is 1,500-2,000 feet. On the north flank of Mine Peak where the fault cuts the Bluff Formation slickensides have hade (K.B-14»2) well-displayed a of 75°­ The inference is that the latest movement of rock along the Rhyolite fault was "normal” and was later than the vulcanism; earlier left-lateral strike-slip movement offset the pre- Cenozoic rocks. Although well exposed where it cuts sedimentary and the Rhyolite fault is difficult to trace metamorphic rocks, in the volcanic rocks to the west. The alignment of French­ man Canyon suggests that it may well be the locus of the west end of the fault. Wind Canyon fault .--The Wind Canyon fault is roughly parallel to the Rhyolite fault. Near the junction of Wind Cretaceous rocks the fault Canyon and Spar Valley, along have a strike separation about 2,000 feet. To the west the fault has offset the volcanic rock as well as the Eagle Peak Syenite; strike separation along this part of the fault is about 400 feet. the discrepancy between the strike of Again, separation rocks of Cretaceous and Tertiary age indicates that there could have been at least two periods of movement along the faults (l) a pre-eruptive left-lateral strike-slip movement; (2) a post-eruptive "normal” movement, south side down. The east-west fault at the Silver Eagle mine on the west flank of the Eagles may be a continuation or a branch of the Wind Canyon fault. On the topographic map of a fluorspar prospect in the upper reaches of Wind Canyon, Gillerman (1953, p. 66, fig. showed the of the Wind to be 8) dip Canyon fault 75° NNW.; yet his mapping of the fault trace and of the mineralized vein along it showed the fault to be near-vertical. Farther east, at the outcrop of Cretaceous rock, the fault is prob­ ably vertical or near-vertical. Eagle Spring faultfault.—-West of Eagle Spring the Chispa Summit Formation is separated from the Bluff, and Cox, Finlay formations by a rhyolite dike which intruded along the Eagle Spring fault. The strike separation along this part of the fault is 1.0-1.5 miles. East of Eagle Spring the Hueco and Bluff formations have been brought into juxtaposition along the fault; the rhyolite that intruded along the fault dis- about of appears 3,000 feet east Eagle Spring. Gillerman (1953, P* 46, pi. l) concluded that just west of Eagle Spring the Eagle Spring fault turns abruptly south­ east, then turns east again to pass along the southern flank Lone Hill. From the of point of junction just west of Eagle Spring he showed the Lone Hill fault leading eastward north of Lone Hill and the southern flank of the of along outcrop Permian limestone. Because I consider the Eagle Spring fault to be which a strike-slip fault, necessarily implies a reasonably straight trace, my interpretation places the Eagle Spring fault north of Lone Hill and the Lone Hill fault south of Lone Hill (pi. l). It is entirely possible that the Lone Hill fault is also a left-lateral strike-slip fault that disappears west­ ward beneath the terrace gravels and beneath the Devil Ridge thrust. The northwest part that leads to the junction of the Eagle Spring fault could be a later normal fault, with the east side downthrown. Blocks the Lone have ex- along Hill fault, however, may perienced only one general period of movement: a late-Tertiary raised the Lone Hill block relative to the faulting that area south and west. The Lone Hill fault would thus be a normal fault throughout its length. North of Eagle Spring there are several nearly east- west faults that may be left-lateral strike-slip faults. of the Stand of Along one these, Stage fault Gillerman (1953, p. 46, pi. l), there is fluorspar mineralization. Folds of Eagle Mountains There is a small, overturned, isoclinal fold (K.2-14.5) in Precambrian rock several hundred feet north of the end of the road that leads to the old mine in the Precambrian rocks. There are also crenulations and chevron folds in the phyllite members of the mixed units (Flawn, 1953, p. 49). The large hill of Permian rock northeast of Eagle breached Spring is a faulted and roughly east-west, doubly plunging anticline. Its anticlinal character is not evident because the north flank of the fold has been largely eroded. There are small north-northwest plunging folds in the Buda Limestone about 500 feet north of the point at which Creek cuts the westernmost of Carpenter through outcrop Buda. The axis of these folds parallels the strike of the beds. A poorly exposed northeast-trending syncline lies west of the northwest end of Espy Ridge. This fold is indicated in blocks which by opposing dips isolated in the Buda, Espy, and Eagle Mountain formations are exposed. In the Cox (M-15) just east of the windmills at the old Yarbro an anti- place, asymmetrical, south-southeast-plunging the cline was created along leading edge of a northeast- moving thrust block. The southwest flank has a dip of 19°; the steep northeast flank dips 58°. In structural form the of the anomalous largest lime­ stone blocks of the Bluff Formation on the south flank of the Eagles between Broad and Cottonwood is a canyons, southeast-plunging syncline. This fold may not have orig­ inated during Laramide compression; it may have been created the during the emplacement of block. There seems to be a broad fold east-southeast-trending in the lower of the volcanic rock that out in part crops Cottonwood Canyon. At the mouth of the the south canyon limb of the fold under the terrace at dips gravel an angle of about 25°. Much of this "folding" may have been the result of adjustment of overburden during the outpouring of lava. Without additional I would hesitate to that evidence, certify took after the vulcanism. folding place during or The dip of Cretaceous rocks peripheral to the Eagle Moun tains and of the overlying volcanic rock is generally centre­ clinal. In describing the structure, which trends northwest, the Mountain Gillerman (1953* P» 40) proposed name Eagle of the fold is it is shown syncline. The trend northwest; T in sections C-C f and D-D plate 1. Because the Cretaceous , tilted the Laramide there is dis rocks were during orogeny, a parity in most places between their dip and that of the over­ lying volcanic rock. Normal Faults of Eagle Mountains The primary trend of the normal faults of the Eagle Moun tains is north-northeast; throws are generally less than 200 feet. An exception to this generalization is the northwest- fault. As determined subsurface data from trending Mine by mine shafts and diamond drill holes in the area just south of the Rhyolite fault, the Mine fault splits into two faults? one branch dips 30°-50° SW. and the other dips 60°-77° SW* two are as (Gillerman, 1953, p. 44)* The shown a single fault on plate 1. This fault extends southeast along Spar Valley, where it is largely covered by alluvium and terrace There the lower of the is the gravel. part Finlay opposite lower part of the Espy; rhyolite has intruded in places fault. along the Northwest of the Rhyolite fault, along which late move­ ment offset the Mine fault about 200 feet, the Mine fault runs north-northwest to the Lone Hill fault. Stratigraphic throw increases northward where Cox and in Espy are juxta­ position along the fault. Rhyolite has intruded along the fault and elsewhere in the north area just of the Rhyolite fault. The nearly east-west strike-slip faults described earlier had a later, post-volcanic "normal" movement. This is indicated by the offset of the volcanic rocks along the Rhyolite and Wind Canyon faults. There may have been move­ ment along the other east-west faults at this time. Just south of Eagle Bluff, at the junction of the Eagle Mountains and the Indio a fault truncates the Mountains, northwest-trending rocks of Bramblett Ridge as well as the folded rocks of the Yucca and Bluff formations to the north­ east. The sense of relative motion along this fault is not known. If the block of limestone cropping out along the south flank of Eagle Bluff is in place, the north side of the fault would be down relative to the south side. This is opposite to the relative movement along other nearly east- west faults with which this fault is roughly parallel; it means that the area north to the Wind Canyon fault is a graben which could have originated by subsidence that attended the late stages of volcanic activity. There is, however, no indication that the fault just south of Eagle to the northeast. Bluff cuts Wyche Ridge The Eagle Mountains are probably bounded on the north­ east and the southwest normal faults whose by displacement can only be estimated by the depth of the bolson fill as determined by deep water-wells. Late movement along or at least parallel to these boundary faults is indicated by the northwest-trending faults (N.O-9-B) in the terrace gravel southwest of the mouth of Snowline Canyon. STRUCTURAL FEATURES OF INDIO MOUNTAINS Structure of the Indio Mountains is dominated by north­ northwest-trending thrust faults and a major northwest- trending normal fault, the Indio fault, that extends almost the length of the mountains. East of the Indio fault, the entire Cretaceous section is exposed; it is repeated in places by thrust faults but only slightly broken by normal faults. West of the Indio fault, the Yucca Formation pre­ and the fault dominates, "broken-glass” pattern and ex­ tremely irregular topography are in marked contrast to the relatively persistent strike valleys and ridges that characterize the area east of the Indio fault. The structural interpretation of the Indio Mountains is different from that previously published (DeFord, 1958b, p. 64-65, 68-69) DeFord and his students believed that the • overthrust both the Bennett fault to along (R, S, T-13 16) and the Squaw fault (P to V-12 to 17) was from east to west in to west-to-east overthrust along faults on the opposition west side of the mountains. They also believed that the block east the block immediately west and the immediately of Indio fault were continuous until they were separated by the faulting. If this were so, the strata exposed in Lost Valley should then have been continuous with those that are now truncated by the Indio fault near the mouth of Snake Canyon. All agree that the Indio fault is a Tertiary normal fault that the west block down. If this is a DeFord f s dropped fact, hypothesis fails to account for the position of the strata in Lost Valley. They are set too far back to the southeast. Because the beds dip generally south, normal faulting along the Indio fault would have offset the beds to the north along the fault instead of to the south. The normal faulting along the Indio fault displaced the volcanic rocks in the Flat Top area with respect to correla­ tive rocks along Green River to the east. Although several high ridges now separate these outcrops, the Tertiary lavas were presumably coextensive. The dip of the Indio fault is about 75° SW. (Bostwick, 1953, p. 46). Several lines of evidence indicate that the block west of the Indio fault is an overthrust sheet: (l) The west block is much more intensively faulted than the block to the east, and the topography and the fault pattern are much more irregular. (2) There is a marked difference in degree of erosion between the two blocks. Along most of the Indio fault, erosion has removed all but a part of the Bluff and Cox formations from the west block. Just east of the fault, formations as young as Buda are still present. The differ­ ence in amount of erosion is difficult to explain unless the east block was less broken and was protected from erosion by a thrust sheet that has since been removed. (3) Less conclusive but also suggestive is the lack of volcanic rock on the east block. If the two blocks were once continuous and covered with volcanic rock, remnants of this rock might be expected on both sides of the Indio fault instead of only on the west side. The evidence is strong that the west block is part of an overthrust sheet that moved generally from west to east. The east part of the sheet is preserved between the Squaw fault and the Willoughby fault. The west part of the over- thrust sheet lies west of the Indio fault. Between the Indio fault and the Squaw fault the overthrust sheet has been eroded from the central block. Thrust Faults of Indio Mountains Willoughby fault of widespread cover of fault.¦—Because the volcanic rock, alluvium, and terrace gravel, positive corre­ lation of the Devil Ridge fault at Devil Ridge with thrust faults to the southeast is not feasible. It seems probable that in the valley east of Eagle Bluff the trace of the Devil Ridge thrust leads east- southeast from beneath the volcanic rock to a point near the mouth of the the trace turns south. It first valley where becomes visible in the Indio Mountains at the east end of Oxford Ridge where the trace is the contact between the over­ lying Yucca and underlying Chispa Summit formations. This, that the Willoughby fault is the south- then, suggests southeast extension of the Devil Ridge fault. There is no doubt that the Chispa Summit dips beneath the Yucca. Throughout the length of Willoughby Ridge, the trace of the Willoughby fault is the contact between the Yucca and Espy formations. About a mile south-southeast of Willoughby Ridge and just north of the Indio Pass road, the trace is again visible where a small outcrop (5.4-l?«2) of overturned Espy Limestone protrudes through the thrust sheet. Still farther south-southeast, gray, rough-weathering limestone to overlie the Summit. This seems Chispa outcrop of limestone was tentatively identified as Finlay, but only on the basis of gross lithology. No diagnostic fossils were found there. From this point south, the trace of the Willoughby fault is hidden. fault the fault from north- Squaw fault.--Dip of Squaw ranges east to east-southeast. Comparing its trace with the trace of the Willoughby fault (sections E-E f F-F f G-G ? H-H T ,, ,, pi. l), I have concluded that they are but parts of a single fault. Northward the trace disappears beneath terrace to the south it terminates the gravel; (V.4-16.7) against Indio fault. The fairly straight of the north half course of the trace indicates that that part of the fault plane dips relatively steeply. It flattens to the south, however, as shown by the sinuous trace. Near where the road just south of the mouth of Snake Canyon passes through a barbed-wire fence high on a gravel hill (V.3-16.3)» there is a good view of the Yucca thrust the Espy. There, conglomeratic redbeds overlie beds of upon the fault gray limestone; plane dips 10°-15° SSE. Northward, where the strike gradually swings northwest then back to north, the dip is northeast or east. The thrust sheet is of but rocks composed largely Yucca, of it. Near as young as Finlay (S, T-17.3) make up part the north end of a small fenster Willoughby Ridge (Q.5-15*3) ex­ poses Chispa Summit beneath the thrust sheet. The most prominent topographic features of the northern Indio Mountains are part of the thrust sheet. Squaw Peak, about 5>400 feet above sea level, is composed of steeply- dipping beds of red-brown Yucca conglomerate. Viewed from the northeast or southwest, Squaw Peak appears to be a mesa; from the northwest or southeast, its true ”knife edge” pro­ file is evident. Southward along the trace, the Squaw fault, a diagonal fault, truncates increasingly younger Cretaceous rock. The color and lithologic contrast of the Yucca and the younger beds make that part of the trace clearly evident. To the north, where the trace lies entirely within the Yucca, its position is indicated primarily by a line of truncation of beds in the overridden block. Folding of the fault plane (section F-F f pi. l) may , have been partly contemporaneous with or entirely later than the thrust faulting. Bennett fault .--The trace of the Bennett fault roughly parallels the trace of the Squaw fault. To the north, the Bennett fault terminates against the Indio fault; and dis­ placement decreases to the southeast where the fault dies out into an anticline (U.O-16.8) half a mile west of East . Ridge This fault is a diagonal fault, for it truncates strata of the Yucca, Bluff, Cox, Finlay, Benevides, and Espy formations. The dip of the fault plane probably averages 30°“40° at the surface. The ostensible horizontal displace­ ment of the Bluff strata the fault is about by 6,000 feet, which was caused by the northeast block overriding the southwest block. Movement could have been the entirely of dip-slip type (Bostwick, 1953 > P» 27-32). Borrega fault fault is not evident in the fault.--The Borrega field because its trace is almost the Yucca entirely within Formation. The fault to extend the length of the appears but in several its trace is obscure. It mountains, places is not certain, for example, that the thrust fault that leads south-southwest from a point near the termination of Red Mountain fault is part of the Borrega fault. The trend of the trace is N. 20° W. The of average dip the is but fault variable, a generally steep dip, i.e., 30°­ 40° SW., is indicated by the relatively straight trace. The Borrega fault is one of several west of the Indio fault that have overthrust the Yucca unknown but probably short distances to the northeast. fault.--Another fault Red Mountain fault relatively high-angle along which beds have been overthrust to the northeast is the Red Mountain fault. Its trace is roughly parallel to the trace of the Borrega fault, but the Red Mountain fault is only about three miles long. The overthrust sheet is composed largely of the red-brown sandstone, siltstone, and conglomerate of the Yucca, but near the north end of the limestone of the Bluff and fault, sandstone of the Cox formations form part of it. They, in turn, appear to have been covered by beds of the Yucca which moved northeastward along a gently southwest-dipping thrust fault. The overthrust may have been at some angle to the of of the principal direction transport overriding block, because as the overriding block moved northeast, it was per­ haps subject to lateral compressive stress by unyielding structural elements along its margins. Minor thrust faults the Indio Mountains, in faults.--In as in* 'i"" in lai-n i 1 i . i.i i iriinim r other parts of the map area, the incompetent rock of the Yucca responded readily to the compressive stresses imposed on the region during the Laramide orogeny. Thrust faults with minor movement are abundant in the Indio Mountains. The traces of these minor faults, most of which trend at very small angles to the strike of the beds, are recognized primarily by truncation of beds along the fault. These minor thrust faults abound in the overthrust sheet west of the Indio fault, and opposing movement, i.e., movement to the west well to the is common. as as east, Northwest of Evans Peak, movement along steeply dipping faults to have been northwest in appears general opposition to the northeastward movement of the Borrega and Red Moun­ tain faults. Northwest of the Indio ranch overthrust an house, sheet of Yucca (R, S, T-13, 14) appears to have moved west­ ward . minor faults A number of relatively thrust displaced the Yucca in the area north of Squaw Peak and south of Ox­ ford Ridge. The overthrusting was generally southwest, south, or southeast. The tight folding along Bramblett Ridge and the more gentle folding to the northeast is associated with minor thrust-type displacement of strata of the Bluff and Cox for­ mations . Strike-Slip Faults of Indio Mountains I have recognized no strike-slip faults in the Indio Mountains. Some of the so-called normal faults that have a general eastward trend may actually be strike-slip faults. Folds of Indio Mountains Folds are more numerous in the Indio Mountains than in other parts of the map area. Fold axes are generally perpendicular to the inferred direction of the greatest prin cipal stresses of the thrust faulting with which, indeed, many of the folds are closely associated. The eastern part of the overthrust sheet between the traces of the and Squaw Willoughby faults contains a multitude of small and large folds. A southeast-plunging synclinorium lies immediately and to southwest of roughly parallel Willoughby Ridge (Q, on a R-15* 16). Superimposed it are number of smaller folds, some of which are overturned to the northeast. The syncli­ norium extends south-southeastward along the eastern margin of the mountains, but only the west limb is exposed. The anomalous dips in the easternmost exposures of the Yucca, Bluff, Cox, and Finlay formations that the west limb compose doubtless reflect folds similar to those in the Yucca and Bluff formations in the of the synclinorium to the nose north. South of well the folds less Squaw are well-expressed, perhaps because of faulting contemporaneous with the folding, or because of later faulting, or both. Eroded folds in the east of the overthrust sheet part north of Oxford form some of the most distinctive topographic features of the Indio Mountains. Bramblett Ridge is the steeply-dipping and in places overturned east limb of an anti­ cline whose western limb has been dropped down by the Indio fault (section E-E 1 pi. l). Bramblett Ridge is also the , west limb of a sharp, faulted syncline whose axis parallels the ridge. To the northeast, the highest point in the Indio Mountains, about 6,000 feet above sea level, is the summit of the east limb of the breached north-northwest-trending Horse Peak anticline. The limbs are made of beds of re- up sistant limestone of the Bluff Formation, which dip away from the axis at angles of 15°-20° Erosion has exposed Yucca in . the core of the fold. The axis of Oxford syncline is roughly parallel to Ox­ ford Ridge. Interestingly, both of these features trend northeast, i.e., their trend is almost perpendicular to that of the other folds of the Oxford A area. greatest principal stress oriented northwest-southeast was associated with this as well as with the minor thrust faults that also folding parallel Oxford Ridge, These features may be another indica­ tion that unyielding structural elements marginal to the over thrust sheet influenced its folding. Northeastward, along the margin of the Indio Mountains, strata of the Bluff and Yucca formations have been deformed in a series of minor anticlines and synclines. These folds reflect the of this to the of may proximity area leading edge the overthrust sheet. The block that was overridden by the Willoughby (Devil Ridge) thrust fault is exposed between the Indio fault and the Squaw fault and east of the Squaw fault from place to place along the eastern margin of the mountains. Willoughby Ridge is the west-dipping limb of an over­ turned syncline; presumably it is a drag fold along the was as Willoughby thrust fault that developed the overriding block moved northeast (section F-F f pi. l). The trace of , the of this fold lies within the of the axial plane outcrop poorly-exposed Chispa Summit Formation northeast of the ridge. On the dip slope of Willoughby Ridge the Espy has been broken by a small thrust fault; movement to the north­ east created a small overturned, drag anticline. A gentle syncline that appears to plunge northwest is inferred to be in the block beneath the eastern part of the overthrust sheet; a gentle anticline lie may immediately to the east (section F-F T pi. l) . , The area that is bounded on the west by the Indio fault and on the east by Squaw fault, i.e., the underthrust block of the Willoughby fault, contains a number of small folds. There are tight, steeply-plunging, northwest-trending folds in the Bluff and Yucca formations just southwest of Squaw Peak. Near Squaw Spring several small drag folds in the Bluff and Cox formations are associated with the Bennett fault. The trend of the fold between axes ranges north and west; more folds exist than could be shown on plate 1. About 0.7 mile northeast of Palmas well (abandoned) is the breached anticline into which the Bennett fault dies as it leads south. The north-northwest-trending fold is ex­ pressed in the Espy Limestone; the limbs dip about 20°-25°* The fold axes are offset by an east-west transverse fault that lies just north of the road. South of the fault the axes are offset to the west several hundred feet. Other small folds of similar trend are expressed in the Espy in this general area. A window in the easternmost overthrust sheet, about 0.7 mile west of Escondido well, exposes the Espy, Eagle Moun­ tains, and Buda formations in a south-plunging anticline upon which are superimposed small tight folds. Along the west limb the Espy has been thrust over the Buda and in places along the east limb the Buda is overturned. The distinctive the brown-weathering Eagle Mountains Sandstone is exposed in center of the fold; along the east limb, the Bluff of the overthrust sheet is in contact with the Buda of the underly­ ing block. The large anticline and the smaller, superimposed folds as well as other small folds nearby reflect the general east-west alignment of the greatest principal compressive stresses the Laramide during orogeny. The western part of the overthrust sheet, which includes that part of the mountains west of the Indio fault, is char­ acterized by a maze of faults and folds, although the north- northwest trend of the structural elements is still dominant. Just north of Oxford Draw and west of the Borrega fault, beds of the Yucca have been tightly folded and displaced by minor thrust faults. Folds plunge north-northwest as well as south-southeast. About a mile and a half south of Oxford Draw and just west of the Borrega fault, there is a broad north-plunging anticline in the Yucca Formation. Due east of this fold and immediately west of the Indio the of the overthrust sheet fault, Yucca has been displaced by minor thrust faults, along which the overthrusting was to west-northwest. This movement the west or was accompanied by folding which produced 15°-20° centroclinal dips in the Yucca. The basin (5.7-14»3) has a maximum diameter of about 1,200 feet. The Bluff Formation in the outcrop west and northwest of the Indio ranch house has been intensely folded. Grossly, the structure consists of a north-northwest-trending syncline with Toward the the an axial fault. south, syncline is over­ turned to the southwest. A west-northwest transverse fault abruptly terminates the trend. A tight, north-northeast-plunging anticline in the Bluff Formation and a south-plunging anticline in the Cox Sandstone extend from beneath the overthrust sheet at the north end of Red Mountain. The tight anticline in the Cox is particularly well exposed high on the cliff on the south side of the draw that runs north of Red Mountain. A fault, perhaps a thrust fault, lies between the two folds; the trace of this fault is coincident with the Yucca-Bluff contact. To the northwest, despite the faults and small many folds in the Yucca, a series of broad anticlinal folds are offset more obscure transverse faults. The Yucca by along the west flank of the Indio Mountains north of Red Mountain, dips west or southwest® East of the point at which the Red Mountain fault ter­ minates against a southeast-trending fault, the east limb of a south-plunging asymmetrical anticline (Vos-15®4) dips at angles approaching 80°* The asymmetrical Lost Valley syncline just north of the Rio Grande is the largest fold of the western part of the overthrust sheet o Strata of the west limb commonly dip 50°­ 60°; those of the east limb, 2s°-30°. Normal faults have beds the axis of the displaced along syncline (section H-H T , of the there is pi. l). East syncline probably an equally large south-plunging anticline pl l)o o Lost Valley syncline is the northward extension of a large, doubly-plunging synclinal horst; the northern part of it overlooks the Lost Valley area and is referred to by local inhabitants as Sierra Bosque Bonito. Atwill (i960, pi. l) showed a marked difference in trend in the axis of the Lost Valley syncline and the axis of the large syncline of Sierra Bosque Benito. He also showed horizontal separation of the axes that approaches 4*ooo feet. Frantzen (1958, p 9 15) be­ lieved that the offset resulted from fault that a accompanied southeast tilting of that part of the syncline north of the the of the west limb of the river; strata syncline, however, continue across the river with very slight horizontal separa­ tion. Near the river, the strata of the east limb of the syncline are largely covered. I suggest that the axis of the syncline of Sierra Bosque Bonito is actually farther east than shown on previous maps, and that there is little or no offset of the axis north and south of the river, although there the axis may bend sharply. Along the west margin of the mountains just north of the Rio Grande, small but tight folds in the Yucca are related to which faults along overthrusting was northwest® A gentle down-warping along the southeast margin of the mountains is indicated by an east dip of about 15° in the volcanic rocks of Tertiary age® The gravel (QTg) just north of Campo Bonito (W.2-16.7) dips s°-6° SE O Bostwick (1953, p. 42-44) considered the northeast-and east-dipping strata of the block between the trace of the Indio fault and the trace of the Squaw fault to be the east limb of a large, northwest-trending anticline which he called the Indio Mountains flexure. Before faulting the strata west of the trace of the Indio fault were continuous with those to the Bostwick believed. He therefore assumed that the east, west limb of the flexure is hidden beneath the bolson fill west of the mountains. I suggest that the western of part the overthrust sheet may conceal the axis as well as the west limb of the Indio Mountains flexure; in other words, the west limb is in the underthrust block beneath the west half of the Indio Mountains, Normal Faults of Indio Mountains Indio fault .--The Indio fault runs northwest through the Indio Mountains! along most of its 13•5-mile trace it fault-line is marked by a southwest-facing, resequent, scarp. To the northwest, the trace is approximately parallel to the strike of the south of the Indio ranch house, however, beds; the fault cuts across the beds at an ever-greater angle. At the mouth of the trace of the fault is at Snake Canyon, an of 60° to the strike of the beds. The relatively angle straight trace indicates that the fault has a steep dip. In centroclinal fold Yucca the vicinity of the in the (5.7-14»3), the dip of the fault plane is about 75° SW. (Bostwick, 1953, p. 46) • It is not feasible to calculate the stratigraphic sep­ aration, because the stratigraphic position of the Yucca out- east and west of the fault are both uncertain. The crop structure sections, however, indicate as much as 7,000 feet of separation in places (section F-F f pi. 1); it varies , widely along the 13•5 mile course of the fault. The Indio fault displaces the volcanic rock of the Indio Mountains. Rocks of the Garren Group along the east margin of the mountains are separated by several high ridges from those in the Flat Top area in the center of the mountains. Presumably the volcanic rocks in the Flat Top area were lowered to their present position by movement along the Indio fault• I think it likely that Cottonwood Canyon owes its origin to the Indio fault. It is aligned properly to be a northward extension, and the south or southeast-dipping volcanic rock exposed in the walls of the canyon appear to be offset, west side down. Bramblett area the normal Ridge area.--Displacement along fault of Bramblett is about immediately northeast Ridge 1,100 feet. This dropped the southwest-dipping beds of sandstone of the Cox on the east against near-vertical limestone strata of the Bluff Formation on the west. Lost Valley syncline.syncline.--Steeply-dipping normal faults and near the axis of the Lost roughly parallel to Valley syn­ cline bound a small horst about 800 feet wide. Displacement the along the west fault is about 1,200 feet; along east fault, about 700 feet (H-H f pi. l). , Marginal faultsfaults.--There is little doubt that the west the mountains is bounded normal fault« A margin of by a relatively high scarp marks the straight line of contact be­ tween the high-standing Cretaceous rocks of the Indio Moun­ tains and the topographically lower bolson fill and terrace gravel. In some places, beds of the fill immediately adja­ cent to the margin of the mountains dip as much as 25° toward the mountains ; the dip indicates movement along the boundary fault since the fill was deposited. Along the east margin of the mountains there is no fault similar to that the west side. The Greta- scarp along ceous rocks and the Tertiary volcanic rocks seem to dip the terrace and bolson gradually beneath gravel fill along Green River. There is, in fact, one place where the out­ crop of Tertiary volcanic rock extends eastward across Green River into the Van Horn Mountains. the Green River Probably bolson (Twiss, 1959b, p. 127-128) is a half-graben, bounded on the east by a normal fault and on the west by a mono­ cline . The southern of the bolson part Eagle (Twiss, 1959b, p. 126) may also be a half-graben, for there is no evidence of the northeast of the Indio a boundary fault along margin Mountains. The half-graben to the south may change to a northward where graben southeast-trending Cretaceous strata of the Eagle Mountains appear to have been sharply truncated. Movement later than the deposition of the bolson fill is indicated by the varied dips in the clearly stratified fill that is well exposed east-northeast of the Bramblett ranch house. Normal faulting of Quaternary is attested age by offset of the Qg2 terrace gravel west of the Indio Moun­ tains. Near-vertical faults in the bolson fill (W. 5-17.0) southeast of Campo Bonito offset the strata several feet. Hills series of Washboard (T.3-11-3), a arcuate cor­ rugations, symmetrical in cross-section, trend a few degrees south of In the field that east* my impression was the area was once a ’’normal” Qg3 terrace gravel resting on bolson fill, and that the corrugations represent offset along a series of arcuate, parallel normal faultso Several north­ northeast-trending faults have also cut the rock in the Wash­ board Hills. It is possible that this anomalous feature is the result of creep of the fill and overlying gravel south- southwest along the "floor” of the graben. To the north­ east, a high relatively abrupt scarp in the bolson fill and the Qgl terrace gravel may be a breakaway scarp. BOLSONS One bolson and two breached bolsons (DeFord and Bridges, flank the of the 1959, p® 294) highlands map area. Eagle bolson borders the ranges on the north, northeast, and east. Red Light bolson along the west and southwest flanks of the and Green River bolson east of the southern Indio ranges Mountains have been breached by the Rio Grande and its trib­ utaries o Twiss (1959b, p. 126-128) named the Eagle and Green River bolsons. They are separated by a threshold of vol­ canic rock and are bounded on the east normal faults. by The eastward dip of the volcanic rock and of the over­ lying ancient gravel as well as the lack of a fault scarp along the east margin of the Indio Mountains suggest that the Green River bolson may be a half-graben, bounded on the west by a monocline, The same be true of the southern part of the Eagle may bolson, because most of the east flank of the northern Indio Mountains seems to lack normal faults® From Oxford major of rock Ridge north, however, ridges Cretaceous are sharply and of the bolson be bounded truncated; this part Eagle may on the west by a normal fault. Espy (1961) reported that the Carrizo Mountain Forma­ tion crops out in the valley southeast of Hot wells (north­ west of the Wyche ranch house and west of the Southern I searched for this but did not find it. Pacific). outcrop Its in Flat would indicate that a threshold presence Eagle of Precambrian basement rock separates Eagle bolson from a similar bolson to the northwest. Whether Eagle bolson is flanked by normal faults or whether it is a half-graben or simply a syncline is not known. Water wells in Eagle bolson along the Southern Pacific as deep thousand feet are bottomed in the bolson fill. as a In the northwestern part of Red Light bolson, water wells as as feet bottomed in the bolson fill. The are deep 650 outliers of Cretaceous rock west of the main part of Devil Ridge and east of the Quitman Mountains suggest that at least the extreme northwestern part of Red Light bolson may be synclinal or that it may be bounded on the northeast by a homocline. Farther south along the west margin of the Indio Moun­ tains, a high-angle west-dipping normal fault almost cer­ tainly bounds Red Light bolson on the east® Rocks of Creta­ ceous age that crop out just west of Red Light Draw about 2 miles north of the Rio Grande suggest that the west margin of the southern part of Red Light bolson may be a homocline. The bolsons and the do not present drainage basins exactly coincide; for example, Green River bolson includes all of Green Valley and the southeastern part of Eagle Flat® TECTONIC ANALYSIS Major overthrust movement in the Eagle Mountains and vicinity was east or northeast; fold axes as well as the normal faults trend north-northwest to northwest. major Many of the folds are asymmetrical to the east or northeast; a few to the west or southwest. The trend of structural features changes from north-northwest to northwest near the of the Mountains and Indio Mountains® junction Eagle In the Devil Ridge area, the direction of the greatest stress during the Laramide was northeast. principal orogeny The more easterly orientation of the greatest principal stress in the Indio Mountains was the result of local field in irregularities in the stress that, turn, reflected the influence of such diverse factors as the configuration of the basement, size and shape of the body of rock being deformed (which was controlled by the configuration of the Chihuahua trough and the adjacent Diablo platform), and in­ homogeneities of the rock being deformed. Based on estimated movement along the faults, total horizontal transport of strata long the Devil Ridge and Red Hills faults was about 35*000 feet, or 6.7 miles. This rather well with the estimate of Smith (1940, 630­ agrees p. 631) of 7.5 miles that was based on geometric reconstruc­ tions. Were it not for the strong possibility that there has been offset of the trace of the Devil Ridge thrust fault be­ tween the Devil Ridge area and the Eagle Spring area, the distance between these two exposures measured perpendicu­ larly to the strike of the fault would give an indication of minimum horizontal movement of strata. Minor, counter-thrust faults are an interesting feature of the structure of the Indio Mountains. Presumably the Bennett fault as well as the several unnamed faults in the Yucca east of Red Mountain, are faults along which movement was generally opposite to the principal eastward movement of the wonders this is not overriding block. Actually, one why more common (DeFord, prologue, this report). Do the thrust faults penetrate to the Precambrian? This question cannot be certainly answered with present data, but several comments be made* The Van Horn Sand- pertinent may stone and the Hazel Formation are not rigid, tectonically the other thrust the strong masses* On hand, no fault in map area brings rock older than Cretaceous to the surface* Within the region, there is much relatively incompetent rock in the section overlying the Precambrian basement rock® Noteworthy among these is the gypsum of uncertain age at the base of the exposed Cretaceous section near Cuchillo Parado and the that is of the Permian section of the gypsum part Malone Mountains (Albritton, 1938, p. 1754)® Dill (1961, p» at the south end of the Sierra de Ven­ 46) reported gypsum he concluded that the of the is either early tana; age gypsum Cretaceous or pre-Cretaceous. Gypsum may well exist in the subsurface in the Eagle Mountains and vicinity, and it or the shale in the Yucca may have served as a glide plane for a d/collement structure. Surely, most if not all of the thrust faults with seem­ ingly minor displacement do not reach the basement. The fault planes probably flatten rapidly downward and become bedding-plane faults at depth. Folds and thrust faults are commonly associated because they result from the orientation of the stress same field, that is, the greatest and median principal stress are in the horizontal plane. Fold axes are perpendicular to the direction of the greatest principal stress. Asymmetrical folds are common throughout the Eagle Moun­ tains and vicinity. Most, but not all, are asymmetrical to the east or northeast. DeSitter (1956, p. 239-246) has sug­ gested that asymmetry of folds arises mainly (l) from a thin­ ning of the folded strata along a basin margin and (2) from an original difference in elevation between the limbs of a fold. The thinning of the strata of the Chihuahua trough toward the Diablo platform is well documented. Because the radius of curvature of a fold is directly proportional to the thickness of the strata involved in the folding, the limb of the fold nearest the a shorter radius platform will have and thus steeper dip. Where original difference in ele­ a an vation exists between flanks of a fold, as when an area bor­ dering a trough is uplifted as folding commences, the steep limb of the resulting asymmetrical fold will be toward the trough. If, however, the strata of the trough were uplifted relative to the marginal area, the asymmetry of the folds would be toward the marginal area. Undoubtedly, such factors as these plus irregularities of the basement, pre-existing folds in older strata, competence and incompetence of strata, and others, interrelate in a complex way to control the direc­ tion and degree of asymmetry. the and Strike-slip faults should develop when maximum minimum principal stress directions are in the horizontal plane and the median principal stress direction is vertical (Anderson, 1951)• Thus, for a given orientation of the greatest principal stress, thrust faults should develop near the surface where the minimum principal stress is vertical, faults the and strike-slip should develop at depth where overburden becomes to minimum sufficiently great cause the principal stress direction to reorient to the horizontal plane. Significantly, some of the strike-slip faults in the and the Eagle Mountains vicinity were developed in over­ ridden block, on which the overburden might have been suffi­ ciently great to place the median principal stress in the vertical position. The strike-slip faults in the overriding block may have originated as tension joints during folding; with continued compression strata along these tension joints may have moved horizontally. Daugherty (1959* p* 29) recog­ nized tension joints in the Sierra Pilares o The roughly parallel alignment of normal fault trends with trends of earlier folds and thrust faults has been widely recognized in the region (Frantzen, 1958, p. 22-23; Vest, 1959* p. 39)0 This could result from a simple re­ orientation of the stress field relaxation of during compres­ sion wherein the direction of greatest principal stress would become the direction of minimum principal stress. In normal stress from the faulting, the greatest principal results weight of the overburden; the direction is thus vertical. The interpretation of the structural geology of the Indio Mountains that I have presented infers that the Devil thrust fault extends south-southeast of the area Ridge map and roughly parallel to the Rio Grande but with its trace hidden beneath the terrace gravel and alluvium along the river. It may, in fact, be the fault whose trace lies along the east flank of the Sierra de los Fresnos (Allen, This that the Sierra 1957, pi. l)• necessarily means Pilares as well as the Sierra de los Fresnos, have been thrust some distance to the east and that the thrust in faults mapped the Sierra Pilares are thrust faults in an overthrust sheet. to and Hill According Moody (1956, p. 1223-1224, fig® 12) the "Texas lineament" fault zone is basically a large- scale left-lateral strike-slip fault and the folds of the Malone the Mountains, Quitman Mountains, and Devil Ridge are second-order drag folds related to this fault. Their figure 12 showed a series of northwest-trending right-lateral strike slip faults across Devil Ridge which I do not recognize. The faults to the southeast in the Eagle Mountains along which there may have been strike-slip movement trend nearly east-west; movement along them would have been left-lateral, These could, according to the hypothesis of Moody and Hill (1956, p. 1213, fig. 5), be third-order left-lateral strike- slip faults. Geologists who have worked in the region in Hudspeth and Culberson counties, Texas, through which passes the large-scale left-lateral strike-slip fault of Moody and Hill, have not reported evidence to support large-scale lateral movement along this fault; nor do Moody and Hill® They cite of and the trace of the only the high angle dip straight fault as evidence for strike-slip movement; these are also characteristics of high angle normal or reverse (thrust) many faults. On small-scale mosaic air photographs, scale about Tobin Aerial east- an 1:72,000, prepared by Edgar Surveys, lineation extends more than 20 miles from southeast-trending the west flank of the Indio Mountains north of Oxford Draw the Summit the east of the Van Horn to Chispa area on flank Mountains* The significance of this lineation is not known, but it seems unlikely that it is merely an accidental align­ ment of surface features and shades of color on the air photographs* Presumably it could be a major zone of frac­ ture . GEOLOGIC HISTORY The relatively few outcrops of pre-Mesozoic rocks in the area record but a fraction of the geologic events map that occurred time. there during pre-Mesozoic If, however, the regional geology of Trans-Pecos is considered, it is to the and possible place Eagle Mountains vicinity in context with such well-documented pre-Mesozoic tectonic fea­ tures as the Texas craton, Diablo platform. Van Horn uplift. Marathon geosyncline, Ouachita foldbelt, and elements of the classic Permian basin such as the Delaware basin and Capitan reef« PRECAMBRIAN TIME The earliest geologic event recorded in rocks exposed in the map area as well as in Trans-Pecos Texas was the depo­ sition of the sediments of the Carrizo Mountain Formation, i.e., quartzo-feldspathic sandstone, shale, and limestone, in a geosyncline within the largely granitic and granodioritic Texas craton (Flawn, 1962, p 0 20, 22). Flawn believed that the over-all homogeneity of the metasedimentary rock of the Carrizo Mountain Formation indicates that it is a single sedi­ series. Because the area of the Carrizo Moun­ mentary outcrop tain Formation is relatively small and because subsurface the in the is the control on basement region scanty, shape, and areal extent of this geosyncline is unknown. orientation, 339 The were to and geosynclinal deposits subjected folding regional progressive metamorphism of low to medium grade; intrusion by rhyolite, then diorite followed (Flawn, 1962, tbl. 3)* The grade of the metamorphism increases from north­ west to southeast. In the Carrizo Mountains and Eagle Moun­ tains low grade metamorphism is indicated by the green-schist facies in which many sedimentary structures are preserved, whereas, in the Van Horn Mountains to the medium- southeast, grade metamorphism is indicated by the amphibolite facies in which most sedimentary structures have been obliterated. The sills of diorite that intruded the Carrizo Mountain Formation of the Eagle Mountains and elsewhere, were con­ verted to amphibolite by a second period of deformation. During this second orogeny the geosynclinal deposits of the Carrizo Mountain Formation were cataclastically and retro­ gressively metamorphosed, while they were overthrust north­ ward along the Streeruwitz fault into juxtaposition with the Allamoore Formation. The Allamoore Formation is a thick of sequence cherty Flawn limestone, volcanic rock, and talc phyllite which (1962, p. 33) preferred to consider as contemporaneous with the Carrizo Mountain Formation. Neither the base nor the of the is its top Allamoore exposed; relationship to the Carrizo Mountain Formation must be inferred. The Hazel, which unconformably overlies the Allamoore, is a subaqueous, perhaps intermentane red sandstone with synorogenic conglomerate that contains fragments of the Allamoore Formation as well as fragments of granite, the latter presumably derived from the craton (King, 1953, p. 127)* The Allamoore was metamorphosed along the zone of thrust and both the Allamoore and Hazel were faulting, severely deformed for a distance of several miles north of the fault zone (King and Flawn, 1953? P» 20) 0 The deformed and metamorphosed geosynclinal prism of rocks of Precambrian age has been designated the Van Horn mobile belt (Flawn, 1956, p* 32-36) Flawn later suggested • that f,deformed belt*’ or "orogen” might be a better name for this feature; DeFord (1958b, p, 60) suggested "tectonic belt*" the Flawn (1956, p. 33, 34, 68) originally considered to be than and marginal to the Texas geosyncline younger craton, but because the age of the metamorphosed rhyolite of the Carrizo Mountain Formation in the Carrizo Mountains and the of the pegmatite and middle-rank schist of the age formation in the Van Horn Mountains is about the same same the of the and in the as age granite pegmatite Llano area and the subsurface Texas craton (1,100 million Wasser years, burg, and others, 1961, p. 48), Flawn (1962, p. 37) later suggested that the Van Horn orogen was within and part of the Texas craton* PRE-PERMIAN PALEOZOIC TIME During the Cambrian Period, most of Texas, including Horn erosion. Rock of latest the Van uplift, was undergoing Cambrian is known in the Llano uplift as well as in the age El Paso and in the Van Horn area the of the area, age red, unfossiliferous, arkosic, cross-bedded, coarse-grained Van Horn Sandstone is uncertain. It seems to be a relatively local continental to deposit; King (1953* p. 95) preferred its designate age as "pre-Cambrian (?)." The Bliss Sandstone thin-bedded, quartzose (?) of Early Ordovician age, 115-120 feet thick and well exposed on Beach the of Mountain, records transgression an epicontinental sea. The Van Horn region was not a site of continuous deposition for El Lime- throughout the Ordovician, however, the Paso stone, a 1,115-foot sequence of thick-bedded dolomitic lime­ stone and dolomitic sandstone, lies disconformably on the Bliss (?) at Beach Mountain. The El in turn, is over- Paso, lain by the dolomitic and cherty Montoya Limestone of Late Ordovician ageo and Silurian, Devonian, Mississippian, Pennsylvanian rocks crop out in the Trans-Pecos region, and an even more nearly complete section is present in the subsurface. The map area and surrounding region were no doubt receiving sed­ imentation during much of the pre-Permian part of the Paleo­ zoic Era. A series of Pennsylvanian orogenies, the last of which in Trans-Pecos Texas extended into earliest Permian time, deformed the sediments of the Marathon geosyncline and up­ lifted and gently folded the Van Horn region to the north- erosion removed much of the earlier Paleo­ west* Subsequent zoic at least, on the northeast side of the rock; map area, erosion was sufficient to expose and probably partly remove rock of Precambrian age* The major structural elements that were created late in the Pennsylvanian Period and early in the Permian Period con­ trolled sedimentation in the region throughout the remainder of the Permian Period* Named from Van Horn the structural features eastward, inherited tectonism are the Diablo from Pennsylvanian-Permian platform, the Delaware basin, the Central Basin platform, and the Midland basin. These subordinate elements of the are greater Permian basin of west Texas and southeast New Mexico. The Mountains are on the southwest flank of the Van Eagle Horn uplift, a grossly domical high on the larger Diablo platform® PERMIAN TIME The early Permian sea that covered western Trans-Pecos Texas transgressed over an uneven erosional surface; the Powwow Conglomerate, a basal siliciclastic filled unit, on topographic lows this irregular pre-Permian surface® Recorded within the Powwow is the transition from a continental fluvial environment to an epineritic environment of deposition. The very finely crystalline fossiliferous limestone or micrite of the Hueco records the gradual advance of the neritic Wolfcamp sea onto the Diablo platform. Several thousand feet of rock of Leonard and Guadalupe deposited on the Hueco Limestone the age probably were in map area concomitant with the development of the classic Permian reefs to the north, east, and southeast, and, with the development of the accompanying basin, reef, and marginal shelf facies. During the Ochoa Epoch, the map area together with marginal shelf areas, rose above water as the Permian sea regressed toward Mexico. According to King (1942, p® 538): The final event of Permian time was the disappear­ ance waters West of evaporite-depositing from the Texas region, and the spreading over it of the last formation of the Ochoa series, a thin sheet of red beds. After this, deposition ceased in the area until later Triassic time • EARLY MESOZOIC TIME Erosion that followed the disappearance of the Permian sea from Trans-Pecos Texas eventually reduced the land to a low, irregular surface which R. T. Hill (1901, p. 363-36?) called the ,TWichita Paleoplain.The transgression of the Mexican sea over this vast, low-lying erosional surface be­ gan in Aptian time and continued uninterruptedly into the Early Cretaceous Epoch (Albritton, 1938, p. 1754, 1765; Huffington, 1943, P* 997)• CRETACEOUS PERIOD The Diablo platform and the differentially subsiding inundated Chihuahua trough to the southwest, which was by the Mexican the tectonic elements that controlled sea, were sedimentation in the map area during the Cretaceous Period. Comanche Epoch The Yucca Formation, a siliciclastic and calciclastic both marine and in­ nearshore deposit probably continental, cludes the oldest Cretaceous rock exposed in the map area. of the Mexican the of the During the advance sea, regolith pre-Cretaceous surface was incorporated in the heterogeneous material that composes the Yucca. Transgression of the sea was so rapid that there was not time for cleaning and sort­ ing the sediments to any marked degree. Continued transgression brought into the map area for the first time since the Permian Period, the neritic environ ment in which the limestone of the Bluff Formation was de­ posited. Conditions were somewhat unstable, however, as indicated by the sandy, oolitic, reef character of the lower the to the part of Bluff compared homogeneous, very finely crystalline Orbitolina-bearing limestone of the upper part of the formation. During a slow regression of the sea, marked perhaps by minor regressive and transgressive move­ ments, the map area was for a long time the locus of a shore or nearshore environment in which reworked material was brought in and further reworked and cleaned. These environ­ mental conditions accompanied by little or no tectonism were Trans-Pecos widespread throughout Texas (fig. 5), and the Cox Sandstone that resulted is characteristically a super- mature orthoquartzite. The Cox Sandstone is the youngest Cretaceous formation that rests on Paleozoic and older rocks; thus it marks the culmination of the of the transgression Mexican sea over the Diablo platform. most The Finlay Limestone represents, thus far, the ex­ tensive transgression (fig. 5) of the Mexican sea into Trans- Pecos Texas. This transgression brought into the map area a marginal neritic sea in which rudistid and caprinid reefs or banks developed. The increasingly brief interruptions of carbonate depo- Moun­ sition, represented by the thin Benevides and Eagle tains formations, as well as the homogeneity of the very limestone the and finely crystalline (micrite) of Espy Buda formations, reflect increasing tectonic stability during the Comanche Epoch. Once the Diablo platform was inundated, the Mexican sea Cretaceous invading Trans-Pecos Texas merged with the sea advancing onto the continental shelf to the east. Westward, however, there is no certainty that the Mex­ ican sea and Pacific Cretaceous sea ever merged. Brunson (1954, p. 60-61) stated s West of Sierra Blanca the sea during Cox-Finlay into Arizona the time extended through Mexican Geosyn­ cline. Ransome (1904, 70) collected fossils from the Cintura formation in the p. Bisbee, Arizona, area horizons. that are suggestive of Fredericksburg • .• Stanton (1909, p. 416) wrote, as did Anderson (1938, that there is little resemblance of the fauna p. 78), of the Shasta series of California and the fauna of the Comanche series of the Southwest. On the other hand, Imlay (1939, p« 425) believed in a possible interocean connection through the Sonoran portal dur­ ing this time. According to Stoyanow (1949, pp• 58­northwestern Mexico and southeastern Arizona 59), were reached several times by different Cretaceous seas. The Arizona faunas indicate times of Indo- Pacific communication and times of Tethyan-Atlantic communication. He found no interchange of fauna be­tween the west and the east at the same time. Gulf Epoch The lithologically varied rock that constitutes the Summit Formation Chispa was deposited in a changeable neritic sea as well as along the shore in stagnant There lagoons. may or may not have been a brief period of subaerial erosion between the Comanche and the Gulf epochs. In the map area, erosion has removed that part of the Gulf Series than Late but sedimentation younger Turonian, have continued there until even into Tertiary time. As may the sea retreated, well before the end of the Cretaceous Period, nearshore marine deposition gave way to fluvial and deltaic and this to deposition, continental deposition. LARAMIDE OROGENY The thrust faults and folds of the Eagle Mountains and of the intense deformation that vicinity record a part accom panied the Laramide orogeny. In the map area, the Chispa Summit Formation is the youngest formation now exposed that in deformation. was certainly involved the The time of culmination of the Laramide was orogeny later than the youngest Cretaceous rock and earlier than the oldest volcanic rock of the Rim Rock Country. This places it as post-Campanian (probably Maestrichtian or later) and pre-Oligocene; that is, between latest Cretaceous and latest Eocene. CENOZOIC ERA Although the early part of the Cenozoic Era was a time of intense structural deformation, erosion was probably suf­ ficient to differences in surface elevations of prevent more than a thousand feet. By the time the first volcanic rocks were laid down in the the maximum differences in map area, elevation of the surface were about 500 feet. The irregular distribution of the volcanic-rock units of the Eagle Moun­ tains no doubt partly reflects the uneven erosion surface over which the lava flowed# The north-northwest to northwest trend of Laramide structural features that is so evident today, must have con­ trolled the topography of the erosional surface on which the volcanic rock was deposited.. One can envision a pre­ vulcanism surface dominated by a series of strike ridges and valleys* If this were the orientation and shape of at so, least the earliest lava flows were no doubt strongly influ­ enced the there by prevailing topographic grain. Although linear of flows are no outcrops ancient lava aligned roughly perpendicularly to the direction of Laramide compression in the there in the Indio Mountains Eagle Mountains, are and in the Rim Rock country. The linear shape and general north- northwest to northwest orientation was emphasized by the later block faulting, most of which was parallel to the earlier Laramide structural trend, but the basic control of the earliest flows was exerted by the ridge and valley topog raphy of the pre-vulcanism surface. Vulcanism Widespread vulcanism in the the region during Oligocene Epoch, probably centered in the Davis Mountains area, spread a blanket of extrusive igneous rock over much of Trans-Pecos Texas. Few vents through which the volcanic rocks reached the surface have been identified, perhaps because these rocks still cover large areas. The volcanic outbursts that spread tuff, welded tuff, and basalt over the Indio Mountains area were trachyte, cyclic. Because no vents are evident there and because the igneous-rock section so closely resembles that of the Van Horn the of the volcanic material was source Mountains, probably to the east. Moreover, the Pantera Trachyte thins westward. The thick section of volcanic breccia and flow breccia in the Eagle Mountains must have had local source, although a no vents have been positively identified. Intrusion During the eruption of the material that now forms the lower rhyolite of the Eagle Mountains, the country rock was intruded by numerous sills and dikes of the rhyolite. Some time later, after the emplacement of the trachyte porphyry and the upper rhyolite and perhaps additional rock that has since been the rocks of the Mountains were eroded, Eagle intruded the by Eagle Peak Syenite. The relative ages of the intrusion of the Eagle Peak the diabase and the (Trd) that Syenite, (Tdd), rhyolite gen erally intruded along east-west faults are uncertain, as is the relationship of the intrusion of those rocks to the time of normal faulting# Although in this discussion the intru­ sive rocks are considered first, much of the normal faulting may have preceded the intrusion of the igneous rock. Cer­ tainly, it seems that the rhyolite and diabase intrusive bodies in the Eagle Spring area, for example, intruded along the fractures. The same well be true of all pre-existing may the dikes as well as of the stock of Eagle Peak Syenite. Whether these fractures, joints or faults, were formed during the episode of Tertiary block-faulting, is not known. It is entirely possible that the diabase dikes and the late rhyo­ lite dikes antedated the emplacement of the Eagle Peak Syenite. Folding In a few places in the region, folds in volcanic rocks indicate that there late was or post-vulcanism compressive tectonism. Perhaps the strongest evidence for this late period of folding is the Colquitt syncline in the northern part of the Rim Rock country. In this fold, rock of Oligo­ cene age is folded concordantly with the Cox and Finlay for­ mations of Cretaceous age (Twiss, 1959b, p. 149). In the map area, perhaps the strongest evidence for post-vulcanism compression is the change in attitude of the lower part of the upper rhyolite in Cottonwood Canyon. This, several in the volcanic rock of the and gentler warps map area may, however, have resulted from subsidence that accom­ panied the extrusion of vast quantities of igneous rock. Block Faulting During the Laramide orogeny western Trans-Pecos Texas and northern Chihuahua were near sea level, but about the middle of the Oligocene Epoch, the region was uplifted several thousands of feet and block-faulted (DeFord and Bridges, 1959, p. 292, 293). In the northern Rim Rock a basalt flow lies country within the lower part of the Tarantula Gravel, the gravel that resulted from erosion of the high scarp created by the Rim Rock normal fault. The Rim Rock fault either postdated with the of be- or was contemporaneous late episode folding, cause the Tarantula Gravel covers part of the Colquitt syn­ cline The Rim Rock (DeFord and Bridges, 1959, p» 286-293)® fault, a major structural feature of the Rim Rock country, is earlier than the late igneous activity represented by the basalt interbedded within the Tarantula; the fault is also earlier than the late compressive episode during which the volcanic rock and Cretaceous rock of the Colquitt syn­ cline were folded. Some of the intrusive rocks of the Eagle Mountains and faults date vicinity may have penetrated along normal that but their could have from mid-Tertiary time, emplacement been earlier. As in the northern Rim Rock the differences in country, elevation created the to fault- by mid-late-Tertiary block ing in the Eagle Mountains and vicinity led to the formation of an old gravel that is probably correlative with the Taran­ tula Gravel. In the map area, the old gravel (QTg) was in the Indio where recognized only Mountains, it probably resulted from erosion of the created the Indio scarp by fault. One wonders why the old gravel is not seen elsewhere in the area. There are three alternatives: it may be sub­ merged beneath the fill, it may have been eroded, or it may never have been deposited. Although the normal faults have several thousand feet movement was of separation, along the faults probably inter­ mittent and slow enough to allow erosion to maintain differ­ ences in elevation of several hundred to a thousand feet. The faults that have displaced the bolson fill and later terrace show that normal gravel the faulting was long-lived; it may still be active. From the time of the earliest the biock-faulting, topo­ to be graphic depressions or grabens began filled by debris eroded from the adjacent highlands. Within the closed basins such as Grayton and Eagle Flat and Salt Basin this process continues today; within the basins, material eroded from open the highlands is in halting migration to the Gulf of Mexico, Evolution of Drainage the Laramide the had Following orogeny region probably a trellis drainage pattern with a prominent northwest trend parallel to the fold axes as well as to the hogbacks and ridges created by thrust faults. With the creation of the intermontane lowlands by block-faulting, drainage became closed, and as the fill inundated higher and higher parts of the mountains the drainage developed a radial pattern. When the intermontane lowlands were breached and an open drainage system again established, downcutting began which has continued to the present day. As the streams cut down­ ward, they were able to maintain their courses and so super­ posed the old drainage on the harder Cretaceous rocks as they were exhumed. This, then, resulted in much of the drainage in the mountains, especially that tributary to Red Light Draw, trending at a high angle to the strike of the beds. The drainage in Spar Valley appears to be partly super- or posed and partly consequent subsequent; it is superposed where it cuts through the Cox to flow out into Eagle Flat and to consequent or subsequent where it flows southeast parallel the high ridge to the north. In the Carpenter Canyon area, the drainage of Carpenter Creek and other roughly parallel draws, is superposed, but the drainage of the large draw along the southwest flank of Espy Ridge is consequent or sub- it cuts the to flow sequent, except where across ridge out into Eagle Flat* It is superposed at that point. Two parts of the map area are in closed drainage sys­ tems; (l) that part that drains into Grayton Lake and (2) that into Flat that part drains Eagle Flat, although Eagle drainage has been captured by the Salt Flat closed-drainage system through the water gap that separates the Carrizo Moun­ tains from the Van Horn Mountains. Headward erosion of Green River will eventually capture Eagle Flat drainage and may capture Salt Flat drainage as well. Active downcutting is in progress both north and south of the divide at the head of Green River well drainage as as in Eagle Flat southeast of Hot Wells and northwest of the house. Wyche ranch The downstream change in course of Green River from south to west is interesting; the acute angle that it makes with the Rio Grande points abnormally upstream instead of downstream. That part of the channel that makes the swing to the west have been inherited from the time when the may basin was closed and flow in that of the channel part was east instead of west. The anomalous course of the channel could stem from localized normal faulting that forced the channel to change its course to circumvent a relatively up­ raised block. Also, it is possible that the Green River channel was originally created at a time when the ancestral Green River flowed westward through the mountains into Red Light bolson. In the Devil Ridge area, many of the tributaries paral­ lel the strike of the beds, and on a small scale this must resemble the drainage pattern of much of Trans-Pecos Texas and after the Laramide No doubt the vast during orogeny. outpouring of volcanic material during the Oligocene Epoch altered the of much of the as the area drastically drainage flows and ash falls blocked or completely filled valleys. The water gap in Back Ridge and that in Speck Ridge a probably represent erosion by superposed stream. On the other hand, headward erosion of Red Hills Arroyo and Goat of the Arroyo has captured some drainage of Eagle Flat These and other tributaries of (Smith, 1939, p. 131-134)® Red Light Draw will probably capture much more. Terrace Development Following the breaching of Red Light bolson and the establishment of an open drainage system, removal of the bolson fill and exhuming of the adjacent highlands began. This probably anti-dated slightly the beginning of downcut­ the Hueco bolson dated ting of by Strain (1959, p. 377) as between late Kansan and medial Illinoian time. Since that time, adjustment, or lack of adjustment, between such diverse factors as climate, tectonic activity, and hardness of the have resulted in of the bolson The rock, terracing fill. gravel-capped terraces represent higher, former levels of stream flow. The series of three principal terraces and several intermediate ones that are present from place to place, alternation of and represent an relatively stable unstable conditions of erosion and deposition. The smooth, gravel capped, upper surfaces of the terraces were developed when the controlling factors were in such nice adjustment as to allow the creation of a broad surface on which aggradation and degradation were approximately in balance. A change in one or more of the controlling factors caused the streams to seek a graded condition at a lower level and thus create terrace. a gravel-capped Alternation of arid and less arid climate is widely accepted as the dominant factor in terrace development in the southwest, but opinion has been divided as to whether, for example, the downcutting coincides with arid or less arid climate. It be that the climate in all may fairly said present but the highest parts of Trans-Pecos Texas is arid. Further, observation of a flash flood, or of the after-effects of one, in the mountains during or following a thunderstorm is that is The convincing proof downcutting occurring today. same is true of arroyos in the valley flats, even in the closed basins where erosion has become so severe drainage in recent that ranchers invest thousands of dollars years in diversion dams which are thrown across the in an arroyos effort to stem the removal of the soil and basin fill. I am convinced, therefore, that downcutting may accom­ pany an arid climate; I can only infer that a less arid climate would more stable conditions under which promote would not channel trenching occur. has Schulenberg (1957, p. 70-73) succinctly summarized the conditions that obtain during the arid and the less arid parts of the cycle. I paraphrase and supplement his remarks in the following paragraphs. During the arid part of the cycle, annual rainfall is slight but individual rainstorms are torrential; stream flow, is intermittent but violent. The Rio Grande therefore, maintains essentially continuous flow but with short periods of flash flooding. Vegetation is sparse, even on the moun­ tain slopes, because of thin or absent soil and low rainfall. The flash floods and lack of enhance vegetation erosion, but the coarse material removed from the mountains is depos­ ited near the base of the mountains where the gradient of the mate- tributary stream suddenly decreases. The larger coarse rial is deposited in the drainage channels; the smaller coarse material is deposited on the broad alluvial fans. Much of the silt and fine sand comes to rest on the down­ stream of the tributaries and on the Rio floodplains larger Grande floodplain. Because the Rio Grande receives only- fine material, it has sufficient energy, especially in time of flash flooding, to incise its channel. This downcutting, in turn, drops the local base level of the tributaries, and erode downward in an attempt to maintain a graded pro- they file. the less arid part of the annual rainfall During cycle, is showers less greater, individual are torrential, and some of the tributaries to the Rio Grande are perennial, thus in- volume. soil creasing its Greater rainfall enhances develop ment, and this together with the increased moisture results in a denser cover of vegetation on mountain slopes. Although the size and quantity of the material eroded from the mountains is less than that during the more arid part of the cycle, more of the material reaches the Rio Grande because of increased competency of the tributary streams. The greater load imposed on the river decreases its erosive capacity, and because it ceases to erode its channel, a temporary base level is established for its trib­ in their utaries. They, turn, expend energy in lateral ero­ sion and The tributaries planation. develop a widespread and complex anastomose drainage pattern as they plane off a surface which broad, gently sloping on they spread a gravel veneer* The areal extent as well as the thickness of the terrace gravel are determined in part by the duration of this less arid part of the cycle. The difference in elevation between the terrace gravels is determined by the length of the arid part of the cycle. The silt and fine sand that now cover the lowest flood- will be less arid plains replaced by gravel during the part of the when load is carried the Rio cycle a greater by Grande. The fine material is thus a forerunner of a gravel-capped terrace that will eventually be created at this level as ter­ race development progresses. ECONOMIC GEOLOGY WATER Surface Water Surface the Mountains and drainage throughout Eagle vicinity is intermittent, lasting only for short periods following infrequent rains. During these periods surface water drains into four basins: Green Valley, Eagle Flat, Grayton Lake, and Red Light Valley. Eagle Flat drainage has been captured by Salt Flat through the water gap near Scott which Crossing. Grayton Lake (C, D-6,7) is a playa receives water from a small part of the map area plus a much larger more than 100 north and east of Sierra area, square miles, Blanca. Green River and Red Light Draw are the principal drainages in their respective valleys and are the largest Rio Grande tributaries in the area. map During relatively wet years, water will stand for some weeks or even months in Grayton Lake and in a low area just west of Scott Crossing and southeast of the Wyche ranch house. Following exceptionally heavy downpours water simply backs over the low southeast of the house be- area up Wyche cause of the effective bottleneck created by the narrow drainage channel between the Carrizo Mountains and the Van Horn Mountains. The Rio Grande is the principal drainage in the area. 361 Although its flow is intermittent, scattered holes of water persist in its channel throughout most of the year. In time of heavy flood, flow in the salt cedar-choked channel is so restricted that channel overflow and flooding of surrounding lowland is common. Rain falls in the region customarily in sudden down- which in but pours, result short-lived volumetrically large surface runoff. To retain much of this surface water as as possible, many U-shaped earthen stock tanks have been built across drainage channels in the low areas. In some narrow in the rock and concrete dams have canyons mountains, also been constructed to retain surface runoff. Stock tanks and dams lose their effectiveness when sed­ iment fills in behind them. Whereas stock tanks are easily cleaned by bulldozer or dragline, the dams across the narrow canyons in the mountains are usually so inaccessible that cleaning must be done by hand. Stock tanks, on the other hand, are especially subject to breaching unless proper overflow spillways are constructed. Water in the Rio Grande is withdrawn for irrigation on the Bramblett and Guerra farms, but, understandably, the Rio Grande is not a dependable source of water. earthen Low, spreader dams, thrown across incipient gullies to retard erosion, are particularly numerous in the the Southern low, sparsely vegetated area just south of Pacific. Ground Water The economy of the region depends upon an adequate supply of ground water. There are no artesian wells, and are few and characterized low flow rates. Ground springs by water is lifted to the surface through wells equipped with windmills• AquifersAquifers.--The most productive aquifer is the gravel, sand, and silt of Tertiary and Quaternary age that fill the intermontane basins and floor the in the mountains. canyons Ground water is also produced, however, from sandstone and limestone of Cretaceous and from volcanic rock of Ter­ age tiary age. Wells producing from the intermontane basin fill, however, are characteristically stronger than wells produc­ from other ing aquifers. Wells65 produce water the map Wells.--Some wells ground in area; the majority are equipped with windmills, but a few are pumped by butane-powered engines. The recent advent of rural electrification into the region will result in a number of wells being pumped electrically. The Southern Pacific Company drilled wells along its right of way many years ago. A well at Torbert, 5 miles northwest of Hot Wells, was reported by Baker (l93sc, p, 375) to have a water level 723 feet beneath the surface an or elevation of 3,620 feet above sea level, The wells at Hot Wells are better known; they are 1,000 feet deep and produce water at 110° F. Water level in the early 1930 T s was approx­ imately 640 feet beneath the surface or at an elevation of T 3,610 feet above sea level. During the 1920 s, these thermal wells supported a small resort-type hotel featuring hot baths (Espy, 1957). Presumably the water is heated by close or proximity to still-hot igneous rocks by gases originating therefrom. The Southern Pacific removed the last of the pumping equipment from the Hot Wells site in 1959. A log of one of the wells shows it to be bottomed in basin fill at a depth of 1,000 feet. The two former city wells at Sierra also Blanca, 1,000 feet deep, are reported to be bottomed in basin fill. These wells are located on the Melbarth ranch immediately south­ east of Sierra Blanca. are and They electrically pumped their yield, measured in the early 1940 T was 35-40 gallons s, minute. per One other well should be mentioned; in Little Hill on the Espy ranch. Deep well was drilled for water in the early 1920 f s to a depth of 1,000 feet. The standard drill­ ing rig was left over the well and used to it and to pump work it over; power was supplied by a gasoline engine. The well was last used in the 1930 f but the still stands. s, rig Springs .--Springs in the area all have low flow rates or are dry. Eagle Spring (H.7-11*8) is the best known, be- from about to 1882 it was the site of an overland cause 1854 stage stand. The spring is located in a structurally complex zone, intruded by rhyolite, just south of the Eagle Spring and ranch house. A nearby spring, small unnamed, is located near the steeply dipping beds of Bluff limestone just up­ stream from the old coal shaft in Coal Mine Arroyo (Espy, I960). is miles north- Squaw Spring (5.3-15*0) approximately 1.5 northeast of the Indio ranch headquarters and about 2 miles southeast of Squaw Peak. There are reports of hundreds of head of cattle watering at Squaw Spring in the space of a few hours during drives across the mountains, but the spring is small and weak. It flows from the base of the now near very east-dipping limestone of the Bluff Formation. Mesquite Spring (X.3-17*8) is on the east bank of Green River approximately mile and half north of the point at aa which Green River swings sharply west to enter the Rio Grande. The water issues from basin fill. Two small springs (X.4-14*5) near the mouth of Box Can­ yon on the Bramblett ranch just a few feet north of the Rio Grande channel yield water from recent alluvium. Mayfield Spring is located just above the point where Arroyo Escuido crosses the western flank of the Indio Moun­ tains (Bramblett, I960); it produces from the Yucca Formation. I did not visit this spring, and its exact location is un­ known to me. It is not shown on plate 1. Oxford Springs (Q.3-13*9) are also well known; they are east of the nearby ranch house and just north of Squaw just Peak along an east-west access route through the mountains. well The springs are weak, although a nearby that probably taps the same source is reasonably strong and produces good water. Cottonwood Spring (M.B-12.7) is located at the head of Cottonwood Canyon, some distance up from the canyon floor. from volcanic rock. The water drains downward It produces volcanic rock and into a small se­ through underlying pool cluded in a niche in a tree-shaded rocky outcrop at the level of the main channel in the canyon. Cypress Spring (L-13) is in the bed of Cypress Creek a short distance west of the Marine ranch house in the Eagle Mountains. Volcanic rock also supplies water to this rela­ tively small spring, which is now protected by a concrete enclosure. Indian Springs (J.6-8.8) is the victim of cleaning attempts; it is now completely filled in and dry. It is within a few feet of Indian Spring well on the Speck ranch northwest of Black Butte; this well produces excellent just from the most the water Espy Limestone, likely from same source that once fed the spring. Carpenter Spring (K.3-12.7), near the head of Carpenter Creek, is about two and a half miles south-southeast of Eagle Spring. Water seemingly issues from the Chispa Summit Forma­ tion. There is a small windmill at the spring; a nearby, large stock tank has been created by damming Carpenter Creek. Panther Spring (J.5-ll«4) produces from the lower rhyo­ lite along the northeast flank of Panther Peak. Qualitythe is potable with a Quality.--Water throughout area few exceptions. It is all satisfactory for stock and for irrigation. Water analyses are available for city well no. Sierra and for of the wells at Hot Wells 2, Blanca, one (tbl. 28) . Recharge and movement only source movement.— Precipitation is the of recharge, and average rainfall recorded at the nearest weather station averaged only 8.2 inches over the 10-year period, 1949-1958. Because the evaporative rate and per­ centage of run-off characteristic of the area are high, most of the precipitation is unavailable for recharge. The movement of ground water is generally from the higher areas toward the intermontane basins. Water pene­ trating the topographically high volcanic rocks and intru­ sive igneous rocks of the Eagle Mountains percolates down­ ward rapidly through the abundant fractures and joints. Restricted small reservoirs exist, however, in the frac may tured volcanic rock. Table 28.--Water Analysis, PPM Silica (Si0)2 Iron (Fe) Calcium (Ca) Magnesium (Mg) Sodium (Na) Potassium (K) Bicarbonate (HCO^ ) Sulfate (SO. )4 Chloride (Cl) Fluoride (F) Nitrate (NO^) Dissolved solids Total hardness as CaCO^ pH Not determined Broadhurst, Sundstrom, and Baker (1935c, p. 371) Sierra Hot Wells 20 18.86 1,,1 -•— 68 7.38 19 2.40 496 162.15 22 * 340 244.6 3 73 128.6 468 30.88 5 -.3 16 4.63 1,655 599.8 /\ 248 \f .8 w /\ 7­ Weaver (1949, p. 152) Movement of subsurface water in the sedimentary rock of the mountain masses is principally controlled by structure and to lesser extent by lithologic variation. Movement is no doubt both aided and abetted by the many faults. Ground water movement in Green Valley should be both northward and southward from the threshold of volcanic away rock that crops out just south of Double Wells ranch house and separates bolson from Green River bolson. Eagle some In Red Light Valley, barring unknown subsurface obstruction, water in the subsurface should move southeast­ ward toward the center and presumably the lowest part of the basin. development.--In the highlands Recommendations for development the sand and that and draws are gravel partly fill the canyons the best source of water. Water wells drilled high on the flanks of the find little Eagles, however, will probably water in the highly fractured volcanic rock, especially if rainfall has been light. There is much water in the pore spaces of the gravel, sand, and silt of the bolson fill in the intermontane areas. Although water wells in the bolson fill must go deeper than those in the the wells in the have highlands, fill a corre­ volume rate of flow because of the spondingly larger larger volume of the reservoir. The maintaining of accurate well records and well performance records would be a first step in the systematic resources area. development of the ground-water of the map SOIL It has been said that soil ,fis the great bridge between and the it is if the inanimate living"; surely an important, not the most important, natural resource of any region. There two with that are soil reports accompanying maps include the Eagle Mountains and vicinity. Carter and others (1934) made a reconnaissance soil survey of Trans-Pecos Texas and 10 soil in the Eagle Mountains and vicin­ mapped types ity, the most common of which were silty clay loam, gravelly loam, very gravelly loam, and rough stony land. In 1950 the district supervisors of the High Point Conservation District, which includes the map area, published a District Program and Work Plan. In this informative booklet are three very district-wide showing (l) soil (2) sites, maps groups, range and (3) range condition. The following brief discussion of soils in the Eagle Mountains and vicinity is based on the booklet (p. 26-60). Soils of the area are largely immature and thus closely related to the bedrock from which they are derived. They are typical of an arid climate inasmuch as they are thin, cal­ careous, and characteristically underlain by caliche. Wide­ spread bare rock and rough stony land are intermingled with immature residual soils containing abundant rock fragments. rainfall in the mature Despite relatively heavy mountains, soils occur there only in small, scattered patches. The at the base of the mountains are characterized by slopes more extensive but still relatively thin, calcareous soils. Still lower on the upper reaches of the valleys and flats are gravelly fine sands and loams. Alluvial soils, fine sandy loams, silty clay loams, and fine wind-blown sand occur farther down the valleys. The floors of the inter­ montane basins have and alkaline deep, usually gypsiferous soils. The highlands of the Eagle Mountains and Indio Moun­ tains are largely rough stony and broken land with non- calcareous soil material. Green River Valley and the south- half of Red have ern Light Valley shallow, medium-textured, gravelly soils. Grayton Lake area and the easternmost por­ tion of Eagle Flat have deep, medium-textured, permeable soils. There are shallow, medium-textured, gravelly perme­ able soils in the northern half of Red Light Valley and near the base of the mountains in Eagle Flat. Devil Ridge is characterized by rough stony and broken land and calcareous soil. The low flats in the north and northwest part of the covered area are by deep, fine-textured, permeable soils. Range condition is determined by the percent of the Ex- original, or climax, vegetation which is still present. cellent condition is achieved soil range by good fertility, high rate of water intake, high water holding capacity, low and The four rate of erosion, large forage production. range condition categories are: - to of Excellent 75 100 percent original plant cover present. Good -50 to 75 percent of original plant cover present . - Fair 25 to 50 percent of original plant cover present. - Poor 0 to 25 percent of original plant cover present. The soils of the Eagle Mountains and vicinity generally lack the desirable qualities listed and are classified "Poor” with few scattered rated as "Fair." a areas largely as Excessive runoff is enhanced by lack of soil cover and sparse vegetation. Vegetation has been reduced by drought and and the result is the of overgrazing, incising steep- walled arroyos into formerly level alluvial plains. This severe downcutting is widespread and has scarred the low­ lands throughout the area. Cultivation is minor in the Eagle Mountains and vicinity but now-barren areas may be brought under cultivation in the future, as in Lobo Flat and Wildhorse Flat to the east. made the comments the Hagelstein (i960) following concerning areas possibility of irrigating such as Eagle Flat; it would depend upon the depth of the water .. .. .. because . with pumping costs as high as they are (butane) it would be necessary to have fairly shallow .. water. As it stands there are few cash that are profitable because of this situation. Cotton is the and there are no allotments in that now, very crops new crop, county. As far as the soil is concerned, it would be suitable water could be found at a . . . _lf shallow enough depth to be economically feasible to pro duce crops and Tf natural gas could be obtained, and Lf at that time a market could be established for crops other than the area would be suitable for irri­ cotton, gation. These are a lot of Ifs, but they could be worked out in the future if our population con­ ... tinues to increase . • areas such as that will be . developed. are being in Wildhorse and Lobo Splendid crops grown areas of Culberson County, but not without careful prepara­ tion of the alkaline desert soil. Vogel (1961) described the necessary soil treatment and preparation as follows: Normally the first step in converting desert soil to farm land in this area is an operation called root-plowing. This is done with a f,catTt pulling a blade from 14 TT to 20 ,T below the surface, thus slicing the roots of the bushes. These bushes are then piled and burned. A land-plane is then used to smooth out the bumps in the land. The rows are then laid off with the desired fall for proper water penetration. The two secrets to growing crops instead of bushes are water and fertilizer. Normally, three or four acre-feet of water together with 100 to 250 pounds of nitro­gen and 40 to 100 pounds of phosphate per are acre re­quired for a cotton crop in this area. Other crops in their fertilizer vary water and requirements. Of other course, things enter in, such as cultivation, insecticide applications, etc. LEAD, ZINC, COPPER, AND SILVER Mountains and been searched for The Eagle vicinity have metallic minerals since white men first settled in the region. Little is known of these early prospectors, but evidence of their activities meets the on hand. eye every Mineralization is widespread, but commercial deposits are few. Despite much exploration, much of it individual and has been but of haphazard, there little production metallic minerals. It must be admitted, however, that only since the mid-1940 T s has the geology of the region been known in any detail. Based on the accompanying map, plate 1, admittedly regional, and larger-scale maps of parts of the area, such as those in Gillerman’s (1953) report, a well planned and di­ rected exploration program might uncover worthwhile prospects. In the Indio Mountains most of the prospecting and min- have been in the of the Indio ranch ing vicinity headquarters. lead mine Carpenter Exploration Company’s Purple Sage (U.4“ 14.3) was the most notable effort. The prospect never became more than a prospect, because lead percentage decreased instead drastically with depth of increasing as expected. The operation extended over the period 1948-1952 no ore was ever shipped. The Black Diamond mine unsuccess­ (T.6-15*3) explored, lead and mineralization in the of fully, copper conglomerate the Yucca Formation about a mile southeast of the Indio ranch headquarters. Numerous pits and shafts in the vicinity occur prospect of the Black Diamond mine. Most of them explored copper or manganese mineralization in the conglomeratic beds of the Yucca Formation. lists these and Allday (1953, pi. IX) prospects pits shafts and keys them to locations indicated on plate 1 of this report. Gillerman described the (1953, P« 52-53) briefly pros­ pecting for and mining of metallic minerals in the Eagle Moun tains. The following description of those prospects and activities is based largely on Gillerman’s report. Two adits (K.7-14*4) have been driven into hill com­ a of the Precambrian Carrizo Mountain Formation. posed They explored veins of copper, lead, zinc, and silver minerals just south of the large east-west Rhyolite fault. No posi­ tive record of production or shipment of ore is available. A 2-foot shaft (M.3-14*3) was sunk in the northeastern part of sec. 4&, block 68, T. 9, near the mouth of Spar Val­ ley, to exploit copper mineralization along a small fault. Approximately 5 tons of copper ore were shipped from this s. prospect in the early 1920 f Lead, zinc, copper, and silver minerals occur at the Black Hills deposit (M-9) (Dick Love mine) northwest of the mouth of Snowline Canyon on the southwest side of the Eagle Mountains. Underground workings at the mine are said to be two shafts and an adit are visible at the sur­ extensive; Henderson recorded face. (1925, p. 215; 1927, p. 609) small of ore from the in 1922 and 1923. shipments deposit silver The Silver Eagle lead and deposit (M-9), just southeast of the Black Hill deposit, was discovered in 1940. The U. S. Bureau of Mines trenched, drilled, and sampled the deposit for zinc (Dennis, 1946b) during 1943-1944® Two shafts were sunk along a vein, but no production from the deposit was recorded. A filled shaft of Silver largely (N.b-10.3) east the Eagle deposit was sunk on a vein in a diabase dike. Minerals identified in the dump galena, are sphalerite, hemimorphite, chalcopyrite, calcite, and quartz. FLUORSPAR Fluorspar occurs widely in the Eagle Mountains as re- and fissure Evans placement deposits. (1946) report on these fluorspar deposits resulted in the later work of Gillerman (1953) 'who investigated the deposits for the U. S« Gillerman described in Geological Survey. (1953, p. 53-92) detail the of the following aspects Eagle Mountain fluorspar district: and history ownership, origin and paragenesis, structural and stratigraphic control of ore bodies, and re- His the serves. maps show workings and deposits as they T existed in the mid-1940 s. He also made recommendations concerning future exploration for fluorspar in the district. The summary of fluorspar in the Eagle Mountains is following based largely on his report. Although extensive development of fluorspar began there in had been known since 1919* Fluorspar 1942, its presence in Texas in the Mountains in was first produced Eagle 1924 ore (Davis, 1927, p. 68) when 46 tons of were produced but not immediately shipped. Mining activity during the pro­ ductive years, 1942-1950 inclusive, was centered in the Spar Valley area; during this period slightly more than 15,000 tons short were produced. Gillerman investigated numerous other deposits in the one general vicinity of Spar Valley, such as the near Eagle Spring. The Rocky Ridge deposit (M-9), some 3*5 miles south west of the mill site in Spar Valley, seemed most promising. This deposit was discovered in 1943, but it was not until 1952 that an access road was built to it. Some exploratory development followed, but no fluorspar has been produced. A small flotation mill, with an original capacity of 50 tons per day increased in 1948 to 80 tons per day, was completed in January 1945, and was used through 1950 in processing ore from the Spar Valley deposit. The mill has since been dismantled and shipped elsewhere. Calcite, ankerite, quartz, and small amounts of pyrite. hematite, and limonite are associated with the fluorspar. and both Appreciable quantities of sulphide minerals barite, commonly associated with western fluorspar deposits, are absent. The fissure occur deposits chiefly along east-trending and northeast-trending faults in rhyolite whereas the replace ment are in limestone and limestone. Fault deposits sandy gouge and breccia have also been mineralized by replacement. Gillerman estimated reserves in the (1953, p# 90-91) Spar Valley area at 50,000 tons of measured, indicated, and inferred ore, and those at Rocky Ridge at 37,000 tons. Other smaller were estimated to tons. He deposits total 20,500 considered a total of more than 100,000 tons of fluorspar ore containing a minimum of 30 percent to be a conservative estimate of the reserves in the Mountain Eagle fluorspar district. Because all fluorspar deposits, with the exception of that near Eagle Spring, are near the Eagle Peak Syenite stock, Gillerman recommended the area within a two-mile radius of the stock for future prospecting. He suggested that east-trending and northeast-trending faults and associ­ ated subparallel faults within this zone should be carefully examined as well as limestone beds similar to those in Spar Valley that have been replaced by fluorspar. BARITE the of Evans (1946, p. 109) reported occurrence barite in the Eagle Mountains: Barite in thin veins locally containing lead and copper minerals occurs in Cretaceous limestone, sand­ stones, and conglomerates in rough foothills of the Mountains in the southeastern Eagle part of Hudspeth County ... I believe this is the barite prospect (T.O-13.1) that is on the Bramblett ranch approximately half a mile east of the point at which Arroyo Escudo enters the low foothills along the western of the Indio margin Mountains, approximately 4 miles S. 40° W. from Squaw Peak. Barite occurs in the dark red-brown sandstone and chert- of the Yucca which strikes N. pebble conglomerate Formation, 25° E. and dips 20°-35° NW. At one outcrop, barite appears to be a fracture filling 5-6 feet wide along a fault trend­ ing S. 35° E. Nearby, barite is interbedded with sandstone of the Yucca Formation and exposed for 40-50 feet along strike. This deposit is worthy of further investigation in spite of its relative inaccessibility. Actually, the cost of a road, roughly parallel to Arroyo Escudo, into the area from the river road would not be prohibitive; much of this access road could be built on gravel-capped stream terraces, and only half a mile of the rugged foothill country would have to be traversed. PETROLEUM of from The prospect petroleum production the map area is fair. Because of the volume and type of sedimentary rock, the area should be considered carefully and, if pos­ sible, it should be explored by drilling before it is con­ demned . Certain characteristics of the Eagle Mountains and vicinity are those usually associated with non-productive areas. They are: 1. Precambrian rocks at the surface. 2. Igneous intrusions. 3. Extensive fractures. areas more There are, however, producing with one or of the above conditions. These seemingly adverse characteristics, then, must be balanced against favorable features to evalu­ ate the region realistically. thickens The stratigraphic section, which drastically southwestward into the Chihuahua trough away from the Diablo platform, offers the possibility of a petroliferous Pennsyl­ vanian and Permian section within reach of the drill. Other Paleozoic rocks be The Permian limestone in may present. the Eagle Mountains is fossiliferous and characterized by a strong petroliferous odor on fresh surface. The Cretaceous rocks cannot be totally ignored. Al- thin Cretaceous section that out though the relatively crops on the platform is largely non-petroliferous, a test drilled at the western edge of the platform (Brice f s Fee No. 4) found free oil in the Buda Limestone (West Texas Geological Society, 1941, p. 1537)0 Furthermore, the bituminous shale of the Summit Formation serve source beds in the sub- as Chispa might surface. Powell that there is (i960) reported approximately feet of shale at the base of the Summit 1,600 black Chispa just a few miles southwest of the map area. An attractive area in which to test the Cretaceous and Paleozoic section is Red Light Valley between the Eagle Mountains-Indio trend and the Quitman Mountains to the west. This valley is the part of the map area farthest from the Diablo platform. Thickened Cretaceous and Paleozoic forma­ tions underlie it. may Southwestward in the Chihuahua trough Jurassic strata be conformably subjacent to Cretaceous strata. The may lower part of the Jurassic Malone Formation in the Malone Mountains northwest of Sierra Blanca consists of limestone conglomerate, impure limestone, sandy shale, and thin-bedded limestone sandstone; the upper part, of black with sandy beds near the top (Albritton, 1938, p. 1754) This somewhat • clastic section is a fossiliferous near-shore facies, but farther south it may change abruptly to a deeper water facies. That it might be a source of petroleum is problematical. The only known surface indication of petroleum in the subsurface is an unsubstantiated report of oil films on water in some of the tanks of the western part of the map area. About 10-11 a 700-900 foot well drilled for years ago water in Cedar Canyon, on the east flank of the southern Quitman Mountains, had a showing of gas sufficient to set off a flurry of leasing activity. Interest died almost im­ mediately, however, and has not been revived. URANIUM Eargle (1956, p. 21) cited a minor occurrence of uranium in the Eagle Mountains: Several channel samples of coal from abandoned mines in the Eagle Ford shale of Late Cretaceous age in the Eagle Mountains about half a mile southwest of Eagle Spring, southeastern Hudspeth County, averaged 0.002 percent eU, and the ash from the coal contained an of 0.004 average percent U. Eargle also cited the occurrence of uranium minerals associated with the Permian Hueco Limestone in the Hueco Moun tains, El Paso and Hudspeth counties: Carnotite and minor amounts of tyuyamunite coat boulders and fractures in the calichified colluvium and, to a lesser extent, joint surfaces of limestone bedrock from 10 to 25 feet below the surface of the ground. ... Zones of the mineral-coated material within the body of the colluvium are as much as 20 feet long . . but . feet less in thickness. Most are in average only 5 or the lower layers of the colluvium the coarse bouldery - bed and the underlying less-indurated bed of caliche and pebbles. The occurrence on the north and northeast flanks of the Permian Eagle Mountains of two relatively large outcrops of Hueco Limestone suggest that there deposits similar to those described by Eargle in the Hueco Mountains might be expected Uranium minerals might also be associated with Cretace­ ous limestone, as coatings on joint surfaces or as deposits in weathered also cavities, for Eargle (1956, p. 7-11) cited an occurrence of this type in the general region, the King Mountain area in Upton County. In both the Hueco and King Mountain areas, the deposits of uranium minerals are intimately related to the modern ground surface, indicating that the uranium was probably leached by surficial waters from overlying or nearby mate­ rial and concentrated in its present position (Eargle, 1956, p. 20). If the topographically high mass of igneous rock in the Eagle Mountains carries uranium minerals in even minute quantities, which is possible, secondary concentration may have localized them in favorable low-lying host beds. Also, leaching of the black shale of the Chispa Summit Formation, in which the coal near Eagle Spring occurs, may have resulted in secondary near-surface accumulations down­ slope from the shale outcrops. COAL Thin beds of coal occur in the Eagle Spring area in black shale the Formation. steeply-dipping of Chispa Summit Coal was probably first mined at this locality late in the 19th century. Sometime to Schmitz (1885, 371-393) in- prior 1885, p. spected the coal-bearing beds near Eagle Spring and reported; The coal-bearing rocks, roughly estimated are about 3,000 feet thick, and could be observed for about 1 to 1 l/2 miles in length • . . He found four of the of which seams coal, largest was f,Big No. II =. From 20 inches to feet seam, 7 (average 3 l/2 feet)." He further said: Of the four veins above mentioned, only No. II has been developed namely, by a shaft to the depth of - feet. The the vein in the mine from 230 dip of ranges 60° to 80°. In the part of the shaft (which upper could be examined to a depth of 130 feet) the vein shows from 3 to 5 feet of coal and is said to thin out, in some places below, to but 20 inches of coal. The lower 100 feet of the mine was filled with water at the time of visit, as it had not been worked for about my a The coal of No. II is bituminous coal of fair year. and excellent for It has quality cooking purposes. a conchoidal fracture. Samples of coal taken about 70 feet below the mouth of the shaft gave the following analysis; Moisture, 3»537 Volatile combustible matter, ....30.843 Fixed carbon. 50.694 Ash,............................14.926 100.000 The coals of the other outcrops are similar to those of No. 11. Hill mentioned the coal Sierra Blanca (1887, p. 62) near in his summary of the state of knowledge of the geology of Texas as of 1 January 1886. mention coal made Specific of near Eagle Spring was by Ashburner in the annual of the mineral (1887, p. 349) report resources of the United States for the 1886: year There have been shipped about 100 tons of coal from the bed at Eagle Springs, 20 miles east of Sierra Blanca. Ashburner (1888, p. 359) commented briefly on the Eagle Spring region in the annual report for the next year: Mr. W. M. of El Paso, states that the Chandler, Eagle Springs mines, in El Paso County, have been leased to Dr. John Arthur of Kansas City, who has con­tracted to mine 30 tons of coal per day. No coal was produced at the mines in 1887. Mr. Chandler states that he does not consider that there is any very large amount of coal in El Paso county. In Ashburner 1888, (p. 367-368) reported: About 110 miles east of El Paso is the /Eagle/ Spring mine which is operated by Fink, Arthur and Co. A four foot bed of coal is reported to have been struck at 150 feet below the surface. Baker (1927, 22, 67) described the coal as ,Talmost p. semi-anthracite” and reported that the coal on the old dump was ,Tstill fresh, shiny, and hard.” On firing, Baker found that the tested left considerable ash. samples Baker (1935c, p. 316) later reported; Trinity Cretaceous Coal Coal occurs along an about 1 mile southwest arroyo of Eagle Spring, on the northeast flank of the Eagle Mountains in southeastern Hudspeth County. The bed strikes N 75° W and dips 82° N 15° E. In the old tunnel is thickness of feet or more of coal. This exposed a 3 coal, after being exposed to the weather in the old dump is and for thirty years or more, still hard, dense, shiny, somewhat metamorphosed, bituminous, and contains of ash.a large percentage No is available.analysis The complicated structure at the locality is apt to make the coal expensive to mine and the bed difficult to fol­ low . The main operations were in Coal Mine arroyo about 0.7 mile southwest of Eagle Spring. Although still accessible to Baker in 1922, the shaft had caved and was inaccessible to Gillerman in 1944-1946 during his investigation of the the consisted area. He reported (1953> p. 53) that workings j of a shaft 200 feet deep with drifts at 100 and 200 feet, and to work the in that the last attempt deposit was 1927. In 1959 the old, nearly filled shaft (then less than 6 feet deep), the remains of a wooden hand-windlass, a large- diameter metal pipe rising vertically from the ground adja­ cent the and a dump of coal tailings were the surface shaft, indications of former operations. A 30-to 40-foot shaft, clearly more recently worked, is 2,000 feet east-northeast of the older shaft. QUARTZ There is a deposit of massive white quartz (K.7-13*8) in the rocks of Precambrian on the northeast flank of age This is of the Eagle Mountains. deposit a potential source quartz that, when crushed and ground, could be used for abrasive backing of "flint" sandpapers. Powdered and silt- also and for the size quartz are used for scouring compounds coarser metal polishes. SAND AND GRAVEL Sand and gravel occur in great quantities in the Eagle Mountains and vicinity in terraces along streams, in allu­ vial fans, and in bolson deposits. Gravel-capped stream terraces flank Green River and Red Light Draw and the many arroyos leading from the west and southwest slopes of the mountains. There are many smaller streams within mountains. gravel-capped terraces along the Large Quaternary alluvial fans at the mouths of the major - canyons Spar Valley, Carpenter, Horse, Frenchman, Snowline, - and Cottonwood canyons constitute a major source of sand and gravel. Tertiary sand and gravel are the main constitu­ ents of the bolson deposits flanking the mountains. Sand-size material is largely composed of quartz, but gravel may be composed of fragments of Permian and Cretace­ ous limestone, Cretaceous conglomerate and sandstone, and and tra- Tertiary welded tuff, rhyolite, syenite, diabase, chyte. Much of the volcanic rock in the gravel is so highly fractured that it would not be suitable for a concrete aggre­ gate. Moreover, some rhyolite gravel is known to react with cement during setting, causing structural defects in concrete. The composition of the gravel depends, however, largely on its location; there is but little volcanic rock in the gravel along the western margin of the southern Indio Mountains, whereas gravel derived from the Eagle Mountains contains abun­ dant volcanic material. Considerable wind-blown sand lies a mile or two south of the Southern Pacific tracks on either side of the road into the wind-blown sand has also accumulated Speck ranch; just north of Grayton Lake. As the region is relatively inaccessible and lacks in- sand and used for construction dustries, gravel are local only. DIMENSION STONE Large-scale use of raw material in the Eagle Mountains and vicinity for dimension stone is unlikely. There are other sources of raw material closer to the small communities in the region where construction is limited that there is so little need of flagging for steps, sidewalks, and platforms or for curbing and paving stone. Limestone of the Bluff, Finlay, Espy, and Buda forma­ tions as well as sandstone of the Yucca, Bluff, Cox, and sources Eagle Mountains formations are, however, possible of dimension stone. Also, the white marble of Permian age half a mile north of Eagle Spring (Baker, 1935c, p. 238) and Precambrian estimated to be from the thick metaquartzite to feet be utilized for this 3,200 3*400 thick, might pur­ pose. Volcanic rock in the area is too highly fractured, for the most part, to serve as dimension stone. LIMESTONE Gillson and others (i960, p. 123) emphasized the large quantity of carbonate rock produced in the United States? The aggregate tonnage of the basic raw material, the carbonate rock, is so enormous that it exceeds the tonnage of any metallic ore mined and quarried in the United and in turn is exceeded States, only by coal, sand and and water the gravel, by in gross tonnage pro­duced by the mineral industry in this country. There are large volumes of Wolfcamp and Comanche lime­ stones in the Eagle Mountains and vicinity. These might well prove to be satisfactory sources for one or more of the many applications described by Gillson: manufacture of ce­ concrete ment and lime, aggregate, road metal, fluxing stone, agriculture, manufacture of glass, carbide, sugar, and paper, and dimension stone. There is much limestone within a few miles of the Southern Pacific tracks. RHYOLITE A well known quarry belonging to Gifford-Hill Co., Inc., is located just north of U. S. Highway 80 and the Texas and Pacific Railroad about 8-9 miles west of Van Horn. This company has long quarried and crushed metamorphosed Precam­ brian rhyolite for use as railroad ballast, road metal, rip- rap, and roofing granules. The crushed rhyolite is so ex­ tremely angular that it makes a stable aggregate when it is packed. This is a desirable characteristic in its several uses as a load-bearing agent. Although not so close to a railroad as in the Gifford- Hill quarry, some of the Tertiary volcanic rock in the Eagle Mountains and Indio Mountains might be utilized in similar The minimum haul to the Southern Pacific tracks would ways. be miles. approximately 5 TUFF OR PUMICITE Ground pumice and tuff (pumicite) are used in metal polishes, sweeping compounds, tooth paste, mechanics hand and in and soap, cleaning, scouring, polishing compounds. They may also be used as an abrasive in rubber erasers and for abrading and polishing hard rubber and fiber board (Chandler, 1956, p. 717)o Just east of the Palmas well in the Indio Mountains and west of the Tertiary volcanic-rock scarp, there are small valleys of tuff between small hogbacks of welded tuff. Some of this tuff, e.g., that in a zone approximately 345-365 feet above the base of MS 4» is almost pure glass. There are other of tuff in the Indio Mountains in the Flat outcrops Top area and just north of MS 4» Fine-grained, homogeneous tuff the Mountains and is notably absent in Eagle Devil Ridge. The inaccessibility of the Indio Mountains tuff deposits make them of doubtful commercial value. PERLITE Gillerman (1953» P» 35) cited the occurrence of perlite in the Panther Bluff-Eagle Spring area: Spherulitic rhyolite occurs in the area southwest of Carpenter Spring and thin beds of pitchstone, vitro­phyre, and perlite are present near Eagle Spring and Panther Bluff. The glassy rocks are of small areal ex­tent and are interbedded with the flows and flow breccias. I measured a section at Panther Peak and explored the regions near it and Carpenter Spring but did not find the glass referred to by Gillerman. This lends to his emphasis remark about small areal extent. Small masses of intrusive glass (K.O-10,3) high on the mountains west of Panther Peak were shown to me by Jack tests that the is not Hayter, but laboratory proved glass perlitic» Volcanic glass occurs in isolated and restricted masses in the Indio Mountains, but it also is not perlitic. BAT GUANO In two localities in the Eagle Mountains, weathering along joints in massive rhyolite has produced caves that con­ tain a little bat guano. There are three caves at the base of Eagle Bluff (N.­ the distinctive cliffs of at 13.1), light-colored rhyolite the east and southeast margins of the Eagle Mountains. The largest of these caves is perhaps 20-25 feet high and ex­ tends back only 50 feet into the cliff. Two caves (J.6-9«3) at the base of the vertical rhyolite cliffs on the west flank of the Eagle Mountains just north of Indian Springs mill are also little more than shelters. They are shallower but are considerably higher at the en­ trance than those at Eagle Bluff. Small volume and relative inaccessibility make the guano deposits of doubtful commercial value. SILICIFIED WOOD The silicified wood in the and Summit Yucca, Cox, Chispa formations of Cretaceous and in the age Tertiary-Quaternary sand and silt lacks the proportions of iron oxide, manganese oxide, and carbon that are responsible for the brilliant colors of silicified wood of the Petrified Forest, Arizona. The fragments of silicified wood in the map area are drab shades of black or brown. FOSSILS In numerous areas fossils weather out in great abun­ dance. Many of these would find a ready market in rock distinctive are ammonites shops. Especially (fragments), large gastropods such as Tylostoma sp o and nerinids, large several of heart-shaped clams, and genera echinoids. SEMI-PRECIOUS STONES The conglomeratic zones of the Yucca Formation contain abundant chert and vein quartz granules and pebbles. The chert is and and the commonly black, white, red, vein quartz is characteristically white or pale pink. Chalcedony anygdules occur in much of the flow rock, especially that just west of Green River, and much of it is in and medium It is faintly banded light light gray. com­ monly botryoidal. Such ornamental stones a considerable traditionally form portion of the stock of roadside rock shops, and the most dis tinctive stones are utilized in inexpensive jewelry. APPENDIX 394 GEOGRAPHY Climate Regionalits 267,339 miles extending Regional.--Texas, square across 13°5 T of longitude and 10°40 f of latitude, has mari­ climates. The continental time, continental, and mountain and mountain types are normal climatic types, but the mari­ time climate is modified by surges of continental air (Blood, The climate of the Mountains and vicin­ 1960, p. l). Eagle ity is the mountain type, which prevails only in Trans-Pecos Texas and is characterized by cool, dry winters and hot, rela­ summers. tively moist The worst drouth in recorded history in Texas occurred during the years 1950-1956 inclusive, when 244 of Texas’ 254 counties were declared disaster areas (Blood, 1960, p. 3)* The drouth brought tremendous financial loss and reduction of natural the effects were evident resources; still vividly in the Mountains in where there Eagle area 1960 were large numbers of dead trees and remains of cattle. LocalLocalo--Climatic characteristics vary throughout the Eagle Mountains area because of a difference in elevation in excess of 4,360 feet. The information to follow is based on data collected a over ten-year period, 1949-1958 (inclusive), by the Van Horn station of the U. S. Weather Bureau, currently at the Sinclair service station on located McVay U 0 S 9 Highway 80. This station is in Salt Basin about 18 miles airline from Eagle Peak and at an elevation of 4,050 feet above sea level. Records of the Van Horn station indicate but do not define the climate in the map area, where extreme tempera­ tures are higher and lower than those recorded at Van Horn and where rainfall (in the higher parts) is greater. The average annual temperature is 63*2° F.; December, the coldest month, averages 45*2° F., and July, the warmest and month, averages 80.7° F. December, January, February temperatures average 40°-50° F,; March and November tempera- and tures average 50°-60° F.; April October temperatures average 60°-70° F.; May, June, July, August, and September than F. average higher 70° In the the ten-year period, 1949-1958, maximum tempera­ ture recorded at Van Horn was 108° F®, and the minimum tem­ perature was -5° F. During the years of this period for which records are available, there were an average of 219 days between the last spring day and first fall day of tem­ perature below 32° F., 234 days between the last spring day and first fall day of temperature below 28° F®, and 269 days between the last spring day and first fall day of temperature below 24° F. The rate of change of temperature with change in eleva­ tion is least in January and greatest in June, July, and August. The monthly values in °F. feet of are, per 1,000 .2 .2 6l,6 64.4 62.2 63.4 64.8 62.6 63.I 63.4 63.4 63 63 Annual days 3M record. . 6 10 Dec 44.0 47.5 47.5 43.5 39.1 44. 48.4 44. 47.5 46.0 45.2 s 6 -t 2M °F, °0 than daily — 11 ov, 54 51.8 50.6 48. 53 54.0 47.8 48.7 54.0 51.4 IN N less Oct. 60.6 68,5 66.0 63.3 62.7 65.5 63.9 66.0 60,7 61.1 63.8 detailed 1M e TEXAS entered. ept 72.6 73.6 74.8 72,4 73.9 78.5 73.7 74. 71.6 73.3 73.9 for TEMPERATURES, S is 1M Data 3 COUNTY Aug. 77.2 (79.6)* 81. 82.0 80,1 80.0 77.4 77.7 79. 81,5 79.6 value AVERAGE .0M .3 .0 July 79.8 78.2 77.3 82.3 79.5 81,4 84.O 80,7 82 83 79 9 CULBERSON average Climatological ANNUAL June 80.0 79.7 81.0 80,4 83.8 80.4 78,2 81,5 78,6 82.7 80.6 years if AND 8M HORN, May 72.0 70.8 71.5 70.2 69.8 71.5 70.3 74.9 68. 72.4 71.2 missing; Monthly nine VAN MONTHLY other AT * Apr. 59.2 64.7 60.0 61.1 63.3 67.8 6 60.2 61.0 62.4 62.3 63 See 2M for 29» record Mar. 56.8 56.1 52.6 50.5 58.2 54.6 55.4 55.9 56. 50.8 54.7 TABLE days missing. August Feb, 48.0 52.0 47.8 47.6 46.2 51.3 45-6 46.6 56.7 50.1 49.8 of more is • an or 35.3 50.0 42.9 49.0 49-5 47.9 41.8 484 50.4 42.3 45.8 ] J • record AverageAverage One Year 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 Ten Year M* change in elevations January 2.5 July 4«75 February 3»5 August 4*75 March 4 September 4 April 4»25 October 3*5 May 4»25 November 3 June 4-75 December 3 Thus, in the Eagle Mountains at an elevation of 6,500 feet above feet Van sea level, nearly 2,500 higher than Horn, January temperatures might average some 6° F. lower than those in Van Horn. In June, July, and August, temperatures at 6,500 feet in the Eagles might average almost 12° F. less than those in Van Horn. Rainfall is geographically and chronologically variable. Total rainfall in 1949 was 16 inches, dropped to 6.8 inches in 1950, and did not total over 10 inches annually until 1957 when it reached 12.4 inches. It is noteworthy that almost 65 of rainfall in percent the area occurs in the period July-October inclusive, when the warm, moist southeasterly or westerly winds sweep over the area. There is a slightly moist mid-winter period, Jan­ uary, when snow falls. This mid-winter moisture is preceded and followed by dryer periods. The following comments are based on weather of maps Kincer (1941, p. 727-736, 742, 743) and of Visher (1954, p. 6.82 5.28 5-35 6.19 6.75 8.20 5,66 9o29 8.20 16.00 12.43 Annual TT Dec. 0.78 0.00 0.09 0.27 0.04 Oc45 0.18 0.00 0.19 T T Nov 0.00 0,00 0.42 0.01 0.00 0.02 0.45 0d4 0.10 INCHES, 0 4293 82 IN © 00 Oct, 1.63 0,14 0.03 0.00 2.17 0.15 2.19 1.09 01 2 T ept, ,28 2.34 0.62 0.73 0.04 0.83 0.51 0,39 4o39 1.31 3 TEXAS S PRECIPITATION, Aug. 1.19 0.00 1.00 0.29 1,05 2.91 0.60 1.41 1.96 0.65 1.11 COUNTY, TOTAL July 2.08 2.21 1,46 1,78 1.93 0.18 3.71 1,50 1.91 1.12 lc79 CULBERSON June 0.43 1.17 0.1? 0.46 0.66 0.45 0.09 0.35 0.27 0.44 0.45 ANNUAL • T AND May 1*79 0.66 0.40 0,17 0.96 0.04 0.12 0.54 0,64 53 0 HORN, T VAN Apr. 1.15 0.71 0,41 0,41 0.08 1.11 0.01 T 0.69 o MONTHLY .46 AT Mar. 0.02 0,60 0.35 0.04 0.54 0.00 0.00 0.21 0.50 0,23 30« T TT 0.28 0.06 0.14 0.08 0.04 0.92 0.98 0.56 0.31 TABLE Feb. ® an T 3-39 0.17 0.10 0.16 0.14 0.97 0.25 0.12 1.51 0.68 J Ten Year Average Year 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 is 157, 159, l60) Average annual snowfall 2-5 inches, and o the forms a of 1-10 snow cover an average only days per year. Thunderstorms occur, on an average, on 20-30 days of the and hail an of 1-2 occurs on year, average days per year. relative in Average humidity July averages 60-70 percent at 8 a. m. (EST), is 35-40 percent at noon (local time), and at 8 p. m. (EST), 30-35 percent. The average annual number of clear is 200-220, and the annual number of days average Normal annual cloudy days is 40-60, evaporation from pans is 90 inches. The surface wind 10 mph; the average velocity is average at 6 a. m. (local time), normally the hour of least wind, is 6-8 mph and the average at 3 p* m. (local time), normally the hour of most wind, is 12 mph. Normal surface wind direc­ tion in is either from southeast January or northwest, and the normal surface wind direction in July is from the south­ east. Classification Classification.--According to the climatological classi­ fication system proposed by Kdppen and reviewed by Haurwitz and Austin (1944, 109-130), Van Horn is classified as P» f,B¥k n dry, desert, with a mean annual temperature less - than 64»4° F* Vegetation The Eagle Mountains are in the northern part of the Chihuahuan Desert, and much of the flora in the map area is typical of a desert region. The vegetation is varied, how- because the elevation from feet to ever, ranges 3,150 7,510 feet above sea level. Three life zones represented in are the area: Lower Sonoran to 4,500 feet; Upper Sonoran, 4,500 feet to 6,500 feet; and Transition, 6,500 feet to 7,500 feet (Arnberger, 1952, p. 5, 7)® The area is subject to extended dry periods, during which vegetation visibly deteriorates, but a single severe thunderstorm is sufficient to restore much of it and cause unexpectedly brilliant blossoms to appear. Herbs and shrubs .--The most characteristic shrubs of the low areas are the evergreen creosote bush (Larrea tridentata) and the tar bush or black brush (Flourensia cernua). Locally ,? TT the creosote bush is called greasewood, but the greasewood of the geologist, Sarcobatus verminculatus, has never been found in Texas* Creosote bush is a typical xerophyte or desert plant, but Sarcobatus is a phreatophyte, a deep- rooted plant which is supplied with water from the water table or the layer of soil just above it (Janssen, 1953jP* 32-33) • One of the most distinctive shrubs of the region is the ocotillo or flamingsword (Fouquieria splendens). It is a cluster of unbranching, slender, thorny limbs up to 20 feet a long which are leafless, except just after rain when bright leaves cover the limbs. The leaves turn brown and green fall off as soon as the soil dries. In April-May flame-red blossoms appear at the tip of each limb, and when the red blossoms happen to coincide with green leaves covering the limbs, the ocotillo is indeed a splendid sight. This shrub does occur on flats, but it flourishes on rocky slopes such as alluvial fans and gravel terraces. Two species of yucca were recognized: the broadleaf yucca (Yucca torreyi) and the narrowleaf yucca (Y. elata). These the there is grow widely throughout area; an especi­ ally dense stand on sandy soil 1-2 miles south of the South­ ern Pacific tracks along the road to the Speck ranch. Other common species of the lily family are beargrass or basket-grass (Nolina erumpens) and sotol (Dasylirion lelophyllum). Sotol and beargrass do not occur with yucca on the sandy flats but are restricted to rocky slopes. When split open the starchy heart of the sotol yields whole- stock food that in time of drouth some may supplant grass (Bailey, 1905, p» 3l)« Lechuguilla (Agave lechuguilla), the indicator plant of the Chihuahuan is on it Desert, übiquitous rocky slopes; seems to flourish equally well on quartzitic sandstone, rock. of the limestone, and volcanic In many parts area lechuguilla completely covers the rocks with a dense blanket of short, stiff, sharp-pointed leaves. Although a constant menace to the unwary foot traveler, it is not a totally use­ less Deer thrive on it, and within each leaf of the plant. smooth fibers that has lechuguilla is a bundle of strong, proved suitable for weaving into matting and for making twine and rope (Bailey, 1905, p. 30-31). Pricklypear (Opuntia engelmanni) is common as is the distinctive and colorful purple-tinged pricklypear (_O. mac­ rocentra) The unusual blind pricklypear (0. rufida) so . t called because of its lack of long thorns on the flattened also identified. was stems, An even more striking member of the cactus family is the walkingstick or buckhorn cholla (0. imbricata) which t bears bright red to purple blossoms in May and June. A re­ lated cactus, also characterized by the structurally intri­ . cate cylindrical joints, is the tasajillo (_O. leptocaulis) Other members of the cactus family identified were devilshead cactus (Echinocactus horizonthalonius)» fishook cactus (Jj. uncinatus) Mexican rose pricklypear (Opuntia , pottsii) and strawberry cactus or pitaya (Echino cereus stramineus). Growing most commonly in large clumps, the bears a dark red sweet and pitaya fruit, juicy when ripe, that has a flavor akin to that of strawberry or raspberry. Closely related to the pitaya is the rainbow cactus (E. rigidissimus), a distinctive barrel-shaped cactus with bands of white alternating red and spines encircling the stem* Because of its bright yellow blossom, it is also known as yellow pitaya. Mormon tea (Ephedra sp.), a distinctive member of the jointfir family, was used by the Utah pioneers in preparing a and because of its tannin and alkoloid content was brew, used as a medicine by the Indians (Dodge, 1954> p. 34)» Desert holly or algerita (Berberis trifoliolata) is com' mon on rocky slopes of the uplands. Its bright red berries are often used for making jelly, and its yellow blossoms add a welcome dash of color to the landscape. Ceniza of the is (Leucophyllum sp.), figwort family, on limestone it has lilac-violet or widespread slopes; indigo blue blossoms. White-blossom pricklepoppy (Argemone alba), purple- verbena rabbit- blossom wild (Verbena bipinnatifida), and brush (Chrysothamnus nauseasus) are common. Allthorn (Koeberlinia spinosa) is easily one of the most distinctive plants of the area, for it is a formid­ able mass of rigid thorns. The plants are few and easily avoided. Cat claw (Acacia greggi), commonly grows in thickets along arroyos at lower elevations. Its sharp, claw-like thorns viciously lacerate flesh and tear clothing, and it may form completely impenetrable barriers. Its one redeem­ ing feature is that its white blossoms are an important source of nectar for honeybees. Resurrection plant (Selaginella lepidophylla) is com­ mon in protected niches of limestone outcrops. During dry months it is brown and dehydrated, its fronds curled dead. after a how­ - tightly seemingly Immediately rain, wide its fronds and turns a rich ever, it spreads green. Grasses. The gramas (Bouteloua sp.) abound throughout the area and are the most valuable in- range grasses. They clude sideoats grama (B. curtipendula), blue grama (B. gracilis) , black grama (JB. eriopoda) , hairy grama (B. hirsuta) and chino grama (B. ramosa or breviseta). t Sideoats grama (B. curtipendula) is common on ridges and mountains, chino grama (B. ramosa or breviseta) on terraces, and blue grama (B. gracilis), sand dropseed (Sporobolus cryptandrus)» and bluestem (Andropogon sp.) on the lowlands. grow In the draws bluestem (Andropogon sp.), awn cottontop (Trichachne californica), sideoats grama (B. curtipendula)» and black grama (B. eriopoda) are typical. Tabosa grass (Hilaria mutica) and burro grass characterize adobe flats. (Scleropogon brevifolius) Trees .--There are truly large trees in the but no area, several species reach 15-20 feet in height. Abundant along water courses in low areas is desertwillow or desert catalpa It boasts (Chilopsis linearis). pink-lavender blossoms during April-August, which are eventually replaced by long, slender seed pods. Cottonwoods (Populus fremontii) are restricted to moist areas and are not common in the Eagle Mountains and vicin­ ity. Specifically, they occur mainly at Hot Wells and in small stands the Rio Grande. The cottonwood which along one Cottonwood is now dead. gave Canyon its name Mesquite (Pro sopis .juliflora) is common in relatively moist, low areas. It is a great moisture thief and thrives at the of and shrubs. Burrows expense surrounding grass in his (1910, p. 392), reporting on geological investigations in northeastern referred to the usefulness of Chihuahua, me squite: Wood for camp-fires, however, is not lacking, the chief that most useful supply being mesquite, of desert shrubs, which during the period of its fruiting in June and July is also capable of afording food for man and beast. It is said that the aborigines always became fat at this period, subsisting entirely on the sweet succulent pods. These are still made use of by the Mexicans who grind the bean together with the pod into called mesquitamal, which is both palat­ a paste able and nutritious. Although not a native of the United States, the salt cedar or tamarisk (Tamarix gallica) has flourished in the southwest following its introduction by the U. S. Department of Agriculture during 1899-1915 (Dodge, 1954, p. 71)• It thrives in hot, dry areas on saline soils. In the Eagle Mountains and vicinity it abounds along the banks and in the channel of the Rio Grande. It so completely chokes the river channel between the Indio and Quitman mountains that river flow is seriously impeded and flooding of farm land ensues even following moderately heavy rains. In the higher mountains "cedar” (Juniperus sp.), piflon pine (Pinus cembroides) and gray oak (Quercus grisea) occur, mainly in canyons. The oak may reach 20-25 feet in height and the junipers and pines 15-20 feet. The recent drouth (1950-1956) took a heavy toll of these trees. Two specific references in the literature to the flora of the Eagle Mountains are known. The earliest was by who in Havard, (1885, p. 492), a physician served expedi­ tions for the exploration of Texas under the command of Major General William R. Livermore, Chief Engineer Officer, Department of Texas, in the summer and fall of 1881 and 1883 . Havard the flora in the inspected vicinity of Eagle Spring and, in addition to some of the species already men­ tioned, recorded the presence of the followings choke cherry (Prunus capuli)» hollygrape (Berberis fremontii), mountain mahogany (Cercocarpus parvlfolius), Texas adolphia (Adolphia infesta), alum root (Heuchera rubescens), sage­ brush or wormwood (Artemisia frigida). and trumped phlox (Gilia aggregata) . Carter and Cory (1932, p. 30) cite the following shrub association as occurring in the Davis, Chisos, Bofecillos, Chinati, Tierra Vieja, and Eagle mountains; yucca (Yucca sp.), sumac (Rhus sp.), buckeye (Ungnadia sp.), persimmon (Diospyros sp.), and cat claw (Acacia sp.). Wildlife Wildlife in this northern part of the Chihuahuan Desert is abundant and varied, and much of it is remarkably special­ ized to insure survival. At noon on a typically hot mid­ summer day, with atmospheric temperature 110° F. and ground surface temperature 150° F., very likely the only moving over- thing will be a turkey buzzard optimistically circling head. In late afternoon and early evening, however, animals begin to move about, and many roam throughout the hours of darkness until the rapidly-increasing heat from the early them to retire to se­ morning sun forces again some cool, cluded niche. Mammals.--The Mammalsblacktailed jackrabbit (Lepus californicus) is the most conspicuous and characteristic animal of Trans- Pecos. In the constant struggle for survival, this fecund, swift, desert-wise animal has been remarkably successful. Supervisors of the High Point Conservation District on the voracious of (1950, p. 66) reported appetite jack­ rabbit s : It has been determined by feeding tests that 15 jack rabbits consume as much valuable food as 1 sheep, and 74 jack rabbits as much as one cow. The Super­ visors of the High Point Conservation District believe that 15 jack rabbits will destroy, in the form of seed­ lings, as much feed as one cow would consume. The jackrabbit is technically a hare, but this desert - region does have a bona fide rabbit the desert cottontail (Sylvilagus auduboni) The abundance of jackrabbits can be attributed directly to the large-scale extermination of coyotes (Canis latrans) by government trappers and bounty hunters. Only three of these desert wolves were seen alive in approximately 11 months in the field, although dozens of their carcasses hung on fence Stomach of posts. analyses 8,263 coyotes were reported by the Supervisors of the High Point Conservation District (1950, p. 67). The diet consisted of average rabbits, 32 per­cent; rodents, 17*5 percent; domestic livestock, 14 percent; deer, 3*5 percent; birds, 3 percent; insects, 1 fruit and 2 and de­ percent; vegetables, percent; animal 26 cayed remains, percent. This record indicates that the coyote is effective in rodent control and is essential to nature’s balance between wild creatures. skunks Striped (Mephitis mephitis), porcupines (Erethizon dorsatum), badgers (Taxidea taxus), kit fox (Vulpes macrotis), and ringtail cats (Bassariscus astutus) are also present, although not in great numbers. Rock squirrels (Citellus variegatus) inhabit the higher- canyons and rocky slopes whereas another rodent, the little kangaroo rat (Dipodomys sp.), is a common sight at night on roads across the lower parts of the region. The antelope ground squirrel (Citellus interpres) inhabits the rocky washes and rocky soil of alluvial fans. Prairie dogs (Cynomys ludovicianus) have built a "dog town, 1' covering an acre or so, just north of the head of Green River. The desert mule deer (Odocoileus hemionus crooki) is a common sight in the mountains. They are usually alone or in groups of two’s or three’s, but herds of up to twenty of these alert, graceful animals have been seen. Their large ears, black-tipped tail, and large size (adult bucks will characteristics. average 150 pounds) are outstanding Some 40 javelina (Pecari ta.jacu) were turned loose on the Bramblett ranch about 1956 by the State Fish and Game Commission, but I did not encounter these animals. Prefer­ ring thick underbrush, they probably frequent the tangled growth that abounds along most of the drainage courses where their dark gray color would not be outstanding. A frequent visitor, although probably not a regular inhabitant, of the area is the mountain lion or cougar (Fells concolor) It is also called panther, puma, or catamount u (Olin, 1954)* Each year a local trapper, using dogs, kills several in the Eagle Mountains and vicinity. A likely spot to catch the big cats is, appropriately enough, Panther Peak, southeast of Eagle Spring where there are abundant ledges and small cavities in the rhyolite. The lions are extremely wary of man and are rarely seen except in a blind encounter. These 100 200 swift, agile predators, weighing from to pounds, prey on calves and lambs. One night’s foraging by a large lion cost a rancher several thousand dollars. may I a lookout for the desert kept sharp bighorn sheep (Ovis canadensis mexicana), because these rapidly disappearing monarchs of the high peaks have been reported in the Sierra Diablo to the north. An unsubstantiated also cited report several in Devil Ridge and Eagle Mountains; I saw none, how­ ever . Reptiles,Reptile s, insects, and spiders A great variety of reptiles flourish in the region, but I will cite only the most common. The most distinctive the mountain lizard, boomer, or collared lizard (Crotaphytus collaris), is a com­ mon sight. The whiptail lizards (Cnemidophorus sp.) are even more abundant; some five species inhabit the region (Axtell, 1959). Man’s traditional nemesis of the desert, the rattle­ snake, is common in the area. I recognized only two species: the gray, western, diamond-back (Grotalus atrox) and the olive-green black-tail (Grotalus molossus). I saw scorpions and millipeds of unknown species, centipedes (Scolopendra sp.), and tarantulas (Avicularia sp.) but not in great numbers. Ants and wasps are also common but present no unusual problem. Mosquitoes abound where there is standing water near the Rio Grande, but they do not frequent the higher mountains. Birdsbuzzards or turkey vultures (Cathartes Birds.--Turkey aura) are by far the most distinctive and common birds in Trans-Pecos. They are the garbage collectors of desert and and devour all carrion. highway, for they eagerly efficiently Graceful in flight, they may soar for hours at a time, from thermal scarcely flapping a wing as they move thermal to off the earth below. It is to become rising warm easy slightly irritated by their ever-closer spirals of inspection while one is eating lunch or examining photographs; they seem never to cease hoping that the next moment or the next step may be the last. Another familiar sight in the desert country of the southwest is the roadrunner (Geococcyx californianus), "the cuckoo that runs” (Peterson, 1960, p. 123). They are re­ luctant to take wing and will use their long legs in an all- out effort to outrun an automobile. When walking in large washes in low country, one is apt to flush from a large juniper a great horned owl (Bubo virginianus). aerobatics the The spectacular of nighthawk (Chordeiles minor) be observed in the late afternoon and early may evening near tanks and ponds. Quail are abundant in the region, or at least have been since rainfall has been suf­ ficient to water in most of the tanks. The scaled or keep blue quail (Callipepla squamata) are recognized by their "scaly markings on breast and back and a bushy white crest or 1 cotton top’." Gambel T s quail are recognized by their "short dark plume curing forward from crown . . male has . an interesting black and white face pattern, a black patch on his buffy belly" (Peterson, 1960, p. 75)» White winged dove (Zenaida asiatica) are easily identi­ fied by the large white patch on each wing. Near water they, and the common mourning dove (Zenaidura sp.) are seen often. Culture Ranch roads, barbed-wire and sheep-wire fences and corrals, windmills and accompanying steel or concrete tanks, earthen stock behind spreader dams, tanks, small reservoirs rock and concrete dams in some of the narrower draws and and ranch the canyons, buildings, usually adobe, are prin­ cipal elements of culture in this sparsely settled region. In addition, the Southern Pacific tracks border the area on the and the lines of the Rio Grande Electric north, power Cooperative, Inc., now extend to most of the ranches of the region. have been Two small landing strips for light planes Rio Grande: one at the Bennett new Adobe bladed near the some 6 miles southeast of the mouth of Green headquarters of Bramblett and the other just east (W.B-13.9) the River, ranch headquarters. head ore Old powder houses, frames, rails, buckets, and other relics of bygone activity abandoned vehicles, The around abandoned mine shafts and adits. litter the area ruins of small stone huts are also common near old mining several foundation slabs (L.2-13.7) camps. In Spar Valley all that remain of what was once a of large buildings are mill. U. S, Bureau of Mines trenches are relatively large and are numerous in the fluorspar areas, prospect pits widely scattered throughout the area. An American standard cable-tool rig (F.4-9.7)* used to drill and later to pump the 1,000-foot Deep well (water), flank of Little Hill southeast of the stands on the north the well old Jolly place. Because of high lifting costs, has not been used since the mid-1930 T s (Espy, I960), U. S. Coast and Geodetic U S. Geological Survey and o and Survey bench marks and triangulation stations accompany- reference and azimuth marks are located throughout ing the stations and bench marks are region; triangulation plotted on plate 1. Widely scattered through the area are wooden stakes 1.5 inches in square cross section, each bearing a numbered metal disc nailed to the top. These markers were set by oil company - crews surveying or geophysical. The old stage road of the Overland-Chihuahua trails, in use from approximately 1854 to 1882, crosses the area roughly east-west Because erosion has been concentrated (pi. l) . along the old ruts and formed water courses which, in turn, became zones of slightly denser vegetation, the old road is clearly visible on aerial photographs, although less evident on the ground. The ruins of Eagle Spring stage stand (H.­ adobe and the 11.8), a crumbling building remains of the old rock are just 0.4 mile north of Eagle Spring. A corral, historical marker, erected the State of Texas gray granite by in 1936 at the site of the Eagle Spring stage stand, has the following inscription; Eagle Springs Stage Stand A station (1854-1882) for the stage coaches and wagon trains of the Overland-Chihuahua Trails, which East the linked the to pioneer West, brought heartening mail and and and the passengers, supplies, quickened life of this remote region, then far out on the lonely fringes of frontier civilization. Economy Hudspeth County, the third largest county in Texas with an area of 4,523 square miles, has large cattle, sheep, and goat ranches. Its irrigated farms along the Rio Grande are less important economically. The labor force on each ranch or farm is usually small and consists of processed Mexican nationals or braceros, and on occasion, unprocessed Mexican nationals or wet-backs. In the there is small- map area, scale farming along the Rio Grande on the Guerra and Bramblett ranches where the crops are cotton, grain, and truck. The fluorspar mines in the Eagle Mountains and other active have small mining operations in years past employed a number of and contributed to the local No people economy. mining operations are currently in progress in the map area, but the talc of U. S. expanding mining industry just north Highway 80 is an important economic asset to the county. The economy of most of Trans-Pecos is based, ultimately, on an unpredictable factor rainfall. This is indeed true - for the Eagle Mountains and vicinity. The farmers, however, must consider another equally unpredictable factor -summer hail. Crop insurance against hail damage, although expensive, is considered almost a necessity. Population is known to be a reasonably accurate economic trend indicator. The 1950 census recorded 4,298 residents in Hudspeth County, whereas the 1960 census showed only or a 3,343 residents, decrease of approximately 22 percent (Texas Almanac, 1961-1962). This indicates a deteriorating economic picture, no doubt related in some measure to the to severe drouth, 1950 1956. Substantial income is derived by many ranchers through the leasing of their ranches for deer hunting. The standard rate is for the $lOO per gun eight-day season, and often as many as 20-30 hunters will lease one ranch. Table 31.--U. S. Geological Survey air photographs, GSLU series, of the Eagle Mountains and vicinity Index photographs (scale 1:62,500) of the area are shown == below in juxtaposition (SB Sierra Blanca; EM proper Eagle = = == Mount ain s; AL Allamocre; LO Lobo ; CH Chispa; NE north­ . east; SE = southeast, and so on) SB, NW SB, NE SB, S¥ SB, SE AL, SW AL, SE EM2,NW EM2,NE EM1,NW EM1,NE LO,NW EMSE EM SW EMSE SW 2, 1, 1,LO, EM4,NE CH3 NW , The vertical, stereo-paired air photographs of the Eagle Mountains and vicinity, series GSLU, are 9 by 9 inches, scale 1:23,600, and were flown in 1950. Each line of numbers below is a flight line reading west to east. The flight lines listed in from north to are sequence south, and the numbers are inclusive. 1. 7-148 to 7-150 11. 7-45 to 7-32 2. 7-128 to 7-124 12. 3-184 to 3-197 3. 7-71 to 7-80 13. 3-164 to 3-152 4. 5-47 to 5-58 14. 3-118 to 3-130 5• 5-32 to 5-19 15. 3-107 to 3-96 6. 4-184 to 4-198 16. 7-05 to 7-14, 3-76 7. 4-163 to 4-148 17. 3-60 to 3-52 8. 4-113 to 4-128 18. 3-22 to 3-29 9. 4-90 to 4-77 19. 3-16 to 3-15 10. to 4-45 4-58 MEASURED SECTIONS Sixteen stratigraphic sections were measured, and their locations are shown on plate 1. Several abbreviations were used in these describing sections; they are: AB feet above the base of the measured section. ~ Md median - nv near vertical - nvb no visible bedding. - Terms of the Wentworth scale are used to describe the grain size of siliciclastic rocks, and the terms of Folk (1959, P» 16) are used to describe the grain size of the carbonate rocks. Rock-color terms are from the Rock-Color Chart (Goddard, and others, 1951); Munsell numbers accompany those rock-color terms in the color only that are repeated chart. The descriptions of joints follow an invariant pat­ tern: (l) strike, (2) dip, and (3) spacing. The bedding thickness terms of McKee and Weir (1953) are used. Measured Section 1 Formations Yucca Formation Locations C.2-2.8; northeast flank. Yucca Mesa, second nose from east end, 1.25 miles south-southeast of J. R. Love ranch house. GSLU Series photo no. 7-75* Measured; June 26-29, 1957, by Underwood and Yeager with Brunton compass and 5-foot staff. Thickness, feet Unit Description Feet above base CRETACEOUS Bluff F0rmati0n................................n0t measured Yucca Formation 101. Covered zone, probably shale or thin-bedded limestone; forms rubble-covered north slope... 1168.5­ 100. red (5R6/2) to medium Limestone, pale gray, weathers grayish pink to grayish orange pink subconchoi­ (5K7/2); very finely crystalline; dal thin smooth fracture; compact; bedded; weathered surface; 0.5-2 ft sharp edged blocks form distinct ledge; nearest resist­ant bed to massive Bluff limestone above. 5 1163.5­ 99-Covered zone, probably thin-bedded limestone; forms moderate to steep rubble-covered north 6 Slope ......a.0»....0«....0.it:.....e.».ee.» 1099.5-.5 98. Limestone, medium dark gray weathers light gray; very finely crystalline; abundant fos­sils and fossil fragments, mostly pelecypods, conchoidal high-spired gastropods; fracture; thin weathered compact; bedded; fairly smooth surface with some solution 1-2 ft channels; blocks form 1edge5*...........g.......... 8 97. Covered zone, as in unit 101 27 1066.5-.5 medium weathers 96. Sandstone, light gray, light to olive gray (5Y6/l)> well-sorted, angular subangular, polished and frosted fine-grained quartz; flecks of iron oxide throughout (Md 0.15 mm); calcite veins; iron oxide coats some fracture surfaces; compact; laminated, also smooth thin bedded; fairly irregular weathered surface; forms slight ledge.».». ¦...«..«. 95. Covered zone, as in unit1014 1060.5­ 94* medium dark weathers Limestone, gray, very light gray; very finely crystalline; abundant calcite veinlets stained with iron oxide; sub­ conchoidal fractures, some coated with iron weathered oxide; compact; nvb; fairly smooth surface; forms slight ledge 3 1057.5­ Covered in unit 93* zone, as 101 7 1050.5­ 92. Limestone, medium light gray; weathers very light gray; very finely crystalline; unidenti­fied fossil fragments; compact; thin to thick bedded; smooth weathered surface; weathers to indistinct ledge; lower 2 ft somewhat nod­ ular 6 ........so.....e.e..»».».»....••« t>oa o • s• • ••••> profile; forms 5 344.5­ 32. Limestone, medium gray, weathers grayish orange; very finely crystalline; calcite most fracture surfaces coated with veinlets; iron oxide; compact; nvb; joints: N52°E, nv to 7C°NW, 2 ft; weathers to rough surface of solution grooves; locally forms massive but to1 ledge laterally weathers ft angu­ lar b1CCk5.0e.00».®0®e00800»®...»..00»..0 6 338.5­ 31. Limestone, light olive gray (5Y5/2) grading upward to medium gray; weathers grayish to orange yellowish gray (5Y7/2); very finely crystalline; lower part has much iron oxide on angular fracture surfaces; calcite vein- lets in upper part; compact; nvb; weathers to gentle largely covered 510pe..... 3»5 335-338.5 30. Limestone, pale red (5H6/2), weathers grayish orange; very finely crystalline; abundant calcite stained with iron veinlets, oxide; subconchoidal fracture surfaces, many coated with iron oxide; compact; nvb; joints: E-W, 45°N, 3 ft; indistinct; weathers to moder­ately rough surface; forms resistant ledge..o. 2*5 o...»<>..00.0.0.0.0*. 332.5­ 29* Limestone, variegated, mainly medium light red weathers gray and pale (5R6/2), light olive gray (516/l); largely very finely crys­talline with few streaks of cal- some sparry cite; iron oxide on many fracture surfaces cal- which are more angular than conchoidal; otte veinlets; compact; thick bedded; joints: E-W, 45°N, 3 ft; weathers 1-5 ft boulders; to weathered surface scarred by solution chan­nels and very rough and sharp; marks prominent 11 steepening of slope; forms ledge ..»...©•© 321.5-.5 28. Covered zone, probably shale or thin-bedded limestone; forms moderate north 510pe.... 15 306.5­ 27. Limestone, medium gray to brownish gray (SYR4/1) with small irregular flecks of mod­ erate reddish orange limestone, weathers very cal- light gray (N 8); very finely crystalline; cite veinlets stained with iron con- oxide, and choidal fracture surfaces of which are many coated with iron oxide; compact; nvb; weath­ers to 1-4 ft angular blocks characterized by solution grooves on weathered surface; forms slight ledge 6 300.5­ 26, Covered probably shale or thin-bedded zone, limestone; moderate north slope littered with limestone debris, 12 .eoa.aa.oe.*©®©©©©©.®*.. 288.5-5 © 25* Limestone, pale yellowish brown grading up to pale reddish brown; weathers moderate orange pink (SYRB/4); very finely crystalline, and vague very finely crystalline intraclasts (Md 1.4 mm) cemented with clear calcite; cal­otte veinlets; fracture surfaces conchoidal and coated with iron oxide; compact; thin bedded (indistinct); weathers to 0.5-1 ft angular blocks; weathered surface rough; forms slight ledge •••••••«••••••••11 • 277.5-88.5 24* Covered probably shale or thin-bedded zone, limestone; forms gentle limestone debris-covered north 510pe......«©««««.9 268.5­ 23. Limestone, medium light gray grading laterally and to red weathers vertically pale (5R6/2); grayish orange pink (10R8/2); very finely crystalline, except upper foot which is one lens of limestone conglomerate, with light brown (5Y6/4) very finely crystalline matrix surrounding poorly sorted, subangular to sub-rounded, medium dark gray limestone fragments (Md 3 mm) up to 8 mm; abundant calcite vein-lets; iron oxide coated conchoidal fracture surfaces; thin to thick bedded; iron oxide concentrations on rough weathered surface; fOrmS ledge»e®*«®»®o®»®»®«®ao»a»»o»»a»*»» 10 258.5­ 22. Covered zone, thin-bedded limestone or shale; forms medium north slope ......•• • 50 208.5­ 5 21. Limestone, pale yellowish brown, weathers grayish orange; very finely crystalline; iron oxide-stained calcite veinlets; conchoidal fracture; compact; nvb; weathers to slightly rounded irregular shaped 0.5-1 ft blocks; surface generally rough with raised iron oxide coated deposits; forms slightly resist­antunat.6 202.5-208.5 20. Covered zone, probably thin-bedded limestone or shale 11 ........ 191.5­ brown with 19. Limestone, pale yellowish irregu­lar patches of pale red (5R6/2); aphanocrys­talline; conchoidal fracture; compact; nvb; joints; N5O°E, nv, 0.5-1 ft; bedding plane slickensides and silicification indicative of intraformational weathers to deformation; blocks, elongate perpendicular bedding; to surface smooth to rough; forms ledge..... 1 190.5­ 18. Covered zone, probably thin-bedded lime­ stone..... 3.5 187-190.5 1?. Limestone, pale brown, weathers yellowish gray (5Y7/2); very finely crystalline; calcite veinlets; slight oxide coating on con­weath­ choidal fracture surfaces; compact; nvb; ers to jagged irregularly shaped 1-4 ft blocks; forms resistant . 1 ........... locally ledge 186-187 16. Covered thin-bedded limestone zone, probably or shale; gentle slope..©.®.®*)®..®©.®*..© 149.5­ 15. Limestone matrix is light olive conglomerate, gray (5Y6/l), finely crystalline, com- very limestone that weathers medium dark pact gray; gravel is limestone, medium dark gray, to 1 in®; aphanocrystalline, ranging up gravel decreases weathers grayish orange; conglomerate in size and density upward; near top, dark gray limestone contains very finely crystalline scattered fragments of pale yellowish brown limestone up to granule size (Md 0.7 mm); iron oxide on fracture surfaces; has stained calcite weathers to ft veinlets; compact; nvb; 0.5-1 blocks with surface; forms resistant rough bed.»®»®»e»®9®».®**»9»»®oe®.o®o®oo»oo««»» » 5 0 149-149.5 14. Covered zone, probably thin-bedded limestone Or Shale .. ...... 30 5 .®eooe..*®®.990.»oo.a.o». 145.5­ 13. Limestone, moderate red and pale brown, weathers brown and to light (SYR6/4) grayish orange pink (10R8/2) respectively; very calcite finely crystalline; veinlets, light-colored as well as iron oxide-stained; com-weathers in. blocks pact; nvb; to angular 2-6 with rough surface; slightly resistant... 9»5 136-145.5 12. Limestone conglomerate, as in unit 15 ** *© 1 13 5-136 11. Covered zone, probably thin-bedded limestone or shale; forms rubble covered north 510pe. »0®00099..*00*»900®»90.a«9.0©0«00®. 8 127-135 10. Limestone conglomerate, as in unit 15*... 2 125-127 9* Limestone, pale red (10R6/2) grading laterally and to weathers upward pale yellowish brown; iron grayish orange; very finely crystalline; oxide stained calcite veinlets; conchoidal fracture surfaces coated with iron oxide; com­ pact; nvb; weathers to 0.5-3 ft blocks with rough, uneven surfaces; some show iron oxide break in concentrations; holds up first major Slop€.*e«.«o***.*o*.o*9e.eoeaeee»o9e>aa**e I*7*s 107*5-125 8. weathers Limestone, medium gray, yellowish gray (5Y7/2); very finely crystalline; slightly argillaceous and arenaceous with scattered fine-to very fine-grained poorly sorted, subangular quartz fragments; compact; thin indistinctly bedded; joints? N55°E, nv, 1-2 ft; weathers to o*s-1 ft angular blocks, some smooth surfaced, others with solution forms grooves and pits; only slight ledges on north slope*®o«®®o®®«»oo®c*o«e«*s**oe» 17 90.5-107*5 7* Covered slope, probably shale or thin-bedded limestone; gentle north slope..•••••••««• 5 85*5-90.5 6. Limestone, yellowish gray (5Y7/2) with grada­tional zones of light brown (SYR6/4)? weathers and yellowish gray (SYB/l) light brown (SYR6/4); finely crystalline; abundant calcite vein- very lets; some iron oxide on conchoidal fracture surfaces; compact; nvb; weathers to small sub- angular blocks whose upper surfaces show solu­ tion grooves with polygonal arrangement; raised ridges of iron oxide stained siliceous-calcareous vein filling occur on some surfaces; unit con­ sists of 10-ft covered 7-ft ledge, slope (prob­ably thin-bedded limestone), and 10-ft ledge 27 .....e.o.e.0..............0......... 58.5-85*5 5* Limestone, medium light gray; weathers pale brown; medium crystalline; very arenaceous; sub- medium-grained, well sorted, angular to angular frosted quartz grains comprise 30-40 percent of rock; iron oxide coats fracture sur­ faces; compact; thin bedded; quartz grains in sharp relief on weathered surface that also has solution forms grooves; slight blocky 0.0. ........... ledge... ..0.....e.(i.e..... 55*5-58*5 4* Covered zone, probably shale or thin-bedded limestone; gentle 510pe...... 3»5 forms north 52-55*5 3* Limestone, medium light gray; weathers yellow­ ish gray (5Y7/2); medium crystalline; argil­ laceous; calcite veinlets, larger veins of dirty yellow calcite; compact; nvb; weathers to boulders 0.5 ft to 3 ft in diameter; weathered surface generally smooth except where weathering grooves with polygonal pat­ tern occur; forms slight ledge; 28-48 ft AB largely covered; upper 4 ft medium gray, aphanocrystalline; compact; joints similar to unit 1; forms slight ledge « 29 23-52 2. Covered zone, probably shale or thin-bedded^ limestone.... 20 o ........................... 3^23 1. Limestone, light brownish gray; weathers pink­ ish gray; very finely crystalline; minute fos­iron oxide-stained calcite sil fragments; veinlets; some fracture surfaces coated with iron oxide; compact; nvb; near-vertical vague joints, N4B°E; weathers to boulders 0,5-2 ft in diameter; weathered surface smooth; forms ledge 3 ...... . ... o... . o.... .....00.0.. o 0.0 . 013 Total. 1199*5 A 1 1UV"iUm..0.e.............0...0.9...9«e.0.0....n0t 1116aSUT6d Measured Yucca F0rmati0n............ thickness, 1199*5 Total thickness, MS 1...,. 1199*5 Measured Section 2 Formation: Trachyte (Ttr) and Tuff (Ttu), Garren Group Locations U.B-15*5; west flank of Flat Top, about half a mile south of north end; lowest near exposure base of slope, east of road; Evans ranch; GSLU Series, photo no. 7-12. Measured? July 30-31? 1957, by Underwood and Yeager with hand level and 6-foot steel tape. Thickness, feet Unit Description Feet above base TERTIARY Garren Group Trachyte (Ttr) red 7. Microporphyritic quartz trachyte, pale brownish (10R6/2), light gray, yellowish gray (5Y7/2), very light gray, pinkish gray, pale red (10R6/2); weathers moderate orange pink, pale brown, grayish orange pink; aphanitic groundmass encloses micropheno­ crysts of euhedral to anhedral feldspar and some quartz, up to 2 5 mm (Md 0.5 mm); from & about 355-470 ft AB, elongate filled ves­ icles give rock pronounced flow structure; from about 570 ft AB, rock more compact than below; also conchoidal fracture; weathers to smooth surface; forms steep slope, much of which is covered; upper 25 feet forms per­sistent 1edge.............©0®©*..®.®..®..© 313 307-620 Tot a 313 1...... Tuff (Ttu) - 6. Tuff, grayish orange pink moderate orange pink (5YR7.5/3); white, grayish orange pink weathers (10R8/2), very pale orange; same; both crystal and lithic tuff; about 251 ft AB tuff is gypsiferous; only upper 12 feet is calcareous; nvb; about 260 ft AB, indis­tinct flow basal contact structure; sharp; upper gradational; laterally per- contact sistent; forms steep, west-facing, rubble-covered 510pe...........©©......®......©.. 51 256-307 weath­ 5* Gravelly sandstone, yellowish gray; ers grayish orange pink (SYR7/2); very poorly sorted; angular rounded, to coarse- grained quartz sandstone with calcareous cement; gravel fractures is quartzite, rounded-subrounded pebbles of volcanic rock, clay, chert and cobbles and boulders of limestone, Yucca-like conglomerate, Cox-like sandstone up to 3 ft; laterally per­ sistent but largely covered; forms steep west—facing slope...®®..©®.*.®©©©©©..*©©.. 12 244-256 4« Covered zone, probably tuff 182 62-244 3 . Tuff, very pale orange, white; weathers same; made slightly conglomeratic by scat­ tered granules of chert; very fine-grained; loosely consolidated; nvb; weathers to smooth, rounded surface; grades upward to 2-ft fine-grained white tuff; slightly more resistant ...o. o . e . 7 55-62 2. Covered zone, probably tuff....•••••••••©• 30-55 1. Tuff, yellowish gray; weathers same; silt size; homogeneous; finely laminated; platy; distinguished by cylinders of same material about 1 in. in diameter, 1-2 in. long, and perpendicular to bedding; tuff cylinders have small hole in center, 3 mm deep, 3 mm in diameter; unit forms moderate slope to w est.®3o o^3o Total.©©.®..©©.®. AllUViUm.*»*»®»»»»9eo»»®»o«»®e®e90o*o®9esosoo*eI10'b 10163.SUT6(1 Measured thickness, Garren Gr0up..«..».00.»...« 620 Trachyte (Ttr)..»....... 313 Tuff (Ttu)©•«•ce.o'•»*.© 307 Total thickness, Mo 2»»d®® 3i®®o*®o»©«®90o»o«®ooe»oo«ia®ooo#» 732-745 8. Covered zone, probably same as upper part Ot Unit l\-8 686-732 7. Quartz trachyte microporphyry, medium light light brownish weathers gray, gray; grayish of feld­ orange pink (SYE7/2); microphenocrysts and to spar some quartz up 3.0 mm (Md 0.75 mm) in aphanitic matrix; abundant vesicles filled with very pale orange material; compact, con­ choidal fracture; largely rubble-covered slope 39 o...... 647-686 6. Covered probably tuff or non-resistant zone, rhyolite; forms slight break in east-dipping slope. 15 .... O ...00.9 9.............O ......... 632-647 unit 5. Covered zone, probably as in 4*...... 35 597-632 red 4. Microporphyritic quartz trachyte, pale light medium gray; weathers pale red purple, (10R6/2), pale brown grayish orange pink; - aphanitic matrix encloses microphenocrysts of to 2 feldspar and some quartz up mm (Md 0.75 mm); compact; subconchoidal fracture; intensely fractured; weathers to angular but smooth-surfaced blocks; forms rough, talus-covered Slope. 0e.e0e0.0.».e0e.»..».e*0«0e*»®.*»... 72 525-597 T0ta1..e0«.e»«... 512 Tuff (Ttu) Covered 12 5 3. zone, probably tuff, west-facing slope 400-525 2. Trachyte, pale brown, brownish gray and medium light gray olivine basalt; trachyte is aphanitic, some of it is vesicular; some contains ellip­ soidal masses of calcite and an unidentified moderate yellow green mineral that ranges up to 30 mm (Md 1.25 mm); 275-320 ft AB, olivine 2 basalt; compact; joints: N45°W, 75°S¥, ft; forms blocky ledge; unit forms west-facing much of which is covered 169 • •••« slope, 231-400 1. Tuff, white, light greenish gray, and pinkish gray; weathers same; fine grained grading up to silt, then up to fine grained; friable to varied degrees; becomes calcareous upward; ft AB has dark sample at 30 5 percent minerals; largely covered with gravel 231 0-231 TotEl p25 •atttttteniit AIIUVIUHI . »DOt Hl© & SUP 6 d ».*.0.e.e.»..«e*0.»»<»...0...»09..».0. Measured thickness, Garren Group 0 ® 1037 Trachyte (Ttr. 512 Tuff (Ttu)s2s Total thickness, MS 3 1037 Measured Section 4 Formations: Tuff (Ttu), Pantera Trachyte, and Hogeye Tuff, Garren Group Location: U.4-17.0; near point where road from Palmas well (abandoned) east to Green River crosses first igneous rock; east stream bank, about 1,000 feet south of road; Evans ranch; GSLU no. Series, photo 7-13• Measured: Underwood and Cleaves with July 31> 1958, by Bruntcn and 5-foot staff. compass Thickness, feet Unit Description Feet above base TERTIARY Garren Group Trachyte (Ttr) not measured Tuff (Ttu) 6. Tuff, white to yellowish gray (5Y7/2); weathers same; largely covered except for lower 18 feet; glassy matrix encloses eu­ hedral to anhedral feldspar microphenocrysts up to 2*2 mm (Md 0.4 mm); scattered grains black metallic mineral; calcareous near base; ob- joints: S6O°E, 80°SW, 0.5-2 ft; N33°E, scure; friable; weathers to smooth, rounded surfaces; stream bed 510-512 ft AB; above this point, tuff forms gentle, west-facing covered slope that leads up to overlying resistant trachyte..... 168 499-667 iotal*e«<»«»*»i# 16 8 Pantera Trachyte (Tp) welded red 5. Trachytic tuff, pale (10R6/2), grayish red (5R4/2), pale reddish brown; weathers light brown (SYR6/4), pale reddish brown, moderate orange pink (10R7/4); eu­anhedral hedral and feldspar microphenocrysts filled range up to 3*o mm (Md 0.2 mm); elongate, conchoidal fracture; vesicles; compact; joints: N7O°E, 86°N¥, 2-3 ft; S3O°E, 66°S¥, 3-6 ft; weathers to smooth surface; forms distinctive cap and dip slope of low, west-facing hog­back 39 460-499 4. Trachytic welded tuff, medium light gray, light dark medium weathers brownish gray, gray, gray; gray, grayish orange pink (SYR7/2); angular of to microphenocrysts feldspar up 4mm (Md 0.25 mm) in aphanitic matrix; scattered grains of black metallic mineral as well as iron orange oxide; indistinct flow structure; compact, ex­cept lower 2-3 ft; nvb, but largely covered; 1-2 joints: N7O°E, 70°N¥, 1-2 ft; S45°E, 67°SW, ft; weathers to smooth surface; forms largely covered west-facing slope beneath resistant above 6 cap 454-460 Total 45 Hogeye Tuff tuff Upper (Thtu) 3. Tuff, light gray, light greenish gray (SGYB/l), weathers yellowish gray (SYB/l); same; largely covered terrace but at ft AB by gravel, 347 and 347 ft Ab are of exposures friable, sugary, silt-size tuff; at 347 ft AB tuff is finely laminated; no visible phenocrysts in aphanitic matrix; at 365 ft AB, calcareous tuff with abundant clear calcite to spheres up 0.5 mm (Md 0.15 mm); weathered crust firm but rock friable beneath; joints at 320 ft AB: N7O°W, 80°SW, 1-6 in.; N4O°E, 85°NW, 2-4 in.; joints at 347 ft AB: S6B°E, 80°SW, 1-4 in.; forms dip slope to east to stream channel, then west facing slope up to more resistant tuff of units 4 and 5 251 . 203-454 Middle trachyte (Thtr) 2. Rhyolitic-trachytic welded tuff, pale red (10R6/2), pale yellowish brown, pale red (5R6/2), grayish orange pink (SYR7/2), light brown (STR7/2), light brown (SYR6/4); weathers pale red (10R6/2, 5R6/2), grayish red (5R4/2); aphanitic matrix enclosed microphenocrysts of feldspar and some quartz up to 1.75 mm (Md 0.3 mm); elongate vesicles mark indistinct flow structure; vesicles filled with light- colored material; nvb; compact, brittle; in­tensely fractured; calcite-fidled fractures; subconchoidal fracture; lower 18 ft forms distinct ledge; another, 120-172 ft AB; 172 ft AB is top of west-facing ledge.... 171 32-203 Lower tuff (Thtl) 1. Covered zone, steep west-facing rubble- covered slope extending up from stream bed; probably tuff........... 32 ............. 0-32 Tuff Total, Hogeye 454 Alluvium. .not measured ......... Measured thickness, Garren Group..., 667 Tuff (Ttu)............. 168 Pantera Trachyte (Tp) 45 Hogeye Tuff., 454 Upper tuff (Thtu).. 251 Middle trachyte (Thtr).. 171 Lower tuff (Tht1)...... 32 . Total MS 4 66? thickness, ...... ...... Measured Section 5 Formations; Benevides Formation, Finlay Limestone 100 north of Indio road at Location; S.B-15.8; yards Pass westernmost outcrop of Finlay along road; a east-northeast of west- a quarter of mile steep, Evans GSLU facing scarp of Cox Sandstone; ranch; Series, photo no. 3-127* Measured; August 12-13, 1958, by Underwood and Cleaves with Brunton and 5-foot staff. compass Thickness, feet Unit Description Feet above base CRETACEOUS Limestone ....not measured Espy Benevides Formation 21. Sandstone, very pale orange and pinkish weathers light brownish gray and very gray; - pale orange pale yellowish brown; quartz, fine-to medium-grained, angular to sub- angular, poorly moderately sorted, to frosted and polished; intergranular iron oxide; calcareous cement; compact; thin and cross bedded; joints; E-W, 80°S, 2 ft; lower part has hardened surface; upper part less hard; contact with overlying Espy Limestone covered; unit 21 could be as much as 40 ft thick; laterally persistent angular ledge 2 5 * 497-522 - 20. Sandstone, very pale orange pale yellowish brown, weathers pale brown light brown - (5YR5.5/3); quartz, fine-grained, moderately- to well-sorted; subrounded-subangular; largely frosted; abundant intergranular iron oxide; somewhat argillaceous; calcareous ce­ment; abundant calcite-cemented fine-grained nodules 12 quartz up to cm (Md 5 cm); roughly spherical; randomly scattered; somewhat more abundant near base; unit has no visible bedding; moderately soft at base; more compact upward; calcite-filled fractures; joints in­ distinct; weathers to smooth, rounded surface on which nodules form irregularities; laterally persistent 8 489-497 19• Covered Benevides Finlay contact zone, ­covered; strike valley within this unit; gully at 440 ft AB; probably siltstone with inter- bedded thin-bedded sandstone near top 88 401-489 Total 121 Finlay Limestone 18. Limestone, medium light gray and light brown­ ish gray, weathers very pale orange, pale yellowish brown, pale brown; very finely crystalline limestone with irregular, scat- and of lime­ tered patches stringers coarser to coarser stone, grading upward crystalline limestone; uppermost part has numerous irreg­ular of iron oxide patches (Md 0.25 mm); rudistids-caprinids abundant base; 382­ near 390 ft AB, rudistids-caprinids replaced or stained by iron oxide; float: radiolitids, oysters; calcite-filled veins common; thin to thick bedded; joints: E-¥, 75°S, 1-3 ft; weathers to slightly rounded blocks with karrenfelder surface; forms persistent ridge, and east-facing dip slope into strike valley to Benevides Formation contact......® 30 371-401 17* Covered zone, probably siltstone or thin- bedded limestone; forms bench between re­ sistant limestone units 7 364-371 16. Limestone, medium light gray, weathers pale yellowish brown; coarsely crystalline grading up to medium crystalline limestone; calcite- filled veins and scattered patches of coarse scattered in calcite abundant; pelecypods upper part; uppermost surface abundant iron oxide-replaced rudistids-caprinids; thin bedded; joints: N5O°E, BO°SE, 1-4 ft; weathers to rounded blocks with rough, karren­ felder surface; forms laterally persistent ledge 9 355-364 light brownish medium 15. Limestone, sandy; gray, light gray, weathers grayish orange pink brown - (51R7/2) and pale yellowish grayish orange; medium to coarsely crystalline lime­stone with fine-to medium-grained, subrounded, poorly sorted quartz; abundant intergranular patches of iron oxide; calcite cement; grades upward to darker, less sandy medium crystalline limestone with patches of iron oxide; float, Tylostoma sp., Nerinea sp., Monopleura sp., unit Toucasia sp., Pervinquieria sp.; compact; thin bedded; joints: N2O°E, 80°N¥, 0.5-1 ft; also N4o°¥, 54°S¥, 0.5-1 ft; unit largely to smooth covered; exposed part weathers nearly surface; forms slight ridge 40 315-355 14. Limestone, medium light gray, light brownish gray; weathers very pale orange, very pale - orange grayish orange; very finely crystal­calcite-filled veinlets and line limestone; intervals larger fractures; many covered fossiliferous, with considerable replacement by iron oxide; fossils, lowermost limestone! Monopleura sp., Pecten (Neithea) sp., oysters; fossil float above lowermost bed, Toucasia sp., Pecten (Neithea) irregularis, Monopleura sp., . Requienia sp., Nerinea sp , Tylostoma sp., EoradioLites Enallaster compact; sp., sp.; thin bedded; joints! N73°¥, 50°S¥, 0.5-1 ft; also N-S, 6l°¥, 2 ft; weathers to angular blocks with karrenfelder surface; forms ledge and 20° dip slope to ea5t........... 17 298-315 13. Covered zone, probably siltstone or thin- bedded nodular limestone; forms 25° slope into bench 16 grading 282-298 medium medium 12. Limestone, light gray, gray, and light brownish gray; weathers pale yellow­ish brown - grayish orange, grayish orange pink (SYR7/2), and very pale orange; very fossiliferous finely crystalline limestone; compact; most abundant microfossil, Dictyoconus walnutensis; occurs at base in thick ledge with scattered oyster fragments; joints: N4O°E, 80°NW, 2-8 ft; also E-W, 70°S, 1-6 ft; also Nl5°W, 69°SW, 2-4 ft; 233*5-238 ft AB, massive bed with abundant walnutensis; joints: JD. N25°W, 62°SW, 3 ft; also S75°E, 85°NE, 1-6 ft; 240-243*5 ft AB, thin-bedded limestone weath­ers to crumbly ledge of 4-6 in. blocks and slabs showing extreme karrenfelder surface; abundant fossil fragments in lower part; throughout are iron oxide-impregnated rudistids-caprinids; overlain by thin-thick bedded karrenfelder surfaced limestone with - iron oxide-stained or replaced monopleura; highest D. walnutensis occurs about 268 ft AB; above 264 ft AB thin-bedded limestone, much of it covered, containing rudistids-caprinids, iron oxide-stained or -replaced, and microfos­ sils (miliolids?); marked karrenfelder sur­faces; forms 25° dip slope; lower part forms crest of cuesta 58 224-282 11. Limestone, medium gray, light brownish gray; weathers very pale orange; very finely crys­talline with small patches of coarse calcite; unidentified microfossil fragments; upper 1 ft contains scattered accumulations of Dictyoconus walnutensis; 3 ft from top Exogyra sp. occurs; macrofossil float: Pecten (Neithea) sp., clams, Exogyra texana, Alepes sp., Nerinea sp., Engonoceras sp., Tylostoma sp. occurs at 215 ft AB; compact; nvb; largely covered; weathers to nodules; forms 15° west slope plus 2-3 ft resistant ledge at top.. 31 193-224 10. Limestone, light brownish gray, medium dark and medium weathers gray, gray; very pale orange -pale yellowish brown; very finely crystalline intraclastic limestone containing foraminifers; much iron oxide on fracture surfaces in zone about 180 ft AB; macrofossil float: Tylostoma sp., nerenid, Nerinea sp. Alepes sp., gastropods, clam; zone of Mono- pleura sp., 185-193 ft AB; compact; thin thick bedded; joints: N75°E, 66°SE, 3-4 ft; oriented also N2O°W, 76°S¥, 1-2-3 ft; randomly karrenfelder rough calcite veinlets; surface; blocks form prominent ledge 19 174-193 9. Limestone, pale yellowish brown grading upward to medium weathers and gray; very pale orange medium gray; very finely crystalline limestone; lower part contains foraminifers; fossil float: Tylostoma sp., Alepe s sp., Toucasia texana , caprinid, Tetragramma? sp., oyster, gastropod; compact; thin bedded; calcite veinlets; weath­smooth forms ers to surface, 35° slope except for few blocky ledges 17 157-174 8. weathers Limestone, medium light gray, yellow­ ish (517/2); finely crystalline with abundant patches of calcite, shell frag­ gray very coarse ments and foraminifers; compact; nvb; joints: N65°E, 72°SE, 3 ft; also N35°E, 72°NW, 15 ft; weathers to nodules near base, less so upward; slight karrenfelder surface; few calcite filled fractures; forms 1edge.................... 8 149-157 7. Limestone, medium dark gray; weathers yellow- to ish gray (5Y7/2); aphano-very finely crys­ talline limestone with minute flecks of coarse calcite; contains microfossil shell fragments plus echinoid spines; macrofossil float: Exogyra texana, Toucasia texana Requienia sp., t caprinid, oyster, Enallaster texanus, Enal­laster sp.; compact; nodular; nvb; gradational contact with overlying massive unit; forms largely covered 35° 510pe.... 19 130-149 6. Limestone, medium dark gray; weathers very pale orange; very finely crystalline containing abundant microfossil shell fragments; brittle; very thin bedded; weathers to rounded surface; forms ledge 5 125-130 medium 5. Limestone, light gray, medium gray; - weathers pale yellowish brown grayish orange, unit is few very pale orange; largely covered; exposures very finely crystalline compact are limestone containing scattered foraminifers and abundant shell fragments less than 0.5 mm; uppermost 1 ft is zone of Exogyra texana; fos­sil float below: Proto cardia? Protocardia sp., texana, Exogyra texana, Tylostoma sp., Engonoceras sp., Enaliaster texanus, Enallaster sp.; unit forms steepening slope of about 20° with thin- sparse exposures; probably compact, bedded limestone 72 53-125 4. Sandstone, grayish orange pink (SYR7/2), grayish orange; weathers very pale orange, pale yellow­ish brown grayish orange; fine-grained, poorly - to moderately sorted, subrounded to subangular, largely frosted quartz with calcareous cement; iron lower intergranular oxide; argillaceous; 5 ft somewhat less resistant than upper 3 ft; thin to thick bedded; upper 3 ft cross bedded, Indicating current source of N2O°E; oscillation ripple N45°W, joints: S65°E, marks with strike 1-3 ft; also NlO°E, 1 ft; weathers to nv, nv, angular blocks; upper 3 ft forms ledge.... 8 45-53 3. Covered probably siltstone or thin-bedded zone, limestone; forms 15° west 510pe.... 15 30-45 2. Limestone, medium light gray, weathers very pale orange, pale yellowish brown; very finely crys­talline limestone with shell fragments averaging less than 0.25 some iron oxide accumulation on fracture surfaces; compact; subconchoidal fracture; nvb; joints: N65°E, nv; most are filled with calcite; weathers to 2-6 in. nodules; largely covered; ledge-former 10ca11y..... 21.5 8.5-30 mm; al­ most fissile; breaks into angular fragments 2 mm thick; 1 in. caliche-like zone separates unit from Cox Sandstone below; unit largely forms 1. Siltstone, yellowish gray (5Y7/2); brittle, covered; 35° slope 8.5 0-8.5 Total * 401 Cox Sandstone .not measured 121 Measured thickness, Benevides F0rmati0n........ Limestone 401 Finlay Total thickness, MS 5 522 Measured Section 6 Formations: Buda Limestone, Eagle Mountains Sandstone, Espy Limestone Location: T.4-16.3; base at section is in a strike valley between Finlay and Espy formations, almost a mile south of intersection of Indio Pass road Evans and William and outcrop of Espy Limestone; son ranches; GSLU Series, photo 3-99* no. Measured: Underwood and Cleaves with August 18-19, 1958 by Brunton and 5-foot staff. compass Thickness, feet Unit De script ion Feet above base CRETACEOUS Finlay Limestone (thrust c0ntact)..............n0t measured Buda Limestone 28. Covered level; covered with zone, orange-stained limestone debris; at 1390 ft AB, rough, black Finlay Limestone overlies Buda Limestone in thrust relationship 15 1375-1390 - brownish olive 27* Limestone, light gray light gray, light brownish gray; weathers grayish - orange, very pale orange pale yellowish with brown; very finely crystalline abundant masses of coarser calcite with diameter 0.075 mm or less; almost lithographic; up­ward contains abundant cellular fossil frag­ - ments or near coral, algae, bryozoa; top, abundant microscopic fragments of shells; rock less homogeneous upward; iron oxide coats some fracture surfaces; weathers to indistinct nodular ledges separated by less resistant some karrenfelder surfaces; zones; crest of last hill along section at 1355 ft AB . 46 1329-1375 Covered thin-bedded 26. zone, probably nodular limestone; forms saddle and lower part of east slope to 26 1303-1329 25. Limestone, light brownish gray, light brown- olive weathers - ish gray light gray; very pale orange pale yellowish brown, yellow­ - ish gray (517/2); finely crystalline very with microscopic fragments of shells; fossil float, unidentified solitary coral; compact; conchoidal thin bedded; calcite fracture; veins and veinlets; weathers to smooth nod­ ules which largely cover dip slope to east 65 1238-1303 24» Limestone, light brownish gray, weathers unit covered ex- very pale orange; largely is cept for 1-ft ledge near middle, which basis for description and assumed to be rep­ resentative of covered portion; very finely crystalline; compact; conchoidal fracture; weathers to relatively smooth nodules; forms crest of small hill covered by nodular rubble 20 1218-1238 23. Limestone, light brownish gray (SYR6/l), weathers grayish orange pink (SYR/2); very finely crystalline with numerous microscopic fragments of gastropods, pelecypods, and other fossils; fossil float, lower 23 ft: Pecten (Neithea) sp., Alectryonla cf. carinata Turritella Hemiaster , sp., sp., approximately 1195 ft AB, 1 ft ledge yielded Budaiceras frechi; unit consists of nodular ledges with intervening non-resistant covered calcite veins and veinlets zones; common; compact; thin bedded; weathers to smooth sur­ face; contact with underlying Eagle Mountains Sandstone covered.... 46 1172-1218 Total 218 Eagle Mountains Sandstone 22. Sandstone, yellowish gray (5Y7/2), pale yellowish brown; weathers pale yellowish brown - - brown grayish orange, pale yellowish very pale orange; quartz, very fine-grained, moderately-to well-sorted, subangular to sub-rounded, largely polished; siliceous cement near base; calcareous cement upper part; inter- granular iron oxide; dendrites on some fracture surfaces; compact; largely covered, but must be very thin bedded; forms gentle slope littered with platy fragments that are relatively smooth; some are desert varnished; some exhibit burrows or trails 7 1165-1172 21. Sandstone, yellowish gray (5Y7/2), pale yellow­ish brown grayish orange; weathers pale yel­- lowish brown, pale brown, grayish brown; quartz, very fine-grained, moderately sorted, subangu­lar, frosted and polished with abundant inter- granular iron oxide; calcareous cement; com­ pact; laminated; unit largely covered but probably very thin bedded; weathered surface smooth; characteristically desert varnished; on some are pseudomorphs of iron oxide after with pyrite; forms west-dipping slope littered sandstone fragments 15 ........... 1150-1165 20. Sandstone, grayish orange grading upward to pale yellowish brown; weathers grayish - orange very pale orange, very pale orange; quartz, very fine-grained well-to moderately-sorted, subrounded, polished and frosted, with abundant intergranular iron oxide; calcareous laminated and cement; compact; cross laminated; but thin largely coarse probably very bedded; desert varnish smooth weathered sur­ common; faces exhibit scattered clam impressions, one of which resembles Inoceramus unit forms near-level surface littered with 0.5-2 in. sp.; angular sandstone fragments 14 . 1136-1150 19* Covered zone, probably very thin-bedded sand­ stone; surface littered with desert-varnished sandstone fragments 7 1129-113£ 18. Siltstone, grayish orange grading upward to very fine-grained sandstone, pale yellowish brown grayish orange; weathers pale brown - - grayish orange pink, and grayish orange; quartz, subrounded, moderately-to well- sorted, largely frosted; argillaceous; com- weathered pact; laminated; surface smooth; one weathered surface parallel to bedding is covered with pseudomorphs of iron oxide after pyrite, averaging 0.15 mm; forms near hori­zontal surface... 5 1124-1129 17. Covered as in unit 19 8 1116-1124 zone, 16. Sandstone, pale yellowish brown, pale yellow­ish brown grayish orange; weathers pale- yellowish brown, grayish orange; quartz, very fine-grained, moderately sorted, subrounded, largely frosted; calcareous cement; somewhat argillaceous; contains Haplostiche texana; Exogyra cartledgei; clams averaging 10 mm long occur on bedding planes near middle of unit; compact; laminated, cross laminated; very thin bedded; weathers to smooth surface, much of which is desert lower 1 ft forms varnished; slight ledge 11 1105-1116 - • 15 Sandstone, pale yellowish brown grayish orange, grading up to pale yellowish brown gastropod hash with much iron oxide staining on fracture surfaces, then to light, brownish gray -light olive gray and grayish orange sandy limestone, top pale yellowish and at to brown sandstone; weathers over-all grayish orange; lower sandstone is very fine grained, moderately sorted, subrounded, largely frosted quartz with calcareous cement and platy shell fragments, probably pelecypods; overlying hash 1-2 ft and composed of turritellid-like gastro­ pods in medium crystalline limestone matrix; to to medium grades upward sandy very finely crystalline limestones with scattered pelecy­ pod fragments; at about 1101 fine- ft AB, very grained platy sandstone contains Haplostiche texana which weather differentially on ex­ posed bedding surfaces; just above is zone of Exogyra cartledgei; compact; very thin bedded and laminated; contact with underlying Espy Limestone covered; much of unit covered with limestone debris 11 1094-1105 Total 78 . Limestone Espy 14-Limestone, light brownish gray, pale yellowish brown; weathers grayish orange, very pale orange; very finely crystalline limestone with iron oxide coating subconchoidal fracture sur­ faces; calcite veins and veinlets; fossil float, in lower rudistids-caprinids part, Gryphaea washitaensis, Turritella sp., Alepe s sp., Tylostoma and Enallaster sp. above; thin­ sp., thick bedded; compact; joints: S7O°E, nv, 4-6 81°NW, 3 ft; weathers to angu­ ft; N24°E, lar blocks with karrenfelder surface; forms low ridge above low-lying outcrop of Eagle Mountains Sandstone to east 123 971-1094 13 . Covered zone, probably thin-bedded limestone; forms bench with gentle west slope above first ledge, east bank of gully; fossil float, Pecten (Neithea) sp., Pholadomya sancti-sabae; caprinids; Tylostoma sp., Turritella sp., Enallast er 31 sp 940-971 12. Limestone, light brownish light olive gray ­weathers gray; very pale orange; very finely crystalline with numerous microscopic shell fragments, pelecypods some of which are or gastropods; some iron oxide stains; calcite veinlets; compact; nvb; joints: S7O°E, 86° S¥; weathers to nodules; slight karrenfelder first weathering; forms distinct ledge, east side gully... 5 935-940 11. Covered thin-bedded zone, probably limestone; extends down 10-15° slope to gully at 900 ft AB, and across to first ledge, east bank; fossil float, east bank: Pecten (Neithea) Pecten sp., (Neithea) georgetownensls » Gryphaea washitaensi s, Tylostoma sp., Turritella Enallaster 120 sp., sp 815-93 5 10. Limestone, light brownish gray, medium light gray; weathers very pale orange, very pale - orange pale yellowish brown; very finely with intraclastic texture crystalline common; matrix is coarser calcite; microscopic frag­ments of shells occur near bottom and top; fossil float, lower 15 ft: Pecten (Neithea) Enallaster sp., Alepes sp., sp., Tetragramma sp.; approximately 735-770 ft AB, large calcite some iron oxide on rudistids; veins; fracture surfaces; compact; thin-thick bedded; joints: S5°E, 55°S¥, 2 ft; N4O°E, 62°NW, 6-8 lower part weathers to nodules; ft; upper part weathers to large angular blocks; karren­felder surfaces; forms dip slope of small ridge east of high ridge to west 110 705-815 9. Covered zone, probably thin-bedded limestone; small 1 ft above base of ft exposed ledge 14 unit is pale yellowish brown, very finely crys­talline fossiliferous limestone; shell frag­ ments to 8-10 fossil float forms range up mm; unit! Pecten (Neithea) roemeri, Pecten (Neithea)georgetownensis, nerinid, heart clam, Turritella? Tylostoma sp ., sp., Alepes? sp., Salenia Enallaster sp., sp., Tetragramma sp.; Drakeoceras drakei found on slope within 5 ft of base of unit, Cymatoceras found on slope about midpoint of unit 35 670-705 8. Limestone, medium light gray; weathers gray­ - ish orange very pale orange; very finely crystalline lowermost and uppermost parts abundantly fossiliferous; shell fragments from 0.15 mm-20 range mm; apparently pelecy­pods; some iron oxide accumulation on frac­ ture surfaces; calcite veins; dense; compact; thin-bedded; nodular; moderately resistant unit; forms dip slope 6 664-670 ?. Limestone, pale yellowish brown, medium light gray, medium gray, light brownish gray, and medium dark gray; weathers pale yellowish - brown very pale orange, very pale orange, pale yellowish grayish orange; - brown lower 11 ft covered but like probably upper part; rocks very finely crystalline; lower exposed contain considerable iron oxide in masses averaging 0.4 mm; relatively unfossiliferous alternate with zones of abundant micro- zones fossil float lower scopic fragments of fossils; 12 ft: Lima cf. L. wacoensis, Gryphaea wash­itaensis , Pecten Theithea) georgetownensis, Pecten~XNeithea) sp., gastropod, Drakeoceras? Enallaster sp., Holectypus cf. H. planatus, sp., Holectypus Salenia fossil 585­ sp., sp.; float, 615 ft AB: Pecten (Neithea) georgetownensis, Pecten (Neithea) sp . Gryphaea washitaensis , , Lima cf. L, wacoensis, Pholadomya sp., Tylostoma sp., Alepes sp., Volvulina sp., Tetragramma sp., Salenia Enallaster sp., sp., Pedinopsis sp.; fossil above 615 ft AB, Alepes? float, sp., Enallaster sp.; foraminifers occur in rock Just east ridge crest at about 633 ft AB; compact; thick about ft AB: thin to bedded; Joints, 588 S5B°E, nv, 1 ft; about 633 ft AB: N2o°¥, 75°S¥, 1-2 ft; N7B°¥, nv, 2-3 ft; nodular; forms series of nodular ledges; main ridge crest at 615 ft AB with east dipping slope beyond; on dip slope, karrenfelder surface common.... 119 545-644 6. Limestone, light brownish gray, medium light gray; weathers very light gray, very pale orange; very finely crystalline; lower part and about 5-15 ft AB have iron oxide-stained calcareous shell fragments: unstained calcite veinlets and veins; abundant pelecypods lower 4-5 ft; 458-461 ft AB, fossil float: Nerinea? sp., Holectypus sp.; 485-515 ft AB, fossil float: Pe ct en (Neithea) georgetownensis, Pholadomya cf. sancti-sabae, Tylostoma sp., Holectypus sp., Enallaster sp.; 523-532 ft AB, fossil float: Gryphaea washitaensis , Pervinquierea Enallaster Hemiaster sp., sp., sp., Holectypus cf. H. planatus: thin to near thick bedded; compact; Joints base: NB2°E, 83°NW, irregular; forms ledge and overlying 35-40° west-dipping slope, much of it covered; rough, irregular surface...... 87 458-545 5. Limestone, medium light gray, medium gray, light brownish weathers gray; very pale orange and grayish orange pink; very finely crystalline; more homogeneous upward; calcite veins and veinlets; compact; thin-bedded; weathers to moderately rough surface; forms 30°-35° slope covered by 2-6 in. limestone fragments; some larger blocks; forms slope between lower­ most massive ledge and overlying massive units 61 397-458 4* Limestone, brownish gray -light brownish gray; weathers pale yellowish brown brown - yellowish very pale orange; finely crystalline; upper part very calcgeneous; ite veinlets; compact; thick bedded; joints: S59°E, 63°SW, N55°E, 75°NW, 0.5-3 ft; karrenfelder and pale very homo-thin to 1-4 ft; weath­ ered surface; forms first major distinct ledge on west slope 23 374-397 3. Limestone, medium dark gray grading upward to medium gray, weathers grayish orange - very pale orange; very finely crystalline; calcite veins common; homogeneous; compact; thin to thick bedded; grades into more massive overlying unit; slight degree karren­ felder weathering; weathers to angular­subangular pebbles, cobbles, boulders; 30° slope largely covered; some resistant ledges out in 89 crop upper part 285-374 2. Covered 25°-30° covered with sub- zone, slope rounded pebbles and cobbles of gray limestone; some boulders; probably shale interbedded with thin-bedded limestone such as at 210 ft AB; medium light gray, very finely crystalline, compact, homogeneous 192 limestone 93-285 1. Limestone, yellowish gray, light brownish gray -light olive gray, medium light gray; weathers - very pale orange, grayish orange very pale orange; very finely crystalline limestone; at 30 ft AB, microfossils occur averaging 0.35 Colonial coral, Mortonlceras n. P. equidistans; at about ft AB float: mm; 85 Pecten (Neithea) texanus, sp., and Pervinquieria aff. iron oxide coats fracture surfaces midway of unit; compact; becomes more homogeneous upward; thin bedded; nodular; some 0,5”! ft resistant ledges; contact with under­lying Benevides covered as is lower 30 ft; may be marl or silt stone ... 93 0-93 Total 1094 Benevides Formation .not measured Measured thickness, Buda Limestone 218 Eagle Mountains Sandstone.. 78 Espy Limestone..,.. 1094 Total thickness, MS 6 1390 Measured Section 7 Formation: Powwow Conglomerate Member, Hueco Limestone Location: L.2-15*2; northeast slope of easternmost hill of Permian about feet west of road from rock, 1,200 Hot Wells to Eagle Mountain ranch; dip is north- section about a third of the east; began way up­slope and measured downslope; Wyche ranch; GSLU Series, photo no. 4-80. Measured: April 23, 1959# by Underwood and Hamilton with Brunton and 5-foot staff. compass Thickness, feet Unit Feet above base De scription Alluvium. .not measured . PERMIAN Member Powwow Conglomerate 19. Sandstone, light gray; weathers to pinkish gray; thin bedded; fine grained; polished to and frosted, angular subrounded, poorly cad- sorted, closely packed quartz grains; die cement: compact; about a foot exposed at base of northeast slope; covered by valley fi11.................. 2 ........... .. 165.9-167.9 18. Covered probably shale or thin-bedded zone, limestone; forms northeast-sloping limestone rubble-covered surface 2.4 ........... 163.5-165.9 17* Calcarenite, over-all light olive gray with (5Y6/l) pale red (5R6/2), grayish red (5R4/2) and dark gray fragments: weathers to medium thin bedded: light gray; very coarse subrounded to grained, rounded, moderately well sorted, closely packed limestone fragments with scattered medium-grained, angular, pol­ished quartz grains; scarce gravel-size lime­stone fragments; compact; weathers to rounded surface; unit largely covered 4*5 159-163.5 16. Covered probably shale; limestone zone, rubble covers 30° slope to northeast 15 144-159 15. Sandstone, conglomeratic, very light gray; weathers light olive gray (5Y6/l); cross bedded and thick bedded; largely coarse, to angular subrounded, poorly sorted, frosted and polished quartz grains; con­tains grayish red (5R4/2) and dusky yellow green, coarse-to granule-size (Md 3*o mm) mudstone and coarse-to pebble-size black chert; becomes less conglomeratic upward; calcite cement; compact; well indurated; weathers to rounded surface 6 138-144 Covered shale 14. zone, apparently grayish pink to mudstone with calcareous nodules; forms 30° slope to northeast 11 * 127-138 medium dark to medium 13. Limestone, gray gray; weathers yellowish gray (5Y7/2); finely crystalline; arenaceous; compact, with sub­conchoidal fracture; thin to thick bedded; to 1 abundant joints? N65°E, nv, 4-in. ft; becomes nodular upward; irregular patches of limestone....... grayish yellow 3 >3 123.7-127 12. Limestone, medium light gray; weathers yel­lowish gray (5Y7/2); very finely crystalline; with semi-conchoidal iron compact fracture; oxide coating on some fracture surfaces; highly fractured near base becoming less so upward; weathers to blocky surface near base - to more rounded surface upward; forms 30° northeast 8 510pe.... 115.7-123.7 11. Covered zone, probably shale; forms 25° rubble-covered slope to northeast 7 108.7-115•7 10. Calcirudite; medium light gray with clastic fragments of grayish red (5R4/2), dusky yellow green gray; and medium dark weathers light olive (5Y5/2); thin bedded; gray very sorted coarse to granule grained; moderately particles nearly equidimensional, rounded, closely packed with calcareous cement; few thin calcite veins parallel bedding; forms 2.7 slight ledge 106-108.7 to 9. Sandstone, very light gray; weathers yel­lowish gray (5Y7/2) with iron oxide stain on weathered surfaces; medium grained; angular to subangular; moderately well sorted; many grains lower 2 are frosted; argillaceous; ft marly; compact; thin bedded; highly fractured; weathers to smooth surface; forms 25° slope to northeast 7 99-106 8. Limestone, sandy, medium dark gray, weathers light gray; medium to coarsely crystalline; sand grains, subrounded-rounded fine-to medium-grained quartz; becomes argillaceous upward; compact; conchoidal fracture; laminated at base becoming thin bedded upward; weathers to smooth surface; forms slight ledge 4 95-99 7. Limestone, medium dark gray, weathers medium light gray; finely crystalline; slightly sandy; contains localization of iron dark oxide, yellowish orange; compact; fracture; conchoidal thin bedded; weathers to slightly rough sur­ face; forms slight ledge 2 93-95 6. Covered probably limestone; forms nodular zone, limestone-covered 30° slope to northeast.. 13 80-93 5- Limestone, dark gray, weathers to medium light gray; finely crystalline; grades upward to very finely crystalline; compact; conchoidal fracture; some minute veinlets filled with secondary calcite; thin bedded; weathers to slightly nodular surface; 30° slope forms to northeast 10 70-80 brown but 4• Limestone, commonly pale yellowish and also variegated with grayish yellow green olive limestone intimately associated with light irregular concentration of grayish red purple (SRP4/2) mudstone; finely to non-calcareous fractures medium crystalline; compact; minute filled with sparry calcite; weathers to irregu­lar surface; forms slightly blocky 30° slope to NE 20 50-70 red thin con­ 3. Siltstone, pale (5R6/2); bedded; tains abundant pinkish gray calcareous nodules, subround to roughly spherical, subangular, 0.5 mm 35 mm maximum diameter (Md 5 to 10 mm); forms 30° slope to NE, largely covered.... 8 42-50 - 2. Limestone, medium light gray with irregular concentration grayish red (5R4/2) of argilla­ ceous limestone; micrite weathers over-all light gray; very finely crystalline; compact; conchoidal fracture; weathers to smooth surface; forms 30° slope to northeast......... 15 27-42 1. Covered forms to northeast zone, 30° slope covered with angular, callche-covered limestone rubble 27 0-27 Total 167*9 Hueco Limestone (fault contact). not measured Measured Powwow thickness. Conglomerate........ 16?.9 ............. Total thickness, MS ?••••••• 167.9 Measured Section 8 Formation: Benevides Formation Location: M. 6-14-3; isolated northeast-facing scarp just northeast of Wyche Ridge and about three quarters of a mile northwest of old Yarbro wells; Wyche ranch; GSLU Series, photo no. 4-55­ Measured: May 6, 1959* by Underwood and Hamilton with Brunton and 5-foot staff. compass Thickness, feet Unit Description Feet above base TERTIARY 5. Lower rhyolite sill, aphanitic except for minute phenocrysts of potash feldspar; resistant.... 25 54-79 CRETACEOUS Benevides Formation 4-Sandstone, yellowish gray (5Y7/2), light olive gray (5Y6/l), white, pinkish gray; weathers yellowish gray (5Y7/2), grayish orange pink (SYR7/2), pinkish gray, pale red pale brown (10R5-5/3); reddish fine­ - to grained, poorly sorted, subangular sub- rounded, largely frosted quartz; argilla­calcareous to ceous; cement; grades upward sandstone, fine-to medium-grained, moderate- to well ly sorted, subangular to subrounded, frosted and polished quartz; clay and cal­careous cement decrease near upward; top, widespread iron oxide stain, siliceous and iron oxide cement, grains polished; thin bedded; weathers to fairly smooth surface; forms series of small 1edge5.............. 23 31" 54 3-Sandstone, calcareous; medium light gray; weathers olive light gray (5Y6/1); ranges to but from fine-coarse-grained, largely sorted fine-grained, poorly (bimodal) subangu­lar to subrounded, frosted and polished quartz; a few shell fragments, probably pelecypods; well faint indurated, thick bedded, cross bedding; 2-in. siltstone break midway; weathered sur­ forms face smooth; blocky 1edge........•.• 3»8 27.2-31 2. Sandstone, limey; medium gray, medium light weathers same as gray; very pale orange; unit 3 except no trace of shell fragments; f upper part harder than lower; in lower part, abundant iron oxide stain 1.2 ................. 26-27.2 1. olive Siltstone, sandy; gray (5Y4/l)> black; weathers sand is yellowish gray (5Y7/2); very fine-grained, moderately sorted, subrounded lower 22 ft contain about nine beds quartz; of nodular sandy limestone, which average 1 ft thick or less; nodules are rounded; aver­ age 4-6 in.; limestone is medium light gray, weathers yellowish gray (5Y7/2) and is very finely crystalline; sand is fine-grained quartz; unit forms 45° slope; nodular zones out as 26 crop slight ledges Total 54 .not measured Alluvium. Measured Benevides F0rmati0n........ 54 thickness, ....... Lower rhyolite sill. 25 8...... Total thickness, MS 79 Measured Section 9 Formations: Bluff Formation, Yucca Formation Location: 5.7-14*7; base of section is just east of Indio fault and north of road from Indio ranch house to Squaw Spring; about three-quarters of a mile north-northwest of the Indio ranch house; Evans ranch; GSLU Series, photo no. 3-125• Measured: June 8-12, 1959, by Underwood and Kirksey with Brunton and 5-foot staff. compass Thickness, feet Unit Feet above base Description CRETACEOUS Cox Sandstone ....not measured Bluff Formation 65* Limestone, medium light gray, light olive medium dark and medium gray (5Y6/l), gray gray; weathers pale yellowish brown, light gray, and yellowish gray (SYB/l); very finely crystalline limestone; compact; subconchoidal fracture; thin bedded; nodular; 2708-2710 ft AB small pelecypod fragments; float, 2710­2740 ft AB : Porocystis globularis, Tapes sp., clams; 2752 ft AB, Orbitolina texana, but not abundant; first abundant ft , zone, AB;float, 2740-2765ft AB: Anatinasp., Trigonia sp., Tapes sp., Pect'en (Neithea) sp., Cyprlmerla sp., Tylostoma sp., unidentified pelecypods, Porocystis globularis; float, 2765-2781: Anat ina Trigonia Pecten sp., sp., (Neithea) sp., Torocystis globularis, Hemi­aster sp., Enallaster sp., Holectypus sp., unidentified pelecypods; above 2781 ft AB, beds more resistant; platy weathering prom­ inent, 2805-2815 ft AB; at 2815 ft AB zone of distinctive pelletiferous limestone; unit forms 20° slope with series of ledges..... 127 2690-2817 64* Covered probably thin-bedded sandstone zone, 20° west slope covered with sand- or limestone; stone and limestone fragments up to 15 cm (Md .12 mm)•».... 2 5 •**»...» 2665-2690 63. Sandstone, yellowish gray (517/2), pale yellow­ish brown, and white; pale yellowish weathers fine-to medium- brown, yellowish gray (5Y7/2); grained, subangular, moderately sorted, polished and quartz with calcareous cement and frosted, scalloped intergranular patches of iron oxide; lower part compact and laminated; becomes and darker and less slightly coarser upward well sorted; about 2640 ft AB, 4 ft very fine- grained fossiliferous sandstone occurs; shell fragments unidentified; white and fine to near medium grained top; grains largely polished: calcareous cement; unit is thin bedded at base, thick bedded upward; cross bedded; weathered surface smooth; forms series of ledges.... 36 2629-2665 62. Covered unit forms bed of and zone; gully extends 15-20 ft east bank........,*... 77 up 2552-2629 61. Limestone, medium light gray grading upward to very fine-to fine-grained quartz sand­stone, dark yellowish brown and medium light gray; very finely crystalline with minor amount of silt and sand size angular quartz; some iron oxide conchoidal fracture: staining; half of unit is fine to fine upper very grained, moderately well sorted, subangular to subrounded quartz sandstone with calcareous cement; sparsely fossiliferous, probably lower pelecypod fragments; part of sandstone contains abundant iron oxide coating grains; joints: N23°W, 64°S¥, 0.5-2 ft, also N2?°E, 1 ft, also 1 ft; unit is com­ nv, N67°E, nv, pact; thin bedded; largely covered but visible in gully beneath terrace gravel; forms dip slope and west bank of gully 18 2534-2552 and 60. Sandstone, pinkish gray very pale orange; weathers and pale - pinkish gray very orange pale yellowish brown; fine-to medium-grained, subangular, well sorted, largely polished iron oxide quartz; intergranular common; slightly more abundant toward top; compact; nvb; severely fractured; joints: N7O°E, 65°SE, 4 in.; forms last knoll before dip to smooth-surfaced slope to gully; weathers covered to 2400 ft angular blocks; largely AB; thin zone nodular limestone at about 2390 ft AB; crest of knoll, 2420 ft AB; dip covered 2425 ft AB to top of slope largely unit near bank of gully 154 2380-2534 59* Limestone, medium light gray; weathers pale yellowish brown; very finely crystalline limestone containing abundant shell frag­ ments; caprinids, clams, gastropods; caprinid zone, 2370-2378 ft AB; veinlets, calcite some iron oxide stained; compact; thin bedded; nodular; forms ledge and slight break before sandstone 14 .. 2366-2380 58. Sandstone, moderate yellowish brown and very pale orange pale yellowish brown; weathers - dusky brown, moderate brown pale brown, (SYR4/4); and grayish orange pink (SYR7/2); fine-to medium-grained quartz, subrounded, to moderately-well-sorted, largely frosted; calcareous cement; abundant intergranular iron oxide near base but decreases upward; few oolites near base; unit becomes slightly coarser and more poorly sorted near top where cross bedding outlined by placered, dark minerals and iron thin oxide; bedded; forms dip slope to east and small upper 11 ft covered but probably thin- bedded d5t0ne................ 68 san • •.• 2298-2366 57* Limestone, medium light gray and medium gray; weathers pale yellowish brown and very pale orange; arenaceous, oolitic, and fossiliferous lime- very finely crystalline stone; sand is fine grained, poorly sorted, subrounded quartz; oolites average 0,25 mm; pelecypod shell fragments; severely frac­tured with abundant calcite veins; weathers to to rough surface; grades upward compact, unfossiliferous thin-bedded limestone with subconchoidal fracture; uppermost 5 ft covered; unit forms dip slope 17 2281-2298 and and fine­ 56. Limestone, arenaceous oolitic, limestone medium grained quartz sandstone; weathered surface shows distinctive light gray; light brown (SYR6/4) differentially weathered laminations of fine-grained quartz; sandstone and medium weath­ very pale orange light gray; and ers grayish orange pale yellowish brown; oolitic unit predominantly arenaceous lime- with oolites ranging from 0.15 mm to stone, 1.8 mm, and from oblate ellipsoidal to dis- to coidal; very finely crystalline coarsely crystalline matrix; sandstone is fine-grained, subangular, well-sorted, frosted and polished quartz; lower beds contain intergranular iron is thin-thick oxide; unit, over-all, compact; bedded; crest of major ridge occurs at 2,195 ft AB and is sandstone; marked dip slope begins at 2,220 ft A8.......®.®*...»<>....* 131 2150-2281 55 • Sandstone and oolitic limestone; medium light gray, pale yellowish brown; weathers grayish orange pink (SYR7/2); fine-grained, subrounded frosted well sorted, largely quartz; mm, few scattered oolites less than 0.3 and fine shell fragments; argillaceous; calcareous cement, becomes less argillaceous, more oolitic upward; about midway of unit rounded nearly spherical oolites reach 0.7 mm (Md 0.4 shell also about mm); fragments present; 2140 rock is almost with ft AB, entirely oolites, subordinate shell fragments in a calcareous matrix; near top oolites are somewhat fewer and occur in a fine-to medium-grained quartz sandstone; argillaceous; calcareous cement; unit is thin to thick bedded; cross bedded; distinctive dark brown discoloration on some surfaces; forms continuous series of e e. ledges 29 .....«.. 2121-2150 54« Sandstone, very light gray and medium light gray; weathers pale yellowish brown and - grayish orange pink pale brown; fine-to medium-grained, moderately sorted, subangular. dark chert largely frosted quartz; grains of and metallic minerals give rock salt and pepper appearance; at 2062 ft AB, 1-2 ft pelecypod shell hash; fossil float collected from lower 16 ft includes clams, gastropods, Exogyra quit­manensis; about 2066 ft AB 1-2 ft limestone with Exogyra quitmanensis, large cardium-like and Trigonia sp.; float from 2068-2078 clams, ft AB included clams, Trigonia sp., and Tylos­toma sp.; above 2078 ft AB, arenaceous limestone which grades into thin bedcted calcareous sand­stone at top similar to that at base; contains scattered pelecypod fragments; unit partially covered; thin bedded; forms 1075° slope with few ledges ........ 75 204^-2121 53* Limestone, medium light gray, weathers same and pale yellowish brown; arenaceous, very finely crystalline limestone; lower Ift has abundant small (25—30 mm) pelecypod fragments and 2ft a few large oyster fragments; about from base and extending upward 4ft large oysters, Exogyra quitmanensis, occur; maximum observed shell length -13 cm; smaller pelecy­ also collected as float were pods present; clams, gastropods, and Anatina sp.; at 2030 ft AB, 3 ft zone of rudistids -caprinids plus other shell fragments; nodular weathering zone 2033-2036 ft AB; colonial coral, Actinas­trea sp. occurs just above; 2041-2046 ft AB, dense zone of pelecypod shell fragments, Exogyra sp. and rudistids -caprinids; unit compact; thick bedded; weathers relatively to smooth surface; forms permanent ledges.... 25 2021-2046 Yucca Formation medium and 52. Sandstone, light gray yellowish gray (517/2); pale weathers brown and moderate brown (SYR4/4); fine-grained, moderately sorted, subangular to subrounded, polished and frosted calcareous base quartz, cement; near iron oxide marks laminations; compact; thin bedded; upper 17 ft covered but probably thin-bedded sandstone; unit weathers to smooth surface; forms small then west to saddle, 25° slope up massive overlying limestone.. 67 1954-2021 and 51. Sandstone, very light gray, pinkish gray; weathers light gray and pale yellowish brownish brown; fine-to medium-grained, subangular, moderately-to well-sorted, polished and frosted quartz with abundant intergranular patches of iron oxide; calcareous cement; com­pact; thin bedded; cross bedded base; at weathers to smooth surface; forms series of prominent blocky ledges 51 ....... ...........0 1903-1954 50, light olive (5Y6/1) and medium Sandstone, gray weathers and light gray; very pale orange pale brown; much of unit is covered and may be siltstone calcareous near base, sandstone is containing nodules; fine-grained, well-sorted, subangular-subrounded, largely frosted quartz, with scattered minute intergranular patches of iron oxide less than 0.1 calcareous ce­ mm; ment; compact; at 1897 ft AB is 2 ft medium light gray sandstone like that at base; unit forms 20° slope covered with sandstone debris and a few calcareous nodules 27 1876-1903 olive and weathers 49. Sandstone, light gray (5Y6/l)» pinkish gray very pale orange; grayish - orange pink (SYR7/2), very pale orange moderate grayish orange and yellowish brown; well- fine-to medium-grained, moderately-to sorted, subangular, frosted and polished calcareous scattered inter- quartz; cement; granular patches of iron oxide abundant; thin bedded; slope largely covered to 1830 ft AB; above unit is cross bedded, more resistant; iron oxide accumulations on fracture surfaces; fine laminations marked by iron oxide accumu­above­ lations; fine grained, compact, at top; non-resistant portion, unit forms series of blocky l6dges>>>9>tii«)iii«i*i«iii>ie>< OP 1821-1876 48. Limestone, medium light gray grading upward to grayish orange pink (SYR7/2); weathers very pale orange and grayish orange; arenaceous very finely crystalline limestone containing oyster fragments; thin bedded; weathers to 4 to 6 in. nodules; becomes less arenaceous to thick upward; thin bedded; compact; good ledge forms; large oysters, Exogyra quitmanen­sis, occurs 1810-1811.5 and 1812.5-1814 ft AB; turritellid-like gastropods and colonial corals at about occur at persistent bedding plane 1812.5 ft AB; joints: NlO°E, 76°NW, 1-4 ft also 4-6 ft; karrenfelder weath­ S43°E, 74°Sw, 2 ft weathers ering surface; upper arenaceous, grayish orange and contains abundant oyster fragments; calcite veins prominent; unit thins to 2 ft within 100-200 ft laterally 13 1808-1821 47-Sandstone, yellowish gray (5Y7/2), grading upward to olive gray (5Y4/l) siltstone and brown sand- pale yellowish (10YR6/2) marly stone; weathers same; lower sandstone fine- to medium-grained, moderately sorted sub- rounded, largely frosted quartz; argillaceous; calcareous cement abundant oyster fragments, some as large as 45 mm but most averaging 5-10 mm; overlying siltstone compact; brittle; con­choidal fracture; iron oxide coats fracture surfaces; non-calcareous; covered from 1795­but 1808, probably very fine-grained marly sandstone that occurs at top; very thin bedded; lower sandstone forms ledge; upper part non-resistant 31 1777-1808 46. Siltstone, pale red (5R6/2) with rounded, very finely crystalline limestone nodules up to 60 mm; pale red-grayish red (5R5/2), dark greenish gray (SGY4/1), and medium dark gray; some nodules show intraclastic texture; 6 in. very fine-grained grayish orange (10YR7/4) quartz sandstone at 1745 ft AB; grades upward to greenish siltstone; unit non-resistant; surface covered with calcareous nodules and limestone debris 39 1738-1777 to 45* Sandstone, pinkish gray grading upward very pale orange; weathers pale red (10R6/2); fine-to medium-grained, moderately sorted, subangular, frosted and polished quartz; argillaceous; abundant intergranular patches of iron oxide; calcareous cement; compact; thin and cross bedded, somewhat platy up­ ward; weathers to smooth surfaced blocks; upper part forms ridge 31 1707-1738 44« Siltstone, pale red (5R6/2) containing a few limestone nodules (Md 20 mm); largely covered; 1 ft stringer of pale yellowish brown sand­stone at 1681 ft AB; fine-grained, moderately frosted sorted, subangular, largely quartz; argillaceous; few scattered patches and vein-lets of iron oxide; 1693-1694 ft AB limestone; yellowish gray (517/2) limestone granule and to pebble conglomerate grading upward, yellowish gray (517/2) containing siltstone scattered and occasional pebbles of limestone; granules unit forms west 510pe...... 41 1666-1707 43* Sandstone, pinkish gray and pale brown, weathers and grayish orange pink (10R8/2) grayish red (10R4/2); part, lower fine- medium-grained, moderately sorted, subangu­lar, largely frosted, argillaceous quartz sandstone; patches intergranular scattered iron oxide; somewhat fisable, upper part fine-grained, compact, laminated abundant iron oxide; calcareous and iron oxide ce­ ment, unit is thin bedded, cross bedded; weathers to smooth surface; forms slight on west ledges slope 8 1658-1666 42. Siltstone, largely covered; contains lime­stone nodules to forms up 90 mm (Md 30 mm); small saddle; flat sandstone fragments, 1-2 in., cover surface 56 1602-1658 41. Sandstone, intercalated grayish orange pink (SYE7/2) gray grading upward and brownish to very light gray; weathers pale brown grayish - - brown and pale brown light brown; lower 5 ft is well fine-grained, sorted, subangular, iron oxide coated quartz; calcite cement; cross bedded; thin bedded; compact; forms prominent ridge of oblong blocks; slope covered, 1562-1579 ft AB; above is fine-to medium-grained, moderately sorted, subangular, frosted and polished quartz sandstone; with abundant patches of iron oxide; compact; massive; weathers to blocks; of cuesta top at 1593 ft AB 44 T'ssß-1602 - red with cal­ 40. Siltstone, grayish pink pale careous nodules as in unit 46; conglomerate; 1518-1520 ft AB; pale brown; finely crystal­line, limestone granules in medium-to quartz sandstone matrix; thin coarse-grained bedded, cross bedded, and compact; forms 25° west slope covered with sandstone debris and calcareous n0du1e5............ 54 1504-1558 39* Sandstone, very light gray, weathers grayish well orange pink (SYR7/2); fine-grained, frosted sorted, subangular, polished and scattered of iron oxide; cal- quartz: patches cite cement; compact; thin bedded; weathers to smooth surface; forms slight ledge 4 1500-1504 red lime­ 38. Siltstone, pale (5R6/2); contains stone nodules as in unit 46; red gray; grades upward to greenish gray (50Y6/1) siltstone, thin to red siltstone as above; west slope with sandstone debris and limestone nodules 16 1484-1500 olive 37. Sandstone, yellowish gray (SYB/l), light gray (5Y6/1), very pale orange, grayish orange pink (SYR7/2), and yellowish gray (5Y7/2); weathers grayish orange pink (51H7/2), pale red (10R6/2), and pale brown; 6-8 in. limestone granule conglomerate at base; siltstone with calcareous nodules 1432-1443 AB; sandstone is fine-to medium-grained, poorly to well sorted, subangular, polished and frosted quartz; near base, sandstone is argillaceous; becomes friable then compact near top; most sandstone beds com­ pact, thin bedded; joints: S6O°E, 81°SW, 2-6 in.; weathers to smooth surface; unit forms prom­ inent change in slope 63 1421-1484 36. Siltstone, pale red (SYR6/2) with scattered calcareous nodules up to 50 mm (Md 10-15 mm); forms gentle west slope 2 5 1396-1421 35* Sandstone, very light gray and light brownish and red gray, pale (5R6/2), siltstone; weathers pale yellowish brown, pale brown and brownish sandstone is fine-to medium-grained, gray; moderately-well-sorted, subangular, pol- to ished and frosted calcite cement; quartz; argillaceous, less so upward; compact; thin bedded and cross bedded; weathers to smooth surface; shale zone forms abrupt break in west 510pe.... 25 1371-1396 medium 34* Sandstone, yellowish gray (5Y7/2), brownish light gray, gray, very light gray, and greenish gray (SGY6/l) and interbedded pale red (5R6/2) siltstone; weathers light brownish brownish and olive gray, gray, gray (5X4/1); sandstone is fine-to medium-grained, well sorted, polished and frosted, subangular quartz; calcite cement; siltstone compact; contains calcareous nodules to 50 mm (Md up 10-15 mm); at 128? ft AB 6-8 ft limestone (Md 3-5 mm) with medium-to conglomerate coarse-grained quartz sandstone matrix; at 1327 ft AB, peculiar granule conglomerate with gravel fraction consisting of clay and calcareous nodules; popcorn-like appearance; sandstone and conglomerate beds form series of small ledges on 10°-15° west 510pe..... 110 1261-1371 red and 33* Siltstone, pale (5R6/2), stringers of light brownish brownish­ thin-bedded, gray, olive gray, and light gray (5Y6/1), compact, fine-to very fine-grained, well sorted cal­otte-cemented quartz sandstone; lower 20 ft of siltstone contains limestone nodules to up 180 mm (Md 15-20 mm) and distinctive black masses botryoidal concretionary (Goethite or psilomelane) with calcareous filling; 4 in.­ thick limestone granule conglomerate 1250 ft AB; unit forms bench 44 1217-1261 32. Sandstone, fine-grained, and pale red (10R6/2) silt stone; sandstone, light brownish gray and very light gray, weathers pale brown and light brown (SYR6/4); lowermost sandstone is well sorted subangular largely frosted, calcite silica, and iron oxide cemented; compact, at thick bedded; ledge former; 1161, 6 ft siltstone containing irregular aphanocrys­talline limestone nodules up to 50 mm; above sandstone first is argil­ siltstone, quartz laceous and medium grained, then fine well grained, compact, moderately sorted, and subangular, polished frosted, argil­ laceous; weathers to smooth surface; forms slight ledge 62 1155-1217 31. Covered zone, probably siltstone; forms Small flat,»®«oo»oeo»©o»oi»oo»98ooo»®o««o 8 1147-1155 30. Sandstone, a minor interbed of siltstone; very light gray, light gray, and light brownish gray; weathers pale brown, grayish orange pink (SYR7/2) and grayish red (5R4/2); medium-grained, moderately sorted, subangu­ lar, frosted and polished quartz; argilla­ceous, calcite cement; grades upward to com­ pact, fine-grained, well sorted, subrounded polished quartz sandstone; becomes medium grained, then grades upward to very fine- grained, subangular, moderately sorted, polished and frosted quartz sandstone; silica and iron oxide unit calcite, cement; cross bedded; weathers to smooth surface; upper 2 ft severely fractured; spherically weathered; forms slight cuesta 15 1132-1147 29* Siltstone, pale red (10R6/2); variety of - irregular, subangular subrounded, lime­stone nodules; some with distinct intra­clastic texture; nodules are medium to dark gray, pale yellowish brown, and grayish red (5R4/2); up to 80 mm (Md 25-30 mm); nodules restricted to lower and litter half, surface; unit forms topographic low between Small CU€StaS*sa*oe»«e*o9oe9**eooo»*»oaa 20 1112-1132 28. Conglomerate, limestone and chert granule- pebble, grading upward to fine-medium­grained sandstone and coarse siltstone; and light gray, very light gray, grayish red weathers (5R4/2); grayish orange pink (10R8/2), pale brown (SYRS/2), grayish red medium (5R4/2); gravel fraction 50 percent also pale gray aphanocrystalline limestone, that red chert, pink and milky quartz ranges in diameter to 80 mm (Md 3-5 mm); subrounded­ rounded; matrix medium-grained quartz sand­stone; 2-ft conglomerate forms discontinuous ledge; medium-grained, poorly sorted, subangu­lar, largely frosted argillaceous and calcite cemented quartz sandstone grades to dense, well fine-grained, sorted, subangular, largely polished quartz sandstone with joints; N47°E, nv, 4-6 ft, also S4O°E, 59°SW, 1-6 ft; at 1103 ft AB, 9 ft siltstone section with thin non-resistant beds; alternating resistant, resistant siltstone very dense; calcite, silica, iron oxide cemented; conchoidal frac­ture; weathers to smooth surfaced platy, 4-8 in* blOCkS*oe9®®®o9**o»o©B®*®*®»Bo®o9atie 17 1095-1112 red weathers 27* Siltstone, sandy; grayish (5R4/2); grayish pink pale red; sand is very fine - grained quartz, subangular, moderately sorted; calcite and iron oxide cement; contains brown­ ish gray aphanocrystalline limestone nodules up to 110 mm (Md 30-40 mm); subrounded; irreg­ular shape; at 1092 ft AB, 2-3 in. discontinu­ ous, grayish orange weathering light brown stream (SYR6/4) aphanocrystalline limestone; ft AB*»o®®09®ae»®»e»««ooaoooo» 13 bed, 1092 1084-1095 26. Sandstone, fine-to medium-grained and con­glomerate; sandstones grayish orange pink (SYR7/2), very light gray, greenish gray (SGY6/1), grayish red, very pale orange and light brownish weather brown and gray; pale red 1 dark grayish (5R4/2); ft limestone, chert granule and pebble conglomerate at base (Md 2*5 mm); matrix is fine-to medium- abundant oxide grained quartz sandstone; iron accumulation; very drab appearance; forms slight ledge; sequence of compact, resistant fine-to calcite medium-grained hard, cemented quartz sandstones of different colors; grains to frosted and subangular subrounded, polished, moderately to well sorted; 13 ft covered zone at 1042 ft AB; pale red siltstone (5R6/2) with calcareous concretions; 1067 ft AB is crest of small west-facing cuesta; at 1070 ft AB 1 ft conglomerate of pale red, gray and white chert, milky and rose quartz, and dark limestone (15-20%) to 95 mm (Md 7 mm); rounded to subrounded; sandstone sparsely up conglomeratic to top of unit; beyond 1067 0 65 ft AB, forms east dipping slope.•• 0 •o «• 25* Siltstone, calcareous; grayish orange pink (10R8/2); weathers same; lower 7 ft contains calcareous nodules, medium light gray; with fine-to fine­ aphanocrystalline, very grained quartz grains; up to 40 mm (Md 5-10 subrounded to mm); irregular; subangular; grades upward to white (N9) calcareous clay- stone; containing nodules as below, but rang-and less ft fine­ ing up to 70 mm abundant; 7 to grained, angular subangular, largely pol­ished quartz sandstone begins at 1000 ft AB; placers of iron oxide particles gives laminated forms cross appearance; compact; bedded; on slight ledge west-facing 25° slope; upper 12 ft brittle, variegated slightly calcareous pale red purple and grayish yellow green; breaks readily into angular fragments (Md 205 mm); shale zones in unit largely COVered«»o»0»®o<»e*B»oooocoooooo9*ooooo»o 3 7 982-1019 24» Conglomeratic sandstone, and conglomerate and interbedded sandstone; and very light gray medium weathers light gray; same; black, gray, white, and pale red chert and pink and milky quartz subrounded-rounded granules and pebbles in a matrix of angular, subangular, poorly sorted, largely frosted medium-to coarse-grained quartzitic sandstone cemented by silica and interbedded with fine-to medium- calcite; grained, subangular-angular moderately sorted, frosted and polished quartz sandstone; com- thin pact; bedded; pale red (5R6/2) siltstone, lenticular zones 950-960 ft AB; conglomerate in upper 20 ft dense, contain relatively more black and pale red chert and are somewhat coarser (Md 5 mm) than lower conglomerates; largely smooth weathered surfaces; upper coarser sandstones and conglomerates form slight l6dg6St4i»«o«9>«*i9««9i9»9«4@i4e) 5? 23. Conglomerate, grading upward to medium-to medium to coarse-grained sandstone; light gray weathers medium to pink- very light gray; gray ish gray; conglomerate gravel fraction 50% limestone, also contains pink quartz, and pale and black limestone some red, white, chert; orange (10YR7/11); maximum size 80 mm grayish to (Md 5-7 mm); rounded subrounded; sandy, medium limestone matrix; aphanocrystalline gray to sandstone with grades upward quartz subangular­subrounded, moderately sorted, largely polished grains; calcite cement; 2 ft of conglomerate at and at 925 ft AB 5 thin base, 883-884 ft AB, bedded; compact; unit begins half way upslope, forms crest and dip slope; sandstone weathers to smooth surface»«»G«o»ooo®o«ooa<»oo‘i»oo 60 865-925 22* Siltstone, pale red (10R6/2); weathers same; fractures to angular fragments averaging 3-5 mm; brittle; contains abundant variegated medium light gray and grayish red (5R4/2) irregular, limestone compact, aphanocrystalline nodules, that weather out and cover 25° slope to West..*<*.»®»®.®«o. 31 «•«•». 834-865 21. Conglomerate, grading upward to fine-and medium- grained sandstone; very light gray; weathers grayish orange pink (SYR?/2); conglomerate pink and milky quartz, dark gray, pale red and white chert, and light limestone to 45 mm (Md gray up subrounded to 5-7 mm); poorly sorted, rounded; matrix medium-grained quartz sandstone; moder­ ately sorted, subangular to subrounded; calca­to reous cement; grades upward compact, non­ conglomeratic fine-grained subangular to sub- rounded, well sorted, largely polished quartz sandstone, followed by moderately sorted, subangular-subrounded, largely frosted, medium- grained quartz sandstone; all cemented by calcite, some possibly silica; uppermost 5 ft again fine-2-ft 27 ft grained sandstone, conglomerate above base of unit; forms stream bed 836 ft 800^834 20. Siltstone, interbedded with very fine-grained red s sandstone; pale (5R6/2), weather pale brown; siltstone contains abundant cal- thin bedded; careous nodules (Md 20 mm), roughly spherical but very irregular; nodules range from grayish below to medium red (5R4/2) light gray above; quartz sandstone well sorted; subangular to subrounded, largely frosted; unit forms small shelf near base, 20° west-dipping slope above; ft covered with limestone upper 15 largely nodules littering surface 42 758-800 to medium-and fine­ 19* Conglomerate grading upward grained sandstone; over-all grayish pink-pale red; sandstone is grayish pink (SRB/2) grading upward to light brownish gray; weathers grayish pink (SYR7/2) to pale red (5R6/2); con- orange is to sub­ glomerate poorly sorted, subangular rounded, pink and milky quartz, pale red, white, and chert, and 5$ dark limestone gray gray ranging up to 80 mm (Md 5 mm) in a medium-to coarse-grained quartz sandstone matrix; sand­ stone is argillaceous, moderately to well sorted, subangular subrounded, largely to frosted and white and pink quartz pale red, white, and black chert cemented by calcite and slightly fisable; uppermost sandstone is com­ pact, fine-to medium-grained pink and milky and red well sorted sub- quartz pale chert, angular, frosted and polished; tightly packed; lower part of unit forms east-dipping slope, stream bed at 740 ft AB and upper part forms west-dipping slope; sandstone weathered sur­ faces SmO0theo»»®»........c...... 56 16. Sandstone and conglomeratic sandstone; pale red and weathers (5R6/2) grayish pink, grayish pink and pale red (5R6/2) ; fine-to medium-grained, to moderately sorted, subangular subrounded, largely frosted quartz cemented by calcite and iron oxide; weakly indurated; crumbly; grades upward to covered interval, probably siltstone; about 536 ft AB, 7 ft conglomeratic sandstone similar to lower part unit 15; forms slight ledge; grades upward medium-grained argil­ to laceous, moderately sorted, subangular, largely frosted pink and white quartz sandstone, black and red calcite pale chert; cement; poorly indurated and friable on fresh surface; forms west—dipping siope»»«»eoooci®*»o»®o»»eB»o 517-549 15® Sandstone, conglomeratic, alternating with sand­stone; very light gray and grayish pink-pale weathers and red red; very light gray pale (5R6/2); basal conglomeratic sandstone; sparse gravel fraction is pale red to black chert and pink and white quartz, chert, and dark aphano­crystalline limestone up to 70 mm (Md 3-5 mm); matrix is poorly sorted, rounded to subrounded; fine to calcite-cemented sandstone coarse very of sub­ (Md 0.25-1005 mm) composed poorly sorted, angular, largely frosted, quartz and a few black chert grains; lenses of conglomeratic sandstone alternate with lenses of fine-to medium-grained sandstone composed of moderate to poorly sorted, angular to subangular, largely frosted pink and white quartz and black and pale red chert; ce­ mented by iron oxide and calcite or by calcite; ft AB sandstone about 448 conglomeratic with gravel ranging up to 80 mm (Md 5“7 mm) with 10­15$ limestone; cuesta; grades upward forms small at forms small cuesta; about 468 ft AB conglomeratic sandstone containing 20$ limestone ranging up to 75 mm (Md 7-10 mm); forms slope to bottom of creek, then part of east bank; from about 490 ft to sparsely conglomeratic sandstone 452 ft AB; AB, 16 ft poorly sorted, fine-to medium-grained, friable subangular, largely frosted sandstone; some calcite cement; uppermost 11 ft is very sparsely conglomeratic, poorly sorted, argilla­ ceous medium-to very coarse-grained, subangular to subrounded, largely frosted pink and white quartz with scattered black and pale red chert; silica and calcite cement; well indurated and compact; cross bedded and thin bedded where visible; weathers to uneven surface; forms ledges. 106 silt­ 14. Sandstone, conglomeratic; sandstone, and stone; very light gray, grayish pink, and weathers grayish red (5R4/2); very light gray, pale red (10R6/2), and grayish pink; largely sandstone and sand- alternating conglomeratic stone; part conglomeratic sandstone, lower fine-grained, moderately sorted, subangular, largely frosted, pink and clear quartz; cal­ otte and iron oxide cement; grades upward to conglomeratic containing chert, sandstone unit 12 quartz and limestone pebbles as in ex- size cept median grain 10 mm; largest fragments are these lower zones weather to limestone; smooth surface; form 70° east-dipping slope; uppermost part of unit is siltstone, brittle, breaks into angular fragments (Md 10 mm); road about crosses Squaw Spring 393-411 ft AB. 50 361-411 13* Sandstone and conglomeratic sandstone; pale weathers red-grayish pink and very light gray, brownish gray and pale brown; sandstone is fine-grained, moderately sorted, subangular, largely frosted pink and clear quartz; calcite and iron oxide cement; well indurated; compact; lower part distinctly laminated; very smooth weathered surface; forms 20° east-dipping slope; thin scattered lenses of conglomeratic sandstone occur in gravel fraction upper part; is red to black chert and and white pale pink to 18 quartz ranging up mm (Md 2*5 mm); poorly sorted, subrounded to rounded; matrix is argil­laceous, calcite-cemented, fine-to very coarse- grained sandstone, made up of very poorly to sorted, subangular subrounded, largely pol­ished quartz grains some grains of and fine pale red and black chert; several small scat­tered patches of apparently copper mineraliza­ tion 17 344-361 12. Conglomerate, gravel fraction pale red (5H6/2) and black chert, pale pink and white quartz, and discoid aphanocrystalline gray limestone ranging from sand size to 110 mm (Md 25-30 mm); poorly sorted; rounded; matrix is medium-to very fine-grained argillaceous calcite-cemented sandstone made up of poorly sorted, subangular to subrounded frosted and polished quartz grains and a few grains of black and pale red chert; moderately well indurated; nvb; limestone forms about 15 % of gravel fraction; no fusulinids were seen in limestone frag­ ments; weathers to uneven surface; forms cuesta 330-344 slight 1A 11. Covered zone, probably shale; 20° W sloping debris-covered surface 5 325-330 10. Sandstone, conglomeratic and sandstone; very to red weathers light gray pale (5R6/2), brownish to brownish light gray gray; gravel fraction is scattered pale red to black chert, and pink and white quartz up to 30 mm (Md 5-7 matrix is mm); poorly sorted, subrounded; medium-to-coarse, subangular subrounded, to largely frosted clear and pink quartz grains with a few black chert grains; calcite and iron oxide cement; grades upward to medium- grained sandstone composed of moderately frosted clear and sorted, subangular, largely pink quartz grains and a few grains of black and pale red chert; contains widely scattered chert pebbles; unit has no visible bedding; well indurated; weathers to smooth hard, surface; forms slight cuesta and dip Slope..«e.a..o»oao®»®e®»oo»®»*a«oo®»*o«. 1A 311-325 9® Sandstone and sandy siltstones pale red (5R6/2), weathers brownish and medium dark gray gray; fine-to medium-grained sandstone made up of moderately sorted, subangular to subrounded, largely frosted pink and clear quartz and a few scattered grains of pale red and black chert; becomes slightly argillaceous upward; calcite cement; well indurated below, moderately in­durated abovt; weathers to smooth surface; forms 20° slope covered with 0.3-o*s ft blocks; siltstone 229-305 ft AB, sandy containing very poorly sorted, subangular-subrounded dark aphanocrystalline limestone particles up to pebble size (Md I*s mm); limestone nodules litter sloTDe»®®o»®o®oooo»9o®®»o*®®®®oas)» 0 291-311 8* Conglomerate, conglomeratic sandstone, and sandstone; very light gray and pale red (10R6/2), weathers light brownish gray and ft of pale red (5R6/2); 23 conglomerate, gravel fraction pale red and black chert, pink and white quartz, and discoidal aphano­ limestone fusu­ crystalline (some containing from sand size to lines) ranging 150 mm; to poorly sorted, rounded; matrix is medium- very coarse-grained argillaceous calcite-ce­mented sandstone made of up poorly sorted, subangular to subrounded frosted and pol­ished quartz grains and a few grains of black and pale red chert; moderately well in­ durated; nvb; weathers to smooth surface; forms slight ledges and crest of small hill followed by 13 ft of slightly argillaceous medium­calcite-cemented sandstone made of grained up frosted moderately sorted, subangular, largely quartz grains and a few dark chert grains; to well indurated; 12 ft conglomerate similar lowest 23 ft except maximum size gravel is 40 mm and limestone less abundant (25%)} upper­ most 14 ft medium-to coarse-grained largely silica-cemented sandstone and slightly con­ glomeratic sandstone made up of poorly sorted, subangular, frosted clear and pink quartz and scattered grains of pale red and black chert; very well indurated; compact; gravel fraction ranges up to 15-70 mm (Md 3-5 mm); unit forms east 62 slope»®9oooo®»®®«o»jo»o®j»ooooo««®» 229-291 7• Sandstone, conglomeratic and sandstone; very light gray, weathers pale red-grayish pink, pink and medium gray; lenses of conglomeratic sandstone and sandstone; gravel fraction is poorly sorted, rounded, pale red and black chert and white and pink quartz fragments rang-to 110 matrix is medium­ ing up mm (Md 5-10 mm); to coarse-grained argillaceous sandstone made up of poorly sorted, subangular to subrounded, largely frosted quartz grains with scattered grains of black and pale red chert; calcite ce­ ment; moderately well indurated; thin bedded; cross bedded; weathers to smooth surface; forms to slight ledge; grades upward conglomeratic sandstone dark containing 15% aphanocrystalline limestone pebbles up 15-20 mm in diameter; to to this grades upward 19 ft of medium-grained sandstone made up of poorly sorted, subangular, clear and largely frosted pink quartz grains with scattered grains of black and gray chert; calcite cement; some iron oxide staining; well hard®®*®®**®®®®®®®®®®®®®®®®®® 27 indurated, 202-229 6. Sandstone, conglomeratic and sandstone; very to brownish light gray grayish pink, weathers to red fraction gray pale (5R6/2); gravel rounded brown to black chert and white quartz ranging from sand size to 70 mm (Md 8-10 mm); matrix is medium-to coarse-grained argil­ very laceous sandstone containing very poorly sorted, subangular subrounded, largely frosted quartz to grains with scattered grains of black and pale red oxide well chert; calcite, iron cement; very indurated; compact; interbedded with lenses of fine-to coarse-grained argillaceous sandstone made up of poorly sorted, subangular to sub-rounded largely frosted quartz grains, with a few scattered black and pale red chert grains; sandstone moderately indurated and fairly com­ pact; unit is thin bedded and cross bedded; smooth forms weathers to surface; slight ledges on west slope»»e»o»«»»»®o«*eo<5o®o 13 189-202 5. Sandstone: pale red (SR6/2), weathers grayish red to moder­ (5R4/2); medium-grained, poorly to ately sorted, angular subrounded, largely frosted quartz with scattered chert grains; mod- well indurated and thick erately fairly compact; bedded; joints: N75°E, 72°SE, 1-2 ft; weathers to 10° slope; roughly spherical calcareous nodules up to 5»5 cm occur at about 159-161 ft AB; at about 185 ft AB, 1-ft bed conglomeratic sandstone containing scattered dark chert and light colored rounded quartz granules and peb­bles in a matrix similar to rock above and below ®o©«o#9®®(*o©o®o®o®ooo®oo9 41 148-189 4• Conglomerate and medium to very coarse grained sandstone; over-all grayish pink-pale red, weathers red is pale (5R6/2); gravel fraction largely brown to black chert, and pink and white quartz ranging from sand size to 20 mm well indurated (Md 2-4 mm); calcite cement; and weathers to 5-10 cm blocks and forms hard; in to medium- slight break slope; grades upward sub- grained, moderately sorted, subangular and rounded, largely frosted, quartz grains ce­ mented by iron oxide and calcite; well indurated about ft AB becomes and compact; 120 unit dark heavily conglomeratic; contains up to 20$ aphanocrystalline limestone fragments and 10$ also chert fine-grained sandstone fragments, and quartz as below, ranging from sand size to 200 mm (Md 10 mm); gravel poorly sorted; well indurated calcite and iron rounded, by oxide cement; matrix very poorly sorted fine- to cross very coarse-grained sandstone; vague bedding; few thin sandstone lenses relatively 2-6 also free of gravel; joints: N45°E, nv, ft, S65°E, 72°SW, 2-4 ft; gully at about 130 ft AB; decreases in abundance and near top gravel size (maximum 25 mm), little limestone and SandstOme***»»«»*a**»e**«eooao»ea«»ao*s« 34 114-148 3. Sandstone; pale red (5R6/2), weathers same; medium-grained, well sorted, subangular to subrounded, frosted quartz; calcite and iron oxide cement; moderately well indurated and smooth weathered forms compact; nvb; surface; 20° dip slope to east and small flat; becomes more resistant upward... 8 106-114 2. Conglomerate and medium to very coarse-grained to sandstone; pinkish gray grayish pink-pale red, weathers pale red (10R6/2 and SR/62); gravel fraction is 80$ brown to black chert and pink and white quartz, 10$ brown siltstone, and 10$ gray aphanocrystalline limestone ranging from sandsize to 100 mm (Md 10 mm); very poorly sorted, subrounded to rounded, percent limestone and gravel size decreases upward but uppermost 10 ft resembles lower part; matrix is calcite- cemented medium-to very coarse-grained sand­stone; about midway of unit, 14-18 ft medium- to very coarse-grained non-conglomeratic quartz sandstone, moderately poorly sorted, sub- to angular to subrounded; largely frosted; calcite cemented; very coarse sandstone slightly fri­well able; over-all, unit indurated, compact; thick bedded near base, thin bedded upward; vague cross bedding; joints? N?O°E, nv, 2-3 ft; caliche-like material fills joints in lower 10 ft; weathers to rough, block ledge with relatively smooth 5urface........... 67 39-106 1. Conglomerate and medium-to coarse-grained red weathers sandstone, pale (5R6/2), pale brown; gravel fraction is brown to black chert and white and pink quartz ranging from sand size to 30 mm (Md 5 mm); very poorly sorted and subrounded to rounded sandstone is quartz, moderately sorted, subrounded, calcite cement; iron oxide coats frosted; some well indurated and surfaces; compact; nvb; smooth weathered surface; forms 15° west slope; upper 1$ ft largely covered but ..<.... •o *..» probably conglomerate 39 • »• •B0• 0-39 Total*o©o©®®«oo 2021 Fault within Yucca Measured Bluff Formation«#•• 796 •0«•• « thickness, Yucca Format ion.•••• 0 •oo• 2021 Total thi ckne ss MS 9*®°®®®®®*®®*®®*®°®®®°®®* 281 , V Measured Section 10 Formation; Cox Sandstone Locations S,B-15*4; base of section at Bluff-Cox contact about 2,000 feet northwest of point where Indio Pass road traverses steep, west-facing scarp; Evans ranch; GSLU Series, photo no. 3-126. Measured; June 12-13, 15, 1959, by Underwood and Kirksey with Brunton compass and 5-foot staff. Thickness, feet Unit Feet above base Description CRETACEOUS Finlay Limestone » •.•.not measured «... o Cox Sandstone 20. Sandstone, very pale orange, moderate orange pink (10R7/4), white, and grayish orange pink (10R8/2); weathers grayish orange pink (SYR7/2), pale brown and light brownish lower gray; largely covered, 50 ft and upper 127 ft, but probably shale or thin-bedded sandstone; exposed sandstone fine-to coarse-grained, moderately well- to well-sorted, subangular, frosted and polished quartz; argillaceous near base; cement largely siliceous; compact, slightly calcareous near base; 25° sandstone debris- covered below slope 1115 ft AB; compact, cross-bedded on thin-bedded, dip slope 1115-1140 ft AB ; crest of ridge at 1115 ft AB ; composed of slightly conglomeratic medium-to coarse-grained sandstone with scattered white chert pebbles; low ridge crest at bed of 1215 ft AB; gully at 1265 unit weathers to smooth ft AB; surface, with thin outer commonly hardened, layer Of qUartZ grains .........a. 242 1025-1267 ......e«.ao«. medium weathers 19- Limestone, light gray; light gray; very finely crystalline limestone; calcite veins and veinlets, some iron oxide- stained; shell fragments, largely pelecypod; fossil floats Exogyra cf. L texana, Toucasia Gryphaea washitaensis, sp., caprinid, 1 ft contains oyster frag­ pelecypod; upper ments; compact; thin bedded; thins-thickens along strike; forms slight 1edge........ 1016-1025 18. Conglomerate and fine-to medium-grained quartz sandstone, grayish orange pink (10R8/2), and white, pinkish gray, very light gray, light gray; weathers grayish orange, yellowish gray (5Y7/0), grayish orange pink (10R8/2), pale brown and pinkish gray; conglomerate con- of dark sists of rounded gravel fraction chert, and a little dark limestone pink quartz, up to 15 mm (Md 3-5 mm) in a matrix of medium- to grained quartz sandstone; grades upward sandstone with subangular, polished and frosted, and commonly well-sorted quartz with of iron intergranular patches oxide; 835-840 to ft AB, a few chert pebbles up 10 mm; siliceous above; compact; thin bedded unit series of and covered forms ridges slopes; bed of at 922 ft AB; largely covered gully above; weathered surface smooth but commonly has hard 21 j) veneer. .....e0...©........©.©.© 803-1016 17. Siltstone, pale reddish brown; weathers same; breaks to nearly equidimensional fragments 10-25 mm in length; somewhat brittle; nvb; covered sandstone debris..• » 28 • largely by 775-803 and 16. Sandstone, white, pinkish gray very pale weathers pinkish gray with solution orange; cavities filled by black leichen, moderate brown (SYR3/4)> light brown (SYR6/4), and fine-to pale brown; medium-grained, angular-frosted and subangular, well sorted, polished, quartz with abundant intergranular accumula­ tions of iron oxide; siliceous cement; com- unit forms pact; thin bedded; cross bedded; dip slope with a few prominent ledges; a few 10 mm chert pebbles occur in ledge about 705“ 715 ft AB; and somewhat more in ledge 720-725 covered from ft AB ft AB; slope largely 740 hard up; weathered surface smooth, veneer t)ciS0»«o«o<»oo®»»oo»ooooe®ffl©i»o«©®#o®o 105 670-775 15. Sandstone, pinkish gray, pale brown, pale red (5H6/2), light brown (SYR6/4)* very pale orange; weathers grayish orange pink (5Y7/2), brownish pinkish gray, pale brown, and gray; largely fine grained but some layers range up to medium grained; quartz subangular to angu­to well polished and lar, moderately sorted, frosted, with abundant intergranular accumula­tions of iron oxide; cement siliceous and slightly calcareous; compact, thin and cross distinctive brown sandstone bedded; 0.5 ft pale at 578 ft AB and 1-2 ft pale red sandstone at ft ABs 611 ft AB; joints in ledge, 583-586 Nl5°W, 72°SW, 4“6 in.; also E-W, 79°S, 4 in.; 670 ft AB summit of high point ; weathers to smooth surface with commonly hardened black leichen weathered cav­ filling small -122 548-670 14. Limestone, slightly arenaceous and lutaceous, pale yellowish brown-grayish orange and very weathers pale orange-pale yellowish brown; grayish orange; sand grains are quartz, range up to medium grained; black mineral occurs in few small isolated patches and as vein fill- distinctive calcite ings; crystalline cavity fillings, usually circular and about 1 in. diameter; some calcite veins; compact, thin bedded where visible; persistent laterally; joints? S?O°E, 77°SW, 4 in.; weathered sur­ face has 6 glazed appearance....®..®...... 542-548 to • 13 Sandstone grading upward siltstone; pinkish medium gray, yellowish gray (5Y7/2), light and weathers gray, greenish gray (50Y6/1); pinkish gray, pale yellowish brown, grayish brown and olive lower light gray (5Y6/l); part is sandstone, fine-to medium-grained, moder­ately well-sorted, subangular, largely polished with siliceous to quartz cement; grades upward fine-grained, moderately sorted, subangular with siliceous and calcareous cement quartz and abundant accumulations intergranular of iron oxide, followed by 0.5 ft of fine-grained, poorly sorted, subangular argillaceous, silica-of unit cemented quartz sandstone; upper part is dense with abun­ arenaceous siltstone, very dant masses of iron oxide and intergranular siliceous cement; unit compact, thin bedded; distinct shelf and west forms slight slope; covered with sandstone and largely orange-limestone debris; weathered surface weathering 00th... 2 9 s m ©...©.tt&oo®*.*®.*©® «0.0.««©»*•©. 513-542 and 12. Sandstone, pinkish gray, white, grayish orange pink (10K8/2); weathers yellowish brownish and gray (5Y7/2), light gray, gray­ish orange pink (SYH7/2); hard, fine-grained silica subangular, moderately sorted, polished and calcite-cemented quartz sandstone with few of iron intergranular masses oxide; grades up­ward to less dense fine-to medium-grained less well sorted, subangular argillaceous abundant quartz sandstone with intergranular iron oxide and with calcareous cement; black metallic mineral also present in stringers and irregular patches; unit thin and cross bedded, lower part also laminated; near base of iron oxide 0.5-I*o ft elliptical stains lower 10 occur; ft forms prominent ledge; above is 10°-15° slope with small ledges; sharp break in slope occurs at 510 ft AB; weathered surface smooth, hardened in lower 56 .......o . part 457-513 11. Sandstone, very pale orange, and white; weathers very pale orange-pale yellowish lower fine- brown and very pale orange; part grained, subangular angular, moderately to sorted, polished and frosted quartz sandstone with abundant Intergranular masses of iron oxide and calcareous cement; grades upward to ledge forming,fine-grained, subangular, well sorted, frosted and polished quartz sandstone with abundant intergranular masses of iron oxide; calcareous cement; thin to very thin bedded; 15° slope above and below above slignt leuge, 440-442 ft AB; slope ledge largely covered with sandstone debris; weathered surface 5m00th....... 27 430-457 10. Sandstone, pinkish gray, white, grayish orange pink (10R8/0), very pale orange- and weathers pinkish gray pale yellowish brown; with and without flecks of black iron oxide; brownish fine- pale brown, and light gray; grained, angular subangular, moderately to sorted, polished and frosted quartz with iron calcare­ abundant intergranular oxide; thick bedded at ous cement; compact; base; thin and cross bedded upward; joints i weathers to smooth S6O°E, 86°SW, 9-10 in.; surface; forms prominent 1edge.......... 23 407-430 9. Sandstone, very light gray, white, and pink­ ish gray; weathers light brownish gray; gray-and ish orange pink (SYR7/2), very light gray, moderate - grayish orange pink orange pink (5YR7.5/3); fine-to medium-grained grading upward to fine-grained sugary sandstone; well quartz grains, angular-subangular, sorted, and siliceous ce­ largely polished; calcareous ment; abundant pale reddish brown intergranu­lar iron oxide; few limonite concretions 10­ 15 mm in lowest 2 ft; 302-384 ft AB, coarse, slightly conglomeratic quartz sandstone, poorly sorted, subrounded to rounded, frosted and and polished,calcite-cemented clear pink and dark chert; chert and lime- quartz sparse stone gravel ranges up to pebble size; over­lain by sandstone similar to basal part of unit, but near top becomes platy; compact, thin and cross bedded, becoming thick-thin to thin beddea near lowest 2 ft forms very top; ledge; overlain by 25° slope to ledges above; weathers to smooth 5urface.............. 66 341-407 8. Covered zone, probably thin-bedded sandstone and siltstone; forms distinct shelf, then 15°-20° slope up to overlying resistant sand­ stone; slope platy, 15 covered with mm sand­ stone fragments 30 311-341 abundant flecks 7. Sandstone, pinkish gray with of pale reddish brown iron oxide; weathers - grayish orange pink moderate orange pink (sm. 5/3) and very pale orange; fine-grained. subangular, moderately well sorted, polished and frosted quartz with calcareous cement; abundant inter- perhaps some silica near base; granular iron oxide causing speckled appear­ance; lower part contains 10-15 long masses which result in cavities weathered of clay, on surface; unit compact; thin and cross bedded, especially upper 8 ft; lower 7 ft and upper 8 ft more resistant than middle part, which forms 10° slope 28 283-311 6. Sandstone, white, pale red (10R6/2), and and rocks pinkish gray; white pinkish gray have abundant moderate reddish brown iron oxide; weathers moderate orange pink (10R8/2), and red fine-to pale brown, pale (10R6/2); sorted medium-grained, subangular, well calcareous slightly argillaceous quartz; cement 272-273 ft AB pale red very fine-to fine-grained quartz sandstone; moderately sorted, subangular to subrounded, calcare­forms similar ous cement; ledge; upper part to lower part but less well sorted; upper part forms both ledge; have abundant inter- granular iron oxide; compact; thin bedded; weathers to smooth surface; except for ledges, unit forms 25° slope largely covered with sandstone debris 28 255-283 5. Sandstone, pinkish gray with speckled appear­ance caused by abundance of moderate reddish brown iron oxide; weathers pale red (10R6/2), brownish brownish fine­ - light gray gray; to grained, angular subangular, moderately and silica- sorted, largely polished calcite- cemented quartz with abundant intergranular iron oxide; slightly argillaceous; compact; thin to very thin bedded; cross bedded at 240 ft AB; distinctive contorted bedding occurs in lower 15-20 ft and upper 1 ft; to joints: NBO°E, 65°SE, 3-6 in.; weathers smooth surface; forms series of resistant ledges of different thicknesses......*.. 35 220-255 4. Sandstone, very fine-to medium-grained, siltstone, and limestone; sandstone, grayish orange pink (10R8/2), white, light brownish - brownish gray, pinkish gray, and light gray gray; upper speckled; lower and sandstone weathers brownish gray (SYRB/1), pale yellow­ ish brown (10YR6/2), brownish gray, pinkish and brownish gray, pale brown, light gray; siltstone, greenish gray (SGY6/1), weathers light olive gray (5Y6/l) and pale brown; most of unit is fine to medium grained, moderately sorted, subangular quartz sandstone, argilla­ calcareous but some sili­ ceous, with largely ceous cement at thick bedded and top; very cross bedded at base; thin bedded upward; also joints: N5°E, 75°NW, 6 in.; S7O°E, 75°SW, 8 in.; at 131 ft AB, 8-in. brownish gray, very well fine-grained quartz sandstone; subrounded, sorted; calcareous cemented; 176-179 ft AB at 186 greenish gray; compact siltstone; ft AB, nodular foraminiferal very finely crystalline at 1 limestone; 196 ft AB, ft coarsely crys­talline fossiliferous lime- silty and sandy stone, poorly exposed in 35° slope; very brittle; fossil float, Tylostoma sp.; 3-4 in. limestone granule conglomeratic fine-to at 212 medium-grained quartz sandstone ft AB; unit forms 25°-30° largely covered slope with a few resistant ledges 110 110-220 3. Sandstone, fine-to medium-grained and silt­stone; sandstone, very pale orange-pale yellow­ish brown; speckled pinkish gray; weathers light brownish gray and pinkish gray; silt­ stone, pale red (10R6/2), light olive gray (5*6/1); weathers light brownish gray; sand­stone, quartz well sorted, subangular, argil­laceous largely frosted; calcareous cement; lower 6-7 ft, thin bedded, laminated at bottom; somewhat fiiable at top; forms small ledges; 25° slope above largely covered, probably thin-bedded sandstone and non­ resistant siltstone; calcareous nodules up to 60 mm occur on slope 70 ft AB; compact; 6-in. olive gray exposed 105 siltstone at ft AB otherwise covered . on slope 61 2. Limestone, medium light gray, weathers light brownish gray; very finely crystalline; silty; conchoidal fracture; hard; thin bedded; weathers to smooth surface; forms small ledge 1 48-49 1. Sandstone, white and very light gray; weathers pale yellowish brown, light brown­ish gray-brownish gray; fine-to fine- very grained quartz, poorly sorted, subangular, frosted and polished; calcareous cement; compact; thin to very thin bedded; cross bedded; lower 15 ft covered, may be pale red siltstone; ft prominent ledge, 35-36 ft A.B; 1 weathers to smooth 5urface.............. 48 0-48 T0ta1.......... 1267 Bluff Formation. .not measured ............ Measured thickness, Cox Sandstone..., 126? Total thickness, MS 10 126? Measured Section 11 Formation: Espy Limestone Location: G.O-8.8; section begins at base of small ridge about three-fourths of a mile east-southeast of Triple tanks and three-fourths of a mile west-southwest of Little Hill; Espy ranch; GSLU no. Series, photo 4-192. Measured: August 2, September 22-23, 1959, by Underwood with Brunton and 5-foot staff. compass Thickness, feet Unit Description Feet above base CRETACEOUS Eagle Mountains Sandstone not measured Limestone Espy 34* Limestone, light gray, light olive gray-olive medium gray (5Y5/l), light gray, weathers pale yellowish brown, brownish gray-olive gray; very finely crystalline; intraclastic texture; fossiliferous; abun­ dant pelecypods, gastropods; number of fossils increases upward, as does compact­ ness of rock; thin bedded; near base weathered surface shows pinpoint-size algal pits; forms slight break in southwest ...... SlOpe.#«.»B.e©.»»9.»»»»®o»®o«.w9a. 5 2188-2193 33• Covered zone, probably thin-bedded sandy limestone; forms distinct shelf sloping s°~ 10° southwest; surface covered with 0.5­ ........ 1 in. sandy limestone fragments. 4 2184-2188 32. Calclithite, pinkish gray; weathers grayish orange pink (SYR7/2); fine-to medium- grained, poorly sorted, rounded to sub- rounded largely very finely crystalline limestone; some fossil fragments, probably pelecypods; thin bedded; faintly laminated; weathers to smooth surface; forms uppermost distinct ledge of northeast-facing cuesta; crest of cuesta at 2183 ft AB 5 «... 2179-2184 31. Covered zone, probably thin-bedded limestone; forms shelf, gently sloping northeast, cov­ ered with 0.5-1 in. limestone fragments. 10 2169-2179 30. Limestone, very light gray; weathers pale yellowish brown; intraclasts of very finely and fossil crystalline limestone fragments in a calcite matrix; abundant coarser caprinids; iron oxide coats some fracture thin to thick surfaces; compact; bedded; somewhat nodular; forms distinct ledge.. 6 2163-2169 29* Limestone, pale yellowish brown, medium brownish medium light gray-light gray, gray; weathers pale yellowish brown-grayish orange, medium light gray; grades upward from intra­ clastic and fossiliferous limestone to very fossiliferous limestone finely crystalline with irregular patches of coarse calcite; some marly interbeds; some iron oxide stain in in lower part; gastropods predominate fossil float: Nerinoides upper part; sp.; Toucasia sp., caprinids; pelecypod; compact; thin bedded; forms 10° northwest slope with only few distinct ledges; 2154-2155 ft AB, abundant gastropods, some turritella­llke.a 28 ...... a....... ......... ........... 2135-2163 28. Limestone, light brownish gray, medium light gray; weathers light olive gray (5Y6/l); very pale orange; very finely crystalline intra­clasts (Md 0.2 mm) in coarse calcite matrix; grades upward to very finely crystalline lime­stone with irregular patches of coarse cal­ cite; some calcite veinlets; thin bedded; ° Joints t N6B E, 82°NW, 4-6 ft; Nl5°W, 76°NE, weathers to smooth forms 2-4 ft; surface; only slight ledge 5 713072135 27. Largely covered; probably marl or thin-bedded limestone; 2114-2118 ft AB, light brownish gray-light olive gray, very finely crystalline intraclasts and fossil fragments (pelecypod and foraminifers) in a very finely crystalline and coarse calcite matrix; fossil float: abun­ dant Nerinoides sp., caprinid; unit forms dis­ tinct shelf covered with 0.5­ northeast-sloping 1 in. angular to subangular limestone frag­ment5................................... 26 2104-2130 26. Limestone, medium light gray-light brownish gray, weathers grayish orange-very pale orange; intraclastic and fossiliferous very finely crystalline limestone; contains pelecypod frag­ ments and foraminifers; calcite veinlets; joints; N6B°E, nv, 2-3 ft; nodular; forms distinct ledge.. 4 2100-2104 25. Covered zone, probably marl or thin-bedded limestone; valley flat covered with mesquite and creosote bush; on low terrace at about 1930 ft AB found Mortoniceras base of sp.; cuesta to southwest is ft AB 2,093 204 1896-2100 24. Limestone, medium gray-light brownish gray; weathers very pale orange; very finely crys­talline limestone, with calcite veins and veinlets; some iron oxide stain in upper part; lower 2 ft forms rough ledge that caps cuesta; above forms dip slope on which iron oxide concretions and pseudomorphs after pyrite occur as float; fossil float this zone; nautiloid, Pervinquieria sp., echinoids, Tetragramma sp•, and Pedinopsis 5p...... 48 1848-1896 23. Covered probably siltstone or thin- zone, bedded limestone; surface covered with 0.5 in. angular-subangular limestone pebbles and scattered iron oxide pseudomorphs after pyrite; fossil float; Lima Pecten wacoensis, (Neithea) georgetownensis, Alepes Tylos­ sp., toma sp., Pervinquieria sp., Enallaster sp., Tetragramma sp.; forms distinct shelf... 16 1832-1848 22. Limestone, medium light gray-brownish gray; weathers very pale orange; very finely crys­talline with unidentified microfossils; calcite also some pelecypod macrofossils; veinlets; compact; thin bedded; weathers to smooth forms small at base nodules; ledge of cuesta 1 1831-1832 21. siltstone or thin- Covered zone, probably bedded limestone; forms valley flat covered with and creosote bush mesquite 169 1662-1831 20. Limestone, light brownish gray-light olive weathers gray; light gray; very finely crys­abundant micro-and macrofossil talline, with fragments, largely pelecypod; scattered irreg­ular patches of iron oxide; compact; calcite veinlets; forms slight ledge with relatively smooth weathered surface 2 1660-1662 medium brownish 19. Limestone, light gray-light gray; weathers very light gray, light gray; very finely crystalline with macroscopic fossil fragments, largely oysters; unit forms cuesta, series of relatively resistant ledges, less resistant zones covered alternating with with to rounded limestone angular fragments, Md 1 in.; iron oxide on fracture surfaces appears about 1620 ft AB; crest of cuesta at 1602 ft AB; at 1640 ft AB, pelecypods abun­dant; fossil float, near top of unit and low on dip slope of cuesta; Pecten (Meithea) sp., oryphaea echmoid*. sp., ................ 148 1512-1660 18. Covered zone; probably siltstone or thin-bedded forms limestone; valley flat covered r with mesquite and creosote bush; at about 1490 ft AB, covered northeast slope of next cuesta begins 797 ... 715-1512 17. Limestone, medium light gray-light brownish weathers gray; very light gray; very finely crystalline with micro-and macroscopic fossil fossil fragments, largely oysters; content increases upward; some iron oxide stain in upper part; compact; thin bedded; smooth weathered surface; unit is a series of rela­ tively resistant nodular limestone beds inter- bedded with nonresistant covered beds; resist­ant and non-resistant beds average 4-5 ft; top of cuesta at 625 ft AB 126 ... . 589-715 16. Limestone, medium light gray-light brownish weathers gray; very pale orange; very finely crystalline; iron oxide coats some fracture surfaces; scattered oyster fragments; brittle; highly fractured; thin bedded; forms irregular blocky ledges with marly interbeds on north­ east face second cuesta along line of section 40 549-589 15* Covered zone, probably marl or thin-bedded limestone; forms 10°-15° northeast slope covered with 0.25-4 in. limestone frag­ments. 22 527-549 14* Limestone, color as in unit 16; very finely crystalline with abundant microscopic fos­sil fragments, also some macroscopic oyster fragments; less fossiliferous upward; com­pact; thin bedded; smooth weathered surface; forms three distinct nodular ledges and def­inite change in slope 19 508-527 13• Limestone, similar to unit 16; unit has thin marl interbeds; fossil float; Pecten (Neithea) texanus, Pecten (Neithea) georgetov;nensis , Pecten (Neithea) sp., Cyprimeria sp., Gryphaea vashitaensis, Tylostoma sp., Mortoniceras sp., Mortoniceras cf. wintoni, Drakeoceras cf. . drakei , _D. cf gabrielensis , Drakeo ceras sp., Paracymatoceras texanum, Pervinquieria cf. trinodosa, Eutrephoceras sp., Enallaster sp., Epiaster elegans, Holaster simplex; forms shelf with slight northeast slope; much of upper part covered; iron oxide psuedomorphs after pyrite slope 30 occur on 478-508 12. Covered zone, probably marl or thin-bedded limestone; forms shelf with slight northeast slope; surface covered with subangular 0.25­ 4 in. limestone fragments; fossil float: Gryphaea washitaensis t Pecten (Neithea) sp., Tylostoma sp., Alepes sp., gastropod, Para­ cymatoceras sp., Mortoniceras sp., Epiaster ............ ......... 19 459-478 11. similar to unit fossil float: Limestone, 16; Pecten (Neithea) sp., Mortoniceras sp.; calcite veinlets; thin bedded; nodular; forms slight ledge; smooth weathered 5ur­face.................................... 0.5 458.5-459 10. Covered zone, probably marl or thin-bedded limestone; forms gently northeast-sloping bench covered with 0.5-5 in. limestone frag­ ments and iron oxide pseudomorphs after pyrite; fossil float: Alepes sp., Para­cymatoceras texanum, Drakeoceras kummeli, Hoiaster simplex 25 ................«. 433.5-438.5 brownish olive 9. Limestone, light gray-light weathers gray; very pale orange; very finely crystalline with abundant micro-and macro­scopic fossils, largely oysters and gastro­pods; calcite veins common; compact; thin bedded; forms small ledge at boundary with valley fill. 0.55 ...... ..............•••.••a. 433-433*5 8. Covered zone, probably marl or thin-bedded limestone; forms valley flat covered with mesquite and creosote bush 287 .............. 146-433 7. Limestone, medium light gray-light brownish gray; weathers very pale orange; very finely crystalline; homogeneous; dense; calcite vein- lets; a few scattered large Pecten sp.; thin interbedded with covered bedded, nodular; beds of siltstone or marl; forms crest of ridge and ls°-20° southwest 510pe....... 79 67-146 6. Covered as in unit 8...* 11 56-67 zone, 5. Limestone, as in unit 7; scattered flecks of iron oxide; a few oyster fragments; forms ........................0... slight ledge q 51-56 4. Covered zone, similar to unit 8, except it forms bench covered with 0.25-0.75 in. ............. angular limestone fragments 19 32-51 3. Limestone, similar to unit 5; fossil float; Pecten (Neithea) sp., Gryphaea washitaensis; forms gentle northeast slope J7 25-32 2. Covered zone, probably marl or thin-bedded limestone; forms bench covered with angular 0.25-1.0 in. limestone fragments and iron oxide pseudomorphs after pyrite 19 £-25 1* Limestone, color similar to unit 7; very finely crystalline with considerable micro­scopic fossil fragments; also, some oysters in lower part; fossil float; Feeten (Neithea) sp., pelecypods (oysters), Nerinoides sp., Tylostoma sp., Engonoceras sp., Pervinquieria Mortoniceras iron oxide on fracture sp., sp.; surfaces; compact; thin bedded; weathers to 0.5-4 in. angular fragments; forms small ledge just above valley flat; thin marl break between upper and lower parts .......... 6 . OTg TotaLeeeoottt) 2193 Alluvium* o»o«o»9»oa»«®o»«oooue<>8»®®»»not measured •£!«*••»• Measured thickness, Espy Lime stone 2193 .......0 ••. Total thickness, MS 2193 Measured Section 11a Formation: Espy Limestone Location: G.b-8.8; base of section about a mile and a half south-southeast of Triple tanks and a little more than a mile and a half northeast of the Speck no. ranch house; Espy ranch; GSLU Series, photo 4-192. Measured: September 24, 1959, by Underwood with Brunton and 5-foot staff. compass Thickness, feet Unit Description Feet above base Alluvium not measured CRETACEOUS Espy Limestone medium 7. Limestone, yellowish gray (5Y7/2), light light brownish weathers - gray gray; very pale orange; very finely crystalline with micro-and macroscopic fossil frag­ ments; fossil float: near base, Proto­cardia sp., Pe cten (Neithea) sp., Exogyra Plesioturrilites ~ sp Trigonia stolleyi, brazoensis, Plesioturrilites sp., Para­cymatoceras 150-165 Haplostiche sp.; ft AB, texana, Kingena wacoensis, Protocardia texana Cyprimeria sp., Pecten (N~eithea) , texanus; compact; conchoidal fracture; thin bedded; largely gravel covered, 165-175 ft AB; forms slightly uneven, gentle southwest 510pe....a...........0.00............... 53 127-180 6. Limestone, light olive gray (5Y6/l); weathers brown - pale yellowish grayish orange; very with abundant micro-and finely crystalline fossil fossil macroscopic fragments; float: Gryphaea washitaensis, Pecten (Neithea) sp., Enallaster sp., Holectypus sp.; compact; conchoidal fracture; thin bedded; weathers to nodules... 123-12? - 5. Limestone, light brownish gray light olive - gray; weathers pale yellowish brown very pale orange; very finely crystalline; homo- fossil geneous; very few microscopic frag­ments; fossil float: Cymatoceras sp.; Enallaster sp., Holectypus sp.; compact; conchoidal fracture; thin bedded; weathers to smooth nodules....0...12 111-123 4. Limestone, yellowish gray (5Y7/2) and medium light gray; weathers pale yellowish brown and very pale orange; medium crystalline with scattered irregular patches of very finely crystalline limestone; unidentified fossil fragments; iron oxide pseudomorphs after pyrite; compact; conchoidal fracture; thin bedded; yellow calcite fills near-vertical joints, NlO°E; smooth weathered surface. 2 109-111 3* weathers Limestone, yellowish gray (5Y7/2); very pale orange; very finely crystalline with microscopic fossil fragments; also Haplostiche texana; fossil float: Pecten (Keitheal texanus, Pedinopsis sp.; upper part more fossiliferous; compact; con­choidal fracture; thin bedded; weathers to smooth surface nodules.... 23 86-109 2. Covered zone, probably thin-bedded lime­ stone 8 14. • - 1. Limestone, medium light gray light brownish weathers light gray; very gray; very finely crystalline with scattered foraminifers and other microscopic fossil fragments; a few macroscopic fossil fragments, probably oysters; some iron oxide stain; compact; conchoidal fracture; thin bedded; weathers to angular to subangular fragments surface bears algal whose P3.t5....9e«9......0...0..9..a.c.«tt...... 2 o^2 Total..lBo Alluvium. ••«••••••••••••••«• .not measured ISO Liraestone......»... • Measured thickness, Espy 180 MS 11a ••••• • Total thickness, Measured Section 12 Formations: Buda Limestone, Eagle Mountains Sandstone Location: G.7-8.7; begins near top MS 11a; a little less than a mile and a half northeast of the Speck ranch house and several hundred feet north of east-west Speck-Espy boundary fence. Measured: September 23-24, 1959, by Underwood with Brunton and compass 5-foot staff. Thickness, feet Unit Description Feet above base CRETACEOUS Chispa Summit Formation not measured Buda Limestone 23. Limestone, light brownish gray changing to medium dark gray near top: weathers very light gray, pale yellowish brown- very pale orange; very finely crystalline with abundant clear, near-spherical calcite bodies and some microscopic fossil frag­ ments; upper part has very finely crys­talline intraclasts (Md 0.15 mm) with scattered foraminifers and abundant iron oxide stain on fracture surfaces; compact; becomes more brittle upward; resistant ledges characterized by karrenfelder sur­face; less resistant parts weather to smooth-surfaced of massive nodules; top ledge at 355 ft AB 37 .... 332-369 22. Limestone, light brownish gray; weathers very light gray, very pale orange; very with abundant clear finely crystalline cal­otte particles of varied shape and size (Md 0.075 mm); micro-and macroscopic unidentified fossil floats fragments; fossil Tylostoma sp., Turritella sp., Budaiceras traces of iron oxide stain fractured on sp., surfaces; compact, conchoidal fracture; weathers to smooth-surfaced nodules; forms series of slightly resistant ledges inter-bedded with less resistant beds of marl, silt­stone or small crest at 195 limestone; ridge ft AB, another at 250 ft AB; at 211 ft AB, 1 ft ledge contains oysters and Pecten (Neithea) sp. 183 ...........0... “' 149-332 21. Covered zone, but probably limestone similar to unit 22; covered with light gray-weathering limestone Cyprimeria Turritella Enallaster 20. Limestone, fossil float? fragments (Md 1 in.); Anchura sp., Trigonia sp., sp., sp., Tylostoma sp., Budaiceras sp., sp., echinoid... 134-149 light gray; weathers pale yellow­ ish brown-very pale medium orange; with abundant intergranular flecks oxide; calcite veins and veinlets; thin bedded; weathers to smooth but surface; fossil float, either from or covered zone below? Pholadomya crystalline of iron compact; irregular this unit sp.; Pecten (Neithea) sp., pelecypod, Tylostoma Anchura Enallaster unit forms sp., sp., sp.; nearly-horizontal shelf 4 130-134 Total.. 239 . Eagle Mountains Sandstone 19» Limestone, sandy; pale yellowish brown-very pale orange; weathers light gray and pale yellowish brown-grayish orange; lower 1-1.5 ft very finely crystalline intraclasts (Md 1.5 mm) with abundant macrofossils, probably oysters; almost a hash; sand is very fine-grained, well-sorted, angular quartz; grades upward to cross-laminated limy very fine grained quartz sandstone, much of which is desert varnished; unfossiliferous; entire unit compact, thin bedded; forms near- horizontal 5urface.........0....®....... 3 127-130 18. Covered zone, probably siltstone or thin- bedded sandstone; surface littered with of Buda Limestone 23 fragments 104-127 17. Sandstone, limy; very light gray-pinkish weathers gray; pale yellowish brown; very fine-to fine-grained, well sorted, sub- rounded polished and frosted quartz; some intergranular flecks of iron oxide; fos­sils: abundant small clams (Md 7.5 mm); one specimen of Pinna sp.; joints: N4O°E, nv, 6 in.; smooth weathered surface; unit forms near-horizontal surface. 3 101-104 16. Covered thin bedded sandstone; zone; probably surface littered with fragments of Buda Lilmest one 13 .............0................. 82-101 15. Sandstone, limy; medium light gray-light brownish weathers brownish gray; gray; very fine-grained well sorted, subrounded­subangular quartz; thin bedded, cross lam­inated; weathers to smooth surface; crops out as slight ledge on horizontal sur­faCe. ©©«...®.0.®®*00.0..00»®*........... 4 78-82 14* Covered zone, probably thin bedded sandstone; surface covered with 0.25-0.5 in. sandstone fragments with desert varnish... 14 nv, N3O°W, nv; faint bedding plane slickensides; smooth weathered surface; forms laterally persist­ ent . ledge 4 60-64 12. Covered zone, as in unit 14............. 14 11. Sandstone, limy; pale yellowish brown-very pale orange; weathers grayish brown; very 13. Sandstone, light brownish gray; weathers pale brown; well to sorted, subangular ished and frosted quartz; compact; homogeneous; thin inated; joints? N53°E, 64-78 gray-light olive very fine-grained, subrounded, pol- calcareous cement; bedded and lam- and fine-grained, well-sorted, subangular frosted rounded, polished and quartz; small clams in unit iron oxide as 17; thin bedded and many grains; compact; to sub- contains coats lam­ inated; weathered surface smooth; clam bur­ rows(?) common; unit is largely covered but considerable float litters surface...... 3 43-46 10. Covered zone, as in unit 14•••••••«••«. . 8 35-43 9. Sandstone, light olive gray; weathers pale yellowish brown-grayish orange; very fine- grained, moderately sorted, subangular, largely polished quartz; siliceous cement; compact; thin bedded and laminated; under­ side of one sample exhibits indistinct flute casts; color of unit is in sharp contrast to those nearby.. 3 32-35 8. Sandstone, pale yellowish brown-grayish orange, plae yellowish brown-very pale orange; weathers moderate yellowish brown and pale yellowish brown; very fine-grained, moderately sorted, subangular, polished and frosted quartz; argillaceous; calcareous ce­ ment, grades upward to fossiliferous very fine-grained quartz sandstone similar to that below; fossils are small clams as in units 11 and 17; compact; thin bedded; lower part cross laminated; smooth weathered sur­ face*.... . 1 28-32 7. Covered zone, probably thin-bedded sand­ stone or siltstone; surface littered with 0.5 to 1 in. desert-varnished sandstone fragm0nt5....a.0»ta«©«00*..0..*0...0®.».9 7 19-28 6. Siltstone, light olive gray and pale yellow­ ish brown; weathers moderate yellowish brown; coarse, moderately sorted, angular to sub- angular, largely polished quartz silt; abun­ dant intergranular iron oxide; calcareous cement; compact; thin bedded and cross lam­ inated; weathered surface smooth and desert varnished; poorly exposed. 5 14-19 Covered in unit 5* zone,as 7 3 11-14 4« Calcarenite, dark yellowish brown; weathers moderate brown (SYR4/4) pale brown; fine­ - grained, poorly sorted, subangular to sub- rounded, limestone with some quartz and sand size fossil fragments; highly fossil­ - iferous a pelecypod (oyster) hash; Exogyra cartledgei, Gryphaea graysonana; very thin bedded; platy; well indurated; weathered surface uridulditory 1 ...0.....,.*...,...,,. 10-11 3* Sandstone, moderate brown; weathers moderate yellowish brown, very fine-grained, well sorted, rounded to subrounded quartz; argil­ laceous; calcareous and iron oxide cement; a few oyster fragments accompanied by un­usual iron oxide concentration; very thin bedded; laminated, smooth weathered 5ur­faCe...ft.a«..0...a.»ft«e..«0.e...e.0..4,.. 4 6-io 2. Covered zone, as in unit 7 4 2-6 1. Sandstone, medium gray; weathers moderate sub- yellowish brown; very fine-grained, rounded, moderately sorted, frosted and polished quartz; pale blue green abundant flecks calcareous (Md 0.075 mm); cement; compact; thin bedded; cross laminated; calcite veinlets; clam burrows(?); smooth weathered 5urface..i....i..i0*.....ii..i 2 o^2 T0ta1.,., 130 Espy Limestone not measured Measured thickness, Buda Limestone. 239 Eagle Mountains Sandstone 130 Total thickness, MS 12 369 Measured Section 13 Formation: Trachyte porphyry (Ttp) Location: J.6-9»4| base of section is contact between lower rhyolite and trachyte porphyry at steep cliffs on the west flank of the Eagles; near end of road that runs through water gap in Speck Ridge and up gravel terrace to base of cliff; Speck ranch; GSLU Series, photo no. 4-118, 119* Measured: October 1, 1959, by 6-foot steel tape. Underwood with hand level and Thickness, feet Unit Description Feet above base TERTIARY Trachyte porphyry 4. Trachyte porphyry, ranges from grayish red (10R4/2) to pale reddish brown; changes near top to medium light gray, then to pale red; uppermost rock variegated pink­ ish gray, pale red (10R6/2) and white; unit weathers over-all grayish red (10R4/2) to pale reddish brown; many surfaces coated by black desert varnish; aphanitic matrix with euhedral to anhedral feldspar pheno­ crysts ranging up to 7.0 mm (Ms 2.5 mm); feldspar white to moderate orange pink (10R7/4); scattered quartz phenocrysts; rock weathers readily; much secondary iron oxide; compact; weathers to smooth- surfaced angular blocks . ... o « 609 134-743 3. Trachyte porphyry, grayish orange with irregular pale red (5R6/2) splotches; weathers moderate brown (SYR4/4) and gray­ ish red matrix feld­ (10R4/2); aphanitic; spar phenocrysts highly altered and in­distinct; liesegang bands common; compact; weathers to angular blocks with smooth sur­ face; forms steep slope. 84 50-134 ...... 2. Trachyte porphyry, grayish red (5R4/2), pale (5R6/2); pale brown, red weathers matrix grayish red (10R4/2); aphanitic with white euhedral to anhedral feldspar to 2.0 phenocrysts ranging up 5 mm (Md less abundant mm); phenocrysts smaller, and less distinct near top; compact; weathers to angular blocks with smooth surface; forms steep 510pe.............. 4-50 1. Trachyte porphyry, medium light gray; weathers medium dark gray; aphanitic matrix; white, euhedral to anhedral feldspar pheno­crysts range up to 4 (Md 2*5 mm); mm com­ pact; sub conchoidal fracture; weathers to smooth, rounded surface; forms steep slope; below is 6-foot transitional zone with lower rhyolite in which frag­ occurzone ments of trachyte porphyry and lower rhyolite (not included in this 5ecti0n)...... .... 4 0-4 T0ta1.743 Lower rhyolite not measured Measured thickness, trachyte porphyry... 743 Total thickness, MS 13 743 Measured Section 14 Formation: Hueco Limestone Location; K.9-14*8; base of section on northeast flank of hill of Permian limestone; just west of road Mountains from Hot wells to Eagle ranch; Wyche ranch; GSLU Series, photo 4-80. no. Measured: October 9? 1959? by Underwood with Brunton compass and 5-foot staff. Thickness, feet Unit Description Feet above base CRETACEOUS lueca Fcrmati0n..©.©......©©*.®..............n0t measured PERMIAN Hueco Limestone medium dark weathers to 9* Limestone, gray; light olive (5Y6/1) and light gray; gray finely very finely crystalline; to occa­ sional irregular nodules and patches of chert; at 879 ft AB 1-ft zone of tabular, irregular chert masses; 891-893 ft AB irreg­ular masses of chert up to 8 in., maximum diameter; above 965 ft AB, irregular chert masses common; 890-893 ft AB contains abun­ dant silicified echinoid spines, gastropods and brachicpods; 993-994 ft AB, silicified gastropods up to 2 in. in diameter; approx­silicified imately 1050 ft AB, solitary cup corals(?); from fetid odor fresh frac­ ture surface throughout; compact, but some­what thin bedded; nonresistant brittle; covered at zones, probably shale, 818-822, 826-830, 832-838 and 841-850 ft AB; largely covered above 960 ft AB; weathers to rela­tively smooth surface; forms part and upper 20°-30° southwest slope of hi11...... »•. 311 749-1060 8. Limestone, medium dark gray; weathers a very distinctive very light gray; very finely crys­talline; fetid odor on fresh fracture surface; laminated near weathers to smooth platy; top; solution surface only slightly marred by cav­ ities; forms minor ledge on 15 o *-20° northeast SlOpe»oe»o»©®6io**».»ei« 22 zone, 1500-1522 light brownish weathers 63* Sandstone, gray; pale yellowish brown; very fine-grained mod­erately sorted, subrounded, largely polished quartz; calcareous and siliceous cement; intergranular iron oxide; thin bedded; poorly exposed on 20° dip slope; weathers to smooth surface 4 ........ 149^-1500 weathers unit 62. Sandstone, pinkish gray; as 67; fine-to medium-grained moderately sorted, and frosted sili­ subangular, polished quartz; ceous cement near base; about midway, siliceous and calcareous cement; conglomeratic near base where scattered granules of chert and clay (Md 3 mm); weathered surface pitted with cav­ities where granules weathered out; otherwise weathered surface smooth; thin to thick bedded; near base weathers to rounded boulders; from is 1-2 in. sandstone 1330 ft AB, slope frag­ men 1273-1496 not 61. Sandstone, as in unit 62, but conglomeratic; calcareous and siliceous cement; several con­ centrations of dark reddish brown iron oxide up to 5 mm; cross bedded; weathers to smooth- surfaced somewhat rounded boulders or forms unit ledge; some surfaces pitted, as in 62, where iron oxide concentrations weather OUt. 18 ...........a. .........oe«.a. eosa.no® 1255-1273 medium weathers 60. Sandy limestone, light gray; (5Y7/2); medium to coarsely yellowish gray limestone with scattered fine- crystalline and hash grained quartz oyster fragments; a near the base; thin weathers to very bedded; rough surface; forms first resistant ledge on southwest dip slope of Cox beyond DEVIL surface covered with triangulation station; limestone float 4 »<><>•••••• 1251-1255 59. Covered probably shale or thin-bedded zone, forms first saddle southwest of limestone; DEVIL surface covered triangulation station; with limestone float 12 1239-1251 weathers 58. Sandstone, pinkish gray; very light and red fine-to fine- gray pale (5R4/2); very grained, well sorted, subangular, largely polished quartz; siliceous cement; intergran­ ular iron oxide; argillaceous; cross bedded; lower thin part bedded, nonresistant; grades upward to resistant ledge that caps high point on which DEVIL triangulation station is located; weathers to smooth surface.. 106 1133-1239 57. Sandstone, pinkish gray; weathers pale brown; fine-grained, well sorted, subangular, largely polished quartz; slightly argillaceous; sili­ceous cement; intergranular iron oxide; thin to thick bedded; cross bedded; forms resistant ledge, intensely jointed; weathers to smooth surface................................. 21 1112-1133 56. Covered zone, probably thin-bedded sandstone; forms 30° northeast-facing slope littered . with fragments of sandstone, 1-2 in. 20 ... 1092-1112 55 • Limestone, medium light gray to light brown­ish weathers gray; very pale orange; very finely crystalline; oyster fragments through­out; macrofossils: Pecten (Neithea) irregu­ laris , Exogyra texana; thin to thick bedded: at ft AB is of nodular zone base; 10?2 top uppermost resistant unit; rock above is less resistant; rough weathered surfaces..... 42 1050-1092 . 54* Covered zone, in unit 56......... 15 as ••« 1035-1050 53. as in unit 58, except nvb; con- Sandstone, choidal fracture..... 32 1003-103 5 medium 52. Sandy and silty limestone, light gray; weathers yellowish gray (5Y7/2); very limestone with of finely crystalline grains fine-to very fine-grained quartz sand and quartz silt; iron oxide coats fracture sur­ faces; fossil float; Actaeonella sp., Nerinea sp., Tylostoma sp., caprinid; oysters; thin to thick bedded; weathered surface smooth to ............0..9.e0....0»»0.»». 96.. rough 19 984-1003 51. Covered zone, probably thin-bedded sandstone; 25° northeast-facing slope littered with 0.5­2 in. sandstone fragments...#..... 28 956-984 50. Sandy and silty limestone, grayish orange; weathers pale yellowish brown; very finely crystalline limestone with scattered grains of very fine quartz sand and quartz silt; and veinlets of black min- irregular patches eral (manganese?); weathered surface pitted; .. e forms slight ledge 2 954-956 49 Covered as in unit 51 24 • zone, 930-954 48. Sandstone, pinkish gray; weathers grayish lower orange pink (SYR7/2); part as in lower part unit 58, except that cement both cal­ careous and siliceous; grades up to very fine- to fine-grained, moderately sorted, sub-rounded, largely polished quartz; laminations marked by iron oxide; some copper carbonate (green) stain; platy; along unit con- strike tains a lens of sorted con- gray, very poorly glomeratic sandstone; upper part unit of largely covered 34 896-930 Covered as in unit 51 16 * 47* zone, 880-896 46. Sandstone, light olive gray (5Y6/l); weathers pale yellowish brown; very fine- well to sub- grained, sorted, subangular rounded; calcareous cement; argillaceous; thin bedded; weathers to smooth compact; surface: forms 20° northeast-facing slope.. 2 878-880 same as unit cement is 45* Sandstone, 58, except both calcareous and siliceous; thin to thick bedded; red-brown iron oxide coats fracture surfaces and bedding and cross- bedding planes; forms ledge.... 9 869-878 44* Covered as in unit 51••••......... 55 zone, 814-869 43* Sandstone, as in unit 45; middle part thin bedded and cross bedded; pitted weathered surface 16 798-814 42. Covered as in unit 51......... 8 ..., zone, 790-798 41. Sandstone, as in unit 45* except weathered surface not pitted; forms series of ledgeS.. 53 ...........»e......0..00e..09... 737-790 40. Covered zone, as in unit 51 •••••••»••••• 24 713-737 39* Sandstone, as in unit 58; pitted weathered surface....... 11 702-713 38. Covered zone, as in unit 51 37 702 in unit for very pale calcareous 37. Sandstone, as 58 except 9-ft, orange, highly and argil­laceous, very fine-grained quartz sandstone that begins 620 ft AB; at 653 ft AB sandstone laminated to very thin bedded, cross bedded, with cross beds 5-6 ft long and 2.5 ft high; weathered surface of upper 4 ft pitted; one 2-in. limestone pebble still in places; unit forms series of blocky 1edge5........... 52 613-665 36. Covered zone, as in unit 51»«» 9 604-613 35. Sandstone, as in unit 58, except not argil­also laceous; joints; N-S, 0.5-1 ft, nv; NB3 °W, I—2 ft, nv. J ......0.00.00.00e0.... 601-604 34• Covered zone, as in unit 51••• »<»........ 13 588-601 33• Limestone, medium light gray; weathers yellowish gray (5Y7/2); very finely crys­ talline with a few scattered fine- to medium- grained quartz grains; scattered unidentified fossil fragments; weathers to pitted surface; forms a ledge 3 585-588 32. Sandstone, lower part as in unit 58, but with scattered chert pebbles 0.5 in. or smaller; iron oxide stain along joints; grades up to 3 ft of variegated pale yellowish brown-pale red fine (10R6/2) very sandy, very finely crystalline limestone 11 574-585 31. Sandstone, resistant ledge at base as in unit to moder­ 58; grades upward medium-grained, to ately sorted, subangular subrounded, pol­ished and frosted with scattered chert quartz pebbles; siliceous and calcareous cement; pitted weathered surface with 0.25-0.50 in. cavities; lower part forms first resistant ledge that can be traced some distance along Strike. 15 .....................eeoo.«.oe«o. '559^74 30. Covered zone, as in unit 51••••.•.•••.•• 45 514-559 29» Sandstone, grayish orange pink (10R8/2); weathers pinkish gray; fine-grained, well sorted, subangular largely polished quartz with abundant intergranular red-brown iron weathers oxide; thin bedded and cross bedded; to smooth surface; forms 1edge.......... 8 506-514 28. Covered zone, as in unit 515 at 478 ft AB, grayish yellow green, fine-grained, calcare­ous quartz sandstone crops out through rubble on slope 39 ..a.0.*.«..0»0»>»0aae.09®0 467-506 27. as in unit 58; forms distinct Sandstone, ledge 2 * o•• 465-467 26. Sandstone, very pale orange, pinkish gray; weathers very light gray; medium-grained, well sorted, subangular to subrounded, frosted quartz; argillaceous; calcareous cement; some­ what friable at 405 has be- base; by ft AB, come resistant sandstone as in unit 58; unit on . is sparsely exposed slope.. 76 389-465 25* Covered zone, as in unit 51••• ••• «» ••29 o. . 360-389 weathers 2U» Sandstone, grayish orange pink well light brownish gray; fine-grained, to sorted, subangular subrounded; argilla­ceous; calcareous cement; abundant red- brown intergranular iron oxide; thin bedded; exposed only in small gu11y......»...••. 9 351-360 23 • Covered zone, probably thin-bedded sandstone; forms 10°-18° northeast-facing slope covered with sandstone fragments up to 3 in. in diameter * • 13 338-351 22. Sandstone, pale brown with irregular zones of greenish gray (5G6/l); very fine-to fine- grained, well sorted, subrounded, polished and frosted calcareous com- quartz; cement; pact; weathers to smooth surface; exposed only in small gully.. 10 328-338 21. Sandstone, pinkish gray; weathers very light gray; fine-grained, moderately sorted, sub- rounded, polished and frosted quartz; somewhat argillaceous; calcareous and siliceous cement; red-brown scattered particles intergranular iron oxide; grades upward to light greenish unit forms series gray fine-grained sandstone; ledges 8 320-328 20. Covered zone, probably thin-bedded sandstone; with forms 10° slope to northeast, covered 0.5-2 ft blocks of sandstone 26 ............ 294-320 19* Sandstone, as in unit 21; thin bedded and bedded cross 45 249-294 olive weathers 18. Siltstone, light gray (5Y5/2); pale yellowish brown; calcareous and siliceous cement; compact conchoidal fracture; thin bedded; slightly rounded surface; weathers to forms resistant ledge 1 248-249 in unit 17. Covered zone, as 20, except fragments of sandstone range from 0.5-4 in.......46 . 202-248 16. olive weathers Limestone, light gray; light gray; very finely crystalline limestone; upper part has irregular, scattered patches of orange iron oxide; above base 1-2 ft, limestone has abundant oyster(?) fragments up to about 12 mm (Md 2.5 mm); middle part of unit very brittle. 26 ............ ...a... .....a....... 176-202 15* Covered zone, probably thin-bedded sandstone or shale; forms gentle slope to northeast; covered with in. of largely 0.5-1 fragments sandstone... 23 153-176 14* Sandstone, pale yellowish brown; weathers pale brown; very fine-grained, well sorted, subrounded quartz; calcareous cement; thin bedded; weathers to smooth surface; forms resistant ledge of angular b10ck5....... 2 151-153 Covered in unit lower of 13. zone, as 15; part slope more gentle c...42 . .................. 109-151 12. Sandstone, pale yellowish brown-grayish orange; weathers pale brown; very fine-to subrounded fine-grained moderately sorted, to rounded quartz; calcareous cement; compact; thin bedded; joints: S7O°W, 0.5 ft, nv; also forms 1 S25°W, 0.5 ft, nv; slight ledge... 108-109 11. Covered zone, as in unit 13....... . 8 ....o 100-108 10. Limestone, medium light gray; weathers light gray; very finely crystalline with abundant microscopic fragments of oysters; fossil float: Tylostoma sp.; calcite veins up to 1 in. wide; rough weathered surface; forms slight ledge. 2 98-100 9. Covered zone, probably thin-bedded sandstone and shale; gentle northeast slope covered with fragments of sandstone and some lime­stone, 0.5-1 in. in diameter 25 .........o•• 73-98 8. Limestone, olive gray (5Y4/1)-light olive gray (5Y6/l)j very finely crystalline with scattered very fine to fine grains of quartz; intergranular orange iron oxide; conchoidal fracture; forms slight ledge of angUlar­blOCkS... 1 .....................««o....... “72^73 Covered as in unit 28 7. zone, 9•*••••. •..•. •. 44-72 6. Limestone, medium gray-light olive gray (516/1); weathers light gray; very finely crystalline with scattered grains of very fine quartz; intergranular iron oxide; micro­fossils; conchoidal fracture; thin bedded; becomes more brittle upward; karren on weathered surface; forms two ledges separated by covered zone 3 41-44 5. Covered probably shale; forms gentle zone, northeast slope covered with 0.5-4 in. angular fragments of limestone 7 34-41 4. Limestone as in unit 6; joints; N5O°E, 0.5-1 in. nv 2 32-34 3* Covered zone, as in unit 5........ •.... 10 . 22-32 2. weathers Sandstone, light gray; very pale to orange; very fine-grained fine-grained, well sorted, subangular to subrounded quartz; calcareous cement; slightly coarser, more argillaceous above; forms slight ledge.. 2 20-22 1. Covered zone, as in unit 5 ••••••••••••• • 20 0-20 T0ta1.......... 1737 Bluff Limestone • . .not measured Measured thickness, Finlay Limestone 588 «... Cox Sandstone 1737 ............ Total "thickn6ss MS 18....................... REFERENCES Adams, G. 8., 1953, Stratigraphy of southern Indio Mountains, Texas! Texas Univ. M.A. 64 Hudspeth County, thesis, p., 13 pis. Adams, J. E., 1929, Triassic of west Texas! Am. Assoc. Petro­ v. no. leum Geologists Bull., 13, 8, p. 1045-1055® , 1944, Highest structural point in Texas: Am. Assoc. Petroleum Geologists Bull., v® 28, no. 4, p» 562­ 564. Adkins, W. 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M.A. 116 8 20 this. thesis, p., 7 figs., pis., The vita has been removed from the digitized version of this document. The vita has been removed from the digitized version of this document. GEOLOGIC MAP OF EAGLE MOUNTAINS AND VICINITY, HUDSPETH COUNTY,TEXAS . STRUCTURE CONTOUR MAP OF TOP OF PRECAMBRIAN, EAGLE MOUNTAINS AND VICINITY