• GEOLOGY OF YELLOW HILL QUADRANGLE BREWSTER COUNTY, TEXAS ! GEOLOGY OF YELLOW HILL QUADRANGLE BREWSTER COUNTY, TEXAS by RICHARD PATRICK McCULLOH, B.S. THESIS Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of MASTER OF ARTS __... THE UNIVERSITY OF TEXAS AT AUSTIN August 1977 F R 0 N T I S P I E C E View east toward xenolith of Boquillas Limestone in stepped portion of Black Ridge sill, west of Black Ridge PREFACE Many people have contributed to this thesis. As an aspiring • vertebrate paleontologist who found himself with a thesis area that con­tains only a few small outcrops of Tertiary sedimentary rocks, when more extensive Tertiary deposits and associated Tertiary vertebrate faunas had been hoped for, I especially benefited from the assistance of others. The members of my thesis committee deserve special mention for their help and guidance. Dr. John A. Wilson suggested the problem, and served as supervisor of the project. I benefited much from his ex­perience and familiarity with areas adjacent to mine. Dr. Stephen E. Clagaugh helped with many matters, and was especially helpful with ig­neous and contact-metamorphic rocks. He also provided NASA funds avail­able to him for the purchase of enlargements of air photos of the thesis area. Dr. Leon E. Long improved the quality ·of the writing with his scrupulous editing. I am extremely grateful to Billy Pat and Sadie McKinney for their untiring assistance and support during the weeks spent in the field, and for providing field accommodations during the summer of 1974. Ron and Shirley Willard, of the Study Butte Store, Study Butte, Texas, were also helpful. Jim and Margaret Stevens assisted me on many occa­sions in the field. Glenn Hatcher helped with some 9f the field work, and shared use of a departmental jeep during the summer of 1974. I thank Dr. Virgil Barnes of the University of Texas Bureau of Economic Geology for providing me with the Burea~'s air photo cover­age of the thesis area for field mapping. I also thank J. W. Macon of the Bureau for his instruction in, and permission for the use of Bureau projection equipment, and for his counsel .regarding transferral of the map to the topographic base. Terlingua Ranch road maps were given to me by Jack North of Terlingua Ranch, and by Urban Engineering, of Corpus Christi, Texas. Many of the faculty and staff of the Department of Geological. Sciences of The University of Texas at Austin gave of their ' time and assistance during the course of this work. Dr. Fred McDowell instructed v me in basic techniques of sample preparation and mineral separation for potassium-argon dating, and performed the dating of my samples. Dr . • Daniel S. Barker was helpful in interpreting igneous rocks, and p·ro­vided chemical data for sills on Mesa de Anguila and Sierra Aguja. He also provided use of a computer program for obtaining norms from chem­ical analyses. G. Karl Hoops performed the chemical analyses. Dr. Lynton S. Land performed the oxygen isotope analyses of a chert sample, and helped interpret them. Dr. Keith Young identified fossils collected from the thesis area. Drs. Robert L. Folk and Earle F. McBride helped with their counsel in matters of sedimentary petrography. Discussions with Dr. W. R. Muehlberger regarding structural geology, and with Dr. R. 0. Kehle regarding sill emplacement, were also helpful. The following fellow graduate students deserve thanks for their help. Don Parker offered continual support, instructed me in sample preparation for chemical analysis, and provided norms for rocks analyzed. Steve McLean instructed me in the use of X-ray diffraction equipment. Jack Droddy X-rayed many contact-metamorphic samples, determined their mineralogy, and was helpful in interpreting these rocks. Tom Grimshaw was helpful in matters of base map screening and map reproduction. I benefited from many discussions with officemate Jack Donaho, and with John Bumgardner. Art Busbey and John Johnson, III, the student editor, contributed to my mental health on several occasions. I gratefully acknowledge the assistance of all these people, but I am solely responsible for any shortcomings of this work. I would like to thank the Awards Committee and the Geology Foundation of the Department for their continued financial assistance in the form of teaching assistantships, generous field support, and a fellowship. Finally, I thank my parents, Bob and Gracelyn Mcculloh, for whom I feel my deepest gratitude. This thesis represents a conglomeration of the skills, work and ideas of many people in addition to those of the author; I hope that I have presented the final product as a coherent whole. This thesis was submitted to the committee in March 1977. Richard Patrick McCulloh August ·1977 GEOLOGY OF YELLOW HILL QUADRANGLE, BREWSTER COUNTY, TEXAS by Richard Patrick McCulloh A B S T R A C T The Yellow Hill Quadrangle is between the Bofecillos, Davis, and Chisos volcanic centers of Trans-Pecos Texas. Tertiary volcaniclas­tic deposits occur only as small outliers on downthrown sides of faults. Erosion has removed the thin Tertiary section to expose the 1365 m sec­tion of Cretaceous limestones, which are mostly biomicrite (wackestone). Tertiary diabase sills have extensively intruded upper Cretaceous lime­s tone, but intruded the entire interval from .the upper Comanchean Buda Limestone to the middle Oligocene Mitchell Mes~ Tuff, inclusive. The sills form part of a belt of mafic sills which extends into Mexico as much as 70 km to the southeast. vii C 0 i'~ T E N T S TEXT Introduction . ................... I! "' • ~ ............ • • • .. 1•• ••• -••••• " " • • • • Climate and Vegetation................................................ 1 Land Use . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . .. . . . . . • . . . . . . . . • . . . . . . . . . . . 4 Geologic Setting.................................................. 4 Previous Work. . . . . . . . . . . . . . . . • . . • . . . . . . . . . . . . . . . . . . . . . . . . • . . . . • . . 5 Purpose and Methods of Investigation............................. 7 Physiography. . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . 8 Physiographic Expression of Intrusive Rocks..................... 8 Physiographic Expression of Sedimentary and Volcanic Rocks....... 8 Effect of Structure on Physiography.............................. 10 Drainage......................................................... 11 Elevation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Surficial Deposits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Stratigraphy and Petrography................ . . . . . . . . . . . . . . . . . . . . . . . . 13 General Statement................................................ 13 Cretaceous System................................................ 14 Comanchean Series.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Trinity Group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Glen Rose Limestone..................................... 14 Name and Type Section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Distribution, Thickness, and Lithology............... 14 Petrography........................................ . . 20 Fossils. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Correlation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Depositional Environment ............................. 22 Fredericksburg Group ....................................... 23 Maxon Sands tone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Telephone Canyon Formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Name and Type Section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Distribution, Thickness, and Lithology............... 23 Fossils and Correlation.............................. 24 Regional Features and Depositional Environment ....... 24 Del Carmen Limestone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Name and Typ~ Section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Distribution, Thickness, and Lithology............... 24 Petrography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Fossils ................... "..................... '11: 25 •• ll!l •••••• Regional Features and Depositional Environment ....•.• 25 Wash.i ta Group . ........... , ............ , . ~ .. ~ .. " . , ") "' • 26 It ...... ., ..." ... •• Sue Peaks Formation. • . . . . . • . .. • . . • . . • . . . . . . . . • . . .. . • • . • • . . 26 viii • Nanie and Type Section.....•.••..•...........••.•..•.• 26 Distribution, Thickness, and Lithology..• •. • ~ ······ · · 26 Petrography. . .. . . . . . . . . • . . • . . . . . . • . . . . . . . . . . . . . • . . • . • . 2 7 Fossils .......................... * ..... . ................. 27 Regional Features and Depositional Environment ...... • 28 Santa Elena Limes tone ..................... ~ . .. . . . . . . . .. . . . • . 28 Name and Type Section..... . . . . . . . . . . • . . . . . . . . • . . . . . . . 28 Distribution, Thickness, and Lithology ...... ......... 28 Petrography......................... . ................ 29 Fossils..... ..................... .................... . 30 Regional Features and Depositional Environment ....... 30 Del_Rio Clay. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Name and Type Section.... . ................... . ....... 30 Distribution, Thickness, and Lithology............... 30 Petrography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Fossils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Regional Features and Depositional Environment ....... 32 Buda Limestone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Name and Type Section................................ 33 Distribution, Thickness, and Lithology............... 33 Petrography.......................................... 33 Fossils.............................................. 35 Regional Features and Depositional Environment ....... 36 Gulfian Series. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Terlingua Group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Boquillas Formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Name an.d Type Section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Distribution, Thickness, and Lithology ............... 37 Petrography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Fossils.............................................. 38 Regional Features and Depositional Environment ....... 39 Pen Formation........................................... 39 Name and Type Section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Distribution, Thickness, and Lithology ............... 40 Pe trography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Fossils...... . ....................................... 41 Regional Features and Depositional Environment ....... 41 Tertiary System.................................................. 42 Eocene Series. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Buck Hill Group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Pruett Formation........................................ 42 Name and Type Section................................ 42 Distribution, Thickness, and Lithology............... 42 Pe trography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Fossils.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Regional Features and Depositional Environment ....... 46 Oligocene Series. . . . . . . . . . . . . . . . . • . . . . . . • . . .. . . . . . . . . . . . . . . . . . . 48 Mitchell Mes a Tuff. . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . • . . . . . 48 Nanie and Type Section............. . .... . .. . .. . .. . .... 48 • Distribution, Thickness _, and Lithology................ 49 Petrography... • .. . .. • . . • . .. .. . . • . . .. . . • . .. • .. • • . • • . . • . . . . • • . 49 Regional Features and Age... . • . . • . • • • . . . • . . .. • . . • .. . • . • • 49 • Quaternary System........ .. ..... . ...... . .,............ .. ........... . .. ...... 51 Quaternary Alluvium.. . • • . .. • . .. • .. . • . . . . . • • . . . .. • . . .. . . • . • .. . . . 51 Distribution, Thickness , and Lithology.•....••..•... 51 & Petrography...... • . • .. . • . . .. . . . • . • .. . . . . . . . • .. . .. . . . • . . • . . 51 Depa sitional Environment. . .. . . . . . . . . . . . . . . . . . • .. . . . . • . . 52 Intrusive Igneous Rocks.... . ........................ . ........... . ... 53 Diabase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • • . . . . . • . . . . . . . 53 Plugs ............._. . . . . . . . . . . . . . .. . . . . . . . . . . . • . . . . . . . . . . . . . . 53 Dikes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Diabase Sills. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Field Relations.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Petrography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 _Chemical-Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Hydrothermal Alteration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Vent Agglomerate... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Related Bodies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Agua Fria Quadrangle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Big Bend National Park ............ ············:········· 72 Mexico ................................. ·-.......-. . . . . . . . . 72 Intrusions in areas to north and northwest .............. 72 Origin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 ,. Age .........-............................................... 73 Rhyolite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 ?Analcite Syenite............................................. 76 Contact Metamorphic Rocks........................................... 77 Contact Metamorphic Rocks Associated With Diabase Intrusions ..... 77 Field Observations...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Mineralogy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Contact Metamorphic Rocks Associated With Trap-Door Dome ......... 80 Field Observations .......-. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Mineralogy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Chert Breccia Knobs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Field Relations... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Petrography.............................. . . . . . . . . . . . . . . . . . . . . . . . . 87 Stable Oxygen Isotopes and Temperature of Formation.............. 89 Origin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Paragenesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Structural Geology.................................................. 96 Fo1ds ..........................._. • . • • • . . . . . .. . . • . • . . . • . • . . . • . . . . . . . 96 Trap-Door Dome_. . . . . . . . . . . . . . . . • . . . . .. . . . . . . . . . . . . • . . . . . . . . . . . • . 100 Faults ........ ............... ......•..........................•... 103 Time of Faulting...... ......................................... 108 • Joints ...............•........................................... 108 ' . ~ Clastic Dik.e .............•........•.• ; . . . . . . . . . . • . . . . . .. • . . . . . • . . . 109 Ve ins •.•..•.•.•..•• ., ........... -. ., • . . • . • . . • • . • . • . . . • .. . . • . • . • . • . • . . • 111 Structural History................_. .•••.....• ,., ...•..••..•..•...... 112 Economic Geol.ogy .......................•......................... ~ .. 115 Water. . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Construetion Material. . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Ranching. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 Land Sales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 Ore Deposits ..................................................... 116 PetroleUill........................................................ 117 Wax. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 References Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Vita................................................................ 126 TABLES Table 1. Generalized Stratigraphic Classification in Yellow Hill Quadrangle............................... . . . . . . . . . . . . . . . . 19 2. Chemical Analyses and CIPW Norms for Two Samples of Diabase from Yellow Hill Quadrangle ....................... 66 3. K-Ar Data, Diabase Sills, Yellow Hill and Agua Fria Quadrangles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 4. .Possible Temperatures of Silica-Laden Fluid Which Silicified Breccia.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Frontispiece -PHOTOGRAPH: View East Toward Xenolith of Boquillas Limestone in Stepped Portion of Black Ridge Sill, West of Black Ridge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii Figure 1. MAP: Location of Yellow Yill Quadrangle..................... 2 2. DIAGRAM: Location Scheme Used in Text...................... . 3 3. MAP: Areas Mapped in Big Bend Region ....................... . 6 4. PHOTOGRAPH: View Southeast Across Block-Faulted Northeast Flank of Terlingua-Solitario Anticline............. 9 5. MEASURED SECTION: Composite Columnar Section; Generalized Stratigraphy and Petrography in Yellow Hill Quadrangle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . 15 • 6. MEASURED SECTION: (a) Section Measured Across Rim of Solitario ........................ 16 -18 (b) Del Rio Section Measured Near Hill 3740 (SC).................... 18 7. PHOTOGRAPH: Hand Specimens of Limestone Pebble Conglomerate and Limestone Breccia Within Buda Limestone........... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 34 8. PHOTOGRAPH: Diabase Sill Intruding Buda Limestone Exposed in Terlingua Creek ........ ····:······················ 57 9. PHOTOGRAPH: Cooling Joints in Sill in Boquillas Limestone.................................................... 58 10. PHOTOGRAPH: Xenolith of Prue'tt Tuff in Diabase Sill Southwest of Agua Fria Mountain ......................... 60 11. PHOTOGRAPH: Partly Coalesced Fingers at Edge of Small Sill in Boquillas Limestone in Terlingua Creek Bed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 12. PHOTOGRAPH: Two Sills in Boquillas Limestone • Connected by Discordant Sheet, South of Pink's Peak. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 13. PHOTOGRAPH: Hand Specimens of Vent Agglomerate From Diabase Plug Near Hill 3412 (EC) ........................ 70 14. MAP: Larger Mafic Sills in Yellow Hill Quadrangle and Areas to the Southeast. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 15. PHOTOGRAPH: Hand Specimens of Contact-Metamorphosed Boquillas Limestone from Uppermost Part of Roof of Trap-Door Dorne. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 16. PHOTOGRAPH: Chert Breccia Knob Protruding from Dip Slope, East Flank of Solitario ........................... 85 17. PHOTOGRAPH: Hand Specimens Collected from Chert Breccia Knob. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 18. MAP: Breccia Knobs and Dike Near Buda-Boquillas Contact ...................................................... 87 19. DIAGRAM: Paragenetic Sequence for Chert Breccias ............ 93 -94 20. MAP: Structure Map of a Part of West Texas .................. 97 21. PHOTOGRAPH: Northeast-Plunging Syncline in Downdropped Fault Block on Northeast Flank of Terlingua-Solitario Anticline. . . . . . . . . . . . .. . . . . . . • . . . . . . . • . . . • . • . . . . . . . . . . . . . . . . . . 101 22. PHOTOGRAPHS: (a) Hill 3412 (EC), a Trap-Door Dome in Boquillas Limestone with (b) "Tail" Extending to Northeast ...................................................... 102 23. PHOTOGRAPH: Resequent Fault Line Scarp Along East Side of Hill 3252 (EC)~ a Sill-Capped Mesa ....................... 104 24. PHOTOGRAPH: Intersecting Faults on Northeast Flank of Terlingua-Solitario Anticline. . . . . . . . . . . . . . . . . . . . . . . • . . . . . 105 25. MAP: Faults on Northeast Flank of Terlingua-Solitario Anticline.......... ................................. 106 26. DIAGRAM: Strike-Frequency Diagram of 46 Joints, Mostly in Boquillas Limes tone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 27. PHOTOGRAPH: Vein 2 m wide in Santa Elena Limestone in Canyon in Rim of Solitario................................ 113 Plate 1. MAP: Geologic Map of Yellow Hill Quadrangle, With Structure Sections .................................. pocket 2. MAP: New Terlingua Ranch Access Roads in Yellow Yill Quadrangle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . pocket • •. .. • ,. / INTRODUCTION The Yellow Hill Quadrangle~ though once difficult to enter, has recently been made accessible through the completion of many new sub­division roads. The quadrangle is located in southwest Brewster County, Texas, between 29°22'30tt and 29°30'00" north latitude and 103°37'30" and 103°45'00" west longitude (Fig. 1). The approximately 170 sq km area contains no towns or highways, and access is via county and ranch roads, and jeep trails. Access to the southeastern part of the quadrangle is via South County Road. Access to the east flank of the Solitario, or Blue Range, in the western part of the area is via Solitario-Sawmill Road. Both of these roads extend north of Highway 170. The northeast part of the area is accessible via North County and Hen Egg Roads, the latter of which extends west from Highway 118. Trails and smaller, less improved roads lead to other places (Plate II) . Locations are given according to the scheme shown in Fig. 2, with numbers referring to spot elevations shown on the topographic base (Plate I). Thus, "Hill 3092 (EC)" refers to the hill, in the east-cen­tral ninth of the quadrangle, whose top is at 3092 ft elevation. Climate and Vegetation The area is characterized by a semi-arid climate with low rain­ fall (mean annual 37 cm) and sparse vegetation. Plants include the ubiquitous lechuguilla (Agave lecheguilla), which is most abundant on high, flat surfaces such as sill-capped mesas. Greasewood (Larrea divaricata) grows abundantly on low flat terraces and is accompanied in some places by abundant huisache (Acacia farnesiana). False agave (Hechtia scariosa), candelilla (Euphorbia antisyphilitica), and the hairy resurrection plant (Selaginella pilifera) are abundant on Comanchean limestones of the Blue Range and Terlingua-Solitario anticline. Mesquite (Prosopis juliflora) and catclaw ·(Mimosa biuncifera) are abun­dant along intermittent streams. Other plants in the area include oco­tillo (Fouquieria splendens), sotol (Dasylirion leiophyllum) , guayacan 1 10 20 30 40 50 0 Miles 0 10 20 30 40 50 -.-i::::=--r::::=--Ki Io m • ters Fig. 1. Location of Yellow Hill Quadrangle . • • NW NC NE WC c EC SW SC SE Fig. 2. Location scheme used in text. (Porlieria angustifolia), mormon tea (Ephedra), tasajillo (Opuntia lepto­caulis), Spanish Dagger and soaptree yucca (Yucca), cholla (Opuntia im­bricata), allthorn (Koeberlinia spinosa), leatherstem (Jatropha dioca var. graminea), foothill basketgrass (Nolina erumpens), and different types of prickly pear (Opuntia). Several types of short grasses grow in clumps. Among the small cacti are button cactus (Epithelantha micro­meris), rainbow cactus (Echinocereus dasyacanthus), Graham dog-cactus (Opuntia grahamii), livingrock cactus (Ariocarpus fissuratus), eagle's­claw cactus (Echinocactus horizonthalonius), and others. Pitaya (Echin­ocereus stramineus) and purple pitaya (Echinocereus dubius) cacti grow in hemisperical clusters. Small flowers, which appear along draws in the spring, include spike phacelia (Phacelia congesta), large flower nemophila (Nemophila phacelioides), margined perityle (Perityle vaseyi), Big Bend bluebonnet (Lupinus havardii), and bracted paintbrush (Castille­ -~latebracteata). Desert baileya (Baileya multiradiata) is also common during the summer, and yellow-trumpet-flower (Tecoma staus), limoncillo (Pectis angustifolia), and Parry ruellia? (Ruellia parryi) grow through­out the summer on weathered diabase detritus . • Land Use The area has been sparsely settled in the past with ranching • providing the main source of income, although some prospecting has been done on the eastern flank of the Solitario. The extreme north-central part of the area is on the Agua Fria Ranch, which raises cattle and un­til recently, goats. The rest comprises a part of the Terlingua Ranch, a land-sale operation. The Terlingua Ranch has been subdivided into approximately 5,000 10-acre plots, over 95 percent of which had already been sold to private owners by the summer of 1974. New unimproved roads have been pushed through most of the Yellow Hill Quadrangle by Terlingua Ranch owners to reach the many 10-acre plots (Plate II). Very few of the plots have actually been settled to date; less than one percent have buildings on them. Geologic Setting Most of the Yellow Hill Quadrangle is underlain by Cretaceous limestone. Tertiary diabase sills, which intrude the limestone, cover most of the rest of the area. Cretaceous shale is locally extensive where it has been faulted down against limestone. Tertiary sandstone, tuff, and ash-flow tuff are found only in the northernmost part of the area, where they and Cretaceous shale are also intruded by Tertiary dia­base sills. Small Tertiary intrusive plugs and chert breccias of prob­able hydrothermal origin occur in some of the Cretaceous limestone. Comanchean rocks are exposed on the eastern flank of the Soli­tario and on the Terlingua-Solitario anticline in the western and south­western parts of the area; they are also exposed along the Yellow Hill fault in the central part of the area and in the bed of Terlingua Creek in the northeast. Gulfian rocks crop out over much of the eastern three-fourths of the area except on the Terlingua-Solitario anticline in the southwest. The Tertiary diabase sills occur in a northwest-trend­ing, structurally controlled belt in the northeastern half of the area. The quadrangle lies at the western edge of Udden's (1907) Sun­ken Block, which is bounded by Mesa de Anguila on the southwest and the Sierra del Carmen on the northeast (Moon, 1953). Structural features • in the area trend predominantly northwest. The area is located between the Davis Mountains to the north, the Chisos Mountains to the southeast, • and the Bofecillos Mountains to the southwest, which comprise three of the major volcanic centers in the Big Bend region. Volcanic deposits which originated from these centers thin toward the area from each direc­tion. Faults trend northwest, with the exception of some on the Ter­lingua-Solitario anticline which trend northeast. Faults cut all units mapped except Quaternary alluvium. The Solitario is a dome formed by igneous, probably laccolithic, intrusion (Baker, 1935; Lonsdale, 1940; Herrin, 1958; Corry, 1972). Faults and joints give the area a distinct northwest grain which is reflected in the topography and drainage. Previous Work Fig. 3 shows the location of previous work done in the southern Big Bend area. The Yellow Hill Quadrangle had not previously been mapped in detail, but was included in an area mapped in much broader scale by Lonsdale (1940). Lonsdale studied only the igneous rocks. Yates and Thompson (1959) mapped the Terlingua Quicksilver 4istrict to the south of the Yellow Hill Quadrangle. Sellards and others (1931, 1933) were the first to prepare more than a reconnaissance map of the Solitario dome. Herrin (1958) made the first detailed map of the Solitario, and Corry (1972) modified this map and incorporated it into his map of the Solitario 7~-minute Quadrangle which adjoins the Yellow Hill Quadrangle on the west. McKnight (1970) mapped the Bofecillos Mountains area which adjoins the Solitario on the southwest. Moon (1953) mapped the Agua Fria Quadrangle to the north and northeast. The Pruett Formation (Tertiary) and its vertebrate faunas in this quadrangle have been studied by Ste­vens~ al. (in preparation), and by Wilson (1974, 1977). Hatcher (in preparation) mapped an area within the southwestern portion of the Agua Fria Quadrangle, to the north of the Yellow Hill Quadrangle. The Tas­cotal Mesa Quadrangle to the northwest was mapped by Erickson (1953). Goldich and Elms (1949) first described, named, and mapped the Buck Hill • Volcanic Group, named after the Buck Hill Quadrangle, which adjoins the International Boundary and Water Commisaion 1955 Fig. 3. Index to areas mapped, Big Bend Region, Texas. After McKnight (1968) . • Agua Fria Quadrangle on the east. To the southeast of all the above areas lies Big Bend National Park, mapped by Maxwell and others (1967). Purpose and Methods ~Investigation The purpose of this study was to prepare a geologic map of the Yellow Hill Quadrangle, to describe its geology, and to search for vertebrate fossil localities in the area. I spent approximately ten weeks in the field during the summer of 1974, and additionally, one week during March 1975 and one week in May 1975. Information was recorded on 9 inch x 9 inch air photographs at a scale of approximately 1:60,000. This infonnation was transferred to a mylar positive of the U.S.G.S. topographic base map of the area, which was made from a film negative of the base obtained from the U.S.G.S. The final geologic map was repro­duced at a scale of 1:24,000, the scale of the topographic base. New Terlingua Ranch access roads were plotted (Plate II) from • maps prepared by Urban Engineering (Corpus Christi, Texas) of proposed roads. These provisional maps were p-repared before the roads were con­structed, and they do not everywhere depict the real roads. Samples were collected from all units mapped, and thin sections of samples representative of most units were cut. Chemical analyses and K-Ar ages were determined for two diabase sill rocks. Sections were measured with Brunton compass and jacob staff. One biostratigraphic zone was treated as a mappable unit. Undifferentiated stream alluvium is the only mapped Quaternary unit. Quaternary terrace gravels in the area are thin and restricted. The most extensive of these gravels occurs as a calichified crust, gen­erally less than ~ m thick, which forms a thin veneer over the Boquillas and Pen Formations. This "caprock" consists of pebbles, cobbles, and boulders of Boquillas flaggy limestone, and some diabase fragments, all cemented by caliche. I could locate contacts between underlying bedrock units beneath the more extensive areas of gravel by checking draws and gullies. Though some of these contacts are uncertain, I have chosen to make the map a bedrock map. Consequently, an inferred underlying Boquil­las Pen contact is mapped rather than the thin Quaternary -gravel cover . • PHYSIOGRAPHY Semi-arid climate and sparse vegetat~on make structure and -iithology the principal controls on physiography in the Yellow Hill Quad­rangle and surrounding areas. The topography is ro_ugh but varies from blocky where faulted, to smooth on dip slopes of limestones and diabase sills, to jagged where extensively dissected, especially on incompetent shaley and marly outcrops. Physiographic Expression~ Intrusive .Rocks Flat-topped mesas occur where resistant, Tertiary diabase sills have been uncovered by erosion. These mesas may have nearly straight edges in plan view where sills are adjacent to faults; otherwise they are irregular because of stream dissection. In the southeast part of the area, small diabase plugs and diabase sill outliers cap conical hills of Boquillas Limestone, as at Pink's Peak, Hill 3308 (SE), Hill 3664 (SC) and Hill 3538 (SE) (Plate I). Smaller diabase plugs, which occur in an approximate northwest trend through the area, stick up as small knobs above Boquillas Limestone and Pen Shale. Diabase dikes weather out in only slight relief because they are short and thin. Plugs of other intrusive igneous rock types may appear as knobs or may have very little topographic expression. One roughly cir­cular outcrop of a rhyolitic, rock fragment-rich rock interpreted to be a vent on the northeast flank of the Solitario is expressed as an amphi­theater. The margins of the body are more resistant to erosion than the interior and weather into reddish crags. Some red and white chert brec­cias occur as small plug-like knobs which project above the Buda Lime­stone on the eastern flank of the Solitario. Physiographic Expression of Sedimentary and Volcanic Rocks Resistant lower Cretaceous limestones, especially the Santa Elena, form dip slopes in the western and southwestern parts of the area. These are most extensive on the eastern flank of the Solitario, where they form the backs of stream-dissected cuestas. On the Terlingua­ • 8 Solitario anticline an extensive dip-slope surface of the Santa Elena is displaced by a network of intersecting normal faults (Fig. 4) . Out­liers of Buda Limestone and Del Rio Clay occur on downfaulted blocks of Santa Elena Limestone on this anticline. Downfaulting has made synclines of two Buda outliers, Hill 3735 (SC) and Hill 3850 (SW), whi~h stand out in relief (Plate I). Less-resistant, lower Cretaceous marls and shales of the Telephone Canyon, Sue Peaks, and Del Rio Formations, form slopes beneath the limestones on the eastern flank of the Solitario and the northeastern flank of the Terlingua-Solitario anticline. Fig. 4. View southeast across block-faulted northeast flank of Terlingua-Solitario anticline, where faults show as displacements of dissected dip slope surface of Santa Elena Limestone. Northeast-trending grabens and horsts intersect northwest-trending ones. Small Del Rio Clay outlier is preserved on downdropped block, left center. Figs. 24 and 25 include same area. • Upper Cretaceous Boquillas flaggy limestone which is exposed over much of the eastern three-fourths of the area is extensively dis­ • sected by intermittent streams. The alternation of beds varying in re­sistance to erosion has resulted in diverse topographic types, including jagged or badlands-like, low hills and swales, low cuestas, and flat areas. Yellow Hill consists of Boquillas Limestone which stands above flat-lying Boquillas and Buda surroundings; it is probably the remnant of a once diabase-capped mesa. An outcrop of contact-metamorphosed Bo­quillas, thought to overlie a concealed trap-door intrusive, forms an anomalous dome with elliptical shape. Because of alteration of the al­ternating hard and soft layers, and jointing, it weathers into forms resembling stacks and chimneys. The shaley upper Cretaceous Pen Formation, where areally ex­tensive, weathers into either low mounds or rugged badlands, with a few knife-edge ridges where small divides are being destroyed~ The Pen also forms slopes with the overlying sandstones of the Tertiary Pruett Forma­tion in the northern part of the area, beneath more resistant diabase or Mitchell Mesa Rhyolite. Effect of Structure on Physiography Faulting has created diverse topographic forms. Graben and horst terranes occur where Boquillas and Pen have been juxtaposed by faults, forming resequent fault line scarps which stand out boldly. On the Terlingua-Solitario anticline, northwest-trending grabens and horsts are cross-cut by northeast-trending ones, creating a field of jostled, rectangular fault blocks strikingly displayed on the Santa Elena dip slope (Fig. 4). The Yellow Hill Fault is expressed by a northeast-facing cuesta of Buda Limestone overlying Del Rio Clay on the upthr·own side of the fault. This cuesta outlines a resequent fault line scarp against the lower, Boquillas Limestone on the downthrown side. Rectangular joint patterns etch out prominently on dip slopes of Santa Elena Limestone. Some cuestas of Boquillas have saw-toothed edges formed by intersecting joint sets. Folds in the area stand out in bold relief regardless of • size, because of surrounding faults and differential weathering . 11 • • Drainage The Yellow Hill Qua.~rangle is in the drainage basin of the middle Rio Grande. The drainage is controlled by structure, is predom­inantly dendritic, and has an overall southeasterly direction. On the east flank of the Solitario the drainage pattern is radial. On the Ter­lingua-Solitario anticline, because of the gridwork of intersecting faults, the drainage approximates a rectangular northeast-northwest pat­tern. All streams are intermittent. The largest, Terlingua Creek, is a south-southeast flowing tributary of the Rio Grande. It is joined by Alamo de Cesario Creek a few kilometers to the north of the area, and is eventually fed by all other streams in the area. Immediately after receiving heavy rains, Terlingua Creek runs high and hard through its twisting canyon, and may continue to run for several days. The other major stream in the area is Saltgrass Draw, which flows southeasterly and joins Terlingua Creek to the east. Elevation The overall slope is to the southeast. A maximum elevation of 5,006 ft (1,526 m) is located in the west-central part of the area on the east rim of the Solitario. A minimum elevation of less than 2,840 ft (866 m) is reached in the southeast part of the area east of Pink's Peak, and also in Saltgrass Draw. Surficial Deposits No well-developed soils were observed; cover consists of talus and terrace gravel. Talus consists of angular, joint-bounded blocks o.f diabase, and angular rubble of weathered Boquillas limestone fragments held together by finer material and caliche, ·like the terrace gravel pre­viously mentioned. The thin terrace gravel covers the Boquillas and Pen Formations at elevations of 3,000-3,200 ft on terrace remnants in the east-central and southeast parts of the area (Plate I). A few small out­liers of terrace gravel deposits occur in the southeastern corner of the quadrangle, in the vicinity of Pink's Peak. These gravels contain much • • • locally derived basaltic and diabasic material. The only soil-like deposit is several centimeter-deep, brown, sandy decomposed diabase developed by in situ weathering in some small areas along drainages on the southernmost diabase sill. This along with numerous joints filled by caliche indicate that this is the only sill that has experienced significant weathering . • STRATIGRAPHY AND PETROGRAPHY General Statement The maximum total thickness of the sedimentary and volcanic rocks exposed in the area is about 1,415 m. The distribution of forma­tions is shown on the geologic map (Plate I). Stratigraphic nomenclature used in this report for Cretaceous formations is that established by Maxwell ~al. (1967). Lower Creta­ceous units consist of limestone which is predominantly biomicrite (wackestone); the thin Del Rio Clay is an exception. These units are exposed in the western and southern parts of the map area, on the east flank of the Solitario and on the Terlingua-Solitario anticline, and locally along Terlingua Creek (NE) and the Yellow Hill fault. In the Yellow Hill Quadrangle, the Comanchean section is approximately 760 m thick. Meager fossil data do not permit faunal correlation of lower • Cretaceous units with the same or equivalent units in other areas, so that lithology and position in the sequence are the principal criteria for correlation. Corry (1972, Table 7) shows partial faunal correlation for the lower Cretaceous of the Solitario with that of Big Bend National Park (Maxwell et al., 1967) and northern Coahuila (Smith, 1970). Upper Cretaceous formations consist of limestone composed of clayey foraminifer biomicrite (wackestone), and of calcitic foraminifer­al clay-shale. These units are exposed over most of the northeastern three-fourths of the area. The estimated thickness of the Gulfian sec­tion is 600 m. Tertiary rocks consist of · sandstone, conglomerate, tuff with minor caliche limestone nodules, and welded ash-flow tuff. Outcrops are small and lie on downthrown sides of faults, in the extreme northern part of the quadrangle. The Tertiary section is markedly thin compared to that of surrounding areas, because of Tertiary non-deposition and perhaps erosion. The thickness of Tertiary strata in the quadrangle is· estimated to be 20-50 m. Alluvium in stream channels consists of silt, sand, and .. 13 • limestone pebbles, cobbles, and boulders. It is thickest in the canyons which drain the east flank of the Solitario, where it is pr~bably up to 15 m thick. In those canyons cut into the rim of the Solitario, the alluvium consists almost entirely of Comanchean limestone boulders. Thin terrace gravel covers areas in the central part of the area, and in small parts of the southwestern and southeastern parts of the area. The gravel is generally less than ~ m thick, and consists of caliche-cemented Gulfian limestone fragments. Strata in the map area generally have dips of less than 30°. Those strata having greatest dips are Comanchean limestones on the eas­tern rim of the Solitario, and Gulfian limestones wherever they are faulted, domed, or intruded. A generalized composite columnar section is shown in Fig. 5. A more detailed stratigraphic section of Lower Cretaceous formations, measured across the rim of the Solitario, is Fig. 6. Table 1 gives time­ • stratigraphic and rock-stratigraphic classification of units. The petrographic terminology used in this report is that of Folk (1976, 1974a) and Dunham (1962) . • Cretaceous System Comanchean Series TRINITY GROUP Glen Rose Limestone Name and type section--R. T. Hill (1891) named the Glen Rose from rocks exposed along the Paluxy River near Glen Rose in Somervell County, Texas. Here the formation has a characteristic "stairstep" appearance because it consists of differentially weathered, alternating beds of resistant limestone and less-resistant marl. Distribution, thickness and lithology--The only exposures of the Glen Rose in the Yellow Hill Quadrangle are along the inner rim of the Soli­tario (Plate I). Here the formation shows the stairstep topography with • • 1600 1400 1200 1000 800 600 400 ,. 200 t Fig. 5. Generalized Stratigraphy and Petrography of Yellow Hill Quadrangl_e Meters Pruett Formation: limestone-pebble conglomerate (at base), - "=='--r:_--..,_---~ calclithite sandstone, and tuff .__~..=-:_._-_-_-.-..=­ ---.-----... :-:~?::.= -.,....----~­ Pen Formation: calcitic, fossiliferous clay-shale, with ~=-=-:.~~-=~ a few thin chalky limestone beds ~~~~ ~~~~~ ~~-=-~~ ...____..,...__:-1,----.._ lnocerarnus undulatoRlicatus Zone Yucca Formation ___.____4T"_ ~-:.=:~~~ ~=-~:2~~ ..J _. ...... ~ ..... Mitchell Mesa Tuff: rhyolitic welded ash flow tuff T San Vicente Member Fizzle Flat Lentil - • • 300 290 280 250 240 230 220 210 200 '\ 190 180 • 170 160 150 130 120 ]~~~~~~~ 110 --­ II I I I I I I I I l 11 111 100 ~='::c::c';:r::::r:::C::C~:;r:;r)J 90-~­ 70 60 --::-_:_:._~_:_=--~--_--r so ­ ~----­ 40 -,....__ ------------~ -­ ~~~--g:-c:y, yel2.ut.t ~1ea-:~e:-i:-.~ ~;:ic~~d ?eli:-::: ?oC ::,i.:nnic:-ice i:.ri::!l b.:tls ~-5 ..:.rn c :1:.~k. Co ru:ai:t:-: ~or.;a 5~.itrcptJC.s. :-'1.3r.:ion-gn.y pe leC:fl:lOd biomic=i~e ;.ric:i Eew i :ic::r·.,al:i lina ~~nd C::a~ked foraminife:-1:-iom.ic:-ir:e: ~ed~ :m or !.~s,:; ~eds l =m co !~ ~: ·J-: bic'.:'Lin.a ~·~­ G;ay, Light gray <.1ea:hering, p.L(lcy-nodular biomicr.;_t~ p.Ji~:i. in:acbeds of :naroon-gr3y. bec!3 J cm or L~ss : oran~e-1Jray W'iac:-..~!":.n~. crys cal:i;it! lime­ ;-t:l:!cypods , ~:eds < l c:11 co l m, containin~ !ri31mia, ~.:<0gy;a7 , He:ni:tscei:, ~?. forami:iifer3; tJith incerbeds of !:laroon-~c:iy, c r1scall.ine li:nes : ;;ine 20 cm co 1.5 m thick. ~~ -Fig. 6. (a) Section measured across rim of Solitario. meters • 600 590 J,~ ~·~i~i~~!ij nodular and lenticular chert; nodules randcr1ly ~~=i~~0j'j 1 =11°1l I I rI .u,".'. '·""""'"' ''"'" '"""·1 " ,.,,,,, 580 ­ 570 _j::c;:i:;:i=;::t~~~::c:::;::c:::;:c~ ~.ls;;;!l.s;!t!;;:tg~o~oeL~-~~-Ql globular chert nodules ~'~~iJ-ifiiiii!~~i!:5[iJ,i :'lodula c · ~~-~ beds-o~-j rudb ts; bedding, 550 ­ 540 I I I I I I I I .thick, in layers within 3 m incerv.ib­ 530 -520 ­510 ­500 - 490 1\ 480 ­ I l I I II I I I \ • 470 -~1......,1"""'".,......l...............ll '-r-I~l.....,_._1........,........,.-jI 460 ­ covered cha.rt to l :i:; c':!ert-:!!p.!.acede.!.liptical chl!rt aureol~ oblique to •.tith chert in 3 cm thick bands Z-15 cm long, < 6 c:n i:r.osc bees ~ 1 m; >ome a.3 ~hin as-) ~:n 450 ­ • 440 430 111113-25 cm beds nodular lt!d 1 beds JO cm 1~--:=:z::..._~-==:::::::' light gray, ="'\ 4 c:n beds 420 .lue i?2aks f"o l"l!lacion--south!!asr. -~ ;;po~ ~ak.s consist o 380 ........................................................-.-'................-r-1 ~O cm co '1 m beds 370 cylindrical a.'ld ~lobular wh.ice cher-: nodules, 360 I I I I I I I I I J \)elecypocis ::iotds < 10 c:n; sou:e placy:nodular silty !Jiomicri:a ::ieds ..........r~...,_.r1-r->-1~...,_.11..,.......1r'-r'-1~ 1 ~1..,.......1~1..,.......1~...,_..11~.,_.11..,.......r~ I 1 I I I I I I I 320 -l-'-..-'-..-~-'-r-....,_1.....,...-r'1...,.....1..,.....1...,....,I l I I I \ \ I I I I I 1 I I I l I I 340 -h-'1 330 -+. 310 ­ ~-:::::::::-~ 300 -.c--e--=--....:c::a Fig. 6. (a) (continued). line of .'!tea.sured .;ec:ion gray, ;rellor.r •.te.athering, ~l;,ty-nodular pelecypod bio:nicri a "1hich contains rudists, scmonicu . gascro;:ods , achinoids , and other fossils. 3!!ds are Z-5 cm thick and becom• thinner , cissila coward cot>. Contains i ncerbeds oc gray, dra:i s teel gray r.reachering. biomicrice 5-20 cm th ic!<.. lenticular chert to l.3 ;n, chert-repla.::!!d rudiscs ?lacy-nodular :udist biornicrit o ~ aJo\ <;:, c:i O> c.J cylindri.cal ::h.erc nodules; :>om• may 'le r-eplaced rudiscs pelec7pcc3 White to pinlti•h, ir&Y weatherin&, parely recrystallized rudut biosicriu White to pinltiah., Gray weachertn1, rudbt-buring cryn&llin• li-tone Gray Ndist biomicrit• Light gray , dark 1ray wutherina crystalline limutone gray, light gray wutherilla C'Udi•t biollicrite Gray, li~ht gray weathering biomicrite with maroon specks and macerated foH il debris . Beds < l cm to l m pinch and swell laterally. In places has necvork of horizontal and vereical, doubly-tapered cracks . • .. • meters 60 ~Li::escone--bct:cvm surface has ~ur:-ows 2-8 co r ioCli.: ::':!. t ~ ••1 !:.'l '1!3 roon '.la::iatite specks and streaX.S, 11mon1t e-stained i:udi3 c3 beds 3/4 ::i 2 m; chert aureoles consi:>t of replacinkish, gr:iy •eathe:-ing, par::ly recrystallized ~dist biomicrite • • Table 1. Generalized stratigraphic classif ication in • Yellow Hill Quadrangle. • • • .. alternating hard and soft limestone beds. The base of the Glen Rose lies outside the quadrangle to the west, in the Solitario Quadrangle . Corry (1972) separated the Glen Rose in the Solitario from an underlying unit which he identified as the Yucca Formation, because of its greater terrigenous content and greater dolomitization. The Yucca overlies the basal Cretaceous Shutup conglomerate, which overlies with angular un­ conformity the Paleozoic section of the interior of the Solitario. The Shutup Conglomerate markes the inner edge of the rim of the Solitario. In the Yellow Hill Quadrangle, the Glen Rose Limestone consists of about 274 m of alternating dark maroon-gray limestone ledges and gray to light gray to yellowish, platy-nodular to fissile "marl." Along much of their outcrop, the resistant ledges have been recrystallized, prob­ably during outcrop weathering, and are finely to coarsely crystalline limestone.· These ledges are massive or have beds greater than 20 cm thick, but some are platy like the lighter colored marly beds . Petrography--The resistant limestone ledges consist of dolomitized, packed pelecypod or foraminifer biomicrite. The micrite matrix contains minor quartz silt and very fine sand. Cavities are filled with finely to very coarsely crystalline sparry calcite. There are three varieties of dolomite: (1) medium crys'talline dolomite, which lines spar-filled cavities, seams, and some fossil molds (2) aphanocrystalline replacement dolomite, which has replaced parts of the micrite matrix (3) sparse, single rhombs or ghosts of rhombs, finely to medium crystalline, in micrite matrix, that could represent primary dolomite. The first two varieties of dolomite have associated with them a limonite strain, while the micrite matrix contains clusters of aphanocrystalline hematite crystals, indica~ing along with textural relations a non-primary origin of these dolomite types, probably associated with migrating sub­surface fluids. Some of the medium crystalline dolomite shows limonite zoning. • • .. • In addition to pelecypods and Foraminifera, the ledges con­tain abundant quantities of an allochem type that can resemble algal coats, or lumpy intraclasts grading into non-uniform-sized "pellets," which grade into smaller clots within the micrite matrix. Allochems are predominantly packed. Some of the pelecypods .have become spar-filled molds, perhaps a result of neomorphism, but the absence of preserved fibrous structure makes solution-cavity fill another possibility. On weathered outcrops these fossils show partial replacement by silica. The micrite matrix contains some rosettes of length-slow chalcedony, most of which are less than 0.2 rrnn in diameter. The platy-nodular "marl" is clayey, sparse pelecypod or fora­minifer biomicrite (wackestone) with minor quartz silt and very fine sand. Sparry calcite fills foraminifer chambers and other cavities. The micrite matrix contains isolated patches of either finely to medium crystalline pseudospar or neomorphosed shell fragments. Some shell fragments have minor amounts of chert in cavities. Minor amounts of megaquartz occur as cylindrical bodies 0.02 nun in diameter and may be filled borings. In both limestone types, clusters of aphanocrystalline hematite up to 0.5 mm are concentrated in cracks and at the edges of echinoderm and pelecypod fragments. The resistant ledges have limonite associated with dolomite, while the "marl" has limonite localized along microstylo­lites. This indicates that meteoric diagenesis was responsible for the non-depositional textural feature.s noted, though sparry calcite and dolomite could have formed in subsurface diagenetic realms (Folk, 1974a). Fossils--The following fossils were identified from the Glen Rose: Trigonia sp. Exogyra sp. ? Lunatia sp. ? Hemiaster sp. Orbitolina texana Romer Dictyoconus sp . • Unidentified fossils include bryozoans, ostracods, gastropods, a crinoid columnal, a possible green alga, a possible rudist, and many • Foraminifera. Orbitolina texana and Exogyra sp. ? are the most abundant fos­sils. Orbitolina is found mostly in marly beds within the interval 85­215 m above the base of the Glen Rose. Exogyra sp. ? is abundant in the ledges but no specimens were found which had weathered out. In beds lacking Orbitolina, the most abundant Foraminifera are miliolids, Dic­tyoconus sp. and types resembling globigerinids. Other Foraminifera in­ clude types that are uniserial, uniserial-biserial, and biserial (cf. Textularia sp.). Adkins (1933) identified some fossils collected from the Glen Rose in the Solitario, and Herrin-(1958) gives the most complete faunal list for this unit. Correlation--The above fossils alone are insufficient for precise corre­lation, but together with lithostratigraphic correlation of the sequence of superjacent formations permit correlation with the Glen Rose described by Maxwell~ al. (1967) in Big Bend National Park, less than 20 km away. The thickness in the Yellow Hill Quadrangle (274 m) is much greater than that given by these authors for the Park (600 ft= 183 m). Depositional Environment--The lithology and fossils of the Glen Rose sup­port the interpretation of Smith (1970), who concluded that the formation was deposited in a shallow marine environment subject to periodic influx­es of terrigenous fines which produced the marly beds. The alternation of beds of clayey, platy-nodular limestone and dolomitized limestone is like the central Texas Glen Rose sequence, which has been interpreted to consist of subtidal and supratidal deposits, respectively (Dawe, 1967; Rice, 1968). The texture, structures, and fossils are consistent with this interpretation, though almost all the dolomite in the resistant beds cannot be called "supratidal dolomite." If the dolomite was originally "supratidal," it was remobilized and redeposited as replacement dolomite after burial . • FREDERICKSBURG GROUP • Maxon Sandstone The Maxon Sandstone is not present in the Yellow Hill Quadran­gle. P. B. King (1930) named the formation from exposures at Maxon Sta­tion at the eastern edge of the Marathon Basin. It has also been repor­ted from the Santiago Peak Quadrangle (Eifler, 1943) and the Hood Spring Quadrangle (Graves, 1954). McAnulty (1955) described possible Maxon from the Catedral Mountain Quadrangle. Maxwell et al. (1967) report that it is exposed about l~ mi (2.4 km) southeast of Persimmon Gap (Big Bed Park) and that, while it is not recognizable in the Park, more cal­careous facies equivalents may be present above the Glen Rose. This plus the absence of the Maxon in the Yel.low Hill Quadrangle indicate that the outcrop southeast of Persimmon Gap is probably .close to the southern limit of the unit caused by both southward thinning (Graves, 1954) and southward facies change (King, 1937; Eifler, 1943; Graves, 1954; Maxwell~ al., 1967). According to Smith (1970), the Maxon Sand­stone was deposited in a southw~rd regression from the Marathon region when this region was uplifted at the end of Glen Rose deposition. Telephone Canyon Fonnation Name and type section--Maxwell et al. (1967) na~ed the Telephone Canyon from exposures in Telephone Canyon along Heath Creek in the Sierra del Caballo Muerto of the Sierra del Cannen. Here the formation is thin­ bedded nodular limestone and marl which fonns a slope between the Glen Rose below, and the cliff-forming Del Carmen Limestone above. Maxwell et al. found the average thickness of the Telephone Canyon to be 75 ft (22.5 m) in Big Bend National Park. Distribution, thickness, and lithology--In the Yellow Hill Quadrangle, .. the Telephone Canyon is exposed only along the inner rim of the Solitario . Here, as at the type locality, it weathers to form a slope between the • Glen Rose and Del Carmen. The formation lies conformably on the Glen • Rose Limestone, and consists of about 33 m of gray to yellow, clayey platy-nodular pelecypod biomicrite (packstone-wackestone). Beds are 1-5 cm thick. Fossils and correlation--A few species of unidentified pelecypods and gastropods were collected from the Telephone Canyon. Its lithology and position in the stratigraphic sequence are the basis for correlation with the Telephone Canyon Formation in Big Bend National Park. Regional features and depositional environment--Smith (1970) traced the Telephone Canyon Formation to the southeast from the type locality into Mexico, where, along the southern Sierra del Carmen, it changes facies to become a part of the Devils River Formation. He gives the average thickness of the Telephone Canyon as 130 ft (39 m) in northern Coahuila, which is about twice that in the Park (Maxwell~ al., 1967). The lith­ .. ology Smith described as consisting of nodular, marly lime wackestones separated by Gryphaea packstones (interpreted as oyster biostromes) and shell fragment wackestones, containing less clay toward the southeastern limit. He concluded that the Telephone Canyon was deposited in a shal­ low sea which received fine terrigenous elastic material from the Mara­thon Uplift, and that inundation of this source ended Telephone Canyon deposition. Del Carmen Limestone Name and type section--Maxwell ~al. (1967) named the Del Carmen from exposures in fault blocks of the Sierra del Carmen. Here it is ma~sive, gray, "fine-to medium-crystalline limestone" containing nodular and lenticular chert, varying amounts of rudists and some "marly layers." They found the Del Carmen to be 103-107 m (338-350 ft) thick at measured section localities in the eastern part of Big Bend National Park, and 142 m (465 ft) thick in Santa Elena Canyon to the west. Distribution, thickness, and lithology--In the Yellow Hill Quadrangle, • • the Del Carmen is exposed only within the rim of the Solitario, but it is a major part of this feature. The upper and lower parts of the for­mation have beds that range from a few centimeters to 1-2 m, and vary in thickness laterally. The middle part of the formation contains several massive ledges up to 12 m thick. Overall, the Del Carmen is a ledge­former. It lies conformably on the Telephone Canyon Formation and is conformably overlain by the Sue Peaks Formation. The Del Carmen consists of about 136 m (446 ft) of rudist biomicrite (wackestone-packstone), and has several layers which contain nodular and lenticular chert bodies. The chert is of replacement origin, because beds in the limestone pass uninterrupted through the chert bodies. Macrofossils are partly silici­ fied, and some of the chert bodies may be silicified hash of macerated fossils. Some chert was seen in vertical stringers and with vertical parting. Del Carmen outcrops commonly show recrystallization to various grades of crystalline limestone, and one sample collected about 8 m be­ low the top of the formation contains dolomite. Petrography--Neomorphism is the dominant diagenetic effect seen in thin section. Fossils have been neomorphosed to sparry calcite, and the mi­crite matrix has become microspar and pseudospar. Some patches inter­preted as finely to medium crystalline pseudospar may actually be shell fragments which have been neomorphosed. Limonite occurs as clusters or seams associated with cavities, pseudospar crystals, or stylolites. All of these features indicate that diagenesis took place in a meteoric realm, though this could be an effect of sampling and outcrop weathering. Fossils--No fossils were collected from the Del Carmen, for, while they are abundant, they do not weather out well and are hard to ·remove. Macro­fossils seen on outcrop consist of rudists and other pelecypods, while microfossils seen in thin section consist of ostracods and a few types of Foraminifera. Regional features and depositional environment--Smith (1970) has summar­ized the distribution of f acies within the Fredericksburg Group for • • central and southwest Texas and northern Coahuila, Mexico. To the east of Big Bend National Park, in northernmost Coahuila, the Del Carmen .. becomes part of the lower Devils River Formation, and further east chan­ ges facies to become rudist limestone of the West Nueces Formation and evaporites and black clayey limestone of the McKnight Formation. In Coahuila to the south, the Del Carmen undergoes facies change into the Aurora Lime Mudstone. In the Big Bend area, the formation thickens west­ ward between the Sierra del Carmen on the east and Santa Elena Canyon and the Yellow Hill Quadrangle on the west. The formation contains ru­ dist bioherms in the Black Gap area of the Sierra del Carmen (St. John, 1965; Smith, 1970). The lithology and distribution of the Del Carmen indicate deposition on a shallow shelf. WASHITA GROUP • Sue Peaks Formation Name and type section--Maxwell et al. (1967) named the Sue Peaks from exposures in the Sierra del Carmen. Here and at other outcrops in Big Bend National Park, the formation could be subdivided into a lower shale member and an upper limestone member, partly correlative with the Kia­michi and the Duck Creek Formations, respectively. In the Park, the fonnation is about 76 m (250 ft) thick. The lower shale member consists of about 23 m (75 ft) of "yellowish-gray and buff marly shale with a few beds of similarly colored thin, marly, nodular limestone." The up­per limestone member consists of a 6 m (20 ft) ledge of "massive gray limestone," overlain by 47 m (155 ft) of "thin, gray, nodular limestone beds and some yellowish-gray shale." Distribution, thickness, and lithology--Exposures of the Sue Peaks in the Yellow Hill Quadrangle are limited to the rim of the Solitario. Here, as at localities in Big Bend National Park, the formation forms a slope • between the massive ledges of Del Carmen Limestone below and Santa Elena • Limestone above. The Sue Peaks lies conformably on the Del Carmen, is only about 22 m (72 ft) thick, and is not divisible into lower shale and • upper limestone members. It consists of gray, platy-nodular clayey pele­ cypod biomicrite (wackestone) having 2-5 cm thick interbeds of gray bio­ micrite that weather drab steel gray. A sample from one of the latter beds contains patches of extremely coarsely crystalline calcite up to 1 cm across, in which are disseminated a few crystals of "limpid" dolomite. Toward the top the formation becomes almost fissile, with beds less than 1 cm thick. The base is generally covered by colluvium and/or alluvium. Petrography--The platy-nodular biomicrite which composes most of the Sue Peaks interval was examined in thin section. Meteoric diagenetic effects are most prominent--the clayey micrite matrix has been partly neomor­ phosed to microspar, and pelecypod shell fragments have been dissolved and filled with sparry calcite. Some shells may have neomorphosed to spar, but, if so, no relict structure is preserved. There are a few isolated fine to coarse crystals of possible pseudospar, but these could also be neomorphosed pelecypod shell "prisms." 0.005-0.1 mm masses of very finely crystalline pyrtte crystals show alteration to hematite at the edges, and hematite clusters of similar size presumably were origin­ ally pyrite. The entire matrix is tinted by a limonite stain. Fossils--The following fossils were identified from the Sue Peaks Forma­ tion: Adkinsites bravoensis (Bose) Hemiaster cf. H. whitei Enallaster cf. E. texana Neithea cf. N. duplicosta (Romer) Tylostoma sp. Protocardia ? • Unidentified fossils include pelecypods, gastropods, a rudist and an ostracod. Fossils are abundant in the Sue Peaks and weather out • readily. • 28 • Regional features and depositional environment--The lithology and fossils of the Sue Peaks indicate deposition on a shallow shelf which was receiv­ • ing fine terrigenous elastic material. In Coahuila the Sue Peaks changes facies eastward into the upper Devils River Formation, and to the south becomes part of the Aurora Lime Mudstone (Smith, 1970). According to Smith, the Sue Peaks thins northward to the southern Marathon Uplift, and received elastic input from source areas to the west. Santa Elena Limestone Name and type section--Maxwell et al. (1967) named the Santa Elena Lime­ stone from exposures at the mouth of Santa Elena Canyon in Big Bend Na­ tional Park. The massive cliff-former here consists of about 226 m (740 ft) of cherty rudist limestone in 2.4-3.0 m (8-10 ft) beds. The rudists are commonly silicified, and chert is more nodular than in the Del Car­ men. The Santa Elena Canyon section is the thickest and least disturbed section measured by Maxwell et al. The Santa Elena section in the Sierra - del Carmen (Sierra del Caballo Muerto) has a thickness of about 160 m (525 ft). Distribution, thickness, and lithology--The Santa Elena is exposed over much of the southwestern half of the Yellow Hill Quadrangle, on the flanks of the Solitario dome and Terlingua-Solitario anticline. In the section measured across the rim of the Solitario, the Santa Elena has a thickness of about 236 m (774 ft), and conformably overlies the Sue Peaks. It is the massive, cliff-forming unit that caps the rim, and over most of its outcrop area forms a dissected dip-slope surface on which faults are easily discernible. The Santa Elena is rudist biomicrite (wackestone) containing layers of lenticular and nodular replacement chert. Beds are mostly 1/3-1/2 m thick, but range from 5 cm to 2 m. Bedding planes show •. the undulosity so characteristic of all the lower Cretaceous limestones . Chert occurs as irregular nodules, which may have oblong, globular or dumbell shapes. Hematite or limonite stain can make the otherwise gray, • white, or black chert brownish or reddish. Macrofossils, mostly rudists, • are at least partly silicified and commonly show brownish or reddish stains. In some places there are silicification aureoles, which may • have centered around a shell hash. One sample collected near the top of the Santa Elena contains finely disseminated dolomite. The Santa Elena commonly shows solution pockets, and there are small caves in a few places near the juncture of the Solitario and Terlingua-Solitario anticline. Petrography--The sparse rudist biomicrite also contains abundant Foramin­ ifera, possibly miliolids, along with pelecypods and other fossils, and a trace of silt. Neomorphic effects are many: micrite has become micro­ spar and finely to medium crystalline pseudospar in places, and foramin­ ifer tests are neomorphosed. Pelecypod and rudist shell fragments are partly replaced by chalcedony. The micrite/microspar matrix contains some 0.3-0.75 Illlil eye-like clots of hematite, which appear as maroon specks in hand specimens. Hematite also occurs as seams beneath micro­ stylolites, and 10-30 µm clusters of aphanocrystalline crystals. Kaolin­ - ite, in one sample, occurs in small (less than 1 Illlil) pockets and cracks, some of which are within foraminifer tests and pelecypod or rudist shells, and in places is associated with pseudospar. Neomorphic effects indicate meteoric diagenesis, and the pre­ sence of kaolinite may indicate acid leaching. One chert nodule was examined in thin section. It is mostly 5-20 µm chert consisting of clear or white spheres in a brown groundmass. Chalcedony spheres (0.02-0.05 Illlil) replace fossils, and have mostly brown centers and clear intersections. According to R. L. Folk (personal com­ munication, 1976), the brown areas contain minute liquid-filled bubbles. Some megaquartz lies between chalcedony spheres. There are a few undi­ gested 0.1-0.3 Illlil patches of micrite in the chert matrix; other calcite has been deposited in cracks, probably during outcrop weathering. The • replacement nature of the chert is indicated by (a) beds passing through nodules, and (b) relict fibrous structure in pelecypod shells, which • rules out solution-cavity fill . • 30 • Fo~sils--The only fossils identified from the Santa Elena are Toucasia sp. indet. and Neithea sp. indet. Unidentified fossils include other • rudists and pelecypods, Foramin.ifera (miliolids?) and an ostracod. The Santa Elena, like the Del Carmen, yields few collectable fossils. Regional features and depositional environment--In Big Bend National Park the Santa Elena thickens westward, ranging from about 160 m in the Sierra del Carmen to 226 m at the mouth of Santa Elena Canyon. It thick­ ens further to the north and west to about 236 m in the Yellow Hill Quad­ rangle. Smith (1970) has shown that the Santa Elena thins southward in­ to northern Coahuila, where it becomes part of the Aurora Lime Mudstone to the south and the upper Devils River Formation at the edge of the Maverick Basin to the southeast. According to Smith, the lower part of the Santa Elena grades southward into the upper part of the underlying, southward thickening Sue Peaks. Smith also interprets the Santa Elena as a shallow shelf deposit that offlaps to the south. Del Rio Clay Name and type section--Hill and Vaughan (1898) named the Del Rio from exposures at Del Rio, Texas. Here it consists of greenish clay with thin limestone and sandstone interbeds. This report follows Maxwell et al. (1967) in using the name Del Rio, though the correlative Grayson Marl has priority, becasue the formation is similar to the unit mapped as Del Rio in the Park. Distribution, thickness, and lithology--The Del Rio is exposed along the flanks of the Solitario and Terlingua-Solitario anticline. In some pla­ces on this anticline, outliers of Del Rio are preserved within downdropped fault blocks. The only other Del Rio exposures are along the Yellow Hill Fault. The Del Rio forms a slope separating the Santa Elena Limestone below from the Buda Limestone above, and consists of gray clay-shale con­ "' taining 1-10 cm thick interbeds of very fine-grained calclithite sand­ • s.tone and siltstone, and 5-30 cm thick interbeds of nodular, wavy-bedded • Buda-like biomicrite. The limestone is most abundant near the top of the Del Rio, where it increases toward a gradational contact with the overlying Buda. The Del Rio also contains fibrous gypsum veinlets 1 cm wide that are both oblique and parallel to bedding. The Del Rio was measured at two places, on the east flank of the Solitario where the thickness is about 18 m (59 ft), and on the north bank of Salt Grass Draw near Hill 3740 (SC), where the base is covered but the minimum thickness is 47 m (154 ft) (Fig. 6). Petrography--In thin section, the gray clay-shale shows no visible lamin­ ation, but the clay has good preferred orientation. It contains less than 10 percent silt (quartz and feldspar) and has 0.001-0.05 mm clusters of pyrite crystals, some of which are altering to hematite. Hematite also occurs as 0.02-0.15 mm botryoidal clusters of crystals which aver­ age 5-10 µm. Limonite is abundant as a matrix stain, as numerous 0.02 mm clusters, and in lenses up to 0.2 x 1.0 mm, and is responsible for the brown weathering color. The clay contains few Foraminifera (globi­ - gerinids?), and is devoid of burrows or other disturbance structures. One of the sandstone inte=beds was examined; it is brown, very fine-grained, laminated, tight, calcitic, mature, cellophane-bearing glauconitic calclithite. The sparry calcite cement is poikilotopic, and contains suspended 0.025 mm rhombs of "limpid" dolomite. Dolomite also fills tests of the uniserial foraminifer Cribratina texana, which is abun­ dant on some bedding planes. Some elongate grains show preferred orien­ tation parallel to laminae, and some are imbricated. Soft grains are com­ pacted and penetrated by other grains. Other fossils include biserial Foraminifera, and echinoderm and mollusc shell fragments. Limestone rock fragments predominate, but some rock fragments may be igneous. Leucoxene is a minor framework constituent. The rock contains some clusters of hematite and pyrite. • The thin limestone interbeds are similar to the overlying Buda Limestone, discussed below . .. • Fossils--Only two fossils were identified from the Del Rio: Cribratina • texana (Conrad) from the sandstone interbeds, and Exogyra cartledgei (Bose)* from near the top of the formation. According to Maxwell et al. (1967) Exogyra cartledgei is characteristic of the upper Del Rio. Regional features and depositional environment--In the Big Bend area, the thickness of the Del Rio varies. This has been attributed to uplift and erosion before (Smith, 1970), during (St. John, 1975; Maxwell et al., 1967) or possibly after (Maxwell~ al., 1967) Del Rio deposition. Smith (1970) reports that in northernmost Coahuila the Del Rio lies disconfor­mably on the underlying formation, the top of which is "commonly iron-stained and bored by clams." The Del Rio thins northward and westward to only a few meters across the Devils River rudist limestone trend. In the subsurface in the Maverick basin, it has a thickness of about 120 m (400 ft), and Smith sees this as evidence of pre-Del Rio topography. In the Black Gap area (St. John, 1965), termination of the lower Del Rio and overlap by the upper Del Rio indicate uplift during Del Rio deposi­tion. St. John's isopach map of the Del Rio shows that it thins toward a high which existed where the Rio Grande is today. The Del Rio also varies in thickness in areas adjacent to the Yellow Hill Quadrangle. It is 40 m (131 ft) thick at Gray Hill in the Agua Fria Quadrangle to the north, and 56 m (185 ft), 30 m (100 ft) and 24 m (80 ft) in the Terlingua district to the south (Yates and Thompson, 1959). Erickson (1953) reported a thickness of about 20 m (65 ft) from the Tascotal Mesa Quadrangle to the northwest. Herrin (1958) reported a thickness of 38 m (125 ft) from the Lefthand Shutup of the Solitario. The Del Rio was not closely studied in the Yellow Hill Quadrangle, but it seems probable that the above features are indicative of uplift and intermittent erosion in different areas before and during Del Rio deposi­tion. The lithology, thickness, and areal distribution of the Del Rio • indicate that it was deposited as a shallow shelf mud. The sandstone interbeds may represent periodic turbidity current deposits . • • • Buda Limestone Name and ~section--Vaughan (1900) named the Buda from expo~ures of glauconite-bearing, fossiliferous crystalline limestone at Buda in central Texas. Distribution, thickness, and lithology--The Buda Limestone is exposed on the flanks of the Solitario and Terlingua-Solitario anticline, along the Yellow Hill fault, and along Terlingua Creek. It consists of dense very light gray, nodular sparse foraminifer biomicrite (wackestone) in beds 0.5 to 0.75 m thick, with individual nodular layers averaging 5-6 m thick. It has medium crystalline calcite veinlets cutting it oblique to bedding. Bedding planes are undulose, giving it the nodular appearance. It wea­thers to a bleached white or dull gray, and forms a prominent ledge above the underlying Del Rio Clay. On the east flank of the Solitario, the • Buda is about 16 m (52 ft) thick. Petrography--The sparse foraminifer biomicrite shows some dismicritic ... texture. Disturbed areas are filled with aphanocrystalline to finely crystalline rhombic sparry calcite, perhaps because of patchy neomorphism. Aphanocrystalline to medium crystalline hematite is disseminated through­out the micrite matrix. Limestone pebble conglomerate or limestone breccia is found within the Buda Limestone at two localities on the east and northeast flanks of the Solitario. The conglomerate/breccia occurs as small pockets within the normal Buda lithology, and is pinkish or pink and white compared to the white, porcelaneous Buda. At both localities the conglomerate/breccia consists almost en­tirely of limestone rock fragments, with a few fragments of coarse chert, cemented by very finely to very coarsely crystalline sparry calcite (Fig. 7). On the northeast flank of the Solitario, the rock is a conglomerate with rounded pebbles and granules, and cobbles and boulders up to 0.3 m. Many of these rock fragments are tlat, and some are oriented, whereas others are edgewise. The limestone rock fragments here consist mostly Fig. 7. Hand specimen of limestone breccia within Buda Limestone on east flank of Solitario (left), and three hand specimens of limestone pebble conglom­erate within the Buda on the northeast flank of the Solitario (right). These rocks differ strikingly from typical porcelaneous Buda bio­micrite. • - • of Comanchean limestone, but a few are caliche, calcite spar, or dolo­mite. Other rock fragments are limonitic and hematitic, glauconite­bearing sandstone, and few igneous rock fragments. There is also much reworked Inoceramus prisms and other pelecypod trash. On the east flank of the Solitario, the rock is a limestone breccia, with a great variety of limestone rock fragments ranging from crystalline and dolomitized limestone to normal Comanchean limestone. It also contains a few quartz grains. The sparry caicite at both localities has rhombic crystal habit and lacks meniscus or pendulous cement/framework relations. For the smal­lest size mode (4-16 um), rhombic habit has been interpreted to indicate cementation in a meteoric diagenetic realm (Folk, 1974b), and the cement/ framework relations and larger crystals indicate a phreatic environment. The conglomerate on the northeast flank of the Solitario shows equant to bladed, very finely to medium crystalline crusts on framework grains, among coarsely to very coarsely crystalline random equant spar, and shows some grain compaction. The breccia from the east flank of the Solitario has pyrite altered to hematite, and the calcite cement shows several size modes between very finely and coarsely crystalline, suggesting possible neomorphism of the cement during recent vadose weathering. The field relations, areal extent, geometry, and petrography indicate a solution, cavern-collapse, sinkhole type of origin. Another possibility, suggested by the rounded pebbles at the northern locality, and by the variety of limestone rock fragments at both localities, is that the pockets of conglomerate/breccia are remnants of sediment trans­ported by streams which drained the Solitario during an earlier stage of uplift and/or erosion. Fossils--The only fossils identified from the Buda are Neithea subalpina (Bose) and Budaiceras hyatti (Shattuck)*. Unidentified fossils include gastropods, pelecypods, globigerinids and other Foraminifera, echinoderm fragments, and an ostracod. Like the Del Carmen and Santa Elena, the Buda yields few collectable, identifiable fossils. Regional features and depositional environment--In Big Bend National Park and eastward to Del Rio, the presence of a middle marly unit within the Buda makes the formation divisible into three members (Maxwell et al., (1967). This division can also be made in northern Coahuila, but dis­appears to the south (Smith, 1970). The Buda in the Yellow Hill Quad­ rangle lacks the three-fold subdivision, but a middle marly member is reported from the Buck Hill (Goldich and Elms, 1949), Tascotal Mesa (Erickson, 1953), and Santiago Peak (Eifler, 1943) Quadrangles to the north. The Buda shows local thickness variation attributed in part to pre-Gulfian erosion by Maxwell et _al. (1967). In the Yellow Hill Quad­rangle and surrounding areas, the Buda thickens toward the south, from 16-21 m (52-69 ft) in the Yellow Hill, Tascotal Mesa, and Agua Fria Quad­rangles (Erickson, 1953; Moon, 1953) to 21-35 m (70-115 ft) in the Park and the Terlingua district (Maxwell et al., 1967; Yates and Thompson, .. 1959). The lithology and fossils of the Buda indicate shallow shelf deposition. Gulfian Series TEP.LINGUA GROUP Boquillas Formation Name and type section--Udden (1907) named the Boquillas Formation from exposures at the old Boquillas post office, about 7!2 mi (12 km) northwest of the present-day town of Boquillas (Maxwell~ al., 1967). Boquillas Formation and Terlingua Group are here used in the sense of Maxwell et al. (1967), who included the lower member of Udden's (1907) Terlingua Beds in their upper member of the Boquillas. These authors divided the Bo­quillas into two members, the Ernst, lower, and San Vicente, upper, which are separated in the Park by an erosion surface. The Ernst overlies the • Buda Limestone and consists of "silty limestone flags, siltston~, and • calcareous clay." The San Vicente is a "flaggy chalk-marl unit," with • - • • basal beds very similar to the underlying Ernst. The clay content in­creases up section toward a gradational contact with the overlying Pen Formation. In the Park, the Ernst-San Vicente contact is placed at the top of the Coilopoceras Zone, about 30 m (100 ft) above the Allocrioceras Zone. The contact between the San Vicente and the Pen Formation is placed 5-6 m (15-20 ft) above the Inoceramus undulatoplicatus Zone. In Big Bend National Park, the Boquillas Formation averages about 244-259 m (800-850 ft) thick (Maxwell~ al., 1967). Distribution, thickness, and lithology--The Boquillas Foramtion is exposed over much of the northeastern two-thirds of the Yellow Hill Quadrangle. The Ernst and San Vicente Members are not distinguished in this report, because almost all of the Boquillas exposed is Ernst. The Boquillas consists mostly of gray, laminated, clayey, sparse foraminifer biomicrite (wackestone) in beds a few to several centimeters thick, separated by laminae of calcitic clay. The flaggy biomicrite wea­thers to a bleached yellow color. -The Allocrioceras Zone is a dense, brown-weathering bed about 10 cm thick which contains abundant Allocrio­ceras hazzardi and an unidentified straight cephalopod. It supports a dense growth of lechuguilla, which with its dark color make the bed darker than surrounding Boquillas on air photos. Over much of the highly-dis­sected outcrop area of the Boquillas, outliers of this bed represent the highest part of the Boquillas exposed. The bed is shown on Plate I by the symbol "cccc." Beds of the upper Boquillas are also gray, laminated, clayey, sparse foraminifer biomicrite but contain more clay and are fissile. The Fizzle Flat Lentil described by Moon (1953) in the Agua Fria Quadrangle, overlies the flaggy biomicrite sequence of the lower San Vicente, is fis­sile and weathers yellow. Outcrops of this rock are almost devoid of vegetation. Overlying the Fizzle Flat is fissile, gray-weathering clayey foraminifer biomicrite which grades upward into calcitic clay-shale. Sev­eral chalky, white-weathering biomicrite beds are within the shale just below the base of the Pen Formation. One of these represents the Inocera­mus undulatoplicatus Zone, but no I. undulatoplicatus were found at the few available outcrops. The thickness of the Boquillas was not measured because no­where is a complete section exposed~ Moon (1953) measured a complete sec­tion in the bed of Terlingua Creek (southern Agua Fria and northeastern Yellow Hill Quadrangles), and the parts of his "Boquillas-Terlingua units" corresponding to the Boquillas as used here have a combined thickness of about 216 m (708 ft). Erickson (1953) estimated the maximum thickness of the Boquillas to be about 183 m (600 ft) in the Tascotal Mesa Quadran­gle to the northwest. McKnight (1970) measured a thickness of 305 m in the Bofecillos Mountains to the southwest. Yates and Thompson (1959) re­ported thicknesses of about 3-5 m (1000 ft) and 332m (1090 ft) for the interval beneath the "Inoceramus undulatoplicatus gray beds" in the Ter­lingua district to the south. In the Yellow Hill Quadrangle, the Boquil­las is probably 250-350 m thick . • Petrography--The clayey, sparse foraminifer biomicrite contains much pele­cypod shell debris, chiefly Inoceramus prisms, in addition to abundant Foraminifera. Foraminifer tests are filled with equant to bladed, very finely to coarsely crystalline rhombic sparry calcite cement. Laminae are discontinuous; most are a function of clay content, but some result from packing of foraminifer tests. The micrite matrix is stained by limonite, and contains 5-500 µm aggregates of aphano-to finely crystal­line hematite and botryoidal aggregates of spherical clusters of crystals. The brown-weathering bed containing Allocrioceras hazzardi shows greater neomorphic effects than other Boquillas samples. The matrix is microspar with 0.5 nun long pods of micrite that are parallel to lamina­tion. Areas of very finely to medium crystalline rhombic spar in the matrix are diffuse at edges, and were probably formed by patchy neomor­phism. Length-slow chalcedony partly replaces some macrofossils. The rhombic sparry calcite cement and neomorphic effects indi­cate that diagenesis was predominantly meteoric. Fossils--The following fossils were identified from the Boquillas: Durania? • Pycnodonta congesta (Conrad) Allocrioceras hazzardi Young Inoceramus cf. I. pictus Sowerby* Inoceramus spp. Baculites sp. Unidentified fossils include Foraminifera (pelagic, uniserial and biserial), ostracods, pelecypods, an annnonite and a straight cephalo­pod. Globigerinids are the predominant Foraminifera, but uniserial and · biserial types are also abundant near the top of the Boquillas. The un­identified ammonite may be Peroniceras sp.; it was found with unidentified pelecypods, tracks and burrows just below the Fizzle Flat Lentil in a brown-weathering bed, the appropriate position for the Peroniceras beds of Maxwell et al. (1967). One of the Inoceramus species collected from the upper Ernst Member correlates with the second member of the Austin • Chalk of central Texas (Keith Young, personal communication, 1975) . Regional features and depositional environment--As already noted, the Boquillas shows an overall southward thickening in areas adjacent to the .... . Yellow Hill Quadrangle. The lithology and fossils of the Boquillas indicate deposition on a shallow shelf (which was part of the Coahuila Platform) subject to intermittent influx of terrigenous fines. Pen Formation Name and type section--Maxwell et al. (1967) named the Pen Formation from exposures at the Chisos Pen, north of the Chisos Mountains in Bed Bend National Park. The Pen "includes Udden's (1907a, pp. 33-41) middle and upper members of his Terlingua Beds and the unit called Terlingua equiva­lent by Adkins (1933, pp. 270-271, 451-452)." In the Park, the Pen con­sists of about 67 to 213 m (219 to 700 ft) of yellowish-weathering cal­ • citic clay containing beds of chalk and sandstone, and abundant calcareous concretions . • Distribution, thickness, and lithology--The Pen is exposed mostly in down­ dropped fault blocks within the eastern half of the quadrangle. It wea­thers to form slopes or badlands. The formation lies conformably and with gradational contact on the underlying Boquillas, and consists of gray, calcitic, foraminiferal clay-shale. In a few places, the Pen con­tains thin chalky limestone beds. The thickness of the Pen was not measured because even a good partial section is not exposed. It seems probable that only the lower Pen is exposed in the quadrangle; outcrops within fault blocks can be traced into areas where the base of the Pen is exposed. Also, the Ter­tiary Pruett Formation, which unconformably overlies the Pen, has cut out the Aguja Formation and part of the Pen, although the Aguja may be absent because of recent erosion in places where the Pruett also is missing. The Aguja unconformably overlies the Pen in areas immediately to the north, east, and south. Finally, sandy facies of the Pen, which charac­terize the upper part of the formation (Maxwell~ al., 1967; McKnight, 1970), are absent. The thickness of the Pen varies greatly in surrounding areas. - It is absent in the Tascotal Mesa Quadrangle_, where the Pruett unconfor­mably overlies the Boquillas (Erickson, 1953). In the southern Agua Fria Quadrangle, the Pen is about 102 m (335 ft) thick, the thickness measured by Moon (1953) for the middle and upper members of his "Upper Boquillas­Terlingua Unit." In the Bofecillos Mountains, McKnight (1970) reported a thickness of about 61 m or 200 ft, although in places the Pen has been cut out completely, and Tertiary rocks overlie the Boquillas. In the Terlingua district, Yates and Thompson (1959) reported a thickness of about 305 m (1000 ft) for the Terlingua Clay, which includes part of the upper Boquillas as used here. Variations in thickness result from ero­sion during pre-Aguja and pre-Pruett times, but the Pen still shows an overall southward thickening . • Petrography--The calcitic foraminiferal clay-shale contains fine, discon­tinuous laminae. Most laminae result from variation in clay content, and in some places laminar patches of clayey micrite alternate with • 41 ... calcitic clay. A few laminae are formed by packing of foraminifer tests along bedding planes. The . ~lay contains minor amounts of quartz and feldspar silt, detrital opaque minerals and pelecypod shell fragments. A sample collected north of Hill 3412 (EC) contains clay intraclasts 0.1-0.7 1Illll long, which contribute to lamination. Foraminifer tests are filled with sparry calcite cement, which can be rhombic or have radiaxial extinction. Some 25 wm dolomite cry­stals occur in the sample that has the clay clasts, along with isolated crystals of medium crystalline sparry calcite. Aphanocrystalline hema­tite is arranged in 10-200 µm aggregates, and limonite stains the calcit­ic clay matrix. An authigenic opaque mineral, probably pyrite, occurs in some foraminifer ·tests, and shows alteration to hematite at edges. A chalky limestone bed sampled from the Pen is a sparse fora­minifer biomicrosparite (wackestone). It contains globigerinid Foramini­fera, Inoceramus prisms, and possible ostracods. The matrix is entirely neomorphosed to microspar, and foraminifer tests are filled with sparry calcite that can be rhombic or have radiaxial extinction. Aggregates 15-200 µm of aphanocrystalline hematite occur in the matrix and complete­ly fill some foraminifer tests. Limonite stains the matrix. Fossils--The only fossil identified from the Pen is a fragment of Inocera­mus sp. from the limestone bed described above. Unidentified fossils are the globigerinid Foraminifera and possible ostracods. Fossils are extremely rare in the few good Pen exposures in the quadrangle. • Regional features and depositional environment--Like the underlying Bo­quillas, the Pen thickens southward in areas surrounding the Yellow Hill Quadrangle. The lithology and fossils of the Pen indicate deposition on a shallow shelf which was subject to relat1vely continuous influx of abun­dant' terrigenous fines. The Boquillas-Pen sequence thus records an in­creasing supply of fine elastic material to the Coahuila Platform. • • Tertiary System Eocene Series ... BUCK HILL GROUP Pruett Formation Name and type section--Goldich and Elms (1949) named the Pruett from ex­ posures near the Pruett Ranch in the north-central part of the Buck Hill Quadrangle. There the Pruett consists mostly of tuff, but "includes conglomerate, tuffaceous sandstone and breccia, and tuffaceous freshwater limestone," and intercalated trachyte, basalt and andesite flows. In the Buck Hill Quadrangle, the Pruett unconformably overlies the Boquillas and in some places the Pen, and is 274-305 m (900-1000 ft) thick. Distribution, thickness, and lithology--The Pruett is exposed only in the extreme northern part of the quadrangle, as small outliers on down­thrown sides of faults. It lies unconformably on the Pen Formation, and with the Pen forms a· slope beneath the Mitchell Mesa Tuff and/or diabase sills. In the Yellow Hill Quadrangle, the Pruett consists of a basal limestone-pebble and cobble conglomerate, overlain by calclithite sand­ stone with minor amounts of interbedded gray and maroon tuff and benton­ ite. One maroon bentonite bed in the Pruett north of Hill 3964 (NC) con­tains white chalky limestone nodules. The thickness of the Pruett was not measured because the sec­tion is incomplete, and outcrops are small and isolated. Many outcrops consist only of basal conglomerate and the sandstone immediately above, but some include sandstone, tuff, and bentonite up to the overlying Mitchell Mesa Tuff. Less than one kilometer north of the quadrangle, 0.75 km east of Mesquite Tank in the Agua Fria Quadrangle, a thin sec­tion of Pruett capped by Mitchell Mesa is exposed in a cliff above the • Pen Formation. The estimated thickness of the Pruett there is 9 m and includes about 3 m of basal conglomerate. The estimated thickness of the Pruett in the Yellow Hill Quadrangle is 10 to 40 m; this variation in thickness may have been produced by recent erosion. • .... • • • Pe.trography--Six Pruett sandstone/conglomerate samples were taken above the basal conglomerate from two localities. These samples range from fine-grained sandstone to sandy granule conglomerate, and consist of light grayish-brown, calcitic, tight, submature to mature calclithite. Only two samples are faintly laminated; the rest lack lamination, but all show some imbrication and preferred orientation of elongate grains. The samples also show some grain compaction, indicated by line contacts and penetratio_ns. The most abundant grain type comprises carbonate rock fragments, which are mostly derived from ·cretaceous limestone. Coman­chean limestone predominates, but the amount of Boquillas rock fragments, including Inoceramus prisms, is significant. Some samples contain few pelsparite and other fragments possibly derived from Paleozoic limestone. The next most abundant grain type is quartz, very little of which is re­cognizable as volcanic. All samples contain a few fragments of chalce­dony and/or megaquartz, as well as chert. All samples contain some feld­spar, and some contain sandstone and siltstone rock fragments, magnetite, biotite, glauconite, and silicified volcanic rock fragments. The cement is very finely to coarsely crystalline mosaic sparry calcite, some of which is rhombic, indicating cementation in a meteoric diagenetic realm (Folk, 1974a). Hematite occurs as aggregates 10-300 µm, and stains parts of the cement. One sample contains 10-750 µm aggregates of aphanocrystal­line (?) pyrite. Some of the hematite and pyrite may not be authigenic, but instead derived from Cretaceous limestone fragments. One of the chalky limestone nodules from a bentonite bed in the Pruett outlier north of Black Ridge was examined in thin section. It is a sandy burrowed dismicrite (=sandy burrowed lime mudstone). Most dis­tinct ovoid burrows are 1-1.5 mm across, but they can range from 0.1 mm to about 4 mm across. Other disturbed areas are less distinct, except for cracks, and may be related to desiccation. All of these voids a~e filled by very finely to coarsely crystalline, rhombic sparry calcite cement. Terrigenous material ranges from silt to fine sand, and is fair­ly uniformly distributed throughout the micrite matrix. Sand and silt consist of quartz, some of which is volcanic and euhedral, volcanic rock fragments, some of which are silicified, and opaque minerals (leucoxene, I hematite, and possibly magnetite). Some possible carbonate rock frag­ments may be indistinct burrows. Fossils--The only fossils found in the Pruett in the Yellow Hill Quadran­gle are unidentified high-spired gastropods less than 5 nun long, collec­ted from lower Pruett sandstone. In the Agua Fria Quadrangle, the thick­er Pruett section has yielded abundant fossil material consisting mostly of terrestrial vertebrates. Sandstone and conglomerate contain land mam­ mals, including primates, rodents, amynodont rhinoceroses~ titanotheres, a horse and artiodactyls; and semi-aquatic reptiles, consisting of croco­ diles and turtles. The basal Tertiary conglomerate, where granular, con­tains teeth of primates, rodents, a bat, and other mammals; and gar scales and turtle scutes. Tuffaceous limestone contains Planorbis sp., Gonio­ basis sp.; and molds of freshwater ostracods (Moon, 1953), as well as well-preserved remains of turtles. Calcitic tuffs have yielded stein­ • kerns of a low-spired Helix-like gastropod (Moon, 1953). J. A. Wilson is studying the vertebrate fossils of the Pruett Formation in the Agua Fria Quadrangle, and has compiled the following faunal list for the Whistler Squat local fauna, which consists of fossils collected from the basal Pruett conglomerate and lower Pruett sandstone and conglomerate: Pristichampsus sp. Peratherium cf. P. Knighti Peratherium cf. P. marsupium ? Diacodon cf. Q. bridgerensis Scenopagus priscus Scenopagus curtidens Talpavus cf. T. nitidus Nyctitherium velox cf. Pantolestes Centetodon sp. cf. C. pulcher • Centetodon sp. B. Chiroptera Microsyops sp . • 45 Notharctus sp. Uintasorex sp. Omomys cf. 0. carteri ? Hemiacodon sp. Stylinodon sp. Thisbemys plicatus Microparamys minutus Lophioparamys sp. Paramyid indet. Mysops boskeyi Prolapsus sibilatoris Prolapsus junctionensis Prolapsus sp. indet. . Proviverra sp. .. Carnivore indet . ? Orohippus sp. Sthenodectes australis ? Epitriplophus sp . ... Diacodexis sp. Malaquiferus sp. Leptoreodon marshi Wood's (1973) study of the rodents of this fauna suggested a middle Eo­cene, probably early Bridgerian age for the lower Pruett. Based on the entire fauna, Wilson (1974) indicated an early Uintan age for the lower Pruett. The upper Pruett Formation has also yielded vertebrate fossils from localities in the Devil's Graveyard in the Agua Fria Quadrangle. This fauna is being studied by J. A. Wilson and includes the following taxa: Leptotomus cf . .!.:· kayi Mahgarita st~vensi • Haenodon cf. H. crucians proviverrine (new genus) Sthenodectes australis Amynodon sp. Protoreodon p~milus •. ? Poabromylus sp. Together these fossils indicate very late Eocene age (J. A. Wilson, per­sonal communication, March, 1977). A detailed discussion of Pruett vertebrates is being prepared by st·evens et al. (in preparation). Regional features and depositional environment--The Pruett thins drasti­ cally between the Yellow Hill Quadrangle and areas to the north. Lava flows which were used to separate Pruett from Duff in the Buck Hill Quad­ rangle (Goldich and Elms, 1949) pinch out before reaching the Tascotal Mesa and Agua Fria Quadrangles (Stevens~ al., 1975). These authors will propose a new formation name for the Pruett-Duff sequence s.outh of .. where the lavas disappear . Strata mapped as "Pruett-Duff" by Erickson (1953) in the Tas­cotal Mesa Quadrangle have been shown to be entirely Pruett by Wilson (personal communication, 1976). These strata are continuous with the fossiliferous Pruett strata in the Devil's Graveyard in the Agua Fria Quadrangle. Erickson's cross-sections show no regional thinning of Pru­ett and Duff toward the Solitario, and one section (p. 1372) which shows thinning on the northeast flank of the Solitario is based on his inter­pretation of the Tertiary strata as Pruett-Duff. In the Agua Fria Quadrangle, Moon (1953) estimated the thick­ness of the Buck Hill Group, which he thought to be mostly Pruett with perhaps some Duff Tuff, to be about 305 m (1000 ft). The Buck Hill there consists mostly of calcitic tuff, but includes conglomerate, sandstone, breccia, clay-tuff, limestone, and a unit referred to as "yellow conglom­erate." The basal conglomerate is about 3-15 m (10-50 ft) thick and con­tains no local igneous material. Sandstone above the basal conglomerate • contains abundant volcanic quartz, much of which occurs as euhedral bi­ pyramids. This same sandstone locally contains abundant petrified wood, " including logs up to 18-24 m (60-80 ft) and few stumps in growth position. Above the sandstone is maroon and gray bentonite with interbedded sandstone. The Upper Pruett consists mostly of calcitic tuff, but in­ cludes tuff-pebble conglomerate and sandstone in channels. Some of the tuff is cross-bedded, indicating sub~l~l~l deposition, and possi­ ble point bar sequences have been recognized in it (J. B. Stevens, per­ sonal communication, 1974). These Pruett deposits are 8-11 km (5-7 mi) north of outcrops in the Yellow Hill Quadrangle. A detailed discussion of the sedimentology, petrography, and paleoecology ·of the Pruett in the Agua Fria Quadrangle is being prepared by Stevens et al. (in preparation). In the Bofecillos Mountains area, the Chisos Formation, which is approximately correlative with the Pruett and Duff Formations, has a maximum thickness of about 320 m (1060 ft), not including the basal con­glomerate; the Jeff Conglomerate, thought to be correlative with the basal conglomerate of the Pruett, is about 6 m (20 ft) thick or less, and locally contains igneous pebbles (McKnight, 1970). In Big Bend National Park, the thickest section of Chisos For­ • mation is about 1070 m (3500 ft) in the Chisos Mountains (Maxwell et al., 1967). Here the Chisos Formation contains three to four lava members, as well as conglomerate, sandstone, and tuff. - The Pruett Formation and correlative deposits thin toward the Yellow Hill Quadrangle from all directions, to an estimated 10-40 m, be­cause of nondeposition and erosion. Considerable Tertiary erosion is in­dicated by the fact that the overlying Mitchell Mesa Tuff lies above the Duff Tuff in areas to the north and northwest, while in one place in the southern Agua Fria Quadrangle the Mitchell Mesa lies only a few meters above the basal conglomerate of the Pruett. Pruett sandstones in the Yellow Hill Quadrangle are calclithites with little interbedded tuff, while in surrounding areas correlative rocks contain abundant volcanic material and interbedded lavas. The Pruett section in the Yellow Hill Quadrangle is a thin tongue of lower Pruett. It is possible that some Pruett was removed by .. pre-Mitchell Mesa erosion. The Pruett in the hills southwest of Agua Fria Mountain is lower Pruett (J. B. Stevens, personal communication, 1974). The Agua Fria intrusive did not influence Pruett deposition, because ma­terial derived from the intrusive occurs only in Quaternary alluvium • (J.B. Stevens, personal communication, 1974), and Moon's (1953) map shows the northwest edge of the intrusive cutting the Pruett, indicating a post-Pruett age for this feature. The abundant fragments of Comanchean limestone in the basal Pruett conglomerate and overlying sandstone indi­cate that the high position of the Solitario and Terlingua-Solitario anti­cline had been established by the time of Pruett deposition, and influ­enced thickness and lithology as much as distance from volcanic centers. McKnig~t (1970) and Corry (1972) have concluded that the dome and anti­cline had formed by the time of the deposition of the Chisos Formation, because the Chisos pinches out against the flanks of these structures. The lithology, structures, fauna, and regional features indi-· cate that Pruett deposition was predominantly fluvial, and partly lacus­trine. McKnight (1970) concluded that, because of the great areal extent and little relief at the base of the basal conglomerate and Jeff, that these deposits were laid down by streams near base level on a vast pedi­ment surface. The abundance of conglomerate containing pebbles and cob­bles almost exclusively of well-rounded limestone fragments also supports this idea. Cross-bedded sandstone, tuff, and conglomerate were probably deposited by braided and low-sinuosity meandering streams. Limestone was deposited in lakes which collected a rain of tuff, and limestone nodules formed in shallow ponds and lakes. Wood (1973) suggests that increasing aridity with time during lower Pruett deposition may be indicated by an increase in the rodent genus Mysops~ from five percent of the rodents sampled from the basal conglomerate to 51 percent of the rodents in the Whistler Squat local fauna. Oligocene Series Mitchell Mesa Tuff Name and type section--Goldich and Elms (1949) named the Mitchell Mesa • Rhyolite from the unit capping Mitchell Mesa in the northwestern corner • of the Buck Hill Quadrangle. The rhyolite has a pink groundmass in which are phenocrysts of quartz and chatoyant, tabular anorthoclase that weather • 49 out in relief. The rock also contains 11gray vesiculated areas and ... red inclusions of baked and silicified ash.'' The Mitchell Mesa weathers to form a resistant ledge above the Duff Tuff. At the south edge of Mitchell Mesa, the rhyolite is about 12 m (38 ft) thick. Distribution, thickness, and lithology--The Mitchell Mesa is exposed only in the extreme northernmost part of the quadrangle, the southern edge of outcrops southwest of Agua Fria Mountain in the Agua Fria Quadrangle. It weathers to form a resistant ledge above softer rocks of the Pruett For­mation, and consists of rhyolitic, welded ash-flow tuff that contains pumice rock fragments and abundant phenocrysts of anorthoclase and quartz. Its thickness was not measured, but is probably less than 10 m (33 ft). Petrography--The Mitchell Mesa Tuff is rhyolitic, devitrified, vitric­crystal-lithic ash-flow tuff porphyry. The light salmon pink groundmass has a microeutaxitic appearance in plane light, which is perhaps a vague, remnant shard texture. It consists mostly of laths, wedges or rectangles 1-5 µm long, of quartz pseudomorphous after tridymite and cristobalite. Some 20-30 µm crystals are scattered through the finer ones, and crystals as large as 50 µm help make up the groundmass. Phenocrysts consist mostly of broad laths of anorthoclase, with some euhedral, embayed quartz crystals 0.2-4.0 mm long. There is a trace of hornblende, 0.1-0.5 mm crystals, altered to hematite at edges. An opaque mineral is probably pyrite altered to hematite at edges, and may represent the precursor of hematite aggregates. Fibrous, devitrified pumice rock fragments are up to several centimeters long, and contain spherical voids 0.03-0.4 mm across. The larger rock fragments have a few voids several millimeters across filled with anorthoclase crystals a few millimeters long. In hand specimen, the rock fragments show preferred orientation and some appear stretched, giv­ing the tuff a barely noticeable eutaxitic texture . • • Regional features and age--Burt {1970) has done the most detailed study of I the Mitchell Mesa to date. He estimated that the ash-flow sheet originally covered more than 3100 sq mi of Trans-Pecos Texas and Chihuahua, and con­ cluded that the source was in the Chinati Mountains based on petrographic and field evidence. Burt distinguished two cooling units within the Mitchell Mesa and correlative Brite Ignimbrite, the lower of which occurs in the Yellow Hill Quadrangle and surrounding areas. This lower unit has a maximum thickness of 70 m (230 ft) midway between the Cath.edral Mountain Quadrangle, where it is 18-41 m (58-134 ft) thick (McAnulty, 1955), and Bandera Mesa, and thins to the north and south. The minimum thickness is less than one meter on South Lajitas Mesa. The Mitchell Mesa Tuff is about 12 m (38 ft) thick in the type area in the Buck Hill Quadrangle (Goldich and Elms, 1949), but Graham (1942) measured thicknesses of 2lm (70 ft) and 46 m (150 ft) in areas west of there. In the Tascotal Mesa Quadrangle, the tuff is 0-21 m (0-70 ft) thick (Erickson, 1953). Moon (1953) gave no measurement for the unit's thickness in the Agua Fria Quadrangle. In the Bofecillos Mountains area, the Mitchell Mesa is mostly 6-11 m (20-35 ft) thick (McKnight, 1970). The Mitchell Mesa was correlated with the Brite Ignimbrite of the Vieja Group in Rim Rock Country by Ramsey (~961), who extended fur­ther to ncrth and west the minimum area known to have been covered by the ash-flow sheet. In the Vieja, the tuff lies above the Oligoce_ne Capote Mountain Tuff. In the Buck Hill and Tascotal Mesa Quadrangles, the tuff rests on the Oligocene Duff Tuff. In the Agua Fria and Yellow Hill Quad­rangles, the Mitchell Mesa lies on upper Eocene lower Pruett deposits, and at one locality is within about 10 m of the Cretaceous Pen Formation. In addition to thinning southward, the unit thins and pinches out on the flanks of the Solitario and of domes of the Contrabando low­land to the south (Maxwell and Dietrich, 1970; McKnight, 1970). The vir­tual absence of Mitchell Mesa Tuff in the Yellow Hill Quadrangle reflects this pinchout on the flanks of the Solitario more than regional southward thinning. Several p9tassium-argon ages have been determined from Mitchell • Mesa/Brite samples. Evernden et al. (1964) listed an age of 29.7 m. y. " for the Brite. Wilson et al. (1968) cited this age and gave additional ages of 33.9 ± 1.8 m. y. for the Mitchell Mesa and 33.0 ± 1.1 m. y. for the Brite. F. W. McDowell (1976, personal communication) determined an age of 31.4 ± 0.5 m. Y~ for the Mitchell Mesa _, which was the average of ages obtained from five samples, including a sample of Mitchell Mesa from the Agua Fria Quadrangle, which by itself gave a K-Ar age of 32.3 m. y. McDowell's ages are taken to indicate an age of 31-32 m. y. for the ash-flow tuff. Quaternary System Quaternary Alluvium Distribution, thickness, and lithology--The distribution of alluvium is shown on the geologic map (Plate I) and is limited to beds of streams and their tributaries, except for the thin unmapped terrace gravels previous­ly mention~d (p. 7, 11-12, 14). Alluvium consists mostly of Cretaceous .. . limestone pebbles, cobbles and boulders, either predominantly Comanchean or Gulfian depending on the bedrock on which it lies. The finer grained alluvium is mostly sandy gravel, and forms thick deposits in some places which weather to form smooth slopes. Where large boulders predominate, as in canyons cut into the rim of the Solitario, the alluvium resembles chaotic rip-rap. The maximum thickness of alluvium is estimated to be about 15 m, southeast of a rhyolite plug near the Lefthand Shutup. Petrography--A sample of the unmapped caliche-cemented terrace gravel, collected north of Hill 3412 (EC), was examined in thin section. It is a limestone pebble conglomerate and sandy flat-pebble conglomerate, ce­mented by vuggy caliche; the conglomeratic equivalent of a pale yellow, extremely porous, submature calclithite. It consists of Boquillas lime­stone pebbles, granules and sand suspended in a porous, spongy caliche­micrite matrix. There are trace quantities ?f sandstone and volcanic rock fragments. The caliche-micrite is full of irregular, jagged voids • ranging from about 5 µm to several millimeters long. Tabular voids are " preferentially concentrated along the upper sides of flat limestone frag­ments. The fragments show an overall preferred orientation parallel to the surface on which they were deposited, but some fragments are edge­wise, which with the seeming lack of grain support give slabbed specimens ..,. an "exploded" appearance. The frothy-looking matrix suggests that the caliche-micrite may be the product of meniscus cementation in large pore spaces that were initially bridged by pendulous cement, which explains the concentration of tabular voids along upper surfaces of flat pebbles. Some patches of microspar occur between the more closely packed clasts; these areas were probably cemented earliest and neomorphosed first. Lim­onite locally stains parts of clasts and matrix. Depositional environment--Stream alluvium was and is deposited by ephem­eral streams. The thin unmapped terrace gravels are outliers of dissec­ted pediment deposits, probably deposited by shallow braided streams and sheetfloods after heavy rains. "· • I N T R U S I V E I G N E 0 U S R 0 CK S DIABASE The most abundant~ areally extensive, and volumetrically impor­tant intrusive ieneous bodies are diabase sills; only two of the intrusive masses mapped are not diabase. A few diabase intrusives are plugs or dikes. Bodies of hydrothermally altered diabase are discussed separately in the section entitled "Hydrothermal Alteration." Plugs Most plugs are small and are exposed on the northeast flank of the Terlingua-Solitario anticline. Their distribution is structurally controlled by the strike of the Boquillas Formation. A plug less than two meters across and too small to map was found about l~ km southeast of Hill 3743 (NE). A plug about 0.3 km across with an associated dike .intrudes the Boquillas north of Hill 3412 (EC). Another diabase plug the same size intrudes Del Rio, Buda, and Boquillas on the northeast flank of the Soli­tario. This plug has faults on its south side, bounding a sliver of Buda overlain by Boquillas. A few small, irregular diabase plugs intrude the Pen Formation; one of these is southeast of Hill 3252 (EC); the others are east of Pink's Peak. Some plugs have yet to be uncovered by erosion. One of these is in an area characterized by gray, resistant hills of baked and contact­metamorphosed Boquillas limestone, about 2~ km north-northeast of Pink's Peak. One of the hills has a small outcrop of diabase at its top, as well as a small dike on both west and east sides. Other diabase bodies in this area consist of dikes, and a partly unroofed sill about ~km east of the quadrangle. The above features make it likely that all of these hills are covered plugs. Another covered plug was found about 1 ~m northwest of Hill • 3215 (EC); it is a small hill that projects above its flat surroundings, • consisting of gray~ contact-metamorphosed Boquillas limestone with 53 .. ... • • abundant analcite spherules. At least two plugs were definitely derived from tops of sills. The north side of the top of Pink's Peak is occupied by a plug, while an outlier of the upper of two diabase sills is exposed around the base of the peak. The relatively large area covered by the sills compared to the area of the plug indicates that the plug was derived from the sills rather than vice-versa. Well-developed, equant polygon columnar joints occur on the north side of the plug; columnar joints are less well-developed. in other parts. The joints range from straight to twisted and curved in cross-section. Some joint faces show plumose patterns indicative of ten­sion, while others are longitudinally grooved. The plug rock shows both finer basaltic and coarser diabasic textures, with the finest occurring where columnar joints are best developed. Hill 3308 (SE) to the south also has a plug with columnar joints a_t its top and an outlier of the same sill exposed around its base. The plug and sill outcrops are con­tinuous with each other on the south side of the peak, showing that the plug was derived from the top of the sill. The small size of most diabase plugs makes it likely that they are irregularitieB on the upper surfaces of sills, like the irregularities on the top surface of the uncovered sill northeast of Black Ridge. Dikes Dikes are few, generally less than ~ m wide, short, have north­westerly trends, and intrude Boquillas and Pen Formations. The dike associated with the plug north of Hill 3412 (EC) trends north-northwest in the Boquillas. The dike consists of two segments, one north of the plug and the other to the south. The maximum length of the dike is about 1.1 km, and the width is less than !2 m. A dike about ~ km long and less than ~ m wide intrudes the Pen Formation on the downthrown side of the fault east of Hill 3308 (SE). The dike has a north-northwesterly trend, and much of it is actually broken into short en echelon segments. Dikes in the Boquillas north-northeast of Pink's Peak were men­tioned earlier; these are only a few meters long, less than ~ m wide, and SS probably represent small apophyses filling cracks in the country rock above plugs that are still unexposed. In this same area is one dike that is about ~ km long and up to a few meters wide. This dike has an overall northwesterly trend, but is sinuous in plan view, perhaps indicating coa­lescence of originally distinct~ echelon dike segments. A small dike and plug intrude the Fizzle Flat Lentil of the Boquillas Formation just outside the east boundary of the quadrangle, east-northeast of Pink's Peak. The dike is about 1/6 km long and less than ~ m wide, and has an east-northeasterly trend. No dikes were found which could have been feeders for the sills. A possible feeder was found exposed in the west wall of Terlingua Creek in the Agua Fria Quadrangle, about 1 km to the north. Here the uppermost and thinnest of three sills is joined by a dike from above. • Diabase Sills Field .Relations Diabase sills are exposed in a north-northwest-trending belt in the eastern part of the quadrangle. Distribution is controlled by the outcrop area of Upper Cretaceous rocks, principally the Boquillas Forma­tion. The area occupied by sills is roughly seven percent of the quad­rangle area, and amounts to about 12.5 sq km or 4.5 sq mi. Sills intrude all formations between the Buda Limestone and the Mitchell Mesa Tuff, inclusive. In general, sills are found in lower Boquillas in the south, and in upper Boquillas and Pen in the north. In the northernmost part of the quadrangle, a sill intrudes Pen, Pruett and Mitchell Mesa (Plate I). Just to the north, in the area southwest of Agua Fria Mountain, sills intrude the Pruett, and probably intrude Mitchell Mesa Tuff, but contacts with the Mitchell Mesa are not well enough exposed to be conclusive. Different rock types put different constraints on the sills. The Buda Limestone has very sharp, though undulose, bedding, is hard and dense, and is thus favorable for concordant intrusion. Only two sills are found in the Buda: one in Terlingua Creek (Fig. 8), and the other a hy­ drothermally altered diabase sill associated with a plug on the southeast flank of the Solitario. The hardness of the Buda, along with the hard­ ness and massive character of the underlying section may account for the Buda being the lower limit of concordant intrusion, and the Santa Elena the lowest observed level of diabase intrusion, and for the relative scarcity of intrusive bodies in these rocks. The flaggy, clayey, well-bedded Boquillas Limestone is the most favorable unit for concordant intrusion. Sills in the Boquillas have sharp contacts that parallel bedding over large areas. Sills have well-developed cooling joints (Fig. 9), and the best developed cross­sections are hexagons stretched in one direction. The Pen Formation is the next most abundantly intruded unit but sills in the Pen are the least concordant, because of the scarcity • of competent beds . • Sills intruding the Pruett can be traced to outcrops southwest of Agua Fria Mountain. Here concordant and discordant masses intercon­nect to form a continuous network occupying different levels within the .~ Pruett. This network consists of a series of sills at different levels with connecting sheets, and irregular discordant bodies. Some sills appear swollen, and have columnar joints with equant polygon cross­sections; smaller pods have joints that show an anastomosing pattern in plan view, and appear platy. Sills show local discordances, of which there are several types. The most common type is an oblique discordant sheet that connects sill segments at different levels, in many places giving the entire body a "monoclinal" appearance. Examples of this occur i.n several places along the west edge of Black Ridge. Another example is the sill east of, and connected to, the sill capping Hill 3743 (NE). A connecting sheet between two sills is exposed in a stream bed south of Hill 3308 (SE) . Such dis­cordances occur in many places in the area southwest of Agua Fria Moun­tain to the north. Probably the best example of this type of discordance is where the Black Ridge sill, most of which lies above the Allocrioceras Zone in the Boquillas, turns downward to a level below this zone at its • Fig. 8. View southwest toward diabase sill intruding Buda Limestone exposed in wall of Terlingua Creek, northeast part of quadrangle. Fig. 9. Cooling joints in sill in Boquillas Limestone northeast of Black Ridge, viewed from northwest. 59 northwest end. Examples of other types of discordances are the undulatory top ... surface of the sill northeast of Black Ridge; a "zig-zag," stepped por­ ' tion of the Black Ridge sill exposed in the south wall of the stream bed southeast of Hill 3704 (NC); and the large, inclined sheet that cuts Pen, Pruett, and Mitchell Mesa in the north. Bradley (1965) believes that discordances in diabase sills reflect the inverse of the paleotopographic surface of the land above the area intruded, and are isostatic. R. 0. Kehle (personal communication, 1975) interprets discordances which connect sills as the coalescence of separate sills intruded at different levels. Stress conditions at the edges are such that coalescence represents the path of least resistance. A combination of these ideas seems necessary to explain stepped sills, in which separate concordant segments are abundant and short. Intrusive contacts are· characterized by baking and contact metamorphism of the country rock, and by chilling of sill margins. Chilled zones are generally the most highly weathered parts of sills. Other, less common contact features include xenoliths, apophyses, and "leaves" of bedded country rock pried up during lateral emplacement of sills. Such "leaves" occur in Terlingua Creek where a sill intrudes the Buda, and at the southeast edge of the sill outcrop at Hill 3252 (EC). Xenoliths are rare, but where present they are conspicuous. They are baked and metamorphosed, but show no signs of assimilation. The most spectacular xenolith is the large, tilted block of Boquillas lime­stone within the stepped portion of the Black Ridge sill (Frontispiece). The rectangular block was detached along joint planes. In one of the sills exposed in Terlingua Creek, angular, irregular Boquillas limestone fragments resemble sedimentary rip-up clasts. A similar xenolith over 1 m long was found in the stream bed west of Hill 3630 (NC). One of the .. thick sills in the Pruett in the Agua Fria Quadrangle contains a large xenolith of Pruett tuff with a petrified tree trunk in growth position • (Fig. 10). • Some of the Boquillas outliers on the sill northeast of Black Ridge could be large xenoliths sunk into the sill rather than lying on it. in diabase sill southwest of Agua Fria Mountain, north of Yellow Hill Quadrangle. A petrified tree trunk is in place at the lower right edge of the xenolith, beneath the man's feet. Boquillas outliers on the southernmost sill are not xenoliths but varia­tion in dip indicates differential movement within the large sheet be­ tween the two sill levels. Apophyses are of various types, including small, irregular blebs, as in the Pen northeast of Black Ridge, dikelets one to several centimeters thick, and "fingers." Dikelets are found mostly at bottom contacts, as in the wind gap cut through the sill northeast of Black Ridge. Small finger-like apophyses are found in Boquillas limestone be­neath the stepped portion of the Black Ridge sill. The fingers of dia­base are parallel to bedding, and have cross section diameters of a few millimeters to a few centimeters. These fingers probably are fillings of conduits preferentially dissolved along bedding planes by escaping volatile substances. 61 Cross sections of large scale fingers can be seen in Terlingua Creek east of Hill 3310 (NE), at the edge of a small sill (Fig. 11). This •t could indicate that the sills were emplaced by coalescence of peripheral fingers, as described by Pollard~ al. (1975) though the orientation of the section with respect to the sill's edge is unknown. At other places where sill edges are seen in cross-section, as in the draw cutting the northwest edge of the Black Ridge sill, there is no evidence for fingered emplacement. I estimate the thickness of major sills capping mesas to be less than about 12 m or 40 ft. The southernmost sill is noticeably thicker than other sills, and its maximum thickness near Pink's Peak is probably no more than 50 m (164 ft). Thickness estimates for three sills exposed in Terlingua Creek are as follows: 6m or 20 ft for the lowest; 3 m (10 ft) or less for the next higher sill; and less than 2 m or 7 ft for the high­ .. est. Moon (1953) reported a sill 50 ft (15 m) thick in his measured sec­ .. tion in Terlingua Creek, but this seems an overly thick measurement. The sills are multi-leveled. Good examples are in Terlingua Creek,. where both Buda and Boquillas are intruded; at Hill 3743 (NE), and in the vicinity of Pink's Peak (Fig. 12). East of Pink's Feak the lower of the two sills is exposed in the west wall of a fault scarp. Petrography Terminology used for diabase textures is that of Moorhouse (1959): diabasic or doleritic refers to optically continuous pyroxene patches interstitial to a mesh of plagioclase laths; ophitic refers to optically continuous pyroxene patches that poikilitically enclose plagio­ clase laths. Subdiabasic or subdoleritic, and subophitic textures are like the above two textures, respectively, except that the pyroxene pat­ ches consist of aggregates not optically continuous. The predominant texture seen in sill samples is subdiabasic; • some are diabasic and a few are ophitic. Most sills consist of diabase porphyry, and have plagioclase phenocrysts whose mean size increases up • section to the north from about 1 cm in the Black Ridge sill to greater 1 than 4 cm long in parts of the sill that cuts the Mitchell Mesa Tuff, Fig. 11. Cross-sectional view from southwest of large scale, partly coalesced, tabular fingers at the edge of a small sill exposed in the bed of Terlingua Creek. Coalescence of the slightly offset fingers would produce a short discordant segment of the sill. Fig. 12. View south from Pink's Peak of two sills, left and right, separated by Boquillas Limestone. A stream (center) has exposed a slightly discordant sheet which connects the two sills. • • • and up to as much as 8 cm long in a sill in the Agua Fria Quadrangle. The grain size of plagioclase laths is 1 mm or less, based on 21 samples. Diabase in the northern sills contains vesicles filled with analcite and calcite. Mineral percentages were determined by visual estimate in thin section. Essential minerals are plagioclase, titanaugite, and olivine. Plagioclase averages about 68 percent, ranging from about 35 to 88 per­cent, and comprises both subhedral laths and larger phenocrysts. In about one-fourth of the samples, plagioclase laths show preferred orien­tation. In one-third of the samples, the more equant crystals and pheno­crysts are sharply zoned. Titanaugite is the only pyroxene, is generally anhedral, and averages about 18 percent of diabase samples. It generally ranges from about four to 25 percent, but one plug j.ust east of the southern edge of the quadrangle contains about 50 percent pyroxene. In thin section, a third of the samples show distinct concentric zoning of pyroxene. Olivine occurs both as phenocrysts up to 4 rrnn long, and in the groundmass. It averages about 7 percent, ranging from about four to 14 percent. In about a third of the samples, olivine shows distinct zoning; in many of these samples the zoned crystals have centers with abundant inclusions of magnetite surrounded by clear rims. Olivine is mostly anhedral, though some samples have a few euhedral phenocrysts. Accessory minerals comprise magnetite, apatite, and possibly pyrite, with magnetite predominating. Magnetite is probably titanomag­netite, and constitutes about 4-6 percent of diabase. Crystals are most­ly euhedral and range in size from a few to 50 µm; they occur in aggre­gates mostly 0.6 to 1.5 rrnn across, but which range from 0.15 to 4 mm, based on eight samples. Apatite averages less than one percent and consists of needles 0.1-1.5 mm long x 2-20 µm wide, poikilitically enclosed in plagioclase. It occurs in most of 21 samples . Late stage minerals consist mostly of analcite and other zeo­lites. Analcite is mostly interstitial to plagioclase laths, but also occurs as vesicle fillings. Most vesicles are filled completely by • f; • • • ' analcite; some vesicles are lined by euhedral analcite crystals and filled by other zeolites or calcite. A few samples have some euhedral analcite crystals in the groundmass, indicating that earliest analcite preceded latest plagioclase crystallization. All but two samples con­ tain analcite, in amounts ranging from about one to ten percent, and averaging about three percent. Other zeolites are fibrous and mostly length-slow, and occur as clusters or vesicle fillings up to about 1 cm in diameter, and frac­ture fillings. Only a few samples, all from northern sills, contain them, in amounts ranging from one to five percent. Most samples contain some amount of the following alteration products: chlorophaeite or bowlingite after olivine, serpentine along cracks in olivine, saussurite after plagioclase, and sericite along cracks in some plagioclase phenocrysts. About a fifth of the samples contain deuteric biotite, in amounts ranging from a trace to about three percent. No evidence of layering or crystal settling was seen in the field, in hand specimens, or in thin sections, and only the southernmost sill is thick enough, greater than about 30 m or 100 ft, to possibly have experienced settling effects (Walker, 1961). A few concentric bands do appear in one small part of this sill on air photos, and detailed sam­pling in this area from bottom to top might reveal layers that vary in concentrations of cumulate minerals. Chemical Data I follow Parker (1977) in usage of the terms hawaiite, mugearite and basalt: hawaiite has Anso-An30 normative plagioclase and differentia­tion index 30-45; mugearite has Anso normative plagioclase and. dif­ferentiation index <30. Priority is given to normative plagioclase com­position. Two chemical analyses of diabase were performed by G. Karl Hoops (Table 2). Samples were taken at the northwest end of Black Ridge and the faulted eastern edge of the southernmost sill; analyses show both • ~ TABLE 2. Chemical Analyses and CIPW Norms for Two Samples of Diabase from Yellow Hill Quadrangle SAMPLE .... Si02 Ti02 Al203 N .u Fe203 ~I FeO MnO ) ~1