Browsing by Subject "Geology -- Texas -- Van Horn Mountains"

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Geologic map of Van Horn Mountains, Texas
(University of Texas at Austin. Bureau of Economic Geology, 1959) Twiss, Page C., 1929-
About 10 miles south of Van Horn, Texas, the Van Horn Mountains rise abruptly above an intermontane plain and extend southward to the Sierra Vieja. The area lies primarily in the southwestern part of Culberson County but extends into Hudspeth and Jeff Davis counties. The mountains owe their present topographic form to late Tertiary block-faulting. Structurally, they are a northward-trending horst which is flanked on the east and west by intermontane basins. The basin east of the mountains is herein named "lobo bolson": although structurally separate, it is topographically a southern extension of Salt Basin. An outcrop (30 46.5 N., 105 55.5' W.) of Tertiary volcanic rock separates two structural basins west of the mountains: the southern basin is herein named "Green River bolson." Topographically, the northern part of the northern basin is a southeastern extension of Eagle Flat. In addition to the major, generally north-south high-angle normal faults that define the horst, many normal faults of lesser magnitude cut the highland mass of the Van Horns; they are either nearly parallel to or at large angles to the strike of the horst. In the southern part of the area are three thrust faults of Laramide age. Older Comanchean strata on the west are thrust over younger Cretaceous strata on the east. These faults occurred before the outflow of lava in the Davis Mountains. The northern segment of the mountains, where Precambrian metasedimentary rocks are exposed, is within the southern boundary of the Van Horn uplift. Rocks of Permian, Cretaceous, and Tertiary age dip generally to the south at about 4 degrees. The area lies along the junction of the Chihuahua tectonic belt on the west and Diablo Platform on the east. The Tertiary volcanic rocks are part of the Davis Mountains lava field, which is the greatest exposure of volcanic rock in Texas. Stream flow of the region is intermittent. Salt Basin, collecting water from Eagle Flat and from the northern and eastern parts of the Van Horn Mountains, is a prominent basin of interior drainage. Water flows northward from Lobo Flat to Salt Basin through Wildhorse Draw. After cloudbursts, water from Eagle Flat flows through the gap between the north end of the Van Horns and the south end of the Carrizo Mountains and joins Wildhorse Draw. Green River, which empties into the Rio Grande, receives water from the western part of the area. Hence, the eastern intermontane basins are being filled with sediment, while outcrops of the Green River bolson are being eroded and the sediment transported to the Gulf of Mexico. In Lobo Flat as much as 1,200 feet of alluvium covers the bedrock, and at the head of Green River, the alluvium is at least 1,100 feet thick. Previous workers have contributed much to the knowledge of the area. Baker (1927) included the Van Horn Mountains in his regional study of Trans-Pecos Texas. Flawn (in King and Flawn, 1953) mapped the Precambrian rocks exposed in the Carrizo Mountains and in the northwestern and northeastern sectors of the Van Horns; summaries of his petrographic descriptions are included in this report. The southern third of the area was mapped by The University of Texas graduate students C. G. Havard, R. I. Ellsworth, E. R. Kennedy, and R. N. Throop in 1948 and P. Braithwaite and D. R. Frantzen in 1957. This study is part of a Bureau of Economic Geology project of geologic mapping in Trans-Pecos Texas. The writer gratefully acknowledges the valuable suggestions of Professor R. K. DeFord and Dr. John T. Lonsdale on stratigraphic and petrographic problems, respectively. Dr. K. P. Young identified the Cretaceous fossils. John W. Trammell ably assisted in the field in 1957, and Charles E. Clark in 1956. This report is part of a doctoral dissertation prepared under the supervision of Professor DeFord. Although the base of this unit is covered, the writer believes that it overlies the Hueco limestone; the dip of these beds is similar to that of the Hueco beds to the north. This unit is about 60 feet thick. Limestone member (Pv!)The basal member of the Victorio Peak grades upward into a medium- to thick-bedded, unfossiliferous, dolomitic limestone. On fresh surfaces, the color grades upward from brownish-black to medium gray; it weathers to pale orange. Bedding is well developed; the beds are from 6 to 15 inches thick. Differential weathering produces a horizontal fluting parallel to bedding. The rock is finely crystalline and composed of clear crystalline calcite patches scattered throughout a finely crystalline dolomite matrix. About 180 feet were measured in the Van Horn Mountains; no upper contact was observed. This unit is similar to the limestone member (Pv1) of the Victorio Peak formation that crops out in the Wylie Mountains (Hay-Roe, 1957). The age is probably Leonard.
Pegmatites of the Van Horn Mountains, Texas
(University of Texas at Austin. Bureau of Economic Geology, 1951) Flawn, Peter Tyrell
Zoned and unzoned perthite-quartz-plagioclase-muscovite pegmatites in the form of tabular bodies, irregular bodies with tabular branches, irregular masses, elongate lenses, lit-par-lit zones, and small augen and stringers are distributed throughout the Precambrian metasedimentary rocks of the Mica Mine area, Van Horn Mountains, Texas. Zoned bodies have a core of erthite and quartz and a plagioclase-quartz-perthite-muscovite wall zone. The pegmatites contain numerous schist inclusions, and some show evidence of contamination by biotite schist and amphibolite. Intimate penetration of the host rock by pegmatite fluids was accomplished by a combination of dilation and digestion. Dilation was effected by injection pressure and (to an unknown degree) orogenic stresses. Crystallization of a solution containing a large excess of potash took place in a closed or restricted system where a delicate balance of solubility factors was maintained for long periods or in a solution of low viscosity, thus facilitating growth of large crystals about a limited number of centers. Zoning and textural relationships are accounted for under these conditions. The importance of horizon and mode of emplacement in the formation of pegmatite textures and shapes is emphasized. A review of the granitization, palingenesis or anatexis, open-system (or aqueous), and magmatic theories of pegmatite origin shows that the features of Mica Mine pegmatites are best explained by the magmatic theory. The Mica Mine area has possibilities of exploitation for feldspar and scrap mica.
The Van Horn Mountains caldera, Trans-Pecos, Texas : geology and development of a small (10 - km sq.) ash-flow caldera
(University of Texas at Austin. Bureau of Economic Geology, 1986) Henry, Christopher D.
The Van Horn Mountains caldera is a small (~10-km2) igneous center in the Trans-Pecos volcanic province. The caldera formed 37 to 38 mya during eruption of the first of two ash-flow tuffs related to the caldera. Part of the first tuff ponded within the caldera; the lower marker horizon of the Chambers Tuff in the Sierra Vieja south of the Van Horn Mountains is probably the correlative outflow tuff. Following collapse, the caldera was partly filled by a heterogeneous assemblage of air-fall tuff, tuffaceous sediment, and possible ash-flow tuff. Contemporaneous with this filling, a rhyolite porphyry was emplaced in the middle of the caldera. The porphyry is comparable to the resurgent dome of other calderas but does not uplift the intruded rocks. An intrusive-extrusive complex of basalt, trachyte, and minor rhyolite was emplaced just outside the eastern margin of the caldera; flows from this complex spilled over the rim and into the caldera. Eruption of a second major ash-flow tuff, herein named the High Lonesome Tuff, may have led to additional collapse.Collapse occurred dominantly along a single, major, nearly circular fracture zone. Embayments in the fracture zone were influenced by precaldera, largely Laramide structures. Large, commonly brecciated blocks of wall rocks slumped into the caldera and were incorporated into caldera fill. Precaldera topography channeled most of the ash-flow tuff to the south, through a low area in the caldera wall.The 10-km2 area of the caldera and the no more than 10-km total volume of erupted ash-flow tuff make it one of the smallest ash-flow calderas worldwide and more comparable to collapse features associated with stratovolcanoes. Nevertheless, it shows all the features typical of much larger volcanic centers.No known ore deposits are associated with the caldera, but the rhyolite porphyry is hydrothermally altered, exhibiting silicification and sericitization typical of many porphyry molybdenum deposits. The apparent lack of deposits may be a function of the small size of the magmatic system, lack of sufficient trace element enrichment by differentiation, or lack of exposure resulting from minimal erosion.