The-west-dipping, north-northwest-trending Black Peak fault and associated monoclines which form the western flank of the Marathon dome, are well exposed near Del Norte Gap. Field mapping shows that the dip of the Black Peak fault increases downward from zero to 80 degrees. A northeast-trending right lateral fault cuts the hanging wall of the Black Peak fault at Del Norte Gap. The Black Peak fault has greater displacement south of the gap than north of the gap. Several north and northwest-trending normal faults cut the Cochran Mountains. The folding and faulting took place after the deposition of the upper Boquillas Limestone and before the deposition of Quaternary gravels. Vertical uplift of the Marathon dome during the Laramide orogeny produced the Black Peak fault and associated features. Normal faults later cut the area. The structural features near Del Norte Gap correspond well to previously described analytical and experimental configuration of features produced by differential vertical movement of basement blocks and previously described examples of vertical tectonics. / text

Sedimentology, stratigraphy, and stable-carbon isotopy were used to reconstruct geologic and climatic events on the Texas southern High Plains from ca. 13,000 yr B.P. to the present. The alluvial sediments in Yellowhouse and Blackwater Draws were used to construct the geologic history. The oldest valley alluvium comprises the > 13,000-yr-B.P. fluvial sediments that were incised and buried by fluvial and lacustrine sediments dating ca. 13,000 to 4900 yr B.P. Lacustrine waters changed from oligotrophic to eutrophic and finally calcalitrophic. Regional valley erosion at 4900 yr B.P. developed a widespread disconformity within the Yellowhouse Draw formation, which separates lower fluvial and lacustrine sediments (ca. 13,000-4900 yr B.P.) from the overlying sediments dating 4900 yr B.P. to present. After 4900 yr B.P., intermittent streams and eolian processes deposited several meters of sand the length of each valley. Cienegas returned to downstream reaches of both draws after 1500-2000 yr B.P. Methods were developed to extract purified collagen residues and hydroxyproline from heavily contaminated fossil bones. Reliable δ¹³C measurements on collgen require isolation of single amino acids, whereas less specific purifications may yield accurate bone collagen ¹⁴C dates. Collagenous residues were extracted from 13,000-200-yr-B.P. fossil bison bones from the Lubbock Lack Site at Lubbock, Texas, and δ¹³C values were determined. Collagen δ¹³C values changed from -8 per mil at 200 yr B.P. to -10 per mil at 4900 yr B.P. and to -17 per mil at 12,500 yr B.P. The δ¹³C changes imply that the Lubbock area grasslands contained 30 to 40 percent C₄ grass biomass at 12,500 yr B.P. in contrast to the 95 percent C₄ grass biomass in today's grasslands. The stratigraphic and isotopic results gave similar paleoecological histories for the Texas southern High Plains. At 12,500 yr B.P. permanent streams existed and grasslands may have resembled those in the northern central Great Plains today. The climate warmed gradually, and the water table dropped until 5000 yr B.P. when a major hydrologic shift occurred. After 4900 yr B.P., modern climatic depositional and vegetation communities were developed. Geomorphic thresholds apparently controlled the regional disconformities, depositional events, and pedogenetic episodes. Climatic change was the ultimate cause of stratigraphic changes, but individual geologic events were not coeval with any similar climatic shift.

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A detailed structural analysis was conducted of the Warwick Hills at the northeast tip of the doubly-plunging Dagger Flat anticlinorium, Marathon Basin, west Texas. Field work delineated a folded duplex structure composed of three horses. Thrust transport was towards the northwest and resulted in a hinterland-dipping duplex. Initial thrusting In the Warwick Hills shortened the area by 2.2:1 (54%). Post-thrusting, the duplex underwent nearly isoclinal folding creating two anticlines and a syncline, second-order folds to the Dagger Flat anticlinoium. Folding combined with thrusting brought the total shortening of the rock package to 6.5:1 (85%). Earlier estimates gave a shortening for the Warwick Hills of 3:1. Finally, the folded duplex was extended by oblique tear faulting that offset the folded thrusts accommodating extension of the major folds in a northeast direction. These tear faults occurred post-plunging of the folds and were the last deformational movements that affected the Warwick Hills. The Ordovician Maravillas and Devonian Caballos Formations acted in the Warwick Hills as a structurally competent couplet. Addition or subtraction of this couplet, or units in this couplet, controlled the location of the major and minor thrusts, the style and shape of folds, and the location of the fold hinges. Bounding the couplet are incompetent shales of the Ordovician Woods Hollow and the Mississippian Tesnus Formations. Thrusts in the Warwick Hills duplex have a basal décollement in the Woods Hollow shale and ramp up through the Maravillas/Caballos couplet with an upper décollement in the Tesnus shale. The entire duplex was primarily folded by flexural slip (i.e. concentric folds) as evidenced by slickensides oriented parallel to bedding and perpendicular to fold axes, the constant thickness of the competent layers and the change in fold shape with depth. Fold wavelength, as determined from the couplet in the lowest thrust sheet, averages ~1,300 m and the average fold axis for the Warwick Hills, as determined stereographically, plunges ~54° N90°E. Shale in the Woods Hollow and Tesnus Formations bounding the couplet, flowed passively during folding into the cavities that were created by the bending of the more competent units. Lower and upper boundaries of disharmonic folding developed in the Woods Hollow and Tesnus Formations respectively. Unique to this area when compared to the rest of the anticlinorium are the presence of tightly folded thrusts and steep east-trending fold axes. The anticlinorium plunges in the Warwick Hills because it drapes off a down-to-the-northeast basement fault. Folds were "dragged" or diverted to the east during thrusting of the duplex over this transversely-oriented paleotopographic fault scarp, or were diverted subsequent to thrusting of the duplex by strike-slip movements at depth along the basement fault. / text

Seven maps depicting general geology, physical properties, environmental geology, soils, soil thickness, slope, and fracture intensity describe the land in the Lake Travis vicinity. The maps, descriptive text, and interpretive tables provide the basis for evaluations of land-use capability. The general geologic map shows variations in bedrock, in surface deposits, and in the structural-geometrical relations of the units. It is a basic data source for constructing and interpreting the other maps. The physical properties map presents qualitative engineering characteristics of substrate and surface materials. Most of the Lake Travis vicinity is underlain by carbonate rocks that are generally stable foundations for construction. The environmental geologic map is based on variations in processes, landforms, and surface and bedrock materials. Process units represent areas subject to flooding and mass wasting as well as loci of aquifer recharge. These areas will sustain only limited use without detrimental environmental effects. Material-landform units represent areas that have constraints to land use; however, with preventive or corrective engineering the land could probably sustain a variety of uses. Soils, soil thickness, slope, and fracture intensity maps present data at a smaller scale than that of the general geologic, physical properties, and environmental geologic maps. Soil cover is generally thin or absent. Most of the land is moderately steeply to very steeply sloping. High-density fracture zones that allow rapid water infiltration occur in some of the area. The land-use capability map constructed to depict suitability for septic tank operation shows that most of the Lake Travis vicinity is generally unsuited for this use. The current land use map delimits areas of population pressure and attendant competitive uses of the land. These pressures intensify the need for enlightened planning so that land use will be compatible with limitations shown or implied on the basic maps. / text

The Middle-Ordovician Simpson group, deposited in a shallow epicontinental sea, is composed of a sequence of interbedded limestone, shale, and smaller amounts of sandstone. By means of electric and radioactivity logs the group is divided into eight zones traceable across Pecos County, except for its absence, over the Fort Stockton uplift area, caused by post-Simpson erosion. Each zone thins persistently eastward toward the Texas Peninsula, which separated the West Texas basin from the Oklahoma basin of deposition. A northwestward increase in sand content of each zone indicates that the source area for the sand was to the northwest. In Pecos County oil is produced from the Simpson at present in only a small north-central area. / text

Detailed field and petrographic study of the Precambrian Rough Ridge Formation of the Packsaddle Schist documents a more complex tectonic history for the Llano Uplift than has previously been reported. The Rough Ridge Formation was affected by two metamorphic events and four phases of deformation. Mineral assemblages in pelitic and mafic lithologies are characteristic of the amphibolite facies, and the presence of cordierite in preference to almandine indicate low pressure during part of the metamorphism. Inclusions of staurolite in a Mn-rich garnet, coupled with the absence of staurolite elsewhere in the rock suggest that pressure may have ranged from medium to low during metamorphism. Textures indicate that post-tectonic recrystallization occurred as a consequence of either slow cooling or of reheating after dynamothermal events. The first phase of deformation (D1) was a complete transposition of original sedimentary layers, with a pervasive foliation (S1) forming parallel to the axial planes of isoclinal folds. D2 is characterized by small folds and a crenulation cleavage S2. D3 formed a crenulation cleavage S3. D4 formed the prominent folds in the area, with a pervasive axial planar cleavage S4. Isolated occurrences of a pre-S1 metamorphic foliation are evidence of still earlier deformation. In one pelitic unit, oriented inclusions show that garnet growth was post-D2, while cordierite growth was post-D4. Growth and recrystallization of micas, quartz and feldspar span all the deformations. The timing of deformation events is bracketed by the pre- to syn-tectonic intrusion of the 1167 +/- 15 m.y. old Red Mountain Gneiss, and the post-tectonic intrusion of a 1080 +/- 15 m.y. old melarhyolite dike. The results of this study demonstrate that rocks of the Llano Uplift have undergone a complex tectonic and metamorphic history similar to that seen in other Grenville Age rocks of North America. / text

The petrologic study of cave sediments is a new field whose history dates back only about 30 years. Most previous work has been done in Europe, with very little in North America. This is the first petrologic study of sediments of Texas caves. Sediments from the 11 Central Texas caves investigated provide information on the diagenetic sequence of authigenic calcite and collophane, and add to the knowledge of the occurrence and distribution of red clays and dolomite silts. The sediments from Fyllan Cave in Travis County place a maximum date of mid-Pleistocene on the existing Colorado River terraces. X-ray analysis of clays in sediments from three caves indicates a decline in kaolinite content, confirming a drying tendency for the last 8000 years. / text

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An ash-flow sheet, to which the names Mitchell Mesa Rhyolite and Brite Ignimbrite have been applied, crops out prominently in Presidio and western Brewster Counties, Texas. Because of its great areal extent it is the most important unit for correlation in the Tertiary volcanic field of southern Trans-Pecos Texas, and it should bear a single name. Priority and widespread use in published literature support the name Mitchell Mesa. The ash-flow sheet is divisible into two cooling-units. The lower, a simple cooling-unit that grades locally into a compound cooling-unit, is a vitric-crystal rhyolitic ash-flow tuff with 15 to 25 percent opalescent alkali feldspar and bipyramidal quartz phenocrysts as long as 4 mm in a light brownish gray, grayish pink, or light gray vesiculated groundmass. The lower cooling-unit ranges in thickness from about 230 feet immediately north of Pinto Canyon to 2 feet at South Lajitas Mesa. The upper, simple cooling-unit is a vitric-lithic ash-flow tuff with as much as 20 percent lithic fragments in a very light gray to brownish gray groundmass containing about 10 percent non-opalescent alkali feldspar and quartz phenocrysts. The upper unit ranges in thickness from 60 to 100 feet. Its only outcrops are overlain by Petan Basalt north and northeast of Pinto Canyon. Except in a few places, the pyroclastic texture of the lower cooling-unit was obliterated by vapor-phase crystallization. Any tridymite and cristobalite originally present were subsequently converted to quartz. Four whole-rock chemical analyses of samples from widely separated localities are similar, showing only minor variations in K₂o and Na₂o. The alkali feldspar phenocrysts are richer in Na₂O and poorer in K₂O than the whole rock. Therefore the feldspar in the groundmass is more potassic than that in the phenocrysts. Foreign inclusions are most abundant in outcrops of Mitchell Mesa Rhyolite closest to the Chinati Mountains. Immediately north of the mountains, a separate ash-flow tuff is present beneath the Mitchell Mesa Rhyolite. This and other evidence leads to the conclusion that the Chinati Mountains area was the source of the ash-flow sheet. / text