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

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

Not available / text

This paper presents a zonation of the Austin chalk and the Burditt marl, divisions of the Austin group in the Jonah quadrangle, Williamson County, Texas. The Austin chalk consists of the Inoceramus subquadratus Schlüter faunizone, Gryphaea wratheri Stephenson faunizone, Inoceramus undulatoplicatus Roemer faunizone, Hemiaster texanus Roemer faunizone, Exogyra laeviuscula Roemer faunizone, and the Exogyra tigrina Stephenson epibole. The Burditt marl contains the Ostrea centerensis Stephenson faunizone. The Austin-Taylor contact has been traced from the southeastern to the northeastern corner of the area. The relations of the Taylor marl and some of the Terrace gravels are discussed. The outcrop, lithology, and paleontology of the Eagle Ford shale, which underlies the Austin group, are also included. / text

The Llano Uplift of central Texas contains the largest exposure of Mesoproterozoic rocks along southern Laurentia and is thus crucial to the understanding of orogenesis and plate reconstructions along a portion of one of the largest orogens in the world. Most of the current understanding of the Mesoproterozoic tectonic evolution of southern Laurentia comes from the southeastern portion of the Llano Uplift. To fully characterize the tectonic evolution Llano Uplift, detailed mapping is necessary in the less-studied western Llano Uplift. The Mesoproterozoic Llano Uplift exposes mid-crustal, poly-deformed and metamorphosed schists and gneisses and abundant pre- to post-tectonic granites through an erosional window of Phanerozoic sedimentary rocks. Three lithologic groups were mapped in the western Llano Uplift, from structural highest to lowest these are the Valley Spring Gneiss (VSG), Lost Creek Gneiss (LCG) and Packsaddle Schist (PS). The VSG consists of pelitic schists and pink quartzofeldspathic schists and gneisses. The LCG is a thick, homogeneous package of medium- to coarse-grained augen granite gneiss, interpreted to be a deformed, coarse-grained, porphyritic pluton. The PS consists of a heterogeneous package of interlayered quartzofeldspathic gneisses, amphibolites and minor marbles. These lithologies are consistent with the PS and VSG domains described in the southeastern Llano Uplift (Mosher, 1998; Reese et al., 2000). The exotic Coal Creek Domain (CCD) of the southeastern Llano Uplift is not observed in the western Llano Uplift. The western Llano Uplift, including the VSG, LCG and PS, records a deformational history that resulted in multiple fold generations (F1-F5) and is characterized by a penetrative axial planar foliation (S1-S5). F2s are isoclinal folds of S0 (primary layering) and S1 that locally fold F1 axial planes and have steeply plunging and generally easterly trending hinge lines. F3 folds are locally developed, nearly colinear and coplanar with F2s, tight to open, and fold all previous structures (F1/F2) and fabrics (S1/S2). F4s are open folds with northeast-trending axial traces that occur on a regional-scale. F5s are open to tight folds of all previous structures, with hinge lines that are primarily southeast trending and steeply plunging. S0 to S3 orientations vary from north to east dipping because of reorientation by younger folds. S4 foliations strike to the northeast and S5 foliations are northwest striking and nearly vertically dipping. Late left-lateral shear zones (D6) with generally an easterly trend and boudinage affects the VSG, LCG and VSG in this study area and is commonly associated with unfoliated granite material. Four generations of intrusive granitic sills and dikes are documented and provide relative and absolute age constraints on deformation. The oldest recognized deformation (D1-D3) is constrained between 1253 +5/-3 Ma and 1126 +5/-4 Ma (Roback, et al., 1999). D4 and D5 deformation are constrained between 1126 +5/-4 Ma and 1076 ± 5 Ma (Roback, et al., 1999). Although a change in metamorphic conditions is documented to have occurred between D2 and D3, metamorphic fabrics and assemblages indicate granulite facies conditions during D1, D2 and D3. Amphibolite facies metamorphism occurred during D4 and presumably D5. Deformation in the eastern Llano Uplift has a similar polyphase deformational history to that recorded here for the western Llano Uplift. Deformation in the eastern Llano Uplift is similarly constrained between ca. 1238 to 1091 Ma. In addition, the youngest fold generation (F5) can be directly correlated in orientation and timing from the western to the eastern Llano Uplift, and is constrained between ca. 1119 and 1091 Ma in the eastern uplift. Both the western and eastern Llano Uplift contain late shear zones and extensional structures. Structural differences between the western and eastern Llano Uplift include differences in style and orientation of all but the latest (D5 and D6) structures. In addition, dip of fabrics and, therefore, structural stacking of lithologic domains is opposite, and no mylonite zones were identified in the west. In conclusion, the lithologic domains appear to correlated across the Llano Uplift based upon gross lithologic similarities and the tectonic evolution is similar to the well-studied eastern Llano Uplift, though the kinematics and orientations differ. These conclusions may require that the kinematics of deformation in the southeastern uplift were controlled by the presence of the exotic island arc terrane (CCD) whereas the kinematics of deformation in the western uplift were controlled by continent-continent collision. / text