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GEOLOGIC QUADRANGLE MAP NO. 49 Geology of the Pedernales Falls Quadrangle, Blanco County, Texas

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GEOLOGIC QUADRANGLE MAP NO. 49 Geology of the Pedernales Falls Quadrangle, Blanco County, Texas BUREAU OF ECONOMIC GEOLOGY THE UNIVERSITY OF TEXAS AT AUSTIN AUSTIN, TEXAS 78712 W. L. FISHER, Director GEOLOGIC QUADRANGLE MAP NO. 49 Geology of the Pedernales Falls Quadrangle, Blanco County, Texas By VIRGIL E. BARNES November 1982 THE UNIVERSITY OF TEXAS AT AUSTIN TO ACCOMPANY MAP-GEOLOGIC BUREAU OF ECONOMIC GEOLOGY QUADRANGLE MAP NO. 49 GEOLOGY OF THE PEDERNALES FALLS QUADRANGLE, BLANCO COUNTY, TEXAS Virgil E. Barnes 1982 CONTENTS General setting .. .. .. ............... 2 Cretaceous System (Lower Cretaceous) . 10 Geologic formations ......... .. ... 2 Trinity Group . 10 Paleozoic rocks ........... .. .. 2 Travis Peak Formation . 10 Cambrian System (Upper Cambrian) . 2 Sycamore Sand . 10 Moore Hollow Group ....... 2 Hammett Shale and Cow Creek Wilberns Formation . ........ 2 Limestone . 10 San Saba Member . .. .... 2 Shingle Hills Formation . 10 Ordovician System (Lower Ordovician) .. 3 Hensen Sand Member . 10 Ellenburger Group . ... .. .... 3 Glen Rose Limestone Member . 10 Tanyard Formation .. ... .... 3 Cenozoic rocks . 11 Threadgill Member ..... ... 3 Quaternary System . 11 Staendebach Member ....... 3 Pleistocene Series . 11 Gorman Formation ...... 4 Terrace deposits . 11 Honeycut Formation ......... 5 Recent Series . 11 Devonian system . .. .... .... 7 Alluvium . 11 Stribling Formation .... .. .. 7 Subsurface geology . 11 Devonian-Mississippian rocks . ...... 8 Mineral resources . 12 Joint fillings ...... ... ... 8 Construction materials . 12 Houy Formation ....... .. .. 8 Building stone . 12 Mississippian System ......... .... 9 Crushed stone . 12 Chappel Limestone ... .. .... 9 Sand and gravel . 12 Barnett Formation ........... 9 Road material . 13 Pennsylvanian System ... .... .. 9 Tripoli . 13 Marble Falls Limestone .. .. .. 9 Water ............. .... .. .. 13 Smithwick Formation? . .. ... 9 References . 14 Mesozoic rocks . 10 Appendix . 15 2 Bureau of Economic Geology, The University of Texas at Austin GENERAL SETTING Pedernales Falls quadrangle is in the south­ Pedernales River leaves the quadrangle to 1, 4 7 5 eastern part of the Llano region and is entirely feet at the top of a hill 1 mile east of the northwest within the Pedernales ~iver basin. Most of the area corner of the quadrangle. within the quadrangle drains directly to Pedernales Pedernales Falls quadrangle is high on the River by way of Cottonwood Creek, Millers Creek, Brock Hollow and its tributary Rough Hollow, southeastern side of the Llano uplift. Paleozoic Russell Hollow, and numerous unnamed drainages. rocks occupy about 23 square miles; the rest of the A narrow strip across the northern part of the quadrangle is occupied by Cretaceous rocks and quadrangle drains northward to Cypress Creek, and some alluvium. The Paleozoic rocks, interrupted by a narrow strip south of Pedernales River along the several northeastward-trending faults, dip about 8 eastern edge of the quadrangle is drained by Flat or 9 degrees southeastward. The relatively flat­ Creek and a few unnamed drainages. lying Cretaceous rocks dip eastward about 16 feet The geology of the Pedernales Falls quadrangle per mile. is shown on a U.S. Geological Survey 7.5-minute Much of the Pedernales Falls quadrangle was topographic quadrangle map and is the fifteenth mapped geologically by Barnes and Lincoln Warren geologic map in the Llano region to appear on a in 1942-1944; the remainder was mapped and a modern 1:24,000 scale, 20-foot contour interval gravity survey made during 1947-1948. Discussions base. The relief in the quadrangle is about 670 feet; of stratigraphic, structural, economic, and geo­ elevations range from about 805 feet where physical problems are in cited references. GEOLOGIC FORMATIONS PALEOZOIC ROCKS quadrangle maps to the west (Barnes, 1952a, 1952b), Barnes and Bell (1954) proposed a change CAMBRIAN SYSTEM (UPPER CAMBRIAN) in nomenclature to bring Wilberns terminology into conformity with Ellenburger terminology. The Moore Hollow Group names "Pedernales Dolomite" and "San Saba Limestone" on the maps and in the texts of those For a summary of information on Cambrian quadrangles are no longer used. Instead, these rocks of Central Texas, the reader is referred to rocks are included in the San Saba-the top Bell and Barnes (1961). Barnes and Bell (1977) member of the Wilberns Formation. Where dolo­ published a comprehensive work on these rocks, mite and limestone are mapped separately, they are and the thicknesses for the Cambrian units within shown as dolomitic and calcitic facies of the San the Pedernales Falls quadrangle were taken from Saba Member and are comparable in rank to the thickness maps in that publication. Thickness of dolomitic and calcitic facies mapped in the over­ the Riley Formation ranges from 760 to 800 feet; lying Threadgill Member and to other units of the thickness of the Wilberns Formation ranges from Ellenburger Group. 600 to 650 feet. The dolomitic facies of the San Saba Member crops out in the northwestern part of the quad­ Wilberns Formation rangle west of a northeastward-trending fault. The San Saba is mostly cherty, fine-grained dolomite; San Saba Member.-Subsequent to the publica­ noncherty, coarse-grained dolomite occupies a tion of the Blowout and North Grape Creek small area at the southern tip of the outcrop. Geology of the Pedernales Falls Quadrangle, Blanco County, Texas 3 No complete section of the San Saba Member grained cherty dolomite. Chert, characteristic of has been measured nearby, but in view of a the Staendebach, is mostly porcelaneous, white to thickness of 610 feet in the Blumberg No. 1 off-white, dolomoldic to compact, in part oolitic, Wagner well to the south (Barnes, 1959, pls. 1, 3), and in part fossiliferous. The sections of Tanyard an estimated 440 feet in the Johnson City quad­ rock on the Johnson City quadrangle have been rangle to the west (Barnes, 1963), and thinning of described by Cloud and Barnes (1948, p. 337-341); the member northward, it is estimated that the San since this publication is out of print, description of Saba within the Pedernales Falls quadrangle ranges the sections is included in the appendix. Descrip­ from about 400 to 520 feet in thickness. tion of the upper 126 feet of the Staendebach Fossils identified by W. C. Bell, Department of Member in a measured section within the Geological Sciences, The University of Texas at Pedernales Falls quadrangle along the north side of Austin, and P. E. Cloud, U. S. Geological Survey, Pedernales River is also included in the appendix. are listed as follows: Fossils are common, especially near the top of the member. Fossils in 13 collections made from Locality 2-46A­ Staendebach chert were identified by W. C. Bell, Finkelnburgia sp. Department of Geological Sciences, The University Scaevogyra cf. S. elevata Whitfield of Texas at Austin; P. E. Cloud, U. S. Geological Saukiinid trilobite aff. Calvinella Survey; R. H. Flower, New Mexico Institute of Locality 2-46B­ Mining and Technology; and J. H. Johnson, Colo­ Finkelnburgia sp. rado School of Mines. These are: Scaevogyra cf. S. swezeyi Whitfield Locality 2-38B­ ORDOVICIAN SYSTEM (LOWER ORDOVICIAN) Ophileta Nautiloid(?) Ellenburger Group Locality 2-39B­ Rhabdoporella Tanyard Formation Locality 2-4lC- Ozarkina cf. 0. complanata Ulrich & Bridge Threadgill Member.-The upper part of the Schizopea, two species, one of which is gigantic Threadgill Member interrupted by Cretaceous over­ Ophileta sp. lap crops out along three drains in the north­ Helicotoma ? western part of the Pedernales Falls quadrangle. Poorly preserved cephalopods including Only the dolomitic facies is present. The dolomite Clarkoceras? is coarse grained, light gray, and noncherty except Locality 2-4lD- locally where some medium light-gray to brownish­ Rhabdoporella sp. gray quartzose chert occurs. The total thickness is Helicotoma cf. H. uniangulata probably about the same as on the Johnson City Ophileta sp. quadrangle, where 242 feet of Threadgill Member Paraplethopeltis was measured. Lytospira? Staendebach Member.-The Staendebach Cystid plates Member interrupted by one fault and Cretaceous Locality 2-47 A- overlap crops out in a mile-wide belt in the Ozarkina typica Ulrich & Bridge northwestern part of the Pedernales Falls quad­ Locality 2-47B- rangle, and the upper portion of the member Helicotoma cf. H. uniangulata occurs in another outcrop in a fault block 1.5 miles Ozarkina cf 0 . typica Ulrich & Bridge to the southeast. The thickness of the Staendebach Ophileta sp. has not been measured within the quadrangle, but Brevicone cephalopods on the basis of 405 feet in the Blumberg No. 1 Locality 2-48A- Wagner well to the south and 415 feet measured in Helicotoma cf. H. uniangulata (Hall) the Johnson City quadrangle to the west, it is Ophileta sp. probably about 410 feet thick. Except for out­ Gasconadia putilla (Sardeson) crops of aphanitic limestone at the top of the Sinuopea cf. humerosa Butts Staendebach, the member is dominantly fine- Archinacella sp. cf. Ectenoceras 4 Bureau of Economic Geology, The University of Texas at Austin Locality 2-48B­ limestone and dolomite in the Gorman is almost as "Helico toma" sp. common within the Pedernales Falls quadrangle as Ophileta shown in the Round Mountain quadrangle (Barnes, Ectenoceras 1978). Organism suggesting a trepostomatas bryozoan Dolomite in the Gorman is mostly micro­ Locality 2-48C­ granular, some is fine grained, and rarely is a "Helicotoma" sp. medium-grained bed seen. It is various shades of Ozarkina typica Ulrich & Bridge light to medium gray; much of the microgranular Ribeiria
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