r/ r J1'1:i• t:. ;;;;;;;;;;;;;;; ;;;;;;;;;;;;;;; C'I • Q. BUREAU OF ECONOMIC GEOLOGY 0 0 THE UNIVERSITY OF TEXAS AT AUSTIN VI Q. AUSTIN, TEXAS 78712 r:Q ct n.J :IE ..D ..J 0 W. L. F1seER, Director r-"I w r:Q Cl Lt) U"J rn co r-"I a""' n.J Cl . GEOLOGIC QUADRANGLE MAP NO. 48 - .,,.....""' 0 M -0 Cl""' Geology of the Marble Falls Quadrangle, Burnet and Llano Counties, Texas By VIRGILE. 8A.RNBI November 1982 THE UNIVERSITY OF TEXAS AT AUSTIN TO ACCOMPANY MAP-GEOLOGIC BUREAU OF ECONOMIC GEOLOGY QUADRANGLE MAP NO. 48 GEOLOGY OF THE MARBLE FALLS QUADRANGLE, BURNET AND LLANO COUNTIES, TEXAS VIRGIL E. BARNES 1982 CONTENTS General setting . 2 Crack fillings ............... 8 Geologic formations . 2 Houy Formation ............ 8 Precambrian rocks . 2 Ives Breccia Member ........ 8 Igneous rocks . 2 Doublehorn Shale Member .... 8 Town Mountain Granite . 2 Mississippian System .............. 8 Paleozoic rocks . 3 Chappel Limestone ........... 8 Cambrian System . 3 Barnett Formation ........... 8 Moore Hollow Group . 3 Pennsylvanian System ............. 9 Riley Formation . 3 Unnamed phosphorite ......... 9 Hickory Sandstone Member . 3 Marble Falls Limestone ........ 9 Cap Mountain Limestone Member 3 Smithwick Formation ......... 9 Lion Mountain Sandstone Member 3 Mesozoic rocks . 10 Wilberns Formation . 4 Cretaceous System (Lower Cretaceous) . 10 Welge Sandstone Member . 4 Trinity Group . 10 Morgan Creek Limestone Member 4 Shingle Hills Formation . 10 Point Peak Member . 4 Hensen Sand Member . 10 San Saba Member . 4 Post-Smithwick? Mesozoic? diabase ..... 10 Ordovician System (Lower Ordovician) . 5 Cenozoic rocks ...................•. 10 Ellenburger Group . 5 Quaternary deposits .............. 10 Tanyard Formation . 5 Terrace deposits and colluvium .. 10 Threadgill Member . 5 Alluvium ............... 10 Staendebach Member . 5 Subsurface geology ................... 11 Gorman Formation . 5 Mineral resources .................... 11 Honeycut Formation . 6 Construction materials .............. 11 Ordovician System (Upper Ordovician) . 6 Dimension stone .......... 11 Burnam Limestone . 6 Road materials ............ 13 Silurian System . 7 Sand and gravel ........... 13 .... , Starcke Limestone . 7 High-calcium limestone .............. 13 ""'(JI Devonian System . 7 Water .......................... 14 Stribling Formation . 7 Lead and zinc .................... 14 ,}5 Mississippian-Devonian rocks . 8 References ........................ 15 ..,. ~ L ~ ~~ (,t GL Vif'<"') C..uv Bureau of Economic Geology, The University of Texas at Austin GENERAL SETTING The geology of the Marble Falls quadrangle is Colorado River drainage basin. The Colorado flows shown on a U. S. Geological Survey 7 .5-minute eastward across the middle of the quadrangle topographic quadrangle map and is the fourteenth through a series of lakes. Tributaries from the geologic map in the Llano region to appear on a north are Elm Creek draining into Lake Lyndon B. modern 1:24,000 scale, 20-foot contour interval Johnson and Deep Creek and Backbone Creek base. The relief in the quadrangle is about 520 feet; draining into Lake Marble Falls. Tributaries of elevations range from 681 feet, pool level of Lake Backbone Creek are Williams Creek, Dry Creek and Travis, to about 1,200 feet at the south border of its branch Dry Branch, Sparerib Creek, Coldspring the quadrangle toward the western side. Creek, and Whitman Branch. Tributaries from the Marble Falls quadrangle is in the eastern part of south are Slickrock Creek and Horseshoe Creek the Llano region, high on the eastern side of the draining into Lake Lyndon B. Johnson and Tiger Llano uplift. Precambrian rocks, in part covered by Creek, Varnhagen Creek, and Flatrock Creek and lakes and some alluvium, occupy about 26 square its branch Little Flatrock Creek draining into Lake miles. Except for some Quaternary deposits and, Marble Falls. A small area in the southeastern part near the south border, some Cretaceous Hensen of the quadrangle drains to Lake Travis mostly by Sand, the rest of the quadrangle is underlain by way of Double Horn Creek. Paleozoic rocks ranging from Cambrian to Pennsyl­ The Marble Falls quadrangle was mapped geo­ vanian in age. These rocks are mostly southeast of logically by Barnes during 1951-1952 assisted by the Marble Falls fault which trends southwest­ W. A. Anderson, T. M. Anderson, and John northeast and divides the quadrangle into two Twining. Discussions of stratigraphic, structural, subequal parts. economic, and geophysical problems are in cited Marble Falls quadrangle is entirely within the references. GEOLOGIC FORMATIONS PRECAMBRIAN ROCKS Marble Falls quadrangle is mapped by Keppel as coarse-grained massive granite with no mineral IGNEOUS ROCKS parallelism. Goldich (1941) made a chemical and petro­ Town Mountain Granite graphic examination of granite in the Granite Mountain pluton in the Marble Falls quadrangle Town Mountain Granite of the Granite Moun­ and in the Lone Grove pluton to the northwest in tain pluton occupies the northwestern part of the the Kingsland quadrangle (see Barnes, 1976, for Marble Falls quadrangle, is faulted against Paleo­ locality). Sandell and Goldich (1943) listed the zoic rocks to the southeast, and passes under rarer metallic constituents in the same rocks. Paleozoic rocks to the northeast. Keppel (1940) Zartman (1964) made a geochronologic study of mapped variations in grain size and textures of the the Lone Grove pluton and found that the granite Llano region plutons, and his map of these plutons has a rubidium-strontium age of 1,000 ± 40 million shows areas designated as coarse-grained porphy­ years. Zartman (1965) determined the isotopic ritic, coarse-grained, medium-grained, and fine­ composition of lead in microcline from the same grained granite. All of the granite within the rock. He concluded that this and other granite 2 Geology of the Marble Falls Quadrangle, Burnet and Llano Counties, Texas analyzed "was probably obtained from a rather vated. The Hickory where exposed within the uniform source, which is possibly related to some Marble Falls quadrangle appears to be considerably deep crustal or upper mantle layer ... " thinner than the 340 feet measured in the Morgan Exfoliation domes are common and include Creek area to the northwest and the 276 feet Granite Mountain, Kennison Rock, School Rock, measured in the White Creek section to the Goose Rock, Sow Rock, Hickory Rock, Corner southwest (Barnes and Bell, 1977; Barnes, 1952a). Rock, and numerous other smaller occurrences. The thickness of the Hickory within the Marble Falls Similar domes forming monadnocks were present quadrangle, as shown on an isopach map in the on the surface of the Precambrian when the former publication, ranges from 260 to 300 feet. Cambrian sea invaded central Texas. Two such Cap Mountain Limestone Member.-Steeply exhumed monadnocks are in the northeastern part dipping Cap Mountain Limestone crops out south of the Marble Falls quadrangle, one of which is tall of Alvin Wirtz Dam in a fault sliver off Marble Falls enough to reach the base of the Cap Mountain fault. Cap Mountain Limestone also crops out in Limestone. Another exhumed monadnock just to the northeastern part of the Marble Falls quad­ the north on the Longhorn Cavern quadrangle is rangle. overlapped by Cap Mountain Limestone mineral­ The lower part of the Cap Mountain is sandy, ized by galena and sphalerite. brown, and in part glauconitic; upward, the lime­ In the vicinity of Kennison rock, many of the stone is less sandy and in part light to medium large tabular feldspar phenocrysts are horizontal gray. The upper part of the Cap Mountain is and dark basic-appearing inclusions are vertical. somewhat fossiliferous and glauconitic grading The phenocrysts are of about the same density as upward to the overlying Lion Mountain Sandstone the rock as a whole and probably represent the Member. The lower contact, likewise, is gradational true direction of flow structure. The dark inclu­ to the Hickory Sandstone Member beneath and is sions, which are denser than the rock as a whole, chosen at a distinct topographic and vegetational may have settled and in settling changed their break. The silty and argillaceous upper part of the orientation to vertical. If this interpretation is Hickory Sandstone weathers to a bench; the correct, the use of dark inclusions as a criterion in calcareous sandstone and sandy limestone at the estimating the shape of granite plutons may lead to base of the Cap Mountain, as well as the limestone error. above, are resistant to weathering and mostly form The granite is composed predominantly of rocky, steep slopes. The Cap Mountain supports microcline, plagioclase, quartz, and some biotite cedar and a variety of thorny plants; the Hickory and hornblende. Accessory minerals are scarce and Sandstone supports deciduous oaks, and where consist mostly of magnetite, sphene, apatite, argillaceous, mesquite is common. zircon, and allanite. The thickness of the Cap Mountain Limestone PALEOZOIC ROCKS was not measured in the Marble Falls quadrangle but thicknesses of 204 feet in the Morgan Creek CAMBRIAN SYSTEM area to the northwest, 411 feet in the Riley Mountains to the west, and 497 feet in the White Moore Hollow Group Creek section to the southwest were measured (Barnes and Bell, 1977; Barnes 1952a). An isopach Riley Formation map of the Cap Mountain in the former publica­ tion shows it to range in thickness from 390 to 4 70 Hickory Sandstone Member.-The Hickory feet within the Marble Falls quadrangle. Sandstone Member crops out mostly in the north­ Lion Mountain Sandstone Member.-Two small eastern part of the quadrangle where it rests on an outcrops of Lion Mountain Sandstone are present uneven Precambrian surface with monadnocks within the Marble Falls quadrangle, one in the reaching as high as the base of the overlying Cap narrow fault block south of Alvin Wirtz Dam and Mountain Limestone. Slaughter Mountain, a mesa­ the other in the northeastern part of the quad­ like feature, is capped by Hickory Sandstone. A rangle. The latter forms a narrow bench distinctly fault sliver of Hickory Sandstone is mapped south­ visible on the aerial photograph used in mapping east of Alvin Wirtz Dam, and nearby Castle the outcrop.