BUREAU OF ECONOMIC GEOLOGY THE UNIVERSITY OF TEXAS AT AUSTIN AUSTIN, TEXAS 78712 Peter T.Flawn,Director Geologic Quadrangle MapNo. 38: Austin West,Travis County, Texas By P.U.Rodda,L.E.Garner,andG.L.Dawe April 1970 THEUNIVERSITYOF TEXASAT AUSTIN TO ACCOMPANY MAP GEOLOGIC BUREAU OF ECONOMICGEOLOGY QUADRANGLE MAP NO.38 Geology of the Austin West Quadrangle, Travis County, Texas PETER U. RODDA 1970 Contents Page Abstract _ 2 Introduction 2 Setting .. 2 Previous work 2 Acknowledgments 2 Stratigraphy 3 Introduction 3 Cretaceous rocks 3 Glen Rose Formation 3 Member 1 3 Member 2 3 Member 3 3 Member 4 3 Member 5 3 Walnut Formation 3 Bull CreekMember 4 Bee Cave Member 4 EdwardsFormation 4 Member 1 - 4 Member 2 _ 4 Member 3 5 Member 4 5 Georgetown Formation 5 DelRio Clay - 5 Buda Formation 6 Eagle Ford Formation 6 Austin Group — 6 Atco Formation - 6 Vinson Formation 7 Quaternary rocks 7 ColoradoRiver terracedeposits 7 Tributary terrace deposits 7 Alluvium - _ 7 Structure - - 8 Faults 8 Joints - 8 Folds - 8 Engineering geology andlanduse 8 Rock and mineral resources 9 Construction materials 9 EdwardsFormation 9 Alluvial deposits 9 Ground water _ 10 References cited 10 Abstract The rocks exposed in the Austin West quadrangle are from northwest tosoutheast,andsix terracedeposits (Sand Cretaceous marine limestones and clays and Quaternary Beach, Riverview,First Street, Sixth Street, Capitol, and alluvial deposits. The Cretaceous rocks dip gentlyeastward Asylum) consisting mostly of sand and gravelparallel the and are broken by onelarge (Mount Bonnell) fault and river and occupy successively higher topographic posi- numerous small, northeast-trending faults comprising the tions; the deposits are more extensixe east of the Mount Balcones fault zone. Most faults are downthrown to the Bonnell fault. Other alluvial deposits are developed along east;total displacement across thefault zoneisabout 1,000 tributary streams. feet. Most of the quadrangle is west of the fault zone in The limestone units generally are stable and resistant, the highly dissected hill country; the rocks consist mostly are difficult to excavate,have mostly moderate tohigh per- of interbedded hard andsoft limestone,dolomite,andmarl meability, and pose few engineeringproblems; they are (Glen Rose, Walnut, and Edwards Formations). In the sources of crushed stone and other building materials;one Balcones fault zonelimestoneunits (Edwards,Georgetown, unit (Edwards Formation) is an important aquifer. The and Buda Formations) and clay units (DelRio Clay and clay units are weak,unstable,impermeable, and corrosive Eagle Ford Formation) are complexly faulted and moder- and generally require special engineeringdesigns. Alluvial ately to strongly dissected. In the southeast corner of the deposits are easily excavated, have moderate to low sta- quadrangle limestone (Austin Group) and clay (Eagle bility and strength,have highpermeability, and mayneed FordFormation) cropout at the westmargin of the black- special designs for large structures; the deposits are im- land prairie. The Colorado River flows across the area portant aquifers and sources of sand and gravel. Introduction The Austin West quadrangle is the first in a series of of the coastalplain.Major tributaries of the ColoradoRiver 1:24,000-scalegeologic maps of the Austin areadesigned in this quadrangle include east-flowing Barton Creek and to provide basic geologic datafor planners,engineers,and south-flowingBull—Creek andShoalCreek. others. Previous workwork. Only papers of direct interest to the The geology wasmapped on vertical aerial photographs Austin West quadrangle are cited. Hill (1890) and Hill at a scaleof 1:20,000 and transferred to theU.S.Geologi- and Vaughan (1898, 1902) discussed stratigraphy, struc- cal Survey, 7.5-minute, topographic base map. Geologic ture, and mineral resources of the Austin area.The report and engineering datacompiledfor eachmapunit aresum- by Hill and Vaughan (1902) is the most comprehensive, marizedin—theaccompanying table. including a geologic map of the Austin areaat a scale of SettingSetting. The Austin area includes portions of several 1:125.000. Papers of more restricted scope are cited at physiographic regions; it extends from the dissected rem- appropriateplacesin—the text. nants of the Edwards Plateau (the hill country) eastward AcknowledgmentsAcknowledgments. The author is indebted to many across the blackland prairie to the edge of the post-oak people for aid in preparing the map and report. Peter sand belt of the coastal plain. The boundary between the T.Flawn,Director,Bureauof Economic Geology,provided coastal plain and the hill country is the complex Balcones theinitial incentivefor theprojectandhas givencontinued fault zoneandBalcones escarpment, an abrupt topographic guidance. L. E. Garner, Bureau of Economic Geology, riseat the westmarginof thefault zone. andK.P.Young,Department of Geological Sciences, The Most of the Austin West quadrangle is in the strongly University of Texas at Austin,have given valuable assist- dissected, westernhill country, but the map area extends ancein allphases of the work.Geologic mappingand strat- eastward across the Balcones escarpment and the weakly igraphic data were provided byL. E.Garner (alluvial de- to moderately dissected Balcones fault zoneinto the west posits), K.P.Young (Austin Group), L. G. Dawe (Glen margin of the gently rolling blackland prairie.Elevations Rose Formation), and W. N. McAnulty, Jr. Engineering range from 420 feet to 780 feet east of the escarpmentand data were generously provided by Frank Bryant & Asso- to 1,050 country. from 490 feet feet inthehill ciates, TheAustin Westquadrangle isentirely within the drain- Trinity Engineering Testing Corporation, Snowden Texas Highway ageof theColoradoRiver, whichflows across the areafrom and Meyer, the City of Austin, and the Department; especially grateful northwest to southeast. West of the Balcones escarpment the writer is to Frank theriver meanders areentrenched in adeep,narrow,lime- Bryant, M. W. Stapp, Kirby Meyer, James F. Todd, and Werchan, stone valley. For about 5 miles east of the escarpment the Albert Ullrich for many courtesies. Leroy U.S. river forms abroad shallow loop across theBalcones fault Soil Conservation Service, provided valuable information zone; steep limestone bluffs along the south (cut) bank on soilsof theAustin area.The cartography is by Barbara contrast with the series of alluvial terraces rising inbroad Hartmann and wasprepared under the direction of J. W. stepsto thenorth.East of themap areatheriver meanders Macon. Josephine Casey and ElizabethT.Moore provided in abroad alluvial valley excavated in the clay and sand editorial assistance. Geologyof theAustin West Quadrangle, TravisCounty, Texas 3 Stratigraphy IntroductionIntroduction.— Rocks exposedin the Austin West quad- surrounding limestone.) Immediately below the Corbula rangle are marine limestone and clay of Cretaceous age bedis adistinctivehighly fossiliferouslimestone containing and alluvial gravel, sand, silt, andclay of Quaternary age. abundant Porocystis globularis, a sub-sphericalplant fossil Cretaceous units generally strike northeast and dip gently about 25 mm in diameter, Orbitolina texana, a discoidal eastward,but intheBalcones fault zonedips arecommonly foraminifer about 5 mm in diameter, and other megafos- varied. The exposed Cretaceous units have an aggregate sils. Porocystis is abundant in the Glen Rose and rare in thickness of about 1,150 feet, most of which is limestone. the Bee Cave Member of the Walnut; Orbitolina occurs Fossils are commonin most of the Cretaceous units and onlyintheGlen— Rose. are useful guides to thestratigraphy. The common mega- Member 2.2 Member 2 is about 120 feet thick and con- fossils of theAustin areaareillustrated inAdkins (1928), sists of gray to tan, thin to thick interbeds of fine- to me- Hill and Vaughan (1902), and Stanton (1947).Regional dium-grained limestone, sandy limestone, burrowed no- aspects of Cretaceous stratigraphy arediscussed in Adkins dular limestone, and marl. Hard, thick-bedded granular (1933),Hill (1901),andYoung (1967). limestones generally form prominent ledges. Megafossils arecommonin—marly limestones andmarls. CretaceousRocks Member 33. Member 3 is about 70 feet thick and con- sists mostly of thin- to medium-bedded, nodular weather- Glen Rose Formation ing, fine-grained gray-brown dolomite and dolomitic lime- The Glen Rose Formation crops out in thenorthwestern stone andburrowed, grayto tan limestone and marly lime- two-thirds of the quadrangle and is the oldest and most stone.Dolomite beds comprise about one-thirdof themem- extensive rock unit. In the Austin area the Glen Rose berand aremore— commonin thelowerpart. consists of about 700 feet of mostly thinly interbeddedhard Member 44. Member 4 is about 120 feet thick and gen- and soft limestone, dolomite, and marl. The beds vary in erallyresemblesMember 2. Theunit consists ofgrayto tan, their resistance to erosion,forming a distinctive stair-step commonly burrowed, thin- to thick-bedded, fine- to me- topography. The upper 450 feet of the formation crop out dium-grainedlimestone and marly limestone. IntheAustin in this quadrangle; lower units are exposed to the west. West quadrangle Member 4 forms steep slopes, well ex- Five informal members have been mapped, primarily on posed in Westlake Hills and in the Mount Bonnell-Rull the relative abundance of thin dolomite beds. The upper Creek area.Megafossils areabundant inmanybeds; hard, and middle members are highly dolomitic