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 and the others granular, ledge-forming limestones commonly contain much less so. The oldest member crops out along the Col- abundant miliolid— Foraminifera. orado River at the western edge of the map area, and Member 55. The youngest member of the Glen Rose younger members occupy successively higher topographic consistsof about 100 feet of thin-bedded,gray-brown,fine- levels.The members persistbut thin to the west. grained, porous dolomite and burrowed, dolomitic lime- Soils developed ontheGlenRose aremostly thin,brown- stone. The upper 10 to 20 feet are deeply weathered,soft, ish-gray, calcareous, gravelly clay loams (Brackett soils), and pulverulent. In the Mount Bonnell area many beds with lesser amounts of mostly thin, brownish, calcareous inMember 5 contain pockets of pale blueto whitecelestite. and noncalcareous clays (Crawford, Denton, San Saba, Member 5 forms relatively gentle slopes between the andTarrantsoils). steeper slopes of Member 4 and the resistant limestone The Glen Rose supports growths of oak, juniper,hack- ledges of the overlyingBull Creek Member of the Walnut berry, and other trees. Oaks are more abundant on the Formation. The lower part of Member 5 is sparsely vege- morepermeablebeds, junipers moreabundant on clayey, tated and forms a light-colored band on aerial photo- less permeable—limestones. graphs. The contactbetween Member 5 and theBull Creek Member 11. Thismember includes all the GlenRose be- is poorlyexposed. tween the basal Cretaceous sands and the Corbula bed, a The Mount Barker section of Feray et al. (1949, pp. widespread thin bed, or series of beds, of hard orange- 23-29, pi. 6) includes descriptions of Members 4 and 5. brown limestone containing abundant specimens of the Member 4 is also well exposed in the cut on Bull Creek small elongate clam Corbula harveyi (Hill, 1893). Total road below Cat Mountain.Members 2 to 5 are exposedin thickness of Member 1is 250 to 300 feet, but only the road cuts andslopesbelow St.StephensSchool. upper 30 feet crop out in the Austin West quadrangle. Member 1is exposed along Lake Austin in the western Walnut Formation part of themap area.The upper 30 feet consists of grayto tan, nodular to thin-bedded, burrowed limestone, marly In Central Texas the name Walnut Formation has been limestone, and marl. (Filled tubular burrows made by applied traditionally to oyster-shell marls and marly lime- crustaceans and other invertebrates are common features stones that overlie theGlenRose orPaluxyFormations and of manylimestones in the Austin area. The burrow filling underlie the nodular Comanche Peak Limestone or the generallyhas acoarser texture and a darker color than the massive rudist-bearing Edwards Formation. Based on re- 4 BureauofEconomic Geology, The University ofTexas gional relationships the Walnut Formation of the Austin Cretaceous rocks but now extinct (Rodda et al., 1966, pp. areahasbeen subdivided into fivemembers, threeof which 5-8). The Edwards caps the high topography west of the have been recognized in the Austin West quadrangle Mount Bonnell fault and crops out extensively in the Bal- (Moore,1961, 1964). These are, inascending order, the cones fault zoneto the east. A complete Edwards section is Bull Creek, Bee Cave,and Cedar Park Members. The Bull not exposed in theAustin area, but regional relationships Creek and Bee CaveMembers have contrasting lithologies suggest that the Edwards is about 300 feet thick in the and have been mapped separately. The type localities of Austin West quadrangle (FisherandRodda,1969; Tucker, both of these units are in the Austin West quadrangle 1962). In the Austin West quadrangle, the Edwards has (Moore, 1961, 1964). The Cedar Park is here included in been subdivided into four informal members on the basis theEdwardsFormation because of similarphysical charac- of lithology. terand topographicexpression. Edwards soils are red-brown, dark brown, and dark Walnut soils are dark brown to brownish-gray, shallow gray, mostly thin, calcareous and noncalcareous clays to deep,calcareous clay and clay loam (Brackett and Tar- (Crawford, San Saba, Speck, and Tarrant soils). Vege- rant soils). — tation onthe Edwards includes live oak, post oak, juniper, BullCreek MemberMember. This unit forms a prominent topo- elm,hackberry,— and persimmon. graphic bench high in the landscape west of the Mount Member 11. Thisunit has an estimated thickness of 200 Bonnell fault. The Bull Creek Member consists of 35 feet feet and consists of thin- to thick-bedded, gray-brown of mostly hard, resistant, thin- to thick-bedded, gray to porous dolomite and dolomitic limestone, and hard, gray tan, fine- to medium-grained, commonly burrowed lime- to tan, fine- to medium-grained limestone commonly con- stone. Much of the limestone is composed of shell frag- taining rudists. Gray to black nodular chert is common ments; miliolid Foraminifera are abundant in somebeds. and is especially abundant in the dolomite beds. Thelower The type locality of the Bull Creek is a cut on City Park 60 feet of Member 1is exposed mostly west of the Mount Road, 0.4 mile west of Ranch Road 2222 (Bull Creek Bonnell fault north of the Colorado River and in Westlake Road) (Moore, 1961). The contact with the overlying Hills. The upper 110 feet of Member 1crop out only east Bee CaveMember is sharp;the upper surface of the Bull of the mainfault;principal exposuresareinthehighbluffs Creekisbored,pitted,iron-stained,andcase-hardened. The of the Colorado River between Bee Creek and Zilker Park, borings were made by Cretaceous marine clams. Vegeta- in bluffs of Barton Creek, and inother areas southwest of tionon theBullCreek ismainly an oak-juniper association TomMiller Dam. An estimated 30 feet of the lower part similar to that ontheGlenRose. of Member 1do not crop out in the map area.The lower Bee Cave MemberMember.— The Bee Cave Member is 30 feet part of Member 1is well exposedin the old quarry in the thick andconsists of soft,grayto tan, extensivelyburrowed, northeast corner of the quadrangle and in a cut on the mostly nodular, fine-grained limestone, marly limestone, West Loop north of Bee Cave Road. Hard, fine-grained, andmarl. The unitis marly at thebase and grades upward burrowed, dolomitic limestone at the base is overlain by to nodular limestone. The contact between the Bee Cave cherty, thin- to thick-bedded rudist limestone, dolomite, al.,1966, 253; and Edwards is gradational and is placed at the transition and dolomiticlimestone- (Roddaet p. Young, from nodular limestone below to the hard, resistant, bur- 1963b, p.127).A to 10-foot-thick solution-collapse zone rowed, fine-grained, commonly dolomitic limestone of the is exposed in this interval in the cut north of Bee Cave Edwards Formation. Megafossils are common in the Bee Road. The upper part of Member 1is well exposedin the CaveMember. The oysters Exogyra teocana and Gryphaea Colorado Riverbluffs below TomMiller Dam, in cuts along mucronata are especially abundant in the lower part; a Red Bud Trail, and in bluffs along Barton Creek. Thin- distinctive, 1 to 2 mm-long, conical foraminifer, Dictyo- to thick-bedded, fine- to medium-grained limestones with conus walnutensis,is abundant in a narrow zone nearthe rudistsgrade upward to dolomiticlimestones and dolomites top of the unit.The typelocality of the Bee CaveMember with abundant chert. The top of Member 1is markedby a is a cut on Bee Cave Road, 0.3 mile west of the intersec- 20-foot-thick, iron-stained, cavernous, solution-collapse tion with St. Stephens School Road (Moore,1961). The zone containing brecciated limestone, dolomite, chert, Bee Cave is well exposedalong Balcones Drive between coarsely crystalline calcite,and residualred clay. This zone Northland Drive and North Hills Drive and on the west is widespread in Central Texas and indicates the former slope of Mount Barker (Feray et al.,1949, pp.23-24, 29- extent of athick gypsum-anhydriteunit (KirschbergEvap- -30, pis. 6, 7).Juniper is especially abundant on this mem- orite) (Fisher and Rodda, 1969). Collapse is due to re- ber. moval of evaporite by solution.Much of the evaporite was removed during or shortly after deposition of the overly- 2; Edwards Formation ing Member little or no evaporite remains in the map area.Many of the numerous caves of the Austin areaare TheEdwards Formation is characterizedbyrudist lime- in thissolution-collapse zone. stones, dolomite, nodular chert, and solution-collapse fea- Member 22.— Member 2is 40 feet thick and consists of tures (Fisher andRodda, 1969; Rodda et al., 1966). Ru- a few feet of gray to tan, thin-bedded,fine-grained, cherty dists are elongate, conical bivalves (clams) common in dolomitic limestone at the base, overlain by interbedded Geology of the Austin WestQuadrangle, TravisCounty, Texas 5
gray to tan, thin- to thick-bedded, fine- to coarse-grained town Formation is placed at a slight topographic bench cherty limestone with rudists, miliolid Foraminifera and above which the limestones are more nodular and marly, other shells, and fine-grained, thin-bedded flaggy lime- the fossils, especially oysters, are more varied and abun- stone. The lower beds have been folded and fractured by dant, and juniper is increasingly abundant. The change collapseinto the evaporite-solution zone of Member 1.The generally is not strongly marked, but between the West base ofMember 2 is markedby aprominentlayer of elon- Loop and Barton Creek the contact is sharp and theupper gate, gray chert nodules. Nodular chert is common in the surface of Member 4 is bored, pitted, and iron-stained. lower part of the unit; the upper part generally is chert- Regional implications of the Edwards-Georgetown trans- free.Member 2 gradesupward into thenodular andflaggy ition are discussed by Martin (1961), Rose (in press), limestones of Member 3. Member 2 crops out extensively Tucker (1962),and Wilbert (1967). in the Balcones fault zone; it is well exposed in cuts on Red BudTrailand in thebluffs of Barton Creek and forms GeorgetownFormation broad, high, chert-strewn surfaces eastward from the TheGeorgetown consistsmostly gray Mount Bonnellfaultsouthof theColoradoRiver. of thin interbeds of to nodular, limestone, — mostly tan, richly fossiliferous, fine-grained Member 33. This unit consistsof 10 to 15 feet of limestone, soft, nodular,fossiliferous,burrowed,marly limestone and marly and marl. Fossil oysters are varied and abundant;manybeds arecomposedmainly of oystershells. marl. Pleuromya knowltoni (Hill,1893), a small elongate Gryphaea washitaensis, the large Exogyra walkeri, and clam about 40 mm long, is the most abundant megafossil Arctostrea carinaia are common to abundant; the latter and is characteristic of the unit. The oyster Exogyra two are diagnostic of the Georgetown.Other oysters,clams, texana and other mollusks and echinoids areless common. snails, ammonites, echinoids,and the brachiopod Kingena Locally the nodular limestone and marl are interbedded wacoensis are common. The Georgetown generally has the with, or grade laterally to, fine-grained, flaggy limestone; same distribution as Edwards Member 4 but is topograph- Member grades upward to flaggy Member 3 limestones of ically higher. nearly complete Georgetown sectionis ex- 4. The narrow outcrop of Member 3 extends over a wide A posed in the railroad cut & G. of area in the fault zone.In the Rollingwood area good ex- of the I. N. RR. north West 6th Street (Feray et al., 1949, pp. 45-48, pi. 11). posures are in cuts alongBeeCave Road and alonga small Thickness the Georgetown in quad- valley northeast of Pickwick Lane.It is also well exposedin of the Austin West rangle is generally about 55 feet but varies from 40 to 60 parts of the Barton Creek drainage and in a small aban- feet. This variationinthickness implies either an erosional doned quarry mile south of Eanes School. Member 3 % unconformity below the Georgetown or facies change be- characteristically supports a thick growth of juniper.This tween the Georgetown and upper Edwards, or possibly a unitis correlative withtheKiamichiFormation to thenorth combination of the two. Theproblem is aregional one and east (Ferayetal.,1949; Tucker,1962; Wilbert,1967) be- . yond the scope of this project. The regional stratigraphy Member 44.— Member 4 is 40 feet consists thick and of the Georgetown and the natureof the Edwards-George- mostly of gray to tan, fine-grained, thin- to thick-bedded town transition are discussed byMartin (1961),Rose (in limestone, dolomitic limestone, Flaggy lime- and dolomite. press), Tucker (1962), and Wilbert (1967). The contact stoneand thininterbeds of limestone and marl atthebaseof with the overlying Del Rio Clay is gradational but abrupt by hard, fine-grained, the unit are overlain a 3-foot bedof andis poorlyexposed. rudist limestone (the Austin marble) Above this is a 6- . Soils on theGeorgetown aremostly dark brown to dark to 10-foot sequence of dolomite,dolomitic limestone,and a gray, thin to thick, calcareous clay and clay loam (Brack- thin solution-collapse zone. The upper 10 to 15 feet of ett, Denton, and Tarrant soils) The Georgetown supports Member 4 areslightly nodular, hard, mostly fine-grained, . an oak-juniper vegetation with junipers especially abun- thin- to medium-bedded limestone with minorinterbeds of dant. dark gray marl. These upper beds contain fossil oysters (Gryphaea washitaensis), brachiopod Kingena waco- the Del Rio Clay ensis, and, in the northern outcrops, the large ammonite Eopachjdiscus brazoense.The upper part ofMember 4in- The Del Rio is composed of dark olive or bluish-grayto cludes beds previously considered part of the Georgetown yellow-brown, pyritic, gypsiferous clay and marl and is Formation but here mapped with the Edwards because of about 75 feet thick in the Austin West quadrangle. A few similar physical characteristics. South of the Colorado thin siltstone and sandstone beds are presentnear the top River Member 4 crops out in theBalcones fault zone from of the unit. The rams-horn oyster Exogyra arietina is the Mount Bonnell fault to Zilker Park and ismost exten- characteristic of the Del Rio and occurs profusely in the sive in the Rollingwood community. North of the river lower part. The oyster Gryphaea roemeriis commonnear Member 4 crops out discontinuously from JohnsonBranch the top of the unit; other megafossils are less common. north to the old quarry on the Highland Park School Gypsum (selenite) is common as scattered crystals and grounds. AsectioninJohnsonBranchis describedinFeray thin fracture fillings. The Del Rio crops out extensively etal. (1949,pp.47-49,pi. 11). north and eastof Barton Creek and north of the Colorado The contact of Member 4 with the overlying George- River from Shoal Creek west to the Mount Bonnell fault. 6 Bureau ofEconomic Geology,The University of Texas
Outcrops are poorly exposed; the clay fails by slump and Eagle Ford Formation slopes. creep even on shallow Below caps of Buda Lime- The Eagle Ford Formation is about 40 feet thick and slopes stone the Del Rio Clay forms steep to moderate consistsmostly of dark gray,massive,calcareous clay, with mantledbyBuda limestonerubble whichsupports agrowth about 9 feet of thin interbeds of silty and sandy, flaggy acover of live oak andjuniper.Elsewhere the DelRio has limestone, clay, and bentonite near the middle of the unit. trees. sectionsof the of grassandscattered mesquite Partial This limestone unit has been named the Bouldin Member are Del Rio along Shoal Creek and Barton Springs Road by Adkins and Lozo (1951). The Eagle Ford map unit in Feray et (1949,pp.49-51,pis.12, 13). described al. includes theunderlyingPepper Shale,adark graytoblack, Del Riosoils aremostly dark grayto yellow-brown,deep laminated, fissile, noncalcareous plastic clay about 3 feet to clays (Ferris,Heiden, shallow, calcareous and Houston thick.The top of the Eagle Ford is marked byabout 3 feet Black soils).Soils on theDel Rioslopes covered withlime- of interbedded,burrowedmarl,marly limestone,and chalk, are shallow, calcareous, stone rubble brownish-gray, grav- which forms areceding slope beneath the harder massive elly clayloams (Brackettsoils). limestone of the Austin Group. The Austin-Eagle Ford contact is lithologically gradational in the Austin West BudaFormation quadrangle (Feray et al., 1949). Fossil oysters, clams (especially Inoceramus), are common in In theAustin West quadrangle the Buda Formation con- and ammonites some beds. Principal outcrops of the Eagle Ford Forma- sists of 35 feet of hard, resistant, fine-grained, burrowed, edge quadrangle. Eagle Ford glauconitic, shell-fragment limestone. The lower half is tion are at the east of the The forms broad flat topography northof 35thStreet from east marly, nodular, and less resistant and forms a receding of Bull Creek Road to Camp Mabry and westof South La- slope beneath the harder upper part. The contact of the mar Boulevard. Two thin outliers of Eagle Ford cap low Buda with the overlyingPepper Shale is sharpbut poorly hills the westernpart fault zone. A complete sec- exposed. The Buda thins northward. On the outcrop in of the tion of the Eagle Ford is exposedon WestBouldin Creek freshly broken surfaces of Buda are characteristically col- eastof SouthLamar Boulevard (Feray etal.,1949,pp.51- ored shades of yellow to pink. The Buda is richly fossili- -55, pi. 14) (The Eagle Ford along part of Bouldin Creek ferous, but specimens are difficult to extract from thehard . was inadvertently omitted from the printed map.) The limestone. The lower and upper partshave distinct fossil Eagle Ford thickens to the north and continues to thin assemblages. Budaiceras is characteristic of The ammonite ; Pepper Buda, southof Austin the Shale does not extendsouth of thelower and thelarge,roughscallop Pectenroemeri Travis County. is typicalof the upperBuda. The oyster Gryphaea roemeri Soilsof theEagleFordare darkbrown tobrownish-gray, is abundant in the basal Buda and in the top of the under- thin to thick,calcareous silty clay clayloam (Austin, lying Del Rio Clay. Hixon (1959) and Martin (1961, and Brackett, Eddy, and Stephen soils) Eagle Ford outcrops 1967) discussed the regional stratigraphy of the Buda; . generally support a cover of grass withscattered mesquite Shattuck (1903) and Whitney (1911) illustrated a variety of Buda fossils. andhackberry. crops out over a area The Buda wide in theAustin West AustinGroup quadrangle and generally forms vertical scarps or steep slopes above the Del Rio Clay. Duringnormal erosionof The Austin Group consists of about 350 feet of chalk, the Buda large and small blocks of the highly fractured limestone,marly limestone, and marl. In the Austin area limestone are slowly detached from the scarp faces and the Austin Group has been subdivided into six separately slump or creep down the Del Rio slopes. The Buda forms mapped formations. Parts of the two lowest formations highbluffs along Shoal Creek and crops out north of 35th (Atco and Vinson) crop out in small areas in the Austin Street and west to the Mount Bonnell fault. South of the West quadrangle; these units are much more extensive to Colorado River the Buda caps the bluff above Barton the east and south. The Austin units generally form mod- Springs Road and forms southwest-trending bluffs high erately dissected, light-colored slopes with growths of live above the east side of Barton Creek. Scattered outcrops oak and juniper.Soils ontheAustin Group aremostly dark extend to the westernpartof Rollingwood and south to the brown tobrownish-gray, thick to thin,calcareous silty clay West Loop. The type locality of the Buda is along Shoal and clayloam (Austin,— Brackett,Eddy, andStephensoils). Creek north of 29th Street just east of the Austin West Atco Formation.Formation The Atco Formation, the lowest sub- quadrangle. Sections on Shoal Creek, Bouldin Creek, and division of the Austin Group, is about 125 feet thick and South Lamar Boulevard are described byMartin (1961, is composedof grayto white,thin- to thick-bedded,massive 1967). toslightly nodular,fine-grainedlimestone,chalk,andmarly Soils on the Buda Formation are mostly dark brown to limestone; megafossils are uncommon. The lower 60 feet red-brown, shallow to deep, calcareous and noncalcareous andupper20 feet of theAtco cropout infault blocks inthe clays (Crawford and Tarrant soils). Live oaks, juniper, southeast corner of the map area. Along West Bouldin elm, hackberry,and persimmon are commonon the Buda Creek the basal Atco forms bluffs above the less resistant outcrop. Eagle Ford. In the southeast fault block about 20 feet of Geologyof theAustin WestQuadrangle,Travis County, Texas 7 upper Atco underlies the Vinson Formation; the Atco- The Sand Beach and Riverview terrace deposits were Vinson contact is gradational. An outlier of basal Atco frequently flooded before regulation of Colorado River crops out above theEagle Fordin thehighest part of Camp flow by a series of dams. The upper surface of the First Mabry. — Street terrace deposit is the level of the highest recorded Vinson FormationFormation. The Vinson Formation is about floods (1869,1935) (Dalrymple etal.,1939). 75 feet thick and consists mainly of gray to white,thin- to More extensive discussions of the Colorado River ter- thick-bedded,massivechalk. TheVinson is softer andless race deposits arein Hill and Vaughan (1898), Urbanec resistant than the Atco Formation. The lower 50 feet of (1963) ,Weber (1968),and Weeks (1945). the Vinson crop out in afault block in the extreme south- Soils on the Asylum, Capitol, and Sixth Street deposits east corner of the Austin West quadrangle. Small fossil aremostly brown-red, noncalcareous sandy loam, gravelly oysters are the only common megafossils in the Vinson in sandy loam, and loamy fine sand (Chancy, Dougherty, this area. and Travis soils). Soils on the First Street deposit are mostly brown, calcareous, sandyloams (Hardeman soils). Quaternary Rocks Soils ontheRiverview and SandBeachdeposits aremostly — light brown to gray-brown,calcareous silty clayloam, fine Colorado River terrace depositsdeposits. Six separateColorado sandy loam, and fine sand (Lincoln, Norwood,and Yahola River terrace deposits are mapped in the Austin West soils). — quadrangle. From highest to lowest (oldest to youngest) Tributary terrace deposits.deposits Alluvial terrace deposits these are: Asylum, Capitol, SixthStreet,First Street,River- are well developed on Barton Creek, Bull Creek, Shoal view, and Sand Beach terrace deposits. These units are Creek, and other smaller creeks in the Austin West quad- most extensive eastof the Mount Bonnell fault and arepro- rangle.The terrace deposits consist of gray to tan, mostly gressively broader and more extensive from lowest to unconsolidated,limestone and chert gravel and sand, and highest. East of the mainfault, in the Austin West quad- calcareous silt and clay, derivedfrom thesurrounding Cre- rangle, the Asylum and Capitol deposits are only on the taceous rocks. Locally, the sand and gravel are cemented north side of theriver.Much of the westernpartof Austin by caliche. Thickness of the terrace deposits ishighly vari- is built on these terrace deposits. West of the mainfault able; maximum thickness is about 20 feet. There—are two the three highest units arepreserved as isolated erosional distinct levels of terrace deposits onBarton Creek inthe remnants onbothsidesof theriver. Zilker Park areaand west of theMount Bonnell fault. The The terrace deposits are composed of mostly unconsoli- higher terrace deposits are generally broader and more dated gravel, sand, silt,and clay derivedfrom Cretaceous dissected. On the westpart of Barton Creek the upper ter- andpre-Cretaceousrocks to the west.The gravel is mainly race unit is composed mostly of wedge-shapedpoint bars limestone and chert with minor amounts of igneous and that thicken downstream and were deposited on down- metamorphic rocks. Much of the original limestone has stream-dipping,rock-cut benches.The twolevelshavebeen been removed by solution, resulting in ahigh proportion mappedas asingleterraceunit. of chert. Gravel is moreabundant at the base of each ter- Also included with the tributary terrace deposits are raceunit andgradesto finer materialupward. local, high, gravel deposits between the Colorado River The Asylum and Capitol deposits have ahigher propor- and the West Loop. These small deposits are only partly tion of gravel than thelower units. The higher gravels are related to modern streams, and theyprobably include ma- cemented locally with calcium carbonate. The Asylum, terial shed locally from east-facinglimestone scarps. Capitol, and Sixth Street deposits are characteristically Soils on the tributary terrace deposits aremostly dark deposits typically yel- colored orangebrown; thelower are brown to dark gray,calcareous, clayloam, silty clay, and low brown. clay(Frio, Volente,andSanSabasoils). The upper terrace deposits are generally surfaces of the Alluvium.Alluvium— This unit includes frequently flooded, un- flat or gently undulating;lower surfaces are irregular, re- consolidated gravel, sand, and silt in the modern channels flectingthe buriedbedrock topography.IntheAustin West al- quadrangle the Asylum, Capitol, and Sixth Street deposits of the tributary streams. Maximum thickness of the is 20 included this unit is al- have maximum thicknesses of 25 to 30 feet; maximum luvium about feet. Also in thickness of eachlower unit is about 40 feet. The Asylum luvial material immediately below TomMiller Dam that and Capitol deposits are highly dissected, with extensive was deposited by the flood following the dam failure of exposuresof bedrock. The Sixth Street deposit is slightly 1900 (Taylor, 1900). Modern sediments inLake Austin dissected, and a narrowbedrock outcrop is exposedlocally areas much as 60 feet thick and werederived mostly from between the SixthStreet and First Street units. The First siltingof thelakebehind theearly Austindams prior to the Street, Riverview,and Sand Beach deposits are relatively construction of Mansfield Dam (Lake Travis) and Tom undissected and no bedrock is exposed between the units. Miller Dam (Taylor,1900,1924). 8 BureauofEconomic Geology,The University of Texas Structure
The general structure of the Austin area is a gently planes generally dip between 55° and 75°. About 60 per- dippinghomocline broken by onelarge fault (Mount Bon- cent of thefaults are downthrown to the east,about 40 per- nell fault) and numerous small faults comprising the Bal- cent to the west.Total stratigraphic displacement across the cones fault zone. The broad regional dip is to the south- Balcones fault zonein the Austin West quadrangleis about east, but in the Austin area the general dip is northeast 1,000 feet. From south to north in the fault zone faults into the Round Rock syncline (Tucker, 1962). Dips gen- striking about N. 10° E. transfer the displacement west- erally are low in the Austin West quadrangle. The Glen ward acrossthe fault zone. Rose, Walnut, and basal Edwards Formations west of the TheMount Bonnell fault is the major structural feature Mount Bonnell fault generally dip northeast at about 20 of the area.The fault trends N.40° E. and dips east. The feet per mile. Drag on the Mount Bonnell fault changes fault is downthrown to the east, and maximumnet slip is this to an east dip of 50 feet per mileinto the fault.Inthe about 600 feet atHucks Sloughbelow Mount Bonnell. Dis- Balcones fault zonethegeneraldipis east-northeast atabout placement decreases northward and southward to about 50 feet per mile. Locally, in fault blocks and in solution- 450 feet at the south and east boundaries of the quad- collapse zones, dips vary greatly. East of the Austin West rangle. The Mount Bonnell fault extends south into Hays quadrangle the regional dip increases to between 100 and County and north into the Austin East quadrangle where 150 feet per mile. The structure of the Austin areaisdis- the displacement is taken up by other en echelon faults. cussed in Muehlberger and Kurie (1956) and Dunaway JointsJoints.— The two pairs of dominant joint sets of this area (1962). are reflected in the trends of the incised meanders of the FaultsFaults.— The Balcones fault zoneis abelt of northeast- Colorado River and Barton Creek. The more prominent trending dip-slip normal faults. Most of the faults strike pair strike N. 40° E., parallel to the Balcones faults, and aboutN.40° E.;displacement on thefaults mapped in the N. 45° W.; both sets extend over the map area.In the Austin West quadrangle rangesfrom about 600 feet on the northern and westernpart of the quadrangle a secondary Mount Bonnell fault to a few feet onthesmallest faults. The pair strikeN.10° W. and N.80° E.; to the east the strikes largest fault other than the Mount Bonnell fault has amax- of these two joint sets change progressively to the sets imum netslip of about 150 feet; most mapped faults have striking N.40° E.andN.45° W. (Muehlberger andKurie, net slips of less than 50 feet. Fault traces generally are 1956). The joints are mainly vertical tension fractures, covered,and faults withless than10 feet of netslip, though withshear— fractures nearfaults and solution-collapse zones. numerous, are difficult to map away from road cuts and FFolds.olds -Folds associated with solution-collapse zones in creek bluffs. North of theColorado River most fault traces theEdwards Formation have amplitudes up to 20 feet and are concealed by residential development and terrace de- are well exposedin the highriver bluffs below TomMiller posits; small faults are well exposed at Barton Springs, Dam and in cuts on Red BudTrail. Many Balcones faults CampbellsHole, and elsewhere along Barton Creek. Fault have producedsmalldragfolds.
Engineering Geology and Land Use
Engineering data for the Austin area have been com- special foundation support generally is not needed. Infil- piled from information generously provided by private tration capacity varies. Coarse-grained limestones, dolo- firms and government agencies. Test data and general mites,and cavernouszones (Edwards Formation andparts engineering evaluations for each geologic unit are pre- of the Glen Rose and Walnut Formations) arehighlyper- sented in table 1. This table presents general engineering meable,andintermittentsprings in theseunits causedrain- characteristics of the mapunits as background forland-use age problems. Absorption of sewage tank effluent can be planning; it does not eliminate the need for detailed site high, but possible contaminationof undergroundand sur- studies. Test data include unit weight, moisture content, face water supply isahazard. Cavernsand sinkholes in the seismic velocity, compressive— strength, plasticity index, Edwards Formation mayrequire grouting or other fill ma- and absorption pressure absorption swell. General engi- terial. Fine-grained and marly limestones (parts of the neering evaluations concern slope stability, foundation Glen Rose, Walnut,and Edwards Formations, theGeorge- characteristics, excavation characteristics,and infiltration town and Buda Formations, and the Austin Group) gen- capacity. erally are much less permeable, but eventhese limestones Limestone units (Glen Rose, Walnut, Edwards, George- commonly are highly fractured and permeable to depths town, Buda, Atco, and Vinson Formations) pose few con- of afew feetbelow naturalslopesurfaces. struction problems; they generally are stable even onrel- The clay units (Del Rio Clay and Eagle Ford Forma- atively steep slopes. Excavation generally is difficult, but tion) may pose severe building problems and generally parts of softer units (Glen Rose and Walnut Formations require special engineeringdesigns. These clayunits gen- andAustin Group) arerippable.Bearingcapacityishigh; erally areunstable even on shallow slopes. Excavation is Geologyof theAustin WestQuadrangle, Travis County,Texas 9 relatively easy. Bearing capacities arelow and they have adequate absorption of septic tank effluent, but saturated high shrink-swell values. Infiltration capacity is low, gen- unitsmayhavelow fluid acceptance.Topographicallylower erallyinadequatefor septictank effluent. units arefrequently orpermanently saturated.Higherunits Slope stability of alluvial deposits (Colorado River ter- mayhavelocal, perched watertables;building sites exca- race deposits, tributary terrace deposits, and alluvium) vated intheseunitsmayhaverecurring drainageproblems. ranges from high to low depending on water saturation, Corrosion characteristics of map units can be classified topographic position, degree of cementation of the sand generally. The clays (DelRio and Eagle Ford) have low and gravel, and nature of the underlying bedrock. Satur- permeability and tend to be highly corrosive; the more ated, uncemented deposits on steep slopes above bedrock permeable limestones (Glen Rose, Walnut, Edwards, clays are least stable; unsaturated, cemented deposits on Georgetown, Buda, Austin) and alluvial deposits are low slopes above limestone bedrock are most stable.Most slightly to moderately corrosive. Corrosiveness is strongly alluvial depositsintheAustin West quadrangle arebetween affected byother factors. The use of different metals con- the extremes. Excavation is relatively easy even for light nected together, ora single kindof metal buried in two or equipment, but saturated deposits poseproblems of stabil- more kinds of rock or soil, produces a corrosive battery ity and water removal. Bearing capacity is low to mod- effect. The nature of the backfill is important; poorly erate; shrink-swell values are low to high depending on drained or poorly aerated earth materials are more cor- clay content. Large structures may require special engi- rosive. Cathodic protection is the only safe treatment for neeringdesigns.Infiltration capacity generallyis high with metals; use of plasticpipe will reduce corrosionproblems.
RockMineralResourcesand
ConstructionMaterials reclaimed for residentialbuilding sites,but oneoldvertical TheAustin areahaslarge resources of construction ma- brick kilnstillstands. terials.At various timescrushed limestone,dimensionlime- The Edwards Formation was also the source of dimen- extensively in 1900, and stone,lime and cementraw materials,sandand gravel,and sion stone used Austin before al., 1947; 1941). brick clay have been produced in the Austin West quad- sparingly since then (Barnes et Nickell, rangle (Burchard, 1910;Nickell,1961). In the westernpartof the city small quarries wereopened along and Although substantial reservesof these materials exist in in the lower part of Member 4 Johnson Branch west toward Red Bud Trail and scattered places south the Austin West quadrangle, few of them arelikely to be in kinds of stone wereproduced, producedin thenear future.The reserves arelargelyin the of the Colorado River. Two same quarry: thin-bedded, fine- east and southeast parts of the quadrangle, an areaeither commonly from the a dense,massive, fine- within the Austin city limits or being rapidly urbanized. grained, dense limestone flags and grained (the marble) containingabund- Limestone andsand andgravel deposits the western limestone Austin in part marble of the areaprobably will beutilized for road construction ant fossil shells of the rudist Toucasia. The Austin is about 3 feet thick and overlies several feet of the flaggy and otherlocal use. — EdwardsFormationFormation. The EdwardsFormation contains limestone beds. large reserves of durable, high purity limestone suitable Near the turn of the century Portland cement was pro- for avarietyof construction,chemical, andindustrial uses duced fromGeorgetownLimestone andDel Rio Clayinthe (Rodda et al., 1966). Edwards limestones are unmatched east-centralpart ofthe— maparea. intheAustin area for purityanddurability, andfew other Alluvial deposits.depositsSand andgravelhave been produced limestones have been quarried.The calciumcarbonate con- at various times in theAustin West quadrangle.The lower tent of Edwards Member 2 generally exceeds 97 percent; two Colorado River terrace units have been worked exten- laterally equivalent beds are used for lime-making and sively; higher Colorado terrace units have been worked other purposes atMcNeil and Georgetown. Edwards Mem- locally. The lower units contain proportionatelyless gravel ber 1also contains highpurity limestone and much dolo- than the upper units, and grain size generally decreases mite. Chert nodules generally are abundant in Edwards down river.Recent production of Colorado River alluvial dolomites; high silica content makes the dolomite unsuit- deposits in the AustinWest quadrangle has been from the able forcommercialproduction. dredging of the modern channel below Tom Miller Dam. For many years alarge quarry in the northeast corner A large amount of sand and gravel wereproduced from of the map areaproduced high-purity crushed stone from a Barton Creek terrace deposit south of Barton Springs. the lower part of Edwards Member 1.The quarry site has The pit has been filled and converted to a residential sub- been reclaimed for commercial, residential, and school division. Small amounts of sand and gravelhave been ob- developments.Many years ago,in the westernpart of Aus- tained elsewhere inthe tributary terrace deposits. West of tin near TomMiller Dam, limestone of EdwardsMember the Mount Bonnell fault the Barton Creek terrace deposits 2 wasburned tomakelime. The quarryhaslong sincebeen contain significant reserves of limestone sand and gravel. 10 BureauofEconomic Geology, The Universityof Texas
Brick clay was obtained from a low Colorado terrace far southas Onion Creek inHays County. The flow of Bar- deposit on the south side of the river near the east edge ton Creek infiltrates theporous lower Edwards east of the of the map area.The clay was fired at comparatively low Mount Bonnell fault and is amajor contributor to the Ed- temperatures, producing a cream-colored, relatively soft wards—Barton Springs reservoir.Barton Springs flow from brick. These operations were suspendedmany years ago, fractures in upperEdwards limestone associated with the and used "Austin common" brick is now in great demand. northeast-dipping Barton Springs fault. Net slip on the fault is about 30 feet. Other springs nearBarton Springs, Ground Water and springs at the base of Deep Eddy bluff (Hill and Vaughan, 1898), are supplied by the same underground In theAustin areaground water is a source of waterfor reservoir. Wells in the lower Edwards have supplied water some small communities,for rural domestic and stock use, for Rollingwood community and other areas south of the and for recreation (Mount et al., 1967; Hill andVaughan, Colorado River. Water quality in the Edwards varies 1898). The principal aquifers are the Edwards Forma- widely, and in some areas the water is unusable for most tionand thebasal Cretaceoussands. purposes (Mount etal.,1967). The basalsands crop out to the west along the shores of Dolomitic units of the Glen Rose Formation are minor Lake Travis; they lap out westward against the Llano aquifers and locally supply small amounts of water of rel- Uplift and dip eastward beneath theAustin area (Barnes, atively poor quality. Following high rainfall, water flows 1948; Fisher and Rodda, 1966). Wells in the Austin West from outcropsof GlenRosedolomite,and theseintermittent quadrangle have reached the sands at elevations of 200 springsposehomesite drainageproblems. to —1,200 feet (Arnow, 1957). Water quality is variable The lowest Colorado River terrace deposits commonly but generallypotable. are saturated and can supply large amounts of water to The Edwards Formation, especially the lower dolomitic wells. Main recharge is from the river. Other alluvial de- member, supplies water to several wells in this area and posits contain small and seasonally variable water supplies; is the source of Barton Springs. Underflow in the lower higher ColoradoRiver and tributary terrace deposits sup- Edwards moves down dip to the springs from at least as ply small amounts of water from perched watertables.
References Cited
Adkins,W. S. (1928) Handbook of Texas Cretaceous fossils:Univ. (1893) The invertebrate paleontology of the Trinity TexasBull.2838, 385pp.,37 pis. Division: Biol.Soc. Washington Proc, vol. 8, pp. 9-40, 8 pis. (1933) TheMesozoic systems in Texas,in Sellards, E. (1901) Geography and geology of theBlack and Grand H., Adkins, W. S.,and Plummer, F. 8., The geology of Texas, Prairies, Texas: U. S. Geol. Survey 21st Ann Rept., pt. 7, Vol. I,Stratigraphy: Univ. Texas Bull. 3232 (Aug. 22, 1932), 666 pp. pp. 239-518. ,and Vaughan, T.W. (1898) Geology of theEdwards ,and Lozo, F. E. (1951) Stratigraphy of the Woodbine Plateau andRio GrandePlain,adjacent to Austin and SanAn- and Eagle Ford, Waco area, Texas,in The Woodbine and ad- tonio, Texas,withreferences to the occurrence of underground jacent strata of the Waco area of Central Texas: Southern waters: U. S. Geol.Survey 18th Ann. Rept., pt. 2, pp. 193-322. Methodist Univ., Fondren Sci. Ser. 4, pp. 101-164, 6 pis. ,and (1902) Descriptionof the Austin quad- Arnow, Ted (1957) Records of wells in Tr'avis County, Texas: rangle: U. S. Geol.Survey Geol. Atlas, AustinFolio (No.42), TexasBd.Water Engrs.Bull.5708, 129pp. 8 pp. Barnes, V. E. (1948) Ouachita facies in Central Texas: Univ. Hixon,S.B. (1959 ) Faciesandpetrography of theBudaLimestone Texas,Bur. Econ. Geology Rept. Inv.No. 2, 15 pp. of Texas and northern Mexico: Univ. Texas, Austin, M. A. ,Dawson,R. F., and Parkinson, G. A. (1947) Build- thesis,151pp. (unpublished). ing stones of Central Texas: Univ. Texas Pub. 4246 (Dec. 8, Martin,K. G. (1961) Washita Group stratigraphy, south-central 1942), 198pp. Texas:Univ. Texas,Austin, M.A.thesis,83 pp. (unpublished). Burchard,E.F.(1910) Structuralmaterialsavailablein the vicin- (1967) Stratigraphy of the Buda Limestone, south- ity of Austin, Texas: U. S. Geol.Survey Bull. 430,pp. 292-316. central Texas,in Hendricks, Leo, cd., Comanchean (Lower Dalrymple, Tate, et al. (1939) Major Texas floods of 1935: U. S. Cretaceous) stratigraphy andpaleontology of Texas: Soc. Econ. Geol. Survey Water-Supply Paper 796-G, pp. 223-284. Paleontologists and Mineralogists, Permian Basin Sec, Pub. Dunaway, W. E. (1962) Structure of Cretaceous rocks of centra] 67-8, pp.286-299. Travis County, Texas: Univ.Texas,Austin, M.A. thesis, 61pp. Moore, C. H., Jr. (1961) Stratigraphy of the Walnut Formation, (unpublished). south-central Texas: Texas Jour. Sci., vol. 13, pp. 17-^lO. Feray, D. E., et al. (1949) Some Cretaceous sections in the vicin- (1964) Stratigraphy of the Fredericksburg Division, ity of Austin, Texas,in Shreveport Geol.Soc. Guidebook,17th south-central Texas: Univ. Texas, Bur. Econ. Geology Rept. annual field trip,pp. 19-64, pis.1-17. Inv.No.52, 48 pp. Fisher, W. L., and Rodda, P. U. (1966) Nomenclaturerevision Mount,J. R., Rayner,F. A., Shamburger, V.M.,Peckham,R. C., of basal Cretaceous rocks between the Colorado and Red andOsborne, F. L. (1967) Reconnaissanceinvestigation of the Rivers, Texas: Univ. Texas,Bur. Econ. GeologyRept.Inv.No. ground-water resources of the Colorado River basin, Texas: 58, 20 pp. Texas Water Dev.Bd.Rept.51,107 pp. ,and (1969) Edwards Formation (Lower Muehlberger, W. R., and Kurie, A. E. (1956) Fracture study of Cretaceous), Texas: Dolomitization in a carbonate platform central Travis County, Texas, a preliminary statement: Trans. system: Bull. Amer. Assoc. Petrol. Geol., vol. 53, pp. 55-72 Gulf Coast Assoc. Geol.Socs., vol. 6,pp. 43-49. (reprinted as Univ. Texas, Bur. Econ. Geology Geol. Circ. Nickell, C. O. (1941) Report on the mineral resources of Travis 69-1). County, Texas: Univ. Texas, Bur. Econ. Geology Mm. Res. Hill, R. T. (1890) A brief description of the Cretaceous rocks of Survey Circ. 39,19 pp. Texas and their economic value:Texas Geol.Survey, Ist Ann. Rodda,P.U., Fisher, W. L., Payne, W. R., and Schofield,D. A. Rept. (1889), pp. 103-141. (1966) Limestone and dolomite resources, Lower Cretaceous Geologyof theAustinWest Quadrangle, TravisCounty,Texas 11
rocks, Texas:Univ. Texas,Bur. Econ. Geology Rept.Inv.No. Weeks, A. W. (1945) Quaternary deposits of Texas CoastalPlain 56, 286 pp. betweenBrazos RiverandRioGrande:Bull.Amer.Petr.Geol., Rose, P. R. (In press) Edwards Group, surface and subsurface, vol.29,pp.1693-1720. Central Texas:Univ. Texas,Bur. Econ. GeologyRept. Inv.72. Whitney, F.L. (1911) Fauna of theBudaLimestone:Univ. Texas Shattuck, G. B. (1903) The Mollusca of the Buda Limestone: Bull.184,55 pp., 13pis. U.S.Geol.SurveyBull.205, 94 pp.,27 pis. Wilbert, W. P. (1967) Stratigraphy of the GeorgetownFormation, Stanton, T.W. (1947) Studiesof someComanche pelecypods and Central Texas,in Hendricks, Leo, cd., Comanchean (Lower gastropods: U. S.Geol.SurveyProf.Paper211, 256 pp., 67 pis. Cretaceous) stratigraphyand paleontology ofTexas:Soc. Econ. Taylor, T.U. (1900) The Austin dam: U. S. Geol.Survey Water- Paleontologists and Mineralogists, Permian Basin Sec, Pub. Supply Paper40, 52 pp. 67-8, pp.256-285. (1924) Silting of thelakeat Austin,Texas: Univ.Texas Young, K. P. (1963a) Upper Cretaceous ammonities from the Bull.2439,23 pp. Gulf Coast of the United States: Univ. Texas Pub. 6304, 373 Tucker,D. R. (1962) Subsurface Lower Cretaceous stratigraphy, pp., 82 pis. CentralTexas,in Contributions to the geology of South Texas: (1963b) Mesozoic history, Llano region, in Barnes, South Texas Geol. Soc, San Antonio, pp. 177-216. V. E., et al., Geology of Llano region and Austin area: Univ. Urbanec, D. A. (1963) Stream terraces and relateddeposits in the Texas,Bur.Econ. Geology Guidebook5,pp. 98-106, 125-131. Austin area, Texas: Univ. Texas,Austin, M. A. thesis, 93 pp. (1967) Comanche Series (Cretaceous), south-central (unpublished). Texas, in Hendricks, Leo, cd., Comanchean (Lower Creta- Weber, G.E. (1968) Geology of the fluvialdeposits of the Colorado ceous) stratigraphy and paleontology of Texas: Soc. Econ. River valley, Central Texas: Univ. Texas,Austin,M.A. thesis, Paleontologists and Mineralogists, Permian Basin Sec, Pub. 119pp.(unpublished). 67-8, pp.8-29. B€6~ 6<£3 3 TABLE 1. GENERAL CHARACTERISTICS AND ENGINEERING PROPERTIES OF GEOLOGICAL UNITS, AUSTIN WEST QUADRANGLE. Cvy. 2.
ENVIRONMENTAL GEOLOGIC CHARACTERISTICS ENGINEERINGTEST DATA
a'D \ H co M ~ " w w _ co , O v o H 9 £ H . W O " 3^ 81 1-1 W> *J " co £ *? SLOPE ROCK AND I— l £ m H M $— STABILITY CHARACTERISTICS CHARACTERISTICS CAPACITY CO !> to R w 3 oS v ' ' < en I— l « O IT. RESOURCES ME-i -°S o^ 3 oi !"* H v H (0 — £s s D - co Generally high with ade- Minor source of sand 0. 1 Bearing capacity quate absorption of and gravel. Yields to Moderate to low-- -81 3 1,000 0.5 4 to 60 0.5 600 Uncansolidated gravel, sand, silt, and clay of tributary streams; includes flood-depositedmaterial immediately below commonly Readily excavatedby moderate to low; septic tank effluent; small variable 16 Alluvium Qal 0-<£o water to to to to 5. (mostly to to Tom Miller Dam. light machinery shrink-swell low may be saturated supplies of water; (mostly saturated 107 70 2,500 2.5 20 to 60) A 75 8.400 to high seasonally andhave supply subject to under low fluid acceptance drought 3.0) Deposits In the past supplied Colorado River Terrace capacity Mostly unconsolidated, yellow- to orange-brown gravel, sand, silt, and clay; consists mostly of Cretaceous limestone Bearing Generally high with ade- much sand and SandBeach Usb 0-40 moderate to low; -0.5 and chert fragments with minor amounts of older igneous, sedimentary, and metamorphic rocks. Gravel more quate absorption of gravel;now mostly Riverview Qrv (older) units, large structures 82 2 1, 000 to 8 to 66 0.03 400 0-40 common inhigher andmore abundant nearbase of each unit. Upper surface of First Street terrace Readily excavatedby septic tank effluent; urbanized. May 0. 1 First Street Qfs Moderate to low may require to to to 7.3 (mostly to to 0-40 is level of maximum recorded flood(1869, 1935). Lower three units partly or completely floodedabove TomMiller lightmachinery lower units may be yield substantial to Sixth Street special 123 35 2,500 3. 5 (mostly 10 to 40) V 5 8.400 Qss 0-25 Dam. Lower two units were frequently 'flooded prior to regulation of flow by Colorado River dams. Terrace design; saturated andhave supplies of water, Capitol Qca 0-30 deposits growths shrink-swell low under above flood level support of live oak and post oak. low fluid acceptance more in lower 4.0) Asylum to Qas 0-25 moderate terrace deposits Potential source of sand and gravel; Mostly unconsolidated, light gray to tan, gravel, sand, silt, clay; locally Bearing capacity and consists of derived limestone and chert High; adequate absorp- some previous 82 3 1,000 0.7 1. 0 7 to 55 0.0 200 ributary Deposits gravel and calcareous silt and clay. terraces Creek, Creek, Creek, Readily excavatedby moderate to low; i lerrace Ut 0- uroup Austin white, thick-bedded, Kvs bl) Gray to thin- to massive chalk. Vmson Formation Forms light-colored, moderately dissected Difficult to excavate None in this area; 10 Low to moderate; 4,000 slopes with live oak andjuniper. Thick- with light equip- Bearing capacity potentialcement 87 10 to 10 to 40 400 High marginal to adequate 0.1 ment; generally high; shrink-swell raw material to to to (marly ness incomplete; onlylower part ofeach absorption septic 250 to to Gray to white, thin- to thick-bedded, massive to slightly nodular, canbe ripped with generally low of elsewhere in 123 30 6,000 units) unit exposed. tank effluent (mostly n.8 1.400 Atco Formation X.at 80 fine-grainedlimestone, marly limestone, and chalk. heavy equipment Austin area over 40) Bearing capacity 1 to 300 Low to moderate; Moderately easy to generally low; may (clays mostly excavate; Upper part is dark gray clay; middle part consists of thin interbeds of sandy and flaggy limestone, chalk, clay, and decreases with lime- need special Low;generally inade- 80 10 2,000 0.7 under 10; -8 to 70 0. 1 800 increasing stone Eagle Ford Formation Kel 4 Generally high but Bearing capacity Gray to tan, hard, fine-grained, glauconitic, fragment slightly generally high, shell limestone. Lower part less resistant and nodular may fail at edges Excavation difficult; 8,000 60 a yellowish Commonly butmay be low 2 rsuda ormation Kbu 5b weathering. Inoutcrop fresh surfaces to pink. forms steep slopes above the Del Rio Clay. of steep slopes generally requires Low * to -* * None to /\ Live oak, juniper, elm, andhackberry are common on at outcrop edge this unit. above weakDel blasting 11,000 420 Rio Clay above slopes of Del Rio Clay Dark gray to olive brown, pyritic, gypsiferous, calcareous clay containing abundant Exogyra arietina (ram's horn Low; decreases oyster). exposed steep slopes 3 20 Poorly in to shallow below Buda limestone. Del Rio slopes readily fail by slide with increasing Moderately easy to Bearing capacity low; 2, Low; inadequate 100 10 000 1.5 to to 4.7 4,400 Del Kio Olay Kdr (b and creep; slopes commonly covered with a thin layer of Buda limestone rubble which supports typically lime- moisture content; excavate light structures need with absorption of septic None to to to to 20 56 to to stone vegetation of live oak and juniper. Elsewhere the Del Rio supports only a cover of grass and scattered mes- fails when wet on special support; machinery tank effluent 123 30 4,500 2.5 (mostly R 4. 6.600 quite trees. shallow slopes high shrink-swell (mostly under 10) 30 to 50) 40 Thin interbeds of gray to tan, nodular weathering, hard, fine-grained limestone, marly limestone, and marl, contain- Excavation difficult; Bearing capacity high; 7, 000 to a ueorgetown ormation rigt bb ing abundant fossil shells. Forms moderate to shallow slopes above more resistant Edwards limestone. Supports High blasting generally no special support Low None to 300 -x- M.' a limestone vegetation with juniper especially abundant. required needed 9,000 (mostly over 100) Mostly hard, dense, gray to tan, thick- to thin-bedded, fine-grained lime- Ked 4U 4 stone; soft, near middle; dolomitic lower part flaggy bedded. Source of high-grade Mostly soft, nodular weathering, gray to tan, marly limestone, with abun- crushed stone; Edwards i) ormation Ked 10 3 dant growth of juniper. large quarry in Member 4 Forms weakly to deeply dissected Low in fine-grained northeast map area hard, light tan, medium-grained, limestones, Mostly gray to fine- to thin- to thick- topography mostly east of Mount Excavation difficult dense recently urbanized. 95 Member 3 bedded limestone; thin beds mostly fine grained, flaggy; thicker beds Bonnell fault. 1, 2, Bearing capacity high; moderate to high Lower Ked. was 7,000 Ked 40 Members High to moderate; blast- to 2 coarser grained with abundant rudist fragments and miliolid foramini- and 4 generally support growths ing generally no special support in coarse-grained source of dimension to re- limestones, 300 Member 2 fers. Chert nodules inlower third. of oak, juniper, hackberry, per- quired needed dolo- stone (Austin Marble 11,000 (mostly simmon, plants. mites, and andpaving flags). 200) Mostly medium-bedded, gray-brown, and other over Member 1 thin- to porous dolomite and dolo- cavernous zones Ked, is important mitic limestone, tan, and gray to fine- to medium-grained rudist aquifer; source of is.cd Bearing capacity 3,000 50 walnut .Formation bolt, gray to tan, nodular weathering, fine-grainedlimestone, Excavationmoder- Low to moderate; 5U marly limestone, and marl with abundant fossil shells. Moderate moderate to high; to to ribc steep, juniper-covered ate to difficult, absorption septic Minor source of Bee Cave Member Forms light-colored, slopes on high topography west of Mount Bonnell fault. to special support of * 7, 000 probably rippable tank effluent probably road material 200 high generally not (esti- (esti- in part marginal needed mated) mated) riard, dense, gray to tan, to grained, 7, 000 100 fine- medium- thin- to thick-bedded limestone; shell fragments and miliolid Excavation diffi- Bull Creek Member Kbkc ib foraminifers common. prominent Bearing capacity high; to to Forms bench on high topography west of main fault. Limestone vegetation of High cult; blasting Low to moderate Minor source of live oak, juniper, no special support 11, 000 ■* 300 hackberry, and other plants. required road material needed (esti- [esti- mated) mated) Mostly thm-bedded, gray-brown, KSr 100 fine-grained, porous dolomite; upper 5 10 to 20 feet pulverulent. menKose a ormation *gr Gray to tan, mostly thin- to thick-bedded, fine- to medium-grained 4 IZO lime- Member 5 stone and marly limestone. Many beds with fossil shells. Bearing capacity uray brown to tan, thm dolomite, generally high to Member 4 Kgr tv interbeds of dolomitic limestone, lime- moderately deeply source 3,000 90 3 stone, Forms to dissected High Excavation diffi- moderate; thin, weak High in dolomitic Minor of road andmarly limestone. to to- hill country west of main fault. cult to moderate; units; to material; dolomitic Member 3 to marl beds canbe moderate LO.OOO 250 U-ray to tan, thin to thick interbeds of fine- to medium- grained limestone, Typifiedby stair-step topography blasting commonly to units members are Kgr L 17 Derived partly from penetrometer correlation bearing curves of the Texas Highway Department ~ *Test not applicable or test data not available