G 4032 T3 cs 2005 C6 GEOL MAPS Miscellaneous Map No. 43 eologic Map of the West Half of the Taylor, Texas, 30 x 60 Minute Quadrangle: Central Texas Urban Corridor, Encompassing Round Rock, Georgetown, Salado, Briggs, Liberty Hill, and Leander

Edward W. Collins

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Bureau of Economic Geology Scott W. Tinker, Director John A. and Katherine G. Jackson School of Geosciences The University of Texas at Austin Austin, Texas 78713-8924

2005 Miscellaneous Map No. 43

Geologic Map of the West Half of the Taylor, Texas, 30 x 60 Minute Quadrangle: Central Texas Urban Corridor, Encompassing Round Rock, Georgetown, Salado, Briggs, Liberty Hill, and Leander

Edward W. Collins

Bureau of Economic Geology Scott W. Tinker, Director John A. and Katherine G. Jackson School of Geosciences The University of Texas at Austin Austin, Texas 78713-8924

2005 ( CONTENTS

ABSTRACT ...... l INTRODUCTION ...... 1 Methods ...... 2 Previous Work ...... 3 GEOLOGIC SUMMARY ...... 4 Lithostratigraphy ...... 5 Faults ...... 9 Resources ...... 9 ACKNOWLEDGMENTS ...... 13 REFERENCES ...... 13

Figures

1. Location and setting of map area, which includes most of the northern Edwards aquifer segment ...... 2

2. Open-file geologic quadrangle maps that compose study area ...... 3

3. Generalized stratigraphic column and geologic resources ...... 6

4. Location of cross section lines and northern Edwards aquifer, chart showing lateral changes of lithostratigraphy, and cross sections illustrating Edwards aquifer rocks and faults ...... 10

5. Edwards aquifer outcrop belt, location and stratigraphic position of springs, and structure contours on top of subsurface Edwards aquifer strata ...... 12 ABSTRACT A 1:100,000-scale surface geologic map of the west half of the Taylor, Texas, 30 x 60 minute quadrangle shows the areal distribution of bedrock and surficial geologic units and faults for this corridor, which is undergoing rapid urban and suburban growth. The map was constructed using field mapping, aerial photographs, existing maps, and 1 :24,000-scale open-file geologic maps that were digitized, coded, and compiled in a Geographic Information System (GIS). The map illustrates outcrop belts of Lower and Upper Cretaceous strata representing about 2,000 ft of shelf deposition, Quaternary terrace deposits and stream alluvium, and upper Tertiary faults of the Balcones Fault Zone. Three Lower Cretaceous carbonate units, the Comanche Peak, Edwards, and Georgetown Formations, compose the prolific northern segment of the Edwards aquifer and its recharge zone. Important resources of the region include groundwater; rock for aggregate, building stone, and lime; and gravel. Knowledge of faults and contrasting rock types and physical attributes of the geologic units within the map area aids land-use decisions, such as planning construction projects, designing foundations, and meeting demands for construction materials.

Keywords: Balcones Fault Zorte, Central Texas, Edwards aquifer, environmental and urban geology, geologic map

INTRODUCTION Miscellaneous Map No. 43 describes the of the Central Texas areas where geological ·physical geology of a Central Texas area considerations are key to managing and undergoing rapid urban and suburban growth. planning the use of land and water resources, The study area lies within the Interstate 35 as well as to responsible, cost-effective and U.S. Highway 183 corridor north of Austin, construction (Flawn, 1965; Woodruff, 1979). Texas, and the area encompassing Round Information provided by the map and this Rock, Georgetown, Salado, Briggs, Liberty Hill, report is intended for a diverse audience com­ and Leander (fig. 1), as well as parts of north­ prising professionals in geology, hydrology, western Travis, western Williamson, southern engineering, urban planning, archeology, and Bell, and northeastern Burnet Counties, part of related fields, as well as laypersons and the regionally important Edwards aquifer's students, all who have varying levels of know­ north segment and recharge zone, and part of ledge of geology. The map can help users the Balcones Fault Zone. identify geologic features that impact ac­ An objective of this report is to provide tivities related to planning land use, designing basic geologic information on the 1: 100,000- construction projects, and managing ground­ scale geologic map constructed for this study, water resources. Applied uses of the map and which, in turn, is a useful source of geological this report include (1) identifying aquifer information about this part of the Central Texas recharge areas; (2) characterizing attributes and urban-growth corridor. Population increases heterogeneities within aquifer strata; (3) identi­ within this corridor have created demands on ying faults; (4) assisting water-management Earth resources and have caused increases in decisions on groundwater flow and aquifer re­ construction activities. The map area is typical sponse to pumpage and recharge; (5) providing 0 100 mi

0 150 km

EDWARDS PLATEAU

.. . Study area "l, .

Aquifer Outcrop Belts 0 50 mi Barton Springs D Northern Edwards segment 0 80 km • Edwards segment • San Antonio Edwards segment D Hill Country Trinity !(//J Southern Edwards Plateau Carrizo-Wilcox

0Ac3614(b3)c Figure 1. Location and setting of map area, which includes most of the northern Edwards aquifer segment.

Methods information on land-use activities such as plan­ ning and permitting of construction, design of The study consisted of (1) reviewing and foundations, and location of landfills and other compiling existing geologic literature and waste-disposal sites; and (6) meeting demands interpretations, (2) studying and interpreting for local construction materials. aerial photographs, (3) mapping geologic units

2 98'00' 97'30' and faults drawn as solid lines are relatively t-----.-----.----..-----+ 31 '00' I I I more distinct in the field and on photographs I I I Briggs I Ding I Youngsport I Salado than those drawn as dashed lines. Dotted lines Dong show where faults are covered by alluvium. I I I ----+---+----r---- Previous Work 1 I I Mahomet I Florence I g~~:r~ I Jarrell The study benefited from, and builds on, I I I many previous geologic investigations done -----1---1----L--- within and near the study area. A regional I I I 1 :250,000-scale map that encompasses the Li~r1 ~Y I Le~~er IGeorgetowri Weir project area exists but is not digital (Proctor I I I and others, 197 4). Previous maps of differ­ -- -,-I - -1--I _II___ - ent parts of the study area include works by Adkins and Arick (1930), Marks (1950), I I Round I Nameless I Leander I Rock I Hutto Walls (1950), Ward (1950), Gordon (1951), I I I Tydlaska (1951), Hartwig (1952), Arrington '----~---~---~------'- 30'30' 0 20 mi (1954), Atchinson (1954), Rogers (1963), QAd4428x 0 20 km Moore (1964), and Collins (1987). These workers, and others cited later, also established Figure 2. Names of 16 open-file geologic quadrangle useful stratigraphic relationships that were maps, scale I :24,000, that compose the study area. used in this study. and structural elements in the field using stan­ The work of Garner and Young (1976) and dard techniques, and (4) preparing geologic Garner and others (1976), which discusses the work maps of the study area. Sixteen open-file environmental geology of Austin, immediately geologic maps, scale 1:24,000, of the study area south of the map area, and the work of were constructed between 1997 and 1999 as Woodruff ( 197 5) southwest of the map area, part of the Texas STATEMAP program (fig. 2). provided information about land use and geol­ Black-and-white aerial photography used in ogy in the region. The earlier studies address the study included a variety of photographs aspects of land use related to stratigraphy, taken between 1950 and 1995, having scales structure, topographic conditions, soils, surface between 1 :20,000 and 1 :63,500. Photographs drainage, rock types and their properties, mi­ were viewed in stereo, and a zoom transfer neral and water resources, and vegetation ap­ scope was used to transfer some of the geologic plicable to much of the Central Texas region. data interpreted on the photographs to the In this region complex geologic conditions are 1 :24,000-scale base maps. Sixteen open-file the setting for dramatic processes related to maps (fig. 2), scale 1:24,000, were digitized and stress on the uses of land and water related to assembled into a seamless data set for rapid urban and suburban growth (Woodruff production of the 1: 100,000-scale map (map). and Collins, 2001a). This study also benefited Digital map data also appear in an ARCINFO­ from information on landscape, stratigraphy, format database (Tremblay and others, 2001). faulting, water resources, springs, construction The region is generally vegetated enough materials, karst, and issues related to urbani­ to make mapping difficult, and public access zation and geology of the mapped area and is typically limited to public roads and public nearby Austin that has been presented in a areas at Lake Georgetown and Stillhouse number of Austin Geological Society guide­ Hollow Lake. Geologic unit contacts and faults books (Woodruff and others, 1985; Young and are portrayed on the map by solid and dashed Woodruff, 1985; Snyder and others, 1986; lines to reflect the relative certainty with which Yelderman, 1987; Collins and Laubach, 1990; features can be located in the field and ob­ Woodruff and Sherrod, 1996; Woodruff and served on aerial photographs. Unit contacts Collins, 2001b).

3 GEOLOGIC SUMMARY Geology of the map area is dominated by study area, the rock record indicates that Cretaceous strata that dip -1° eastward and the Cretaceous volcanism coincided with upper Tertiary Balcones Fault Zone (map). In general, Austin and lower Taylor deposition (Ewing the surface and near-surface geology of the and Caran, 1982; Sohl and others, 1991). During study area records a series of geologic events the latter part of the period, Taylor deposits that span -110 million years, from Early experienced a gradual increase in terrigenous Cretaceous time to the present. The west part sediment influx. of the study area is marked by the outcrop belts Between 24 and 5 million years ago (mostly of Lower Cretaceous units: the Glen Rose, Miocene), faulting along the Balcones zone Paluxy (minor), Walnut, Comanche Peak, caused the west part of the map area, and the Edwards, and Georgetown Formations. These region west of it, to be uplifted relative to the units are often associated with hilly range­ east part of the map area. Normal faults of land, rocky soils, and weathered surface the Balcones Fault Zone cut Cretaceous rocks bedrock. Rocks of these Lower Cretaceous and generally follow the north-northeast units, deposited mostly in marine-shelf and regional strike of the Cretaceous outcrop belt shelf-margin settings, include limestone, and structural grain of the buried Paleozoic dolomitic limestone, dolomite, argillaceous Ouachita fold and thrust belt (Sellards and limestone, marl, and some minor sandstone Baker, 1934; Weeks, 1945; Flawn and others, (Moore, 1964, 1996; Rodda and others, 1966; 1961; Murray, 1961; Ewing, 1991a, b; Collins Wilbert, 1967; Young, 1967; Stricklin and and Hovorka, 1997). Balcones faults, marking others, 1971; Rose, 1972; McFarlan and Menes, the edge of the Texas coastal plain, may be a 1991; Kerans and Loucks, 2002). During the manifestation of gulfward extension, flexure, Early Cretaceous, a peninsula and islands and tilting along the perimeter of the Gulf of existed west of the map area near the Llano Mexico. Most movement on the Balcones Fault region; there, sediments were being deposited Zone is thought to have occurred during the in fluvial, shorezone, and nearshore marine late Oligocene or early Miocene (Weeks, 1945). environments (Young, 1972). Timing of fault movement corresponds roughly The east part of the map comprises outcrop to regionally extensive Basin and Range belts of the Upper Cretaceous Del Rio, Buda, tectonism that affected western North America, and Eagle Ford Formations and Austin and although it is unknown whether Balcones Taylor Groups. This eastern map area typically faulting and the distant deformation of western is used as farmland. Relatively thick clay-rich North America are related. Another possible soils cover most bedrock. The Upper Creta­ origin of the extension that formed Balcones ceous sedimentation record for the study faults is downdip slippage on Jurassic salt. area indicates that cyclic sea-level fluctu­ A series of normal faults composing the ations affected a broad, generally low-relief Milano and Mexia zones that lie -30 to 40 mi marine shelf area (Hayward and Brown, 1967; east, parallel to the Balcones zone, may be Young, 1967; Sohl and others, 1991). Regional breakaways that formed above the edge of Jur­ regression and transgression during the begin­ assic salt, marking updip termination of thin­ ning of the Late Cretaceous resulted in depo­ skinned extension (Jackson, 1982). Balcones sition of Del Rio clay- and mud-rich deposits faults probably did not form solely by slip on and Buda limestone. After subaerial exposure salt because they lie west of the salt pinch-out, of the Buda, an episode of shelf inundation although this deformation may have coincided coincided with Eagle Ford shale deposition. with and contributed to Balcones faulting. Eagle Ford shale deposition was followed Balcones faulting and related uplift on the by a period of abundant chalk and carbonate regional upthrown fault block, including the deposition of the Austin Group. Outside the Edwards Plateau, caused increased stream

4 gradients that resulted in increased rates of compnstng Paluxy lowstand siliciclastic erosion and incision of strata (Young, 1972). sediments, Walnut and Comanche Peak rocks Possibly during the late Pliocene to early representing transgressive facies, and Edwards Pleistocene, ~1.6 million years ago, stream deposits of highstand carbonate platform gradients lowered to a gradient at or near the facies. Moore ( 1996) interpreted Georgetown prefaulting gradient, and relative erosion rates deposits, overlying Edwards rocks, to be on the two sides of the fault zone may have another 3'd-order depositional sequence. Upper decreased (Young, 1972). Cretaceous depositional sequences that overlie Several notable streams flow eastward Georgetown rocks contain (1) Del Rio shale­ across the study area: Lampasas River, Salado Buda limestone; (2) Eagle Ford shale, which is Creek, North Fork San Gabriel River, South bounded by condensed zones (Adkins, 1932; Fork San Gabriel River, and Brushy Creek. Qua­ Feray, 1949); (3) Austin Group limestone and ternary terrace and channel alluvium asso­ chalk; and (4) Taylor Group marl. ciated with these streams and their tributaries The Glen Rose Formation, consisting of overlie older bedrock units. Remnant Tertiary limestone, argillaceous limestone, and dolo­ to Quaternary alluvial deposits locally cap mitic limestone with wackestone, packstone, bedrock deposits at elevations higher than and lesser grainstone textures, constitutes deposits associated with the modern streams. the oldest Lower Cretaceous strata exposed in Springs are relatively common in the study the study area. Only the upper 200 ft of area, particularly within the strata of the this ~800-ft-thick unit are at the surface. Edwards aquifer. Springs and streams of the Characteristic Glen Rose stair-step topography area have been an important element in both caused by alternating resistant and recessive the history and settlement of the region beds results from the common, upward­ (Scarbrough, 1973). These same streams and shoaling, subtidal to tidal-flat, cyclic deposition springs, as well as the region's aquifers, con­ (Stricklin and others, 1971; Moore and Bebout, tinue to be important resources of the region. 1989). Fossils include mollusks, rudistids, oy­ sters, echinoids, and the foraminifer Orbitolina. Lithostratigraphy Dinosaur tracks have been found locally The Lower and Upper Cretaceous rocks (Moore and Bebout, 1989; Moore, 1996). Some that dominate the map area record ~2,000 ft strata exhibit vuggy porosity and karst fea­ of marine shelf deposition that spanned tures. An interval of one to three thin beds ~30 Ma (fig. 3). Affected by cyclic relative containing the bivalve Corbula informally di­ sea-level fluctuations, these rocks represent vides the lower and upper Glen Rose Forma­ seven 3'd-order depositional sequences that tion throughout Central Texas (Stricklin and extended from Albian through Campanian others, 1971; Pittman, 1989). Only upper Glen chronostratigraphic stages, although de­ Rose rocks are at the surface in the map area. tailed sequence chronostratigraphy for Recent soil studies within Glen Rose terrain Upper Cretaceous deposits of the study area outside the map area indicate that soils has not been done (Yurewicz and others, overlying some limestone beds are degraded 1993; Moore, 1996; Amsbury, 2002; Kerans and nearly impervious, whereas other soils and Loucks, 2002). In general, the sequences overlying more argillaceous beds are thicker contain deposits of transgressive facies and have more water-infiltration and water­ overlain by highstand facies, and these se­ holding properties (Marsh and Marsh, 1994; quences are bounded by unconformities. Woodruff and others, 1994; Wilding, 1997). Moore (1996) noted that upper Glen Rose The Paluxy Formation is a thin, <10-ft-thick rocks within the study area represent highstand interval of quartz sandstone that is interbedded carbonate-platform facies of a 3'd-order with limestone. It overlies Glen Rose rocks and depositional sequence. Glen Rose sediments interfingers with lower Walnut strata at the are overlain by another depositional sequence northwest part of the map area. Moore and

5 System Stage Unit Stream alluvium Sand Quaternary Terrace alluvium and } gravel Quaternary Older alluvium to Tertiary resource

Campian Taylor

-SB

Upper Cretaceous Santonian

Austin Potential building stone and lime resource

Coniacian

SB Turonian Eagle Ford Swelling clays SB-:::::::= Buda Cenomanian Del Rio Swelling } clays SB

Georgetown

Potential SB building Edwards stone, Edwards aquifer aggregate, and lime resource Comanche Peak Lower - < Keys Albian Cretaceous ,-, Valley - - WhiteStoile :; c _ Q.eQ.ar_ P11r~ (ii Bee Cave ~ ------Bull Creek SB - ---.;:::-Paluxy Figure 3. Generalized stratigraphic col­ umn and geologic resources. SB=3'd-order Glen Upper ft m depositional sequence boundary. Strati­ Rose Trinity 100130 graphic relationships, chronostratigraphic aquifer stages, and 3'd-order sequence boundaries modified from Moore ( 1964; 1996), Salvador and Muneton ( 1989), and Yurewicz and 0 0 others ( 1992). QAd4427x

6 Martin (1966) and Moore (1996) demonstrated strata are considered the aquifer's most porous that the thin Paluxy deposits of the map area rocks (Senger and others, 1990; Jones, 2002). mark the edge of a Lobate-shaped elastic depos­ The Edwards Formation comprises it having a geometry that is typical of a small massive to thin-bedded limestone, dolomitic delta that may have developed southward limestone, dolomite, and minor argillaceous from the main area of Paluxy deposition. limestone that have wackestone, packstone, Walnut Formation limestone, argillaceous and grainstone textures. The unit thins from limestone, and marl overlie the Glen Rose ~300 to 90 ft northward across the map and minor Paluxy deposits. Walnut deposits area. Rodda and others ( 1970) described the represent a transgressive facies and were Edwards in the Austin area as a forma­ subdivided into six members by Moore tion consisting of four informal members (1964; 1996): the Bull Creek limestone, Bee differentiated on the basis of lithology (from Cave marl, Cedar Park limestone, Whitestone base to top): (1) a lower interval of chert-rich, limestone, Keys Valley marl, and the upper thin- to thick-bedded, porous dolomite and marl. Individual members are ~30 to 50 ft dolomitic limestone and limestone commonly thick. Walnut carbonate rocks exhibit mud­ with rudists; (2) a unit of interbedded thin- to stone, wackestone, and packstone textures. thick-bedded cherty limestone, containing Common fossils include oysters, clams, fossils that include rudists and miliolid echinoids, and gastropods. In the southwest Foraminifera, and thin-bedded flaggy lime­ part of the map area and beyond, the stone; (3) a unit of nodular, fossiliferous, Whitestone and Keys Valley members and burrowed, argillaceous limestone and marl; the younger Comanche Peak Formation and ( 4) an upper interval of thin- to thick­ interfinger with Edwards Limestone (map, bedded limestone, dolomitic limestone, and inset diagram). Moore (1996) noted that the dolomite. Rodda and others (l 970) also re­ lower Walnut members thin and onlap Glen ported the upper 20 ft of member 1 to consist Rose strata toward the southwest, onto the San of an iron-stained, cavernous, solution-collapse Marcos Arch. Following Rose's (1972) inter­ zone containing brecciated limestone, dolo­ pretation, rocks equivalent to the lower Walnut mite, chert, coarsely crystalline calcite, and southwest of the Colorado River are part of the residual red clay. Lozo and others (1959) and Edwards stratal sequence. Walnut rocks are Moore ( 1996) demonstrated that the lower not considered aquifer units of the Northern part of the Edwards Formation interfingers Edwards aquifer, although limestone intervals with Walnut and Comanche Peak strata of the Walnut could locally contain water or northward from Austin. Rose (1972) elevated allow some recharge to the aquifer. the Edwards to group status with two form­ The Comanche Peak Formation consists of ations for the region south of the Colorado nodular, fossiliferous limestone with wacke­ River; he reported, however, that north of the stone and packstone textures. It interfingers Colorado River the Edwards should retain its with Edwards limestone in the southwest part single formational rank. The Edwards Form­ of the study area. It is typically between 40 ation is not subdivided in the map area. and 70 ft thick, thickening toward the north Vuggy textures, collapse breccias, cavern and pinching out southwest of the map area. systems, chert, and local rudistids are charac­ In the southwest map area, the thin interval teristic of the unit (Rodda and others, 1970). of Comanche Peak limestone is mapped with The Georgetown Formation consists of fos­ Walnut and Edwards rocks as an undivided siliferous limestone, argillaceous limestone, unit (plate) because absence of outcrops pre­ and minor marl (Wilbert, 1967). These car­ vented proper mapping of the thin formation bonate wackestones, packstones, and grain­ as a separate unit. Comanche Peak rocks make stones compose the upper unit of the Northern up the lower part of the Edwards aquifer north­ Edwards aquifer. Georgetown deposits thicken ern segment, although Edwards Formation northward across the study area from ~60 to

7 110 ft. Diagnostic fossils include bivalves The Eagle Ford Formation consists of Kingena wacoensis and Gryphaea washitaensis. three lithologic intervals: a lower calcareous Vuggy porosity occurs within some beds, but shale; a middle flaggy, silty limestone to cal­ vugs in Georgetown rocks are not as common careous siltstone; and an upper shale. Garner as they are in Edwards rocks. and Young (1976) reported that several thin, Overlying Georgetown rocks, the Del Rio <3-inch-thick bentonite beds occur in the Formation consists of ~65 ft of calcareous, middle part of the unit in the Austin area. fossiliferous claystone to mudstone. It serves Eagle Ford deposits are ~23 ft thick in Travis as the confining bed for the Edwards aquifer. County and thicken northward to ~65 ft in Pyrite and gypsum are common. Fossils of Williamson County. Adkins (1932), Feray Ilymatogyra arietina (formerly Exogyra arietina) (1949), and Young (1985) reported that a shale are common. Garner and Young (1976) reported condensed zone, a few feet thick and having that unweathered Del Rio clay is composed of phosphate nodules, bounds upper Eagle Ford kaolinite, illite, and lesser amounts of mont­ Formation at its contact with the Austin Group. morillonite. During weathering, illite weathers The ~ 140-ft-thick strata interval that to montmorillonite. Weathered Del Rio clay consists of the Del Rio, Buda, and Eagle Ford therefore contains only small quantities of illite Formations exhibits physical characteristics and greater amounts of montmorillonite that are particularly important to construction (Garner and Young, 1976). Del Rio deposits are practices. Clay-rich Del Rio and Eagle Ford usually poorly exposed in slopes below the strata and their soils exhibit relatively poor more erosion-resistant Buda Formation. slope stability and foundation strength, and the The Buda Formation in the map area con­ clays have expansive properties that cause sists of a lower, slightly glauconitic, fos­ swelling when wet and shrinking when dry siliferous limestone and an upper, hard, (Garner and Young, 1976). Buda Formation resistant, burrowed, fossiliferous, shell­ limestones that cap slopes of the Del Rio can fragment limestone (Martin, 1967). The sometimes be unstable because of the poor formation thins northward across the area from slope stability of the Del Rio clay. ~30 to <3 ft. Arrington (1954) reported Buda The Austin Group, also called the Austin limestone to be absent at several places north Chalk, represents ~360 to 425 ft of thin- to thick­ of the San Gabriel River in Williamson County. bedded chalk, marl, and limestone deposited He interpreted the area of missing Buda strata in an open shelf setting (Marks, 1950; Garner to be the result of pre-Eagle Ford erosion on a and Young, 1976; Young and Woodruff, 1985). structurally high area. Undivided Quaternary Young (1985) described six formations in the surficial material covers much of the area, so it Austin Group: the Atco, Vinson, Jonah, is also possible that the thin Buda was eroded Dessau, Burditt, and Pflugerville. He also de­ from the area during the Quaternary (Senger scribed a strata! sequence he called the Austin and others, 1990). Regional studies indicate that Division, which includes rocks between the the upper part of the Buda was eroded before disconformity at the top of the Eagle Ford and Eagle Ford deposition (Adkins, 1932; Hayward the disconformity at the base of the Taylor and Brown, 1967; Martin, 1967). Truncated Group Pecan Gap unit. Young considered the Buda limestone is overlain by a thin, less than Austin Division to be a genetic unit that in­ a few feet thick, black shale interval that has cludes Austin Group strata and an upper been reported as the Pepper Shale, a shale facies claystone unit, the Sprinkle Formation, which equivalent to the Woodbine Formation of the is typically mapped with the Taylor Group. East Texas basin region (Adkins, 1932; Feray Austin and Taylor strata in the map area are and Young, 1949; Hayward and Brown, 1967; not well exposed. Austin Group rocks are ex­ Martin, 1967). In the study area the thin Pepper posed locally in stream beds, but Taylor strata shale interval is rarely exposed and is mapped are rare. Much of the land within the outcrop with Eagle Ford shale deposits. belts of these units is farmland. For this study,

8 the Austin Group is undivided. Clay-rich depo­ of the map area, maximum throw of the area's sits overlying limestone of the upper Austin largest fault, the Mount Bonnell Fault, is nearly Group are mapped as part of the lower Taylor 600 ft (Collins and Woodruff, 2001). In the Group. map area maximum displacements of larger Quaternary alluvial deposits are gener­ faults are between 50 and 150 ft. ally associated with modern streams, al­ Large cross faults that strike subperpendi­ though some local remnant, older terrace cular to the north-northeast-striking structures deposits (Quaternary-Tertiary?) exist at high­ were not recognized in the map area during er elevations. In the Edwards outcrop belt this or earlier investigations (Collins, 1987; and westward, streams have incised narrow Senger and others, 1990). However, some valleys (Senger and others, 1990). Here, Qua­ minor, smaller-scale faults strike westward. ternary alluvial deposits are thin and not Joint analysis along the San Gabriel River near areally extensive. Downstream from the Ed­ Georgetown indicates that most joints follow wards outcrop belt, broad alluvial surfaces, two regional sets (Collins, 1987). Joints of one which consist of terraces associated with ac­ set strike northward between 340° and 020°. A tive streams, as well as older, remnant terraces, second set of joints trend westward between are well developed. Some of the terrace de­ 260° and 300°. Other joint sets may exist locally posits are at least 35 ft thick and may be within the map area because Muehlberger thicker in some locations. Seeps and springs (1990) noted several other regional joint sets in commonly occur at the contact between bed­ the nearby Austin area. rock and terrace deposits (Senger and others, 1990). Most of the seeps probably discharge Resources groundwater accumulated by surface infiltra­ tion into the porous alluvial sand and gravel. Throughout Central Texas, construction materials such as rock aggregate, building Faults stone, cement, and sand and gravel are The Balcones Fault Zone is a regional zone generally in demand owing to population of normal faults along the perimeter of the Gulf growth and urbanization. In the map area Coast Basin (fig. 1 and map). This fault zone Edwards and Georgetown rocks are quarried extends from near Del Rio, east-northeastward at relatively large-scale operations for ag­ to San Antonio, where the zone bends gregate and building stone. Smaller limestone northward through New Braunfels, Austin, pits that occur throughout rural areas are used Georgetown, and Waco and continues toward mostly for road construction and maintenance. Dallas (Murray, 1961; Reasor and Collins, 1988; Terrace deposits in the map area provide a local Ewing, 1991; Collins, 1995; Collins and Ho­ potential source of sand and gravel resources. vorka, 1997). Faults composing the zone are In other parts of Central Texas, lime for cement either more common or more pronounced is quarried from Edwards and Austin between Uvalde and Georgetown. In ge­ limestones, suggesting that these units in the neral, faults in the map area strike north­ map area are a potential lime resource (McBride northeastward and dip between 40° and 80°. and others, 1992). Net throw across the fault zone is down Groundwater resources of the study area toward the east, although faults dip both include Edwards and Trinity aquifers (Klempt eastward and westward. Fault intensity and and others, 1975, 1976; Brune and Duffin, 1983; fault-zone structural relief decrease north­ Maclay and Small, 1986; Senger and others, ward from Austin, where fault zone structural 1990; Jones, 2002). Groundwater from the relief is ~ 1,600 ft (Collins and others, 2002). Trinity aquifer (Glen Rose and older rocks), At the north boundary of the study area in which underlies the Edwards aquifer, is used southern Bell County, the fault zone's struc­ primarily in the west part of the study area, tural relief is ~600 ft (fig. 4). In Austin, south where Edwards strata are absent or too thin to

9 (b) Southwest Northeast HAYS I TRAVIS I WILLIAMSON BELL co co co co

en ::::> Austin Group Map area ffifil c..O

::::>UJa..~ I ------.==.-:::.-.;::~::-1Eagle Ford Fm 5 lfuiM:Eii::::::;D~e:::;l:::;R:;:io~F:=o=rm=a==:t=io=n==i Datum Georgetown Formation }Ed d aq~ffers en Edwards Limestone strata ::::> a::O Comanche Peak F UJ UJ ~ ~ Walnut Formation 500-ft --' UJ vertical gap 0a:: distance ~GlenRose Formation / ft m 20or60

mi 0L.-±,o o 20 km

(c) 1 1' r=--~~--

2 ..._C_____ ---..:K~wg:.._r ------=------~ ,,K" j 3 ..___c====____ .A rr:n 3' 4 ~ .__C:__~-=--=---=--=- 1!-t-=:_=:_-=-~=K~w::::::::9:r.__r=:_:~~- =_=-~· ~ 5~~ 5' I ~ - J,.__~ --~

Figure 4. (a) Location of ~ map area (patterned) and 6~ 6' cross-section lines. The map study area includes the area across cross sections 1- 5. (b) Generalized lithostratig­ 7 raphy of study area. Minor 7' Paluxy deposits of northwest ft edge of map area not shown. 400+-r-, Ku Stratigraphy data from vari­ O 0 2 mi ous sources, including Moore V. E. x 10 ( 1964; 1996), Rose ( 1972), and Garner and Young ( 1976). Undivided Upper Cretaceous strata (c) Cross sections illustrating Edwards aquifer; undivided Edwards aquifer strata and Comanche Peak, Edwards, and Georgetown strata faults. Cross sections 1- 5 are IKwgrl Walnut and Glen Rose strata within the map area. QAd864(a)cx

10 produce significant amounts of groundwater ated with larger-displacement faults influen­ (Senger and others, 1990). In the area of the ces hydrologic properties, and (3) aquifer per­ Edwards aquifer, groundwater from the meability is either unchanged or enhanced underlying Trinity aquifer is rarely used. parallel to faults and in many cases decreased This northern segment of the Edwards perpendicular to faults. These characteristics aquifer comprises Comanche Peak, Edwards, most likely also apply to the northern Edwards and Georgetown rocks. Vuggy textures, voids aquifer in the map area. in collapse breccias, and cavern systems in Locations of springs discharging from Ed­ Edwards strata are characteristic of the unit and wards aquifer strata were studied by Collins account for most of the significant porosity in and Woodruff (unpublished poster data, 2002) the limestones that compose most of the aqui­ for the northern Edwards aquifer region in fer (Abbott, 1973). The confined, subsurface order for stratigraphic positions of the springs part of the aquifer ranges from -420 ft in cen­ within the aquifer strata to be reviewed. tral Travis County to -260 ft in southern Sixty springs were identified for the map Bell County (Collins and others, 2002). Faults area (fig. 5). These springs represent larger control the structural position of the porous springs of the area and do not include the limestone units, similar to other parts of the many small springs and seeps that occur Edwards aquifer (Collins and Hovorka, 1997; throughout the region. Two of the larger Collins, 2000; Ferrill and others, 2004). Faults springs are artesian and appear to be asso­ can serve as conduits for groundwater flow, ciated with faults. Thirty-four springs dis­ and at some locations faults may displace charge near the contact between the Edwards porous beds against relatively less porous beds, and underlying Comanche Peak Formations, thus causing abrupt changes in groundwater verifying porosity differences that exist be­ flow paths. In their evaluation of the structural tween these units. Only one spring was iden­ framework of the San Antonio segment of the tified within Georgetown strata. Seventeen Edwards aquifer recharge zone bordering San springs discharge within Edwards strata, Antonio, Ferrill and others (2004) concluded which contain the more porous strata of the that (1) constriction of groundwater flow paths aquifer rocks. Seven springs were also iden­ may increase with the increase of fault-segment tified within interfingering Edwards, Coman­ connectivity associated with large fault dis­ che Peak, and Walnut strata overlying Glen placements, (2) fault-zone deformation associ- Rose rocks at the southwest part of the map.

11 97' 30'

1+ + 0 +1' 31 ° ~ ~

0 B mi °"' 0 Bkm Contour interval 100 ft

°"' ~ Briggs ~ 2+ + °"'°"' 01! Oo,'.::ot

N ~ Q • ~

~

Liberty Hill 4r

~ Leander

Outcrop belt of undivided Comanche Peak, Edwards, 1-1' Cross-section location • Structure data from well D and Georgetown Formations Edwards aquifer springs ~ Lithofacies changes ~ Artesian spring near fault l!h.- Within interfingering Edwards, Outcrop belt of undivided and interfingering Walnut, D Comanche Peak, Edwards, and Georgetown Comanche Peak, and Walnut strata Formations; Comanche Peak and some Walnut °'" Near contact between Edwards lithofacies pinch out toward the south-southwest and Comanche Peak Formations e,._ Spring discharging from Quaternary O>. Within Edwards Formation alluvium or Upper Cretaceous strata -600 - Elevation of the base of the Oel Rio Formation (approximate top of the Edwards aquifer [ft]) ~ Within Georgetown Formation -l!i_ Faults mapped at ground surface; most faults dip eastward; selected west-dipping faults indicated by bars. Number Interpretations and compilation by E. W. Collins and C. M. Woodruff, Jr. (Collins indicates throw (ft) at selected location. and others, 2002) + Monocline QAd760(a2)x Figure 5. Edwards aquifer outcrop belt, location and stratigraphic pos1t1on of springs, selected fault displacements, and structure contours on top of subsurface Edwards aquifer strata. Modified from unpublished poster data compiled by E. W. Collins and C. M. Woodruff, Jr. (Collins and others, 2002).

12 ACKNOWLEDGMENTS Work on this map was funded partly implied, of the U.S. Government. Initial map by the U.S. Geological Survey STATEMAP digitizing for GIS database creation was by program under cooperative agreement no. Liying Xu and Rachel Waldinger, under the 01HQAG0039. Support from the Texas Water supervision of Thomas A. Tremblay, Senior GIS Development Board is also gratefully acknow­ Analyst. Map and text benefited from reviews ledged. Views and conclusions contained in by S. C. Ruppel, P. R. Knox, Ian Duncan, J. A. this map and text are those of the author and Raney, and Renaud Bouroullec. Map graphics should not be interpreted as necessarily repre­ and text illustrations were by John T. Ames. senting the official policies, either expressed or Lana Dieterich edited and designed the text. REFERENCES

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14 Association of Geological Societies Transactions, Tremblay, T. A., Dale, Sarah, and Andrews, John, 2002, v. 39, p. 247-264. Digital data set for Geologic Map of the West Half Proctor, C. V., Jr., Brown, T. E., Waechter, N. B., Aronow, of the Taylor, Texas, 30 x 60 Minute Quadrangle: Saul, and Barnes, Y. E., 1974, Seguin sheet: The Central Texas Urban Corridor, Encompassing Round University of Texas at Austin, Bureau of Economic Rock, Georgetown, Salado, Briggs, Liberty Hill, and Geology Geologic Atlas of Texas, V. E. Barnes, project Leander, Texas: The University of Texas at Austin, director, scale 1:250,000, reprinted 200 I. Bureau of Economic Geology, Open-File Digital Data Reaser, D. F., and Collins, E. W., 1988, Style of faults and Set. associated fractures in Austin Chalk, northern Tydlaska, L. J., 1951 , Geology of Palm Valley quadrangle, extension of the Balcones Fault Zone, central Texas: Williamson County, Texas: University of Texas, Gulf Coast Association of Geological Societies Austin, Master's thesis, 56 p. Transactions, v. 38, p. 267-276. Walls, Billy, 1950, Geology of the Bell Gin quadrangle, Rodda, P. U., Fisher, W. L., Payne, W. R., and Schofield, Williamson County, Texas: University of Texas, D. A., 1966, Limestone and dolomite resources, Austin, Master's thesis, 56 p. Lower Cretaceous rocks, Texas: University of Texas, Ward, D. L., 1950, Geology of area immediately west of Austin, Bureau of Economic Geology Report of Georgetown, Williamson County, Texas: University Investigations No. 56, 286 p. of Texas, Austin, Master's thesis, 47 p. Rodda, P. U., Garner, L. E., and Dawe, G. L., 1970, Austin Weeks, A. W., 1945, Balcones, Luling, and Mexia fault West, Travis County, Texas: The University of Texas zones in Texas: American Association of Petroleum at Austin Bureau of Economic Geology Geologic Geologists Bulletin, v. 29, no. 12, p. 1733- 1737. Quadrangle Map 38, 11 p., scale I :24,000. Wilbert, W. P., 1967, Stratigraphy of the Georgetown Rogers, C. W., 1963, Structural geology of Round Rock Formation, Central Texas, in Hendricks, Leo, ed., Quadrangle, Williamson County, Texas: University Comanchean (Lower Cretaceous) stratigraphy and of Texas, Austin, Master's thesis, 49 p. paleontology of Texas: Society of Economic Pale­ Rose, P. R., 1972, Edwards Group, surface and subsurface, ontologists and Mineralogists, Permian Basin Central Texas: The University of Texas at Austin, Section, Publication No. 67-8, p. 256-285. Bureau of Economic Geology Report of Investi­ Wilding, L. P., 1997, A reappraisal of the Brackett soil gations No. 74, 198 p. series, in Woodruff, C. 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II, structural and economic geology: ___ 1979, Land resource overview of the capital area University of Texas, Austin, Bureau of Economic planning council region, Texas-a nontechnical Geology, Bulletin No. 340 l, 884 p. guide: The University of Texas at Austin, Bureau of Senger, R. K., Collins, E. W., and Kreitler, C. W., 1990, Economic Geology, 29 p., 4 pl. Hydrogeology of the northern segment of the Woodruff, C. M., Jr., and Collins, E. W., 2001 a, Preface, Edwards aquifer, Austin region: The University of in Woodruff, C. M., Jr., and Collins, E. W., Texas at Austin, Bureau of Economic Geology Report coordinators, Austin, Texas, and beyond-geology of Investigations No. 192, 58 p. and environment, a field excursion in memory of Snyder, F. R. , De La Garza, Laura, and Woodruff, C. M., L. 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15 Woodruff, C. M ., Jr., Wilding. L. P., and Marsh, W. M., ___ 1972, Mesozoic history, Llano region, in Barnes, 1994, Bedrock, soils, and microtopography of a test V. E. , Bell, W. C., Clabaugh, S. E., Cloud, P. E., Jr., area within the Barton Creek watershed, western McGehee, R. V. , Rodda, P. U. , and Young, Keith, Travis County, Texas, in Johns, D. A. , and Woodruff, Geology of the Llano region and Austin area: The C. M., Jr. , eds., Edwards aquifer- water quality and University of Texas at Austin, Bureau of Economic land development in the Austin area, Texas: field Geology, Guidebook No. 13, p. 41-46. Young, Keith, 1985, The Austin division of Central Texas, trip guidebook: Gulf Coast Association of Geological in Young, Keith, and Woodruff, C. M ., eds., Austin Societies 44•h Annual Convention, Austin, Texas, Chalk in its type area- stratigraphy and structure: p. 37-41. Austin Geological Society, Guidebook 7, p. 3-52. Yelderman, J. C., Jr., coordinator, 1987, Hydrogeology of Young, Keith, and Woodruff, C. M ., 1985, Austin Chalk the Edwards aquifer, northern Balcones and Washita in its type area- stratigraphy and structure: Austin Prairie segments: Austin Geological Society Guide­ Geological Society, Guidebook 7, 88 p. book 11 , 91 p. Yurewicz, D. A., Marler, T. B., Meyerholtz, K. A., Siroky, Young, Keith, 1967, Comanche Series (Cretaceous), south F. X., 1993, Early Cretaceous carbonate plat­ central Texas, in Hendricks, Leo, ed., Comanchean form, north rim of the Gulf of Mexico, Mississip­ (lower Cretaceous) stratigraphy and paleontology pi and Louisiana, in Simo, J. A. , Scott, R. W. , and of Texas: Society of Economic Paleontologists and Masse, J.-P., Cretaceous carbonate platforms: Ameri­ Mineralogists, Permian Basin Section, Publication can Association Petroleum Geologists Memoir 56, No. 67-8, p. 9- 30. p. 81 - 109.

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