Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Hays County, Texas

Home , Balcones Fault, Edwards Group, Edwards Plateau, Georgetown Formation, San Marcos Springs

GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC CHARACTERISTICS OF THE EDWARDS AQUIFER OUTCROP, HAYS COUNTY, TEXAS

U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 95-4265

Prepared in cooperation with the EDWARDS UNDERGROUND WATER DISTRICT Blank Page GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC CHARACTERISTICS OF THE EDWARDS AQUIFER OUTCROP, HAYS COUNTY, TEXAS

By John A. Hanson and Ted A. Small

U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 95–4265

Prepared in cooperation with the EDWARDS UNDERGROUND WATER DISTRICT

Austin, Texas 1995 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Acting Director

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

For additional information write to: Copies of this report can be purchased from:

District Chief U.S. Geological Survey U.S. Geological Survey Branch of Information Services 8027 Cameron Rd. Box 25286 Austin, TX 78754–4733 Denver, CO 80225–0286

ii CONTENTS

Abstract ...... 1 Introduction ...... 1 Methods of Investigation ...... 3 Acknowledgments ...... 3 Geologic Framework ...... 5 General Features ...... 5 Stratigraphy ...... 5 Hydrogeologic Characteristics ...... 7 General Features ...... 7 Porosity and Permeability ...... 8 Summary ...... 9 References Cited ...... 9

PLATE

[Plate is in pocket]

1. Map showing hydrogeologic subdivisions of the Edwards aquifer outcrop, Hays County, Texas

FIGURE

1. Map showing location of the study area ...... 2

TABLE

1. Summary of the lithologic and hydrologic properties of the hydrogeologic subdivisions of the Edwards aquifer outcrop, Hays County, Texas ...... 4

CONTENTS iii Blank Page Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Hays County, Texas

By John A. Hanson and Ted A. Small

Abstract INTRODUCTION

All of the hydrogeologic subdivisions within Located in the Lower Cretaceous Kainer and the Edwards aquifer outcrop in Hays County have Person Formations of the Edwards Group (Rose, 1972) some porosity and permeability. The most porous and the overlying Georgetown Formation, the Edwards and permeable appear to be hydrogeologic sub- aquifer is one of the most permeable and productive car- division VI, the Kirschberg evaporite member of bonate aquifers in the nation. The dissolution-modified, faulted limestone aquifer (Buszka and others, 1990) is the Kainer Formation; hydrogeologic subdivision the sole source of public water supply for San Antonio III, the leached and collapsed members, undivided; and is the major source of water for Hays County. and hydrogeologic subdivision II, the cyclic and Most recharge to the Edwards aquifer occurs in marine members, undivided, of the Person Forma- counties immediately west of Bexar County (fig. 1). tion. The two types of porosity in the Edwards Streams cross Edwards aquifer outcrops (the recharge aquifer outcrop are fabric selective, which is zone) in the Balcones fault zone and lose much, if not related to depositional or diagenetic elements and all, of their flow to faults, fractures, sinkholes, and typically exists in specific stratigraphic horizons; caves in the outcrop. After entering the aquifer, the and not fabric selective, which can exist in any water moves east to points of discharge in Bexar County lithostratigraphic horizon. Permeability, the capac- (mostly municipal wells) and then northeast, parallel or ity of porous rock to transmit water, depends on the almost parallel to the northeast-trending Balcones faults physical properties of the rock such as size, shape, into Comal and Hays Counties, where it is discharged and distribution of pores, and fissuring and dissolu- by wells and springs. tion. Additional recharge to the Edwards aquifer is from Edwards aquifer outcrops in the Balcones fault Two faults, San Marcos Springs and zone in northern Bexar County and southern Comal and Mustang Branch, completely, or almost com- Hays Counties. The rugged, scenic limestone hills of the pletely, offset the Edwards aquifer by juxtaposing Edwards aquifer outcrops are the site of rapidly Edwards aquifer limestone against nearly imper- encroaching residential and commercial development. meable upper confining units along parts of their Kipp and others (1993, p. 1) report a substantial traces across Hays County. These faults are increase in development activities over the Edwards thought to be barriers, or partial barriers, to ground- recharge zone during 1992–93. They also note that water flow where the beds are juxtaposed. increased development brings a greater threat of contamination to the Edwards aquifer. The aquifer could be In Hays County, the Edwards aquifer proba- contaminated from spills or leakage of hazardous mate- bly is most vulnerable to surface contamination in rials; or runoff from the rapidly developing urban areas the rapidly urbanizing areas on the Edwards aquifer that surround, or are built on, the intensely faulted and outcrop. Contamination can result from spills or fractured, karstic limestone outcrops characteristic of leakage of hazardous materials; or runoff on the the recharge zone. Furthermore, some of the hydrogeo- intensely faulted and fractured, karstic limestone logic subdivisions that compose the Edwards aquifer outcrops characteristic of the recharge zone. have greater effective porosity and permeability than

Abstract 1 98o 07’30"

HAYS COUNTY

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97 o52’30" 30o 07’30"

C EdwardsPlateau y Cre pr e k ess C O re io n NC e On o A k 94 45’ L B R I V E R 30o

Plain

San Marcos Platform E

V Base modified from University of Texas, I Bureau of Economic Geology, 1979, R 1981a, b, 1983 Coastal

O Purga C to ry N C A ree L o k 29 52’30" B San Marcos

Gulf EXPLANATION

STUDY AREA

EDWARDS AQUIFER OUTCROP

0 5 10 MILES TEXAS

San Antonio

HAYS COUNTY NE O COMAL Z COUNTY T L U BEXAR A COUNTY F S E N C O BAL

Figure 1. Location of the study area.

2 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Hays County, Texas others, and in areas where they crop out, might provide member of the Kainer Formation, which caps several efficient avenues for contaminants to enter the aquifer. hills in southwestern, north-central, and northeastern According to Buszka (1987, p. 2), "Carbonate aquifers Hays County, can be identified on aerial photographs by such as the Edwards are readily susceptible to ground- a pattern of concentric rings of sparse vegetation grow- water contamination where the presence of pollutants ing on the differentially weathered limestone. The coincides with the outcrop of the aquifer." In Hays regional dense member of the Person Formation can be County, the Edwards aquifer probably is most vulnerable to surface contamination in the rapidly urbanizing recognized on aerial photographs as small, light-to- areas on the Edwards aquifer outcrop. almost-white areas. The U.S. Geological Survey, in cooperation with Well logs and geologic map data were compiled the Edwards Underground Water District, mapped the and used in mapping the hydrogeologic subdivisions Edwards aquifer outcrop and described its hydrogeo- of the Edwards aquifer in the study area. The thick- logic characteristics (porosity and permeability) to doc- nesses of the hydrogeologic subdivisions that compose ument conditions pertinent to movement and the Edwards aquifer were determined from well logs in contamination of ground water. This report describes and adjacent to the aquifer outcrop in Hays County. the geologic framework and hydrogeologic characteristics of the Edwards aquifer outcrop in Hays County. The upper member of the Lower Cretaceous Glen Rose This information will help to provide a better under- Limestone, the lower confining unit (table 1), was standing of the processes controlling the spatial distri- mapped adjacent to the Edwards aquifer outcrop along bution of recharge and the flow of water into the the northwestern boundary of the study area (pl. 1). aquifer. The information also will help determine the The upper confining units, which include the Upper areas of the recharge zone that are most susceptible to Cretaceous Del Rio Clay, Buda Limestone, Eagle Ford potential contamination from land-use practices. Group, Austin Group, and the Navarro and Taylor Groups, undivided, were mapped along the eastern and Methods of Investigation southeastern boundary of the study area. The hydrogeologic subdivisions (table 1) Faults were identified in the field by stratigraphic modified from Maclay and Small (1976) were used to displacement and characteristics related to faulting, map the Edwards aquifer outcrop. The stratigraphic such as zones of fault gouge composed of soils that nomenclature and descriptions of Rose (1972) were greatly resemble caliche, or relatively thick, vein-like used in mapping the geologic units for the San Marcos masses of subhedral to anhedral calcite crystals. Steeply platform (fig. 1). The carbonate-rock classification system of Dunham (1962) was used for the lithologic inclined strata, uncommon in the relatively flat-lying descriptions. Member, hydrogeologic subdivision, and Edwards aquifer outcrop, typically represent drag- porosity/permeability type were determined at the out- folding related to faulting. The presence of cedar elm crop. The porosity type follows the sedimentary carbon- trees, which tend to grow along faults, perhaps as a ate classification system of Choquette and Pray (1970). result of enhanced downward movement of water along The hydrogeologic subdivisions of the Edwards aquifer the fault planes, also was used to indicate faults. outcrop in Hays County are shown on plate 1. Recent aerial photographs were used to locate Acknowledgments roads and excavations that could provide outcrop exposures for field examination and for orientation in the Special thanks are extended to Dean Word, Dean morphologically similar Edwards aquifer outcrops. In Word Industries, Joe Rogers, and Centex Industries for addition, stratigraphic information was ascertained by access to their quarries. Also, thanks are extended to Dr. inspection of surficial expressions and features as indicated by the following examples. The basal nodular P.R. Rose and Dr. Charles Woodruff, Jr., for their help- member of the Kainer Formation supports a dense ful comments during a visit to two quarries near San growth of juniper and oak trees and can be recognized Marcos, and to the many property owners who granted on aerial photographs by the dark trace that encircles the permission to enter their property, supplied informa- hills of the overlying dolomitic member. The dolomitic tion, and aided in collecting the field data.

INTRODUCTION 3 Table 1. Summary of the lithologic and hydrologic properties of the hydrogeologic subdivisions of the Edwards aquifer outcrop, Hays County, Texas

[Hydrogeologic subdivisions modified from Maclay and Small (1976); groups, formations, and members modified from Rose (1972); lithology modified from Dunham (1962); and porosity type modified from Choquette and Pray (1970). CU, confining unit; AQ, aquifer]

Group, Hydro- Hydrogeologic Thickness Field Cavern Porosity/ formation, logic Lithology subdivision (feet) identification development permeability type or member function

Navarro and Taylor CU 600 Clay; chalky limestone Gray-brown clay; None Low porosity/ Groups, undivided marly limestone low permeability Austin Group CU; 130 – 150 White to gray limestone White, chalky None Low porosity/ rarely AQ limestone; low permeability; Gryphaea aucella rare water production from fractures Upper Eagle Ford Group CU 30 – 50 Brown, flaggy, sandy Thin flagstones; None Primary porosity lost/ confining shale and argillaceous petroliferous low permeability units limestone

Upper Cretaceous Buda Limestone CU 40 – 50 Buff, light gray, Porcelaneous Minor surface karst Low porosity/ dense mudstone limestone low permeability Del Rio Clay CU 40 – 50 Blue-green to Fossiliferous; None None; primary upper yellow-brown clay Ilymatogyra confining unit arietina Georgetown Formation CU 10 – 40 Gray to light tan Marker fossil: None Low porosity/ I marly limestone Waconella low permeability wacoensis Cyclic and AQ 80 – 100 Mudstone to packstone; Boxwork vugs; light Many caves; might be Laterally extensive; marine miliolid grainstone; tan, massive; some associated with earlier both fabric and not members, chert Toucasia and karst development fabric/one of the more II undivided Caprinid porous and permeable of the subdivisions

Leached and AQ 80 – 100 Crystalline limestone; Bioturbated iron- Extensive lateral Majority not fabric/ collapsed mudstone to stained beds development; large probably the most members, grainstone; chert; separated by rooms permeable of the III undivided collapsed breccia massive limestone subdivisions Person Formation beds; Montastrea (?) sp. Regional CU 20 – 24 Dense, argillaceous Wispy iron-oxide None; only vertical Not fabric/ IV dense mudstone stains fracture enlargement low permeability; member vertical barrier Grainstone AQ 50 – 60 Miliolid grainstone; White crossbedded Few caves Not fabric/ Edwards aquifer member mudstone to grainstone; recrystallization reduces V wackestone; chert Toucasia and permeability Edwards Group Edwards Lower Cretaceous Lower Turritella Kirschberg AQ 50 – 60 Crystalline limestone; Boxwork voids, with Probably extensive cave Majority fabric/one of the evaporite chalky mudstone; neospar and development more porous and VI member chert travertine frame permeable of the subdivisions Dolomitic AQ 110 – 130 Mudstone to Massively bedded Caves related to Mostly not fabric; some VII member grainstone; crystalline light gray, Toucasia structure or bedding bedding-plane fabric/

Kainer Formation Kainer limestone; chert abundant planes locally permeable Basal nodular Karst 50 – 60 Shaly, nodular Massive, nodular and Few caves Fabric/low permeability member AQ; limestone; mottled, Exogyra VIII not karst mudstone texana CU and miliolid grainstone Upper member of the CU; 350 – 500 Yellowish tan, thinly Stair-step Some surface cave Some water production at Lower Glen Rose Limestone evaporite bedded limestone and topography; development evaporite beds/ confining beds marl alternating relatively impermeable unit AQ limestone and marl

4 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Hays County, Texas GEOLOGIC FRAMEWORK stone against nearly impermeable upper confining units along parts of their traces across Hays County. These General Features faults are thought to be barriers, or partial barriers, to ground-water flow where the beds are juxtaposed. A regional dip of about 20 feet per mile (ft/mi) Geomorphic expression of faulting on the to the southeast for Cretaceous strata on the Edwards upthrown fault blocks is indicated on topographic maps Plateau (fig. 1) in northwestern Hays County was by the branching of subsequent valleys normal to the reported by DeCook (1963, p. 43). According to consequent valleys, forming a "T-square" morphology DeCook (1963, p. 43), the Comal/Hays County line lies of the valleys. The formation of the consequent valleys along the approximate axis of the San Marcos platform; resulted from the drop in base level of the downthrown however, Rose (1972, fig. 16) places the axis in Comal block, which initiated headward erosion on the escarp- County. Northeast-trending faults of the Balcones fault ment. The development of the subsequent valleys possi- zone cross the entire county, but are more numerous in bly is the result of faults structurally weakening the the southeastern part of the county. According to consequent valley slopes creating the T-square pattern DeCook (1963), the San Marcos Springs fault (pl. 1) normal to the natural course of headward erosion forms a prominent part of the escarpment separating the (Thornbury, 1962). Gulf Coastal Plain from the Edwards Plateau (fig. 1). Balcones faults are en echelon, high-angle, nor- Stratigraphy mal, and mostly downthrown to the southeast (down-to- the-Gulf Coast); however, a few faults are downthrown The Edwards Group (table 1) is about 440 to 534 to the northwest. Sellards and Baker (1934, p. 56–57) feet (ft) thick in Hays County. The Edwards Limestone described the zone of faults consisting principally of the of DeCook (1963) is roughly equivalent to the Edwards Comal Springs and San Marcos Springs faults as "* * * Group of Rose (1972). DeCook (1963, p. 27) reported a a highly complicated zone of faulting characterized thickness in the Edwards Limestone of about 350 ft in a by numerous faults, some of which are of large throw. well in San Marcos and a thickness of about 400 ft in a * * * The individual faults have in the main the same well in northeast Hays County. In 1990, the Edwards trend, although there are many faults of divergent trend, Underground Water District reported a thickness of resulting in numerous small and irregularly shaped fault about 480 ft in the Edwards Group in wells drilled near blocks, so that the fault pattern as a whole is very com- San Marcos Springs (pl. 1). According to DeCook plex." DeCook (1963, p. 47) noted that dips of the faults (1963), the Edwards Limestone in Hays County prima- were not easily obtained and where the dips have been rily consists of "***light-gray, brittle, thick-bedded to observed, they commonly exceed 60 degrees and their massive limestone, commonly dolomitic, containing steepness is indicated further by the generally straight minor beds of argillaceous or siliceous limestone and traces of faults across uneven land surfaces. Grimshaw calcareous shale. Bedded or nodular chert and flint and Woodruff (1986, p. 72) stated, "The fact that fault characterize much of the formation." This is useful traces are not at all influenced by topography indicates information when mapping the outcrop of the Edwards that all faults are vertical or nearly so." Group. Massive, nodular limestone beds at the lower DeCook (1963, p. 44) reported that at least seven part of the Kainer Formation overlie the alternating marl major faults in Hays County are traceable for many and limestone beds of the upper member of the Glen miles and have relatively great displacements. DeCook Rose Limestone in Hays County (DeCook, 1963). The (1963, p. 44) further noted at least 70 more faults of upper member of the Glen Rose Limestone is identified lesser length with generally less displacement. The by its characteristic stair-step topography caused by the seven major faults are Comal Springs, San Marcos differential weathering of the nonresistant marl and Springs, Kyle, Mustang Branch, Hidden Valley, resistant limestone and dolomite beds (Stricklin and Wimberley, and Tom Creek (pl. 1). At least three others, 1971, p. 23). other faults—Academy, Mountain City, and Bat Cave The major formal lithostratigraphic units of the (pl. 1)—probably could be included in the list of major Edwards aquifer are the Kainer, Person, and George- faults. Two faults, San Marcos Springs and Mustang town Formations. The Kainer and Person Formations of Branch, completely, or almost completely, offset the the Edwards Group were divided into seven informal Edwards aquifer by juxtaposing Edwards aquifer lime- members by Rose (1972). Maclay and Small (1976)

GEOLOGIC FRAMEWORK 5 combined Rose’s seven members with the Georgetown Kainer Formation. The grainstone member is 50 to 60 ft Formation to form the group of eight informal hydro- thick and primarily is dense, miliolid grainstone geologic subdivisions of the Edwards aquifer identified cemented with calcite spar; however, patches of mud- on plate 1. The Georgetown Formation is not known stone to wackestone are scattered throughout (table 1). to yield water in the study area. However, because well Chert nodules generally are rare in this member. drillers historically have relied on the Georgetown Locally, Toucasias and stubby spar-filled turritella Formation to indicate the top of the Edwards aquifer, gastropods are common near the top of the member. the formation is considered part of the aquifer. Except Chondrodonta is found in approximately the same for the Georgetown Formation, the strata that compose stratigraphic interval as Toucasia, but is not common. the Edwards aquifer were deposited in shallow to very The Person Formation (Rose, 1972, p. 19) is shallow marine water (Rose, 1972) and reflect deposi- about 180 to 224 ft thick in Hays County (table 1). tional environments resulting from slight changes in The lithology of the Person Formation ranges from water level, water chemistry, temperature, and circula- mudstone to layers of locally intensely burrowed tion. These factors caused subtle to not-so-subtle varia- mudstone to grainstone to crystalline limestone. The tions in the overall lithology of the various members regional dense member is the lowermost member of the and some variations within the individual members. Person Formation and consists of dense, argillaceous The Kainer Formation (Rose, 1972, p. 19) is mudstone, with distinctive wispy shale partings. The about 260 to 310 ft thick in Hays County (table 1). combination of the grainstone member (Kainer Forma- The lithology of the Kainer Formation ranges from tion) and the regional dense member (Person Forma- mudstone to miliolid grainstone to crystalline lime- tion) forms a very distinctive mapping horizon. stone. The lowermost unit, the basal nodular member, The leached and collapsed members, undivided, is 50 to 60 ft thick and generally is a dense, shaly, overlie the regional dense member and were mapped as fossiliferous, nodular limestone, mudstone, and some one unit because they could not be distinguished as sep- miliolid grainstone. The fossil oyster Exogyra texana is arate members. The lithology of the leached and col- scattered erratically throughout the member and is lapsed members, undivided, ranges from variably abundant locally. The tan-yellow clay layer at the base burrowed mudstone to grainstone with intervals of crys- of the basal nodular member might be equivalent to the talline limestone; chert lenses are common as well. The Walnut Clay described by DeCook (1963). collapsed zones common in this member were caused The next higher member, the 110- to 130-ft-thick by the collapse of the overlying limestone into the voids dolomitic member, is mostly dense crystalline lime- created by early dissolution of the thin evaporite layers stone with occasional zones of grainstone and layers of and lenses (Rose, 1972, p. 55). variably burrowed, and occasionally strongly dissolu- The cyclic and marine members, undivided, tioned mudstone. Chert nodules and thin discontinuous also were mapped as one unit. According to Rose (1972, beds of chert are scattered throughout the member. p. 71), the cyclic member and much of the marine mem- Rudists, typically Toucasia, are common locally near ber were eroded from the axis of the San Marcos plat- the top of the member. Chondrodonta, a thick-shelled form before deposition of the Georgetown Formation. pelecypod, also is found near the top of the dolomitic The remaining part of the marine member consists of member, but is not common. medium thick to thick beds of variably honeycombed The Kirschberg evaporite member is 50 to 60 ft mudstone and fossiliferous packstone and lenses of thick and consists mostly of crystalline limestone and miliolid grainstone and chert nodules. DeCook (1963) chalky mudstone with chert nodules and lenses. This reported that in an area northwest of San Marcos the member lacks the collapse features common to the uppermost part of the Edwards limestone is character- Kirschberg evaporite on the Edwards Plateau, which ized by massive honeycombed rudistid-bearing might indicate that less evaporite was deposited on the biostromes. Rocks of this description were identified San Marcos platform. However, boxwork structure in a quarry about 2 miles (mi) north of San Marcos thought to represent dissolution of evaporites is evident where there is an excellent exposure near the contact of in thinly layered beds of crystalline limestone in the the marine member with the overlying Georgetown western part of the county. Formation. Locally, the marine member is bored at the The grainstone member overlies the Kirschberg contact. About 5 ft below this contact is a honeycombed evaporite member and is the uppermost member of the zone. Some of the vugs in this honeycombed zone have

6 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Hays County, Texas a semi-rectangular outline, similar to those shown by taceous rocks that overlie the Austin Group, and are Stricklin and others (1971, p. 54), as boxwork struc- mostly calcareous clayey chalky limestone. tures. P.R. Rose (Geologist, oral commun., 1994) Field identification of the various members in the suggests that these boxwork vugs might represent pref- Kainer and Person Formations was based on their char- erential dissolution of clasts of collapse breccia formed acteristic lithologies and fossils (table 1). Red clay soil by the collapse of overlying layers into voids left by the that resembles the "terra rossa" of Pleistocene age, dissolution of underlying evaporites. Caprinids (a type described by Young (1986, p. 63) as a diagenetically of fossil rudistid), some of which are larger than those altered paleosol, commonly is evident in outcrops of the generally seen on the San Marcos platform, locally are Edwards Group, but rarely in the Glen Rose Limestone common in the rudist biostrome just below the honey- or in the clays, marls, or limestones of the upper confin- comb zone. Toucasias locally are common near the con- ing units. According to Young (1986, p. 65), the red tact of the marine member with the Georgetown clay soil indicates that lithology was important in the Formation. development of central Texas terra rossa. Red clay soil The Georgetown Formation, which overlies the was observed more often in the Person Formation than Edwards Group, was deposited on the eroded surface of in the Kainer Formation, but locally it is common in the the Person Formation in deeper water than was charac- dolomitic member and Kirschberg evaporite member, teristic for most of the Edwards Group deposition and has been reported in core and drill cuttings in the (Rose, 1972, p. 71). The Georgetown Formation gener- Kainer Formation. Large accumulations of terra rossa ally is a marly, fossiliferous limestone, usually contain- relative to the volume of pore space in the limestone ing ammonites, oyster-like clams, and the brachiopod decrease the effective porosity and thereby decrease the Waconella wacoensis, formerly Kingena wacoensis permeability. (Roemer), which is an excellent marker fossil for the Young (1986, p. 65) observed that karstification Georgetown Formation (table 1). Exposures of the of the Person Formation was more thorough than karsti- Georgetown Formation are common and range from 10 fication of the Kainer Formation on the San Marcos to 40 ft thick. platform. Because of the lithologic similarities between the leached and collapsed members, undivided, and the The Upper Cretaceous Del Rio Clay, Buda Lime- cyclic and marine members, undivided, of the Person stone, Eagle Ford Group, Austin Group, and Navarro Formation, the contact between the two sometimes is and Taylor Groups, undivided, overlie the Georgetown difficult to determine. In such areas, the approximate Formation (table 1). The Del Rio Clay is a dark blue- stratigraphic thickness was used to identify the unit and green to yellow-brown, variably gypsiferous clay com- locate the approximate contact. A unique colonial coral, monly containing pecten-type fossil clams and an abun- identified as Montastrea sp. (Finsley, 1989), was dance of the fossil oyster Ilymatogyra arietina, formerly observed in the lower to middle part of the leached and Exogyra arietina (Roemer). These fossil oysters are collapsed members, undivided, and could serve as a known locally as "rams horns." The Buda Limestone guide fossil. consists of dense, variably nodular, sublithographic or "porcelaneous" limestone (Sellards and others, 1933, p. HYDROGEOLOGIC CHARACTERISTICS 397); and buff, light-gray mudstone, commonly containing calcispheres and tiny calcite-filled fractures. General Features The Eagle Ford Group overlies the Buda Limestone and consists of thin flagstones of brown, flaggy sandy shale Major factors controlling porosity and permea- and argillaceous limestone. Some of the freshly frac- bility in the Edwards aquifer outcrop are faulting, tured flagstones (thin brittle slabs) emit a petroliferous stratification, and karstification—a form of diagenesis odor. Because the Eagle Ford Group is dark brown in resulting from extensive dissolution of limestone. the subsurface, local water-well drillers commonly refer Zones of faulted, fractured, and dissolutioned lime- to this shale as lignite. The Austin Group overlies the stone, along with layers of burrowed, honeycombed, Eagle Ford Group and consists of chalky, variably and occasionally cavernous limestone, are common marly, generally fossiliferous limestone, commonly in the Edwards aquifer outcrop. containing the fossil oyster Gryphaea aucella. The The karst features of the Edwards Group rocks Navarro and Taylor Groups, undivided, are Upper Cre- in Hays County are characterized by resistant terrain

HYDROGEOLOGIC CHARACTERISTICS 7 of dense limestone, sparsely dotted with sinkholes permeability observed in the field within the subdivi- and caves, which can greatly enhance porosity and sions are discussed in ascending order. permeability. Although not necessarily representative Hydrogeologic subdivision VIII (basal nodular of previous geologic times, the present-day dry- member) has interparticle porosity but little permeabil- subhumid climate (Thornthwaite, 1952) (rainfall 33.79 ity in the miliolid grainstone and nodular limestone inches per year [in/yr]; Bader and others, 1993, table beds. Cavern porosity and permeability associated with 3.1) is not favorable for rapid karst development. caves in this subdivision characterize several caves in a small area in Comal County, but these properties were Porosity and Permeability not seen in Hays County. This subdivision is locally, but not regionally, porous or permeable. According to Choquette and Pray (1970, p. 212), Hydrogeologic subdivision VII (dolomitic mem- porosity in sedimentary carbonates is either fabric ber) has little visible porosity or permeability in the selective or not fabric selective. Fabric selective poros- dense crystalline limestone. Interparticle (breccia) ity is related directly to the depositional or diagenetic porosity and permeability and fracture porosity and per- fabric elements of a sediment and typically is controlled meability associated with faulting are common locally. by lithostratigraphic horizon. Not fabric selective Vuggy porosity and permeability also are common porosity is not related to depositional or diagenetic fab- locally in the burrowed zones. ric elements of a sediment and can exist in any litho- Hydrogeologic subdivision VI (Kirschberg stratigraphic horizon. evaporite member) generally has common to abundant intercrystalline porosity in the chalky mudstone, and Choquette and Pray (1970, p. 222) designated locally abundant vuggy porosity and permeability prob- seven types of carbonate porosity that are "extremely ably associated with faulting and evaporite dissolution common and volumetrically important." Five of these (Maclay and Small, 1976). This subdivision has both (interparticle, intraparticle, intercrystalline, moldic, and fabric selective and not fabric selective porosity, and fenestral) generally are fabric selective, and two (frac- appears to be the most porous and permeable subdivi- ture and vuggy) are not fabric selective. According to sion in the Kainer Formation. Choquette and Pray (1970, p. 223–224), breccia poros- Hydrogeologic subdivision V (grainstone mem- ity is a type of interparticle porosity and can be either ber) has widely separated interparticle and intraparticle fabric selective or not fabric selective. Other types of porosity and little permeability in the dense, tightly porosity in the Edwards aquifer outcrop are channel and cemented miliolid grainstone; and local fracture poros- cavern, both of which are not fabric selective; and bur- ity and permeability associated with faulting. Other- row, which can be either fabric selective or not fabric wise, this subdivision has little porosity or permeability. selective. Choquette and Pray (1970, p. 250) noted that Hydrogeologic subdivision IV (regional dense vugs and channels are similar in that neither is fabric member) has little porosity or permeability except for selective. Vugs and channels differ in shape; "vug" is some fracture porosity and permeability associated with used to describe the more equidimensional pores; faulting. This subdivision probably is the least porous or whereas, "channel" is used to describe markedly elon- permeable subdivision and locally might be a confining gated pores or irregular openings with a marked elonga- unit within the Edwards aquifer. tion in one or two dimensions. Hydrogeologic subdivision III (leached and col- Permeability is the capacity of a porous rock to lapsed members, undivided) has vuggy and burrow transmit water. According to Ford and Williams (1989, porosity and permeability associated with burrowed p. 130), permeability depends on the physical properties zones; breccia and cavern porosity and permeability of the rock, particularly pore size, shape, and distribu- associated with collapsed zones resulting from dissolution. Ford and Williams (1989, p. 150) further state that, tion of evaporites; and fracture porosity and permeabil- "As a consequence of the effects of fissuring and differ- ity associated with faulting. This subdivision probably ential solution, permeability may be greater in some is the most porous and permeable of the subdivisions directions than in others, as well as in certain preferred and, thus, the most susceptible to contamination from stratigraphic horizons." The eight hydrogeologic subdi- surface sources. visions of the Edwards aquifer, the names of the corre- Hydrogeologic subdivision II (cyclic and marine sponding members, and the type of porosity and members, undivided) has moldic and vuggy porosity

8 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Hays County, Texas and permeability associated with fossiliferous zones are similar in lithology and appearance, whereas others and dissolution of evaporites, and fracture porosity and are more distinctive. permeability associated with faulting. San Marcos The major factors controlling porosity and per- Springs issue from openings believed to be in this sub- meability in the Edwards aquifer outcrop are faulting, division along the San Marcos Springs fault (William F. stratification, and karstification. Karst features in the Guyton and Associates, 1979). Field observations indi- outcrop, which can greatly enhance porosity and perme- cate that this subdivision has only slightly less porosity ability, include sinkholes and caves. The two types of and permeability than subdivision III. porosity in the Edwards aquifer outcrop are fabric selec- Hydrogeologic subdivision I (Georgetown For- tive, which is related to depositional or diagenetic mation) has almost no visible porosity or permeability. elements and typically exists in specific stratigraphic According to DeCook (1963, p. 40), the shale, marl, horizons; and not fabric selective, which can exist in and compact limestones of the Georgetown Formation any lithostratigraphic horizon. Permeability, the capac- are relatively impermeable, and the formation acts as ity of porous rock to transmit water, depends on the an upper confining unit for water in the Edwards lime- physical properties of the rock such as size, shape, and stone. distribution of pores, and fissuring and dissolution. The Edwards aquifer hydrogeologic subdivisions VI SUMMARY (Kirschberg evaporite member of the Kainer Forma- tion), III (leached and collapsed members, undivided, of The Edwards aquifer, sole source of public water the Person Formation), and II (cyclic and marine mem- supply for San Antonio, is the major source of water for bers, undivided, of the Person Formation) appear to be Hays County. The aquifer primarily consists of dissolu- the most porous and permeable. Hydrogeologic subdi- tion-modified, faulted limestone. The Edwards aquifer vision III probably is the most susceptible to contamina- is recharged in its outcrop area in the Balcones fault tion from surface sources. zone. In Hays County, the Edwards aquifer probably is REFERENCES CITED most vulnerable to surface contamination in the rapidly urbanizing areas on the Edwards aquifer outcrop. Con- Bader, R.W., Walthour, S.D., Waugh, J.R., 1993, Edwards tamination can result from spills or leakage of hazard- aquifer hydrogeologic status report for 1992: Edwards ous materials; or runoff on the intensely faulted and Underground Water District Report 93–05, 71 p. fractured, karstic limestone outcrops characteristic of Buszka, P.M., 1987, Relation of water chemistry of the Edwards aquifer to hydrogeology and land use, San the recharge zone. Antonio region, Texas: U.S. Geological Survey Water- Northeast-trending faults of the Balcones fault Resources Investigations Report 87–4116, 100 p. zone cross Hays County, but are more numerous in the Buszka, P.M., Zaugg, S.D., Werner, M.G., 1990, southeastern part of the county. San Marcos Springs Determination of trace concentrations of volatile fault forms a part of the escarpment separating the Gulf organic compounds in ground water using closed- Coastal Plain from the Edwards Plateau. San Marcos loop stripping, Edwards aquifer, Texas: Bulletin Springs fault and Mustang Branch fault completely, or of Environmental Contamination and Toxicology, almost completely, offset the Edwards aquifer by juxta- p. 507–515. posing Edwards aquifer limestone against nearly imper- Choquette, P.W., and Pray, L.C., 1970, Geologic meable upper confining units along parts of their traces nomenclature and classification of porosity in across Hays County. These faults are thought to be bar- sedimentary carbonates: American Association of riers, or partial barriers, to ground-water flow where the Petroleum Geologists Bulletin, v. 54, no. 2, p. 207–250. beds are juxtaposed. DeCook, K.J., 1963, Geology and ground-water resources of Hays County, Texas: U.S. Geological Survey Water- The Kainer and Person Formations of the Supply Paper 1612, 72 p. Edwards Group and the overlying Georgetown Forma- Dunham, R.J., 1962, Classification of carbonate rocks tion compose the Edwards aquifer. The Kainer and the according to depositional texture, in Classification of Person Formations consist of seven informal members. Carbonate Rocks Symposium: American Association of These members, together with the overlying George- Petroleum Geologists Memoir 1, p. 108–121. town Formation, form the eight informal hydrogeologic Finsley, Charles, 1989, A field guide to fossils of Texas: subdivisions of the aquifer. Some formation members Austin, Texas Monthly Press, 189 p.

SUMMARY 9 Ford, D.C., and Williams, P.W., 1989, Karst geomorphology Stricklin, F.L., Jr., Smith, C.I., and Lozo, F.E., 1971, and hydrology: London, Chapman and Hall, 601 p. Stratigraphy of Lower Cretaceous Trinity deposits of Grimshaw, T.W., and Woodruff, C.M., Jr., 1986, Structural central Texas: Austin, University of Texas, Bureau of style in an en echelon fault system, Balcones fault zone, Economic Geology Report of Investigations 71, 63 p. central Texas—geomorphologic and hydrologic Thornbury, W.D., 1962, Principles of geomorphology: New implications, in Abbott, P.L., and Woodruff, C.M., Jr., York, John Wiley and Sons, 617 p. eds., The Balcones escarpment, central Texas: Thornthwaite, C.W., 1952, Evapotranspiration in the Geological Society of America, p. 71–76. hydrologic cycle, in The physical and economic Kipp, G.K., Farrington, P.T., and Albach, M.J., 1993, Urban foundation of natural resources, v. II, The physical basis development on the Edwards aquifer recharge zone: of water supply and its principal uses: U.S. Congress, Edwards Underground Water District Report 93–09, House of Representatives, Committee on Interior and 40 p. Insular Affairs, p. 25–35. University of Texas, Bureau of Economic Geology, 1979, Maclay, R.W., and Small, T.A., 1976, Progress report on Geologic atlas of Texas, Seguin sheet: Austin, scale geology of the Edwards aquifer, San Antonio area, 1:250,000. Texas, and preliminary interpretation of borehole _____1981a, Geologic atlas of Texas, Austin sheet: Austin, geophysical and laboratory data on carbonate rocks: scale 1:250,000. U.S. Geological Survey Open-File Report 76–627, 65 p. _____1981b, Geologic atlas of Texas, Llano sheet: Austin, Rose, P.R., 1972, Edwards Group, surface and subsurface, scale 1:250,000. central Texas: Austin, University of Texas, Bureau of _____1983, Geologic atlas of Texas, San Antonio sheet: Economic Geology Report of Investigations 74, 198 p. Austin, scale 1:250,000. Sellards, E.H., Adkins, W.S., and Plummer, F.B., 1933, The William F. Guyton and Associates, 1979, Geohydrology of geology of Texas, v. 1, Stratigraphy: Austin, University Comal, San Marcos, and Hueco Springs: Texas of Texas, Bureau of Economic Geology Bulletin 3232, Department of Water-Resources Report 234, 85 p. 1,007 p. Young, Keith, 1986, The Pleistocene terra rossa of central Sellards, E.H., and Baker, C.L., 1934, The geology of Texas, Texas, in Abbott, P.L., and Woodruff, C.M., Jr., eds., v. 2, Structural and economic geology: Austin, The Balcones escarpment—geology, hydrology, University of Texas, Bureau of Economic Geology ecology and social development in central Texas: Bulletin 3401, 884 p. Geological Society of America, p. 63–70.

10 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Hays County, Texas U.S. OEPA RTI.1 E>'\l O F 11-IE I.NTERIO R PREPAR£0 IN COOPEAATlON 1\'llll TltE WATER-RESOURCES INVEsnGATIONS REPORT 95-4265 U.S. GEOLOGICAL SURVEY EDWARDS UNDERG ROUND \ \~ll:R DISTRICT 1 1 ~-d ~rologoc n~bd~:_oo~.l_".!..' ~&J~·~~=:::::.J'~r_;.~ ___ ol .... l !o"•oh-0.-... K... ~ ,....,

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EXPLANATION

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e.. ,..,.., , ,.,.,.. U . SG~• ' Sorm 1 ~ 1 .00D ...... g~o. MAP SHOWING HYD ROGEOLOGIC SUBDIVISIONS OF THE EDWARDS AQUIFER OUTCRO P, HAYS COUNTY, TEXAS uo..-.... J,.., ...... I.. !O M.. Pr.,.U.oo.t.... l' By John A. Hanson and Ted A. Small 1995 U.S. DEPARTME T OF THE INTERIOR PREPARED IN COOPERAT ION WITl-f lHE WATER-RESOUR ES INVESTIGATIONS REPORT 95-4265 U.S. GEOLOGICAL SURVEY EDWARDS U DERGROUND WATER DISTRICT Hydrog,,oiogic subdivisions of the EdwJrds JCjuifer outcrop - PLATE 1 Hanson, JA, and Smal l, T.A., 1995. Geolog1c lrameworK and hydrogeolOgic charactenst1cs ol the Edwards aqu1ler outcrop, Hays County, Tex3s

97"' 67'30"

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EXPLANATION

UPPE R CONFINING UNITS, 2 0 V GRAINSTONE MEMBER ,_ UNDIVIDED a: w .. ~ "'z !!, ::> :::; VI KIRSCHBERG EV APORITE :::> => a: "':::> AUSTIN GROUP "'=> 0 &l MEMBER 0 (!) 0 (!) [2 w z w <..) " a: 5;1 z <( 0 "'0 ,_ u: EAGLE FORD GROUP ,_ a:"' a: w VII DOLOM ITIC MEMBER w <( z 98002'30" w z a: <( a: 0 <..) s: s: (.) (.) 0 0 ~ a: w w a: a: BUDA LIMESTONE w VII I BASAL NODULAR MEMBER w w :;: Q. Q. Q. Q. g :::> 2. UPPER MEMBER OF THE GLEN ROSE DEL RIO CLAY ~ LIMESTONE (LOWER CONFINING UNIT)

~ I GEORGETOWN FORMATION 29"60' a: z HYDROGEOLOGIC SUBDIVISIONS I • VIII·· Scale 1:75 000 w 0 Modifi ed from Maclay and Small (1976) u. ;:: 0 2 3 MILE S 5 II CYCLIC AN D MAR INE ..:::> :::; MEMBERS, UNDIVIDED l Sl 0 "a: cr 0 ~PRIMA RY FAULT··U, upthrown; "'0 u. Ill LEACHED AND COLLAPSED D D, downthrovJn a: "0 2 MEMBERS, UNDIVIDED <( "'a: 0 s:

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Base modilred jr om U.S. Geologrcal Survey 1:24.000 quadrangles MAP SHOWING HYDROGEOLOGIC SUBDIVISIONS OF THE EDWARDS AQUIFER OUTCROP, HAYS COUNTY, TEXAS Umversal Transverse Mercator ProJection. zone 14 By John A. Hanson and Ted A. Small 1995