Geologic Map of the Edwards Aquifer Recharge Zone, South-Central Texas

38'30" 23'00" 07'30" 52'00" 36'30" 21'00" 05'30" ° ° ° ° ° ° ° 30 30 30 29 29 29 29

ood Cottonw Creek

Clark and Small (1997), Hanson (1995), Small and Clark (2000), Hanson (1994), zone

Geology compiled and modified from Clark (2003), Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government For sale by U.S. Geological Survey Information Services CO 80225 Denver, Box 25286, Federal Center, 1-888-ASK-USGS ArcInfo coverages and a PDF for this map are available at http://pubs.usgs.gov

fault fault Small and others (1996), Stein Ozuna (1995).

Manuscript approved for publication December 28, 2004

S Balcones U SCIENTIFIC INVESTIGATIONS MAP 2873 SCIENTIFIC INVESTIGATIONS D.E., Ozuna, G.B., Cole, J.C., Clark, A.K., Small, T.A., and Morris, R.R., compilers, 2005, Geologic map of the Edwards aquifer recharge zone, south- central Texas: U.S. Geological Survey Scientific Investigations Map 2873, scale 1:200,000.

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I-1 U H S aquifer, San Antonio area, Texas, and preliminary interpretation of borehole geophysical and laboratory data on carbonate rocks: U.S. Geological Survey Open-File Report 76–627, 65 p. geochronology of igneous intrusions from Uvalde County, Texas—Defining a more precise eruption history for the southern Balcones Volcanic Province: U.S. Geological Survey Open-File Report 2004–1031, 33 p. Maverick basin and Devils River trend, Uvalde Real Counties, Texas: Arlington, Texas, University of Texas at Arlington M.A. thesis, 217 p. Maverick basin and Devils River trend, Uvalde Real Counties, Texas, C.L., ed., Stratigraphy and structure of the Maverick basin Devils River trend, Lower Cretaceous, southwest Texas—A field guide and related papers: South Texas Geological Society, p. 3–33. of Texas at Austin, Bureau Economic Geology Report Investigations 74, 198 p. characteristics of the Edwards aquifer outcrop, Medina County, Texas: U.S. Geological Survey Water-Resources Investigations Report 00–4195, 10 p., 1 sheet. characteristics of the Edwards aquifer outcrop, Comal County, Texas: U.S. Geological Survey Water-Resources Investigations Report 94–4117, 10 p., 1 sheet, scale 1:75,000. hydrogeologic characteristics of the Edwards aquifer outcrop (Barton Springs Segment), northeastern Hays and southwestern Travis Counties, Texas: U.S. Geological Survey Water-Resources Investigations Report 96–4306, 15 p., 1 sheet, scale 1:75,000. electromagnetic and magnetic survey of the Seco Creek area, Medina Uvalde Counties, Texas: U.S. Geological Survey Open-File Report 03–226, 47 p. Uvalde Counties, Texas: U.S. Geological Survey Open-File Report 02–49. Available only at URL http://pubs.usgs.gov/of/2002/ofr-02-0049. characteristics of the Edwards aquifer recharge zone, Bexar County, Texas: U.S. Geological Survey Water-Resources Investigations Report 95–4030, 8 p., 1 sheet, scale 1:75,000. Park 35 Circle, Austin, TX 78753. Accessed October 15, 2003. Hendricks, Leo, ed., Comanchean (Lower Cretaceous) stratigraphy and paleontology of Texas: Society Economic Paleontologists and Mineralogists, Permian Basin Section, Publication 67–8, p. 9–29. Kg Miggins, D.P., Blome, C.D., and Smith, D.V., 2004, Preliminary Miller, B.C., 1983, Physical stratigraphy and facies analysis, Lower Cretaceous Miller, B.C., 1984, Physical stratigraphy and facies analysis, Lower Cretaceous, Rose, P.R., 1972, Edwards Group, surface and subsurface, central Texas: University Small, T.A., and Clark, A.K., 2000, Geologic framework hydrogeologic Small, T.A., and Hanson, J.A., 1994, Geologic framework hydrogeologic Small, T.A., Hanson, J.A., and Hauwert, N.M., 1996, Geologic framework Smith, B.D., D.V., Hill, P.L., and Labson, V.F., 2003, Helicopter Smith, D.V., B.D., and Hill, P.L., 2002, Aeromagnetic survey of Medina Stein, W.G., and Ozuna, G.B., 1995, Geologic framework hydrogeologic Texas Commission on Environmental Quality, 2003, www.tceq.state.tx.us, 12100 Young, Keith, 1967, Comanche Series (Cretaceous), south-central Texas, Maclay, R.W., and Small, T.A., 1976, Progress report on the geology of Edwards

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REFERENCES CITED a S

ACKNOWLEDGMENTS

Trinity Group (upper part) U

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Alternating beds of yellowish-tan, medium-bedded limestone and

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Devine

mudstone to grainstone with thin solution zones and chert beds (Clark, 2003); thickness 20–60 ft (Miller, 1984). Clark (2003, p. 5) stated that the lower unit is equivalent to basal nodular member of Kainer Formation, San Marcos platform facies. Lower unit is a lower confining unit of the Edwards aquifer, except for areas that are higher in porosity and permeability due to solution enhancement of bedding planes. Formation thickness 140–320 ft n mudstone to grainstone. Also contains laminated pellet and packstone (Miller, 1983). Thin solution zones and chert beds, but no significant cavern development. Thickness about 60–80 ft confining unit, is gray, thick-bedded, burrowed, shell-fragment wackestone, packstone, and grainstone; thickness 120–260 ft. Identified in the field as a bearing, gray wackestone. Classified as having mostly nonfabric- selective porosity and low permeability, due to only minor cavern formation directly associated with fracture dissolution. Lower unit is composed of thin-bedded, Conformably underlies the West Nueces Formation in Maverick basin and the Kainer Formation in San Marcos platform, but unconformably underlies the Devils River Formation in trend (Miller, 1984). Lower confining unit of the Edwards aquifer in south-central Texas. argillaceous limestone with minor evaporite layers. Surface cavern development associated with faults and fractures some water production at evaporite beds have been noted, but are considered a rare occurrence. Field identification is commonly associated with (1) stair-step topography that forms through differential erosion of the alternating limestone and marl beds, (2) the presence of fossilized ripple marks and sparse casts of Protocardia texana Classified hydrologically as having mostly nonfabric-selective porosity and generally low permeability (Small Clark, 2000; 2003). Thickness 350–800 ft Displacement amount in feet Environmental Quality (2003) o

i 5

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Glen Rose Limestone, upper member (Lower Cretaceous) West Nueces Formation (Lower Cretaceous) Contact Fault Edwards aquifer recharge zone Cave or sink Spring

i D 235 ft This study was financially supported by the National Cooperative Geologic Texas: Austin, Texas, The University of Texas Ph.D. dissertation, 122 p. Edwards aquifer, Uvalde County, Texas: U.S. Geological Survey Water-Resources Investigations Report 03–4010, 17 p., 1 sheet. upper confining unit, and hydrogeologic characteristics of the Edwards aquifer, south-central Uvalde County, Texas: U.S. Geological Survey Water-Resources Investigations Report 97–4094, 11 p., 1 sheet, scale 1:75,000. quadrangle—Geologic framework of an urban-growth corridor along the Edwards aquifer, south-central Texas: University of Texas, Bureau Economic Geology Miscellaneous Map 39, 28 p., 1 sheet, scale 1:100,000. characteristics of the Edwards aquifer outcrop, Hays County, Texas: U.S. Geological Survey Water-Resources Investigations Report 95–4265, 10 p., 1 sheet, scale 1:75,000. nomenclature, southern Edwards Plateau, southwest Texas: Gulf Coast Association of Geological Societies Transactions, v. 14, p. 285–306. Quaternary Upper Cretaceous Lower Cretaceous Kkg Kwn Kgru Kmkl U

LAKE Mapping Program (NCGMP). We thank Dave Moore (USGS) and John Waugh (San Antonio Water System) for their critical reviews of the map and text. In addition, we appreciate the past efforts of San Antonio Water System, Edwards Aquifer Authority, Texas Bureau of Economic Geology, and other water-related governing agencies for their support and guidance of geologic mapping in south-central Texas. Abbott, P.L., 1973, The Edwards Limestone in the Balcones fault zone, south-central Clark, A.K., 2003, Geologic framework and hydrogeologic characteristics of the Clark, A.K., and Small, T.A., 1997, Geologic framework of the Edwards aquifer Collins, E.W., 2000, Geologic map of the New Braunfels, Texas, 30 Hanson, J.A., and Small, T.A., 1995, Geologic framework hydrogeologic Lozo, F.E., Jr., and Smith, C.I., 1964, Revision of Comanche Cretaceous stratigraphic ult MEDINA

Kplc 3

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Alluvium Upper confining units, undivided Navarro and Taylor Groups, undivided Austin Group Eagle Ford Group Buda Limestone Del Rio Clay Georgetown Formation Cyclic and marine member Leached and collapsed member Regional dense member Grainstone member Kirschberg evaporite member Dolomitic member Basal nodular member Upper member of the Glen Rose Limestone i e Toucasia

d d

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i AQUIFER RECHARGE ZONE, SOUTH-CENTRAL TEXAS GEOLOGIC MAP OF THE EDWARDS

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o ° MEDINA COUNTY —Mudstone to grainstone and chert-bearing, M H Maverick basin facies 99 99 Devils River trend facies

Hondo

—Dark, laminated, fissile mudstone identified by its t

—Brown to tan, thin-bedded mudstone, wackestone, l Charles D. Blome, Jason R. Faith, Diana E. Pedraza, George B. Ozuna, James C. Cole, Allan K. Clark, Ted A. Small, and Robert R. Charles D. Blome, Jason R. Faith, Diana E. Pedraza, George B. Ozuna, James C. Cole, Allan K. Clark, Ted

, shell-fragment wackestones and grainstones containing

f k

Kdr

Formal subdivision of the Devils River Formation has been lacking A helicopter electromagnetic survey flown in the Seco Creek area Thickness 540–670 ft in Medina and Uvalde Counties, of which e Exogyra texana

Kgru r

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mudstone to wackestone; minor lateral cavern development at surface and nonfabric-selective porosity. Identified in the field as gray nodular mudstone, containing black rotund bodies and Upper part of formation has extensive cavern development and abundant caprinids, monopleurids, and requieniids. Highly dissolutioned and brecciated, the middle part has vuggy porosity abundant chert, with numerous sinkholes and some cavern development. Solution-enlarged fractures are present in the relatively massive, nodular, burrowed mudstone near base of formation. Upper part of formation is classified as having fabric-selective porosity, which grades downward into more nonfabric-selective porosity near base. Small porosity and permeability are related to solution-enlarged fractures in lowermost part of formation. to date. Geologic maps of the Texas Bureau Economic Geology show the Devils River Formation subdivided into upper and lower units, whereas all USGS mapping to date (including this map) shows the Devils River Formation as a single undifferentiated unit. According to Clark (2003), most workers since Lozo and Smith (1964) have recognized that the Devils River Formation lacks sufficient marker beds to permit detailed or widespread subdivision. in Medina and Uvalde Counties (Smith others, 2003) clearly shows the Devils River as two separate units on basis of contrasts in resistivity. Clark (2003), in mapping Uvalde County, also demonstrated that the informal basal nodular member of Kainer Formation could be traced from the San Marcos platform into Maverick basin. Recent fieldwork by a number of authors (Clark, Blome, and Faith) now shows that at least the informal basal nodular and dolomitic members of the Kainer Formation may be laterally traceable from the San Marcos platform into Devils River trend and even into the deeper water Maverick basin. This lateral continuity of the lowermost units was first observed by Rose (1972) and revisited Miller (1983). Nevertheless, local aquifer researchers have not to date accepted the informal subdivisions throughout Edwards Group. Hydrostratigraphic subdivision of the lower part Devils River Formation in Medina and Uvalde Counties mapping of Maverick basin lithologies in Kinney County are ongoing. Clark (2003, p. 4, 10) believed the lower 20–70 ft to be equivalent the basal nodular hydrostratigraphic member of Kainer Formation, San Marcos Platform facies Peak is typified by grainstone grading downward to light-gray, fossiliferous mudstone; about 75 ft thick. Minor karst and limited fracture enlargement classifies this unit as having both fabric- and nonfabric-selective porosity, except for areas of high dissolution. Lower part of the Salmon Peak is a thick, chert-bearing, massive lime mudstone to grainstone; average thickness 310 ft. Lower part has mostly nonfabric-selective porosity associated with minor karst development and solution along fractures. Total thickness averages 385 ft packstone, and grainstone. Most porous permeable unit within the McKnight Formation, with collapse breccia, no cavern development, and very high, mostly fabric-selective porosity where dissolution of evaporite layers occurs. Contact between the upper and middle units of the McKnight is gradational, just as upper unit conformable with the overlying Salmon Peak Formation. A horizon of bored rip-up clasts described as a conformable “conglomeratic zone” by Lozo and Smith (1964) underlies the Salmon Peak Formation and is considered part of the upper McKnight (Clark, 2003). Thickness 100–160 ft petroliferous odor and vegetative band on aerial photographs. Very low permeability, no notable cavern development, and classified as having mostly nonfabric-selective porosity. Considered a confining unit by Clark (2003, p. 5). Thickness about 40 ft and and is locally water bearing through dissolution along bedding planes (Stein and Ozuna, 1995; Clark, 2003). Thickness 20–70 ft miliolid rudists and chert. Middle of formation consists recrystallized brecciated mudstones that grade downward into alternating beds of vuggy spar and chert-bearing wackestone grainstone. Lower 120–250 ft contains sparry limestone and nodular, burrowed mudstone to wackestone, with gastropods, directly related to faults, fractures, and bedding planes; thus, considered nonfabric-selective porosity except where solution along bedding planes yields water (Stein and Ozuna, 1995; Small Clark, 2000). Thickness 110–140 ft crystalline limestone. Massive-bedded dolomitic member weathers light gray in outcrop and has abundant e

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CORRELATION OF MAP UNITS 9

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MAVERICK BASIN i lR

o a n i b a S k B e e r C co from mudstone to formation is fossiliferous; typified by rudistid-rich mudstones and wackestones that grade into intertidal and supratidal dolomitic mudstones with evaporites and groups include oysters and gastropods (Rose, 1972; Abbott, 1973). Chert occurs throughout unit in varying amounts and is locally abundant (Collins, 2000). The Kainer’s limestone and dolomitic limestone represent cyclic subtidal to tidal-flat depositional environments (Rose, 1972; Abbott, 1973). Thickness 250 to more than 300 ft mudstone to wackestone. Crossbedding and ripple marks occur in grainstone; cavern development is rare to nonexistent throughout. Unit is classified as having nonfabric-selective porosity and low permeability due to recrystallization (Stein and Ozuna, 1995; Small Clark, 2000). Thickness 50–60 ft chalky mudstone, and chert; fossils uncommon. Identified by boxwork voids with neospar and travertine framing. Extensive cavern development throughout unit makes the Kirschberg one of most porous (majority fabric selective) and permeable members of the Edwards aquifer (Stein and Ozuna, 1995). Average thickness 50–60 ft Edwards aquifer. Reddish-brown and gray to light-tan, marly limestone with biomicritic texture; commonly contains the brachiopod wacoensis washitaensis covered by vegetation and soil in some areas. The Georgetown is considered an upper confining unit, has very low porosity and permeability, and has little or no karstification cavern development (Stein and Ozuna, 1995). Thickness 2–20 ft; generally thins from northeast to southwest grainstone, and crystalline limestone. Also contains collapsed breccia, dolomitized biomicrite, burrowed mudstone, and stromatolitic limestone. Chert is locally abundant and common fossils include pelecypods, gastropods, and rudistids (Collins, 2000). The Person’s limestone, dolomitic and dolomite reflect shallow subtidal to tidal-flat cyclic depositional environments (Rose, 1972; Abbott, 1973). Thickness 170–180 ft miliolid scattered hydrologically because of the large number subsurface caverns associated with incipient karstification. Very permeable laterally extensive, fabric and nonfabric-selective porosity (Small Hanson, 1994; Stein and Ozuna, 1995). Thickness 10–100 ft grainstone, with chert, extensive collapsed breccia, and isolated stromatolitic limestone. Identified in the field by bioturbated iron- stained beds separated by massive limestone beds, and presence of the fossil coral San Marcos platform facies. Classified as having nonfabric-selective porosity and very high permeability rates (Small Hanson, 1994). Thickness 70–100 ft susceptible to erosion within the Edwards Group; also considered a vertical barrier to flow throughout the Edwards aquifer. Small and Clark (2000, p. 4) noted the occurrence of wispy iron-oxide stains for field identification purposes. Very few caverns have been found, but vertical fracture enlargement does occur locally in the nonfabric- selective, low-permeability unit (Small and Hanson, 1994). Thickness 16–24 ft an Intrusive igneous rocks, undifferentiated Upper confining units, undivided Austin Group Eagle Ford Group Buda Limestone Del Rio Clay Georgetown Formation Salmon Peak Formation Upper unit of the McKnight Formation Middle unit of the McKnight Formation Lower unit of the McKnight Formation West Nueces Formation Upper member of the Glen Rose Limestone Upper member of the Glen Rose Limestone Bl Grainstone member Kirschberg evaporite member Cyclic and marine member Leached and collapsed member Regional dense member LOWER CRETACEOUS STRATA AND EDWARDS GROUP Georgetown Formation (Lower Cretaceous) Person Formation (Lower Cretaceous) Kainer Formation (Lower Cretaceous) Ka Ka Ki

Ka r Ku Kb Kg e Kef Kdr Ksa iv Kwn R Kmkl Kgru l Kmku a Kmkm Sabin The lithostratigraphy of the Edwards aquifer varies from northeast to southwest In the San Marcos platform, Person and Kainer Formations of Edwards Kg Kg Kkg Kplc Kkke Kprd Kpcm 27

due to the three fluctuating depositional environments: San Marcos platform, Devils River trend, and the Maverick basin. The Edwards Group of San Marcos platform was deposited in shallow to very marine waters and is divided into seven hydrostratigraphic units, the eighth unit being overlying Georgetown Formation. The San Marcos platform facies grades abruptly into the Devils River trend facies in northern Medina County. The is represented by two lithostratigraphic units: the Devils River Formation, historically considered indivisible, and the overlying Georgetown Formation. The lateral transition from the Devils River Formation to deeper water West Nueces, McKnight, and Salmon Peak Formations of the Maverick basin facies is highly gradational and variable along their assumed boundaries (Clark, 2003) Group were divided into seven informal lithostratigraphic units by Rose (1972). These units, in addition to the Georgetown Formation, were further modified by Maclay and Small (1976) into eight informal hydrostratigraphic members that formed the Edwards aquifer strata 1 H S Group

Edwards co Creek Ki Little Blan (upper part)

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UPPER CRETACEOUS STRATA

DESCRIPTION OF MAP UNITS US Ksa

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normally found around rivers and streams; inundated regularly. Gravel mostly contains limestone and minor chert. Undivided terrace deposits included within unit clay. Pebbles and cobbles in the Leona are predominantly limestone with some chert. Coarser gravels are present near base of formation; silt increases up-section. The Leona is locally a prolific ground-water source and has rare cavern development variable (low to high) porosity due to the poorly sorted gravels. Locally, silty and clayey cement significantly reduces the permeability. In general, formation is thickest near stream channels or older abandoned meander channels. Thickness a few feet to 80 ft coarse-grained ultramafic and hypabyssal rocks that occur as surface exposures or in subsurface as dikes, plugs, and shallow intrusions. Ultramafic field is centered in Uvalde County and extends to the west of map area into Kinney County. Igneous rocks are represented by five rock types—alkali basalt, melilite-olivine nephelinite, olivine nephelinite, nepheline basinite, and phonolite (Miggins others, 2004). others, 2002) revealed more than 200 shallow igneous intrusive bodies, whereas fewer than 70 have been mapped. An a geochronologic study (Miggins and others, 2004) showed at least two distinct phases of magmatic activity in Uvalde County. First phase intrusive activity occurred approximately 82–80 m.y. ago, whereas younger intrusive rocks (phonolites) were emplaced 74–72 m.y. ago mapping of much the recharge zone concentrated on Edwards Group, the main aquifer-bearing unit of Edwards aquifer system, and, in some areas, the upper confining units were consolidated. Areas mapped as upper confining unit include any or all of the following map units: (Buda Limestone), and similarity makes the Navarro and Taylor difficult to differentiate in many areas. Mapped only in the San Marcos platform, where unit contains gray to brown clay and marly limestone. Very low porosity and permeability, with no cavern development. The groups are sometimes considered confining units in the Edwards aquifer region (Hanson and Small, 1995). Thickness averages 600 ft Massive, gray to white, chalky marly, fossiliferous mudstone. Identified in the field as white, chalky limestone, commonly containing the fossil oyster as a confining unit; however, ground water is locally associated with fractures. Scarce cavern development and low porosity permeability. Thickness 130–350 ft argillaceous limestone identified in the field as thin flagstone with a petroliferous odor. Strata weather easily and form flat to gently rolling topography. Primary porosity has been lost, rendering very low permeability rates in the Eagle Ford Group. Known as lignite by local drillers. No cavern development or fossils are evident in this group. Thickness 30–150 ft throughout the Edwards aquifer region dense mudstone. Identified in the field as nodular, porcelaneous limestone with calcite-filled veins and no common fossils. Limestone beds in the upper part of Buda are generally hard and dense may exhibit conchoidal fracturing and a porcelaneous texture when broken. Limestone beds in the lower part of Buda tend to be chalky (Collins, 2000). Regionally considered a confining unit. Unit has minor surface karst with low porosity and permeability (Small Hanson, 1994). Thickness 40–90 ft Edwards aquifer. Small and Hanson (1994, p. 6) described it as a blue- green to yellow-brown, variably gypsiferous clay containing iron nodules, abundant pectin-type fossil clams, and the oyster Ilymatogyra arietina Minor, thin lenticular beds of highly calcareous siltstone may also occur. Unweathered Del Rio Clay is composed of kaolinite, illite, and lesser amounts of montmorillonite (Collins, 2000). Secondary gypsum occurs as fracture fillings in clay-rich exposures near igneous bodies (Clark, 2003). The Del Rio has no recognized cavern development and no significant porosity or permeability. Directly overlies the Lower Cretaceous formations in many areas. Thickness about 40–110 ft

Alluvium (Quaternary) Leona Formation (Quaternary) Igneous intrusive rocks, undifferentiated (Upper Cretaceous) Upper confining units, undivided (Upper Cretaceous) Navarro and Taylor Groups, undivided (Upper Cretaceous) Austin Group (Upper Cretaceous) Eagle Ford Group (Upper Cretaceous) Buda Limestone (Upper Cretaceous) Del Rio Clay (Upper Cretaceous) o

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aquifer recharge zone, upgradient of the regional drainage, State Texas has defined the contributing zone as “the area or watershed where runoff from precipitation ﬂows downgradient to the recharge zone of Edwards aquifer” (Texas Commission on Environmental Quality, 2003). The jurisdictional extent of the Edwards aquifer recharge zone as identified and provided online by the TCEQ was slightly modified for illustrative purposes in the current compilation map. o S

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Efforts by a National Cooperative Geologic Mapping Program project to compile Geologic maps of previous workers typically show a vague, dashed boundary The Edwards aquifer serves the domestic, industrial, and agricultural needs of The complex geology of the recharge area includes lithologic units assignable to The Edwards aquifer is structurally complex due to the numerous high-angle Geologic mapping of the Edwards recharge area by U.S. Geological Survey has Geologic maps of previous workers typically showed a straight, dashed, northwest- The State of Texas officially defined the recharge area and contributing zones s e W the geology of Edwards aquifer recharge area in south-central Texas have helped to refine the hydrostratigraphy of one most permeable and productive carbonate aquifers in the United States. The complex geology of recharge area includes lithologic units assignable to the Lower Cretaceous Edwards Group and related and the Georgetown Formation, which is underlain by Lower Cretaceous Glen Rose Limestone (lower confining unit) and overlain by the Upper Cretaceous Del Rio Clay, Buda Limestone, and Eagle Ford, Austin, Taylor, Navarro Groups (upper confining units). The rock units constituting the Edwards aquifer in northeastern part of recharge area (San Marcos platform facies) include the Kainer, Person, and Georgetown Formations. The Kainer and Person Formations are subdivided into seven informal hydrostratigraphic members. The central part of the recharge area (Devils River trend facies) contains reefal facies lithologies of the Lower Cretaceous Devils River Formation and the overlying Georgetown Formation. The western part of recharge area contains Edwards Group lithologies of deeper water origin (Maverick basin facies) composed of the West Nueces, McKnight, Salmon Peak, and Georgetown Formations. separating San Marcos platform and Devils River trend exposures because the River Formation was historically interpreted as discontinuous and lithologically variable. However, current field studies of the San Marcos platform–Devils River trend transition area reveal that the facies boundary is distinct and follows topographic expression in some areas. We believe that future work will show of the hydrostratigraphic units belonging to the Kainer Formation, particularly informal basal nodular and dolomitic members, occur in the Devils River trend and deeper water Maverick basin units. Hydrostratigraphic subdivision of the lower part Devils River Formation in Medina and Uvalde Counties mapping of Maverick basin lithologies in Kinney County are ongoing. about 2 million people and is the primary source of water for San Antonio, Texas, Nation’s eighth largest city. Northeast of San Antonio, along the Balcones fault zone, the Edwards aquifer discharges from major artesian springs and provides unique habitats for at least eight endangered species. Ground-water development, especially near San Antonio, has significantly diminished spring flow during recent periods of below-normal rainfall. the Lower Cretaceous Edwards Group and related units Georgetown Formation, which is underlain by the Lower Cretaceous Glen Rose Limestone (lower confining unit) and overlain by the Upper Cretaceous Del Rio Clay, Buda Limestone, and Eagle Ford, Austin, Taylor, Navarro Groups (upper confining units). The rock units constituting the Edwards aquifer in northeastern part of recharge area (San Marcos platform facies) include the Kainer, Person, and Georgetown Formations. The Kainer and Person Formations of the Edwards Group were divided into seven informal lithostratigraphic units by Rose (1972). These were later categorized into seven informal hydrostratigraphic members with the overlying Georgetown Formation considered the eighth member of Edwards aquifer (Maclay and Small, 1976). The central part of the recharge area (Devils River trend facies) contains reefal facies lithologies of the Lower Cretaceous Devils River Formation and overlying Georgetown Formation. The western part of the recharge area contains Edwards Group lithologies of deeper water origin (Maverick basin facies) composed the West Nueces, McKnight, Salmon Peak, and Georgetown Formations (Clark, 2003). normal faults of the Balcones fault zone, which trend, on average, to northeast, are downthrown to the southeast, and are linked by scattered cross-faults. This network of overlapping faults has a significant impact on the geohydrology of Edwards aquifer. The relationship between recharge, ground-water flow paths, fault characteristics, and productivity/sustainability of the aquifer must be better understood if regional planners are to meet future water needs of south-central Texas. been conducted at 1:24,000 scale and compiled for each county. This map compilation was constructed from the original mapping by Clark and Small (1997), Clark (2003), Hanson and Small (1995), Printed on recycled paper Base modified from U.S. Geological Survey digital data (1:24,000-scale county boundaries and hydrography). Universal Transverse Mercator projection, zone 14, North American Datum 1983 38'30" 23'00" 07'30" 52'00" 36'30" 21'00" 05'30" ° ° ° ° ° ° ° 30 30 30 29 29 29 29