Ft44 Structure Map of the San Antonio Segment of the Edwards Aquifer and Balcones Fault Zone, South-Central Texas
(.p {f--, 168 Ft44 rf'(r, -
Miscellaneous Map No. 38
Structure Map of the San Antonio Segment of the Edwards Aquifer and Balcones Fault Zone, South-Central Texas: Structural Framework of a Major Limestone Aquifer: Kinney, Uvalde, Medina, Bexar, Comal, and Hays Counties
Edward W. Collins and Susan D. Hovorka
Oklahoma Geological Survey RARY
1997
Bureau of Economic Geology Noel Tyler, Director The University of Texas at Austin Austin, Texas 78713-8924 Contents Introduction ...... l Previous Work ...... l Methods ...... 2 Geologic Setting ...... 2 Structural Framework of the Aquifer ...... 5 Sununary ...... I 0 Acknowledgments ...... l 0 References ...... 11
Figures l. Regional setting and location of study area along the Balcones Fault Zone ...... l 2. Stratigraphy of the Edwards Group ...... 3 3. Structural, hydrologic, and facies setting of the Edwards aquifer ...... 4 4. Simplified structure of Edwards aquifer strata showing throw of faults ...... 6 5. North-south structural cross sections A-A' through E-E' of Edwards strata composing Edwards aquifer and recharge zone and of Edwards strata downdip of the freskaline water interface ...... 7 6. Schematic block diagram of relay ramp ...... 9 Introduction
Normal faults of the regional Balcones Fault 0 200mi Zone are the principal structural control on the I '1 I Edwards limestone aquifer and recharge zone, 0 300km currently the sole source aquifer of San Antonio, Texas. The San Antonio segment of the aquifer, extending over an area of -3,000 mi2 in Kinney, Uvalde, Medina, Bexar, Comal, and Hays Counties TEXAS (fig. 1 and map), is the main water resource for residential, agricultural, and industrial use in Bexar, Medina, and Uvalde Counties. Discharge from the aquifer in Comal and Hays Counties feeds springs that are tourist attractions and that have been shown to be critical habitats of endangered species. The map of the aquifer depicts the subsurface structure of the base of the Del Rio Formation (approximate top of the confined part of the aqui GULF OF MEXICO fer), the Edwards Group outcrop belt (aquifer recharge area and unconfined part of the aquifer), faults, large relay ramps, and the approximate interface between fresh and saline water. This 0Aa2130c report summarizes key elements of the aquifer's FIGURE I. Regional setting and location of study area structural framework and describes structural attri along the Balcones Fault Zone. Faults (solid dark lines) butes that affect aquifer recharge, ground-water delineating fault zones are schematic. flow, areal extent, and depth (Hovorka and others, 1995). Faults and large relay ramps control the structural position ofthe aquifer strata. Some large faults may act as barriers or partial barriers to subsurface structure maps of the Edwards aqui ground-water flow. Smaller faults and associated fer were published by K.lempt and others ( 1979) joints form local and regional ground-water and Maclay and Small ( 1986). We used an updated conduits. Because relay ramps are areas of greater subsurface data base and recent surface geologic stratal continuity and numerous smaller displace maps to illustrate the structure of the Balcones ment faults, they can be preferential pathways of Fault Zone and Edwards aquifer at a scale that ground-water flow. provides more detail than that of previous stud ies. We are thus able to make new interpretations on fault occurrence, throw, and geometry. Previous Work Many surface geologic maps of parts of the Edwards outcrop belt and adjacent areas were Our structure of the aquifer (map) builds on reviewed forth is study, such as regional 1:250,000- many previous studies of the structure and stra scale maps by Brown and others ( 1974), Proctor tigraphy of the Edwards aquifer, particularly the and others ( 197 4 ), and Waechter and others ( 1977). investigation of Edwards Group stratigraphy and More-detailed maps and interpretations that we facies by Rose ( 1972), the descriptions of lithol studied include those by George ( 1952), Holt ogies within the aquifer by Abbott (1973) and (1956), Bills (1957), King (1957), Noyes (1957), Maclay and Small ( 1986), and the hydrogeologic Whitney (1957), Arnow (1959), Reeves and Lee aquifer cross sections by Small ( 1986). Previous (1962), DeCook (1963), Cooper (1964), Abbott ( 1966, 1973), Grimshaw (1970, 1976), Newcomb geology of the Edwards Group on the west half of ( 1971 ), Shaw ( 197 4 ), Waddell ( 197 6), Grimshaw the map (west of Lake Medina) was compiled and Woodruff (1986), Waterreus (1992), Stein from 1 :250,000-scale maps by Brown and (1993), Small and Hanson (1994), and Stein and others (1974), Waechter and others (1977), and Ozuna ( 1995). Surface geologic maps, constructed Gustavson and Wermund ( 1985). The apparently at 1 :24,000 scale by the senior author, were also greater abundance of faults in the east part of the used (Baumgardner and Collins, 1991; Collins outcrop belt reflects the greater detail of source and others, 199la, b; Raney and Collins, 1991; maps compiled of that area. The areal boundaries Collins, l 992a-d, 1993b-h, 1994a-e, 1995a-j). of igneous plugs, based on their mapped geom etry in outcrop, might not represent their geometry in the subsurface. The location of subsurface Cre Methods taceous igneous rocks is based on work by Ewing (1990). Cross sections presented herein general Outcrop geology and subsurface interpretations ize the faults to near-vertical structures because were compiled on I: I 00,000-scale base maps of the small scale and vertical exaggeration of and then reduced to the published 1:250,000 scale the sections. (map). The subsurface structure on the top of the Edwards aquifer (base of the Del Rio For mation) was interpreted according to data from Geologic Setting about 1,000 wells, in addition to previously reported data on outcropping geologic units and Edwards Group strata compose three major thicknesses ofstrata overlying the Edwards Group. depositional belts that trend northwestward across The major source of data is the stratigraphic the San Antonio segment of the aquifer: Maverick interpretation of well logs collected from files of Basin, Devils River trend, and the San Marcos the Edwards Underground Water District (EUWD Platform (figs. 2 and 3 ). Person and Kainer [now the Edwards Aquifer Authority]) and the Formations (Rose, 1972) are found on the San surface casing division of the Texas Water De Marcos Platform, and the Devils River Formation velopment Board (TWDB). In addition, published (Rose, 1972) was deposited on the platform well log and stratigraphic data (Rose, 1972; Small, margin. The West Nueces, McKnight, and Salmon 1986; Schultz, 1992, 1993, 1994) and unpublished Peak Formations were deposited in Maverick well data (TWDB/EUWD files) were compiled. Basin (Smith, 1964). Each formation is made up Major structural discontinuities were identified of a characteristic suite of interbedded lithologies by means of I 00-ft contour intervals. The approx having distinctive hydrologic and petrophysical imate interface between fresh and saline water properties. The Kain er and Person Formations are is from Brown and others ( 1992) and Schultz characterized by high-frequency depositional (1993, 1994). cycles that include subtidal wackestones and The Edwards outcrop section of the map was packstones, grain-dominated packstones, grain compiled from various sources. Outcrop geology stones, subtidal gypsum (replaced by calcite), and shown on the east half of the map (east of Lake dolomitic tidal-flat wackestones and grainstones Medina) was compiled mostly from 1:24,000- (Hovorka and others, 1996). The Devils River scale maps by the senior author (Baumgardner and Formation was identified by Rose ( 1972) where Collins, 1991; Collins and others, 1991 a, b; Raney the thick wackestone that defines the base of the and Collins, 1991; Collins, 1992a-d, l 993b-h, Person Formation becomes difficult to identify in l 994a-e, 1995a-k). In addition, the surface geol outcrop. Core and log examinations show that the ogy of southern Hays County was modified from Devils River Formation is lithologically similar 1:250,000-scale mapping (Proctor and others, to the cyclic Kainer and Person Formations across 1974) and 1:24,000-scale mapping (Grimshaw, much of its mapped extent, becoming more 1976; Grimshaw and Woodruff, 1986). Outcrop lithologically homogeneous near the edge of
2 Maverick Basin Devils River trend San Marcos Platform
Buda Limestone
Del Rio Formation
Georgetown Formation
10 c .2 iii 9 E... 0 Salmon u.. Peak c 8 0 Formation ...Cl) ------.§ . ------a..CD iii Regional 7 E... dense member ------0------u..... CD > Grainstone 6b a: member
6a c .2 iii E... 0 u..... CD c 4 .i'ij -·-·-·-·-·-·-·-·-·--...... ------~ 3 West Nueces Formation Basal 2 nodular member
Basal transgressive
Glen Rose Formation Cyclic units defined for this study
Thick evaporites I~:~:~ j Abundant grainstones ~ --- - Sequence boundary ------Flooding surface 0Aa7667c
FIGURE 2. Stratigraphy of the Edwards Group. Formation names assigned to the Maverick Basin, the Devils River trend, and the San Marcos Platform from Rose (1972). Cyclic units from Hovorka and others (1994).
3 100° w sa· w I '""- , "
0 20 mi -29• N O 30 km
Basinal facies Devils River (grainstone) Platform tacies
• Outcrop • Outcrop • Outcrop /Fault
.Aquifer ~Aquifer
j Saline ~Saline Saline [< .•.. l ~:} < :I 0Aa7883c
FIGURE 3. Structural, hydrologic, and fades setting of the Edwards aquifer (from Hovorka and others, 1995). The interface between fresh and saline waters (Schultz, 1994) defines the downdlp extent of the freshwater aquifer. Three major depositional fades belts that trend oblique to the aquifer are (1) Maverick Basin, (2) Devils River trend, and (3) San Marcos Platform. Fault locations derived from Brown and others (1974), Proctor and others (1974), and Waechter and others (1977).
Maverick Basin (Hovorka and others, 1996). The Edwards strata within the San Marcos Platform Maverick Basin facies were deposited in subtidal, facies thicken from -500 ft at the outcrop belt to slightly deeper water environments (Hovorka -650 ft downdip in southern Bexar County and others, 1996). The West Nueces Formation is (Hovorka and others, 1996). In Medina and composed of low-porosity subtidal wackestones eastern Uvalde Counties, the Devils River Forma and packstones and some grainstones near the top. tion thickens from -500 ft at the outcrop belt to The overlying McKnight Formation was originally -800 ft in southwestern Medina County. The composed ofhighly cyclic subtidal dark, organic Maverick Basin facies in western Uvalde and rich, argillaceous limestones and subtidal gyp eastern Kinney Counties, 600 to 700 ft thick with sum. Gypsum has dissolved from the aquifer and in the outcrop belt, thickens to> 1,000 ft in south is preserved only in the saline part of the Edwards central Kinney County. The Edwards Group out Group. The Salmon Peak Formation is composed crop belt approximately defines the main recharge of a thick interval of massive, burrowed, miliolid area and unconfined part of the aquifer. The packstone that coarsens upward. subsurface Edwards Group represents most of
4 the confined aquifer. Georgetown limestones occurredduringthelateOligoceneorearlyMiocene overlying Edwards limestones on the San Marcos (Weeks, 1945). Platform compose ~60 ft of low-porosity strata The Edwards aquifer is prolific because it that are commonly grouped with the aquifer. Del combines high-matrix porosity, which allows the Rio clay overlies the aquifer strata (fig. 2). Beneath aquifer to store tremendous volumes of water the Edwards aquifer lie limestone and marl of the (>200,000,000 acre-ft) of water (Hovorka and Glen Rose Formation. The upper part of the Glen others, 1996), with well-developed fracture and Rose Formation has permeabilities generally lower karstic conduit systems (Veni, 1987) that allow than those of Edwards strata, and the upper strata rapid movement of water. The high porosity re of the Glen Rose are thought to define the base flects the depositional and diagenetic evolution of of the Edwards aquifer in this area (Maclay and the limestone and dolomite facies that constitute Small, 1986). the aquifer (Hovorka and others, 1996). The dia The aquifer and recharge zone lie within the genetic evolution of porosity in the aquifer has Balcones Fault Zone, a regional zone of normal been strongly influenced by Miocene uplift of the faults along the perimeter of the Gulf Coast Basin Edwards strata along the Balcones fault system (fig. 1 and map). The fault zone, one of the main and development of the freshwater aquifer (Ellis, structural features of Central Texas, extends from 1986; Deike, 1990). The permeability structure near Del Rio east-northeastward to San Antonio, of the aquifer is even more closely related to where the zone bends northward through New Balcones faulting because conduits and solution Braunfels, Austin, Georgetown, and Waco and features such as caves reflect interaction among continues toward Dallas (Murray, 1961; Ewing, fractures, variable solubility of the cyclic strata, 1991 ). Normal faults composing the zone are either and the evolving hydrologic system (Hovorka and more common or more pronounced between others, 1995). High hydraulic conductivity can Uvalde and Georgetown, an area that coincides be related to the proximity of wells to lineaments with the Balcones escarpment, a prominent fault (Alexander, 1990) and to fractured and faulted line scarp that is an area of large offset across the intervals in wells (Hovorka and othe~s, 1995). fault zone. The fault zone closely follows the trend Apertures of many fractures have been enlarged of the buried Paleozoic Ouachita fold and thrust by solution. Although caves and fractures make belt (Flawn and others, 1961 ). Balcones faults up only a small percentage of total aquifer marking the edge of the Texas Coastal Plain are a porosity, they have hydraulic conductivities about manifestation of gulfward extension, flexure, and two orders of magnitude higher than those of tilting along the perimeter of the Gulf of Mexico. unfractured, porous Edwards limestones (Hovorka Most displacement on the Balcones Fault Zone and others, 1995).
Structural Framework of the Aquifer
The structural position of Edwards aquifer strata eastern Kinney and western Uvalde Counties, the is controlled by normal faults and associated subcrop aquifer strata form a relatively narrow gentle folds (Collins, l 995k). Faults cut Edwards band -2.5 to -5 mi wide (figs. 4 and 5a; map). Strata throughout the aquifer, and some of the Larger displacement faults do not appear to be as larger faults partly bound the unconfined aquifer common within the aquifer in eastern Kinney and recharge area (figs. 4 and 5; map). In eastern County as they are toward the east (figs. 4 and 5), inney County, Edwards strata crop out across although subsurface data are sparser in Kinney ) the Edwards Plateau (north of the Balcones Fault County than in the eastern aquifer areas. In Zone) and the fault-zone area where the strata Medina, Bexar, Comal, and western Hays Coun gently dip a few degrees into the subsurface. In ties, the generally <10-mi-wide Edwards outcrop
5 K!NNev -- · - -TlivALoe· - · - · - -- · ------TMEolNA - - I c
N / °' / / ...... / -...... / ~ Fresh-5aline ' wat8f' interface Fault throw - 15 - 100ft (- 4.5 - 30 m) - 100 - 30011(- 30 - 00m) c· - 300 - 50011 (- 90 - 150m) - - 500 - >80011( - 150 - 240m) +-- Relay ramp 0 10mi H StruciuraJ high I I I 0 15km _.;:ll ...- Contour line; elevation m feet Contour Interval 50011 (150 m) I ::: =·u:=1 Edwards outcrop ~
FIGURE 4. Simplified structure of Edwards aquifer strata showing throw of faults. Structural cross sections A-A' through E-E' shown in figure 5. MCR • Medina County relay ramp; SAR • San Antonio relay ramp; SMR • San Marcos relay ramp. 0 (a) A A' (b) B B' North-northeast South-southwest Northwest Southeast
&~ SL-+~~4--+-+.... r--'Hii~~ -150 ft - 350 ft - 500 ft -12oft \ . • (- 46 m) (-106 m) (-150 m) - 250 ft (-36 m) -300 ft .(-76m) -220ft -SOOft • Fresh-saline (-90 m) (-150m) -400ft<- 67 ml. water interface (- 122 m) - 350 ft (- 106 m)
(c) C C' North-northwest South-southeast
-220ft • (- 67 m) - 500 ft • (-150 m) • - 200 ft - 100 ft (-30m (-61 m) • • -170 ft - 210 ft (- 52 m) (- 64 m) • - 520 ft (- 158 m) • - 350 ft (- 106 m) (d) D D' North-northwest South-southeast .·. ··:·.·.·.. · <:-:-
Edwards strata composing aquifer ~ and recharge zone - 170 ft Edwards strata downdip of aquifer -BOOft (-52m) • (- 244 m) • - 450 ft (- 137 m) - 500 ft Igneous intrusion - 120 ft (-150 m) • (- 36 m) • -100 ft Selected fault throw in feet •(-30 m) and (meters) (e) E E' ft m SL Sea level Northwest Southeast ~feoo ~oooft ~~(- 55 m) (- 49 m) * * * * 0 -340ft -850ft -120ft -100ft 0 8000m (- 104 m)(- 259 m) (- 36 m) (- 30 m) Vertical exaggeration 1Ox 0Aa9544c
FIGURE S. North-south structural cross sections A-A' through E-E' (a through e, respectively) of Edwards strata composing Edwards aquifer and recharge zone and of Edwards strata downdip of the fresh-saline water interface. Asterisks indicate faults that have> 100 ft of throw. Fault throw approximate. Fault dipsare simplified as near-vertical structures because of scale and vertical exaggeration of the sections. Lines of section shown in figure 4.
7 belt (unconfined aquifer and recharge area) is subsurface to determine whether these faults have locally absent because of erosion and faulting of listric geometries and sole out into shale with depth the strata (fig. I and map). From central Uvalde or whether they displace the sub-Mesozoic County and across Medina County, the confined unconformity and the underlying Ouachita rocks, aquifer's width ranges from -9 to -30 mi, and as reported by Ewing ( 1991 ). structural relief of the aquifer strata is as much Most faults within the aquifer study region strike as -1,850 ft (figs. 4 and 5b, c ). In Bexar County N40° to 70°E and dip southeastward, although some the confined aquifer's width is between -4 and faults dip northwestward. Some subsidiary faults -20 mi, narrowing toward the northeast, and faults strike northwestward, northward, and eastward. have caused> 1,400 ft of structural relief (figs. 4 The largest faults have throws in excess of 800 ft, and 5d). In Comal County the confined aquifer is although smaller displacement faults are more narrow, mostly
8 individual fault strands, and displacement de stresses in the hanging wall of normal faults. creases laterally toward the fault tips. Some of the Transfer cross faults having oblique slip may larger southeast-dipping faults are associated with also form. northwest-dipping antithetic faults that bound Within the Balcones Fault Zone, relay ramps narrow grabens -3,000 to -4,000 ft wide. South of different sizes may lie between closely spaced and southeast of the Edwards outcrop belt, many (< 1 mi) en echelon faults (Collins, 1993a), as well faults are inferred in the subsurface that gener as between widely spaced (-6 to -12 mi) large ally have map patterns similar to those of the displacement faults (Grimshaw, 1976; Grimshaw outcrop belt. and Woodruff, 1986; Collins, 1995k). Grimshaw Other features of the regional structural frame and Woodruff ( 1986) interpreted a 6-mi-wide left work of the Edwards aquifer are fault-related relay step (off northeast edge of map) of the outcrop ramps (figs. 4 and 6). Relay ramps, also called belt at San Marcos to be a northeast-dipping ramp transfer zones, are structures that can form be structure (San Marcos ramp) between large tween the tips oftwo en echelon normal faults dip displacement faults. In northwest San Antonio, the ping in the same direction (Larsen, 1988; Peacock fault zone and the Edwards outcrop belt have an and Sanderson, 1991, 1994). The ramp connects -8-mi right step of the largest displacement faults the hanging-wall and footwall blocks of the faults. and a southwest-dipping relay ramp (San Antonio Displacement from one fault is transferred across ramp) that has formed between the large faults the relay ramp to the other fault. Because relay (fig. 4). Stratal folding within the San Marcos and ramps are strained zones produced by shearing and San Antonio ramps may be enhanced because both block rotation during slip along the overlapping ramps lie along the gently dipping flanks of the faults (Larsen, 1988), they may be cut by addi broad, northwest-trending San Marcos Platform. tional subsidiary cross faults that have different In northern Medina County an -12-mi right step strikes and numerous fault and joint intersections. of the Edwards outcrop belt marks another relay Some of the cross faults within relay ramps are ramp (Medina County ramp). The Medina County probably release faults (Destro, 1995) that exhibit ramp (fig. 4) appears to be composed of at least normal dip slip and form to release the bending three smaller -4-mi-wide relay ramps~
FIGURE 6. Schematic: block diagram ofrelay ramp (modified from Peac:oc:k and Sanderson, 1994, and from Hovorka and others, 1995). Folded aquifer strata in ramp might be fractured and faulted and serve as highly transmissive pathways where offset on major faults bas otherwise reduced aquifer continuity.
9 Summary
Normal faults and related gentle folds of the outcrop belt in Comal, Bexar, and eastern Medina Balcones Fault Zone are the main structural control Counties have, however, determined that the fault on Edwards strata and associated limestones that zone consists ofseveral large 2- to 7-mi-wide fault compose the Edwards aquifer. The confined part blocks bounded by faults having throws between ofthe aquifer, as wide as-30 mi in Medina County, -100 and -850 ft. Smaller fault blocks and faults has as much as 1,850 ft of cumulative structural lie within the larger fault blocks. relief caused primarily by Balcones faulting. In Relay ramps represent areas within the fault zone contrast, in Comal County the confined aquifer is where aquifer continuity is relatively good because mostly Acknowledgments Data used for this report were collected in The map and text benefited from the helpful part during geologic and hydrologic investiga comments of reviewers L. Edwin Garner, tions of the Edwards aquifer for the Edwards Martin P. A. Jackson, Stephen E. Laubach, and Underground Water District under contract Robert E. Mace. Tucker F. Hentz was technical no. 93-17-FO, Alan R. Dutton, Principal Investi editor. Susan Krepps is responsible for the structure gator. Additional structural data discussed herein map graphics, under the supervision of Joel L. were collected during field mapping for the U.S. Lardon, Senior Computer Illustrator. Cartography Geological Survey STATEMAP program under of an early version of the map was done by contract no. 1434-94-A-1255, Jay A. Raney, Patrice A. Porter. Text illustrations were drawn by Principal Investigator. The views and conclu KerzaA. Prewitt, Jana S. Robinson, Maria Saenz, sions contained herein are those of the authors and David M. Stephens, under the supervision of and should not be interpreted as necessarily Richard L. Dillon, chief cartographer. Others representing official policies, either expressed contributing to the production of this report were or implied, of the United States government. Susan Lloyd, word processing and design, and Erika M. Boghici assisted with the data base and Bobby Duncan, editing. Lana Dieterich co digitized an early version of the structure map. ordinated production. 10 References Abbott, P. L., 1966, Geology of Canyon Dam area, Comal _____ l 993a, Fracture zones between overlapping County, Texas: University of Texas, Austin, Master's en echelon fault strands: outcrop analogs within the thesis, 146 p. Balcones Fault Zone, Central Texas: Gulf Coast _____ 1973, The Edwards Limestone in the Balcones Association of Geological Societies Transactions, v. 43, Fault Zone, South-Central Texas: The University of p. 77-85. Texas at Austin, Ph.D. dissertation, 122 p. l 993b, Geologic map of the Bat Cave Alexander, K. 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