U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5149

Prepared in cooperation with the U.S. Army Corps of Engineers

Geologic Framework, Structure, and Hydrogeologic 

Characteristics of the Knippa Gap Area in Eastern Uvalde and 

Western Medina Counties, Texas
Front cover, Photograph showing outcrop of eroded and dissected extinct volcano located adjacent to the

Frio River and east of the city of Uvalde in Uvalde County, Texas. Basalt is remnant of volcanic neck, and 

adjacent to the basalt is pyroclastic material ranging from fine-grained material to volcanic bombs, which 

formed the slope of the volcano. Photograph taken January 12, 2012, by Robert R. Morris.

Back cover:

Top, Photograph showing sandstone ledges in the Escondido Formation located approximately 7 miles

south-southeast of the community of D’Hanis in Medina County, Texas. Photograph taken April 13, 2012, by 

Robert R. Morris.

Bottom, Photograph showing cave in fractured Austin Group rock located approximately 9.3 miles

northeast of city of Uvalde in Uvalde County, Texas. Photograph taken August 7, 2012, by Robert R. Morris.
Geologic Framework, Structure, and 

Hydrogeologic Characteristics of the 

Knippa Gap Area in Eastern Uvalde and 

Western Medina Counties, Texas

By Allan K. Clark, Diana E. Pedraza, and Robert R. Morris

Prepared in cooperation with the U.S. Army Corps of Engineers

Scientific Investigations Report 2013–5149

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

Suggested citation:

Clark, A.K., Pedraza, D.E., and Morris, R.R., 2013, Geologic framework, structure, and hydrogeologic characteristics 

of the Knippa Gap area in eastern Uvalde and western Medina Counties, Texas: U.S. Geological Survey Scientific 

Investigations Report 2013–5149, 35 p., 1 pl., http://pubs.usgs.gov/sir/2013/5149/. 

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iii

Acknowledgments

This study was completed on private ranchland, and the authors would like to extend their 

thanks to John Hoyt and others from the Edwards Aquifer Authority (EAA) for facilitating access 

to the study area. In addition, grateful appreciation is extended to the land owners for granting 

access to their land, which made completion of this investigation possible.

v

Contents

Acknowledgments .......................................................................................................................................iii

Abstract ..........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Purpose and Scope .............................................................................................................................3

Description of the Study Area ...........................................................................................................4

Previous Studies ..................................................................................................................................4

Methods of Investigation .............................................................................................................................8

Geologic Interpretation Methods ......................................................................................................8

Geodatabase Development and Data Management ...................................................................10

Geologic Framework, Structure, and Hydrogeologic Characteristics ...............................................11

Geologic Framework .........................................................................................................................11

Lower Cretaceous Series ........................................................................................................11

Upper Cretaceous Series ........................................................................................................12

Upper Cretaceous Series Igneous Rocks .............................................................................15

Eocene Rocks ............................................................................................................................15

Pliocene to Pleistocene Series Rocks ..................................................................................15

Structure ..............................................................................................................................................18

Hydrogeologic Cross-Section A–A’ .......................................................................................19

Hydrogeologic Cross-Section B–B’ .......................................................................................19

Hydrogeologic Characteristics ........................................................................................................19

Summary .......................................................................................................................................................23

References Cited .........................................................................................................................................24

Appendix 1. Wells Included in a Geodatabase of Geologic Framework, Structure, and 

Hydrogeologic Characteristics of the Knippa Gap Area in Eastern Uvalde 

and Western Medina Counties, Texas ...........................................................................29

Plate

[In pocket]

Plate 1. Geologic framework, structure, and hydrogeologic characteristics of the Knippa 

Gap area in eastern Uvalde and western Medina Counties, Texas 

Figures

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

2. Map showing regional depositional provinces, boundary of the Balcones Fault 

Zone, and the Balcones escarpment .......................................................................................3

3. Map showing physiographic provinces of Texas ...................................................................5
vi

4. Diagram showing summary of the lithologic and hydrogeologic characteristics 

of the stratigraphic units of the Devils River Trend in eastern Uvalde and western 

Medina Counties, Texas ..............................................................................................................6

5. Diagram showing summary of the lithologic and hydrogeologic characteristics 

of the stratigraphic units of the Maverick Basin in eastern Uvalde County, Texas ..........7

6. Aerial photograph showing a linear fault identified by the vegetation growing 

along its surface expression ......................................................................................................8

7. Aerial photograph showing circular bands of vegetation that correlate with 

isolated exposures of igneous rock in an area composed primarily of alluvium...............9

8. Photograph showing the abundance of Ilymatogyra arietina in the Del Rio Clay 

located in northeastern Uvalde County, Texas .....................................................................13

9. Photograph showing contact between Buda Limestone (hillside) and Del Rio 

Clay (bed of creek) located on the west side of bridge at tributary to Seco Creek 

in northwestern Medina County, Texas .................................................................................13

10. Photograph showing exposed Austin Group in road cut excavation located on 

south side of U.S. Highway 90 and approximately midway between Sabinal and 

Knippa Texas, eastern Uvalde County, Tex. ...........................................................................14

11. Photograph showing Inoceramus sp. fossil that is typical of the Austin Group 

located in the bed of the Leona River approximately 3 miles south-southeast of 

city of Uvalde in Uvalde County, Texas ..................................................................................14

12. Photograph showing sandstone ledges in the Escondido Formation located 

approximately 7 miles south-southeast of the community of D’Hanis in Medina 

County, Texas ..............................................................................................................................16

13. Photograph showing outcrop of eroded and dissected extinct volcano located 

adjacent to the Frio River and east of the city of Uvalde in Uvalde County, Texas .........16

14. Photograph showing columnar jointing common in igneous rocks located 

adjacent to the Frio River and east of the city of Uvalde in Uvalde County, Texas .........17

15. Photograph showing caliche-cemented Uvalde Gravel located approximately 

5.3 miles southeast of the town of D’Hanis in Medina County, Texas ...............................17

16. Diagram showing idealized view of the relay-ramp structure that dips to the 

northeast from the Uvalde Salient in Uvalde County, the relay-ramp structure 

that dips to the southwest from Medina County, and the inferred direction of 

groundwater flow paths towards the Knippa Gap structural low (trough) ......................18

17. Photograph showing channel porosity developed along a bedding plane in the 

Austin Group located near the intermittent Leona Springs 2 on the Leona River 

in Uvalde County, Texas ............................................................................................................21

18. Photograph showing cave in fractured Austin Group rock located approximately 

9.3 miles northeast of city of Uvalde in Uvalde County, Texas ...........................................21
vii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Concentrations of chemical constituents in water given in milligrams per liter (mg/L).

Geologic Framework, Structure, and Hydrogeologic 

Characteristics of the Knippa Gap Area in Eastern Uvalde 

and Western Medina Counties, Texas

By Allan K. Clark, Diana E. Pedraza, and Robert R. Morris

Abstract

The Edwards aquifer is the primary source of potable 

water for the San Antonio area in south-central Texas. The 

Knippa Gap was postulated to channel or restrict flow in the 

Edwards aquifer in eastern Uvalde County, and its existence 

was based on a series of numerical simulations of groundwater 

flow in the aquifer. To better understand the function of the 

area known as the Knippa Gap as it pertains to its geology 

and structure, the geologic framework, structure, and 

hydrogeologic characteristics of the area were evaluated by 

the U.S. Geological Survey in cooperation with the U.S. Army 

Corps of Engineers—Fort Worth District. 

The principal structural feature in the San Antonio area 

is the Balcones Fault Zone, which is the result of Miocene 

age faulting. In Medina County, the faulting of the Balcones 

Fault Zone has produced a relay-ramp structure that dips to 

the southwest from the Edwards aquifer recharge zone and 

extends westward and below land surface from Seco Creek. 

Groundwater flow paths in the Edwards aquifer are 

influenced by faulting and geologic structure. Some faults 

act as barriers to groundwater flow paths where the aquifer 

is offset by 50 percent or more and result in flow moving 

parallel to the fault. The effectiveness of a fault as a barrier 

to flow changes as the amount of fault displacement changes. 

The structurally complex area of the Balcones Fault Zone 

contains relay ramps, which form in extensional fault systems 

to allow for deformation changes along the fault block. In 

Medina County, the faulting of the Balcones Fault Zone has 

produced a relay-ramp structure that dips to the southwest 

from the Edwards aquifer recharge zone. Groundwater 

moving down the relay ramp in northern Medina County 

flows downgradient (downdip) to the structural low (trough) 

from the northeast to the southwest. In Uvalde County, the 

beds dip from a structural high known as the Uvalde Salient. 

This results in groundwater moving from the structural high 

and downgradient (dip) towards a structural low (trough) to 

the northeast. These two opposing structural dips result in a 

subsurface structural low (trough) locally referred to as the 

Knippa Gap. This trough is located in eastern Uvalde County 

beneath the towns of Knippa and Sabinal.

By using data that were compiled and collected 

for this study and previous studies, a revised map was 

constructed depicting the geologic framework, structure, 

and hydrogeologic characteristics of the Knippa Gap area 

in eastern Uvalde and western Medina Counties, Tex. The 

map also shows the interpreted structural dip directions and 

interpreted location of a structural low (trough) in the area 

known as the Knippa Gap. 

Introduction

The Edwards aquifer is the primary source of potable 

water for the San Antonio area (fig. 1) in south-central Texas 

(Edwards Aquifer Authority, 2013; Hovorka and others, 1996). 

San Antonio is currently the seventh largest city in the United 

States and second largest city in Texas (U.S. Census Bureau, 

2012a). The population of south-central Texas is projected to 

increase by 34 percent between 2010 and 2030 (Texas Water 

Development Board, 2010). Given this projected growth, the 

ability to effectively manage sources of potable water is a 

major concern for local and regional water-resource managers.

Computer simulations of the Edwards aquifer are among 

the tools local and regional water-resource managers use to 

manage sources of potable water in south-central Texas. The 

accuracy of computer simulations (Maclay and Land 1988; 

Kuniansky and Holligan, 1994; Lindgren and others, 2004; 

Lindgren, 2006) of the Edwards aquifer can be improved by 

incorporating more detailed information pertaining to the 

geology and geologic structure controlling groundwater 

flow paths of the system. This is particularly true for a part 

of the Edwards aquifer locally known as the Knippa Gap in 

the western part of the Edwards aquifer near Knippa, Tex. 

(fig. 1). 

The Knippa Gap was postulated by Maclay and Land 

(1988) to channel or restrict flow in the Edwards aquifer in 

eastern Uvalde County (fig. 1), and its existence was based on 

a series of numerical simulations of groundwater flow in the 

aquifer. Those numerical simulations were made in an attempt 

to synthesize the geologic, structural, and hydrogeologic 

characteristics of the aquifer (Maclay and Land, 1988). The 
Figure 1. Location of study area.

D'Hanis

Utopia

Laguna Del Rio

Brackettville

Uvalde

Sabinal

San Antonio

Knippa

Leakey

Rio Grande

Nueces River

Medina River

San Antonio

River

Nueces River

West Nueces

River

Nueces River

Leona

River

Dry Frio

River

Frio River

Seco

Creek

Sabin

a

l

River

Ho

ndo

Creek

Cibolo

Creek

Guadalupe River

Frio River

San Migue l Creek

EDWARDS

COUNTY

VAL VERDE COUNTY REAL COUNTY

KERR COUNTY

BANDERA

COUNTY

KINNEY COUNTY

MAVERICK COUNTY ZAVALA COUNTY

UVALDE COUNTYMEDINA COUNTY

FRIO COUNTY

DIMMIT COUNTY

LA SALLE COUNTY

McMULLEN COUNTY

ATASCOSA COUNTY

BEXAR COUNTY

BLANCO COUNTY

COMAL COUNTY

KARNES C

WILSON COUNTY

GONZALES COUNTY

Leona Springs 1

Leona Springs 2

29°30'29°00'

99°45' 99°00' 98°15'

Base modified from U.S. Geological Survey 1:250,000-scale digital dataUniversal Transverse Mercator projection, zone 14North American Datum of 1983

T E X A S

Study area

UVALDE

COUNTY

MEDINA

COUNTY

Trini

aqui

BALCONES FAULT ZONE

B

CO

EXPLANATION

Edwards aquifer (George and others, 2011) Outcrop SubcropStudy boundaryApproximate location of the Uvalde SalientSpring and name Leona Springs 1

0 10 20 30 40 MILES0 10 20 30 40 KILOMETERS
Introduction 3

Knippa Gap occurs near the boundary of the Devils River 

Trend and Maverick Basin depositional provinces (fig. 2). 

Maclay and Land (1988, p. A43) provide a detailed description 

of the Knippa Gap, referring to it as a “narrow opening 

within an extensive, complex barrier system.” Maclay and 

Land (1988, p. A43) go on to explain that groundwater flows 

through the Knippa Gap originating from what they referred to 

as “the western storage unit” and “the western Medina storage 

units,” historical terms referring to the San Antonio segment of 

the Edwards aquifer recharge zone (Maclay and Land, 1988). 

Maclay and Land (1988, p. A43) explain that the groundwater 

flows “southward and downdip toward the southernmost 

part of the [Edwards] aquifer in southeastern Uvalde and 

southwestern Medina Counties,” and that the Knippa Gap 

“occurs in the Devils River [Formation] and lies near the lesspermeable rocks of the Maverick Basin.”

To better understand the function of the area known as 

the Knippa Gap as it pertains to its geology and structure, 

the geologic framework, structure, and hydrogeologic 

characteristics of the area were evaluated by the U.S. 

Geological Survey (USGS) in cooperation with the U.S. Army 

Corps of Engineers (USACE) —Fort Worth District. 

Purpose and Scope

The purpose of this report is to describe the geologic 

framework, structure, and hydrogeologic characteristics of 

Figure 2. Regional depositional provinces, boundary of the Balcones Fault Zone, and the Balcones escarpment. Figure modified from 

Abbott and Woodruff, 1986.

Central

Texas

Platform

Ancestral Gulf Of Mexico

Present–day

Gulf Of Mexico

Stuart City Reef Ouachita Structural Belt

Devils River Trend

Maverick

Basin

San Marcos

Arch

Llano

Uplift

San Marcos

Platform

Tectonic Hinge Line

0 50 100 150 200 MILES

0 10050 150 200 KILOMETERS

Depositional province

 Devils River Trend

 Maverick Basin

 San Marcos Platform

Boundary of Balcones Fault Zone

Balcones escarpment

EXPLANATION

$
4 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

the Knippa Gap area of the Devils River Trend and Maverick 

Basin (fig. 2) in eastern Uvalde and western Medina Counties, 

Tex. This report includes a surficial geologic map developed 

on the basis of previous investigations and information 

obtained during field mapping. The map includes two 

hydrogeologic cross sections of the study area. A description 

of the geodatabase developed to manage project data and 

spatially relate the data is included in the report. 

Description of the Study Area

The study area (fig. 1) is in eastern Uvalde and western 

Medina Counties, south-central Texas, and is located at the 

northern boundary of the Coastal Plain physiographic province 

(fig. 3) (Fenneman, 1931). The northern boundary of the study 

area coincides generally with the surficial exposure of the 

Edwards aquifer (fig. 1) outcrop in the Balcones Fault Zone 

(fig. 2). The Balcones Fault Zone crosses through the study 

area in a series of parallel to subparallel, vertical to nearly 

vertical, en echelon, normal faults that are predominantly 

downthrown to the south and southeast (Sellards and others, 

1932). The Balcones Fault Zone lies on the boundary between 

the western part of the Coastal Plain section of the Coastal 

Plain physiographic province and the Edwards Plateau section 

of the Great Plains physiographic province (fig. 3) (Fenneman 

and Johnson, 1946).

The most prominent topographic feature of the Balcones 

Fault Zone is the Balcones escarpment, a topographic 

escarpment in south-central Texas (Sellards, 1919). Based on 

data obtained for this study, change in land-surface altitude 

across the Balcones Fault Zone is approximately 500 feet (ft); 

ranging from an altitude of approximately 1,150 ft at the inner 

edge of the western part of the Coastal Plain to an altitude 

of approximately 1,650 ft near the southern margin of the 

Edwards Plateau. The land surface in the study area is nearly 

horizontal to gently sloping with elevated areas separated by 

broad, level valleys, and each of the valleys are incised by 

streams (Stevens and Richmond, 1970; Dittmar and others, 

1977).

The rocks within the study area are sedimentary and 

igneous and range in age from the Lower Cretaceous to the 

Pleistocene. The sedimentary rocks are limestone, chalk, 

gravel, sand, silt, and clay. The igneous rocks are ultramafic 

to intermediate in composition. The formations present in 

the study area from bottom to top are: Glen Rose Limestone, 

West Nueces Formation, McKnight Formation, Salmon Peak 

Formation, Devils River Formation, Del Rio Clay, Buda 

Limestone, Eagle Ford Group, Austin Group, Anacacho 

Limestone, Escondido Formation, Indio Formation, Uvalde 

Gravel, and Leona Formation (figs. 4–5). The specific 

formation present in the study area varies east to west from 

the Devils River Trend to the Maverick Basin depositional 

province (figs. 4–5). In the study area, the Edwards aquifer 

is formed within the Devils River Formation, Salmon Peak 

Formation, McKnight Formation, and West Nueces Formation.

Previous Studies

Numerous reports have been published pertaining to 

the geology and groundwater resources of Uvalde and 

Medina Counties, Tex. Shumard (1860) named the Austin 

Formation for outcrops in Travis County, Tex., which 

subsequently redefined the stratigraphic location by White 

and Hill (1887). The Escondido Formation was first named 

and defined by Dumble (1892). Hill and Vaughn (1898) 

named and described the extent of the Edward Limestone 

for the Edwards Plateau, Nueces and Uvalde quadrangles, 

southwestern Texas of which it is the chief component of the 

scarps and mesas. Hill and Vaughn (1898) also named the 

Del Rio Clay and described its location for outcrops near Del 

Rio, Tex., and named and described the Anacacho Formation. 

Vaughan credited Hill with defining and naming the Buda 

Limestone for outcrops near Buda, Tex., and Eagle Ford 

Formation for outcrops near the old community of Eagle 

Ford, Dallas County, Tex. (Vaughan, 1900b). The Devils 

River Formation was named by Udden (1907) for the 

limestone between the Glen Rose Limestone and the Del 

Rio Clay. Liddle (1918) was the first to discuss the geology 

and mineral resources of Medina County, Tex. The Indio 

Formation was designated by Trowbridge (1923) for 

outcrops on the Indio Ranch, Maverick and Dimmit Counties, 

Tex. Sayre (1936) separated the Georgetown Formation 

from the Edwards Limestone, documenting that they were 

distinct formations composed primarily of limestone. The 

Austin Chalk was assigned Group status by Wilmarth (1938). 

Lozo and Smith (1964) substantially revised the stratigraphic 

nomenclature of the various geologic units associated with the 

Edwards Limestone, resulting in an improved understanding 

of the geologic structure. The surface geology in the Knippa 

area was mapped by Barnes (1974); Barnes also redefined 

the Eagle Ford Group, separating the Boquillas Formation 

(not in study area) from the Eagle Ford Group (Barnes, 

1977). The transition zone from the Maverick Basin to the 

Devils River Trend was mapped by Miller (1983) who also 

mapped the basal nodular member of the Devils River and 

West Nueces Formations and subdivided the McKnight 

Formation into lower, middle, and upper members in the 

north-central part of Uvalde County. Clark and Small (1997) 

and Clark (2003) investigated the geologic framework and 

hydrogeologic characteristics of the Edwards aquifer in 

Uvalde County, Tex.

Although sedimentary rocks are most common in the 

study area, the existence of igneous rocks has been known 

for more than 100 years (Vaughan, 1900a). Lonsdale (1927) 

completed field and laboratory analyses of the igneous rocks 

outcropping in central Texas and published thicknesses of 

formations within Uvalde County, Tex. Smith and others 

(2003) published an aeromagnetic survey of Medina and 

Uvalde Counties to delineate subsurface igneous bodies. The 

igneous provenance was delineated and briefly explained by 

Smith and others (2008).
Figure 3. Physiographic provinces of Texas. 

36°00'

31°30'

27°00'

94°30'

99°00'

103°30'

GREAT PLAINS

COASTAL PLAIN

Edwards Plateau

UVALDE 

COUNTY

MEDINA

COUNTY

BEXAR

COUNTY GULF OF MEXICO

EXPLANATION

Physiographic prov and Johnson, 19 Great Plains Coastal PlainPhysiographic sect

Edwards Plateau 

Study boundaryBalcones Fault Zon

0 50 100 MIL0 50 100 KILOMETERS

Base modified from U.S. Geological Survey 1:7,000,000-scale digital dataPhysiographic regions data from Albers Conic Equal area projectionNorth American Datum of 1983

Study area
Figure 4. Summary of the lithologic and hydrogeologic characteristics of the stratigraphic units of the Devils River Trend in eastern Uvalde and west[Groups and formations modified from Dumble (1892), Hill and Vaughn (1898), Hill (1901), Udden (1907), Wilmarth (1938), Lozo and Smith (1964), Humphreys (1984), Miller lithology modified from Dunham (1962); and porosity type modified from Choquette and Pray (1970)] Se- ries Group/formation Thickness (feet) Lithology Field identification Hydrologic unit Hydrologic function Cavern development 

Pleistocene Leona Formation 0–120 Red-reddish gray silt, fine gravels Unconsolidated gravels in and adjacent to streambeds Leona aquifer Aquifer None Pliocene Uvalde Gravel 0–20 Loose pebbles, cobbles, gravels. Gravels often cemented with caliche Large gravels in areas of higher relief often cemented with caliche None None None Upper Cretaceous

Navarro 

Group Escondido Formation 200–875 Fine-grained sandstone, with interbedded shale, clay, and pyroclastic material Brown, fine-grained sandstone, locally fossiliferous

Upper confining unit to the Edwards aquifer

Confining unit None 

Taylor 

Group Anacacho Limestone 188 Massive mudstone to packstone, with interbedded bentonitic clay White to gray packstone, with thick sequences of bentonitic clays Confining unit None 

Austin Group 246 Massive, chalky to marly, fossiliferous mudstone White, chalky limestone; Gryphaea aucella, Inoceramus sp. Confining unit; aquifer where connected to Edwards by faults/ fractures Minor along fracture/ faults 

Eagle Ford Group 71 Brown, flaggy, sandy shale and argillaceous limestone

Thin brown flagstones; 

petroliferous, fossil fragments and fossils of Inoceramus sp.

Confining unit None

Washita Group

Buda Limestone 72 Buff to light-gray, dense mudstone 

Porcelaneous limestone, 

nodular, iron nodules, iron staining, iron replacement of fossils

Confining unit Minor surface karst 

Del Rio Clay 70 Blue-green to yellow-brown clay Fossiliferous; Ilymatogyra arietina Confining unit None 

Lower Cretaceous

Devils River Formation Undivided 270 Upper

Light-grey wackestone, 

packstone, miliolid grainstone, rudist boundstone, shell fragments, chert nodules and beds

Most fossils (caprinids, 

monopleurids, and requieniids) near top. Highly dissolutioned and brecciated, with vuggy porosity and most chert near middle. Relatively massive and burrowed, with little chert near base.

Edwards aquifer Aquifer

Most extensive near top 

with some sinkhole development near middle and some cavern development associated with solutionally enlarged fractures near base

Fredricksburg Group 260 Lower Miliolid, shell fragmented, mudstone to grainstone, chert; mudstone to grainstone burrowed Basal nodular 20–70 Nodular, burrowed mudstone to wackestone miliolids, gastropods, and Exogyra texana Nodular gray mudstone, black rotund bodies Confining unit; aquifer where solutionally enhanced Minor, primarily near contact with Glen Rose Limestone 

Trinity 

Group Glen Rose Limestone 900–1,530 Tan, alternating layers of medium- bedded limestone and argillaceous limestone. Stairstep topography, tan mudstones, marly, thin bedded, Tylostoma sp., Turritella sp., Protocardia 

texana

Trinity aquifer, 

lower 

confining 

unit to the 

Edwards 

aquifer

Confining unit to 

Edwards aquifer; aquifer where evaporite beds present

Minor, related to 

fractures, faults, and bedding plane
Figure 5. Summary of the lithologic and hydrogeologic characteristics of the stratigraphic units of the Maverick Basin in eastern Uvalde County, Texa[Groups and formations modified from Hill and Vaughn (1898), Hill (1901), Wilmarth (1938), Greenwood (1956), Lozo and Smith (1964), Barnes (1977), Humphreys (1984), and from Dunham (1962); and porosity type modified from Choquette and Pray (1970); *, not present in study area]

Series Group/formation Thickness (feet) Lithology Field identification Hydro- logic unit Hydrologic function Cavern development 

Pleistocene Leona Formation 0–120 Red-reddish gray silt, fine gravels Unconsolidated gravels in and adjacent to streambeds Leona aquifer Aquifer None Pliocene Uvalde Gravel 0–20 Loose pebbles, cobbles and gravels often cemented with caliche Large gravels in areas of higher relief often cemented with caliche None None None Eocene

Wilcox 

Group Indio Formation 150–200 Lenticular, thin-clay bed, gray to buff sandstone, sandy shale Thin beds of lignite and bog iron ore None None None 

Upper Cretaceous

Navarro 

Group Escondido Formation 200–875 Fine-grained sandstone, with interbedded shale, clay, and 

pyroclastic material

Brown, fine-grained sandstone, locally 

fossiliferous

Upper confining unit to the Edwards aquiferConfining unit None 

Taylor 

Group Anacacho Limestone 188 Massive mudstone to packstone, with interbedded bentonitic clay White to gray packstone, with thick sequences of bentonitic clays Confining unit None 

Austin Group 246 Massive, chalky to marly, fossiliferous mudstone White, chalky limestone, with locally abundant Gryphaea aucella, Inoceramus sp. Confining unit; aquifer where connected to Edwards by faults/ fractures Minor along fracture

faults 

Eagle Ford Group 71 Brown, flaggy, sandy shale and argillaceous limestone Thin flagstones; petroliferous, fossil fragments and fossils of Inoceramus sp. Confining unit None 

Washita Group

Buda Limestone 72 Buff to light-gray, dense mudstone Porcelaneous limestone, iron nodules, iron staining, iron replacement of fossils Confining unit Minor surface karst Del Rio Clay 70 Blue-green to yellow-brown clay Fossiliferous; Ilymatogyra arietina Confining unit None 

Lower Cretaceous

Salmon Peak 

FormationUpper unit 75 Wackestone that grades upward into grainstone Light-gray wackestone to grainstone, with abundant fossil fragments

Edwards aquifer

Aquifer

Minor karst, associat

with solutionenlarged fractures 

Lower unit 340 Thick, massive lime mudstone, grainstone, and chert Massive, gray mudstone Aquifer Minor karst, associat

with solution- enlarged fractures 

Fredricksburg Group

McKnight 

FormationUpper unit 120 Brownish, thin-bedded, pelleted, mudstone to grainstone Brown, thin-bedded mudstone to grainstone Aquifer Negligible 

Middle unit* * * * * * Lower unit* * * * * * 

West Nueces 

FormationUndivided* * * * * * 

Basal nodular unit 70 Nodular, burrowed mudstone to wackestone miliolids, gastropods, and Exogyra texana Nodular gray mudstone, black rotund bodies Confining unit; aquifer where solutionally enhanced Minor, primarily nea

contact Trinity aquifer 

Trinity 

Group Glen Rose Limestone 900–1,530 Tan, alternating layers of medium-bedded limestone and 

argillaceous limestone

Stairstep topography, tan mudstones, marly, 

thin bedded, Tylostoma sp., Turritella

sp., Protocardia texana

Trinity 

aquifer 

Confining unit to Edwards 

aquifer; aquifer where evaporite beds present

Minor, associated wi

solution-enlarged fractures
8 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

Sayre (1936) observed recharge to the Edwards 

aquifer in Uvalde and Medina Counties occurs from 

stream losses and calculated an annual amount of recharge 

from these counties. Sayre (1936) also speculated that 

water was moving in the Edwards aquifer eastward from 

Medina County into Bexar County. Sayre (1942) made 

the fundamental observations on the regional extent of 

circulation within the Edwards aquifer and indicated that 

groundwater flow was regionally eastward through the 

confined aquifer from the recharge areas in Uvalde and 

Medina Counties to natural discharge sites at the major 

springs in Comal and Hays Counties. Welder and Reeves 

(1962) described the groundwater resources and mapped 

the geology of Uvalde County, reported that the principal 

structural feature is the Balcones Fault Zone with regional 

dip to the south-southeast of approximately 100 feet per 

mile, and delineated a prominent, subsurface structural 

high known as the Uvalde Salient. The western limit of 

the Knippa Gap was defined by Maclay and Land (1988) 

as the Uvalde Salient. Green and others (2006) reported on 

the water levels, structural geology, and water chemistry 

of the Edwards aquifer in Kinney and Uvalde Counties to 

develop an updated conceptual model of the groundwater 

system and to clearly define the relation between areas of 

groundwater storage in the Edwards aquifer of Uvalde and 

areas to the east.

Methods of Investigation

Geologic Interpretation Methods

Previous geologic maps and reports (Maclay and 

Land, 1988; Clark and Small, 1997; Clark, 2003; Clark and 

Journey, 2006; Smith and others, 2003) covering parts of 

Uvalde and Medina Counties were used as the starting point 

for documenting the geologic framework and hydrogeologic 

characteristics of the study area. Previous interpretations of 

the origin and age of the formations present in Uvalde and 

Medina Counties were applied to the study area. The surface 

geology was mapped by using stratigraphy and nomenclature 

of previous reports (Shumard, 1860; White and Hill, 1887; 

Dumble, 1892; Hill and Vaughn, 1898; Udden, 1907; Liddle, 

1918; Trowbridge, 1923; Sayre, 1936; Wilmarth, 1938; Lozo 

and Smith, 1964; Barnes, 1974; Barnes, 1977; Miller, 1983). 

Helicopter electromagnetic survey (HEM) data (Smith and 

others, 2003, 2008) were used to help depict the geology and 

to aid in identifying faults.

Historical aerial photography acquired in 1954 (U.S. 

Department of Agriculture, 1954) was examined to observe 

vegetation and drainage patterns that might reveal possible 

changes in surficial geology, the locations of fractures and 

faults, and to identify any linear features that might reveal 

geologic structure (fig. 6). Visible surficial delineations of 

Figure 6. Aerial photograph showing a linear fault identified by the vegetation growing along its surface expression (U.S. Department 

of Agriculture, 1954). 

Fault

Fault

Fault

Fault

0 1,000 FEET
Methods of Investigation 9

geologic features such as fractures and faults often 

contain denser vegetation compared to the adjacent terrain 

and can be identified from vegetation changes on aerial 

photographs (Singhal and Gupta, 1999). Figure 6 provides 

an example of a linear fault identified by the vegetation 

growing along its surface expression. Changes in rock type 

at formational contacts also can result in changes in 

vegetation. Figure 7 provides an example of circular bands of 

vegetation that correlate with isolated exposures of igneous 

rock in an area composed primarily of alluvium. Surficial 

features were traced from aerial photographs and transferred 

to USGS 1:24,000-scale 7.5-minute quadrangle topographic 

maps. 

Historical borehole geophysical data were compiled for 

174 wells in the study area (appendix 1). Formational contacts 

in the subsurface were identified where possible on the basis 

of changes in the natural gamma response evident from the 

geophysical logs. Changes in natural gamma typically indicate 

changes in the lithology and can be used to identify the tops 

and bases of the formations. The combined data then were 

digitized, and a feature class was developed representing the 

geology. 

Figure 7. Aerial photograph showing circular bands of vegetation that correlate with isolated exposures of igneous rock in an area 

composed primarily of alluvium (U.S. Department of Agriculture, 1954).

Igneous

0 1,000 FEET
10 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

During January through August 2012, field mapping 

was done to further identify geologic contacts, thicknesses 

of exposed formations, and faults and fractures. The field 

mapping data were augmented with data obtained from 

previous reports, HEM surveys, aerial photographs, and 

historical borehole geophysical data. The geophysical data 

were historically collected by the USGS or obtained over 

many years from either the Texas Water Development Board 

or Edwards Aquifer Authority. Hardcopies of these data are 

maintained at the USGS office in San Antonio, Texas. 

Compiled data were used to aid in the interpretation of 

features mapped in the field such as outcrops incised by a 

stream or offset by faulting. During field mapping, geologic 

and hydrogeologic features were identified, and the latitude, 

longitude, and altitude of contacts and faults were recorded by 

using hand-held Global Positioning System (GPS) units. 

Dunham’s (1962) carbonate-rock classification and 

Choquette and Pray’s (1970) porosity classification systems 

were used to describe the rock outcrops. This approach is 

consistent with work done to describe rock outcrops by 

previous investigators in south-central Texas (Maclay and 

Small, 1984; Stein and Ozuna, 1995; Clark and Small, 1997; 

Clark, 2003; Blome and others, 2005). Dunham’s (1962) 

carbonate-rock classification, based on the arrangement and 

ratio of fine to coarse sediments, was used to describe the 

facies, aspect, appearance, and characteristics of rock units, 

which usually reflect the diagenesis of its origin (Bates and 

Jackson, 1987). The porosity classifications of Choquette and 

Pray (1970) were used to characterize the porosity as fabric 

selective or nonfabric selective. Fabric selective porosity is 

secondary porosity that preferentially developed along specific 

sedimentary structures, strata, or mineralogy (Choquette and 

Pray, 1970). Moldic porosity is an example of fabric selective 

porosity; moldic porosity was formed where shell material 

dissolved, leaving a void. Nonfabric selective porosity is 

secondary porosity that developed without the influence of 

sedimentary structures and is not associated with the original 

sedimentary or diagenetic processes but is caused by other 

geologic processes such as fracturing and faulting (Choquette 

and Pray, 1970). Vug porosity, which formed as a result of 

the diagenetic process, is an example of nonfabric selective 

porosity.

Geodatabase Development and Data 

Management

The data collected and plotted on USGS 1:24,000-

scale 7.5-minute quadrangle topographic maps during the 

field mapping were scanned, merged, and digitized to create a 

Geographic Information System (GIS) data layer covering the 

study area. All GIS data were geo-referenced to a Universal 

Transverse Mercator (UTM) projection using North American 

Datum of 1983 (NAD 83). 

To manage the data collected or complied for the study, 

a geodatabase was developed for project use. A geodatabase 

is a collection of spatial and tabular data that allows users 

to associate data with physical and spatial features (Zeiler, 

1999). The geodatabase is composed of four core database 

objects: feature datasets, feature classes, relation classes, and 

attribute tables. Feature classes in the geodatabase are spatial 

layers of similar vector type data (point, line, or polygon). 

A feature dataset manages a grouping of feature classes and 

uses a defined geographic or projected coordinate system for 

all the data managed and grouped under the feature dataset. 

Relation classes allow for data stored in feature classes to be 

linked with tabular information stored in attribute tables. The 

geodatabase for this study was based on the Environmental 

Systems Research Institute (ESRI) ArcGIS 10.0 personal 

geodatabase platform (Environmental Systems Research 

Institute, Inc., 2013). 

The project geodatabase includes a single feature dataset 

containing geospatial data that describes the surface geology 

for the study area. The Surface_Data feature dataset contains 

five feature classes that spatially represent the study area 

boundary, mapped geology, faults, wells, and geologic crosssection traces. In addition, there are two attribute tables (tbl_

GeologyInfo and tbl_WellInfo) that contain well and geologic 

information and are associated with the well feature class. The 

attribute tables are linked to the well feature class by relation 

classes based on the well identification field. To uniquely 

identify each well, wells were assigned a State well number 

using the Texas Water Development Board (TWDB) Ground 

Water Data System (Nordstrom and Quincy, 1999). Some 

wells were missing a complete State well number and were 

assigned two to three characters at the end of the well number 

sequence to distinguish wells from other wells (appendix 1). 

Well and borehole geophysical information were compiled 

from hardcopy files or downloaded from existing digital 

databases hosted by various Federal, State, and local agencies.

An interactive approach can be taken to explore these 

data in the geodatabase when using GIS or through traditional 

database queries. In GIS, these data can be viewed spatially 

and in combination with other relevant geospatial data 

layers such as aerial imagery, administrative boundaries, and 

hydrography. The data can also be analyzed spatially in GIS 

to aid in identifying distribution patterns, data gaps, spatial 

relations, and to create cartographic products.

Data formats and database schemas from different source 

agencies are not structured the same, so the final geodatabase 

schema was structured to capture only pertinent information 

related to the study. Queries were done on well entry records 

to identify duplicate records based on well number, geographic 

location, or both. Simple cross-check queries were performed 

to ensure the number of records between related tables 

remained consistent. The data and metadata created for use 

in this study conform to Federal Geographic Data Committee 

(FGDC) standards (Federal Geographic Data Committee, 

2012). The metadata document and describe the contents of 

the geodatabase such as title, abstract, publication date, author, 

geographic extent, projection information, process steps, and 

attribute label definitions. Metadata were reviewed by using 

the USGS Geospatial Metadata Validation Service (U.S. 

Geological Survey, 2013) and by USGS personnel.
Geologic Framework, Structure, and Hydrogeologic Characteristics 11

Geologic Framework, Structure, and 

Hydrogeologic Characteristics

By using all of the data that were compiled and collected 

for this study, a map (pl. 1) was constructed depicting 

the geologic framework, structure, and hydrogeologic 

characteristics of the Knippa Gap area in eastern Uvalde and 

western Medina Counties, Tex. Part A of plate 1 is a geologic 

map depicting the geologic formations in the study area 

(excluding gravel deposits, which were plotted separately in 

part B of plate 1), overlain with contacts between formations, 

faults, fractures, and facies changes. The inferred structural 

dip is also featured in part A of plate 1, and the location 

of the Knippa Gap structural low (trough) is delineated. 

Hydrogeologic cross-sections A–A′ and B–B′ are identified, 

along with the locations of wells from which borehole 

geophysical data were obtained.

Gravel deposits are shown in part B of plate 1 without 

the underlying geology in order to highlight the amount of the 

study area that is covered by the gravels. The gravel covered 

land surface historically has been an obstacle for analyzing the 

geology and structure of the Knippa Gap area. 

The major hydrogeologic feature of the study area, the 

San Antonio segment of the Edwards aquifer, is depicted 

in part C of plate 1. The approximate downdip limit of the 

freshwater zone (Schultz, 1994) of the Edwards aquifer is 

shown. 

The two hydrogeologic cross sections (parts D and 

E of plate 1) were constructed by using the geophysical 

borehole data compiled for the study. Part D of plate 1 depicts 

hydrogeologic cross-section A–A′, which trends southwest 

to northeast across the study area. Part E of plate 1 depicts 

hydrogeologic cross-section B–B′, which trends northwest to 

southeast across the study area. The locations of the wells used 

to construct each hydrogeologic cross section are included, 

and vertical faults are delineated as well as the Edwards 

aquifer at depth.

Geologic Framework

In eastern Uvalde and western Medina Counties, 

sedimentary rocks from the Lower Cretaceous to the 

Pleistocene are exposed, along with intrusive igneous rocks 

of Upper Cretaceous age (Smith and others, 2003). The 

sedimentary rocks are composed of limestone, chalk, gravel, 

sand, silt, shale, and clay. The igneous rocks are ultramafic 

to intermediate in composition (Lonsdale, 1927; Sellards, 

1932). Volcanic activity during the Cretaceous period resulted 

in pyroclastic deposits and basaltic flows (Lonsdale, 1927; 

Sellards and others, 1932) in the study area.

Lower Cretaceous Series 

The Glen Rose Limestone is part of the Trinity Group 

(figs. 4–5) and was deposited during the Lower Cretaceous 

period. According to Miller (1984), the Glen Rose Limestone 

unconformably underlies the Devils River Formation (pl. 1, 

fig. 4) in the Devils River Trend (fig. 2) in the eastern part of 

the study area and conformably underlies the West Nueces 

Formation in the Maverick Basin (fig. 2) in the western part of 

the study area. The Glen Rose Limestone varies in thickness 

from approximately 900 ft to approximately 1,530 ft in Uvalde 

County (Welder and Reeves, 1962) and is composed mostly of 

alternating layers of limestone and argillaceous limestone.

The Lower Cretaceous Devils River Formation (fig. 4) 

was unconformably deposited over the Glen Rose Limestone 

(Miller, 1984). In the study area, the total thickness of the 

Devils River Formation was calculated to be approximately 

600 ft based on geophysical logs and is composed primarily 

of miliolid, pellet, rudist, shell-fragment, lime grainstone 

and wackestones locally dolomitized, brecciated, and chert 

bearing (Lozo and Smith, 1964). The limestone is also locally 

dolomitized, brecciated, and chert bearing (Lozo and Smith, 

1964). 

The Devils River Formation lacks sufficient marker beds 

that effectively preclude a more detailed, regional subdivision, 

or differentiation (Lozo and Smith, 1964) other than the 

regionally extensive basal unit of mixed sediments informally 

referred to as the basal nodular member (Miller, 1984; Clark, 

2003) and an informal recognition of lower and upper parts of 

the Devils River Formation (Collins, 2000). 

The regional basal nodular member (figs. 4–5), which 

is approximately 20–70 ft thick, is a distinctively nodular, 

burrowed, sequence of dolomitic mudstone to wackestone 

(Clark, 2003) in the Devils River Formation (fig. 4) and West 

Nueces Formation (fig. 5). The basal nodular member contains 

miliolids, gastropods, and the pelecypod Exogyra texana. The 

basal nodular member is overlain by a burrowed unit (Miller, 

1984) that contains a thin- to massive-bedded mudstone to 

grainstone, approximately 30–40 ft thick, and is named for 

its distinctive massive, resistant layers of porous, burrowed 

limestone (Rose, 1972). The mudstone becomes more thickly 

bedded and less burrowed progressively upward through the 

unit. Above the burrowed unit thickness of the dolomitic unit 

is approximately 260 ft, and is a wackestone to grainstone, 

containing miliolid, shell fragments, and gastropods. In 

addition, the dolomitic unit lithologically ranges from shellfragmented, mudstone to wackestone, and beds of rudist and 

small rudist bioherms are common (Miller, 1984). Chert, 

which first occurs approximately 20 ft above the basal nodular 

member, can be found as nodules or beds throughout the 

Devils River, West Nueces, McKnight, and Salmon Peak 

Formations (figs. 4–5) (Rose, 1972). 

The upper 270 ft of the Devils River Formation consists 

of light-gray wackestone, packstone, miliolid grainstone, and 

rudist boundstone. Rudists, shell fragments, chert nodules, and 

thin chert beds are scattered throughout and are commonly 

abundant (Miller, 1984). Caprinid fossils, along with other 

varieties of rudists, are present locally, commonly form 

boundstone, and have often been replaced by chert (Miller, 

1984).

The edges of the western part of the study area coincide 

with the eastern limit of the Maverick Basin. The formations 
12 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

within the Maverick Basin are the West Nueces, McKnight, 

and Salmon Peak Formations (fig. 5) (Lozo and Smith, 1964). 

In the study area, only the basal nodular member of the 

West Nueces Formation, the upper part of the McKnight 

Formation, and the Salmon Peak Formation are present 

(pl. 1, fig. 5). The upper part of the West Nueces, lower part 

of the McKnight Formation, and the middle part of the 

McKnight Formation are not present in the study area because 

of changes in the shape of the Maverick Basin during the 

depositional sequence.

The only part of the West Nueces Formation present in 

the study area is the 70-ft thick (Clark, 2003) basal nodular 

member (pl. 1, fig. 5). As in other areas, the basal nodular 

member of the West Nueces Formation is a distinctively 

nodular, burrowed mudstone to wackestone (Miller, 1984) 

containing miliolids, gastropods, and the Exogyra texana

(Clark, 2003). 

According to Miller (1984), the McKnight Formation, 

which overlies the West Nueces Formation, can be subdivided 

into informal lower, middle, and upper units (fig. 5). The 

lower and middle units are not present in the study area but 

are included in figure 5 to help in understanding the complex 

depositional history of the area.

In surface exposures approximately 20 miles (mi) 

west-northwest of the northern boundary of the study area, 

the upper McKnight unit (fig. 5) is approximately 120 ft 

thick. The upper McKnight is composed of brownish, thinbedded, pelleted mudstone to grainstone. The contact between 

the upper McKnight unit and the overlying Salmon Peak 

Formation is gradational and conformable. A conglomeratic 

zone, consisting of bored rip-up clasts, lies just below the 

Salmon Peak Formation (Lozo and Smith, 1964). Lozo and 

Smith (1964) believed this horizon of rip-up clasts represented 

a gap in deposition at the base of the Salmon Peak Formation. 

The lower part of the Salmon Peak Formation (fig. 5) is 

up to 340 ft thick in the Maverick Basin and consists mostly 

of thick-bedded to massive pelagic-derived mudstones 

that grade [shoal] upward into a crossbedded, rudist shell 

grainstone containing chert in the upper 165 ft (Lozo and 

Smith, 1964; Humphreys, 1984). The upper unit of the Salmon 

Peak Formation is up to 75 ft thick in the Maverick Basin and 

consists mostly of grainstones and wackestones with bioclastic 

grainstones near the top part. 

Upper Cretaceous Series

In the study area, the Del Rio Clay, Buda Limestone, 

Eagle Ford Group, Austin Group, Anacacho Limestone, and 

Escondido Formations (pl. 1, figs. 4–5) were deposited during 

the Upper Cretaceous. The sedimentary units are composed of 

shale, clay, sand, limestone, and igneous pyroclastic/detrital 

material. The Upper Cretaceous age igneous rocks that are 

common in the study area are both intrusive and extrusive 

types (Ewing and Barker, 1986).

The Del Rio Clay of the Washita Group (pl. 1, figs. 4–5) 

directly overlies the Lower Cretaceous formations in Uvalde 

and Medina Counties. In the study area, the Del Rio Clay 

is approximately 70 ft thick (based on data from 153 wells 

in the study area) and consists of mostly dark blue-green 

and yellow-to-brown clay with thin beds of impervious 

limestone. Iron nodules are locally abundant and the index 

fossil Ilymatogyra arietina (fig. 8) is abundant throughout. 

Secondary gypsum commonly occurs as fracture fillings in 

Del Rio Clay outcrops near igneous bodies. Exposures of 

the Del Rio Clay are found north of Uvalde and adjacent to 

U.S. Highway 83 (fig. 1). The contact between the Del Rio 

Clay and the overlying Buda Limestone (pl. 1, figs. 4–5) 

is distinct and easily recognized, with the Buda Limestone 

blocks slumping (fig. 9) down the hillsides over the Del Rio 

Clay outcrops. 

The thickness of the Buda Limestone of the Washita 

Group averages 72 ft in the study area based on data from 

106 well logs. The Buda Limestone is a buff to light-gray, 

dense mudstone, weathering from a smooth gray to a grayishwhite, nodular surface exhibiting smooth conchoidal fractures 

(Adkins, 1932; Clark, 2003). In the study area, the formation 

commonly contains iron nodules, iron staining, and fossils 

replaced by iron. The lower contact of the Buda Limestone 

with the Del Rio Clay is unconformable as is the upper contact 

of the Buda Limestone with the Eagle Ford Group (Martin, 

1967) (pl. 1, figs. 4–5). Bored rip-up clasts of Del Rio Clay 

can be found in the lower few inches of the Buda Limestone 

(Kittenbrink, 1983) located north of Uvalde; however, these 

clasts are not common. 

The Eagle Ford Group (pl. 1, figs. 4–5) is approximately 

71 ft thick in the study area, based on 72 well logs and field 

mapping in 2012 by the authors. The Eagle Ford Group 

consists of brown, flaggy, sandy shale and argillaceous 

limestone (Trevino, 1988). In the study area, this Group 

contains fossil fragments and fossils of Inoceramus sp. Some 

of these freshly fractured flagstones emit a petroliferous 

odor. The upper contact with the overlying Austin Group 

(pl. 1) is gradational. According to Freeman (1961), there is 

a transitional zone of up to 5 ft in thickness where the Eagle 

Ford Group beds resemble the Austin Group. 

The Austin Group (pl. 1, figs. 4–5, fig. 10) is 

approximately 246 ft thick in the study area based on the 

average thicknesses recorded on 45 well logs. The Austin 

Group consists of massive, chalky, locally marly, generally 

fossiliferous mudstone that commonly contains the fossil 

oyster Gryphaea aucella, and Inoceramus sp. (figs. 4–5, 

fig. 11). Field observations indicate that Austin Group outcrops 

located near igneous bodies are commonly a grainstone with 

reworked pyroclastic or detrital igneous material, abundant 

fossil fragments, iron nodules, and fractures. The fractures 

often contain void-filling calcite, sometimes in the form 

of dogtooth spar. In many areas, the Austin Group and the 

Anacacho Limestone are indistinguishable, probably because 

the two units were deposited contemporaneously, resulting 

in a lateral gradation. In addition, these two units were likely 

modified by igneous activity, which possibly heated the 

limestones (Clark and Small, 1997).
Geologic Framework, Structure, and Hydrogeologic Characteristics 13

Figure 8. The abundance of Ilymatogyra arietina in the Del Rio Clay located in northeastern Uvalde County, Texas. Photograph taken 

April 23, 2012.

Figure 9. Contact between Buda Limestone (hillside) and Del Rio Clay (bed of creek) located on the west side of bridge at tributary to 

Seco Creek in northwestern Medina County, Texas. Photograph taken December 13, 2011.
14 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

Figure 10. Exposed Austin Group in road cut excavation located on south side of U.S. Highway 90 and approximately midway between 

Sabinal and Knippa Texas, eastern Uvalde County, Tex. Photograph taken May 29, 2012.

Figure 11. Inoceramus sp. fossil that is typical of the Austin Group located in the bed of the Leona River approximately 3 miles southsoutheast of city of Uvalde in Uvalde County, Texas. Photograph taken January 12, 2012.
Geologic Framework, Structure, and Hydrogeologic Characteristics 15

Contemporaneous with Upper Cretaceous igneous 

activity was the development of fringing reefs around the 

volcanoes or uplifts caused by intrusion of the igneous 

material (Luttrell, 1977). According to Wilson (1983), these 

fringing reefs represent patchy biostromes on bathymetric 

highs and atoll reefs or bank deposits around seamounts 

caused by the igneous intrusions. According to Ewing (2011), 

the fringing reefs coalesced into regional sheets and formed 

the Anacacho Limestone (pl. 1, figs. 4–5) that interfingers 

with and overlies the Austin Group in the study area. The 

Anacacho Limestone is a massive mudstone to packstone with 

interbedded bentonitic clay (Welder and Reeves, 1962; Clark, 

2003) having an average thickness of 188 ft in the study area 

based on logs from 22 wells. 

According to Hazzard (1956), the lower part of the 

Anacacho Limestone contains beds that are rusty-red and 

clay-rich, which probably indicates the presence of waterlain pyroclastics. Hazzard (1956) also states that some of the 

limestone beds are coquina. The formation contains heavy oil/

asphalt that has been mined west of the city of Uvalde since 

1891 (Getzendaner, 1931). In the study area, the Anacacho 

Limestone and all underlying formations are intruded or cut by 

igneous rocks.

The Escondido Formation (pl. 1, figs. 4–5, fig. 12) 

conformably overlies the Anacacho Limestone, is between 

200 ft and 875 ft thick in the study area (Barnes, 1977), and 

consists of fine-grained, friable, calcareous, and argillaceous 

sandstone, with interbedded shale, clay, and pyroclastic 

material (Welder and Reeves, 1962). Ewing (2011) states 

“This formation contains a thick set of resistive sandstones [in 

geophysical logs] in the heavy oil province, perhaps forming 

a shoreline or wave-dominated delta sequence (see Pisasale, 

1980). Asphaltic sandstones of the Escondido Formation 

crop out along the Nueces River in southern Uvalde and 

northernmost Zavala Counties.”

Upper Cretaceous Series Igneous Rocks

Upper Cretaceous magmatic activity (Miggins and others, 

2004) resulted in the emplacement of numerous and widely 

scattered igneous bodies from intrusive (dikes, sills, stocks, 

plugs, and possibly laccoliths) and extrusive (submarine and 

possible subaerial volcanoes) events (pl. 1, fig. 13) (Sellards 

and others, 1932; Smith and others, 2008). The igneous bodies 

intrude all of the Cretaceous age formations in the study 

area (Sellards and others, 1932). The Uvalde igneous field 

consists of fine- to coarse-grained, ultramafic and hypabyssal 

rocks (Miggins and others, 2004). Igneous activity might 

have been accompanied by pyroclastic events and lava flows 

(Ewing and Barker, 1986). These igneous bodies usually 

have columnar jointing, which forms perpendicular to the 

cooling surface (fig. 14). Recent radiogenic dating identifies 

two distinct phases of magmatic activity. The first phase of 

intrusions occurred approximately 82–80 million years ago, 

and the second phase, consisting of phonolites, occurred 

approximately 74–72 million years ago (Miggins and others, 

2004). 

The igneous activity was probably caused by changes in 

mantle-convection regimes (Ewing and Barker, 1986), from 

compressional forces associated with the Laramide orogeny 

(Ewing, 1991), or a combination thereof. Any of the factors 

could have reactivated zones of weakness in the underlying 

Ouachita Structural Belt (fig. 2) (Wilson, 1977). The Ouachita 

Structural Belt formed during the late Paleozoic collision of 

two continents (Poole and others, 2005). Regardless of the 

exact process, this igneous activity caused a regional uplift 

that is locally referred to as the Uvalde Salient (Welder and 

Reeves, 1962), and resulted in the subaerial exposure of 

igneous rocks during the Upper Cretaceous time (Ewing and 

Barker, 1986). According to Ewing and Barker (1986), the 

area of volcanic activity forms the axis of the Uvalde Salient 

referred to as the “Frio River Line.” The “Frio River Line” is 

not shown on the map. The igneous rocks are not shown on 

figures 4–5 because they penetrate, and are present, in all of 

the rocks stratigraphically below the Escondido Formation.

Eocene Rocks

The Indio Formation (pl. 1, fig. 5) unconformably 

overlies the Escondido Formation and is between 150 and 200 

ft thick according to Welder and Reeves (1962). Welder and 

Reeves (1962) further state the Indio Formation is nonmarine 

in origin, was deposited in streams or deltas (Sellards and 

others, 1932), and contains lenticular, thin-bedded clay, 

sandy shale, and grey to buff sandstone. In some locations, 

the formation contains lignite and bog iron ore. Generally, 

the sandstone is friable and poorly cemented, but in some 

locations, the Formation contains better carbonate cement with 

cross bedding (Welder and Reeves, 1962). 

Pliocene to Pleistocene Series Rocks

The Pliocene- to Pleistocene-age Uvalde Gravels and 

Leona Formation (pl. 1, figs. 4–5) (Sellards and others, 1932) 

were produced by the erosion of the Edwards Plateau. These 

units are present at the surface in most of the study area 

(Barnes, 1974).

The Uvalde Gravels (fig. 15) are Pliocene- to Pleistoceneage (Sellards and others, 1932) and commonly occur as a 

veneer of sediments at the land surface and rarely exceed 

20 ft in thickness (Barnes, 1974). The Uvalde Gravels were 

deposited by streams flowing from the Edwards Plateau 

with the fine-grained material being removed by subsequent 

winnowing (Sayer, 1936). The remaining gravels are loose 

pebbles, cobbles, or gravels. Some of the gravels are weakly 

cemented by caliche (Sayre, 1936; Welder and Reeves, 1962). 

After deposition, structural uplift, and ongoing erosion in the 

area caused dissection of the Uvalde gravels, which left the 

gravels topographically higher compared to the base of the 

current stream channels (Barnes, 1974). 
16 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

Figure 12. Sandstone ledges in the Escondido Formation located approximately 7 miles south-southeast of the community of D’Hanis in 

Medina County, Texas. Photograph taken April 13, 2012.

Figure 13. Outcrop of eroded and dissected extinct volcano located adjacent to the Frio River and east of the city of Uvalde in Uvalde 

County, Texas. Basalt is remnant of volcanic neck, and adjacent to the basalt is pyroclastic material ranging from fine-grained material 

to volcanic bombs, which formed the slope of the volcano. Photograph taken January 12, 2012. 

Area shown on figure 14
Geologic Framework, Structure, and Hydrogeologic Characteristics 17

Figure 14. Columnar jointing common in igneous rocks located adjacent to the Frio River and east of the city of Uvalde in Uvalde 

County, Texas. Photograph taken January 12, 2012. 

Figure 15. Caliche-cemented Uvalde Gravel located approximately 5.3 miles southeast of the town of D’Hanis in Medina County, Texas. 

Photograph taken April 13, 2012.

Approximately 5 Feet
18 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

According to Hill and Vaughn (1898), the Leona 

Formation (figs. 4–5) is formed by the gravels forming the first 

terrace of the Nueces and Leona Rivers, below the level of the 

Uvalde Gravel, and also for the flood-plain deposit extending 

westward from the city of Uvalde on the Leona River to the 

Nueces River. Hill and Vaughn (1898) extended the name 

Leona Formation to include the terrace deposits along all the 

principal streams of Texas (Sellards and others, 1932). Welder 

and Reeves (1962) included “a smaller terrace, some 10 to 20 

ft below the Leona terrace level” and the younger alluvium. 

According to Sellards and others (1932), the Leona Formation 

is Pleistocene in age. Welder and Reeves (1962) reported a 

maximum thickness for the Leona Formation of 35 ft. Sellards 

and others (1932) reported that the Leona Formation is 

composed of red and reddish gray silt and fine gravel with a 

thickness of 0 to 120 ft.

Structure

The principal structural feature in Uvalde and Medina 

Counties is the Balcones Fault Zone (fig. 2), which is the 

result of Miocene age faulting (Weeks, 1945). As is typical 

elsewhere in the Balcones Fault Zone, most of the faults in 

the study area are of high-angle, en echelon, normal faults, 

that are vertical or nearly vertical and downthrown to the 

southeast (George, 1952). Although most of the faults in 

Uvalde County are similar to those in Medina County (Holt, 

1959) and Bexar County (Arnow, 1959), individual fault 

displacements typically are less in Uvalde County, where 

the maximum displacement across the entire Balcones 

Fault Zone is approximately 700 ft (Welder and Reeves, 

1962). The Balcones Fault Zone in Uvalde County probably 

contains smaller fault displacements because the igneous rock 

resulted in more stability and formed an area not as easily 

affected by faulting. Many of the relatively minor faults, with 

displacements of 20 ft or less, are difficult to identify in the 

field. Some faults are actually shatter zones—not the single, 

sharp breaks implied by individual lines on maps. 

The structurally complex area of the Balcones Fault Zone 

contains relay ramps (fig. 16) (Hovorka and others, 1996). 

Several reports go into a more detailed description of relay 

ramps and relay ramp development; these include but are 

not limited to: Clark and Journey (2006), Ferrill and others 

(2003), Hovorka and others (1998), and Collins and Hovorka 

(1997). Relay ramps form in extensional fault systems to 

allow for deformation changes along the fault block (Clark 

and Journey, 2006). Ramp structures connect the footwall of a 

fault segment to the stratigraphically higher segment (hanging 

wall) of the overlapping fault. As stress (extension) occurs and 

strain along the ramp increases, rotation and internal fracturing 

occur along the relay ramp (Trudgill, 2002). Subsequently, 

continuing extension produces cross faults within the relay 

ramp structure. In Medina County, the faulting of the Balcones 

Fault Zone has produced a relay-ramp structure that dips to 

the southwest from the Edwards aquifer recharge zone and 

extends westward and below land surface from Seco Creek 

Figure 16. Idealized view of the relay-ramp structure that dips to the northeast from the Uvalde Salient in Uvalde County, the relayramp structure that dips to the southwest from Medina County, and the inferred direction of groundwater flow paths towards the Knippa 

Gap structural low (trough). 

HW

FW

HW

Uvalde

Salient

Knippa Gap

relay ramp relay ramp

HW

FW

FW

Edwards

aquifer recharge zone

EXPLANATION

Inferred structural feature

Inferred groundwater flow direction

Fault

Hanging wall of a fault

Footwall of a fault

HW

FW

Modified from Clark and Journey, 2006, fig. 5
Geologic Framework, Structure, and Hydrogeologic Characteristics 19

(Clark and Journey, 2006). In Uvalde County, beds dip from 

the Uvalde Salient towards the northeast. These two opposing 

structural dips result in a subsurface structural low (trough) in 

the area known as the Knippa Gap located near the towns of 

Knippa and Sabinal, Uvalde County (fig. 1).

Hydrogeologic Cross-Section A–A’ 

Hydrogeologic cross-section A–A′ (pl. 1, part D), 

developed for this study, is located in the southern part of the 

study area extending from the southwest near well YP–69–51–

606 to the northeast near well TD–69–38–905b. The location 

and trend of this hydrogeologic cross section was selected 

to cross the area defined as the Knippa Gap by Maclay and 

Land (1988). The hydrogeologic cross section shows a distinct 

structural low (trough) near the center of the hydrogeologic 

cross section with opposing downfaulting on either side of the 

trough. 

The land-surface altitude along the hydrogeologic cross 

section ranges from approximately 850 to 960 ft. The altitude 

of the top of the Edwards aquifer ranges from approximately 

525 to -150 ft at wells TD–69–38–905a and YP–69–44–902. 

The average thickness (appendix 1) of the Edwards aquifer 

(590 ft).is depicted on the geologic cross section. Fault 

locations were determined from field mapping coupled with 

well data. Dips of the fault blocks along the section were 

assumed to be zero unless either well control or surface 

geology indicated otherwise. Some dips were determined 

by having multiple wells within the fault blocks. Dips of 

some fault blocks were determined based on dips observed 

at the surface outcrops. Cross-section A–A′ shows fault parts 

for six fault blocks, stratigraphic and structural relations of 

the Edwards aquifer in the subsurface, the location of nine 

wells, apparent dips, and the interpreted range in altitude of 

the top and bottom of the Edwards aquifer in the subsurface. 

The hydrogeologic cross section runs roughly parallel to 

the Balcones Fault Zone. Fault block 19 is assumed to be 

horizontal based on data from one well and surface geology. 

Fault block 19a is a horst (upthrown geologic structure) with 

no dip relative to fault blocks 19 and 16. Fault block 16 has an 

apparent dip angle trending towards the east based on the data 

from the two wells. Fault block 16a is a horst that has resulted 

in the Edwards aquifer being juxtaposed against the upper 

confining unit of fault blocks 15 and 16. The dip of fault block 

16a is assumed to be zero because of lack of well and surface 

control. The western part of fault block 15 dips toward the 

east. There is essentially no dip between well YP–69–44–902 

and well YP–69–45–701a. The central part of fault block 15 

dips towards the west based on data from wells YP–69–45–

701a and YP–69–45–5xx. Fault block 15 appears to indicate a 

structural low (trough) in an area defined by Maclay and Land 

(1988) as the Knippa Gap. Fault block 14 is up relative to fault 

block 15 and down relative to fault block 20 with no dip based 

on surface mapping and data from one well. Fault block 20 

dips to the west based on data from two wells coupled with 

data from the surface mapping. 

Hydrogeologic Cross-Section B–B’

Hydrogeologic cross-section B–B′ (pl. 1, part E) is 

located near the center of the study area and extends from 

the north from near well YP–69–36–602, through the town 

of Sabinal, Tex., and terminates near well YP–69–53–2xx 

in the south. The location and trend of this hydrogeologic 

cross section were chosen because it crosses through the area 

defined by Maclay and Land (1988) as the Knippa Gap. The 

land-surface altitude along the hydrogeologic cross section 

ranges from about 1,100 to 850 ft above NAVD 88. The 

altitude of the top of the Edwards aquifer in cross-section 

B–B′ ranges from 925 to -325 ft at wells YP–69–36–602 and 

YP–69–45–8xx. The average thickness (appendix 1) of the 

Edwards aquifer (590 ft) is depicted on the geologic cross 

section. Hydrogeologic cross-section B–B′ shows faults 

controlling parts of 18 fault blocks, stratigraphic and structural 

relations of the Edwards aquifer in the subsurface, the location 

of seven wells, apparent dips, and the variability of interpreted 

range in altitude of the top and bottom of the Edwards aquifer 

in the subsurface. Fault locations were determined from field 

mapping coupled with well data. Dips of the fault blocks are 

assumed to be zero because of lack of sufficient geologic 

control. Faults are primarily downthrown to the south along 

this hydrogeologic cross section in typical en echelon fashion. 

The hydrogeologic cross section runs perpendicular to the 

structural low (trough) identified by this study, and through 

the area defined by Maclay and Land (1988) as the Knippa 

Gap. Fault blocks 1 and 2 are in the Del Rio Clay outcrop. 

Fault block 3 and 4 are in the outcrop of the Austin Group. 

Fault blocks 5 and 6 are downthrown to the south. Fault 

blocks 7–10 form a horst (geologic feature) identified based 

on surface mapping, consequently, the Edwards aquifer is 

uplifted and juxtaposed against the upper confining units. 

Fault blocks 11–17 continue the en-echelon downthrown 

pattern to the south. Fault block 18 is upthrown, juxtaposing 

a part of the Edwards aquifer against the upper confining 

unit.

Hydrogeologic Characteristics

The Leona aquifer of the Leona Formation supplies water 

to wells in several parts of Uvalde and Medina Counties, 

primarily for domestic and livestock use (figs. 4–5) (Welder 

and Reeves, 1962). Based on field observations, the Leona 

aquifer contains fabric selective interparticle porosity with the 

thickest sections located near the center of valleys and forming 

the most productive parts of the aquifer (Welder and Reeves, 

1962). The gravels forming the Leona aquifer generally are 

considered unconsolidated. Clay overlies much of the gravels 

in the Leona aquifer and confines the groundwater resource in 

many places; therefore, the interaction between groundwater 

and surface water is often restricted (Welder and Reeves, 

1962) and, according to Holt (1959), where the Leona aquifer 

outcrops, the aquifer is recharged by precipitation, streamflow, 

and flow from springs flowing from other aquifers. 
20 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

The authors noted during field reconnaissance that the 

Uvalde Gravel (figs. 4–5) has fabric selective interparticle 

porosity. Caliche generally forms the cement binding the 

gravels together. Precipitation occurring on the outcrop 

infiltrates rapidly through these gravels into underlying 

formations or moves downgradient into the Leona aquifer 

in stream valleys. Therefore, in the higher topographic areas 

between the streambeds, the gravel unit is believed to be 

nonwater-bearing (Welder and Reeves, 1962). 

Field reconnaissance showed that the Indio Formation 

(figs. 4–5) contains fabric selective interparticle porosity 

in the sands. The sands are generally poorly cemented with 

carbonate cement (Welder and Reeves, 1962). According to 

Welder and Reeves (1962), the Indio Formation yields small 

quantities of water mainly for domestic and livestock uses 

and the quality of water produced from the formation varies 

greatly spatially, and in many areas, the formation yields only 

saline water containing greater than 1,000 milligrams per liter 

of dissolved solids.

According to Barker and Ardis (1996, p. B53), the upper 

confining unit to the Edwards aquifer is formed by the Del 

Rio Clay, Buda Limestone, Eagle Ford Group, Austin Group, 

Anacacho Limestone, and Escondido Formation (figs. 4–5), 

and “yield little or no water or a very small amount of water to 

mostly shallow dug wells.” These units form a regional barrier 

to vertical groundwater flow; however, several formations 

within this regional barrier have very good localized hydraulic 

connection with the deeper Edwards aquifer. Springs flow 

from the Buda Limestone and Austin Group in or near the 

study area that appear to discharge water from the Edwards 

aquifer (Clark and Small, 1997; Green and others, 2006). 

This groundwater moves vertically up through and along 

fractures or faults and discharges into surficial drainages 

and probably discharges into the Leona aquifer in the 

subsurface through similar faults and fractures (Green and 

others, 2006).

According to Welder and Reeves (1962), the Escondido 

Formation (figs. 4–5) yields small quantities of saline 

water in Uvalde and Medina Counties. Getzendaner (1931) 

stated that many of the wells completed in the Escondido 

Formation were dry. Field observations by the authors 

showed that the sandstones in the formation contain fabric 

selective interparticle porosity and secondary nonfabric 

selective fracture porosity. Clays and shales of the formation 

contain fabric selective porosity. The sandstones are partially 

cemented by carbonates. Field mapping indicated the 

occurrence of asphalt in the outcrop area appears to form a 

vertical barrier to the downward movement of water, and 

subsequently, many of the stock ponds in the Escondido 

Formation seem to be located in areas containing higher 

concentrations of asphalt.

Getzendaner (1931) reported that many of the wells 

drilled into the Anacacho Limestone were dry. Holt (1959) 

stated that the Anacacho Limestone (figs. 4–5) yields water 

for domestic purposes only. Groundwater flows vertically up 

through faults and fractures from the Edwards aquifer and 

discharges from the Anacacho Limestone and into the Leona 

River (fig. 1, Leona Springs 2) (Clark and Small, 1997). 

From field observations, it was determined that the porosity 

in the limestone portions of the Anacacho Limestone is fabric 

selective moldic porosity and secondary nonfabric selective 

fracture and bedding plane porosity. Welder and Reeves 

(1962) stated that the formation must be cased off during 

drilling because the bentonitic clays in the formation absorb 

water and swell. 

Welder and Reeves (1962) stated that the Austin Group 

yields small to moderate amounts of water to wells near the 

outcrop (figs. 4–5). Banta and Clark (2012) state that water 

in the Austin Group is derived from many sources including 

direct infiltration of precipitation, infiltration along stream 

channels, and artesian water from the underlying Edwards 

aquifer. In addition, Welder and Reeves (1962) stated that 

several wells completed in the Austin Group near the city of 

Uvalde produce large quantities of water, and Holt (1959) 

reported several wells completed in the Austin Group 

produced moderate amounts of water. Getzendaner (1931) 

stated that many of the wells drilled into the Austin Group 

were dry. Field observations by the authors indicated that 

porosity in the Austin Group is both fabric selective and 

nonfabric selective. The fabric selective porosity is mainly in 

the form of moldic and interparticle porosity. The observed 

nonfabric selective porosity is in the form of fractures, 

bedding plane openings, channels (fig. 17), and caves (fig. 18). 

According to Poston and Chen (1991), the permeability of the 

Austin Group is 0.01 to 0.001 millidarcy in the rock and 10 to 

100 times greater in the fractures. 

Leona Springs 1 (figs. 1 and 17) and Leona Springs 2 

(fig. 1) in the outcrop of the Austin Group, discharge 

groundwater, which flows upward from the underlying 

Edwards aquifer through faults and fractures (Clark and Small, 

1997). Green and others (2006) state that the supply of water 

to many of the springs in the vicinity of the study area are 

also from water moving up through faults and fractures from 

the underlying Edwards aquifer under artesian conditions and 

discharging from the Austin Group. These springs are similar 

to springs discharging from the Austin Group in Bexar County. 

Banta and Clark (2012) also state that artesian water moves 

through faults, fractures, or conduits from the underlying 

Edwards aquifer, and that the volume of discharge from the 

Austin Group is often related to the proximity to the fault, 

fracture, or conduit supplying the water. Banta and Clark 

(2012) based the conclusions that the volume of discharge 

increases near faults on changes in water levels and pump tests 

in wells near faults when compared to ones more distant from 

faults. Discharge from these springs provides flow to river 

systems, such as the Leona River, or into the gravels of the 

Leona Formation (Green and others, 2006).

The Eagle Ford Group yields small quantities of 

saline water (figs. 4–5) (Welder and Reeves, 1962). Field 

observations for this study indicated that the Eagle Ford Group 

contained both fabric selective interparticle porosity and 

nonfabric selective fracture porosity. 
Geologic Framework, Structure, and Hydrogeologic Characteristics 21

Figure 17. Channel porosity developed along a bedding plane in the Austin Group located near the intermittent Leona Springs 2 on the 

Leona River in Uvalde County, Texas. Photograph taken January 12, 2012.

Figure 18. Cave in fractured Austin Group rock located approximately 9.3 miles northeast of city of Uvalde in Uvalde County, Texas. 

Photograph taken August 7, 2012.
22 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

According to Welder and Reeves (1962), near the city 

of Uvalde, the Buda Limestone supplies moderate quantities 

of water for domestic and irrigation uses (figs. 4–5). Porosity 

in the Buda Limestone appears to be related to nonfabric 

selective fractures. Getzendaner (1931) stated that water 

produced from the Buda Limestone is related to the close 

proximity of faults. Several springs issue from the Buda 

Limestone near the study area, and the source of the water is 

believed to originate from the underlying Edwards aquifer 

(Green and others, 2006). Water in the Edwards aquifer flows 

upward along and through faults and fractures under artesian 

conditions and discharges from the Buda Limestone (Green 

and others, 2006).

Welder and Reeves (1962) reported that the formation 

currently (2013) referred to as the Del Rio Clay is not known 

to yield water in Uvalde County (figs. 4–5). The Del Rio Clay 

contains little porosity based on its lithology. The authors 

noted during field observations that in surface outcrop the 

formation is often used by ranchers for developing stock 

ponds. 

The Edwards aquifer is a major aquifer in south-central 

Texas (figs. 4–5) (George and others, 2011). In the study 

area, the Edwards aquifer is composed of the Devils River 

Formation in the Devils River Trend and the West Nueces 

Formation, upper McKnight Formation, and Salmon Peak 

Formation in the Maverick Basin (figs. 4–5) (Clark and Small, 

1997). According to Maclay and Small (1976), the Edwards 

aquifer has relatively large porosity and permeability, in 

part, from the development and redistribution of secondary 

porosity. 

Effective porosity in the Edwards aquifer outcrop is a 

result of both fabric and nonfabric selective porosity (Small 

and Clark, 2000). Maclay and Small (1976) state that the 

effective porosity in the Edwards aquifer is closely associated 

with large permeability such as is common with karst features. 

Porosity-test data reported by Small and Maclay (1982), 

determined by mercury injection, indicate that the Edwards 

aquifer contains porosities ranging from 5 to 35 percent. For 

a detailed description of the geology and hydrogeology of the 

Edwards aquifer refer to Maclay and Small (1976, 1984) and 

Small and Maclay (1982).

The Edwards aquifer in the Devils River Trend is formed 

in the Devils River Formation and is one of the most porous 

and permeable units in the study area (fig. 4) (Clark and Small, 

1997). The permeable zones generally are associated with 

solutionally enlarged bedding planes and fractures. Although 

most of the permeability probably results from fractures, 

the aquifer also contains nonfabric selective bedding plane, 

fracture, breccia, vug, channel, and cave porosity; in addition, 

the aquifer contains fabric selective, intercrystalline, burrow, 

and moldic porosity (Clark, 2003). The vertical fractures near 

the top of the Edwards aquifer in the Devils River Formation 

provide the most effective paths for recharge, and where 

subaerially exposed, these fractures greatly increase the rate of 

recharge per a given precipitation event (Welder and Reeves, 

1962). 

The upper part of the Edwards aquifer in the Devils River 

Formation contains fabric selective moldic, and nonfabric 

selective bedding plane, fracture, vug, channel, and cave 

porosity (fig. 4) (Maclay and Land, 1988; Clark, 2003). 

The middle part of the Edwards aquifer in the Devils River 

Formation contains nonfabric selective, interconnected vug 

porosity making the middle section of the unit permeable; 

although probably less so than the upper part of the unit 

(Clark, 2003). The lower part of the Edwards aquifer in the 

Devils River Formation contains both fabric selective burrows 

and intercrystalline porosity and nonfabric selective porosity 

in the forms of vugs and fractures. 

The Edwards aquifer of the Maverick Basin facies is 

formed in the Salmon Peak Formation, upper unit of the 

McKnight Formation, and basal nodular unit of the West 

Nueces Formation (fig. 5) (Clark and Small, 1997). The 

permeability of the Edwards aquifer in the Salmon Peak 

Formation (fig. 5) decreases stratigraphically with depth. The 

porosity appears to be the result of nonfabric selective porosity 

in the form of solutionally enlarged fractures, bedding planes, 

and some cavern development (Clark, 2003). The lowermost 

250 ft of this unit has relatively minor nonfabric selective vug 

porosity (less than 3 percent) and nonfabric selective fracture 

porosity with little permeability (Small and Maclay, 1982). 

The upper unit of the McKnight Formation in the 

Edwards aquifer contains some fabric selective moldic 

porosity that evolved from the removal of evaporites and 

subsequent breccia resulting from the collapse of weakened 

bedding. The upper unit of the McKnight Formation also 

contains nonfabric selective fracture and porosity (Clark and 

Small, 1997). 

Only the basal nodular unit of the West Nueces 

Formation is present in the study area (fig. 5). The basal 

nodular unit of the West Nueces Formation contains nonfabric 

selective fractures with a small amount of porosity (less than 3 

percent) (Sieh, 1975).

The Trinity aquifer is considered a major aquifer by 

the Texas Water Development Board (figs. 4–5) (George 

and others, 2011). The Trinity aquifer is recharged by lateral 

subsurface flow from the Edwards Plateau, infiltration of 

precipitation, and seepage of surface water (Barker and Ardis, 

1996). Although the Trinity aquifer is a major aquifer in 

south-central Texas, it is considered a lower confining unit 

to the Edwards aquifer because the Trinity aquifer is less 

permeable than the Edwards aquifer, and the potential of the 

Trinity aquifer to provide and transmit water is appreciably 

less than that of the Edwards aquifer (Barker and Ardis, 1996). 

Ashworth (1983) subdivided the Trinity aquifer into the 

upper, middle, and lower aquifers. The upper Trinity aquifer 

underlies the Edwards aquifer. According to Ashworth (1983), 

few wells are completed in the upper Trinity aquifer because 

it generally produces small quantities of poor quality water. 

In south-central Texas, the upper Trinity aquifer is formed in 

the upper member of the Glen Rose Limestone (Ashworth, 

1983). According to Ashworth (1983), the upper Trinity 

aquifer is separated from the middle Trinity aquifer by thin, 
Summary 23

hydraulically tight interbeds. Porosity in the upper Trinity 

aquifer is primarily in the form of nonfabric selective fractures 

and caves that formed along solutionally enlarged fractures 

(Clark, 2004). It also contains fabric selective boxwork, 

breccia, and moldic porosity in certain intervals associated 

with variations in lithology (Clark, 2004). 

Getzendaner (1931, p. 104) reported that “Many small 

wells get their water from the serpentines.” According to 

Welder and Reeves (1962), many of the wells in Uvalde 

County penetrate basalt and serpentine (not shown on figs. 4 

or 5) and further reported that wells containing several 

hundred feet of serpentine were unproductive; however, in 

wells that have several tens of feet of serpentine but open 

to other aquifers, production was unaffected. A few wells 

produce a small amount of water from basalt (Welder and 

Reeves, 1962). The “igneous swarm” in Uvalde and Medina 

Counties probably reduced the transmissivity of the Edwards 

aquifer on a regional scale (Smith and others, 2002) and 

influence of groundwater flow and quality (Blome and others, 

2004). The porosity and permeability of these intrusive rocks 

are decreased by secondary minerals deposited within the 

primary porosity (Blome and others, 2004). Volcanic plugs, 

dikes, and sills are the most common form of the igneous 

masses (Welder and Reeves, 1962). These features result from 

the intrusion and emplacement of molten material; therefore, 

the porosity in the basalts is from nonfabric selective fractures 

and is related to cooling and contraction of the magma bodies 

(figs. 14–15) (Clark, 2003).

Groundwater flow paths in the Edwards aquifer are 

influenced by faulting and geologic structure (fig. 16). Faulting 

associated with the Balcones Fault Zone affect groundwater 

flow paths by (1) forming a barrier to flow which results 

in water moving parallel to the fault plane (Maclay, 1995), 

(2) increasing flow across the fault plane because of fracturing 

and juxtaposing porous and permeable units, or (3) no effect 

on the groundwater flow paths. Maclay (1995) stated that 

(based on Small,1986) faults could be barriers to groundwater 

flow paths where the aquifer is offset by 50 percent or more 

and result in flow moving parallel to the fault. According to 

Clark and Journey (2006, p. 2), “the amount of displacement 

along a particular fault tends to vary, and thus the effectiveness 

of a fault as a barrier to flow probably changes along the 

fault plane. Near a fault tip [where a fault ends no barrier to 

flow exists; as displacement down the fault plane increases, 

the effectiveness of the fault as a barrier to flow increases.” 

The structural high, known as the Uvalde Salient, results in 

groundwater moving off of the high and downgradient (dip) 

towards a structural low (trough) to the east (pl. 1, fig. 16). 

Groundwater moving down the relay ramp in northern Medina 

County flows downgradient (dip) to the structural low (trough) 

from the northeast to the southwest (pl. 1, fig. 16) (Clark and 

Journey, 2006). Clark and Journey (2006) reported up to four 

separate fault bounded flow paths moving down the northern 

Medina relay ramp towards the southwest. The two opposing 

flow paths join within the structural low (trough) and turn 

south flowing through the area known as the Knippa Gap 

(Maclay and Land, 1988). According to Maclay (1995), south 

of the towns of Knippa and Sabinal is an area of minimal fault 

displacement (plate 1, parts A and D) that was designated by 

Maclay and Land (1988) as the Knippa Gap. 

Summary

The Edwards aquifer is the primary source of potable 

water for the San Antonio area in south-central Texas. To 

better understand the function of the area known as the Knippa 

Gap as it pertains to its geology and structure, the geologic 

framework, structure, and hydrogeologic characteristics 

of the area were evaluated by the U.S. Geological Survey 

(USGS) in cooperation with the U.S. Army Corps of 

Engineers (USACE)—Fort Worth District. The purpose of 

this report is to describe the geologic framework, structure, 

and hydrogeologic characteristics of the Knippa Gap area of 

the Devils River Trend and Maverick Basin in eastern Uvalde 

and western Medina Counties, Texas. The Knippa Gap was 

postulated to channel or restrict flow in the Edwards aquifer in 

eastern Uvalde County; its existence was based on a series of 

numerical simulations of groundwater flow in the aquifer. 

The rocks within the study area are sedimentary and 

igneous and range in age from the Lower Cretaceous to the 

Pleistocene. The sedimentary rocks are limestone, chalk, 

gravel, sand, silt, and clay. The igneous rocks are ultramafic to 

intermediate in composition.

By using all of the data that were compiled and collected 

for this study, a map was constructed depicting the geologic 

framework, structure, and hydrogeologic characteristics of 

the Knippa Gap area in eastern Uvalde and western Medina 

Counties, Tex. The map also shows the interpreted structural 

dip directions and interpreted location of a structural low 

(trough) in the area known as the Knippa Gap. 

The geologic framework of the study area is composed 

of sedimentary rocks forming the Devils River Trend and 

the Maverick Basin. The Lower Cretaceous Devils River 

Trend is formed in the Devils River Formation. The Lower 

Cretaceous Maverick Basin rocks are formed in the West 

Nueces Formation, McKnight Formation, and Salmon Peak 

Formation. Overlying rocks of both depositional provinces are 

the Upper Cretaceous Del Rio Clay, Buda Limestone, Eagle 

Ford Group, Austin Group, Anacacho Limestone, Escondido 

Formation, the Eocene Indio Formation, the Pliocene Uvalde 

Gravel, and the Pleistocene Leona Formation. 

Episodes of Upper Cretaceous magmatic activity resulted 

in the emplacement of numerous and widely scattered igneous 

bodies from intrusive and extrusive events. The igneous bodies 

are located in all of the formations in the study area.

The principal structural feature in Uvalde and Medina 

Counties is the Balcones Fault Zone, which is the result 

of Miocene age faulting. The structurally complex area of 

the Balcones Fault Zone contains relay ramps that form in 

extensional fault systems to allow for deformation changes 

along the fault block. In Medina County, the faulting of the 
24 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

Balcones Fault Zone has produced a relay-ramp structure that 

dips to the southwest from the Edwards aquifer recharge zone 

and extends westward. In Uvalde County, beds dip from the 

Uvalde Salient towards the northeast. These two opposing 

structural dips result in a subsurface structural low (trough) 

feature in the area known as the Knippa Gap, which is located 

near the towns of Knippa and Sabinal in Uvalde County.

Hydrogeologic cross-section A–A′, developed for 

this study, is located in the southern part of the study area 

extending from the southwest near well YP–69–51–606 to the 

northeast near well TD–69–38–905b. The location and trend 

of this hydrogeologic cross section was selected to cross the 

area defined as the Knippa Gap by previous investigators. The 

geologic cross section shows a distinct structural low (trough) 

near the center of the hydrogeologic cross section with 

opposing downfaulting on either side of the trough. 

Hydrogeologic cross-section B–B′ is located near the 

center of the study area, and extends from the north to the 

south, through the town of Sabinal, Tex. The location and 

trend of this hydrogeologic cross section was chosen because 

it crosses through the area defined as the Knippa Gap.

The Leona aquifer supplies water to wells in several parts 

of the Uvalde and Medina Counties primarily for domestic and 

livestock uses. The Uvalde Gravel is not known to yield water 

in the study area. The Indio Formation yields small quantities 

of water mainly for domestic and livestock uses. 

The upper confining unit to the Edwards aquifer is 

formed by the Del Rio Clay, Buda Limestone, Eagle Ford 

Group, Austin Group, Anacacho Limestone, and Escondido 

Formation. These units form a regional barrier to vertical 

groundwater flow; however, several formations within this 

regional barrier have very good localized hydraulic connection 

with the deeper Edwards aquifer. 

The Edwards aquifer is a major aquifer in south-central 

Texas. In the study area, the Edwards aquifer is composed of 

the Devils River Formation in the Devils River Trend, and 

the West Nueces Formation, and upper McKnight Formation, 

and Salmon Peak Formation in the Maverick Basin. The 

Edwards aquifer has relatively large porosity and permeability, 

in part from the development and redistribution of secondary 

porosity.

Groundwater flow paths in the Edwards aquifer are 

influenced by faulting and geologic structure. Faults could be 

barriers to groundwater flow paths where the aquifer is offset 

by 50 percent or more and result in flow moving parallel to 

the fault. The effectiveness of a fault as a barrier to flow 

probably changes along the fault plane as the amount of 

displacement changes. The structural high, known as the 

Uvalde Salient, results in groundwater moving off of the high 

and downgradient (dip) towards a structural low (trough) 

to the east. Groundwater moving down the relay ramp in 

northern Medina County flows downgradient (dip) to the 

structural low (trough) from the northeast to the southwest 

bounded by large displacement. The two opposing flow paths 

join within the structural low (trough) and turn south flowing 

through the area known as the Knippa Gap. 

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Appendix 1—Wells Included in a Geodatabase of Geologic 

Framework, Structure, and Hydrogeologic Characteristics of 

the Knippa Gap Area in Eastern Uvalde and Western Medina 

Counties, Texas

Appendix 1 31

Appendix 1

Appendix 1. Wells included in a geodatabase of geologic framework, structure, and hydrogeologic characteristics of the Knippa Gap 

Area in Eastern Uvalde and Western Medina Counties, Texas.—Continued

[--, not unavailable]

Well identifier 

(state well 

number)

Longitude 

(decimal 

degrees)

Latitude 

(decimal 

degrees)

Land 

surface 

altitude 

(feet) 

Depth from 

land surface 

to the top of 

the Edwards 

Group 

(feet)

Altitude of 

the top of 

the Edwards 

Group 

(feet)

Depth from 

land surface 

to the top of 

the Glen Rose 

Limestone 

(feet)

Approximate 

altitude of 

the top of the 

Glen Rose 

Group 

(feet)

Thickness 

of the 

Edwards 

Group 

(feet)

TD–68–31–1xxa1 99.3453 29.4639 1,110 220 890 -- -- --

TD–68–31–1xxb1 99.3719 29.4681 1,139 140 999 -- -- --

TD–68–31–1xxc1 99.3722 29.4689 1,141 130 1,011 -- -- --

TD–69–38–101 99.3384 29.4691 1,140 95 1,045 -- -- --

TD–69–38–103 99.3584 29.4727 1,190 155 1,035 -- -- --

TD–69–38–1xxa1 99.3739 29.4711 1,144 110 1,034 -- -- --

TD–69–38–1xxb1 99.3672 29.4689 1,148 140 1,008 -- -- --

TD–69–38–905a 99.2806 29.4013 977 462 515 1,000 -23 538

TD–69–38–905b 99.2795 29.4022 977 443 534 993 -16 550

TD–69–45–101 99.4683 29.3689 1,005 838 167 -- -- --

TD–69–45–102 99.4975 29.3561 1,005 913 92 -- -- --

TD–69–45–103 99.4647 29.3458 975 925 50 1,475 -500 550

TD–69–45–104 99.4619 29.3353 960 1,160 -200 -- -- --

TD–69–45–105 99.4631 29.3675 998 785 213 -- -- --

TD–69–45–108 99.4672 29.3597 987 -- -- -- -- --

TD–69–45–109 99.4672 29.3603 988 -- -- -- -- --

TD–69–45–201 99.4306 29.3739 1,002 758 244 -- -- --

TD–69–45–202 99.4219 29.3594 1,022 895 127 -- -- --

TD–69–45–203 99.4556 29.3628 983 850 133 1,400 -417 550

TD–69–45–301 99.3908 29.3686 985 972 13 -- -- --

TD–69–45–302 99.3878 29.3497 991 985 6 -- -- --

TD–69–45–303 99.3825 29.3594 1,005 1,010 -5 -- -- --

TD–69–46–101 99.3462 29.3344 1,052 1,065 -13 1,645 -593 580

TD–69–46–102 99.3445 29.3638 1,079 -- -- -- -- --

TD–69–46–1xx1 99.3492 29.3728 1,054 -- -- -- -- --

TD–69–46–404 99.3606 29.3333 1,033 1,106 -73 -- -- --

TD–69–46–405 99.3492 29.2950 980 1,280 -300 1,855 -875 575

TD–69–46–6xx1 99.2867 29.3130 865 -- -- -- -- --

TD–69–46–8xxa1 99.3311 29.2594 945 1,347 -402 1,923 -978 576

TD–69–46–8xxb1 99.3325 29.2717 970 1,200 -230 1,770 -800 570

YP–69–36–602 99.5281 29.4216 1,094 170 924 720 374 550

YP–69–36–800 99.5437 29.4108 1,084 182 902 732 352 550

YP–69–36–904 99.5233 29.4086 1,074 168 906 785 289 617

YP–69–36–9xx1 99.5303 29.4138 1,084 164 920 800 284 636

YP–69–37–402 99.4731 29.4533 1,158 220 938 700 458 480

YP–69–42–606 99.7525 29.2939 1,013 280 733 -- -- --

YP–69–42–902 99.7519 29.2544 994 0 994 600 394 600

YP–69–42–911 99.7617 29.2886 1,003 195 808 745 258 550

YP–69–42–9xx1 99.7553 29.2608 1,068 -- -- -- -- --

Appendix 1. Wells included in a geodatabase of geologic framework, structure, and hydrogeologic characteristics of the Knippa Gap 

Area in Eastern Uvalde and Western Medina Counties, Texas.

[--, not unavailable]
32 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

Appendix 1. Wells included in a geodatabase of geologic framework, structure, and hydrogeologic characteristics of the Knippa Gap 

Area in Eastern Uvalde and Western Medina Counties, Texas.—Continued

[--, not unavailable]

Well identifier 

(state well 

number)

Longitude 

(decimal 

degrees)

Latitude 

(decimal 

degrees)

Land 

surface 

altitude 

(feet) 

Depth from 

land surface 

to the top of 

the Edwards 

Group 

(feet)

Altitude of 

the top of 

the Edwards 

Group 

(feet)

Depth from 

land surface 

to the top of 

the Glen Rose 

Limestone 

(feet)

Approximate 

altitude of 

the top of the 

Glen Rose 

Group 

(feet)

Thickness 

of the 

Edwards 

Group 

(feet)

YP–69–43–103 99.7103 29.3480 1,089 102 987 -- -- --

YP–69–43–105 99.7211 29.3361 1,075 235 840 -- -- --

YP–69–43–106 99.7401 29.3555 1,082 72 1,010 672 410 600

YP–69–43–108 99.7442 29.3458 1,062 60 1,002 -- -- --

YP–69–43–109 99.7419 29.3506 1,081 145 936 -- -- --

YP–69–43–202 99.6953 29.3586 1,068 0 1,068 730 338 730

YP–69–43–205 99.6792 29.3514 1,063 90 973 -- -- --

YP–69–43–2xx1 99.6948 29.3361 1,047 119 928 719 328 600

YP–69–43–301a 99.6506 29.3653 1,067 190 877 715 352 525

YP–69–43–301b 99.6545 29.3608 1,044 50 994 713 331 663

YP–69–43–302 99.6389 29.3472 1,034 222 812 772 262 550

YP–69–43–303 99.6342 29.3511 1,034 268 766 818 216 550

YP–69–43–305 99.6461 29.3614 1,059 94 965 -- -- --

YP–69–43–306 99.6281 29.3586 1,033 310 723 860 173 550

YP–69–43–307 99.6303 29.3447 1,026 300 726 -- -- --

YP–69–43–308 99.6558 29.3497 1,037 145 892 -- -- --

YP–69–43–3xxa1 99.6478 29.3355 1,032 294 738 804 228 510

YP–69–43–3xxc1 99.6561 29.3486 1,038 155 883 -- -- --

YP–69–43–402 99.7450 29.3194 1,042 210 832 -- -- --

YP–69–43–403 99.7337 29.3225 1,052 256 796 860 192 604

YP–69–43–408 99.7139 29.3289 1,073 345 728 -- -- --

YP–69–43–409 99.7301 29.3241 1,054 242 812 870 184 628

YP–69–43–410 99.7325 29.3247 1,055 278 777 -- -- --

YP–69–43–4xxa1 99.7439 29.3314 1,070 154 916 -- -- --

YP–69–43–4xxb1 99.7353 29.3297 1,051 175 876 750 301 575

YP–69–43–4xxc1 99.7286 29.3292 1,062 225 837 -- -- --

YP–69–43–4xxd1 99.7339 29.3253 1,044 220 824 795 249 575

YP–69–43–4xxe1 99.7483 29.3067 1,045 158 887 -- -- --

YP–69–43–4xxf1 99.7258 29.3003 1,038 223 815 -- -- --

YP–69–43–502 99.6853 29.3300 1,031 126 905 -- -- --

YP–69–43–503 99.6897 29.2919 987 335 652 935 52 600

YP–69–43–601 99.6633 29.3211 1,019 320 699 870 149 550

YP–69–43–603 99.6367 29.2958 985 1,016 -31 -- -- --

YP–69–43–605 99.6348 29.2922 978 680 298 1,275 -297 595

YP–69–43–606 99.6406 29.3111 1,006 201 805 776 230 575

YP–69–43–6xx1 99.6406 29.3183 1,002 -- -- -- -- --

YP–69–43–7xxa1 99.7258 29.2886 1,078 230 848 -- -- --

YP–69–43–7xxb1 99.7442 29.2778 1,023 -- -- -- -- --

YP–69–43–7xxc1 99.7117 29.2658 974 220 754 820 154 600
Appendix 1 33

Appendix 1. Wells included in a geodatabase of geologic framework, structure, and hydrogeologic characteristics of the Knippa Gap 

Area in Eastern Uvalde and Western Medina Counties, Texas.—Continued

[--, not unavailable]

Well identifier 

(state well 

number)

Longitude 

(decimal 

degrees)

Latitude 

(decimal 

degrees)

Land 

surface 

altitude 

(feet) 

Depth from 

land surface 

to the top of 

the Edwards 

Group 

(feet)

Altitude of 

the top of 

the Edwards 

Group 

(feet)

Depth from 

land surface 

to the top of 

the Glen Rose 

Limestone 

(feet)

Approximate 

altitude of 

the top of the 

Glen Rose 

Group 

(feet)

Thickness 

of the 

Edwards 

Group 

(feet)

YP–69–43–7xxd1 99.7117 29.2761 966 360 606 887 79 527

YP–69–43–802 99.6822 29.2628 952 477 475 -- -- --

YP–69–43–803 99.6758 29.2675 903 492 411 1,072 -169 580

YP–69–43–804 99.6934 29.2761 974 358 616 1,000 -26 642

YP–69–43–902 99.6336 29.2764 956 794 162 1,394 -438 600

YP–69–43–905 99.6578 29.2689 960 605 355 -- -- --

YP–69–43–908 99.6494 29.2639 948 654 294 -- -- --

YP–69–43–909 99.6489 29.2536 941 845 96 -- -- --

YP–69–43–912 99.6281 29.2675 950 842 108 -- -- --

YP–69–43–916 99.6475 29.2817 971 690 281 1,330 -359 640

YP–69–43–917 99.6453 29.2508 936 880 56 1,480 -544 600

YP–69–43–9xx1 99.6358 29.2656 948 800 148 1,400 -452 600

YP–69–44–101 99.6208 29.3717 1,053 350 703 890 163 540

YP–69–44–102 99.6167 29.3617 1,040 360 680 -- -- --

YP–69–44–103 99.6222 29.3469 1,020 390 630 -- -- --

YP–69–44–105 99.6250 29.3358 1,012 468 544 -- -- --

YP–69–44–106 99.6117 29.3508 1,013 478 535 -- -- --

YP–69–44–109 99.5864 29.3378 998 740 258 1,340 -342 600

YP–69–44–110 99.5969 29.3347 1,007 -- -- -- -- --

YP–69–44–201 99.5836 29.3611 1,020 595 425 -- -- --

YP–69–44–203 99.5714 29.3658 1,012 601 411 1,201 -189 600

YP–69–44–204 99.5803 29.3456 1,004 650 354 1,250 -246 600

YP–69–44–301 99.5136 29.3533 1,003 930 73 1,480 -477 550

YP–69–44–302 99.5197 29.3400 988 910 78 1,460 -472 550

YP–69–44–303 99.5217 29.3533 1,002 920 82 -- -- --

YP–69–44–304 99.5294 29.3381 985 -- -- -- -- --

YP–69–44–305 99.5228 29.3644 1,010 745 265 -- -- --

YP–69–44–401 99.5917 29.3261 993 784 209 -- -- --

YP–69–44–402 99.5936 29.3300 998 635 363 1,185 -187 550

YP–69–44–404 99.6119 29.3292 1,006 520 486 -- -- --

YP–69–44–405 99.6108 29.3108 994 685 309 1,285 -291 600

YP–69–44–406 99.6153 29.3186 1,000 515 485 -- -- --

YP–69–44–407 99.5878 29.3072 977 -- -- -- -- --

YP–69–44–408 99.6103 29.2956 975 726 249 1,276 -301 550

YP–69–44–4xxa1 99.6233 29.2925 972 403 569 1,003 -31 600

YP–69–44–4xxb1 99.6208 29.2964 971 509 462 1,129 -158 620

YP–69–44–502 99.5750 29.3144 973 870 103 -- -- --

YP–69–44–503 99.5828 29.3128 972 -- -- -- -- --

YP–69–44–5xx1 99.5694 29.3244 972 -- -- -- -- --
34 Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area

Appendix 1. Wells included in a geodatabase of geologic framework, structure, and hydrogeologic characteristics of the Knippa Gap 

Area in Eastern Uvalde and Western Medina Counties, Texas.—Continued

[--, not unavailable]

Well identifier 

(state well 

number)

Longitude 

(decimal 

degrees)

Latitude 

(decimal 

degrees)

Land 

surface 

altitude 

(feet) 

Depth from 

land surface 

to the top of 

the Edwards 

Group 

(feet)

Altitude of 

the top of 

the Edwards 

Group 

(feet)

Depth from 

land surface 

to the top of 

the Glen Rose 

Limestone 

(feet)

Approximate 

altitude of 

the top of the 

Glen Rose 

Group 

(feet)

Thickness 

of the 

Edwards 

Group 

(feet)

YP–69–44–601 99.5297 29.3147 1,002 1,030 -28 1,555 -553 525

YP–69–44–703 99.5864 29.2575 936 918 18 1,605 -669 687

YP–69–44–704a 99.6112 29.2550 922 1,080 -158 1,800 -878 720

YP–69–44–704b 99.6111 29.2550 922 1,080 -158 -- -- --

YP–69–44–806a 99.5425 29.2600 895 944 -49 1,610 -715 666

YP–69–44–806b 99.5425 29.2600 895 955 -60 -- -- --

YP–69–44–807 99.5644 29.2683 926 880 46 -- -- --

YP–69–44–808a 99.5725 29.2753 936 836 100 1,558 -622 722

YP–69–44–808b 99.5725 29.2753 936 855 81 1,405 -469 550

YP–69–44–8xx1 99.5670 29.2905 943 866 77 1,534 -591 668

YP–69–44–902 99.5056 29.2700 892 1,050 -158 1,598 -706 548

YP–69–44–9xx1 99.5011 29.2792 931 1,075 -144 1,625 -694 550

YP–69–45–400 99.4698 29.3197 953 920 33 1,470 -517 550

YP–69–45–402 99.4831 29.3114 933 921 12 1,471 -538 550

YP–69–45–405 99.4686 29.3272 953 1,020 -67 1,570 -617 550

YP–69–45–406 99.4700 29.3194 948 914 34 1,464 -516 550

YP–69–45–4xx1 99.4653 29.3067 931 -- -- -- -- --

YP–69–45–501 99.4514 29.3128 932 988 -56 -- -- --

YP–69–45–504 99.4567 29.2939 914 1,116 -202 -- -- --

YP–69–45–5xx1 99.4528 29.3016 920 932 -12 1,507 -587 575

YP–69–45–701a 99.4787 29.2850 912 1,056 -144 1,644 -732 588

YP–69–45–701b 99.4786 29.2850 912 1,062 -150 -- -- --

YP–69–45–702 99.4911 29.2675 893 1,143 -250 -- -- --

YP–69–45–703 99.4786 29.2644 882 1,139 -257 -- -- --

YP–69–45–704 99.4664 29.2878 901 1,190 -289 -- -- --

YP–69–45–7xx1 99.4817 29.2858 916 -- -- -- -- --

YP–69–45–802 99.4558 29.2881 903 1,190 -287 -- -- --

YP–69–45–8xx1 99.4362 29.2700 886 1,211 -325 1,830 -944 619

YP–69–46–102 99.3442 29.3636 1,079 956 123 1,481 -402 525

YP–69–50–309 99.7548 29.2486 972 64 908 664 308 600

YP–69–51–102 99.7317 29.2225 955 165 790 -- -- --

YP–69–51–104 99.7473 29.2363 943 68 875 668 275 600

YP–69–51–1xxa1 99.7417 29.2330 968 130 838 812 156 682

YP–69–51–1xxb1 99.7347 29.2433 982 115 867 715 267 600

YP–69–51–1xxc1 99.7326 29.2141 940 125 815 845 95 720

YP–69–51–203 99.6858 29.2131 950 326 624 -- -- --

YP–69–51–204 99.6953 29.2136 963 420 543 1,020 -57 600

YP–69–51–205 99.6931 29.2158 958 -- -- -- -- --

YP–69–51–2xxa1 99.6853 29.2414 912 -- -- -- -- --
Appendix 1 35

Publishing support provided by

Lafayette Publishing Service Center

Information regarding water resources in Texas is available at 

http://tx.usgs.gov/

Appendix 1. Wells included in a geodatabase of geologic framework, structure, and hydrogeologic characteristics of the Knippa Gap 

Area in Eastern Uvalde and Western Medina Counties, Texas.—Continued

[--, not unavailable]

Well identifier 

(state well 

number)

Longitude 

(decimal 

degrees)

Latitude 

(decimal 

degrees)

Land 

surface 

altitude 

(feet) 

Depth from 

land surface 

to the top of 

the Edwards 

Group 

(feet)

Altitude of 

the top of 

the Edwards 

Group 

(feet)

Depth from 

land surface 

to the top of 

the Glen Rose 

Limestone 

(feet)

Approximate 

altitude of 

the top of the 

Glen Rose 

Group 

(feet)

Thickness 

of the 

Edwards 

Group 

(feet)

YP–69–51–2xxb1 99.6856 29.2372 965 -- -- -- -- --

YP–69–51–2xxc1 99.6850 29.2361 972 -- -- -- -- --

YP–69–51–2xxd1 99.6811 29.2306 912 661 251 1,271 -359 610

YP–69–51–2xxe1 99.6744 29.2136 906 -- -- -- -- --

YP–69–51–301a 99.6484 29.2255 905 442 463 1,042 -137 600

YP–69–51–301b 99.6486 29.2247 905 450 455 1,000 -95 550

YP–69–51–408 99.7370 29.1827 890 120 770 720 170 600

YP–69–51–5xx1 99.6717 29.1986 881 145 736 765 116 620

YP–69–51–602 99.6356 29.1755 866 594 272 1,055 -189 461

YP–69–51–606 99.6325 29.1933 876 758 118 1,318 -442 560

YP–69–51–6xxa1 99.6350 29.2039 882 200 682 -- -- --

YP–69–51–6xxb1 99.6320 29.1939 873 680 193 1,280 -407 600

YP–69–51–6xxc1 99.6669 29.2036 888 -- -- -- -- --

YP–69–52–201 99.5784 29.2191 882 896 -14 1,344 -462 448

YP–69–52–403 99.6164 29.1986 875 844 31 1,430 -555 586

YP–69–52–7xx1 99.5906 29.1369 833 1,570 -737 2,455 -1,622 885

YP–69–52–9xx1 99.5395 29.1594 813 1,536 -723 2,136 -1,323 600

YP–69–53–2xx1 99.4189 29.2358 824 1,115 -291 1,800 -976 685

Average thickness of the Edwards Group 590

1

Final State well number is pending assignment by the Texas Water Development Board.

Clark and others—Geologic Framework, Structure, and Hydrogeologic Characteristics of the Knippa Gap Area—SIR 2013–5149

Printed on recycled paper

I SBN 978- 1- 4113-3687- 2

9 7 8 1 4 11 336872