U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3418

Prepared in cooperation with the Edwards Aquifer Authority

Geologic Framework and Hydrostratigraphy of the Edwards 

and Trinity Aquifers Within Hays County, Texas
Cover. A, Photograph showing the Blanco River Valley, looking north from Little Arkansas Road, Hays County, Texas (photograph by Allan 

K. Clark, U.S. Geological Survey, February 27, 2018). B, Photograph showing a low water crossing on the Blanco River, looking south from 

Little Arkansas Road, Hays County, Texas (photograph by Allan K. Clark, U.S. Geological Survey, February 27, 2018). C, Photograph showing 

swimming hole on Cypress Creek in Blue Hole Regional Park, Wimberley, Hays County, Texas (photograph by Allan K. Clark, U.S. Geological 

Survey, February 27, 2018).

A

B

C
Geologic Framework and Hydrostratigraphy 

of the Edwards and Trinity Aquifers Within 

Hays County, Texas

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

Prepared in cooperation with the Edwards Aquifer Authority

Pamphlet to accompany

Scientific Investigations Map 3418

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit https://store.usgs.gov.

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

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Clark, A.K., Pedraza, D.E., and Morris, R.R., 2018, Geologic framework and hydrostratigraphy of the Edwards and 

Trinity aquifers within Hays County, Texas: U.S. Geological Survey Scientific Investigations Map 3418, 1 sheet,

scale 1:24,000, pamphlet, https://doi.org/10.3133/sim3418.

ISSN 2329-1311 (print)

ISSN 2329-132X (online)

ISBN 978-1-4113-4262-0
iii

Acknowledgments

The authors thank Brian Hunt and Brian Smith of the Barton Springs Edwards Aquifer 

Groundwater Conservation District and Alex Broun of the Hays Trinity Groundwater Conservation 

District for their assistance. Mr. Bob Cocke is thanked for his patience and assistance in 

accessing areas on the Blanco River on multiple occasions. The authors are grateful for the 

assistance of the numerous land owners and managers of private and public lands who provided 

access to their properties for this study. 

The authors would also like to thank Charles D. Blome and Lynne Fahlquist, U.S. Geological 

Survey, for their insightful technical reviews of this report.
iv
v

Contents

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

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

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

Purpose and Scope ..............................................................................................................................1

Description of Study Area ...................................................................................................................1

Methods of Investigation.....................................................................................................................2

Geologic Framework .....................................................................................................................................2

Trinity Group...........................................................................................................................................2

Edwards Group......................................................................................................................................3

Washita, Eagle Ford, Austin, and Taylor Groups .............................................................................3

Structure.................................................................................................................................................3

Hydrostratigraphy..........................................................................................................................................4

Upper Confining Unit of the Edwards Aquifer..................................................................................4

Edwards and Trinity Aquifers..............................................................................................................4

Edwards Aquifer...........................................................................................................................4

Trinity Aquifer ...............................................................................................................................4

Upper Zone of the Trinity Aquifer.....................................................................................5

Middle Zone of the Trinity Aquifer....................................................................................5

Hammett HSU Confining Unit............................................................................................5

Lower Zone of the Trinity Aquifer.....................................................................................5

Implications of Hydrostratigraphic Characteristics and Geologic Structure

on Groundwater Recharge and Flow Paths .......................................................................5

Summary..........................................................................................................................................................6

References Cited ...........................................................................................................................................7

Sheet

[https://doi.org/10.3133/sim3418]

 Geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers

within Hays County, Texas
vi

Figures

[On sheet]

1. Map showing location of the study area relative to the State of Texas, Hays

County, the outcrops of the Edwards and Trinity aquifers, and the Balcones

fault zone

2. Chart showing summary of the geologic framework and hydrostratigraphy of

the Edwards and Trinity aquifers within Hays County, Texas

3. Photograph showing conglomerates within the Sycamore Sand at Hammetts

Crossing, Travis County, Texas 

4. Photograph showing bedding plane porosity that developed at the junction

between the bedding plane and a fracture within the Cow Creek Limestone

 at Hammetts Crossing, Travis County, Texas 

5. Photograph showing marker bed known as the Loriolia Bed, found 80 feet below

the contact of the Glen Rose Limestone and the Kainer Formation

6. Photograph showing Jacobs Well Spring, which forms the headwaters of

Cypress Creek north of Wimberley, Texas

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2

) 0.3861 square mile (mi2)

U.S. customary units to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 2.590 square kilometer (km2)

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Datum

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.
vii
viii
Geologic Framework and Hydrostratigraphy of the 

Edwards and Trinity Aquifers Within Hays County, Texas

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

Abstract

The Edwards and Trinity aquifers are classified as major 

aquifers by the Texas Water Development Board and are 

major sources of water in south-central Texas, where Hays 

County is located. Detailed maps and descriptions of the 

geologic framework and hydrostratigraphic units (HSUs) of 

these karstic aquifers in Hays County are needed for water 

managers to effectively manage groundwater resources in 

the area. During 2016–18, the U.S. Geological Survey, in 

cooperation with the Edwards Aquifer Authority, documented 

the geologic framework and hydrostratigraphy of the 

Edwards and Trinity aquifers for a large part of Hays County, 

characterizing approximately 560 square miles of the county. 

The report includes a 1:24,000-scale hydrostratigraphic map 

and descriptions of the geology and HSUs in the study area. 

In addition, parts of the adjacent upper confining unit to the 

Edwards aquifer are described. 

The rocks exposed within the study area are within 

outcrops of the Trinity and Edwards Groups and the overlying 

Washita, Eagle Ford, Austin, and Taylor Groups. The rocks 

are sedimentary and formed during the Cretaceous age. The 

principal structural feature in Hays County is the Balcones 

fault zone, which is the result of late Oligocene and early 

Miocene age high-angle normal faulting and fracturing. 

Hydrostratigraphically, the exposed rocks represent a section 

of the upper confining unit to the Edwards aquifer, the 

Edwards aquifer, the upper zone of the Trinity aquifer, the 

middle zone of the Trinity aquifer, and the upper part of the 

lower zone of the Trinity aquifer. Complexity in the aquifer 

system results from a combination of the original depositional 

history, bioturbation, primary and secondary porosity, 

diagenesis, fracturing, and faulting. 

Introduction

The Texas Water Development Board classifies the 

karstic Edwards and Trinity aquifers (fig. 1) as major sources 

of water in south-central Texas, where Hays County is located 

(George and others, 2011). Hays County was the third-fastestgrowing county in the Nation in 2016, as well as the fastest 

growing county in the Nation with a population of at least 

100,000 people (U.S. Census Bureau, 2016). To effectively 

manage available water resources, water managers need 

detailed maps and descriptions of the geologic framework 

and hydrostratigraphic units (HSUs) of the aquifers in Hays 

County. The geologic framework and hydrostratigraphy of the 

Edwards and Trinity aquifers largely control groundwater flow 

paths and storage in the aquifers (Kuniansky and Ardis, 2004). 

During 2016–18, the U.S. Geological Survey (USGS), in 

cooperation with the Edwards Aquifer Authority, documented 

the geologic framework and hydrostratigraphy of the Edwards 

and Trinity aquifers within Hays County at a 1:24,000 scale. In 

2016, the USGS published similar hydrogeologic assessments 

of the Edwards and Trinity aquifers for different study areas in 

south-central Texas (Clark and others, 2016a, b). Descriptions 

of the geologic framework and HSUs in this report were 

modified from those in Clark and others (2016a, b).

Purpose and Scope

The purpose of this report is to present the geologic 

framework and hydrostratigraphy of the Edwards and 

Trinity aquifers within Hays County (fig. 1). The HSUs that 

compose the Edwards and Trinity aquifers were mapped to 

aid in the understanding of groundwater recharge, discharge, 

and flow paths. The scope of the report is focused on the 

geologic framework and hydrostratigraphy of the outcrops 

and hydrostratigraphy of the Edwards and Trinity aquifers 

within Hays County (fig. 1). Descriptions of the geologic 

framework and hydrostratigraphy of the Edwards and Trinity 

aquifers within Hays County are provided, as well as a 

detailed map of the hydrostratigraphy at a 1:24,000 scale. 

Compared to the level of detail available in existing geologic 

maps, the geologic map in this report was prepared at a finer 

scale to aid water managers as they work to anticipate and 

mitigate issues related to changing land use and increasing 

groundwater demands.

Description of Study Area

The study area consists of about 560 square miles (mi2) 

of Hays County, which is northeast of San Antonio, Tex., and 

southwest of Austin, Tex. (fig. 1). The rocks exposed within 

the study area are within outcrops of the Trinity and Edwards 
2 Geologic Framework and Hydrostratigraphy of the Edwards and Trinity Aquifers Within Hays County, Texas

Groups and the overlying Washita, Eagle Ford, Austin, and 

Taylor Groups (Barker and Ardis, 1996) (fig. 2). The rocks 

in the study area are primarily sedimentary carbonates that 

formed during the Cretaceous age (Barker and Ardis, 1996). 

Karst features in the study area include sinkholes, caves, and 

underground streams that allow rapid infiltration of surface 

waters to the subsurface (Veni, 1988). Faulting in the study 

area is late Oligocene to early Miocene age (Weeks, 1945b) 

and is an extensional fault system known as the Balcones fault 

zone (Hill, 1900). The Balcones fault zone trends, generally, 

southwest to northeast in south-central Texas (Maclay and 

Small, 1986). The faults are vertical to near vertical with 

normal throw, are en echelon, and are mostly downthrown to 

the southeast (Hill, 1900; Maclay and Small, 1986).

Methods of Investigation

Geological data and information from previous reports 

(Hanson and Small, 1995; Stein and Ozuna, 1995; Clark, 

2003, 2004; Clark and others, 2009; Blome and Clark, 

2014; Clark and others, 2016a, b) were reviewed to assist 

in field mapping. During 2016–18, geologic framework and 

hydrostratigraphic mapping was completed on public and 

private land in Hays County. Field-mapping techniques were 

consistent with those used in previous similar studies (Clark, 

2003; Clark and Morris, 2015; Clark and others, 2016a, b) 

and guided by the use of Global Positioning System (GPS) 

units and digital maps and geologic mapping applications 

installed on a tablet computer; field-mapping observations 

were recorded onsite by using a tablet computer loaded with 

geospatially registered 7.5-minute USGS topographic maps. 

Locations of visible and interpreted geologic contacts, faults 

and fractures, marker units, and other areas of interest were 

recorded by using an integrated third generation (3G) network 

assisted GPS receiver on the tablet computer. In areas without 

cellular service, positions were determined by using a handheld compass and triangulation techniques. Faults identified 

in the field were based on observed and inferred stratigraphic 

offsets. Strikes and dips of faults and fractures were noted 

where possible. Bedding attitudes of fractures and faults were 

obtained by using a hand-held compass or the tablet computer 

compass application. The locations of springs and sinkholes 

were obtained from field mapping, topographic maps, and the 

previous geologic mapping report produced by Hanson and 

Small (1995). The data obtained by using the tablet computer 

compass application were independently cross verified on 

a regular basis with data obtained by using a hand-held 

compass. The field data were transferred by using ArcGIS 

Desktop version 10.3.1 (Esri, 2016), quality checked by 

comparison with original draft maps, and then used to examine 

the geologic framework and develop the hydrostratigraphic 

map of the study area. Digital data of the geographical extent 

of the surficial hydrostratigraphic units and faulting within the 

study area and associated metadata are available for download 

in a companion data release (Pedraza and others, 2018).

Lithologic descriptions, HSU names, and porosity type 

are consistent with those used previous publications such as 

Clark and others (2016a, b). The descriptions of the geologic 

framework and hydrostratigraphy in this report were adapted 

for the study area from Clark and others (2016a, b). Formal 

geologic names are consistent with those in the National 

Geologic Map Database (U.S. Geological Survey, 2018). 

Informal geologic and HSU names are consistent with those 

used in previous publications (Maclay and Small, 1976; Clark 

and others, 2009, 2016a, b; Blome and Clark, 2014) (fig. 2). 

The thicknesses of the mapped lithostratigraphic members and 

HSUs were determined from field observations. Thickness 

variations are caused by variations in local depositional and 

erosional conditions. Porosity varies in each lithostratigraphic 

unit and is dependent on the unit’s original depositional 

environment, lithology, structural history, and diagenesis. 

HSUs were identified as either fabric selective or not fabric 

selective on the basis of variations in the amount and type of 

porosity visually evident in the outcrop. 

Lithologic descriptions follow the classification system 

of Dunham (1962). Porosity descriptions are based on the 

sedimentary carbonate classification system of Choquette and 

Pray (1970). Descriptions of clastic rocks were done under the 

classification scale of Wentworth (1922).

HSUs were identified on the basis of the type of porosity 

visually evident in the outcrop. Porosity varies in each 

lithostratigraphic member, depending on the unit’s original 

depositional environment, lithology, structural history, and 

diagenesis. Porosity types were described as either fabric 

selective or not fabric selective in accordance with the 

sedimentary carbonate classification system of Choquette and 

Pray (1970).

Geologic Framework

The rocks within the study area are Cretaceous age, 

sedimentary, and primarily composed of sands, silts, clays, 

and limestones (fig. 2). Stratigraphically (ascending order), 

the geologic units are the Trinity Group (Sycamore Sand,

Hammett Shale, Cow Creek Limestone, Hensell Sand, and 

the lower and upper members of the Glen Rose Limestone), 

Edwards Group (Kainer and Person Formations), Washita 

Group (Georgetown Formation, Del Rio Clay, and Buda 

Limestone), Eagle Ford Group, Austin Group, and Taylor 

Group (Pecan Gap Chalk).

Trinity Group

The Early Cretaceous to late Early Cretaceous Trinity 

Group (Imlay, 1940) contains shale, mudstone to grainstone, 

boundstone, evaporites, sandstone, siltstone, conglomerates, 

and argillaceous limestone (fig. 2). The sediments were 

deposited on a large, shallow marine carbonate platform 

(Comanche shelf) as clastic-carbonate “couplets” during 
Geologic Framework 3

three marine transgressional events (Lozo and Stricklin, 

1956; Stricklin and others, 1971). These three distinct 

“couplets” contain sediments that formed (1) the Hosston 

(Sycamore Sand [Hill, 1901], fig. 3) and Sligo Formations 

(Imlay, 1940); (2) the Hammett Shale (Hill, 1901; Lozo and 

Stricklin, 1956) and the Cow Creek Limestone (Hill, 1901) 

(fig. 4); and (3) the Hensell Sand (Hill, 1901) and the Glen 

Rose Limestone (Hill, 1891) (fig. 5). According to Wierman 

and others (2010) both the Hosston Formation and the 

stratigraphically higher, but thinner, Sligo Formation thin 

and pinch out to the northwest. The Sycamore Sand, which is 

the outcrop equivalent to the Hosston Formation, appears at 

the surface in the bed of the Pedernales River in the northern 

part of the study area. The Sligo Formation pinches out in the 

subsurface farther to the southeast and is not found except at 

depth (for example, in well logs) in the study area; therefore, 

the Sligo Formation is not shown in figure 2, and it is not 

discussed further in this report. 

In the study area, the thickness of the Trinity Group thins 

from the south to the north (Barker and Ardis, 1996). Although 

all formations in the outcrop thin toward the north, the most 

pronounced thinning is found in the lower member of the Glen 

Rose Limestone, which is 195 feet (ft) thick in the south and 

thins to 130 ft thick near the Pedernales River in the north 

(fig. 1). Descriptions of each of the geologic units and their 

associated lithology are shown in figure 2. Additional geologic 

and ichnofossil descriptions are provided in Clark and others 

(2016a, b) and Clark and Morris (2017).

Edwards Group

The late Early Cretaceous Edwards Group (fig. 2) 

is composed of mudstone to grainstone, shales, and chert 

deposited in an open marine to supratidal flats environment 

(Rose, 1972; Maclay and Small, 1986) during separate marine 

transgressions. A marine transgression during the Early 

Cretaceous resulted in the deposition of the Kainer Formation 

(Rose, 1972). The Person Formation was deposited during a 

subsequent marine transgression (Rose, 1972). The Edwards 

Group formed on the landward margin of the Comanche 

shelf, which was sheltered from storm waves and deep 

ocean currents by the Stuart City reef trend in the ancestral 

Gulf of Mexico (Barker and Ardis, 1996; Clark and others, 

2006) (fig. 1). Descriptions of each of the geologic units and 

their associated lithology are shown (fig. 2) and discussed 

in further detail in Clark and others (2016a, b) and Clark 

and Morris (2017). 

Washita, Eagle Ford, Austin, and Taylor Groups

Following the deposition of the Edwards Group was 

tectonic uplift, subaerial exposure, and erosion in the area that 

is now south-central Texas. This area was then once again 

submerged during the late Early Cretaceous when another 

marine transgression resulted in the deposition of shales, 

mudstones, and wackestones that formed the Georgetown 

Formation (Richardson, 1904) of the Washita Group (fig. 2). 

Much of the Georgetown Formation was subsequently 

removed during a period of marine regression (Rose, 1972). 

The Del Rio Clay of the Washita Group (fig. 2), which 

contains clay and packstone, was deposited during a marine 

transgression during the early Late Cretaceous when the Stuart 

City reef (to the south) was breached. Continued deposition 

of sediments in shallow subtidal to intertidal zones resulted in 

the mudstone and wackestone that formed the Buda Limestone 

(Grunig and others, 1977) of the Washita Group (fig. 2). The 

Late Cretaceous Eagle Ford Group (fig. 2) was deposited 

as sandy shale and argillaceous limestone in a lagoonal to 

marine, open shelf environment (Grunig and others, 1977). As 

the marine transgression continued in the Late Cretaceous, an 

open, shallow shelf developed, and sediments were deposited 

far from shore, resulting in the mudstones and wackestones 

that form the Austin Group (fig. 2) (Grunig and others, 1977). 

The uppermost stratigraphic unit exposed in the study area is 

the Late Cretaceous Pecan Gap Chalk of the Taylor Group (fig. 

2). The Pecan Gap Chalk is composed of marl and calcareous 

clay and was deposited in an open marine environment 

following a marine transgression (Ellisor and Teagle, 1934). 

A description of the stratigraphy, lithology, index fossils, and 

stratigraphic thickness is provided (fig. 2), with additional 

detailed discussion available in Clark and others (2016b). 

Structure

The principal structural feature in Hays County is 

the Balcones fault zone (fig. 1), which is the result of late 

Oligocene and early Miocene age (Weeks, 1945b) extensional 

faulting (Weeks, 1945a, b; Galloway and others, 2000, 2011; 

Rose, 2016, 2017) and fracturing. The Balcones faulting 

resulted from the eastern Edwards Plateau uplift (Rose, 2017). 

In the Balcones fault zone, most of the faults in the study area 

are high-angle to vertical, en echelon, normal faults that are 

downthrown to the southeast (George, 1952). As is typical 

with extensional fault zones, the Balcones fault zone includes 

horst and graben structures (Pantea and others, 2014). The 

Balcones fault zone is considered dormant (Ewing, 2005). 

Relay ramp structures are also common within an 

extensional fault system (Ferrill and Morris, 2007). Ramp 

structures can be relatively small features that extend less 

than a few miles or relatively large features that extend tens 

of miles. Relay ramps form in extensional fault systems to 

accommodate rock fabric stress relief and an increase in 

deformation (Clark and Journey, 2006). Ramp structures 

link the footwall of a fault segment with the hanging wall 

of an overlapping fault segment (Collins, 1995; Clark and 

Journey, 2006; Hunt and others, 2015). As extension occurs, 

the increased strain on the rock fabric causes faulting that 

results in the formation of relay ramps with rotation and 

internal fracturing occurring along the relay ramps (Trudgill, 

2002; Ferrill and Morris, 2007). Continued extension results 

in the formation of cross faults within the relay ramp structure 
4 Geologic Framework and Hydrostratigraphy of the Edwards and Trinity Aquifers Within Hays County, Texas

(Trudgill, 2002). Some examples of reports documenting relay 

ramp structures within the Balcones fault zone include Collins 

(1995), Clark and Journey (2006), Clark and others (2013), 

and Hunt and others (2015).

Hydrostratigraphy

Hydrostratigraphically, the rocks exposed in the study 

area represent a section of the upper confining unit to the 

Edwards aquifer, the Edwards aquifer, the upper zone of the 

Trinity aquifer, the middle zone of the Trinity aquifer, and 

the upper part of the lower zone of the Trinity aquifer (fig. 2). 

Descriptions of the HSUs, thicknesses, hydrologic function, 

porosity type, and field identification are provided (fig. 2) and 

are described further in Clark and others (2016b) and Clark 

and Morris (2017).

Upper Confining Unit of the Edwards Aquifer

The rocks that form the upper confining unit to the 

Edwards aquifer are (top to bottom) the Pecan Gap Chalk, 

Austin Group, Eagle Ford Group, Buda Limestone, and Del 

Rio Clay (Maclay and Small, 1976; Hanson and Small, 1995) 

(fig. 2). Because the formations and groups are generally 

categorized as a confining unit to the Edwards aquifer and not 

as separate water-bearing aquifers, the lithologic terms (group, 

formation, limestone, and clay) will be used to describe both 

the geologic framework and the hydrologic characteristics 

of the HSU being discussed. The rocks that form the 

upper confining unit to the Edwards aquifer do not supply 

appreciable amounts of water to wells in the study area except 

for the Austin Group (Petitt and George, 1956) and parts of 

the Georgetown Formation (Stein and Ozuna, 1995); for this 

reason, of the units that compose the upper confining unit to 

the Edwards aquifer, only the Austin Group and Georgetown 

Formation will be described. 

The Austin Group supplies water to several springs in 

Uvalde, Medina, and Bexar Counties, as well as to some 

domestic and irrigation wells (Garza, 1962; Arnow, 1963; 

Banta and Clark, 2012). The most notable springs supplied 

by the Austin Group are San Pedro Spring and San Antonio 

Springs in Bexar County (fig. 1). The most prolific wells and 

springs within the Austin Group likely tap water that moves 

up faults and fractures under artesian conditions from the 

underlying Edwards aquifer (Livingston and others, 1936; 

Veni, 1988; Banta and Clark, 2012). San Marcos Springs in 

Hays County, which is the second-largest spring system in 

Texas, issues from faults that juxtapose the Edwards aquifer 

against the Austin Group (Hanson and Small, 1995).

Although commonly considered part of the Edwards 

aquifer, the Georgetown Formation is part of the upper 

confining unit to the Edwards aquifer (George, 1952; Maclay 

and Small, 1976; Stein and Ozuna, 1995). Maclay and Small 

(1976) listed the Georgetown Formation as HSU I of the 

Edwards aquifer because the formation was the unit where 

drillers would set casing before drilling through to the waterbearing units in the underlying Edwards aquifer. 

Edwards and Trinity Aquifers

In the study area, the Edwards aquifer is within the 

Edwards Group, and the Trinity aquifer is within the Trinity 

Group. The Edwards and Trinity aquifers contain enhanced 

secondary porosity that is developed along bedding planes, 

fractures, and caves (Maclay and Small, 1983; Veni, 1987, 

1988, 1994a; Johnson and others, 2002; Ferrill and others, 

2003; Gary and others, 2011). The Edwards and Trinity 

aquifers have been considered separate aquifers on the basis of 

differences in permeability (Hammond, 1984; Kuniansky and 

Ardis, 2004); other assessments have shown that the Edwards 

aquifer and the upper part of the upper zone of the Trinity 

aquifer might function as a single aquifer (Johnson and others, 

2002; Clark, 2003; Clark and others, 2009; Hunt and others, 

2016). Barker and Ardis (1996) also stated that recharge to the 

Edwards aquifer from the underlying Trinity aquifer occurs 

from diffuse upward leakage. A detailed description of the 

HSUs, thicknesses, hydrologic function, porosity type, and 

field identification is provided (fig. 2).

Edwards Aquifer

Maclay and Small (1976) subdivided the Edwards aquifer 

into HSUs I to VIII (fig. 2). In the informal naming convention 

used by Maclay and Small (1976), the Georgetown Formation 

of the Washita Group was designated as HSU I. Different 

layers that Maclay and Small (1976) discerned in the Person 

Formation of the Edwards Group were designated as HSUs II, 

III, and IV, and different layers of the Kainer Formation of the 

Edwards Group were designated as HSUs V, VI, VII, and VIII.

Trinity Aquifer

Ashworth (1983) subdivided the Trinity aquifer into 

upper, middle, and lower zones (fig. 2). The upper zone of 

the Trinity aquifer yields water from the upper member of 

the Glen Rose Limestone. The middle zone of the Trinity 

aquifer yields water from the lower member of the Glen Rose 

Limestone, the Hensell Sand, and the Cow Creek Limestone. 

The regionally extensive Hammett HSU forms a confining 

unit between the middle and lower zones of the Trinity aquifer 

(Ashworth, 1983; Wierman and others, 2010; Clark and 

others, 2016b). The lower zone of the Trinity aquifer yields 

water from the Sycamore Sand of the Hosston Formation. The 

only outcrop of the Sycamore Sand in Hays County is along 

the Pedernales River in the far northern corner of the county 

(Ashworth, 1983).
Hydrostratigraphy 5

From field observations the authors believe that there 

may be a difference in permeability between the beds in the 

upper and middle zones of the Trinity aquifer depending 

on the presence or absence of argillaceous limestone. 

Argillaceous limestone beds (marl beds) appear to store 

water for extended periods of time and slowly release it 

into fractures or into the contact between beds. From these 

observations the authors believe that significant quantities 

of water may be stored in argillaceous limestone beds in the 

vadose and phreatic zones of the aquifer.

Upper Zone of the Trinity Aquifer

The upper zone of the Trinity aquifer was informally 

subdivided into five HSUs in Bexar County by Clark (2003). 

In the study area, however, there are only four HSUs present 

(listed from top to bottom): Camp Bullis, upper evaporite, 

fossiliferous, and lower evaporite. Although not mapped in 

Hays County, the Cavernous HSU (upper part of the upper 

zone of the Trinity aquifer) found in Bexar County and parts 

of Comal County (Clark and others, 2016b) may exist in 

Hays County in areas that are covered by outcrops of the 

Edwards Group. The possible existence of the Cavernous 

HSU in Hays County is based on a few outcrop observations 

in deeper canyons that dissect through the Edwards aquifer 

and into the underlying upper zone of the Trinity aquifer. 

Additional evidence for the existence of at least a portion of 

the Cavernous HSU in Hays County comes from aquifer tests 

carried out by Hunt and others (2016). 

Middle Zone of the Trinity Aquifer

The middle zone of the Trinity aquifer is composed of 

the Bulverde, Little Blanco, Twin Sisters, Doeppenschmidt, 

Rust, Honey Creek, Hensell, and Cow Creek HSUs (Clark and 

Morris, 2015) (fig. 2). Across the study area south to north 

the various HSUs thin, and the Bulverde, Little Blanco, Twin 

Sisters, and Doeppenschmidt grade into the Rust HSU because 

of changes in the depositional environment. Underlying the 

Cow Creek HSU is the regional confining unit, the Hammett 

HSU, which separates the middle and lower zones of the 

Trinity aquifer.

Several large springs likely issue out of the Honey Creek 

HSU in Hays County, most notably Jacobs Well Spring (figs. 

1 and 6) (Brune, 1975). Also, Pleasant Valley spring (fig. 

1) was recently documented as the Trinity aquifer spring in 

south-central Texas with the largest discharge (Watson and 

others, 2014); its discharge was measured as ranging from 9 

to 18 cubic feet per second (Barton Springs Edwards Aquifer 

Conservation District, 2013). It is likely that the water that 

flows out of Pleasant Valley spring and Jacobs Well Spring 

originates from the underlying Cow Creek HSU and that 

the water flows upward under artesian conditions through 

fractures, faults, and conduits into the Honey Creek HSU and 

ultimately to land surface (Barton Springs Edwards Aquifer 

Conservation District, 2013).

Hammett HSU Confining Unit

Overlying the lower zone of the Trinity aquifer is the 

confining Hammett HSU, which likely restricts the downward 

flow of groundwater and results in contact springs near the 

base of the Cow Creek HSU in some locations (Clark and 

Morris, 2017). In addition to likely restricting downward flow 

in some locations, the Hammett HSU also likely restricts the 

upward flow where the groundwater has a relatively high 

hydraulic head and would otherwise flow towards land surface 

(Clark and others, 2016a, b).

Lower Zone of the Trinity Aquifer

The only exposed part of the lower zone of the Trinity 

aquifer is the Sycamore HSU in the far northern corner of 

Hays County. The Sycamore HSU where exposed along 

the Pedernales River is probably a point of recharge for the 

lower zone of the Trinity aquifer (Wierman and others, 2010). 

Ashworth (1983) and Ashworth and others (2001) stated 

that the primary source of recharge to the lower zone of the 

Trinity aquifer is leakage across the Hammett HSU through 

faults and fractures. 

Implications of Hydrostratigraphic 

Characteristics and Geologic Structure on 

Groundwater Recharge and Flow Paths 

Groundwater recharge and flow paths in the study 

area are influenced not only by the hydrostratigraphic 

characteristics of the individual HSUs but also by faults and 

fractures and geologic structure. Faulting and the resulting 

structures (grabens and horsts) common in fault zones 

like the Balcones fault zone may increase the potential of 

controlling or altering local groundwater flow (Pantea and 

others, 2014) by juxtaposing permeable and less permeable 

lithologies against one another. Dye-tracing studies by 

Johnson and others (2010) indicate that the permeable zones in 

juxtaposed members might be narrow; however, if cavernous 

permeability is present then all available water might be 

transmitted at or through the fault. When juxtaposed against 

zones with relatively more permeability, zones with relatively 

less permeability might act as a barrier to groundwater flow 

(Stein and Ozuna, 1995). 

Current (2018) and past studies, as well as field 

observations, have shown that the groundwater flow paths 

between the Edwards and Trinity aquifers and the various 

HSUs are complex. The source of complexity in the 

aquifer system results from a combination of the original 

depositional history, bioturbation, primary and secondary 

porosity, diagenesis, fracturing, and faulting (Clark and 

others, 2016b). The combination of these factors has resulted 

in the development of modified porosity, permeability, and 

transmissivity within and between the aquifers (Clark and 

others, 2016b). Faulting has produced highly fractured areas 
6 Geologic Framework and Hydrostratigraphy of the Edwards and Trinity Aquifers Within Hays County, Texas

that have allowed for the rapid infiltration of water and 

subsequently formed solutionally enhanced fractures, bedding 

planes, channels, and caves that are highly permeable and 

transmissive (Clark and others, 2016b). The juxtaposition of 

the aquifers and the HSUs caused by faulting has resulted in 

areas of interconnectedness between the Edwards and Trinity 

aquifers and the various HSUs that form the aquifers (Clark, 

2003, 2004; Clark and others, 2009; Pantea and others, 2014; 

Hunt and others, 2015). 

According to Ferrill and Morris (2003), faults within 

the Edwards Group are more dilatant (open) than those in the 

Glen Rose Limestone because the Edwards Group lithologies 

are more competent. Ferrill and Morris (2003) also stated that 

fault deformation increases permeability at or near faults. Fault 

permeability in the Glen Rose Limestone is heterogeneous 

and affects groundwater flow both parallel and perpendicular 

to faults (Ferrill and others, 2003). Clay smear and calcite 

deposition can affect cross-fault flow and may inhibit the 

flow if the deposit is appreciable (Ferrill and others, 2003). 

Solution-enlarged fractures and conduits might also form 

parallel to the dip of relay ramps in the Edwards Group (Clark 

and Journey, 2006) because of the northeast to southwest 

extension. The effects of relay ramps on groundwater flow 

paths within the Edwards and Trinity aquifers are documented 

in Maclay and Small (1983), Groschen (1996), Clark and 

Journey (2006), Clark and others (2013), Pantea and others 

(2014), and Hunt and others (2015).

According to Veni (1988), cave formation is strongly 

guided by secondary fractures that form as a result of faulting, 

rather than by the actual fault plane. Clark and Journey (2006) 

stated that the fractures generally are parallel or perpendicular 

to the main fault trend of the Balcones fault zone. Faulting 

affects cave development through fractures that form because 

of extension perpendicular to the Balcones fault zone (Clark 

and Journey, 2006). As extension of the series of en echelon, 

mainly down-to-the-coast faults occurred, the material had to 

occupy a larger area, resulting in extension perpendicular to 

the fault zone (Clark and Journey, 2006). 

Summary

During 2016–18, the U.S. Geological Survey, in 

cooperation with the Edwards Aquifer Authority, mapped the 

geologic framework and hydrostratigraphy of the Edwards and 

Trinity aquifers within Hays County, Texas. The Texas Water 

Development Board classifies the karstic Edwards and Trinity 

aquifers as major sources of water in south-central Texas. The 

geologic framework and hydrostratigraphy of the Edwards and 

Trinity aquifers largely control groundwater flow paths and 

storage in the aquifers, and therefore, refined mapping will aid 

in anticipating and mitigating issues related to changing land 

use and increasing groundwater demands. 

The purpose of this report is to present the geologic 

framework and hydrostratigraphy of the Edwards and Trinity 

aquifers within Hays County, Tex. The HSUs were mapped 

to improve the understanding of groundwater recharge, 

groundwater flow paths, and discharge. In addition, this 

report contains a detailed map at a scale of 1:24,000 of the 

hydrostratigraphy of the Edwards and Trinity aquifers in 

Hays County.

The rocks exposed within the approximately 560 square 

miles of the study area are within the outcrops of the Trinity 

and Edwards Groups and the overlying Washita, Eagle Ford, 

Austin, and Taylor Groups in Hays County. The rocks are 

sedimentary and formed during the Cretaceous age. Karst areas 

in the study area are characterized by sinkholes, caves, and 

underground streams that allow rapid infiltration of surface 

waters to the subsurface. Faulting in the study area is primarily 

from an Oligocene and early Miocene age extensional fault 

system known as the Balcones fault zone. The Balcones 

fault zone generally trends southwest to northeast in southcentral Texas. The faults are vertical to near vertical with 

normal throw, are en echelon, and are mostly downthrown 

to the southeast.

Hydrostratigraphically, the rocks exposed in the study area 

represent a section of the upper confining unit to the Edwards 

aquifer, the Edwards aquifer, the upper zone of the Trinity 

aquifer, the middle zone of the Trinity aquifer, and the upper 

part of the lower zone of the Trinity aquifer. 

The rocks that form the upper confining unit to the 

Edwards aquifer are (top to bottom) the Pecan Gap Chalk, 

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

Clay, and Georgetown Formation. Although commonly referred 

to as part of the Edwards aquifer, the Georgetown Formation 

is part of the upper confining unit to the Edwards aquifer. 

Previous investigators informally designated the Georgetown 

Formation as hydrostratigraphic unit (HSU) I; different layers 

of the Person Formation of the Edwards Group as HSUs II, 

III, and IV; and different layers of the Kainer Formation of the 

Edwards Group as HSUs V, VI, VII, and VIII.

The upper zone of the Trinity aquifer was informally 

subdivided by previous investigators into five HSUs in Bexar 

County. In the study area, however, there are only four HSUs 

present (listed top to bottom): Camp Bullis, upper evaporite, 

fossiliferous, and lower evaporite. Although not mapped in 

Hays County, the Cavernous HSU (upper part of the upper 

zone of the Trinity aquifer) found in Bexar County and parts 

of Comal County may exist in Hays County in areas that are 

covered by outcrops of the Edwards Group. The middle zone of 

the Trinity aquifer is composed of the Bulverde, Little Blanco, 

Twin Sisters, Doeppenschmidt, Rust, Honey Creek, Hensell, 

and Cow Creek HSUs. Underlying the Cow Creek HSU is the 

regional confining unit, the Hammett HSU, which separates 

the middle and lower zones of the Trinity aquifer. The only 

exposed portion of the lower zone of the Trinity aquifer in the 

study area is the Sycamore HSU in the far northern corner of 

Hays County. 
References Cited 7

Groundwater recharge and flow paths in the study 

area are influenced not only by the hydrostratigraphic 

characteristics of the individual HSUs but also by faults and 

fractures and geologic structure. Faulting and the resulting 

structures (grabens and horsts) common in fault zones like the 

Balcones fault zone may increase the potential of controlling 

or altering local groundwater flow by juxtaposing permeable 

and less permeable lithologies against one another. 

References Cited 

Amsbury, D.L., 1974, Stratigraphic petrology of lower and 

middle Trinity rocks on the San Marcos platform, southcentral Texas, in Perkins, B.F., ed., Aspects of Trinity 

geology: Geoscience and Man, v. 8, p. 1–35.

Arnow, Ted, 1963, Ground-water geology of Bexar County, 

Texas: U.S. Geological Survey Water-Supply Paper 1588, 

36 p., accessed July 25, 2018, at https://pubs.usgs.gov/

wsp/1588/report.pdf.

Ashworth, J.B., 1983, Ground-water availability of the Lower 

Cretaceous formations in the Hill Country of south-central 

Texas: Texas Department of Water Resources Report 

273, 172 p., accessed July 25, 2018, at http://eahcp.org/

documents/1983_Ashworth_GroundWaterAvailability.pdf.

Ashworth, J.B., Stein, W.G., Donnelly, A.C.A., Persky, K., 

and Jones, J.P., 2001, The lower Trinity aquifer of Bandera 

and Kerr Counties, Texas: Plateau Regional Water Planning 

Group and Texas Water Development Board, 128 p., 

accessed July 25, 2018, at https://www.twdb.texas.gov/

publications/reports/contracted_reports/doc/0704830695_

RegionJ/Reference_LowerTrinity.pdf.

Banta, J.R., and Clark, A.K., 2012, Groundwater levels 

and water-quality observations pertaining to the Austin 

Group, Bexar County, Texas, 2009–11: U.S. Geological 

Survey Scientific Investigations Report 2012–5278, 18 p., 

2 apps., accessed July 25, 2018, at https://pubs.usgs.gov/

sir/2012/5278/.

Barker, R.A., and Ardis, A.F., 1996, Hydrogeological 

framework of the Edwards-Trinity aquifer system, westcentral Texas: U.S. Geological Survey Professional Paper 

1421–B, 61 p., accessed July 25, 2018, at http://pubs.

er.usgs.gov/publication/pp1421B.

Barnes, V.E., 1976, Geologic atlas of Texas, Crystal CityEagle Pass sheet, Dolan Hoye Eargle memorial edition: 

Austin, University of Texas-Austin, Bureau of Economic 

Geology, 1 sheet, scale 1:250,000.

Barnes, V.E., and Rose, P.R., 1981, Geologic atlas of Texas, 

Llano sheet: Austin, University of Texas, Bureau of 

Economic Geology, scale 1:250,000.

Barton Springs Edwards Aquifer Conservation District, 2013, 

Pleasant Valley spring: A newly documented karst spring of 

the Texas Hill Country Trinity aquifer: Accessed February 

23, 2018, at http://bseacd.org/2015/07/pleasant-valleyspring/.

Blome, C.D., and Clark, A.K., 2014, Key subsurface data help 

to refine Trinity aquifer hydrostratigraphic units, southcentral Texas: U.S. Geological Survey Data Series 768, 

1 sheet. [Also available at https://pubs.usgs.gov/ds/768/.]

Brune, Gunnar, 1975, Major and historical springs of Texas: 

Texas Water Development Board Report 189, 95 p., 

accessed July 25, 2018, at https://www.twdb.texas.gov/

publications/reports/numbered_reports/doc/R189/R189.pdf.

Choquette, P.W., and Pray, L.C., 1970, Geologic nomenclature 

and classification of porosity in sedimentary carbonates: 

American Association of Petroleum Geologists Bulletin, 

v. 54, no. 2, p. 207–250.

Clark, A.K., 2003, Geologic framework and hydrogeologic 

features of the Glen Rose Limestone, Camp Bullis Training 

Site, Bexar County, Texas: U.S. Geological Survey WaterResources Investigations Report 03–4081, 9 p., 1 pl., scale 

1:24,000, accessed July 20, 2018, at https://pubs.usgs.gov/

wri/wri034081/.

Clark, A.K., 2004, Geologic framework and hydrogeologic 

characteristics of the Glen Rose Limestone, Camp Stanley 

Storage Activity, Bexar County, Texas: U.S. Geological 

Survey Scientific Investigations Map 2831, scale 1:24,000, 

1 p., 1 pl., accessed July 20, 2018, at http://pubs.usgs.gov/

sim/2005/2831/pdf/sim2831.pdf.

Clark, A.K., Blome, C.D., and Morris, R.R., 2014, Geologic 

framework and hydrostratigraphy of Guadalupe River State 

Park and Honey Creek State Natural Area, Kendall and 

Comal Counties, Texas: U.S. Geological Survey Scientific 

Investigations Map 3303, 8 p., 1 sheet, scale 1:24,000, 

accessed July 20, 2018, at https://pubs.usgs.gov/sim/3303/.

Clark, A.K., Faith, J.R., Blome, C.D., and Pedraza, D.E., 2006, 

Geologic map of the Edwards aquifer in northern Medina 

and northeastern Uvalde Counties, south-central Texas: 

U.S. Geological Survey Open-File Report 2006–1372, 

23 p., 1 pl., accessed July 20, 2018, at https://pubs.er.usgs.

gov/publication/ofr20061372.

Clark, A.K., Golab, J.A., and Morris, R.R., 2016a, Geologic 

framework, hydrostratigraphy, and ichnology of the Blanco, 

Payton, and Rough Hollow 7.5-minute quadrangles, Blanco, 

Comal, Hays, and Kendall Counties, Texas: U.S. Geological 

Survey Scientific Investigations Map 3363, 21 p., 1 sheet, 

scale 1:24,000, accessed July 20, 2018, at https://pubs.

er.usgs.gov/publication/sim3363.
8 Geologic Framework and Hydrostratigraphy of the Edwards and Trinity Aquifers Within Hays County, Texas

Clark, A.K., Golab, J.A., and Morris, R.R., 2016b, Geologic 

framework and hydrostratigraphy of the Edwards and 

Trinity aquifers within northern Bexar and Comal Counties, 

Texas: U.S. Geological Survey Scientific Investigations 

Map 3366, 28 p., 1 sheet, scale 1:24,000, accessed July 20, 

2018, at https://pubs.er.usgs.gov/publication/sim3366.

Clark, A.K., and Journey, C.A., 2006, Flow paths in the 

Edwards aquifer, northern Medina and northeastern 

Uvalde Counties, Texas, based on hydrologic identification 

and geochemical characterization and simulation: 

U.S. Geological Survey Scientific Investigations Report 

2006–5200, 48 p., accessed July 20, 2018, at https://pubs.

usgs.gov/sir/2006/5200/.

Clark, A.K., and Morris, R.R., 2015, Geologic and 

hydrostratigraphic map of the Anhalt, Fischer, and Spring 

Branch 7.5-minute quadrangles, Blanco, Comal, and 

Kendall Counties, Texas: U.S. Geological Survey Scientific 

Investigations Map 3333, 13 p., 1 sheet, scale 1:50,000, 

accessed July 20, 2018, at https://pubs.usgs.gov/sim/3333/.

Clark, A.K., and Morris, R.R., 2017, Bedrock geology and 

hydrostratigraphy of the Edwards and Trinity aquifers 

within the Driftwood and Wimberley 7.5-minute 

quadrangles, Hays and Comal Counties, Texas: U.S. 

Geological Survey Scientific Investigations Map 3386, 

12 p., 1 sheet, scale 1:24,000, accessed July 20, 2018, at 

https://doi.org/10.3133/sim3386.

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., accessed July 

20, 2018, at http://pubs.usgs.gov/sir/2013/5149/.

Clark, A.R., Blome, C.D., and Faith, J.R., 2009, Map showing 

the geology and hydrostratigraphy of the Edwards aquifer 

catchment area, northern Bexar County, south-central Texas: 

U.S. Geological Survey Open-File Report 2009–1008, 

24 p., 1 pl., scale 1:50,000, accessed July 20, 2018, at http://

pubs.usgs.gov/of/2009/1008/.

Collins, E.W., 1995, Structural framework of the Edwards 

aquifer, Balcones fault zone, central Texas: Gulf Coast 

Association of Geological Societies Transactions, v. 45, p. 

135–142, accessed September 4, 2018, at http://archives.

datapages.com/data/gcags/data/045/045001/0135.htm?q=%

2BtextStrip%3Abexar+textStrip%3Ashale+%2ByearSort%

3A%5B1980+TO+2013%5D.

Douglas, R.C., 1960, The foraminiferal genus Orbitolina in 

North America: U.S. Geological Survey Professional Paper 

333, 52 p., accessed July 25, 2018, at https://pubs.er.usgs.

gov/publication/pp333.

Dunham, R.J., 1962, Classification of carbonate rocks 

according to depositional texture, in Ham, W.E., 

Classification of carbonate rocks symposium: American 

Association of Petroleum Geologists Memoir 1, p. 108–121.

Ellisor, A.C., and Teagle, John, 1934, Correlation of Pecan 

Gap Chalk in Texas: American Association of Petroleum 

Geologists, v. 18, no. 11, p. 1506–1536.

Esri, 2016, ArcGIS Desktop—Release 10: Redlands, Calif., 

Esri, accessed February 18, 2016, at http://www.esri.com.

Ewing, T.E., 2005, Phanerozoic development of the Llano 

Uplift: South Texas Geological Society Bulletin, May issue, 

p. 15–25.

Ferrill, D.A., and Morris, A.P., 2007, Fault zone deformation 

controlled by carbonate mechanical stratigraphy, Balcones 

fault system, Texas: American Association of Petroleum 

Geologists Bulletin, v. 92, no. 3, p. 359–380, accessed 

July 25, 2018, at http://archives.datapages.com/data/

bulletns/2008/03mar/BLTN07066/IMAGES/BLTN07066.

PDF.

Ferrill, D.A., and Morris, A.P., 2003, Dilational normal faults: 

Journal of Structural Geology, v. 25, no. 2, p. 183–196, 

accessed September 4, 2018, at https://doi.org/10.1016/

S0191-8141(02)00029-9. 

Ferrill, D.A., Sims, D.W., Morris, A.P., Waiting, D.J., and 

Franklin, N.M., 2003, Structural controls on the Edwards 

aquifer/Trinity aquifer interface in the Camp Bullis 

quadrangle, Texas: Edwards Aquifer Authority and U.S. 

Army Corps of Engineers, prepared by Southwest Research 

Institute, San Antonio, Tex., variously paged.

Galloway, W.E., Ganey-Curry, P.F., Li, X., and Buffler, R.T., 

2000, Cenozoic depositional history of the Gulf of Mexico 

Basin: American Association of Petroleum Geologists 

Bulletin, v. 84, p. 1743–1774.

Galloway, W.E., Whiteaker, T.L., and Ganey-Curry, Patricia, 

2011, History of Cenozoic North American drainage basin 

evolution, sediment yield, and accumulation in the Gulf of 

Mexico basin: Geosphere, v. 7, no. 4, p. 938–973, accessed 

July 24, 2018, at https://doi.org/10.1130/GES00647.1.

Gary, Marcus, Veni, George, Shade, Beverly, and Gary, Robin, 

2011, Spatial and temporal recharge variability related to 

groundwater interconnection of the Edwards and Trinity 

aquifers, Camp Bullis, Bexar and Comal Counties, Texas, 

in Interconnection of the Trinity (Glen Rose) and Edwards 

aquifers along the Balcones fault zone and related topics: 

Karst Conservation Initiative Proceedings, February 

17, 2011, p. 6–10, accessed February 9, 2016, at http://

www.speleogenesis.info/directory/karstbase/publication.

php?id=9869
References Cited 9

Garza, S., 1962, Recharge, discharge, and changes in groundwater storage in the Edwards and associated limestones, 

San Antonio area, Texas—A progress report of studies, 

1955–59: Texas Water Development Board Bulletin 6201, 

42 p., accessed July 25, 2018, at http://www.twdb.texas.gov/

publications/reports/bulletins/doc/B6201.pdf.

George, P.G., Mace, R.E., and Petrossian, Rima, 2011, 

Aquifers of Texas: Texas Water Development Board Report 

380, 172 p., accessed July 25, 2018, at https://www.twdb.

texas.gov/publications/reports/numbered_reports/doc/

R380_AquifersofTexas.pdf?d=23236.398949342703.

George, W.O., 1952, Geology and ground-water resources of 

Comal County, Texas, with sections on Surface-water runoff

by Breeding, S.D., and Chemical character of the water 

by Hastings, W.W.: U.S. Geological Survey Water Supply 

Paper 1138, 126 p., 3 pls., accessed July 25, 2018, at https://

pubs.er.usgs.gov/publication/wsp1138.

Groschen, G.E., 1996, Hydrogeologic factors that affect the 

flow path of water in selected zones of the Edwards aquifer, 

San Antonio region, Texas: U.S. Geological Survey WaterResources Investigations Report 96–4046, accessed July 25, 

2018, at https://pubs.er.usgs.gov/publication/wri964046.

Grunig, D., Young, K., Jordan, M.A., and Parker, D.L., 1977, 

Guidebook to the geology of Travis County: University of 

Texas, accessed December 4, 2017, at http://www.library.

utexas.edu/geo/ggtc/ch1.html.

Hammond, W.W., Jr., 1984, Hydrogeology of the Lower Glen 

Rose aquifer, south-central Texas: Austin, University of 

Texas, Ph.D. dissertation, 245 p. 

Hanson, J.A., and Small, T.A., 1995, Geologic framework 

and hydrogeologic characteristics of the Edwards aquifer 

outcrop, Hays County, Texas: U.S. Geological Survey 

Water-Resources Investigations Report 95–4265, 10 p., 

accessed July 25, 2018, at https://pubs.usgs.gov/wri/wri95-

4265/pdf/wri95-4265.pdf.

Hill, R.T., 1891, The Comanche Series of the Texas-Arkansas 

region: Geological Society of America Bulletin 2, p. 

503–528. 

Hill, R.T., 1900, Physical geography of the Texas region: U.S. 

Geological Survey, Topographic Atlas of the United States, 

folio 3.

Hill, R.T., 1901, Geography and geology of the Black and 

Grand Prairies, Texas, with detailed descriptions of the 

Cretaceous formations and special reference to artesian 

waters, in Walcott, C.D., ed., Twenty-first annual report 

of the United States Geological Survey to the Secretary of 

the Interior, 1899–1900: Part VII—Texas: U.S. Geological 

Survey Annual Report 21.

Hunt, B.B., Andrews, A.G., and Smith, B.A., 2016, Hydraulic 

conductivity testing in the Edwards and Trinity aquifers 

using multiport monitor well systems, Hays County, central 

Texas: Barton Springs/Edwards Aquifer Conservation 

District, Report of Investigations 2016–0831, 39 p., 

accessed July 25, 2018, at https://bseacd.org/uploads/Huntet-al.-2016_Slug-Testing_FINAL.pdf.

Hunt, B.B., Smith, B.A., Andrews, A., Wierman, D.A., Broun, 

A.S., and Gary, M.O., 2015, Relay ramp structures and their 

influence on groundwater flow in the Edwards and Trinity 

aquifers, Hays and Travis Counties, central Texas: National 

Cave and Karst Institute, symposium 4, 14th Sinkhole 

Conference, 12 p., accessed July 25, 2018, at https://www.

researchgate.net/publication/281558159_Relay_Ramp_

Structures_and_Their_Influence_on_Groundwater_Flow_

in_the_Edwards_and_Trinity_Aquifers_Hays_and_Travis_

Counties_Central_Texas.

Imlay, R.W., 1940, Lower Cretaceous and Jurassic formation 

of southern Arkansas and their oil and gas possibilities: 

Arkansas Geological Survey Information Circular, no. 12, 

64 p.

Imlay, R.W., 1945, Subsurface Lower Cretaceous formations 

of south Texas: American Association of Petroleum 

Geologists Bulletin, v. 29, no. 10, p. 1416–1469.

Inden, R.F., 1974, Lithofacies and depositional model for a 

Trinity Cretaceous sequence, central Texas, in Perkins, 

B.F., and Amsbury, D.L., eds., Aspects of Trinity geology: 

Geoscience and Man, v. 8, p. 37–52.

Johnson, Steve, Esquilin, Roberto, Mahula, David, Thompson, 

Emily, Mireles, Jesse, Gloyd, Ron, Sterzenback, Joseph, 

Hoyt, John, and Schindel, Geary, 2002, Hydrogeologic data 

report for 2001: Edwards Aquifer Authority, 134 p. 

Johnson, S., Schindel, G., and Veni, G., 2010, Tracing 

groundwater flow paths in the Edwards aquifer recharge 

zone, Panther Springs Creek Basin, northern Bexar County, 

Texas: Edwards Aquifer Authority Report 10–01, 112 p.

Kuniansky, E.L., and Ardis, A.F., 2004, Hydrogeology and 

ground-water flow in the Edwards-Trinity aquifer system, 

west-central Texas—Regional aquifer-system analysis—

Edwards-Trinity: U.S. Geological Survey Professional 

Paper 1421–C, 78 p. [Also available at https://pubs.er.usgs.

gov/publication/pp1421C.]

Livingston, P., Sayre, A.N., and White, W.N., 1936, Water 

resources of the Edwards Limestone in the San Antonio 

area, Texas: U.S. Geological Survey Water-Supply Paper 

773B, 70 p., accessed July 25, 2018, at https://pubs.er.usgs.

gov/publication/wsp773B.
10 Geologic Framework and Hydrostratigraphy of the Edwards and Trinity Aquifers Within Hays County, Texas

Lozo, F.E., and Stricklin, F.L., Jr., 1956, Stratigraphic notes 

on the outcrop basal Cretaceous, central Texas: Gulf Coast 

Association of Geological Societies Transactions, v. 6, 

p. 67–78.

Maclay, R.W., and Small, T.A., 1976, Progress report on 

geology of the Edwards aquifer, San Antonio area, Texas, 

and preliminary interpretation of borehole geophysical and 

laboratory data on carbonate rocks: U.S. Geological Survey 

Open-File Report 76–627, 65 p. [Also available at https://

pubs.er.usgs.gov/publication/ofr76627.]

Maclay, R.W., and Small, T.A., 1983, Hydrostratigraphic 

subdivisions and fault barriers of the Edwards aquifer, 

south-central Texas: Journal of Hydrology, v. 61, 

p. 127–146.

Maclay, R.W., and Small, T.A., 1986, Carbonate geology and 

hydrology of the Edwards aquifer in the San Antonio area, 

Texas: Texas Water Development Board, Report 296, 90 

p., accessed July 25, 2018, at http://www.twdb.texas.gov/

publications/reports/numbered_reports/doc/R296/R296.pdf.

Pantea, M.P., Blome, C.D., and Clark, A.K., 2014, Threedimensional model of the hydrostratigraphy and structure 

of the area in and around the U.S. Army-Camp Stanley 

Storage Activity Area, northern Bexar County, Texas: 

U.S. Geological Survey Scientific Investigations Report 

2014–5074, 13 p. [Also available at https://doi.org/10.3133/

sir20145074.]

Pedraza, D.E., Clark, A.K., and Morris, R.R., 2018, Geospatial 

dataset of the geologic framework and hydrostratigraphy 

of the Edwards and Trinity aquifers within Hays County, 

Texas, at 1:24,000 scale: U.S. Geological Survey data 

release, https://doi.org/10.5066/P9IEJHMH.

Perkins, B.F., 1974, Paleoecology of a rudist reef complex in 

the Comanche Cretaceous Glen Rose Limestone of central 

Texas, in Perkins, B.F., and Amsbury, D.L., eds., Aspects of 

Trinity geology: Geoscience and Man, v. 8, p. 131–173.

Petitt, B.M., Jr., and George, W.O., 1956, Ground-water 

resources of the San Antonio area, Texas: Texas Board 

Water Engineers Bulletin 5608, v. 1, 80 p.; v. 2, pt. 1, 

252 p.; v. 2, pt. 2, 285 p.; v. 2, pt. 3, 335 p. 

Richardson, G.B., 1904, Report of a reconnaissance in 

Trans-Pecos Texas, north of the Texas and Pacific Railway: 

University of Texas, Mineral Survey Bulletin, no. 9, 119 p., 

accessed July 25, 2018, at https://repositories.lib.utexas.edu/

handle/2152/24408.

Rose, P.R., 1972, Edwards Group, surface and subsurface, 

central Texas: Austin, University of Texas, Bureau of 

Economic Geology Report of Investigations 74, 198 p.

Rose, P.R., 2016, Late Cretaceous and Tertiary burial history, 

central Texas: Gulf Coast Association of Geological 

Societies Journal, v. 5, p. 141–179, accessed July 25, 

2018, at http://www.gcags.org/Journal/2016.GCAGS.

Journal/2016.GCAGS.Journal.v5.09.p141-179.Rose.pdf. 

Rose, P.R., 2017, Regional stratigraphy of the Edwards Group 

and associated formations of Texas (Lower Cretaceous, 

Comanchean): In defense of the classic view: Gulf Coast 

Association of Geological Societies Journal, v. 6, p. 

111–134, accessed July 25, 2018, at http://www.gcags.org/

Journal/2017.GCAGS.Journal/2017.GCAGS.Journal.v6.08.

p111-134.Rose.pdf. 

Stein, W.G., and Ozuna, G.B., 1995, Geologic framework 

and hydrogeologic characteristics of the Edwards Aquifer 

recharge zone, Bexar County, Texas: U.S. Geological 

Survey Water-Resources Investigations Report 95–4030, 

8 p. [Also available at https://pubs.er.usgs.gov/publication/

wri954030.]

Stricklin, F.L., Jr., and Smith, C.I., 1973, Environmental 

reconstruction of a carbonate beach complex, Cow Creek 

(Lower Cretaceous) Formation of central Texas: Geological 

Society of America Bulletin, v. 84, no. 4, p. 1349–1368.

Stricklin, F.L., Jr., Smith, C.I., and Lozo, F.E., 1971, 

Stratigraphy of lower Cretaceous Trinity deposits of central 

Texas: Austin, University of Texas, Bureau of Economic 

Geology Report of Investigations 71, 63 p. 

Trudgill, B.D., 2002, Structural controls on drainage 

development in the Canyonlands grabens of southeast 

Utah, USA, in Underhill, J.R., and Trudgill, B.D., eds., 

The structure and stratigraphy of rift systems: American 

Association of Petroleum Geologists Bulletin Special Issue, 

v. 86, no. 6, p. 1095–1112, accessed July 25, 2018, at http://

archives.datapages.com/data/bulletns/2002/06jun/1095/

images/02_1095.pdf.

U.S. Census Bureau, 2016, Five of the Nation’s eleven fastestgrowing cities are in Texas, Census Bureau reports: U.S. 

Census Bureau, accessed July 12, 2016, at http://www.

census.gov/newsroom/press-releases/2016/cb16-81.html.

U.S. Geological Survey, 2018, National Geologic Map 

Database, Geolex search: Accessed July 23, 2018, at http://

ngmdb.usgs.gov/Geolex/search.

Veni, G., 1987, Fracture permeability: Implications on 

cave and sinkhole development and their environmental 

assessments, in Beck, B.F., and Wilson, W.L., eds., Karst 

hydrogeology: Engineering and environmental applications: 

Boston, A.A. Balkema, p. 101–105.

Veni, G., 1988, The caves of Bexar County (2d ed.): Austin, 

Tex., Texas Memorial Museum Speleological Monographs 

2, 300 p. 
References Cited 11

Veni, G., 1994, Geomorphology, hydrology, geochemistry, 

and evolution of the karstic Lower Glen Rose aquifer, 

south-central Texas: Pennsylvania State University, Ph.D. 

dissertation, 712 p.

Watson, J.A., Hunt, B.B., Gary, M.O., Wierman, D.A., and 

Smith, B.A., 2014, Potentiometric surface investigation 

of the middle Trinity aquifer in western Hays County, 

Texas: Austin, Texas, Barton Springs/Edwards Aquifer 

Conservation District Report of Investigations 2014–1002, 

21 p., accessed July 25, 2018, at http://bseacd.org/uploads/

Watson_et_al_BSEACD_RI2014_1002_FINAL.pdf.

Weeks, A.W., 1945a, Oakville, Cuero, and Goliad Formations 

of Texas Coastal Plain between Brazos River and Rio 

Grande: American Association of Petroleum Geologists 

Bulletin, v. 29, p. 1721–1732.

Weeks, A.W., 1945b, Balcones, Luling and Mexia fault zones 

in Texas: American Association of Petroleum Geologists 

Bulletin, v. 29, no. 12, p. 1733–1737.

Wentworth, C.K., 1922, A scale of grade and class terms for 

clastic sediments: Journal of Geology, v. 31, p. 377–392.

Whitney, M.I., 1952, Some zone marker fossils of the Glen 

Rose Formation of central Texas: Journal of Paleontology, 

v. 26, no. 1, p. 65–73. 

Wierman, D.A., Broun, A.S., and Hunt, B.B., 2010, 

Hydrogeologic atlas of the Hill Country Trinity aquifer, 

Blanco, Hays, and Travis Counties, central Texas: Prepared 

by the Hays-Trinity, Barton Springs/Edwards Aquifer, 

and Blanco-Pedernales Groundwater Conservation 

Districts, 17 pls. [DVD], accessed July 25, 2018, at https://

repositories.lib.utexas.edu/handle/2152/8977.
12 Geologic Framework and Hydrostratigraphy of the Edwards and Trinity Aquifers Within Hays County, Texas

For more information about this publication, contact

Director, Texas Water Science Center

U.S. Geological Survey

1505 Ferguson Lane

Austin, TX 78754–4501 

For additional information visit https://www.usgs.gov/centers/tx-water.

Publishing support provided by

Lafayette Publishing Service Center
Clark and others—Geologic Framework and Hydrostratigraphy of the Edwards and Trinity Aquifers Within Hays County, Texas—SIM 3418

I SBN 978- 1- 4113- 4262-0

9 7 8 1 4 1 1 3 426 2 0

ISSN 2329-1311 (print)

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3418 Printed on recycled paper