Bedrock Geology and Hydrostratigraphy of the 

Edwards and Trinity Aquifers Within the Driftwood 

and Wimberley 7.5-Minute Quadrangles, Hays and 

Comal Counties, Texas

Pamphlet to accompany

Scientific Investigations Map 3386

U.S. Department of the Interior

U.S. Geological Survey

Bedrock Geology and Hydrostratigraphy 

of the Edwards and Trinity Aquifers Within 

the Driftwood and Wimberley 7.5-Minute 

Quadrangles, Hays and Comal Counties, 

Texas

By Allan K. Clark and Robert R. Morris

Pamphlet to accompany

Scientific Investigations Map 3386

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

William H. Werkheiser, Acting Director

U.S. Geological Survey, Reston, Virginia: 2017

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Suggested citation:

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

ScienceBase citation:

Clark, A.K., Pedraza, D.E., and Morris, R.R., 2017, Data release of the 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 data release, https://doi.org/10.5066/F76D5RXQ.

ISSN 2329-132X (online)
iii

Contents

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

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

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

Description of Study Area ...................................................................................................................3

Methods of Investigation.....................................................................................................................3

Geology............................................................................................................................................................4

Trinity Group...........................................................................................................................................4

Edwards Group......................................................................................................................................4

Faulting ...................................................................................................................................................5

Hydrostratigraphy..........................................................................................................................................5

Edwards Aquifer....................................................................................................................................7

Trinity Aquifer ........................................................................................................................................7

Upper Zone of the Trinity Aquifer..............................................................................................7

Middle Zone of the Trinity Aquifer.............................................................................................8

Hydrologic Characteristics of Structure...........................................................................................8

Summary..........................................................................................................................................................8

Acknowledgments.........................................................................................................................................9

References Cited..........................................................................................................................................10

Sheet

1. Bedrock geology and hydrostratigraphy of the Edwards and Trinity aquifers

within the Driftwood and Wimberley 7.5-minute quadrangles, Hays and Comal

Counties, Texas ........................................................................................................................ link

Figures

1. Location of the study area...........................................................................................................2

2. Photograph showing rudist in the basal nodular member of the Kainer Formation .........3

3. Photograph showing chert nodules in the dolomitic member of the Kainer Formation...6

4. Photograph showing laminations in the grainstone member of the Kainer Formation,

some fenestral porosity near the top of the boulder ..............................................................6

5. Photograph showing solution channels in the hydrostratigraphic units VIII (basal

nodular member/Walnut Clay)....................................................................................................7

6. Photograph of tepee structures at the base of the fossiliferous hydrostratigraphic

units.................................................................................................................................................8
iv

Table

1. Summary of the geologic framework, hydrostratigraphy, ichnology, of the Edwards 

and Trinity aquifers in the Driftwood and Wimberley 7.5-minute quadrangles in Hays 

and Comal Counties, Tex....................................................................................................................5

Conversion Factors

International System of Units to Inch/Pound

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)

Inch/Pound 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)

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.
Bedrock Geology and Hydrostratigraphy of the Edwards 

and Trinity Aquifers Within the Driftwood and Wimberley 

7.5-Minute Quadrangles, Hays and Comal Counties, Texas

By Allan K. Clark and Robert R. Morris

Abstract

The Edwards and Trinity aquifers are major sources of water 

in south-central Texas and are both classified as major aquifers 

by the State of Texas. The population in Hays and Comal Counties is rapidly growing, increasing demands on the area’s water 

resources. To help effectively manage the water resources in the 

area, refined maps and descriptions of the geologic structures 

and hydrostratigraphic units of the aquifers are needed. This 

report presents the detailed 1:24,000-scale bedrock hydrostratigraphic map as well as names and descriptions of the geologic 

and hydrostratigraphic units of the Driftwood and Wimberley 

7.5-minute quadrangles in Hays and Comal Counties, Texas.

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, and the middle zone of the Trinity aquifer. In the study 

area, the Edwards aquifer is composed of the Georgetown Formation and the rocks forming the Edwards Group. The Trinity 

aquifer is composed of the rocks forming the Trinity Group. 

The Edwards and Trinity aquifers are karstic with high secondary porosity along bedding and fractures. The Del Rio Clay is a 

confining unit above the Edwards aquifer and does not supply 

appreciable amounts of water to wells in the study area.

The hydrologic connection between the Edwards and 

Trinity aquifers and the various hydrostratigraphic units is complex because the aquifer system is a combination of the original 

Cretaceous depositional environment, bioturbation, primary and 

secondary porosity, diagenesis, and fracturing of the area from 

Miocene faulting. All of these factors have resulted in development of modified porosity, permeability, and transmissivity 

within and between the aquifers. Faulting produced highly 

fractured areas which allowed for rapid infiltration of water and 

subsequently formed solutionally enhanced fractures, bedding 

planes, channels, and caves that are highly permeable and transmissive. Because of faulting the juxtaposition of the aquifers 

and hydrostratigraphic units has resulted in areas of interconnectedness between the Edwards and Trinity aquifers and the 

various hydrostratigraphic units that form the aquifers.

Introduction

The Edwards and Trinity aquifers (fig. 1) are major 

sources of water in south-central Texas (Kuniansky and 

Ardis, 2004) and are both classified as major aquifers by 

the State of Texas (George and others, 2011). The population of the central Texas area in Hays and Comal Counties is rapidly growing, increasing demands on the area’s 

water resources (U.S. Census Bureau, 2016). To effectively 

manage the water resources in the area, detailed maps and 

descriptions of the geologic structures and hydrostratigraphic units (HSUs) of the aquifers in Hays and Comal 

Counties, Tex., are needed. Groundwater flow and storage 

in the Edwards and Trinity aquifers are largely controlled 

by the aquifers’ structure and hydrostratigraphy, and 

therefore, refined information about these features can help 

in anticipating and mitigating issues related to changing 

land use and increasing groundwater demands. In 2016, 

the U.S. Geological Survey (USGS) mapped the bedrock 

geology and hydrostratigraphy of the Edwards and Trinity 

aquifers within the Driftwood and Wimberley 7.5-minute 

quadrangles in Hays and Comal Counties, Tex. Descriptions of the bedrock geology and HSUs used in this report 

are modified from those of Clark and others (2016).

The karstic geologic setting of Hays and Comal 

Counties underscores the need for updated hydrostratigraphic information. For example, the dissolution of the 

carbonate rocks composing the Edwards and Trinity aquifers results in distinctive landforms rich in both springs 

and karst features (caves, sinkholes, and other visible 

areas of solution-enlarged porosity). Porosity developed 

in carbonate rocks can have an appreciable effect on the 

hydrostratigraphic characteristics of the formations and 

can create focused points of recharge (caves and sinkholes) 

and discharge (seeps and springs) (Hanson and Small, 

1995; U.S. Geological Survey, 2011). The same porosity 

that can focus recharge can also result in an aquifer that is 

highly susceptible to contamination because storm-water 

runoff is quickly transferred to the subsurface (Ryan and 

Meiman, 1996).
2 Bedrock Geology and Hydrostratigraphy of the Edwards and Trinity Aquifers, Texas

Figure 1. Location of the study area.

§ ̈¦35

§ ̈¦35

§ ̈¦35

§ ̈¦410

§ ̈¦37

£¤281

£¤290

£¤281

£¤281

UV46

§ ̈¦10

§ ̈¦10

UV16

UV173

§ ̈¦10

£¤290

1604

Blanco River

River

Little Blanco

Medina

Guadalupe

River

River

Medina

San Antonio

River

River

Honey

Creek

San 

Antonio

San

Marcos

Bandera

Round

Mountian

Wimberley

Saint 

Hedwig

Kyle

Bulverde

Somerset

Blanco

New Braunfels

Johnson

City

Buda

Boerne

Dripping

Springs

Twin 

Sisters

Pipe Creek

Medina

Lake

Canyon

Lake

KENDALL

COUNTY

HAYS

COUNTY

COMAL

COUNTY

BLANCO

COUNTY

BANDERA

COUNTY

BEXAR

COUNTY

BALCONES FAULT ZONE

97°52'30"

98°15'00"

98°37'30"

99°00'00"

99°22'30"

30°22'30"

30°00'00"

29°37'30"

29°15'00"

0 5 10 15 20

0 10 20 5 15 KILOMETERS

MILES

EXPLANATION

Edwards aquifer

Trinity aquifer

USGS 7.5-minute quadrangle 

 study area boundary

Balcones fault zone

Base modified from U.S. Geological Survey digital 

data at 1:100,000-scale

Universal Transverse Mercator, zone 14

North American Datum of 1983

Comal Springs

Government Canyon

State Natural

Area

Camp 

Bullis

Camp 

Stanley

Magic Springs

Ouachita Structural Belt

TEXAS

BALCONES FAULT ZONE

Trinity aquifer

Edwards aquifer

COMANCHE SHELF

San Marcos

Platform

Study area

Wimberley 

Quadrangle

Ancestral Gulf Of Mexico

Present–day

Gulf Of Mexico

Stuart City Reef

Trend

Wimberley 

Quadrangle

Driftwood Quadrangle
Introduction 3

Purpose and Scope

The purpose of this report is to present the bedrock 

geology and hydrostratigraphy of the Edwards and Trinity 

aquifers within the Driftwood and Wimberley 7.5-minute 

quadrangles in Hays and Comal Counties, Tex. The hydrostratigraphic units were mapped to help improve the understanding 

of groundwater recharge, discharge, and groundwater flowpaths. 

The scope of the report is focused on bedrock geology and 

hydrostratigraphy of the Edwards and Trinity aquifers within the 

Driftwood and Wimberley 7.5-minute quadrangles in Hays and 

Comal Counties, Tex. (fig. 1). This report presents the detailed 

1:24,000 hydrostratigraphic map as well as names and descriptions of the bedrock geologic and hydrostratigraphic units in the 

study area. The boundary of the study area coincides with the 

quadrangle boundaries (fig.1).

Description of Study Area

The rocks within the approximately 128 square mile (mi2) 

study area are sedimentary and karstic and range in age from 

Early to Late Cretaceous. Karstic areas are characterized by 

sinkholes, caves, and underground streams that result in rapid 

infiltration of surface waters to the subsurface and rapidly flowing groundwater through entrenched paths. Lower Cretaceous 

rocks form the Trinity and Edwards Groups, and Upper 

Cretaceous rocks form the Georgetown and Del Rio Formations 

of the Washita Group (Barker and Ardis, 1996) (fig. 2). The 

Miocene age Balcones fault zone is the primary structural 

feature within the study area. The fault zone is an extensional 

system of faults that generally trends southwest to northeast in 

south-central Texas. 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

The methods used in this study were similar to those 

used in Hanson and Small (1995), Stein and Ozuna (1995), 

Clark (2003, 2004), Clark and others (2009, 2016), and Clark 

and Morris (2015). Geological data and previous reports were 

reviewed to assist in field mapping. During 2016, geologic 

and hydrostratigraphic mapping was carried out in Hays 

and Comal Counties, Tex., on public and private land. Field 

mapping was aided by the use of global positioning system 

(GPS) units, tablet-based digital maps, and geologic mapping applications. Observations were recorded on-site using a 

tablet computer loaded with geospatially registered 7.5-minute 

USGS topographic maps. Locations of visible contacts, faults 

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

recorded using the integrated 3G-assisted GPS on the tablet 

computer. Faults identified in the field were based on observed 

and inferred stratigraphic offsets. Strikes and dips of faults 

and fractures also were also noted where possible. Bedding 

attitudes of fractures and faults were obtained using a handheld compass or the tablet-computer compass application. 

Figure 2. Photograph showing rudist in the basal nodular member of the Kainer Formation.
4 Bedrock Geology and Hydrostratigraphy of the Edwards and Trinity Aquifers, Texas

Lidar and digital elevation model (DEM) data were used 

to try to determine the location of sinkholes and springs but 

neither method supplied reliable location data. Therefore 

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). Data 

obtained by the use of the tablet-computer compass application were cross-verified on a regular basis with data obtained 

with the handheld compass. The field data were transferred 

using ArcGIS ArcMap version 10.3.1 (Esri, 2016), quality 

checked by comparison with original draft maps, and then 

used to develop the geologic and hydrostratigraphic map of 

the study area.

The descriptions of the bedrock geology and hydrostratigraphy in this report were adapted for the study area 

from Clark and Morris (2015) and Clark and others (2016). 

Formal geologic names are consistent with those in the 

U.S. Geological Survey National Geologic Map Database 

(U.S. Geological Survey, 2016). Informal geologic and hydrostratigraphic names are consistent with those used in previous 

publications (Maclay and Small, 1976; Clark and others, 2009; 

Clark and others, 2014, 2016).

Lithologic descriptions follow the classification system 

of Dunham (1962) and porosity descriptions are based on the 

sedimentary carbonate classification system of Choquette and 

Pray (1970). Thicknesses used in this report are from field 

observations in the study area. Thickness variations within a 

unit result from differences 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 based on 

variations in the amount and type of porosity visually evident 

in the outcrop. Sedimentological features, paleontology, and 

ichnofossils (Hantzschel, 1962) were examined and described 

on site. Ichnofossils were described using morphology, surface 

textures, and type of burrow-fill (for example, Pemberton and 

Frey, 1982; Hasiotis and Mitchell, 1993).

Geology

The Trinity Group (Imlay, 1940) rocks were deposited 

during the Early Cretaceous on a large, shallow-marine carbonate platform (Comanche shelf) (fig. 1), as clastic-carbonate 

“couplets” during three marine transgressional events (Lozo and 

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

“couplets” deposited sediments that formed (1) the Hosston and 

Sligo Formations (Imlay, 1940); (2) the Hammett Shale (Lozo 

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

and (3) the Hensell Sand (Hill, 1901) as well as the lower and 

upper members of the Glen Rose Limestone (Hill, 1891) which 

are the focus of this report. The Hosston, Sligo, Hammett Shale, 

Cow Creek Limestone, and Hensell Sand of the Trinity Group 

are not present in surface exposures in the study area.

The Lower Cretaceous Edwards Group (Rose, 1972) 

rocks were deposited in an open marine to supratidal flats 

environment (Rose, 1972; Maclay and Small, 1986) during 

two marine transgressions. The rocks that compose the 

Edwards Group were deposited 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 (Clark and others, 2006) (fig. 1).

Following tectonic uplift, subaerial exposure, and erosion near the end of the Early Cretaceous, south-central Texas 

was again submerged during the Late Cretaceous by a marine 

transgression that resulted in deposition of the Georgetown 

Formation of the Washita Group (Richardson, 1904). Much of 

the Georgetown Formation was subsequently removed during 

a period of marine regression. The Stuart City reef was then 

breached during a subsequent transgressive episode resulting 

in deposition of the Del Rio Clay of the Washita Group that 

continued through the deposition of the Buda Limestone (not 

present in the study area) of the Washita Group.

Trinity Group

The Trinity Group contains shale, mudstone to grainstone, boundstone, evaporites, sandstone, and argillaceous 

limestone and in this study area is represented by the Hosston 

Formation, Sligo Formation, Hammett Shale (Lozo and 

Stricklin, 1956), Cow Creek Limestone, Hensell Sand (Barnes, 

1976), and Glen Rose Limestone (table 1). The Hosston, 

Sligo, Hammett Shale, Cow Creek Limestone, and Hensell 

Sand of the Trinity Group are not present in surface exposures 

in the study area, and subsequently, will not be discussed further in this report. In the study area, the lower member of the 

Glen Rose Limestone is typically about 195 feet (ft) thick. The 

lower member is formed by (least abundant to most abundant) 

evaporites, miliolid grainstone, shale, boundstone, mudstone 

to grainstone, and argillaceous wackestone to packstone 

(table 1). The upper member of the Glen Rose Limestone is 

typically about 400 ft thick. The upper member is made up of 

evaporites, mudstone to packstone, miliolid grainstone, and 

argillaceous limestone. Descriptions of each of the geologic 

units and their associated lithology and ichnofossils are shown 

in table 1. For additional geologic and ichnofossil descriptions, 

see Clark and others (2016).

Edwards Group

The Edwards Group, which overlies the Trinity Group, 

is composed of shale, dolomite, chert, evaporites, packstone, 

grainstone, miliolid grainstone, wackestone, and mudstone. In 

the study area, the Edwards Group is composed of the Kainer 

and Person Formations. The Kainer Formation is subdivided 

into the following informal (bottom to top): the basal nodular 

(figs.2 and 3) (Walnut Clay [Hill, 1891]), dolomitic (fig. 4), 

Kirschberg evaporite, and grainstone (fig. 5) (Rose, 1972; 

Maclay and Small, 1976). Although there is disagreement 

between various authors on the exact area for the transition of 
Hydrostratigraphy 5

Table 1. Summary of the geologic framework, hydrostratigraphy, 

ichnology, of the Edwards and Trinity aquifers in the Driftwood and 

Wimberley 7.5-minute quadrangles in Hays and Comal Counties, Tex. 

(Click here to access a full-size, high-resolution image).

the basal nodular member into the Walnut Clay. Based on field 

observations, the basal nodular member to the south of the 

study area is primarily a shaley, nodular, burrowed mudstone to 

grainstone with dolomite (table 1). In the study area, the basal 

nodular begins to contain interspersed shale, clay, and oyster 

(Ceratosteon texana) beds. This change in lithology, which 

occurs north of Farm to Market Road 32, is considered by the 

authors to be the area of transition between the basal nodular 

and Walnut Clay. The Person Formation is subdivided into 

three informal units (bottom to top): (1) the regional dense, 

(2) leached and collapsed (undivided), and (3) cyclic and marine 

(undivided) members (Maclay and Small, 1976). In the study 

area, the Edwards Group is typically about 400 ft thick.

The Kainer Formation is composed of (in increasing 

abundance) shale, dolomite, chert, packstone, grainstone, 

miliolid grainstone, wackestone, and mudstone. Chert is found 

throughout the formation. The Person Formation is composed 

of (in increasing abundance) shale, grainstone, miliolid 

grainstone, wackestone, packstone, and mudstone. The Person 

Formation contains chert throughout. Descriptions of each of 

the geologic units with associated lithology and ichnofossils 

are shown in table 1. For additional geologic and ichnofossil 

descriptions, see Clark and others (2016).

Faulting

The principal structural feature in Hays and Comal 

Counties is the Balcones fault zone, which is the result of 

Miocene faulting (Weeks, 1945) and fracturing (fig. 1). As is 

typical elsewhere 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 with any normal fault extensional system, this also includes 

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

The primary orientation of mapped fractures and faults 

in the study area is southwest to northeast between 45 and 

50 degrees azimuth. The secondary fractures trend obliquely 

to the Balcones fault zone at approximately 145–150 degrees 

azimuth. Variations in strikes and dips of the faults in the 

outcrop result from stress-strain relations between the different 

lithologies (Trudgill, 2002; Ferrill and others, 2003; Clark and 

others, 2014).

Orbitolina texana

(Roemer, 1852), 

limestone beds thicker 

and more resistive 

than overlying and 

underlying patch reefs 

formed on rudist, reefal 

talus 

Thick argillaceous beds, 

thin shale beds, Orbitolina 

texana (Roemer, 1852), 

contains ponds and 

seeps, often little 

vegetation, steeper slopes 

often with “badlands” type 

weathering, thinner in areas 

where patch reefs are 

present in the underlying 

Doeppenschmidt HSU

Forms stair-step 

topography with soils,

Orbitolina texana

(Roemer, 1852)

BU, VUG, 

FR, BP,

BR, CV

FR, CV

IP, BU, 

FR, BP,

CV

IP, MO,

VUG, FR,

BR, CV 

IP, MO,

BU, BP, 

FR, CV 

BU, BP, 

FR, 

occasional

CV

IP, MO, 

BU, BR 

MO, BR, 

BP, FR

MO, BU, 

BP, FR

IP

IP, MO,

BU, BP, 

FR, CV 

IP, FR, CV

IP, MO, 

BU, BP, 

FR, CH, 

CV

MO, BU,

FR, CV 

IP, IC,

MO, BU,

VUG, FR,

BP, CV

Aquifer

Confining

Aquifer

Aquifer

Aquifer

Aquifer,

confining unit in 

areas without 

caves 

Confining

Semi-confining

Aquifer

Semi-confining

Aquifer

Semi-confining;

Confining 

shale beds

Aquifer

Semi-confining

Aquifer

Confining

Confining

Aquifer

70–90

20

40

40–50

90–120

40

10

30–40

(typically 30)

30–40

(typically 30)

30–40

(typically 30)

40–80

(typically 40)

40–70

(typically 40)

45–60

(typically 55)

120–130

230

III

IV

V

VI

VII

VIII

Camp Bullis 

Fossiliferous 

Lower

evaporite 

Little Blanco 

Twin Sisters

Doeppenschmidt

Rust

Honey Creek 

Bulverde

II

Middle zone of the Trinity aquifer Upper zone of the Trinity aquifer

Edwards aquifer

Trinity aquifer

Wackestone, packstone, grainstone, 

boundstone, burrows; caprinid, miliolid, 

Orbitolina texana (Roemer, 1852), 

Toucasia sp., Trigonia sp., Turritella sp., 

various corals, pectens, shell fragments

Alternating beds of argillaceous wackestone, 

packstone; mudstone, wackestone, 

packstone, grainstone, milliolid grainstone; 

monopleurid, Nerinia sp., Orbitolina 

texana (Roemer, 1852), Tylostoma sp., 

and oysters, pectens, and pelecypods 

Mudstone, wackestone, packstone, 

grainstone, boundstone, argillaceous 

wackestone and packstone, miliolid 

grainstone; caprinid, Toucasia sp.

Argillaceous wackestone, shale; Orbitolina

texana (Roemer, 1852), gastropods, 

pelecypods

Mudstone, wackestone, argillaceous 

wackestone, boundstone; caprinid, 

monopleurid, Toucasia sp., 

Orbitolina texana (Roemer, 1852), 

gastropods, pectens, pelecypods

Wackestone, grainstone, argillaceous 

wackestone, shale, evaporites;

monopleurid, Toucasia sp., 

Macraster sp., Nerinia sp., Orbitolina 

texana (Roemer, 1852), Porocystis 

golobularis, Salenia texana, gastropods, 

pecten, and pelecypods

Dissolved evaporites, highly altered 

crystalline limestone and chalky mudstone, 

breccia, boxwork voids; Corbula beds

Alternating wackestone, packstone,

miliolid grainstone, argillaceous limestone,

mudstone, silty mudstone at base; 

Hemiaster sp., Neithea sp., Orbitolina minuta

(Douglas, 1960), Porocystis golobularis, 

Protocardia texana, Tapes decepta, 

Turritella sp., gastropods, mollusks;

(section not subdivided into an upper and 

lower unit as in Bexar and Comal Counties)

Alternating beds of burrowed wackestone, 

packstone, miliolid grainstone, argillaceous

limestone

Shaley, nodular limestone, burrowed 

mudstone, wackestone, packstone, miliolid 

grainstone, dolomite, contains dark, spherical

textural features locally known as black 

rotund bodies (BRBs); caprinid, Ceratostreon

[Exogyra] texana, miliolid, gastropods; area 

transitions to basal nodular containing oyster 

beds and shale 

Chert (absent in lower 20 ft), dolomitic 

mudstone, wackestone, packstone, 

grainstone 

Highly altered crystalline limestone, chalky

mudstone, occasional grainstone associated

with tidal channels; chert (beds and nodules);

coarse-grained spar, breccia and travertine,

dissolution has removed all evaporites in 

the study area

Miliolid, skeletal fragmented grainstone,

mudstone, wackestone; chert (beds and 

nodules); cross-bedded and ripple marked 

Dense, shaley, mudstone, wackestone, 

oyster-shell mudstone and wackestone,

iron staining, chert 

Recrystallized limestone, mudstone, 

wackestone, packstone, grainstone; chert 

(bedded and large nodules); iron-stained, 

stromatolitic, Toucasia sp., Montastrea 

roemeriana, oysters

Cyclic and

marine

 (undivided) 

Leached and

collapsed

(undivided) 

Regional dense Grainstone Kirschberg evaporite Dolomitc *

Early Cretaceous

Lower

Cretaceous

Trinity 

Upper

Glen Rose Limestone Kainer

Edwards

Person Georgetown

Late Cretaceous

Washita

80–90

20–30

*

Fossiliferous blue-green to yellow-brown 

clay, packstone, iron nodules; Ilymatogyra 

arietina 

*

Confining 

unit

Period

Epoch

Group

Formation

Member

(formal and informal)

Lithology and ichnology

Hydrologic unit

Hydrostratigraphic unit1

(HSU)

Thickness2

(outcrop in the 

study area,

 in feet) 

Hydrologic

function 

Porosity

type Field identification

Map abbreviations

and color

Geology Hydrostratigraphy

Table 1. Summary of bedrock geology and hydrostratigraphy of the Edwards and Trinity aquifers within the Driftwood and Wimberley 7.5-minute quadrangles, Hays and Comal Counties, Tex. 

1

Informal

2

Thickness range based on field mapping in the study area

Thick beds of 

wackestone, packstone,

grainstone; corals, 

caprinid, Trigonia sp., 

cliff forming; outcrop 

often contains large 

limestone float with large 

channel and moldic

porosity, caves and 

springs 

Limestone beds thicker

and more resistive to 

erosion than overlying and 

underlying units, Orbitolina

texana (Roemer, 1852), 

patch reefs

Salinia texana bed 

immediately below 

Corbula bed, abundant 

fossils including 

Porocystis golobularis,

Orbitolina texana 

(Roemer, 1852), 

Macraster sp., Nerinia sp., 

pecten, gastropods, 

pelecypods

Weathers to an orangish 

red with a pebbly texture, 

often has less cedar 

growth and thicker grasses,

boxwork porosity, 

Corbula sp., spring and 

seeps

Limestone and 

argillaceous limestone,

Orbitolina minuta 

(Douglas, 1960)

Alternating beds of

limestone and argillaceous

limestone, fossils rare, 

stair-step topography

Massive, nodular and 

mottled limestone, BRBs 

and orange wisps, 

Ceratostreon 

[Exogyra] texana, seeps 

and springs, ferns 

growing near contact of 

underlying unit

Massively bedded light 

gray, Toucasia sp., 

abundant

Boxwork porosity with 

neospar and travertine 

frame

Cross-bedded, ripple 

marks, miliolid 

grainstone

Wispy iron-oxide stains,

thin bedded, often white 

in aerial photographs

Bioturbated iron-stained

beds separated by 

massive limestone beds; 

stromatolitic limestone,

Montastrea roemeriana

I

Del Rio 

Clay 

Black dendrites, iron

nodules and staining,

Plesioturrilites brazoensis,

Waconella wacoensis

Thin graded cycles; 

massive beds to relatively 

thin beds; cross-beds, 

caprinids

Clay, holds water,

fossiliferous; Ilymatogyra

arietina

Reddish-brown, gray to light tan shaley

mudstone and wackestone, black dendrites,

iron nodules, iron staining; Plesioturrilites 

brazoensis, Waconella wacoensis 

Kplc

Kprd

Kkg

Kkke

Kkd

Kkbn

Kgrle

Kgrf

Kgrcb

Kgrb

Kgrlb

Kgrts

Kgrd

Kgrr

Kgrhc

Kdr

Kpcm

Kg

Pelletal limestone, mudstone, miliolid

grainstone, packstone, chert (bedded and 

large nodules); caprinid, cross-bedded 

MO

MO, BU,

VUG, BP,

FR, CV

40–50 None

[Period, Epoch, Group, Formation, members, and lithology modified from Imlay (1945), Whitney (1952), Lozo and Stricklin (1956), Stricklin and others (1971), Rose (1972), Stricklin and Smith (1973), Amsbury 

(1974), Inden (1974), Perkins (1974), Clark and others (2009, 2013, 2014), Weirman and others (2010), Blome and Clark (2014), and the U.S. Geological Survey National Geologic Map Database, GEOLEX 

(http://ngmdb.usgs.gov/Geoloex.html); Orbitolina minuta (Douglas, 1960), Orbitolina texana (Roemer, 1852); aquifers from Maclay and Small (1976), and Ashworth (1983); thickness from outcrop, Clark and 

others (2009, 2014), Weirman and others (2010); hydrologic function modified from outcrop (Clark and others, 2009, 2013, 2014; Weirman and others, 2010; Clark and Morris, 2015); porosity types modified from 

Choquette and Pray (1970). 

Fabric selective: IP, Interparticle porosity; IC, Intercrystalline porosity; SH, Shelter porosity; MO, Moldic porosity; BU, Burrowed porosity; FE, Fenestral; BP, Bedding plane porosity.

Not-fabric selective: FR, Fracture porosity; CH, Channel porosity; BR, Breccia; VUG, vug porosity; CV, Cave porosity; *no further subdivision; **not present in the study area] Basal nodular

Kwc

Walnut Clay

*

Kgrue

Dissolved evaporites, highly altered

crystalline limestone and chalky mudstone,

breccia, boxwork voids

IP, MO,

BU, BR Aquifer Upper 10evaporite Spring and seeps

**

*

Kgrue Upper evaporite 

I

Del Rio Clay Kdr

Upper Glen Rose

Limestone

Upper Confining

Unit (UCU)

Lower Glen Rose

Limestone

Glen Rose

Limestone

Kainer

Person

Georgetown

Grainstone

Kirschberg evaporite

Dolomitic

Member 

Formation (Formal and informal)

Informal 

hydrostratigraphic 

unit

EXPLANATION OF HYDROSTRATIGRAPHIC UNITS

II

III

IV

V

VI

VII

VIII

Camp Bullis

Fossiliferous

Lower evaporite

Bulverde

Little Blanco

Twin Sisters

Doeppenschmidt

Rust

Honey Creek

Kg

Kpcm

Kplc

Kprd

Kkg

Kkke

Kkd

Kgrcb

Kgrf

Kgrle

Kgrb

Kgrlb

Kgrts

Kgrd

Kgrr

Kgrhc

Basal nodular

Cyclic and marine, undivided

Leached and collapsed, undivided

Regional dense member

Kwc

Walnut Kkbn Clay

Hydrostratigraphy

Hydrostratigraphically, the 

rocks exposed in the study area 

represent a section of the Edwards 

aquifer, the upper zone of the Trinity 

aquifer, and the middle zone of the 

Trinity aquifer (table 1). In the study 

area, the Edwards aquifer yields 

water from rocks of the Edwards 

Group and the Trinity aquifer is 

formed in rocks of the Trinity Group. 

The Edwards and Trinity aquifers are 
6 Bedrock Geology and Hydrostratigraphy of the Edwards and Trinity Aquifers, Texas

Figure 3. Photograph showing chert nodules in the dolomitic member of the Kainer Formation. The 

photograph also shows vug and moldic porosity.

Figure 4. Photograph showing laminations in the grainstone member of the Kainer Formation, some

fenestral porosity near the top of the boulder. The thickness of the gray boulder is approximately 60 cm.
Hydrostratigraphy 7

karstic with high secondary porosity that is developed along 

bedding and fractures (Maclay and Small, 1983; Johnson and 

others, 2002; Ferrill and others, 2003; Gary and others, 2011). 

The descriptions of the hydrostratigraphy and porosity of individual (HSUs) shown on figure 2 are modified and expanded 

from Choquette and Pray (1970), Maclay and Small (1976), 

Stein and Ozuna (1995), Clark and others (2009), Blome and 

Clark (2014), Clark and Morris (2015), and Clark and others (2016). For additional descriptions of the upper confining 

unit, Edwards aquifer, and Trinity aquifer see Clark and others 

(2016).

The Del Rio Clay is a confining unit above the Edwards 

aquifer and does not supply appreciable amounts of water 

to wells in the study area (fig. 2). Because the formation is a 

confining unit and not water-bearing, it will not be discussed 

further in this report.

Edwards Aquifer

The Edwards aquifer was subdivided into HSUs I to VIII by 

Maclay and Small (1976) (fig. 2). The Georgetown Formation of 

the Washita Group contains HSU I. The Person Formation of the 

Edwards Group contains HSUs II (cyclic and marine members 

[undivided]), III (leached and collapsed members [undivided]), 

and IV (regional dense member); the Kainer Formation of the 

Edwards Group contains HSUs V (grainstone member), VI 

(Kirschberg evaporite member), VII (dolomitic member), and 

VIII (basal nodular member/Walnut Clay) (fig. 6).

Trinity Aquifer

Ashworth (1983) subdivided the Trinity aquifer into 

upper, middle, and lower aquifer units (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 latter two formations 

are not exposed in the field area. The regionally extensive 

Hammett Shale forms a confining unit between the middle 

and lower zones of the Trinity aquifer. The Hammett Shale 

and the lower zone of the Trinity aquifer, which consists of 

the Sligo and Hosston Formations, do not crop out in the 

study area.

Upper Zone of the Trinity Aquifer

The upper zone of the Trinity aquifer was informally 

subdivided into five HSUs in Bexar County (fig. 2) by Clark 

(2003) and was subsequently renamed (informally) by Clark 

and others (2009). In the study area, however, there are only 

three HSUs present (top to bottom): the Camp Bullis, fossiliferous (fig.6), and lower evaporite.

Figure 5. Photograph showing solution channels (to the left and right of the hammer) in the 

hydrostratigraphic units VIII (basal nodular member/Walnut Clay).
8 Bedrock Geology and Hydrostratigraphy of the Edwards and Trinity Aquifers, Texas

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). Underlying the Cow Creek is the 

regional confining unit, the Hammett HSU, which separates 

the middle and lower zones of the Trinity aquifer. In the 

study area, only the Bulverde, Little Blanco, Twin Sisters, 

Doeppenschmidt, Rust, and Honey Creek crop out. The lower 

zone of the Trinity aquifer does not crop out in the study area.

Hydrologic Characteristics of Structure

From field observations and previous studies, it is apparent that the hydrologic connection between the Edwards and 

Trinity aquifers and the various HSUs is complex. The source 

of complexity in the aquifer system results from a combination 

of the original depositional history, bioturbation, primary and 

secondary porosity, diagenesis, and fracturing of the area from 

faulting. All of these factors have resulted in the development 

of modified porosity, permeability, and transmissivity within 

and between the aquifers. The original depositional sediments 

have produced lithified layers of shales, impure limestones, 

and limestones which in turn have varying types of porosity 

related to biological activity and to subsequent diagenesis. 

Faulting has produced highly fractured areas 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. The juxtaposition of the aquifers and HSUs due to faulting has resulted in 

areas of interconnectedness between the Edwards and Trinity 

aquifers and the various HSUs that form the aquifers. For 

additional descriptions of how structure potentially affects the 

local hydrogeology, see Clark and others (2016).

Summary

The Edwards and Trinity aquifers are major sources of 

water in south-central Texas and are both classified as major 

aquifers by the State of Texas. The population of the central 

Texas area in Hays and Comal Counties is rapidly growing, 

increasing demands on the area’s water resources. To help 

effectively manage the water resources in the area, refined 

maps and descriptions of the geologic structures and HSUs of 

the aquifers in the Driftwood and Wimberley 7.5-minute quadrangles in Hays and Comal Counties, Tex., are needed.

The purpose of this report is to present the geology and 

hydrostratigraphy of the Driftwood and Wimberley 7.5-minute 

quadrangles in Hays and Comal Counties, Tex., as a detailed 

1:24,000-scale hydrostratigraphic map with names and 

Figure 6. Photograph of tepee structures at the base of the fossiliferous hydrostratigraphic units. 

The laminations are approximately 3 cm thick.
Acknowledgments 9

descriptions of the geologic and hydrostratigraphic units in the 

study area. The report includes the approximately 128 square 

miles (mi2

) within the outcrops of the Edwards and Trinity 

aquifers in the Driftwood and Wimberley 7.5-minute quadrangles in Hays and Comal Counties, Tex.

The rocks within the study area are karstic, sedimentary, 

and range in age from Early to Late Cretaceous. Karstic areas 

are characterized by sinkholes, caves, and underground streams 

that result in rapid recharge of surface waters infiltrating into 

the subsurface and rapidly flowing through entrenched groundwater flowpaths. Lower Cretaceous rocks include the Trinity 

and Edwards Groups, and Upper Cretaceous rocks include the 

Georgetown Formation of the Washita Group.

The Trinity Group contains shale, mudstone to grainstone, boundstone, evaporites, sandstone, and argillaceous 

limestone and in this study area is represented by the Hosston 

Formation, Sligo Formation, Hammett Shale, Cow Creek 

Limestone, Hensell Sand, and Glen Rose Limestone The 

Hosston, Sligo, Hammett Shale, Cow Creek Limestone, 

Hensell Sand Formations of the Trinity Group are not present 

in surface exposures in the study area.

The Edwards Group, which overlies the Trinity Group, 

is composed of shale, dolomite, chert, evaporites, packstone, 

grainstone, miliolid grainstone, wackestone, and mudstone. In 

the study area, the Edwards Group is composed of the Kainer 

and Person Formations. The Kainer Formation is subdivided 

into the following informal (bottom to top) the basal nodular/

Walnut Clay, dolomitic, Kirschberg evaporite, and grainstone. 

The Person Formation is subdivided into the following informal units (bottom to top): (1) the regional dense, (2) leached 

and collapsed (undivided), and (3) cyclic and marine 

(undivided) members.

The Miocene Balcones fault zone is the primary structural feature within the study area and is an extensional system 

of faults that generally trends southwest to northeast across 

south-central 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, and the middle zone of the Trinity aquifer. 

In the study area the Edwards aquifer is composed of the 

Georgetown Formation and of the rocks forming the Edwards 

Group. The Trinity aquifer is composed of the rocks forming the Trinity Group. The Edwards and Trinity aquifers are 

karstic with high secondary porosity along bedding and fractures. The Del Rio Clay is a confining unit above the Edwards 

aquifer and does not supply appreciable amounts of water to 

wells in the study area.

The Edwards aquifer was subdivided into HSUs I to VIII. 

The Georgetown Formation of the Washita Group contains 

HSU I. The Person Formation of the Edwards Group contains 

HSU units II (cyclic and marine members [undivided]), 

III (leached and collapsed members [undivided]), and IV 

(regional dense member); the Kainer Formation of the 

Edwards Group contains hydrostratigraphic units V (grainstone member), VI (Kirschberg evaporite member), VII (dolomitic member), and VIII (basal nodular member/Walnut Clay).

The Trinity aquifer is separated into upper, middle, and 

lower aquifer units (zones). The upper zone of the Trinity 

aquifer is in the upper member of the Glen Rose Limestone. 

The middle zone of the Trinity aquifer is formed in the lower 

member of the Glen Rose Limestone, Hensell Sand, and Cow 

Creek Limestone. The regionally extensive Hammett Shale 

forms a confining unit between the middle and lower zones of 

the Trinity aquifer. The Hammett Shale and the lower zone of 

the Trinity aquifer, which consists of the Sligo and Hosston 

Formations, do not crop out in the study area.

The upper zone of the Trinity aquifer was first informally 

subdivided into five hydrostratigraphic units in Bexar County. 

In the study area, however, only three HSUs are present (top 

to bottom): the Camp Bullis, fossiliferous, and lower evaporite. The middle zone of the Trinity aquifer originally was 

informally subdivided into nine HSUs in Bexar and Comal 

Counties, but, only the Bulverde, Little Blanco, Twin Sisters, 

Doeppenschmidt, Rust, and Honey Creek are present in outcrop in the study area.

From field observations and previous studies, the hydrologic connection between the Edwards and Trinity aquifers 

and the various HSUs is complex. The source of complexity 

in the aquifer system is a combination of the original depositional history, bioturbation, primary and secondary porosity, 

diagenesis, and fracturing of the area from faulting. All of 

these factors have resulted in the development of modified 

porosity, permeability, and transmissivity within and between 

the aquifers. The original depositional sediments have produced layers of shales, impure limestones, and limestones that 

in turn have varying types of porosity related to biological 

activity and to subsequent diagenesis. Faulting has resulted in 

highly fractured areas that have allowed for the rapid infiltration of water and subsequently formed solutionally enhanced 

fractures, bedding planes, channels, and caves which are 

highly permeable and transmissive. The juxtaposition of the 

aquifers and hydrostratigraphic units because of faulting has 

resulted in areas of interconnectedness between the Edwards 

and Trinity aquifers and the various hydrostratigraphic units 

that form the aquifers.

Acknowledgments

The authors would like to recognize the landowners 

and managers of private and public lands in Hays and Comal 

Counties, Texas, who provided access to their property for 

this study.
10 Bedrock Geology and Hydrostratigraphy of the Edwards and Trinity Aquifers, Texas

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For more information concerning this publication, contact

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This publication is available online at

https://doi.org/10.3133/sim3386

Clark, A.K. and Morris, R.R.—Bedrock Geology and Hydrostratigraphy of the Edwards and Trinity Aquifers, Texas—Scientific Investigations Map 3386

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3386