U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3105

Geologic Map of the Edwards Aquifer and Related Rocks in 

Northeastern Kinney and Southernmost Edwards Counties, 

South-Central Texas
 

 

 

 

COVER:

Fractured and jointed Salmon Peak Limestone exposed in 10-m-deep cut, Tularosa Road, about 

22 km (14 mi) northeast of Brackettville in Salmon Peak quadrangle. Hills on distant horizon 

capped by Salmon Peak Limestone. Photograph by D.W. Moore, May 2005.

 
Geologic Map of the Edwards Aquifer and 

Related Rocks in Northeastern Kinney 

and Southernmost Edwards Counties, 

South-Central Texas

By David W. Moore

Pamphlet to accompany

Scientific Investigations Map 3105

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation: 

Moore, D.W., 2010, Geologic map of the Edwards aquifer and related rocks in northeastern Kinney and 

southernmost Edwards Counties, south-central Texas: U.S. Geological Survey Scientific Investigations 

Map 3105, scale 1:50,000, 18-p. pamphlet. [Available at URL http://pubs.usgs.gov/sim/3105]

ISBN 978-1-4113-26637
iii

Contents

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

Paleogeography.............................................................................................................................................2

Structure..........................................................................................................................................................6

Methods...........................................................................................................................................................7

Terminology.....................................................................................................................................................9

Description of Map Units............................................................................................................................10

Acknowledgments.......................................................................................................................................15

References Cited..........................................................................................................................................15

Figures

1. Area of Edwards aquifer; extent of geologic map shown by red box..................................1

2. Nine 7.5-minute quadrangles and the western parts of three other

 quadrangles included in the geologic map area.....................................................................2

3. Paleogeography (regional depositional setting) of Comanchean and

 Gulfian rocks. Present-day counties and selected cities are shown..................................3

4. Paleogeography of Early Cretaceous Epoch (late Albian subage

 about 101 Ma)................................................................................................................................4

5. Fractured and jointed Salmon Peak Limestone exposed in 10–m–deep cut,

 Tularosa Road, about 22 km (14 mi) northeast of Brackettville in Salmon Peak

 quadrangle.....................................................................................................................................6

6. Type section and type locality of the West Nueces Formation (Kwn) at

 bluffs of the West Nueces River, northern Salmon Peak quadrangle .................................7

7. Rock units of the Edwards aquifer. Telephoto view northward toward 

West Nueces River and type locality ........................................................................................8

8. Gamma ray log of 1,055 ft. deep drill hole RD–70–55–2AA ...................................map sheet

Table

Summary of lithologic and hydrologic properties of the stratigraphic units 

in northeastern Kinney and southernmost Edwards Counties, 

south-central Texas .........................................................................................................................5
iv

Conversion Factors

Multiply By To obtain

 Length

 centimeter 0.3937 inch (in)

 meter 3.281 foot (ft)

 kilometer 0.621 mile (mi)

Vertical coordinate information is referenced to the National Geodectic Vertical Datum of 1929

North American Datum of 1983 (NAD 83)

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

The Edwards aquifer is the most prolific carbonate 

aquifer in south-central Texas. It supplies water to agriculture, 

industry, military, natural ecosystems, and potable water to 

more than two million people in the region. Comanchean and 

Gulfian Series Cretaceous strata, consisting of the 200-m-thick 

Edwards aquifer and the confining rock units above and below 

it, dip gently and thicken southward (seaward) in northeastern 

Kinney and southernmost Edwards Counties (fig. 1). These 

shallow marine shelf carbonate strata total about 400 m in 

thickness and were divided into lithostratigraphic formations that were mapped at 1:24,000 scale in nine quadrangles 

and parts of three more (fig. 2). Previously published smallscale geologic maps that include Kinney County are those of 

Bennett and Sayre (1962), Lozo and Smith (1964), and the 

Geologic Atlas of Texas, 1977.

Normal faults, mostly downdropped to the southeast, 

trend northeastward across the area. They lie within a western 

extension of the Balcones fault zone. Other structural features 

include a few broad, open folds and several solution collapse 

features. Basaltic igneous rock bosses and dikes intrude the 

limestone and mudstone strata, recording Late Cretaceous 

volcanism, perhaps in shallow seas (Fowler, 1956; Greenwood, 1956). The northern part of the map area is the hilly 

Edwards Plateau cut by streams into resistant, Upper Cretaceous limestone. The plateau is bounded on the south by the 

Balcones Escarpment. The west Gulf Coastal Plain lies south 

of the escarpment.

The purpose of this report is to provide a detailed geologic map that can assist in the management of groundwater 

resources in Kinney County. Knowing the nature of the rock 

strata in the recharge and confined zones of the Edwards 

aquifer is necessary for understanding water volumes and flow 

in the aquifer. 

Geologic Map of the Edwards Aquifer and Related 

Rocks in Northeastern Kinney and Southernmost 

Edwards Counties, South-Central Texas

By David W. Moore

Area of Edwards Aquifer; extent of geologic map shown by red box 

Figure 1. Area of 

Edwards aquifer; 

extent of geologic 

map shown by red 

box.
2 Geologic map of the Edwards aquifer, northeastern Kinney and southernmost Edwards Counties, Texas 

Paleogeography

A sketch of Cretaceous paleogeography of southcentral Texas helps explain the origin of the mapped rockstratigraphic units and their distinguishing characteristics. 

Regional studies have produced a consensus paleogeography 

(fig. 3) of a shallow marine, carbonate sediment environment 

that includes intertidal flats, bays, reefs, and open shallow 

seas (Young, 1959; Dunham, 1962; Fisher and Rodda, 1969; 

Stricklin and others, 1971; Enos, 1974; Rose, 1972; Coates, 

1973; Bebout, 1974; Perkins, 1974; Bay, 1977; Smith, 1981; 

Miller, 1984; Humphreys, 1984; Scott, 1990). Investigations of 

modern carbonate depositional facies (for example, Ginsburg, 

1957; Scholle and others, 1983) and Cretaceous microfacies 

using rock slabs and thin-sections (Miller, 1983) have refined 

the paleogeographic model, as has oil-exploration research, 

including geophysical logging, seismic profiling, petrography, 

and paleontology. Understanding the paleogeography of the 

original depositional environments helps explain variation in 

lithofacies, permeability, and transmissivity of the mapped 

rock units; these factors affect flow of groundwater in the 

Edwards aquifer. Diagenesis, also important (Hovorka and 

others, 1994), is not considered here. 

Figure 2. Nine 7.5-minute quadrangles and the western parts of three 

other quadrangles included in the geologic map area. Geologic mapping 

was done at 1:24,000 scale.

EDWARDS COUNTY

Kickapoo

Caverns Bull Waterhole

Pinto Mtn. Brackettville NE

Salmon 

Peak

Brackettville

Brackettville

Elm Mtn. Turkey Mtn.

KINNEY COUNTY

West

Nueces

River

UVALDE COUNTY

Montell Laguna Mustang Waterh Mustang Waterhole

29°37’30” 100°30’ 100°07’30”

29°15’

90

Brackettville-Laguna Rd.

Edwards Plateau

Balcones Escarpment

West Gulf Coastal Plain

KINNEY Silver Lake COUNTY

29°30’

29°22’30”

334
Paleogeography 3

BANDERA

Ancestral Gulf of Mexico

Existing

Gulf of Mexico

VAL VERDE EDWARDS

UVALDE

ZAVALA

DIMMIT

KERR

GILLISPIE

KENDALL COMAL

BLANCO TRAVIS

HAYS BASTROP

WILLIAMSON

KINNEY

LA SALLE

FRIO

MEDINA

MAVERICK

COMANCHE

SHELF

COAHUILA

PLATFORM

paleoslope

STUART CITY

REEF

0 50 100 150 MILES

Laredo

Austin

MAVERICK

BASIN

DEVILS RIVER TREND

0 50 100 150 200 KILOMETERS

REAL

BEXAR

San

Antonio SAN MARCOS

PLATFORM

SAN MARCOS

ARCH

Figure 3. Paleogeography (regional depositional setting) of Comanchean and Gulfian 

rocks. Present-day counties and selected cities are shown. Red box locates the geologic 

map. Modified from Rose (1972).

In Late Triassic to Middle Jurassic time, the supercontinent Pangaea began fragmenting, rifting the lithosphere 

and opening the subtropical Tethys seaway and proto-Gulf 

of Mexico (Winker and Buffler, 1988). Rifting continued in 

Late Jurassic and Early Cretaceous time. The Gulf oceanic 

lithosphere cooled and subsided (Dunbar and Sawyer, 1987), 

and a rise in mid-Cretaceous global sea level forced seas 

northward onto the continental margin and formed a shallow sea 

on the Comanche shelf (fig. 3). Kinney County was located 

on that shelf and in the Maverick Basin (figs. 3, 4), whose 

intrashelf location is inferred from relatively thick Cretaceous 

strata in Maverick County, Texas (Winter, 1961). North of 

the basin stood a platform margin, the Devils River trend, 

containing abundant rudistid reefs and shoals (fig. 3). West of 

the basin was the Coahuila (Mexico) platform and east of it 

was the San Marcos platform (fig. 4). The south margin of the 

basin was delimited by the Stuart City reef in late Albian and 

early Cenomanian time (Scott, 1990); that reef separated the 

Comanche shelf from the proto-Gulf of Mexico (Winter, 1961; 

Rose, 1972). A reconstruction of the continental paleogeography about 101 Ma is shown (fig. 4). Shallow, subtropical 

seas sporadically covered both platforms and the intrashelf 

Maverick Basin during mid-Cretaceous and Late Cretaceous 

time (Young, 1986). The oldest mapped unit is the Glen 

Rose Formation (Kgr) of early Albian age, about 112 to 108 

Ma. Most of the rock units mapped in Kinney County are of 

middle Albian through Santonian ages, about 108 to 84 Ma 

(table 1). 
4 Geologic map of the Edwards aquifer, northeastern Kinney and southernmost Edwards Counties, Texas 

Carbonate deposition, interrupted by erosional intervals, 

prevailed during Early and Late Cretaceous during which the 

Trinity, Fredericksburg and Washita Groups, and Eagle Ford 

and Austin Formations were deposited (Scott and others, 1999). 

Carbonate-forming mollusks, especially rudistid bivalves, 

bryozoa, foraminifera, stromatoporoids, echinoderms, corals, 

other invertebrates, and possibly planktonic algae flourished 

in warm epicontinental seas (Friedrich and others, 2008) under 

a warm, equable climate (Barron, 1983; Fassell and Bralower, 

1999). Carbonate mud, biogenic skeletal debris, and evaporites that accumulated on the sea floor and reef flanks were 

the original sediment of the Comanchean and Gulfian rocks. 

Production of micrite in Pleistocene coralline algal crusts by

in vivo calcified coccoid cyanobacteria has been documented 

and, by analogy, is suggested as an origin of microcrystalline 

cements in fossil reefal limestone (Kaźmierczak and Iryu, 

1999). Water depth on the Comanche shelf varied owing to sea 

level fluctuation, buildup of carbonate mounds or reefs, and 

differences in crustal stability of the San Marcos arch and the 

Maverick Basin (Loucks, 1977). 

Cretaceous paleogeography was essential to forming the 

distinctive, mappable, rock-stratigraphic units depicted on the 

geologic map. For example, the middle member of the McKnight 

Formation (Kmm) is almost entirely covered by a band of 

dense vegetation that flourishes on dark-gray, organic-rich, 

thin beds of mudstone and shale. This distinctive rock unit 

resulted from its deposition in relatively deep, low-energy, perhaps anoxic lagoonal waters on the intrashelf Maverick Basin 

(Lindgren and others, 2004). The lower member of the McKnight (Kml) contains evaporite minerals (subsurface) and, on 

outcrop, collapse breccia caused by dissolution of those minerals; the evaporites perhaps formed on tidal mudflats (Miller, 

1984) or on a subtidal shelf (Lehmann and others, 1998). In 

contrast, the Devils River Limestone (Kdvr) in northern part of 

map area (fig. 3), contains shell-fragment mudstone, wackestone, and rudist-fragment wackestone, indicating high-energy 

water on reefs and shoals. Resistant, massive beds of dense 

carbonate rock of the unit form characteristic smooth hills 

in the northern part of the map area. Petrography of several 

formations (Miller, 1983) revealed differences in the prevailing sizes of carbonate grains, which reflect variation in water 

energy and biota of mid-Cretaceous depositional environments in bays, open marine, intertidal areas, closed alkaline 

lagoons, shoals, reefs, and tidal flats (Winter, 1961; Lozo and 

Smith, 1964; Rose, 1972). These depositional environments 

shifted about as sea level fluctuated through time. In a marine 

environment, in principle, the prevalent wave, current, thermal 

energy, salinity, and oxygen availability determine grain 

size, sorting, composition, and bedding type of the sediment. 

Another key factor in this regard is the biota, like foraminifera, 

Figure 4. Paleogeography of Early Cretaceous Epoch (late Albian subage about 

101 Ma). Intrashelf Maverick Basin shown in red letters. Ma, million years ago. 

Accessed on February 15, 2006, at: http://www.cretaceousfossils.com.

Fort Worth

 Basin

Llano

Islands

Chihuahua

 Basin Coahulla

Platform San Marcos

 Platform

Valles-San Luis Potosi

 Platform

Golden Lane

 Platform Great Yucatan

 Bank

Guerroro-Oaxaca

 Platform

Tucumcari

 Basin

Stuart City

 reef trend

Appalachian

 thrust belt

Ozark

domeMississippi

embayment

Sioux

ridge

Canadian Shield

 region

Skull Creek

 seaway

 Idaho

batholith

Mogollon

highlands

Cordilleran

thrust belt

 Sevier

thrust belt

Cordilleran

thrust belt

 Early Cretaceous Epoch

(late Albian subage, about 101 Ma)

Maverick

 Basin
Table 1. Summary of lithologic and hydrologic properties of the stratigraphic units in northeastern Kinney and southernmost Edwards Counties, sou

 Age1(numbers = Ma) Series Formation or member Thickness m (ft) Lithology Field identification Poros

perm

CRETACEOUS

LATE

SantonianConiacian

GULF

Austin (Kau)

 113(370)

 Relatively soft, argillaceous, chalky, medium-bedded limestone; interbeds of marl (earthy

 mixture of clay and calcium carbonate). Contains coccoliths, foraminifera Inoceramus and Gryphaea shells; poorly exposed 

Chalky lime mudstone; poorly exposed over most of  map area; exposed in old quarry of Fort Clark Springs; forms level land covered by thick caliche 

Low to mo ity and p

Turonian Eagle Ford (Kef) 39 (130) Flaggy, argillaceous limestone, calcareous mudstone and siltstone, some silicic sandstone, and silty marl. Weathers to pastel-colors of pink, light brown, reddish brown; contains sparse Inoceramus shells Weathers on low-relief land surface as pastels of brown, pink, yellow flagstone pieces cemented by caliche Primary po

 low perm

Cenomanian

late

COMANCHE

Buda(Kbu)

 30–48(100–160)

 Dense, thick-bedded, argillaceous, microcrystalline, limestone (micrite) and locally dolostone.

 Contains fine fragments of mollusk shell 

Distinctive white and beige north-facing ledge of thick beds of dense limestone; conspicuous, 2-m thick bed of white limestone at top 

Low porosi permeab

middle

Del Rio (Kdr)

 25–27(85–90)

 Calcareous shale and mudstone, contains abundant Ilymatogyra arietina and pecten-like

 shells and foraminifera. Regionally persistent unit. Only mapped unit that is predominately clayey 

Brown, earthy slope below a light-colored limestone bench or ledge; abundant small “ram’s horn” oysters 

Negligible;  upper co to Edwar

early

Devils River (Kdvr)

 Salmon Peak (Ksp) 

 90–95(295–310)

 Thick-bedded lime mudstone (micrite); recrystallized and dolomitized locally; porcela-

 neous, conchoidal fracture; some shell fragment wackestone; abundant large chert nodules in upper part 

Steep, light-colored (white) conical hills; resistant, slightly dissected uplands; bedding appears  massive on outcrop 

Both fabric  fabric se to high p low to hi ability 

EARLY

Albian

late

McKnight (Km)

 upper member  (Kmu) 

 24–30(80–100)

Laminated to medium beds and flaggy bedding; brecciated in places, few thick chert laminae; locally contorted bedding (solution collapse) 

Weathers as limestone cliffs (buttresses) in river bluffs 

Fabric sele fabric se

middle

middle member (Kmm) 

 6–12(20–40)

 Dark, laminated mudstone and petroliferous shale and clayey lime mudstone Laminated to thin bedded, thick vegetation growth;

 vegetated band 9 m thick on hills 

< 3 percent 

 lower member  (Kml) 

 24–30(80–100)

Thin-bedded Gryphaea fragment and pellet grainstone, micrite, and laminae of chert that  pinch out; few thin brecciated beds; some open folds of local extent; solution zones 

Thin-bedded lime mudstone (micrite); petroliferous odor 

Vuggy and  porosity miability 

 43

(140) 

 Medium to thick-bedded mudstone and recrystallized limestone, burrowed, shell-fragment

 wackestone, oysters, miliolids, gastropods 

Crops out on lower slopes of narrow canyons in hill country bluffs along West Nueces River; distinctly nodular 

5-10 percen porosity permiabiNegligible 

 21(70) 

 Nodular, burrowed lime mudstone, shell fragments, “heart clams,” Exogyra texana,

 miliolids, Lunatia

earlyGlen Rose (Kgr)

 Notdeterminedin map area

 Thin alternating beds of limestone, dolostone, and marl Alternating thin ledges and earthy intervals; poorly

 exposed 

1For discussion of placement of the boundaries between Comanchean-Gulfian provincial chronostratigraphic units and Albian-Cenomanian ages based on biostratigraphic zonation, see Mancini (1979). 2From Clark (2003).

99.6

92

89

 WestNueces (Kwn) Basalnodular 

Edwards aquifer
6 Geologic map of the Edwards aquifer, northeastern Kinney and southernmost Edwards Counties, Texas 

calcareous algae, mollusks, and echinoids. These ecological 

elements and subsequent diagenesis produced the lithofacies 

seen in the mapped rock-stratigraphic units. In this study, 

although lithofacies were not mapped, they contributed to the 

distinctiveness of mapped rock-stratigraphic units. 

Following lithification of the sediments, Neogene 

epeiorogenic uplift energized streams and rivers, causing 

erosion of the strata, normal faulting, sculpting of the plateau, 

escarpment, and shaping of the coastal plain present in the 

map area today. 

Structure

Extending across the map are en echelon, northeast-trending, mainly down-to-southeast, high-angle normal faults. They 

are a western extension of the Balcones fault zone that developed during Neogene uplift of the Edwards Plateau (Ewing, 

1991). When mapped using aerial photographs (National 

Aerial Photography Program (NAPP), infrared, approximately 

1:40,000 scale, dated Sept. 2, 1996), faults appeared numerous 

in the Del Rio Formation (Kdr), Buda Limestone (Kbu), and 

Eagle Ford Formation (Kef) and less common in the Salmon 

Peak Limestone (Ksp). I believe that this apparent difference 

may not be that fewer faults exist in the Salmon Peak. It may 

be that the bedding of the other formations is more sharply 

defined, making the faults more easily detected on aerial 

photographs. The massive and uniform beds of limestone in 

the Salmon Peak exhibit few clear faults on the aerial photographs. If this speculation is valid, faults may be just as 

numerous in the Salmon Peak Limestone as in the aforementioned three formations that overlie it. 

In the field, only two minor faults were seen in bluffs of 

the West Nueces River. They offset beds of the West Nueces 

Formation by only 1–2 m. Generally elsewhere in the map 

area, estimated dip slips of 1–7 m on faults are less than many 

displacements farther east in the Balcones fault zone near San 

Antonio. Trending northeastward in the central part of Kinney 

County are two broad folds having an estimated closure of 

< 10 m. In addition, a monoclinal structure dips at 4–5 degrees 

southwestward 6–8 km northwest of Salmon Peak (north of 

Ranch Road 334 and West Nueces River).

Thick beds of limestone units that compose the Edwards 

aquifer, the Salmon Peak, McKnight, and West Nueces Formations, are brittle and jointed. Although I did not systematically 

analyze the abundant joints evident in the rocks, I believe the 

joints probably formed in Neogene time under tensional stress 

caused by subsidence of the oceanic crust to the south relative 

to a more stable continental crust to the north. Jointed Salmon 

Peak Limestone is well exposed in a 10-m-deep road cut on 

Tularosa Road, about 6 km north of the Texas Ranch Road 344 

(Brackettville-Laguna Road) junction (fig. 5). In this exposure, 

several solution cavities as large as 1 m across are developed 

along high-angle joints and at joint-plane bedding-plane junctions and at junctions of variously oriented joint sets. On aerial 

photographs, the Salmon Peak Limestone displays extensive 

thin, dark lineaments that may manifest such sets of fractures 

or joints. If so, and if the mapped area is representative of the 

larger Edwards aquifer recharge zone farther east, rock units 

of the aquifer may be more densely fractured than generally 

recognized. This suggests the possibility that fracture flow of 

groundwater may be significant in the aquifer.

Figure 5. Fractured and jointed Salmon Peak Limestone exposed in 10-m deep 

cut, Tularosa Road, about 22 km (14 mi) northeast of Brackettville in Salmon Peak 

quadrangle. Hills on distant horizon capped by Salmon Peak Limestone. Photograph by D.W. Moore, May 2005.
Methods 7

Methods

Mapping of rock-stratigraphic units was accomplished 

chiefly by use of aerial photography, after establishing the 

boundaries between the units in key field reference sections. 

In this largely privately owned region, accessible areas and 

reference sections included: (1) Shahan Ranch in the Pinto 

Mountain quadrangle; (2) “The River Road” that leads to 

cliff exposures along the West Nueces River (see index map 

on main map). This road connects the Tularosa Road (Ranch 

Road 3199) and Brackettville-Laguna Road (Ranch Road 334) 

and accesses Lozo and Smith’s (1964) type sections for the 

West Nueces Formation and reference section of the McKnight 

Formation (figs. 6, 7). (Location of the sections on map shown 

by WN I, WN II, and M 1 symbols on bluffs of West Nueces 

River); (3) Bar-N Ranch south of the West Nueces River in 

southeastern Salmon Peak quadrangle; (4) Las Moras Mountain; (5) Kickapoo Cavern State Park; (6) Fort Clark Springs; 

and (7) exposures along public roads.

Most mapping was done using National Aerial Photography Project (NAPP) color-infrared aerial photographs, at 

1:40,000 scale, after they were stereographically oriented 

on a Kern PG–2 analog photogrammetric stereo plotter by a 

technician. Geologic contacts, faults, escarpments, and collapse features were interpreted and transferred by pantograph 

to 1:24,000-scale topographic maps, which then were scanned 

and digitized. Some bedding attitudes were estimated from 

oriented stereographic models; these are depicted on the geologic map as a dashed bedding symbol that differs from those 

attitudes that were measured directly with inclinometer, which 

are depicted using a solid bedding symbol.

Figure 6. Type section and type locality of the West Nueces Formation (Kwn) at bluffs of the West Nueces River, northern Salmon 

Peak quadrangle (see type locality symbols in east-central part of map). Ledges in foreground are thin-bedded grainstone and 

micrite of the lower member of the McKnight Formation (Kml). Buttress-like cliffs in background are thick-bedded wackestone and 

lime mudstone (micrite) of the upper part of the informal “basal nodular unit” of the West Nueces Formation (Kwn). Kmm, middle 

member of McKnight Formation, showing characteristic vegetation band; Kmu, upper member of McKnight. Photograph by D.W. 

Moore, May 2005.

Kmu

Kmm

Kwn

Kml Kml
8 Geologic map of the Edwards aquifer, northeastern Kinney and southernmost Edwards Counties, Texas 

The naming convention of Lozo and Smith (1964) was 

used for rock units making up the Edwards aquifer. Names of 

units above and below the Edwards aquifer follow conventional nomenclature in south-central Texas (table 1). The Devils 

River Limestone (Kdvr) was mapped in the Devils River trend 

(fig. 1), which extends along the north edge of the map area. 

The Kdvr unit was mapped where I could not differentiate 

the West Nueces (Kwn), McKnight (Km), and Salmon Peak 

Formations (Ksp). The mapped rock units of the Comanchean 

Series contain the Edwards aquifer, which is included within 

the West Nueces, McKnight, and Salmon Peak Formations. 

These units and other rock-stratigraphic units above and below 

them, make up the geologic map. 

In describing and naming carbonate rocks, two schemes 

are used. The first, standard definitions of rock types (Bates 

and Jackson, 1980) were applied during field examination. 

The second scheme, that of Dunham (1962), assigns carbonate 

rock names based on depositional texture (carbonate microfacies). Miller (1984) determined these microscopically and I 

used his names for some rock units in the Description of Map 

Units section on the map sheet.

River

Kmu

Kmm

Kml

Kwn

West Nueces River

Ksp

Figure 7. Rock units of the Edwards aquifer. Telephoto view northward toward West Nueces River and type locality (Lozo and 

Smith, 1964) of West Nueces Formation (Kwn). Kml, Kmm, Kmu are lower, middle, and upper members of McKnight Formation 

respectively. Ksp, Salmon Peak Limestone. Photograph by D.W. Moore, September 2005.
Terminology 9

Terminology

[Descriptions of colors from Munsell Color, 1975]

Caliche—Light-colored, commonly white (10YR 8/2), grayish-orange-pink (10R 8/2), very pale orange (10YR 8/2), 

locally soft, porous, and punky, also hard, massive and laminated calcium carbonate formed either in the 

soil-forming zone (pedogenic carbonate) or below the soil zone by precipitation from groundwater. In the 

map area, caliche is an extensive deposit 0.3–2 m thick, which engulfs angular pieces of bedrock on outcrops, 

forms thick laminations on bedrock, and cements alluvial gravel and colluvium in meters-thick aprons adjacent 

to streams and arroyos. On the coastal plain, it is ubiquitous and cements residuum and alluvium. 

Carbonate rock names—Two schemes were used. First scheme: standard field definitions (Bates and Jackson, 1980), 

for example, lime mudstone, or simply mudstone, made mostly of carbonate silt or clay particles. Second 

scheme: Dunham’s (1962) rock classification based on sedimentary depositional textures seen through microscopic examination of thin sections (by Miller, 1984). For example, a preponderance of microscopic calcareous silt and clay particles in mudstone (field term) yields the term microcrystalline limestone, (micrite). I 

named some rocks based on Miller’s (1984) microscope work that uses the Dunham scheme.

Colors—Colors of unconsolidated deposits were compared to standard color chips (Munsell Color, 1975). Colors of 

rock were compared to a rock-color chart, Geological Society of America (1970).

Euxinic—Depositional environment of restricted circulation and stagnant or anaerobic conditions.

Facies—Areally restricted part of a stratigraphic unit that exhibits lithologic and paleontological characters different 

from those in another part of the same unit (Bates and Jackson, 1980).

Marl—Soft rock, chiefly commingled clay and calcium carbonate formed under marine conditions. For the purposes of 

this report, marl includes poorly consolidated calcareous claystone and impure, argillaceous limestone.

Pantograph—Mechanical system of fulcrums and a pencil connected to the moveable arm of an analog photogrammetric stereoscope that moves in concert with a stereoscope. The pencil traces paths of those movements onto 

a paper base map. The pantograph transfers contacts between rock-stratigraphic units from their location on an 

aerial photograph to their precise location on a topographic base map.

Gradational boundary—Boundary on the geologic map depicted by a change in color of map units and gradational 

contact symbol (see Explanation on map). This type of boundary indicates where details on the ground are lost 

that allowed detailed mapping of units and, across the boundary, only a general, undifferentiated unit is identifiable. For example, upper (Kmu), middle (Kmm), and lower (Kml) McKnight units were mappable as far 

as a gradational boundary, and across it only the McKnight (Km) Formation, undifferentiated, was recognized 

and mapped. Also a gradational boundary was mapped where the West Nueces (Kwn), McKnight, and Salmon 

Peak (Ksp) units were no longer differentiated, and a correlative unit, the Devils River Limestone (Kdvr), was 

mapped instead. 
10 Geologic map of the Edwards aquifer, northeastern Kinney and southernmost Edwards Counties, Texas 

Description of Map Units

[Color of units from Munsell Color, 1975]

Qal Alluvium (Holocene and late Pleistocene)—White (10YR 8/2) and very pale brown (10YR 8/3) silt, 

sand, granules, pebbles, and cobbles. The unit weathers gray (10YR 6/1), is mostly unconsolidated, 

and is composed of subrounded clasts, predominantly limestone and minor chert. Present in channels, floodplains, on low terraces, and mapped as sheetwash alluvium in large sinkholes and closed 

topographic depressions. In hilly terrain (northern part of area) the unit is caliche-cemented, laminar 

in places, sandy, angular limestone and minor chert gravel; 1–3 m thick adjacent to channels, grading to colluvium upward on hillslopes. Forms point bars of rounded cobbles and pebbles adjacent 

to channels of West Nueces River (northeast part of map area). On nearly flat land, as in the upper 

valley of Pinto Creek (2 km southwest of Alamo Village, on Ranch Road 674 in west-central part 

of map), unit is grayish-brown (10YR 4.5/2) and brown (10YR 5/3) calcareous, silty clay, clay, and 

fine sand cut by channels 2–3 m deep; channels expose abundant limestone pebbles in lower part. 

On broad flats, unit is highly calcareous and clayey and contains rounded gravel; unit absorbs and 

releases runoff. Base covered; estimated thickness 0.3–5 m

Qc Colluvium (Holocene and late Pleistocene)—Thin colluvium covers much of the map area except the 

steepest hillslopes in the northern area. Only exceptionally thick colluvium was mapped, generally 

on 4–5 percent slopes on flanks of small valleys. Silty clay with abundant gray limestone pebbles, 

cobbles, and angular fragments on surface. Cementing the colluvium on nearly flat land (southern 

half of the area) is caliche, 0.5–1 m thick, which is mostly soft and punky, and capped locally by 

wavy laminar hard surface; caliche forms rinds 2–4 mm thick on limestone and chert clasts. As 

developed on Devils River (Kdvr) and Salmon Peak (Ksp) Limestones in the hilly northern part of 

map area, bedrock crops out upslope of concave-upward footslopes, which are covered by colluvium made of pebbly clay and silty soil that supports grass and abundant shrubs. Thickness of unit 

2–5 m

Qls Landslide deposit (Holocene and late Pleistocene)—Jumbled soil material, talus, and blocks of 

limestone, siltstone, and basic igneous rock on the flanks of Turkey Mountain. Thickness estimated 

2–10 m

Qaf Fan alluvium (Holocene and late Pleistocene)—Similar to alluvium (Qal) compositionally and texturally except deposits form fans at the mouths of small watersheds in the central part of map. Thickness estimated 2–6 m

Qt Terrace alluvium (late Pleistocene)—Alluvium in terraces 5–10 m higher than modern channel of 

West Nueces River. Brown (10YR 5/3) clast-supported, rounded limestone pebble and cobble 

gravel, and sand, silt, and clay matrix. Apparently mostly planar bedded; calcite-cemented, resistant 

ledges locally. Common erosional scarps (depicted on map) in unit record scouring of alluvium 

during floods of West Nueces River. Base covered. Estimated thickness 3–8 m 

Ql Leona Formation (Pleistocene)—Grayish-brown (10YR 8/2) and light-brownish-gray (10 YR 6/2) 

calcareous, clayey, silty, and sandy unconsolidated alluvium containing limestone granules and 

pebbles, and caliche fragments, which are abundant in the lower part of unit. Soft masses of calcium 

carbonate are present in the uppermost 1–2 m. The unit underlies wide terraces that slope southward at <1 percent gradient; terraces are a few meters higher than Pinto Creek and Elm Creek 

(western part of map area). The unit also underlies narrow terraces adjacent to, and 2–4 m higher 

than, the Holocene floodplains of Las Moras Creek and Lindsey Creek (south edge of map area). 

The unit probably is coeval with some terrace alluvium (Qt) mapped along the West Nueces River. 

The Leona records pre-Holocene fluvial aggradation as streams widened their valleys in the upper 

coastal plain south of the Balcones Escarpment (Sayre, 1936). Unit is widely exposed 50 km east 

of Kinney County along the Nueces and Leona Rivers in Uvalde County. In the early 20th century, 

wells dug in the Leona yielded sufficient water for domestic use or livestock, but reportedly were 

dry during severe droughts (Bennett and Sayre, 1962; Vaughan, 1900). The Leona thus may be an 

accessible, shallow aquifer in wet years. Base of unit covered; in the mapped area, estimated maximum thickness 5 m. Unit pinches out away from streams

Ki Igneous instrusive rocks (Late Cretaceous, Campanian)—Very dark gray (5Y 3/1) alkaline mafic 

rock of basaltic affinity present as bosses, dikes, sills, and some possible laccoliths. Rock weathers 
Description of Map Units 11

dark yellowish brown (10 YR 4/4). Forms erosional, isolated hills (Pinto, Las Moras, Turkey, and 

Elm Mountains) that rise above the upper coastal plain. Dikes appear as dark, narrow lineaments on 

aerial photographs; on the ground they weather recessively and are hidden by dark soil and dense 

vegetation. Centimeter-sized pieces of basalt were dug from soil over a dike at one locality. In these 

samples, olivine(?) is altered to pseudomorphs of amber-colored iddingsite, an oriented micaceous 

mineral, and brown, hydrous iron oxide (limonite and goethite?). Spencer (1969) classified similar 

rocks in the Uvalde region as melilite-olivine nephelinite, olivine nephelinite, analcite phonolite, olivine basalt, and nepheline basanite, in order of decreasing abundance. Greenwood (1956) 

described volcanic mudflows and the mineralogy of an alkali peridotite intrusion into the Del Rio 

Formation 18 km S62°E from Turkey Mountain (a few kilometers east of the map area in Uvalde 

County). Miggins and others (2004) obtained 40Ar/39Ar ages of 82–80 Ma and 74–72 Ma on similar 

rocks in Uvalde County. Dike thickness 1–3 m; sill thicknesses estimated as 5–10 m, possibly 

>10 m in places

Kau Austin Chalk (Upper Cretaceous, Santonian-Coniacian)—Relatively soft, argillaceous, chalky, 

medium- to thick-bedded micritic limestone, and mudstone intercalated by very thin beds of marl. 

Poorly exposed on coastal plain under a 1- to 1.5-m thick caliche hardpan. Best exposed at Fort 

Clark Springs (at Brackettville) in an 8-m-high wall of the old quarry, now an outdoor amphitheater, 180 m south of the Las Moras Springs. Inoceramid and nonspecific pelecypod shell fragments 

were observed in the quarry; Inoceramus sp. and Gryphaea sp. were observed in the map area. 

Workers report that the Austin Chalk in central Texas contains coccoliths, planktonic foraminifera, 

oyster benthic fauna (for example, Exogyra), Inoceramus sp. (Czerniakowski and others, 1984) 

and the oyster Gryphaea aucella (Bennett and Sayre, 1962; Blome and others, 2005); Bardack 

(1968) reported the holostean long-snouted fish Belonostomus sp. Exposures of Austin Chalk near 

Bracketville contain sparse, dark-brown, framboids 0.3–1 cm in diameter which are subspherical, 

densely clustered, intergrown cubes, pyritohedrons, and octahedrons of limonite? or goethite? 

pseudomorphs after pyrite. Sparse framboids on the light-colored weathered surface of the Austin 

helps distinguish it from exposures of the underlying Eagle Ford (Kef) Formation, which apparently lacks many framboids. It is unclear what the pyrite indicates about the Austin Chalk depositional environment. The origin of the framboidal pyrite is debatable; pyrite framboids may form 

in oxygen-depleted, pelagic or benthic water columns during sedimentation or may be diagenetic 

(Wilkin and Barnes, 1997). Modern, microscopic pyrite is common in recent sediments, microbial 

mats, and soils (Schelble and others, 2003). The Austin-Eagle Ford contact is obscure on the plain in 

the southern part of the map area, although Freeman (1961) reported it obvious in Val Verde County, 

west of Kinney County. The contact was interpreted as unconformable in Kinney County (Bennett 

and Sayre, 1962), and as conformable in southwestern Kinney County (Smith, 1981) and in Val 

Verde County (Freeman, 1961; Lock and Peschier, 2006). According to Hovorka and Nance (1994) 

the unit was deposited as nannoplanktonic oozes in water < 200 m deep, north and west of the 

San Marcos platform during a world-wide Coniacian-Santonian sea-level highstand. Unit thickens 

toward center of Maverick Basin (fig. 3). Measured thickness of unit near southeastern corner of 

map area from well log RP–70–55–2AA at Highway 90 and West Fork Turkey Creek, 1.4 km south 

of south edge of geologic map, is 113 m (see fig. 8 on map)

Kef Eagle Ford Formation (Upper Cretaceous, Turonian)—Flaggy, argillaceous limestone, calcareous 

mudstone and siltstone, in very thin beds and thick (<1 cm) laminae; some calcareous, very finegrained, silicic sandstone, and silty marl. Unit appears as shallow exposures of flaggy rock pastelcolored, “pink,” “tan,” and “pale red” on the coastal plain surface; unit is partly obscured by caliche 

hardpan. Color-chart colors are: pink (5YR 7/4), light-brown (7.5YR 6/4), light-brownish-gray 

(10YR 6/2), and reddish-brown (5YR 5/3), reddish-yellow (7.5YR 6/8), and yellow (10YR 8/4). 

Depositional textures include very finely crystalline limestone, lime mudstone, and peloid wackestone of about 0.06 mm-diameter microspheres, in lime mud matrix. The microspheres may be 

of cyanobacterial origin (Duque-Botero and Maurrasse, 2005). Unit contains Inoceramus sp. as 

large as 6 cm diameter, echinoids, shark teeth, and fish teeth. A few, small exposures in roadcuts 

show planar shale laminations 0.5–5 cm thick and beds as thick as 30 cm. Hummocky bedding, not 

exposed in Kinney County and well exposed in Val Verde County (60 km west of Brackettville), 

was interpreted as contourites formed in moderately deep water on the upper continental slope 

during an exceptionally high sea stand (Lock and Peschier, 2006) and interpreted by Trevino and 

Smith (2002) as tidal flat or shallow shelf deposits; unit contains slump folded beds and turbidites in 
12 Geologic map of the Edwards aquifer, northeastern Kinney and southernmost Edwards Counties, Texas 

Val Verde County (Lock and Peschier, 2006). Interbedded marine limestone, calcareous mudstone 

and siltstone, and very fine-grained sandstone suggest deposition below storm wave-base. Smith 

(1981) assigned an age of late Turonian to Langtry Member of Boquillas Formation. The Boquillas 

is mapped in west Texas and is correlative to the Eagle Ford in Kinney County. The basal contact 

of the unit on the Buda Limestone (Kbu) was not observed in northeastern Kinney County; it was 

interpreted by Lock and Peschier (2006) to be sharp and unconformable in Val Verde County, west 

of the map area. Thickness about 42 m in drill hole RP–70–55–2AA at Highway 90 and West Fork 

Turkey Creek, 1.4 km south of south edge of geologic map (see fig. 8 on map and note at bottom 

right of geologic map border)

Kbu Buda Limestone (Upper Cretaceous, Turonian-Cenomanian)—White (10YR 8/2), pale yellow 

(2.5Y 7/4), and light-gray (10YR 7/2), locally very pale orange (10YR 8/2) dense, argillaceous, 

thick-bedded, microcrystalline limestone (lime mudstone), intrasparite, and locally dolostone. Unit 

weathers olive gray (5YR 3/2); smooth to conchoidal fracture; contains silt-size iron oxide particles 

and scattered sub-millimeter size fragments of mollusk shell. Typically, the unit is a sequence of 

resistant, planar 0.5- to 1.5-m-thick beds that dip southward and crop out in a north-facing cuesta. 

A 2-m-thick white limestone bed at the base of this sequence is conspicuous and can be traced 

east-west across the entire map area. The outcrop caps the brown, earthy, low slope of the underlying Del Rio Formation (Kdr). Basal contact on Del Rio is sharp; possible depositional environment 

is middle to outer shelf, open-marine environments (Denison and others, 2003). The CenomanianTuronian boundary probably is present within the unit, based on foraminifera in the Buda in the Big 

Bend region 400 km west of the map area (Frush and Eicher, 1975). Thickness 25–40 m along strike 

in map area, thickening westward and southward. Thickness about 30.5 m in drill hole RP–70–55–

2AA at Highway 90 and West Fork Turkey Creek, 1.4 km south of south edge of geologic map (see 

fig. 8 on plate and note at bottom right of geologic map border)

Kdr Del Rio Formation (Upper Cretaceous, early Cenomanian-late Albian)—Formerly mapped as 

Grayson Marl and Grayson Formation (Bennett and Sayre, 1962), from outcrops in Grayson 

County, Texas (Cragin, 1894). Unit crops out as light-yellowish-brown and brownish-yellow, silty, 

calcareous shale and mudstone in an earthy, low-angle, generally north-facing slope; base of unit 

is generally covered. Drill hole samples of unit are dark-bluish-gray, dense, plastic, laminated clay 

and claystone. Exposures contain 5–20-cm-thick beds of calcite-cemented claystone and laminations of iron-oxide and calcite-cemented, fine-grained sandstone; gypsiferous in places. Contains 

abundant, 2–3 cm-long “ram’s horn” marine oysters Ilymatogyra arietina (previously Exogyra 

arietina), Texigryphaea graysonana, pectens, and Globigerina foraminifera. Other fossils reported 

in the unit in central Texas and Maverick basin (not seen in map area) include: echinoids (starfish), 

Ptychodus sp. (teeth of shell-crushing shark or ray), Cretolamna (shark teeth), and ostracodes. The 

unit contains the Graysonites ammonoid fauna (G. lozoi, G. adkinsi), which define the AlbianCenomanian boundary in the Gulf region (Mancini, 1979). The Albian-Cenomanian boundary is in 

the basal 1.5 m of Grayson Formation (Del Rio equivalent) near Waco, Texas, based on the presence 

of the planktonic foraminiferan Rotalipora evoluta (Pessagno, 1969). Researchers have interpreted 

depositional environments of the Del Rio Formation as 1) modest water-depth tempestites (Lock, 

2008), 2) shallow near-shore, muddy siliclastic marine transgressive (Jones, 1993), and 3) open 

shelf, moderately deep water, low wave and current energy (Rose, 1972). The formation is extensive 

in southern-central and eastern Texas, on the San Marcos platform, and in southern Oklahoma and 

is the only unit in the Fredericksburg-Washita stratigraphic interval composed chiefly of clay. The 

Del Rio is part of the upper confining unit of the underlying Edwards aquifer (table 1). Outcrop 

thickness 25–27 m; thickness from well logs in Grass Valley is 29 m (Roberto Esquilin, written 

commun., 2007); thickness is 40 m in a 1055 ft-deep well (RP–70–55–2AA, drilled April 29, 1982) 

3.0 km southeast of junction of U.S. Highway 90 and State Road 1572 (to Spofford) and 60 m south 

of U.S. Highway 90, in Odlaw quadrangle, 1.4 km south of south edge of geologic map (see fig. 8 

on map for gamma log); unit is >76 m thick in southwestern Zavala County, southeast of Kinney 

County (fig. 3)

Ksp Salmon Peak Limestone (Lower Cretaceous, upper Albian)—Very pale orange (10YR 8/2; fresh break), 

porcelaneous microcrystalline limestone (micrite) to finely crystalline limestone (recrystallized?), 

conchoidal fracture; weathers gray (10 YR 5/1) and medium olive gray (5Y 5/1). Abundant chert 

nodules 5–25 cm across exposed discontinuously along horizons, which is typical in northeastern 
References Cited 13

Kinney County and in the “lower unit” at Chalk Bluff type section in western Uvalde County 

(Lozo and Smith, 1964). Lower part is thick-bedded (0.5–1.5 m) globigerinid lime mudstone. Unit 

includes partly burrowed, miliolid packstone. The contact with the underlying McKnight Formation 

was interpreted as disconformable by Lozo and Smith (1964). Unit name was proposed (Lozo and 

Smith, 1964) to replace the “Georgetown Limestone” and was named after Salmon Peak located 

30 km northeast of Brackettville (see map). Unit represents reef margin and basin facies (for a 

detailed facies and cyclical stratigraphic analysis of this unit, see Zahm and others, 1995; Humphreys, 1984). Forms light-toned, whitish, conical hills, northeast part of map area. Is upper unit of 

Edwards aquifer. Thickness about 115 m at Chalk Bluff type section, Uvalde County, 37 km east of 

Brackettville. Eroded thickness in map area typically 90–95 m; more than about 215 m in subsurface of Maverick Basin, Maverick County, south of map area

Km McKnight Formation, undivided (Lower Cretaceous, Albian)—Laminated and thin-bedded, lightgray (10YR 7/2) lime mudstone, petroliferous shale, and cyclic, subtidal, dark organic-rich argillaceous limestone; mild fetid odor on fresh break. Locally developed breccia and collapsed bedding, probably caused by dissolution of evaporite minerals. Depositional environment of the unit 

was basinal, calm, sometimes euxinic, hypersaline, and evaporitic (Rose, 1972; Sheu and Burkart, 

1982); at other times carbonate-forming, probably in a low energy, restricted lagoon. Studies of 

marine mud deposits (Schieber and others, 2007; Macquaker and others, 2007) concluded that currents and burrowing by diminutive macrofauna are active at many mud-water interfaces, suggesting 

some energy there. This unit and its informal members (described below) are the middle part of the 

Edwards aquifer. Thickness was reported as 42.7 m by Lozo and Smith (1964) at their Bitter Ranch 

type section (see symbol on map, Salmon Peak quadrangle). Unit is about 91 m thick in a drill hole 

(International Boundary and Water Commission ID–22 core test, 6 km WNW of Del Rio, Val Verde 

County, Tex.). Thickness in northeast part of map area 50–61 m

Kmu Upper member—Light gray (10YR 7/2), very pale brown (10YR 8/4), mottles of yellow (10YR 7/6) 

and brownish yellow (10YR 6/6), wisps of yellow (10YR 7/6), dense micrite; weathers gray (10YR 

5/1). Mostly thin-bedded, some thick beds (0.4–0.6 m), porcelaneous, conchoidal fracture. Unit contains evaporite minerals in the subsurface, as inferred from geophysical logs. Chert laminae present; 

bedding is locally brecciated and collapsed. Thin-bedded, some thick-bedded pellet lime wackestone 

and packstone and thin beds of silty, lime mudstone. In places unit forms a subtle bench below 

the uniform hillslopes eroded in the overlying Salmon Peak Limestone (Ksp). Evaporite minerals 

(subsurface only) suggest tidal mudflat and hypersaline, shallow lagoonal depositional environments 

(Miller, 1984). Thickness 24–30 m 

Kmm Middle member—White (10YR 8/1, 10YR 8/2) dense microcrystalline limestone (micrite or lime 

mudstone), weathers gray (10YR 6/1); petroliferous odor on fresh break, conchoidal fracture. Very 

thinly bedded (flaggy) and laminated dark-gray, carbonaceous, somewhat petroliferous, lime mudstone; organic rich; locally common chert laminae about 1 cm thick; local thin beds of shell hash. 

Brecciated in places, probably owing to dissolution of beds of evaporitic minerals, causing localized 

collapse. Some micrite beds contain thin pieces of oyster shell. Within an area less than 4 km from 

Salmon Peak (north-central part of map), this middle member forms a conspicuous horizontal vegetation band about 7 m thick; mapping of unit within this area is certain. Northeast of Salmon Peak 

the vegetation band divides into two and three thinner bands, making the upper and lower limits of 

the unit less certain. Unit was deposited in a lagoonal environment (Miller, 1984), which was hypersaline at times. Thickness 6–12 m

Kml Lower member—Very pale brown (10YR 8/3) and white (10YR 8/2) lime mudstone (micrite). Some 

rock in the unit is uniform and porcelaneous and breaks smoothly or conchoidally; other rock is 

micro-vuggy, porous, recrystallized, very finely crystalline, thin-bedded (5–30 cm thick) limestone 

that weathers gray (10YR 6/1) and contains common to abundant 1-cm-thick chert laminae repeating in sets that are as much as 30 cm thick; typical lime mudstone beds are 20–40 cm thick. Scarce, 

local oyster packstone beds (coquina beds of oysters 2–4 cm long). Thin-bedded pellet and shellfragment grainstone. Locally brecciated beds are attributed to dissolution of evaporite minerals. Unit 

accumulated on an evaporite-dominated tidal mudflat and in shallow lagoons (Miller, 1983). Unit 

apparently intertongues with upper part of underlying West Nueces Formation (Kwn) northeastward 

from Salmon Peak (northeast part of map). Thickness 24–30 m

Kwn West Nueces Formation (Lower Cretaceous, Albian)—Light gray (10YR 7/1) nodular limestone. 

Lower 21 m is a thick-bedded, distinctively nodular limestone, not mapped separately, but shown in 

Description of Map Units 13
14 Geologic map of the Edwards aquifer, northeastern Kinney and southernmost Edwards Counties, Texas 

table 1; this regionally persistent part is known informally as “basal nodular unit” and “basal transgressive unit.” Depositional texture is chiefly a nodular, shell-fragment wackestone that contains 

common, dark-brown spherules (0.2–0.5 mm diameter) and about 3 percent calcite spar void fillings. The nodular appearance may result from recent weathering of the clayey limestone, originally 

lime mud, that underwent thorough burrowing by organisms. A few thick (0.5–1 m) interbeds of 

pinkish-gray (7.5YR N/4) grainstone are interspersed in the nodular limestone. Unit is jointed and 

contains common, small (<1 m) solution cavities in well-exposed, kilometer-long cliffs of the West 

Nueces River in the Salmon Peak quadrangle (fig. 6); in the cliffs, only one normal fault of 1-m 

displacement was evident. Unit contains common oyster fragments, Ceratostreon texanum (Exogyra 

texana), heart clams, and Texigryphaea, an oyster. Lozo and Smith’s type sections in bluffs of West 

Nueces River shown on geologic map by symbols WN I and WN II. The formation is the lowest 

part of the Edwards aquifer (table 1). Unit overlies Glen Rose Formation (Kgr), possibly conformably in most of Maverick Basin, although in the wider region, basal contact is generally nonconformable. The base was not observed in the map area. Thickness of unit according to Lozo and 

Smith (1964) is about 44 m; thickness as mapped in this report 36–43 m

Kdvr Devils River Limestone (Lower Cretaceous, Albian)—Unit is a carbonate ramp reefal facies of the 

Devils River trend, laterally equivalent to the Salmon Peak (Ksp), McKnight (Km), and West 

Nueces (Kwn) Formations (Maverick Basin facies). Unit is white (10YR 8/2) dense, microcrystalline limestone (lime mudstone), grainstone, and packstone that weathers light gray (10YR 7/2) and 

gray (10YR 5/1). Outcrops on hillslopes in northern part of map area commonly are pocked, pitted, 

and show fluted dissolution features and are topped by scattered chert nodules. Also includes laterally persistent, 1.5- to 2-m-thick beds of micrite; skeletal grainstone, and rudist packstone. Miliolid, 

pellet, rudist, shell-fragment lime grainstone and wackestone are locally recrystallized, dolomitized, 

and brecciated (Lozo and Smith, 1964). Lower 15–18 m is nodular, shell-fragment wackestone 

containing heart clams and Ceratostreon texanum (Exogyra texana). This part correlates to basal 

nodular unit of West Nueces Formation (Kwn) (Miller, 1984). Pastel-colored (pink, tan, pale red, 

and yellow), concentrically layered, oblate and semi-spherical nodules of chert are abundant in parts 

of unit; nodules are 1–3 cm in diameter, 2–20 cm long. Unit is exposed in northern part of map area, 

an area of mature, dendritic, stream patterns and smoothly rounded hills and small canyons. Unit 

was deposited north of the Maverick Basin in high-energy reefs, shoals, and marine carbonate banks 

(Devils River trend; fig. 3) and at times in shallow to deep subtidal settings (Zahn and others 1995). 

The Devils River Limestone was mapped where the Salmon Peak, McKnight, and West Nueces Formations were indistinguishable; the unit makes up the entire Edwards aquifer north of the Maverick 

Basin. Eroded thickness about 67 m

Kgr Glen Rose Formation (Lower Cretaceous, lower Albian)—Alternating thin to medium beds of 

relatively resistant limestone, dolostone, and soft claystone and marl. Unit is poorly exposed to 

concealed on low-angle slopes and flats in the east-central part of map area immediately north of 

the West Nueces River and in the valleys of Sycamore and Live Oak Creeks (northeast-central part 

of map). The uppermost beds probably crop out locally in the channel of the West Nueces River 

(Ted Small, U.S. Geological Survey, oral commun., 2005). Samples there were ambiguous as to 

lithostratigraphic unit. The distinct basal nodular unit of the West Nueces Formation (Kwn), which 

overlies the Glen Rose, is clearly exposed at the mouth of, and in the valley walls of, Chalk Creek, 

2 km west of Salmon Peak, where a previous map (Lozo and Smith, 1964) erroneously depicts 

Glen Rose Formation. Other areas where the Glen Rose Formation should crop out (based on a 44 

m thickness of the overlying West Nueces Formation) are obscured by colluvium and alluvium and 

the unit was mapped based on float. Unit accumulated in Trinity time (early Albian, see table 1.) in 

intertidal to restricted shallow marine, low wave energy and current energy on a carbonate ramp that 

prograded seaward, built upward, and evolved into a rudist reef complex that fringed the Comanche 

Shelf (Rose, 1972; Bay, 1977). The Glen Rose is the lower confining unit of the Edwards aquifer 

(table 1). Thickness of the unit undetermined in map area, approximately 335–473 m thick in a well1 

in western Kinney County

1

Well G–2 Richardson Oil Co. Martin Rose no. 1, and Well T–1 Austral Oil and Exploration Co. Wardlaw-Whitehead Est. no. A–1 (from Bennett and Sayre, 

1962, pl. 3).
References Cited 15 

Acknowledgments

I thank Roberto Esquilin, Edwards Aquifer Authority, 

San Antonio, Texas, for providing gamma-ray logs of water 

wells. Ted Small (deceased 2007), Allan Clark, and Jason 

Faith, geologists with the Water Resources Discipline of the 

U.S. Geological Survey (USGS), San Antonio, and Grant 

Snyder, hydrogeologist, shared their knowledge of the Lower 

Cretaceous stratigraphy and Edwards aquifer; Robert Morris 

(USGS) provided aerial photographs of Kinney County. Jason 

Faith (USGS) assisted in the field. I thank George Ozuna, 

Office Chief, USGS Texas Water Science Center, San Antonio, for use of office, library, and vehicles. Jim Messerich, 

photogrammetric technician, USGS Denver, assisted with 

stereographic photogrammetric work. Paco Van Sistine, Scott 

Snyders, Lyndsay Hazen, and Tom Adamson (USGS) digitally 

formatted the geologic map and base map. Thanks to John 

Boerschig of Katey, Texas, and local ranchers Tully Shahan, 

Chuck Hall, and Mike Harrell for their interest in the mapping 

and allowing access to their land.

This work was funded by the USGS National Cooperative Geologic Mapping Program, Peter Lyttle, Program Coordinator, and the Mid-continental Carbonate Aquifer Project, 

Chuck Blome, Project Chief. 

References Cited

Bardack, David, 1968, Belonostomus sp., the first holostean 

from the Austin Chalk (Cretaceous) Texas: Journal of Paleontology v. 62, p. 1307–1309.

Barron, E.J., 1983, A warm, equable Cretaceous—The nature 

of the problem: Earth-Science Reviews, v. 19, p. 305–338.

Bates, R.L., and Jackson, J.A., 1980, Glossary of geology (2nd 

ed.); Falls Church, Virginia, American Geological Institute, 

751 p.

Bay, T.A., Jr., 1977, Lower Cretaceous stratigraphic models 

from Texas and Mexico, in Bebout, D.G., and Loucks, R.G., 

eds., Cretaceous carbonates of Texas and Mexico, applications to subsurface exploration: Austin, University of Texas 

at Austin, Report of Investigations no. 89, p. 12–30.

Bebout, D.G., 1974, Cretaceous Stuart City shelf margin of 

south Texas: Gulf Coast Association of Geological Societies 

Transactions, v. 24, p. 138–159.

Bennett, R.R., and Sayre, A.N., 1962, Geology and groundwater resources of Kinney County, Texas: Texas Water Commission Bulletin 6216, 163 p.

Blome, C.D., Faith, J.R., Pedraza, D.E., Ozuna, G.B., Cole, 

J.C., Clark, A.K., Small, T.A., and Morris, R.R., 2005, 

Geologic map of the Edwards aquifer recharge zone, southcentral Texas: U.S. Geological Survey Scientific Investigations Map 2873, scale 1:200,000.

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

characteristics of the Edwards aquifer, Uvalde County, 

Texas: U.S. Geological Survey Water Resources Investigation Report 03–4010, 17 p.

Coates, A.G., 1973, Cretaceous Tethyan coral-rudist biogeography related to the evolution of the Atlantic Ocean, in

Organisms and continents through time: London, The Palaeontological Association Special Paper in Palaeontology no. 

12, p. 169–174.

Cragin, F.W., 1894, Choctaw and Grayson terranes of the 

Arietina: Colorado College Studies, v. 5, p. 43–48.

Czerniakowski, L.A., Lohmann, K.C., and Wilson, J.L., 1984, 

Closed-system marine burial diagenesis—Isotopic data 

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18 Geologic map of the Edwards aquifer, northeastern Kinney and southernmost Edwards Counties, Texas 

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Moore—Geologic Map of the Edwards Aquifer, Northeastern Kinney and Southernmost Edwards Counties, Texas —Scientific Investigations Map 3105

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