MW–10

A A'

U D

D U

D U

D U

D U D U

MW–8

MW–6

MW–19

MW–12

MW–H

Kgr(l)

Kgr(u)D

Kgr(u)E

Kgr(u)D

Kgr(l)

Kgr(u)E

Kgr(u)D

Kgr(u)D

Kgr(u)E

Kgr(u)E

Kgr(u)E

Kgr(u)E

MW–G

Sect oi n B–B'

Salado Creek

1,300

1,200

1,250

1,400

1,350

1,000

1,100

1,050

1,150

1,300

1,200

1,250

1,400

1,350

1,000

1,100

1,050

1,150

ALTITUDE, IN FEET

ABOVE NGVD 29

ALTITUDE, IN FEET

ABOVE NGVD 29

D U

B'

MW–12

MW–1

MW–5

MW–3

Kgr(l)

Kgr(u)E

Kgr(u)D

Kgr(u)E

Kgr(u)D

Kgr(u)E

Sect oi n A–A'

Salado Creek

1,300

1,200

1,250

1,400

1,350

1,000

1,100

1,050

1,150

1,300

1,200

1,250

1,400

1,350

1,000

1,100

1,050

1,150

ALTITUDE, IN FEET

ABOVE NGVD 29

ALTITUDE, IN FEET

ABOVE NGVD 29

B

ctivity and immediately adjacent area, Bexar County, Texas.

1300

1350

1350

1300

1350

1250

1250

1350

1400

2 MILES

MW–10

LOWER MEMBER

INTERVAL B

FAULT—Dashed where inferred

U, upthrown; D, downthrown

SECTION TRANSECT

MONITOR WELL AND NUMBER

HIGH PRIORITY KARST FEATURES—

Locations are approximate (George Veni,

George Veni & Associates, written

commun., 2003)

EXPLANATION

VIII

EDWARDS A

QUIFER

U

D

Kgr(l)

Kgr(u)E

Kgr(u)D

Kgr(u)A

INTERVAL C

INTERVAL D

INTERVAL E

INTERVAL A

Kgr(u)B

Kgr(u)C

Kkb

TRINITY A

QUIFER

UPPER MEMBER

GLEN ROSE LIMESTONE

KAINER FORM

ATION

UPPER

ZONE MIDDLE

ZONE

EDWARDS GROUP

TOPOGRAPHIC CONTOUR—Shows

altitude of land surface. Contour interval

50 feet. Datum is NGVD 29

1200

B B'

BASAL NODULAR

MEMBER

Kgr(l)

Kgr(u)B

r(u)B

Kgr(u)B

Kgr(u)B

Kgr(u)B

Kgr(u)C

Kgr(u)C

Kgr(u)C

Kgr(u)D

ed from U.S. Geological Survey

adrangles 

ransverse Mercator projection 

tum NAD 27

SCIENTIFIC INVESTIGATIONS MAP 2831

Clark, A.K., 2004, Geologic framework and hydrogeologic characteristics of the

Glen Rose Limestone, Camp Stanley Storage Activity, Bexar County, Texas

IN COOPERATION WITH THE

U.S. ARMY

The observed porosity and permeability of the hydrogeologic subdivisions in the study

area are described below, from youngest to oldest:

The porosity of Interval A primarily is associated with fractures and caves and is notfabric selective. The porosity is interconnected, thus the interval is more permeable relative to Interval B. The interval likely contains avenues for recharge, although its small

areal extent limits its relevance in terms of the potential for recharge.

Interval B contains little porosity and permeability. Not-fabric selective fracture porosity appears to be the dominant type with some minor cave development along solutionally

enlarged fractures. Most fractures have little solution enlargement. This interval acts as a

confining unit, except in the few places where cave development has occurred.

Interval C contains fabric selective boxwork porosity and breccia porosity associated

with collapse resulting from solution of evaporites. This interval tends to channel water

laterally to discharge at springs and seeps. This interval commonly is covered with soil

and vegetation.

The porosity and permeability of Interval D generally are low. However, fabric selective, moldic porosity occurs in the biostrome near the top of the interval. The biostrome

also contains not-fabric selective porosity associated with vugs, fractures, and caves. This

porosity appears interconnected, thus making the biostrome one of the more permeable

zones of the study area. The major part of Interval D is dominated by not-fabric selective

porosity, primarily in the form of fractures and caves. Permeability is relatively high in the

few areas in areas where fractures or caves have been solutionally enlarged.

Like Interval C, Interval E contains fabric selective porosity in the form of moldic

boxwork structures and collapse breccia associated with the dissolution of evaporites. This

interval acts as a lateral conduit for flow, as reflected by the numerous seeps and springs

that appear within its exposed outcrop.

The 20 to 30 feet of lower Glen Rose Limestone exposed in the study area contains

not-fabric selective porosity associated with fractures and caves, and in some places, fabric selective moldic porosity. Many of the karst features at the CSSA mapped by Veni

(George Veni, George Veni & Associates, written commun., 2003) occur where the lower

Glen Rose Limestone is exposed along Salado Creek. The lower Glen Rose Limestone

probably is one of the more permeable units in the study area.

SUMMARY

The Camp Stanley Storage Activity (CSSA) overlies the Trinity aquifer in northern

Bexar County, Tex. The Glen Rose Limestone comprises the upper zone and the upper

part of the middle zone of the Trinity aquifer and crops out in the study area. During 2003,

the USGS, in cooperation with the U.S. Army, mapped the hydrogeologic subdivisions

and faults of the Glen Rose Limestone at the CSSA and immediately adjacent area (Camp

Stanley study area) to help resource managers improve their understanding of the distribution of porosity and permeability of the outcropping rocks, and thus the conditions for

recharge and the potential for contaminants to enter the Glen Rose Limestone.

The rocks of the Glen Rose Limestone are fossiliferous limestones, alternating and

interfingered with mudstone, wackestone, packstone, and grainstone; and where present,

more massive carbonates are interbedded and interfingered with evaporites and marls.

High-angle normal faults in the study area are part of the Miocene-age Balcones fault

zone, an extensional system of faults that generally trends southwest to northeast in the

region. Although faults are the dominant structural feature of the region, they appear to

have little effect on recharge and in some places might act as barriers to ground-water

flow. Two primary sets of fractures were identified at CSSA, the larger oriented northwest

to southeast, perpendicular to the trend of the Balcones fault zone, and the smaller oriented southwest to northeast, parallel to the Balcones fault zone. In contrast to faults, fractures associated with the Balcones fault zone probably facilitate recharge.

The Glen Rose Limestone is subdivided (informally) into an upper and a lower member. On the basis of previous mapping by the USGS at adjacent Camp Bullis, the upper

Glen Rose Limestone in the study area has been subdivided into five mappable intervals,

designated (youngest to oldest) A through E.

Intervals A and B (alternating and interfingering medium-bedded mudstone to packstone, with solution zones locally) compose the upper 150 to 270 feet of the upper Glen

Rose Limestone at the Camp Stanley study area. Interval A occurs only in a small area in

the southern part of the CSSA, where it ranges from about 30 to 40 feet thick. The only

distinguishing factor in the field between the two intervals is the greater number of caves

present within Interval A. The porosity of Interval A, which primarily is associated with

fractures and caves (not-fabric selective), is interconnected and thus the interval is more

permeable relative to Interval B. Interval B contains little porosity and permeability.

Interval C is a solution zone 10 to 20 feet thick that originally was an evaporite bed.

The interval contains fabric selective boxwork porosity and breccia porosity and tends to

channel water laterally, as reflected by seeps and springs.

Interval D (alternating beds of wackestone, packstone and marl) is 135 to 180 feet

thick. The porosity and permeability of Interval D generally are low, although fabric

selective, vug, fracture, cavern, and moldic porosity that is interconnected occurs in a 30-

to 40-foot-thick biostrome near the top of the interval, which makes the biostrome one of

the more permeable zones of the study area.

Interval E is a 7- to 10-foot-thick solution zone that, like Interval C, originally was an

evaporite bed. Also like Interval C, the interval contains fabric selective boxwork porosity

and breccia porosity and tends to channel water laterally, as reflected by numerous seeps

and springs.

The lower Glen Rose Limestone (massive mudstone to grainstone) exposed in the

study area contains not-fabric selective porosity associated with fractures and caves and

probably is one of the more permeable units in the study area.

REFERENCES

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

the Hill Country of south-central Texas: Texas Department of Water Resources

Report 273, 172 p.

Barker, R.A., and Ardis, A.F., 1996, Hydrogeologic framework of the Edwards-Trinity

aquifer system, west-central Texas: U.S. Geological Survey Professional Paper

1421–B, 61 p.

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

porosity in sedimentary carbonates: American Association of Petroleum Geologists

Bulletin, v. 54, no. 2, p. 207–250.

Clark, A. K., 2003, Geologic framework and hydrogeologic characteristics of the Glen

Rose Limestone, Camp Bullis Training Site, Bexar County, Texas: U.S. Geological

Survey Water-Resources Investigations Report 03-4081, 9 p., 1 pl.

Dunham, R.J., 1962, Classification of carbonate rocks according to depositional texture,

in Classification of Carbonate Rocks Symposium: American Association of Petroleum Geologists Memoir 1, p. 108–121.

Forgotson, J.M., Jr., 1956, A correlation and regional stratigraphic analysis of the formations of the Trinity Group of the Comanchean Cretaceous of the Gulf Coastal

Plain; and the genesis and petrology of the Ferry Creek Anhydrite: Transactions of

the Gulf Coast Association of Geological Societies, v. 6, p. 91–108.

GldSfI2003Ghd M282003URL

The lowermost mapped unit in the study area is the lower member of the Glen Rose

Limestone (hereinafter, lower Glen Rose Limestone), which from well logs appears to be

about 320 to 340 feet thick at the CSSA. About 20 to 30 (vertical) feet of the unit is

exposed in the central part of the CSSA along the bed of Salado Creek (fig. 1). Although

mostly covered with alluvium and vegetation, the exposed rocks of the unit are a massive

mudstone to grainstone.

The upper Glen Rose Limestone has been subdivided into five mappable intervals

(Clark, 2003) that extend across the CSSA. These intervals (designated A through E) are

described below, from oldest to youngest:

Interval E is a solution zone 7 to 10 feet thick that originally was an evaporite bed.

Evaporites were subsequently dissolved leaving behind a calcareous mudstone. Often this

interval can be detected in caliper logs because it tends to wash out during the drilling

process. The Corbula bed, which is a very thin-bedded grainstone, lies at the base of

Interval E and marks the top of the lower Glen Rose Limestone. Typically the Corbula

bed is found as float (rock fragments displaced from site of origin) because of its more

resistant nature relative to the surrounding calcareous mudstone. In the outcrop, Interval E

appears as a yellow-to-white calcareous mud, and in some places, contains gray sparite

with boxwork structures. Interval E forms broad, gentle valleys as a result of differences

in rates of erosion between Interval E and the overlying Interval D and underlying lower

Glen Rose Limestone. This interval also contains numerous species of fossils including

the very large gastropod Nerinea romeri (Whitney), Orbitolina texana (Roemer), Porocystis globularis (Giebel), and Turritella sp. in addition to numerous species of pelecypods,

gastropods, shell fragments, and worm tubes.

Interval D is 135 to 180 feet thick and is composed of alternating beds of wackestone,

packstone, and marl. Near the base of Interval D, about 15 to 20 feet above the Corbula

bed of Interval E, is a second marker bed, a thin-bedded, silty mudstone that has a “platy”

appearance. About 95 feet above the base of Interval D is a thick-bedded biostrome, 30 to

40 feet thick, composed of Caprinuloidea sp. and mudstone. The biostrome is overlain by

10 to 30 feet of alternating, thin- to medium-bedded wackestone and packstone. Abundant

Orbitolina texana (Roemer), Porocystis globularis (Giebel), Tapes decepta (Hill), Protocardia texana (Conrad), Turritella sp., Hemiaster sp., and various fossils and fossil fragments can be found throughout Interval D.

Interval C is a solution zone 10 to 20 feet thick that, like Interval E, originally was an

evaporite bed. It is composed of yellow-to-white calcareous mud interspersed with thin

layers of mudstone. Some boxwork structure appears in cavities where evaporites have

been dissolved. Unlike Interval E, this interval contains few fossils in both diversity and

abundance.

Intervals B and A compose the upper 150 to 270 feet of the upper Glen Rose Limestone. Both intervals are composed of alternating and interfingering medium-bedded mudstone to packstone, with solution zones locally. The solution zones were evaporite beds

that have been dissolved. Intervals B and A are indistinguishable on the basis of lithology.

The distinguishing factor in the field between the two intervals is the greater number of

caves present within Interval A. Both intervals are relatively devoid of fossils. Interval B

ranges from 120 to 150 feet thick at CSSA. Interval A occurs only in a small area in the

southern part of the CSSA, where it ranges from about 30 to 40 feet thick. In the Camp

Stanley study area south of the CSSA (at Camp Bullis), the total section of Interval A (as

much as 120 feet) is present. The contact between Interval A and the overlying rocks of

the Kainer Formation is conformable.

The basal nodular member of the Kainer Formation (hydrostratigraphic zone VIII of

Maclay [1995]), is exposed on a hilltop at the very southern part of the study area. The

unit consists of shaley, nodular limestone, mudstone, and miliolid grainstone and is 50 to

60 feet thick in northern Bexar County (Stein and Ozuna, 1995).

HYDROGEOLOGIC CHARACTERISTICS

Faults and Fractures

High-angle normal faults are the dominant structural feature of the region; however,

faults appear to have little effect on recharge and in some places might act as barriers to

ground-water flow. The development of relay ramps would have an appreciable effect on

ground-water flowpaths on a local scale, allowing water to move downdip at oblique

angles to southwest-northeast trending faults. If a breech has developed in a ramp, flow

might be parallel to the breech.

Fractures associated with the Balcones fault zone probably facilitate recharge to the

upper and middle zones of the Trinity aquifer. The fractures trending northwest to southeast likely are the principal avenues of recharge to the aquifer as a result of extension perpendicular to the Balcones fault zone. Cave development probably is concentrated along

extensional fractures.

Observed Porosity and Permeability

The porosity and permeability of the rocks of the Glen Rose Limestone generally are

lower than the porosity and permeability of the rocks of the Edwards Group, which form

the Edwards aquifer to the south of the study area. Relatively greater permeability in the

mapped hydrogeologic subdivisions of the Glen Rose Limestone tends to be related to

lithology, zones of higher primary porosity, solutionally enlarged fractures, and karst features (primarily caves).

Fewer karst features were observed at the CSSA than at adjacent Camp Bullis. Preliminary results of a karst inventory at CSSA (George Veni, George Veni & Associates,

written commun., 2003) indicate that most karst features occur near the top of Interval D

and the top of the lower Glen Rose Limestone (fig. 1).

west [320

nal forces in the

ough seven

east through

to south and

ng. Both sec

not influenced

Stratigraphy

The Glen Rose Limestone (table 1) conformably overlies the Bexar Shale member of

the Pearsall Formation (not exposed in the study area). The contact is gradational and

interfingering. The 60- to 70-foot-thick Bexar Shale member typically is a mixture of dark

mudstone, clay, and shale (Barker and Ardis, 1996). The Glen Rose Limestone is subdivided (informally) into an upper and lower member. The two members are separated by a

regionally extensive marker bed known as the “Corbula bed” (Whitney, 1952), which is a

grainstone composed of the small fossil clam Corbula martinae.

[Group, formations, and members modified from Forgotson (1956), Rose (1972), Ashworth (1983); hydrogeologic

subdivisions (aquifers, zones, intervals) from Maclay (1995), Barker and Ardis (1996), Clark (2003); lithologic terminology

modified from Dunham (1962); and porosity type modified from Choquette and Pray (1970). AQ, aquifer; CU, confining

unit; *, not exposed in study area]

1 Referred to as “hydrostratigraphic zone” VIII by Maclay (1995).

Group,

formation,

member

Hydrogeologic

subdivision

Hydrologic

function

Thickness

(feet)

Lithology

Field

identification

Karst

features

Porosity type/

permeability

Edwards Group

Kainer Formation

Basal nodular

member

Edwards

aquifer

VIII

1

AQ karst;

CU not karst

50–60 Shaley, nodular

limestone, mudstone, and miliolid grainstone

Massive, nodular, and

mottled; abundant

gastropods and Exogyra texana

Large lateral

caves at surface

Fabric; stratigraphically controlled/large conduit flow at

surface

Glen Rose Limestone

Upper member

Trinity aquifer

Upper zone

Interval A

AQ associated

with fractures and

karst;

CU if not

2

30–120 Alternating and

interfingering

medium-bedded

mudstone, wackestone, and packstone with solution zones locally

Near contact with

Edwards Group; stairstep topography;

devoid of fossils;

evaporite beds locally

Some caves

below contact with

Edwards

Group.

Caves associated with

fractures and

bedding

planes

At surface, not-fabric selective

fracture and cavern porosity;

probably very permeable

near contact with Edwards

Group, which decreases with

depth, breccia porosity associated with collapse resulting

from dissolution of evaporite

beds

Interval B

CU;

AQ associated

with fractures and

karst

120–150 Alternating and

interfingering

mudstones, clays,

wackestone, and

packstones

Devoid of fossils; stairstep topography;

evaporite bed at top of

zone

Few caves Fabric selective interparticle

porosity and not-fabric selective fracture and some cavernous porosity; generally

low permeability away from

caves and solutionally

enlarged fractures

Interval C

AQ 10–20 Yellow-to-white

calcareous mud

and vuggy mudstone

Yellow calcareous mud,

with springs and

seeps; topography

tends to be flat; few

No known

caves

Fabric selective, breccia and

moldic (boxwork) porosity

Interval D

AQ associated

with bioherms;

CU above and

below

bioherms

135–180 Alternating beds of

wackestone,

packstone, and

marl; thin-bedded “platy” mudstone; thickbedded biostromes locally

Profuse Orbitolina texana; “platy” mudstone

bed; generally low

relief; stair-step

topography not well

defined

Relative abundance of

caves related

to fractures

and bedding

planes.

Both fabric and not-fabric

selective porosity; vug, fracture, cavern, and moldic

porosity within biostrome; in

lower 90 feet very low porosity, primarily fracture related.

Interval E

AQ 7–10 Yellow-to-white

calcareous mud

and vuggy mudstone

Yellow-to-white calcareous mud, with springs

and seeps; topography

tends to be flat;

Corbula bed at base

separating upper and

lower Glen Rose Limestone; numerous fossils

No known

caves

Fabric selective, breccia and

moldic (boxwork) porosity

Lower member

Middle zone

AQ in

bioherms and

evaporite

bed; also

associated

with karst

and fractures;

CU elsewhere

320–340 Thick-bedded mudstone; thin- to

medium-bedded

mudstone, wackestone, packstone,

and grainstone

Massive mudstone Relatively

more caves

than in other

subdivisions

except interval D

Both fabric and not-fabric

selective porosity; large fracture, cavern, and moldic

porosity within bioherms;

mudstones and marls primarily fracture porosity; low

permeability in mudstones

and marls; good permeability

in bioherms

Pearsall

Formation

Bexar

Shale

member

CU 60–70 Dark mudstone,

clay, and shale

* * *

Table 1. Summary of the lithologic and hydrogeologic properties of the hydrogeologic subdivisions

of the Glen Rose Limestone and associated units that crop out, Camp Stanley Storage Activity and

immediately adjacent area, Bexar County, Texas.

2 30-40 feet thick at Camp Stanley Storage Activity.

fossils