[Home] 

[Table of Contents] [Next Section]

Background Information Report

Soils and Geology

Soils

In general, soils at CSSA are thin, dark-colored, gravely clays and loams. The soil types are strongly influenced by topography and the underlying limestone. All soil classifications used for this report are taken from the USDA Soil Conservation Service (now the Natural Resource Conservation Service (NRCS) soil survey series for Bexar County, Texas (USDA, 1991). Figure 3 shows the number of acres at CSSA each soil type covers. Figure 4 shows the eight soil types occurring at CSSA.

Brackett Soils

Brackett (BrE) soils occur over 12.8 percent (512.5 acres) of the CSSA lands. These soils cover a large portion of the east pasture and the inner cantonment at CSSA. These soils occur on slopes of 12 to 30 percent, such as those found on Steele, McFarland, and Schasse Hills, as well as Taylor Ridge. These loamy and clayey soils are thin (about 4 inches thick), grayish-brown, and strongly calcareous. Gravel and cobblestone lithics occur at the surface and shallow subsurface. The soils can develop over soft limestone and are underlain by hard limestone, which gives the slopes a stairstep appearance. Topographic relief associated with Brackett soils is expressed as steep, cone-shaped hills with “saddles” between them. Brackett soils are nonarable and best suited to native grasses (USDA, 1991).

Tarrant Soils

At CSSA, the Tarrant soils occur along the outer edges of the Salada Creek floodplain. The soils are thin and form over hard, fractured limestone. The surface layer is usually about 10 inches thick and is a dark grayish-brown, calcare­ous, clay loam with scattered gravel and cobblestones within, and on the surface layer. Two types of Tarrant soils occur at CSSA: Tarrant association, gently undulating, and Tarrant association, rolling.

The gently undulating Tarrant association (TaB), gently undulating areas are typical of prairie and plateau topo­graphy. They occur primarily in areas not occupied by streams, such as the north-central area of the inner cantonment, as well as the west sides of Steele and Wells Hills and on the hills north of the inner cantonment. This soil type covers approximately 14.3 percent (572.6 acres) of CSSA. The soils are dark colored, very shal­low, calcareous, and clayey, and are best suited for native grasses and range use.

Rolling Tarrant association (TaC) is found on the eastern sides of Anderson and Schasse Hills, in areas not occupied by streambeds. This soil type occurs over only 1.3 percent (52.1 acres) of CSSA lands. The slopes tend to have a gradient of 5 to 15 percent. The soils are dark colored, very shallow, clayey, weakly calcareous, and typically more stony than Tarrant association, gently undulating.

Brackett-Tarrant Association

Brackett-Tarrant association soils (Bte) cover 24.9 percent (997.0 acres) of CSSA. The soils are formed on hills with 8 to 30 percent slopes and consist primarily of soils that developed over limestone. At CSSA, this soil type is found north of the inner cantonment, in the north pasture. The slopes of ridges are Tarrant soils which are clayey, calcareous, and very dark grayish-brown. The Brackett soils are light grayish-brown and calcareous. Tarrant soils make up 65 percent of the association and Brackett make up 20 percent. The soils are not suited to crops, and stones and topography made the use of machinery difficult (USDA, 1991).

Crawford and Bexar Stony Soils

Crawford and Bexar Stony soils (Cb) occupy portions of both the inner and outer cantonments, for a total of approximately 16.9 percent (676.7 acres) of CSSA. They occur in broad, nearly level to gently undulating areas with slopes of 0 to 5 percent. The soils are stony, very dark gray to dark reddish brown, noncalcareous clay, about 8 inches thick. Bexar soils range from a cherty clay loam to gravely loam. The soils are nonarable and suited for native grass, such as Texas winter grass, little bluestem, sideoats grama, and buffalo grass.

Trinity and Frio Soils

The Trinity and Frio soils (Tf) cover approximately 8.8 percent (352.4 acres) of CSSA. The soils frequently subjected to flooding, are the main channel soils for Salado Creek and a large tributary that joins the creek in southwestern CSSA. Some areas are subject to thin sediment depositions, while other areas are scoured. Channels are poorly defined and are of small capacity. Trinity soils are 3 to 5 feet deep and composed of clayey to gravely loam. Frio soils are a dark grayish-brown clay loam, 3 to 4 feet deep. Vegetation may consist of elm, hackberry, oak, mesquite, and other thorny shrubs, Texas wintergrass, Johnson grass, buffalo grass, Bermuda grass, and annual weeds.

Krum Complex

The Krum Complex soils (Kr) make up the remaining soils covering the streambeds and floodplains, approximately 20.0 percent (800.8 acres) of CSSA. The soils are dark grayish-brown or very dark grayish-brown, calcareous, and approximately 30 inches thick. The soils developed from slope allu­vium of the limestone prairies. They occur on slopes of 2 to 5 percent and occupy "foot" slopes below Tarrant and Brackett soils. The Krum Complex soils receive sediments and runoff from higher elevation soils and are highly prone to hydraulic erosion if unprotected.

Lewisville Silty Clay

A minor soil type found at CSSA is the Lewisville silty clay (LvB) found on slopes of 1 to 3 percent. This soil type covers only 1.0 percent (40.0 acres) of CSSA. It typically occu­pies long, narrow, sloping areas separating nearly level terraces from upland soils. It can be found in small areas south of Dietz Elkhorn Road and north of the inner cantonment boundary around Moyer Road. Surface soils are dark grayish-brown and about 20 inches thick. This is a highly productive soil but is also susceptible to hydraulic erosion if unprotected.

Physiography

CSSA is characterized by a rolling terrain of hills and valleys in which nearly flat-lying limestone formations have been eroded and dissected by streams draining to the east and southeast. Normal faulting has occurred near the central area and the south­eastern boundary of the installation. Regionally however, two major trends of fractures extend northwest-southeast and northeast-southwest. Faulting in the lime­stone units has juxtaposed strata of different ages, but fault scarps and traces are almost absent because the similar calcareous lithologies weather similarly. The faults are northeast-southwest trending, but most are not as continuous as the frac­tures. Soil cover is relatively thin, and bedrock is exposed in most areas other than stream valleys.

River and stream dissection of limestone is the major surface feature at CSSA. Most major rivers and streams originating in the Edwards Plateau to the northwest of CSSA tend to follow the northwest-southeast regional fracture patterns. Resis­tive limestone beds crop out as topographic highs, but none of these beds form buttes or mesas. Rather, the predominant physiography is hills and "saddles" which lead to stream valleys. Topographic relief across the area ranges from about 1,100 feet to 1,500 feet above sea level.

Sinkholes and caverns are present on the surface and in the subsurface, primar­ily in areas where porous and fractured limestone formations are exposed. The sinkholes and caves result from dissolution of limestone and gypsum by infiltrating surface water. Commonly, fractures are enlarged by moving groundwater which enhances porosity and permeability. Fractures, fault scarps, and karstic low-lying areas can be recognized on aerial photographs.

Geology

Information is summarized from the following sources. Groundwater movement and characteristics are discussed in the Texas Water Development Board (TWDB) report 273 (Ashworth, 1983). Specific information on the hydrogeology of the lower Glen Rose aquifer is given in a Ph.D. dissertation (Hammond, 1984). Hydrogeology around and within the Camp Bullis area is discussed in an unpublished master’s thesis (Wattereus, 1992).

Stratigraphy. The oldest and deepest known rocks in the study area are Paleozoic age (225 to 570 million years ago) schists of the Ouachita structural belt. They underlie the predominant carbonate lithology of the Edwards Plateau. The Cretaceous-age sediments were deposited as onlapping sequences on a submerged marine plain and, according to well logs and outcrop observations, these sediments thicken to the southeast. Table 1 summarizes the Cretaceous System stratigraphy. They represent the Trinity Group Travis Peak Formation shallow marine deposits. The Travis Peak Formation attains a maximum thickness of about 940 feet and is divided into five members, in ascending order: the Hosston Sand, the Sligo Lime­stone, the Hammett Shale, the Cow Creek Limestone, and the Hensell Sand. Overlying the Travis Peak Formation, but still a part of the Cretaceous-age Trinity Group, is the Glen Rose Limestone.

The Hosston Sand is generally composed of conglomer­ate, sandstone, and claystone, becoming increasingly more dolomitic and shaley downdip to the southeast. The Sligo Limestone exists downdip where the Hosston grades into a sandy lime­stone. Overlaying the Sligo is the Hammett Shale, which has an average thickness of 60 feet. It is composed of dark blue to gray fossiliferous, calcareous, and dolomitic shale. It pinches out north of the study area and attains a maximum thickness of 80 feet to the south.

Above the Hammett Shale is the Cow Creek Limestone. It is a massive fossilif­erous, white to gray, shaley to dolomitic limestone that attains a maximum thickness of 90 feet downdip in the area. At CSSA, it averages about 80 feet in thickness.

The youngest member of the Travis Peak Formation is the Hensell Sand, locally known as the Bexar Shale. The shale thickness averages from 80 to 150 feet. It is composed of silty dolomite, marl, calcare­ous shale, and shaley limestone, and thins by interfingering into the Glen Rose Formation.

The upper member of the Trinity Group is the Glen Rose Limestone. The Glen Rose Limestone was deposited over the Travis Peak Bexar Shale and represents a thick sequence of shallow water marine shelf deposits. This formation is divided into upper and lower members. At CSSA, the Glen Rose is exposed at the surface and in stream valleys. Figure 5 shows the outcrop locations of the Glen Rose members at CSSA and the surrounding area.

The upper Glen Rose consists of beds of blue shale, limestone, and marly limestone with occasional gypsum beds (Hammon, 1984). Based on well log information, the thickness of the upper member reaches 500 feet in the Bexar County. The thickness of this member at CSSA has not been determined, but it is estimated from well logs to be between 20 and 150 feet.

The lower Glen Rose, underlying the upper Glen Rose, consists of a massive fossiliferous limestone, grading upward into thin beds of limestone, marl, and shale (Ashworth, 1983). The lower member, according to area well logs, is approximately 300 feet thick in the CSSA area. Based on published maps of the region, only the upper Glen Rose was thought to outcrop at CSSA. However, based on geological information obtained during project work on contract F33615-89-D-4003 (order 67), it is believed that the lower Glen Rose also outcrops at CSSA.

The boundary between the upper and lower members of the Glen Rose Limestone is defined by a widespread fossil stratigraphic market known as the Corbula bed (Whitney, 1952). The Corbula bed is 0.5 to 5 feet thick and contains small pelecypod clamshells, which are three to five millimeters in diameter. Presence of the Corbula fossil indicates a slightly more saline depositional environment than fossils found above and below the Corbula. A gypsum bed has also been identified close to the Corbula bed.

Fredricksburg Group sediments, including the Edwards Formation, overlie the Glen Rose Formation in many areas as erosional remnants outcropping as topographic highs. For this report, the Fredricksburg Group limestones will not be discussed because of the lack of outcrop in the immediate vicinity of CSSA.

Structure. The predominant structural features in the area are regional vertical fractures, the regional dip, and the Balcones fault zone (escarpment). The regional fractures are the result of faulting in the Cretaceous sediments and in the deeper Paleozoic rocks. The two sets of fracture patterns trend northwest-southeast and northeast-southwest across the region. The regional dip is to the east and southeast at a grade of about 100 feet per mile near the fault zone in Bexar and Comal Counties, decreasing to 10 to 15 feet per mile northwest of CSSA.

The Balcones fault zone is a series of high-angle normal faults that generally trend northeast and southwest. Total displacement in northwest Bexar County is approximately 1,200 feet. The faulting is a result of structural weakness in the underlying Paleo­zoic rocks and subsidence in the Gulf of Mexico basin to the southeast. The down­drop blocks outcrop as progressively younger strata from northwest to southeast across the fault zone. In addition to major faulting along the Balcones fault zone, numerous minor northwest-southeast-trending faults also occur. These faults are laterally discontinuous and their displacement is small. Figure 6 and Figure 7 show cross-sections prepared with data from surface geophysical, soil gas, mapping installation of two Glen Rose monitoring wells, ground penetrating radar and seismic surveys performed under AL/OEB 67 in 1995.

Cross section A-A’ (Figure 6) shows the regional dip of beds interpreted between well logs at 10 to 12 degrees to the south-southeast. The two fault zones shown in the geologic map (Figure 5) are depicted in this section, as is the small graben-type offset interpreted from seismic reflection survey data (shot points 310 to 325) about 1,100 feet south of well 16. A fourth area of faulting at the south end of the section is a fault or fault zone located offsite in the area of The Dominion, about 1.7 miles south of CSSA. This fault correlates with one of the faults shown on a cross section extending north and south of CSSA over 8 miles along I-10 (Simpson Company and Guyton Associates, 1993). Because this fault is interpreted between two wells which show an offset in elevations of the top of the Bexar Shale, the fault is shown on this diagram as a zone in which the exact surface location is not known. The fault is included in the CSSA cross section A-A’ to verify the presence of faults with displacement up to the southeast as well as many faults along the Balcones fault zone which have displacement down to the southeast.

Of particular interest is the area between and beneath CSSA wells 16, MW1, MW2, and CSSA well 1. The northern fault zone shown in the geologic map is represented in this area, as are the two source areas of groundwater contamination, SWMUs B-3 and O-1. The faulting in this aera, though not of great magnitude such as 100 feet of throw that would displace a relatively impermeable shale against a more permeable limestone, is significant in that pathways for groundwater contamination are more complex than in unfaulted areas. Even with the vertical exaggeration of 20x horizontal, the diagram shows the short horizontal distance (400 to 1,200 feet) compared to the depths (at least 150 feet to groundwater) over which PCE, TCE, and DCE probably have traveled between SWMUs O-1 and B-3 to well 16. When the karstic geology regime is taken into account, in which contaminants can migrate along solution enhanced fractures as well as faults, joints, and bedding planes, determination of the exact pathways that the dense nonaqueous phase liquid contaminants (DNAPLs) PCE and TCE have migrated becomes more difficult if not impossible.

Southwest-to-northeast cross section (B-B’) indicated a displacement between the Poetchke well and CSSA well 11. More detailed examination of geophysical well logs did not find such displacement, and Figure 7 shows the resulting interpretation.

Looking at the subsurface geology along strike on cross section B-B’ shows a slight dip of bedding planes to the southwest. The section shows one fault between well 2 and PH1, which correlates to an interpreted fault from surface mapping and the GPR profiling. A minor displacement of about 4 feet is noted between elevations of the Bexar Shale. The northern fault zone is interpreted as southeast of well 2 on the line of section and just northeast of well 16 (see Figure 6 for reference). Because the line of section is about 35 degrees from parallel along the northern fault zone, the width of the fault zone looks much wider in Figure 7 than in Figure 6. Interestingly, the April 1996 water level measurements between wells 2 and MW1 also show a decrease in potentiometric surfaces. When viewed from well 2 to MW1 along the projection, the direction of this decrease is to the southeast.