GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC CHARACTERISTICS OF THE GLEN ROSE LIMESTONE, CAMP STANLEY STORAGE ACTIVITY, BEXAR COUNTY, TEXAS

The Trinity aquifer system is a regional water source for the hill country of south-central Texas. The upper zone of the Trinity aquifer is composed of the upper Glen Rose Limestone member and the upper part of the middle zone of the Trinity aquifer is composed of the lower Glen Rose Limestone member. Based on mapping to the east on the Camp Bullis military instillation, the upper Glen Rose Limestone member has been subdivided into 5 intervals (A-E). Of these intervals, E and C, which are calcareous muds with boxwork structures and collapse breccia, are responsible for much of the near surface lateral movement of ground-water within the study area. Both of these intervals were evaporite beds that have undergone evaporite dissolution and have good fabric-selective porosity. A biostrome in interval D appears to have high porosity and permeability associated with fractures (not-fabric-selective porosity) and molds (fabric-selective porosity), which allow for substantial ground-water recharge from precipitation and run-off. Significant recharge probably occurs where the biostrome occurs at the surface. Interval A, which is a very permeable interval east at Camp Bullis (Clark, 2003), is mostly absent at CSSA due to erosion and plays a very minor role in recharge at/or near CSSA. Faulting within and adjacent to CSSA is associated with the northeast-southwest extensional Balcones fault zone. This fault zone has resulted in high-angle normal down to the coast faulting. Throw on most faults is 10 to 20 feet. Also present in the study area are fractures that are both parallel and perpendicular to the Balcones fault zone. The northwest-southeast fractures, which are perpendicular to the Balcones fault trend, appear to be the most permeable, probably due to east-west extension. This report describes the hydrogeologic units exposed at the Camp Stanley Storage Activity in Bexar County, Texas and presents a map of these units. Also presented is information about faults and fractures in the study area.