The geologic formations in the vicinity of Lackland Air Force Base and Medina Base consist of sedimentary rocks of Cretaceous, Tertiary, and Quaternary age. The lithology of these formations consists primarily of limestone and marl, with lesser amounts of gravel, sand, silt, shale, and clay. The formations that comprise the geologic setting at Lackland Air Force Base and Medina Base include the Navarro Group of Late Cretaceous age, the Midway Group and Uvalde Gravel of Tertiary age, and the Leona Formation and fluviatile terrace deposits of Quaternary age. The hydrologic investigation revealed that the Navarro and Midway Groups are not known to yield water to wells in Bexar County. Each stream-terrace deposit of the Leona Formation and the fluviatile terrace deposits are separate water-bearing units. At Lackland Air Force Base, shallow ground water was determined to be present in the Leona Formation and the flow probably is toward Leon Creek. At Medina Base, the fluviatile terrace deposits are present along both banks of Medio Creek and probably are separate water-bearing units. Furthermore, ground-water flow in each of these units probably is toward Medio Creek. The Uvalde Gravel, present only at and west of Medina Base, is not a source of shallow ground water. However, during periods of precipitation, the formation can readily absorb precipitation and surface runoff. Water drains quickly through the formation because of its topographic position and substantial hydraulic conductivity.
Additional publication details
USGS Numbered Series
Reconnaissance investigation of the geology and hydrogeology of Lackland Air Force Base, San Antonio, Texas
Water-Resources Investigations Report
U.S. Dept. of the Interior, U.S. Geological Survey ;
ESIC, Open-File Reports Section [distributor],