Geologic framework and hydrogeologic characteristics in the southern part of the Rancho Diana Natural Area, northern Bexar County, Texas, 2008-10

The area designated by the city of San Antonio as the Rancho Diana Natural Area is in northern Bexar County, near San Antonio, Texas. During 2008-10, the U.S. Geological Survey, in cooperation with the city of San Antonio, documented the geologic framework and mapped the hydrogeologic characteristics for the southern part of the Rancho Diana Natural Area. The geologic framework of the study area and its hydrogeologic characteristics were documented using field observations and information from previously published reports. Many of the geologic and hydrogeologic features were found by making field observations through the dense vegetation along gridlines spaced approximately 25 feet apart and documenting the features as they were located. Surface geologic features were identified and hydrogeologic features such as caves, sinkholes, and areas of solutionally enlarged porosity were located using hand-held Global Positioning System units. The location data were used to create a map of the hydrogeologic subdivisions and the location of karst features. The outcrops of the Edwards and Trinity aquifer recharge zones were mapped by using hydrogeologic subdivisions modified from previous reports. All rocks exposed within the study area are of sedimentary origin and Lower Cretaceous in age. The valley floor is formed in the cavernous member of the upper Glen Rose Limestone of the Trinity Group. The hills are composed of the basal nodular member, dolomitic member, Kirschberg evaporite member, and grainstone member of the Kainer Formation of the Edwards Group. Field observations made during this study of the exposed formations and members indicate that the formations and members typically are composed of mudstones, wackestones, packstones, grainstones, and argillaceous limestones, along with marls. The upper Glen Rose Limestone is approximately 410 to 450 feet thick but only the upper 70 feet is exposed in the study area. The Kainer Formation is approximately 255 feet thick in the study area and is composed of, in ascending order, the basal nodular member, dolomitic member, Kirschberg evaporite member, and grainstone member. The Edwards and Trinity aquifers contain a combination of fabric-selective and not-fabric-selective porosities. Porosity types observed in the study area that can increase the effective porosity and increase permeability include solutionally enlarged caves, sinkholes, fractures, bedding planes, channels, molds and vugs. Caves found during hydrogeologic mapping might have been spring discharge points, but sufficient downcutting over geologic time in the rocks has occurred so that springs discharge at lower elevations near the creek channel. The mapped caves, sinkholes, and other areas of solutionally enlarged porosity might facilitate recharge during large storm events when runoff occurs on the hillsides; additional areally distributed recharge in the study area occurs as a result of infiltration.