MAPPING GEOLOGIC ELEMENTS IMPORTANT TO LAND USE AND WATER MANAGEMENT, CENTRAL TEXAS AND WEST TEXAS URBAN-GROWTH AREAS

New geologic maps of the Austin and San Antonio urban corridors within the Edwards and Hill Country Trinity aquifers of Central Texas and El Paso–Hueco Bolson region of West Texas figure prominently in planning land use, designing construction projects, and managing groundwater resources. Maps enabling users to identify geologic features that impact those activities are intended for a diverse audience and are at a scale of 1:100,000. Applied uses of these maps include (a) identifying aquifer recharge boundaries; (b) characterizing attributes and heterogeneities within aquifer strata; (c) identifying faults; (d) assisting water-management decisions on groundwater flow and aquifer response to pumpage and recharge; (e) providing information on land-use activities such as planning and permitting of construction, designing of foundations, and locating of landfills and other waste-disposal sites; and (f) meeting demands for local construction materials. Maps of the Central Texas study areas illustrate Cretaceous limestone, marl, and shale units that represent >2,000 ft of shelf and shelf-margin deposition. Normal faults of the Balcones Fault Zone cut these strata and control the structural position of porous limestone units that compose the prolific Edwards aquifer and its recharge zone. Occurrences of Del Rio clay and Eagle Ford clay, shale, marl, and bentonite are important in construction because of potential problems related to shrinking and swelling of clays in these units. Limestone is being quarried for aggregate, cement, and building stone. Maps of the West Texas study area illustrate the geology of the Hueco Bolson’s piedmont and basin floor, part of the Rio Grande valley and valley border, and highlands that bound the Hueco Bolson. Stratigraphy includes Precambrian through Tertiary bedrock in the mountain areas and upper Tertiary to Quaternary basin-fill and surficial deposits of the mountain flanks, Hueco Bolson, and Rio Grande valley. Quaternary faults cut basin-fill and surficial deposits along the mountain flanks and within the basin. These faults at least partly controlled the basin’s sedimentation and are important to consider in land-use planning and construction and in evaluation of groundwater resources. The bolson aquifer contains coarser grained fluvial deposits. Aggregate, cement, and gravel are quarried from the map area.