GEOLOGIC MAPPING OF THE NORTH AUSTIN URBAN CORRIDOR AND NORTHERN EDWARDS AQUIFER FOR THE TEXAS STATEMAP PROGRAM

A new 1:100,000-scale geologic map of the north Austin urban corridor provides basic geologic information necessary for land-use planning, construction practices, and management of water and land resources in an area that is undergoing increased urban development, population growth, and exploitation of natural resources. The map is intended for a diverse audience and has a variety of applied uses such as (a) identifying aquifer recharge boundaries, (b) recognizing attributes and variations within aquifer strata, (c) making water-management decisions related to groundwater flow and aquifer response for pumpage and recharge, (d) conducting land-use activities such as planning and permitting construction projects, designing foundations, locating landfills and other waste-disposal sites, and (e) locating sources of construction materials. The map was constructed through reviewing existing maps, field mapping, interpreting aerial photographs, constructing sixteen 1:24,000-scale open-file maps, and digitizing and compiling the open-file maps in a geographic information system.

Across the map area normal faults of the Balcones Fault Zone displace Cretaceous limestone, marl, and shale that represent >2,000 ft of shelf and shelf-margin deposition. Faults strike northeastward to north-northeastward, and the composite structural offset of southeast-dipping and northwest-dipping faults is down –to –the southeast. Across central Austin the composite structural relief is ~1,600 ft. Composite structural relief and fault intensity decrease northward from Austin. At the northern boundary of the north Austin study area the composite structural relief is ~600 ft. Faults control the structural position of the porous limestone units that compose the Edwards aquifer. They can serve as conduits for groundwater flow, although at some locations faults may displace porous beds against relatively less porous beds, thus causing abrupt changes in groundwater flow paths. Occurrences of Del Rio clay and Eagle Ford clay, shale, marl, and bentonite are important to construction practices because of potential problems related to shrinking and swelling of clays in these units and their associated soils. Limestone is actively being quarried for aggregate, cement, and building stone.