GEOLOGIC MAPPING IN CENTRAL TEXAS AQUIFER AREAS AND POPULATED CORRIDORS FOR THE TEXAS STATEMAP PROGRAM

Some of the recent geologic mapping for the Texas STATEMAP program has been conducted in the populated corridors and associated aquifers north and east of Austin, Texas, and along an important segment of an alluvial aquifer within the Brazos River Valley in east-central Texas. Examples of maps and related information for these areas will be presented. These maps and related data provide basic geologic framework data to aid in managing water and earth resources, planning land use, identifying aquifer recharge areas, identifying sources of aggregate and other earth resources, and recognizing areas having stratigraphic units and associated soils prone to foundation problems. The 1:100,000-scale map and cross sections of the north Austin corridor illustrate the framework of the three units composing the northern Edwards aquifer within a part of the Balcones fault zone, the areal extent of formations that contain swelling clay, and units containing possible aggregate and sand and gravel resources. Sixteen 1:24,000-scale maps of the east Austin corridor are planned for digitization and compilation of a 1:100,000-scale map and cross sections that will portray this area's geologic framework. Geologic elements of particular interest include the formations composing the Carrizo-Wilcox aquifer, formations associated with swelling clays, and units that have been mined for lignite, clay, and sand and gravel. Cross sections and 1:24,000-scale maps of the Brazos River Valley will help delineate the framework of the Brazos alluvial aquifer along a river segment that intersects and is hydrologically connected to four sandstone aquifers: the Carrizo-Wilcox, Queen City, Sparta, and Jackson-Yegua. Brazos alluvial deposits are as thick as 60 ft and typically consist of a lower, relatively coarser grained interval that is between 30 and 5 ft thick. Terrace deposits are relatively thin. Some terraces are isolated remnants, although some are stratigraphically connected to the thicker alluvial aquifer areas.