GEOLOGIC MAP OF THE SOUTHEAST PART OF THE AUSTIN, TEXAS, 30 × 60 MINUTE QUADRANGLE: CENTRAL TEXAS POPULATION CORRIDOR ENCOMPASSING BASTROP AND SMITHVILLE

Recent geologic mapping for the Texas STATEMAP program has been conducted in the population/transportation corridor southeast of Austin, Texas—an urban to rural area experiencing population growth and increased development. The geologic map and related data for this area present basic geologic framework information that will aid in managing Earth and water resources, planning land use, and designing construction projects. Geology of the area consists mostly of Paleocene through Eocene mud- and sand-rich units representing marine, deltaic, and fluvial deposition during repetitive cycles of transgression and regression. Minor Upper Cretaceous marine marl and calcareous mud deposits also occur in the western study area. Remnant, topographically high gravel deposits (upper Tertiary to Quaternary) and Quaternary terrace deposits of the well-defined, 2- to 4-mi-wide valley of the southeast-flowing Colorado River also occur. Bedrock units typically exhibit eastward regional dips of less than 2 degrees, although within the east half of the area, strata are cut by the northeast-striking Milano Fault Zone, which is composed of normal faults that form multiple graben. Aquifer units include sandstones of the Simsboro and Calvert Bluff (both of the Wilcox Group), Carrizo, Queen City, and Sparta Formations. The base of fresh groundwater across most of the map area is in the deepest aquifer, the Carrizo-Wilcox, where depths to fresh groundwater increase eastward (away from the Carrizo-Wilcox outcrop belt) and generally range between 200 and 1,600 ft. Lignite intervals exist within some of the Eocene units. They have been periodically mined, and some mined lands have been reclaimed. Mining of gravel from terrace deposits has met some of the demand from past development of the area. Abandoned gravel pits are of environmental concern because they have the potential of being used for illegal dumping, possibly impairing surface-water and groundwater quality. Paleocene and Upper Cretaceous clay-rich units are associated with some swelling-clay soils that may require construction designs and techniques that are different from techniques applied in sandy soils associated with sandstone units.