Geologic Framework of the Northern Edwards Aquifer, Central Texas

The northern segment of the Edwards aquifer comprises Lower Cretaceous Comanche Peak, Edwards, and Georgetown strata of central Travis, Williamson, and southern Bell Counties, Texas (Fig. 1). Geologic and structural-contour maps and cross sections were constructed to illustrate geologic elements, including the aquifer outcrop belt/recharge area, faults, aquifer stratigraphy, and aquifer-thickness variations. These geologic interpretations support applications 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, and (d) providing information necessary for land-use planning and permitting construction projects. This study entailed a review and compilation of existing data and interpretations by the authors and by previous workers and new efforts at constructing cross sections and maps. Geologic aspects important to the aquifer\u27s framework that are discussed herein include (a) variations in the thickness of the aquifer strata, (b) lateral lithostratigraphic changes, and (c) location and displacements of faults. Across the study area, normal faults of the Balcones Fault Zone displace Cretaceous limestone, dolomite, marl, and shale that represent \u3e2,000 ft of shelf and shelf-margin deposition. Composing a commonly prolific part of the aquifer, Edwards limestone, dolomitic limestone, and dolomite are often more porous than the other aquifer strata, Georgetown and Comanche Peak limestone and argillaceous limestone (Fig. 1b). The composite thickness of the aquifer units within the confined, subsurface part of the aquifer ranges from about 420 ft in central Travis County to about 260 ft in southern Bell County. From central Travis County northward to southern Bell County the highly porous Edwards Group strata thin from as much as sim.gif (57 bytes)370 to sim.gif (57 bytes)90 ft, whereas the thickness of Georgetown strata increases toward the nort