GEOLOGIC MAP COMPILATION OF THE EDWARDS AQUIFER RECHARGE AREA, SOUTH-CENTRAL TEXAS, AND ITS IMPLICATIONS TO EDWARDS GROUP HYDROSTRATIGRAPHY

Efforts by a National Cooperative Geologic Mapping Program project to compile the geology of the Edwards aquifer recharge area in south-central Texas have helped refine the hydrostratigraphy of one of the most permeable and productive carbonate aquifers in the United States. The complex geology of the recharge area includes lithologic units assignable to the Lower Cretaceous Edwards Group and the Georgetown Formation, which is underlain by the Glen Rose Limestone (lower confining unit) and overlain by the Upper Cretaceous Del Rio Clay, Buda Limestone, and Eagle Ford and Austin Groups (upper confining units). The geology of the Edwards Group in the northeast part of the recharge area (San Marcos Platform facies) is characterized by the Kainer, Person and Georgetown Formations, which are subdivided into 8 informal hydrostratigraphic units. The central part of the recharge area (Devils River Trend facies) contains reefal facies lithologies of the Devils River Formation and overlying Georgetown Formation, and the western part of the recharge area contains lithologies of deeper-water origin (Maverick Basin facies) composed of the West Nueces, McKnight, and Salmon Peak Formations.

Previous geologic maps typically showed a vague boundary separating San Marcos Platform and Devil River Trend exposures. This was due, in part, to inaccessibility and poor exposure, but largely it was because the Devils River Formation exposures were interpreted as discontinuous and lithologically variable. However, recent field studies of the San Marcos Platform/Devils River Trend transition zone show that many of the hydrostratigraphic units belonging to the Kainer Formation, particularly the basal nodular and dolomitic members, can be traced into the Devils River Formation and even into deeper-water Maverick Basin units. The hydrostratigraphic subdivision of the lower part of the Devils River Formation in Medina and Uvalde Counties and mapping of Maverick Basin lithologies in Kinney County is ongoing.