SIM 2873: Introduction
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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 to 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 related units and the Georgetown Formation, which is underlain by the Lower Cretaceous Glen Rose Limestone (lower confining unit) and overlain by the Upper Cretaceous Del Rio Clay, Buda Limestone, and Eagle Ford, Austin, and Taylor and Navarro Groups (upper confining units). The rock units comprising the Edwards aquifer in the northeastern part of the recharge area (San Marcos platform facies) include the Kainer, Person, and Georgetown Formations. The Kainer and Person Formations are subdivided into seven informal hydrostratigraphic members. The central part of the recharge area (Devils River trend facies) contains reefal-facies lithologies of the Lower Cretaceous Devils River Formation and the overlying Georgetown Formation. The western part of the recharge area contains Edwards Group lithologies of deeper water origin (Maverick basin facies) composed of the West Nueces, McKnight, Salmon Peak, and Georgetown Formations.
Geologic maps of previous workers typically show a vague, dashed boundary separating San Marcos platform and Devils River trend exposures because the Devils River Formation was historically interpreted as discontinuous and lithologically variable. However, current field studies of the San Marcos platform/Devils River trend transition area reveal that the facies boundary is distinct and follows topographic expression in some areas. We believe that future work will show that some of the hydrostratigraphic units belonging to the Kainer Formation, particularly the informal basal nodular and dolomitic members, occur in the Devils River trend and deeper water Maverick basin units. 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 are ongoing.
This data set is intended to illustrate the detailed geographic extent of the stratigraphic units and faulting for the Edwards aquifer recharge zone in south- central Texas.
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