DETAILED MAPPING OF THE GEOLOGIC CONTROLS ON KARST DEVELOPMENT IN THE CENTRAL EDWARDS PLATEAU

The Edwards Plateau is composed largely of Cretaceous carbonate rocks at the surface, with Permian and Pennsylvanian sedimentary facies below, followed by Cambro-Ordovician sandstone, and finally the basement layer of Precambrian igneous and metamorphic rocks. The sedimentary facies deposited during the Pennsylvanian period were mostly limestone from reef deposits as the region was covered by shallow seas much of the time 300 million years ago. The area has been fractured by the Fort Chadbourne fault system, allowing for possible recharge zones to form through to the Edwards Aquifer. The Cretaceous limestones are made up of the following formations: the lowermost (or oldest) is the Georgetown limestone, then the Glen Rose limestone, Buda limestone, and the Boquillas formation. The large amount of soluble limestone makes the Edwards Plateau region an ideal location for karst environments to form. However, overlaying the Cretaceous Limestone deposits is the Del Rio Clay, derived from a layer of Miocene or Pliocene alluvial deposit that has been weathered away, which hinders the development of karst features in the Edwards Plateau. There have been noteworthy karst features discovered in the area, namely the Sonora Caverns, but further investigations into karst features have been slowed by the region's interest in oil and gas exploration, and groundwater preservation. While there is a large amount of core data available, little has been done to create a comprehensive understanding of those areas not rich in oil. These areas could contain important information on recharge locations for the Edwards Aquifer that supplies much of the West Texas region with its water. A thorough investigation of the Cretaceous stratigraphy in the Edwards Plateau, using both core and field data, will be used to better understand the geologic and hydrologic controls on karst development in the central Edwards Plateau near and around Sonora. A more comprehensive map of the surface and subsurface geology, and potential locations of undiscovered caves or sinkholes will be developed. Expanding the knowledge of the location and attitudes of sinkholes, joints, springs, and other karst features which influence groundwater recharge could benefit water conservation efforts as well as influence water management strategies.