Geologic Setting Of The Edwards And Trinity Aquifers Of Central And South Texas – Interactions Among Tectonics, Carbonate Deposition, And Groundwater Flow

Abstract

The geologic evolution of the Edwards and Trinity aquifers has had a profound impact on the physics of flow and therefore the options for resource management of the prolific but heavily used water supply. Clastics, carbonates, and evaporites of the Trinity and the Edwards Groups were deposited on the craton during early parts of Cretaceous flooding of the North American continent. High accommodation coupled with gentle warping across the Maverick Basin and the San Marcos Arch accounts for the internal geometry of the thick sedimentary wedge. High-frequency cycle patterns include subtidal rocks, grainrich shoal facies, reef buildups, subaqueous gypsum, and stacked tidal flat deposits that are repeated through the sequence. Subtidal rocks were extensively dolomitized, and gypsum underwent multiple episodes of calcitization and dissolution. Miocene deformation oblique to Cretaceous trends created down-to-the coast en echelon faults and grabens of the Balcones fault zone as well as strong hydraulic gradient. Groundwater capture through fracture systems resulted in regional integration of the Edwards aquifer groundwater flow system perpendicular to surface water drainage. Interaction between fresh water and brine discharged from the Gulf Coast sedimentary wedge drove intensive karstification that developed along preferred zones defined by structure and sedimentary fabrics. Water Resources and Geologic Context The Edwards aquifer of the Balcones Fault zone is the source of Comal, San Marcos, and Barton Springs as well as numerous smaller springs (fig. 1) which provide habitat for endangered species and flow to downstream users and habitats. The Edwards aquifer is also the principle water supply for urban users in San Antonio and smaller populations, and to rural and agricultural users. The Edwards Trinity aquifer of the Edwards Plateau is supplies abundant springs that source creeks and rivers and provides water resource for rural uses and growing communities. Water use in the Trinity aquifer may be closely related to recharge to the Edwards aquifer both by supporting surface water flow and an incompletely understood cross-formational flow (Mace and others, 2000). The faulted karstic carbonate Edwards aquifer has been extensively studied but the complexities of flow through the aquifer remain inadequately understood. The less deformed Trinity Aquifer includes sandstones and carbonates and has been less extensively studied. This paper provides a context for integration of current and ongoing geophysical investigations into the growing understanding of flow in these aquifers by reviewing and highlighting interactions between Cretaceous depositional history, structural setting, and karst evolution. Heterogeneous development of the carbonate platform Depositional setting During the early Cretaceous, regional transgression across the North American craton resulted in a complex of dominantly carbonate facies (fig. 2). The Stuart City reef marks the break between the