Geologic History and Hydrogeologic Setting of the Edwards-Trinity Aquifer System, Westcentral Texas
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The Edwards-Trinity aquifer system under- lies about 42,000 square miles of west-central Texas. Nearly flat-lying, mostly Comanche (Lower Cretaceous) strata of the aquifer system thin northwestward atop massive pre-Cretaceous rocks that are comparatively impermeable and structurally complex. From predominately terrige- nous clastic sediments in the east and fluvial- deltaic (terrestrial) deposits in the west, the rocks of early Trinitian age grade upward into supratidal evaporitic and dolomitic strata, intertidal lime- stone and dolostone, and shallow-marine, open- shelf, and reefal strata of late Trinitian, Fredericks- burgian, and Washitan age. A thick, downfaulted remnant of mostly open-marine strata of Eaglefor- dian through Navarroan age composes a small, southeastern part of the aquifer system. The Trinity Group was deposited atop a roll- ing peneplain of pre-Cretaceous rocks during three predominately transgressive cycles of sedimenta- tion that encroached upon the Llano uplift. The Fredericksburg and Washita Groups were depos- ited above the Trinity Group mostly in the lee of the Stuart City reef trend, a shelf margin ridge that sheltered depositional environments in the study area. The Washita Group subsequently was cov- ered with thick, mostly fine-grained Gulf strata. During late Oligocene through early Miocene time, large-scale normal faulting formed the Balcones fault zone, where the Cretaceous strata were downfaulted, intensively fractured, and differentially rotated within a series of northeast- trending fault blocks. In addition to fracturing the rocks in the fault zone and extending the depth of freshwater diagenesis, the faulting vertically displaced the terrain, which steepened hydraulic gradients and maintained relatively high flow velocities near the surface. A shallow regime of dynamic ground-water flow evolved that pro- moted dissolution and enhanced the transmissivity of the Edwards Group in the Balcones fault zone. Cementation, recrystallization, and mineral replacement caused by deeper,