Hydrogeologic Framework of the Edwards-Trinity Aquifer System, West-Central Texas

The Edwards-Trinity aquifer system underlies about 42,000 square miles of west-central Texas. Nearly flat-lying Comanche (mostly Lower Cretaceous) and Gulf (Upper Cretaceous) strata of the aquifer system thin northwestward atop generally massive pre-Cretaceous rocks that are comparatively impermeable and structurally complex. From predominately terrigenous clastic sediments in the east and terrestrial deposits in the west, the rocks of early Trinitian (Comanchean) age grade upward into supratidal and intertidal evaporitic and dolomitic rocks and shallow-marine, lagoonal, and basinal carbonate strata of late Trinitian, Fredericksburgian, and Washitan (Comanchean) age. A thick, downfaulted remnant of mostly open-shelf sediments of Eagle- fordian through Navarroan (Gulfian) age confines a small, southeast- ern part of the aquifer system. While clastic deposition prevailed upon alluvial plains inland of a westward-advancing Cretaceous sea, offshore environments were dominated by the biogenic accumulation of calcium carbonate in warm, generally clear seawater. The Trinity strata were deposited as the sea encroached upon the Llano uplift, the most prominent feature on a rolling peneplain composed of folded and faulted pre-Cretaceous rocks. The Fredericksburg and Washita strata mostly formed above the Llano uplift, on a carbonate platform sheltered from storm waves and deep ocean currents by the Stuart City reef trend. Subsequently, the entire study area was blanketed with mostly argillaceous sediments of the Eagle Ford, Austin, Taylor, and Navarro Groups. During late Oligocene through early Miocene time, large-scale normal faulting formed the Balcones fault zone, where the Cretaceous strata were displaced vertically, fractured intensively, and rotated dif- ferentially within a series of southwest-to-northeast trending fault blocks. Ground-water flow shifted toward the northeast in response to rejuvenated hydraulic gradients and high-angle barrier faults that blocked southeastward flow. Subsurface conduits lengthened i