Abstract
The formations of the Fredericksburg and lower Washita Groups in Uvalde and Real Counties, Texas, were deposited on a broad platform behind the Stuart City reef trend. Nine distinct lithofacies and corresponding depositional environments occur within this lower Cretaceous depositional regime: (1) nodular and burrowed mollusk lime wackestone lithofacies representing an open-marine platform, low wave-energy environment; (2) rudist, shell fragment, miliolid wackestone to packstone lithofacies representing a partially restricted to open-marine platform, low to moderate wave- and current-energy environment; (3) rudist, shell fragment, miliolid, grainstone to wackestone lithofacies representing an open-marine platform and bank, moderate to high wave- and current-energy environment; (4) evaporite collapse breccia lithofacies representing a hypersaline platform interior environment; (5) miliolid, shell fragment wackestone and Texigryphaea bed lithofacies representing a platform interior, shallow subtidal to intertidal transitional environment; (6) alternating thin evaporite collapse breccia with laminated mudstone and pellet packstone lithofacies representing an evaporite-dominated tidal mud flat environment; (7) carbonaceous lime mudstone lithofacies representing a platform interior euxenic lagoonal environment; (8) pellet intraclast lime wackestone to packstone lithofacies representing a restricted platform interior environment; and (9) pelagic lime mudstone to wackestone lithofacies representing a deep, open to slightly restricted platform interior environment.
The Fredericksburg-early Washita depositional history is characterized by 2 distinct cycles of deposition. The first cycle is interpreted as an initial marine transgression over the partially eroded Glen Rose tidal flat deposits, followed by a period of regional shoaling, culminating with the deposition of the Kirschberg Evaporite within the Devils River formation and northward, and deposition of the lower McKnight evaporite-dominated tidal mud flat sediments to the south. The second cycle of deposition is interpreted as a gradual marine transgression culminating in the deposition of open-marine platform rudist bank facies (upper Devils River) and partially restricted to open deep platform interior deposits (Salmon Peak) prior to middle Washita emergence and erosion forming the pre-Del Rio unconformity.
