Stratigraphic Revision, Edwards Group (Lower Cretaceous) of Texas, and Regional Surface-Subsurface Synthesis: ABSTRACT

Abstract

Porous carbonate rocks called form a widespread aquifer in the Edwards Plateau region of west-central Texas and prolific petroleum reservoirs beneath the coastal plain of south-central Texas. Recently completed surface mapping in the Edwards Plateau and subsurface mapping in south-central Texas permit correlation of Edwards rocks in both areas, and allows the entire central Texas region to be viewed as an evolving carbonate depositional complex. To better express this surface-subsurface synthesis, a new stratigraphic nomenclature is proposed: Edwards is elevated to group status, 2 new outcropping formations are recognized in the Edwards Plateau, and 2 new formations are defined in the subsurface of south-central Texas. Previously unverified disconformities at an near the top of the Edwards have been documented and a boundary that was considered to be disconformable is now interpreted to be conformable. Regional structural-depositional elements that influenced Edwards deposition are reflected by lithofacies and isopach patterns. Dominant elements on the Comanche shelf formed a vast sediment trap in which shallow marine and tidal flat carbonates and evaporites accumulated preferentially over the Central Texas platform, protected by the Stuart City reef on the southeast, the Devils River bank on the southwest, and the broad North Texas shelf-lagoon on the northwest and northeast. Edwards and associated rocks record the progressive inundation of the Central Texas platform. Porosity in the subsurface Edwards is related chiefly to early exposure on positive tectonic elements, whereas porosity in the surface and near-surface Edwards is related chiefly to the present geomorphic cycle. End_of_Article - Last_Page 361------------