Edwards Group, surface and subsurface, central Texas

A thick sequence of Lower Cretaceous limestone and dolomite, known traditionally and informally as Edwards," covers much of west-central Texas and composes one of the dominant physiographic elements of the State, the Edwards Plateau. In this region the base of the Edwards is an important fresh-water aquifer. Along the inner edge of the Gulf Coastal Plain, in the Balcones fault zone, the Edwards is also exposed but severely faulted and altered by ground water. South and east of the Balcones fault zone these same rocks dip gently gulfward beneath the coastal plain. Here the Edwards is an important target for oil exploration, with nearly 400 million equivalent barrels of oil and gas located or produced to date. Stratigraphic relationships between the subsurface Edwards of this area and the Edwards of the eastern Edwards Plateau have never been securely established. The purposes of this report are (1) to delineate the stratigraphic framework of Edwards and associated rocks in the areas of the coastal plain and eastern Edwards Plateau; (2) to relate this framework with other work to the northeast and southwest; (3) to use the resulting regional framework to reconstruct a generalized paleogeography and depositional history of these rocks in Texas; and (4) to outline and explain the several diagenetic processes that have affected Edwards rocks.