Sandy beds of the Wilcox Group in Texas are underlain and overlain by clays of the Midway and Claiborne Groups. The Wilcox is divided by a persistent shale wedge. Contrasting delta systems described as high-constructive and high-destructive exist in the divisions. Merged high constructive deltas characterize the lower, while the upper Wilcox was deposited as a high destructive system. Maximum sand development occurs downdip parallel to regional depositional strike in the delta facies. Further downdip, shales replace sands, and sediments are (abnormally high pressured) geopressured.
The structure o5 the updip port[on is that of a stable shelf. Downdip, greatly thickened beds are cut by large faults. These faults and the resultant deposits are recognized as being the result of a previously unnamed phenomenon--here termed hydrothermal tectonism. This process is keyed to the thermal diagenesis of the clay mineral montmorillonite and resultant transfer of overburden pressure from the rock framework to the contained fluid.