ABSTRACT

A thick sequence of carbonate rocks, here designated the Atascosa Group, was deposited on the continental shelf of southwest Texas immediately above the Pearsall formation of early Cretaneous age and ended with the deposition of the Del Rio. A reef complex, the Stuart City formation, developed at the edge of the undaform behind which the Edwards and Glen Rose formations were deposited. On the clinoform, the seaward side of the reef complex, a monotonous sequence of fine-grained limestone was deposited.

The undaform includes two provinces: (1) the San Marcos platform, a relatively stable area, and (2) the Maverick basin, subsiding more rapidly than the San Marcos platform.

On the San Marcos platform the Edwards is divided into an "A" (upper) and a "B" (lower) unit, and these are separated by a shalier unit, which is easily recognized from the electrical logs. The Edwards "A" and "B" units both change facies in the Maverick basin. The upper part of the Edwards "B" is correlated with the McKnight formation, and the Edwards "A" is correlated with the newly named Pryor member of the Georgetown formation.

The Georgetown (except for the Pryor member), the Del Rio, the Buda, and the Eagleford formations blanket the entire undaform.