ABSTRACT

The Cretaceous of west-central Texas consists of a basal quartz sand, the Antlers, and an overlying carbonate sequence. The Antlers is laterally equivalent to both Fredericksburg and Trinity sequences to the northeast. Its southern limit of utility is at the latitude of the Brady Mountains, Menard County. The carbonate sequence above the the Antlers can be divided into two geologically distinct areas; a northern area, generally coincident with the Callahan Divide, and southern area which is the northern part of the Edwards Plateau proper.

The carbonate sequence on the Callahan Divide may be divided into a basal, nodular, marly unit and an overlying massive rudistid bearing limestone. The basal unit has characteristics of, and is laterally equivalent to, the Walnut and Comanche Peak formations of central Texas. These units cannot be recognized consistently and the sequence is termed the "Walnut-Comanche Peak undifferentiated." The overlying massive rudistid bearing limestone is an extension of the Edwards of north-central Texas and the term Edwards is used across the Callahan Divide.

In the northern Edwards Plateau, the Edwards replaces the Walnut-Comanche Peak facies and constitutes the entire Fredericksburg interval. The term Edwards can be properly extended south into the Edwards Plateau until its upper boundary is no longer determinable, that is to say, at the position of the Devils River Limestone in southern Edwards and Val Verde counties. The top of the Edwards is an unconformity from Edwards and Val Verde counties northward to an east-west line running through central Irion, Tom Green and Concho counties. North of this line the contact appears to be conformable. The basal beds of the overlying post-Edwards section provide a stratigraphic datum that can be extended northward from the Edwards Plateau to the Callahan Divide just south of Abilene in Nolan County.