Revision of Comanche Cretaceous Stratigraphic Nomenclature, Southern Edwards Plateau, Southwest Texas

Abstract

ABSTRACT The Devils River and correlative Lower Cretaceous rock units in the southern Plateau of Texas are divisible into three geologically distinct areas on the basis of different facies developments. These are referred to as northern, central, and southern areas, and the stratigraphic nomenclature is revised accordingly. The northern area sequence, regionally characterized by ammonite-bearing marly mudstones changing facies southward into rudist-miliolid limestones, is divisible by a disconformity into two formations. New names are under consideration pending additional fieldwork. The central area sequence, assigned to the Devils River proper and confined to a narrow band encircling the southern or Maverick basin area, is represented by the combined and undivided rudist-miliolid limestones of the northern units. Udden's concept of the stratigraphic interval is retained, but the area of application is restricted. The southern area sequence is marked by the abrupt southward facies change of the undivided Devils River carbonate section into the three units distinctive of the Maverick basin: a lower wackestone unit, the so called Edwards Formation; the evaporitic Kiamichi Formation (also called McKnight); and an upper lime mudstone unit previously identified as the Georgetown Formation. Suppression of the older names is advocated, and replacement nomenclature is proposed.