Pre-Selma Upper Cretaceous Stratigraphy of Western Alabama

Abstract

The pre-Selma Upper Cretaceous deposits of western Alabama have long been divided into the Tuscaloosa and Eutaw formations but recent field work shows that six stratigraphic units of formational rank can be mapped: Eutaw formation McShan formation Tuscaloosa group Gordo formation Coker formation Eoline formation Cottondale formation The contacts separating these units are unconformable with the possible exception of that between the Cottondale and the Eoline formations. The Cottondale formation consists of cross-bedded fine to coarse, somewhat gravelly sand and carbonaceous clay that locally contains a rich flora. In origin it may be entirely continental or in part marine. The Eoline formation is a marine formation which commonly consists of cross-bedded glauconitic sand overlain by laminated somewhat carbonaceous clay. The Coker formation consists of highly cross-bedded and lenticular beds of sand and ferruginous clay, and is probably non-marine. The Gordo formation also consists of cross-bedded sand and mottled ferruginous and carbonaceous clay, but is the only formation of the Tuscaloosa group that contains large amounts of gravel. Throughout the area thus far studied in detail most of the gravel is chert. The formation is probably non-marine. The McShan formation is marine and has generally been included with the Eutaw formation, but an important and widespread unconformity separates it from the restricted Eutaw. It consists of finely glauconitic very fine sand and laminated sandy clay. The Eutaw formation, in contrast to the McShan, contains much coarser glauconite and much more compact gray shale. The top member of the Eutaw is the oldest Cretaceous bed that contains abundant fossil shells on the outcrop. All six formations have been recognized in wells as far downdip as Lauderdale and Neshoba counties, Mississippi.