ABSTRACT

Upper Eocene and Lower Oligocene strata in the eastern Gulf Coast region are represented by western, clastic-dominated deposits of Mississippi and eastern, carbonate-dominated deposits of Georgia and northern Florida. These lithofacies intergrade and intertongue across Alabama. The complexity of the lithofacies changes in the region has caused problems in the recognition of geological ages. The stratigraphic position of the Eocene-Oligocene boundary as determined by macrofossils and benthic microfossils is different from that position indicated by planktonic foraminifers. The four members of the Yazoo Clay (North Twistwood Creek Clay, Cocoa Sand, Pachuta Marl and Shubuta Clay Members), the lateral equivalent of the Shubuta Clay Member (Crystal River Formation), and the overlying Red Bluff Clay/Bumpnose Limestone and their associated foraminifers were studied from exposures at St. Stephens Quarry in Washington, County, Alabama, at Little Stave Creek in Clarke County, Alabama, and on the Chickasawhay River in Wayne County, Mississippi. Based on vertical distributions of planktonic foraminifers at these localities, the Eocene-Oligocene boundary occurs at the top of the Yazoo Clay. The lowermost two members of the Yazoo Clay were not placed in any specific foraminiferal biozones at this time due to the paucity of planktonic foraminifers in these beds, but these units have been assigned to the Globigerinatheka semiinvoluta Interval Zone by Barker in Blow (1979). The distributions of planktonic foraminifers in the Pachuta Marl and Shubuta Clay Members at both localities allow assignment of these units to the Globorotalia cerroazulensis (s.l.) Interval Zone, while the Red Bluff Clay/Bumpnose Limestone is assigned to the Pseudohastigerina micra Interval Zone.