ABSTRACT

The stratigraphic position of the Eocene-Oligocene boundary in southwest Alabama, based on macrofossils, differs from that based on planktonic foraminifers. To resolve the dilemma, the Yazoo and Red Bluff Clays and the associated foraminifers were studied from St. Stephens quarry, Washington County, Alabama. Based on planktonic foraminiferal vertical distribution at this locality, the Eocene-Oligocene boundary occurs at the top of the Shubuta Member of the Yazoo Clay. Cribrohantkenina inflata (Howe), Hantkenina alabamensis Cushman, Hantkenina longispina Cushman and the Globorotalia cerroazulensis group become extinct, and Globigerina ampliapertura Bolli and Globigerina gortanii (Borsetti) first appear near this horizon. This planktonic foraminiferal vertical distribution allows assignment of the upper Yazoo Clay to the Globorotalia cerroazulensis (s.l.) Interval Zone and the Red Bluff Clay to the Cassigerinella chipolensis -- Pseudohastigerina micra Interval Zone.