The Eocene-Oligocene boundary in the central Gulf coastal plain has been placed traditionally at the contact of the Shubuta Member of the Yazoo Formation and the Red Bluff Formation, or the contact of the Shubuta and the facies equivalents with the Bumpnose formation. Calcareous nannofossils were examined from six upper Eocene to lower Oligocene localities in Alabama and Mississippi. The Shubuta, Red Bluff, and equivalents have a very similar calcareous nannofossil flora, and both are in Martini's Zone NP21. However, from the base of the Shubuta up through the Red Bluff, 10 calcareous nannofossil extinction horizons can be used to subdivide the lower part of Zone NP21. Discoaster saipanensis Bramlette and Riedel, D. barbadiensis Tan Sin Hok, and Reticulofenestra reticulat (Gartner and Smith), which become extinct at or near the top of Zone NP20, are only rarely present in the 27 Shubuta samples examined, are poorly preserved, and are assumed to have been reworked. Below the Shubuta lies the Pachuta Marl Member of the Yazoo, which was examined at one locality in Mississippi and two in Alabama, and although the flora is poorly preserved, contains significant numbers of all three Eocene species.

If the Eocene-Oligocene boundary is assumed to correspond to the Shubuta-Red Bluff contact, this boundary, at least in the Gulf coastal plain, cannot be recognized using traditional calcareous nannofossil markers, because of its inclusion within Zone NP21. This contact, however, appears to coincide with the last occurrence of the planktonic foraminifer Globorotalia cerroazulensis s.l.; the extinction of Hantkenina spp. may occur slightly below the contact. The extinction of Discoaster saipanensis below that of the planktonic foraminifers has also been observed on several legs of the Deep Sea Drilling Project, where this offset is no more than a few meters. At the Red Bluff type locality, the separation approximates 65 ft (20 m). Clearly, in the study area, the extinctions of G. cerroa ulensis and D. saipanensis do not define the same horizon.

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