Paleogene formations extending from eastern Alabama to Arkansas have been assigned to internationally recognized biostratigraphic zones based on calcareous nannoplankton fossils. All zones from NP 1 to NP 24 are present in the study area, with the exception of NP 11 (within the early Eocene) and NP 23 (within the early Oligocene).

Gulf Coast formations correlate to international standard chronostratigraphic stages as follows. The Clayton and Porters Creek formations correlate to the Danian Stage. The Naheola Formation, Salt Mountain Limestone, Nanafalia Formation, Tuscahoma Sand, and lower Hatchetigbee Formation (in part) correlate to the Selandian Stage. The Hatchetigbee (in part) and lower Tallahatta correlate to the Ypresian Stage; the upper Tallahatta and most of the Lisbon and Cook Mountain Formations correlate to the Lutetian Stage. The upper Lisbon and Cook Mountain formations, the Gosport Sand, Moodys Branch, and lowermost Yazoo formations correlate to the Bartonian Stage. The Crystal River and most of the Yazoo Formation correlate to the Priabonian Stage. The uppermost Yazoo Formation, the Bumpnose Lim stone, Red Bluff, Forest Hill, and Mint Spring Formations, Marianna and Glendon Limestones, and Byram and Bucatunna Formations correlate to the Rupelian Stage. The Chickasawhay Limestone and Paynes Hammock Formation span the Rupelian-Chattian boundary.

The Paleocene-Eocene boundary (approximately the NP 9-NP 10 boundary) is therefore within the lower Hatchetigbee Formation (Bashi Marl Member). The Eocene-Oligocene boundary (within lowermost NP 21) is within the uppermost Yazoo Formation (Shubuta Clay Member) in places, and within the lowermost Bumpnose, Red Bluff, and Forest Hill Formations and Marianna Limestone elsewhere in the region.

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