Definition of the Eocene-Oligocene boundary in the eastern Gulf Coast region is complicated because the upper Eocene and lower Oligocene strata in Mississippi consist of clastic-dominated deposits whereas time-equivalent strata in south-central Alabama are comprised primarily of carbonates. These strata intertongue across southwestern Alabama. Based on vertical planktonic foraminiferal distribution in these strata, the Eocene-Oligocene boundary occurs at the top of the Shubuta Member of the Yazoo Clay in Mississippi and southwestern Alabama and at the top of the Crystal River Formation in south-central Alabama. The Pachuta and Shubuta Members of the Yazoo and Crystal River are assigned to the upper Priabonian Globorotalia cerroazulensis (s.1.) Interval zone. The Red Bluff Clay and Bumpnose Limestone are placed in the lower Rupelian Pseudohastigerina micra Interval zone. Vertical compositional and diversity changes are apparent in the foraminiferal populations and indicate a rise in sea level through the Priabonian with a decrease in water depth in the lower Rupelian. The decrease in water depth might be attributed to a drop in sea level and/or to progradation of the Forest Hill delta across Mississippi into Alabama. Lower Yazoo sediments accumulated in an inner to middle neritic shelf paleoenvironment, and the upper Yazoo and Crystal River marls are middle to outer shelf deposits. The Red Bluff and Bumpnose sediments were deposited under middle to inner shelf conditions. West to east foraminiferal compositional and diversity trends are apparent. During th Priabonian and lower Rupelian, water depths in Mississippi exceeded those of south-central Alabama.

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