OLIGOCENE FORAMINIFERAL PALEOECOLOGY AND SEQUENCE STRATIGRAPHY OF ALABAMA AND MISSISSIPPI: A FIRST LOOK

The biostratigraphy and sequence stratigraphy of the Oligocene rocks in the eastern Gulf Coastal Plain has been well documented over the past 30 years in the Little Stave Creek section, the St. Stephens Quarry section and in the St. Stephens core of southwestern Alabama. Four depositional sequences are identified: the Bumpnose-Red Bluff sequence (TE3.3; BRB), the Mint Spring-Marianna-Glendon sequence (TO1.1; MMG), the Byram-Bucatunna sequence (TO1.2; BB), and the Chickasawhay Sequence (TO2.1; C). An additional Oligocene sequence, the Paynes Hammock sequence (TO2.3; PH), overlies the Chickasawhay sequence elsewhere in Alabama and Mississippi. Oligocene stratigraphic relations are less clear away from southwestern Alabama due to limited exposure. Of particular interest is the MMG sequence where the Mint Spring Formation is considered to represent a transgressive systems tract (TST), the Marianna Limestone represents a “condensed section”, and the Glendon Limestone as a Highstand Systems Tract (HST). The Marianna is an atypical condensed section as its thickness of over 9 m represents active sedimentation. The interpretation of this sequence is further complicated by changes in lithofacies to the West.

Benthic foraminifera were collected from Oligocene outcrops and cores across the eastern Gulf Coastal Plain. Benthic foraminiferal assemblages have been shown to provide valuable paleobathymetric information on sections which can aid in sequence stratigraphic interpretations. Most of the studied samples are assignable to either the Discorbis-Quinqueloculina assemblage or the Nonion assemblage. Samples from the Mint Spring in central and western Mississippi contain abundant specimens of Textularia, Clavulinoides, and Gaudryina and are in sharp contrast to the assemblages in the Mint Spring of southwestern Alabama.

The paleoecologic results generally support the sequence stratigraphic model of southwestern Alabama with the exception of sequence TO1.1 (MMG). The Mint Spring of central and western Mississippi appears to be a unique lithofacies while the Marianna of central and western Mississippi correlates with the Mint Spring of southwestern Alabama. The Marianna Limestone of southwestern Alabama does not have a correlative to the West and may represent a “catch-up” carbonate platform facies.