The Vicksburg Group (Oligocene) is a predominantly carbonate unit that extends in a narrow belt from Rosefield, Louisiana, to western Florida. East of the LaSalle arch (eastern Louisiana) the carbonates of the Vicksburg Group are continental shelf in origin and display a sedimentary strike of approximately east-west.

The carbonate formations of the Vicksburg Group are demonstrated to be facies of one another and to constitute a single sedimentary cycle. The outcrop belt strikes west-northwest across the sedimentary strike displaying changes in the lithofacies of the group.

The formational division of the Vicksburg Group established by Cooke in 1918 and others is clarified, and the scope of the Byram and Glendon Formations is revised to conform to the lithofacies of their types localities. The Byram Formation is redefined to include the silty sands and wackestones of a regressive carbonate shelf/destructional bank facies. The Glendon Formation is restricted to include only the skeletal grainstones and coarse sands of a carbonate shoal/shoreline. The Marianna Formation includes mudstones of an algal mud shelf bottom and silty sands of a back-bank facies. The Mint Spring Formation consists of silty sands of a destructional delta environment and includes those glauconite sands that overlie the prodelta clays of the Forest Hill Formation. The Rosefield Forma ion is probably a chenier plain silty clay with a coquina beach zone of fossils common to the Byram Formation.

Penecontemporaneous or post-Vicksburg erosion on the crest of the Wiggins uplift apparently restricted or removed possible Vicksburgian coral-algal reef or nummulitic bank sediments. Limited subsurface data show that a nummulitic bank did develop on the north flank of the uplift. This bank migrated northward as the Marianna back-bank area shoaled and produced the Nummulites-Lepidocyclina grainstone/sand of the Glendon Formation at its type locality.

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