The upper Claiborne, Jackson, and Vicksburg groups record approximately 13 Ma of deposition within the upper Paleogene section of Mississippi (middle Eocene through lower Oligocene). These strata are characterized by repeated suites of siliciclastic and carbonate lithologies, including sandy, glauconite-bearing shell beds and marls exposed updip (in outcrop), and thick limestones that developed across a downdip paleo-high. These carbonate strata are well known from subsurface mapping efforts, but their significance remains largely unexplored in outcrop-based stratigraphic and paleontological studies. Here, we review and integrate subsurface data from well log and core studies with field-based observations to propose a new, mixed carbonate-siliciclastic depositional model for the upper part of the Paleogene section of Mississippi. Deltaic strata overlie widespread basin-margin unconformities but thin distally to condensed marine shales across the Wiggins Arch. Accordingly, these beds reflect lowstands in sea level and were deposited through the regressive outbuilding of the shoreline into the Mississippi Interior Salt Basin. Glauconitic shell beds occur in association with coastal barrier systems, where they reflect the transgressive reworking of previously deposited sand and bioclastic material. In contrast, carbonate sediments are interpreted to have formed in response to the onshore trapping of siliciclastics, coupled with enhanced carbonate production during relative highstands in sea level. These are characterized by successive foraminiferal-algal grain banks in the subsurface, which thin and grade landward to mollusk-rich shelf mudstone-wackestones (marl and limestone units) in outcrop. Notably, both carbonates and glauconitic shell beds appear to broadly record marine transgression but are expressed differently based on relative shoreline position within major base-level cycles. This model differs from the predominantly siliciclastic style of deposition that characterizes the present-day northern Gulf margin and appears to lack a strong modern analog. It also provides a basis for refined stratigraphic interpretation, with important caveats to sequence stratigraphic models commonly applied in this section, and establishes a framework in which to analyze late Paleogene faunas in the context of evolving, process-related depositional environments.
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