GRADIENT ECOLOGY AND COMMUNITY STRUCTURE OF A MIXED CARBONATE-SILICICLASTIC SHELF, MIDDLE EOCENE (LUTETIAN – BARTONIAN) U.S. GULF COASTAL PLAIN

Middle Eocene strata exposed in the eastern U.S. Gulf Coastal Plain contain a diverse, well-preserved macrofauna that provides an excellent record of Cenozoic climatic and biotic events. Although patterns of diversity have received previous study, the structure and evolution of paleocommunities in this setting have remained largely unexplored in detail. Stratigraphically constrained abundance data are used here to identify distinct biofacies and reconstruct an environmental gradient within the upper Lisbon Formation of Alabama and its equivalents in the Kosciusko and Cook Mountain Formations of Mississippi. These units represent lithofacies within sequence TE2.3, a regional third-order cycle recognized throughout the Gulf and Atlantic Coastal Plains. Deposition through this interval broadly records the evolution of a shallow, mixed carbonate-siliciclastic shelf during the late Lutetian and early Bartonian. Transgressive, relatively coarse siliciclastics are characterized by a turritellid-noetiid assemblage as well as an abundance of the nassariid gastropod Buccitriton. In contrast, highstand carbonates and calcareous sands are dominated by the oyster Cubitostrea sellaeformis alongside a diversity of solitary corals. These assemblages are arrayed along an onshore-offshore gradient in ordination space, with grain size and lithology separating along the primary axis of variation. Accordingly, the disparate expression of biofacies within component systems tracts reflects the interaction of a given taxon’s preferred environment and its expression through the relative sea level cycle. This variability within the upper Lisbon sequence has been previously obscured due to the low temporal resolution afforded in the Gulf Coast Paleogene. Thus, our results provide a framework to test for ecosystem stability in response to environmental change through this dynamic interval of Earth’s history.