MARINE FIRMGROUNDS OF THE MIDDLE EOCENE COOK MOUNTAIN AND LISBON FORMATIONS: UNDERSTANDING SHELF EVOLUTION DURING THE LATE LUTETIAN HIGHSTAND, EASTERN U.S. GULF COASTAL PLAIN

The middle Eocene strata of the eastern U.S. Gulf Coastal Plain were deposited on a shallow, mixed carbonate-clastic shelf during a time of global sea level and climatic fluctuations. The Cook Mountain and “upper” Lisbon Formations of Mississippi and Alabama record transgressive and highstand deposition during parts of a single third-order depositional sequence in the late Lutetian and early Bartonian. Outcrop studies linked to regional correlations reveal regionally persistent firmgrounds associated with flooding surfaces that occur at various stages of relative sea level rise. These surfaces are typically developed on silty clays and are marked by deep networks of exichnial Thalassinoides and crab burrows passively filled with shelly sediment from overlying condensed beds. A maximum flooding surface is recognized at a burrowed or bored contact that underlies the Cook Mountain and its equivalents in Alabama. Cook Mountain carbonates comprise a thick section of sandy marls and oyster beds that can be traced towards a progradational grainstone bank at the shelf margin. A distinct carbonate firmground appears to cap this interval in outcrop, with the overlying section grading upwards into carbonaceous, prodelta muds within the Cockfield genetic unit. Similar textures occur at the upper contact of the Lisbon with the Gosport Sand and have typically been interpreted as evidence of a sequence boundary. While subaerial exposure likely occurred in response to forced regression in proximal settings, we suggest that this portion of the shelf profile was likely never exposed in the eastern Gulf, and that sequence boundaries instead occur as correlative conformities that underly burrowed contacts developed during transgression. Additional mapping of shelf unconformities and lowstand strata is required to better understand the evolution of this classic Paleogene section.