THE FORT POLK MIOCENE VERTEBRATE SITES (LOUISIANA, USA): IMPLICATIONS FOR LOCAL AND GLOBAL PALEOENVIRONMENTAL CHANGES

The terrestrial-mammal-bearing Fort Polk Miocene beds, between 13.9 Ma and 12.6 Ma in age based on magnetostratigraphy, lie within the regressive upper Castor Creek Member, Fleming Formation. The sediments were deposited in a period of global falling sea level and climate changes related to increase in Antarctic glaciation. Erosion of soils resulted in concentration of nodules and terrestrial animal remains in the thin conglomeratic layers from which most fossils used in this study are recovered, using acid dissolution and screening. A Miocene environment like current coastal south Louisiana was originally postulated. Later research indicates that a better analog for some of the localities is south central Louisiana uplands.These are higher than the modern coast and floodplains because of entrenching of rivers when sea level dropped during Pleistocene glaciations.

A 12.8 Ma carbon isotope negative excursion occurring stratigraphically above the major fossil sites may represent a brief but recognizable expansion of C₄ grasses, which were present throughout the range of vertebrate fossil sites, based on preliminary phytolith evidence. Modern local soil carbon isotope values are similar to the pre-excursion results.

A new site, the stratigraphically lowest fossiliferous level, has yielded a mix of terrestrial and marine vertebrates including whale, rodent, and large camel remains. A Louisiana aquiclude which has been correlated in the subsurface with beds bearing the Burkeville Local Fauna (vertebrates) in east Texas is probably the more marine influenced, lower bulk of the Castor Creek Member. The new site might be correlative to either the Texas Burkeville or Cold Spring Local Faunas. The highest main fossiliferous levels are correlated to the Cold Spring, thus constraining the Burkeville to older than the age of the highest main fossiliferous levels at Fort Polk, approximately 13.5 Ma.