Paleomagnetic Constraints on the Western Louisiana Miocene Vertebrate Sites of Fort Polk and Their Paleoenvironments

Paleomagnetic analyses of samples from four cores of sites yielding Miocene (Barstovian Land Mammal Age) terrestrial vertebrates from Fort Polk in western Louisiana have yielded a magnetic reversal pattern that indicates an age for these cores between 13 and 15 Ma. Studies since 1993 have yielded over 6,500 cataloged specimens of Miocene vertebrates, including nine orders of mammals. The sites record a regression from marine (TVOR SE Site, 14.0 Ma) to terrestrial sites (DISC, 13.75 Ma, and TVOR, 13.5 Ma).

Ages from the magnetostratigraphic study place all sites after the middle Miocene climatic optimum, in a cooler period generally marked in the Gulf Coast by marine regression. Large mammals, such as the horse /Cormohipparion /and /Prosynthetoceras /(a protoceratid related to camels), both found at TVOR SE Site, are best correlated with the animals of the Cold Spring Local Fauna of East Texas, not the stratigraphically lower Burkeville Local Fauna. The presence of the horse /Cormohipparion/ at TVOR SE indicates that its fauna is within the Late Barstovian (Ba2), and the faunas of the younger Fort Polk sites are also Late Barstovian. The Fort Polk sites' ages as estimated via magnetostratigraphy are in accordance with a beginning of the Late Barstovian and the appearance in the central Gulf Coast area of the Cold Spring Local Fauna before 14 Ma.