PALYNOSTRATIGRAPHY AND PALEOECOLOGY OF A VERTEBRATE LOCALITY AT THE MISSISSIPPIAN - PENNSYLVANIAN BOUNDARY IN WESTERN KENTUCKY

In many parts of the Illinois Basin, crossbedded, quartz pebble-bearing quartz arenites mark the base of the Pennsylvanian system. In some areas, however, paleosols or coal beds mark the boundary. As similar lithologies occur in the Late Mississippian, determining the age of strata near the Mississippian - Pennsylvanian boundary can be problematic. A good example is found in Hancock County, Kentucky, where 17 m of shales, paleosols, thin coals, siltstones, and sandstones occur between distinctive crossbedded, quartz arenites of the Pennsylvanian Caseyville Formation and Mississippian Limestones of the Buffalo Wallow Formation. Five coal, shale and mudstone samples were collected to palynologically determine a Mississippian or Pennsylvanian age for vertebrate fossils that were discovered at this locality.

The four stratigraphically oldest samples were shales that filled a scour surface. Although these lithologies could be interpreted as an incision with subsequent Pennsylvanian sedimentation along the systemic unconformity, the samples actually contained spore and pollen assemblages of Late Mississippian (Chesterian) age. Hence, the vertebrate remains at the site are Late Mississippian in age. Diagnostic Late Mississippian palynomorph taxa in these samples include: Tripartites, Rotaspora, and Grandispora. Ecologically, these samples are dominated, or co-dominated, by forms ascribed to lycopod trees (Lycospora), seed ferns (Schulzospora), small ferns (Granulatisporites and others), and calamites (Calamospora).

The stratigraphically youngest sample was collected from a thin coal that occurs just above the scour, and 5 m below the overlying quartz arenite. This sample yielded an assemblage of Middle to Late Morrowan age, indicating that the original placement of the boundary at the base of the quartz arenite was incorrect. Diagnostic Pennsylvanian taxa in this sample include: Laevigatosporites, Endosporites and Granasporites medius. Ecologically, sample 5 is strongly dominated by Lycospora pusilla, L. pellucida, and L. granulata, indicating that lycopod trees were the principle vegetation.

Collectively, these results indicate the important role palynology can play in determining the age and paleoecology of sediments near a systemic boundary.