Age and paleo-environmental significance of upper Paleozoic ostracodes from the Appalachian and eastern Illinois Basins

The Greene Formation of the Dunkard Group is the youngest Paleozoic stratigraphic unit in the Appalachian Basin. Given the absence of marine fossils, the age assignment of these strata has been contentious, although plant fossils indicate a position at or near the Pennsylvanian-Permian boundary. The recent recovery of ostracodes from the GIL 30 core in western Kentucky (Illinois Basin), bearing nonmarine Permian fossil associations, is of significance for regional comparisons. The objective of this study, therefore, is to compare the nonmarine ostracode assemblages from the Dunkard to those in the Kentucky core, the goal being accurate age determinations for these rocks. The GIL30 core was recovered from a down-faulted block located near Sturgis, KY and contains strata deposited during the latest Pennsylvanian and earliest Permian. The Pennsylvanian Mattoon Formation comprises mostly siltstone and shale layers with a few layers of coal, Marine fossils such as Lingula and other brachiopods are found, along with plant remains. The overlying Permian Mauzy Formation lacks coal and contains a much higher proportion of non-marine limestone than the Mattoon. A single marine zone yielded fusulinids, which indicate Early Permian age according to R.C. Douglass of the U.S. Geological Survey. We observed three ostracode assemblages in the GIL 30 core: (A) a marine assemblage dominated by Hollinella sp.; (B) a brackish assemblage dominated by Geisina sp., Cavellina nebrascensis, and Velatomorpha sp.; and (C) a freshwater assemblage dominated by Whipplella cuneiformis. The freshwater assemblage in the GIL 30 can be correlated with confidence to the uppermost units of the Greene Formation (e.g., Windy Gap Limestone) of the Dunkard Group in the stratotype area of northeastern West Virginia and southwestern Pennsylvania. These findings confirm an Early Permian age for at least the uppermost Dunkard Group strata.