Evidence for Faunal Tracking of Recurrent Fossil Assemblages In Four Major Marine Zones In the Glenshaw Formation (Upper Pennsylvanian, Appalachian Basin)

The Upper Pennsylvanian Glenshaw Formation contains a series of marine zones that were deposited on the detrital slope of the Appalachian highlands during the last major transgressions from the Midcontinent Sea in eastern North America. These marine zones contain fossil assemblages that can be characterized as biofacies that inhabited a variety of shallow marine environments. The goals of this study were to 1) determine if biofacies in Glenshaw marine zones recurred with a distinctive composition-abundance structure during each marine episode, 2) examine the stratigraphic distribution of these biofacies to determine if they tracked a preferred habitat, 3) examine the distribution of individual genera to determine the degree of association among the constituent taxa.

The results of this study indicate that marine zones in the Glenshaw Formation contain four recurrent biofacies that are positioned along an environmental gradient that represents changing conditions associated with nearshore to offshore settings. These four biofacies re-appear with a distinctive composition-abundance structure tracking a preferred set of environmental conditions related to glacio-eustatic sea-level changes in the Appalachian Basin. A symmetrical (Nearshore-Offshore-Nearshore) pattern is found in only one marine zone, whereas, the asymmetrical (Offshore-Nearshore) biofacies pattern found in three of the four marine zones is attributed to relatively rapid rates of transgression commonly associated with glacio-eustatic cycles.

The eight most abundant genera were non-randomly distributed among the four Glenshaw biofacies, indicating a consistent environmental preference. In contrast, only four of the remaining 15 less abundant genera were non-randomly distributed. The abundant taxa maintain a more consistent membership among the four biofacies by tracking their preferred environment. An inherent aspect of environmental tracking is that species respond independently, leading to variations in the taxonomic structure of faunal assemblages. The fact that nearly half of the genera that compose the four Glenshaw biofacies are randomly distributed illustrates this variation.