PTEROTOCRINUS WING PLATES A RELIABLE BIOSTRATIGRAPHIC TOOL FOR THE ELVIRAN STAGE, CHESTERIAN SERIES, ILLINOIS BASIN
After 31 years of geologic mapping in southern Illinois under the auspices of the COGEOMAP and STATEMAP cooperative USGS mapping programs, useful wing plates of Pterotocrinus have come to light in the Elviran Stage. The most useful wing plates found thus far are Pterotocrinus menardensis (Weller) from the Menard Limestone and Clore Formation, Pterotocrinus tridecibrachiatus (Gutschick) from the Kinkaid Limestone and Pterotocrinus pegasus (Gutschick) from a horizon in the basal Grove Church Formation. These wing plates all occur in the dark gray shales and gray wackestones of the Elvirian Stage.
When mapping the rocks in highly faulted terrain like that of southeastern Illinois, it is difficult to know which limestone or shale unit outcrops especially within the Elviran Stage. These dark gray, argillaceous, fossil wackestones and shales look quite similar. Therefore, the aforementioned wing plates, if found, can help in determining small fault displacements when mapping Elviran carbonate rocks.