THE IREVIKEN AND MULDE CARBON ISOTOPE EXCURSIONS IN THE ILLINOIS BASIN, SOUTHERN ILLINOIS

The St. Clair Member of the Bainbridge Formation in southern Illinois is an approximately 6-9 m thick crinoidal wackestone to packstone bounded by the thin Seventy-Six Shale below and the thick, argillaceous limestone Moccasin Springs Member above. The St. Clair was assigned to the Wenlock based on its macrofauna and some conodont data, but the precise position of global series and stage boundaries was unclear, as was the age of the base of the Moccasin Springs.

Carbon isotope stratigraphy of the St. Clair from an outcrop near Dongola Hollow and from the Schlamer #1 core, both located in Alexander County, Illinois, reveals the presence of the Ireviken (early Sheinwoodian) and Mulde (middle to upper Homerian) carbon isotope excursions. The St. Clair therefore represents essentially the entire Wenlock Series. The base of the Wenlock is slightly below the onset of the Ireviken excursion and therefore falls in the Seventy-Six Shale. The base of the Gorstian lies very near the top of the St. Clair but its exact position remains unclear. The isotope data indicate that the Ireviken and Mulde are stratigraphically adjacent, suggesting that the middle Wenlock is extremely condensed or missing in these sections. Analysis of conodont samples from the outcrop and core is ongoing. Preliminary results indicate that the conodont fauna is abundant (hundreds to over 1,000 elements per kilogram sample), but dominated by coniform taxa. Biostratigraphic data may be able to precisely place the base of the Ludlow.