THE IREVIKEN AND MULDE (SILURIAN, WENLOCK) POSITIVE CARBON ISOTOPE EXCURSIONS IN THE ILLINOIS BASIN, SOUTHERN ILLINOIS

The Ireviken and Mulde positive carbon isotope excursions and associated conodont extinctions are becoming increasingly well known globally, but data from the Illinois Basin are scarce despite its importance as a major midcontinent depocenter. The St. Clair Member of the Bainbridge Formation was traditionally considered to represent the Wenlock based on macrofossils and some scant conodont data. However, the positions of global series and stage boundaries relative to the superjacent Moccasin Springs Mbr. and subjacent Seventy-Six Shale Mbr., as well as more general age constraints on the lower Moccasin Springs Mbr. were unclear.

Carbon isotope and conodont biochemostratigraphy of the St. Clair from the Schlamer #1 core and an outcrop near Lindsey Cemetery, Dongola Hollow (both Alexander County, Illinois) documents the Ireviken (early Sheinwoodian) and Mulde (middle to upper Homerian) positive carbon isotope excursions, indicating that the St. Clair represents approximately the entire Wenlock Series. Globally, the base of the Wenlock falls just beneath the onset of the Ireviken excursion, and is therefore placed in the Seventy-Six Shale. The exact position of the base of the Gorstian is unclear from these data, but likely occurs near the top of the St. Clair. Interestingly, the Ireviken and Mulde are stratigraphically adjacent in these sections, suggesting that the middle Wenlock is absent or extremely condensed. Lastly, a new lithostratigraphic definition for the St. Clair in Illinois simplifies its identification in the field, expands the upper boundary of the member, and also allows the physical boundaries to more closely correspond to Wenlock chronostratigraphic boundaries.