CONODONT BIOSTRATIGRAPHY OF THE SCHLAMER #1 CORE AND REVISIONS TO THE SILURIAN CONODONT ZONATION FOR THE GEOLOGIC TIME SCALE 2020

The Schlamer #1 Core was drilled in Alexander County, Illinois, and contains the Upper Ordovician through nearly the entire Silurian succession present in the area, including the Girardeau Limestone, Sexton Creek Limestone, Seventy-Six Shale, St. Clair Limestone, and Moccasin Springs Formation of the Bainbridge Group as well as the basal few feet of the Bailey Limestone. The core was analyzed for conodont biostratigraphy, graptolite biostratigraphy, and carbon isotope chemostratigraphy, and the Hirnantian (HICE), early Sheinwoodian (Ireviken), Homerian (Mulde), and Ludfordian (Lau) positive carbon isotope excursions are all present in the core.

The conodont fauna in the core includes nearly all of the Silurian kockelellids as well as a range of biostratigraphically useful ozarkodinids. The integration of carbon isotope chemostratigraphy with high-resolution conodont biostratigraphy helps to demonstrate the timing of specific Silurian conodont zonal boundaries with respect to global series and stage boundaries. Additionally, the presence of graptolites in the core, combined with other regional conodont studies, helps to refine cross-correlation between conodonts and graptolites in the Ludlow and Pridoli series. Specifically, we can now demonstrate that the base of the Kockelella crassa conodont Zone does not precisely mark the base of the Ludlow Series, but, in fact, lies slightly below the base of the Ludlow, above the top of the Mulde positive carbon isotope excursion. This core, along with several other conodont studies from the U.S., Europe, and Australia over the past few years, will serve as the backbone of revisions to the global Silurian conodont biozonation that will be utilized in the upcoming GTS 2020 volume.