REGIONAL AND GLOBAL CHRONO- AND BIO-CHEMOSTRATIGRAPHIC CONTRIBUTIONS OF THE SCHLAMER #1 CORE (SILURIAN, ILLINOIS BASIN)

The Schlamer #1 Core (Alexander County, Illinois) penetrates an interval from the Ordovician Girardeau Limestone through the basal Bailey Formation (?Early Devonian). The Sexton Creek Fm., Seventy-Six Fm., St. Clair Fm., and Moccasin Springs Fm. record nearly the entire Silurian succession present in the Illinois Basin. High-resolution conodont, graptolite, and carbon isotope bio-chemostratigraphic analysis of the core allows synthesis of prior works on the Silurian of the Illinois Basin and contributes to regional and global understanding of biotic and geochemical events during the upper Llandovery–Pridoli. The core captures the early Sheinwoodian (Ireviken), Homerian (Mulde), and Ludfordian (Lau) positive carbon isotope excursions. To our knowledge this is the first reported occurrence of all three major Silurian excursions in a single section from North America, and the core therefore provides a “backbone” for bio-chemostratigraphic correlation from the Illinois Basin to diverse regional and global Silurian sections.

The Schlamer #1 conodont fauna contains nearly all Silurian kockellelids and many biostratigraphically informative ozarkodinids, as well as significant graptolites that aid correlation of conodont vs. graptolite biozonation in the Ludlow and Pridoli. Integration with the high-resolution carbon isotope chemostratigraphy facilitates interpretation of conodont biozone boundaries with respect to the timing of the carbon isotope excursions. Most notably, the core demonstrates that the base of the Kockelella crassa conodont zone lies slightly below the base of the Ludlow Series, above the top of the Mulde excursion, rather than precisely correlated with the base of the Ludlow as previously thought. The wealth of integrated conodont and carbon isotope information from the core will help to revise Silurian conodont biozonation for the Geological Time Scale 2020.

Lastly, the core provides a widely applicable regional stratigraphy for the Silurian units. Problematic units are limited to the Sexton Creek Fm. (thought to be correlative with the poorly understood “Brassfield”) and to the question of whether the St. Clair Fm. in its type area of Arkansas is strictly correlative with the St. Clair Fm. of southwestern Illinois and southeastern Missouri.