Integrated δ13Ccarb, conodont, and graptolite biochemostratigraphy of the Silurian from the Illinois Basin and stratigraphic revision of the Bainbridge Group

The Illinois Basin of southwestern Illinois and southeastern Missouri contains one of the most extensive Silurian records in North America and is a potentially critical area for global sea-level and paleoclimatic/paleoceanographic studies. Unfortunately, the Silurian stratigraphy regionally remains understudied and often can only be correlated globally at series level. Silurian strata from southwestern Illinois and southeastern Missouri are currently assigned to the Sexton Creek Limestone, Seventy-Six Shale, St. Clair Limestone, and Moccasin Springs Formation. The latter three units are variously considered members of the Bainbridge Formation or formations within the Bainbridge Group. We advocate the formalization of the term Bainbridge at the group level throughout its area of use, composed of (in ascending order) the Seventy-Six Formation (raised in rank and geographically expanded herein), the St. Clair Formation (stratigraphically expanded herein), and the Moccasin Springs Formation with four members: the Greens Ferry, Sheppard Point, Lithium (proposed herein), and Randol members. The Bainbridge Group is unconformably underlain by the Silurian Sexton Creek Formation and conformably overlain by the ?Pridoli–Devonian Bailey Formation. Herein, additional nomenclatural changes are presented to the Cason Formation of Arkansas as well. Integrated carbon isotope (δ13C), conodont, and graptolite biochemostratigraphy of three localities in the southwestern Illinois and southeastern Missouri area indicate the presence of the early Sheinwoodian “Ireviken,” Homerian “Mulde,” and Ludfordian “Lau” positive carbon isotope excursions in the Bainbridge Group (the first published consecutive occurrence from North America). The stratigraphic revisions presented in this study provide a consistent and widely applicable lithostratigraphic nomenclature for the Bainbridge Group throughout its areal extent. This study also provides the first detailed integrated biochemostratigraphic record of all three major Wenlock–Ludfordian δ13C excursions from this important Laurentian basin, and it enables the first precise correlation of the Bainbridge Group into the global Silurian chronostratigraphic scheme.