DOCUMENTATION OF THE KLONK CARBON ∂13C ISOTOPIC EXCURSION IN THE SILURIAN/DEVONIAN ROCKS OF ILLINOIS AND ITS IMPLICATIONS FOR THE DEPOSITIONAL HISTORY OF INTRACRATONIC BASINS OF THE REGION

Detailed chemostratigraphic, biostratigraphic, and lithostratigraphic study of a deep continuous core from southern Illinois documents for the first time the depositional history of the Wenlock through Pridoli Silurian rocks of the “proto-Illinois” (Vincennes) Basin. Represented by the Bainbridge Group (mostly St. Clair Limestone and Moccasin Springs Formation), the age of these rocks and the overlying Bailey Limestone, along with the location of the Silurian-Devonian boundary have long been controversial. This study resolves major aspects of this debate. Analysis of carbon ∂¹³C isotopic data demonstrates that the early Sheinwoodian or Ireviken (early Wenlock), mid Homerian or Mulde (late Wenlock), mid Ludfordian or Lau (late Ludlow) and Klonk (late Pridoli-early Lochkovian ) ∂¹³C excursions are all represented providing the only documented complete post-Llandovery Silurian section in Illinois. Biostratigraphic data (conodonts, graptolites, and palynomorphs) indicate that the uppermost shaley strata of the Moccasin Springs are latest Silurian (late Pridoli), whereas the lower Bailey is early Devonian, based on palynomorphs. In addition, the biostratigraphic data parallels the development of the Klonk ∂¹³C excursion (latest Silurian - earliest Devonian), placing the Silurian-Devonian boundary at the transitional contact between the Moccasin Springs and Bailey.

The chronostratigraphic documentation of the “proto-Illinois” Basin Silurian has important implications for regional late Silurian depositional patterns. In contrast with basins to the north (Michigan, Williston, Hudson Bay), which exhibit thick sequences of late Silurian evaporites and related sedimentary rocks, age-equivalent rocks in the Illinois Basin are represented by a much thinner interval of argillaceous, normal marine strata representing a sediment-starved basin open to oceanic circulation. These rocks are similar to those found in late Silurian sections across the Southern Midcontinent region, extending from western Tennessee into subsurface West Texas. The late Silurian shelf edge (reef and carbonate bank) to basin center topography of the Illinois Basin was filled mostly with early Devonian sediments in contrast with the Michigan Basin where age-equivalent topography was filled with late Silurian evaporates.