LOWER TO MIDDLE DEVONIAN (PRAGIAN-LOWER GIVETIAN) CONODONT FAUNAS FROM THE CLEAR CREEK, GRAND TOWER AND ST. LAURENT FORMATIONS, SOUTHERN ILLINOIS BASIN, USA

The study of conodont samples of the Clear Creek, Grand Tower and St. Laurent formations in the Grand Tower type section along the Mississippi river (Jackson Co., SW Ill.) and in the Illinois State Geological Survey White County core (White Co., SE Ill.) was conducted to develop detailed biostratigraphic correlations and constrain the timing of 3rd order sea level changes along the southern Ouachita continental margin of Laurussia. Conodonts are abundant in most samples and faunas largely dominated by Latericriodid and Icriodid taxa with varying numbers of Polygnathids.

The upper part of the Clear Creek Formation (Fm.) in White County contains Pragian conodonts, principally Latericriodus claudiae delimiting the deposits to the sulcatus-kindlei zones. In outcrop sections at Grand Tower, the presence of L. robustus and L. alces in the lower Dutch Creek Member of the Grand Tower Fm. documents its upper Emsian age (serotinus Zone). These taxa make their entries 7 meters (m) above the base of the Grand Tower Fm. in the White County core. The Emsian-Eifelian Stage boundary is placed at the first appearance of Polygnathus partitus 7 m above the base of the Dutch Creek in outcrop sections in Jackson County, and correlates with a level 28 m above the base of the Grand Tower Fm. in the White County core section. The upper part of the Dutch Creek Member is of lower Eifelian age (costatus Zone) indicated by the appearances of Icriodus orri, I. angustus and I. stephensoni. The drowning of sandy platform deposits of the Dutch Creek Member by the middle shelf muddy carbonates of the “upper Member” of the Grand Tower Fm. records an Eifelian sea level rise within the australis Zone. The base of the australis Zone in the surface and core sections is placed at the first occurrence of P. pseudofoliatus. Conodonts from deposits within the second marine flooding interval in the upper part of the “upper Member” identify a late Eifelian marine flooding event within the kockelianus zone. A third Middle Devonian flooding surface recorded in both sections occurs at the contact of the Grand Tower and St. Laurent fms. within the ensensis Zone. The lowest Icriodus obliquimarginatus in the lower St. Laurent suggests a basal hemiansatus Zone (Givetian age), followed by the first occurrence of P. timorensis identifying the position of the base of the timorensis Zone.