LOWER-MIDDLE DEVONIAN (PRAGIAN-LOWER GIVETIAN) CONODONT BIOSTRATIGRAPHY, MIDDLE DEVONIAN SEA LEVEL RECORD AND ESTIMATED CONODONT APATITE D18O SEA SURFACE TEMPERATURE CHANGES IN THE SOUTHERN ILLINOIS BASIN, USA

Investigation of the Devonian stratigraphy, conodont biostratigraphy and conodont apatite δ¹⁸O geochemistry of the Clear Creek, Grand Tower and St. Laurent formations was conducted in southern Illinois to develop refined biostratigraphic correlations, constrain the timing of 3^rd order sea level changes and estimate stable oxygen isotopic paleo-temperature changes of Devonian sea surface waters along the southern continental margin of Laurussia. Conodonts from the White County Core indicate a Pragian age for the upper Clear Creek Formation in White County. At Grand Tower (Jackson Co.), conodonts document an upper Emsian age (serotinus Zone) of the lower Dutch Creek Member of the Grand Tower Fm. and place the Emsian-Eifelian Stage boundary 7 m above the base of the Dutch Creek. The remainder of the Dutch Creek is of lower Eifelian age (costatus Zone). Drowning of sandy platform deposits of the Dutch Creek by deeper subtidal muddy carbonates of the “upper Member” of the Grand Tower Fm. records the first Eifelian sea level rise within the australis Zone (first occurrence of P. pseudofoliatus). Conodonts from deposits within a 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 occurs at the contact of the Grand Tower and St. Laurent fms. and falls within the ensensis Zone. The lowest I. obliquimarginatus in the lower St. Laurent suggests a basal hemiansatus Zone (Givetian age), followed by the first occurrence of P. timorensis (timorensisZone).

Analysis Icriodid P₁ elements gives δ¹⁸O values ranging from 17.4 and 21.2 ‰V-SMOW through the middle Grand Tower-St. Laurent fms. suggesting considerable changes in subtropical temperatures and/or changes in local seawater δ¹⁸O values in the southern Illinois Basin during the Eifelian and Early Givetian. Within the mid-upper Grand Tower Formation, δ¹⁸O values decrease by 4‰ then increase by 2.5‰. In the overlying St. Laurent Formation, values decrease by 1‰. If solely due to temperature, these values suggest warming of up to 16°C during the middle Eifelian followed by a 11°C cooling in the Late Eifelian and 4°C warming in the Early Givetian-- swings large enough to implicate changing seawater δ¹⁸O values as an important variable in the δ¹⁸O record.