UPPER EMSIAN TO LOWER GIVETIAN CONODONT BIOSTRATIGRAPHY AND CONODONT APATITE DELTA 18O RESULTS OF THE GRAND TOWER AND SAINT LAURENT FORMATIONS: SOUTHERN ILLINOIS BASIN-REEL FOOT EMBAYMENT

Ages for Lower and Middle Devonian strata of the southern Illinois Basin and Reel Foot Embayment along the Ouachita continental margin of western Laurussia are not well constrained. Studies by Orr and Collinson (1960s) suggested the Emsian-Eifelian and Eifelian-Givetian boundaries were at or near the contacts of the Clear Creek-Grand Tower and the Grand Tower-St. Laurent Formations, respectively. To refine age estimates and more precisely locate stage boundaries, we studied conodonts in samples from the type area of Grand Tower, Illinois. The lower 7 m of the Dutch Creek Member (sandy grainstones) of the Grand Tower Formation yields Latericriocus nevadensis, L. alces and L. robustus characteristic of the Upper Emsian (serotinus Zone). The first appearances of Icriodus orri, I. angustus and I. stephensoni in the uppermost Dutch Creek indicate these strata are within the costatus Zone. Skeletal wackestones and packstones of the overlying “upper Member” of the Grand Tower Formation are separated from the Dutch Creek Member and overlying St. Laurent Formation by marine flooding surfaces. The conodont fauna of the “upper Member” is dominated by the afore-mentioned Icriodus species and indicate the “upper Member” is entirely Eifelian in age. The appearance of Polygnathus pseudofoliatus a few meters above the base of the “upper Member” fixes the base of the australis Zone at that level. The kockelianus and ensensis zones faunas are not present in samples studied so far. The appearance of I. excavatus at the base of the St. Laurent Formation indicates a lowermost Givetian age with the base of the timorensis Zone as marked by the first appearance of I. brevis in the lower Saint Laurent Formation.

Conodont δ¹⁸O values of Icriodid P₁ elements range between 17.4 and 21.2 ‰VSMOW through the section and suggest considerable changes in subtropical Sea Surface Temperatures (SSTs) and/or changes in the isotopic composition of local seawater in the southern Illinois Basin during the Eifelian and Early Givetian. Within the 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.