STRATIGRAPHIC SIGNIFICANCE OF δ¹³C CHEMOSTRATIGRAPHY IN THE UPPER ORDOVICIAN-LOWER SILURIAN OF THE NORTH AMERICAN MIDCONTINENT

Continued chemostratigraphic work, following the discovery of the Hirnantian (uppermost Ordovician) δ¹³C excursion (HICE) in the Girardeau Limestone and Leemon Formation of SE Missouri and SE Illinois, has led to the documentation of the HICE from the lower Mosalem Formation in eastern Iowa and the Schweizer Member of the Wilhelmi Formation in NE Illinois. In both these areas, the HICE show δ¹³C peak values of near 3‰. The age of these poorly fossiliferous formations has long been controversial, but the presence of the HICE indicates a latest Ordovician rather than Early Silurian age. The overlying Tete des Morts Formation in Iowa and the Elwood Formation in Illinois show baseline δ¹³C values between –1 and +0.5‰ indicating an Early Silurian age. In westernmost Ohio (West Milton) samples through the uppermost Richmondian Stage and the lowermost Brassfield Limestone show slightly elevated values of about +2‰ but the middle and upper Brassfield have baseline values between 1 and 1.5‰ and there is no obvious isotope excursion. The Manitoulin Dolomite on Manitoulin Island, Ontario shows δ¹³C values of <0‰. However, on Bruce Peninsula (Hope Bay), the lower 2 m of this unit and the uppermost 1.5 m of the underlying Queenston Formation have values between +2 and +3‰ suggesting the presence of HICE and Hirnantian age. Further studies may confirm the new idea that the pre-Manitoulin post-Queenston Whirlpool Formation was deposited during the Hirnantian sea-level lowstand rather than representing the early phase of the Silurian transgression.