CHRONOSTRATIGRAPHIC SIGNIFICANCE OF d13C CHEMOSTRATIGRAPHY IN THE ORDOVICIAN-SILURIAN BOUNDARY INTERVAL IN THE NORTH AMERICAN MIDCONTINENT

Recent d¹³C investigations of key sections in Missouri, Iowa, Illinois, Ohio, and Ontario have given new insights into the age relations of several formations in the latest Ordovician and Early Silurian in the Midcontinent region. The latest Ordovician (Hirnantian) global positive d¹³C excursion (HICE) has been recorded in the Cape Girardeau and Leemon formations in S.E. Missouri and S.W. Illinois indicating latest Ordovician age. The presence of HICE in the lower Mosalem Formation in eastern Iowa and in the Schweizer Member of the Wilhelmi Formation in N.E. Illinois shows this interval to be of Hirnantian, rather than Early Silurian, age. Attempts to find the HICE in the Centerfield-Belfast interval in S. Ohio between the Richmondian Bull Fork Formation and the mid-Llandoverian Brassfield Limestone have been unsuccessful, which supports the chitinozoan evidence that this interval is Early Silurian rather than Late Ordovician in age. The d¹³C curve through the Manitoulin Dolomite and the uppermost part of the underlying Georgian Bay Formation on Manitoulin Island, Ontario shows rather uniform baseline values between 0‰ and 1‰ and no HICE. However, samples from the lower 2 m of the Manitoulin Dolomite and uppermost 1.5 m of the underlying Queenston Formation at Hope Bay, Bruce Peninsula have elevated d¹³C values between +2‰ and +3‰ indicating the presence of HICE and latest Ordovician age. This suggests that the base of the Manitoulin Dolomite is diachronous and that on Bruce Peninsula, the upper Queenston Formation, the Whirlpool Formation, and the lowermost Manitoulin Dolomite were deposited during the latest Ordovician (Hirnantian) sea level lowstand.