THE LATE MIDDLE ORDOVICIAN (CHATFIELDIAN) GUTTENBERG CARBON ISOTOPE EXCURSION (GICE): A USEFUL TOOL FOR CORRELATION FROM NEW YORK TO MANITOULIN, ONTARIO AND CENTRAL KENTUCKY

Correlation of the classical Middle Ordovician successions in New York and south-central Ontario remains controversial. Although in the past forty years great strides have been made in understanding the bio- and lithofacies relations in the Black River and Trenton Groups (or Simcoe Group), there is still a need to correlate these limestones in a facies independent manner. K-bentonites have been used by various workers for correlation, but the areally extensive Deicke and Millbrig K-bentonites, are thin, difficult to identify chemically, or absent in some regions in New York and southern Ontario. Also, currently available biostratigraphic information is inconclusive and not precise enough for detailed correlation.

A positive carbon isotope excursion in the lower Chatfieldian limestones referred to as the Guttenberg carbon isotope excursion (GICE) is now well known from occurrences in Iowa, Pennsylvania, Kentucky, Tennessee, Illinois, Missouri, Oklahoma, Virginia, Estonia, and Sweden. This excursion is a powerful tool for correlating lower Trenton Group (middle Simcoe Group) limestones where outcrops lack K-bentonites and diagnostic index fossils.

This study reports preliminary data showing the GICE at Dexter and Martinsburg (Roaring Brook), New York and Manitoulin Island (Birch Island), Ontario. The data correlates the Bobcaygeon Formation in Ontario with the Napanee Limestone in New York as well as with the lower Lexington Limestone of Kentucky and coeval strata elsewhere in the Mid-continent. Further, the stratigraphic relationship of the GICE with a thin K-bentonite in the Birch Island section indicates that previous identification of that K-bentonite as the Deicke is correct.