A COMBINED CONODONT AND CARBON ISOTOPE CHRONOSTRATIGRAPHY FOR THE IBEXIAN SERIES OF WESTERN NORTH AMERICA

A profile of stratigraphic variation in marine carbonate carbon isotopic ratios (δ¹³C) has been constructed through the entire Ibexian Series in the type region of western Utah. The profile is comprised of over 700 individual samples from the Notch Peak Formation, the House Limestone, the Fillmore Formation, and the Wah Wah Formation, using the classic Hintze localities and sections developed by James Miller and collaborators. Existing conodont stratigraphies by Miller, Ethington, and others were used to calibrate a combined chronostratigraphic correlation framework for the Ibexian.

Several features in the δ¹³C profile from the Lower Ibexian have been previously recognized, most notably as a sharp positive excursion near the base of the Stairsian Stage and an abrupt negative excursion (the Jose Event) near the base of the Tulean. Two new features, from the youngest Ibexian stage, are reported here. The new features are found as negative δ¹³C excursions against a gently rising long-term trend, near the base and top of the Blackhillsian Stage.

The western Utah δ¹³C profile has proven to be reliable in correlating conodont zone boundaries in other localities with well-known conodont faunas, most notably the El Paso Group. Current research into Lower Ordovician successions in the North American midcontinent suggest that the Ibexian δ¹³C profile may be useful in providing age constraints for notably difficult units with sparse conodont faunas, such as the Cotter Dolomite in southwestern Missouri.