TESTING THE LATE ORDOVICIAN PRE-GICE WARM WATER CARBONATE HYPOTHESIS IN ALABAMA

The Guttenberg Carbon Isotope Excursion (GICE) (uppermost Sandbian-lower Katian, Late Ordovician) has been suggested to represent the transition from a Cambrian-Ordovician greenhouse world to Late Ordovician icehouse world. This transition is thought to coincide with a proposed shift from deposition of warm water carbonate rocks to cool water carbonate rocks in the North American midcontinent. We used oxygen isotopes (d¹⁸O) of conodonts to test the idea that the rocks below the GICE interval represent a consistently warm environment. Conodonts were isolated from samples of the Chickamauga Group collected at the Tidwell Hollow section in Blount County, AL and from the Red Mountain Expressway section in Birmingham, AL. Both successions underlie strata that biostratigraphically correlate with the GICE interval. Conodonts at Tidwell Hollow include Appalachignathus sp. and Phragmodus flexuosus near the base of the Chickamauga Group and Oulodus oregonia and Aphelognathus kimmswickensis near its top. These taxa indicate that the interval studied ranges from the Plectodina aculeata Zone to at least the Plectodina tenuis Zone. Conodonts from the Red Mountain Expressway section were collected only to 22m below the Millbrig K-bentonite, where Belodina compressa and Curtognathus spp. dominate the fauna. At 10m below the Millbrig, Polyplacognathus ramosus occurs and the top of the section contains a fauna that includes Phragmodus undatus and Icriodella superba. These results indicate that the sampled interval ranges from the Belodina compressa Zone to the Phragmodus undatus Zone. If the conodont d¹⁸O values indicate that the rocks below the GICE in Alabama represent a consistently warm environment throughout the Chickamauga Group below the Millbrig K-bentonite Bed, then it supports the idea that the pre-GICE environment was relatively stable, and that the GICE, along with the changes in d¹⁸O in the GICE interval, could reflect a perturbation in the Ordovician climate.