CARBONATE δ13C AND PRELIMINARY CONODONT δ18O FROM THE UPPER STONES RIVER FORMATION AND LOWER NASHVILLE FORMATION (LATE ORDOVICIAN) NEAR GADSDEN, AL

The Guttenburg Carbon Isotope Excursion (GICE) (uppermost Sandbian-lower Katian) has been suggested to represent a phase of the transition from a Cambro-Ordovician greenhouse world to a Late Ordovician icehouse world. This transition includes a shift from deposition of warm water carbonates to cool water carbonates in the North American midcontinent. Samples were analyzed for δ¹³C from the upper Stones River Formation and the lower Nashville Formation near Gadsden, Alabama to test for the presence of the GICE. The Stones River Formation contains the late Sandbian Deicke and Millbrig K-bentonite beds, which are documented to be just below the GICE in many parts of eastern North America. The δ¹³C values increase from 0‰ to +1‰ over the five meters below the Deicke and plateau at just under +1.5‰ throughout the uppermost Stones River Formation. At the contact with the Nashville Formation, δ¹³C values drop and oscillate between +0.5‰ and -1.0‰ through the first 5.5 meters of the lower Nashville Formation. The conodont fauna changes from a shallow water fauna characterized by very robust elements such as Curtognathus spp., Erismodus radicans, and Aphelognathus sp. cf. A. gigas in the carbonate mudstones of the Stones River Formation to a fauna dominated by more delicate forms like Panderodus gracilis and Plectodina aculeata in the interbedded and clay-rich carbonate mudstone and shale of the Nashville Formation. The contact between these units is interpreted as a deepening event and the M4-M5 boundary. The δ¹³C trend in the upper Stones River Formation is GICE-like. However, it is older than expected occurring within the Deicke and Millbrig interval. If it is a record of shifts that forced the GICE elsewhere, then the change at Gadsden is older and ends earlier than usual. There is likely little time missing at the contact between the two formations based on the presence of Plectodina aculeata in the lower Nashville Formation suggesting that it is still within the P. undatus conodont zone, and the M4-M5 boundary is below the GICE in most interpretations. Preliminary oxygen isotope data suggest that there is not much difference in temperature between the Stones River Formation (19.7 – 20.3 ˚C) and the Nashville Formation (19.4-19.6 ˚C), and these temperatures are cooler than reported coeval midcontinent temperatures by about 2-4˚C.