A composite Llandovery δ13Ccarb record from the Michigan Basin, USA

Abstract

Abstract A new high-resolution, composite δ13Ccarb curve for the Michigan Basin, USA, was constructed using two stratigraphically overlapping subsurface cores that span the complete Llandovery interval. The Llandovery succession of the Michigan Basin measures between 140 m and 265 m thick and is represented by three lithostratigraphic groups: the Cataract (Manitoulin formation and Cabot Head Shale), the Burnt Bluff (Lime Island, Byron, and Hendricks formations), and the Manistique (Schoolcraft and Cordell formations). The composite δ13Ccarb curve exhibits five positive carbon isotope excursions (CIEs). Conodont and 87Sr/86Sr data suggest that the CIEs correlate with the Hirnantian, Early Aeronian, Late Aeronian, Valgu, and Ireviken global excursions. The new composite δ13Ccarb curve from the Michigan Basin represents the most stratigraphically complete, single-basin Llandovery record worldwide. The composite δ13Ccarb curve permits the ages of three Llandovery lithostratigraphic groups to be chronostratigraphically determined within ca. 1 Ma resolution. The composite δ13Ccarb curve also implies that the Ordovician–Silurian boundary in the Michigan Basin is located at the top of the Manitoulin formation and not at the base of the Manitoulin formation, which is consistent with recent findings from Manitoulin Island, Ontario. From the regional perspective, the high-resolution composite δ13Ccarb curve provides a valuable tool for delineating the depositional history of the Michigan Basin during the Llandovery. From a global perspective, the new composite δ13Ccarb curve sheds light on the dynamics of the global carbon cycle, particularly during the Rhuddanian, which is rarely preserved in other carbonate basins worldwide.