NEW COMPLEXITIES IN THE EXPRESSION OF LATE ORDOVICIAN CARBON ISOTOPE EXCURSIONS STRADDLING THE TURINIAN-CHATFIELDIAN BOUNDARY INTERVAL

The mid-Caradoc (position of the newer Turinian-Chatfieldian stage boundary) carbon isotope excursion was proposed by Patzkowsky et al. (1997, Geology 25:911-914) to represent an initial cooling event that coincided with the onset of the greenhouse-icehouse transition in the Late Ordovician. Continuing studies of chemostratigraphic d¹³C profiles of marine strata of the Platteville and Decorah formations of Iowa, Illinois, and Minnesota clearly show a more complex structure of multiple positive carbon isotope excursions that were not detected in earlier published works. This complex excursion interval coincided in part with major paleoceanographic changes that resulted in cessation of carbonate accumulation and the development of a starved submarine surface across large areas of the North American epeiric sea. Particular attention is focused on carbonate facies of the Carimona and Castlewood beds (including the Deicke K-bentonite), which have been variously included in the uppermost Platteville or basal Decorah formations in different stratigraphic schemes. The Carimona-Castlewood interval captures a positive carbon isotope excursion (peak d¹³C to +1.5‰ PDB in east-central Iowa) that precedes a larger excursion that was previously recognized in strata of the stratigraphically-higher Guttenberg Member (peak d¹³C to +2.5‰ PDB near the position of the Elkport K-bentonite). Of note, the Carimona-Castlewood carbonate interval (with Deicke K-bentonite) as well as overlying portions of the Spechts Ferry Shale (with Millbrig K-bentonite) are missing across large areas of the Mississippi Valley area in parts of Illinois, Wisconsin, and Iowa (including the initial discovery site for the major excursion). The Carimona-Castlewood excursion interval thins from a maximum thickness of 2 to 3 m in Minnesota southeastward to a starved submarine hardground surface in northern Illinois. In Jackson Co., Iowa, this excursion interval is locally represented by only a few cm of thin stacked packstone tempestites separated by omission surfaces.