INTRA-BASINAL PALEO-REDOX VARIABILITY: A CORE-BASED GEOCHEMICAL AND ISOTOPIC TRANSECT OF THE CLEVELAND SHALE ALONG THE WESTERN EDGE OF THE APPALACHIAN BASIN

Conditions that existed during accumulation of the Devonian Ohio Shale of the Appalachian Basin, and in particular the degree of anoxia, have been debated widely. In this study, a four-core transect that includes the Cleveland Member of the Ohio Shale (Famennian, Upper Devonian) is investigated using geochemical proxies (C, S, and trace-elements) to elucidate paleo-depositional environments. The transect extends from central Kentucky northward into central and northern Ohio, defining variability in the deposition and redox characteristics that existed in the Appalachian Basin during a unique episode in Earth history. Results indicate that for these very organic-carbon rich shales (which contain up to 15% organic carbon), redox conditions likely varied both spatially and temporally. The upper Cleveland contains more organic carbon than does the lower Cleveland, and the overall carbon content decreases toward the northernmost part of our study area (northern Ohio). Associated with variations in organic carbon are redox differences, in that conditions during accumulation of the lower part of the Cleveland Shale may have been slightly better oxygenated (dysoxic) compared with those that existed during accumulation of the upper parts of the unit (anoxic). This is demonstrated by vertical variations in DOPT, V/Cr, and Ni/Co. Redox conditions also vary from core to core, suggesting local fluctuations may have been important; data for the northernmost (or most proximal) core suggest the least reducing conditions. A full assessment of redox-sensitive trace element variability for each core will be presented and compared. Mo contents are generally high in these cores (averaging between 80 and 125 ppm for various cores). Some cores demonstrate a decrease in Mo content in the upper part of the Cleveland Shale whereas others do not, suggesting local control on Mo content rather than a global or basin-wide drawdown of seawater Mo levels during accumulation of the upper Cleveland.