Environmental Reconstruction of Anoxic Marine Systems: Core Black Shales of Upper Pennsylvanian Midcontinent Cyclothems

Environmental analysis of offshore shale members of Upper Pennsylvanian Midcontinent cyclothems was undertaken using a combination of high-resolution X-radiography, petrography, and geochemical analyses. A hierarchical sampling strategy permitted investigation of (1) broad stratigraphic and geographic patterns of compositional variation through analysis of three shale members (Hushpuckney, Stark, and Muncie Creek) in five cores along a proximality gradient, and (2) fine stratigraphic variation within each study unit through continuous cm-scale sampling of dmthick compositional cycles. Offshore shales consist of (1) three major non-clastic components, i.e., organic carbon, Fe-sulfide, and phosphate, which can be effectively proxied by TC, TS, and P2O5, respectively, and (2) a clay-mineral matrix composed predominantly of illite and smectite (or mixedlayer I/S), the relative proportions of which can be proxied by Al2O3/SiO2 ratios. Spatio-temporal variation in sediment fluxes permitted identification of two clastic components: (1) a southernderived smectite-rich component associated with high concentrations of inertinite, and (2) a northern-derived illite-rich component associated with high concentrations of vitrinite. Strong