Petrography and geochemistry of Pennsylvanian black shales in offshore and nearshore stratigraphic settings in Midcontinent and Illinois Basins

Although all Midcontinent Pennsylvanian black shales were once regarded as shallow water deposits, the widespread, phosphatic, conodont-rich black shales that lack benthic fossils and lie stratigraphically between marine transgressive and regressive limestones in a cyclothem (T-R sequence) are now recognized as offshore sediment-starved condensed-section deposits, which accumulated slowly in anoxic water below a pycnocline. In contrast, less widespread phosphate- and conodont-poor black shales that typically overlie coals (or other flooding surfaces), tend to grade vertically and laterally into shoreline/terrestrial deposits and often contain a sparse benthic fauna probably formed under conditions of organic overload in nearshore dysoxic environments. Recently, Coveney et al. (1991) suggested that some widespread, conodont- and Mo-rich black shales interpreted as offshore shales by Heckel (1977) were deposited rapidly in a shallow shoreline environment. This dissertation investigated several black shales of different stratigraphic settings in the Middle-Upper Pennsylvanian succession of midcontinent North America in order to characterize their petrography and geochemistry in an attempt to clarify the emerging depositional models for black shales and to constrain the possibilities as to what the geochemical evidence actually means. The black facies of offshore shales included in this study (from the Midcontinent and Illinois Basins) display lenticular lamination, with lenses composed of a fine “hash” of quartz crystals (that may be compacted radiolarians) within an organic-rich clay matrix containing fine quartz silt. The SEM microfabric of offshore shales has an overall preferred orientation of clay flakes. Geochemical analysis shows that offshore black shales are enriched in metals and have V/(V + Ni) values suggesting an anoxic-to-euxinic depositional environment. In contrast, nearshore shales display variable and often random SEM particle orientation, contain abundant plant fragments and coarser quartz grains, and have significantly lower metal concentrations and V/(V + Ni) values suggesting dysoxic depositional environments. Petrographically and geochemically, the much debated Mecca Quarry black shale fits the characteristics of offshore black shales in the Illinois Basin, and thus was probably deposited in an environment similar to that of other Illinois offshore shales, which is the more variable, shoreline-influenced (as opposed to shoreline-located), nearer-shore edge of the overall offshore black shale environment.