Identification and characterization of the middle devonian portwood member of the new Albany shale in the Appalachian and Illinois basins

The Middle Devonian Portwood Member of the New Albany Shale largely consists of dark grey to black dolomitic shale intervals with prominent interbeds of tan weathering dolomite. It shows thin layers and laminae of dolosiltite, and displays soft sediment deformation at multiple scales. A recent exposure of rocks with Portwood characteristics located in the Illinois Basin has been examined at the shores of Barren Lake, Kentucky. Due to lack of rare correlative fossils in the Portwood, hand specimens and thin sections from Barren River Lake were compared with a collection of samples from the type area in central Kentucky. Lithologically the Barren River Lake samples are a close match with Portwood samples from central Kentucky and differ significantly with regard to carbonate content (higher), sedimentary features (thickness of dolosiltite beds, soft sediment deformation), and provenance (more plutonic quartz) from the Trousdale Member. Middle to Late Devonian black shales of the Appalachian and Illinois Basins contain prominent amounts of silt-sized quartz. Quartz is abundant in many mudstones and can be derived from terriginous clastic or authigenic sources. Detrital quartz was used to gauge provenance by scanning with an electron microscope (SEM) using backscatter (BSE) and cathodoluminescence (CL) techniques. Significant differences were observed in amounts of plutonic and high-grade metamorphic quartz vs. low-grade metamorphic quartz between samples from the Blocher (Trousdale equivalent) and the Portwood Members of the New Albany Shale. Both contain low-grade metamorphic quartz along with authigenic quartz; however, the Portwood contains elevated amounts of igneous quartz determined by using cathodoluminescence. Where described in east-central Kentucky, the Portwood shows high lateral variability in thickness, and consists of several erosion bounded depositional sequences. The Barren River Lake locality, and any Portwood equivalents west of the Cincinnati Arch, suggest that during the Givetian the Cincinnati Arch was not an active tectonic element. Previous interpretations of restricted coastal lagoons and estuaries in east-central Kentucky are reinterpreted that the Givetian Sea consisted of a broad expanse of mostly shallow water environments. Basement fault reactivation may have generated local highs and depressions, and along with eustatic sea level changes accounts for the pronounced lateral variability seen in the Portwood.