AN OCCURRENCE OF THE MIDDLE DEVONIAN PORTWOOD MEMBER OF THE NEW ALBANY SHALE IN THE ILLINOIS BASIN

The Middle Devonian Portwood Member is largely a dark grey to black dolomitic shale interval with prominent interbeds of tan weathering dolomite. It shows thin layers and lenses of dolosiltite, and displays soft sediment deformation at multiple scales. The Portwood has been described from outcrops in central Kentucky and, based on fossil content, appears to be correlative with the Givetian Tully Limestone in New York. In central Kentucky, the Portwood overlies the Middle Devonian Boyle Dolomite and is overlain by the Givetian to Frasnian Trousdale Shale member.

Whereas the Portwood sensu stricto is only known from exposures in the Appalachian Basin, a new exposure of rocks with Portwood characteristics has been examined at the shores of Barren River Lake/KY, located in the Illinois Basin. Because biostratigraphically relevant fossils are rare 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) and sedimentary features (thickness of dolosiltite beds, soft sediment deformation) from the Trousdale member. The latter is the only other dolomitic black shale interval of Devonian age in the region.

Where described in central Kentucky, the Portwood shows high lateral variability in thickness, and consists of several erosion-bounded depositional sequences. In that context, finding likely Portwood equivalents west of the Cincinnati Arch suggests that during the Givetian the Cincinnati Arch was not an active tectonic element, and that the Givetian sea consisted of a broad expanse of mostly shallow water environments. Basement faulting may have generated and reactivated local highs and depressions, and in concert with eustatic sea level changes it may have given rise to the pronounced lateral variability seen in the Portwood.