Identification of the Middle Devonian Portwood Member of the New Albany Shale in the Illinois Basin

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 lenses of dolosiltite, and displays soft sediment deformation at multiple scales. An exposure of rocks with Portwood characteristics has been examined in Barren River Lake/KY, a locality in the Illinois Basin. Due to a lack of index fossils in the Portwood, hand specimens and thin sections from Barren River Lake were compared petrographically 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 igneous quartz) from the overlying Trousdale/Blocher members.

Silt sized detrital quartz is a common constituent in Middle to Late Devonian black shales of the Appalachian and Illinois Basins, and the provenance of this quartz can be examined via scanned color cathodoluminescence (CL). Significant differences in CL characteristics were observed between samples from the Trousdale/Blocher interval and the Portwood Member of the New Albany shale. Both intervals contain metamorphic and igneous quartz, in addition to authigenic quartz cement. The Portwood, however, contains significantly more igneous quartz than is observed in the Blocher/Trousdale interval.

Where described in east-central KY, the Portwood shows high lateral variability in thickness, and consists of several erosion bounded depositional sequences. Its continuation to the Barren River Lake area and other areas west of the Cincinnati Arch suggests that during the Givetian the Cincinnati Arch was not an active tectonic element. Previous interpretations of restricted coastal lagoons and estuaries in east-central KY are reinterpreted to suggest 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 these, together with eustatic sea level changes probably account for the pronounced lateral variability seen in the Portwood.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California