NEW FINDINGS REGARDING MAJOR ORDOVICIAN, SILURIAN, AND DEVONIAN UNCONFORMITIES IN THE EASTERN VALLEY AND RIDGE PROVINCE OF VIRGINIA, TENNESSEE, GEORGIA, AND ALABAMA

Our understanding of the regional relationships of several important unconformities owes much to the work of John Dennison and his students over the decades. Recent fieldwork in the eastern Valley and Ridge from Virginia to Alabama has added to this understanding. In exposures of Ordovician redbeds of the Bays Formation and its equivalents, there are unconformities beneath pebbly sandstones that might be thought to be correlative were it not for the presence of the mineralogically distinct Deicke and Millbrig K-bentonites (altered tephras) from Roanoke, Virginia to Gadsden, Alabama. These sandstones have unconformable relations with the underlying strata (redbeds in Alabama, Georgia, and Virginia, to non-red shallow shelf to deeper water shaly carbonates in Virginia). The sandstones variously occur above or below the K-bentonites, or they enclose the K-bentonites, suggesting a complex interplay of tectonics and eustasy.

Upsection, in exposures near Pumpkin Center, Tennessee, a 10-12 m thick white quartz arenite was mapped 60 years ago by Bob Neuman of the USGS as a Bays Formation sandstone; it herein is reinterpreted as an isolated exposure of the “upper” Tuscarora (Clinch) Sandstone based on the presence of a 2-4 m thick fossiliferous limestone that separates the underlying redbeds of the Bays and the white quartz arenite. Fossils in the limestone include brachiopods tentatively identified as Rafinesquina? sp. and thus later Ordovician; additional biostratigraphic work is planned. About 3.5 km NE both the fossiliferous limestone and white quartz arenite are missing, and the Devonian Chattanooga Shale unconformably overlies the Deicke K-bentonite, which itself overlies several hundred meters of Bays Formation redbeds.

This patchy distribution of Ordovician limestone and Silurian sandstone, and the value of K-bentonites in helping to work out the often complex stratigraphic relationships among the unconformities of this region, is not unlike the stratigraphic relations of Silurian and Devonian units that John Dennison worked out at a number of exposures in southwestern Virginia and elsewhere in this region, for example, the patchy distribution of the Devonian Oriskany Sandstone, and the importance of the Tioga, Belpre, and Center Hill K-bentonites in the Devonian Millboro Shale and its equivalents.