WHAT IS THE ORIGIN OF THE MIDDLE ORDOVICIAN SEVIER BASIN, SOUTHERN APPALACHIANS?

The southern Appalachian Middle Ordovician Sevier basin has been considered a foreland basin for many decades, but instead it appears to be a cannibalized, complex foreland basin. Conglomerates within the Sevier Shale are dominantly composed of limestone clasts with lesser amounts of sandstone, dolostone, and basement clasts. These clasts were derived from Precambrian through Ordovician, rifted margin and platform strata from the east, and are devoid of volcanic material that should be expected to be derived from the island arcs that were being accreted to the east. This may indicate that the basin was not juxtaposed against the orogenic core, which zircon data also indicate, or that there was a barrier to foreland deposition. The lack of an extensive deep-water (black shale) facies in the Sevier basin suggests it was recycled by a propagating foreland fold-and-thrust belt or preserved farther east under the Blue Ridge – Piedmont (BRP) thrust sheet. Regional seismic reflection data do not indicate preservation of Sevier Shale beneath the BRP. The large volume of calcareous material and the preserved thickness of Sevier Shale (1,981 m) indicate only the western portion of the basin is preserved because the composition and high maturity of the conglomerates are inconsistent with a slowly subsiding basin that did not drop below the carbonate compensation depth.

We suggest loading from obducting arcs created a westward propagating foreland fold-and-thrust belt that broke the platform. In our model, platform sedimentary rocks and basement in earlier foreland basins are cannibalized and recycled. These would have sourced the Middle Ordovician conglomerates, while creating a topographic barrier that prevented synorogenic-derived volcanic clasts and Taconic-age zircons from reaching the foreland basin.