ALTERNATIVE TECTONIC MODEL FOR THE DEVELOPMENT OF THE SOUTHERN APPALACHIAN BLOUNTIAN FORELAND CLASTIC WEDGE

Deposition in the Blountian foreland basin (BFB) (AL to VA) began 466-464 Ma and diachronously drowned the Cambrian-Ordovician carbonate shelf. The BFB was initially filled with black shales grading upward into turbidites and submarine fans prograding from nearby SE sources. The BFB is conventionally interpreted as a peripheral basin in a foredeep or back-bulge setting resulting from exotic arc obduction (A-type subduction) atop the Laurentian margin. In contrast, we suggest that the BFB resulted from the opposite subduction polarity (B-type) and is a retroarc basin associated with more proximal back-arc basins formed atop the outer Laurentian margin. This interpretation is based on the following constraints. First, BFB detritus was sourced only from uplifted Laurentian strata with no evidence for volcanic or deep-water sedimentary sources. Second, there is no evidence that an Ordovician deformation front ultimately advanced beneath, into or onto the BFB, e.g., no transported Ordovician allochthons or known Ordovician thrusts along the SE BFB margin over a strike distance of >500 km, in dramatic contrast with Taconic basins in the central and N. Appalachians formed during A-type subduction. Third, the BFB formed cratonward of a large Early-Middle Ordovician (480-460 Ma) volcanic/sedimentary extensional back-arc basin preserved in the eastern Blue Ridge and Inner Piedmont which developed on the seaward edge of the Laurentian plate. Fourth, K-bentonites within the BFB are temporally and geochemically similar to large explosive felsic ash flow sheets and hypabyssal intrusives in the eastern Blue Ridge. Fifth, thrust sheets restoring outboard of the BFB (Talladega belt) show no evidence for significant uplift, metamorphism, deformation or erosional breaching during development of the BFB. Reactivation and inversion of earlier, rift-related marginal and intraplate basement faults in the middle and proximal foreland during Middle Ordovician back-arc extension could easily provide marginal uplifts resulting in subsidence and filling of the BFB. Palinspastic reconstruction of the Middle Ordovician margin indicates that the “Blountian highland” source area lay to the NW of the palinspastic positions of the easternmost Alleghanian foreland thrust sheets and western Blue Ridge-Talladega belt allochthon.