EOLIAN INFLUENCE IN QUARTZ ARENITES AND CONGLOMERATES OF THE ORDOVICIAN BLOUNT MOLASSE IN THE APPALACHIAN BASIN, VIRGINIA TO ALABAMA

Compositionally mature, medium to coarse-grained quartz arenites, and granule to pebble conglomerates of the Ordovician Bays Formation and related units of the early Taconian (Sandbian to Katian; ~458-452 Ma) Blount molasse of the southern Valley and Ridge province, contain sedimentary structures (planar bedding, tabular and trough crossbedding, current and adhesion ripples, shelly debris in pebble lags with normal and reverse grading, slump structures on oversteepened ripple crests and troughs, channel structures, Skolithos) that indicate accumulation in nearshore, beach, and coastal plain settings. These sands and gravels were delivered to Laurentian coastal areas via braided stream and fan delta complexes that prograded over the now nearly filled southern Taconic foredeep. Bimodal textures in several samples point to eolian reworking of these sands in the final or near-final depositional environment, but a lack of obvious dunes combined with the total absence of stabilizing vegetation in these pre-Silurian sediments suggests that the sand may have been blowing about on very broad beaches and associated sandy tidal flats. The larger grains are polymictic and well rounded, the smaller grains are typically subrounded to subangular monocrystalline quartz, and a few samples contain shelly laminae. The bimodality suggests eolian sorting prior to final deposition, which for the sands with shelly debris was likely as strand lines on a beach or sand flat.

The principal quartzose units in the Blount molasse in which bimodality is present are (1) the Walker Mountain Sandstone (western and southwestern Virginia); (2) the “middle sandstone member” and unnamed thinner units (northeast Tennessee); (3) unnamed conglomerates (Georgia and Tennessee), and (4) the Colvin Mountain Sandstone (Alabama). Individual beds are < 1 m to > 20 m thick, and total thickness of arenites is from < 1 m to > 50 m. Recurring pebble types (vermicular chlorite in vein quartz, specular hematite inclusions in vein quartz, fibrous chalcedony, jasper, silicified ooids) together with sandstone detrital modes imply a source terrane that included appreciable thicknesses of older sedimentary rocks and lesser areas where hydrothermal deposits and pegmatites, schists, and plagioclase-bearing igneous or gneissic rocks were exposed and being eroded.