PETROLOGY AND SIGNIFICANCE OF QUARTZ ARENITES AND CONGLOMERATES IN THE ORDOVICIAN BLOUNT MOLASSE FROM VIRGINIA TO ALABAMA, SOUTHERN APPALACHIANS

Compositionally mature quartz arenites and granule conglomerates in the Bays Formation, the main unit of the early Taconian (~460 - 450 Ma) Blount molasse of the southern Valley and Ridge province, are the Walker Mountain Sandstone (Virginia), the “middle sandstone member” and unnamed thinner units (Tennessee), and the Colvin Mountain Sandstone and unnamed conglomerates (Georgia and Alabama). 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) point to accumulation in nearshore, beach, and coastal plain settings. The coarse sands and gravels came to the coastal region via braided stream and fan delta complexes, and bimodal textures indicate some eolian reworking prior to sealevel rise and final deposition. 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 and specular hematite inclusions in vein quartz; fibrous chalcedony, jasper, and silicified ooids) and sandstone detrital modes imply a source of older sedimentary rocks, hydrothermal deposits and pegmatites, schists, and plagioclase-bearing granitic or gneissic rocks. Altered polycrystalline quartz grains suggest extended weathering during storage as alluvium prior to remobilization and final deposition. Lithologic similarity suggests contemporaneity but the units are diachronous. Temporal and areal shifts in the depocenters of gravel progradation into the Blount foredeep are evident because the Deicke and Millbrig K-bentonites are older than the unnamed thin granule conglomerate along Horn, Rocky Face, and Hamilton Mountains near Dalton, Georgia, contemporaneous with the Colvin Mountain Sandstone, and younger than the Walker Mountain and “middle” sandstones. So, the sandstones do not overlie the same unconformity, but different and local disconformities that formed in response to tectonic activity along the basin margin. Like most of the Silurian and Devonian basin-margin unconformities in this region, these do not extend across the full width of the associated foredeep, so their significance as sequence boundaries in the basin is questionable.