Despite agreement among most workers concerning the Late Proterozoic to early Paleozoic age of Blue Ridge and Inner Piedmont metasandstones, the depositional setting of these thick (10’s of kilometers) and areally extensive siliciclastic units is often based on preconceptions concerning the timing and nature of Paleozoic orogenesis in the southern Appalachians. Data from southern Appalachian crystalline core samples provide new constraints on the timing of deposition, provenance, and basin architecture. Two contrasting tectonic settings, a Late Proterozoic rifted margin or an Ordovician accretionary wedge, are the favored models for deposition of the psammitic and pelitic material found throughout the crystalline core. New geochronologic and geochemical data, in addition to field relationships, favor deposition in one or more large rift basins between the Late Proterozoic breakup of Rodinia and Ordovician volcanic activity.

Detrital zircon ages and geochemical data indicate that Blue Ridge and western Inner Piedmont metasandstones were derived from similar Grenvillian felsic continental sources. Zircon cores are mostly ~1100-1250 Ma with either late Grenville (~1.0-1.1 Ga) or, less commonly, Paleozoic (~350 Ma) rim ages. The presence of Neoproterozoic as well as Grenvillian zircons in eastern Inner Piedmont samples reflects a unique influx of sediment associated with the closure of the ocean basin and docking of the Pan-African Carolina (Avalon) terrane. Additionally, Ordovician detrital zircons in samples from the central and eastern margin of the Inner Piedmont suggest deposition between the Taconic and Acadian orogenic events in a new Laurentian margin rift basin.