The Middle Ordovician Fincastle conglomerate north of Roanoke, Virginia and its implications for Blue Ridge tectonism

Abstract

Clasts within the Fincastle conglomerate indicate that more than 15,000 ft of structural relief had developed between the axis of the Blue Ridge anticlinorium and the trough of the Salem synclinorium by Middle Ordovician time. The Fincastle conglomerate is restricted to the northeast depression of the Salem synclinorium, south of the Southern and Central Appalachian boundary. Evidence indicates the Fincastle conglomerate is a northeastern lithofacies of the Bays Formation. The Fincastle has an average thickness of 48 to 69 ft, a maximum thickness of 300 ft, and is absent to the southwest where it has fined to siltstone and shale. Each conglomerate zone may grade up into diamictite or graywacke which, in turn, grades upward into shale. Clasts of well-foliated basement gneiss, vein quartz, quartz pebble conglomerate, sandstone and carbonate clasts were derived from the complementary Blue Ridge anticlinorium and perhaps the Piedmont. Calcite-cemented sandstone, sandstone conglomerate and micritic, black limestone clasts were probably derived from within the basin. The deposit is characterized by subrounded to rounded, elongate clasts (up to 17 in long) in a graywacke matrix. The Fincastle conglomerate was deposited in broad, shallow, marine channels. Sedimentary features indicate it was resedimented from a stream environment during flooding or high river stages. The textural and compositional immaturity of the Fincastle sediments indicate they bypassed the nearshore marine environment. The Fincastle conglomerate indicates erosion of a local culmination on the Blue Ridge anticlinorium or a geanticline in Middle Ordovician time to the southeast in the vicinity of the Goose Creek window.