The Stratigraphic Relationship of the Catoctin, Unicoi and Harpers Formations Within a Fault-bounded Rift Basin in Central Virginia

Abstract

Sediments deposited within a rift basin that is now situated in the Blue Ridge region of central Virginia include spilitic metabasalt, which exhibit pillow structures, alluvial slump deposits, fluvial channel sands, deltaic siltstones and shales. The basin is bounded on its northwestern flank by granulites of the Pedlar Formation and by augen gneiss of the Ductile Deformation Zone (Bartholomew,1972) to the southeast. The sedimentary package rests unconformably upon the Pedlar Formation and underlies a thrust contact below the Ductile Deformation Zone. The ten separate facies that are contained within the basin illustrate the tectonic association between active faulting, volcanism, and the deposition of shallow water sediments. This belt of sediments is a linear collection of late Precambrian -- early Cambrian rift-basin facies. Vertical and horizontal gradational transitions are evident from field mapping.The sedimentary facies model implies that the depositional environments were contained within a basin of high vertical relief. The intertongued relationship of lenticular, discontinuous sediments causes stratigraphic problems. Lithological comparisons of the rocks within the basin are made to the Catoctin, Unicoi and Harpers Formations. However, the facies model proposed to explain the basin does not attempt to delineate formations. The sediments are to be considered as a distinct deposition of continental alluvial elastics and volcanics contained by the crystalline basement complexes on either side.