NEOPROTEROZOIC TUFFS AND LAVAS IN THE WESTERN BLUE RIDGE OF SW VA-AN UPDATE
Host sedimentary sequences have been assigned to Chilhowee, Ashe and Wills Ridge Formations by previous USGS mappers. Preliminary geochemical studies indicate the metavolcanics in each thrust block show many similarities to CIMP volcanics from the Catoctin Formation in NOVA. Neoproterozoic volcanics in the Pilot and Sylvatus quads may have similar contact relationships to post glacial cap carbonate (marbles) as previously noted in the Fauquier Fm. Relating section locations to amount of transport on the Blue Ridge thrust southwest of Roanoke puts depocenters in the most southwestern Neoproterozoic basins a significant distance to the SE of the Blue Ridge. Facies range from proximal shelf to distal fan delta deposits. Eastern belts are “off the chart” with regard to Simpson’s Chilhowee facies diagrams. This may explain Simpson‛s conclusion that the Unicoi in far southwestern Virginia was deposited on attenuated continental crust. Differences in volcanic sequences support Rankin‛s thesis of diverse volcanic stratigraphy in separate rift basins NE and SW of Roanoke. This includes Rankin‛s hypothesis that major concentrations of felsic volcanic rocks are largely restricted to sites at major bends in Appalachian structural trends particularly one just southwest of Roanoke and one in the Grayson Highlands along the joint VA - NC - TN border.