NEOPROTEROZOIC TUFFS AND LAVAS IN THE WESTERN BLUE RIDGE OF SW VA-AN UPDATE

A regional stratigraphic study accompanying USGS-VDMME STATEMAP I 81 corridor quadrangle mapping in SW VA sheds new light on rift to drift facies transitions noted in Neoproterozoic sedimentary and volcanic rocks exposed in Blue Ridge thrust blocks along the SE edge of the Appalachian Valley. New mapping and facies analysis have delineated greenschist meta-volcanic rocks in separate but most likely time equivalent volcanic sequences discernible just below the base of the Paleozoic rocks from central Virginia to Tennessee. These volcanic rocks include a bimodal pyroclastic suite in the lower 100 meters of the Neoproterozoic northeast of Roanoke. SW of Roanoke felsic pyroclastics overlie diamictite breccia in the lower 100 meters of section. Felsic metavolcanics are overlain by laterally persistent, basaltic flows several hundred meters above the base of the Neoproterozoic on the southwest side of a major fracture zone SW of Roanoke.

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.