THE RAZOR RIDGE VOLCANIC CENTER AND IMPLICATIONS FOR THE MOUNT ROGERS AND KONNAROCK FORMATIONS

The Neoproterozoic Mount Rogers (MR) and Konnarock Formations (KF) comprise a succession of clastic, volcanic and glaciogenic rocks that extends 60-km along-strike in the western Blue Ridge of NC-TN­-VA from Mountain City, TN to Speedwell, VA. The rocks nonconformably overlie Mesoproterozoic basement, and are overlain by the lower Cambrian Chilhowee Group. The ~760 Ma MR is up to ~3 km thick and separated into a lower clastic unit with minor basalt and an upper volcanic section comprised of several peralkaline, high-silica (>75% SiO₂), porphyritic rhyolites. The KF, interpreted to overlie the MR, consists of maroon laminated mudstone locally with dropstones, arkoses, and polylithic diamictites. A glaciolacustrine environment has been proposed for the KF.

New mapping of the northeasternmost extent of the MR and KF, the Razor Ridge volcanic center east of Troutdale, VA, reveals a continuous section of the MR and KF and several key stratigraphic relations. Stratigraphy of the lower MR is variable and consists of coarse-grained arkose, conglomerate and shale resting nonconformably on Mesoproterozoic rocks. Locally, amygdaloidal basalt occurs at the base of the MR. Rhyolites of the Razor Ridge are uniformly light grayish purple and porphyritic with alkali feldspar, quartz and plagioclase phenocrysts; zones of abundant fiamme, and volcaniclastic boulder conglomerate are locally present. Maroon laminated mudstone and lesser boulder conglomerate of the KF stratigraphically overlie MR rhyolites. A rhyolite sheet from the Razor Ridge volcanic center extends into the middle part of the KF and indicates the KF was locally deposited contemporaneous with MR at ~760 Ma. West of Troutdale, volcaniclastic rocks of the lower MR grade into the KF. The top of the KF is disconformably overlain by conglomerates of the Unicoi Formation.

Interbedded MR rhyolite with KF indicates that glaciation was contemporaneous with ~760 Ma volcanic activity and extension. Additional Neoproterozoic rift-related clastic rocks of the Grandfather Mountain (NC), Meechum River and Swift Run (VA) Formations, and A-type plutonic rocks of the bimodal Crossnore-Robertson River Igneous Suite (750-680 Ma) suggest a ~600 km rift basin comparable in scale to the East African rift, superposed with areas of significant rhyolitic volcanism.