REDEFINING BOUNDARIES IN THE NEOPROTEROZOIC LOWER MOUNT ROGERS FORMATION THROUGH DETAILED MAPPING OF RHYOLITE CONTACTS

The Mount Rogers Formation (MRF) in SW Virginia records Neoproterozoic volcanism and sedimentation during intracontinental rifting of the eastern Laurentian margin of Rodinia at ~760-750 Ma, with the opening of the Iapetus Ocean occurring ~200 Ma later. The MRF nonconformably overlies Mesoproterozoic basement granitoids, formed during the Grenville orogeny. The MRF is divided into two distinct units; the upper MRF consists predominantly of rhyolites and ash flow sheets, while the lower MRF consists of interbedded bimodal volcanic rocks and clastic sedimentary alluvial fan deposits (arkose and conglomerate). Our previous 1:24,000-scale mapping has focused on differentiating the stratigraphy of the lower MRF, covering portions of four quadrangles in SW Virginia and North Carolina. The current project extends the previous mapping to the northeast. Abundant conglomerates around Grayson Highlands State Park become less evident to the northeast and rhyolites dominate as the outcrop belt of the lower MRF narrows into the Stone Mountain fault. This study focuses on the contacts between the upper and lower MRF, and the lower MRF and Mesoproterozoic basement. Evidence for deformation along the basement contact proves consistent with our previous interpretations of the basement-cover contact as a sheared unconformity. Between rhyolites of the lower and upper MRF we encountered a previously undescribed rhyolite, which we informally refer to as the Bearpen rhyolite. This rock consists of a light to dark maroon matrix with abundant K-feldspar phenocrysts and lesser quartz and plagioclase. K-feldspar phenocrysts form unique clusters, and flow banding is distinctive at the outcrop scale. This research will describe the contact relationships between the lower MRF and the Bearpen rhyolite, as well as characteristics of deformation along the basement-cover contact.