GLACIERS AND VOLCANISM: INVESTIGATING THE CONTACT BETWEEN THE NEOPROTEROZOIC KONNAROCK AND MOUNT ROGERS FORMATIONS, SW VA

The Neoproterozoic Konnarock Formation (KF) and Mount Rogers Formation (MRF) of southwest Virginia present a complex geologic setting in which the interactions of the two formations are not entirely understood. As the glaciogenic deposits of the KF have previously been suggested to have formed during the 717-635 Ma Sturtian “Snowball Earth" glaciation, determining the age of the KF and its relationship to the underlying MRF is of global geologic significance. The MRF is a bimodal-volcanic, rift-related relic of the Neoproterozoic Era that formed during initial rifting of the supercontinent Rodinia, ~760 -750 million years ago. Late-stage rhyolitic eruptions and basaltic dike intrusions may aid in bracketing the age of the KF. KF strata overlie the main MRF volcanic sequence along a contact that has been interpreted as either discontinuous (e.g., unconformable or faulted) or continuous (e.g., conformable) with the underlying rocks. Variations in interpretations stem from extensive weathering of the rocks, poor exposure between outcrops, and the intricate assortment of rock types at the contact, including laminated mudstone of the lower KF, arkose, conglomerate, rhyolite and rhyolitic tuff, and greenstone. Late-stage eruptions of rhyolite appear to have caused disruptions in the laminated mudstones of the lower KF, supporting the conformable interpretation. Disruptions include convolutely deformed bedding and “baking" of the mudstones. In this study, we seek to determine if these relationships provide evidence for concurrent glaciation and volcanism during the deposition of the lower KF mudstone, and to compare the nature of the younger rhyolitic flare-up to the earlier main sequence of MRF volcanism. Preliminary work for this project involved fieldwork in the Rocky Hollow area (Grayson County, VA) for detailed mapping of the MRF-KF contact and to collect samples of the different rock types associated with the contact. Our ongoing investigations include thin section petrography to describe all the rock types and obtaining whole-rock geochemistry of the late-stage rhyolite for comparison with the main MRF rhyolites.