DIFFERENTIATING THE UNDIFFERENTIATED: MAPPING COMPLEX NEOPROTEROZOIC VOLCANIC AND GLACIOGENIC STRATIGRAPHY IN THE MOUNT ROGERS AREA, SW VA

Evidence of Cryogenian glaciation is found around the world; however, the triggers responsible for the onset of such intense climate change are complex. A number of glacial episodes, including those of the Neoproterozoic, occur during or soon after major episodes of continental rifting. One such example occurred during initial rifting of Rodinia along the eastern margin of Laurentia (present-day coordinates), which began with intracontinental rifting approximately 780-750 Ma. Evidence for this event is found in rift-related volcanic and sedimentary deposits of the Mount Rogers Formation (MRF) in SW Virginia and NW North Carolina. The MRF, a series of rhyolitic lavas, pyroclastic deposits, basalts and boulder conglomerates, is overlain by glacial deposits of the Konnarock Formation (KF). Representing some of the best evidence of Neoproterozoic glaciation along this margin, the KF consists of maroon, rhythmically layered, mudstone containing dropstones, arkose, conglomerate and diamictite. The age of the KF is uncertain, only bracketed between the youngest dated rhyolite of the MRF (~750 Ma) and the overlying sandstone of the latest Neoproterozoic to Early Cambrian Unicoi Formation. If the age of the KF is more accurately bracketed, its relationship to the global “Snowball Earth” episodes [i.e. Sturtian (715-700 Ma) and the Marinoan (680-635 Ma)] may be determined.

The purpose of our study is to better document the stratigraphic relationships and age constraints between the MRF volcanics and the overlying glacial deposits in SW VA. This region was mapped in detail by Rankin (1993), although in recent years other workers have revised some of his initial structural and stratigraphic interpretations. On Rankin’s map, the area of our research is shown simply as undifferentiated Mount Rogers Formation. Thus far, our mapping of this area has revealed a transitional relationship between MRF conglomerate and KF diamictite, as well as apparent interlayering of rhyolite and KF deposits (as previously suggested by Merschat et al., 2014). This implies that volcanism was still occurring during the glaciation event. If so, the KF is closer in age to the MRF than previously assumed, and therefore represents either a pre-Sturtian global event or a more local or regional glaciation.