BRACKETING THE AGE OF NEOPROTEROZOIC GLACIATION IN THE VIRGINIA BLUE RIDGE

The most intense period of glaciation in Earth’s history occurred during the Cryogenian Period of the Proterozoic Eon. Multiple glaciation events occurred during the Cryogenian, significantly the Sturtian (717-660 Ma) and the Marinoan (~635 Ma) glaciations. It has been suggested that during these extreme events even the oceans were frozen, leading to the term “Snowball Earth.” The Konnarock Formation (KF), in the Appalachian Blue Ridge of SW VA, is composed of glacial sedimentary rocks deposited during the Cryogenian Period. An exact age for the KF is unknown, but may be approximated by bracketing the age between the underlying rift-related felsic volcanic rocks of the Mount Rogers Formation (MRF) (760-750 Ma) and overlying rift-to-drift strata of the Chilhowie Group (latest Neoproterozoic to Cambrian). Rankin (1993) interpreted the basal contact of the KF as unconformable, although mapped it as faulted against the MRF throughout most of its map area. Yet, Merschat et al. (2014) recognized rhyolite interlayered with the KF, which leads to the possibility that the contact is conformable with the underlying MRF. The nature of the KF-MRF contact is significant in determining whether the glacial deposits represent an older regional event or were formed during a global “Snowball Earth” episode.

We are currently mapping the KF-MRF contact in detail to investigate whether the contact is conformable, unconformable or faulted. To date our work has been focused in an area that Rankin (1993) interpreted as faulted. In a number of traverses across the contact we have observed a seemingly consistent stratigraphic sequence, from underlying rhyolite and volcanic breccia of the MRF, into mudstone, pebbly arkose containing rhyolitic clasts, and diamictite of the KF. Fault fabrics are not evident on the outcrop and hand sample scale. Thus far, our observations call into question the interpretation of a faulted contact in this area, and suggest the contact may be conformable.