RAPID PROGRESSION FROM INTRACONTINENTAL VOLCANISM TO GLACIATION: EVIDENCE FROM NEOPROTEROZOIC STRATA IN THE BLUE RIDGE OF SW VA

The Snowball Earth hypothesis arose from the recognition that glacial sedimentary deposits dated in the range of 635-715 million years ago can be found around the globe. This time in geologic history is now understood to have been the most intense and widespread period of glaciation the Earth has yet experienced. Intriguingly, the Snowball Earth episodes were preceded by intense volcanism as the supercontinent of Rodinia began to break up, and were closely followed by the “Cambrian explosion” of multicellular life on Earth; therefore, there is much interest in determining the triggers for the glaciation and its subsequent effects on the climate and environment. Two major global glaciations have been well documented during the Neoproterozoic , the Sturtian (715-680 Ma) and the Marinoan (680-635 Ma) events. A third, older glacial period, the Kaigas glaciation, has been hypothesized to have also been a global event; however, the significance and even the existence of this event is disputed.

Surprisingly few recent studies have recognized the potential importance of the Neoproterozoic glacial deposits of eastern North America. In particular, the Konnarock Formation in southwest Virginia may have been deposited in response to global glacial events. The age of the Konnarock Formation is uncertain, only bracketed between the youngest rhyolites of the underlying Mount Rogers Formation at ~750 Ma, and overlying sandstones of the latest Neoproterozoic – earliest Cambrian Unicoi Formation. Recently we have focused on exploring the nature of the contact with the underlying volcanic rocks. Observations that suggest the KF conformably overlies the MRF are: 1) lack of fault fabrics along the contact between the KF and MRF, although this contact had been previously mapped as a fault throughout much of the region; 2) a local source of MRF volcanic lithic clasts in conglomerates in the KF; and 3) mafic dikes that cut both the KF and MRF. A conformable contact between the KF and MRF contact suggests that glaciation closely followed volcanism and intracontinental rifting in this region.