VOLCANOES TO GLACIERS: NEOPROTEROZOIC RIFT BASINS IN A PALEOZOIC OROGEN, SW VIRGINIA

The most extensive record of Neoproterozoic glaciation in the eastern U.S. is found in the Appalachian Blue Ridge of SW Virginia. Glaciolacustrine sedimentary rocks of the Konnarock Formation (KF) are commonly assumed to represent deposits of the “Snowball Earth” Sturtian glacial episode (~717-660 Ma), although the age range of the glacial deposits remains uncertain. The KF overlies bimodal volcanic rocks of the Mount Rogers Formation (MRF) that formed during volcanism and fault basin development during early stages of Rodinian intracratonic rifting. The youngest dated rhyolites in the MRF are ~ 749 Ma, although a small ‘outlier’ of rhyolite mapped within the KF is reported to yield an age of ~751 Ma. The minimum age of the KF is constrained by overlying rift-to-drift sedimentary rocks, which are likely younger than 563 Ma. 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 began as volcanism and intracontinental rifting continued in this region. We have mapped the KF in contact with two distinct volcanogenic bodies – one of flow-banded rhyolite resembling the ~753 Ma Whitetop member of the MRF, the other consisting of porphyritic rhyolite, tuff, and volcaniclastic sandstone or conglomerate, intruded by basaltic and rhyolitic dikes. Basal rhythmites of the KF are also intruded by mafic dikes, and in one key location, by a hypabyssal felsic dike. Based on field relationships, we hypothesize that the latter volcanic sequence and initial KF deposits were deposited on an uneven rift-basin topography, possibly against the Whitetop Rhyolite uplifted in the footwall of basin-bounding faults.