NEOPROTEROZOIC ALLUVIAL FAN SEDIMENTATION IN THE MOUNT ROGERS FORMATION, SW VIRGINA

The Neoproterozoic Mount Rogers Formation (MRF) in SW Virginia records volcanism and sedimentation during initial, aborted rifting of the Laurentian portion of Rodinia, ~760 Ma. Successful rifting and opening of Iapetus occurred some 200 million years later, and is evident from volcanic deposits that outcrop from central Virginia to Maritime Canada. The MRF, however, provides a rare look at tectonism, volcanism, and sediment deposition related to the earlier event. The upper MRF constitutes an eruptive center dominated by rhyolite lavas and ash-flow sheets. The lower MRF, which nonconformably overlies Mesoproterozoic basement, comprises a assemblage of bimodal volcanic rocks intermixed with arkosic conglomerate and sandstone. Although low-grade metamorphism and related deformation during Paleozoic orogenesis have partially obscured original stratigraphic and structural relationships, the lower MRF preserves evidence that the sedimentary protoliths represent alluvial fan deposits formed during synsedimentary rifting. Detailed mapping and outcrop studies show that the deposits conform to a number of criteria for alluvial fan deposits, including (but not limited to):

• Sediments deposited relatively close to their source area. The majority of clasts in the conglomerates derive from the lower MRF rhyolite and underlying basement granitoids.
• Poorly sorted, compositionally immature sediments, with poorly rounded clasts reflecting short distance of transport.
• Presence of inverse grading, as in lower parts of debris flow deposits.
• Major changes in lateral and vertical facies, and generally lenticular to wedge-shaped geometry.

The upper MRF is overlain by glaciogenic deposits of the Konnarock Fm., which have been related to the Neoproterozoic ‘Snowball Earth’ event. Although the lower MRF was deposited during an earlier part of the Cryogenian Period, to date we have not found evidence for glacial deposits in the study area.