INITIAL RIFTING OF THE RODINIAN SUPERCONTINENT: PRELIMINARY EVIDENCE FROM THE LOWER MOUNT ROGERS FORMATION, SOUTHWESTERN VIRGINIA

The Mount Rogers Formation (MRF) in southwestern Virginia records volcanism and sedimentation during initial, aborted rifting of the Laurentian portion of Rodinia, ~760 million years ago. 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, appears to contain a rare vestige of surface deposits related to the earlier event. The upper MRF constitutes an eruptive center dominated by rhyolite lavas and ash-flow sheets, and U-Pb zircon dating of the middle rhyolite member has yielded ages of ~760 Ma. The lower, clastic-dominated MRF has received less attention, but we have recently begun a program of study to differentiate this unit and reconstruct its ancient environment. Herein we present a preliminary assessment based on initial field mapping and petrographic analysis.

The >760 Ma lower Mount Rogers Formation contains a sequence of intermixed clastic sedimentary and bimodal volcanic rocks that sits nonconformably on Mesoproterozoic (Grenville) crystalline basement. Despite chlorite-grade metamorphism and locally intense foliation development, identification of protoliths is generally straightforward. In the immediate study area, a distinctive maroon porphyritic rhyolite (Fees Rhyolite Member) occurs at or near the base of the lower MRF. The rhyolite is stratigraphically overlain by feldspathic sandstone and pyritic quartzite, followed by vesicular basalt and plagioclase-phyric basalt porphyry with “turkey-track” texture. The sequence is capped by polymict conglomerate intermixed with feldspathic to lithic wacke. The conglomerate ranges from grain- to matrix-supported, and commonly contains cobble-sized to boulder-sized clasts. Framework grain composition is dominated by basement granitoids, porphyritic rhyolite, and basalt, all of which appear to be locally derived from the underlying rocks, with less abundant sandstone, vein quartz, and epidosite. The conglomerate presently dips underneath the stratigraphically lower rocks, indicating that the sequence is overturned. Map distribution of facies suggests that periods of volcanic activity alternated with uplift, erosion, and alluvial fan deposition, probably related to rift-related faulting.