REASSESSMENT OF STRATIGRAPHY IN A NEOPROTEROZOIC INTRACONTINENTAL RIFT: THE VOLCANOGENIC MOUNT ROGERS FORMATION, SW VA

Early stages of breakup of the Rodinian supercontinent along the eastern Laurentian margin are preserved in the Blue Ridge of SW Virginia as ca. 760-750 Ma bimodal volcanic rocks and clastic sedimentary deposits of the Neoproterozoic Mount Rogers Formation (MRF). The stratigraphy and inferred eruptive sequence of the MRF were described in detail by Rankin (1993), who divided the complex into an “upper” section comprised of rhyolite lavas and ash-flow sheets, and a “lower” section consisting of bimodal basalt/rhyolite volcanics interlayered with sedimentary rocks. Based on field relationships, rocks of the lower MRF were inferred to be older. However, recent U-Pb zircon ages reported for rhyolites in the upper and lower MRF (Tollo et al., 2012) show that their ages largely overlap within the error bars of the analyses; therefore, the sequence of eruption is uncertain.

Coarse clastic sedimentary deposits in the lower MRF are dominated by arkosic sandstone and conglomerate. The poorly sorted, compositionally immature sediments, rapid changes in lateral and vertical facies, and generally lenticular geometry suggest deposition as alluvial fan deposits. Polymict conglomerates are dominated by boulder- to cobble-sized clasts of rhyolite and granitoids, assumed to be locally derived. Recent laser ablation ICP-MS Zircon U-Pb analyses of rhyolite clasts from MRF conglomerates indicate three dominant age populations: ~780-775 Ma, ~760 Ma, and ~753 Ma. The 760-753 Ma ages are consistent with ages of underlying and overlying rhyolites. Locally, the conglomerates contain clasts that are petrographically similar to rhyolites in the upper MRF. Recognition that the conglomerates must be younger than the overlying rocks requires reassessment of the stratigraphy and structural relationships within the MRF.