THE NEOPROTEROZOIC-MESOPROTEROZOIC UNCONFORMITIES IN THE VIRGINIA BLUE RIDGE

A vast unconformity separates the Mesoproterozoic basement complex from the overlying Neoproterozoic cover rocks in the Virginia Blue Ridge. Traditionally, this unconformity has been interpreted as the herald of Iapetan rifting, with many of the overlying cover units inferred to be correlative. We now recognize two temporally distinct unconformities between Mesoproterozoic and Neoproterozoic rocks in the Blue Ridge. The older, 680 to 720 Ma, unconformity occurs at the base of the Lynchburg, Fauquier, and Mechum River sequences. Overlying units include both non-marine and marine deposits that form thick arkosic sequences (1-5 km) derived from local sources. The Mechum River and Lynchburg units are nearly identical, however sediment transport indicators reveal that they formed in different depocenters separated by a basement high. The younger unconformity occurs at the contact between Mesoproterozoic basement and the overlying Swift Run, Catoctin, and Chilhowee sequences. The siliciclastic Swift Run Formation formed contemporaneously with the extrusion of mafic lavas in the Catoctin Formation between 560 and 570 Ma. Locally, these units accumulated in half-grabens bounded by master faults to the west. Some of the basin-bounding structures may have developed as low-angle detachment faults. This unconformity stepped to the west as these tectonic basins were filled. The older unconformity formed in response to an episode of continental extension and magmatism that included significant vertical tectonics in the Blue Ridge. This event occurred at least 100 million years prior to Iapetan rifting and may be related to far-field changes in Laurentian plate kinematics. Uplift and erosion prior to the formation of the latest Neoproterozoic rocks likely removed much of the older Neoproterozoic sequence. The younger Iapetan unconformity may have developed in response to the passing of a thermal bulge at the onset of normal faulting and then progressively migrated westward as the uplifted rift shoulder retreated.