THE LATE NEOPROTEROZOIC COVER SEQUENCE IN THE EASTERN BLUE RIDGE, VIRGINIA: PROGRESS AND PROBLEMS IN OUR UNDERSTANDING OF IAPETAN RIFTING

Late Neoproterozoic metasedimentary cover rocks in the eastern Blue Ridge of Virginia form a 1 to >6 km thick package that records the breakup of Rodinia and opening of the Iapetus Ocean. However, the stratigraphic nomenclature associated with these rocks is complicated by a series of local names and tenuous along-strike correlations. Field relations with older A-type 750 to 700 Ma granitoids and detrital zircon data indicate that this sequence was deposited after 725 to 700 Ma, however the paucity of datable volcanic rocks in this sequence makes precise temporal control difficult.

In the eastern Blue Ridge of northern and central Virginia, the Lynchburg Group unconformably overlies the Mesoproterozoic basement complex and underlies the 570 to 550 Ma metabasalts of the Catoctin Formation. The Lynchburg Group includes both non-marine and marine units that were sourced from the nearby granitic basement complex. The base of the Lynchburg Group is commonly coarse-grained arkosic wacke interbedded with graphitic siltstone. The locally restricted Rockfish Conglomerate forms a distinctive unit that was deposited in an erosional trough at the base of the Lynchburg Group. Outsized clasts of granitic basement and rhythmically bedded strata are interpreted as dropstones associated with ice-rafting processes seaward of a tidewater glacier. The marine glaciogenic deposits preserved in the Rockfish Conglomerate may be related to either the Sturtian (~700 Ma) or the Marinoan glaciation (655-635 Ma). Previous workers have interpreted the strata at the top of the Lynchburg Group to correlate with the Swift Run Formation from the western Blue Ridge. In the western Blue Ridge alluvial rocks of the Swift Run Formation are interlayered with metabasalts of the overlying Catoctin Formation. In the eastern Blue Ridge the contact between the Lynchburg Group and Catoctin is an unconformity and distinct Swift Run lithologies are absent. The aggregate thickness of the Lynchburg Group requires significant vertical tectonism to create the needed accommodation space, and the entire Lynchburg Group may fill a pre-Iapetan aulacogen.