STRUCTURAL AND STRATIGRAPHIC ANALYSIS OF THE BASEMENT-COVER UNCONFORMITY AND OVERLYING LYNCHBURG GROUP, EASTERN BLUE RIDGE PROVINCE, CENTRAL VIRGINIA

The late Neoproterozoic metasedimentary Lynchburg Group in the eastern Blue Ridge geologic province crops out from northern to southern Virginia and records the pre-Iapetan rifting of Rodinia. The Lynchburg Group unconformably overlies both the Mesoproterozoic basement complex and a suite of Cryogenian A-type granites from which much of its material is derived. Based on data collected from Culpeper County and the Rockfish River further south, previous workers have constructed a widely accepted stratigraphic column. From the bottom this consists of the geographically restricted basal Rockfish Conglomerate in the south and Bunker Hill and Monumental Mills Formations in the north. These formations are overlain by the laterally continuous Thorofare Mountain, Ball Mountain, and Charlottesville Formations. The along strike relationships of these upper formations however are tenuous at best as the area between the original field locations has been the subject of very little work.

Through the mapping of field data from southern Albemarle and Nelson Counties the contact between the Lynchburg Group and the underlying basement granites has been found to rise over a kilometer through the stratigraphic sequence. Ten kilometers north of the Rockfish River area the basement complex comes in contact with the Ball Mountain Formation as opposed to the Thorofare Mountain Formation as had been originally hypothesized. The Thorofare Mountain Formation pinches out along strike and is geographically isolated from its type localities in Culpeper. With this data a new stratigraphic column can be made in which the Thorofare Mountain Formation is divided into northern and southern sections, each within a distinct basin defined by different stratigraphic sequences.