STRATIGRAPHIC AND STRUCTURAL CONSIDERATIONS IN THE NEOPROTEROZOIC LYNCHBURG GROUP AND ROCKFISH CONGLOMERATE, EASTERN BLUE RIDGE, CENTRAL VIRGINIA

Neoproterozoic metasedimentary rocks in the eastern Blue Ridge of Virginia form a thick siliciclastic sequence and provide key evidence for rifting in southeastern Laurentia. The stratigraphic nomenclature for these rocks involves a series of local names (i.e. Fauquier, Lynchburg, Moneta, Ashe, Alligator Back, Wills Ridge) that have been correlated along strike for hundreds of kilometers.

In central Virginia, the Lynchburg Group forms a 2 to 4 km thick sequence in the eastern Blue Ridge. Locally, the Rockfish Conglomerate is a distinctive unit at the base of the Lynchburg Group. The Rockfish Conglomerate is a cobble to boulder conglomerate interlayered with pebbly arkosic sandstone and is characterized by outsized clasts and rhythmically bedded strata. Outsized clasts of granitic basement are interpreted as dropstones associated with ice-rafting processes seaward of a tidewater glacier. New geologic mapping indicates that the Rockfish Conglomerate is a geographically restricted lenticular unit up to 200 m thick that was deposited in an erosional trough above the Mesoproterozoic granitoid basement complex. A finer-grained sequence of interlayered turbiditic arkosic sandstone, siltstone, and graphitic mudstone overlies the Rockfish Conglomerate. Gabbroic dikes and sills intrude the metasedimentary sequence.

Beds in the Rockfish Conglomerate and overlying Lynchburg Group strata are overturned and dip steeply to the NW. A greenschist-facies foliation is variably developed in the both basement and cover sequence. Strain analysis, at both the outcrop and thin-section scale, indicates modest to moderate strain in the Lynchburg Group (R_XZ= 1.3 to 2.6). Retrodeformation of the Rockfish Conglomerate, taking into account the strain preserved in these rocks, indicates the lenticular trough was more U-shaped prior to Paleozoic deformation.