STRATIGRAPHY AND STRUCTURAL GEOMETRY OF THE NEOPROTEROZOIC ROCKFISH CONGLOMERATE, CENTRAL VIRGINIA BLUE RIDGE

Neoproterozoic metasedimentary rocks in the central Appalachians provide key evidence for rifting in southeastern Laurentia and the subsequent opening of the Iapetus Ocean. The Neoproterozoic Lynchburg Group forms a thick rift sequence in the eastern Blue Ridge, Virginia. In central Virginia, the Rockfish Conglomerate is a distinctive unit at the base of the Lynchburg Group that some researchers posit to have a glaciogenic origin.

The Rockfish Conglomerate is a cobble to boulder conglomerate interlayered with pebbly arkosic sandstone and is collectively characterized by outsized clasts (up to 80 cm) 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 and was deposited in a trough that unconformably overlies the Mesoproterozoic granitoid basement complex. A finer-grained sequence of interlayered arkosic sandstone, siltstone, and graphitic mudstone overlies the Rockfish Conglomerate. Gabbroic dikes and sills intrude the metasedimentary sequence. The marine glaciogenic deposits preserved in the Rockfish Conglomerate may be related to the Marinoan glaciation (655-635 Ma).

Beds in the Rockfish Conglomerate and overlying Lynchburg Group strata are overturned and dip steeply to the NW. A greenschist facies foliation is well-developed in the both basement and cover sequence and is subparallel to bedding. Fabric analysis of coarse-grained clasts in the Rockfish Conglomerate yields mean aspect ratios of 1.5 to 2.5, significantly greater than aspect ratios in the sand-sized matrix, consistent with strain partitioning between the coarse-grained clasts and matrix. A set of cm-scale oblique reverse high-strain zones cut the penetrative foliation.