A proglacial origin for the upper Proterozoic Rockfish Conglomerate, central Virginia, U.S.A.

Abstract

The upper Proterozoic Rockfish Conglomerate is a lenticular unit of stratified pebbly sandstone and conglomerate as much as 500 m thick at the base of the Lynchburg Group in central Virginia. It has been interpreted as a deep-water resedimented deposit, but glacial influence is suggested by the presence of dropstones and coarse-grained rhythmites. The coarsest deposits in the Rockfish Conglomerate are confined to the lower 30 m and consist of stratified pebbly sandstones containing outsized clasts and lenses of cobble conglomerate. They are interpreted as subaqueous glacial outwash deposited on a delta front. Coarse-grained, rhythmically-bedded sandstones containing thin, laterally persistent pebbly horizons make up the bulk of the unit and probably accumulated by a combination of mass flow and ice-rafting processes seaward of the delta front. Overlying this rhythmite facies is a thin horizon of dropstone-bearing turbidites. The Rockfish Conglomerate may reflect local progradation of highland glaciers across the margin of the Lynchburg basin during the initial stages of late Proterozoic rifting. Recognition of glaciogenic rocks in the Lynchburg Group provides further evidence for regional glaciation in the southern Appalachians during late Proterozoic time. Glaciation in the southern Appalachian Blue Ridge may correlate with the worldwide Vendian glacial event, compatible with geochronologic data that indicate rifting between 690-570 Ma in the southern Appalachians.