COARSE EPICLASTIC AND VOLCANICLASTIC METASEDIMENTARY ROCKS DEPOSITED ALONG THE LATE PROTEROZOIC/MIDDLE PROTEROZOIC BOUNDARY IN SW VA

STATEMAP Program detailed mapping along the Blue Ridge from James River in Amherst Co. to Floyd Co. in SW VA indicates that the boundary between Late and Middle Proterozoic rocks is both an unconformity and a major zone of tectonic detachment. Coarse epiclastic and volcaniclastic deposits along the boundary are largely relics of the original depositional geometry which have been preserved through periods of tectonic inversion and southwesterly increasing thrust displacement along the boundary in the Eastern Blue Ridge Sequence.

Relict conglomeratic beds persist along the boundary between Late and Middle Proterozoic rocks as far southwest as Smith Mountain Lake. In the lake area, conglomerate beds occur at the base of the Moneta Formation, a bimodal metavolcanic-volcaniclastic sequence at the base of the Lynchburg Group. The map pattern of the Moneta shows a cross section of a tilted “steer's head” rift with the Moneta volcanics concentrated in the deeper part of the rift flanked along strike of the boundary to the NE and SW by lithic conglomerate beds in the Ashe Formation(Conley 1985).

The Ashe Fm. is overlain by Alligator Back Fm. SW of Smith Mountain Lake. Metavolcanic and metasedimentary rocks in the Alligator Back closely resemble distal marine clastics and submarine volcanics of the Oronoko Belt of the Western Blue Ridge sequence (Catoctin Fm). The Alligator Back contains several stratified submarine volcanic complexes in the Boones Mill area.

The Wills Ridge Formation (Rankin, 1993) is the cover sequence on the Gossan Lead thrust sheet traced from near Boones Mill SW across the Blue Ridge escarpment. It contains metaclastic rocks that Rankin has correlated with the Lynchburg Group. On the Boones Mill quadrangle, NE of the Blue Ridge escarpment, the Wills Ridge has a basal conglomerate section full of Laurentian basement gneiss blocks and felsic metavolcanics. This “Swift Run-like” section is absent on the plateau where clast-supported conglomerates are noted as “uncommon “ in the Sylvatus area by Whisonant and Tso and also by Rankin near Mount Rogers. Units structurally overlying the Wills Ridge in far southwestern Virginia include a large allochthon of deepwater metaclastics, amphibolites and ultramafic schists associated with the “Ashe Metamorphic Suite”, part of a suspect terrane in North Carolina.