Upper Proterozoic Glaciogenic Rift-Valley Sedimentation: Upper Mount Rogers Formation, Southwestern Virginia
Julia M. G. Miller
Glaciogenic sediments are common as graben fills in upper Proterozoic sequences of North America. One example is the upper Mount Rogers Formation (MRF) in southwestern Virginia, which accumulated in a rift valley prior to formation of the proto-Atlantic Ocean. The upper MRF is up to 1,200 m thick and crops out on two thrust sheets. It generally overlies about 1,500 m of rhyolitic and basaltic volcanic rocks with some sandstone and conglomerate (lower and middle MRF). It is overlain disconformably by areally extensive siliciclastic rocks, the Chilhowee Group, which probably record miogeoclinal conditions.
Trough cross-bedded conglomerates with interbedded rhyolite flows probably underlie the upper MRF in the northeast of the outcrop area; these record alluvial fan sedimentation and contemporaneous volcanism. Elsewhere, initial upper MRF sediments are thick argillites with interbeds of arkosic arenite and wacke and rare diamictite. Normal grading, disturbed bedding, convolute lamination, and abundant argillite rip-up clasts imply deposition by turbidity currents and mass flow. The argillite is commonly well laminated: silt to clay, varvelike laminae, 1 mm to 3 cm thick, suggest glaciolacustrine deposition. Dropstones must have formed in ice-contact lakes. Predominantly structureless diamictite, 0-400 m thick, caps the sequence in most places, and thins toward the northeast. It is interp eted as lodgment till that indicates glacial advance in the south. Glaciogenic sediments such as these may have been common graben-fill sediments during certain periods of geologic time.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.