3 - The Neoproterozoic Konnarock Formation, southwestern Virginia, USA: glaciolacustrine facies in a continental rift

Summary

Abstract

The Konnarock Formation (new name, Rankin, 1993) comprises about 1100 m of diamictite, sandstone and argillite and crops out within a limited area in the Blue Ridge thrust belt of southwestern Virginia, USA. The formation overlies thick, bimodal volcanic rocks, approximately 760 Ma, with interbedded sandstone and conglomerate (Mount Rogers Formation; Rankin, 1993), or orthogneisses about 1 Ga. It underlies areally extensive Lower Cambrian sandstones and shales. The Konnarock Formation was deposited on the North American craton, most likely during an aborted phase of rifting which preceded continental separation and formation of the Iapetus Ocean.

Argillite, including coarse and fine laminites and massive mudstone, occurs at the base of the formation and may be up to about 650 m thick. This sequence records seasonal sedimentation in a thermally stratified, ice-contact lake. Fine laminites (varvites) were largely deposited from overflows and interflows. Turbidity currents deposited coarser laminae and graded beds. Dropstones and till pellets were dropped from floating ice. Massive and graded sandstone and massive or bedded diamictite are interbedded with the argillite and become more abundant higher in the section. They were deposited in the lake by turbidity currents, density-modified grain flows, and debris flows. Liquefied flows resedimented mixed sandstone and argillite units, and post-depositional liquefaction modified some beds.