Shearing and Kolking Phenomena in Fluvial Sediments Old Crow, Yukon Territory, Canada

Abstract

ABSTRACT Features attributed to shearing and kolking are exposed in fluvial sediments of the Old Crow River. Folded laminations attributed to fluid shear occur in the upper portion of an 80 cm thick coset of predominantly type B ripple-drift cross-laminated silts. Neither liquefaction nor upward porewater seepage are considered to have been necessary prerequisites for folding. Rather, rapid sedimentation produced sediments weak enough to be folded by fluid shear. Kolking features consist of angular, undercut erosional scours in silty sand and laminated organic silty sands. A kolk is a form of macroturbulence consisting of an intermittent vortex capable of entraining debris which ordinary tractive and hydraulic lifting forces cannot entrain. Kolks have been associated with a steep energy gradient, a low suspended sediment concentration, and dunes. Kolks inferred here probably were not associated with dunes. A steep energy gradient is unlikely unless the kolks were associated with a sudden flood surge caused perhaps by the release of an ice jam.