Depositional environments during a glaciolacustrine to marine transition in the Richardson and Rae River basin, N.W.T.

Abstract

This paper describes the principal glaciolacustrine deposits of a sequence of overlapping ice-marginal lakes known as glacial Lake Richardson and its transition to a postglacial sea following the retreat of a lobe of the Laurentide ice sheet from the Richardson and Rae River basin. The glaciolacustrine to marine transition is characterized by an upward-fining sequence of facies deposited in proglacial lacustrine environments, succeeded by an upward-coarsening trend corresponding to a regressive marine sequence. A composite stratigraphic section includes (a) basal diamicton, (b) stratified subaqueous sand and gravel, (c) glaciolacustrine silty clay rhythmites, (d) massive marine silty clay, and (e) littoral marine sands. The sudden passage from lacustrine to marine environments is reflected in the abrupt change in depositional mechanisms. Turbidity currents prevailed in the proglacial lake as inferred from thick sequences of varve-like couplets, whereas the salinity of marine waters, impeding the sedimentation of fines by density underflows, led to the deposition of massive structureless marine silty clays by flocculation.