ABSTRACT: Long, dendritic, water-terminating esker systems: their implications for Laurentide deglacial meltwater drainage and ice dynamics

CGRG Bibliography of Canadian Geomorphology
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Abstract

Systematic analyzes of Laurentide eskers provide logical constraints on (i) the environmental conditions required for esker formation and preservation, (ii) the extent of the deglacialmeltwater drainage system and (iii) the style of deglaciation. Despite the fact that some Laurentide eskers are nearly unbroken for several hundred kilometers, most are asserted to be segmental and to have formed time-transgressively at the ice margin in short R-channels as the ice retreated. By contrast, continuity can be inferred from regional geomorphic andsedimentologic trends for long, dendritic esker systems that terminated in standing water. Regional trends in ridge continuity, paleoflow variability, clast characteristics (lithology and roundness) and slope suggest that these eskers were formed synchronously in long, closed R-channels; R-channel length is conservatively recorded by esker length. The presence of gravel facies arranged into ridge-scale macroforms implies sedimentation by powerful flows down non-uniform R- channels. Laterally-extensive, vertically-stacked gravel-sand couplets transitional to downflow rhythmites record unsteady flow and a supra-subglacial meltwater connection. The presence of a proglacial water body prevented winter conduit closure, and stagnant ice favoured long, dendritic esker preservation. Long, dendritic, water-terminating eskers record long R-channels draining regionally stagnant ice. With the equilibrium line at or above a flat, stagnant ice mass, downwasting occurred in a regionally-extensive ablation zone. High volumes of supraglacial meltwater passed through a karstic plumbing system to the bed. Flow in this integrated system was driven to the ice margin by hydraulic head; a potentiometric surface steeper than the ice surface slope (flat) developed under non-steady state conditions due to the poor efficiency of meltwater transmission through the ice plumbing system back from the margin. As the water pressure in R-channels is inversely related to water discharge, larger R-channels captured water from smaller channels effecting a dendritic R-channel system. The proglacial water body kept R-channels open in the winter and permitted reoccupation of the R-channels over multiple years. Calving in deep water may have resulted in conditionally-active ice at the margin.

Bibliography of Canadian Geomorphology