DYNAMICS OF THE LAURENTIDE ICE-SHEET IN HUDSON-BAY, CANADA

Interpretation of sidescan sonar and high resolution sub-bottom seismic data from Hudson Bay has led to the development of a model for final disintegration of the Laurentide Ice Sheet. The well-preserved geomorphic features observed from the bay floor are similar to subaerially exposed glaciogenic features observed around the perimeter of Hudson Bay, and can be related to the following three paleoenvironments: subglacial erosion/deposition, ice margin deposition and reworking, and post-glacial reworking and burial. Subglacial features observed include fluted terrains with superimposed rogen moraines, relief attributed to ice surging, eskers, subglacial channels and dead ice topography. De Geer moraines and recessional moraine complexes indicate the mechanism and orientation of ice breakup/retreat. The orientations of these features are locally consistent with regional trends indicating the overall direction of ice sheet flow and retreat during the last phase of the Laurentide Ice Sheet. Subglacial channel complexes incised into the till and bedrock, and high energy bedforms deposited on the till surface occur in present day water depths down to 160 m. Due to isostatic depression these could have been as much as 315 m deeper at the time of formation. They indicate a wet-based glacier with high energy subglacial drainage during the late phases of ice retreat. The sidescan sonar data also reveal numerous zones of distinctive morphology attributed to ice breakup and post-glacial processes. Cross-cutting relationships allow the determination of the relative ages of these zones. From oldest to youngest these zones are interpreted to represent scouring of the bedrock and overburden by the remnants of the ice sheet, seafloor scouring by tabular icebergs which moved in a tidally induced(?) arcuate pattern or under the influence of massive glacial lake outflows, development of dead ice topography under residual ice masses, and random iceberg scouring by smaller icebergs restricted to present day water depths of less than 180 m. Recently published reconstructions of the collapse of the Laurentide Ice Sheet, based on dated Quaternary sections on land, suggest that the bay was rapidly deglaciated in approximately 400 years. The well-preserved and distinctive glaciogenic and stratigraphic indicators observed from the bay floor suggest that they were formed by a succession of mechanisms which were too short lived to obliterate the underlying evidence of deglaciation. This supports the interpretation of extremely rapid deglaciation of Hudson Bay and points out that subsequent marine processes have not been sufficient to obliterate the glacial features. © 1990.