THE LATE QUATERNARY STRATIGRAPHIC RECORD NORTHWEST OF MONTREAL: REGIONAL ICE SHEET DYNAMICS AND ICE STREAM ACTIVITY

Evidence of complex ice sheet dynamics characterized by abrupt changes in ice flow direction and convergent ice flow patterns are apparent throughout the upper St. Lawrence Valley suggesting that ice stream activity was a key process during the Late Wisconsinan in that region. Recent subsurface investigations of buried valleys near Montreal have provided a stratigraphic record of this complex glacial dynamics. A 78 m thick stratigraphic succession has been continuously drilled and its 2D architecture defined by seismic reflection. It consists, from base to top, of proximal glaciolacustrine sediments, two superposed and contrasting till sheets (Argenteuil and Oka tills) of inferred Late Wisconsinan age, and Champlain Sea sediments. The glacial sediments of this sequence record an ice advance toward south (Argenteuil Till) followed by an abrupt ice flow shift toward the southwest (Oka Till). Compositional and geomorphic data indicate that Oka Till is ubiquitous in the region and had a strong regional imprint in terms of glacial landform development. The analysis of a regional Digital Elevation Model (DEM) in combination with published ice-flow indicators shows convergent flow patterns from the Ottawa-Montreal-Adirondacks regions toward the Lake Ontario basin . These are locally crosscut by southward-trending ice flow features, thus suggesting that fast southwestward flow within the Ontario Lobe was abruptly shut down and replaced briefly by southward ice flow. The ice stream was most likely initiated in the Lake Ontario basin and its catchment area migrated northeastward into the Montreal region. Southward flow in the upper St. Lawrence Valley was seemingly favoured during full glacial conditions as well as during late deglaciation, as a post-ice stream re-equilibration mechanism.