THE LAST PHASES OF THE LAURENTIDE ICE SHEET MARGIN IN THE ST. LAWRENCE ESTUARY AND GULF AND ADJACENT AREAS

The St. Lawrence Estuary and Gulf area forms an inner continental shelf marked by the Laurentian Channel extending from the mouth of the Saguenay Fjord to the continental slope. During the retreat of the Laurentide Ice Sheet (LIS) margin, the area formed a large calving bay dominated by a high influx of glacial ice, meltwater and sediment through rapid ice flow. However, the retreat patterns and dynamics of the LIS margin still remain poorly documented in this region of northeastern North America, mostly because their geomorphic record lies on the seafloor. Recent investigations in the region using swath bathymetry imagery and terrestrial LiDAR imagery reveal new information on the late-glacial and deglacial history of the LIS by the observation of (1) mega-scale glacial lineations (MSGLs) associated with the St. Lawrence Ice Stream and (2) series of grounding-zone wedges (GZWs), morainal banks and ice-contact submarine fans extending laterally along the shelf and coastal zone that record episodic phases of retreat of the marine-based LIS margin, following its rapid retreat in the deeper waters of the Laurentian Channel after 14.8 cal ka BP. The occurrence of GZWs along distinct isobaths indicates that bathymetry exerted a strong control on ice stabilization during deglaciation by reducing the relative water depth at the ice margin and thereby the buoyancy and rate of iceberg calving, but ice-marginal re-advances over some GZWs suggest an additional response to climate-driven forcing. Following these events, the LIS margin retreated rapidly to the coastal slopes of the Québec-North-Shore, where a bedrock escarpment promoted a phase of very rapid ice flow that carved swarms of large glacial grooves distributed discontinuously over a distance of >500 km. As the ice margin stabilized further upslope, submarine fans aggraded to form sandur-deltas as large volumes of sediments were being delivered and relative sea-level was rapidly falling due to initial glacio-isostatic rebound.