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Abstract

Swath bathymetry data and seismic profiles collected in the NW Gulf of St Lawrence reveal a series of wedge-shaped depositional systems interpreted as grounding zone wedges (GZWs). Some segments of the GZWs change locally to form frontal moraines, or morainal banks, and subaqueous ice-contact fans, reflecting changes in either the nature of the ice margin or the rate of sediment input. These grounding zones (GZ) of the ice margin extend laterally along three isobaths at depths of 180 (GZ1), 120 (GZ2) and 80 (GZ3) m (±20 m) along the Québec North Shore shelf, the 120 m-deep GZ2 system being traceable over a distance of >300 km. Associated GZWs can occur in three geometries along a same isobath system: curvilinear, lobate and shelf-break. GZ systems were built during three distinct stages of stabilization of the marine-based southeastern margin of the Laurentide Ice Sheet following its rapid retreat across the deeper waters of the Laurentian Channel in the Gulf of St Lawrence after 14.8 cal ka BP. The occurrence of GZ 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. However, fluctuations and re-advances of the ice margin are also recorded by the overprinting of a portion of the GZ2 system by the younger GZ3 system, potentially suggesting an additional response to climate-driven forcing.

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