Débordement du Courant glaciaire du Saint-Laurent sur la bordure nord du moyen estuaire pendant le Bølling-Allerød et réponse de la marge de l'Inlandsis laurentidien à la Période froide intra-Allerød au sud du Québec, Canada

The middle and upper Estuary regions of the St. Lawrence River include the Laurentian highlands to the north and the Appalachian piedmont and uplands to the south. From a synthesis of the literature, the glacial dynamics of these regions are the result of: 1) the changing position of an ice dispersion centre in the Québec-Labrador Dome; 2) the upstream retreat of a calving bay from the Gulf of St. Lawrence to the middle Estuary at the origin of the rapid inland extension of the St. Lawrence Ice Stream (StLIS) of the Laurentide Ice Sheet (LIS); 3) the progressive separation of appalachian ices from the LIS; and 4) an increasing topographiccontrol on ice flows. The LIS ice flowed southeastward over the area independently of structural topography during the Late Glacial Maximum or closely thereafter. From published and original data, the StLIS eventually reached the lower part of the middle estuary as amoving-inland head of ice flow convergence. The ice diversion accelerated the regional-scaleablation on both sides of the estuary likely under the warmer Bølling climate. A generalizedthinning in the middle estuary is inferred to cause the northward ice-flow reversal of theAppalachian ice divide to expand into the studied region, consequently initiating the NorthwardIce Overflow (NIO) on the Laurentian margin, up to elevations of 750 to 950 m in the BeaupréBasin. This likely took place between the end of the Bølling and the early Allerød.Based on this study, the northern part of the region records a detailed Late Glacial sequence ofice flows and retreat events. After the initiation of the NIO, the overflow ice in the BeaupréBasin thinned by at least 100 m during part of the Allerød. The Beaupré Group of Moraines andthe Savane Moraine in the Laurentians provide evidence of a significant subsequent slowdownin the glacial retreat. These moraines are attributed to the Intra-Allerød Cold Period (IACP)between approximately 13.250 and 13.050 cal ka BP, based on their older age position relativeto: 1) the Saint-Narcisse Moraine of early Younger Dryas; 2) the deglaciation of a lake basinwhose basal sediments are dated at 12.750 cal ka BP; and 3) the local marine invasion(Goldthwait Sea) prior to 13-12.9 ± 0.1 cal ka BP. Laterally to these moraines, the glacialmegalineations of the Montmorency massif and Gouffre Valley represent a major glacial flowtoward the SSE from the Québec-Labrador Dome, which did not override the Beaupré Basin.This LIS flow is closely followed by