St. Lawrence Ice Stream overflow on the northern edge of the middle Estuary during the Bølling-Allerød and response of the Laurentide Ice Sheet margin to the Intra-Allerød Cold Period in southern 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 topographic control on ice flows. The LIS ice flowed southeastward over the area independently of structural topography during the Glacial Maximum or closely thereafter. From published and original data, the StLIS eventually reached the lower part of the middle estuary as a moving-inland head of ice flow convergence. The ice diversion accelerated the regional-scale ablation on both sides of the estuary likely under the warmer Bølling climate. A generalized thinning in the middle estuary is inferred to cause the northward ice-flow reversal of the Appalachian ice divide to expand into the studied region, consequently initiating the Northward Ice Overflow (NIO) on the Laurentian margin, up to elevations of 750 to 950 m in the Beaupré Basin. 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 of ice flows and retreat events. After the initiation of the NIO, the overflow ice in the Beaupré Basin thinned by at least 100 m during part of the Allerød. The Beaupré Group of Moraines and the Savane Moraine in the Laurentians provide evidence of a significant subsequent slowdown in 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 relative to: 1) the Saint-Narcisse Moraine of early Younger Dryas; 2) the deglaciation of a lake basin whose 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 glacial megalineations of the Montmorency massif and Gouffre Valley represent a major glacial flow toward the SSE from the Québec-Labrador Dome which did not override the Beaupré Basin. This LIS flow is closely followed by a reactivated StLIS which flowed into small valleys and lowlands of the north shore of the middle and upper estuary from the Québec City region to the Malbaie Valley (glacial striations and the Escarpment and South-Éboulements moraines). The LIS flow episode toward the SSE probably began at the start of the IACP. At least two moraine belts (Cherry River–Mégantic and Mount Ham) are tentatively associated with the IACP in southern Québec and thus would have a climatic origin.These new findings confirm a much shorter duration for the sequence of late glacial events in the region than was proposed in previous models as well as continuity between the early StLIS in the Gulf (the Laurentian Channel Ice Stream) and the late StLIS of the middle estuary. The IACP slowdown of the glacial retreat opens the possibility of correlation at the scale of the southeastern margin of the LIS.

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