PATTERN OF THE LAURENTIDE ICE SHEET RETREAT ON THE UNGAVA PENINSULA (NUNAVIK, CANADA): IMPLICATIONS FOR DEGLACIATION MODELS AND ICE-DAMMED LAKE DEVELOPMENT

The understanding of the dynamics of the Labrador Sector of the Laurentide ice sheet during the deglaciation of the Ungava Peninsula (northern Quebec) is complicated by the faint glacial geomorphological imprint and dominant unmoulded till plains. Although the deglaciation of the Hudson Strait region is well understood, details on the ice retreat pattern remain poorly documented on the adjacent Ungava mainland. Here we mapped glacial surficial deposits and associated landforms, and measured the elevation of glacial lake strandlines and deltas to gain insights on the deglaciation. Ice-flow indicators (striations) and the trend of eskers provide key information on the evolution of ice flows and patterns of ice retreat. Results show that low-elevations areas and associated valleys show evidence for an active deglaciation typical of warm-based ice conditions, which had a significant impact on the onset of the deglaciation. The early deglaciation of the Deception Bay region (NNE Ungava) was key to the formation of glacial lakes. The south-south-west retreat of the ice margin caused the development of two temporary ice-dammed lakes. In the north, glacial Lake Gatin first formed and subsequent ice-withdrawal led to the development of Lake Foucault. Lake Gatin displays evidence of lake level stages between 444 m and 282 m while those of Lake Foucault range between 413 m and 224 m. While the lakes coexisted, they never shared a connexion as their respective lake levels and extents were controlled by opening of outlets resulting from the deglaciation of the Foucault River area. We also used strandline elevations along a north-south transect extending through the Watts and Françoys-Malherbe lakes to determine the regional (southerly) tilt (0.82 m/km) and develop a digital terrain model (DTM) correcting for glacial isostatic adjustment (GIA) deformation, which yields robust meltwater volumes. In the upper plateau areas, the lack of well-developed glacial landforms and overall rare occurrence of meltwater channels and short-length eskers suggest cold-based ice conditions throughout the deglaciation. Ice-flow reconstructions suggest that the late ice dynamics were strongly controlled by the regional topography (position of the drainage divide) and the opening of Hudson Strait.