Deglaciation landforms and C-14 chronology of the Lac Guillaume-Delisle area, eastern Hudson Bay: A report on field evidence

This paper presents new field data pertaining to the deglaciation of the Lac Guillaume-Delisle area, aiming to establish its emplacement in time and space between the breaching of the Laurentide Ice Sheet (LIS) over Hudson Bay and the final ice disintegration over Northern Québec and Labrador. The deglaciation of the east coast of Hudson Bay started at 8200 cal BP in the south-western sector of the Lac Guillaume-Delisle estuary, when the marine limit was at 271 m above sea level (asl). By 8000 cal BP, frontal moraines and proximal glacio-marine sediments were deposited in the transverse valleys along the coastal cuesta ridges at a mean elevation of 250 m asl. They are associated with a regional topographic pinning event of the western margin of the Quebec-Labrador Ice Sheet (QLIS) on the high coastal relief. At that time, the coastal relief encouraged a change in deglaciation mode from fast calving in the open and deep Tyrrell Sea to a tidewater ice-sheet controlled by the topography. The ice front thereafter retreated eastward under very large and fast crustal unloading where ice-contact deltas fed by short sandar and eskers were deposited between 7900 and 7400 cal BP (between 240 and 195 m asl), as the ice front was receding inland and lost contact with Tyrrell Sea. A series of deposits and landforms characterize this modification in the sedimentation regime when the ice-sheet margin changed from tidewater to terrestrial. Dating shows that the deglaciation of the western margin of QLIS on land was very fast, i.e. faster than when the front of the ice-sheet was in contact with the Tyrrell Sea. As the fall of the relative sea-level continued, a large volume of paraglacial sediments was deposited in the river valleys making up vast fluvial terraces and deltas between 6500 and 5800 cal BP (between 125 and 100 m asl). Since then, land emergence has continued at a rate of about 1.3 m/century. Downcutting by rivers, post-glacial deltaic sedimentation and shoreline changes still continue today.