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Caroline Lavoie, GRC Geocičncies Marines (Spain)
Michel Allard, Centre d?études nordiques (Canada)
Philip R. Hill, Natural Resources Canada (Canada)
Denis Duhamel, Centre d?études nordiques (Canada)
Patrick Lajeunesse, Centre d?études nordiques (Canada)
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We present geomorphological evidence of the depositional patterns and environmental changes at Lac Guillaume-Delisle (east coast of Hudson Bay, Canada) during the early Holocene deglaciation. Lac Guillaume-Delisle is a stable structural estuary contained in a Precambrian east-west oriented graben. Stratigraphic studies, mapping of emerged drift facies and landforms, mapping of sea bottom sediments and re-evaluated radiocarbon dating provide new key data to explain the dynamics of the western QLIS margin retreat throughout successive ice-stillstands, continuous relative sea-level (RSL) fall, river discharge (ablation on land), and its final ablation inland. After the breakup of the Laurentide Ice Sheet (LIS) over Hudson Bay, a first stillstand of the retreating ice-front occurred at 8200 cal BP in the southwestern sector of the estuary. Then, when the regional Tyrrell Sea (precursor of Hudson Bay) level was around 250 m a.s.l. along the coastal cuesta ridges and on the south-eastern side of the estuary, ice-contact and glacio-marine deposits were laid as subaqueous frontal moraines and subaqueous outwash fans and trains in the transverse valleys. This belt of deposits oriented north-south is interpreted as resulting from a second stillstand of the glacial front anchored approximately to 8000 cal BP on high coastal relief. The ice front thereafter retreated eastward as suggested by a few series of DeGeer moraines and the Tyrrell Sea followed the retreating ice front up to a distance of 83 km inland into glacial valleys.
The ice finally disappeared to the east of the estuary (between Lac Guillaume-Delisle and Lac à l'Eau-Claire) by 7500 cal BP. Postglacial emergence and abundant glacio-marine and glacio-fluvial sediments left by the glacier in the glacial valleys created conditions favourable for fluvial-deltaic activity and the build-up of marine landforms when the emergence rate changed from about 9.6 m/century to about 1.3 m/century around 6300 cal BP. New fieldwork data support the idea of a very rapid retreat of the QLIS of the LIS from the east coast of Hudson Bay until the final disintegration in central Québec-Labrador. The new chronological framework highlights spuriously old radiocarbon dates on marine shells collected in ice-contact glacio-marine deposits.
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