Abstract

The three-dimensional configuration of the Quaternary sediments in the Gulf of St. Lawrence is described based on analysis of 8000 km of high-resolution seismic reflection data complemented by analysis of seven piston cores. Till and or ice-contact deposits, glaciomarine sediments, postglacial basinal muds, and bank-top lagoonal sediments grading to sands and gravels make up the stratigraphic succession. Numerous thick accumulations of ice-contact - morainal deposits and "till tongues" indicate the position of former ice margins. The sequence of breakup of the last glacial ice in the gulf has been interpreted on the basis of these data and associated accelerator mass spectroscopy 14C dates. Grounded ice extended beyond Cabot Strait before 14.3 ka and retreated rapidly to north of Anticosti Island by 13.7 ka. Local residual piedmont lobes of glacial ice within the Cape Breton Channel and Baie des Chaleurs persisted longer, until about 12.2 ka. Ancestral hanging valleys formed the Cape Breton Channel and Baie des Chaleurs and served as conduits for ice lobes that flowed into the deeper Laurentian Channel. As many as four superimposed (Late Wisconsinan) glaciogenic sequences, up to 190 m in composite thickness, occur at the mouths of these (hanging) valleys. The thickest glacial sections were deposited on the southwestward slope of the Laurentian Channel. The surface of the till - ice-contact sediments between 440 and 100 m below present sea level has been extensively modified by iceberg scouring. The deeper limit (440 m) marks the maximum draft of icebergs, which was in large part determined by the thickness of the calving ice front. The disappearance of iceberg scour marks at 100 m is interpreted to have resulted from erosion of the glacial deposits by a Late Wisconsinan transgression whose low stand is suggested by a well-developed terrace on parts of the Magdalen Plateau at a present water depth of 110 m. A piston core that penetrated sediments overlying the uppermost (Late Wisconsinan) till in the Cape Breton Channel is interpreted to represent a deposit of lagoonal or shallow-marine sediments with localized deposits in excess of 65 m thickness. The depositional style of the postglacial deposits suggests sufficient bottom currents to erode sediments on the Magdalen Plateau.

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