QUATERNARY SEDIMENTATION AND MARINE PLACERS ALONG THE NORTH SHORE, GULF OF ST-LAWRENCE

Offshore areas, along the North Shore of the St. Lawrence Estuary, hav e major lithostratigraphic and seismostratigraphic units that relate t o the advance or retreat of the Late Wisconsinan Ice Sheet, subsequent marine transgression or regression, and reworking of postglacial depo sits. Glacial diamicton and glaciomarine units (acoustic units 1 and 2 ) were emplaced between > 18 and 14.5 ka, by basal meltout or ice-marg inal sedimentation; they reflect ice-proximal sedimentation associated with ice-terminal stillstands. Deep-water muds (acoustic unit 3) repr esent ice-distal accumulation of glaciomarine sediment from glaciofluv ial plumes between 13.5 and 11 ka. After this time exceptionally thick nearshore coarse-grained deltaic and estuarine successions (acoustic unit 4) were deposited. The uppermost postglacial sediment (acoustic u nit 5) forms the seabed and reflects a reworking phase concomitant wit h a lowering sea level and ablating Late Wisconsinan ice sheets. Glaci oisostatic rebound, which occurred about 23 ka to the present, uplifte d glacial and marine deposits and resulted in extensive reworking and production of modern placers. Heavy-mineral concentrations vary as fol lows: terrestrial tills, 9-20%; modem storm-berm and delta top deposit s, 43-60%; delta slope deposits, 25 -55 %; and deep (170 + m) offshore sediments, 0-2%. Three stages occurred in marine placer formation: (1 ) 6700 BP, fluvial discharge was high, and fluvial-dominated deltas we re built; marine limit was 30 m asl, with progradation of deltas and d elivery of sediments with at most 2% heavy minerals; (2) 5200 BP, fluv ial discharge was reduced; marine limit was 15 m asl, deltaic sediment s were reworked, increasing heavy mineral concentration to 2-8%; (3) 2 800 BP, fluvial input was greatly reduced, waves and tides were more i nfluential, a strong littoral current system developed, causing signif icant reworking of nearshore sediments, heavy mineral concentrations i ncreased, with values exceeding 20% locally. Mass budget calculations show that the second-cycle reworked sediment (acoustic unit 5) is a po tential economic target (1 km3, or 1700 Mt). If 7% (using atomic weigh ts) of this target sediment is ilmenite (FeTiO3), then 27 Mt of titani um may be available.