SEQUENCE-ANALYSIS OF UPPER PLEISTOCENE (WISCONSINAN) GLACIOLACUSTRINEDEPOSITS OF THE NORTH-SHORE BLUFFS OF LAKE-ONTARIO, CANADA

Up to 90 m of glaciolacustrine Upper Pleistocene (Wisconsinan) sedimen ts are well exposed along the north shore of Lake Ontario (Scarborough and Bowmanville Bluffs), and contain critical information on the effe cts of glaciers on sedimentation in large proglacial lakes, and on the deglaciation history of the Great Lakes region of North America, The sediments vary greatly, but the overall vertical trend is of a diamict unit at the base, changing upward into distal clay-rich rhythmites, p roximal sand-rich rhythmites, and well washed, deltaic and/or coastal sands, In some areas, gravelly units are interbedded between the basal diamict and the distal rhythmites; they are interpreted as subaqueous outwash-fan deposits, The glaciolacustrine sediments of these bluffs include two different assemblages: one related to overland glaciofluvi ally derived sediments (''lacustrine assemblage''), and another direct ly related to a glacier with subaqueous terminus (''glacial assemblage ''), The basic concepts of sequence stratigraphy help in unraveling th ese complex deposits, if conceptually adapted to accommodate the ''gla cial assemblage'' where a ''stand'' is not necessarily controlled by w ater level but rather by the relative position of the sediment injecti on point in the basin, Thus, ''lowstand'' (lowstand-equivalent systems tract) subaqueous outwash-fan deposits of the ''glacial assemblage'' may form in ice-dammed lakes, while, at the same time, ''highstand'' ( highstand systems tract) facies of the ''lacustrine assemblage'' are d eposited in other parts the basin, Furthermore, as the glacier retreat s, subaqueous outwash deposits may be overlain abruptly by deep-basin, clay-rich lacustrine rhythmites, locally forming a forced transgressi ve facies sequence without any change in water level. Opening and clos ing of outlets by the glacier also causes rapid changes in lake level, leading to development of shelf-margin systems tracts and forced-regr ession systems tracts for the ''lacustrine assemblage''.