Journal Of Sedimentary Research, Vol. B65, No.3(1995), 388-400

I.P. MARTINI and M.E. BROOKFIELD

Department of Land Resource Science, University of Guelph, Guelph, Ontario NlG 2Wl Canada

Up to 90 m of glaciolacustrine Upper Pleistocene (Wisconsinan) sediments are well exposed along the north shore of Lake Ontario (Scarborough and Bowmanville Bluffs), and contain critical information on the effects 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, proximal 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 de- posits. The glaciolacustrine sediments of these bluffs include two different assemblages: one related to overland glaciofluvially derived sediments ("lacustrine assemblage"), and another directly related to a glacier with subaqueous terminus ("glacial assemblage"). The basic concepts of sequence stratigraphy help in unraveling these complex deposits, if conceptually adapted to accommodate the "glacial assemblage" where a "stand" is not necessarily controlled by water level but rather by the relative position of the sediment injection 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 deposited in other parts the basin. Furthermore, as the glacier retreats, subaqueous outwash deposits may be overlain abruptly by deep- basin, clay-rich lacustrine rhythmites, locally forming a forced transgressive facies sequence without any change in water level. Opening and closing of outlets by the glacier also causes rapid changes in lake level, leading to development of shelf-margin systems tracts and forced-regression systems tracts for the "lacustrine assemblage".