LATE-GLACIAL SEDIMENTATION AND HISTORY OF THE LAKE NIPIGON BASIN, ONTARIO

The Lake Nipigon basin lies north of the Lake Superior basin and was t he hydrological link between glacial Lake Agassiz and the Great Lakes during part of the last deglaciation. A sequence of glaciolacustrine s ediments, composed mainly of silt-clay rhythmites and sand, was deposi ted in the offshore waters of glacial Lake Nipigon by overflow from La ke Agassiz and meltwater from the retreating glacier margin. Sections from six long sediment cores and four lake bluff exposures reveal a sa ndy (early deglacial) lower section that is overlain by 300 to 850 sil t-clay rhythmites (varves). Deposition of these varves, as well as coa rser sediment along the western shore, began after 9200 BP, as the gla cial margin retreated northward along the continental divide that sepa rated the Nipigon basin from the higher Lake Agassiz basin to the west . The absence of ice rafted clasts in the rhythmites suggests-that the ice had retreated from the lake by the time they were deposited. On t he basis of their elevation in relation to the lowest raised beach at West Bay, which formed about 9000 BP, most rhythmites probably were de posited between 9000 and 8000 BP. Species of arboreal pollen are prese nt in early postglacial sediments bf the Nipigon-Superior lowlands, su ggesting that the Lake Nipigon region became colonized by coniferous a nd deciduous forests soon after deglaciation. The presence of non-arbo real pollen species suggest that these forests were interspersed with open meadows and grasslands, similar to today's floral assemblages. Fo ssil molluscs recovered from glaciolacustrine sand exposed along the e astern side of the basin suggest that the limnological characteristics of late glacial Lake Nipigon were similar to those of today.