A LATER DEGLACATION OF NORTHWESTERN ONTARIO: IMPLICATIONS FOR EASTERN LAKE AGASSIZ DRAINAGE

Deglaciation in northwestern Ontario was later than originally supposed based on new radiocarbon ages from west of Thunder Bay, Ontario. The existing paradigm for eastern drainage from glacial Lake Agassiz is through a variety of spillway channels incised across the sub-continental drainage divide separating the Great Lakes from Hudson Bay. The chronology of spillway channels has only been assumed from bulk radiocarbon dates on moraines that are not directly traced to the channels in northwestern Ontario. It is the opening of the Kashabowie-Seine channel that delivered meltwater into the Great Lakes system that has been linked to large discharges of freshwater to the North Atlantic triggering the Younger Dryas cold period. However, this channel's geomorphology and lack of boulders is dissimilar to the southern and northwestern outlet channels of Lake Agassiz. Consequently, we have begun mapping and dating sequential ice margin positions along the drainage divide to determine when Lake Agassiz could have drained to the east. Sample locations associated with the Steep Rock and Brule Moraines were from small lakes high in the landscape, above the elevation of the isostatically depressed Kashabowie-Seine channel. The general stratigraphy indicates short-lived glaciolacustrine lakes giving way to organically productive lakes. At Third Lake this transition is at 10,000 ± 75 (ETH-28946) above ~200 rhythmites, whereas at Crawfish Lake the transition is at 10,190 ± 40 (Beta-195959). These preliminary results indicate that deglaciation of the Kashabowie-Seine channel area was approximately ~800 years later than previously thought, which implies that any drainage eastward from Lake Agassiz through the Kaministikwia-Seine channel is considerably younger than previously supposed. The hypothesis that drainage from Lake Agassiz's eastern outlets caused the Younger Dryas cold period is thus problematic.