DEGLACIATION WEST OF THUNDER BAY, ONTARIO: A CONUNDRUM FOR EASTERN LAKE AGASSIZ DRAINAGE

New radiocarbon ages from west of Thunder Bay, Ontario suggests that deglaciation was later than originally supposed. 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 basin from Hudson Bay. The spillway channels are mostly in quite inaccessible regions of northwestern Ontario and their chronology has only been assumed on bulk radiocarbon dates from moraines that cannot be directly traced to the channels. It is the opening of the Kashabowie-Seine channel that has been linked to large discharges of freshwater to the North Atlantic triggering the Younger Dryas cold period. However, this channel is considerably smaller and lacks boulders in comparison to the southern and northwestern outlet channels of Lake Agassiz. Hence, our initial strategy for determining the timing of eastern drainage from Lake Agassiz is mapping and dating sequential ice margin positions along the same drainage divide. Sample locations associated with the Steep Rock and Brule Moraines were from small lakes high in the landscape, above the elevation of 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 is thus problematic.