SOUTHWESTERN LAURENTIDE ICE STREAMS DURING FINAL DEGLACIATION

At 10 C14 ka BP, southwestern Laurentide lobes were under topographic control in the Red River valley and Lake Superior basin. Subsequent ice streaming commonly unconfined by topography dominated most sedimentation across northern Manitoba and northern Ontario. Initially, one ice stream in Manitoba and four in Ontario flowed NE to SW. In Manitoba, the Pas Moraine, a radiating fine-grained till deposit derived from nearby, is more detached from later ice stream features than is the tendency in Ontario. Recurring features in the remaining SW-flowing ice streams are radiating ice flow, thick fine-grained exotic till, large radiating eskers, large moraines commonly consisting of coarse sand and gravel, and immense glaciofluvial deposits on the flanks. In Lake Winnipeg, one set of thick varves records a few decades of anomalous sedimentation correlated to one moraine to the north. In NW Ontario, four moraines indicate ice stream resurgence at half-millennium intervals, to some degree recorded in varves. In Lake Superior, two sets of thick varves correlate to two moraines to the north, indicating ice stream resurgence with recurrence of centuries and duration of decades. In two areas of Ontario, younger south to southeastward ice flow cross-cuts evidence for southwestward flow. The Matheson event, straddling the Québec border, is seen as a regional south-southeastward re-direction of ice flow, associated with the glaciofluvial Harricana moraine on its eastern flank, but not necessarily a feature that should be regarded as an ice stream. The Winisk ice stream 1) beheaded the northwestern Ontario ice stream, 2) coincided in time with the Manitoba ice stream, 3) was channeled by topography in the north, 4) transported red till from Hudson Bay to as far as Ogoki River, 5) deposited a few meters of till with remarkable lateral compositional homogeneity as well as fabrics and lower contact boulder pavement striations parallel to overlying streamlined terrain, and 6) includes an esker that ultimately extends southwestward to the Agutua-Nakina moraine, indicating the ice stream formed in an ice sheet with its margin at this moraine. Varying ice steams thus dominated ice flow in the final retreat of the southwestern Laurentide ice sheet, thus playing a factor in thinning the ice mass, leading to its rapid breakup in Hudson Bay.