Late-stage phases of glacial Lake Ojibway in the central Abitibi region, eastern Canada

Abstract

The decay of the Laurentide ice sheet southern margin during the last deglaciation led to the development of Lake Ojibway that covered large expanses of northeastern Ontario and northwestern Quebec. The history of Ojibway lake phases is poorly detailed mainly because of the physical configuration of the lake basin and the dominance of fine-grained glaciolacustrine sediments that prevent the formation of well-developed and extensive sandy strandlines. Here we use a complex sequence of relict terraces carved in glaciolacustrine rhythmites to document the evolution of Lake Ojibway in northwestern Quebec. Specifically, lake levels were constrained by measuring the elevation of 154 raised wave-cut scarps present in the eastern Lake Abitibi region. Results provide evidence for four distinct shorelines with elevations of 299, 289, 282, and 272m (±1m) at the latitude of La Sarre. The highest lake level documented appears to be linked to one of the two known (Kinojévis) phases of Lake Ojibway, while the three other lake levels project well below the main outlet system that controlled the elevation of the lake during the deglaciation. The elevation, uplift gradients, and areal extent of these lower shorelines suggest that the two intermediate lake levels likely formed during late stages of the deglaciation, following abrupt drawdowns of the lake's surface. The fourth and lowest shoreline is associated with a postglacial lake that developed after the complete withdrawal of Ojibway water from the region. These low-elevation shorelines bring new evidence for significant changes in the areal extent and depth of Lake Ojibway near the end of the deglaciation. Although the origin of these late-stage phases remains unspecified, the associated drawdowns likely implied routing events into newly deglaciated regions and/or (subglacial) meltwater discharges into the North Atlantic.