ABSTRACT: Insights on the events surrounding the drainage of Lake Ojibway based on James Bay Lowlands stratigraphic sequences.

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Abstract

The general northward ice retreat in NE Ontario and NW Québec was concomitant with the development of glacial Lake Ojibway. The deglaciation of this area was characterized by the scission of ice margin into Hudson and New-Québec ice domes, late-glacial (Cochrane) readvances into the Lake Objiway basin around final deglaciation time, which culminated with drainage of Lake Ojibway and subsequent incursion of post-glacial Tyrrell Sea at ~8 ka. Classical paleogeographic reconstructions indicate that this drainage occurred through a collapse of residual ice (e.g. Dyke and Prest, 1987) and recent glaciological modeling suggests that the lake may have drained subglacially prior to before final collapse (e,g, Clarke et al., 2004). Here we focus on the drainage of Lake Ojibway by documenting late- and postglacial sediment sequences exposed along Broadback and Rupert rivers in the James Bay lowlands, a region which has the particularity of lying near the final resting position of the ice margin at the end of the deglaciation. Our stratigraphic investigations record the uppermost part of deglacial sequence, which consists of (1) a carbonate-bearing readvance till, (2) Ojibway varves, (3) a 50 cm-thick horizon composed of thinly laminated reddish and grey silt beds containing abundant rounded clay balls and disseminated clasts, overlain by (4) Tyrrell Sea sediments. The horizon marking the contact between glaciolacustrine and glaciomarine sediments is here interpreted to reflect the abrupt drainage of Lake Ojibway. Radiocarbon dating of mollusks and foraminifers extracted from the uppermost part of the drainage horizon yielded ages of 7.64 and 8.02 14C ka BP. Detailed micropaleontological investigation reveals quite surprisingly that the upper varve sequence contains foraminifers and bivalve and ostracod fragments. This material in currently being analyzed for further 14C dating and for paleoenvironmental conditions (d13C and d18O). The results should provide additional information on the timing of event and on the mechanism of drainage. Marine shells dated at 8.01 14C ka BP were also collected from a Cochrane till exposure along the nearby Harricana River, thereby suggesting possible subglacial exchanges between Lake Ojibway and Tyrrell Sea. These results, albeit preliminary, suggest that final deglaciation of the area was more complex than anticipated and also indicate that the James Bay region may have served as an important pathway through which Lake Ojibway waters drained shortly before marine incursion

Bibliography of Canadian Geomorphology