ESTABLISHING A SIGNATURE IN LAKE CORES FOR THE DRAINAGE OF LAKE OJIBWAY, CANADA

Glacial meltwater systems can create large scale events such as rapid drainage of meltwater lakes that can produce spectacular geomorphic signatures. For this case study we examine the sediment sequence associated with the demise of glacial Lake Ojibway from a small pond, South Fraserdale Lake (N 49.4622, W 81.3124, 229 m), located south of the Hudson Bay. Visual sediment descriptions, density, magnetic susceptibility and grain size data constrain the stratigraphy. Grain size data collected at 2 cm intervals ranged from 79.9 - 1.19 µm and correlate to the visual changes. There are five main units in the section of core examined; unit 7 is a basal glacigenic varve package that grades upward into unit 6, a disturbed varve section containing rip-up clasts. Unit 6 has discernable but poorly defined bedding composed of small lenses and laminae of silt and clay, and has a trimodal grain distribution ranging from clay to very fine silt. Unit 5 is finely laminated and grades into unit 4, a massive mottled section. Unit 6 contains the same grain size distributions as units 4 and 5 but also includes two other fractions, course clay and fine silt. A second erosional contact separates unit 3 from 4, and the contact appears subaerially exposed. Unit 3, a fine sand, infills cracks and contains small organic fragments. The thickness of unit 3 varies locally between cores. Tentatively, unit 6 is interpreted to be a drainage signature as evidenced by the rip-up clasts and grain size change. Grain size above unit 6 suggest that after the drainage event ended and terrestrial processes added sediment to the basin. The lakes that persist today are depressions in the paleo-lake bed. As the drainage occurred, fine grained material in suspension was redistributed and settled out. Unit 5 reflects sedimentation similar to that occurring in the present basin. The present day basin may have remained full, allowing sedimentation to continue, preserving the event. Preliminary data show this stratigraphic signature is present in 3 out of 5 lakes examined in a 130 km radius. The signature of rapid drainage events must be considered carefully because other anomalies may occur in the section, causing confusion for the actual drainage.