Changes in Stratigraphy Across the Lake Ojibway Basin, Implications for Lake Drainage Record

Changes in Stratigraphy Across the Lake Ojibway Basin, Implications for Lake Drainage Record Lake Ojibway's formation, sedimentation and drainage history was influenced by the retreat of the Laurentide ice sheet. Eleven Lake cores were taken from a geographic area ranging from the Cochran area in central Ontario 240km south east into western Québec to establish a varve chronology to help assess the timing of its final drainage as it is implicated in the 8200 yr event. Differences in stratigraphy have been found across the basin. In three sites interior to the Cochran re-advance limit (Paulen, 2001) the stratigraphy is: a till unit, at the bottom, grades into an ice proximal varve unit which thins upward and gives way to a pellet unit of fine grained silts and clays above which a finely laminated sediment becomes massive. Outside the Cochran re- advance limit the sequence is: an ice proximal varve unit containing contorted sections above which disturbed sediments become finely laminated or massive until a contact with darker organic "modern" sediments. A sudden drainage event across the entire basin should leave a distinct signature. The new cores east of the Cochran re-advance act as a test and allow the generation of two new hypotheses: (1) the finely laminated to massive units common in both sets of cores below the darker organics is the drainage stratigraphy or (2) the finely laminated to massive units are the post drainage signature representing the settling out after drainage, making the contact between the varved glacial sediments and the finely laminated sediments the drainage signature. This data may also place the Cochran re-advance closer to the drainage event because the stratigraphy between regions is similar above. It is possible that the Cochran could have been the triggering mechanism for the drainage event. If true, it could have ramifications for the size of the basin and geometry of the ice sheet, putting the ice margin further south and making the basin smaller until final collapse.