GLACIOLACUSTRINE SEDIMENTATION AND CHRONOLOGY OF THE WESTERN THUNDER BAY LOWLAND, NORTHWEST ONTARIO

The deglacial chronology of the Laurentide Ice Sheet (LIS) within northwest Ontario is critical to understand the possible hydrologic linkage between glacial Lake Agassiz and the upper Great Lakes between 11.0 and 8.0 ka ¹⁴C yrs BP (13.0 to 8.9 ka cal yrs BP).  Proposed outburst floods from Lake Agassiz through the Great Lakes to the North Atlantic Ocean have been implicated in the reduction of North Atlantic thermohaline circulation and pegged as a cause for the Younger Dryas cold event (11 – 10 ka ¹⁴C yrs BP; 13 – 11.4 ka cal yrs BP). Deglacial chronology and glacial stratigraphy are also important in determining the response of the LIS to the Younger Dryas cold period (i.e., regional readvance).  The Marks Moraine west of Thunder Bay, Ontario has been previously correlated to the well-dated 10 ka ¹⁴C yrs BP ice margin positions on the southern end of the Superior Basin.  Inside the Marks Moraine, glaciolacustrine sediments lie above and below brown-red Superior Lobe diamicton.  Cutbank exposures along the Whitefish River and lacustrine cores from inside and outside the Marks Moraine were examined in order to discern the glacial chronology.  Results from the Harstone Cut exposure (particle size analysis, loss on ignition, and magnetic susceptibility) show that at least 250 varves are present below Superior Lobe brown-red diamiction and 160 varves are present above diamiction from Mokomon Lake (~20 km to the north), although the basal diamicton was not reached in Mokomon Lake.  Varves below diamicton indicate the Superior Lobe advanced into, and subsequently overrode, a glacial lake (glacial Lake O'Connor).  These varves differ significantly from offshore deglacial Lake Superior varves - glacial Lake O'Connor summer beds are coarser (~15 µm) and rippled sand beds are present, indicating a proximal sediment source (i.e. the Superior Lobe).  Varves above Superior Lobe diamicton are also distinct - they show more classic rhythmic sedimentation and are finer than those of Lake O'Connor.  Glacial Lake O'Connor sediments indicate an advancing Superior Lobe within the lowland west of Thunder Bay and that the Superior Lobe was present in the lowland >250 years before the western end of the lowland was overridden.  This indicates that the LIS was within the Superior Basin prior to the Superior Lobe advance, which is at odds with current interpretations.