Seismic stratigraphy of Thunder Bay and the Isle Royale region of Lake Superior.

Abstract

Glacial Lake Agassiz was the largest of the proglacial lakes to have formed in North America during retreat of the Laurentide Ice Sheet. The extent of Lake Agassiz was controlled by a combination of factors including topography, ice position and degree of isostatic rebound. Most changes in the outlet location of the lake involved a catastrophic release of water. Evidence for some of these catastrophic floods are visible in both terrestrial and sub‐lacustrine records. An influx of freshwater from Lake Agassiz to the North Atlantic from such floods has been suggested as a possible trigger for the initiation of the Younger Dryas cold interval. A series of airgun single‐channel seismic‐reflection profiles from Thunder Bay and the adjacent Isle Royal Trough of Lake Superior suggest that water from Glacial Lake Agassiz did not catastrophically release into Lake Superior at this locality, as has been previously suggested. Seismic surveys from a region further north within Lake Superior identified features of a younger catastrophic release of water. The features associated with the younger event include bedrock scour and large debris fans, neither of which are present in the Thunder Bay area. Given the similarity in geomorphic conditions at the two locations, similar features are expected if the same type of event occurred. Therefore, the absence of such features suggests that Lake Agassiz did not drain catastrophically through Thunder Bay. Instead of catastrophic outflow features in the seismic records, a thick package of distinct units occur in the Isle Royale trough, which suggest a complex history of ice retreat and readvance. Trough parallel profiles show a massive seismic package, which grades into more individually distinguishable units in the direction of general ice retreat.