DRAINAGE OF GLACIAL LAKE AGASSIZ INTO THE SUPERIOR BASIN

Glacial Lake Agassiz used a succession of different outlets during its history, some thought to be eastward, through the Lake Superior basin. Two major episodes of eastward drainage, one through the Thunder Bay area at about 12.9 cal ka, and one through the Nipigon area at about 11.3 cal ka. The older episode has been implicated in global climate change at the beginning of the Younger Dryas (YD), due to flooding of the North Atlantic with fresh water and suppression of Atlantic meridional overturning circulation. New high-resolution seismic-reflection data from Lake Superior bear on the history of Lake Agassiz discharge into the basin. The data for Thunder Bay and adjacent Lake Superior, the purported route for Lake Agassiz discharge at the beginning of the YD, do not support high discharge flows into the basin then. No evidence of deltaic sedimentation, bedrock incision, erosion of bay-mouth sills, or deposition of deepwater subaqueous fans were observed. Complete removal of all evidence of high-magnitude discharge at the beginning of the YD by the subsequent Marquette readvance of ice into the basin seems unlikely. In contrast, the seismic data provide abundant evidence for younger Lake Agassiz discharge farther to the northeast. These data are consistent with terrestrial evidence of post-Marquette (post 11.5 cal ka), high-volume, eastward drainage of Lake Agassiz through the area around Lake Nipigon. Several embayments of Lake Superior in this area are underlain by deep bedrock channels that are not apparent in the modern bathymetry. Lakeward of these incised channels, where bedrock descends into the Superior basin, are distinct deposits that we interpret as subaqueous fans, related to high discharges into the basin. The deposits are laterally discontinuous and distinctly fan shaped. Internally, they are composed of a lower chaotic unit and an upper weakly stratified unit, from which we infer an early, high discharge phase, followed by waning flows. These features, incised bedrock channels and deepwater subaqueous fans, are considered diagnostic of high-magnitude discharge into the Superior basin. Similar features do not occur in the Thunder Bay area, so, although drainage at the beginning of the Younger Dryas may have been eastward, it was not associated with catastrophic flooding.