CGRG Bibliography of Canadian Geomorphology
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Author : Cutler, P.M.; Colgan, P.M.; Mickelson, D.M.; and MacAyeal, D.R.
Date : 2000.
Title : Influence of the Great Lakes on the advance of the southern Laurentide Ice Sheet at the Last Glacial Maximum.
Publication : 2000 Geological Society of America, Annual Meeting -- November 12-16, Reno, Nevada, USA.
Issue :
Page(s) :
Abstract
The Laurentide Ice Sheet encountered the Great Lakes as ice moved towards its last-glacial-maximum (LGM) extent. When ice entered a lake, calving became a major factor in the ice-sheet mass balance. The rate of mass loss from calving was related to water depth; deeper lakes like Lake Superior presented the greatest obstacle to southward advance. The bathymetry of the Great Lakes therefore offered a geographically diverse influence on calving, and must have been consequential in the timing and extent of advance around the southern margin. Countering the bedrock-controlled influence on water depth was moraine-shoal development at the ice-sheet margin. Shoal migration through sediment recycling and addition of fresh material from upstream offered a means of locally diminishing the water depth, thus reducing calving losses and promoting ice advance. We use a time-dependent flowband model that treats calving, moraine-shoal development, isostacy, and lake-outlet dynamics to explore the interplay between these factors and their influence on behavior of ice lobes in the Great Lakes region. The model uses finite-element methods, with a horizontal node spacing of 15 km. Despite uncertainties in pre-LGM climate, our simulations show that glacier ice would have had difficulty traversing Lake Superior. Advance through the central portion of the lake was potentially delayed for a few thousand years compared to ice at similar latitudes without a deep-water margin. Moraine-shoal development was initially of minimal consequence in Lake Superior because fresh-sediment supply from upstream was impeded by long-lived frozen-bed conditions over uplands to the north. Only when the bed thawed and climate approached LGM conditions did ice inputs outweigh losses to permit advance out of the lake basin. Results suggest that the lack of ice occupation in Wisconsin's Driftless Area probably resulted from the presence of Lake Superior to the north. More broadly, we speculate that more extensive pre-Wisconsinan glaciations were enhanced by shallower Great Lakes.
Bibliography of Canadian Geomorphology