A COMPARISON BETWEEN PROGLACIAL SUBAERIAL SEDIMENT DEBRIS FLOWS AT THE MATANUSKA GLACIER, ALASKA, AND POSSIBLE SIMILAR DEPOSITS IN WISCONSIN

Subaerial sediment debris-flow deposits ("flowtills" or "till flows") were analyzed at the Matanuska Glacier, Alaska, during the summer of 2002 following the protocol established by Lawson (1976). The following summer, diamictons in portions of thirteen counties in Wisconsin and interpreted as subaerial sediment-flow deposits also were analyzed following the same protocol to determine if they had similar properties to those at the Matanuska. The Wisconsin diamictons are much coarser (72% sand, 18% silt and 10% clay, s=15 %, 11% and 9% respectively) than those that occur at the Matanuska Glacier (dominated by silt) and exhibit much weaker flow signatures than those at the Matanuska Glacier. Also, the fabric strengths of Wisconsin diamictons are similar to those at the Matanuska (S1 values at each location fall in the range of 0.50 to 0.70). The composition of the measured fabric pebbles in the Wisconsin diamictons averaged 43% sedimentary, 29% metamorphic, and 28% igneous rock types, and their shapes were dominated by discs followed by rods, blades then spheres (42%, 24%, 23%, 11% respectively). A combination of necessary bed conditions for glaciohydraulic supercooling (Larson et al., in press) coupled with textural and fabric analyses suggests that locally similar conditions conductive to basal freeze-on and debris release as sediment flows may have existed along the margin of the Laurentide Ice Sheet. Ice flowing through the Lake Michigan and Lake Superior basins during the last glacial maximum could have incorporated debris into its bed as basal meltwater supercooled and froze along its base. Similar processes have been observed at the Matanuska Glacier and other active ice margins throughout the world. Glaciohydraulic supercooling has been observed at the Matanuska Glacier as well as other glaciers and it has been suggested (Lawson et al., 1998) that the process produces thick sections of debris-laden basal ice which upon melting results in sediment flows. The abundance of sediment-flow deposits in Wisconsin suggests that glaciohyraulic supercooling and basal freeze-on may also have existed locally along the margin of the Laurentide Ice Sheet.