NUMERICAL MODELING OF SUBGLACIAL SEDIMENT DEFORMATION - IMPLICATIONS FOR THE BEHAVIOR OF THE LAKE-MICHIGAN LOBE, LAURENTIDE ICE-SHEET

We apply a numerical model of the late Wisconsin (circa 20,000 years B .P.) Lake Michigan Lobe (LML), Laurentide Ice Sheet, to investigate ho w fine-grained subglacial sediment might influence lobe behavior, part icularly rapid millennial-scale marginal oscillations observed in the geologic record. Over the Canadian Shield, we assume a rigid bed (''ha rd bedded'') basal boundary condition. In areas overlain by fine-grain ed sediment (''soft bedded''), the base of the ice is coupled to a def ormable sediment layer, using a rate-dependent stress-strain law. Geot echnical tests of clay-rich till deposited by the LML provide control for sediment theologic parameters. Simulated cross-sectional profiles are consistent with reconstructions from geologic evidence. Time-depen dent simulations suggest that a soft-bedded lobe could have reached st eady state in about 20,000 years or less, in contrast to 50,000 to 60, 000 years for an otherwise identical hard-bedded lobe. A soft-bedded l obe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial-scale shifts in accumulation o r ablation as a nonsliding hard-bedded lobe, but in both cases the res ponse is slower than that indicated by the geologic record. Results su ggest that while strong millennial-scale changes in accumulation and a blation can produce responses in hard-bedded or soft-bedded ice that a re consistent with the geologic record, changes in subglacial sediment viscosity, even relatively modest changes (whether independent or in conjunction with climate change), might more readily account for mille nnial- and submillennial-scale fluctuations of the lobe margin. These observations do not exclude a role for sliding, but they do provide so me perspective from which to evaluate relative contributions of the va rious processes that influence lobe behavior.