Numerical modeling of advective transport of saturated deforming sediment beneath the Lake Michigan Lobe, Laurentide Ice Sheet

We investigated the dynamics of advective transport of saturated, fine-grained deforming sediment beneath the Late Wisconsinan Lake Michigan Lobe (LML) of the Laurentide Ice Sheet using a numerical flowline model. Sediment rheology is modeled using a general constitutive law for mass movement of fine-grained material. Geotechnical tests on selected tills deposited by the LML in northern Illinois suggest an effective viscosity of about 5 × 109 Pa-s for fully saturated sediment and mild nonlinearity in the stress-strain relation. Numerical calculations of sediment flux, based on conservative parameters and assumptions, yield estimates on the order of 100 m3/y/meter width of the lobe, suggesting that advective transport could account directly for at least 25% of the total sediment flux estimated on the basis of field evidence. Less restrictive parameters and assumptions would increase sediment flux attributable to both advective transport as well as other ice-velocity-dependent sediment transport processes. Modeling results also suggest that maximum advective transport rates might be associated with viscosities up to an order of magnitude higher than the viscosity exhibited by fully saturated sediment. These results thus suggest that subglacial sediment deformation could account for a substantial portion of total sediment flux in areas with fine-grained glacial till, and that transport rates are strongly sensitive to the subglacial hydrologic processes that control basal water pressure and sediment viscosity.