Mobilization, Movement and Deposition of Active Subaerial Sediment Flows, Matanuska Glacier, Alaska

Abstract

Subaerial sediment flow is the predominant process depositing diamictons at the terminus of Matanuska Glacier. Flows originate where sediments overlie glacier ice. Ablation of ice exposed in slopes disaggregates the overlying sediment and mixes it with meltwater and debris released simultaneously. This material generally flows only after its strength is further reduced by excess pore pressures and seepage pressures generated by meltwater from thawing ice. Moving sediment flows show reasonably systematic changes in physical attributes such as dimensions, texture, flow rates, density and erosional action, and in grain support and transport mechanisms that can be related to changes in the water content of their matrix material. At lowest water contents, flows support grains by their strength and move through shear in a thin zone at their base. Increased thickness of the zone in shear and deformation of other types accompany increased water contents, with grain interference and collisions, localized liquefaction and fluidization, transient turbulence, and bedload traction and saltation operating simultaneously in such moving flows. At highest water contents, flows appear fully liquefied. The fluidity of the sediment flow and the amount of water in the sediment flow channel determine the degree of preservation of the source flow's properties and the depositional morphology. Because mobilization of a sediment flow destroys the glacial sedimentary properties of its sediment source and, further, because the mechanics of transport and deposition develop new "non-glacial" properties in this sediment, the diamicton deposited in the glacial environment by sediment flow should not be called till.