DES MOINES LOBE TILL COMPOSITION AND TEXTURE AS AN INDICATOR OF FLOW DYNAMICS

The lack of lithological and textural variation of the Des Moines Lobe (DML) till exposed in southwestern Minnesota indicates decoupling of basal ice from its sediment bed. DML till contains an abundance of Pierre Shale, which can be used as a distinctive indicator lithology. From the source of the shale in Manitoba to the field area in western Minnesota, the DML till develops its textural and lithologic composition. As the DML lobe continues to advance into Minnesota, the topographic highs of the Coteau des Prairies and the Alexandria Moraine confine the lobe. Within this topographic confinement, the till exhibits no systematic change in either lithologic or textural composition along the central axis of the lobe, despite increasing transport distances of greater than 150 km. This observation indicates the till is not being diluted by the progressive incorporation of underlying material nor comminuted by englacial or subglacial shearing. However, the shale content and texture of the till deposited along the margins of the DML rapidly changes as the ice advances over mild adverse slopes indicating both dilution and comminution rapidly occur. Consolidation tests of the DML till (Hooyer and Iverson, 2002) suggest that the basal water pressure was near the ice overburden pressure. We test the hypothesis that topographic constraints restrict the flow of basal meltwater along the axis of the lobe, yielding higher water pressures to decouple the ice from the bed and effectively decreasing shear strain within the bed. Along the DML margins, where the ice is in contact with the bed, basal processes of comminution and dilution operate effectively resulting in a compositionally and texturally variable till.