TRANSVERSE RIDGES AND INDICATOR DISPERSAL: IMPLICATIONS FOR COMPOSITIONAL VARIABILITY IN DES MOINES LOBE TILL, MINNESOTA, USA

A series of sub-parallel, low relief, till ridges orientated transverse to ice flow associated with the lateral margins of the Des Moines Lobe (DML) in Minnesota are underlain by a till sheet of variable composition. Similar till ridges are well documented along the southwestern margin of the Laurentide Ice Sheet throughout the mid-continent. These ridges extend over large aerial extents (up to 1000s of km²) on broad flat terrain, frequently located adjacent to topographic highs. The subtle swells of the till ridges create a washboard pattern across the landscape and are commonly referred to as washboard, corrugated, or minor moraines. Washboard moraines have been attributed to sub-glacial extrusion of fine-grained, water-saturated till into basal crevasses of stagnant ice, or meltout of englacial till concentrated along shear plains of active ice.

Recent mapping has delineated a spatial relationship between washboard moraines and the shale content of DML till. Till of the DML contains a distinctive indicator lithology, Cretaceous Pierre Shale, that can be used to evaluate change in till sedimentology. Samples of till from three separate investigations were combined to evaluate the percentage of shale in the 1-2 mm fraction. Preliminary modeling results indicate (1) shale content is uniform along the central axis of the lobe, (2) shale content rapidly decreases near the margins of the lobe, representing the greatest spatial compositional variation and coincident with the occurrence of washboard moraines, and (3) the shale content further decreases as the DML advanced up the adverse slopes of the Coteau des Prairie and Alexandria Moraine. The integrated patterns between washboard moraines and zones of rapid change in shale content suggest that compositional variability is the result of processes acting during formation of a single till unit. We propose that the high shale content along the axis of the DML is due to a lack of erosion of the underlying substrate and consequent pure advection of shale-bearing debris by the lobe. The shale content of till at the margins of the lobe rapidly decreases due to higher rates of incorporation of non-shale bearing debris.