T192 Post-Glacial Landscape Evolution: Landforms and Processes in Alpine and Sub-Alpine Areas

Tuesday, 6 November 2012
1:30 PM - 5:30 PM
207BC

KIRK BRYAN AWARD: FABRICS AND PATTERNS OF TILL DEFORMATION BENEATH GLACIERS

Paper number: T192-12
Program: Technical Sessions
Time: 4:25 PM – 4:55 PM

Authors

Abstract

Till beneath glaciers commonly deforms, affecting their slip dynamics, sediment transport, and resultant landforms. Particle fabrics of basal tills of the geologic record have been used for decades to infer aspects of till deformation and genesis but with interpretations predicated on consensus rather than experimentation. Experiments with a large, custom-built ring-shear device both disprove conventional wisdom, such as the Jeffery model of particle rotation as applied to tills, and demonstrate the utility of till fabric based on anisotropy of magnetic susceptibility (AMS). Fabrics derived from the three principal magnetic susceptibilities of intact till specimens provide a reproducible and three-dimensional indicator of the magnitude, direction, and style of deformation that is superior to traditional grain fabrics.

AMS-based fabrics measured by densely sampling basal till sheets of the U.S. Midwest indicate that till deformed in patches, commonly with strongly divergent or convergent shear and steeply-inclined shear planes. Deformation usually did not extend to depths of more than a few decimeters below the ice and was accompanied by progressive till aggradation. These observations are fully consistent with steep gradients in pore-water pressure measured in the beds of modern glaciers and do not support the deep, pervasive, simple shear to high strains assumed in some models.

AMS fabrics of drumlins of the Green Bay Lobe in southeast Wisconsin and of low-relief, transverse (washboard) moraines of the Des Moines Lobe in central Iowa help illuminate the genesis of these landforms. AMS fabrics are not symmetrically disposed about drumlin long axes but indicate bed shear 7-25° to the southeast, with shear-plane orientations not related to local drumlin slopes. Drumlin sculpting was likely the result of erosion of a till layer deformed previously by more southeasterly flowing ice. AMS till fabrics of moraine ridges indicate that they formed by upward shearing and extrusion of till into basal crevasses, demonstrating surge-like motion of the Des Moines Lobe.