DEFORMATION OF THE DOUGLAS TILL, NORTHWESTERN WISCONSIN

Some former ice sheets may have moved primarily by shearing their till beds, which would have affected their dynamic response to climate variability. This hypothesis for glacier flow requires that basal till be sheared to high strains (>100). Till fabric measurements may provide information about strain magnitude, and thus help test this model as applied to past ice sheets.

This study focuses on the Douglas till, a clay-rich basal till exposed in bluffs along the south shore of Lake Superior in northwestern Wisconsin. This till was deformed to an unknown extent beneath the Superior Lobe of the Laurentide Ice Sheet and contains unusual pebble fabrics that lie transverse (NW-SE) to the regional ice-flow direction (NE-SW). Transverse fabrics are not consistent with results of laboratory experiments in which fabric development was measured in samples of Douglas till sheared to strains as high as 714 in a ring-shear device. These results indicated that strong flow-parallel fabrics developed with increasing strain, with no transverse fabric development, contrary to theory developed for viscous fluids. We measured fabrics resulting from the anisotropy of magnetic susceptibility (AMS) of multiple samples in multiple profiles of Douglas till at 0.2 m intervals. In the longest profile, sand-particle fabrics and microshear orientations were also measured. AMS fabric strengths are generally strong (70% of S₁ eigenvalues ranged from 0.80-0.98). Orientations vary with depth and change from transverse to parallel to the regional glacier flow direction over lateral distances as small as a few meters. Sand-particle and AMS fabrics were similarly oriented. Strong fabrics indicate that the Douglas till has been sheared to a strain of at least 10 over much of its thickness. Variable fabric strengths and orientations with depth suggest that the till was deposited by progressive bed accretion and deformation due to lodgement. Patchy bed deformation with flow divergence around less deformable areas of the bed may explain large lateral variations in fabric direction over short distances. These measurements indicate bed deformation to be considerably more complex than the unidirectional simple shear that is usually assumed.