Magnetic fabrics of the Douglas Till of the Superior lobe: exploring bed-deformation kinematics

摘要

The Superior lobe of the Laurentide ice sheet, during its last advance into northwestern Wisconsin, may have moved and transported sediment primarily through deep, widespread shear of its bed to high strains (>100). To study the kinematics of this deformation, we measured the anisotropy of magnetic susceptibility of 3750 intact samples of its basal till-the Douglas member of the Miller Creek formation-collected from eight profiles at 0.2 m depth increments. A benchmark for interpretations was provided by results of ring-shear experiments conducted on the Douglas Till, which provided fabric characteristics for the case of horizontal simple shear. Orientations Of Maximum susceptibility (k(1)) were controlled by preferred orientations of silt-sized magnetite grains and were similar to sand-grain orientations measured in thin sections. Strengths of fabrics formed by orientations of k(1) indicate that most of the till sampled was deformed but to variable strains. Deformation averaged over the 20 km spanned by the study approximated horizontal simple shear. However, large differences in k(1) fabric azimuths (>45 degrees) over lateral distances of meters to tens of meters indicate highly heterogeneous bed deformation, perhaps focused in anastomosing zones with associated divergent and convergent till shear. This interpretation is supported by orientations of principal susceptibilities that imply, in many cases, either that shear planes were steeply dipping, particularly transverse to the shearing direction, or that there were major components of pure shear. Variations in k(1) fabric azimuth with depth indicate that most of the till thickness did not shear simultaneously; rather, till accumulated at the bed as shear direction changed in response to temporally shifting zones of shallow deformation (<1 m). This heterogeneous, temporally variable deformation of the bed differs from many applications of the bed-deformation model but is consistent with subglacial measurements at modern glaciers.