Deformation of the Douglas till, northwestern Wisconsin

The bed-deformation model asserts that a glacier can move by pervasively shearing its bed to strains sufficiently large to account for most glacier motion (>100). Although commonly invoked, this hypothesis has never been unequivocally tested using the geologic record. In this study, laboratory fabric-strain calibrations are used to evaluate strain magnitude and shear direction in the Douglas till of northwestern Wisconsin and to thereby test elements of the bed-deformation hypothesis. The Douglas till is a clay-rich basal till deposited by a late-Wisconsinan advance of the Superior lobe of the Laurentide ice sheet. This till contains unusual pebble fabrics that lie transverse (NW-SE) to the regional ice-flow direction (NE-SW), as indicated by flutes.; Anisotropy of magnetic susceptibility (AMS) was measured along eight vertical profiles through the Douglas till at 0.2 m intervals, and AMS fabrics were computed from principal directions of magnetic susceptibility. Sand-particle fabrics and microshear orientations were also measured in one of these profiles. AMS data were interpreted using results of ring-shear experiments, which demonstrated that strong, flow-parallel fabrics (steady-state S1 eigenvalue of 0.83) develop in the Douglas till at a shear strain of ∼20.; AMS fabrics are generally strong (63% of S1 ≥ 0.83), indicating that most of the till has been sheared to a strain ≥ ∼20. Sand-particle and AMS fabrics were similarly oriented, and microshears indicate fabric development was subglacial, rather than in shearing basal ice. Major variations in fabric orientation and strength occur laterally over distances of a few meters and with depth over decimeters, indicating that the till deformed heterogeneously, probably during progressive accretion of till to the bed by lodgement. Strong fabrics transverse to the regional glacier-flow direction are interpreted to reflect shear divergence or convergence in a heterogeneously deforming bed, resulting in local flow directions commonly perpendicular the regional one. These measurements indicate that deep, unidirectional, simple shear of the bed, as is usually assumed in models, was unlikely.