Magnetic fabrics of drumlins of the Green Bay Lobe, southeastern Wisconsin

Abstract

To study if the relief of drumlins of the Green Bay Lobe of the Laurentide Ice Sheet is dominantly the result of depositional or erosional processes, we measured the anisotropy of magnetic susceptibility of ∼2500 samples of the Horicon till from five drumlins in southeastern Wisconsin. These drumlins contain no macroscopic heterogeneities that might otherwise help guide interpretation of deformation kinematics. Ring-shear experiments on this till provided the calibration necessary to relate fabrics based on principal susceptibilities of multiple intact samples to patterns of subglacial till deformation. Fabric strengths, based on orientations of maximum susceptibility and measured from samples collected below the zone of pedogenesis, weaken abruptly at depths shallower than 2 m. The till lacks evidence of systematically divergent or convergent shear related to drumlin shape. Most till has been sheared in directions 6–24° to the east of drumlin long axes, with shear-plane orientations that are unrelated to drumlin morphology. An exception is till at shallow depths (≤2 m) where shearing azimuths and shear-plane orientations agree more consistently with drumlin trends and morphologies. The Horicon till was likely deformed in shear during advance of the lobe. Drumlins formed by erosion of this basal till after glacier flow had shifted to the west, when the Johnstown moraine was being built. A supraglacial till, ≤2 m thick, was draped over some of the drumlins during retreat of the lobe. These results do not support models of drumlin formation in which sediment deposition helps amplify drumlin relief.