MAGNETIC FABRICS OF DRUMLINS OF THE GREEN BAY LOBE

According to some theories, drumlins form due to subglacial deformation of sediment. If so, strain indicators in the sediment should yield deformation patterns that are systematically related to drumlin morphology. Clast fabrics have been used most commonly to make inferences about strain patterns in drumlins but with a wide range of sometimes divergent interpretations. These divergent interpretations reflect, in part, a lack of experimental control on the relationship between the state of strain and resulting fabrics.

We have begun to study fabrics formed by the anisotropy of magnetic susceptibility (AMS) of the Horicon till within selected drumlins of the Green Bay Lobe in south-central Wisconsin. Unlike past fabric studies of drumlins, laboratory deformation experiments that we have conducted with the Horicon till provide a quantitative foundation for inferring strain magnitude and direction from fabrics, specifically from the alignment of principal directions of magnetic susceptibility. Intact till samples were collected from transects in 4 drumlins in Dane, Dodge, and Jefferson counties, by both exploiting existing outcrops and collecting 92 cm cores and sub-sampling them. Coring success has been mixed due to both high groundwater tables in the wet early summer of 2008 and the stoniness of the Horicon till. Results, to date, indicate that weak AMS fabrics, characteristic of very low shear strains (<< 1.0), typify tills within ~1.5 m of the ground surface but below the soil layer. These fabrics are interpreted to reflect supraglacial till deposition. Fabrics are usually stronger at greater depths. These fabrics are consistent with basal till deformation to at least moderate strains (> 1) but in directions and along surfaces that can deviate markedly from the contour of the drumlin. We infer that the fabric of at least some of the drumlins was set by basal till deformation prior to drumlin formation and that drumlinization did not reset the fabric. A tentative conclusion is that these drumlins are largely erosional features formed by processes unrelated to bed deformation, but ongoing core analysis will reveal whether this conclusion is robust.