A LABORATORY AND FIELD STUDY OF THE MICROSTRUCTURAL STRAIN SIGNATURE IN A BASAL TILL, LAKE MICHIGAN LOBE

Landscapes formed by southern lobes of the Laurentide Ice Sheet have been associated frequently with deformation of subglacial sediments. However, the degree to which those sediments (commonly till) have been sheared has never been quantified. Using a large ring-shear device, a basal till of the Lake Michigan Lobe (LML) (Batestown member) was sheared to different prescribed strains that ranged through nearly three orders of magnitude. The alignment of elongate sand-sized particles (microfabric) was determined as a function of strain by measurement of grain orientations in thin section. In addition, the degree of alignment of magnetic particles was quantified by determining the fabric defined by directions of maximum magnetic susceptibility of multiple samples. Laboratory results show that these fabrics become stronger and increasingly parallel to the shearing direction with strain and do not become steady until shear strains of 20-30. Thus, multiple proxies for strain have been developed for application to field studies.

Field samples of the Batestown till were taken from two localities along the LML flow path. Intact samples were extracted at 20 cm intervals along four vertical profiles that spanned the entire till thickness (2.0-3.4 m). Thus far, only magnetic susceptibility analyses have been completed. Based on the laboratory calibrations, field data indicate that significant portions of the Batestown till have been deformed to shear strains of at least 10. Patterns of fabric strength and direction suggest that strain accumulated as the basal till accreted over time rather than during a single deformation event. Comparison with microfabric data and further field studies are needed to test this hypothesis.