USE OF ANISOTROPY OF MAGNETIC SUSCEPTIBILITY TO INFER TILL SHEAR STRAIN IN A DRUMLIN OF THE GREEN BAY LOBE, WISCONSIN

Till contains particles of all sizes from boulders to clay particles. As shear strain increases in till beneath a glacier, so does the degree of particle alignment. This study uses anisotropy of magnetic susceptibility (AMS) to estimate the magnitude of till deformation. AMS describes distortion of an applied magnetic field by aligned magnetic minerals in an intact sample. The direction of maximum magnetic susceptibility is measured for each sample, and using multiple samples we compute an eigenvalue–a measure of the degree to which directions of maximum susceptibility align. Eigenvalues can, in principle, range from 0.33 (random) to 1.0 (perfect alignment). AMS has several advantages over other methods including low cost, speed, and repeatability.

An empirical relationship was developed between particle alignment, as indicated by AMS, and shear strain. This was accomplished using a ring-shear device to shear till to known strains. Sets of 50 till samples were collected after experiments conducted to six shear strains ranging from zero to 100 and analyzed using AMS. Eigenvalues increased with increasing strain until they became steady at shear strains of 20-25.

These results were used to estimate shear-strain magnitude in a drumlin of the Green Bay Lobe of the Laurentide Ice Sheet. AMS fabric results from 11 sampling sites within a transverse cross-section of the drumlin indicate that its till (Horican till) was deformed to shear strains up to 13. The majority of sites indicated lower strains from zero to five. These findings are inconsistent with theories of drumlin formation that invoke pervasive deformation of the glacier bed to large strains as a factor in drumlin genesis. Future work will include extending these measurements to other cross-sections of the drumlin, to other drumlins, and to till that lies between drumlins.