FAULT ORIENTATIONS AS EVIDENCE FOR SUBGLACIAL DEFORMATION IN DRUMLIN FORMATION, NORTHEASTERN MICHIGAN

Countless hypotheses have been proposed to attempt to explain the origin of drumlins, including direct ice erosion, erosion by subglacial flooding, and differential deformation of subglacial sediments. A few miles south of Rogers City, Michigan, a gravel pit exposes the sediments of a drumlin along the entire length of its cross-section. The drumlin is composed of layers of sorted sands, silts, clays and diamicton. The upper several meters are extremely deformed. Below this, the sediments are less deformed to a depth of over 12 m. Here, the clays have deformed plastically, whereas the sands have deformed in a brittle fashion. Using a Brunton compass, fault orientations were measured throughout the sand layers, then plotted using the program Stereonet 4.9.5a. Analysis of the plots showed most fault planes occurred roughly perpendicular to the drumlin orientation, with a few nearly vertical planes roughly aligned with the gravel pit face. The alignment of the faults perpendicular to the drumlin axis seem to reflect direct basal ice deformation, presenting strong evidence that the process that caused the deformation at depth is directly related to the process that formed the drumlin itself.