EVIDENCE FOR RAPID DRUMLIN FORMATION THROUGH INTENSE SEDIMENT DEFORMATION RESULTING FROM INCREASED PORE WATER PRESSURE DURING A SURGE OF THE HURON LOBE

Drumlins composed primarily of sorted material were studied in the Metz drumlin field, south of Rogers City, MI, where the surface relief between drumlins and intervening low areas is between 3-10 m. An exposure along the central long-axis of one drumlin extends 130 m and is over 15 m deep, revealing sediments to a greater depth than the surface relief. The lower package of drumlin sediments are distinct, sorted layers of sand, silt, and clay that are mildly deformed to a depth of over 12 m below the surface. The clay/silt layers have been sheared, hardened, and thrust over overlying sediments, creating a stack of sheared clay/silt layers with intervening very fine sand and coarser sand layers. The upper 2-9 m of sediment consists of a contorted mix of intensely deformed gravel, sand, silt, and diamicton. Truncated beds at the lower contact with the layered sorted sediments indicate that the contact is erosional, with thrust faults on the ice proximal side, and normal faults on the ice distal side of the drumlin. Blocks of sediment, locally quarried from the underlying sediments, are less deformed than the intensely deformed sediments they are contained within. These structures indicate that drumlin formation was initiated by an increase in pore water pressure within the sediments underlying the ice sheet. An increase in pore water pressures would lower the effective normal stresses of the overlying ice and change the deformation style from shear deformation of deep clay/silt layers to contorted deformation of sand, silt, and diamicton layers at the surface to a lower effective depth. The high pore water pressures responsible for this deformation must have initiated a surge of the Huron Lobe during the Greatlakean stage of glaciation. Local ice stagnation features, such as enormous kames and eskers stretching for tens of miles, indicate that stagnation followed the surge. This new evidence may show that entire drumlin fields form by rapid deformation of subglacial sediments related to surges of ice lobes.