FAST FLOW DURING READVANCE OF THE LAKE MICHIGAN LOBE, SOUTHWESTERN MICHIGAN

A rapid readvance of the Lake Michigan Lobe into southwestern Michigan between 14 and 15.5 ka BP driven by high basal pore pressures is interpreted from sediment—landform associations and exposures of deformed subglacial sediment. High basal pore pressures were caused or facilitated by advance into a proglacial lake dammed between the ice margin and higher ground to the east. This advance was probably not climatically driven because the adjacent Saginaw Lobe was retreating at the time.

A narrow, N-S oriented band of drumlins ~10 km wide was produced during the readvance at a distance of approximately 40 km from the terminal Kalamazoo Moraine. These drumlins, which trend east-west to northwest-southeast, occur on the western edge of the Valparaiso upland. The Kalamazoo Moraine is a prominent upland containing parallel ridges interpreted as imbricate thrust sheets and large areas of hummocky topography underlain by thick sand and gravel sequences. Large glaciofluvial fans head at the moraine at the ends of valleys interpreted as tunnel valleys and form a broad outwash-filled lowland to the east.

The substrate over which the ice advanced consisted of lacustrine sediments deposited in the proglacial lake(s) that existed along this margin. Exposures show a 5 to 10 m interval of deformed lacustrine silt and clay interbedded with sand and gravel immediately below the diamicton deposited during the advance. Deformation includes tight, asymmetric to recumbent folds with fold axes both parallel and perpendicular to the direction of ice advance. Thin, undeformed sandy stringers and pods in the diamicton and sharp truncation of the deformed lacustrine beds suggest that the ice was partially decoupled and rapidly sliding on its bed facilitated by high pore pressures. The folds may have been produced before or after the most rapid period of sliding, when the ice was exerting more pressure on the bed. The association of drumlins with subglacial and marginal glaciotectonic deformation suggest that advance of the lobe to the Kalamazoo Moraine was rapid and that high subglacial pore pressures were a factor in this advance.