LATE WISCONSINAN READVANCE OF THE LAKE MICHIGAN LOBE: A POSSIBLE SURGE DRIVEN BY HIGH SUBGLACIAL PORE PRESSURES

The Lake Michigan lobe advanced eastward across western Michigan during the Michigan subepisode, where it met the southwestward moving Saginaw lobe or overrode areas recently deglaciated by the retreat of the Saginaw lobe. The Ganges till was deposited during the first advance. During and after retreat from this advance, proglacial lakes fronted the glacier eastward to the high topography in the vicinity of the Kalamazoo moraine. Lake elevations up to 850 feet indicate that drainage to the south was blocked at some point. Stratigraphic correlations in Van Buren County suggest a sequence of units representing ice-proximal deposition, followed by quiet offshore conditions, and ice-proximal deposition again during a major readvance, possibly to the Kalamazoo moraine. This advance is associated with the formation of drumlins on the western edge of the Valparaiso upland and extensive subglacial deformation of the lacustrine sediments on the crest of the Valparaiso upland. Irregular uplands previously mapped as the Kendall or Valparaiso moraine, may actually be huge ice-thrust masses. Evidence of thrusting persists as far eastward as the proximal margin of the Kalamazoo moraine. The Kalamazoo moraine itself is a broad, collapsed mass of sand and gravel, with extensive glaciofluvial fans beginning at the distal margin. These relationships are consistent with a surge of the lobe driven by high subglacial pore pressures in the underlying lacustrine sediment. High pore pressures were caused by deep proglacial lakes impeding marginal drainage and higher topography in the direction of ice movement, which also restricted subglacial drainage from the ice margin. The surge may have terminated at the Kalamazoo moraine with broad marginal stagnation and a release of subglacial meltwater to form the proglacial outwash fans.