THE ORIGIN OF THE CENTRAL KALAMAZOO RIVER VALLEY: RELATIONSHIPS BETWEEN OUTBURST FLOODS, TUNNEL CHANNELS, ICE DYNAMICS AND REGIONAL DRAINAGE HISTORY IN SOUTHWESTERN MICHIGAN

The Central Kalamazoo River Valley (KRV) is located in southwestern Michigan and occurs as a deeply incised trench over a mile wide and in excess of 300 feet deep. This prominent pysiographic feature is situated in a reentrant formed by the Lake Michigan, Saginaw and Huron-Erie lobes of the Laurentide Ice Sheet. Newly collected geomorphological and stratigraphic information suggests that this valley is the result of a catastrophic outburst flood emanating from the Saginaw lobe of the Laurentide Ice sheet. The irregular and tortuous path of the KRV bisects the Kalamazoo moraine (15,500 ka B.P.) of the Lake Michigan lobe, the most prominent moraine in SW Michigan. Morphological indicators in the valley such as erosional residuals, large bars, cross-cutting relationships and channel size support an origin of rapid incision. Exploratory boreholes and near surface geophysics show the valley to be partially infilled with late Pleistocene and Holocene sediment. The oversize channel terminates at a large gravel fan at Plainwell, Michigan. High elevation erosional terraces near the mouth of the channel indicate that the spillway route from the east was previously established by either proglacial lake spillway incision or superglacial meltwater flow from the Lake Michigan lobe draining from west to east. At the time of valley formation by the westward flowing outburst the Lake Michigan lobe had retreated at least 35 km to the west to the Lake Border moraine (14,000 ka B.P.) or possibly beyond. With the Lake Michigan lobe absent to impede flow path, drainage proceeded southwesterly until draining into Lake Chicago near St. Joseph, Michigan. The source of the outburst appears to have been a system of tunnel channels beneath the Saginaw lobe. Along the KRV, the meltwater flowed beyond the extent of the subglacial channels and became subaerial. Segments believed to be tunnel channels display convex-up flow paths and contain eskers. During the interval represented by the outburst, the Saginaw Lobe appears to have been in a relatively stationary position.