LANDFORMS, CHANNEL MORPHOLOGY AND CROSS-CUTTING RELATIONSHIPS: EVIDENCE FOR TIME-TRANSGRESSIVE FORMATION OF SAGINAW LOBE TUNNEL CHANNELS IN SOUTHERN MICHIGAN

Ten years of mapping, stratigraphic investigation and spatial analysis on a set of radial trenches spanning a 70 km wide area in southern Michigan has lead to the interpretation that these features represent Saginaw Lobe tunnel channels of Laurentide Ice Sheet. Approximately twenty-five NE-SW channels ~25 m deep, 1 km wide and tens of kilometers long occur in sediments 3-180 m thick overlying Mississippian and Pennsylvanian age sedimentary rocks dipping northeasterly to the center of the Michigan Basin. Channels display variable morphologies that include straight walled stream cut channels, crenulated sections that expand and contract and subtle channel sections only recognizable as chains of depressions and lakes in the regional hydrology. Channels longitudinal profiles increase in elevation down glacier at nearly right angles to the regional slope, strongly suggesting they formed under pressurized conditions. Some channels are continuous through hummocky uplands formerly mapped as Saginaw lobe moraines, while other channels terminate at large glaciofluvial fans at former ice marginal positions. In addition to glaciofluvial fans channels occur in association with a myriad of other glacial landforms including drumlins and eskers. Although many of the channels parallel one another, topographic cross sections constructed at right angles to the channels indicate channels were incised to different depths. Further, cross-cutting relationships between channels and landforms provide striking evidence to suggest that long channel systems observed may have formed incrementally, possibly been reoccupied, and may have been operating independent of one another. Although the above evidence suggests channel systems may have formed time-transgressively there is evidence to suggest in a number of instances specific channels may have formed from discrete high-magnitude discharge events.