PALIMPSEST TUNNEL CHANNELS OF THE SAGINAW LOBE

The network of valleys associated with the Saginaw Lobe of the Laurentide Ice Sheet is a striking feature on regional DEMs. These relatively straight valleys begin in the Saginaw Bay lowlands of southern Michigan and follow southwesterly trends, parallel to the presumed flow lines and subglacial hydraulic gradient of the lobe. These aspects and other evidence have led several researchers to interpret the valleys as tunnel channels in the past several years. Disagreement remains, however, with respect to the relative age of the channels, with Fisher et al. (2005) proposing that the channels were eroded during a subglacial megaflood event. Investigations near the southern margin of the Saginaw Lobe at its LGM or near-LGM limit suggest that subglacial discharges producing tunnel channels were repetitive events distributed throughout the history of the lobe. In the vicinity of the Sturgis moraine, near the Indiana-Michigan border, tunnel channels cut into bedrock are both distal to the moraine and developed within the moraine itself, suggesting that they formed originally during an advance terminating well south of the moraine and were later reoccupied when the ice readvanced and formed the moraine. North of the Sturgis moraine, near Kalamazoo, a tunnel channel was delineated by four rotasonic borings to bedrock. This channel is weakly visible in the surface topography, mainly by aligned lakes and wetlands. The borings indicate that the channel was cut into bedrock, filled by outwash, overridden by another advance that deposited a diamicton, and finally capped by more outwash. It seems unlikely that this palimpsest tunnel channel could have been formed at the same time as the deeply incised, well defined channels to the north approaching the Saginaw Bay lowlands. Thus we argue for multiple episodes of tunnel channel genesis associated with multiple advances of the glacier.