EVIDENCE FOR HIGH MAGNITUDE FLOWS IN TUNNEL CHANNELS, SOUTH-CENTRAL MICHIGAN, USA

The glacial landscape in south-central Michigan is a dissected surface with streamlined, residual, and upland blocks surrounded by lowland valleys dominated by outwash. The upland surface has ridges, hills, and hummocks composed of boulder gravel, and is inset with tunnel channels, some with eskers in them. Tunnel channels in south-central Michigan mapped over an area 125 km wide are up to 50 km long, and 2-3 km wide. Overall, the tunnel channels make up a complex network of channels that splay southwards. Ground penetrating radar transects, and hummock exposures located in tunnel channels reveal that hummocks (~100 m diameter) are glaciofluvial bedforms with a supraglacial flow-till veneer. The hummocks within tunnel channels indicate that tunnel channels are a result of channel-wide flows capable of forming large bedforms within boulder gravel. In-phase wavy contacts between glaciofluvial sand and subglacial melt-out till along a tunnel channel wall have a 0.5 m amplitude and 3-5 m wavelength. Clasts from the overlying till penetrate and deform the underlying sand recording recoupling of the ice to its bed. The glaciofluvial sand is interpreted as cavity fills of erosional ripples carved into the glacier’s sole from waning flow within the tunnel channel. It appears from a variety of evidence, that subglacial meltwater played important roles in the evolution of the subglacial landscape in south central Michigan.