Saginaw Lobe tunnel channels (Laurentide Ice Sheet) and their significance in south-central Michigan, USA

Abstract

A network of tunnel channels in southern Michigan records substantial subglacial meltwater activity beneath the Saginaw Lobe of the Laurentide Ice Sheet. The channels are incompletely filled with outwash, contain eskers, and in many places crosscut and continue beyond upland ridges previously mapped as recessional moraines. The presence of the tunnel channels and drumlins on these upland ridges indicate that the ridges are not recessional moraines. Instead, outwash fans and minor ridges record deglaciation in the area. A palimpsest relationship between buried bedrock valleys and tunnel channels records the presence of multiple generations of tunnel channels. Evidence for a subglacial meltwater sheetflow consists of sheets of boulder gravel in upland >15 km wide with a hummocky upper surface. The hummocks transform down flow into erosional remnant drumlins; possibly the result of flow acceleration on a negative slope. Tunnel channels at the distal end of the drumlin swarm suggest collapse of the sheetflow into a channelized flow. The tunnel channels then end at the Sturgis Moraine at the heads of large outwash fans. The observed geomorphic relationships between tunnel channels, moraines, and drumlins in south-central Michigan are applicable to glacial landform studies elsewhere, and indicate the important role of meltwater in landscape evolution.