GEOMORPHOLOGY AND QUATERNARY GEOLOGY OF TUNNEL CHANNELS ALONG THE WESTERN MARGIN OF THE GREEN BAY LOBE, WISCONSIN

Tunnel channels are common along Marine Isotope Stage 2 (MIS 2) ice marginal landscapes of Wisconsin. Although the geomorphic expression of tunnel channels on the modern landscape varies among former lobes of the Laurentide Ice Sheet, their location is typically marked by linear zones of collapse perpendicular to former ice margins. It is difficult to estimate the size tunnel channels were during formation using their modern surface expression because ice subsequently reoccupied the subglacial tunnel. This was buried by up to ~100 meters of pro and supraglacial sediment, which collapsed several thousand years after ice receded from the area.

This poster presents the results of recent geologic mapping and subglacial hydraulic modeling along a portion of the Green Bay Lobe (GBL) in central Wisconsin. Tunnel channels here extend at least 15 km from the Hancock margin, clearly cross the Hancock and Almond margins, and contain kettles that range from ~10 to 40 meters deep. Preliminary results of active and passive seismic surveys indicate the tunnel channels may have reached bedrock up to 100 m below the modern surface. Geologic mapping and Geoprobe coring indicate the upper 20 m of surrounding sediments are predominantly outwash sand and gravel.

Previous work along the western margin of the GBL suggested that formation of tunnel channels resulted from the catastrophic discharge of subglacial lakes, tunnel channels may have been active more than once, and permafrost played a key role in the formation of subglacial water ponding that subsequently drained through tunnel channels. However, our modelling of the subglacial hydraulic potential near the margin of the GBL suggests permafrost was not necessary if an adverse bed slope was approximately 11 times greater than the surface slope. This condition occurs at several locations along the western GBL, and in all instances coincident with the presence of tunnel channels. However, there is strong evidence that permafrost was present as ice advanced to the MIS 2 margin, and permafrost must have been present for buried ice to survive through multiple ice advances in the area. In addition, the melting of buried ice produced the geomorphic expression of tunnel channels in the modern landscape. Therefore, the geomorphic record of the area can provide insight into subglacial processes.