Tunnel Channels of the Saginaw Lobe, Michigan, USA

Introduction
Drainage of meltwater from a glacier occurs at the surface of the glacier, as well as internally and between the ice and its bed. Different components of the meltwater drainage system commonly interact with each other, transferring meltwater from the surface to the base of the glacier, for example. Meltwater processes result in a wide variety of landforms and deposits. These erosional and depositional features are concentrated near and beyond the margin of the ice because ablation (melting) is at its maximum in the marginal zone.

Among the most interesting types of landforms produced by meltwater erosion are tunnel channels, which are also known as tunnel valleys. These are interpreted to be eroded by high-energy meltwater flowing in a tunnel at the base of the glacier. These tunnels are are similar to ones in which eskers are deposited, although they erode downward into the substrate beneath the glacier rather than up into the ice. Despite this difference, eskers do occur in some tunnel channels. Wright (1973) first interpreted valleys of this type in Minnesota as the result of catastrophic discharges of meltwater that was initially impounded under the ice and then was suddenly released by some mechanism to flow through tunnels toward the margins. Hooke and Jennings (2006) suggested that the subglacial lakes were ponded behind a dam of frozen soil and rock under the margin of the glacier (permafrost) until a channel eroded headward from the margin and drained the lake catastrophically. Brennand and Shaw (1994) explain tunnel channels in Ontario as the result of erosion by huge subglacial sheetfloods. Gradual erosion of the valleys over time has also been suggested as mechanism for formation (Mooers, 1989).The purpose of this vignette is to describe the characteristics of tunnel channels in the Saginaw lobe of the Laurentide Ice Sheet in Michigan and to explore different hypotheses for their origin.

Characteristics of tunnel channels
The most diagnostic characteristics of tunnel channels include: (1) dimensions reaching >100 km in length and up to 5 km width, (2) generally straight reaches oriented parallel to the subglacial hydraulic gradient of the ice lobe, (3) longitudinal profiles that can rise in elevation in the downstream direction indicating formation by pressurized meltwater, and (4) irregular side slopes and valley bottoms (Kehew et al., 2007). Despite the generally straight reaches of these valleys, braided, dendritic, and anastomosing networks have been described.

Tunnel channels have highly variable topographic expression due to burial by glacial or post-glacial deposition after their initial formation. One mechanism of burial is the slumping of debris into the valleys as the ice retreats across the landscape. Deposition of sediment during readvances of the ice also provide a mechanism for filling and burial. The degree of burial by younger sediment can range from minor to nearly complete, which would leave in some cases, no visible signs of the valley at land surface.

Tunnel channels of the Saginaw Lobe
The Saginaw Lobe is one of three major lobes of the Laurentide Ice Sheet that coalesced in southern Michigan (Fig. 1). Tunnel channels occur non-continuously from Saginaw Bay to the southern border with Indiana. Kehew and Kozlowski (2007) identified five different types based on the degree of burial, the source (glacial lobe) of sediment that buried the channel, and the presence of eskers in the channels. One group of channels consists of straight reaches oriented in a fan shaped pattern around the central part of the Saginaw Lobe (Fig. 2). Cross-cutting relationships indicate the following sequence of events for the formation of a tunnel channel: (1) subglacial erosion of the channel near the margin of the glacier, most likely by a discrete, high energy flow of meltwater, (2) collapse of ice and debris into the valley during stagnation and/or retreat of the glacier, and (3) gradual meltout of the buried ice causing subsidence to form the modern topographic depression (Figure 3). Re-advance of the Saginaw Lobe or advance of the adjacent Lake Michigan Lobe over terrain vacated by retreat of the Saginaw Lobe prior to stage 3 added additional sediment to the channels, completely burying them in some cases.

Cross cutting relationships can sometimes be used to work out the history of the channels. As shown in Figure 4, after erosion of a channel and collapse of ice and debris into it from the Saginaw Lobe, the Lake Michigan Lobe advanced into the area. Braided outwash fans from the Lake Michigan Lobe grade smoothly grade across the tunnel channel, indicating that it had to have been filled with a combination of collapsed ice and debris. If there had been an open valley at the time, the fans would not grade continuously across it as they do. Later, meltout of the buried ice exposed the modern valley.

Many tunnel channels contain eskers (Fig. 5), suggesting that the tunnels remained active for a relatively long period of time after erosion and were gradually filled with sediment by streams flowing in the tunnel. A bore hole drilled in one of these eskers yielded a fining-upward sequence of sediment suggesting a gradual decline in energy of streams flowing in the channels as the ice stagnated.

Much remains to be learned about these channels including (1) what was depth of erosion into the substrate, (2) how long and continuous are individual valleys, (3) what was the relationship between the channels and specific ice-marginal positions, (4) whether erosion was rapid during catastrophic releases of sub-glacial meltwater or more gradual sustained discharge in tunnels that remained in the same location, and (5) what was source of the meltwater?

• Brennand, T.A. and Shaw, J. 1994. Tunnel channels and associated landforms, south-central Ontario: their implications for ice-sheet hydrology. Canadian Journal of Earth Science 31, 505-522.
• Hooke, R. LeB. and Jennings, C.E. 2006. On the formation of the tunnel channels of the southern Laurentide ice sheet. Quaternary Science Reviews 25, 1364-1372.
• Kehew, A. E. and Kozlowski, A.L. 2007. Tunnel Channels of the Saginaw Lobe, Michigan, USA, in, Johannsson, P., and Sarala, P. (eds.) Applied Quaternary research in the central part of glaciated terrain, Geological Suryvey of Finland, Special Paper 46, pp. 69-77.
• Kehew, A.E., Lord, M.L., & Kozlowski, A.L. 2007. Glacifluvial landforms of erosion. In: Elias, S.A. (Ed.), Encyclopedia of Quaternary Science. Elsevier, 818-831.
• Mooers, H.D. 1989. On the formation of tunnel valleys of the Superior Lobe, central Minnesota. Quaternary Research 32, 24-35.
• Wright, H. E. Jr. 1973. Tunnel Valleys, glacial surges, and subglacial hydrology of the Superior Lobe , Minnesota, In: The Wisconsinan Stage (ed. R. Black, R. Goldthwait, and H. Willman, Geol. Soc. of Amer. Mem.136, 251-276.