TOWARD A UNIFIED EXPLANATION FOR SUBGLACIAL TUNNEL FORMATION

Over the past 50 years, glacial mapping in Minnesota has drawn attention to the poorly understood subglacial, erosional landform known as a tunnel valley, or channel. These broad, prominent troughs have been recognized previously at the margin of the Scandinavian Ice Sheet, and have now been described for nearly all of the mid-continent ice lobes of the Laurentide Ice Sheet. However, there seem to be as many origins proposed as there are researchers studying them. Regarding the source of the water, some hypotheses require a large subglacial reservoir; others, moulins to allow surface water to reach the bed. Regarding the distinctness of the features at the margin, some require a frozen margin through which the tunnels eroded; others, pre-existing channels that are exploited by subglacial water.

There is general agreement that tunnel valleys 1) are subglacial; 2) have an early, erosive stage that then wanes; 3) are most distinct within several kilometers of the ice margin; and 4) are usually straight and have no tributaries, although they may have distributaries.

A recent covered-flume experiment, built to model sub-ice-stream drainage (Catania and Paola, 2001), validates a conceptual model initially mentioned by Boulton and Hindmarsh (1987) and further developed here. It is proposed that the steep pressure gradient associated with the change from subglacial to subaerial water pressure led to the development of channels that originated at the ice margin by sapping. Sapping occurred locally, perhaps controlled by inhomogeneities in the bed. These channels eroded headward; in some cases they captured other headward-eroding streams. When the sapping channels intersected the pressurized subglacial drainage system, they bled off large amounts of water for a short time.

Drainage of a significant portion of the bed could have resulted in ice stagnation. With gradual recharge of the subglacial water system, the sapping and draining events could be repeated as long as the necessary subglacial water pressure gradient redeveloped.

Boulton, G. S., and R.C.A. Hindmarsh, 1987. Sediment deformation beneath glaciers: rheology and geological consequences. JGR v. 92, No. B9, p. 9059 – 9082.

Catania, G., and C. Paola, 2001. Braiding under glass. Geology v. 29, N. 3, p. 259 – 262.