SUBGLACIAL DRAINAGEWAYS PRESERVED ON THE FLOORS OF LAKES SUPERIOR AND MICHIGAN

Combined bathymetric and elevation data show the full extent of a system of deep troughs on the floors of Lakes Superior and Michigan. The south-trending, parallel to gently radiating system of troughs includes the deepest spot in Lake Superior (389 meters below lake level). Some of the troughs are incised more than 200 meters into the generally flat bedrock of the lake bottom in this area, which averages 175 to 200 meters below water level. The troughs are generally between 1 and 2 kilometers wide. The trough walls are straight and steep-sided. The floors of the troughs rise steeply to the south and continue onshore on the Upper Peninsula of Michigan, where they are spatially correlative with large outwash fans of the Munising moraine. Fluvial transport of sand and gravel was most likely driven by the potentiometric gradient imposed by the ice and the fans were deposited at the ice margin. The troughs in the Lake Michigan basin are most likely slightly older but have a similar appearance. However, they are not as deep and appear to have been partially filled with sediment issuing from the retreating ice margin as it moved north. Large, fan-shaped deposits of sand and gravel also appear at the southern extension of the troughs in central Michigan and in Wisconsin.

We propose that the troughs are remnants of subglacial drainageways of the last ice lobe that advanced to central Wisconsin and Michigan and that drainage evolution—rather than climate—controlled the advances of the ice lobe. It had at least three and up to seven oscillations during deglaciation. In fact, the advance and retreat of this ice lobe are thought to have forced late-glacial, northern hemisphere climate change by rerouting meltwater that interrupted thermohaline circulation in the North Atlantic Ocean (Broecker and others, 1988; Colman, 2002). We propose a common origin for similar features described elsewhere in the Great Lakes region, northern Europe, and Antarctica. The trough network described here was created by a process that is widespread but that has not been incorporated into models of ice flow.

REFERENCES

Broecker, W.S., Andree, M., Wolfli, W., Oeschger, H., Bonani, G., Kennett, J., and Peteet, D., 1988, The chronology of the last deglaciation: Implications to the cause of the Younger Dryas event: Paleoceanography, v. 3, no. 1, p. 1-19.

Colman, S.M., 2002, A fresh look at glacial floods: Science, v. 296, no. 5571, p. 1251-1252.