BURIED TUNNEL VALLEYS ACROSS THE UPPER PENINSULA OF MICHIGAN: GROUNDWATER CONDUITS FOR OUTBURST FLOODS FROM GLACIAL LAKE AGASSIZ?

Detailed bathymetry of lakes Superior and Michigan reveal an extensive system of trenches whose dimensions and spatial associations suggest an origin as glacial tunnel valleys, and that they are genetically related. An intervening system of connecting channels (now buried by a 10,000 km2 apron of outwash sediments that grades to the south) likely exists across the breadth of the intervening landmass, Michigan’s eastern Upper Peninsula (U.P.). In this study, we identify buried valleys and surmise that they carried substantial groundwater flow from Lake Minong (proto-Lake Superior) to Lakes Chippewa (proto-Lake Michigan) and Stanley (proto-Lake Huron) ~10.5-8.5 ka as Lake Agassiz spilled billions of cubic meters of water into Lake Minong. To test the hypothesis that such a system existed, we: 1) used digital elevation models (DEM’s) and bathymetric data to identify and study candidate buried tunnel valleys; and 2) conducted gravimetric surveys to delineate buried valleys; and 3) collected and processed OSL samples from dunes occurring within or just leeward of candidate valleys to determine the timing of events. One gravity survey of particular significance was completed in the vicinity of the Kingston Plains. It delineates a valley incised into bedrock that is ~70 m deep, up to 2 km wide that likely spans the U.P. from Lake Superior to Lake Michigan, now buried by a southward-thinning apron of outwash sediments. The valley is spatially correlative with tunnel valleys on the floors of Lakes Superior and Michigan. Southeast of the Kingston Plains, it intersects with the headwaters of the subaerial, southeast-trending valley of the Fox River, which is conspicuously underfit within a broad valley and contains several parabolic dunes. OSL ages of these dunes are consistent with large-scale sapping, and headward extension of valleys subsequent to periods of presumed massive southward flux of groundwater 10.5-8.5 ka.