A BURIED WATERFALL ON A BACKWARDS MISSISSIPPI: HOW ICE-SHEET-DRIVEN DRAINAGE REVERSAL AND ISOSTATIC ADJUSTMENT ACTIVATED LANDSCAPE EVOLUTION IN THE UPPER MIDWEST

Glacial isostatic adjustment warps the land surface, tilting lakeshores and coastlines, and modulating sea level. Based on depth-to-bedrock data from beneath the upper Mississippi River and its former courses, we find that its buried bedrock long profile contains a 50-meter-high knickpoint at the head of a 300-kilometer-long over deepening. We propose that this is a partial cast of the peripheral bulge – the ring of isostatically-raised solid Earth – of the Laurentide Ice Sheet. In order for this incision to occur, large quantities of sediment-starved meltwater would need to enter the upper Mississippi valley. This happened only during the initial advance (or possibly advances) of the Laurentide Ice Sheet into the upper Midwest, which dammed formerly north-flowing rivers into lakes that catastrophically overtopped their drainage divides. Following drainage integration, abundant sediments from glacial erosion traveled along with the meltwater, causing valleys to aggrade during times of ice advance. Therefore, this peripheral bulge should record fossil glacial isostatic adjustment – and hence, information on ice-sheet geometry – from the early Pleistocene, between 2.6 and 0.8 million years before present. To better date this glacial advance and initial incision, we suggest possible correlations with (1) magnetically reversed tills in western Wisconsin that may contain datable paleosols, (2) changes in the Gulf of Mexico oxygen-isotope record that should be sensitive to the addition of drainage area to the Mississippi, or (3) potential cosmogenic-burial dating on sediments in caves along the upper Mississippi valley.