QUATERNARY INCISION OF THE UPPER MISSISSIPPI RIVER ACROSS A GLACIAL FOREBULGE
The Upper Mississippi River flows gently through a flat-floored alluviated valley that lies between the high bluffs of the Paleozoic Plateau along the borders of Minnesota, Wisconsin, Iowa, and Illinois. Beneath the modern Upper Mississippi downstream of St. Paul lies a hundreds-of-kilometers-long overdeepening in its bedrock valley profile. Frye (1963) explained this flat-to-reversed bedrock valley gradient by invoking incision across an ice-marginal forebulge. While this hypothesis has seen little attention in recent decades, data and computational methods to evaluate it have advanced. We use a state-of-the-art model of geophysical response to the changing ice masses of the last glacial cycle to generate a time series of flexural isostatic uplift and subsidence and geoid deflection in the Upper Mississippi River catchment. The modeled glacial forebulge is supported by GPS data (Sella et al., 2007) that show significant subsidence centered around the Paleozoic Plateau region as the present remnant of the forebulge relaxes. This subsidence pattern is partly responsible for the alluviated flat-bottomed valleys of the Mississippi River corridor through the Paleozoic Plateau. However, for a few thousand years during deglaciation, this region instead experienced rock uplift at rates of over a centimeter per year, comparable to the most active modern orogens. Simultaneously, large outburst floods from proglacial lakes provided torrents of sediment-poor water that were capable of scouring deep valleys. We couple the modeled forebulge rock uplift and subsidence patterns with meltwater chronologies and longer-term Laurentide Ice Sheet histories as inputs to a model of fluvial incision and alluviation. Results suggest that in early glacial cycles, the river bevels its bedrock floor during forebulge growth and the valley fills with alluvium as the forebulge decays. In later cycles, similar patterns of rock uplift and subsidence result only in scour and fill of alluvium. This is consistent with field evidence; in addition, the modeled bedrock long profile is similar to that of the modern Mississippi. The generations of Laurentide Ice Sheets and their geophysical and hydrologic effects are therefore responsible for sculpting this distinctive gash in the Midwestern landscape.