LANDSCAPE EVOLUTION OF THE MINNESOTA RIVER VALLEY

Some authors suggest that Glacial Lake Agassiz discharged water out its southern outlet during the Early Lake (11,900-11,800 B.P.), Cass and Lockhart (11,600-10,800 B.P.), and Emerson (9,900-9,500 B.P.) phases to help carve the Glacial River Warren trench, which is now occupied by the underfit Minnesota River. Cores were collected from post-catastrophic flood fluvial sediments and alluvial fans in three different reaches of the valley. These relatively sandy fluvial sediments bear a paleosol complex near its terrace surface, and is overlain by prograding alluvial fans containing their own paleosol complexes. Multiple radiocarbon dates on wood and conifer needles from these three sites along different reaches of the Minnesota valley indicate that the River Warren's catastrophic flooding ceased prior to 10,400 B.P. The sandy fluvial deposits were aggrading from before 10,400 B.P. up to a time bracketed between 9,300 B.P. and 8,800-7,900 B.P. The paleosol complex developed at or near the surface of the fluvial unit was overlain by prograding, alluvial fans sometime after 8,800-7,900 B.P. This paleosol complex continued developing in the finer-grained basal fan sediments and ranged in age from 8,800-5,400 B.P. The fans contain a second stratigraphically higher paleosol complex that ranges from 4,500-2,000 B.P., and a third, still younger soil complex at or near the present day land surface. These data indicate that Glacial River Warren and associated catastrophic flooding down the Minnesota valley predates any Emerson Phase discharge through Lake Agassiz's southern outlet (ca. 9,900-9,600 B.P).