LATE MIS 3 ONSET TO LARGE-SCALE AGGRADATION ON THE UPPER MISSISSIPPI RIVER VALLEY, USA
We present new chronologic control on the onset of aggradation of outwash in the upper Mississippi River valley. We collected rotosonic cores of sediment from the Savanna Terrace, the highest depositional terrace along the upper Mississippi River associated with the Last Glacial Maximum (LGM), at two sites between La Crosse, WI, and Prairie du Chien, WI. The Savanna Terrace occurs in the mouths of tributaries to the Mississippi River, and is composed largely of slackwater sediment that was deposited in response to outwash aggrading on the mainstem river. Both cores penetrated ~47 m to bedrock. The slackwater sediment in the cores contained abundant plant macrofossils to within a few meters of bedrock. Radiocarbon ages from plant macrofossils show that large-scale aggradation of outwash had commenced by 34.5 ka and had filled the valley above the level of the modern floodplain prior to 21.6 ka.
This indicates that incision prior to late Marine Isotope Stage (MIS) 3 had largely removed alluvial sediment associated with previous glaciations; that glaciers had advanced from the Superior basin into the headwaters of the Mississippi River basin during MIS 3; and that the floodplain aggraded approximately 42 meters by the LGM. These data provide new insight into the timing of ice advance prior to the LGM and the record of that advance in a major outwash-bearing river that drained the ice sheet, and inform the discussion of sediment and meltwater produced during MIS 3.