ABSENCE OF GLACIOFLUVIAL AND GLACIOLACUSTRINE SEDIMENT THAT PRE-DATES LATE MIS 3 IN SOUTH-CENTRAL AND SOUTHWESTERN WISCONSIN, USA

AMS radiocarbon chronologic data from sediment cores collected to bedrock in south-central and southwestern Wisconsin and along the adjacent reach of the Mississippi River demonstrate a pervasive absence of glaciofluvial and glaciolacustrine sediment pre-dating late MIS 3, either through non-deposition or systemic removal.

An increasing inventory of rotosonic cores in topographic lows and along alluvial corridors have failed to find older sediment, even though these would be ideal settings to preserve pre-MIS 3 material. Cores that largely targeted slackwater or lacustrine settings have consistently contained terrestrial plant macrofossils at or within a few meters of the bedrock surface, providing chronologic control on the age of sediments filling these depocenters. Two cores collected in tributaries of the Mississippi River indicate aggradation commenced along that corridor no earlier than 34.5 ka; four cores in similar depositional environment along the lower Wisconsin River indicate aggradation commenced there slightly prior to 28.6 ka; and two cores from the glacial Lake Wisconsin basin indicate in-filling of that basin started slightly prior to 31.2 ka. These data are supported by published studies constraining the extent of MIS 3 ice in east-central Wisconsin and stratigraphic data from east-central Wisconsin.

Our data highlight some of the challenges of using the terrestrial record to understand MIS 8-6 glacial dynamics and landscape evolution during MIS 5-3. These include: the geometry of the MIS 8-6 ice sheet in the area of the Green Bay and Lake Michigan lowlands; the pervasive absence of pre-late MIS 3 glaciofluvial and glaciolacustrine sediment in the region despite the widespread presence of pre-MIS 3 loess; drainage basin configuration for the area encompassed by the Wisconsin River during MIS 5-3; and the spatial and temporal extent of sediment fill in the Wisconsin and upper Mississippi River valleys during MIS 5-3.