LOCAL RESPONSE TO LATE-GLACIAL CONDITIONS ALONG THE KICKAPOO RIVER, SOUTHWEST WISCONSIN

The Kickapoo River in southwest Wisconsin is the largest tributary of the lower Wisconsin River. The meandering planform of the river is deeply incised into the nearly flat-lying Paleozoic sedimentary sequence, and includes several examples of abandoned bedrock meanders. Late Pleistocene and more recent sediment is as much as 30 m thick in the Kickapoo valley; the late Pleistocene sedimentation in the Kickapoo valley is intimately tied to glacial processes and events.

Mainstem streams in the upper Midwest, including the Mississippi and Wisconsin Rivers, contain depositional terraces composed of glacial outwash. Within the unglaciated Driftless Area of Wisconsin, tributary streams were dammed by outwash aggrading on the mainstem stream. This often formed depositional terraces on the tributaries that are limited to within a few kilometers of the confluence. The Kickapoo River, however, contains a terrace that can be mapped more than 50 km upstream of its mouth. Geoprobe coring into the terrace demonstrates that it is comprised of primarily well-sorted sand. Aggradation of this terrace blocked several of the abandoned bedrock meanders along the Kickapoo valley; radiocarbon ages from plant macrofossils retrieved from laminated slackwater sediment in the abandoned meanders demonstrates that they were open lakes for at least the interval of 26.7 to 19.8 ka.

The slope of the terrace in the Kickapoo valley and the elevation of the dated lacustrine sediment indicate that they are not directly related to the glacially-sourced terrace aggradation that was occurring on the Mississippi and lower Wisconsin River at the same time. The headwaters of the Kickapoo River incise into friable Cambrian sandstone. We suggest that landscape instability caused by the harsh climate conditions during the peak of the glaciation mobilized sand down the Kickapoo River, providing a local source for aggradation despite the lack of glacially-derived sediment found on nearby mainstem streams.