LATE PLEISTOCENE AGGRADATION AND DAMMING OF TRIBUTARIES TO THE LOWER WISCONSIN RIVER IN SOUTHWESTERN WISCONSIN

Driftless Area landscapes were affected and shaped by periglacial processes and deposition of glacial sediment sourced from the Laurentide Ice Sheet (LIS) during Marine Isotope Stage 2 (MIS 2), as well as previous glaciations during the Quaternary Period. For example, aggradation of MIS 2 outwash along the lower Wisconsin River valley caused damming and filling of tributaries of the Wisconsin River. Radiocarbon ages from sediment cores collected in these tributaries give new insight into the timing and dynamics of late Pleistocene sedimentation along rivers in southwestern Wisconsin, and provide the basis for observations about late Pleistocene landscape change in southwestern Wisconsin.

Four cores were collected at sites along Mill Creek and Kickapoo River, the latter of which is the largest tributary of the lower Wisconsin River. The cores include lake sediment formed when aggrading outwash dammed the tributary mouths, thus transforming the tributary rivers into lakes. Up to 26 meters of sediment was deposited in the Kickapoo River valley and 17 meters was deposited in Mill Creek valley. A basal lake sediment radiocarbon age from Mill Creek indicates damming and filling with lake sediment by 23.3 - 23.9 cal kyr BP. Radiocarbon ages from abandoned meanders on the Kickapoo River valley, located ~40 km downstream from Mill Creek, indicate the presence of lakes at 20.7 - 21.3 cal kyr BP, 20.6 - 21.0 cal kyr BP, and 19.6 - 20.0 cal kyr BP. Passive seismic data in the Kickapoo River valley show that bedrock is as deep as 40 m, indicating sedimentation commenced some time prior to the oldest radiocarbon date. Bracketing radiocarbon ages from one abandoned paleomeander in the Kickapoo Valley indicate 15.5 m of lake sedimentation over a period of ~1100 cal years.

These ages provide new insight to the geomorphic history of the Driftless Area, and document the presence of deeply incised bedrock valleys that were rapidly infilled with outwash and lake sediment during MIS 2. In addition, these sites provide insight to timing and dynamics of sediment flux from the Green Bay Lobe of the LIS.