ON THE LONG-TERM BEHAVIOR OF THE LAURENTIDE ICE SHEET USING NEWLY IDENTIFIED PRE-ILLINOIAN (>MIS 6) MARGINS

A new series of Pre-Illinoian (>MIS 6) glacial margins were delineated in a region of extensive pre-Last Glacial Maximum till deposits: Nebraska, Kansas, Iowa, Missouri, and Illinois. This area also includes the Mississippi and Missouri rivers. This dataset will aid in understanding the configuration and extent of Laurentide Ice Sheet (LIS) prior to the Illinoian, a topic of growing interest. A better grasp of behavior of the older LIS advances will aid in understanding long-term changes in paleoclimate, shifts between ice centers, and the landscape development of North America during the Quaternary. Recent LIS reconstructions used modelling results and/or extrapolated local and regional boundaries to create continent-scale extents. Such endeavors can be improved with added observations and parameters, like new margins.

The mapping method treated the modern glaciated landscape as an inverse problem and used the Late Wisconsinan (MIS 2) Des Moines Lobe and Illinoian lobes as analogues. The study area has clay-rich till deposits ranging from 0 to +100m. The Pre-Illinoian (>MIS 6) boundaries are based on relative differences between drainage patterns, elevation, fluvial incision, and cross cutting relationships. Flow direction of lobes can also be inferred from the orientation of drainage networks. For instance, lobes that flow down the regional slope will leave a specific shape of drainage network, while lobes that advance ‘up’ regional slope block previously established rivers and create proglacial lakes. The results show 10+ Pre-Illinoian (>MIS 6) margins in the study area. Conversely, these boundaries also include areas that were entirely ice-free. The Driftless Region in SW Wisconsin was never covered by ice while areas in SW Minnesota and NE Iowa were glaciated. In addition, there appears to be a trend in which ice flow from Manitoba was the dominate input during older glacial advances, but an unknown up-ice factor resulted in the start of eastern sourced lobes into SE Iowa via Illinois near the end of the Pre-Illinoian.