LATE WISCONSINAN AGGRADATION AND INCISION HISTORY OF THE UPPER MISSISSIPPI RIVER, USA

The Upper Mississippi River valley served as a pathway for Laurentide Ice Sheet meltwater from the Des Moines, Superior, Green Bay and Lake Michigan lobes during isotope stage 2. The record of meltwater flow down the Mississippi River integrates complex relationships between ice margin fluctuation, glacial lake drainage, meltwater production and sediment supply. Terraces preserved in the Mississippi Valley in Wisconsin, Minnesota, Iowa and Illinois provide an opportunity to understand the fluvial response to glacial and periglacial conditions. However, the chronologic constraint of terrace formation is limited, especially in Minnesota and Wisconsin because of a general lack of organic material found in glaciofluvial deposits due in part to periglacial conditions and tundra vegetation during isotope stage 2. Optical dating provides an opportunity to improve the chronology and here we use 57 optical ages (15 sites) from fluvial and eolian sediments to constrain terrace formation along a ca. 600 km stretch of the Upper Mississippi River from Bay City, Wisconsin to Lomax, Illinois. Ages for the highest terrace (Savanna Terrace) in Wisconsin and Minnesota range from ca. 26 to 22 ka, indicating a major period of aggradation. These age estimates agree with optical and radiocarbon ages from terraces immediately adjacent to the Des Moines and Superior lobes south of St. Paul, Minnesota. After this period of aggradation, optical ages indicate no net aggradation or incision until ca. 18 ka, when over 30 m of incision occurred over a 1-2 kiloyear span (ca. 18-16 ka). This period represents the time interval for the formation of the cut terrace(s) of the Bagley terrace complex. However, discordance between optical ages and radiocarbon ages from wood and gastropods from fluvial sediments and interbedded colluvium from the Driftless Area in Wisconsin and Minnesota make chronologic conclusions cautionary. Optical age overestimation due to partial bleaching of fluvial sediment is a major concern for this study, especially sites proximal to the maximum margin of the Laurentide Ice Sheet. Single-grain optical dating may be needed to fully resolve the chronologic disparity.