CHRONOLOGY OF LATE WISCONSIN FLUVIAL AGGRADATION / INCISION AND EOLIAN ACTIVITY IN THE UPPER MISSISSIPPI VALLEY

The late Wisconsin chronology of fluvial aggradation / incision and eolian activity in the Upper Mississippi Valley (UMV) and tributaries is not completely understood mainly due to the lack of datable organic matter in fluvial and eolian deposits. Optical (OSL) dating of fluvial and eolian sediments in the UMV and tributaries provides a means of determination of a more detailed chronology and an opportunity to contribute to the understanding of the retreat chronology of the Laurentide Ice Sheet (LIS) and meltwater routing. This study presents 26 optical dates on fluvial and eolian quartz sand and two radiocarbon dates from charcoal within fluvial sediments in western Wisconsin, southeastern Minnesota, and northeast Iowa. Samples were taken from the Savanna Terrace (highest late Wisconsin fill terrace), the lower, inset Bagley Terrace complex, and eolian dunes on several Bagley Terrace levels. Optical ages from fluvial sediments range from 23.0 – 13.2 ka. Age overestimation due to partial bleaching of fluvial sediment is a major concern in the UMV, especially due to its paleo-proximity to the LIS. Several UMV fluvial samples exhibit asymmetric equivalent dose distributions, diagnostic of partial bleaching. Use of the minimum age model in analysis of equivalent dose distributions result in better agreement between fluvial ages, existing radiocarbon chronology and optical ages from overlying eolian sediments. The fluvial optical ages suggest incision below the Savanna Terrace occurred after ~21 ka, and Bagley Terrace deposition occurred ~18 – 16 ka. Eolian optical ages, obtained from dunes on several Bagley Terrace levels, range from 15.3 – 13.2 ka. Equivalent doses of eolian samples are normally distributed, indicating complete bleaching prior to deposition. Age estimates from this study support the hypothesis of rapid UMV incision (up to 30 m) below the Savanna Terrace and formation of at least part of the Bagley Terrace complex prior to drainage of glacial Lake Agassiz (<14 ka).