LATE WISCONSIN OPTICAL AGE CHRONOLOGY OF FLUVIAL INCISION AND EOLIAN ACTIVITY IN THE UPPER MISSISSIPPI VALLEY

The general paucity of datable organic matter within late Pleistocene fluvial and eolian sediments of the Upper Mississippi River (UMR) and tributaries has hindered interpretations of its middle-late Wisconsin aggradation and incision history. Additionally, the timing and routing of meltwater through the UMR from the retreating Laurentide Ice Sheet (LIS) is critical in assessing the impact of freshwater forcing on abrupt climate shifts during the last glacial termination. To determine a chronology of fluvial and eolian deposition, we obtained 17 optically stimulated luminescence (OSL) ages on fluvial and eolian quartz sand along the UMR in western Wisconsin. 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 UMR, especially due to its paleo-proximity to the LIS. Several UMR fluvial samples exhibit asymmetric equivalent dose distributions, diagnostic of partial bleaching. Use of minimum age models and the leading edge technique in analysis of equivalent dose distributions result in better agreement between fluvial ages, existing radiocarbon chronology and optical ages from overlying eolian sediments. Fluvial sediments of the Bagley Terrace near Pepin, WI date to 16.7 ka via use of a minimum age model, while an age of 24.2 ka results from use of the weighted mean equivalent dose. Eolian 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 generally support the hypothesis of UMR incision (< ~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). Fluvial ages (e.g., 16.7 ka from Pepin, WI) and minimum-limiting ages from eolian dunes (15.3 – 13.2 ka) on several Bagley Terrace surfaces support this hypothesis.