THE STATUS OF MIS 3 TILLS IN IOWA, USA

Exposures of MIS 3 glacial sediment are thought to paltry within North America as MIS 2 ice buried/eroded older deposits. However, there are MIS 3 glacial diamictons in Iowa both beyond and below MIS 2 deposits. Numerous radiocarbon ages from wood collected from basal till and buried organic horizons returned ages between 45-30ka. These ages show two groupings, one at ~42ka and another at ~30ka which suggests two advances. Differentiating the MIS 3 boundary is problematic as glacial landforms from these advances, e.g., moraines, have been muted or removed by wide-scale periglacial erosion during MIS 2. Due to the mass wasting, remote sensing was utilized to identify a margin using broad glacial geomorphologic signals. Patterns such as river reorganization, drainage network integration, and landscape incision were used to differentiate distinct pre-MIS 2 glacial surfaces. These boundaries were tested using till lithology from boreholes. The MIS 2 till (Dows Fm.) and the MIS 3 tills (Sheldon Creek Fm.) have similar carbonate and shale rich lithologies which distinguishes them from older Pre-Illinoian tills (>MIS 6). These geomorphic and lithologic patterns were used to establish the MIS 3 boundary to the east and west of the Bemis Moraine (MIS 2 terminal moraine) in Iowa. The interpretation of two advances is supported stratigraphically. In a ~25m quarry section on the MIS 2 till plain, MIS 2 ice deposited Dows Fm. till was on Peoria Fm. loess which accumulated on a two till sequence which intern buried the Sangamon Geosol formed in Wolf Creek or Alburnett Fm. tills (>MIS 6). Other cores outside of the Bemis also have two carbonate and shale rich tills above older deposits. Together, these lines of evidence show a lobe of the Laurentide Ice Sheet was south of 42.5˚N during intervals in MIS 3.