REDEFINING THE MIDDLE WISCONSIN SHELDON CREEK BOUNDARY IN NORTH CENTRAL IOWA

The Middle Wisconsin Sheldon Creek glacial advance has long been recognized in Iowa, but certain aspects of its characteristics and distribution are still not well understood. Limited exposures of the Sheldon Creek Formation in Iowa make it difficult to evaluate, as much of its surface is covered by the Late Wisconsin Des Moines Lobe advance. Further complicating identification of the eastern extent are the extensive amount of shallow bedrock and the erosional processes that occurred during the Last Glacial Maximum when the Iowan Surface landform region formed. Obtaining a complete stratigraphic sequence of Sheldon Creek deposits over a paleosol formed in the underlying Pre-Illinoian materials is uncommon. Therefore, differentiating the Sheldon Creek glacial deposits from the much older Pre-Illinoian till can be difficult in this region.

Mapping completed as part of a Natural Resources Conservation Service soil survey update as well as mapping under the USGS STATEMAP program have significantly revised the boundary of the Sheldon Creek Formation in north central Iowa. The eastern margin was previously thought to roughly coincide with the maximum extent of the Des Moines Lobe. Current mapping indicates that the Sheldon Creek boundary extends well beyond its original placement. Drill core descriptions, analytical data, and radiocarbon dates have been used to constrain the stratigraphic framework. Grain-size, clast lithology, and pXRF data are used to differentiate the Sheldon Creek deposits from the older Pre-Illinoian till units. The Des Moines Lobe and Sheldon Creek advances have a similar provenance, therefore identifying the boundary between the Dows and Sheldon Creek Formation deposits is based on landform features and aerial imagery. Geomorphology, LiDAR, soils data, and drainage distribution patterns have been used in conjunction with analytical data to define the eastern margin of the Sheldon Creek advance.