Evidence for Middle Wisconsin Glaciation in North Central Iowa

Newly collected drill core information has extended the eastern boundary of the Middle Wisconsin (OIS 3) Sheldon Creek Formation in north central Iowa into western portions of the Iowan Erosion Surface (IES) landform region. Previously, all till underlying the IES was assumed Pre-Illinoian (> OIS 11) in age. New borings reveal an extensive area to the east of the Des Moines Lobe (OIS 2) where thin, weathered Sheldon Creek Formation till buries a well-developed paleosol, presumably the Sangamon Geosol (~OIS 11-5 in this area).

The Sheldon Creek and Dows Formation basal tills share a northwestern provenance and contain less matrix clay and more shale clasts than the Pre-Illinoian tills on the IES. In the rare instances where a complete weathering profile is preserved, those formed in Pre-Ilinoian till are thicker and more mature than those formed in the Sheldon Creek Formation.

Radiocarbon ages collected from wood and organic-carbon rich zones in and beneath the Sheldon Creek Formation bracket the advance(s) between 26,000 and 41,000 calendar years before present. This is the same time interval that “Des Moines and James Lobe” western provenance slackwater sediments were accumulating in the lower reaches of Middle Mississippi River Valley tributaries and that the bulk of the Roxana Silt was deposited along the Mississippi Valley. Both the timing of the Sheldon Creek Formation advance(s) and its geochemical composition strongly suggest it was a source of Roxana Silt.