GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 283-3
Presentation Time: 9:00 AM-6:30 PM

Evidence for Middle Wisconsin Glaciation in North Central Iowa


KERR, Phillip J., Iowa Geological Survey, IIHR - Hydroscience & Engineering, 340 Trowbridge Hall, Iowa City, IA 52242; Department of Earth and Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242, BETTIS III, E. Arthur, Department of Earth and Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242, TASSIER-SURINE, Stephanie A., Iowa Geological Survey, IIHR- Hydroscience and Engineering, 100 C. Maxwell Stanley Hydraulics Laboratory, Iowa City, IA 52242, QUADE, Deborah J., Iowa Department of Natural Resources, 1023 W. Madison St., Washington, IA 52353, WOIDA, Kathleen, U.S. Dept of Agriculture, Natural Resources Conservation Service, 210 Walnut Street, 693 Federal Bldg, Des Moines, IA 50309 and KILGORE, Susan M., Valley City State University, 101 College St. SW, Valley City, ND 58072, phillip-kerr@uiowa.edu

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.