TIMING OF TWO MIDDLE WISCONSIN GLACIAL ADVANCES INTO NORTH CENTRAL IOWA

Radiocarbon dates collected from wood and organic-rich zones in or beneath the Middle Wisconsin Sheldon Creek Formation till place the Laurentide Ice Sheet in Iowa for two phases during MIS 3. Samples were collected from locations with Sheldon Creek Formation present either beneath the Late Wisconsin Dows Formation (MIS 2) or from sites beyond the Des Moines Lobe. In one field site, Brushy Creek, two Middle Wisconsin tills were observed where the younger unit sat disconformably above the older. Dates from wood within both tills and plant material between the units places the advances more than 10 ka apart. A compilation of different sites reveals the earlier advance is bracketed by ages between 37,000 and 46,000 cal BP while the second advance dates between 30,000 and 35,000 cal BP.

The Middle and Late Wisconsin aged deposits share a similar lithology. They are generally less clayey and contain more Pierre shale clasts than underlying Pre-Illinoian deposits (>MIS 11) due to their provenance northwest of Iowa. During the Last Glacial Maximum, the Des Moines Lobe represented the furthest extent of an ice stream from the Interior Plains. The Middle Wisconsin advances into Iowa likely shared a similar pathway.