AGE, DISTRIBUTION & LITHOLOGY OF THE SHELDON CREEK FORMATION, NORTH CENTRAL IOWA

New data is presented concerning the distribution, lithology, and age of glacial diamicton of the Sheldon Creek Formation of northern Iowa. This mid Wisconsin unit is exposed at the land surface with a thin loess mantle in northwest Iowa (the “Tazewell Till Plain”), and as “extramorainic drift” east of the Des Moines Lobe. The Sheldon Creek Fm. is buried either by glacial diamicton of the Dows Fm. or by Peoria Loess beneath the Des Moines Lobe in north central Iowa. The unit does not appear to exist beneath the Des Moines Lobe south of the Altamont I Moraine.

Radiocarbon ages obtained from wood and organic-rich sediment found below, at the base of, within, and at the top of the Sheldon Creek Fm. indicate that the unit was deposited by at least two Wisconsin glacial advances dating from approximately 40,000 to 26,000 years before present.

The Sheldon Creek and overlying Dows Formations share many lithologic characteristics, with both units containing clasts of Cretaceous Pierre Shale. However, these clasts are more abundant in the Dows Fm. than they are in the Sheldon Creek Fm. The presence of Pierre Shale indicates that both the Des Moines Lobe and the glacial ice that deposited the Sheldon Creek Fm. advanced from the northwest. Current work involves detailed analysis of the lithologic characteristics of the 1-2 mm sand fraction, and the clay mineralogy of these two units.