LITHOLOGIC CHARACTERIZATION OF THE MID-WISCONSINAN SHELDON CREEK FORMATION, NORTHCENTRAL IOWA

The Sheldon Creek Formation is a mid-Wisconsinan till unit found in the subsurface beneath Late Wisconsin Dows Fm. on the Des Moines Lobe (DML) north of the Altamont Moraine in central Iowa. Sheldon Creek Fm. diamicton also occurs beneath a thin mantle of Peoria Loess west of the DML and at the surface east of the DML. Radiocarbon dates constrain deposition of the Sheldon Creek Fm. between ~21,000 and 40,000 ¹⁴C yr B. P. (~25,000-44,000 cal yr B. P.).

In core and outcrop Sheldon Creek and Dows Fm. diamictons share many macroscopic properties. We analyzed a suite of Sheldon Creek and Dows Fm. samples using several techniques, including a lithologic study of the 1-2 mm sand fraction, clay mineralogy, and an X-ray fluorescence spectroscopy (XRF) analysis of major and minor elemental components. Sheldon Creek Fm. tills have greater amounts of carbonate and lower quantities of Cretaceous Pierre Shale in the 1-2 mm sand fraction, and lower expandable clay content than does the Dows Fm. Lithologic differences also exist between the Sheldon Creek Fm. beneath the DML and the unit adjacent to the lobe in northwest Iowa. Sub-Des Moines Lobe samples contain less carbonate and shale, and more crystalline content in the 1-2 mm sand fraction than in the extra-Des Moines Lobe samples. Sub-DML samples also have higher expandable clay and illite percentages than the extra-DML samples.

The lithologic variations and the available radiocarbon dates suggest that the Sheldon Creek Fm. tills may have been deposited by more than one glacial advance, possibly with slightly varying source areas. Slackwater records from the Mississippi River Valley in the St. Louis area that record two periods of accumulation from “western provenance” meltwater beginning at 44,000 ¹⁴C yr B. P. and about 21,000 ¹⁴C yr B. P. may record meltwater from ice lobe(s) that deposited the Sheldon Creek Fm.