MIDDLE WISCONSIN MIS3 GLACIAL ADVANCE IN IOWA: DEFINING THE SHELDON CREEK FORMATION

Iowa has one of the most complete Quaternary continental stratigraphic records, however, the distribution and timing of many events has historically been uncertain. One such event was a Middle Wisconsin advance of the Laurentide Ice Sheet during Marine Isotope Stage 3 (MIS3). During this time, the Sheldon Creek Formation till was deposited. These materials have long been identified and were often termed the ‘Tazewell’ till in earlier literature, but relatively little was known when compared to other glacial advances into Iowa. Chronologic constraints were limited and correlation was problematic until recent studies of the distribution, thickness, timing, and lithology led to more fully characterizing this geologic formation.

Surficial mapping by the Iowa Geological Survey (IGS) over the last ten years has redefined the maximum extent of this glacial advance and provided a robust sample set for study. The Sheldon Creek Formation can now be differentiated from other geologic formations using stratigraphic relationships and lithologic characteristics (grain-size, clay mineralogy, grain counts, and elemental data). Late Wisconsin deposits of the Des Moines Lobe advance had a similar source area, and therefore, similar grain-size and mineralogy. In these settings, stratigraphic relationships are utilized to separate the younger Dows Formation from the Sheldon Creek Formation. Beyond the Des Moines Lobe margin, erosional processes have removed or masked the characteristic geomorphic features of the ice advance (moraines), so lithologic evaluation is the primary methodology for distinguishing the Sheldon Creek materials from the much older Pre-Illinoian till deposits. Radiocarbon ages suggest that two Sheldon Creek advances can be identified in Iowa: the younger Lehigh Advance, which reached its terminal position around 30 kya, and the older Ft. Dodge Advance that had a maximum distribution around 42 kya. Differentiating the two Sheldon Creek advances using lithology alone may be possible, but requires further investigation. Given the newly available data, the IGS proposes the Sheldon Creek Formation as a new geologic unit. Potential candidates for regional correlation are the Verdi and Moland members of the New Ulm Formation of Minnesota.