RECYCLING OLD IDEAS: THE IOWAN, THE ALTONIAN AND THE MIS 3 SHELDON CREEK FORMATION IN IOWA

Evidence for an MIS 3 ice advance into the midwestern U.S. has been recognized for over a century but was largely discounted. The loess record offers some of the most compelling, albeit indirect, evidence for such an advance. The widespread Roxana Silt and equivalents are constrained stratigraphically to a time interval between development of the subjacent Sangamon Geosol (Sangamon interglacial) and deposition of the overlying Peoria Silt (Late Wisconsinan). Eventually, radiocarbon dates gave ages for the Roxana spanning MIS 3; thus, the loess implied derivation from valley-train outwash sourced from an early Wisconsinan advance, dubbed the Altonian Substage. The Altonian displaced the Iowan as the oldest Wisconsin glacial substage, but both of these time-stratigraphic units, compatible with today’s MIS 3, were soon discarded: the Altonian, because till deposits in northern Illinois, thought to be of this age, turned up missing. In Iowa, the Iowan surface east of the Des Moines Lobe was shown to be a regional erosion surface above older pre-Illinoian tills, not undissected topography of a younger “Iowan” glaciation.

Meanwhile, quarry excavations on the Des Moines Lobe (DML) in Iowa had exposed two tills (Sheldon Creek Formation) below the Peoria Silt and the surficial till of the DML. The Sheldon Creek tills are separated by a weakly developed weathering profile and give radiocarbon ages of ~43 and 30 ka, respectively: a nice fit with MIS 3. These two tills are easily recognized by their matrix properties, and tracing shows that they extend beneath the DML and become the surface deposit in small areas east of the lobe and over an extensive area to the west.