STRATIGRAPHIC COMPLEXITY WITHIN THE EARLY AND MIDDLE PLEISTOCENE TILL SEQUENCE IN NEBRASKA AND WESTERN IOWA

The Early Pleistocene till sequence in Nebraska and western Iowa includes two informal stratigraphic units dubbed the C (older) and B (younger) groups. Both groups include multiple tills. The established Middle Pleistocene sequence includes three individual tills designated A1, A2 and A3, youngest to oldest. However, additional tills could be present. For example, the two older A tills, both with normal magnetic remanence, underlie the Lava Creek B ash, and thus should be between 0.78 and 0.63 Ma (MIS 18-16) in age. In northeast Missouri, however, a till with similar lithology in the same stratigraphic position as the A2 is younger than ~0.4 Ma, based on cosmogenic nuclide burial dates. Another till with reversed remanence and similar lithology to the A-type tills has been reported in Nebraska, but if it is a separate unit it is not common.

We completed cosmogenic nuclide burial dating within a paleosol beneath the A2 till at two locations. Preliminary isochron plots from both sites give dates of ~0.8 +/- 0.1 Ma for deposition of the A2. This date is nominally consistent with MIS 16 deposition (just beyond 1-σ error limits), but at face value would imply that both the A2 and A3 were deposited during MIS 18. Nevertheless, these results indicate that two similar tills of different age occupy the same relative stratigraphic position in different regions so that four Middle Pleistocene tills are present in the Midwest. If both the A2 and A3 tills are indeed MIS 18 deposits, a 1.5m paleosol beneath the A2 at one of these sites poses additional complications. This would be an unlikely interstadial soil, and the lithology of the parent material (another till) is inconsistent with that of any known till within the B group. Thus, these results would indicate that there could be additional late Early Pleistocene tills in this region.