Age constraints of the Middle Pleistocene till and loess sequence in northeast Missouri, USA, based on pedostratigraphy within a polygenetic paleosol


Pre-Illinoian (pre-MIS 6) tills in northern Missouri, USA mark the southernmost advances of the early Laurentide Ice Sheet (LIS), so their ages likely correspond to its largest ice volumes. Present age constraints for the two youngest tills in this sequence (Macon and Columbia) are consistent with deposition between MIS 12 and 8, but recognition of a pre-Illinoian loess above these tills now provides better age control. This loess is present near the Missouri River, and at one site it is overlain by three younger loess formations of known age. The oldest of the overlying formations dates to early MIS 6, so the underlying loess is pre-Illinoian. A thick paleosol is preserved within the top of the pre-Illinoian loess, so it is no younger than MIS 8 or early MIS 7. This loess also overlies a thick paleosol at the top of the (youngest) Macon till, which in turn rests upon another mature paleosol within the Columbia till. Therefore, the stratigraphic sequence restricts the minimum age of the Macon till to MIS 10 and that of the subjacent Columbia till to MIS 12. When combined with previous age constraints, these results isolate MIS 8-7 for the pre-Illinoian loess and MIS 10&12, respectively, as the mostly likely ages of these two tills. The presence of MIS 8-7 loess also implies that the LIS reached the upper Missouri River drainage basin at that time, although no corresponding till has been identified. However, there is no evidence of MIS 14 glaciation in the Midcontinent of the U.S. Based on these results, the LIS grew to a larger size during MIS 12&10 than during MIS 8, and was even smaller during MIS 14.


Geography, Geology, and Planning

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Middle Pleistocene loess, Missouri, Paleosols, Pedocomplex, Pedostratigraphy

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