AGE CONSTRAINTS OF THE MIDDLE PLEISTOCENE TILL SEQUENCE IN MISSOURI BASED ON PEDOSTRATIGRAPHY

Existing age constraints for the two youngest Pre-Illinoian (pre-MIS 6) tills recognized in northern Missouri are consistent with both being as young as MIS 8 or as old as MIS 11. However, cosmogenic nuclide burial dates, for the (older) Columbia till, which constrain its maximum age, may have additional uncertainty beyond the formal error limit. A Pre-Illinoian loess preserved along the Missouri River provides additional constraints on the age of these two tills, because a Pre-Illinoian loess can be no younger than about MIS 8, and the loess sequence and pedostratigraphy in this area indicate that this loess is probably an MIS 8 deposit. At one site this loess is the lowest of four which comprise a thick pedocomplex resting directly above a mature interglacial paleosol atop the (youngest) Macon till, and the Macon overlies another mature paleosol within the underlying Columbia till. This sequence restricts the minimum age of the Macon till to MIS 10 and that of the subjacent Columbia till to MIS 12. The presence of probable MIS-8 loess also implies that the Laurentide Ice Sheet reached the Missouri River drainage basin at that time, although a corresponding till has not been identified.