A near synthesis of pre-Illinoian till stratigraphy in the central United States: Iowa, Nebraska and Missouri

Cosmogenic-nuclide burial ages provide relatively precise ages for the three earliest glaciations that reached northeast Missouri: ca. 2.4, 1.3 and 0.76 Ma. Ages of the youngest two glaciations in this area are less precise, but both are between ∼0.2 and 0.4 Ma. The tills in Nebraska and western Iowa have not been dated directly, but are constrained by the ages of interbedded tephra, and these limits are consistent with the burial ages in Missouri for tills in the same relative stratigraphic positions, with one exception. Two of the three normal-polarity tills in Nebraska/western Iowa are present below the 0.64 Ma Lava Creek B tephra, and thus should be between 0.64 and 0.78 Ma in age. However, only one of the normal-polarity tills in northeast Missouri is within this range. This discrepancy indicates either a problem with the age constraints or that the till sequence is more complex than yet recognized.