PRE-ILLINOIAN TILL SEQUENCE IN NORTHERN MISSOURI AND THE GLOBAL CLIMATE RECORD

In northern Missouri 5 pre-Illinoian tills are preserved in direct superposition along the southernmost boundary of the early Laurentide ice sheet, which coincides with the northern margin of the Ozark Dome. The largest pre-Illinoian glaciations reached this same terminus, but were stopped by the uphill grade. These 5 pre-Illinoian glaciations are most likely to be represented by strong climate signals in oceanic sediment.

The youngest 3 tills (McCredie formation) preserve a normal detrital remanent magnetization (DRM) and are presumptively younger than the Matuyama/Brunhes reversal at 0.78 Ma. The respective glaciations may correspond to isotopic stages 14, 16, & 18, but direct dates of the tills are needed for confirmation. The oldest two tills (Moberly and Atlanta formations) preserve a reversed DRM and hence are older than 0.78 Ma. The preglacial paleosol directly beneath the oldest till (Atlanta Formation) recently gave a burial age of ~2.4 Ma, based on the ratio of the cosmogenic isotopes 26Al & 10Be, which is in excellent agreement with estimates of the first continental-scale glaciation in North America inferred from the oceanic record.

The subglacial conditions near the pre-Illinoian ice margin favored deposition over erosion; buried paleosols with thick argillic horizons are often preserved nearly intact atop each till. Thus, the cosmogenic-isotope technique, applied to these paleosols, may provide direct ages of each glaciation. Considering the close agreement for the age of the first glaciation, an excellent correlation between the land and ocean records might be extended through the entire pre-Illinoian.