Glacial stratigraphy and paleomagnetism of late Cenozoic deposits of the north-central United States

The north-central United States preserves one of the best continental records of late Pliocene and early-middle Pleistocene glaciations in the Northern Hemisphere. The glaciogenic sequences of this region consist of multiple tills interbedded with paleosols and volcanic ashes. Here we present results on paleomagnetic measurements and till compositional data from glacial sedimentary sequences in Iowa, Nebraska, Kansas, and Missouri. Periods of normal polarity (Brunhes Chron) and reverse polarity (Matuyama Chron) were identified in the sedimentary sequences investigated. This chronology is further constrained by the presence of three volcanic ashes derived from dated eruptions of the Yellowstone caldera, which indicate that the oldest till was deposited >2.0 Ma. Based on these results we identify three groups of tills representing at least seven pre-Illinoian glaciations: two older groups of reverse-polarity tills containing low and intermediate proportions of crystalline clasts, respectively, and one younger group of normal-polarity tills enriched in crystalline lithologies. The clay mineralogy of the reverse-polarity tills is enriched in kaolinite and depleted in expandable clays relative to the normal-polarity tills, which are also characterized by a minor amount of chlorite. The silt fraction of tills also shows mineralogical contrasts whereby the normal-polarity tills are characterized by increases in calcite, dolomite, and feldspar, whereas the older tills show depletion in these minerals and a relative enrichment in quartz. These petrographic and mineralogic changes are indicative of an increase with time in the areal distribution of unweathered igneous and metamorphic source bedrock.