COMPOSITIONAL CHANGES IN LATE CENOZOIC GLACIAL DEPOSITS OF THE NORTH-CENTRAL U.S.: IMPLICATIONS FOR THE LONG-TERM EVOLUTION OF THE LAURENTIDE ICE SHEET

The north-central U.S. 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 these sedimentary sequences. 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 a low and intermediate amount of clasts and minerals derived from crystalline bedrock, respectively, and one younger group of normal-polarity tills enriched in crystalline materials. Taken together, these petrographic and mineralogic changes may reflect an unroofing sequence of the Canadian Shield that involved the removal of a former saprolite mantle by ice sheet erosion, progressively exposing a greater surface area of unweathered igneous and metamorphic rocks. These data suggest the occurrence of change in the character of the underlying geological substrate of ice sheets during the late Cenozoic. This change in substrate may have caused a fundamental shift in ice sheet dynamics, which may be reflected in the evolution of global ice volumes.