COSMOGENIC-NUCLIDE BURIAL DATING OF EARLY AND MIDDLE PLEISTOCENE GLACIAL SEDIMENTS

Cosmogenic-nuclide burial dating is a means of dating rocks and sediments that have experienced a period of exposure to the cosmic-ray flux at the Earth's surface, followed by burial under sufficient overburden to stop most of the cosmic-ray flux. This method is useful for dating glacial sediment sequences because they commonly include interglacial surficial sediments, paleosols for example, that were buried by emplacement of tills. In this situation, cosmogenic-nuclide burial dating of the buried surficial sediment yields the age of the ice sheet advance that emplaced the overlying till. In addition, the half-lives of the nuclides Al-26 and Be-10, commonly used for burial dating, are well suited to dating events 0.5-3 Ma. We have used this method to date the entire sequence of Laurentide Ice Sheet (LIS) advances in central Missouri; these occurred 2.4 Ma, 1.3 Ma, 0.75 Ma, and twice between ca. 0.5-0.25 Ma. Our goal in this talk is to facilitate the use of this method in other regions of the glaciated central U.S., with the goal of building a more complete picture of LIS formation and evolution. Thus, in this talk we will describe how to effectively use this method to date early and middle Pleistocene glacial sediments. Basically the method is most effective when applied to preglacial or interglacial surficial sediments that have a known provenance and a simple geomorphic history, were exposed for a relatively long time, and were buried relatively deeply by till. If these requirements are not met, burial-dating results will most likely be ambiguous and/or uninterpretable. Our main point will be that the method relies on a thorough understanding of the geologic situation in advance of site selection and sample collection.