APPLICATION OF SEDIMENT GEOCHEMISTRY AND OPTICALLY-STIMULATED LUMINESCENCE DATING TO TILL STRATIGRAPHY IN SOUTHERN MINNESOTA

Much of Minnesota is underlain by a thick (>30 m in many areas) accumulation of glacial tills that have been the subject of stratigraphic investigations for decades. Lithostratigraphic correlation has been successfully applied to a similar sequence of tills in Wisconsin, and is about to be formalized in Minnesota. Here we report on efforts to apply till geochemistry and optically stimulated luminescence age dating (OSL) to these investigations. Till stratigraphy in this region is facilitated by sharp contrasts in provenance across northwestern (carbonate and shale) and northeastern (igneous and iron-rich) source areas. Previous studies have largely relied on clast counts, grain size analysis, and other physical properties such as color to correlate tills. A method based on geochemical analysis may provide an additional means to stratigraphically link tills across the region. We initiated our study at the Salisbury Hill Road (SH) locality in Scott County, Minnesota, which offers exposures of up to five different Quaternary tills that have been previously studied using traditional techniques. We sampled each of the SH tills and prepared the <63 micron sediment fraction for pressed-pellet X-ray fluorescence (XRF) analysis. Our initial XRF results are largely consistent with previous provenance studies based on clast counts. Carbonate-rich samples had elevated Ca and Mg abundances, while crystalline-rich samples had elevated Na and Fe levels. Sandier tills were enriched in silica, while all tills had similar Al and K abundances. Samples high in shale generally have higher S content, but this correlation was not as robust. We continue to test these relationships with analysis of additional exposures in the region. Midcontinent till stratigraphy is hindered by poor age control on buried Pleistocene deposits. The SH exposure also contains a prominent buried glaciofluvial deposit associated with a northeastern source (Henderson Formation) till. We sampled this sand for OSL analysis, which yielded a mean age of 20 ka (calendar). This age is slightly younger than, but largely consistent with other ages reported for associated deposits in the region, suggesting that OSL analysis of buried glaciofluvial deposits may offer additional geochronologic control in ongoing efforts to advance till stratigraphy.