A "COARSENING UPWARD" SEQUENCE WITHIN MIDCONTINENT TILLS SPANNING THE ENTIRE PLEISTOCENE

The matrix texture of glacial tills reflects many things. This research demonstrates that in the mid-continent region relative age is one of these factors. In western Iowa, for example, the sand content increases from ~10% within the oldest (Early Pleistocene) till to nearly 40% within the youngest Middle Pleistocene till. The total sand content is closely correlated with the concentration of igneous rock fragments within the coarsest sand, implying that matrix textures are strongly influenced by erosion across the Precambrian Shield and its condition through successive glaciations. Older glaciations eroded highly weathered regolith with more fines, but over time younger glaciations entrained fresher bedrock. This pattern has been invoked in various models, but most of the empirical evidence probably escaped notice, because detailed work on till stratigraphy has focused mainly on younger tills of similar age with textures that were highly influenced by entrainment of (fine) Great Lakes sediment.

Three tills of the western Des Moines Lobe (Late Pleistocene) in Iowa re-enforce this interpretation. These tills (Middle to Late Wisconsinan) also become sandier with younger age, increasing from ~30 to 50% sand, oldest to youngest. Thus, the trend toward sandier tills spans the entire Pleistocene. Minor deviations in the overall coarsening sequence, however, occur between different age groups so that the entire trend is composed of three separate coarsening subsequences for Early, Middle and Late Pleistocene tills, respectively. The most likely reason for the deviations that separate successive age groups is a progressive westerly shift in ice centers.