UTILIZING TILL GEOCHEMISTRY TO CORRELATE AND DISTINGUISH TILLS IN MINNESOTA, USA

The Quaternary of Minnesota is represented by numerous glaciations, with deposits of the late Wisconsinan preserved at the surface across most of the state. Repeated episodes of deposition and erosion have left a sequence of tills and associated sands of variable thicknesses. Mapping and understanding Quaternary stratigraphy is key to managing groundwater resources in glacial aquifers, a major source of water for Minnesotans.

The Minnesota Geological Survey (MGS) currently analyzes the till texture (grain size) and composition of the very coarse sand (1-2 mm) fraction to distinguish mappable till units. Recently, the MGS began utilizing geochemistry (4-acid leach, ICP-MS, <63 µm) of rotary sonic cores to better understand and correlate till units and associated sand bodies across the state. Previous studies (e.g. Wittkop et al., 2020) have demonstrated that using till matrix geochemistry supplemented with other information is an effective way to trace bedrock sources up ice.

Initial results from till samples (N = 1413) suggests certain elements can be useful in distinguishing provenance. Examples of elements elevated in tills derived from the northwest include Arsenic, Cadmium, Calcium, and Antimony as a result of incorporating varying amounts of carbonate and shale bedrock. Tills sourced from the northeast are elevated in Aluminum, Copper, Iron, and Titanium. Some elements such as sulfur do not correlate with provenance but may suggest oxidation of till. Currently, variations among elements compared to provenance are best shown alongside rotary-sonic cores, tables, and normalized plots. Future work for correlation and distinguishing units includes more multivariate statistical analyses and spatial analysis for various till units.