GEOCHEMICAL TRENDS IN WESTERN ILLINOIS DIAMICTONS

Distinguishing diamictons from differing source areas in western Illinois is commonly challenging because they are so similar in texture and color. Traditionally, stratigraphic units in the area are differentiated primarily by clay mineral composition. To further assist identification, classification, and characterization of units, a geochemical study of these sediments is being developed. Illinois Episode diamicton (Kellerville Till Member of the Glasford Formation) and older pre-Illinoian diamictons (Wolf Creek and Alburnett Formations) were preferentially sampled from unaltered parent material (C and D horizon) in core or at exposures to minimize the effects of weathering. Samples were air-dried, disaggregated, and then sieved. The <63 micron (silt and clay) fraction was analyzed for trace metals by inductively coupled plasma mass spectroscopy (ICP-MS); major oxides were detected by LiBO2 fusion-ICP.

Bivariate oxide-oxide major elemental plots show Kellerville sediments contain higher concentrations of MgO whereas the Wolf Creek and Alburnett Formations are more Al-rich. Total alkalis-silica diagrams also indicate Kellerville sediments have a more basic composition compared to the older tills. Trace metal data reveal further elemental variability between units including high Pb, Zn, Mo, and As values for pre-Illinoian tills compared to Kellerville till. Geochemical trends of the deposits reflect the mineralogy of local bedrock and other overridden materials, however, they also may be influenced by distal sources. Illinois Episode till was deposited by northeastern-source Lake Michigan lobe ice, whereas ice that advanced from the northwest deposited the Wolf Creek and Alburnett diamictons. As we collect more geologic data, geochemical data will complement the clay mineral composition, and trends will assist in modeling glacial history and sediment provenance. The results of this study will have implications for geologic mapping and will assist with understanding till composition and origin.