CHEMICAL AND MINERALOGICAL ANALYSIS OF A SANGAMON SOIL EXPOSURE, SOUTHWESTERN OHIO

The Sangamon soil, which developed between the Illinoian and Wisconsinian glaciations, has been described from many locations in the Midwestern United States. However, quantitative data on the Sangamon from Ohio and Indiana are lacking, except for some published grain size profiles. We analyzed samples from a paleosol exposed near Sharonville, in southwestern Ohio, which we identified as the Sangamon based on its stratigraphic position beneath late-Wisconsinian drift. In our outcrop, the Sangamon is developed on a fining-upward diamicton. The origin of this diamicton is uncertain, but it may be colluvium derived from Illinoian till or the local bedrock. X-ray fluorescence analysis of this diamicton shows that the relative proportions of certain immobile elements such as Nb and Zr decrease with depth through the soil profile, reflecting their relative enrichment due to the selective removal of mobile alkalies and alkaline earths. Enrichment of the clay component of the soil in the lower zone is indicated by a downward increase in Al, suggesting eluviation of the upper part of the unit. Scanning electron micrographs reveal the presence of highly altered primary minerals such as mica and feldspar in the soil zone. Powder X-ray diffraction analysis shows that the upper and lower parts of the unit have different clay mineralogies. The less than 2 micrometer fraction of bedrock shale and the lower, unweathered diamicton, the likely parent materials for the soil, is characterized by well crystallized mica and chlorite, whereas the soil zone consists of randomly interstratified illite/smectite and chlorite/smectite. This difference is consistent with a model of a leached soil resting on the parent diamicton.