MAGNITUDE AND VARIABILITY OF LAST INTERGLACIAL PEDOGENESIS: EVIDENCE FROM SANGAMON GEOSOL MORPHOLOGY, MINERAL WEATHERING, AND GEOCHEMISTRY IN SOUTHERN INDIANA

Documenting the magnitude and variability of morphological expression and mineral weathering of the Sangamon Geosol is useful for understanding long-term pedogenesis, estimating nutrient pools, and for correlation of Quaternary deposits. We present data from nine profiles of the Sangamon Geosol formed in outwash deposited along the Illinois Episode glacial margin in southern Indiana. Profiles were described in pedologic detail; lab analyses include particle size, dithionite Fe, clay mineralogy and 8-63 µm silt mineralogy (XRD) and geochemistry (pXRF). Each site was sampled for OSL determination of the outwash age; preliminary results from three sites indicate deposition 130 ± 10 ka.

The profiles are buried by Peoria Loess deposited ca. 29-16 ka, indicating the soil formed over a minimum of 100 ka. Morphologically, the Sangamon Geosol is a humid climate soil with an eluvial upper solum and an illuvial lower solum. The Geosol is polygenetic resulting from slow aggradation of a mid-Wisconsinan silt that over-thickened the eluvial upper horizons. Bt horizons average 3.5 m ± 0.9 m thick, with total sola thickness averaging 4.4 m ± 1.1 m. Leaching depths > 6 m in all cases routinely extend throughout the thickness of outwash. Maximum clay contents in B horizons average 27 ± 2%, an average of 22% increase relative to parent material. Dithionite soluble Fe averages 3 ± 1% in Bt horizons. Silt-size plagioclase is depleted in Bt horizons, averaging 10 ± 9% loss relative to C horizons, which is supported by differences in paired Bt-C horizon geochemical ratios. Ratios TiO₂/CaO (1.3 ± 1.4) and Zr/Sr (2.8 ± 2.8) are largely due to plagioclase depletion in Bt horizons. Clay minerals show marked depletion of illite in most profiles, averaging 16 ± 15% loss. Illite is replaced by expandable minerals and kaolinite (increasing 9 ± 6%), and through interstratification, including kaolinite/expandable minerals in all profiles. All measures of profile morphology and weathering support the interpretation of progressive mineral weathering producing clays through particle size reduction and neoformation. Variability largely reflects sediment stratification in some profiles. All trends support interpretation of a single interglacial record of pedogenesis. Additional OSL results will allow confirmation and calculation of weathering rates.