Morphology and Weathering Trends of the Sangamon Soil Complex in South-Central Indiana in Relation to Paleodrainage

Abstract

Three paleodrainage groups are proposed for profiles of the Farmdale–Sangamon soil complex in south-central Indiana. The groups (good, intermediate, and poor) are differentiated on the basis of matrix colors and color patterns. Genetic support for the groupings is provided by morphological trends still evident following > 100,000 yr of pedogenesis and burial by late Wisconsinan loess. Depth of carbonate leaching, solum thickness, and argillic horizon thickness all decrease with poorer drainage. Clay mineralogy also reflects paleodrainage. Illite degradation is intense in all profiles, but profiles with good drainage have poorly crystalline, interstratified expandable minerals, while well crystalline smectites dominate in profiles with poor drainage. Remanent aggregation in former A horizons is stronger in more poorly drained profiles, while the effects of structural overprinting from the modern soil increase with better modern drainage. Soil morphology, mineralogy, and parent material–paleolandscape position of Sangamon profiles are all internally consistent with color development under soil hydrological conditions during the last interglacial to glacial transition. The occurrence frequency of each drainage group indicates that the Sangamon soilscape was better drained than now, and morphology and clay mineral evidence suggests that areas with poor drainage did not experience the extreme seasonal groundwater shifts that the modern landscape experiences.