Clay Mineral Weathering in Southern Wisconsin Soils Developed in Loess and in Shale-Derived Till

Abstract Loess is the dominant soil parent material covering the western and central portions of southern Wisconsin. Glacial till derived mainly from Devonian shales and Silurian dolomite is the most extensive parent material in extreme southeastern Wisconsin. The soils developed in loess are coarser in texture, leached of carbonates to greater depths and more acid and contain a higher percentage of zirconium in their coarse silt than those developed in the till. The clay of the soils developed in loess contains a higher proportion of minerals of more advanced weathering indices (montmorillonite, “pedogenic” chlorite and kaolinite) formed by weathering. The clay mineralogy was fairly uniform in the four soils developed from loess, but the two Gray-Brown Podzolic soils were found to contain only about one-half as high a percentage of total clay and one-fourth as high a percentage of medium and fine clay in their A horizons (which were also 7 in. less in thickness) than the A horizons of their prairie soil analogues. The Varna soil developed in the till contains clay with a higher proportion of minerals of less-advanced weathering indices (mafic chlorite and dioctahedral mica), largely inherited from the parent material. There has been some transformation of mica and mafic chlorite to expansible layer silicates and amorphous material in the Varna solum and the ratio of ferrous to total iron decreased both with decreasing particle size (from fine silt through fine clay) and with approach to the soil surface (in the whole clay).