GEOCHEMICAL AND MICROMORPHOLOGICAL EVIDENCE OF SOIL POLYGENESIS IN THE DRIFTLESS AREA OF WISCONSIN

Geochemical and micromorphological assessment of soils forming on dolomite-rich bedrock in the southern Driftless Area (sDA) of Wisconsin reveals a pattern of complex pedogenesis dominated by surficial additions/transformations of wind-blown materials and dolostone epikarst degradation. Total elemental compositional data show strong differentiation between the upper portion of these soils (quartz-rich silt loams) and underlying redder hued clay-rich subsoils (also called the Rountree Formation) abruptly contacting bedrock. The high clay contents of these subsoils (41-83%) suggest that the relatively high purity dolomite of this particular setting cannot be the sole parent material for sDA soils, but rather serves as a foundation for karst sediment accumulation zones and source of high base saturation through gradual dissolution. Conservative tracers in the weathering column (accounted through Ti:Zr ratios) indicate through dilution with depth that both parent materials contributed to the formation of the soils. In the deeper horizons, ‘dilution' of loess materials by selective translocation of Si and Al into dissolving carbonate matrix may have been as great as 95%, as Zr levels approach that of the bedrock parent and the assumption is made that the loess contains significantly higher Zr composition (mean = 415 ppm) than the carbonates (mean = 29 ppm). Thin sections support this concept by showing pervasive intergranular clay intercalation between dolomite grains of the saprolite, suggesting replacement of calcite cements from the original bedrock by pedogenic clays (sandy loam horizons with clay component dominated by fine to very fine clays) derived from the overlying silty materials. The stratigraphy, spatial discontinuity and time-transgressive nature of the Rountree Formation suggest it is a product of long-term pedogenic inputs into the epikarst interface and subsequent dissolution of the carbonate foundation over long episodes of weathering.