Origin of Red Clay over Dolomite in the Loess‐Covered Wisconsin Driftless Uplands

Abstract

Red clays associated with Prairie du Chien and Sinnipee (Galena) dolomites and loess were studied stratigraphically and geomorphically at 10 locations. The clay content (< 2 µm, 82 ± 9.1%; < 0.2 µm, 70 ± 10.4%, n = 15) relative to the overlying loess (< 2 µm, 28 ± 4.4%; < 0.2 µm, 16 ± 4.9, n = 4) and the dolomite insolubles (< 2 µm, 47 ± 11%; < 0.2 µm, 34 ± 9.6%, n = 7) indicate fine clay enrichment. Smectite‐rich oriented clay coatings on dolomite sands suggest illuviation. Undisturbed chertlines from dolomite through red clay bodies necessitate a volume‐for‐volume replacement. Periglacial activity during the Wisconsinan frequently produced loess (i), over a mixed zone of loess and red clay (ii), over in‐situ red clay (iii), and dolomite (iv). For the 1‐to 10µm quartz; euhedral grains indicated by x‐ray diffraction I(100)/(101) = 0.23, 0.54, 1.02, 0.70, respectively; nine trace element contents = 2.15 ± 0.40, 1.61 ± 0.18, 0.78 ± 0.10 relative to dolomite (1.0); and oxygen isotopes expressed as δ¹⁸O = 18.6, 19.7, 26.9, 26.4%. The values for the mixed zone (ii) fall between those for the loess (i) and insitu red clay (iii). Optical data for the morphology of the fine sand fractions (50–250 µm) were corroborative. The silt and sand of in‐situ red clay (protected in karst depressions and by chert lag gravels) are from the dolomite.