Distinguishing soil age and parent material effects on an Ultisol of north-central Wisconsin, USA

A red and brown sandy clay loam layer containing quartz and chert gravel overlying Cambrian sandstone outside the limits of the most recent Pleistocene glaciation in north-central Wisconsin, USA, was interpreted in the field as a thick, well-developed paleoargillic horizon. The degree of soil profile development at this site is strikingly different from that seen in nearby soils of known late-Wisconsinan to Holocene age, suggesting that the soil at the site could be pre-Wisconsinan or even pre-Quaternary. A thin surface mantle of loamy sand was interpreted as a younger deposit; similar sand fills common ice-wedge casts at the site. Interpretation of the sandy clay loam layer is complicated by the possibility that it may include sediments of the Cretaceous or Tertiary Windrow Formation, derived from deeply-weathered pre-Quaternary landscapes. Micromorphological studies demonstrate that the sandy clay loam layer contains abundant illuvial clay, with evidence of more than one illuviation episode. The loamy sand mantle and sand filling ice-wedge casts are similar mineralogically and contain easily-weatherable minerals, whereas the sand in the sandy clay loam layer has a highly restricted and resistant mineralogy similar to Windrow sediments. Many of the K-feldspar grains in the sandy clay loam layer are highly weathered. The soil is classified as an Ultisol, but smectite is abundant in the sandy clay loam clay fraction. The resistant sand mineralogy is possibly inherited from Windrow-like sediments, and cannot be used to infer intense weathering in situ. On the other hand, field relationships clearly indicate that the sandy clay loam argillic horizon largely formed before the permafrost episode represented by the ice-wedge casts. This evidence, together with greater solum thickness and extent of clay illuviation than nearby soils of late-Wisconsinan to Holocene age, indicate that at least one phase of pedogenesis at this site was pre-Wisconsinan.