Subsurface stratigraphy and genesis of pre-Wisconsin paleosols in Whitebreast Creek watershed, south-central Iowa.

摘要

The Quaternary stratigraphy and genesis of the buried paleosols of the Whitebreast Creek Watershed in south-central Iowa are not well understood. Therefore, in a 4.6-km transect of 10 sites land-surface elevations as well as some selected macromorphological, mineralogical, physical, and chemical properties were determined and/or analyzed for the major subsurface materials and the buried pre-Wisconsin paleosols. The results showed that the upland stratigraphy consisted of Wisconsin loess with a Basal Loess (BL) increment over a major soil-stratigraphic unit, a Yarmouth-Sangamon (YS) paleosol. The YS paleosolum is formed mainly in pre-Illinoian till. The chemical data such as total P analysis did not support significant mixing and/or pedoturbation of the YS solum. A weakly developed Wisconsin paleosol in the BL on the erosion surfaces displayed evidence of involution. The Late-Sangamon (LS) sola on the erosion surfaces and loess-covered terraces were polygenetic and formed in multiple parent materials. The materials underlying the pre-Wisconsin paleosols on the terraces indicated occurrence of three distinct deposits, suggesting some major hiatus in the depositional environments of the old valley. The colluvial-alluvial, till-derived sediment, clay sediment, and a sandy sediment composed the depositional units of the terraces. The whole sequence was underlain by Pennsylvanian shale bedrock. The macromorphology, clay mineralogy, elemental analysis, pedogenic free oxides, total-P, total-Ti, and pH suggested a catenary soil-landscape relationship. The clay mineralogy of YS and LS paleosols showed smectites, vermiculites, kaolinites, micas, Fe oxides were dominant with most variation in vermiculites, micas, and Fe oxides. A relatively intensive weathering occurred in the YS solum. Diagenesis due to influxes from the overlying loess confound pedogenic processes. The LS paleosols were moderately weathered as evidenced by the extractable free oxides of Al, Fe, and Mn and their relationships with total elements. The higher concentration of pedogenic free oxides in the footslope of the upland indicated movement of the sesquioxides by pedogenic and geomorphic processes occurred in this catenary system. The macromorphology and Mn