A Model for the Source and Distribution of Loess on the Green Bay Lobe Land Surface, WI

Loess has been recognized on the Green Bay Lobe glacial land surface for nearly 100 years, yet studies on sources, thickness patterns, and effects on soil development are few. We use soil survey (SSURGO) data to map the distribution of loess on the Green Bay Lobe surface and propose a model for the source and timing of loess accumulation. We evaluated county-level soil survey data for potential loess thickness based on Official Series Descriptions and associated soil map units. In the GIS, soil map units were extracted based on potential loess thickness in categories of <50, 50-100, and >100 cm, along with soils formed in eolian sand for 14 south central WI counties. The extracted map units were overlain on a digital elevation model.

Most extensive thick loess (soils with >100 cm loess) occurs 40 to 100 km behind the terminus of the Green Bay Lobe, indicating age of land surface exposure is not the primary control on thickness. The boundary between loess and sandy soils is distinct, and corresponds with the high topography of the Prairie du Chien cuesta. Eolian sand occurs on lower topography underlain by sandy glacial sediments, outwash, and lake sediment, and appears to have been trapped by the escarpment, allowing loess accumulation above. Preliminary mineralogical evidence matches clay and silt of offshore sediment of Glacial Lake Wisconsin with the loess mantle. We propose that the Central WI Sand Plain became a source of loess following deglaciation and drainage of Glacial Lake Wisconsin. New optical ages from central WI indicate eolian activity by W-NW winds between 14-10,600 cal yrs BP (Rawling et al., in press, Geomorphology). Pedogenic impact of the loess mantle includes low density, stone-free soils, high moisture holding capacity, greater SOC storage, and dispersible clays that contributed to development of well expressed Bt horizons.