The Sangamon Paleosol in the Southern Mississippi River Valley: Characteristics and Paleoclimatic Inferences.

The Sangamon is a widespread and distinctive paleosol in the southern Mississippi River Valley (MRV) that developed in the Illinoian Loveland Loess (Loveland) and is a key paleoclimatic indicator of weathering and pedogenesis  at least during the last major interglacial. The Sangamon is best preserved near the east bluff of the MRV where the Loveland is relatively thick and overlain by younger Wisconsin loess (Roxana Silt, Peoria Loess). The Sangamon also is present where the Loveland is thin and/or younger loess is absent. Comparison of the Sangamon in both buried and unburied settings reveals important interpretive details. Where buried, Sangamon solum thickness is greater than, but still comparable to unburied exposures. Burial by younger loess did not assure complete preservation of an uneroded or unmodified Sangamon profile. Erosion and/or mixing commonly truncated or obliterated the original A and upper Bt horizons, both in buried and unburied settings. Paleosol horizons developed in the mixed zone commonly have been  interpreted as belonging to the Sangamon. The mixed zone, however, includes younger sediments (e.g., loess, colluvium) modified by soil- forming processes that affected the truncated Sangamon. Thus, some key Sangamon characteristics (e.g., solum, thickness, argillan development, soil fabric) are influenced by landscape stability during and prior to burial by younger loess. One distinctive feature of the Sangamon and older loess paleosols in the MRV is the presence of a randomly interstratified kaolinite smectite, which commonly indicates the contact between the intact paleosol and younger mixed material. Age data suggest that a stable, uneroded Sangamon profile may represent weathering and pedogenesis between 125 and 55 ka. Paleoclimatic interpretations of the Sangamon must encompass pedologic development, stratigraphic context, and geomorphic setting as well as climate change. Data indicate that the Sangamon is the result of multiple soil forming periods and erosion.