THE ENIGMATIC ROXANA SILT

The Roxana Silt (Roxana) in the Lower Mississippi Valley (LMV) has sedimentologic, mineralogic, pedologic, and chemical characteristics that differentiate it from the overlying Peoria Loess (Peoria) and the underlying Loveland loess (Loveland). Roxana deposition began at ~55 ka, when the Gulf of Mexico coastline was ~25 m lower and tens of kilometers seaward of its present position, and the Laurentide Ice Sheet was absent from or occupied only the far northern Upper Mississippi Basin. Within several thousand years of initial Roxana deposition, the mid-continent was cooler and wetter, possibly being affected by Meteor Crater (~50 ka) ejecta. The contact between the Roxana and the underlying Sangamon paleosol(Sangamon)/Loveland complex in the upper LMV is generally within a few tens of cm, but field and laboratory data indicate that the first 15-20 k.y. of Roxana deposition was slow with formation of an aggrading soil that at most localities is welded to and shares some chemical and mineralogical characteristics with the underlying more developed Sangamon. This period of relatively slow loess deposition was followed by a few-thousand-year period of rapid loess deposition, followed by a final period of slow loess deposition and formation of an aggrading soil, the Farmdale paleosol. Age data for the upper LMV indicate that the Farmdale paleosol began forming in the Roxana around 34 ka and continued to about 26 ka. Chemical and SEM data indicate minimal mineral weathering during Roxana deposition and paleosol formation (~55-26 ka). In the lower LMV (~ south of 34º), Roxana deposition was accompanied and followed by significant erosion, resulting in a “mixed zone” in the stratigraphic position of the Roxana. Mixed-zone composition varies with geologic setting but generally consists any combination of Loveland, Sangamon, Roxana, basal Peoria, and (or) colluvium from these and(or) older units. The Roxana seems to have a combination of properties that make it an important linkage to climate changes during the last interglacial/glacial transition. It may be a useful reference to contemporary changes in the LMV, an area that is still undergoing important sporadic changes during the present glacial/interglacial transition.