PRE-ILLINOIAN LOESS IN CENTRAL MISSOURI

Recent excavations and cores exposed and recovered Pre-Illinoian (pre-MIS 6) loess in central Missouri along the Missouri River valley. At one site this loess is capped by a weakly developed Yarmouth Geosol overlain by MIS-6 Loveland Silt with a typical Sangamon Geosol and then Wisconsinan loess. This Yarmouth solum is distinctly less mature than other Yarmouth profiles which developed within till, but is comparable to the overlying Sangamon profile, which implies that deposition of the oldest loess occurred about one glacial cycle prior to MIS 6.

At a second site cores recovered a sequence of four loess deposits (Peoria-Roxana-Loveland-unnamed) within a gleyed pedocomplex above the youngest Pre-Illinoian till in Missouri. This till has been dated to between 0.25 – 0.4 Ma, with a probable age of ~0.35 Ma (MIS 10). Yarmouth and Sangamon profiles, respectively, are present atop the lower two loesses, demonstrating that the older of these is Pre-Illinoian. The underlying till also preserves a mature interglacial paleosol with an argillic B horizon, leaving MIS 8 as the most likely age of the oldest loess. The presence of this loess at least 140 km west of the Mississippi indicates that its source was the Missouri River valley, although it may correlate to a late Middle Pleistocene loess that is also present along the Mississippi. The latter loess is present in St. Louis County, Missouri where it has been tentatively correlated with the Crowley’s Ridge Silt in Arkansas.