Mid-Wisconsinian stratigraphy and paleoenvironments at the St. Charles site in south-central Iowa

A sequence of loess-mantled sediments exposed along Clanton Creek provides the first evidence of fluvial, pedologic, and biotic environments before the last glacial maximum in south-central Iowa. Two fining-upward fluvial sequences, one inset into the other, are exposed. Radiocarbon ages indicate that the alluvial fills are about 34,000 yr old. Basal gravel in the oldest fill contains well-preserved mammoth (Mammuthus) bones. Sparse seeds from this horizon suggest weedy flood-plain conditions. The younger alluvial fill contains well-preserved pollen, plant macrofossils, and insects. The pollen is dominated by nonarboreal taxa and Pinus, suggesting a prairie border or savanna environment. Vascular-plant and bryophyte macrofossils indicate a variety of aquatic and marsh environments on the flood plain. Insects are mostly sympatric in mixed conifer and hardwood forest that extends along latitude 47[degrees] to 49[degrees] between eastern North Dakota and New England, but forest beetles are rare, and the fauna is dominated by open-ground forms. The site is interpreted as an open flood plain dotted with marshes and oxbow lakes; it was surrounded by open woodland or savanna similar to that in northeastern North Dakota at present. July temperatures at the St. Charles site were probably 3 to 5 C[degrees] cooler than those at present. The pre-loess stratigraphy of the site differs markedly from that of upland sites and demonstrates that correlation between upland and lowland sequences cannot be done without adequate dating.