| Popis: |
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 macofossils 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° to 49° between eastern North Dakota and New England, but forest beetles are rare, and the fauna is dominated by openground 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 north-eastern North Dakota at present. July temperatures at the St. Charles site were probably 3 to 5 C° 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. |
