Abstract

Colluvium on foot slopes in parts of North America and Europe has been attributed to a major mass-wasting episode during the last glacial period. Stratigraphic evidence and 14C ages support this hypothesis for the northern part of the Upper Mississippi Valley. Colluvium in this region grades laterally into, or interfingers with, fluvial sediment beneath the late Wisconsinan Savanna terrace. Colluvial foot slopes are truncated by fluvial surfaces postdating the incision that created the Savanna terrace between 13 and 11 ka. Samples from within the colluvium have 14C ages between 18.6 and 12 ka. Ages of 28.9 and 20.3 ka have been obtained from beneath colluvium, and fluvial sediment inset into colluvial foot slopes has yielded ages between 12.5 and 9.8 ka. Widespread permafrost during the late Wisconsinan glacial maximum may explain the onset of accelerated mass wasting that produced the colluvium, although mass wasting apparently continued for some time during subsequent climatic warming. The results described here imply major, long-term, climatically driven fluctuations in sediment supply from hillslopes to the fluvial system in this region.

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