AGE OF COLLUVIUM INDICATES ACCELERATED LATE WISCONSINAN HILLSLOPE EROSION IN THE UPPER MISSISSIPPI VALLEY

Colluvium on foot slopes in parts of North America and Europe has been attributed to a major mass-wasting episode during the last glacial pe riod. Stratigraphic evidence and C-14 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 b eneath the late Wisconsinan Savanna terrace. Colluvial foot slopes are truncated by fluvial surfaces postdating the incision that created th e Savanna terrace between 13 and 11 ka, Samples from within the colluv ium have C-14 ages between 18.6 and 12 ha. Ages of 28.9 and 20.3 ha ha ve been obtained from beneath colluvium, and fluvial sediment inset in to colluvial foot slopes has yielded ages between 12.5 and 9.8 ha. Wid espread permafrost during the late Wisconsinan glacial maximum may exp lain the onset of accelerated mass wasting that produced the colluvium , although mass wasting apparently continued for some time during subs equent climatic warming. The results described here imply major, long- term, climatically driven fluctuations in sediment supply from hillslo pes to the fluvial system in this region.