EVIDENCE FOR ICE RAFTING IN LAKE SUPERIOR FROM 10,600 TO 8,200 YEARS B.P

Intervals of strongly bimodal grain size distributions exist in early Holocene sediments 21 km off the south shore of Lake Superior near the mouth of the Ontonagon River. The most prominent mode contains clay and fine-silt while the secondary mode includes medium to coarse-grained sand. The sediment count drops to zero between modes (fine sand to very coarse silt). We identified at least six layers with bimodal size distributions over an 80 cm sedimentary interval. Interlayered sediments exhibit a unimodal size distribution containing only fine-silt and clay. Removal of the coarse-grained size fraction from the bimodal distributions gives a sediment distribution remarkably similar to the unimodal sediment. These results suggest that the clay and fine silt was deposited steadily throughout the early Holocene, while the medium to coarse-grained sand was deposited episodically by an entirely different sedimentary process. The middle and late Holocene sediments do not show this bimodal size distribution. Since Lake Agassiz drained into Lake Superior during the early Holocene (about 10,600 to 9,500 calendar years BP), the most likely explanation for the influx of medium to coarse sand is through ice rafting during catastrophic flood events. Such floods occurred when progressively lower outlets opened between Lake Agassiz and the Superior Basin during recession of the glacier. In areas of more rapid sedimentation, the coarse-grained layers may be identified using seismic reflection methods enabling them to be used for correlation as has been done in Lake Huron.

The youngest sediment exhibiting bimodal distribution occurs immediately above an unconformity identified by a discontinuity in paleomagnetic inclination. The magnetic inclination data suggests erosion of 500 to 1,000 years of sediment circa 8,200 B.P. Such a date is younger than most dates for drainage of Lake Agassiz into the Lake Superior Basin, however, light oxygen isotope ratios at 8,600 BP suggest glacially derived waters entered the Huron basin at about this time.