It is conventional wisdom that the last continental ice sheet in North America extended beyond the southern margins of the Great Lakes basins during both early (ca. 75 ka) and late (<30 ka) phases of Wisconsin glaciation. This model hinges on the presence of a regionally extensive diamict, traditionally interpreted as a subglacial till, exposed in the Ontario Basin at Toronto between last interglacial lacustrine sediments and late Wisconsin tills. The discovery of an in situ lacustrine ostracode fauna confirms previous sedimentological and geophysical investigations which concluded that the early Wisconsin diamict is not a subglacial deposit but a stony mud deposited on the floor of an extensive glacial lake. The unit, together with younger mid-Wisconsin sediments, was only overridden by the Laurentide Ice Sheet during the late Wisconsin, after 30 ka. We argue that during the early phase of Wisconsin glaciation the Laurentide Ice Sheet was thin, highly dynamic, and of insufficient volume to extend fully across the Great Lakes basins, a situation more closely in accordance with the marine 18O/16O record and other proxy climatic data that show maximum cooling and global ice volumes at about 20 ka.

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