Late Wisconsin landform distribution and glacier-bed conditions in Wisconsin

Abstract

The late Wisconsin Laurentide Ice Sheet advanced across permafrost and reached its maximum extent in Wisconsin between about 18,000 and 15,000 years ago. Deep permafrost persisted in southern Wisconsin until about 14,000 years ago and in northern Wisconsin until about 13,000 years ago. We suggest that during maximum glaciation a zone about 5 km wide in the south and 20 km wide in the north along the margin of the late Wisconsin glacier was frozen to its bed. Meltwater from farther behind the margin, where the bed was at least locally thawed, cut a series of closely spaced tunnel channels through the frozen-bed zone. These channels most likely formed episodically, and they were the source for much of the meltwater-stream sediment deposited in broad outwash plains beyond the ice margin. Frozen-bed conditions near the margin also likely contributed to increased upward shearing of sediment and the accumulation of thicl supraglacial sediment in northern areas. Up ice from the frozen-bed zone the glacier bed was at least locally thawed in a zone about 75 km wide. Extensive drumlin fields formed in the area of the bed that was thawed. By about 13,000 years ago permafrost melted in northern Wisconsin and thawed-bed conditions probably extended to the ice margin throughout Wisconsin and adjacent areas. After about 13,000 years ago in northern Wisconsin the glacier was sliding on its bed and forming drumlins out to the ice margin, and thick supraglacial sediment no longer accumulated.