THE MARQUETTE ADVANCE: THE LAST GASP OF THE SUPERIOR LOBE IN THE WISCONSIN EPISODE

The Superior lobe of the Laurentide ice sheet underwent major readvances during its final retreat. These fluctuations have left behind a complex stratigraphy of glacial, fluvial, and glaciolacustrine sediment. The last advance to fill the Superior basin, the Marquette, has been recognized by dated ice margins in the Upper Peninsula of Michigan. It extended west far enough to block the eastern outlets of glacial Lake Agassiz, but its southwestern extent had until recently been uncertain. Work by the author since 2001 has filled in this part of the picture.

In the southwestern arm of the Lake Superior basin, deposits of the Marquette advance overlie thick lacustrine sediment of glacial Lake Duluth. The till consists mainly of massive red calcareous clay to silty clay, admixed with a small amount of sand and coarse fragments up to small boulders. It is typically thin, roughly a meter thick, and somewhat patchy. In some exposures, a distinctive yellowish silt bed occurs underneath the red clay till. Little or no glacial lake sediment overlies the till. Where the till overlies red glacial lake clay, the contact is commonly obscure, and the till can easily be mistaken for lake clay. It is so thin that it is basically in the soil zone. When I first studied the area, I called it lacustrine, and explained its lack of bedding as the result of soil processes such as bioturbation. The underlying lake sediment is bedded, but also contains scattered pebbles and boulders, presumably dropped from icebergs. Where the red clay till and underlying silt overlies fine-grained glacial lake sand however, it is distinctive and the contact clear. Distortion of the contact and the underlying silt, and incorporation of the silt into the red clay shows that the red clay is indeed a till, not lacustrine clay. The silt is interpreted as a shallow-water deposit from glacial Lake Superior, which was rising again in advance of the ice.

The elevation of the ice margin shows a low gradient in the study area. This suggests that the shear stress at the base of the ice was very low; perhaps even that the ice was floating in the deepest part of the basin, buttressed mainly along the shore.