EXTENT OF THE SUPERIOR LOBE IN WEST-CENTRAL WISCONSIN DURING THE ILLINOIAN GLACIATION

Till of the River Falls Formation in western Wisconsin is reddish brown, sandy, and derived from the Lake Superior region. Baker and others (1983) proposed that the River Falls Formation till was deposited during the Illinoian Glaciation, and also that a pronounced ice-free embayment existed at that time between the Superior and Chippewa Lobes in the eastern half of Dunn County. This embayment roughly coincided with the modern Red Cedar River valley. Recent work in Chippewa County suggests that the Superior Lobe extended farther east than previously recognized and that most of Dunn County was covered by ice during the Illinoian Glaciation.

River Falls Formation outwash in western Chippewa and Eau Claire Counties is up to 35 m thick, extremely eroded, and enriched in pedogenic clay to depths of 5 m below the land surface (Syverson, in review). This outwash commonly contains ice-proximal cobbles and boulders and nice, large Lake Superior agates. Meltwater from the late Wisconsinan Chippewa Lobe deposited outwash of the Copper Falls Formation in adjacent areas that is cobble poor, is relatively unweathered, and rarely contains Lake Superior agates.

Bement and Syverson (1995) described pre-Wisconsinan proximal outwash and reddish-brown, sandy diamicton in southern Eau Claire County (most likely parts of the River Falls Formation). In addition, the Lake Superior agates in the River Falls Formation outwash of western Chippewa County are typically associated with areas farther to the west/northwest that were formerly covered by the Superior Lobe. During the Illinoian Glaciation, I propose that the Superior Lobe flowed southeastward into the Chippewa Falls area and deposited River Falls Formation till in southwestern Chippewa County. If so, most of Dunn County must have been covered by glacier ice during the Illinoian Glaciation. The thick, agate-rich River Falls Formation outwash unit in western Chippewa and Eau Claire Counties might have been deposited in an interlobate junction as the Superior and Chippewa Lobes wasted back from their maximum ice-margin positions. The lack of River Falls Formation till in eastern Dunn County (part of the proposed ice-free embayment of Baker and others (1983)) probably was caused by intense erosion within the Red Cedar River drainage system.