REDISTRIBUTION OF LOESS DUE TO THAWING OF MIS-2 PERMAFROST?

This discussion of the highly variable and discontinuous loess deposits of western Wisconsin highlights how a spatial approach can provide key insights into paleosystems of loess transportation and deposition. The work is focused on the landscapes near the Chippewa River valley in western Wisconsin, a mix of sandstone bedrock-controlled uplands in the west, and lower-relief, till-covered plains in the east. The valley is floored by outwash and lacks loess. Initial loess deposition here may have been dominated by the valley trains of the Mississippi and Chippewa Rivers.

Evidence exists for a widespread eolian erosion event in this region, likely triggered by thawing permafrost and strong storms, as the MIS-2 was retreating. This event, which must have also been associated with an as-yet-unforested landscape, likely facilitated erosion and redeposition of much of the preexsiting outwash and loess. Much of the sand is present today as dunes and eolian sand in ice-wedge casts, whereas much of the loess was redeposited downwind of large bedrock uplands (protected sites). As a result, the landscape has areas that lack loess immediately adjacent to areas of thick loess, and at some sites the loess thickness exceeds 7 m. These thick loess deposits occur primarily to the east and southeast of the valley, in the lee (SE sides) of large bedrock uplands. They typically extend downwind for 1-6 kms, and follow a clear NW-SE trajectory, indicative of transport on strong NW winds. I argue that these “wind-shadow” deposits take two forms: (1) remnant deposits protected from strong erosive winds; these deposits may date back to the “original” loess depositional event, and (2) secondary loess that was re-entrained from sites in the valley and deposited downwind of the large ridges. The latter is typically coarse-textured and, at sites near the ridge crests, interbedded with eolian sand. Loess on the NW sides of these ridges is thin or nonexistent, and many such sites show evidence of abrasion and erosion, with numerous ventifacted boulders. Most of the smaller ridges were apparently unable to develop a critical wind shadow, and thus lack measurable loess on all sides. Luminescence dates indicate that this period of loess re-deposition may have continued, at least at some sites, into the early Holocene.