WIND-BLOWN SAND AND SILT DEPOSITS IN WESTERN WISCONSIN, USA

The Wisconsin Geological and Natural History Survey is nearing completion of 1:100,000-scale surficial geologic mapping for 11 counties in southwestern and west-central Wisconsin. These counties largely define the unglaciated Driftless Area, which is characterized by deep fluvial incision into nearly flat-lying Paleozoic sedimentary rock. The area includes loess sourced from the Mississippi River and other sources during recent glaciations, as well as aeolian sand mobilized by fluvial incision into friable Cambrian sandstones. These data are being incorporated into a new 1:1,000,000-scale map of windblown sand and silt deposits of Wisconsin.

The thickest loess occurs in the southwestern corner of the state, south of the west-flowing Wisconsin River. Here, broad upland surfaces are capped by the Ordovician Sinnipee Group dolomites. Surfaces as much as 110 m higher than the adjacent Mississippi River are blanketed with 3-7 m of loess, which thins to the east (downwind). Most upland loess in Wisconsin represents the Peoria Member of the Kieler Formation, deposited from ca. 30 to 14 ka.

In the central Driftless Area north of the Wisconsin River, loess thicknesses are highly variable due to irregular preservation on the highly dissected landscape. This stands in stark contrast to areas south of the Wisconsin River, where the much less deeply dissected landscape has retained a more uniformly thick loess cover. Nevertheless, loess sometimes still exceeds 7 m on uplands immediately downwind of the Mississippi River in this central area.

Progressing even farther north, the landscape is emplaced lower in the Paleozoic sedimentary sequence, with rivers incising into the friable Cambrian Wonewoc, Eau Claire, and Mt. Simon sandstones. These mature, quartz-rich formations weather easily and can thus provide abundant sand to the aeolian system. In particular, the La Crosse and Black River valleys are mantled by thick (>10 m) windblown sand with abundant dune forms.