SYSTEMS OF LOESS TRANSPORTATION AND DEPOSITION: EXAMPLES FROM WESTERN WISCONSIN

Loess deposits form via sequential processes of silt formation (usually via glacial grinding), entrainment and transportation by wind, followed by eventual deposition. Of these three suites of processes, contemporary research has been primarily focused on silt entrainment and transportation, as these systems can provide the most insight into paleoenvironments.

Loess in western Wisconsin was derived mainly from the meltwater sluiceways of the Mississippi and Chippewa Rivers that form the western boundaries of the region. Thick loess continuously blankets the flat-lying bluffs east of the Mississippi River. However, within the sandy landscapes of the Chippewa basin proper, loess deposits are much less common, and those that are present have a distinctly patchy distribution. Many areas lack loess entirely. Nonetheless, some loess deposits in the basin can exceed 5 meters in thickness. Explaining this spatial disparity forms the focus of this presentation.

Following loess deposition across the Chippewa basin, much of it was then, later, re-entrained and transported downwind. Various authors have suggested that loess entrainment is facilitated by saltating sand, and in the sandy basin of the Chippewa River there is ample evidence for this process. Dune forms and eolian sand deposits abound on the basin floor, and therefore, in low-relief areas with sandy substrate almost no loess exists. Loess deposits are mainly found to the east and southeast of large bedrock ridges, which are scattered throughout the basin. Here, loess is thick and coarse, quickly thinning to the east and southeast. I argue that loess in these wind-sheltered, “lee” areas was initially deposited on the basin floor and later re-entrained and transported to the east-southeast; some of this loess was deposited downwind of obstructions to the wind. High bedrock ridges form one kind of wind obstruction here, but deep river valleys also operated similarly in this region; loess is thick just downwind of major N-S flowing rivers. In effect, the Chippewa basin will be used as an example of loess “redistribution”.