LOESS SPATIAL CHARACTERISTICS ESTIMATE PALEOWIND PATTERNS DURING AND AFTER LATE GLACIAL MAXIMUM IN WISCONSIN, USA

Loess distribution patterns in the midwestern US have long been used to infer paleowind directions during and after the Late Glacial Maximum (LGM). Landscapes east of north-south trending rivers such as the Mississippi, Wabash and Illinois all have thick loess deposits, pointing to loess transport mainly on westerly winds. Most of these sites, however, are considerable distances from the ice front, and were even farther away in the post-LGM period, when loess generation may have been maximal. Thus, they provide only a limited picture of paleowinds.

The general distribution of MIS-2 aged loess deposits in Wisconsin has long been known. To these data we add thickness and texture information derived from >2100 loess samples from uplands across the state, providing an unparalleled opportunity to add insight into loess distribution and transport systems during and shortly after the LGM.

Wisconsin’s thickest loess is found on its western margins; loess here was mainly derived from the Mississippi River valley train and other western sources. This loess thins and fines rapidly to the east. Many interior areas, e.g., outwash and glaciolacustrine plains, are usually loess-free, as are areas that remained geomorphically unstable long after loess deposition had stopped, such as hummocky moraines. Other areas in Wisconsin, far from the Mississippi River, also have thick loess. Generally, this loess is associated with “interior” source areas such as glaciolacustrine plains, outwash plains, and hummocky moraines. Loess near these source areas almost always thins and gets finer to the southeast, suggesting that depositional systems were driven by northwesterly winds. With the exception of areas immediately near the ice front, which may have been influenced by katabatic winds, most loess in Wisconsin at and after the LGM was deposited on northwesterly winds. This presentation will highlight several examples in support of this conclusion.