CORRELATION AND CHRONOLOGY OF PRE-WISCONSINAN TILL UNITS IN WISCONSIN

Pre-Wisconsinan tills have been identified in southern (the Walworth Fm. and the Capron Mbr. of the Zenda Fm.), western, and north-central [n-c] WI. Paleomagnetism and weathering intensity have been used to infer sediment ages, but the chronology is constrained by limited data (Syverson and Colgan, 2004). This paper will focus on tills in western and n-c WI.

The number of glaciations represented by members of the Pierce and Marathon Fms. is uncertain. Tills of the Pierce Fm. (western WI) and the Marathon Fm. (n-c WI) are the oldest in the region and have a northwesterly provenance. The lowest member of the Marathon Fm., the Wausau Mbr., is extremely weathered and eroded; it might represent an Early Pleistocene glaciation that occurred >>780 ka. The overlying Medford Mbr. of the Marathon Fm., as well as the Hersey Mbr. of the Pierce Fm., are very dark gray to yellowish brown, silty, calcareous, and have reversed remanent magnetization (Baker et al., 1983; Syverson et al., 2005). These units might have been deposited during the same event >420 ka, a time pre-dating the Illinoian Glaciation (MIS-6 & 8). Pierce till has more kaolinite than Marathon till (23% vs. 8%, respectively, Thornburg et al., 2000). Different ice flowlines during the same event might have incorporated different amounts of kaolinite, or using logic from Roy et al. (2004), Pierce till might have been deposited during an older glaciation when more weathered saprolith was available for erosion.

Tills of the River Falls Fm. (western WI) and the Bakerville Mbr. of the Copper Falls Fm. (n-c WI, formerly a member of the Lincoln Fm., see Syverson et al., in press) unconformably overlie the Pierce and Marathon Fms. River Falls and Bakerville tills are reddish-brown, sandy, and contain lithologies from the Lake Superior region. River Falls till has a normal remanent magnetization and a surficial weathering profile that is typically >2.2 m thick (Baker et al., 1983). Although Bakerville till is not as weathered, both the Bakerville and River Falls tills are highly eroded and lack glacial landforms. Based on these observations, the River Falls and Bakerville tills have been tentatively attributed to the Illinoian Glaciation (MIS-6 & 8), although they could have been deposited during other glaciations of the Brunhes Chron (MIS-10, 12, 16, 18, Syverson et al., 2005).