QUESTIONS SURROUNDING PRE-LATE PLEISTOCENE LAKE DEPOSITS ALONG THE LOWER WISCONSIN AND UPPER MISSISSIPPI RIVERS

Numerous tributaries along both the lower Wisconsin and upper Mississippi River valleys contain slackwater lake deposits dating to the MIS 2 glaciation. A smaller number of sites associated with higher, and therefore older, geomorphic surfaces also contain lake sediments. Due to their apparent antiquity, these deposits may help unravel the Pleistocene history of drainage system development in the North American mid-continent; however, initial data from these locations provide as many questions as answers.

Geoprobe cores collected from two sites recovered pre-late Pleistocene lake sediment. A core collected on the Bridgeport strath terrace near Muscoda, WI, contains weakly laminated light yellowish brown (10 YR 6/4) silty clay with 1- to 2-cm rounded dolomite pebbles. Another core collected from a depositional terrace near Lynxville, WI, that is >10 m higher than the MIS 2 Savanna terrace contains laminated dark gray (10 YR 4/1) silty clay interbedded with well-sorted sand. Sediment from both cores has a normal paleomagnetic orientation, suggesting the sediment was most likely deposited at some time after the Brunhes-Matuyama transition at ~781 ka.

These lake deposits are problematic in the context of the current southward-directed Mississippi drainage system: it is difficult to envision a geomorphic process to dam a large lake along these two reaches of the Wisconsin and Mississippi Rivers. Field- and GIS-based data (Carson et al., 2013) suggest that the modern upper Mississippi River was likely part of the St. Lawrence drainage system prior to Quaternary glaciations. Damming of the lower St. Lawrence River by early or middle Quaternary glaciations would provide a plausible mechanism for producing these lake deposits; however, paleomagnetic measurements on samples of lake deposits found along the modern Ohio River have a reversed polarity. Thus, a conundrum currently exists with identifying a geomorphic process to produce these deposits in the past ~781 ka.