RECENT ADVANCES IN STRATIGRAPHY AND CHRONOLOGY OF PRE-WISCONSINAN GLACIAL DEPOSITS IN MINNESOTA

Calcite from marl in interglacial (based on pollen analyses) lake sediment from core in central Minnesota dated by the High Resolution Ur/Th disequilibrium method resulted in minimum age of 200,000 YBP (Edwards personal communication, 2006). The lake sediment overlies Browerville formation till, which appears to be the youngest pre-Wisconsinan till in Minnesota. This suggests that Middle Pleistocene glacial deposits in Minnesota are Marine Isotope Stage (MIS) 8 or older.

Recent mapping has produced thousands of samples from pre-Wisconsinan deposits. Texture analyses and lithologic counts of 1-2 mm coarse-sand grains are used to identify characteristics specific to individual deposits, and when used stratigraphically and with other features (oxidation, leaching, etc.), facilitate correlation. At least 10 distinct pre-Wisconsinan till units have been recognized. Truncated paleosols and oxidation zones in some units confirm that interglacial and possibly interstadial periods separated some ice advances.

²⁶Al and ¹⁰Be cosmogenic-nuclide burial dating on fluvial sediment in 4 core holes from southwestern Minnesota have yielded minimum ages of ~500,000 YBP (Balco and others, 2005). The fluvial sediment overlies carbonate-rich tills that predate the Browerville formation. In one of these core holes a stratigraphically lower fluvial layer, overlying another carbonate-rich till, produced a minimum date of >1 million YBP. These carbonate-rich tills are wide spread in the subsurface throughout Minnesota (X till sequence of Meyer and Knaeble, 1996) and have only a few pre-Wisconsinan units beneath them. Magnetostratigraphy measurements by the Minnesota Geological Survey (UMRB project, open file) on lake sediment in 2 of the cores that did not have the older fluvial layer and by M. Roy (unpublished results) on correlative carbonate-rich tills exposed along the Minnesota River valley in southwest Minnesota are all of normal polarity, suggesting deposition after the Brunhes-Matuyama reversal at 788K YBP.

While some pre-Wisconsinan glacial sediment is older than MIS 19, these results suggest that the majority of the sediment in Minnesota appears to have been deposited during the Middle Pleistocene between MIS stages 8-18.