RI-48 Pre-Late Wisconsinan Till Stratigraphy of North-Central Minnesota

Abstract

Continuous cores of complete Quaternary sections at more than 60 sites in north-central Minnesota yield evidence for at least nine pre-late Wisconsinan tills. Found only in the subsurface, they are of both northwest and northeast provenance. The oldest till-the Mulligan Lake of northeast provenance-does not have a recognized counterpart to the south. Tills of northwest provenance-Wirt Lake, Bigfork, Eagle Bend, Funkley, and Browerville-are correlated with tills recognized earlier in central Minnesota. The younger northeastprovenance tills-Shooks, First Red, and Saum-also are correlated with till units recognized in central Minnesota. The subsurface record indicates alternating northeastern and northwestern advances during each major glaciation in north-central Minnesota. The distinct couplets of northeast- and northwest-source tills imply shifting locations for the Labrador and Keewatin ice centers during the Pleistocene. The Pleistocene stratigraphic sequence of Minnesota is divided into five informal "event" units based on the subsurface record. Evidence for the earliest of these--event V-is limited, but its tills are similar to tills of event X. During event W, ice carrying detritus of Rainy provenance extended from north-central Minnesota into central Minnesota. During this interval, ice carrying detritus of Winnipeg provenance moved south across and probably beyond Minnesota. During event X, ice carrying material of Superior provenance dominated flow from the Labrador center into north-central Minnesota at a time when ice carrying Rainy-provenance detritus apparently did not enter the state. Throughout event X, ice carrying Winnipeg-provenance detritus flowed southeastward into Minnesota, and probably again well south and east of the state. Event Y was marked by a resurgence into central Minnesota of ice carrying Rainy-provenance debris, with ice carrying Winnipeg-provenance debris again flowing into the state in a more southward direction. The final event Z comprises the late Wisconsinan glaciation when Keewatin ice expanded from a direction more to the west of Minnesota (incorporating material of Riding Mountain provenance) than earlier advances of ice carrying Winnipeg-provenance material. Sediments of event Y are probably illinoian or older, whereas those of events X, W, and V are probably pre-illinoian in age. The sedimentary record is evidence that the Keewatin and Labrador ice centers were both phenomena of the Laurentide ice sheet throughout the Pleistocene, although their locations probably shifted considerably through time.