OPTICAL AND RADIOCARBON DATING OF THE NIPISSING TRANSGRESSION, SOUTHEASTERN LAKE SUPERIOR BASIN

An improved early to mid-Holocene chronology of the lake-level rise from the level of the Houghton phase to the peak Nipissing phase of ancestral Lake Superior is important to evaluate the relative roles of isostatic rebound and climate on lake level. Additionally, records from the Lake Superior basin are needed to compare with more highly resolved lake-level paleohydrographs from the Lake Michigan-Huron basin. In the southeastern Lake Superior basin, coastal exposures near Grand Marais, Michigan, reveal sediment and organics associated with vertical aggradation of a barrier beach and back-barrier environment deposited during the Nipissing transgression. These deposits were preserved owing to positive sediment supply along this reach of coast during the transgression from bluff erosion of sandy kame terrace sediments of the Grand Sable Banks and subsequent eastward longshore transport. Eighteen radiocarbon ages from two closely spaced sites west of Grand Marais constrain aggradation of back-barrier (lagoonal) sediments between ca. 7300 and 3900 cal yrs BP. Four optical ages from upper shoreface sediments at Lonesome Point (east of Grand Marais) indicate aggradation from ca. 6900 to 4700 a. Five optical ages from dunes overlying nearshore sediments of the barrier range from ca. 4700 to 3600 a and document lake-level fall from the peak Nipissing level. Radiocarbon and optical ages are generally compatible, especially with regards to the timing of lake-level rise above modern Lake Superior (between ca. 7350 and 6900 a). This timing agrees with radiocarbon ages from previous studies further west at Beaver Lake and at Bay Furnace near Munising, Michigan. An early-mid Holocene lake-level paleohydrograph based on ages from this study and other sites throughout the Lake Superior basin reveals intervals of lake level as much as ca. 10 m above the isostatically controlled outlet elevation at North Bay, Ontario. This work highlights key time intervals for future studies to refine climatic conditions during the early and mid-Holocene in the basin.