PRE-NIPISSING LAKE-LEVEL RISE AT ALPENA, MICHIGAN

The Nipissing phase of ancestral Lakes Michigan, Huron, and Superior was the last extreme high-water stage of the upper Great Lakes. Represented as either a one- or two-phase highstand, the Nipissing phase occurred following a long-term lake-level rise. This pre-Nipissing transgression was primarily an erosional event with only the final stage of the lake-level rise preserved as barrier beaches, spits, and strandplains of beach ridges. South of Alpena, Michigan, late Holocene coastal deposits occur as a strandplain between Devils Lake and Lake Huron. In the landward part of this strandplain, there is a higher elevation platform of Nipissing beach ridges that formed in the final stage of lake-level rise to the Nipissing peak. Vibracores and ground penetrating radar were used to define the architecture of this platform, and optically stimulated luminescence and radiocarbon age determinations were used to constrain the age of the deposits. The pre-Nipissing shoreline transgressed over Devils Lake lagoonal deposits from 6400 to 6100 calendar years BP. A stillstand occurred from 6100 to 4800 calendar years BP, and then the shoreline prograded as lake level continued to rise at a relative rate of about 0.8 cm/yr. This depositional regression produced a thickening wedge of sediment, containing 20 beach ridges. The pre-Nipissing lake-level rise ended 4500 years ago at the Nipissing peak and a relative elevation of 185 m (IGLD 1985).