ANALYSIS OF MIDDLE HOLOCENE (CA. 6000 TO 4500 YEARS AGO) LAKE-LEVEL CHANGE OF ANCESTRAL LAKE SUPERIOR AT THE HURON MOUNTAINS, UPPER MICHIGAN

The early to middle Holocene lake-level rise to the peak water-level elevation of the Nipissing phase of the ancestral upper Great Lakes was primarily an erosional event along most shoreline areas. Only in areas having a positive rate of sediment supply to the shoreline were coastal landforms or sedimentary deposits preserved. These locations typically consist of beach ridges filling embayments or spits extending from headlands; consequently, little is known about lake-level and concomitant shoreline behavior during this rise that ended 4,500 years ago. A few locations along the coasts of Lakes Huron and Michigan and a spit near the outlet of Lake Superior at Sault Ste. Marie, Ontario/Michigan, have been studied with sufficient data to construct a relative paleohydrograph. A coastal embayment within the Huron Mountain Club in the Huron Mountains of upper Michigan contains a series of beach ridges that arc between Jacobsville Sandstone bedrock headlands. The beach ridges isolate Cranberry Bog (a paleo-lagoon) from Lake Superior. Topographic elevations and changes within the strandplain indicate that deposits associated with the rise to and fall from the peak Nipissing phase are preserved in the strandplain. Twenty-seven vibracores were collected along a 1.1-km-long and north-south-oriented transect crossing the strandplain. The transect crosses 40 beach ridges, but only the landward 25 ridges were cored to determine basal foreshore elevations. Seven optically stimulated luminescence (OSL) samples were collected from pits dug into the lakeward toeslope of 5 of the 25 vibracored ridges. The 16th ridge from the landward edge of the transect is the highest in elevation and largest in relief, indicating that it represents the peak Nipissing water-level elevation. Three vibracores, as well as OSL samples, were collected from this ridge. Basal foreshore elevations and OSL age determinations were used to produce a relative paleohydrograph of the area that can be combined with paleohydrographs from other peak Nipissing sites to evaluate lake-level change and glacial isostatic adjustments throughout the upper Great Lakes basin.