EARLY TO MID-HOLOCENE SHORELINE BEHAVIOR ASSOCIATED WITH THE NIPISSING PHASE, SOUTHEASTERN LAKE SUPERIOR

Systematic studies of beach ridge strandplains have established a framework of shoreline behavior during lake-level change along the upper Great Lakes (UGLs) after ~ 4.5 ka. Less is known about the Basin’s coastal history prior to that time. Diverse data (¹⁴C and optical ages, GPR, lake vibracores, outcrop profiles) collected along the southeastern shore of Lake Superior reveal a pattern of coastal response to lake-level rise/change after 7.8 ka. Prior work suggests that collapse of a lake-facing 90 m thick body of glaciofluvial sand and gravel (Grand Sable Banks [GSB]) ~ 5 km west of Grand Marais MI led to emplacement of bluff top parabolic dunes prior to 6.0 ka. Modern topography suggests that easterly littoral deposition stemming from the bluff collapse and its aftermath kept pace with rising lake level (i.e., depositional transgression), building a broad sand platform which today constitutes Lonesome Point, ~10 km east of GSB. Samples of sand taken from the lake-facing bluff at Lonesome Point returned optical ages ranging from 7.8 ka near bluff base to 4.7 ka at its crest, thus capturing a depositional history spanning the Nipissing Phase of the UGLs. An optical age of (5.5 ka) from a beach ridge near the landward edge of the strandplain documents depositional regression during the Nipissing Phase, in a manner similar to that of two other recently documented sites in the UGLs (Alpena, Huron Mountains). Optical ages from parabolic dunes that sit atop and distal to the bluff face post-date the peak of the Nipissing Phase at ~4.5 ka. A two km transect of ground penetrating radar running south from the Lonesome Point bluff is interpreted as revealing four radar facies: basal bedrock; glaciofluvial sediments; Nipissing Phase coastal sediments; and capping eolian sediments. Littoral drift from the initial collapse of GSB influenced other down drift coastal features including initial capture of Muskallonge Lake and possible deposition of a subaqueous sand platform north of Crisp Point. The Nipissing phase coastal sediment package near Lonesome Point is the result of high sediment supply sourced from the GSB