LEGACY OF A TRANSGRESSION: SHORELINE BEHAVIOR ALONG THE SOUTHEASTERN MARGIN OF THE SUPERIOR BASIN IN RESPONSE TO HOLOCENE LAKE-LEVEL CHANGE

Systematic study of beach ridge strand plains within the Upper Great Lakes basin (Lakes Superior, Michigan and Huron) has established a robust mid- to late- Holocene paleohydrograph for those lakes (Johnston et al. 2012). This work has documented lake-level change and shoreline behavior over the past 6 ka, including lake level rise from just after the Houghton Low to the peak Nipissing Phase at ~4.5 ka as well as its subsequent fall to near current levels. Below, we present new data comparing extant geomorphic development along the SE shore of Lake Superior with the above-referenced paleo-hydrograph.

OSL dating of the face of Lonesome Point shows that the Point rapidly prograded and aggraded during the rise of ancestral Lake Superior ~ 7.9 to ~ 4.7 ka. GPR confirms a sequence of shore-facing ravinements within the Point’s subsurface. By ~ 4.5 ka, the near-planar surface of Lonesome Point was nearly coincident with adjacent peak Nipissing features. LiDAR suggests beach ridges formed atop the terrace during a slower rise in lake level after ~6.0 ka. After 4.5, these recently formed beach ridges were overlapped by large parabolic dunes; OSL shows these latter dunes were emplaced between ~ 3.6 and 4.2 ka. Studies of perched dune emplacement on high bluffs along Lake Superior have connected their deflation with the undermining/collapse of bluffs during high/rising lake levels (Anderton and Loope, 1995). It has been inferred from this work that relative bluff stability generally accompanied low/declining lake level. However, recent recognition that some high, sand-dominated bluffs became unstable during both high and low stands suggests that this simple “perched dune model” is not adequate to predict general coastal response to lake level change. Several lake bluff slumps/slides have been ¹⁴C dated at 4.2-3.9 ka, closely following the rapid drop of ancestral Lake Superior after its peak level. Permeable sand underlying the entire lake-facing bluff likely facilitated widespread slumping. Falling base level after ~4.5 ka probably also drove piracy of Sable Creek westward to its present outlet to Lake Superior by ~3.9 ka.