Moraines and late-glacial stratigraphy in central Lake Superior

Abstract

Seismic-reflection surveys of the Isle Royale sub-basin, central Lake Superior, reveal two large end moraines and associated glacial sediments deposited during the last cycle of the Laurentide Ice Sheet in the basin. The Isle Royale moraines directly overlie bedrock and are cored with dense, acoustically massive till intercalated down-ice with acoustically stratified outwash. Till and outwash are overlain by glacial varves, a lower red unit and an upper gray unit. The maximum extent of late Younger Dryas-age readvance into the western Lake Superior basin is uncertain, but it was probably controlled by both ice dynamics and climate. Our data indicate that during retreat from the maximum, the ice paused just long enough to construct the outer of the two moraines, >100 m high, and then retreated to the inner moraine, during which time most of the lower glacial-lacustrine sequence (red varves) was deposited. Retreat from the inner moraine coincided with a marked flux of icebergs at the calving margin and a change to gray varves. Rapid retreat may be related to both an influx of meltwater from Glacial Lake Agassiz about 10,500 cal yr BP and retreat of the calving margin down an adverse slope into the Isle Royale sub-basin.

Early Holocene ice retreat from Isle Royale in the Laurentian Great Lakes constrained with 10Be exposure-age dating

“…2b,3a), and down-ice of the IR2 moraine (Fig. 2b;Breckenridge, 2007;Breckenridge et al, 2004;Colman et al, 2020;Maher, 1977;Raymond et al, 1975;Teller and Mahnic, 1988).…”

“…Lake Agassiz drainage into the Lake Superior basin is associated with the onset of red varve deposition in areas that were ice-free at the time (Fig. 2b; Breckenridge and Johnson, 2009), most likely when lake levels were between Duluth and Minong stages (Breckenridge, 2004;Colman et al, 2020;Teller and Mahnic, 1988). In the western Lake Superior basin, red varves are identified north of Isle Royale, in some inland lakes on Isle Royale's northern margin (Figs.…”

“…However, it is unclear how far ice advanced in the western Lake Superior basin (Fig. 2; Black, 1976;Clayton and Moran, 1982;Colman et al, 2020;Drexler et al, 1983;Farrand and Drexler, 1985;Peterson, 1985). This uncertainty results from a lack of dateable material in Lake Superior sediments, the paucity of well-defined ice-marginal deposits associated with the Marquette Readvance around the western Lake Superior basin, and alternative interpretations for existing 14 C ages and stratigraphy (Colman et al, 2020;Hobbs and Breckenridge, 2011).…”

“…However, the path meltwater took through Lake Superior during this drainage is unclear because ice margin positions remain less well-constrained since there is less recent research and dating in this area (Drexler, 1981;Huber, 1973;Hughes, 1963). Black, 1976;Clayton and Moran, 1982;Drexler et al, 1983;Hughes and Merry, 1978); stippled yellow line (<ii>; Peterson, 1985); cyan dash-dot line (<iii>; Colman et al, 2020). Black dots are existing 14 C data locations (Dalton et al, 2020); Yellow triangles are existing 10 Be locations (Leydet et al, 2018;Lowell et al, 2021;Ullman et al, 2015).…”

Reconstruction of isostatically adjusted paleo-strandlines along the southern margin of the Laurentide Ice Sheet in the Great Lakes, Lake Agassiz, and Champlain Sea basins

“…This difference may indicate Earth's crust beneath the Washburn lake was more deeply depressed, and as ice melted, it rebounded more quickly than the crust beneath the Campbell strand, possibly because of thinner ice in the Agassiz basin and substantially thicker local ice in the deep Superior basin. The thicker Superior basin ice may have led to its persistence until $11.5 ka BP (Clayton and Moran 1982;Clayton 1983;Clayton et al 1985;Drexler et al 1983;Lowell et al 1999;Breckenridge 2007;Colman et al 2020).…”