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.
Bibliographical noteFunding Information:
We thank Captain Mike King and the crew of the RV Blue Heron for all their help with the research cruises on Lake Superior. C.F.M. (Mike) Lewis, Jessica Gary, Dan Gustavson, and Emily Voytek participated in the 2009 cruise and helped with data collection and discussions. John Green discussed with us the bedrock that is likely beneath the lake in the study area. Helpful reviews were provided by Tom Lowell and three anonymous reviewers. Funding for this research was provided by the National Science Foundation under grants OCE-0421091 and OCE-0623607.
- Laurentide Ice Sheet
- Marquette readvance
- North America
- Seismic-reflection stratigraphy
- Subaqueous moraines