ORIGIN OF WESTERN LAKE SUPERIOR’S BURIED RIDGES

High resolution seismic reflection data have revealed several buried ridges in far western Lake Superior that roughly parallel the paleo-bathymetric contour, sitting between 40 and 105 meters below the present day lake surface. The ridges stand as much as three meters high above the paleo-lakefloor and are up to 500 meters wide. They are blanketed by up to 10 meters of sediment and there is little, if any, surface expression of them on the modern day lake floor. Based upon their size and morphology, we suggest that the ridges are paleo-beach ridges deposited in a glacial lake experiencing lake level fluctuations. Understanding how these ridges came to be can help constrain the history of Lake Superior.

Piston cores were collected from the sediments burying the ridges. Four units can be correlated to the high-resolution sub-bottom CHIRP data. The upper six meters of the sampled section consists of massive red clay, interpreted to be Holocene post-glacial sediments associated with the Nemadji River plume. The red clay is underlain by nearly one meter of well sorted sand, deposited during a period of low lake level. An erosional unconformity separates the sand from a half meter of red/grey glaciolacustrine rhythmites. The core terminates in one meter of well sorted sand with clay stringers, which directly overlies the ridge. Our interpretation of the data is that the beach ridges formed in a glacial lake (Lake Minong) that was experiencing lake level fluctuations. Organic samples have been taken from the bottom of unit four and sent for radiocarbon dating.