A Seismic Stratigraphic Study of Western Lake Superior

Abstract

Approximately 1000 km of high resolution seismic and sidescan sonar profiles were collected in the western third of Lake Superior during the 1996 field season. The study area was bounded to the east by a line from Isle Royale to Houghton, Michigan, and extended westward to Duluth, Minnesota. Seven seismic facies are identified on seismic reflection profiles. Each has a distinct acoustic signature, and can be correlated with a representative section of Lake Superior sediments. These units are incorporated into three depositional groups. The oldest sediments, Group A, represent pre-Late Wisconsinan and Late Wisconsinan sedimentation (Isotope stage 4-2). This group contains seismic facies composed of lacustrine clays and till. Group A is followed by Group B, which includes till, red clay, red varved clay and gray varved clay. This group represents Split Rock and Nickerson phase glacial deposits (Isotope stage 2). The youngest group, Group C, contains gray clay, which represents Holocene sedimentation (Isotope stage 1). Sidescan sonar profiles revealed features covering the lake floor including circular depressions and linear grooves interpreted to be iceberg scour marks. Three moraines, found on seismic profiles within the till unit of Group B, could represent the ice margin at the time of Glacial Lake Duluth.