Seismic Reflection Study of Recessional Moraines beneath Lake Superior and Their Relationship to Regional Deglaciation

Abstract

Approximately 8000 km of continuous seismic reflection profiles throughout Lake Superior were examined for evidence of recessional moraines and other ice-margin deposits associated with the retreat of late Wisconsin ice. These features are correlated with the record of glacial-lake evolution in western Lake Superior. An offlapping sequence of glacial and glacial-lacustrine sediments overlying bedrock is recognized in west-central Lake Superior that is progressively younger to the northeast. The sequence underlies more recent glaical-lacustrine and postglacial sediments. Four facies are recognized on the basis of geomorphologic and acoustic properties and are interpreted to represent a southwest-to-northeast assemblage of: proglacial stratified drift (facies A), drift in major end moraines (facies B), till deposited as glacial retreat resumed, or possibly late-stage ablation till (facies C), and basal till (facies D). The prominent moraines of facies B are unusually thick and are believed to mark the ice-margin shorelines of successive major proglacial lakes that formerly occupied parts of western Lake Superior. The moraines are tentatively correlated with Glacial Lake Duluth (unit 1), Glacial Lake Washburn (unit 2), and Glacial Lake Beaver Bay (unit 3), the most prominent of lakes drained via the progressively lower outlets via the Moose Lake/ Brule-St. Croix Rivers, the Huron Mountains, and the Au Train-Whitefish regions, respectively.

Side‐scan targets in Lake Superior—evidence for bedforms and sediment transport

“…The generally transparent unit 1 corresponds to post-glacial sandy clays overlying varved glacial-lacustrine clay. The stratified unit 2 is probably glacial outwash material, and unit 3 is glacial till (Johnson, 1980;Landmesser et a/., 1982;Johnson er al., 1984). The subbottom reflections in the generally well-laminated unit 2 are often disrupted in the ring zone.…”

“…The lake bed is highly reflective in this area at distances of up to 30 km offshore. Seismic reflection studies suggest that these ridges may be glacial moraines (Landmesser et al, 1982), and thus the more reflective lake bed observed here may be due to coarser, glacially derived sediments at the surface. In the offshore reflective zone there are parallel lineated targets and occasional circular targets ('light' rings).…”

“…(1981) suggested that these pockmarks are caused by freshwater flow out of the glacial sands. The ring structures observed here could also be formed by liquids venting from the underlying glacial deposits which are over 100 m thick in places (Landmesser et al, 1982). Thus a ring-shaped depression may be the stable form of a pockmark in this environment.…”

“…There is a prominent 50 m high ridge extending into the lake at the western end of the study area (near 88"5O W). This prominent ridge and several smaller, isolated highs in the deeper portions of the study area are glacial moraines (Johnson et al, 198C;Landmesser et al, 1982).…”

Moraines and late-glacial stratigraphy in central Lake Superior

“…topography of the bedrock surface and the total thickness of sediment above bedrock (Wold and Hutchinson, 1979;Wold et al, 1982). Landmesser et al (1982) used a subset of these data along with 3.5 kHz data that they collected in the central part of the lake to define three recessional moraines and related deposits, which are discussed further here. We collected seismic-reflection data using multiple systems in several areas of Lake Superior in order to better understand the glacial and post-glacial history of Lake Superior and Lake Agassiz.…”