14: Topography and surficial structure of Lake Superior bedrock as based on seismic reflection profiles
Richard J. Wold, Deborah R. Hutchinson, Thomas C. Johnson, 1982. "14: Topography and surficial structure of Lake Superior bedrock as based on seismic reflection profiles", Geology and Tectonics of the Lake Superior Basin, Richard J. Wold, William J. Hinze
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The thickness of the unconsolidated sediment and the topography of the underlying bedrock surface of Lake Superior are interpreted from 8,000 km of high-resolution seismic reflection profiles taken during 1966 and 1967. A depth-to-bedrock map was constructed by combining the isopach map of unconsolidated sediments (prepared from our profiles) with the bathymetric map of the lake (Canadian Hydrographic Service Chart 885, 1973).
Lake Superior can be divided into three morphologic regions on the basis of bathymetry and underlying bedrock: a western region composed of long linear valleys and gentle changes in relief, a central region composed of a single broad bathymetric depression, and an eastern region composed of a complex pattern of linear troughs and ridges.
The western region is dominated by a nearly continuous bathymetric and bedrock valley paralleling the north shore from Thunder Bay, Ontario, to Duluth, Minnesota. The bedrock valley reaches depths of more than 800 m below lake level near Silver Bay, Minnesota, and has more than 500 m of overlying unconsolidated sediments. It probably resulted from differential glacial erosion, where the relatively erodible sandstone of the Proterozoic Bayfield Group comes in contact with Proterozoic rocks of the relatively resistant underlying volcanic rocks of the Keweenawan Supergroup and gabbro of the Duluth Complex.
The central region, separated from the western region by a north-south basement ridge, consists of a broad valley with only 8 to 15 m of unconsolidated sediments overlying the bedrock surface.
The complex pattern of troughs and ridges of the eastern region forms a north-south dendritic pattern with valleys as much as 100 km long but only 5 to 10 km wide. The bedrock surface is more than 600 m below lake level in some places and is overlain by as much as 300 m of unconsolidated sediments. These valleys are probably the result of erosion by subglacial streams. The stream erosion may well have followed a system of shear zones that have been observed in the shore exposures of the underlying Proterozoic sedimentary rocks of the Bayfield Group and Jacobsville Sandstone to the south. Most of these bedrock valleys are truncated about 15 km north of the south shore, where the lake floor rises abruptly to the coastline, although two valleys extend onshore.
In general, the morphology of the bedrock surface in Lake Superior probably reflects the result of scour by glacial and subglacial streams, which in turn were localized by lithologic contacts, pre-existing topography, and shear zones.
Over most of the lake, the acoustic impedance contrast across the contact of unconsolidated sediments with bedrock is high enough so that the seismic energy is reflected with little or no penetration into the bedrock. Despite this, there are some places where layering within the bedrock can be identified and apparent dips determined. Among the structural trends determined from these dips are the following: a southwest-plunging synclinal feature bordering the Bayfield Peninsula; a syncline lying between the Apostle Islands and the Keweenaw Peninsula that probably represents the center of the Lake Superior depositional syncline; a south-plunging syncline located between Michipicoten Island and Superior Shoals; and an apparent southward dip of the bedrock in the southeastern region of the lake.