Irregular ring shaped depressions are widely developed on the floor of Lake Superior. They are typically between 200 and 300 meters across. The troughs that define these rings are typically 10 to 30 meters wide and up to five meters deep. High resolution multibeam bathymetric data show that the lake floor rings are actually chains of individual bathymetric pocks. In Lake Superior bathymetric pocks have been observed as isolated features, irregular polygonal rings, and closely grouped ring networks. The Lake Superior basin contains sediments deposited during the last deglaciation of the basin. The glacial and post-glacial sediments sit atop Precambrian basement rocks. These bathymetric ring features have not been commonly reported elsewhere. Pockmarks, however, have been observed globally and are typically the result of expelled fluids or gases. A series of high resolution, single-channel seismic data sets were collected in addition to high resolution multibeam bathymetry to test the working hypothesis that Lake Superior’s rings are the result of sediment dewatering. The vast depth of the Thunder Bay trough provides an ideal location to study these features, protected from wave base erosion.
The results confirm the existence of three types of dewatering structures associated with the lake floor rings. These features include two deep-sourced and one shallow dewatering structure(s). The evidence suggests that the rings were produced by the expulsion of fluid from the lake floor at, or close to, the termination of glaciolacustrine deposition in the basin. This places the formation of these unusual features around the time of the last eastward overflow from Lake Agassiz directly into Lake Superior. Abrupt lake level change linked to the Agassiz overflow is believed to have triggered basin wide expulsion of pore water leading to the formation of the lake floor rings.
University of Minnesota M.S. thesis. February 2012. Major: Geological sciences. Advisors: Nigel Wattrus and John Swenson. 1 computer file (PDF); ix, 110 pages.
Gustafson, Daniel Jay.
A geophysical investigation of dewatering structures in Western Lake Superior..
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