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Geological Society, London, Special Publications; 1984; v. 15; p. 293-307;
DOI: 10.1144/GSL.SP.1984.015.01.19
© 1984 Geological Society of London

Contourites

The sediment texture of contourites in Lake Superior

J. D. Halfman1 & T. C. Johnson

Limnology Program and Department of Geology, University of Minnesota, Duluth, Minnesota 55812, USA

Previous sedimentological studies in Lake Superior indicated that deep water circulation is sufficiently intense in some regions to influence the deposition and erosion of postglacial clay. This is particularly evident off the Keweenaw Peninsula, where 3.5 kHz profiles, side-scan sonar, and cores reveal wavy bedforms and interbedded muds and sands interpreted to be contourites. This region is a good lacustrine analogue to the marine environment, yet is unique in its lack of interbedded turbidites and significant bioturbation.

The sedimentary texture of the core top (< 2 cm depth in core) samples in the region, determined by pipette and sieve analyses, changes from sand (md = 2.2 phi; 220 µm) at the base of the slope, where the currents are strong, to clay (md = 10.8 phi; 0.6 µm), farther offshore, where the currents are weak. Textural analyses reveal a distinct bi-modal distribution of sand and clay populations, with the sand dominating the sediment at the base of the slope, and clay dominating the sediment further offshore. The silt fraction of the sediment samples was analysed by an electronic particle analyser, which detected changes in the particle-size histograms from coarse-grained (md = 4.79 phi; 36 µm) and positively skewed (0.45) distributions to fine-grained (md = 6.06 phi; 15 µm) and negatively skewed (–0.09) distributions, that correlate to the previous sand and clay textures, respectively. There is a down-core increase in grain-size in two piston cores that suggests that bottom currents decreased in intensity from 9500 years ago to the present.


1 Present address: Duke University Marine Lab., Pivers Island. Beaufort, North Carolina 28516, USA.




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N. J. Wattrus, D. E. Rausch, and J. Cartwright
Soft-sediment deformation in Lake Superior: evidence for an immature Polygonal Fault System?
Geological Society, London, Special Publications, 2003; 216: 323 - 334.
[Abstract] [PDF]