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Nigel J. Wattrus1, Joe A. Cartwright2, Deborah E. Rausch1
(1) University of Minnesota, Duluth, MN
(2) University of Cardiff, Cardiff, United Kingdom

Abstract: Lake Superiors' rings: Clues to the origin of Polygonal Fault Systems

Polygonal Fault Systems (PFS) are a class of soft-sediment structures recently described in Paleogene claystones from the North Sea Basin. They are characterized by a complex polyhedral geometry of intersecting dipping fault surfaces.

The polygonal fault geometry is the result of 3D contraction induced by syneresis in the very fine grained sediments. The presence of such a fault system has serious implications for development strategies. It also has implications for general models of shale compaction and for allied processes such as primary and secondary migration, basin wide fluid flux, and for seal capacity.

Sidescan sonar surveys in Lake Superior reveal the widespread development of ring structures (100-300m diameter) on the lake floor. High resolution seismic reflection surveys over these features show that the underlying fine-grained glacio-lacustrine clays are extensively fractured and faulted. These appear to be linked to the lake floor rings. Multiple tiers of apparently genetically independent fracture systems are widely developed. These features are developed in sediments deposited less than 10,000 years ago as ice was retreating out of the basin. They have been interpreted to be a PFS that has developed in these young sediments. These structures represent an opportunity to study the processes associated with the initiation and growth of PFS.

A survey to study these features was recently collected. High resolution psuedo-3D seismic reflection data, multibeam bathymetry and several 6m piston cores were collected in an area with well developed rings and fracturing. Results from the study will be presented.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana