Extensive geophysical surveying of western Lake Superior has revealed the presence of a system of small extensional faults deforming a 10-15-m-thick interval of glacio-lacustrine clays of early Holocene age over an area of ϳ5000 km 2 . Mapping of these closely spaced faults shows that they have (1) a polygonal planform geometry with a large range of strikes and (2) oblique to orthogonal intersection geometries. The fault system is layer bound and restricted to fine-grained postglacial sediments. The fault system is overlain by an extensive field of 100-400-m-diameter, 1-7-m-deep pockmarks, directly implicating the faults in the process of pockmark formation. The fault system is the youngest and most accurately dated example of a polygonal fault system described to date and the first to be described from a freshwater setting. This example conclusively demonstrates that nontectonic faulting can occur in sediment buried only a few meters and can be accompanied by highly focused fluid flow, a result that has wide implications for compaction processes of fine-grained sediments.
New multi-beam sonar and seismic data collected in Lake Superior document the widespread development of lake-floor rings in fine-grained lake-floor sediments. The multi-beam images reveal that the rings develop as connected clusters and that individual rings have an irregular polygonal appearance. High-resolution seismic data collected with a 28 kHz echo sounder reveal extensive fracturing and faulting in the glacio-lacustrine sediments below the lake-floor. Displacement on the faults is typically normal with throws of less than 50 cm. Three styles of faulting are recognized: (a) monoclinal flexure; (b) graben-like; and (c) conjugate. Zones of acoustic blanking below the faults may be associated with de-watering and mobilization of the sediments. Lateral thickness variation in some horizons suggests that fault and fracture development is linked with lateral movement of sediment. Piston cores collected near lake-floor rings show well-developed fractures and microfaults, suggesting that fracturing and faulting occurs on a wide range of scales. The seismic and lithological characteristics of the glacio-lacustrine section are similar to those of sediments in which Polygonal Fault Systems (PFS) have been described. This suggests that the rings in Lake Superior may be the surface expression of PFS in the near-surface sediments.
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