Evidence of Soft-Sediment Post-Depositional Deformation in Lake Superior

Abstract

In the very fine-grained, gas-poor sediments of Lake Superior, it is possible to collect extremely high resolution images of the subsurface with a 28 kHz echosounder. Penetration depths routinely exceed 20 m. This represents a significant portion of the lake's soft sediment section and makes it possible to image sediments that were deposited during the most recent de-glaciation of the basin. A high-resolution "pseudo-3D" seismic survey recently collected with this system in western Lake Superior provides convincing evidence of buried iceberg scouring in the lakefloor glaciolacustrine sediments. In seismic profiles, the scour appears as localized regions of acoustical blanking that are frequently "U" or "V" -shaped. The scour zones are typically 10s of meters across and up to 6 m deep. In plan-view, they exhibit a curvilinear appearance, frequently running for several kilometers. Features associated with the iceberg scouring include thickening and thinning of beds caused by the building of lateral and terminal berms; and overlying normal faulting due to sediment collapse into the scour marks as the deformed sediment is reconsolidated. Ring-shaped depressions on the modern lake floor are theorized to be the result of sediment dewatering linked to the development of the underlying features.