LAKE AGASSIZ'S ROLE IN THE FORMATION OF LAKE SUPERIOR'S LAKE FLOOR RINGS

Irregular ring-shaped depressions typically between 200 and 300 m across are widely developed on the floor of Lake Superior. They have not been commonly reported elsewhere. The depressions that define these rings are typically between 10 to 30 m wide and up to 5 m deep. They occur both as closely grouped networks and as isolated features. They are believed to be produced by fluid expulsion from the lake floor.

We present results from a geophysical survey conducted in northwestern Lake Superior. The objectives of the survey were to define the geometry of the sub-surface plumbing below the rings and to gain some insight into how these structures developed. The data collected included:

a). Two high-resolution pseudo-3D single-channel seismic reflection datasets, one acquired with a small air gun and a second acquired with a CHIRP profiler;

b). Multibeam and sidescan sonar data;

c). A 9 m piston core that was collected in close proximity to a ring feature, to determine the physical characteristics of the near-surface sediments and to calibrate the seismic data.

The high resolution bathymetric surveying revealed that the rings are made up of pockmarks arranged in irregular polygonal patterns. Increased backscatter in the sonar imagery about the pocks is interpreted as an indicator of the development of a lag-surface. CHIRP sub-bottom profiling across the pocks reveal chimney-like structures below the lake floor pocks. Two distinct drainage structures are observed, a shallow sourced type that is commonly developed below the rings and a deeper sourced type that appears to be linked to the topography of the underlying till surface.

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. We propose a model that argues that the formation of the rings was linked to the overflow event. Overtopping and subsequent down cutting of the lake’s sill at Nadoway Point initiated by the overflow, triggered a dramatic and abrupt drop in lake level that de-stabilized the already overpressured lake floor sediments leading to a basin-wide fluid expulsion event, forming the rings.