Effects of bottom currents and fish on sedimentation in a deep-water, lacustrine environment

Sands and muds accumulating under the influence of apparently strong bottom currents in a deep-water environment of Lake Superior were studied in detail using 3.5-kHz echo sounding; side-scan SONAR; piston, gravity, and box coring; and lake-floor photography. The study area includes a well-defined slope and basin environment. Bottom currents maintain a scoured trough, 20 m deep and 2 km wide, at the base of the slope and modify the shapes of large ringlike depressions that are common in many regions of Lake Superior. Side-scan SONAR records reveal lineations subparallel to the direction of bottom-current flow that are interpreted in different areas to be sand ribbons, slight depressions, or depositional furrows.

The sediments range from silty sand in the scoured trough to sandy clays farther offshore. Radiocarbon dating and ²¹⁰Pb dating show the sediments to be postglacial rather than relic glacial-lacustrine sediments. Grain size distributions are affected by bottom currents and a nearby source of sand. Some sedimentary structures, including plane laminations and interbedded sands and muds, are due to bottom currents. Fish create dish laminations, however, that often obscure the effects of the bottom currents. Fish and fish-scour depressions are common in lake-floor photographs, whereas evidence for bottom currents is not. Biological activity therefore appears to erase traces of intermittent currents.