A preliminary survey of relict shoreface-attached sand ridges in Western Lake Superior

High-resolution multibeam and seismic reflection surveying in western Lake Superior, near the outflow of the Amnicon River, Wisconsin, has shown that the nearshore lake floor appears to consist of till with no recent sediment cover. This surface is partially obscured 3 km offshore by a series of low relief, elongate, sand ridges that are arranged in an en echelon pattern at an oblique angle to the present day shoreline. They are extensively gullied suggesting that they are not presently active. Two sets of elongate features can be identified. The features in first group, Group I, are over 0.5 km wide, rise 3-5 m above the surrounding lake floor and are over 2 km long. The features belonging to the second set, Group II, are not as well developed and are typically smaller than those in Group I. Group II features are typically 0.2-0.3 km wide, 0.4-1.6 km long and rise 1 -3 m above the surrounding lake floor. The most striking difference between the groups is the difference in their orientation with respect to the shoreline. The Group I features run NE-SW while the Group II features are directed approximately N-S. The shoreline runs approximately ENE-WSW so the features in both cases 'open' northeast. The Group I features are believed to be relict drowned shoreface-attached sand ridges that formed when the lake level lay over 10 m below its present day level. These sand ridges formed as the result of currents, associated with strong northeasterly gales, moving the abundant sediment eroded off the shore. The smaller Group II features are believed to be sub-aqueous sand dunes that developed transverse to bottom currents produced by the northeasterly gales. No dates have been obtained for the sand ridges. Based upon their depth and the lake level history curve for Lake Superior we suggest that the ridges formed either 6500-6000 BP, prior to the Nipissing highstand.