Abstract

Examination of recent bathymetric, side-scan sonar and high-resolution seismic data shows that Sable Island and its east and west extensions are surrounded by shoreface-connected ridges. The main ridge field to the south and west of the island contains 80 ridges in water depths of 16 to 40 m. These ridges have an average height, spacing and length of 5.1 m, 1700 m and 8 to 10 km, respectively, and locally occur in a heirarchically superimposed arrangement. Only six ridges occur to the north of the island in water depth of 10 to 25 m. Their average height, spacing and length are 3.8 m, 1900 m and 5 km, respectively. All ridges converge eastward on the shoreline with angles of 70° south of the island and 35° on the north. The smaller ridge sizes, shallower depths and smaller angles of divergence on the north side are probably due to the lower wave energy in this area. The ridges appear to be asymmetric with their steeper face to the west or northwest, but the maximum local slope is generally on the eastern flank (2.1° east versus 1.1° west). The western flank of the ridges is acoustically reflective and is interpreted as being mantled by coarse lag material that represents the top of the Pleistocene. Smaller bed forms, including wave ripples, and two- and three-dimensional megaripples less than 1 m high, can be identified on this surface. The megaripples indicate shoreward flow parallel to the ridge axes. Seismic data show that the ridges are partially erosional and partially constructional. The geometry of the Pleistocene surface beneath the ridges suggests an overall eastward movement of the ridges, probably in response to the combined effects of tidal currents and wave-generated currents associated with northeast-tracking winter storms.

Vibracores collected in 23 to 27 m of water show that the depositional parts of the ridges consist of 2 to 6 cm thick beds in medium to coarse, dominantly quartz, sand, with limited amounts of shelly debris. The majority of the beds are inversely graded, and are organized into coarsening-upward sequences 30 to 90 cm thick. Cross-bedding has not been identified. The uppermost 10 to 20 cm of each core is generally bioturbated more intensely than the remainder of the core. Conceptual models of ridge evolution predict either coarsening- or fining-upward sequences through the ridges.