Abstract

No direct measurements of bottom sediment transport rate, sediment dispersal or the depth of the mobile zone are known to have been made in sands on the outer continental shelves of Canada, yet interpretations of bed mobility have been made without reference to the magnitude or return interval of such events. On Sable Island Bank, interpretations of sand movement have been based on grain size trends, bed form type and orientation, heavy mineral assemblages, and bottom sediment factor analysis. The interpretations suggest that sand migrates in a clockwise fashion around Sable Island, while farther afield, the transport directions appear to be random both in space and time. Recent surveys of sediment mobility on Sable Island Bank suggest that previous interpretations are erroneous. The movement of sediment in water depths greater than 20 m is from the southwest to the northeast in the general direction of the storm tracks, whereas adjacent to Sable Island the transport is predominantly eastwards. The so-called sand waves in the region, used to infer active sediment transport, are indeed shoreface-connected ridges, similar to those observed in the Mid-Atlantic Bight. The significance of such features in terms of sediment mobility and transport direction is quite different from that of sand waves. No active sand waves have been found in the area, although both sand ribbons and moribund two-dimensional megaripples have been found. Circular shell beds, interpreted from side-scan sonograms, indicate a dispersal of shell debris in the down-transport direction. The results of a numeric simulation of sediment transport on Sable Island Bank show that bed load transport takes place almost continuously, but the probability of sediment transport at a given rate decreases with increases in the magnitude of that rate, grain size and water depth. This study indicates that the sediment transport rate at any location can be expressed as a probability density function, and that the return interval of any transport event is defined by that of the major storm events.