INTERNAL TIDES AND BEDFORMS AT THE SHELF EDGE

John Southard astutely observed in 1976 that the “shelf edge is by no means the ‘Pleistocene museum’ it was once believed to be”. In the 25 years since, as academic and industrial research has probed most continental margins of the world, the shelf edge has become recognized as a highly dynamic zone with complex sediment dispersal patterns.

Tides are in many places the dominant agent of sediment transport on shelf edges and in canyons on the adjacent continental slope. Data obtained by Unocal at a deepwater mooring site in Indonesia's Makassar Strait clearly demonstrate the existence of strong, semidiurnal tides flowing along the axis of a submarine canyon at a water depth of 993 m. Maximum current speeds reach about 40 cm/sec during the peak of each fortnightly spring tide. The origin of these internal tidal currents is probably related to the compression of the deepwater tidal wave as it ascends the slope where preservation of momentum forces an increase in shore-perpendicular current velocities. One would expect peak current strength where the canyons terminate at the shelf edge.

3D seismic reflection data cubes of the Makassar continental margin reveal an extensive field of large, low-amplitude bedforms within, between, and in a band shelf-ward of the canyon heads. The bedforms are generally asymmetric with internal onlap geometries that demonstrate accretion up-slope and laterally away from the canyon floor towards the inter-canyon ridges. The bedforms have amplitudes on the order of 10 m and wavelengths on the order of 1 km. Piston cores collected from the bedforms contain dominantly clay mud with widely spaced thin sands.

Details of the bedform distribution and spacing are consistent with an origin by tidal currents directed up the canyons. An interesting speculation: if internal tides move fine-grained sediments from the deep onto the shallower shelf edge, do they also move deepwater fauna onto the outer shelf?