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Keywords:

  • Furrows and ridges;
  • internal waves;
  • NW Australia;
  • sediment waves;
  • 3D seismic

Abstract

This study analyses the three-dimensional geometry of sedimentary features recorded on the modern sea floor and in the shallow subsurface of a shelf to upper slope region offshore Australia that is characterized by a pronounced internal wave regime. The data interpreted comprise an extensive, >12 500 km2 industrial three-dimensional seismic-reflection survey that images the northern part of the Browse Basin, Australian North West Shelf. The most prominent seismic–morphological features on the modern sea floor are submarine terrace escarpments, fault-scarps and incised channels, as well as restricted areas of seismic distortion interpreted as mass wasting deposits. Besides these kilometre-scale sea floor irregularities, smaller bedforms were discovered also, including a multitude of sediment waves with a lateral extent of several kilometres and heights up to 10 m. These sedimentological features generally occur in extensive fields in water depths below 250 m mostly at the foot of submerged terraces, along the scarps of modern faults and along the shelf break between the outer shelf and the upper continental rise. Additional bedforms that characterize the more planar regions of the outer shelf are elongate, north-west/south-east oriented furrows and ridges. The formation of both sediment waves and furrow-ridge systems requires flow velocities between 0·3 m sec−1 and 1·5 m sec−1, which could be generated by oceanic currents, gravity currents or internal waves. In the studied setting, these velocities can be best explained as being generated by bottom currents induced by internal waves, an interpretation that is discussed against oceanographic background data and modelling results. In addition to the documentation of three-dimensional seismic–geomorphological features of the modern sea floor, it was also possible to map kilometre-scale buried sediment wave fields in the seismic volume down to ca 500 ms two-way-time below the present sea floor, indicating the general potential for the preservation of such bedforms in the sedimentary record.