Acoustical characterisation of seafloor sediments and its relationship to active sedimentary processes in Cook Strait, New Zealand

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Abstract
3.5 kHz seismic profiles are used to characterise the seabed in Cook Strait. The various acoustical responses have been classified into nine groups or echo‐types which, together with sediment samples, photographs, and side‐scan sonography, provide an insight into modern erosional and depositional processes operating in the strait. Much of northern Cook Strait is underlain by semi‐consolidated, late Pleistocene sediments that are eroded by strong, tide‐dominated currents even at depths >200 m. Locally, erosion of these deposits is impeded by a lag gravel pavement that occupies much of the 150–350 m deep central strait. The same strong currents effectively transport bedload along the Wellington continental shelf, which is a rocky platform with a patchy veneer of mobile sand and gravel. Outside the main tidal stream, within semiprotected embayments, deposition is manifest by prominent sediment bodies of mud and sand prograding across the inner‐middle shelf. Seaward of the shelf, in southern Cook Strait, the seafloor is dissected by a complex of submarine canyons that appear to syphon off tidally transported sand to the nearby Hikurangi Trough. However, in at least one place, transport is impeded by a slide blocking the canyon axis. Outside submarine canyons, products of gravitational mass movement are not conspicuous, even though Cook Strait lies across a zone of high seismicity. This scarcity of evidence is, in part, attributed to current modification of any such deposits.