NIWA survey reveals deep water canyons off Otago coast

NIWA's has mapped the seabed off the coast of Otago, revealing the structure of nine canyons in the Great South Basin. This survey is providing new information about geological processes in the region as well as clues to what types of life might exist on the seafloor. 

NIWA's research vessel Tangaroa mapped the seabed off the coast of Otago in July this year, revealing the structure of nine canyons in the Great South Basin.

The seabed of the Great South Basin has not been mapped in detail before, and this recent survey is providing new information about geological processes in the region as well as clues to what types of seafloor life might exist in the area.

The highly detailed map represents a survey of 16,000 km2 and shows part of a much larger canyon system. This system ends 1100 km east of New Zealand, in a large deep 'apron-shaped' sediment deposit in the Bounty Trough. It is the second largest sedimentary distribution system in New Zealand. These systems are like underwater river systems, transporting sediment from the coast far out to sea.

NIWA geologist Dr Helen Neil says, "What is really amazing is that we now have an intricate map showing the detail of the many near-shore canyons that form a set of tributaries sending sediment on its journey to the deep sea. We have mapped a series of near-shore canyons along a section of the eastern South Island continental slope. Canyons reach depths of around 750 metres some 5-15 kilometres down the canyon and extend into the North, Central and South channels, which ultimately merge to form Bounty Channel.

"It gives us a far greater idea of the structure of the Otago canyon complex. This entire system has a really long history. Way back 55-60 million years ago when the tectonic rift occurred, this v-shaped notch was created in the seafloor that could act as a distribution system for sediments."

The channel is not distributing sediment in the way it did in the past as the sea level is higher and New Zealand doesn't currently have an aggressive glacial environment. There is also less erosion from the Southern Alps. Most of the sediment that does come down gets trapped in the lakes.

When the sea level is lower and there is aggressive erosion on the alpine fault, larger amounts of sediment flow off the land and through the undersea canyon system.

Canyon, channel, fan systems are important sources of information about climatic events, oceanographic changes, and events onshore. The sedimentary record that accumulates in these systems can give important insights into major tectonic, climatic and sea level changes of the past.

Dr Neil says, "This map not only tells us something about the geological processes that have, and are, taking place, but these data can be used to inform environmental regulation, conservation or resource use."

Government agencies, iwi, regional and district councils, port authorities, conservation groups and industry will able to draw on this information to develop and manage these resources in a more effective and sustainable way.

The survey data will also help provide a basis for any future biodiversity assessment. That assessment could be used to establish an environmental baseline, evaluate the potential environmental effects of any oil drilling, and help develop any special environmental guidelines specific to the area.

The data show hints of gas expulsion and special deep sea ecosystems. "The maps show 'pock marks' along the margin at depths of between 550-875 metres where methane seepage, which supports particular biological communities, typically occurs," says Dr Ashley Rowden, a seabed ecologist at NIWA.

"We know something about the animals that live in the shallow areas of the canyons, but we don't know what is in the really deep parts, particularly in the trough where oil drilling may occur in the future," says Dr Rowden.

Investment in previous seismic surveys has helped encourage uptake of exploration permits by the petroleum industry. This survey of the Great South Basin will provide further data to support current and future exploration of the area.

This survey voyage is funded by Land Information New Zealand (LINZ) as part of the all-of-government Ocean Survey 20/20 programme that LINZ coordinates to provide New Zealand with better knowledge of its ocean territory, including New Zealand's Exclusive Economic Zone, continental shelf and the Ross Sea region.

Detail of the survey coverage of the Otago canyon complex. See the full survey image.
Survey coverage of part of the Otago canyon complex offshore East Coast, South Island extending from Waitaki Canyon in the north to Hoopers Canyon in the south. Image is a sun-illuminated digital elevation model. Depth range 100m (red) to 2000m (purple). NIWA