Melbourne: Scientists have discovered a massive undersea landslide in Australia’s iconic Great Barrier Reef that may have triggered a huge tsunami more than 300,000 years ago.

The remains of the slip, known as the Gloria Knolls Slide, were discovered 75 kilometres off the north Queensland coast while the scientists were working from the Marine National Facility’s blue-water research ship Southern Surveyor. “This is all that remains after a massive collapse of sediment of about 32 cubic kilometres’ volume more than 300,000 years ago," said Robin Beaman, from James Cook University in Australia.

Beaman said a debris field of large blocks, or knolls, and numerous smaller blocks, lies scattered over 30 kilometres from the main landslide remains, into the Queensland Trough, to a depth of 1,350 metres.

“We were amazed to discover this cluster of knolls while 3D multibeam mapping the deep GBR seafloor. In an area of the Queensland Trough that was supposed to be relatively flat were eight knolls, appearing like hills with some over 100 metres high and three kilometres long," said Beaman.

A sediment sample from a knoll at a depth of 1,170 metres identified a remarkable cold-water coral community of both living and fossil cold-water coral species, gorgonian sea whips, bamboo corals, molluscs and stalked barnacles. “The oldest fossil corals recovered off the top of the knoll was 302 thousand years, which means the landslide event that caused these knolls must be older," said Angel Puga-Bernabeu at the University of Granada in Spain.

Modelling the potential tsunami for a sudden ‘mass failure’ on this scale yields a three-dimensional tsunami wave elevation of about 27 metres. However, the wave would likely be dampened significantly by the presence of any coral reefs, researchers said.

Considerably more seabed mapping and sampling is needed to fully assess the tsunami hazard to the Queensland coast posed by these types of underwater landslides. The scientists said one-third of the Great Barrier Reef lies beyond the seaward edge of the shallower reefs, and the discovery of this prominent undersea landslide and its vast debris field in the deep Great Barrier Reef reveals a far more complex landscape than previously known. The study was published in the Marine Geology journal.

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