Wednesday, 25 April 2012

Kimberley’s platform reefs reveal unique formations

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Fringing reefsOn further inspection he found the reef’s sides to be composed of a rock substrate, visible at extreme low tide. Image: B. WilsonMARINE biologist Dr Barry Wilson has proposed a theory to explain the formation of the Kimberley’s fringing coral reefs.

In 2010 he inspected several flat-topped reefs that he says are far too thick to be composed entirely of coral.

He says the Kimberley’s coastal coral reefs are only about 6,000 years old, owing to a 120-metre rise in sea level after the last glacial period; and a gradual continental tilt caused by the subduction of Australia’s tectonic plate.

“The coastline was 50 kilometres out, away from the present coastline,” Dr Wilson said.

“So as a process of this subsidence we find that all the rocky shores of the Kimberley and most of those [offshore] rocks are flat-bedded, really old Paeleozoic sandstones.

“With subsidence some of those have come down to what is now the contemporary inter-tidal zone.”

Referring to Collier Bay’s Montgomery Reef, Dr Wilson said there was “no way” a coral deposition of such magnitude could be only 6,000 years old.

“This platform is way up in the inter-tidal zone—we haven’t got it measured but from biological criteria I believe it’s above half-way up the tidal zone,” he says.

On further inspection he found the reef’s sides to be composed of a rock substrate, visible at extreme low tide.

“There’s a very steep slope on the outer rim and the water pours off that at low tide, like a waterfall all the way around,” he says.

“Up on top is the lagoon, about 300 square kilometres in area, there are lots of corals in that, but there’s no reef growth around the periphery. So how did this thing form?”

He says the lagoon’s sea waters are impounded by walls formed not by corals, but Lithothamnia, a type of algae that excretes calcium carbonate.

It is Dr Wilson’s theory that the reef was originally a terrestrial mesa that became flooded at spring tide as the coastline subsided and sea levels rose, allowing corals to start growing on its flat top about 6,000 years ago.

While the rock substrate’s composition is yet to be examined, Dr Wilson further hypothesises it to be dolomite of stromatolitic origin, similar to the composition of the nearby High Cliffy Islands.

“What that means is Montgomery is not a coral platform reef—it’s a really ancient Proterozoic structure which happens to be flat-topped,” he says.

“Corals now inhabit it but it’s not a coral reef in the strict sense.”

He said his discoveries call for investigation of all of the Kimberley’s other platform reefs.

This story pertains to deliveries in theme 3 of the Kimberley Science and Conservation Strategy.

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