MARINE biologist Dr Barry Wilson has proposed a theory to explain the formation of the Kimberleyâ€™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 Kimberleyâ€™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 Australiaâ€™s tectonic plate.
â€śThe coastline was 50 kilometres out, away from the present coastline,â€ť Dr Wilson said.
â€śSo 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.
â€śWith subsidence some of those have come down to what is now the contemporary inter-tidal zone.â€ť
Referring to Collier Bayâ€™s Montgomery Reef, Dr Wilson said there was â€śno wayâ€ť a coral deposition of such magnitude could be only 6,000 years old.
â€śThis platform is way up in the inter-tidal zoneâ€”we havenâ€™t got it measured but from biological criteria I believe itâ€™s above half-way up the tidal zone,â€ť he says.
On further inspection he found the reefâ€™s sides to be composed of a rock substrate, visible at extreme low tide.
â€śThereâ€™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,â€ť he says.
â€śUp on top is the lagoon, about 300 square kilometres in area, there are lots of corals in that, but thereâ€™s no reef growth around the periphery. So how did this thing form?â€ť
He says the lagoonâ€™s sea waters are impounded by walls formed not by corals, but Lithothamnia, a type of algae that excretes calcium carbonate.
It is Dr Wilsonâ€™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 substrateâ€™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.
â€śWhat that means is Montgomery is not a coral platform reefâ€”itâ€™s a really ancient Proterozoic structure which happens to be flat-topped,â€ť he says.
â€śCorals now inhabit it but itâ€™s not a coral reef in the strict sense.â€ť
He said his discoveries call for investigation of all of the Kimberleyâ€™s other platform reefs.
This story pertains to deliveries in theme 3 of the Kimberley Science and Conservation Strategy.