MARINE biologist Dr Barry Wilson has proposed a theory to explain the formation of the Kimberleyâs 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 Kimberleyâs 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 Australiaâs tectonic plate.
âThe coastline was 50 kilometres out, away from the present coastline,â Dr Wilson said.
âSo 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.
âWith subsidence some of those have come down to what is now the contemporary inter-tidal zone.â
Referring to Collier Bayâs Montgomery Reef, Dr Wilson said there was âno wayâ a coral deposition of such magnitude could be only 6,000 years old.
âThis platform is way up in the inter-tidal zoneâwe havenât got it measured but from biological criteria I believe itâs above half-way up the tidal zone,â he says.
On further inspection he found the reefâs sides to be composed of a rock substrate, visible at extreme low tide.
âThereâs 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 he says.
âUp on top is the lagoon, about 300 square kilometres in area, there are lots of corals in that, but thereâs no reef growth around the periphery. So how did this thing form?â
He says the lagoonâs sea waters are impounded by walls formed not by corals, but Lithothamnia, a type of algae that excretes calcium carbonate.
It is Dr Wilsonâs 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 substrateâs 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.
âWhat that means is Montgomery is not a coral platform reefâitâs a really ancient Proterozoic structure which happens to be flat-topped,â he says.
âCorals now inhabit it but itâs not a coral reef in the strict sense.â
He said his discoveries call for investigation of all of the Kimberleyâs other platform reefs.
This story pertains to deliveries in theme 3 of the Kimberley Science and Conservation Strategy.