Dr Barry Wilson, former Curator of Molluscs at the WA Museum, presented a talk covering the general biogeography of the North West shelf with new information on the extensive coral reefs of the Kimberley Coast. Barry has an in-depth knowledge of this remote area from his many research trips; his most recent book is referenced below.

The illustrated talk introduced us to coral reef development and aspects of how corals have adapted to the special regional conditions in the area.

The Kimberley coast stretches from Broome to the Northern Territory border and includes the many offshore islands and reef systems.

Coral reefs featured historically in the literature of Australia have been dominated by the Great Barrier Reef along the eastern seaboard of Australia and it was not until the 1880s that the existence of a fringing coral reef off Troughton Island off the Kimberley Coast of WA was documented. The first report was made by an officer on the British hydrographic ship “Penguin”. It was noted at the time that the reefs were unusual because the waters were so muddy.

Charles Darwin, writing in the 1840s, drew attention to coral reef development in clear waters in his study of coral atolls. The discovery of coral off the northern west coast of WA was considered unusual because the waters are regarded as ‘muddy’. In fact, coral reefs are now well known throughout the area both connected to the continental coast and associated with the many offshore island reefs and strings of shoals.

Coral reefs (aragonite structures built by living organisms) are of two types; coastal fringing reefs and atoll-like shelf edge reefs. The islands are largely surrounded by fringing reefs. The Ashmore Reef is an example of a shelf edge platform reef. Fringing reefs are different in the Kimberley with the two main factors affecting their development being the geology and the geological history.

The Kimberley coast is a ria-type subsidence coastal margin where current sea level cloaks drowned terrestrial geology. The geology of the area comprises part the Kimberley Basin (Kimberley Block), essentially early Proterozoic fairly flat-lying sediments (mostly sandstones) with volcanic sequences (basalts, dolerites, tuffs, rhyolites) (1500 Ma) and younger limestone margins together with a portion of highly contorted metamorphic rocks of the King Leopold Terrain.

Sea levels have risen from approximately 120 m below current levels after the last glacial maxima about 18-20,000 years ago. These sea level changes have had a major influence on both offshore and onshore bioregions. These regions cover:-

• Slope atolls and platform reefs with oceanic biota where, for instance, the fish and other marine life are those that are widespread throughout huge expanses of oceans and where species show no endemicity. Scott Reef, Seringapatam Reef and the three Rowley Shoals are slope atolls that arise from depths of 300 to 800 m on the continental slope west and north west of the Kimberley coast. Browse, Cartier, Hibernia and Ashmore Reefs are platform reefs arising from the outer margin of the continental shelf. These are all bioherm structures (built of coral and coralline algal skeletal remains) that originated in the Miocene.
• Fringing and platform reefs abundant in the turbid Kimberley bioregion where the biota is continental and has high endemicity (as much as 20% compared to 7% in the whole of Australia).

Modifying characters affecting the Kimberley Bioregions include; high rainfall with major river runoff, terriginous inner shelf sedimentation, macrotidal and very strong and complex marine currents, the subsiding ria coastline with its dissected escarpment, inundation by post glacial transgressions and extensive development of fringing and platform reefs.

The Kimberley fringing reefs are geologically young at ~7000 years (Holocene) and developed mostly on the Proterozoic sandstones providing diverse geomorphology extremely rich in coral species (318 species) and a very rich and diverse genera (63 genera). Fringing reefs are developed as: reefs without rock platforms on sheltered shores and seaward reefs on rock platforms. It has been noted that the corals have adapted to local conditions, with differences in the species types on leeward and windward aspects, as on Maret Island. Here there was the general development of the branching species of the genus Acropora sp corals without a rock platform on the leeward side whilst domal faviids (brain corals of the family Faviidae) developed on exposed wide limestone platforms of the northwest seaward reef front.

The many images included fringing reefs on Turtle and Adele Reefs (a mid-shelf platform Holocene growth on sandstones). Not all reef substrates are on sandstones (of various ages). The East Montalivet group, for instance, were on basalts. Montgomery Island has a younger limestone and dolomitic substrate with stromatolites, and rhodolith banks (a red algae resembling coral). Marine heritage sites have also been noted on this island.

A question arises as to how corals cope with turbid waters? Their success may be connected to the vacuuming/cleansing action of tidal flows. However, corals do not grow in permanently turbid waters and they cease growing if sea level changes are too fast. Periods of coral growth are used to interpret changes in sea levels, several noted from the period 18 to 20,000 years ago when the connectivity between areas of the Northwest Shelf ceased and shoals were submerged.
The Kimberley coral reefs are a varied, species-rich and largely undescribed coral reef province off one of the World’s wilderness area.

Susan Stocklmayer

Footnote- Dr Barry Wilson’s latest book, a copy of which was on view is entitled ‘The biogeography of the Australian North West Shelf, Environmental Change and Life’s Response.’