HOLOCENE GROWTH HISTORY OF A REEF COMPLEX ON A COOL-WATER CARBONATE MARGIN - EASTER GROUP OF THE HOUTMAN ABROLHOS, EASTERN INDIAN-OCEAN

The Houtman Abrolhos reefs, situated on the western continental margin of Australia, occupy a transitional position between cool-water shelf carbonate sediments to the south and more tropical environments to the north. Their existence at the outer limits of the geographical range for coral reef growth is a result of the warm, poleward-flowing Leeuwin Current. Though the modern reefs differ ecologically from tropical reefs, their geological characteristics have been little known until recently. Each of the three island groups in the Abrolhos consists of a central platform of Last Interglacial reefs, about which windward and leeward Holocene reefs have developed asymmetrically. In the Easter Group the subtidal windward reef in the west is ca. 10 m thick and is backed by a leeward-prograding, lagoon sand sheet which is 0-3 m thick. The emergent parts of the leeward reefs in the east consist of an upward-shallowing sequence comprising reef facies, peritidal rudstone facies, and coral rubble storm ridges. This is underlain by over 26 m of Holocene reef facies. Coring and dating of the Holocene reefs (using both U/Th TIMS and C-14 methods) in the Easter Group has shown significantly different lithofacies in the windward and leeward reefs, and has allowed reconstruction of Holocene reef growth and sea-level history, Coralline algal bindstones and interbedded coral framestone facies characterise the relatively slow-growing windward Holocene reefs, whereas the fast-growing leeward reefs consist of coral framestone facies which are dominated by Acropora. The leeward reefs commenced growth about 10,000 U/Th years ago and the Morley reef grew to 0.3 m above present sea level by 6400 U/Th years B.P., recording a relative high sea-level event. This generated Holocene constructional topography characterised by ''blue-hole'' terrain. Windward Holocene reef growth commenced after 8200 U/Th years B.P. following erosion of the windward part of the Last Interglacial platform. High wave energy and competition with macroalgae limited coral growth, and the coralline algal-dominated windward reefs grew more slowly to sea level. The Holocene sea-level record provided by dates from the 26 m core of the Morley reef (a ''keep-up'' reef) is the first such record from the western continental margin of Australia.