Growth fabrics and growth rates of Holocene reefs in the Great Barrier Reef

Brief description

Over the past five years, scientists of the Bureau of Mineral Resources and James Cook University of North Queensland have drilled 61 holes into 24 reefs throughout the Great Barrier Reef Province between 15°30S and 25°50S. Eleven holes penetrated to the Pleistocene and nearly 250 radiocarbon dates have been recorded. Analysis of drill-hole core has delineated five major biosedimentologic facies - coralline algal facies. coral-head framework facies, branching-coral framework facies, detrital carbonate facies, and detrital siliciclastic facies. Latitudinal uniformity in framework facies contrasts markedly with major regional variations in detrital facies, the reefs of the mid-shelf of the central region being dominated by clastic carbonates. Depositional rates of detrital carbonate facies vary between 1-4 m/1000 yr for sand flat progradation , 7-9 m/1000 yr for trade wind storm sedimentation, and 13-18 m/1000 yr for high-energy low-frequency events. Framework growth rates varv from 1- 16 m/1000yr with low rates (2 m/I000 yr) for coralline algae. intermediate rates (up to 7 ml 1000 yr) for coral head facies, and high rates (up to 16 m/lOOO yr) for branching frameworks. Rates of 8-12 m/1000 yr occur in all environments: modes of 7-8 m/1000 yr typify patch reefs , 4-6 m/1000 yr typify windward margins. and 3-9 m/1000 yr typify leeward margins. Fringing reefs usually grow at rates of 1-4 m/1000 yr and are dominated by coral-head facies. Depth to the Pleistocene is generally greater in the central region compared to the northern and southern reefs: reef initiation, however, began at the same time throughout the province (8-9000 yr B. P.). Initial reef growth lagged significantly behind sea-level rise, such that water depths of up to 12 m developed over reef surfaces prior to sea-level stabilisation. However, some reefs in the southern region exhibit no growth lag - initial colonisation and growth maintaining pace with sea-level rise. The growth rate of most reefs decreased markedly as reef surfaces approached stabilised sea level.

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