Internal structure and accretionary history of a nearshore, turbid-zone coral reef: Paluma Shoals, central Great Barrier Reef, Australia

Nearshore environments directly influenced by fluvial sediments and dominated by terrigenoclastic sedimentation are generally considered marginal for coral reef growth. There are, however, a growing number of examples from the modern and fossil record that clearly show that reef-building and terrigenous sedimentation are not mutually exclusive. Although a number of descriptive accounts of nearshore turbid zone reefs have emerged in recent years, our understanding of the accretionary architecture and history of such reefs remains extremely limited. Here we detail the depositional facies and accretionary history of an inshore, turbid-zone coral reef complex, Paluma Shoals, on the central Great Barrier Reef Shelf of Australia. High turbidity (N40 NTU for over 30% of the time) and terrigenous sedimentation — both at levels close to the perceived limits for coral growth — are episodically important influences at Paluma Shoals. Despite these potentially limiting conditions Paluma Shoals exhibits high live coral cover (~50–60%, but locally up to 80%), with the intertidal reef flats compositionally dominated by G. aspera microatolls, and Galaxea sp. and Porites sp. colonies. Seventeen cores from 5 cross-reef transects reveal that the internal reef structure comprises an unconsolidated detrital coral rubble framework, 2.0–2.7 m thick, dominated by fragments of Acropora pulchra, T. frondens and M. mollis. These occur within a mixed terrigenoclastic–carbonate sediment matrix. Basal facies, in particular, contain a high (~60%) proportion of terrigenoclastic sediment. Radiocarbon dating indicates that the reef initiated above Pleistocene clays ~1600 cal yr BP. The present reef flat was emplaced within the last ~150–100 cal yr BP. Within the last ~60 years, areas of the North Shoal have become partially smothered as a result of intertidal sandflat progradation. The depositional sequence at Paluma Shoals shows; 1) that long-term (millennial timescale) reef accretion has occurred under conditions dominated by terrigenoclastic sediment accumulation; 2) that relatively rapid vertical reef growth and progradation can occur in these ‘marginal’ settings; and 3) that these reefs are susceptible to natural episodic mortality events associated with the remobilisation of intertidal and shallow subtidal sediments. This study provides important insights into coral reef development where environmental conditions are perceived to be marginal for hermatypic coral growth