Sedimentary environments of the Central Region of the Great Barrier Reef of Australia

Abstract

The sediments and calcareous organisms on the outer reefal shelf of the Central Region of the Great Barrier Reef were collected and observed by SCUBA diving and research vessel techniques (including underwater television) to understand the production and processes of deposition of the sediment. The carbonate grains are mainly sand and gravel size and solely of skeletal origin. Over the whole area the major CaCO3 producers, in order of decreasing importance are: benthic foraminiferans (chiefly Operculina, Amphistegina, Marginopora, Alveolinella and Cycloclypeus), the calcareous green alga Halimeda, molluscs and corals. Coral abundance is high only close to reefs and submerged rocky substrates. Benthic foraminiferal sands dominate the inter-reef areas i.e. the bulk of the shelf, and Halimeda gravels form an outer shelf band between 60 and 100 m depths. Seven distinct facies are recognised after quantitative analyses of the sediments. These are: A. Shelf edge slope (>120 m depth); B. Shelf edge (with rocky outcrops); C. Outer shelf with high Halimeda (>40%); D. Inter-reef I; E. Inter-reef II ( 100 m depth but >2% pelagics); F. Lee-ward reef talus wedge (<2 km from sea level reefs); G. Lagoonal.