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Reef Island Sediments of the Northern Great Barrier Reef

R. F. McLean and D. R. Stoddart
Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences
Vol. 291, No. 1378, The Northern Great Barrier Reef (Nov. 13, 1978), pp. 101-117
Published by: Royal Society
Stable URL: http://www.jstor.org/stable/75221
Page Count: 18
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Reef Island Sediments of the Northern Great Barrier Reef
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Abstract

The reef islands are composed almost exclusively of bioclastic materials locally supplied from adjacent reef flats and reef crests. No sediment of terrigenous origin was encountered on the islands investigated (except drift pumice). Islands are built either of sand or of gravel, and only rarely mixtures of both sand and gravel. The sands are typically well sorted with a prominent size mode within the medium-coarse size range (0.25-1.00 mm). Major skeletal components include corals, Foraminifera, molluscs and crustose coralline algal fragments, either whole or broken. Most grains show signs of considerable abrasion. The gravels are more homogeneous in composition (mainly corals) but reveal a great range of size, shape and surface characteristics. Elongated clasts, derived from branching corals, provide the main components but corals of other growth forms are present. The sand deposits and gravel deposits are normally spatially discrete on any one reef but mixtures of these two size grades do occur, notably on the Turtles. Reasons for the presence or absence of spatial discrimination are discussed.

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