Exposed limestones of the Northern Province of the Great Barrier Reef

Abstract
The exposed reef limestones occur principally on the inner-shelf reefs and can be separated into two groups — organically cemented (reef-rock) and inorganically cemented (beach-rock, rampart-rock, boulder-rock and phosphate-rock). No examples were found of exposed sub tidal reef framework; the reef-rock exposed is entirely of intertidal origin resulting from incipient encrustration by intertidal corals and coralline algae. Most of the beach-rock, rampart-rock and boulder-rock exposures are intertidal and many show vadose cement fabrics. The cements, chiefly aragonite needles in beach-rock and cryptocrystalline high Mg calcite in ram part and boulderrocks, are thought to be derived from seawater, though the environments of precipitation on windward sides of reefs where rampart-rocks form are quite different from those on the leeward sides where beach-rocks form. Phosphate-rock develops supratidally on the surface of some sand cays. Solutions derived from guano precipitate thin layers of phosphatic cement which bring about the centripetal replacement of carbonate grains.

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