Abstract

Ooids are reported from the Great Barrier Reef Province for the first time. They occur over an area of 340 km 2 as unconsolidated sediments on the floor of the Capricorn Channel, in water depths of 100-120 m, and are believed to have formed in the early Holocene. X-ray diffraction analysis shows the ooids to be composed of high magnesium calcite. This is confirmed by electron probe studies, which also indicate that magnesium occurs in a non-carbonate phase within the chambers of foraminifera that acted as ooid nuclei. Petrographic and scanning electron microscopic studies show that the ooids exhibit well defined concentric, radial, and granular fabrics. All three fabric types occur in the cortex developed around polycrystalline nuclei. A well developed radial fabric occurs in optical continuity with echinoderm fragments that acted as nuclei. Diagenetic alteration of the ooids has resulted first in an overprint of a secondary radial fabric on the primary concentric and radial ones, and secondly in the progressive obliteration of these three fabrics by the growth of an equigranular mosaic, learning to the formation of a structureless ooid. During or after these changes ooid nuclei have been replaced by high-magnesium calcite.

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