Black and brown speckled foraminiferal sand occurs in the western part of the Capricorn Reef Complex and on adjacent parts of the Inner Shallow Shelf in the zone of mixing of terrigenous and carbonate sediments. The colored materials, which are found preferentially on foraminifera tests, are iron and manganese sulphides, hydroxides and/or oxides. The iron and manganese appear to be derived from associated terrigenous sediments and are being reprecipitated under reducing conditions and in the presence of sulphate-reducing bacteria around centers of organic material such as the foraminifera tests in the sediment. Removal of sediment by erosion and the activity of burrowing organisms brings reduced sediment back into the oxidizing zone where sulphides (black) are oxidized to ferric hydroxides and/or oxides (brown).

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