An unconsolidated submarine deposit of iron ooids and pisoids, located offshore the volcanic island Mahengetang, Indonesia, is described herein. The deposit occurs in a shallow-marine setting, in an area characterized by venting of hydrothermal fluids and expulsion of gas across the sediment-water interface. The ooid deposit is essentially pure to a minimum depth of 0.5 m and covers an area of at least 1000 m2. Ooids are composed of concentric accretionary layers of limonite admixed with amorphous silica, precipitated around andesitic rock fragments. The smallest layers forming the ooids are about 10 ± in thickness. The iron-rich precipitate is ∼ 50% iron by weight. Ooid formation is likely promoted by precipitation of iron and silica from exhalative fluids rising up through the substrate. Concentric layering is thought to result from constant agitation of ooids associated with currents and expulsion of gas from the sediment.

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