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A rugged tectonic backbone, deeply weathered and stream-incised, composes the island of Java, Indonesia. Streams laden with fine-grained terrigenous sediment spill out into the marine setting of the Java Sea. Longshore drift moves the fine-grained suspended sediment load from points of discharge parallel to the coastline. Hence, although the Java Sea lies beneath the equator, carbonate sedimentation is essentially inactive along the coast. Reefs and skeletal facies, however, develop and become abundant about 25 km (16 mi) offshore, away from the influence of terrigenous sediments and freshwater plumes from rivers. The reefs, known as the Pulau Seribu Group, occur on the shelf at a depth of 30 to 40 m (100 to 130 ft) and are elongated in an approximately east-west directio . Storms pile up skeletal debris on the reef flats, thus building up islands that develop beaches and locally become vegetated. The skeletal debris is mostly composed of particles of corals, mollusks, echinoid spines, foraminifera, and red and green algae.
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