Modern reefs in the West Java Sea

Abstract

Abstract The Kepulauan Seribu are a group of small coral reefs in the west Java Sea extending in a chain roughly north-south for approximately 40 kilometres. The reefs vary in size from 50 metres to several kilometres in diameter and between 20 and 45 metres in height above the sea floor. There are well developed physiographic zones on all the reefs – sand cay, reef flat, lagoon, ‘growing edge’, reef slope and channel – each with distinctive physical, biological and sedimentological characteristics. A model for growth of the reefs is proposed whereby the reef ‘frame’ grows in its simplest form as a flattened inverted cone. This reef frame of in situ coral colonies only occupies a very small proportion of the total reef sediments; the bulk consists of reef flank and reef flat sediments although they are mostly generated from the growing edges. The sediments on the reefs are chiefly skeletal sands and gravels; the main skeletal components are corals, molluscs, and Halimeda, with smaller proportions of echinoids, foraminifera and red algae. Carbonate mud is present in the deeper parts of the lagoons. Non-skeletal elements are rare. Two major facies units are distinguished on the reefs: a coral-molluscan facies on the reef top where the individual grains are abraded and generally well sorted, and a coral-Halimeda facies on the reef flanks characterised by poor sorting and absence of abrasion of the grains. The adjacent shelf areas consist of silty clays. Comparison of the Kepulauan Seribu reefs with Miocene reefs in Indonesia reveals that the latter are generally larger and of a different type. Red algae and foraminifera are abundant and carbonate mud is ubiquitous. The latter, however, could be formed during diagenesis rather than indicating primary low energy sedimentation.