Halimeda bioherms of the eastern Java Sea, Indonesia

Abstract

Extensive areas of coalescing bioherms composed primarily of disarticulated Halimeda plates occur on a large carbonate bank (Kalukalukuang Bank) located 50 to 70 km east of the central Sunda Shelf margin (eastern Java Sea, Indonesia). High-resolution seismic profiles suggest that these features attain maximum thicknesses of up to 52 m above an acoustically reflective surface interpreted as the top of the Pleistocene. Piston cores and vibracores from the bioherms indicate a composition of Halimeda packstone containing varying amounts of foraminifera-rich carbonate mud. A shell lag occurs at their base above the Pleistocene(?) surface. Seismic stratigraphy of the bioherms suggests that they generally developed as individual mounds that accreted both vertically and laterally until they coalesced to form composite features. Discordant reflectors are common and probably represent morphological modifications associated with high-energy events, current scour, or local mass movement. Halimeda bioherms occur in water depths ≥20 m on both exposed and reef-protected bank margins, unlike the only other recorded modern examples, which occur in the lagoonal facies behind the northern Great Barrier ribbon reefs of Australia. Their presence and rapid growth rates (to 5.9 m/1000 yr as determined from carbon-14 dating of piston core subsamples) are possibly related to upwelling of deep, nutrient-rich, south-moving water from Makassar Strait.