Abstract

This paper reviews the physiographic and climatic conditions under which the Pulau Seribu coral reef system developed. Three shallow cores, from two of the islands, provide insights into vertical variations in composition and diagenesis and into the coral island growth history. The deepest borehole, 32.8 m, bottomed in stiff, siliceous carbonate mudstone of apparent Pleistocene age. Age dating of selected coral debris at various depths indicate that the vertical accumulation was largely achieved during the period 10,000-4500 yrs BP. This rate of 5-10 mm per year matches the initially rapid post Wisconsin sea level rise. Since then coral island growth has been outward. The sediments are coral dominated (50% or more) with only minor red and green algal material and a notable lack of fines. The present island phreatic freshwater lenses are small. Overall cementation is limited but does occur as spar calcite fill of coral calices, and meniscus high Mg calcite in near surface beachrock. Marine cements include fibrous aragonite and stubby bladed high Mg calcite and peloidal cryptocrystalline masses of both. Limited evidence of aragonite dissolution is present throughout the buildups. Top-down degradation of framework constituents is underway which undoubtably will lead to the ultimate modification of primary depositional fabrics. Porosities and permeabilities are very high in the relatively unaltered Holocene carbonate sediment, especially in the coral-rudstone rampart deposits which act as conduits for seawater flushing and recharge of the coral-dominated carbonate buildups.