Holocene sea-level changes and barrier reef formation on an oceanic island, Palau Islands, western Pacific

Internal facies and development of an oceanic island's barrier reef were revealed by the stratigraphical study of six drill cores in Palau Islands, western Pacific. The Holocene reef development is primarily constrained at its foundation by the antecedent topography of Pleistocene substratum, Holocene barrier reef is an increment on the Pleistocene barrier reef, which had been subaerially exposed during glacial stages, About 8300 cal. year BP (calibrated calendar years B.P.), branching Acropora facies initially formed a bank on the seaward side of a Pleistocene limestone surface with a vertical accumulation rate as high as 30 m/ka (ka = 1000 years), After 7200 cal. year BP, when the sea-level rise rate decreased, reef crest facies caught up with the sea surface with an accumulation rate of less than 2.2 m/ka. Corals found in the reef crest facies are similar to the present-day reef crest corals dominated by Acropora digitfera and A. humilis. After the reef crest was formed, bioclastic sand and gravel facies prograded lagoonward of the reef crest and consisted mostly of reef derived materials. The construction of patch reefs postdated the barrier reef fort-nation. The mature barrier reef provided calm conditions inside the lagoon, which then led to the construction of patch reefs and fringing reefs. Sea-level changes deduced from the accumulation curves show rapid rise before 7200 cal. year BP followed by a slight rise of 4 m at its maximum. This change in sea-level rise rate inspired the change in reef facies from branching Acropora to reef crest. (C) 2002 Elsevier Science B.V. All rights reserved.