Los arrecifes coralinos de Golfo Dulce, Costa Rica: aspectos geológicos

Abstract

The Holocene growth history of the fringing reef a Punta Islotes, Golfo Dulce, Pacif coast of Costa Rica, has been reconstruted based on data obtained from nine core holes and four sediment cores collected along four reef front transects. These cores reveal the thicknees of the reef to be at least 9 m. Radiocarbon date indicate that the reef started to grow about 5500 yr. B.P. over an uneven surface of Cretaceous basalt. Three reef facies - that are represented by reef zones on the present day reef surface- were recognized: reef-flat braching coral, fore-reef massive coral, and fore-reef talus sediment facies. Accumulation rates of the reef-flat facies range from 1,6 to 2,4 m/1000 yr (mean and standard desviation= 1,90 +/- 0,31) of the fore-reef facies from 0,9 to 8,3 m/1000 yr (3,82 +/- 2,52), and of the fore-reef talus facies from 0,07 to 0,3 m/1000 yr.Holocene growth of Punta Islotes fringing reef can be divided into four stages: A. Initial stage, 5500-4000 yr B.P, settling of Pocillopora damicornis and formation of a small fringing reef. B. Establishment of the reef, 4000-1500 yr B.P, continuos growth of the braching coral facies, initiation of the massive coral facies, and drop in the accumulation rates of the fore-reef talues facies. C. Rapid vertical growth stage, 1500-500 yr B.P., accumulation rates of 5 to 8,3 m/1000 yr, growth of most of the reef framework. D. Final stage, 500 yr B.P to the presentm decline accumulation rates, first due to an increase in fresh water, and more recently to the presense of terrigenous sediments related to deforestation on the adjacent shores.Today, the coral reef in the inner section of Golfo Dulce are totally dead or with very low percentages of live coral. Both, living and dead corals of those reefs are being bioeroded mainly by the boring bivalves, Lithophaga spp.