GROWTH AND STRUCTURE OF FRINGING REEFS IN A MUDDY ENVIRONMENT, SOUTH THAILAND

Most fringing reefs of the SE coast of Phuket, Thailand, have wide (up to 300 m) intertidal reef flats and narrow (2-5 m) reef fronts that a but the muddy forereef only a few meters deep. These reefs prograde by splitting, toppling, and regeneration of reef-front massive corals, n otably Porites lutea. Splitting of massive corals is greatly aided by the boring action of Upogebia sp. shrimps and by planes of weakness de veloped within the colony skeleton by nestling bivalves. The large siz e of toppled blocks ensures that some of the uppermost polyps are viab le above the soupy forereef sediment surface. Cores indicate that the reefs are essentially tabular features consisting of mainly massive re ef-front corals grown on muddy foundations that have shallowing-upward s sediment characteristics. C-14 ages and leveling data of fossil form er reef-front corals exposed on the reef flat suggest that reef growth started in the mid-Holocene when sea level was at least 0.8 m higher than now. Since then, the rate of lateral reef progradation has averag ed 40 mm/yr. The preservation potential of these reefs is low because of the combination of a lack of organic or inorganic binding of the co rals into a rigid frame, and the unconsolidated nature of the underlyi ng sediments.