Evolution of Miocene organic reefs and functions of algae’s reef-making in Xisha Islands

The data of lithology, mineralogy, lithoehemistry, palaeoorganism-palaeoecology of 455.25 m cores from Xichen-1 well have been obtained, the characteristics, types, models of fossil taphonomy and reef-making actions about different communities of 9 genera and 43 species algae have been discussed, and the evolution process of algae’s ecologic system from community succession to community replacement has been resumed. It is pointed out that organism evolution (phodophyta CorallinaceaeAethesolithon nanhaiensis community replaced by chlorophytsHalimeda, community) are related to palaeo-ocean changes (regression 135.51 m) from late period of earfy Miocene to middle Miocene. This conclusion is identical with the cycle curve of the global sequence, but different from the study of sediment basin area in the northern South China Sea.