Abstract:

Yongle Islands(atoll)is situated in the west of Xisha Archipelago of the South China Sea.It consist of seven bigger separated reefs.The central lagoon of theatoll is about 40min depth.These reef-flats appear a similar in geomorphic features,and can be divided into several zones,including outer-slope,outer-crest,outer-flat,island(sand,gravel or rock island),inner-flat,inner-slope.Each zone has its own organisms and sediment features.Two lines of beachrocks occur outside the Quanfu Island.This indicatesthat the sand island has moved three times at least.Shiyu Island,consisting of cemented rock,appears to be an anticline structure.Obviously,it is a deposi-ting anticline.