COMPARATIVE STUDY OF REEF GEOLOGY BETWEEN BAHAMA BANKS AND XISHA ISLANDS

Comparative study of the reef platforms is made in this paper for the Bahama banks and the Xisha Islands upon depositional characteristics and facies models. Clastic, argillaceous and slope bioclastic limestones are well distributed in the two areas. The deposition of the cements of aragonite and magnesium calcite on the seafloor has made most of the original pores blocked and thus porosity reduced. Bioclastic limestone dominates the beach rocks on the Xisha reefal flat. In the subsurface stratigraphy, however, bioclastic limestone and dolomite predominate. The existence of various algal clutter causes the increase in the porosity of rocks. The Bahama Bank is an isolated carbonate reef platform,consisting of platform margin deposits, slope deposits and reef flat deposits. The Xisha Islands is also an isolated carbonate platform reef in general,and the Yongle-Xuande atoll is the main reef complex in the region. Many types of reefs,including platform-edge reef,tower reef,horseshoe-like reef,atoll,patch reefs etc, are observed there.The Bahama Bankis characterized by cyclical progradation. Thereare two sedimentary types of sequences, however, progradational and erosional,in the steep slope of high energy in the Xisha Islands. The lime-sand island in the Xisha owes its origin to storm surge and prevailing wind.