Some Miocene and Pleistocene corals from Well Drilling CK-2 in Xisha Islands, South China Sea
Scleractinia are the major members of the Mesozoic, Cenozoic, and extant corals. Scleractinian corals are distinguished by their calcareous external skeletons. Scleractinian corals may be divided into two ecological groups: hermatypic (reef-building) and ahermatypic corals. Hermatypic corals are characterized by the presence of vast numbers of zooxanthellae in their endodermal tissues. They are the most common in warm and shallow marine waters of the tropics. Strong sunlight is essential for the vigorous growth of hermatypic corals. Ahermatypic corals lack zooxanthellae and are environmentally less restricted than reef-building corals. It is now generally recognized that, since the Miocene, there are two first-level reef-coral biogeographic provinces, i.e., the Caribbean and Indo-Pacific, in the world. The South China Sea, including Xisha Islands, is attributed to the Indo-Pacific Province. During the Neogene and Quaternary, the Indo-Pacific coral faunas were already rich than those of the Caribbean. The oldest coral-bearing beds are from the early Miocene in Xisha Islands, South China Sea. About 19 genera of Miocene scleractinian corals are recognized from the 364–878.3 m interval in the Well CK-2 core from Chenhang Island, Xisha Islands, South China Sea. They are Acropora, Astreopora, Antillophyllia, Antillia, Astrhelia, Caryophyllia, Cyphastrea, Diploastrea, Echinophyllia, Favia, Favites, Fungia, Galexea, Goniopora, Hydnophora, Montipora, Platygyra, Porites and Turbinaria. Among them, the geological ranges of the genera Echinophyllia, Fungia and Galaxea are from the Miocene to the Quaternary, indicating that the age of the strata that yield these three genera is not older than the Miocene. While the genus Antillia has been recorded from the Eocene to the Miocene and the genus Antillophyllia has been reported from the Oligocene to the Miocene, indicating that the age of the strata that contain these two genera is not younger than Miocene. Moreover, the genus Astrhelia has hitherto been known only from the Miocene. All the above-mentioned evidence suggests a Miocene age. About 21 genera of the Pleistocene scleractinian corals and one genus of Octocorallia are identified from the 21.4–215.6 m interval in the Well CK-2 core. Among them, the geological ranges of Acanthophyllia, Enallopsammia, Fungia, and Galaxea are from the Miocene to the Quaternary. The genera Acoropora, Coenocythus, Cyphastrea, Euphyllia, Montipora, Pavona, Porites, Favites, Goniastrea, Platygyra and Turbinaria have been recorded from the Palaeogene to the Quaternary and the genera Astreopora, Goniopora and Leptoria are the characteristic corals from the Cretaceous to the Quaternary. A few genera, however, afford evidence of considerable weight for the determination of the age of the strata. The three scleractinian corals, i.e., Lobophyllia, Symphyllia, and Trochopsammia, and the octocoral genus Heliopora have hitherto been known only from the Quaternary. The above evidence suggests that the age of the coral assemblage is Quaternary (Pleistocene). Since very few Miocene and Pleistocene corals have been reported previously from the Xisha Islands, a brief description of these fossils is warranted in the paper.