西沙群岛琛科二井中新世和更新世的一些珊瑚

Some Miocene and Pleistocene corals from Well Drilling CK-2 in Xisha Islands, South China Sea

提要西沙群岛琛航岛琛科二井(CK-2)有878.3 m厚的新生代珊瑚礁灰岩(Cenozoic reef limestone)。新生代珊瑚礁灰岩自上而下包括了第四系的全新统和更新统以及新近系的上新统和中新统。其中,中新统厚514.3 m,产19属石珊瑚(Scleractinia corals)。更新统厚215.6 m,产21属石珊瑚(Scleractinia)和1属八射珊瑚(Octocorallia)。珊瑚是营底栖固着生活的动物,在地层断代方面它没有能像一些浮游动物(例如浮游有孔虫等)那么“精准”。但也有一些珊瑚的地质历程相对比较短暂的,它们在地层划分和对比方面还是能起到重要作用。本研究在琛科二井井深364–878.3 m的中新世地层中鉴定出有若干只见于中新世或者常见于中新世的珊瑚,如:星日珊瑚(Astrhelia)、安的列斯珊瑚(Antillophyllia)和大安的列斯珊瑚(Antillia)等。另外在井深21.4–237 m的更新世地层中也发现了数只限于第四纪的珊瑚,如:叶状珊瑚(Lobophyllia)、合叶珊瑚(Symphyllia)、轮沙珊瑚(Trochopsammia)和苍珊瑚(Heliopora)等。上述珊瑚的时代分布与有孔虫生物地层鉴定的结果以及古地磁和同位素年代地层测定的数据完全吻合。珊瑚除了在地层的划分对比方面能够起到一些关键作用外,更重要的是它在恢复和解释古地理、古气候和古生态等方面也都具有非常重要的意义。

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.