【摘要】 西沙群岛灰砂岛是晚玉木冰后期在各环礁的宽广礁坪上堆积的砂岛,堆积物质为礁碎屑和生物砂、砾.它的发育与新构造运动、季风风浪、海平面变化、礁坪的发育和环礁礁湖的变迁等有密切的关系.对它的研究丰富了生物礁的内容,为生物礁经济资源的开发提供有益的资料,并为礁相油气田的研究提供将今论古的依据.更多还原

【Abstract】 A sand cay is typified by a sand island accumulated by biogenetic debris (sand and gravel) on the annular reef flat of the present coral atolls. There are thirty sand cays in the Xisha Islands. They are distributed mainly over Xuande,Yongle and Dongdao atolls etc.. Of the sand cays, the YongXing Island is the bigest one with an area of 1.85km2 and a height of 8m. Generaly, the area of a sand cay is less than 1km2.A complete sand cay is composed of the following three elements:1. a sand terrace, which is accumulated with coral, shell, algae and fora-minifera debris. The sarid terrace being 1-3m above the sea level decides the shape of the sand cay.2. a sand or gravel bar, with a height of 3-6m above sea level and a width from some metres to some hundred metres, circling continually the sand terrace. In a sand cay there is one or a few sand bars extending in parallel. The height of the sand cay is determined by those of sand and gravel bars.3. a lagoon or depression, which is located in the centre of the sand terrace or between two sand bars. When it is dry, it is called a depression; when water fills the depression, then it becomes a lagoon. The salinity of lagoon isup to 50.76‰ in the Shehang Island and 7.33‰ in the Dong Island.A sand cay with the three elements is a complete one. It the three elements occur incompletely in the sand cay, this kind of sand cay is either growing or being destroyed.The biogenetic debris consisting of the sand bar is often well rounded and well sorted, in which the content of the foraminifera is about 42-35%, and that of coral debris is 22-23% together with some shell and algae debris. In the sand bar, scouring cross-stratification has well developed. The biogenetic debris in the sand terrace and the lagoon or the depression is mainly composed of coarse grained elastics of coral, shell, algae and foraminifera etc..Relationship between the sand cay and the NE-going monsoon is so intimate that the configuration of littoral dunes (embryonic sand cay) is subject to the NE monsoon. The sea-level fluctuations and environmental changes as well as the migration of life can be reflected in the sand cay.If the above mentioned three elements are found in ancient reef limestone, then the ancient sand cay can be determined.更多还原