【摘要】 <正> 西沙群岛是我国重要的生物礁区之一,岛屿星罗棋布,类型繁多.研究这些岛屿的类型及其特征,对判断礁区环境演变、礁体组合成生与发展有着重要的思义;对研究古生物礁的比较沉积学亦有现实意义.一、西沙群岛的自然地理概况更多还原

【Abstract】 The evolutiou of islands in the Xisha Archipelago is closely related to the growth of coral reefs.The coral reefs in the studied area can be put into three groups:1. Complex atolls: Consisting of several atolls with a lagoon at the center. Three complex atolls named Yongle, Xuande, Xuandedong (Eastern Xuande) are recognizable.2. Individual atolls: We have found five small atolls grown up on separated foundations with small lagoons at the center.3. Platform reefs: Those are the reefs with flat top. In this group two islands named Jinyin and Zhongjian are included.There are 31 islands in the area, 27 of them are investigated by the authors. Based on the statistic method it can be seen that all islands are grown up on reef flats with the axes parallel to the Jatters.No relation between the distribution of the islands and the size of reef flat has been found. However, most of the islands occur on the lee side of the pre-ferrential wind, this means that the wind has played a great role in the evolution of islands.It is believed that the reef-related islands are products of certain stages of the evolution of coral reefs.Criteria for island classification such as materials, cementation and consoli-dation of the materials, evolutionary stages of island, vegetation and exposure of basements are discussed. Of course, the names used before are also considered. Seven types of islands can be divided: 1. Destructive volcanic islands.2. Destructive Consolidated lime-sand islands.3. Constructive lime-sand islands.4. Transitional lime-sand islands.5. Destructive lime-sand islands.6. Destructive Consolidated lime-gravel islands.7. Destructive lime-gravel islands.Volcanism, wind process and action of tide and wave are main factors controlled the occurence and evolution of the islands mentioned above. Therefore, several models of the evolution of islands have been proposed.Based on the 14C dating, four stages of island formation are inferred as follows:1. Gaojianshi stage: Corresponding to the early Pleistocene about 2 m. y. ago.2. Shidao stage: Corresponding to the late Pleistocene.3. Holocene stage: It is Postujated that many islands are formed during the stage of stable sea level since 7000 y. BP.更多还原