西沙七连屿海滩岩沉积特征和胶结成岩机理研究

The beachrocks, developed on the coast of Qilian Islets and Cays, Xisha Islands, represent a carbonate sedimentary formation on a typical biological coast of coral reef areas, which are of great research significance in understanding the coastal geomorphic development in coral reef area, the carbonate diagenesis in sea-land interactive setting, the paleoclimate, the paleogeography, the neo-tectonics, the sea-level change, and the reef-associated petroleum reservoirs. The proposed project will focus on the sedimentary characteristics and cementation/diagenetic mechanism of beachrocks around Qilian Islets and Cays. By GPS survey, tracing, measuring and mapping, the subaerial and submarine development and distribution of beachrocks will be determined. Using the succession and sedimentologic data of shallow drilling cores and measuring the natural section of beachrock outcrops, the depositional sequences of beachrocks will be established and the depositional background, formational conditions and developmental history of beachrocks will be traced. Based on the cement AMS14C dating, geomicrobiological testing of carbonate sediments, geochemical and Raman spectrum analysis of cementation and diagenetic minerals, the beachrock formation time will be fixed, the presence of organic compounds and microbes will be analyzed cementation and diagenetic characteristics of beachrocks will be determined, and the effects of sediment structure, coastal hydrodynamics and cementation/diagenesis on the formation of beachrocks will be evaluated. According to the field observations and comparative studies on the beachrocks developed in windward and leeward positions, under different hydrodynamic conditions affected by the monsoon, and correspondingly to the different geomorphic and sedimentary facies zones of coral reefs, the relations between beachrock formation and different sedimentary-ecological zonation development of coral reefs will be documented.

七连屿的海滩岩代表了典型生物海岸的碳酸盐沉积建造,其对研究珊瑚岛礁海岸地貌发育、海陆交互环境碳酸盐成岩作用、古气候、古地理、新构造运动、海平面变化以及岛礁稳定性及影响因素具有重要意义。申请课题拟开展七连屿海滩岩的沉积特征和胶结成岩机理研究, 通过GPS定位、追索、测量和填图确定海滩岩的露头和水下发育和分布; 运用浅钻岩芯获得的层序和沉积岩石学资料以及测制露头自然断面来建立海滩岩的沉积层序和追溯其沉积背景、形成条件和发育历史;通过胶结物测年、碳酸盐沉积的地质微生物测试、胶结成岩矿物的地球化学和拉曼光谱研究来确定海滩岩的形成时间、有机质和微生物作用、胶结成岩特征以及沉积物结构、海岸水动力特征和胶结成岩作用对海滩岩形成的控制和稳定性的影响；根据迎风和背风位置、季风影响的水动力变化以及珊瑚礁不同沉积相带对应的海滩岩的实地观测和比较研究分析海滩岩的形成与珊瑚礁不同沉积和生态分带形成和发育的关系。

从700多年前开始发育的西沙七连屿海滩岩是目前西沙群岛保存最好、沉积和早期成岩特征未发生改变、前人未曾开展研究的典型珊瑚礁区海岸带碳酸盐沉积物。研究工作以七连屿四个礁屿的海滩岩为研究对象，利用GPS野外定位详细记录并描述了七连屿不同礁屿海滩岩的野外露头特征；通过偏光显微镜、扫描电子显微镜和拉曼原位观测技术对岩石薄片的观察，确定了七连屿海滩岩的岩石碎屑成分、含量以及沉积物颗粒结构特征；同时借助碳氧同位素分析手段结合偏光显微镜及扫描电子显微镜的观测结果综合探讨了七连屿海滩岩胶结物的形态、成分、形成演化机制与影响因素。相比较于前人对海南岛和西沙其他礁屿海滩岩的研究，该项工作全面系统地描述和确立了西沙礁屿海滩岩的野外分布和沉积层序特征，揭示了礁屿上不同成岩位置海滩岩的沉积物颗粒组成、沉积结构及胶结特征，证实了微生物活动有关的颗粒泥晶化作用在海滩岩的早期成岩过程中起着重要作用，指出早期海水胶结是七连屿海滩岩的最重要胶结作用，普遍形成等厚环边状或簇状的针纤状文石胶结物，阐明了七连屿各岛屿不同位置海滩岩的成因机制和影响其形成和发育的主要因素，并将七连屿海滩岩与邻近地区和其他海域的生物海岸的海滩岩进行对比分析，总结出全球珊瑚礁区海滩岩特征的共性与差异，更进一步突出了海滩岩与其它碳酸盐沉积建造在沉积结构和胶结机制上的差异。海滩岩是海岸带抵御海浪侵蚀的天然屏障，其形成和演化过程以及发育机制的研究成果在沙岛和礁屿的海岸保护、地表碳酸盐岩形成机理、领海基线的确立和南海领土主权的维护方面都具有重要的理论和现实意义。