【作者】 赵强；

【导师】 吴时国； 许红；

【作者基本信息】 中国科学院研究生院（海洋研究所） ， 海洋地质， 2010， 博士

【摘要】 本论文的研究工作基于西沙群岛现代生物礁沉积的科学考察和西琛1井部分钻井岩芯资料进行,研究内容主要包括西沙群岛现代礁区沉积相、钻井沉积相、钻井岩芯地球化学和成岩作用几个方面,主要采用的地球化学的手段包括碳、氧、锶同位素、常量元素、微量元素、电子探针、包裹体和阴极发光等方法。首先,对西沙现代礁区沉积相特征进行了详细的分析和研究,识别出原地珊瑚礁相(礁格架相)、礁缘砾石粘结岩相(礁顶相)、礁坪砂砾岩相、灰沙岛、礁前塌积相、礁后泻湖相和礁前浅海泥砂相几个亚相。利用岩性、古生物和地球化学特征,并结合西沙现代生物礁考察的成果,对西沙群岛海区全部4口生物礁钻井的沉积相进行了较为细致的划分,识别出滨、浅海生物碎屑滩相、礁格架相(包括原地珊瑚格架和礁缘藻粘结岩相)、泻湖相、礁坪相、灰沙岛相等几个亚相。对石岛风成沉积进行了重点考察,利用薄片、扫描电镜、碳、氧同位素、常量元素和微量元素等方法,对其岩石学特征和地球化学特征进行了详细研究。通过与石笋氧同位素曲线的对比,对石岛风成沉积与古土壤的形成年龄进行了精确标定。由老到新,石岛五期风成沙丘沉积分别形成于:58 500 a B. P.前,56 000~48 000 a B. P.,45 500~38 000 a B. P.,36 000~30 500 a B. P.,28 500~25 000 a B. P.;四期化石土壤层由老到新分别形成于:30 500~28 500 a B. P.,38 000~36 000 a B. P.,48 000~45 500 a B. P.,58 500~56 000 a B. P.。结合末次冰期背景、宣德环礁形态特征及东亚季风特征,建立了末次冰期背景下石岛风成沉积的相模式。成壤作用导致Al、Fe、Mn、U元素在土壤中相对富集,而Y、Cr、REE元素的在土壤层中的相对富集似乎与风尘或火山灰有关。碳、氧同位素对成岩作用反应敏感,淡水淋滤作用会导致碳、氧同位素的明显负偏,而白云化作用正好相反。西琛1井钻井岩芯碳、氧同位素具有相同的变化趋势,相关系数达0.88。西琛1井井深116m以下,钻井岩芯的87Sr/86Sr比值比同期大洋水明显偏高,可能与青藏高原的隆升有关。116m以浅,87Sr/86Sr强烈偏负,出现极低值,可能与海底安山质火山的喷发有关。西沙生物礁自中新世发育以来,记录了一系列古海洋、古气候事件:钻井岩芯中的白云岩段在区域上有很好的可比性,其成因与中新世末期的米辛尼亚(Messinian)事件和上新世中期的北极冰川的形成密切相关。西琛1井中新统757~788m碳、氧同位素的偏高与白云化作用无关,可能反映了南极冰盖重大扩张事件。西永1井上新世的钙质超微化石记录了一次迅速的海侵事件。第四系地层中碳、氧同位素曲线整齐划一的负偏与末次冰期的淡水成岩作用有关。上新世末期以来,礁坪相、礁格架相和灰沙岛相逐渐增多,说明环礁发育更为成熟,反应构造沉降趋于稳定。中新统上部白云石晶体的电子探针线扫描图的Ca、Mg含量成波状起伏,显示由内而外白云化程度的差异,此一特征与阴极发光的环带构造相对应。白云石矿物流体包裹体具有高温、宽盐度的特征,并据此提出了地热对流驱动白云化模式。更多还原

【Abstract】 This paper is mainly based on the sedimentary survey of modern coral reef of Xisha Islands and parts of drilling cores of Xichen 1 well. Making use of many geochemistry techniques, such as the carbon, oxygen and strontium isotopes, major elements and trace elements, electron microprobe, inclusion and cathodeluminescence, the modern reef facies and the drilling well facies of Xisha Islands, the geochemistry of drilling core and the diagenesis have been studied.First of all, the characteristics of modern sedimentary facies have been particularly researched, and then , they were used to classify the drilling facies. Seven subfacies, such as in-place reef facies (reef frame facies), gravel bindstone facies in the margin of the reef, reef platform facies, sand cay facies, colluvial fore-ree f facies, backreef lagoon facies and sand-mud facies fore reef in shallow sea, have been identified in the modern sedimentary facies in Xisha Islands waters. Except the colluvial fore-reef facies, the other facies have been identified in the core of four drilling wells in Xisha Islands. And the bioclastic bank on shore and off shore also have been identified in the drilling cores.The aeolianite of Sand Island has been surveyed, by the way of thin section, scanning electron microscope (SEM), carbon and oxygen isotope, major elements and trace elements, the characteristics of petrology and geochemistry. The oxygen isotope curve has been compared with the oxygen isotope curve of cave stalagmites, and the acureate formation time of the aeolian dunes and palaeolsols have been confirmed. Following the time sequence (from older to newer), the five stages of aeolian dunes had been formed during the next five stages: 73 000~58 500 a B. P., 56 000~48 000 a B. P.,45 500~38 000 a B. P.,36 000~30 500 a B. P.,28 500~25 000 a B. P.;and the four stages of palaeolsols had been formed during the next four stages: 30 500~28 500 a B. P.,38 000~36 000 a B. P.,48 000~45500 a B. P.,58 500~56 000 a B. P.. By combining the background of the last glaciation, the morphological feature of Xuande Atolls and the Eastern Asian monsoon, the sedimentary facies of aeolianite has been set up.The pedogenesis make the elements of Al, Fe, Mn and U concentration in the palaeolsols. Although the elements of Y, Cr and REE(rare-earth element) seems have little relationship with pedogenesis. We induce that such element may come from the windblown dust or volcanic ash.The carbon and oxygen isotopes are sensitive to the diagenesis, and they have the same variation trend. In the fresh water environment, they tended to negative bias obviously, and in the dolomization environment, they tend to positive bias obviously. The correlation factor between the carbon and oxygen isotope in Xichen 1 drilling cores is 0.88.Below the depth 116m of Xichen 1 drilling cores, the values of 87Sr/86Sr of drilling cores are obviously higher than the sea water at that time, which reflected the uplift of the Qinghai-Tibet plateau. Above 116m, the value of 87Sr/86Sr occurred many very low values, this may has a close relationship with the submarine volcano activities.According to the lithology, palaeontology and geochemistry characters, all the four drilling cores of Xisha Islands have been classified, and systematic comparative study has been done. We found that many important palaeo-oceanography events have been recorded in drilling cores. The dolomite in the drilling cores could well be compared, and their genesis is close to the Messinian event and formation of Arctic glacier. The carbon and oxygen isotopes looks obviously positive bias during the section 757~788m in Xichen 1 drilling well. This event has little relation with dolomization, we induced that it recorded the important enlargement of Antarctic glacier. The Calcareous nannofossils in Xiyong 1 drilling cores recorded a fast transgression event happened in the early stage of Pliocene. The obvious negative bias of carbon and oxygen isotope in Quaternary stratas recorded the fresh water eluviation event happened in the last glaciation stage. Since the later stage of Pliocene, the contain of reef platform, reef framework and sand cays has gradually increased, which show that the atoll reef became more mature, and the tectonic subsidence became more slowly.The content of Ca and Mg in the dolomite mineral looks fluctuated in the electron spectrum line-scanning-maps, which show the different state dolomization from inside to the outside of the dolomite minerals. We believe that, the difference of dolomization in dolomite mineral coresponded to the zonal structures in the cathodeluminescence figures. According to the fluid inclusion characteristics of dolimite minerals, the geoheat driving dolomization model has been built up.更多还原