南海北部深水盆地渐/中新世生物地层与沉积环境

The primary objectives of IODP Expedition 367/368 to the northern South China Sea (SCS) was to obtain sediment record and drill through the basement on the Continent-Ocean transition (COT), and to probe the evolution model and mechanism on the breakup process from continent rifting, rupturing to oceanic crust formation. The two cruises all obtained precious basement rock-basalt and thick sediments on it, However the biostratigraphy for the brown/red clay deposits just above the basement is general rough and seems not consistent with each other in the two COT sites. This age model for the brown/red layer in the two sites will play a significant role in the better understanding of the evolution mechanism of the rifted margin of the SCS, and also for the better understanding of sedimentary history of the deep SCS. In this study, through biostratigraphic work on planktonic foraminifera, deep-water agglutinated foraminifera, nannofossils and radiolaria and their faunal analyses on the brown/red clay deposits just above the basalts in two sites U1500 and U1502, combined with comparative analysis of different faunal distribution in the two sites, we try to built a more integrated fine biostratipraphy for the red deposits in this region. Furthermore, we will probe the variation of deposit environment corresponding to different sediments and discuss possible influence of regional tectonic movements on the evolution of deposit environment. This study will provide important scientific basis for the study of rifted margin of the SCS and also for the Oil and gas exploration in the deep SCS.

IODP367/368航次的首要目标是在南海北部洋陆转换带（COT）钻穿基底，探讨南海从大陆张裂到洋壳形成这一过程的演化模式与机制。该两个航次分别在南海北部COT的重要站位均钻穿基地，获取了洋壳玄武岩，然而船上初步研究对于紧邻基底之上的红层沉积的生物地层划分却很模糊，难以统一。该段沉积年代地层对于准确认识南海张裂陆缘演化机制将起至关重要的作用。本研究拟选取IODP367/368航次U1500和U1502两个重要站位，重点针对玄武岩之上的红层沉积物，通过相对精细的浮游有孔虫、胶结底栖有孔虫、钙质超微化石和放射虫的群落和年代地层工作，结合两个钻孔的生物群落深入对比分析，进一步较细致地建立该段红层沉积地层的生物年代地层框架，在此基础上进一步探讨深海红层形成所对应的沉积环境变化，分析区域大构造变动对沉积环境演化的影响。本研究将为南海北部陆缘张裂演化研究以及南海深海油气资源勘探提供重要的科学依据。

南海北部深海洋盆红层沉积的生物地层工作对于理解南海沉积环境演化与洋盆变迁有重要意义。通过对南海北部IODP367/368航次U1500、U1502和U1503三个站位玄武岩之上的红层沉积开展了较精细的浮游有孔虫、底栖有孔虫、钙质超微化石群落和锶同位素年代地层分析等研究工作，揭示了南海北部深海沉积的阶段性演化。根据浮游有孔虫和钙质超微化石生物地层和有孔虫Sr同位素年代，发现南海深海红层沉积均发育在约12Ma之前，可能与吕宋海槛的形成之前南海深层洋流的性质密切相关。沉积速率在此前后有明显变化，红层沉积沉积速率明显偏低，之后则有明显升高。在海槛形成前，太平洋深层流可以顺畅进入南海中，海盆深部为较强的氧化环境。海槛形成后深部环流发生显著变化，红层沉积消失。三个钻孔红层沉积红层的沉积厚度从陆架往中央海盆逐渐减薄，反映了沉积物物源主要来自于陆坡，随着往中央海盆水深加深沉积物的供给逐渐减少。三个钻孔红层沉积中的微体化石含量有非常相似的两阶段变化：下部红层沉积，碳酸盐含量高，浮游有孔虫和钙质超微化石丰度相对较高，浮游有孔虫以Catapsydrax dissimilis、Paraglobototalia mayeri、Paraglobototalia nana和Paraglobototalia opima等早中新世/渐新世浮游有孔虫为主。而上部红层沉积有孔虫、超微微体化石的含量非常低接近于缺失，表明受到了相同的物源变化影响所致。放射虫在红层沉积中基本缺失，推测与硅质生物的溶解保存密切相关。U1503钻孔中出现浮游有孔虫地层种的倒转出现现象，并且下部红层中浮游有孔虫多为重结晶壳体，有大量压扁的形变浮游有孔虫壳体出现，结合现代岛礁陆坡区再沉积研究，我们推测南海深海洋盆红层沉积中的浮游有孔虫可能为陆坡再沉积，其指示的年代要比红层沉积明显偏老。本研究为南海深部海盆沉积环境演化认识提供了重要资料。