南海东沙海域浅表层异常沉积层的生物地层年代厘定和形成机制探讨

The deposition process of shallow sediments in Dongsha area (Dongsha Islands and its adjacent area) of the South China Sea displays a very complicated situation. Seabed erosions, outcrops of older strata and allogene deposits are widely distributed in this area. Because previously studies mainly relied on seismic date, more evidences was required to prove the distribution, chronostratigraphic and formation mechanism of these abnormal deposits. Based on the methods of biostratigraphy and community structure analysis of foraminifera and calcareous nannofossils, the Neogene (from Miocene to Pliocene) deposits which were discovered in sea-surface samples of Dongsha area would be analyzed in this study to identify the geological chronology and forming mechanism of these abnormal deposits. The results would reveal the in-suit deposits and allogene deposits, and to reveal the age distribution, direction of odder stratum in Dongsha uplift area. Combining with the sediment microstructure analysis, AMS14C dating and δ18O curves, the chronostratigraphic framework and paleo-environments changes of core DH5 near the northeast of Taiwan canyon would be reconstructed, and the occurrence time, sedimentary types and material sources of these Neogene allogene deposits in this area would be inferred by the Sr-Nd isotope analysis, grain size distribution and mineral constituents. Finally, we will contrast the difference of causing mechanism between the outcrops in Dongsha uplift area and allogene deposits of core DH5. Above results will provide new significant evidences on the studies of stratigraphic division, bottom-water currents, regional tectonics, evolution of sedimentary environment and others foundational geology theories, and also have some significant meanings for further promoting on the oil-gas exploration.

南海东沙海域浅表层沉积过程复杂，海底剥蚀、较老地层出露和深水异地沉积现象广布，前期对上述异常沉积层的研究多以地震剖面分析为主，在其分布、年代及形成机制认识上急需实际站位沉积物样品年代资料补充。本申请拟对东沙海域海底含新近纪 (中新世至上新世)微体古生物(有孔虫和钙质超微)化石的沉积物样品开展生物地层年代学和群落结构研究，结合其他指标，确定这些异常沉积层的年代、分布特征及形成原因。相关研究在有效区分原地和异地沉积基础上，准确划定东沙群岛隆起区较老地层出露的范围、年代及展布规律；结合岩心细微结构观察、AMS14C测年、氧碳同位素分析，重建台湾峡谷东北侧DH5岩心年代地层框架和沉积过程；通过锶钕同位素、粒度和矿物组成分析，查明区域内上述异常沉积层的类型、源区和形成机制差异，为区域地层划分对比、底流搬运路径、物源分析、古环境演化研究等提供更为直观、可靠的新证据，对油气勘探开发等亦具有重要意义。

东沙群岛周边海域浅表层沉积物中异常沉积层广泛发育，这些异常沉积以新老地层化石混合出现，含大量近岸异地搬运种, 或钙质生物化石壳体破损严重为主要特征。采用微体古生物年代地层学和群落结构研究与沉积学相结合方法，可为有效判定其沉积类型、形成年代和形成机理等研究提供重要依据。相关研究对于深入理解从源到汇的沉积过程和水合物等油气资源开发具有重要意义。本项目对东沙群岛周边海域第四纪浅层含异常沉积层的多个表层及柱状样沉积物样品，开展了多门类生物地层年代学与群落结构、粒度组成与岩性分布、氧碳同位素与AMS14C测年等相关分析，综合确定了其沉积类型、形成年代及触发原因等。相关研究查明了东沙群岛周边海域表层沉积物中出现的新近纪露头的沉积年代为中新世至上新世22.5～3.7 Ma，构造隆升和底流剥蚀搬运是其形成的主要原因；确定了台湾峡谷中段DH5岩心中含新近纪化石的异常沉积层为多期次浊流沉积，MIS3以来的气候与海平面变化是该区域重力流沉积发育的主要控制因素；首次在东沙群岛东北部发现的钙质生物壳体强烈溶解现象，指示了水合物分解引起的地层失稳可能是该区域浊流沉积发育的突发诱因；对比研究表明，浊流等异地搬运沉积在东沙群岛东部更为发育，在浊流发育较弱的西南部，底栖有孔虫组合可以作为反映了区域古生产力变化的良好指标；在本研究中所发现和命名的两个新硅藻种类，可被推广应用于古环境演化研究中。相关研究结果进一步理清了该区域内新近纪地层遭受剥蚀出露的范围、台湾峡谷中部重力流沉积的形成与演化过程，对于深入理解重力流沉积的触发机制、底流剥蚀与搬运等沉积作用、水合物分解可能引发的地层失稳等环境效应研究等均具有重要意义。