藏北羌塘盆地侏罗纪黑色岩系地层的全球对比-基于颗石藻化石和高分辨率碳同位素地层学

The marine Jurassic strata from Qiangtang Basin, northern Tibet, are very important in the eastern Tethyan region, especially the coccoliths-bearing black rock series from Shuanghu-Amdo areas which are not only objects about petroleum exploration, but ideal area to discuss the Jurassic palaeo-oceanography. However, it is too difficult to undertake the regional and/or global Jurassic stratigraphic divisions and correlations because of absence of standard ammonites and occurrence of coccoliths only in the middle to upper parts. Therefore, this project will focus on the two sets of coccoliths-bearing black rock series, take on coccoliths identification and their characteristics including abundance, preservation and diversity from the lower to upper parts of the two profiles. In addition, the high-resolution carbon isotopes will be taken to analyze different carbon-isotope values. With regarding to the vertical changes and excursions, the factors will be discussed such as sea-level fluctuate, organic carbon contents, primary productivity, and so on. Based on the characteristics of coccoliths and carbon isotopes, we will study the biostratigraphic zones, especially the calcareous nannofossils-coccoliths, and correlate to those classic and famous profiles of Jurassic black shale in the Europe. Furthermore, the Jurassic strata may be divided in the Shuanghu-Amdo area, which will be useful to correlate to the Yanshiping Group in the Qiangtang Basin. Though detailed field works and experiments, this study can provide more information about Jurassic bio- and chemostratigraphy, heighten the correlation between these marine Jurassic strata in the Tibet and Europe. Additionally, this study can also show the palaeoceangraphy, palaeoclimate and palaeoecology.

藏北羌塘盆地海相侏罗纪地层在整个东特提斯域占有十分重要的地位，尤其是双湖-安多地区侏罗纪含颗石藻的黑色岩系地层，既是研究区油气勘探的重点，也是该时期古海洋学最理想的地层之一。由于缺乏建阶的菊石化石，加上颗石藻化石仅出现在剖面的中上部，从而使得区域乃至全球侏罗纪地层划分与对比极为困难。因此，本项目拟选择两套黑色岩系为研究对象，对颗石藻化石的属种进行鉴定并统计其生态学特征（保存、丰度、分异度等），建立化石分带序列。同时采用高分辨率的碳氧同位素地层学，系统分析碳氧同位素时空分布特征及其受控因素（海平面变化、有机碳埋藏、初级生产力等）。在此基础之上，将古生物与化学地层相结合，与欧洲侏罗纪经典剖面进行对比，进一步划分和对比研究区侏罗纪地层，加强我国海相侏罗纪地层和西欧侏罗纪典型地区的对比；进一步揭示这个时期的古海洋、古气候和生物演化。

藏北羌塘盆地海相侏罗纪地层在整个东特提斯域占有十分重要的地位，尤其是双湖-安多地区侏罗纪含颗石藻的黑色岩系地层，既是研究区油气勘探的重点，又是该时期古海洋学最理想的地层之一。由于缺乏建阶的菊石化石，加上颗石藻化石仅出现在剖面的中上部，从而使得区域乃至全球侏罗纪地层划分与对比极为困难。因此，本项目选择了双湖和安多两套黑色岩系为研究对象，对颗石藻化石的属种进行了鉴定并统计其生态学特征，建立了化石分带序列。同时采用高分辨率的碳氧同位素地层学，系统分析碳氧同位素时空分布特征及其受控因素。在详细的野外地质调查基础上，采集了大量样品，系统开展了古生物学、元素地球化学、有机地球化学和同位素地球化学工作，取得如下重要认识：（1）地层时代的确定：两个剖面发现69个样品含有颗石藻化石，主要为Watznaueria britannica 和大小不等的 W. manivitiae以及新属种：Stephanolithion hexum, Octopodorhabdus decussatus, Biscutum dubium, B. erismatum, Lotharingius, Holococcolith, Schizosphaerella等，并建立了颗石藻化石带：NJ12 Ansulasphaera helvetica， 其时代为Late Bathonian- Callovian期，有可能延伸至basal Oxfordian；（2）古海洋学意义：元素地球化学（含量以及特征比值）以及颗石藻化石特征表明，表层水营养级别逐渐增加，为中等—富营养。这种变化趋势与特提斯洋边缘西北部Callovian-Oxfordian地层相似。同时也表明东特提斯洋与西北部表层水生物生产率增加主要受到气候条件影响。另外，高生物生产率可能是西藏或者其它地方形成中侏罗世富含有机质沉积物的最主要控制因素；（3）有机地球化学特征：有机质主要为藻类，伴有少量的陆源高等植物，有机质类型为Ⅱ型干酪根，成熟度中等，为较好的烃源岩；（4）同位素地球化学：碳同位素曲线图显示，在剖面底部到中部，碳同位素值发生了正偏移（接近2.17‰ PDB）。而氧化-还原较为敏感的元素如Mo，V，Ni，Cr和 U等富集，加上颗石藻Watznaueriaceae勃发，认为该套黑色页岩可能与表层水温度较高、海平面上升以及表层生物生产率增加有关。