四川盆地长宁地区下志留统龙马溪组下段深水页岩旋回地层学研究

The sequence stratigraphy of shales has always been weakly researched in sedimentology. Although the sequence stratigraphy of shales has been investigated through the biostratigraphy and chemostratigraphy correlation in recent years, its genesis mechanism and the dominating factors have been the subjects of only a handful studies. Succession of apparently homogeneous shales can contained significant information in the geological history evolution, and it is a perfect target for cyclostratigraphy research, and the correlations between the sedimentary cyclicity in shales of Lower Lungmachi Formation in Sichuan Basin and the orbital forcing period have been found. Therefore, this study take the black shale in Lower Lungmachi Formation of Lower Silurian in the Changning Area, Sichuan Basin as an example. High-resolution data (MS, TOC) as proxies to conduct a detailed cyclostratigraphic study. Time series analysis will reveal significant sedimentary cycles in Lower Lungmachi Formation which are orbital forcing cycles. The age of Ordovician-Silurian boundary is estimated by anchoring the floating ATS to an U-Pb age of bentonite from the Lungmachi bed, and also the graptolite zones. Two geostatistic methods were applied to find the correlations among multiply geochemistry and geophysics proxies and to test the proxy sensitivity to external climate forcing, the relationship between palaeoclimate cyclicity and eustatic change will be discussed. This study is expected to provide new data for cyclostratigraphy research in Early Paleozoic, and offer new insights for the sequence stratigraphy division of the deepwater shales

页岩的沉积层序特征是沉积学界研究的薄弱领域。目前国内外学者大多通过生物地层对比、化学地层对比来划分页岩地层，对其发育控制要素及沉积成因的分析涉及较少。页岩沉积环境蕴含着丰富的地史演化信息，是理想的旋回地层学研究对象。基于在四川盆地下志留统龙马溪组下段地层中所发现的与天文轨道周期相对应厚度旋回，本课题拟在长宁地区开展页岩的旋回地层研究。通过对高分辨率替代指标进行时间序列分析，找出长、短偏心率，斜率和岁差周期造成的地层旋回厚度，根据前人在长宁双河剖面划分的完整的生物化石带，及斑脱岩层的锆石定年，找到绝对年龄的控制点，建立高精度天文地质年代标尺。查明不同地球化学、地球物理替代指标间的内在联系并判断不同替代指标对气候变化的敏感性，揭示研究区早志留纪古气候、古环境特征及与海平面变化的响应关系。研究结果将为早古生代旋回地层学研究添加新资料，并对页岩地层的旋回划分提供“成因控层”的新认识。

奥陶-志留之交发生了全球大规模冰川事件，并发生了生物大灭绝，碳同位素漂移等重大地质时间，对这些事件的理解离不开精确的年代框架。在华南扬子地区，奥陶五峰组-志留龙马溪组沉积的一套富有机质黑色页岩具有巨大的勘探潜力。对这套页岩开展沉积层序研究，有机质富集主控因素研究，及其与关键地质事件的对应关系具有重要意义。本次研究对川东南地区长宁双河剖面五峰组-龙马溪组下段页岩开展旋回地层学研究。.对获取的高精度TOC数据序列进行MTM频谱分析和多窗口频谱分析，证明地层记录了良好的天文轨道信号。通过405kyr长偏心率滤波结果，建立了双河剖面的“浮动天文年代标尺”，估算出鲁丹阶和赫南特阶的持续时间分别为2.234和1.969Ma。顶界笔石带A. Ascensus带的首现面被厘定为志留系的底界面，为本次研究的锚点，估算出晚奥陶世生物大灭绝事件的持续时间约为0.45Ma，碳同位素漂移事件（HICE）的持续时间约为0.80Ma。从天文学角度验证了轨道周期变化和有机质富集之间的联系，并建立了两个有机质富集因素的对比假设：①有机质的初级生产力是长偏心控制的主要因素。在岁差幅度变化最剧烈的所对应的偏心率最大值处，频繁的气候变化引起了强烈的风化作用，在较强的季风降水作用下产生了含有大量养分的沉积物，导致有机质富集。②有机质的保存条件是长偏心控制轨道力下的关键因素。尽管大量的养分输入促进了季节性高生产力，但在长期干旱条件下，有机质被氧化分解。相反，季节性的减少提供了一个稳定的沉积环境，在偏心率极小的时期，有机物可以不受保存条件的好坏影响，并大量富集。通过对页岩沉积的层序地层划分，探索海平面最大值应与理论模型的偏心率最大值的相位关系，推测出405kyr的TOC滤波曲线和理论天文轨道周期偏心率曲线具有一致（而不是相反）的相位关系，从而得出第一个有机质富集主控因素的假设（偏心率控制初级生产力）更具有客观合理性。