冰川型高频海平面旋回对细粒沉积岩形成的约束：以黔北五峰组-龙马溪组为例

The extraction of palaeoclimate-palaeoenvironmental information from "seemingly homogeneous" fine-grained sedimentary rocks and the study of their genetic mechanism are hot and difficult issues in sedimentology and shale oil and gas exploration. In Northern Guizhou, the Wufeng-Longmaxi fine-grained sedimentary rocks were deposited during the Ordovician-Silurian transition and recorded the processes of high-frequency glacioeustasy. Therefore, the Wufeng-Longmaxi Formations are ideal targets to decipher how high-frequency glacioeustasy controls the formation of fine-grained sedimentary rocks. This study, based on these strata, is started with the clues of coupling relationship among sea-level changes, hydrodynamics and palaeoenvironment. On the base of mm-cm scale lithofacies, analyses of elemental geochemistry, Nd istopes and magnetic susceptibility will be applied. According to the results above, changes of hydrodynamic conditions, evolution of palaeoenvironments, and high-frequency glacioeustasy as well as their spatial distribution will be reconstructed systematically. Subsequently, by analyzing the triggering mechanisms of high-frequency glacioeustasy on hydrodynamic conditions and palaeoenvironments, the constraints of high-frequency glacioeustasy on the formation of fine-grained sedimentary rocks will be clarified, and the depositional model of fine-grained sedimentary rocks controlled by high-frequency glacioeustasy will be established finally.

从“看似均质”的细粒沉积岩中提取气候-环境信息，研究其成因机制，是当今沉积学界和页岩油气勘探领域的热点、难点问题。奥陶纪-志留纪转折期发育的黔北五峰组-龙马溪组下段细粒,记录了全球性的冰川型高频海平面旋回过程，是破解冰川型高频海平面旋回对细粒沉积岩形成约束作用的理想窗口。本申请拟立足该套地层，以海平面变化-水动力-古环境间的耦合关系为线索，在毫米-厘米级岩相划分的基础上，结合元素地球化学、钕同位素和磁化率等测试手段，综合研究细粒沉积岩水动力条件变化和古环境特征演变，系统恢复冰川型高频海平面旋回及空间演化，阐明高频海平面变化对水动力-古环境的驱动机制，明确其对细粒沉积岩形成的约束作用，建立适合冰川型高频海平面变化背景下的细粒沉积岩沉积模式。

奥陶纪-志留纪转折期发育的黔北五峰组-龙马溪组下段细粒,记录了全球性的冰川型高频海平面旋回过程，是破解冰川型高频海平面旋回对细粒沉积岩形成约束作用的理想窗口。研究区目的层分为了7类岩相：生物扰动黏土岩、黑色硅质页岩、生物碎屑岩、黑色页岩、黑白云质粉砂岩、灰色泥质粉砂岩和深灰色泥质粉砂岩。目的层划分为了2个三级层序sq1和sq2，sq1包括海侵体系域和高位体系域，发育浅水泥质陆棚和深水硅质陆棚，平面分布稳定，sq2包括低位体系域、海侵体系域和高位体系域，发育浅水灰（泥）质陆棚、深水硅（泥）质陆棚和浅水粉砂质陆棚，平面上“南浅北深”特征明显。氧化还原参数（Mo和U）表明凯迪阶五峰组表现为从有氧环境到缺氧环境的整体变化特征，而其中的高频变化特征与高频海平面变化旋回相关（2-3个）。赫南特期观音桥组沉积环境为冰川发育鼎盛时期的欠还原环境，随后赫南特期末-鲁丹期初冰川消退造成的大规模海侵事件引起了无氧事件。Mo-U协变关系和Mo/TOC比值（高达32.2）显示整个黔北乃至上扬子地区为较为开放的弱局限环境。Cd/Mo 比值和 Co (ppm) × Mn (wt%) 综合判别图指示上升洋流并未在整个上扬子地区发育，仅在外陆棚发育季节性的上升洋流。沉积速率（Si/Al）的大体变化速率与氧化还原条件具有一致性，即厌氧与低沉积速率相耦合。赫南特期末-鲁丹期初活跃的广西造山运动引起了东部沉积速率的增加。热液明显促进了生产力的繁盛。本研究的结果显示古环境的纵向变化与高频海平面变化密切相关，而在空间上的梯度变化则与古地理和造山运动有关。本研究的技术方法体系可有效应用于其他海相细粒沉积岩。