Abstract: At present, there are disputes on whether the degree of water retention at the turn of Ordovician and Silurian has an impact on the enrichment of organic matter. Based on the division of biostratigraphic framework, the paper analyzes the sedimentological and geochemical characteristics of shale in different periods in western Hubei, and explores the relationship between the degree of water retention and the enrichment of organic matter. The findings show that the turn of Ordovician and Silurian in western Hubei can be divided into three periods from early to late:pre-glacial period (WF2-WF3 graptolite zone), glacial period (WF4 graptolite zone-Guanyinqiao section), and post-glacial period (LM1-LM5 graptolite zone), all in deep water environment. The degree of water retention has no significant impact on the preservation conditions of organic matter; it mainly controls marine productivity and further affects the enrichment of organic matter. The depositional period of the pre-glacial WF2 graptolite zone was in the stage of platform-shelf transition, and the sea level was low. Meanwhile, the surrounding paleo-land and local underwater paleo-uplift were uplifted dramatically, the degree of water retention was the strongest (strong retention), and the nutrient exchange with the open sea was the weakest, resulting in the lowest level of paleoproductivity and the lowest TOC content (2.9% on average). After entering the depositional period of the WF3 graptolite zone, the sea level continued to rise to a high level, and the degree of water retention was weakened significantly (moderate retention). With the increase of paleoproductivity level, the TOC content increased (3.6% on average); although the sea level was the lowest in the glacial period, the rising ocean currents were extremely active and the degree of water retention was the weakest. Plenty of nutrients flowed into the Yangtze Sea Basin with upwelling currents, resulting in the highest level of paleoproductivity and the highest TOC content (4.6% on average); in the post-glacial period, the climate warmed up, Gondwana glacier melted rapidly, the tectonic activities remained stable, the sea level rose significantly to a high level, the degree of water retention was moderate, the paleoproductivity was high, and the TOC content was high (4.0% on average).

Key words: Middle Yangtze, black shale, Wufeng Formation-Longmaxi Formation, degree of water retention, enrichment of organic matter

摘要： 奥陶纪—志留纪之交的水体滞留程度是否对有机质富集产生影响目前还存在一定的争议。在生物地层格架划分的基础上，分析了鄂西地区不同时期页岩沉积学及地球化学特征，探讨了水体滞留程度与有机质富集的关系。研究结果表明：鄂西地区奥陶纪—志留纪之交由早至晚可划分出冰期前（WF2—WF3笔石带）、冰期（WF4笔石带—观音桥段）和冰期后（LM1—LM5笔石带）3个时期，均处于较深的水体环境中。水体滞留程度对有机质保存条件无明显影响，主要控制海洋生产力，进而影响有机质富集：冰期前WF2笔石带沉积期处于台-棚转换期，海平面相对较低，同时周缘古陆和局部水下古隆起剧烈隆升，水体滞留程度最强（强滞留），与外海营养物质交换能力最弱，导致古生产力水平最低，TOC含量最低（平均为2.9%）。进入WF3笔石带沉积期后，海平面持续上升进入高位，水体滞留程度明显减弱（中等滞留），古生产力水平升高，TOC含量随之升高（平均为3.6%）；冰期海平面虽然处于最低位，但上升洋流异常活跃，水体滞留程度最弱，营养物质随上升洋流大量涌入扬子海盆，导致古生产力水平达到最高，TOC含量最高（平均为4.6%）；冰期后，气候回暖，冈瓦纳冰川快速消融，构造活动稳定，海平面大幅上升进入高位，水体滞留程度中等，古生产力水平较高，TOC含量较高（平均为4.0%）。

关键词: 中扬子, 黑色页岩, 五峰组—龙马溪组, 水体滞留程度, 有机质富集