长试管静置沉降实验结果对湖盆细粒沉积纹层成因的启示*

沉积结构与沉积构造的研究是目前细粒沉积学中的重要内容。同时,富有机质泥/页岩的纹层成因也一直是困扰人们的难题。影响湖盆细粒沉积物纹层形成的因素很多,其中盐度、有机质含量是非常重要的因素,尤其是油页岩在淡水与咸水环境均可发育,其纹层的形成与水体盐度有何关系需要深入探讨。通过长试管静置实验,模拟并观察在静水条件下,黏土矿物、富有机质泥质沉积物在淡水、微咸水及咸水3种湖盆水体环境中的沉降过程与沉降速率,通过实验观察绘制沉降过程曲线并计算沉降速率。研究表明: 在淡水中富有机质泥的沉降速率明显高于黏土矿物的沉降速率;而在微咸水与咸水中,黏土矿物的沉降速率明显比富有机质泥要高;同时,2种类型的细粒沉积物在3类水体环境中各自的沉降速率也存在不同的规律。通过分析实验结果,指出在不同水介质条件下,絮凝作用类型的差异与浮力作用的影响是造成细粒沉积速率差异的主要原因。而有机质、黏土矿物及水体盐度共同控制细粒沉积物的沉降速率,一旦这些条件发生变化,那么就容易形成类似或不同的纹层,尤其是有机质丰度与盐度变化时,更容易形成不同成分的纹层。因此,油页岩的形成除需要相对安静水体环境外,还与有机质丰度、黏土矿物含量、絮凝过程及水体盐度变化的综合响应有关,而并非单独与水体盐度相关。另外,受沉积作用影响,湖盆不同区域的细粒沉积构造类型有差异。

Implication of the large-tube settling experiment results on genesis of fine-grained deposition lamination of lacustrine basin

The study on sedimentary texture and structure is an important aspect in fine-grained sedimentology. At the same time,the genesis of laminar structure of organic mudstone or shale has always been a difficult problem. There are many factors affecting laminar structure of fine-grained deposits in a lacustrine basin,among which salinity and organic matter content are very important factors. Especially,oil shale can be developed in fresh or salt water environment. The relationship between the formation of laminar structure and water salinity needs to be further discussed. In this paper,through large-tube settling experiment,the depositional process and settlementation rate of clay minerals and organic rich argillaceous sediments in fresh water,brackish water and salt water were simulated and observed under hydrostatic conditions. Through experimental observation,the settlement process curve was drawn and the settlement rate was calculated. The research shows that the settlementation rate of organic rich mud in fresh water is significantly higher than that of clay minerals;in brackish water and saline water,the sedimentation rate of clay minerals is significantly higher than that of organic rich mud. At the same time,the sedimentation rates of the two types of fine-grained sediments in the three types of water environment also have different laws. By analyzing the experimental results,it is pointed out that the difference of flocculation type and the influence of buoyancy are the main reasons for the difference of fine particle deposition rate under different water medium conditions. Organic matter,clay minerals and water salinity jointly control the sedimentation rate of fine-grained sediments. Once these conditions change,it is easy to form similar or different laminae. In particular, when the abundance and salinity of organic matter change,it is easier to form laminae with different components. Therefore,in addition to the relatively quiet water environment,the formation of oil shale is also related to the abundance of organic matter,the content of clay minerals,the flocculation process and the comprehensive response to the change of water salinity,rather than the water salinity alone. In addition,in different areas of the lacustrine basin,fine-grained sedimentary structure types are different due to different depositional processes.