湖相深水细粒物质的混合沉积作用探讨

摘要

沉积物的混合是自然界的普遍现象,湖盆中广泛发育的深水细粒沉积岩本身就是一类混积岩.以东营凹陷古近系沙河街组三段下亚段—沙河街组四段上亚段细粒沉积岩为研究对象,通过厘米级岩心观察描述、小型沉积构造解剖、岩石薄片观察、X射线衍射全岩矿物分析及微量元素测试等方法,研究细粒物质的产出状态和混合方式;分析湖盆流体、气候及水体性质对细粒物质迁移、混合、沉积的控制作用,尝试探讨细粒物质的混合沉积机制.结果表明,湖相深水细粒沉积岩既有物质成分上的混合,也有结构的混合,且在宏观与微观的不同尺度均存在混合;混合沉积作用有均匀混合、纹层叠置混合、不均匀团块状混合等类型.纹层叠置混合沉积是最主要的混合沉积方式,依据纹层接触关系、所占比例及矿物产出状态可分为泥砂—灰纹层叠置混合型、泥—灰纹层叠置混合型、泥砂粒序—灰纹层叠置混合型、灰—泥—云纹层叠置混合型等.湖相深水细粒物质的混合沉积作用类型多且往往受多个因素同时控制,气候及水动力条件是控制细粒混合沉积岩发育的最主要因素,不同流体(浊流和底流)作用下的细粒沉积物混合作用存在差异.气候引起湖水性质变化及水体分层,从而控制不同类型细粒物质的絮凝沉降及化学沉积.不同构造部位、不同水体深度的混合作用不同;相同构造部位在垂向上也存在混合沉积方式的差异.%The mixing of sediments is a common phenomenon in the nature.Fine-grained sedimentary rocks in deep lake are typical peperites.The authors take the example of fine-grained sedimentary rocks in the Lower Member 3 and Upper Member 4 of Shahejie Formation in Dongying Sag.Through core observation in cm-scale,detailed description of small-sized sedimentary structures,thin section identification,analysis of whole rock X-ray diffraction and trace elements test,fine-grained sediments occurrence and their mixed mode were studied,and the effect of basin fluid,climate and water properties on transfer,mixing and sedimentation of fine-grained rocks was analyzed.The mixing sedimentary mechanism of fine-grained rocks was discussed.The results indicate that fine-grained sedimentary rocks in lacustrine deepwater are mixed in both components and structures under macroscopic and microscopic scales.The mixing sedimentation can be classified into homogeneous,lamina-overlapped and heterogeneous mixings with lamina-overlapped mixing the major type of fine-grained rocks.Based on the lamina contact relations,proportion and mineral occurence,the lamina mixing sedimentation is divided into:Lamina-overlapped mixing of sandmud and limestone,lamina-overlapped mixing of mud and limestone,laminate mixing of mud-sand sequence and limestone,lamina-overlapped mixing of limestone,mud and dolostone.The types of mixing sedimentation of fine-gained sediments are often influenced by multiple factors at the same time,and the climate and the hydrodynamic conditions are the main factors.The mixing sedimentation of fine-grained sediments varied in different fluids (turbidity current and bottom current).The climate causes changes of physical and chemical properties of water as well as layering of water body,which controls different types of flocculating settling and chemical deposition of fine-grained sediments.The mixing sedimentation types vary in different structural positions and water depths,and are even different vertically in the same structural position.