湖相细粒沉积模式——以东营凹陷古近系沙河街组四段上亚段为例

Lacustrine fine-grained depositional model:a case study of the upper submember of the fourth Member of Paleogene Shahejie Formation in Dongying sag

为提出针对湖相细粒沉积岩的沉积模式,以"将今论古"为指导,选取与古近纪东营凹陷湖盆条件及细粒沉积具有较强可对比性的青海湖开展研究,分析细粒沉积组分、岩相、垂向及平面展布特征,总结沉积规律。结果表明:古代与现代湖相细粒沉积均明显受水深影响。组分中,石英及长石随水深增加而含量降低、粒度减小;有机质吸附于黏土矿物而含量增加。其在平面上呈圈层展布。湖相细粒碳酸盐沉积与海相碳酸盐沉积具有可对比性,借用碳酸盐补偿深度(CCD)这一概念来解释水深与碳酸盐的关系。基于组分上述特征,建立受控于"CCD"的湖相细粒沉积模式。在深湖环境("CCD"之下),碳酸盐欠饱和,发育灰黑色页状黏土岩;在半深湖区("溶跃面"之下,"CCD"之上),碳酸盐半沉淀-半溶解,自下而上依次发育深灰色页状灰岩、灰色页状黏土岩、灰色页状灰质混合细粒岩及灰色页状灰岩;在浅湖("溶跃面"之上),碳酸盐过饱和而沉淀,自下而上依次发育灰色页状白云质混合细粒岩、浅灰色页状黏土岩及浅灰色页状灰岩;在滨湖—干涸环境,发育浅灰色页状膏质黏土岩及滩坝粉砂岩。

To propose the depositional model for lacustrine fine-grained sedimentary rocks,Qinghai Lake was chosen as the research object to analyze the components,lithofacies,horizontal and vertical distribution characteristics of fine-grained sediments and summarize deposition laws under the guidance of uniformitaranism,which is comparable with the Paleogene Dongying sag in terms of lake basin conditions and fine-grained sediments.The results show that ancient and recent lacustrine fine-grained sediments are deeply affected by water depth.For the components,the quartz-feldspar content and particle size decreases with increasing water depth;organic matter is adsorbed on clay minerals,and thus its content increases.It presents circle layer distribution in horizontal level.A comparison can be performed between lacustrine fine-grained carbonate deposition and marine carbonate deposition,and the concept of carbonate compensation depth（CCD）is used to illustrate the relationship between water depth and carbonate.Based on the above component characteristics,the lacustrine fine-grained depositional model controlled by CCD was established.In the deep lake environment（below CCD）,carbonate is undersaturated,and gray-black laminated claystone is developed;in the semi-deep lake（below lysocline,above CCD）,carbonate is semi-deposited and semi-dissolved,while dark gray laminated limestone,gray laminated claystone,gray laminated calcareous mixed fine-grained sedimentary rock and gray laminated limestone are developed successively from bottom to top;in the shallow lake（above lysocline）,carbonate is supersaturated and then deposited,while gray laminated dolomitic mixed fine-grained sedimentary rock,light gray laminated claystone and light gray laminated limestone are developed successively from bottom to top;in the lakeshore-dry environment,light gray laminated gypsiferous claystone as well as beach and bar siltstone are developed;turbidity sediment can be found occasionally.