廊固凹陷古近系沙河街组古湖泊环境与有机质富集机制

Palaeolake environment and organic matter enrichment mechanism of Paleogene Shahejie Formation in Langgu Sag

通过对廊固凹陷古近系沙四—沙三下亚段泥岩样品的元素地球化学、有机地球化学和同位素地球化学分析,结合湖泊的发育演化和古气候变化特征,探讨湖泊演化过程中水体盐度、氧化还原条件和生产力变化及其有机质富集机制。廊固凹陷在湖泊形成期(沙四段)湖盆构造活动弱,气候干热,湖泊水体盐度高,易形成盐跃层,使底层水处于稳定的缺氧条件,表层水体古生产力较低,有机质富集程度低;在湖泊鼎盛阶段(沙三下亚段)盆地强烈下陷,气候暖湿,湖泊水体为淡水,底层水体为弱还原环境,由于地表径流带来大量营养物质,表层水古生产力较高。湖相沉积物中有机质的富集程度受古湖泊不同演化阶段生产力和氧化还原条件的控制。沙四段有机质富集主要受底层水缺氧环境控制,沙三下亚段有机质富集与表层水的高生产力密切相关。

According to the analyses of element geochemistry, organic geochemistry and isotope geochemistry of mud- stone samples from the 4th member （Es4） and the lower subsection of the 3rd member （ Es3 ） of the Shahejie Forma- tion in the Langgu Sag, combined with the characteristics of lake evolution and climate change, it was discussed in this paper the changes of salinity, redox condition and productivity as well as the enrichment mechanisms of organic matter. During the formation stage （Es4）, the lake was deposited in an inactive tectonic activity with arid climate, brackish water, stable water column stratification, water bottom anoxia and lower enrichment of organic matter. When the lake developed to the peak stage （ the lower subsection of Es3 ） , the basin subsided strongly with hu- mid climate, fresh water and weak reducing environment in the bottom water. The productivity of surface water was high triggered by a great quantity of nutrients inflowed by direct runoff. The enrichment of organic matter in lacustrine sedimentation was controlled by the productivity and redox conditions during different lake evolution stages. In Es4, the enrichment of organic matter was mainly controlled by the anoxie environment in the bottom water, while in the lower subsection of Es3, it was closely related to the high productivity of surface water.