High Frequency Lake Level Fluctuation And Its Correspondence With Theabundance Of Organic Matter In Lower Nenjiang Formation Of Songliao Basin, NE China

The Songliao Basin suffered a massive transgression during sedimentation of thelower Nenjiang Formation. Well-preserved Cretaceous lacustrine deposits provide aunique opportunity to study lake level fluctuation, which contains abundantinformation about paleoenvironment and paleoclimate. However, the detailed lakelevel fluctuation is still less studied. Compared with SK-1south borehole and othersections in northeast China, on the basis of deposits description from Well J79in thewestern slope of Songliao Basin, a high-resolution lake level record curve and therelationship between lake level and the abundance of total organic carbon werereconstructed based on lithological combination, sedimentological analysis, organicgeochemistry and paleontology of the upper Yaojia Formation to lower NenjiangFormation.The lake has experienced several low-stand and high-stand cycles during thisperiod. The lake level rose during deposition of the lower member1and member2ofNenjiang Formation, but declined gradually in the middle and upper part. The furthersedimentary analysis shows that there are approximately ten major stages of lake levelfluctuation. Compared with SK1borehole in the central depression area and outcropin the southeast uplift area, it reveals that the Well J79lake level fluctuation curve issimilar to other wells and outcrop in long time scale, but the former one is morefrequent. According to the cyclostratigraphic analysis,24sixth order cycles,8fifthorder cycles and2fourth order cycles were identified. We also suggested that the lakelevel fluctuation was dominated by both climate and tectonic, of which, tectoniccontrolled in long time scale, but climate played a more important role in short timescale.A continuous profile of core samples were collected from the Nenjiang Formation J79borehole, aiming to characterize the organic matter in the sourcerock. Based on the analysis of total organic carbon content (TOC) and Rock-Evalpyrolysis, a high-resolution geochemical section was constructed. The result showsthat the abundance of organic matter displays a periodical variation on the profilewhich implies a similar variation in lake level fluctuation and sedimentaryenvironments. Furthermore, compared to the lake level fluctuation curve, it showsperfect correspondence with the abundance of organic matter. The correlation betweenTOC and HI suggests that the abundance of organic matter were likely controlled bythe burial and preservation condition of original sedimentary environments. It isworthwhile to emphasis that the organic rich sediments of the lower part ofNenjiang Formation are good source rocks with high TOC and HI, and the content oforganic matter in central depression area is higher than that in the edge of the basin.