The Explicit Lithofcies Division And Its Paleoclimatic Implications Of The First And The Second Member Of Nenjiang Formation Recovered From SK-1 Scientific Drilling Project

The large amounts of anthropogenic burning of fossil fuels sharply increased the CO₂ level,rising the global temperature.The Earth climatic system is deviating from current glacial into deglacial or even greenhouse state.Researches on the climate state under the conditions of the greenhouse in the geological time can provide unique insights into the future climate evolution.Cretaceous is a typical greenhouse period.The Songliao Basin(SLB)in China has deposited almost the complete Cretaceous strata which provide valuable geological materials for analyzing climatic status of greenhouse.The first member(K₂n¹)and second member(K₂n²)of Nenjiang Formation,deposited in the SLB,is a set of deep lacustrine fine mud deposits,and has well recorded the paleoclimate information.Therefore,this paper took these two members,recovered from the SK-1 drilling core in the SLB,as researching materials.Firstly,the lithofacies of Nenjiang Formation was detailed recognized and classified.Then,with the help of multivariate statistical methods,including principal component analysis and constrained cluster analysis,the paleoclimatic analysis was carried out using climate-sensitive proxies.The mudstone lithofacies of the first and second members of Nenjiang Formation are characterized by black color.In detail,the lithology in K₂n¹is dominated by dark-gray rocks but with green tones,which accounts for more than 72%of whole lithology.The lithology in K₂n² is also dominated by dark-gray and grayish-black rocks,but with higher ratio 91%.The 83%of mudstone in K₂n¹ is dominated by medium and coarse grain.The mudstone in K₂n²with coarse and medium grain size accounts for 97%of the whole lithology.The lithology of K₂n¹ can be divided into nine kinds of mudstone facies and one kind of non-mudstone facies.And the K₂n² is comprised of four kinds of mudstone facies and one kinds of non-mudstone facies.The latter acid-adding experiment indicates that the lithology of K₂n¹contains as high as 49%of calcareous components.While the lithology of K₂n²is mainly comprised of clay or siliceous components,together accounting for 96%of whole lithology.Thus,it is speculated that the lacustrine salinity changed continuously,and the whole climatic status was rather variable during the deposition of K₂n¹.While the lithology of K₂n²show little oscillation,indicating a stable climatic background.The first(K₂n¹)and second(K₂n²)member of Nenjiang Formation can be divided into seven and three lithofacies respectively based on the geochemical datasets.Compared with the chemostratigraphic results,which divided the first(K₂n¹)and second(K₂n²)member of Nenjiang Formation into terrigenous facies,redox facies,salinity facies and productivity facies.The boundaries of salinity facies of K₂n¹are consistent with the previous lithofacies boundaries previously divided by us,indicating that the accumulation of K₂n¹ is mainly affected by salinity conditions.While,the lithofacies boundaries of K₂n²show high consistence with boundaries divided based on redox facies,indicating the lower part of the second member of Nenjiang Formation(K₂n²)is mainly controlled by the redox conditions of deep water of SLB.Through detailed lithological,chemostratigraphic,and paleoclimatic analysis,it is concluded that the first member of Nenjiang Formation(K₂n¹)was deposited under a climatic condition of high frequent oscillation,and the lacustrine salinity changed continuously.While the second member of Nenjiang Formation(K₂n²)was deposited under a rather deep water and show stable climatic background.