Depositional Environments And Inter-annual Paleoclimatic Characteristics Of The Fine-grained Sedimentary Rocks Of The First Member Of The Nenjiang Formation In The Late Cretaceous Songliao Basin

With the increasing of CO2 concentration,the enhancing of the greenhouse effect may force the earth to enter a global greenhouse climate sate.The Late Cretaceous is a Prototypical example of a past greenhouse period,and therefore paleoclimatic and paleoenvironmental studies of this period provide valuable constrains on future climate predictions.Most records of climate in the Late Cretaceous are derived from marine sediments,and knowledge of terrestrial climate changes is limited due to the fragmentary geological records on continents.The Sk1 and Sk2 Continental Scientific Drilling Project in the Songliao Basin provides a nearly continuous terrestrial record of the Late Cretaceous for the reconstruction of paleoenvironments and paleoclimate of the Late Cretaceous.Paleoenvironments of fine grained sediments sedimentation and preservation are favorable for the recording of paleoclimate.The first group of Nenjiang Formation,mostly consist of thick dark laminated fine grained sediments,was deposited during the maximum enlargement of the lake transgression.Sand,silt and clay being taken as the three units,clastic sediments,whose diameter below 2 mm,are classified into six lithologies: sandstone,sandy mudstone,silt,clay siltstone(coarse mud),silty claystone(medium mud)and clay(fine mud).4 sorts of fine grained sediments,silts,clay silts,silty clay and clay,are recognized in the first member of Nenjiang Formation(N1).4 sedimentary structures,massive bedding,normal graded bedding,horizontal bedding and lamination,are detected,as well as microfracture,bioturbidite,flame structure and convolute bedding.5 depositional environments,deep lake,half deeplake,muddy are recognized,and N1 is subdivided into 6 depositional sequences,which are coinsistent with the counting curve variation of S.Each sequence show the transition of deposition environment from shallow to deep water,and of the oxygen content of bottom water from low to high,which might be beneficial for the sequences subdivision and for the building of isochronous frameworks.Detailed paleoclimate information are recorded in laminated Fine Grained Sediments.Precise timescale could be setup vie varve studies of laminated fine grained sediments,which is helpful for the understanding of annual scale climate variation.Varve thickness was influenced by the annual precipitation variation.Varve thickness time series spectra structure is similar to the Modern ENSO spectra structure,with obvious 2-7 year periodicities.Numerical simulation provides the basis for the modern climate prediction,as well as cross-test with paleoclimate reconstruction.CESM is the mostly used model for climate simulation,especially for annual climate variation.CESM is used in this study with paleotopography of 90 Ma and paleoclimate parameters of Cretaceous.Simulation results indicate strong 2-7 year periodicities in the east equatorial SST variation and precipitation variation,which is similar to the modern ENSO as well as the ENSO signals from varved sediments.The coherence of ENSO signals in geological records and numerical simulation conform the existence of ENSO in Cretaceous.