High-freqency Lake-level Fluctuation In Numbers 1 And 2 Of Nenjiang Formation In Songliao Basin And Its Controlling Factors

The Cretaceous is an important "greenhouse" climate period in the Earth's history.The Organic-rich source rocks in member 1 and 2 of Nenjiang Formation in Songliao Basub provide a good continental sedimentary record for the Cretaceous climate change.A large body of evidence suggests that lake anoxic events(LAE2)occur at locations members 1 and 2 of Nenjiang Formation.And it is still a hot scientific hot issue that if there's marine transgression in the deposition period of bottom of Nenjiang Formation.In addition,Some scholars had introduced the term"limno-eustatic" to describe the effect of water volumes that are bound to groundwater and lake storage on sea-level fluctuations and cycles during major greenhouse phases of Earth history.They give the hyposesis that lake-level and sea-level fluctuations should be in an out of phase relation,i.e.a major marine sea-level lowstand should correspond to a lake-level highstand,and vice versa,which suggest that lacustrine mudstone deposite in the highstand period without marine transgression.In this thesis,the number and thickness of cycles of the natural gamma-ray logging from 15 wells of members 1 and 2 of Nenjiang Formation in Songliao Basin include the SK-1 Continental Scientific Drilling are revealed.The Fischer Plots based on these number and thickness of cycles are correlated from well to well to build up three to establish three connecting-well profiles to identify the high-frequency lake-level fluctuation in the whole basin.The lake-level is checked and the paleoenvironment is reconstructed with element contents measured by high reslution X-ray Fluorescense(XRF)core scanning scanner for core from the SK-1 Continental Scientific Drilling.The distributions of elements and elemental ratios as proxies for paleoenvironment.This reconstructed the evolution process of the ancient lake water environment in the Songliao basin in the late Santanian-early Campanian.At the lower part of the number 1 of the Nenjiang formation Al/K curve reflects low chemical weathering condition,which corresponds to the lake-level lowstand and sea-level highstand.Simultaneously,the proxies show a paleoenvironment with high salinity(Sr/Ba),strong reducing(Cr/V,S/Ti)and strong paleoproductivity(Cu/TI).Therefore,it reveals that the sustained marine transgressive event occures in bottom of number 1 of Nenjiang Formation which contols the lake-level and the paleoenvironment of Songliao Basin.After the end of the transgression,in the absence of transgressions,chemical weathering and lake-level changes in the opposite direction.Simultaneously,The trend in change of salinity and oxygen content is opposite to lake-level fluctuation,and the trend of productivity is the same as lake-level fluctuation.The lake-level and sea-level fluctuations show an out of phase relation between thenm.More over,orbital cycles of short eccentricity E2(123ka)and precession P1(22.5Ka)are revealed from the chemical weathering index(Al/K)in number 2 of Nenjiang Formation,indicating that changes of astronomically driven climate controls the lake-level,the paleoenvironment and the out of phase relation between the lake-level ang the sea-level fluctuaions(limno-eustatic).