Zircon U-Pb Chronology Of Cretaceous Yingcheng Formation And Nenjiang Formation In The Songliao Basin

The Songliao Basin,as the largest lacustrine basin in China,has a complete sequence of the Cretaceous non-marine strata.The late Cretaceous Nenjiang Formation in the Songliao Basin has received extensive attention because of its abundant hydrocarbon source rocks and records with the largest lake intrusion and Cretaceous Normal Superchron(CNS).Predecessors only obtained an isotopic age from the lower part of Member 2 of Nenjaing Formation.The Nenjiang Formation / Yaojia Formation boundary is the starting time of the largest lake invasion events and sea transgression events in the basin,and the age is still in dispute because of the lack of accurate isotopic age.There are two major volcanic events in the Yingcheng Formation,which is an important part of the Late Mesozoic magmatism in the Northeast Asia.The time limit of the volcanic event in the Yingcheng Formation is determined to understand the temporal and spatial evolution of the regional tectonic-magmatic events and regional geodynamics process.With the first phase of the implementation of the Cretaceous Continental Scientific Drilling project(CCSD-SK-I),the Late Cretaceous integrated stratigraphy of the Songliao Basin has been completed.The comprehensive chronology of the Early Cretaceous in the Songliao Basin is to be carried out in the second phase of CCSD project(SK-II).The constructed comprehensive chronology can provide basic chronological data for other stratigraphic work in the basin,and allows the precise correlation of the terrestrial Upper Cretaceous of the Songliao Basin to marine strata as well as other continental sequences.The main purpose of the SK-II drilling is to expose the Lower Cretaceous to the basement.SK-II(e)has been drilled the cores from Member 1 and Member 2 of Nenjiang Formation,the lower part of Denglouku Formation,Member 3 and Member 4 of Yingcheng Formation,it provides excellent research materials for this article.We used 6 tuff layers from the core of SK-II(e)and X1-4,Yuewangcheng section and Yaojiachezhan section,and used those samples to carry onSIMS zircon U-Pb dating,and got some important stratigraphic boundaries ages.(1)The boundary of the Member 1 and Member 2 of the Nenjiang Formation is 83.1Ma-83.5Ma,the bottom of Nenjiang Formation deposits at 85.1Ma-85.5Ma.(2)The lower part of Member 3 of the Yingcheng Formation is 111.7Ma.These ages provide an accurate chronological constraint for the lower part of the Nenjiang Formation in the Songliao Basin,which provides important chronological constraints for the tectonic movement event,the end of the CNS and the largest lake invasion event.It provides the basis data for chronology work of the Yingcheng Formation.Moreover,these ages strengthen the correlation between Chinese Cretaceous geological events and global Cretaceous events.