Cretaceous Stratigraphy And Micropalaeontology Of Songliao Basin

Songliao Basin is the largest continental sedimentary basin during late Cretaceous in Asia,where yields the Songhuajiang Biota and Minshui Biota of strong endemic features. Based on the study of microfossils,51 fossil assemblages(or fossil zones) have been set up. According to new data of lithostratigraphy,chronostratigraphy,seismic stratigraphy,logging stratigraphy,magnetostratigraphy and event stratigraphy,the division schemes on Lower/ Upper Cretaceous boundary has been located,and the geological times have been referred to each formation.Under the revised stratigraphic succession,the relation between micropaleontological evolution and lacustrine environmental change has been generally discussed.The major scientific achievements of this study are as follows:1.Important angiospermae pollen fossils such as Quantonenpollis,Cranwellia,Scollardia, Lythraites,Xinjiangpollis,Gothanipollis,Triporopollenites and angiospermae fossils as Platanus septentrionalis,Trapa angulata have been obtained from the Quantou Formation. They are Late Cretaceous in age.The reasonable geologic time of the Quantou Formation may be later than 100Ma deduced by apatite fission tracks and zircon U-Pb analysis.The Lower/Upper Cretaceous boundary in Songliao Basin is,therefore,between the Quantou and Denglouku formations,instead of between the Quantou and Qingshankou formations.2.The dark mudstones and shales in the Member 1 of Qingshankou Formation are of high organic carbon and low diasteranes contents.Organic geochemical characters are illustrated by biomarkers of 28,30-bisnorhopane and gammacerane,and positive excursion of kerogen isotopes.It is suggested that the dark mudstones and shales in the Member 1 of Qingshankou Formation are the products of lake anoxic period.It corresponds to the oceanic anoxic event at Cenomanian-Turonian boundary in Cretaceous.3.Based on the study of biostratigraphy,chronostratigraphy,magnetostratigraphy and event stratigraphy,a correlation of the lacustrine sequence to marine standard has been proposed,i.e.,the Quantou Formation corresponds to Cenomanian stage,Qingshankou Formation correlates to Later Cenomanian-Early Turonian,Yaojia Formation to Later Turonian-Coniacian,Nenjiang Formation to Santonian-Campanian.Sifangtai and Minshui Formations to Maastrichtian. 4.Analysis of lithology,lithofacies and microfossils of Cretaceous in Songliao Basin, shows that shore and shallow lake facies yield the most abundant microfossil groups,and the predelta and deltaic distributary plain facies rich in fossils as well.In the shore and shallow lake areas during the high level periods(such as Qingshankou and Nenjiang sedimentary periods),the lacustrine biota are of high diversity and high abundance.The fossil ornamentations tend to be complicated and multiplicated.During the lower level period(such as Yaojia sedimentary periods),fossil assemblages are of lower diversity and lower abundance. The fossil ornamentations show relatively simple structures.