Late Cretaceous-paleocene Palynological Stratigraphy And Climatic Evolution In LD17 Well Of Songliao Basin

The Songliao Basin is the biggest Cretaceous oil-gas generating lacustrine basin of China,preserving complete Cretaceous strata.It is the potential area which can determine the continental Cretaceous-Paleogene(K-Pg)boundary.Continental Scientific Drilling Project “SK1 Well” has achieved the preliminary outcomes.Its biostratigraphy,magnetostratigraphy and other evidences suggest that the K-Pg boundary lies in the upper part of Member 2 of the Mingshui Formation in Songliao Basin.Sporopollen fossils are the key identification of K-Pg boundary.Above 360.6m depth of SK1 Well,fossils are scarce and through nearly 100m(360.6-263.4m),making the location of the boundary uncertain to some degree.So,it is necessary to gain more information from adjacent boreholes,in order to improve the K-Pg boundary palynological stratigraphy of Songliao Basin.LD17 well is located in the west slope of the Songliao Basin,revealing strata from the Yaojia Formation to the Mingshui Formation.In order to determine the inferred position of the K-Pg boundary and end-Cretaceous climatic and environmental changes,sporopollen fossils in the Mingshui Formation of LD17 well are analyzed in this study.Three zonations are identified in ascending order: Taxodiaceaepollenites-Wodehouseia-Aquilapollenites,Schizaeoisporites-Pinuspollenites-Callistopollenites,and Pinuspollenites-Ulmipollenites-Tricolpollenites zones.The zone 1 lies in the lower-middle part of Member 1 of the Mingshui Formation and may be in the early Maastrichtian;zone 2 lies in the upper part to lower part of Member 2 of the Mingshui Formation and may be midde-late Maastrichtian.Zone 3 lies in the middle to upper part of Member 2 of the Mingshui Formation and may contain K-Pg boundary.It is deduced that an extinction event may happen in the middle to lower part of middle Member 2 of the Mingshui Formation and its upper part presents Cenozoic features.So,K-Pg boundary may be in the middle of Member 2 of the Mingshui Formation and the exactpositon between 209.85-193.54 m.Member 1 of the Mingshui Formation is rich in sporopollen fossils,which are carried by quantitative analysis.The result indicates a subhumid subtropical paleoclimate,with vegetation of theropencedrymion and shrub.A cooling and wetting event might happen in the middle part of Member 1 of the Mingshui Formation(early Maastrichtian).