Ostracod Biostratigraphy From The Member 2 Of Mingshui Formation Of SK1 And Its Constrain On The K/Pg Boundary

As one of the most important extinction events in geological history,the extinction events across the K/Pg boundary have attracted much attention during these years.Most of the records about biological,climatic and environmental changes related to the K/Pg boundary are from marine strata,while the phenomenon from continental strata is still less understood.Therefore,the comparison between marine and non-marine will help us to better understand this event.Songliao Basin is the largest continental basin in China.It has developed a whole set of Late Cretaceous to early Danian strata of continental facies.Successive stratigraphic records of Late Cretaceous-early Danian were obtained from the SK1 cores.A comprehensive study of SK1 includes paleontology,paleomagnetism and cyclostratigraphy etc.It has indicated that the K/Pg boundary might locat in the Member 2 of Mingshui Formation,but the precious position is still uncertain.There are relatively abundant ostracod fossils in the Member 2 of the Mingshui Formation,which can contribute to the studying the biological and environmental changes across K/Pg boundary.In this paper,about 12 genera and 23 species of ostracod were identified from the Member 2 of Mingshui Formation,SK1,and the Limnocythere dectyophora were identified from SK1 for the first time.Based on the characteristics and the vertical distribution of the ostracod,a detailed study of biostratigraphy has been carried out.Theree ostracod zones have been established: 1)Cypridea tuberculorostrata-Candoniella exila assemblage zone(515.51m-334.97m),2)Candona viriosa-Timiriasevia ovata assemblage zone(334.97m-329.36m),3)Limnocythere dectyophora assemblage zone(329.36m-299.09m).By comparing these ostracod assemblages with other area,it is considered that the first assemblage zone is dominated by Mesozoic species,the second assemblage is mixture of Mesozoic and paleogene,and the third assemblage is marked by the first appearance and upward continuation of Limnocythere dectyophora,which is flourished in the Paleogene to Quaternary Period.In addition,Mesozoic species begin to disappear at the bottom 329.36 m of the third assemblage.Therefore,the age of the third assemblage might be Danian.Together with other evidences,the K/Pg boundary of SK1 is preliminarily put at the bottom of the third assemblage zone(329.36m).The miniaturization effect is a widespread evolutionary phenomenon after an extinction event.Based on the quantitative analysis of the size and abundance of the ostracod which is of longer distribution and higher abundance,the phenomenon that some species became smaller near the K/Pg boundary have been abserved,suggesting a possible miniaturization.Based on comprehensive analysis of ostracod across the K/Pg boundary,it is concluded that the disappearance of the ostracod is a gradualprocess.We suggest that the factor for disappearance of the ostracod might be the evolution of the basin itself and the change of the lake environment.The latter factor perhaps also related to the changes of the global climate caused by the Deccan basalt eruption.