Late Cretaceous Calcareous Nannofossil Biostratigraphy In Southern Tibet

The Cretaceous/Paleogene(K/Pg)boundary has important implications for global stratigraphic correlationand biological evolution studies.More than 350 K/Pg sections have been discovered around the world.However,no complete K/Pg boundary record has been found so far in the southern part of eastern Tethys.This study is based on the Zhepure section of the Dingri area in Southern Tibet.The calcareous nannofossils were examined and the biostratigraphic framework was established for the ZhepureShanpo and Jidula Formations providing a reference to determine the K/Pg boundary in the Zhepure section.The main results are as follows:1.Calcareous nannofossilsare abundant in the Zhepure section.A total of 37 species from 25 genera were identified.The cosmopolitan species Watznaueria barnesiae takes the absolute dominance,which is followed by Micula staurophora and Cribrosphaerella ehrenbergii.2.The first occurrences of four marker species(e.g.,Micula murus,Lithraphidites quadratus,Ceratolithoides kamptneri,and Micula prinsii)were recognized,based on which two calcareous nanofossil zones and four subzones(CC25b,CC25 c,CC26a,CC26b)were established.The age of the studied interval is 69 ~ 67.3Ma.3.The first occurrences of Micula prinsii(67.3 Ma)was found in the middle part of the Jidula Formation.The overlaid strata are barren of calcareous nanofossil.It suggests that the K/Pg boundary of this section is very possible to line between the upper part of the Jidula Formation and the basal limestone of Zongpu Formation.The specific positionneeds to be determined using other methods including geochemistry.4.The species richness of calcareous nanofossils is higher in the lower part of the ZhepureShanpo Formation,decreases at the boundary with the overlying Jidula Formation,and becomes barren in the middle of the Jidula Formation.This suggests a shoaling water depth from the top of the ZhepureShanpo Formation.A paleoenvironmental analysis ofthe calcareous nanofossil assemblages indicates a low surface productivity and cold temperature in the surface ocean.