Abstract: Continuous Paleogene marine deposits are outcropped in southern Tibet. Yadong area in southern Tibet was once located in the Tethyan residence sea basin and developed the youngest wellexposed marine sediments (the highest marine layer, HML). The age of the HML stands for the final closing age of the residual basin. The study on HML in this area can provide direct information about the late evolutionary history and the closing age of eastern Tethys. Based on the detailed analysis on the calcareous algae fossils from the Gulupu section in Tüna of Yadong area, eleven species of seven genus of Corallinaceae, four species of four genus of Dasycladaceae, one species of one genus of Codiaceae have been identified and three assemblages have been recognized from the Zongpu Formation and Zhepure Formation, which are ranked from bottom to up as follows: Lithoporella melobesioidesLithophyllum carpathicum assemblage, Lithoporella melobesioidesOvulites margaritula assemblage, Distichoplex biserialisJania denotata assemblage. Based on the study of the characteristics of calcareous algae fossil assemblages, it is inferred that during Late Paleocene to Eocene Tüna region should be located in a shallow shelf environment and the depth of sea water might gradually decrease to be less than 30 m until Late Eocene.

Key words: Yadong in southern Tibet, Paleogene, calcareous algae, sedimentary environment