EARLY TERTIARY CALCAREOUS NANNOFOSSILS FROM SOUTHERN TIBET AND THE CLOSING TIME OF EAST TETHYS IN TIBET

Abstract

The Tertiary marine strata occur mainly in southern Tibet as well as in the Gandies Mountains of Tibet. The Tertiary strata of southern Tibet can be divided lithologically into three formations: the Jidula Formation, Zongpu Formation and Zhepure Formation. The Zhepure Formation is the highest marine stratum in Qinghai Xizang(Tibet)Plateau.Twelve samples containing calcareous nannofossils were collected for this study from the upper shale member of the Zhepure Formation, in Qumiba section, Dingri County of southern Tibet. According to Martini's Zonation (1970), and Okada and Bukry's Zonation (1980), three nannofossil assemblages are established based on the FAD or LAD of some marker species. The first one is Nannotetrina fulgens Chiasmolithus gigas assemblage corresponding to the NP15 zone of Martini Zonation, the second one is Discoaster bifax Chiasmolithus solithus assemblage corresponding to the NP16 zone, and the third one is Sphenolithus pseudoradians Reticulofenestra bisecta assemblage corresponding to the NP17 zone. The third nannofossil assemblage is dated as late Middle Eocene on the basis of nannofossil characters,and the stratum yielding this assemblage is the highest marine bed in Tibet. This indicates that the sea water did not withdrawn from the Qinghai Xizang (Tibet) Plateau until about 38 Ma according to new data of Berggren et al. (1995) on the last occurrence of Discoaster bifax and the first occurrence of Chiasmolithus oamaruensis. The East Tethys closed in Tibet and the Qinghai Tibet Plateau rose up from the ocean dramatically at the same time.