西藏仲巴地区白垩纪末期—始新世早期海相地层

LATEST CRETACEOUS TO EARLY EOCENE MARINE STRATA IN THE ZHONGBA REGION, TIBET

西藏仲巴县北部地区出露有晚白垩世至古近纪的海相地层 ,本次工作新测制了卓勒剖面 ,并对原错江顶剖面上部地层做了再次研究。地层中化石丰富 ,据有孔虫化石研究结果重新厘定曲下组时代为古新世早期、加拉孜组上段属始新世早期 ,认为该区白垩 /古近纪界线位于曲贝亚组与曲下组之间。在这一界面上 ,古新世磨拉石直接覆于晚白垩世的陆棚碳酸盐台地沉积之上 ,其间存在沉积间断 ,为弧前盆地演化后期的重大沉积转变。古新世早期曲下组为近海相磨拉石沉积 ,古新世晚期至始新世早期加拉孜组为残留海盆沉积。加拉孜组顶部为该区最高海相地层 ,其上为冈底斯群的磨拉石不整合覆盖。冈底斯群的时代应晚于始新世中期。

The marine Cretaceous to Eocene stratigraphic sequence crops out in the Zhongba region along the northern side of the Yarlung Zangbo Suture Zone. Two sections involving uppermost Cretaceous to lower Eocene strata have been studied in detail. The Zhuolei Section containing the uppermost Cretaceous to Paleocene strata is a new section recently studied by the authors. The Cuojiangding Section was measured by Liu Cheng-jie et al. (1988) and the interval beds between the uppermost Cretaceous and lower Eocene were re-measured and re-studied as part of this project. The uppermost Cretaceous strata of the Qubeiya Formation in the Zhongba region is predominantly represented by carbonate platform deposits. Coarse-grained sandstone and conglomerate that was named the Quxia Formation were deposited directly on the top of the carbonate deposits. The stratigraphic age of the Quxia Formations provides key information about the location of the Cretaceous/Paleogene boundary. This formation is lack of fossils. By biostratigraphic correlation with the Gamba-Tingri area, the unfossiliferous part of the sequence can be restricted to the correlative beds containing a Rotalia-Smoutina-Lockhartia fauna horizon. Thus, the presence of foraminiferal faunas of Lepidorbitoides and Rotalia-Smoutina-Lockhartia marks the K/E boundary. In previous work, the Jialazi Formation was limited in the Paleocene (Liu Cheng-jie et al., 1988). In this work, larger foraminiferal faunas have been re-studied. The Miscellanea-Daviesina fauna found in the lower and middle parts of the formation indicates a Paleocene age, and its disappearance marks the end of the Paleocene. The upper part of the formation yields Nummulites-Discocyclina fauna of early Eocene age. During the Late Cretaceous the forearc basin along the Yarlung Zangbo suture evolved into a shallow marine basin as a consequence of continental convergence. This is concluded from widespread occurrence of Campanian and Maastrichtian shallow water carbonate platform, which in places encloses rudist reefs and bioclastic shoals with abundant larger foraminifers. In the Zhongba region, the Danian terrigenous conglomerate rests on the Maastrichtian platform carbonates, providing evidence that at the K/E transition time, the Zhongba region was tectonically uplifted, which culminated in a short hiatus in deposition. We suggest that the continental collision between India and Asia was the driving force behind major eustatic sea level drop, widely observed in early Danian time in Tibet. The Zhongba region was a remnant basin during the late Paleocene and early Eocene. It was filled up by molasse deposits since the middle Eocene.