西藏白垩纪—早第三纪有孔虫与特提斯—喜马拉雅海的演化

CRETACEOUS--EARLY TERTIARY FORAMINIFERA 0F XIZANG (TIBET) AND EVOLUTION OF THE TETHYS-HIMALAYAN SEA

由于印度板块向北漂移,使特提斯-喜马拉雅海在白垩纪后期进入了衰退时期.在白垩纪—第三纪的不同阶段中,陆壳的升降运动造成了海平面的变化,形成了不同的生态环境。本文主要依据不同时期不同地区的有孔虫化石组合特征,分析了特提斯-喜马拉雅海在白垩纪至早第三纪的演变过程,揭开了青藏高原隆升的序幕。

Part of the Himalayas which lies between the Tibetan Plateau and theIndian Subcontinent belongs to the eastern part of the Tethyan Tectonic Realm,from where many data had been collected during the last decade (Zhang, 1981;Wan, 1987). It is generally acceptable that the Tibetan Plateau is composed ofdifferent continental blocks which were detached from Gondwana at differentgeological times. 1. Cretaceous Since the beginning of the Cretaceous, the so-called Himalayan Sea wassandwiched between the Indian and Eurasian plates. In the Early Cretaceous(140--100 m.y.B.P.) a deep sea trough extended along the Indus-Yarlun Zangbo(Tsangpo) belt where ophiolites and deep sea silicolites were deposited. TheHimalayan region extending toward the south of the deep trough was then acontinental slope on the north edge of the Indian Plate. The fact that the seawas rather deep is attested by its rare fauna of ammonoids, bivalves and algaein the black shale deposits of great thickness in this region. After the Mid- Cretaceous, the Yarlun Zangbo Trough ceased its sea-floor spreading and en-tered into its closing episoder this is evidenced by the great thickness of flyschdeposits resulted from the encroachment of the Indian Plate upon the EurasianPlate. The Himalayan region became a continental shelf on the northern edgeof the Indian Plate, due to the general elevation of the region. A prolific faunaof ammonoids, bivalves, both planktonic and benthonic foraminifera lived inthis suitable marine environment with deposits of siltstones, mudstones andlimestones. The sea still further shallowed in the latest Cretaceous (Maastrich-tian).Almost all of the planktons disappeared at this time, whereas the largerbenthonic foraminifera Orbitoidae as well as rudistid and inoceramid bivalvesflourished. A widespread regression occurred in the Himalayan Sea at about the endof the Cretaceous. The sea entered into its last evolutionary phase and graduallywithdrew from the area during the regression, although two transgressions wereinvolved in the Paleocene and Early Eocene. 2. Latest Cretaceous to earliest Paleocene regression During the Late Cretaceous, an oceanic environment existed in the Himala-yan region, which connected with the western Tethys and the Indo-PacificOcean, yielding a community of the Globotruncana, Orbitoides and Omphalocyclusfaunas (Wan, 1985; 1987). In the mid-Maastrichtian to early Danian interval,this region is characterized by marine oscillations, with a regression marked bythe deposition of arenaceous facies of varying thickness over the entire region.The age of the sandstones in different areas suggests that the end-Cretaceousregression proceeded from both east and west towards the Gamba--Tingri region.This regression was brought about by the tectonic events which led to the colli-sion of the Indian Plate with the Eurasian Plate, causing a general elevationalong the northern margin of the Indian Plate. The consequent elevation wasevidenced by the formation of both arms of the Himalayan Sea (Fig. 1). TheLower Indus and Baluchistan Basins in the west and the Assam Basin inthe east became shallower during the Middle Maastrichtian; abundant terrestialmaterial emptied into the basins, and the marine biota became restricted indiversity. Following the elevation of the Lower Indus and Baluchistan Basins,the Upper Indus Basin was influenced by the regression in the Late Maastrich-tian. The reaction to the elevation in the Zhongba Basin was even later, wherethe regression phase spanned the Latest Maastrichtian and Early Danian. Theregression did not reach the Gamba-Tingri Basin until the Early Danian, whenit gave rise to the deposition of the Jidula Sandstone. 3. Paleocene After the end-Cretaceous regression, deep water environments ceased to existin the Himalayan Sea. This regression phase was followed by the Paleocenetransgression, producing a continental shelf environment connecting the Indianand Eurasian plates and extending into a narrow belt from the Gamba-TingriBasin westwards to Pakistan and eastwards to Assam along the Himalayas. The e