关键词: 有孔虫古地理, 中—新生代, 西藏特提斯, 演变

Abstract:

Foraminiferal data are the critical record of geological history, they reflect the dynamic changes of geographic patterns and ecologic environments. Various scientific issues, like tectonic evolution and changes of relative positions of plates, are the research topics of geoscientists. Foraminiferal ecology and paleogeographic distribution studies can recognize the peleobiogeographic realms and trace the tectonic evolution. One result of such studies is the recognition that Mesozoic-Cenozoic paleobiogeographic differentiation in Tibet is a dynamic response to the Tibetan-Tethys evolution.
In the Early Jurassic, larger benthic foraminifera Orbitopsella and bivalve Lithioties occurred in southern Tibet, both are thermophilic fauna living in the circum-Tethys shallow water. They were dominant during the Pliensbachian to early Toacian. In the Late Jurassic, psychrophilic Buchia biota emerged in southern Tibet. They are identical to the Buchia and Retroceramus found in the Antarctic, south end of South America, New Zealand and Australia. It implies that the opening of Neothetys, especially the Central Atlantic, pushed the Indian Continent southward from lower to higher latitudes.
The foraminiferal fossils in the Lower Cretaceous are poorly preserved in Tibet. In the mid-Cretaceous, larger benthic foraminifera Orbitolina widely developed in the Lhasa and Qiangtang basins of the Asian Continent. It occurred frequently in the shallow marine belts surrounding the Tethys Ocean during the late Barremian to Cenomanian of the Cretaceous. Its distribution extends to Myanmar in the east and Ladakh to the west. Along the coast of Tethys, it migrated to Japan, Malaysia, Indonesia, Kashmir, Afghanistan, the Middle East, West and South Europe, North Africa and the Caribbean regions. However, it never migrated into the Indian continent where a mixed planktonic and smaller benthic foraminiferal fauna appeared during that time. Orbitolia was also found in the Tanganyika region in East Africa, but it did not migrate eastward into the Indian continent either. Like Orbitolina, a bivalve fauna of Isodomella-Caestocorbula was found in the Lhasa block, and no affinity to the Indian continent was recognized. The distributions of Orbitolina and related biotas provide the basis for the reconstruction of tectonic paleogeography. By the mid-Cretaceous, the Lhasa block had attached to the Eurasian continent. Along the Bangong Lake-Nujiang River Suture, the Tethys closed at the end of the Jurassic when the Lhasa block was located at the south margin of the Eurasian continent. The shallow water faunas like Orbitolina and bivalves could migrate along the east and west margins of the Eurasian continent. Paleobiogeographically, the Lhasa block belongs to the northern temperate realm in the Middle Cretaceous. The Indian continent started to move away from Africa by the early Cretaceous and drifted to the north. It was isolated from the Eurasian continent to the north and Africa to the west by the Neo-Tethys Ocean that was a barrier for migration of shallow water faunas. No Orbitolina has been found on the Indian continent where faunas are shown not to be closely related to the contemporary shallow water biota of the Eurasian continent. The migration of Orbitolina was obstructed by the Tethys Ocean, whereas the deep water environment was favorable to the thriving of planktonic foraminifera such as Ticinella-Rotalipora and Marginotruncana-Globotruncana faunas during the middle to late Cretaceous. Till the latest Cretaceous, deep sea existed between the Indian and Asian continents. There are fundamental differences in the biogeographical features of the two sides of the Yarlung Zangbo suture zone. Larger foraminiferal fauna of Orbitoides-Omphaloceclus lived along the northern margin of the Indian continent, whereas Lepidorbitoides-Pseudorbitoides fauna were restricted in the south margin of the Gangdisê magmatic arc.
Stratigraphic and paleontological evidence documented the dramatic changes in themicrofauna content and sedimentary facies across the Cretaceous-Paleogene boundary. In the Gamba and Zhongba areas in southern Tibet, the boundary is marked by a major disconformity, separating platform carbonates from overlying terrigenous conglomerates and sandstones. Danian Rotalia-Smoutina-Lockhartia fauna was only found at the north margin of the Indian continent; it had disappeared from the Asian continent. The paleobiogeographic affinity was obscured by the terrigenous sandstones and boulder-size conglomerates in the Danian. After the Selandian (Paleocene), the biotic difference clearly ceased. Foraminifera Miscellanea-Daviesina fauna occurred similarly in between both sides of the suture. Therefore, supported by paleobiogeographic evidence, we argue that the initial collision of the Indian and Asian continental blocks should occur in the Danian time (~66-61 Ma). If that is the case, the major biotic and lithofacies changes at the Danian, observed in the Tibetan Tethys and mostly referred to as intrinsic to the eustatic sea level change, were likely driven by continental convergence of the Indian and Asian plates.
In the Early Eocene, one common biogeographic realm was formed across both sides of the Yarlung Zangbo suture zone dominated by the Nummulites-Discocyclina fauna. Foraminiferal geography witnessed the initial collision between the Great Indian continental margin and the Asian continent in the Early Paleocene. Deep sea disappeared and a residual sea remained. The occurrence of Nummulites willcoxi-Globigerina ouachitaensis fauna indicates the uppermost marine sediments with a late Ecocene Priabonian age. Then, the Tethyan Himalaya closed and sea water retreated from southern Tibet during ~35-34 Ma.

Key words: foraminiferal paleogeography, Mesozoic-Cenozoic, Tibetan Tethys, evolution