从有孔虫分析西藏南部白垩纪海平面升降

CRETACEOUS FORAMINIFERA FROM SOUTHERN XIZANG (TIBET): A STUDY ON EUSTATIC CHANGE

白垩纪是地质历史中的最大海侵时期。海平面的升降改变了海洋的物理、化学及生物因素,从而影响了有孔虫的演化与发展。根据对有孔虫丰度、分异度及演化类型的研究,认为西藏白垩纪最大海侵时期形成于赛诺曼期与土仑期的界线附近。此后海平面总的处于下降趋势,只是在康尼亚克期至三冬期海平面又有回升。马斯特里赫特末期有孔虫的大量灭绝反映了白垩纪与第三纪界线事件的影响。

Cretaceous marine environment of southern Xizang (Tibet) was characterized by a major eustatic transgression. The eustatic change altered the physical, chemicaI and bio- logical factors of the sea water, and hence affected the evolution and extinction of the foraminifera. An evolutionary model and an eustatic curve have been proposed after the study of the foraminiferal abundance, diversity and evolutionary pattern. It is suggested that the maximum transgression took place toward the end of Cenomanian. The extinction of the Tibetan foraminifera commenced from the mid- Cenomanian. It was to a degree stepwise in the proceeding of a major eustatic transgression and a locally intensified oxygen minimum layer. A general regressive tendency formed from the Turonian to the end of Cretaceous, although a potential transgression occured during the Coniacian and Santoni- an. A mass extinction of the foraminiferal fauna brought about at the end of Maas- trichtian, it was closely related to the globle regression at the time.