西藏第三纪有孔虫生物地层及地理环境

西藏南部海相第三系自下而上划分为:基堵拉组、宗浦组和遮普惹组。基堵拉组的归属直接关系到白垩——第三系的界线问题。以往在证据不充分的情况下将基堵拉组归于白垩系。本次工作在该组中找到了具时代意义的化石,有双壳类、介形虫、有孔虫等。通过化石群的研究确定了基堵拉组属于古新世丹宁早期。白垩—第三系界线应位于宗山组与基堵拉组之间。通过基堵拉组的横向对比得出了该组在空间上穿时的结论。浮游有孔虫动物群的发现确定了本区最高海相层为遮普惹组上段,时代属于始新世晚期。 西藏第三纪有孔虫类型丰富。据动物群的古生态研究得出了不同时代的有孔虫生物相:丹宁期为Rotalia生物相和Textularia生物相;朗德期为Miscellanea生物相和Ranikotbalia生物相;伊普尔期至路坦丁期包括Orbitolites生物相、Assilina生物相及冈底斯有孔虫生物相;普里亚波期以Globigerina生物相为特征。据有孔虫生物相的特征及氧碳稳定同位素的测试结果综合得出了西藏南部第三纪包括两次海侵旋回,即古新世和始新世旋回。二者又分别包括两回次一级的旋回,即古新世的丹宁期旋回和朗德期旋回;始新世的伊普尔期至路坦丁期旋回和普里亚波期旋回。

Foraminifera Biostratigraphy and Paleogeography of the Tertiary in Tibet

The Tertiary marine beds are generally represented by the Paleocene and Eocene in Tibet. It is of great interest for paleontologists to establish the Tertiary marine sequence of China and to trace the paleogeography, and of greatimportance to date the last phase of the Tethyan transgression in Tibet and the uplift of the Qinghai - Xizang Plateau. I. Foraminifera Biostratigraphy The Tertiary marine beds occur largely in southern Tibet as well as in the Gangdise Mountains in northern Tibet, in-which Foraminifera is one of the most important fossil groups. By the analyses of forarniniferal fauna and their distribution, it is available to make the classification and correlation of the strata, and to discuss some important stratigraphic problems. 昑he Tertiary strata of southern Tibet can be divided lithologically into the Jidula Formation, the Zongpu Formation and the Zhepure Formation. The "Zong-pu Group" was originally named in Gamba region and the "Zhepure Group" in Tingri region which w.as considered as the overlying unit of the "Zongpu Group", and both were referred to the Tertiary.Now, the detailed study of fossil Foraminifera by the writer shows that the lower part shale of the "Zhepure Group" belongs to the Cretaceous and its overlying sandstones and limestones consisting the upper part of,.the group belong to the Tertiary. TLe ?Tertiary marine strata of Zongba region are named the Jialazi Formation. . The age of the Jidula Formation is the key to locate the Cretaceous/Tertiary boundary. By now, some newly found fossils of Bivalvia, Ostracoda and Foraminifera in the Jidula Formation testify that it is a part of the Paleocene. The fossils found in the Jidula Formation are different from those in the underlying Zongshan Formation and are identical with those in the overlying Zongpu Formation. Both contacts are comformable. The change in fossil groups coincides with the evolution of life between the Cretaceous and Tertiary on the globe. Thus, the Cretaceous/Tertiary boundary is just between the Zongshan Formation and the Jidula Formation. Some stratigraphers believed that the Jidula sandstone is synchronic with the Pab sandstone in Pakistan. But deposits formed in transgression or regression tend to be diachronous. It has been demonstrated that the sandstone is referred to the Middle Maestrichtian in the Lower Indus Basin and to the Upper Maestrichtian in the Upper Indus Basin. It extends across the boundary of the Cretaceous and Paleocene in Zongba region, and belongs to the Paleocene in Gamba and Tingri. The age of the sandstone becomes younger and younger along the direction of transgression. As a' result of the writer's study, the uppermost marine beds are represented by the Upper Member of the Zhepure Formation in Gamba region. II. Foraminifera Biofacies The Daman Foraminifera exhibits two biofacies. The Rotalia biofacies covering Gamba, Tingri and Yadong areas develops in the Lower Member of the Zougpu Formation. The character of the fauna reflects a shallow marine environment with low energy conditons. The Textularia biofacies occurs in the Lower Member of the Jialazi Formation in Zongba region. It might indicate a near shore environment with allochthonous material. Distribution of the Landenian foraminifera biofacies is similar to the distribution of Danian. There are also two biofacies. The Miscellanea biofacies develops in Gamba, Tingri and Yadong areas, in the Upper Member of the Zong-pu formation.An open marine realm may be inferred. The depth was about 70-90 meters. The Ranikothalia biofacies is recognizd in the Middle Member of the Jialazi Formation in Zongba region. It shows a shallow marine environment during deposition. The Ypresian-Lutetian Foraminifera can be distinguished into three biofacies. The Orbitolites - Fasciolites biofacies occurs in the Lower Member of the Zhe-pure Formation in Gamba region. The occurrence of Orbitolites and abundant miliolids might indicate a somewhat restricted, sheltered and shallow water environment. The Assilina biofacies widely dis