西藏南部岗巴地区白垩纪中期钙质超微化石带和Cenomanian-Turonian界线

我国西藏南部海相白垩系含有较丰富的钙质超微化石。文中着重研究岗巴地区两个剖面 (即剖面A ,B)Albian Santonian钙质超微化石的分布。根据标志种的存在 ,识别出 5个初现面事件 ,相应地建立 6个钙质超微化石带 ,自下至上是Prediscosphaeracretacea带 ,Eiffellithusturriseiffeli带 ,Lithraphiditesacutum带 ,Gartneragoobliquum带 ,Quadrumgartneri带 ,Lucianorhabduscayeuxii带。同时 ,通过洲际对比 ,建议以G .obliquum初现面作为划分本区Cenomanian和Turonian界线的标志。此外 ,Q .gartneri带和I .cayeuxii带之间缺失多个化石带 ,据此推测Turonian至Santonian期间本区可能存在沉积间断。

THE MIDDLE CRETACEOUS CALCAREOUS NANNOFOSSIL ZONES IN GAMBA AREA, SOUTHERN XIZANG (TIBET), CHINA AND THE CENOMANIAN TURONIAN BOUNDARY

This study deals with calcareous nannofossils of mid Cretaceous at Gamba Area in Southern Xizang (Tibet),China. The rock samples were collected from the Gamba Group of both Sections A and B. On the basis of the characteristic sediments,the Gamba Group can be divided into three stratigraphic units from bottom to top,namely the Gambadongshan, Chaqiela and Gambacunkou formations.Calcareous nannofossils mainly appeared from the uppermost Gambadongshan Formation to Gambacunkou Formation.The preliminary results enable us to propose a detailed nannofossil zonation for the middle Albian to the late Santonian.This zonation scheme consists of six zones and five addition biohorizons.The upper part of Gambacunkou Formation clearly exhibits a stratigraphic hiatus since the top of Quadrum gartneri zone (early Turonian) is immediately overlain by Lucianorhabdus cayeuxii zone (late Santonian).The Cenomanian Turonian boundary is discussed in detail on the basis of calcareous nannofossil data.The first occurrence of Gartnerago obliquum rather closely corresponds with the first appearance datum of Helvetoglobotruncana praehelvetica (foraminifera),thus that they are considered to be very useful to determine the boundary between Cenomanian and Turonian in Southern Xizang (Tibet).