THE MIDDLE CRETACEOUS CALCAREOUS NANNOFOSSIL ZONES INGAMBA AREASOUTHERN XIZANG (TIBET)，CHINA ANDTHE CENOMANIAN-TURONIAN BOUNDARY
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Abstract:
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 characteristicsediments 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 middleAlbian 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 detailon the basis of calcareous nannofossil data．The first occurrence of Gartnerago obliquum rather closely corresponds with thefirst appearance datum of Helvetoglobotruncanapraehelvetica （foraminifera） thus that they are considered to be very usefulto determine the boundary between Cenomanian and Turonian in Southern Xizang （Tibet）．
