藏南定日地区Cenomanian/Turonian界线附近的生物古海洋事件

藏南定日地区在白垩纪中期发育一套浅灰色深灰色的以钙质页岩、泥灰岩及微晶灰岩为主的浅海陆棚相沉积,岩石中除含较高的粘土矿物及陆源石英矿物颗粒以外,还含有十分丰富的有孔虫化石。依据浮游有孔虫Helvetoglobotruncana praehelvetica的首次出现将C/T界线置于样品9922及9923之间,该界线位于Whiteinella archaeocretacea化石带之中。通过定量分析,该区有孔虫的丰度、分异度以及浮游与底栖有孔虫比率、具旋脊与不具旋脊有孔虫的比率等指标在剖面纵向上表现出3个明显的演化阶段,即Rotalipora cushmani带上部、W.archaeocretacea带及H.helvetica带下部,有孔虫动物群的变化特征完整地记录了C/T界线附近古海洋事件的全过程。该次事件中,碳稳定同位素δ¹³C值存在着明显的异常变化——正向偏移,元素地球化学U、Th及K的丰度也表现出明显的异常变化,其丰度值均比标准平均值偏高。所有这些特征均是在全球洋脊迅速扩张这一背景下海平面发生剧烈变化的结果。

BIO-PALAEOOCEANOGRAPHIC EVENTS NEAR THE CENOMANIAN-TURONIAN BOUNDARY IN TINGRI, SOUTHERN TIBET, CHINA

The Middle Cretaceous shallow-sea sediments,represented by a series of light-dark gray marine clastic and carbonate rocks,were well exposed along the Gongzha Section in Tingri,southern Tibet.In those sediments,not only the contents of clay and quartz mineral grains were higher,but also foraminiferal fossils were abundant.The Cenomanian-Turonian boundary,which located between samples 9922 and 9923,was characterized by the first appearance of Helvetoglobotruncana praehelvetica.In this locality,3 obvious evolutionary stages of the upper R.cushmani zone,W.archaeocretacea zone and the lower H.helvetica zone were recognized after the quantitative study on abundance and diversity of foraminiferal fossils,planktonic/benthic and keeled/non-keeled planktonic foraminifera ratios.The characteristics of foraminiferal fauna changed coincide with the bio-palaeooceanographic events near the C/T boundary.During this event,the positive excursion of δ¹³C and the higher contents of U,Th,K isotope compositions from marine rocks were also remarkable features.All characters from microfossils and geochemistry resulted from a strong eustatic change related to the fast expansion of oceanic ridge.