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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Helvetoglobotruncanapraehelvetica的首次出现将C T界线置于样品 992 2及 992 3之间 ,该界线位于Whiteinellaarchaeocretacea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Rotaliporacushmani带上部、W .archaeocre tacea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δ1 3 C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。
In the middle of the Cretaceous, a set of light gray, dark gray shale, calcareous shale, marl and microcrystalline limestone are mainly deposited in the shallow shelf of the continental shelf in the southern Cretaceous. In addition to the higher clay minerals and In addition to terrestrial quartz mineral particles, it also contains very rich foraminifera fossils. Based on the first appearance of the planktonic foraminifera Helvetoglobotruncanapraehelvetica, the C T boundary was placed between samples 992 2 and 992 3, which lies in the fossil zone of the Whiteinellaarchaeocretacea. Through quantitative analysis, the abundance and differentiation of foraminifera and the ratio of planktonic and benthic foraminifera, the ratio of rotifer to non-dorsal foraminifera showed three significant The evolutionary stages, that is, the upper part of the Rotalipora cushmani belt, the Warchaeocre tacea belt and the lower H. Hessheltica belt, and the characteristics of the foraminiferal fauna have completely recorded the whole process of paleoceanographic events near the CT boundary. The δ13C value of carbon stable isotopes shows a significant anomaly-forward migration in this episode, and the abundances of elemental geochemistry U, Th and K also show significant anomalous changes Higher than the standard average. All of these features are the result of dramatic sea level changes in the context of the rapid expansion of the ocean ridge.