白垩纪黑色页岩与海水含氧量变化——以西藏南部为例  

Cretaceous black shale and dissolved oxygen content--- A case study in southern Tibet

作　　者：万晓樵[1] 刘文灿[1] 李国彪[1] 李艳 

机构地区：[1]中国地质大学,北京100083 [2]内蒙古集宁市地质环境监测站,内蒙古集宁012000

出　　处：《中国地质》2003年第1期36-47,共12页Geology in China

基　　金：中国地质调查局1:25万区域地质调查项目(2001300009111);科技部973预研项目(2001CCA01800)联合资助。

摘　　要：黑色页岩具有特殊的沉积学、古生物学和有机地球化学特征,主要表现在生物组合和丰度的变化、有机碳含量的增高与碳稳定同位素的偏移等方面。具有丰富有机质来源的高生产力表层水是黑色页岩中富有机碳沉积物形成与埋藏的先期条件。白垩纪中期黑色页岩在全球广泛分布,是Cenomanian/Turonian期界线事件的最重要标志。在西藏南部的C/T界线处,浮游有孔虫种一级的绝灭速率达34%,底栖有孔虫的绝灭速率达50%;有机碳含量偏高;δ13C出现2.74‰的正向偏移;生物扰动几乎消失,微小钻孔黄铁矿化;有孔虫房室充填有黄铁矿微球丛;诸多其他地球化学指标亦存在不同程度的异常。这些特征说明当时海洋不同深度水层处于不同强度的贫氧至缺氧状态,同时沉积的黑色页岩是缺氧环境中有机碳大量保存的结果。该现象发生的原因是白垩纪中期的海平面上升和气候变化的影响。西藏特提斯海古海洋条件的变化明显受全球和区域性海平面上升的影响,由此形成的白垩纪中期黑色页岩具有全球一致的特征。Black shales have special lithological,paleontological and geochemical features,which are mainly represented by changes in faunal assemblage and abundance,an increase of organic carbon content and positive excursion of carbon stable isotopes.High surface water productivity is of paramount importance in organic matter deposition.Black shales were worldwide during the Mid Cretaceous and well marked the Cenomanian-Turonian transition.In southern Tibet,the extinction rates for planktonic foraminiferal species are34%,and up to50%for the benthos at the C/T boundary.A short positiveδ 13 C excursion that shows a maximum value of2.74 ‰ occurs at the top of the Cenomanian.The bioturbation almost disappeared;microburrows are pyritized;foraminiferal chambers are filled with pyrite microspherical clusters.Other geochemical indicators also show different degrees of anomalies.These features suggest that different degrees of oxygen-poor to oxygen-deficient states occurred in water layers at different depths of the ocean.The shales deposited simultaneously are the result of preservation of abundant organic carbon in the anoxic environment.The causes for the occurrence of this phenomenon are the influences of the sea-level rise and climatic change in the Mid Cretaceous.The changes of the conditions of the Tibetan Tethys Sea were markedly affected by the global and regional sea-level rises and the Mid Cretaceous black shale formed thus have the features consistent with those of the global shales.

关 键 词：海水 白垩纪 黑色页岩 含氧量 西藏 海平面上升 沉积岩 有机地球化学 

分 类 号：P534.53[天文地球—古生物学与地层学]