滇黔北五峰组—龙马溪组页岩沉积环境和古气候地球化学特征

Sedimentary Environment and Paleoclimate Geochemical Characteristics of Shale in the Wufeng and Longmaxi Formations,Northern Yunan-Guizhou Area

为研究奥陶大灭绝与该时期古环境和古地理的联系,对滇黔北地区五峰组—龙马溪组样品进行了有机碳(TOC)、主量元素、微量元素和碳同位素地球化学分析。结果表明,滇黔北地区五峰组—龙马溪组Ba/Al和Ni/Al值的变化在五峰组沉积时期相对稳定,从五峰组沉积时期开始逐渐增加,直到观音桥组沉积时期达到顶峰随之下降,在龙马溪组沉积时期又开始上升,随后逐渐减小并稳定下来,特别在赫南特阶观音桥组时期,生产力出现了跳跃性的波动,即表明奥陶纪末期古海水性质存在重大转变。V/Cr和U/Th比值总体显示出五峰组沉积时期表现为缺氧环境,观音桥组沉积时期表现为厌氧环境,龙马溪组表现为缺氧环境,U-Mo协变模式表明滇黔北地区五峰组沉积时期为强滞留海盆,龙马溪组属于半滞留海盆。化学风化值指数(CIA)表明滇黔北地区温度在五峰组早期就开始缓慢下降,观音桥时期区域温度降至冰期,龙马溪组早期又渐渐恢复,但从龙马溪组到观音桥组至五峰组时期气候处于极不稳定的状态。奥陶纪末期生命大灭绝是在极地冰川发育的背景下,洋流模式、强度及海水的理化性质的改变导致两者受海陆分布格局控制。

To study the relationship between the mass extinction of life in the Ordovician and the paleoenvironment and paleogeography of that period,total organic carbon(TOC),major elements,trace elements and carbon isotope geochemical analyses were carried out on samples from the Wufeng Formation-Longmaxi Formation in the northern Yunan-Guizhou area.The results show that the changes in the Ba/Al and Ni/Al values of the Wufeng FormationLongmaxi Formation indicate that the marine productivity of the Wufeng Formation was relatively stable,starting from the Wufeng Formation and gradually increasing until the Guanyinqiao Formation reached its peak and then decreased.In the Longmaxi Formation,marine productivity began to rise again,then gradually decreased and stabilized.Especially in the Guanyinqiao Formation during the Hirnantian period,there was a jump in productivity in the Guanyinqiao Formation,which indicated a significant change in the nature of the Ordovician seawater.The Vr/Cr ratio and U/Th ratio generally show that the Wufeng,Guanyinqiao and Longmaxi Groups were in anaerobic environments.The U-Mo covariant model shows that the Dianqianbei area was a strong retention basin during the period of the Wufeng Formation,and during the period of Longmaxi Formation it was a semi-retention basin.The chemical weathering index(CIA)indicates that the temperature in northern Yunnan and Guizhou began to decline slowly in the early Wufeng Formation.The regional temperature fell until the glacial period during the Guanyinqiao period.The Longmaxi Formation gradually recovered in the early period,but from the Longmaxi Formation to the Guanyinqiao Formation to the Wufeng Formation the climate was extremely unstable.The mass extinction of life at the end of the Ordovician was caused by changes in the pattern and intensity of ocean currents,and changes in the physical and chemical properties of seawater as polar glaciers developed.Both of these changes were due to the distribution pattern of sea and land.