莲沱砂岩——南华大冰期前气候转冷的沉积记录

湖北宜昌地区出露的莲沱组中上部细屑岩的化学蚀变指数(CLA)介于60～70之间,表明是在寒冷干燥的气候条件下沉积的。比较莲沱组与板溪群中下部沉积岩的主化学成分和 CIA 值表明,板溪群源区遭受过强烈的化学风化,相应的古气候是温暖湿润的,而莲沱组源区化学风化弱,可见两者形成时的古气候环境明显不同。根据 CIA 值推测,地层层序上莲沱组位于板溪群的中上部,莲沱砂岩的 CIA 研究表明南华大冰期前全球气候已经从温暖湿润转变为寒冷干燥,CIA 值继续降低预示着全球性的冰期到表,不久就发生了"雪球地球"事件。

Liantuo sandstones: sedimentary records under cold climate before the Nanhua big glaciations

Chemical Index of Alteration(CIA)values of argillites,from the middle and upper parts of the Liantuo Formation exposed in Yichang region,Hubei province,range from 60 to 70,suggesting that they were deposited under frigid climate.Comparing with major elements and CIA values of the Liantuo Formation,provenance areas of the middle and upper parts of the Banxi Group would undergo more severe weathering under warm and wet climate,while provenance areas of the Liantuo Formation would be less weathered.Therefore,there were two different kinds of climates in which the Liantuo Formation and Banxi Group were deposited. Based on CIA values,the Liantuo Formation can be correlated to the middle and up parts of the Banxi Group.The research from CIA values of the Liantuo sandstones implicates a transition from greenhouse to icehouse,moreover,the upward lowering of CIA values indicates the coming of the global glaciation,and after that the so called "Snowball Earth" began.