论陡山沱组的年代

三峡地区震旦系的三层式地层结构 (冰积层 ,陡山沱组特色的大型带刺疑源类组合层 ,埃迪卡拉生物化石层 )在世界其它地区是常见的。通过生物和化学地层对比我们认为陡山沱组及其化石组合层是在马雷诺恩 ( Marinoan)冰期之后、埃迪卡拉生物群分异之前沉积的。根据这一地层对比方案以及西伯利亚、纳米比亚和阿瓦龙半岛 ( Avalon Peninsula)新元古代地层的精确放射性同位素测年资料 ,陡山沱组的沉积年龄很可能在五亿五千万年至六亿年之间。这一估计要新于以前发表的铷锶法或钾氩法测定的陡山沱组黑色页岩或海绿石年龄。更精确地估算陡山沱组的沉积年龄还有待于在陡山沱组地层中发现火山灰以及对火山灰的精确年龄测定 ,但我们根据地层对比估算的陡山沱组年龄 ,为评估庙河和瓮安陡山沱组生物群的演化意义提供了必要的地层框架。

ON THE AGE OF THE DOUSHANTUO FORMATION

The three fold succession of tillites, rocks containing large acanthomorphic acritarchs,and Ediacaran fossil bearing strata seen in Sinian sections in the Yangtze Gorges area is typical of late Neoproterozoic sections around the world. Bio and chemostratigraphic correlations indicate that the Doushantuo Formation is younger than the widespread Marinoan glacial age and older than most or all diverse Ediacaran assemblages. The three fold stratigraphic framework can be calibrated using precise U Pb age dates from Neoproterozoic sections in Siberia, Namibia, and the Avalon Peninsula, suggesting that the depositional age of the Doushantuo Formation is 550-600 Ma, significantly younger than previously published estimates based mostly on Rb Sr and K Ar dates on black shales and glauconites. More precise constraints of the age of Doushantuo deposition await the discovery and analysis of ash beds in the Doushantuo itself, but our estimate does provide the broad time framework necessary to evaluate the evolutionary significance of exceptionally preserved Doushantuo biotas in Weng'an and Miaohe.