扬子板块埃迪卡拉系(震旦系)陡山沱组层序地层划分与对比

在对扬子板块埃迪卡拉系陡山沱组不同相区的25条代表性剖面野外研究基础上,通过沉积岩石学和岩相序列特征的系统分析,认为陡山沱组沉积时期曾发生3次二级海平面升降事件。依据3个海平面升降转换面,可识别出3个层序底界面:(1)陡山沱组底部与下伏南华系南沱组及其同期层位的冰碛杂砾岩之间的界面;(2)在浅水沉积区陡山沱组中部和上部分别出现喀斯特侵蚀面;(3)在深水沉积区相应层序界面为岩相结构转换面。依据火山灰锆石U-Pb同位素年龄,可将陡山沱组层序地层划分为2个半二级层序或超层序(SS1,SS2和SS3-TST),其中SS1时限为35Ma(635~600Ma),SS2时限为35Ma(600~565Ma),SS3-TST时限为14Ma(565~551Ma)。陡山沱组底部广泛发育的盖帽白云岩底和3个层序内的最大海泛面可以作为4个相对等时面,结合事件沉积标志层,可建立扬子板块陡山沱组从浅水沉积区至深水沉积区等时性二级层序地层划分对比格架。研究结果表明,三峡地区陡山沱组四段式划分方案不适用于整个扬子板块内陡山沱组的区域地层划分和对比。因而,建议扬子板块陡山沱组应该以二级层序地层为基础,结合化学地层和生物地层进行综合划分和对比。陡山沱组新的地层划分对比格架为研究陡山沱组古地理演变和编制该时期高精度的岩相古地理图奠定了基础。

Sequence stratigraphic subdivision and correlation of the Ediacaran (Sinian) Doushantuo Formation of Yangtze Plate,South China

Based on field investigations and detailed analyses of lithofacies of the 18 representative sections from the various sedimentary settings,three second-order eustatic sea level changes and three sequence bottom boundaries are identified within the Ediacaran Doushantuo Formation of the Yangtze Plate,South China. The major sequence boundary(SB1)at the base of the Doushantuo Formation which is marked by the interface between the Cryogenian Nantuo diamictites and overlaying cap dolomite. The two major sequence boundaries(SB2 and SB3) are marked by the karstic surfaces are recognizable within the middle and upper portion of the Doushantuo successions in a shallow-water sedimentary setting. But these unconformities are missing in the deeper water setting,the corresponding sequence boundaries(SB2 and SB3)within the Doushantuo successions in the deeper water setting are represented by abrupt shift of lithofacies stacking patterns. The results demonstrate that the Doushantuo successions consist of two and half second-order sequences or super sequences with time span of ca. 35 Ma for SS1(635-600 Ma)and SS2(600-565 Ma),and ca. 14 Ma for SS3-TST(565-551 Ma). The cap dolomite distributed widely at the bottom of Doushantuo Formation and the maximum flooding surfaces in the sequences are important markers for stratigraphic correlation. Present sequence stratigraphic framework of the Doushantuo Formation can be used for stratigraphic correlation of the Doushantuo successions from the shallow-water facies to the deep-water facies of the Yangtze Plate. Meanwhile,our results indicate that the four members subdivision of the Doushantuo Formation based on its type succession in the Yangtze Gorges area is not practical for the whole Yangtze Plate and generally leads to incorrect stratigraphic correlations and confusions of the Doushantuo successions outside the Yangtze Gorges area. Therefore,it is suggested here that subdivision and correlation of the Doushantuo successions should be based on the second-order sequence stratigraphy together with carbon isotpe chemostratigraphy and biostratigraphy. The new stratigraphy framework of Doushantuo Formation will be a good basis for the palaeogeography evolution research and high-precision lithofacies palaeogeographic mapping.