上扬子区下古生界层序地层格架的初步研究

Sequence-Stratigraphic Framework for the Early Palaeozoic of the Upper-Yangtze Region

上扬子区下古生界在贵州及其邻区发育较为完整。在震旦纪与寒武纪之交的台地淹没事件之后,寒武系形成一个从深水盆地相黑色页岩系到大套台地碳酸盐的沉积序列,其中下寒武统包含5个三级层序而可以进一步归为一个二级层序,中上寒武统构成一个二级层序而且可以进一步划分为7个三级层序。奥陶纪艾家山世末期的都匀运动、奥陶纪末期的与冈瓦纳大陆冰期相响应的全球海平面下降事件以及志留纪末期广西运动的共同作用,造成扬子地台大幅度的古地理变迁以及残留不全的奥陶系和志留系。详尽追踪和对比的结果可以将奥陶系划分为8个三级层序且可以进一步归为一个二级层序,在残留不全的的志留系中可以划分出7个三级层序并可以进一步归为一个二级层序。层序地层格架的建立,可以大致解读许多有趣的地质现象,如寒武纪碳酸盐台地的生长过程,奥陶纪与志留纪之交的“浅海静水”型黑色页岩系的发育特点,志留纪由北而南的海侵所造成的复杂的志留系展布特征等等;同时也意味着许多有待于进一步研究的问题,早寒武世数次快速海侵事件与生物事件的关系,奥陶系顶部特殊的“观音桥灰岩”的成因,奥陶系红花园组海绵生物礁的基本沉积模式等等。

The Early Palaeozoic strata of the upper-Yangtze region are fully developed in Guizhou Province and its adjacent areas. A Cambrian depositional succession from deep-water black shales to shallow-platform carbonate was formed after the obvious drowned events of platforms at the turn from Sinian to Cambrian. The Lower Cambrian can be divided into 5 third-order sequences and still can be grouped into one second-order sequence ; the Middle and Upper Cambrian can be divided into 7 third-order sequences that make up one second-order sequence. Several geological events, i. e. the Duyun movement occurred at the end of the Ordovician Aijiashanian epoch, a large-scale sea-level fall occurred at the end of the Ordovician that is the response to glacial stage of the Gondwana and the Guangxi movement occurred at the end of the Silurian, led to the great variance of palaeogeography and the relic of Ordovician to Silurian strata in the study area. On the basis of the detail tracking and correlation, 8 third-order sequences that make up one second-order sequence can be discerned in the Ordovician strata and 7 third-order sequences that constitute one second-order sequence can be divided in the Silurian strata. The sequence-stratigraphic framework from the Cambrian to the Silurian in the study area reveals lots of interesting geological phenomena such as the growth pattern of the Cambrian carbonate platform, the developmental feature of black shales marked by “shelf-sea and dead-water” in the transitional period from Ordovician to Silurian, the complex distributional feature of the Silurian strata resulted from the southward transgression and so on. And lots of problems needed to further research are also revealed in the sequence-stratigraphic framework： the complex relation between the rapid transgressive events and the bio-diversity events in the Early Cambrian, the origin of the Guanyinqiao limestones in the top part of Ordovician and the basic depositional model of sponge-reef of the Ordovician Honghuayuan Formation etc.