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

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.