Sequence-Stratigraphic Framework and Its Forming Background of Palaeogeography for the Middle Cambrian of the Upper-Yangtze Region

In Upper-Yangtze region, especially in Guizhou Province and its adjacent areas, Middle Cambrian is well developed and is marked by a succession from small-amount limestones to large-amount dolomites. In shallow intra-platform setting of northwestern part of study area, middle Cambrian is made up by both thin muddy and sandy dolomites with trilobite fossils of Douposi Formation and thick dolomites lacking fossils of Loushanguan Group. In deep-water setting of southeastern part of study area, Middle Cambrian includes Duliujiang Formation and lower part of Yangjiawan Formation that are constituted by black shales, muddy shales, muddy limestones and breccia limestones. In transitional region, middle Cambrian can be divided into Kaili and Jialao Formations marked by muddy shales and muddy limestones. Both regularly temporal succession of sediments and spatially distributional pattern of sedimentary facies form a particular sequence-stratigraphic framework that contains three third-order sequences. This sequence-stratigraphic framework not only demonstrates two types of facies-changing surfaces and two types of diachronisms in stratigraphic records but also means a regular deepening-shoaling process of sedimentary environment controlled by cycles of third-order sea-level changes. Series of panel diagrams of sequence-stratigraphic framework and corresponding maps of lithofacies and palaeogeography can overally illustrate temporal evolution succession of sediments and spatial distributional pattern of sedimentary facies for third-order sequences, and can demonstrate forming palaeogeographic setting for third-order sequences. More interestingly, large-scale transgression after a large-scale regression at turn from early to middle Cambrian apparently resulted in expansion of ecological space. With this ecological expanding two famous biota's that are named as the Taijiang Biota in end of early Cambrian and the Kaili Biota of initial stage of middle Cambrian are formed in deeper background of sedimentary environment. These two famous biota's represent another biological explosion after biological explosion in early stage of early Cambrian represented by both the Small-Shelly Biota of initial stage of early Cambrian and the Chengjiang Biota of early of Early Cambrian. Therefore, together with genetic relationship between mass-extinction event genetic relationship between biodiversity event and transgression become an important problem that needs to be studied in future.