Abstract:

 The Sinian System in the Upper-Yangtze region contains the Doushantuo Fm. and the Dengying Fm., the former includes more black shales and the latter is marked by a set of dolomites. From litho-facies succession to meter-scale cycle and from sedimentary-facies succession to third-order sequence, the Sinian System in the shallow-water setting can be divided into five third-order sequences that can be grouped into one second-order sequence. Thirdorder sequences are always made up of a generally upward-shoaling sedimentary-facies succession. For the second-order sequences constituted both by the Doushantuo Fm. with more black shales and the Dengying Fm. that is marked by a set of dolomites, is also an upward-shoaling facies succession.Therefore, there is similar facies-succession fabric between the third-order and the second-order sequences.According to the two types of facies-changing surfaces and two types of diachronisms in the stratigraphic, sequences-stratigraphic divisions at main logged sections become the basis to establish the sequence-stratigraphic framework of Sinian System in the study area. This sequence-stratigraphic framework indicates several features: The Doushantuo Fm. with more deep-water black shales represents a transgression process after the great glaciation period. Different from the changing feature in the Cambrian, the dolomites of the Dengying Fm. with well-preserved primary sedimentary fabrics are characterized by a set of dolomites in the deeper depositional background. This changing feature may mean a particular dolomitization in the Sinian and a problem that is further researched in the future.

Key words: sequence-stratigraphic division, the Sinian System, the Upper-Yangtze region