Sequence-Stratigraphic Framework And Its Palaeogeographical Background For The Sinian System Of The Upper-Yangtze Region

The Sinian system in the upper-Yangtze region contains the Doushantuo Fm. andthe Dengying Fm., the former includes more black shales and the later is marked bya set of dolomites. From litho-facies succession to meter-scale cycle and fromsedimentary-facies succession to third-order sequence, the Sinian system in theshallow-water setting can be divided into five third-order sequences that can begrouped into one second-order sequence. According to two types of facies-changingsurfaces and two types of diachronisms in the stratigraphic records ,sequences-stratigraphic divisions at each main logged section become the basis toestablish the sequence-stratigraphic framework of Sinian system in the study area.This sequence-stratigraphic framework indicates several features: The DoushantuoFm. with more deep-water black shales represents a transgression process after thegreat glaciation period. Different from the changing feature in the Cambrian, thedolomites of the Dengying Fm. with well-preserved primary sedimentary fabrics arealso characterized by a set of dolomites in the deeper depositional background. Thischanging feature may reflect a particular dolomitization in the Sinian and a problemthat need further researched in the future. The Sinian stratigraphic succession from theDoushantuo Fm. to the Dengying Fm , both makes up of a larger-scaleupward-shoaling succession of sedimentary-facies and demonstrates a growth processof ramp carbonate-platform that is the response to the third and the second-ordersea-level changes. This process is clearly illustrated in a series of the panelsequence-stratigraphic framework diagrams and the correspondingpalaeogeographical maps.