HIGH-FREQUENCY SEQUENCES AND SEA-LEVEL OSCILLATIONS IN THE EMEI AREA ON THE WESTERN MARGIN OF THE UPPER YANGTZE PLATFORM

The present paper has emphasized that the accommodation space and sediment supply are a dual control on sequence stratigraphic framework.The Triassic strata in the Emei area on the western margin of the Upper Yangtze platform are divided into two second-order cyclic sequences and nine third-order cyclic sequences on the basis of the detailed study of outcrop sequence stratigraphy and seismic stratigraphy.The following cyclic hierarchy are ascertained;1st order(200-400 Ma),2nd order(10-100 Ma),3rd order (1-10 Ma). 4th order(0.1-0.2 Ma),5th order(0.2-0.5 Ma)and 6th order(±0.02 Ma).
The geodynamical model for basin subsidence during Triassic time has been constructed for the study area.The model has revealed that the Triassic strata in this region have undergone two evolutionary stages;the passive continental margin during the Early and Middle Triassic,and foreland basin during the Late Triassic.Morever the classification scheme for the high-frequency sequences are also suggested in this paper,including the following four types produced in different environment;deltaic mouth bar or shoreline cyclic sequence type;vertical ordering variation type in ripple index;the superimposed dual type of thick-bedded limestones (dolostones) and thin-bedded limestones (dolostones),and the dual type of calcarenite,sandstones and mudstones.
The inner framework of the 3rd-order sequences may be indicated by the accummulation styles and other information about the high-frequency sequences.These results represent the refinement of the high-resloution sequence stratigraphy,and lay the foundations for the quatitative delineation of the global sea-level curves.