Sedimentary Environmental Evolution of the Cenozoic in China Offshore Basins

Key words: 
• China offshore basins /
• basin type /
• sedimentary environment /
• sedimentary system

Abstract: Compared with China's inland, sedimentary environments of the Cenozoic in China offshore basins have the diversity. In different sedimentary basins, there are three kinds of sedimentary environments include continental, paralic and marine facies. Each basin has undergone changes in different sedimentary environments generally. Through studying the China offshore basins controlled by plate tectonic movement, analyzing the basin structure in the evolution of plate movement, combined with the evidence related to sedimentary environment, we could verify the relationship between sedimentary environment and sedimentary system in China offshore basins.
As China offshore was caught in the middle of Eurasian Plate, Pacific Plate and Indian Ocean Plate, its formation and evolution were controlled by these three plates. Western Pacific is a north to south of the trencharcbasin system. Generally bounded by the Taiwan Strait, the East China Sea Basin is a convergent margin with the evolution of conversion or passive continental margin due to the continuing westward subduction by the Pacific Plate or the Philippines Plate. The South China Sea is a passive margin with the evolution of active or convergent margin due to the continuing eastward subduction of South Ocean Shell. So there would have different basins as the evolution of different mechanisms. In this paper, the Cenozoic in China offshore basins were divided into three types including rift basins on internal plate, convergent basins on plate margin and scatter basins on plate margin. Convergent basins on plate margin can be subdivided into rift and depression. Scatter basins on plate margin can be divided into rift, depression and slip.
Different types of sedimentary basins could reflect the evolution of different sedimentary environments. In the main developmental stages of Cenozoic basin, the sedimentary environment of rift on internal plate is relatively simple. It was dominated by continental deposits. The sedimentary environment of plate margin is relatively complex as it near the ocean. Due to the continuing subduction to the Eurasian plate by the Pacific plate and Philippine plate, the convergent basins on plate margin have the characteristics of convergence with the evolution of continental to marine facies. As the continuous expansion of the South China Sea basin, the scatter basins on plate margin have the characteristics of scatter with the evolution of marine to continental facies.
In different basin structure and sedimentary environment, there are different types of sedimentary system. The rift basin has the sedimentary system with accumulation of nearsource. The depression basin has the sedimentary system with accumulation of farsource. As far from the sea, these basins on internal plate which mainly have continental sedimentary system were less affected by the transgression. Those basins near from the sea on plate margin which mainly have paralic or marine sedimentary system were more affected by the transgression. In these basins on plate margin，the convergent basins on plate margin have shallow water body, small slope break and small delta. They were more influenced by the tidal water and paralic strata were well developed. The scatter basins on plate margin have deeper water body, large slope break, large delta and marine strata. 