THE GRAIN SIZE COMPOSITIONS OF THE SURFACE SEDIMENTS IN THE EAST CHINA SEA: INDICATION FOR SEDIMENTARY ENVIRONMENTS
东海陆架中北部沉积物粒度特征及其沉积环境

The surface sediments on the continental shelf of the East China Sea were analyzed for the grain size composition, 14C ages and foraminifera assemblages. The results showed that the shelf sediments consisted of sand, silt, silty sand, sandy silt and sandy mud, according to the nomenclature of Folk (1970), with silty sand dominant. The mean grain sizes of the surface sediments overall became coarser from land to sea, which indicated the controls of sediment sources and hydrodynamic sorting on the grain size. Three depositional zones can be identified by synthesizing the depositional environments and formation processes, including sandy sheet depositional area off the Yangtze River, modern muddy deposition area, and central sandy deposition area. The sandy sheet off the Yangtze River was primarily controlled by the Pacific tidal currents and the storm surges during late deglacial to early Holocene period, and the shapes of sand waves are still changing now. Modern muddy depositional areas are composed of prodelta mud off the Yangtze estuary, Zhemin Coatal mud belt on the inner shelf and southwest mud patch around Cheju Island. These muddy systems have different sediment sources and formation mechanisms. The central sandy sediments had been eroded and re-deposited in transgression process since the Last Glacial Maximum, which determines that their sedimentary environments were not in accordance with present oceanic circulations of the East China Sea.Despite this, a little of fine suspended matter can still deposit in the modern weak hydrodynamic environment.