东海陆架中北部沉积物粒度特征及其沉积环境

THE GRAIN SIZE COMPOSITIONS OF THE SURFACE SEDIMENTS IN THE EAST CHINA SEA:INDICATION FOR SEDIMENTARY ENVIRONMENTS

通过对东海陆架表层沉积物粒级组成、粒度参数、14C年龄和微体古生物组合的综合分析,绘制了东海陆架的沉积物类型分布图;运用Folk等(1970)沉积物分类方法将东海表层沉积物分成砂、粉砂、粉砂质砂、砂质粉砂、砂质泥5种类型,其中粉砂质砂分布最广,砂质泥分布最少;沉积物由陆向海粒度变粗,反映沉积过程中的物源和沉积动力控制作用。根据沉积环境及成因分析,可将东海陆架沉积分为3类:分别是长江口外席状砂沉积区、现代泥质沉积区和陆架中部砂质沉积区。长江口外砂质沉积是全新世冰消期晚期潮流作用及风暴潮流共同作用的产物,是高海平面以来太平洋潮波系统作用下的潮流沙沉积,沙波地貌仍在发生变化。现代泥质沉积区包括长江前三角洲沉积、浙闽沿岸流沉积和济州岛西南泥质沉积三个区域,不同沉积区的成因机制不同。陆架中部砂质沉积是末次冰盛期之后海侵作用下发育的砂质沉积物,在海侵的不同阶段中沉积物被冲刷改造,具有不等时性特征,沉积环境与现代陆架海洋环流的动力特征不一致,现代沉积作用较弱,仅接受悬浮体细粒沉积。

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, modem 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 modem weak hydrodynamic environment.