CHARACTERISTICS OF GRAIN SIZE DISTRIBUTION OF SURFACE SEDIMENTS AND DEPOSITIONAL ENVIRONMENTS IN THE NORTHERN SHELF REGION OF THE SOUTH CHINA SEA

Grain size parameters and their distribution patterns are studied for 244 surface sediments collected from the northern shelf region of the South China Sea.Results show that there is a total of 9 types of sediments distributed in the region,dominated by sand and silt from 0~8Φ,with the deposits of 0-4Φ as the major.The northeastern part of the study area is dominated by coarse and medium sediments,finely sorted with negatively skewness and relatively flat kurtosis,and the southwest area by poorly sorted fine sediments with positively skewness and flat kurtosis.In the transitional zone,however,the sediments along the edge of the continental shelf are characterized by the fine sand moderately sorted with positively skewness and sharp kurtosis.The research area is then divided into three different depositional zones based on Q-type cluster analysis and grain-size trend analysis,i.e. 1) the deposition zone of the Pearl River estuary and the nearshore area of eastern Guangdong Province (Ⅰ district) dominated by the coastal current,and the sediments are mainly terrestrial from the Pearl River,the Hanjiang River and others,such as those from coastal bedrock weathering,denudation of islands,and a small amount of erosioal products from the Dongsha Island.The hydrodynamic conditions are weak in the region; 2) the transitional zone (Ⅱ district) controlled by the branch of the Kuroshio current,South China Sea warm current and coastal current.It is under moderate hydrodynamic conditions,having mixed provenance from the Pearl River,Taiwan bank and Dongsha Islands erosion products; 3) the residual deposits of Taiwan bank and ancient Pearl River delta (Ⅲ district). Under the joint affect of the branch of Kuroshio current,South China Sea warm current,Jiulong River and coastal current,the area is under a strong hydrodynamic conditions and dominated by Late Pleistocene residual deposits mixed with a small amount of modern sediments.