Study On The Characteristics Of Surface Sediment In North Branch Of Changjiang Estuary And Analysis Of Its Sedimentary Environment And Sediment Source

In this paper there were 88 surface sediments taken from the middle to lower section of North Branch of Changjiang Estuary in 2003 and 2007, we measured these samples'grain size, heavy mineral and clay mineral, as well as magnetic parameters. These samples were comprehensive analyzed for surface sediment characteristics and dynamic sediment environment, also the assemblage and distribution of heavy mineral and clay mineral, magnetic parameters. With mathematical statistics analysis, this study explored the complexity of sedimentary environment evolution and the diversity of sediment source.The results were shown as follows:1. Most sediments in the study area are mainly composed of clayey silt, sand and silt sand; the distribution of particle mean size presents finer trend from outer to inner estuary; and majority sediments are poorly sorted-very poorly sorted; the kewness of sediment which distributes in relative deeper water area is mainly positive-very positive; and kurtosis shows platyurtic-mesokutic.2. Based on Pejurp and Folk triangular diagram, North Branch is divided into four sedimentary dynamic zones:sandspits, river channel, tidal sands and tidal inlets. Mica-stable mineral-epidote assemblage mainly distributes over the sandspits and tidal sands, where hydrodynamic condition is relative strong, as well as with coarse grains and relatively lower values of magnetic parameters:xarm,xfd%, xarm/x and xarm/SIRM; the assemblage of hornblende-magnetic-carbonate minerals mainly covers in the river channel and tidal inlets, accompanying with relative lower hydrodynamic condition and finer clayey silt, together with high values of magnetic parameters xarm,xfd%,xarm/x and xarm/SIRM. In addition, the illite and chlorite occupy the dominant composition of clay mineral, montmorillonite comes the second, and kaolinite is the minimum one; Their distribution is changeable, total decreasing from upper to the lower section, meanwhile, the assemblage interlocks and intersperse each other. We thought this situation might be related to the dual tidal currents, the interface between salt water and fresh water, as well as particles characteristics themselves.3. According to Gao-Collins Model, the trend of sediment grain size suggests that the net sediment transportation points into inner estuary mouth, so the sediment of North Branch was carried by both flood and currents from outer offshore. Moreover, the results from synthetically studying the surface sediment characteristics and dynamic sediment environment, as well as the assemblage and distribution of heavy mineral and clay mineral, and the magnetic parameters, also show the sediment source is as same as Changjiang River. It could be concluded that the sediment source of the North Branch mainly originated from Changjiang area.4. Factor and Correlation Analysis indicate that hydrodynamic condition, environmental condition, source rocks and transportation tendency are the determinative factors which control the sediment style, the distribution and combination of heavy mineral, also the variations of mixed sediment magnetic properties.