The Analysis On Sedimentary Environment Of Xinglong Sand, North Branch Of Changjiang Estuary

The research about core XL1, XL2 and section STG on Xinglong Sand in the North Branch of Changjiang estuary have mainly been focused on the modern aggradation and analyzing the sediment process of North Branch of Changjiang estuary through the hydrodynamic condition, while research works on the sedimentary environment. After many sampling in the profile of the XL1, XL2 core and STG section, I have done the particle analysis, the environment magnetic analysis and the macrofossil analysis. Then the main results can be concluded through the above mentioned analysis.1. The grain size composition of Xinglong sand is mainly fine and coarse sand, which reflects the hydrodynamic condition is stronger. The particle parameters and the curves of the typical samples' demonstrate the change of the stronger and weaker hydrodynamic condition in different depth. The rhythm of the grain size change along the depth is distinctness , and it indicates the hydrodynamic change take on obvious pulsatile trait.2. The lower value of χ,SIRM shows the content of magnetic grains in the Xinglong sand is very little. The variety of S-100% between71.9 and 89.1 and the variety of Bcr between 6.3 and 30.2, along with F300% preponderates over 90%, as indicates the ferrous magnetic mineral dominating magnetic properties. The lower value of χarm/χ, χ/SIRM and χfd% shows that the pseudo-single domain(PSD) and multi-domain(MD) dominate the particle of the magnetic mineral, and that the Super- paramagnetism(SP) is less, which also indirectly reflects the hydrodynamic condition is stronger.3. The number of foraminifera is rather more. The most are between several hundreds and thousands, the size of the foraminifera is smaller, the species are few, E.naraensis, A. convexidorsa, A. beccarii vars.etc. dominating the benthic ferminifera, the plank foraminifera is also distributing in Changjiang estuary. The distributing feature of foraminifera indicating that salinity controls the foraminifera's distribution.4. Based on comprehensive analysis of the content variation of grain size, the magnetic analysis, the foraminifera analysis and other analysis, it can be found that the mode of environmental evolvement of the Xinglong sand : tidal channel-tidal current ridge-tidal flat- subtidal zone- intertidal zone- supratidal zone - inning'ssedimentary pattern. So we can speculate on the North branch of Changjiang estuary is analogy with the Xinglong sand's, that is it may follow the pattern of tidal channel-tidal current ridge- subtidal zone- intertidal zone- supratidal zone - inning and at last will combinate with shore. And then, the north branch of Changjiang estuary will silt.5. The formation and evolement of Xinglong sand have something extraordinary with the change of the spot of spring tide of north and south branch of Changjiang estuary, and that the conversion of tidal current from ebb tide to flood tide domination is pivotal factor. Many other factors also affected the Changjiang estuary's alternating change, for example the sea level's rise and the changes of sand from the upriver, which affected by the change of the climate etc., all these are the primary factors of the Changjiang estuary alternating between the south and the north branch.