The Analysis On The Sedimentary Characteristics Of DT Core In Chong Ming Island And The Quaternary Sedimentary Environment Evolution Of North Branch Of Yangtze Estuary

In order to probe the sedimentary environment evolutionary process of Chongming Island and the North Branch of Yangtze Estuary during the Quaternary,this paper,based on the multiple experiments at the core sediment samples in Chongming Island(DT core),such as its grainsized separation and composition,magnetic parameters and magnetic mineral measurement, geochemical and foraminifera analysis,¹⁴C dating measurement,has established the high-resolution sedimentary facies and stratigraphic sequences of the North Branch of Yangtze Estuary since Quaternary.The main results can be concluded as follows:1.DT core sediments are mainly composed of clay and silt.Based on lithology,sedimentary textures and physical sedimentary structure,in accordance with analysis of standard deviation of grain size parameters(σ1),skewness(SK1),and kurtosis(KG) of the core,the DT core can be grouped into seven depositional facies.The analysis of Pejrup triangles showing an upward-fining succession,which presents a tide-dominated sedimentary sequences.2.Susceptibility,non hysteretic remanence,saturation isothermal remanent parameters,the hard remanence HIRM,S-300,S-100,the frequency of susceptibility Xfd%,Xarm/X,SIRM/X shows that the magnetic properties is dominated by ferromagnetic minerals.Profile changes of magnetic characteristics are in response to the dynamic conditions.The correlation analysis between magnetic parameters and sediment grain size,as well as magnetic minerals,indicates that the magnetic minerals exist in different grain-sized particles,magnetic susceptibility varies with the grain size.3.According to the analysia of chemical elements in DT core sediments,the major elements are mainly originated from terrigenous composition,for example,Al,Si,and Rh,Ti,Fe,Zr,Ba, etc.The elements are divided into three groups:the first element group:Al,K,Ti,Fe,Si,Rb,Mn, etc.,on behalf of terrigenous debris deposit,and the abundance of these elements is controlled by the particle size;the second group of elements:Ag,Sn,Ba and so on,is the role of marine biological and sediment regeneration;the third group of elements:Ca,Sr,Zr,on behalf of the biogenic deposit. 4.The DT core is abundant in foraminifera which dominated by Ammonia beccarii vars., Ammonia convexidorsa,Elphidlum magellanicum,Epistominella naraensis.The foraminifera distribution of profile,combined with the foraminifera species and the dominant species,reveals that the sediment core can be divided into six foraminifera assemblages.Foraminifera distribution in DT core was controlled by the hydrodynamic conditions,particularly the tidal current play a leading role in the region.5.Through systematical analyses and comprehensive study of all above sedimentary records, DT core can be divided into seven sedimentary facies since Quaternary,from bottom-up followed by Floodplain;Estuary-shallow sea;Prodelta;Delta front;Tidal sand ridge;Tidal channel; tidal flat;and Soil.The sediment sequences of DT core indicates a clear transgressive-regressive sequences after the Last Glacial Maximum.