Study On The Sedimentary Facies And Environment Evolution Of DT05Core In Kushuiyang, Jiangsu Offshore, Southern Yellow Sea

South Yellow Sea radial sand ridge system contains a lot of information about sea level changes, land and sea interaction, climate changes and environmental evolution. They are the best carrier for studying marine environmental changes after the polar ice cores, deep-sea sediments and loess deposits. Radial sand ridges is a giant sea accumulation by the Yellow River and Yangtze River in the Western Pacific Rim and is a typical product of interaction between land and sea. Kushuiyang is the large type of accumulated sand ridges.The genesis and sedimentary environment changes of the tidal channel since the late Pleistocene are the great significance to understanding dynamic changes of the whole radial tidal sand ridge system.The borehole of11DT05is located on the Kushuiyang sandy ridge area (33°04.475’N,121°48.298’E) in November2011.The depth of this hole footage is36.97m, but the actual core length is22.20m, core rate is60.05%.The core was firstly logged and sampled in lab and a series of analysis including:grain size, XRF were carried out,210Pb, in combination with14C dating. Further, combined with sedimentary facies, the formation and long term evolution of sand ridges was analyzed.The results have shown the11DT05core is mainly composed of silt, with the average of76.13%, followed by sand, an average of19.82%, clay has the lowest content,with the average of3.6%. The average particle size changes between1.78～8.6Φ, average of4.7Φ.The average sorting is1.75, with a poor sorting. The average Skewness is0.58, with a very fine Skewness. The average Kurtosis is1.21, with a mesokurtic distribution. Four sedimentary facies have been distinguished:(1)Tidal sand ridge facies(0-5.69m):It mainly consists of sand silt and light olive gray, intercalated with clay patches and wavy beddings. The average grain size is6.07Φ.(2) Silt clay sedimentation(5.69-6.49m):It consists of silt clay, with yellowish-brown. The average grain size is7.40Φ. Exposed signs including iron mottling and plant-derived fragments are common in this facies.(3) High coastal tidal(6.49-13.51m):Rhythmic couplets between olive gray silt and olive clay dominated the facies.Horizontal laminations and low-angle inclined bedding are common in this interval. The average grain size is6.210. It is considered as intertidal-subtidal sedimentary facies.(4)Middle and low coastal tidal(13.51-22.20m):Rhythmic couplets between olive gray silt and yellowish-brown clay dominated the facies.Wave beddings and lenticular beddings as well as ripple beddings are common in this facies.The average grain size is4.45Φ.Facies in Kushuiyang areas show the borehole experienced middle and low coastal tidalâ†'high coastal tidalâ†'silt clay sedimentationâ†'tidal sand ridge facies since the late Pleistocene.At about60-25kaBP(MIS3.0),South Yellow Sea experienced the Wurm Glacial Stage,global climate was cold,sea-level was low,forming coastal tidal facies in the area.At about25-2kaBP(MIS2.0), the late stage of late Pleistocene,South Yellow Sea experienced the late stage of Wurm Glacial Stage,while global climate became cold again,sea-level fell,forming silt clay sedimentation. Since the middle Holocene epoch, tidal current became the most important hydrodynamic condition in this area, forming tidal sand ridge system.Interpretation of grain size, XRF were carried out,210Pb,14C dating, geochemical elements and comparison with other columns provide evidences for the discimination of these sedimentary enviroment evolution.