Study On The Stratigraphy And Sedimentary Environment Evolution Of Northern Radial Sand Ridges Since The Late Pleistocene,Jiangsu Offshore,Southern Yellow Sea

As a special landform, radial sand ridges contain a lot of information about environment evolution. Studying in radial sand ridges is of great significance to revealing sedimentary environment evolution in the shelf areas of Eastern China,predicting the trend of marine environment evolution.And so is to port development,new energy and tourism resource utilization.Different regions in the radial sand ridges experienced differences in environment evolution. Dabeicao eastern sand ridge,as a typical representative of the radial sand ridges,we can further understand the forming mechanism and development of the whole radial sand ridges with the study of sedimentary environment evolution, genetic mechanism,material sources since the late Pleistocene.Results of the research can also be the guidance of Jisangsu coastal development promoting development of marine economy in Jiangsu.The borehole was obtained in Dabeicao eastern sand ridge areas(Core07SR03,33°32.982’N,12°24.170’E) in December,2007.The depth of the core was70.3m.The core was firstly logged and sampled in August,2011,and after a series of analysis including:grain size,foraminifera,megafossils,mineral,geochemical elements were carried out,in combination with14C dating and interpretation of shallow seismic record,the formation, provenance and long term evolution of Dabeicao eastern sand ridge was analyzed systematically. The results have shown the core is mainly composed of silt,with the average of74.40%,followed by sand,an average of18.47%,clay has the lowest content,with the average of7.13%.The average particle size is4.9φD.The average sorting is1.69,with a poor sorting.The average Skewness is0.22,with a fine Skewness.The average Kurtosis is1.03,with a mesokurtic distribution.Mineral analysis shown light minerals mainly contain quartz and feldspar,and heavy minerals is mianly composed of hornblend,epidote and rock debris.Assemblage type in clay mineral is illite+montmorillonite+kaolinite+chlorite.Illite has the hightest content,with the average of67.61%.The results of geochemical elements and clay minerals shown that material from the Yellow River dominates this area before the late Pleistocene,while the Yellow River and the Yangtze River exerted influence in this area since the Holocene.Six sedimentary facies have been distinguished.(1) Tidal sand ridge facies(0-1.8m):It mainly consists of sand silt,light olive gray,intercalated with clay patches and wavy beddings. The average grain size is3.90φ.Foraminifera are common in euryhaline shallow water.White neverita didyma can be found in this facies.(2) Shallow marine estuaries(1.8-14.0m):It mainly consists of silt,olive gray.The average grain size is4.850. Foraminifera includes Ammonia beccarii, Spiroloculina lucida, Ammonia annectens, Spimloculina subtenuis, Rotaliids, Sigmolilina subtenuis,which are common in euryhaline shallow water.Bivalve fossils and gastropod fossils dwell in shallow water is of a great quantity.(3) Coastal tidal(14.0-17.8m):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 is6.10φ. Bivalve fossils and gastropod fossils include Ostrea (Crassostrea) pesitigris Hanley,Corea,sp.and so on. Megafossils in the interval is a mixture of freshwater and saltwater species. It is considered as a intertidal-subtidal sedimentary facies.(4) Silt clay sedimentation(17.8-20.9m):Silt clay,yellowish-brown,the average grain size is5.270.Exposed signs including iron mottling and plant-derived fragments are common in this facies.Foraminifera and megafossils are unavailable in this facies. (5) Coastal tidal(20.9-52.7m):Silt,olive gray,intercalated with yellowish-gray clay layers. The average grain size is5.20φ.Horizontal laminations and low-angle inclined beddings are common in this interval. Bioturbation and shell fragments can be seen in silt layers.Charcoal can also be seen in some silt layers.Plant roots and iron tuberculosis can be seen in clay layers. Foraminifera includes Ammonia annectens,Sigmolilina subtenuis,Elphidium ibericus, Nonion grateloupi Rotaliids Ammonia grateloupi, Pseudorotalia schroeteriana, Asterowtalia subtrispinosa and so on.(6) Fluvial facies(52.7-70.3m):It mainly consists of fine sand with horizontal laminations.The average grain size is4.00φ. Exposed signs including iron mottling and plant-derived fragments are common in this facies.Foraminifera and megafossils are unavailable in this facies.The results of Sr and Ba show geochemical elements is a good indicator to palaeosalinity.The vertical variation of Sr/Ba,Ca/Mg and W(Na+K) has a significant law,suggesting this area experienced two transgressive-regressive cycles from the bottom to the top.The results of geochemical elements and interpretation of sedimentary facies are in good agreement,so it is feasible to use geochemical elements as a indicator to palaeosalinity and sea level.Facies in Dabeicao eastern sand ridge areas show the borehole experienced fluvial faciesâ†'down-coastal tidalâ†'silt clay sedimentationâ†'upper coastal tidalâ†'shallow marine estuariesâ†'tidal sand ridge since the late Pleistocene.At about60-70kaBP(MIS4),South Yellow Sea experienced the last glaciation-Wurm Glacial Stage.In early Wurm Glacial Stage,global climate was cold,sea-level was low,forming continental strata in the area(F6). During the wurm interglacial stage(MIS3),South Yellow Sea experienced the fluctuation of a significant warming.Sea-level raised during this time.forming marine strata(F5).At about30-16kaBP(MIS2),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(F4).At about10kaBP,South Yellow Sea experienced the epoch of late Holocene, climate became warm,sea-level raised,forming series of marine stratas(coastal tidal-shallow marine estuaries-sand ridges). During the Holocene, tidal current continued transforming underlying sediments and shaping topography of tidal sand ridges.Interpretation of shallow seismic record, foraminifera,megafossils,14C dating, geochemical elements and comparison with other boreholes provide evidence for the discimination of these sedimentary enviroment evolution.