The Late Quaternary Core Recorded Stratigraphy And Environmental Evolution,Southern Margin Of Radial Sand Ridge System,South Yellow Sea

The distribution of sedimentary system and sedimentary facies in coastal zone and continental shelf is mainly affected by sea-level changes,sediment supply and regional tectonic subsidence.During the late Quaternary period,the transgression and regression had happened many times in the eastern coast of China in the background of climate-forced interglacial and glacial cycles and paleo Asian monsoon prevalence.Along the the western coast and shelf of South Yellow Sea,several depositional systems were set up with Quaternary deposits between 100 and 400 meters.These depositional systems include Subei coastal plain,old Yellow river delta and Yangtze river delta,radial sand ridge system,and Yangtze shoal.During the late Quaternary,the eustatic sea-level changes exceeds 150 meters.The frequent eustatic fluctuation exposed and submerged the shelf and coast alternately.The coastline migration controlled the spatial distribution of sedimentary systems in the eastern coast and continental shelf of China.In addition to the factor of sea-level changes,the sediment supply from old Yangtze river and old Yellow river also controls the development of sedimentary systems.Both the factors determined the evolution of sedimentary environment and growth of sedimentary structure in the study area.The study area is distributed between the old Yellow river delta and modern Yangtze river delta,and it is a typical sea-land interaction zone in eastern coast of China.In contrast to the Holocene deposits of Jiangsu coast and continental shelf,the research on Pleistocene is not sufficient.The mechanism of how the late Quaternary sea-level changes and sediment source changes influence the sedimentary systems should be studied.Based on the analysis of strata and sedimentary facies,augmented by grain size,magnetic susceptibility,loss of ignition and trace elements as well as foraminifera and molluscan fossils,four sedimentary facies were identified,and the evolution of sedimentary environment since MIS6 was also reconstructed.By using 14C and OSL datings,the chronological stratigraphic framework was set up.By core correlation,the spatial distribution of sedimentary facies was studied.Based on debris zircon U-Pb age,the sedimentary sources since MIS6 was provinced,too.Through recognizing main erosional interface and major facies change surface,five depositional units were delineated in an ascending order of U1,U2,U3,U4 and U5.By color,lithology,sedimentary structures,fossils,sedimentary sequences,contact relationships and auxiliary analysis of environmental indicators,four types of sedimentary facies were delineated.U1 is between 5.6 to 22.6 meters and dominated by fine sand,fine sand with thin silty clay layers and fine sand and clayey silt interlayers.Tidal bedding is main sedimentary structure in this unit,and dominated by vein bedding.Nassarius variciferus and Meretrix meretrix fossils are founded in this unit,indicating an intertidal to shallow subtidal zone.Foraminifera are mainly composed of Ammonia beccarii,Elphidium advenum and Rosalina brady,indicating a near shore environment.Sr/Ba indicates that this unit is a strong marine layer.U1 is interpreted as tidal ridge facies.U2 is between 22.6 to 47 meters and dominated by silt layers,silty clay and clay interlayers and homogeneous clay layers.Nassarius variciferus is founded in this unit,indicating an environment of intertidal zone or shallow sea.Planktonic foraminifera is increased in this unit,and both deep water and shallow water benthic foraminifera is founded in the sediment,indicating a shallow sea environment.Sr/Ba indicates that this unit is also a strong marine layer.U2 is interpreted as shallow sea facies.U3 is between 47 to 55 meters and dominated by gray or grayish yellow fine sand layers.Tidal bedding is the main sedimentary structure in this unit,and is dominated by vein and wavy bedding.Umbonium thomasi and Nassarius variciferus fossils are founded in this unit,indicating an environment of intertidal zone.There is no planktonic foraminifera in sediment.Ammonia beccarii and Quinqueloculina lamarckiana are main species in benthic foraminifera,indicating a near shore environment with sandy seabed.Sr/Ba indicates this unit is a weak marine layer.U3 is interpreted as tidal flat.U4 is between 55 to 74.5 meters and dominated by gray homogeneous fine sand layers,gray fine sand and brown gray silt or silty sand interlayers.Muddy strip formed in fine sand layers,and displayed as vein bedding.Cerithidea cingulata is founded in this unit,indicating a intertidal zone with fresh water inlet.No planktonic foraminifera was founded in this unit.Ammonia beccarii,Elphidium advenum and Quinqueloculina lamarckiana are main species in benthic foreminifera,indicating an environment of low salinity near shore.The same as U3,Sr/Ba indicates U4 is also a weak marine layer.U4 is interpreted as tidal flat,too.U5 is between 74.5 to 115.6 meters and dominated by medium sand and fine sand.Coarse sand and fine gravel could also be founded in the sediment.23 binary structures are identified,and the thickness of each layer is between 1.5 to 3.5 meters.Scouring structure could be founded frequently.A lot of calcium tuberculosis and carbonized rot wood appeared in this unit.Variety of freshwater gastropod are founded in this unit,indicating a fluvial environment.Sr/Ba in this unit is lower than 1,indicating as a land environment.According to the characteristic of U5,river channel facies and floodplain facies could be distinguished further.But,U5 is mainly composed by river channel facies.What's more,an emergence of a little benthic foraminifera in the upper section of this unit indicates an influenced by tide.Based on the above factors,U5 is interpreted as fluvial facies.By using 14C and OSL dating method,combined with stratigraphic correlation among several boreholes,the sedimentary sequence of core YZ10 was determined.All of Ul,U2 and U3 were deposition of MIS1.Based on 14C dating result,we reckon that U1 was formed since 5.5ka BP,U2 was formed between 8 to 5.5 ka BP,and U3 was formed between 14 to 8 ka BP.Based on 14C dating result and OSL dating result,we reckon that U4 was formed during MIS5.U5 is a thick layer of fluvial facies deposition,and it was formed during MIS6.Evolution of sedimentary environment of study area is reconstructed since MIS6.Fluvial environment was developed during MIS6,and gradually evolved into a tidal influenced river.During MIS5,study area developed as a tidal flat,accepted the seawater cyclical submerged,and the outside region of Yangtze river mouth developed as predelta and delta front facies.During MIS3,the study area evolved into a land environment.During MIS2,the south region of line from Yangzhou to Taizhou to Qutang to Lvsi developed river channel,and the north of this line developed interfluve,and formed a hard clay layer.The transgression of the late glaciers led to the gradual evolution of the study area into coastal tidal flat to shallow sea.After the high sea level period,tidal current disturbed the seabed sediment and developed tidal sand ridge.Age of detrital zircon of core YZ10 sediment is mainly distributed in the four characteristic ranges of 190 to 290Ma,400 to 550Ma,700 to 1000Ma and 1.8 to 2.0Ga.The age spectrum of the debris zircons of the sediments shows that the sediment source of core YZ10 is stable.The content of zircons aged between 700 to 1000Ma is higher than that of the Yellow River,together with the occurrence of zircons formed in Cenozoic era indicates that the main sediment source of study area was from Yangtze River.And the low carbonate content of core YZ10 also support this conclusion.The magnetic susceptibility is closely related to the sedimentary power.The strong dynamic condition is favorable to enrich the coarse particles,and the magnetic susceptibility is increased in this condition.And a weak water dynamic condition is not conductive to enrichment of coarse particulate matter,so the magnetic susceptibility decreased.In core YZ10,the highest magnetic susceptibility appeared in fluvial sediment,followed by other marine sediment.The acid-soluble Sr/Ba ratio has a good effect in distinguishing the land and marine sediment.The ratio higher than 1 represents marine facies,while the ratio lower than 1 represents land facies.Study shows that the marine sediment accumulated in study area are mainly developed under a tidal controlled environment,sea level together with sediment supply are main factors controlling the development of sedimentary system.There is no marine strata formed in MIS3,and the marine strata originally considered to be formed in MIS3 belongs to MIS5 actually.