Post-glacial Sediment Environment Evolution Of The Sub-aqueous Yangtze Delta

The formation and evolution of the sub-aqueous Yangtze delta depositional systemare closely related with regional sea level change and terrestrial sediment supply.Research on the spatial and temporal distributions of the delta depositional system couldcontribute to clarify the following scientific issuses, e.g. the depositional pattern andtransportation mechanism of Changjiang supply materials, the Yangtze delta evolutionmodel and its response to tectonic-climate-sea level fluctuations, different spatial andtemporal scales of geological events and its records in the Yangtze delta strata.Modern sedimentary environment of the Changjiang estuary and its adjacent areawas studied based on the grain-size analysis of3081surficial sediment samples andmodern sedimentary rates derived from²¹⁰Pb dating. And still the evolution of thePostglacial depositional environment in the subaqueous Yangtze delta and relatedpaleoenvironment changes, were discussed according to the lithology, sedimentary faceanalysis, AMS-¹⁴C dating, grain-size analysis, magnetic susceptibility, major and traceelements, TC-TN-TOC, elements contents derived from ITRAX core scanner andseismic data.Research on the modern sedimentary environment was summarized as follows.（1）7types of sediment distributed in this study area were defined, namely sand, siltysand, mud sand,sandy silt, sandy mud,silt and mud, according to the Folk classbased on the grain-size analysis. On this background seafloor classification mapwas compiled and corresponding well the dynamical environment distribution.（2） On the basis of surfacial sediment distribution, five different depositionalenvironments in the present study area, namely the subaqueous delta, the Subei radial sand ridge, Hangzhou Bay, shallow marine shelf and transitional belt, andtheir boundary are defined.（3） The distribution of mud area off the Changjiang estuary was defined based on thedistribution of sediment types and modern sedimentation rates. It was confinedby the Ma-an Island and Shengsi Island in the south,122.8°E in the east and31.2°N in the north. The highest sedimention rate, about6.3cm/yr, occurs on thesedimentary center of the mud area, around31.2°N,122.9°E.Research on the evolution of post-glacial depositional environment in thesub-aqueous Yangtze delta was summarized as follows.（1） Prominent transgressive surface and maximum flooding surface existed in theYangtze sub-aqueous delta strata indicated by seimic profiles. TST depositionalsystem between TS and MFS is composed of TST1，formed during earlyPostglaciation, and TST2. HST depositional system above MFS is composed ofthe earlier HST1and the later HST2.（2） TST1，composed of intertidal-subtidal, upper tidal and estuary upwards, wasinduced by the slow sea level rising after melt-water pulse1A （MWP-1A）,Younger Dryas event and the melt-water pulse1B （MWP-1B） correspondingly.The14m thick high stand system tract TST2, composed of transitionaldepositional facies and shallow marine facies, formed from8.2cal kyr BP to themid-Holocene high sea-level period. HST1, the prodelta sedimentary system,formed after the mid-Holocene high sea-level periods, composed of massive mudthins offshore, pinching out at about5-6km south-east of core CJK08. HST2isabsent around core CJK08, pinching out at water depth30m,7km north-east ofcore CJK08.（3） Results of core CJK08, obtained from the most east of Changjiang subaqueousdelta show a tidal river depositional environment from14.3to13.5cal kyr BP, anestuary system from13.5to8.2cal kyr BP and a transitional-shallow marineenvironment since8.2cal kyr BP. The tidal river depositional system is characterized by the alternations of sand and sand-mud intercalations with lowcontent of S and Cl, and high sedimentation rate of0.73cm/yr. Transitonal andshallow marine depositional system are mainly composed of massive mud withextremely high sedimentation rate about0.93cm/yr during8.2-6.7cal kyr BP andlow sedimentation rate about0.028cm/yr since6.7cal kyr BP. We explain theextremely high sedimentation rate during8.2-6.7cal kyr BP as a result of strongmarine processes mostly dispersed the suspended sediments since the Holocenehigh sea level and the low sedimentation rate since6.7cal kyr BP induced bystarvation of modern sediment supply and southern transport of Changjiangmaterials since2cal kyr BP.（4） Core CJK10, with17.2m thick postglacial sediment, is located around the islandsoff the south of the subaqueous Yangtze delat. Floodplain, shore swamp and tidalflat dominated during11.2-9.6cal kyr BP, with the highest sedimentation rate7.5m/kyr. A hiatus occurred during9.6-4.2cal kyr BP, the transition from tidal flatto subtidal flat, caused probably by lack of sediment supply deduced by theabruptly rising of sea level. Subaqueous delta depositional sedimentary facies,with a thickness of only³m，developed since3.2cal kyr BP.（5） The mean sea level during10.6-9.6cal kyr BP, about36.3-33.2m below modernmean sea level, was reconstructed according to the AMS-¹⁴C dating from intertidalflat layer vs. depth of core CJK08.（6） During12-8.2cal kyr BP deposition center, with thickness thinning from land tosea, was located around the paleo-Changjiang river channel area.8.2-7.5cal kyrBP the whole sub-aqueous delat received rare sediment deposition, whereas thecore CJK08recorded relatively thick sedimentation, about10m.7.5-2.5cal kyrBP deposition center located more landward, between core JS98and core HM03.The depositional thickness, around sub-aqueous delta area, at water depth deeperthan20m, is less than2m during this period. Since2.5cal kyr BP, area betweencore JS98and core HM03exposed. And duiring this period, the deposition center is obviously diveting to southeast indicating that more Changjiang materialssupplied southeastwards and southwards.（7）7.5cal kyr BP, during the Holocene high sea level stands, delta depositionalsystems intiated around the upper apex of the paleo-Changjaing river channel.During7.5-2.5cal kyr BP thick prodelta-prodelta and estuary sand shoaldepositional system developed around the west of Chongming island. Deltadepositional system is not well developed around the subaqueous area, e.g. about5.9cal kyr BP prograding the area around core ZK9, and about4.2cal kyr BParound core CJK10.