The Study Of High Resolution Sequence Stratigraphy Of Holocene In The Yangtze River Delta Area

Delta is located in land-sea interaction zone, and it is particularly sensitive toclimate change, tectonic movement and sea level fluctuation, so it is the best area forsequence stratigraphy research. The subqueous Yangtze delta is the components of theYangtze River delta system as well as the intermediate link of Land and continentalshelf，so the sequence stratigraphy research of subqueous Yangtze delta is useful tosolve the current controversial issues such as the desertization environment of shelfregion in east china sea during the last stage of late Pleistocene and the flow path ofYangtz river at the same time.This paper is based on the integrated survey and evaluation of specific project inmarginal sea of China（908-02-02-05）and China national marine special project forpublic interest―The Technical Research on vulnerability Survey and GeohazardEvaluation of major deltas of china（200805063）‖.We divided sedimentary facies ofboreholes CJK06,CJK07and CJK09using analysis of lithology, grain-size, traceelements, micropaleontology and AMS-14C dating, and fluvial facies, tidal flat facies,estuary facies, shallow sea facies, prodelta facies composed the holocene sediment ofsubqueous Yangtze delta. Combined the core with shallow seismic profile,we made theidentification of sequence boundary and the sequence division of the the holocenesediment, and we also did some research with the age,thickness,the internal sedimentaryfacies and the reflection character of the sequence units. On the basis of sequenceanalysis of boreholes and shallow seismic profile, we carried out high resolutionsequence stratigraphic correlation, and established the Yangtze subaqueous deltaholocene sequence stratigraphy evolution model based on sea level curve.Sequence stratigraphy research showed that the holocene sedimentary strata ofsubqueous Yangtze delta is a sixth-order Ⅰ type sequence.18kaBp, the sea leveldropped to the shelf140m water depth, and the area of Yangtze river delta wereexposed, then incised valley developed.The yellowish green paloosol in the inteuluveand the erosional surface under the incised valley form theⅠtype sequence boundarywhich manifested as widely distributed, rugged strong reflector on the shallow seismicprofile.13~11kaBp,postglacial transgression was present in the area of subqueous Yangtze delta,and reached the maximum until7.5kaBp.The MFS was in the shallow seasediment, and manifested as a clear seismic reflecting interface T2with strong energy，high frequency and good continuity.Since about2kaBp,the shoreline rapidly advancedto the east china sea,and prodelta reached most area of the current subqueous Yangtzedelta.The T1intensive reflecting interface formed because of difference of physicalproperties between the sediment of prodelta and shallow sea,so highstand system tractwas divided into early highstand system tract（E-HST）and late highstand systemtract(L-HST).The holocene stratigraphy was composed of transgressive systemtract(TST),the early highstand system tract(E-HST) and the late highstand systemtract(L-HST),and the evolution process was divided into the following four stages.Stage Ⅰ（13~7.5kaBp）: The sea level rose quickly with global warming, sedimentsupply rate was far lower than the rate of sea level rise, so the retrogradationaltransgressive system tract(TST) formed. It was estimated that the mean sea level risingrate was about10mm/a and the sediment rate of the subqueous Yangtze delta laybetween1.64~7.03mm/a during this period. The transgressive system tract was betweenMFS and SB reflecting interface with paralle or subparallel seismic facies.It had anonlap contact relationship with T3reflecting interface at the bottom,and had toplap orconcordance contact relationship with T2reflecting interface at the top. The thicknessof TST was larger near estuary and got thinner seaward.The maximum thickness wasabout30m from boreholes.The incised valley had a huge accommodation space andsediment supply,so the thickness of TST composed of river,tidal river, estuary, andshallow sea face was bigger than interfluve,which was composed of coastal tidal flatand shallow sea face.StageⅡ（7.5~6kaBp）：The sea level kept rising but more slowly than StageⅠ,andthe sediment rate was just below the mean sea level rising rate,and so the Yangtze riverdelta began to develop in aggradation status forming the early stage of E-HST when thesubqueous Yangtze delta area was in shallow sea environmentStage Ⅲ（6~2kaBp）：The sea level tended to be stableduring this period when theYangtze River carried large amounts of detrital material to form the early highstandsystem tract（E-HST）.It located between T2and T1reflecting interface,and got thinnerseaward as wedge shaped depositional bodies.In the southern sea area,the E-HSTshowed low angle downlap onto the T2or T3reflecting interface with micro oblique seismic reflection structure，but it showed concordance to T2reflecting interface withparallel reflection structure in the northern and middle sea area. The internal reflectorof E-HST showed toplap onto T1or T0(seafloor) reflecting interface at the top. Thedistribution scope（the distance of extension seaward）increased southward due to theamount of sediment supply changed and the the post-depositional erosion ortransformation by ocean tide and other dynamic current with the southward migration ofthe estuary.With the southward migration of the estuary, the E-HST in the northern andmiddle sea area was transformed in the internal structure and surface morphological,and the E-HST got missing at different degree. The core showed the maximumthickness was nearly25m composed mostly of shallow sea sediment.Stage Ⅳ（2~0kaBp）：The sea was relatively stable,and the accommodation spacewas no longer expanding，but the quantity of sediment supply increased rapidly，so thecoastline advanced seaward more rapidly than past nearly5500years to form the latehighstand system tract（L-HST）.The L-HST located between T0and T1reflectinginterface with the same distribution and variation as E-HST,but the the distributionscope（the distance of extension seaward）got samaller than E-HST with few kilometersto20km migration westward of the east boundary. The L-HST has a clear reflectionstructure with strong energy，good continuity and relatively homogeneous period.It hada downlap contact relationship with T1reflecting interface at the bottom and toplapwith T0reflecting interface at the top.During the period of L-HST,the depocenter hadmoved to the southern region of Yangtze subaqueous delta, the maximum thickness ofL-HST was about12m of borehole ECS0702,but other boreholes showed the thicknessof less than10m.On the basis of Sequence stratigraphy analysis of the subqueous Yangtze delta,wedid the high precision comparison between the land boreholes（HQ98,JS98and CM97）and the underwater boreholes used in this paper,then the Yangtze river estuary evolutionmodel was established according to the Walther’s phase-rule. The sedimentaryenvironment horizontal distribution was river, tidal river, estuary, shallow sea from landto sea.The Sediments are mainly from the Yangtze river,and particle size becamesmaller seaward.There was no tidal sand ridge in the Yangtze river estuary. Weconcluded that the the tidal sand ridge found in the boreholes of Hongqiao, Huangqiaoand Jinsha area formed in the time of high sea level,and there would be tidal sand ridge in the northern regions during the whole evolutionary process of Yangtze river deltawith coexistence patterns similar to today’s North Jiangsu tidal sand ridge district andthe modern delta distribution.The progradation rate of Yangtze river delta abruptly became bigger in6kaBp and2kaBp, so the evolutionary process of Yangtze river delta was divided into three stage:aggradating-low progradaing phase (7.5~6kaBp), normal progradating phase(6~2kaBp)and rapid progradaing phase(2kaBp~). The sea level had experienced short-term slowrise from7.5kaBp~6kaBp, the Yangtze river delta promoted slowly in the aggradation-low progradaing state,and the eastern boundary was maintained at about120.5°E. Thenduring the period from6kaBp to2kaBp, sea level rising trend got more weak,and thedelta advanced seward significantly.At the time of4kaBp,the delta front had moved tothe vicinity of120.5°E,and the prodelta reached the vicinity of122°E.Then at the timeof2kaBp, the delta front had passed the location of borehole JS98,and the prodeltareached the vicinity of122.4°E. From2kaBp to now,the delta advanced more rapidlythan other two previous stages.At1kaBp, the delta front had passed the location ofborehole CM97,while the location of borehole JS98developed into land,and theprodelta had psssed the location of borehole CJK11,CJK09and CJK06reaching thevicinity of122.7°E.So far away from1kaBp,the rapid promotion of delta front is moreobvious.The delta front reached the vicinity of122.2°E at present while the location ofborehole CM97developed into land,and the prodelta reached the vicinity of123°E.