Researches On The Structure Of Holocene Sedimentary Strata,Off The Abandoned Yellow River Delta Coast

During the Holocene,the environment of the abandoned Yellow River Delta has changed dramatically.The environment changed in the short term,including the period of transgression,times of sea-level fluctuation,sea-level stable period and the period of erosion after losing the source of sediment,therefore,the formation process of the subaqueous deltas is very typical,making the region become an ideal area to study of the progress of continental-oceanic interaction.In recent decades,the research on the deposition of the abandoned Yellow River coastal shelf has been very deep,and a lot of research data have been accumulated in the past decades.On this basis and the reach of the newly acquired shallow seismic profiles data,this article analyzed the structure of Holocene sedimentary strata off the Abandoned yellow River delta coast.The shallow seismic profiles survey lines has a total length more than 600km,its interpretation shows there a 2 distinct interfaces in the Holocene strata,named T2 interface andTl interface from bottom to top.By comparing with the existing borehole stratigraphic division,and combined with previous studies,it concluded that T2 is a transgressive interface formed during the last deglaciation transgression,TO is the interface formed by the increase of sediment sources in the late Holocene.Accordingly,the Holocene strata are divided into 2 layers,the U2 layer is a transition layer cause of rising sea reformed the Late Pleistocene continental strata during the last deglaciation,U1 is the sediment layer formed under the condition of sediment input was increase in the late Holocene,mainly during the period of the Yellow River flow to northern coast of Jiangsu Province in 1128-1855 years.The internal bedding structure of U1 layer transfers from horizontal to obliquely,its morphology has obvious characteristics of underwater delta deposits.The deposition structure in U1 unit layer is continuous,which could be interpreted as it was formed in the same period.Judging from the borehole data of dating,the U1 unit layer is mainly formed in the late Holocene epoch,that is,nearly in recent 1000 years.The time is highly consistent with the period of time when the Yellow River inputed into the South Yellow Sea.So it can be judged that the main body of the U1 unit layer is the Abandoned Yellow River subaqueous delta deposit.The contact relationship between U1 layer and U2 layer is unconformity,which shows clearly that there is lack of deposition between U1 and U2.Boreholes data also show that there is a discontinuity between the early Holocene and late Holocene in this region.This result indicates that the missing layer is the deposition in the middle Holocene for about 6000 years,it also means that the process of the study area is not continuous.The morphology of the Abandoned Yellow River subaqueous delta is composed of three parts:the nearshore steep slope,the middle section of the wide platform and the distal gently inclined slope.The angle of internal bedding structure of U1 is gradually increased,which is closely related to the changes of the underlying topography.T1 have a 20m small incline Angle within the depth of 20m,and the U1 unit also keep horizontal bedding layer.As the T1 interface becomes more inclined,the internal bedding of U1 unit layer increases.Within the scope of the study,the T1 interface gradually deepened to the east,U1 layer also gradually thickened to the east,which led to deposition center of the Abandoned Yellow River Subaqueous Delta delta leaning to the East.According to the interpretation of seismic profiles,the general scope of the Abandoned Yellow River subaqueous delta was defined,and calculated its deposition amount.The Abandoned Yellow River subaqueous delta extends more than 180km,its area up to 25000km,and extends far more than 122°10'E.With an average deposition thickness is 12.5m,the total deposition amount of the Abandoned Yellow River subaqueous delta is more than 300 billion m3.The distal part of the Abandoned Yellow River subaqueous delta was interrupted by the Radial tide of the South Yellow Sea.