Shallow Sequence Stratigraphy And Sedimentary Evolution Of The Xiyang Tidal Channel In The Radial Sand Ridge Field

Since Marine Isotope Stage(MIS)5,the geomorphic and sedimentary evolution in the coastal ocean of China is mainly controlled by the global cycles of sea-level changes and the complicated land-sea interactions during these cycles.Under this circumstances,the Radial Sand Ridge Field(RSRF)began to develop and it is the giant sandy deposits,which was mainly supplied by the Changjiang,Huanghe and Huaihe Rivers,sediments in the western South Yellow Sea.Meanwhile,it is also the important product of land-sea interactions in the Eastern China.The Xiyang tidal channel is the biggest one in the northern part of the RSRF and located between the tidal flat coast and the tidal sand ridges,therefore,it is the typical area for the study of the sedimentary sequence and evolution in the middle Jiangsu land-sea interaction zones.Based on the first-hand data,comprised of high resolution shallow seismic profiles which could cover the whole Xiyang area basically and core 07SR01 in the southern Xiyang,plus the well-studied cores and sections collected from Xiyang and its adjacent areas,this study finished detailed analyses and correlations on the seismic and core stratigraphy respectively.Meanwhile,in order to remedy the current insufficient studies on cores,we attempted to use the existing knowledge in the nearshore areas of the Abandoned Yellow River estuary,which is located in the north of Xiyang,to extrapolate the sedimentary environments and chronology of the seismic unit 3(U3)in the northern Xiyang,and then,finished the correlations of core and shallow seismic sequences for the whole Xiyang area.In the end,we established the sequence stratigraphy framework of the Xiyang tidal channel preliminary and deduced the history of sedimentary evolution macroscopically.The main results of this study as follows.(1)Five seismic units(U1?U5)could be recognized in the Xiyang tidal channel downward from the seabed and there are consistent differences in the sedimentary sequences between the northern and southern Xiyang.U1,characterized by foreset,parallel and subparallel reflection configurations,is located in the areas in which the erosion and deposition are concomitant and mainly belonged to modern tidal ridges and tidal channel infillings.U1 probably began to develop from AD 1128,since then the Abandoned Yellow River and its sediments influenced the north Jiangsu coast.U2,characterized by complicated cut and fill reflection configurations,is located in the whole area and the thickness of U2 probably increases from north to south.The deposits of U2 are mainly belonged to estuary/shelf tidal sand ridges and tidal channel,and probably began to develop since the Holocene transgression and high sea-level stand.U3,characterized by parallel and subparallel reflection configurations with good continuity,relatively low amplitude,relatively high frequency,is only located in the northern Xiyang and the thickness of U3 decreases from north to south until it wedges out obviously.U3 should be nearshore and neritic muddy deposits and probably developed during 9?5 ka BP.U4,characterized by subaparallel and microwave-shape as principle plus cut and fill reflection configurations,is located in the whole area and mainly developed in the floodplain with rivers and lakes during the sea-level lowstand between the end of MIS 3 and MIS 2.Partial U4 could become the stiff mud layer and is belonged to coastal lacustrine deposits during the Last Deglaciation.Due to the insufficient identifications of U5,the spatial distribution of U5 is still not clear,and the southern Xiyang uncovered that U3 is belonged to marginal bank and riverbed deposits in the tidal estuary and characterized by the similar reflection configurations of U4.(2)The sequence stratigraphy framework of the Xiyang tidal channel since MIS 3 as follows.Type I sequence developed during this period.The top surface of stiff mud layer or the incised surface of paleovalley are the sequence boundary(SB).Above the SB is the Post Glacial sequence(Sql)and the Last Interstadial sequence(Sql)is below the SB.The transgressive system tract(TST)and highstand system tract(HST)could be identified in the Sql.The maximum flooding surface(MFS)is the boundary of TST and HST,and could be put in the nearshore and neritic muddy deposits which comprise the main part of TST in the northern Xiyang.Above the MFS,the HST is mainly comprised of tidal sand ridge and tidal channel deposits.In the southern Xiyang,the MFS should be in the tidal sand ridge and tidal channel deposits.By now,only the HST and forced regressive wedge system tract(FRWST)could be identified in Sq2 and it is difficult to distinguish the lowstand prograding wedge system tract(LPWST)and FRWST in these two sequences.For Sq2,the HST in the southern Xiyang is mainly comprised of marginal bank and riverbed deposits in the tidal estuary,however,it is not clear in the northern Xiyang;the FRWST is comprised of floodplain or small incised valley deposits.(3)The sedimentary evolution history of the Xiyang tidal channel since MIS 3 as follows.During the sea-level highstand of Last Interstadial,marginal bank and riverbed deposits in the tidal estuary developed at ca.39 cal ka BP,and maybe it is similar for the northern Xiyang.At the end of MIS 3,global sea-level began to fall and continental floodplain deposits have developed at ca.26 cal ka BP,and this kind of deposits could continue to develop during the sea-level lowstand of MIS 2,even before the transgressive impact during the Last Deglaciation.During this sea-level lowstand,small incised valley also could develop.The Last Deglaciation began at ca.15 cal ka BP,and the step-like and rapid rise of postglacial sea level occurred as a result of global meltwater pulses.In the Xiyang area,the transgressive surface developed at ca.11.8?11.4 cal ka BP;coastal lacustrine deposits and transgressive basal peat developed with common storm records at ca.11.4?9.6 cal ka BP;this area was totally covered by seawater and became littoral at ca.9.5-9.2 ca ka BP.Since ca.9 ka BP,the Xiyang area became nearshore and neritic environments,however,the northern and southern part of Xiyang area have undergone different progressions of sedimentary evolution.For the northern part,it underwent nearshore-neritic muddy depositional environments and tidal sand ridge/channel successively.The transition age is ca.5 ka BP.However,tidal sand ridge/channel environments continued since ca.9 ka BP in the southern part.The submerged tidal sand ridges developed before AD 1128 and were constantly migrating.During AD 1128 to 1855,due to the southward migration of the Yellow River mouth to the northern Jiangsu coast,the Xiyang area was infilled with a large amount of river-derived sediments,causing the emergence of some of the previously submerged tidal sand ridges.From AD 1855 to the present,the infilled Xiyang area has undergone scouring,resulting in its modern form.The modern Xiyang tidal channel continues to widen and deepen,due both to strong tidal current scouring and anthropogenic activities,however,the general scouring intensity has decreased obviously compare with 1930s.