Discussion On Holocene Infilling Processes Of Incised Valley Of The Yangtze And Its Controlling Mechanisms

Since the Last Deglaciation,sea level has risen globally and coastal landform has been changed accordingly.When sea level stabilized,the material from river basin accumulated in the estuary and formed estuarine-delta zone in the early-middle Holocene.Delta system is regarded as the largest sink of terrestrial source materials in the land-sea interaction zone,which contains rich information of environmental changes,and witnesses the geological and geomorphic evolution.Moreover,because of its superior natural conditions,the river delta has been a favorable area for human beings since the early civilization.At the same time,it also faces the dual pressure caused by drastic human activities and environmental changes.Therefore,to study the evolution process and mechanism of the estuarine-delta system has become the core problem in many disciplines from the systematic perspecoueoutive of the coupling of climate,environment and human activities.The study on patterns of the Holocene delta evolution and the response of the delta to sea level rise and climate change can satisfy the prediction of the future evolution trend of the estuarine-delta environment on a time scale.Although some achievements about the development of the delta have been made,the early studies are inadequate in having ¹⁴C-supported frameworks.This has led to some insufficiencies of deltaic sedimentary facies to be explained reasonably,and sedimentary pattern of estuary of the Early-Middle Holocene is still unclear.Based on database established by a large number of boreholes in the study area,the basal elevation of the Holocene was obtained to help reconstruct the pre-Holocene topography.The results show that the pre-Holocene incised valley opens seaward from ca.15 km in width at the delta apex to 120 km wide in the present river mouth area.The valley,50⁷0 m below present MSL is flanked by a large promontory-like topography?10³0 m below MSL?to the south,i.e.the southern delta plain where Taihu Lake is located,and is beside an extensive lower topography,ca.20 m below the MSL to the north.This 20-m contour extends northward,nearly in parallel to the present coastline,presently part of the northern delta plain?Lixiahe depression?.The topographic features of the lower valley make the sediments distributed in a slightly“asymmetric”state.This valley provides a broader spatial condition for the preservation of sediment and promotes formation of strong tidal currents as seen today.The geometry of incised valley is recognized as a key factor to control the infill of the valley through hydrodynamic conditions.The sediment core HM was taken as targeting the research objectives.The chronological framework,stratigraphic sequence and environmental change were reconstructed by means of particle size,foraminifera,geochemical elements and AMS-¹⁴C dating.The result showed that during 12,100¹0,900 cal.a BP,the sedimentary facies of HM was featured with a sedimentation rate of 1.2 cm/a.Tidal infill is characterized by the lower-salinity foraminiferal species?Ammonia convexidorsa?and lower Sr/Ba value?4.10 in average?.During 10900⁹100 cal.a BP,the estuarine facies was recognized with a sedimentation rate of 1.2 cm/a.The foraminifera species were dominated by those of shallow-water nature and low-salinity?Epistominella naraensis and Ammonia convexidorsa?,and the Sr/Ba value is 5.69 in average.Tidal sand ridge was formed during 9100⁵700 cal.a BP,in which there occurred highest Sr/Ba value?7.21 in average?and lowest foraminifera abundance.This probably resulted from strong tidal currents due to maximal marine transgression of the Early Holocene,to control the development of tidal sand ridge.Since 5700 cal.a BP,deltaic sedimentation had begun,and foraminifera became coastal species again.There were inconsistencies between the temporal distribution of Sr/Ba and foraminiferal abundance,witnessed as the lower-higher-lower value of Sr/Ba vs the lower–higher foraminiferal abundance from the Early-Late Holocene.Sr/Ba value is agreeable to the intensity of weaker-stronger-weaker marine transgression of the Holocene,but not the foraminiferal abundance,the latter was probably to reflect the poor preservation condistion due to stronger coastal erosion and higher-turbid water in the Early-Middle Holocene than that of the Late Holocene.In light of 116 intensively dated sediment cores collected and 7 new cores in the study,the Holocene sediment thickness was defined into 4 phases?11700⁷000 cal.a BP,7000⁴000 cal.a BP,4000²000 cal.a BP and 2000⁰ cal.a BP?in order to examine the spatiotemporal shift of the depocenter of the incised valley.At the same time,age-depth model was established to explore the characteristics of sedimentation rate and its influencing factors.The result demonstrates the depocenter shift landward from Haimen to Zhengjiang in the Early Holocene,and then a shift not only seaward but also southeastward?dextral?.Shoreline movement caused by sea-level rising is the fundamental controlling factor for the depocenter shift back and forth.In inner valley,depocentern shift in dextral direction is not such obvious due to topographic constraint,but to be gradually ES-oriented at/off the valley.It is proposed that the Coriolis Force was the main driving force that was imposed on the strong tidal currents in the study area,to direct the sediment transport in southeast direction.Sedimentation rate measured in this study showed high-low-high phenomenon in the valley.The rapid sea-level rise of the Early Holocene provides a large accommodation space for receiving an ample sediment to fill in the valley.The consequential lower sedimentation rate links the strong coastal erosion closely associated to the maximal stage of marine transgression in the later Early Holocene.And week Asian summer monsoon is responsible for low sedimentation rate.The following higher sedimentation rate signed the fast infill of the valley during the Middle-Late Holocene when the coast moved seaward rapidly with the increasingly enhancement of human activies..There were three main stages in the infillings of the incised valley,i.e.transgressive deposition,estuarine deposition and deltaic deposition.The sediment infillings underlying deltaic sedimentation can be divided into four types:tidal infillings,estuarine infillings?thinner?,tidal sand ridge,estuarine-coast infillings?thicker?.The distribution pattern of particle size is coarse-fine-coarse-fine seaward.The sediment-dynamics was transferred from river,river/tidal mixed to tidal current respectively,and the total capacity of hydrodynamics goes through the process of lower-higher-lower.The special dynamic conditions and sediment transport of the Early Holocene Yangtze estuary made it different from that of the classical estuary model which sediment distribution is coarse-fine-corase.The Yangtze river-mouth sand shoals in the incised valley serve as the skeleton sediment facies for the Holocene delta development,but their formation time may not be connected successively with each other.For example,Hongqiao and Huangqiao river-mouth shoals stopped to grow at ca 4000 years ago.Jinsha and Haimen shoals were formed almost simultaneously at ca 3000 years ago.Chongming and Changxing began to develop during ca.2400²300 years ago.Also,several sub-deltas can be developed at sometimes.The NW-ES alignment of these river mouth sand shoals is the result of the southeast movement of the sediment transport caused by Coriolis Force-driven tidal sedimentation.