The Evolution Of Holocene Sedimentary Environment In Cixi Of Ningbo Coast

The Qiantang Estuary?Hangzhou Bay?is a typical strong tide estuary,is wellknown for its spectacular tide.These years the Yangtze River sediment runs into the east China sea,in addition to accumulating in its own estuary,the Qiantang estuary is also a main destination under the action of coastal currents and tidal currents.The Yangtze River sediment is crucial to the development of the Qiantang estuary landform.During the rapid rise of sea level in the Early-Middle Holocene,like the Yangtze River estuary,the Qiantang River estuary also experienced a rapid filling process in the ancient incised river valley.There is a worth discussing scientific question that the sediments of the much thick stratum are also originated from the Yangtze River or not.It has important scientific significance of this issue,which can help understand the historical characteristics of river sediment discharge into the sea and revealed geomorphologic evolution during the early-middle Holocene.Furthermore,the relevant study could enhance better understanding of delta evolution in association to sea-level rise as well.The CX1 core in Qiantang incised valley is taken as the research object in this issue,the AMS¹⁴ C and OSL age,the lithology,grain size,and geochemical elements of sediments are analysed.This paper aims to determinine the age and stratigraphic framework of the CX1 core and reconstruct the filling process of the Qiantang paleoincised valley under the control of the Holocene sea level,and at the same time,based on the provenance index system of geochemical elements of residual modern river sediments and fully considering the changes of Holocene Yangtze River sediments to the sea,to study the filling process and provenance evolution of Qiantang paleo-incised valley.There has obtained the following preliminary understanding:1)The results of AMS¹⁴ C and OSL ages of CX1 show that this core has recorded the sedimentation evolution process since about 50 cal.kyr BP.The Holocene deposition began at the depth of 50.4 m.The sediment rate was highest after 2.6 cal.kyr BP.,reaching to 6.4¹3.9 mm/ yr,and lowest before 7.7 cal.kyr BP.about 2.2 mm/yr.The lithology,Sr/Ba and Mn/Fe in the acid-soluble sediment as well as the particle size parameters show that,the area experienced river-tidal flat-estuaryshallow marine-modern estuary deposition process with sea-level rise in Holocene.Details as follows:?1?Before 11 cal.kyr BP.,the sediments were mainly poor sorting,dark gray,medium-fine sand.The particle size probability distribution showed a multi-peak distribution,the probability accumulation curve was a threestage type,and the rough cut point moves to the right side.The Sr/Ba value is less than 1,and the Mn / Fe value is the lowest in whole CX1 core.It could be defined stagnant river bad;?2?11⁸ cal.kyr BP.,the sediment particles are extremely fine and uniform,almost composed of dark green silt and clay which enrich organic mud and the average particle size is about 20.4 ?m.The value of Mn/Fe is high,that indicate the prototype sedimentary environment characteristics,so the stratum may be considered as tidal flat facies;?3?During the period of 8⁶ cal.kyr BP.,the interbeds of sand-mud in the layers increased significantly,the proportion of sand grade components increased,and the maturity of the sediment decreased.There are two main peaks in the particle size frequency distribution curve and The volume ratio of jumping particles reaches more than 80%.The Sr/Ba ratio value continues to increase.It can be thought as paleo-estuarine;?4?6² cal.kyr BP.,the average particle size,sorting coefficient and skewness of the sediment become smaller than the lower core.The particle size frequency distribution curve shows a single peak,which is concentrated around 30 ?m.The probability accumulation curve is twostage,with the main skipping phase.In this section,the Sr/Ba and Mn/Fe values increase visibly,which is the highest in the whole CX1 core,and the marine characteristics of the sediment are very significant.So this section is shallow marine deposition;?5?After 2 cal.kyr BP.,the sediment particles are coarse,poor sorting and have low structural maturity.Tidal sediment structure is visible.The particle size frequency distribution curve is multi-peak distribution,and the sediment is mainly carried by jumping.It is very similar to the sedimentary facies of modern Qiantang estuary.2)Geochemical element analysis of the residual sediments from eastern China river shows that the particle size effect is very significant,but Cr/Th and Sc/Th are influenced by this effect weakly.There is a fact that elemental rations Cr/Th and Sc/Th in modern Yangtze river sediments are much higher than that in the Qiantang river,which could has indicated the difference between Yangtze and Qiantang river.However,the Cr/Th and Sc/Th ratios of Yangtze River sediments did not remain unchanged,Cr/Th and Sc/Th of early-middle Holocene Yangtze river sediments are much higher than now.Therefore,this point needs to be fully considered when analyzing the sediment provenance during the geological history.3)the analysis of geochemical elements in the residual sediments of <45 ?m and <4 ?m phrase from CX1 core indicates that the Holocene deposits in this core were mainly provided by the Yangtze River sediments and continental shelf sediments in addition.The sea level rose rapidly before 6 cal.kyr BP.,Yangtze river sediments were carried by the strong marine power to the Qiantang paleo-incised valley of Qiantang River,the early Yangtze sediments in the continental shelf also moved towards the land,the Qiantang paleo-incised valley were filled by sediments rapidly.During 6² cal.kyr BP.high sea-level period,the sediments from Yangtze river were mainly used for the delta landform construction itself,and the sediments transported to the sea decreased correspondingly,resulting in a slowdown of the filling rate of the Qiantang paleo-incised valley.This period some of fine sediments in continental shelf were send to Qiantang estuarine.Since the late Holocene?2 cal.kyr BP.?,the Yangtze River Delta is rapidly advancing to the sea,and large amounts of sediments from the Yangtze river has been carried along by coastal currents and tidal currents,which steadily supplies the Qiantang estuary construction.