Researches On The Post-glacial Sedimentary Evolutions Of The Three Typical Depositions On The Western South Yellow Sea

As a a typical epicontinental sea, the South Yellow Sea (SYS) has received largeamounts of terrestrial material during the Holocene and boasts many types of deposits.Therefore the SYS is an ideal area for researches of sedimentary processes andmechanism of continental shelves. The last tens of years have seen many studies onthe late Quaternary sedimentary records from the coastal to shelfal areas of the SYS,leading to great progress in researches of paleo-environmental evolutions, sea-levelchanges, sediment sources and paleo-current systems. However, there are someproblems to be solved. Among the unsolved problems, the sedimentary records of thefollowing three types of deposits, that is, shelf relict sands, the distal part of the radialtidal sand ridges on the western SYS, and subaqueous delta of the Old Yellow Riverin AD1128-1855, are taken as main research focuses in this study, in order to betterunderstand the post-glacial transgressive process and sedimentation on the westernSYS.Making use of about4100km of high-resolution shallow seismic profiles,drilling cores and vibrocores, this paper carrys out a integrated analyses of the seismicprofiles, sedimentary characteristics of the cores, benthic foraminiferal and ostracodassemblages of the cores, and AMS14C dating and210Pb and137Cs measurements,combined with a comparative research on the latest results concerned in the world, toprobe deeply into the origion and sedimentary evolutions of the relict sands in theearly Holocene, radial tidal sand ridges in the late Holocene, and subaqueous delta ofthe Old Yellow River (AD1128-1855) on the western SYS.This study indicates that the relict sands, which are about0.4m thick, overlie onthe aarly Holocene coastal marsh or tidal flat deposits with an evident erosioninterface in between. The middle and upper parts or sometimes the whole sequence ofthe relict sands have been reworked under modern dynamic conditions. The sedimentation rate varies between0.20~0.30cm/a. The relict sands show a bimodalgrain size distribution pattern in frequency curves. A sharp peak occurs with coarsefraction between3and4φ and a secondary peak with fine fraction of about7φ. In theprobability cumulative curve, the saltation component comes to70%~80%, withabout20%~30%of suspension load. Of the benthic foraminifera and ostracodaassemblages, the reworked relict sands are characterized by the mixing of thenearshore euryhaline shallow water species and deeper water species commonlyfound in inner and middle shelf areas. The erosional interface at the bottom of therelict sands is considered as a regional ravinement surface formed during thetransgression in Early Holocene due to shoreface retreating landwards. The relictsands were accumulated on the transgressive surface during transgression in thedeglaciation period as lag deposits after winnowing and reworking by marinedynamic processes. And the relatively high suspension composition in the relict sandssuggests the input of fine-grained substances during the reworking process. As theconclusion, the relict sands in the study area are interpreted as a kind of palimpsestsediment.This study shows that the distal sand ridges of the radial tidal sand ridges on thewestern SYS have been being active since about2.5cal kyr BP, and their internalreflections dip to the east and northeast downlapping onto the basal boundary of thesand ridges. The dip angles of the internal foresets are generally less than0.2°,mostly convex in shape and tangent to the erosive surface of the sand ridges. The coresection of the distal sand ridges roughly coarsens upward, and in the sandy dedimentsthe saltation load is higher than80%and the suspension load less than20%, with agood sorting. During1128-1855when the old Yellow River entered the South YellowSea from the Jiangsu coast, sedimentation rates in the distal sand ridges increasedsignificantly, from0.28~0.77cm/yr before the river shifting up to1.10cm/yr duringthe event. But the rate decreased to0.23cm/yr after the old Yellow River going backto the Bohai sea again in1855. According to previous researches, the paleo-Yangtzeriver estuary and Dongtai were supposed to be the vertex of the ridial tidal sand ridges in about8ka BP and6ka BP, respectively. The progradations of the sand ridges tothe core site from the paleo-estuary during8-2.5ka BP and from Dongtai during6-2.5ka BP are calculated to be about61.7m/yr and65.9m/yr, respectively. Theprogradation further towards the sea from the distal ridge, however, has reduced toabout4m/yr since about2.5cal kyr BP, which is considered to be caused byweakened tidal action and declined sediment supply in the deeper water.The data of this study evidently reveal compound clinoform morphologies, thatis, a subaerial/subaqueous delta couplet sourced by the OYR in study area. Thesubaerial delta comprises the present-day onshore delta plain and deltafront extendingto the~17m water depth, while the subaqueous delta is composed of a wide, gentlyinclined topset and a relatively narrow, steeply sloping foreset, stretching seawardabout160km away from the shoreline and showing a morphological asymmetry witha generally southward deflection relative to the OYR mouth. The topset is as wide asabout25to100km in water depths of10-25m, but is generally less than3m thick.The foreset is much narrower and thicker, generally as wide as about5-30km in10-45m water depth and as thick as4-16m with a maximum of20m, and showsseaward-dipping internal reflectors (generally0.3-0.5 ). The rollover points(topset-foreset transition) of the subaqueous clinoform range mostly between about10and25m water depth.210Pb and137Cs geochronology indicate that maximumsediment accumulation rates (1.06and2.16cm/yr) correspond to the foreset region,with much lower rates in topset and the surrounding shallow-sea regions (<0.64cm/yr). Marine erosion due to abandonment of the delta after1855and artificial dikebuilding around the abandoned delta coast since the1930’s have impacted thesediment accumulation in the subaerial deltafront and subaqueous delta. Estimates ofsediment budget suggest that about42.7%and12.8%of the total load primarily fromthe OYR (>97%) have accumulated on the subaerial and subaqueous deltas,respectively. Our data also reveal that the subaqueous delta reltated to the OYR isunderlain by a large-scale clinoform as thick as about4to13m with lower-angled(<03 ) internal reflectors dipping seaward than those in the overlying foreset deposits. Based on dates of its uppermost sediments and its surface elevations, the ageof the underlying clinoform is broadly confined to a range between about2.5and9.8cal kyr BP. Energetic marine hydrodynamics (waves, tides and alongshorecoastal-currents) and sufficient sediment supply form the OYR are interpreted to beresponsible for the across-shelf subaqueous delta morphology.