Sedimentary Characteristics And Evolutions Of The Luan River Delta During The Holocene

Sediments in deltas that connect mainlands and oceans archive signials of sea level,climate,neotectonics,dynamics,and human activities.In this paper,four minor studies have been focused.First,we studied the grain size trends and sedimentary dynamics in the modern subaqueous Luanhe River Delta(MSLRD).Afterwards,the Holocene sedimentary characteristics in the MSLRD and surrounding areas were analyzed.Also,the Holoence sedimentary environments were reconstructed and analysised.Thereafter,we establish the evolution process of the Qilihai dune barrier based on the reconstruction of sedimentary sequence.Moreover,the influences of sediment supply due to the eastward shift of Luanhe River and climate changes due to the intensified East Asian winter monnoon(EAWM)on the development of dune barrier are analyzed.At last,we divided the Holocene Luanhe River Delta(LRD)into three sub-deltas and one alluvial-delta using additional ¹⁴C ages of sediment cores and geological profiles collected from previous reports.Influences of sea level changes,climate changes,neotectonics,and human impacts on the evolution of the LRD during the Holocene and historical period are interpreted.Directions of grain size trends in the southern area with water depth<5m are SW,but those of the northern area with water depths<5m are NW and indicating a sediment supply from the delta to the northern sandy coast.Seaward grain size trends were observed facing directly the river mouth.Directions of grain size trends between water depths 5-15m are SW and the intension are stronger than the others.Grain size trends in the southern area with water depths>15m are SW,but those of the northern area with water depths>15m are NE.Tidal current is the dominating dynamic in the MSLRD and fluvial dynamic effects where water depths<5m.However,the grain size trend analysis method can not explain influences of wave dynamics,topography,lag deposits and human impacts except the tidal current and partly river due to the method itself and our design.Transgression and offshore deposits are poorly developed during the Early-to Middle Holocene which are characterized by a thin(<1m)abundant shell fragment silty sand deposits.Fluvial environment formed at open lowland with grass vegetation during the Late Pleistocene to Early Holocene.Pollen assemblages in the Middle Holocene marine sediments cue no river entern the surrounding area.Moreover,a mixing deposit as thick as 9.50m with decreasing of marine facies upwards is observed in the L1.The overlying prodelta facies of MLRD contains convolute bedding and thin very fine sand layer.Seismic profiles illustrate subaquatic sand banks with 1-2 km-width and 3-4m-hight,and 1.5-2.2 km-width swales.Delta prograding body is identified with thichness<2m and partly?5m in the nearshore and swales between sand banks.Qilihai dune barrier contains underlying fluvial and lucusrine facies,inlet facies,tidal flat facies,saltwater marsh facies,and sand dune facies towards the upper boundary.The Qinlihai dune barrier first appears at?7 cal kyr BP and grow little until?3.6 cal kyr BP.During 2.10-1.65 cal kyr BP,the back-barrier was rapidly infilled with the barrier grew up.After 616 cal yr BP,sands were blown from the beach to the barrier and formed the coastal dune overlying the back-barrier sediments.At?3.6 cal kyr BP,Palynological assemblages changed from Quercus-Pinus-Artemisia to Pinus-Quercus-Artemisia and indicating a cool event.This cool event associated with EAWM strenghthing lead to less rainfall,together with vegetation degradation,would increase the probability of Luanhe River migration;thus,the sediment supply to the Qilihai dune barrier increases.Itensified EAWM blow sand from the beach to the back-barrier during the Little Ice Age with vegetation of grassland.In addition,the initial timing of the Little Ice Age in the Qilihai dune barrier and southern Yanshan Mountains is earlier 100 years than most areas in China.Fluvial facies dominated the sediments in the north of Luanhe River.Peat layers are found partly upon the fluvial facies,but no marine sediments are observed.The radiocarbon ages of the underlying fluvial facies and overlying peat layers constraint the fluvial sediment deposited during 13.2-11.2 cal kyr BP,and suggesting Luanhe River flowed through this place.As thick as 10m transgressive marine sediments in the Caofeidian area indicate Luanhe River flowed this area probably during 8?7 cal kyr BP.Then,the LR flowed into the Bohai Sea at Nanbao area,and left a delta which was part upon the old Liuzan-caofeidian delta during 7?3.6 cal kyr BP.Finally,the LR flowed to the Wangtan lobe and wandered between Su River and moder LR since 3.6cal kyr BP.According to the historial documents,the LR has entered the Bohai Sea with at least two distributary channels.These two channels delvier sediments into the Bohai Sea alternately or simultaneous.The west channel was located between the present Daqing River and Xiaoqing River area.The east channel flowed into the Laomigou.The river channels changed frequently during to floods,under the influences of sea level,climate,human impacts,and neotectonics during the historial period.However,the channels were restricted between the present Su River and LR.