Seismic Sequence And Its Evolution Since Mid-late Holocene In And Near The Modern Luanhe River Subaqueous Delta

Luanhe River Delta(LRD)is an important part of the source-sink system on the west coast of Bohai Sea.The study of seismic sequences and related sedimentary processes in the area of Luanhe River subaqueous delta can provide an important basis for source-sink evolution in the west coast of Bohai sea.Based on the approximately 26 km high-resolution shallow seismic profiles data from the coastal area of the modern Luanhe River Delta(LRD),4 seismic horizons(R1R4)and 4 seismic units(SUs0-3)were identified in the mid-late Holocene sequences.R1 is channel form,with lowest amplitude,low continuity and buried depth of>25 m.R2 distributes nearly horizontal,with moderate reflection amplitude,low continuity,and average burial depth of 17 m.R3 shows undulating form,with high amplitude,high continuity and buried depth ranging from 8 to 25 m.R4 has highest reflection amplitude,highest continuity and an average burial depth of 15m.The bottom interface of SUO was not detected,so it is difficult to distinguish internal reflection structure.SU1 takes R1 as the bottom interface and overlies upon SUO,with semi-lenticular shape,low amplitude and semi-transparent to wavy reflection.Sub-interfaces can be identified within a few of units.The middle thickness of SU1 is 2?8 m.SU2 takes R2 as the bottom interface and is distributed nearly horizontal,with middle amplitude and opaque reflection,of what is about 2 m thickness on average.SU3 takes R3 as the bottom interface and R4 as the top interface,with high amplitude and transparent reflection.Sub-interfaces can be clearly identified in some units,and the thickness is 1?3 m.According to the superposition relationship and the reflection characteristics,the interpretation results are as follows:R1 is sequence boundary(SB),corresponding to the last glacial maximum(LGM)and adjacent incised valley.R2 is wave Ravinement surface(wRs),R3 is the seafloor before LRD progradation,possibly corresponding to the maximum flooding surfaces(MFS).R4 is modern seafloor(SF).SUO is the late Pleistocene fluvial and lacustrine sedimentary layer.SU1 is channel filling.SU2 is marine sandy deposition.And SU3 is the argillaceous sedimentation after LRD progradation.From the sequence characteristics and seismic morphology,SU3 can be divided into three regions(?-?).Region ? and ? are thicker,corresponding to two sedimentary centers,with an average thickness of 1 m and 3 m,respectively.SU3 of region ? has parallel or nearly parallel reflection,which is uniform in region ?.SU3 is missing in zone II,where constituted by tidal sand ridges with SU2.Therefore,it is interpreted as LRD progradation deposition and coastal current accumulation deposition,respectivelyIn order to verify the sedimentation model of SU3,10 cm,20 cm and 30 cm from the top of 13 down-core gravity samples were selected for grain size tests.Grain size tests were carried out on 98 surface samples near the LRD area.We select down-core samples in the mud area of region ? and ? for 210Pb and 137Cs test.The results show that the frequency curve main peak falls in 7(p in region ?.Where the second peak concentrated in 2 ?.Distribution of grain size in region ?,however,is opposite,which focus on 2 ?.The next peak is on 7 ?,indicating the two different sedimentary dynamic effects.And the peak move,illustrating this area not only affected by coastal current erosion,but also affected by the Luanhe River.Surface grain size test results show that the sand composition concentrates in the region ?,with the average grain size of 2?3?,accounting for 90%.While the silt clay components of fine particles are distributed in the other two regions,with an average grain size of 5?7 ?,and the proportion of component content reaching over 80%.Therefore,the two regions of SU3 is accumulated by Luanhe River and the coastal current,respectively.And region ? is the coastal current erosion area.The average sediment rate of Z9 is 2.58cm/yr in region ?.However the results of Z7,Z11,Z12 and Z23 are 1.94cm/yr 2.04cm/yr 1.55cm/yr 1.15cm/yr respectively in region ? The bottom age of SU3 in region ? is 1937 AD,while the absolute age of SU3 in region ? cannot be calculated.Therefore region ? and? in LRD subaqueous area is controlled by Luanhe River and coastal current,respectively,while both factors affecting in region ?.Hence the development model of Luanhe River subaqueous delta is mainly controlled by Luanhe River and coastal current,presenting a special depositional model progradation accumulation-coastal current erosion-coastal current accumulation from nearshore to offshore.According to the seismic sequence characteristics of Luanhe River subaqueous delta.Sedimentary evolution since the middle Holocene summarized as follows:sea surface rapidly rising since LGM,filling channels,forming SU1.When wRs had effect in this area,the corresponding ravinement lead to R2,and the original channel filling erosion at the same time.The coastal sediments were forced to move towards the sea during ravinement,forming marine sandy deposition.R3 at the top interface of SU2,is the product of the coastal current,and shows the spatial difference.In the nearshore,protected by LRD deposition(region ? of SU3),it forms the base of progradation.In the offshore(region ? and ?),R3 developed downward and eroded to SUO,resulting in complete loss of R2 in region ?.And there are a large number of tidal ridges using SU2.Region ? is then covered by the diffused sediments carried by the coastal current,resulting in R3 forming the base of the coastal current mud deposits.