Paleogeographic Reconstructions And Sedimentary Response Since Late Pleistocene In The Northwestern Margin Of South China Sea

The sediments preserved along the continental shelf edge have been generally thought to document the evolution history of high-frequency paleoclimate and paleoenvironment,which is the best way to gain our knowledge of geomorphology evolution in the coastal region of the Quaternary,particular during the Last Glacial Cycle?LGC?,and would be beneficial for the exploration of natural gas hydrate in this region.The continental margin of northern South China Sea?NSCS?is located in a low latitude area and unabridged preserved a transgression stratigraphic sequence that developed on the wide continental shelf during a high-frequency sea-level change cycle,the NSCS thus was regarded to be a natural lab for the study of the paleogeomorphic evolution and sedimentary response mechanism controlled by the sea level change during the Milankovitch cycle.In this thesis,I selected the NSCS as the study area and primarily based on global sea level change data,Digital elevation model?DEM?,2D seismic profiles and lithological data of several boreholes,to reconstruct the paleogeomorphic characters of the several main high sea level stages,i.e.,marine isotopic stage?MIS?5,MIS 3,and MIS 1,during the LGC.I also analyzed the sedimentary records on the continental shelf edge during the MIS4,MIS3 and MIS2,particular focused on characters of the seismic sequence,sedimentary facies,and lithology,trying to figure out which kind of factors have controlled the sedimentary response.Morphological dynamic models were also used to describe the information of the Hainan delta in the Beibu Gulf influenced together by the sea level change,paleoclimate,paleoriver,ocean current,and monsoon.This thesis has acquired several achievements as listed in the following paragraphs,based on above analyses.?1?The paleogeomorphic features of the last sub-glacial and interglacial periods in the NCSCS not only show the advance and retreat processes of the coastline,but also record the response processes of the foredeposit and dedeposition of terrigenous debris transported by large rivers.It is suggested that the paleogeomorphic changes in the northwestern margin of the south China sea during the last glacial-interglacial period were drastic and the relationship with the sedimentary response was the closest.The NSCS continental shelf is the region where landscape and sedimentation change occurred in a dramatic way during the last glacial-interglacial period,which would be important for our understanding of the Pleistocene sea level change and sedimentation along the continental shelf.The sea level at the NSCS was at its highest position in the MIS 5e,i.e.,at the 123Kyr B.P.,and the coastal line has retracted landward with a maximum value.In this case,the Beibu Gulf has expended with an increase area of 2.5×10⁴ km²,although the submarine topography changed a little.By contrast,during the glacial period of MIS 4,i.e.,at the 66 Kyr B.P.,the Beibu Gulf and Yinggehai were both occupied by a coastal plain where large river flows and deltaic deposition occurs,resulting in the South China,Hainan Island to be united together during this glacial period.The model suggests that the coastal line of the Yinggehai is located in the area away from present coastal line with a distance of 50-100 km,so is the Beibu Gulf of 110 km during the glacial period of MIS3—they distributed as a type of C around the Hainan island.The Hainan Island was united with the South China with a highest topography,i.e.,the Qiongzhou Strait was not open,therefore,it is highly possible that the Hainan Island has played a very important role in the formation of the Hainan delta.The largest of the SCS during these 4 cycles occurred in the MIS 5e with an area of 3.6×10⁶ km²,and the minimal of the SCS occurred in the glacial stage of MIS2 with an area of 1.6×10⁶ km²;the coastal line has migrated seaward or retract landward during the evolution of the SCS with a maximum range even greater than 600 km.Besides,the paleogeomorphic evolution of the SCS during the last glacial period-interglacial period was also characterized by the progradation and retrogradation of large river systems which recorded sediments transported from source areas to the NSCS.?2?The anaylsis of sequence stratigraphy and residual formation thickness revealed that the sea level change during the LGC controlled the vertical sedimentary development pattern of the continental marginal sea sedimentary system in the northwest of the south China sea and was one of the main factors controlling the formation of the hainan delta.I defined the maximum flooding surface?MFS?of the MIS 3,unconformities R1?56 Kyr B.P.?and R2?65 Kyr B.P.?by the study of the stratigraphic sequence of the Hainan Delta.The R2 unconformity of the Hainan Island could be correlated with MIS 4 unconformity that has found in other continental shelfs all over the world,which was formed at the lowest sea level stage during the last glacial period with a lot of incised channels identified on the R2 surface.The same incised channels were also found on the MFS and R1unconformities,which were formed at the highest sea level stage during MIS 3,and between MIS 3 and MIS 2,respectively.These incised channels extracted from seismic profiles matched very well with river networks dataset generated from DEM,and connected the west coastal region of the Hainan Island with the Beibu Gulf and Yinggehai as the transport channels of sediment provenance.The stratigraphic sequences consist of two units,DU1 and DU2.Among them,the DU2 was defined as the Hainan Delta sediments with R2 as its bottom surface and R1 as its top surface.The sedimentary facies altered with sea level change,altering in a sequence of predeltic facie composed of mud and silt,delta front facie composed of silt and sand deposited in a low energy environment,delta front facie composed of interbedded silt and sand,and submerged distributary channel facie composed of sand deposited in a high energy environment.Such sedimentary facies could be correlated with transgressive systems tract?TST?developed sea level rising stage during MIS4-MIS3,highstand systems tract?HST?of the MIS3,forced regressive falling-stage systems tract?FSST?during MIS3 and R1,and HST of the MIS3-R1.Therefore,it is obvious that sea level change and sediment supply controlled the vertical structures of the Hainan Island.The residual sediments of R1-R0?bottom of seafloor?were mainly distributed in offshore regions west and south of the Hainan Island with a thickness less than 30 m.In general,the thickness of residual sediments of R1-R0increase from northwest to southeast,and the depocenter is located in the southeast margin of the Hainan Delta,close to the Hainan Island but far from the Red River and Vietnam continental margin.The accumulation rate of the delta is usual less than 5.3×10^-4 m/yr while in some areas the accumulation rate can be as high as 1.1×10^-33 m/yr.Compared with the R1-R0,the distribution patterns of residual sediments of R2-R1 has changed with thickness greater than 30 m occupied almost half of the Hainan Delta and in some region the thickness even greater than 60 m,all these thick sediments distributed as a type of C around the Hainan Island.This distribution pattern of the residual sediments of R2-R1 suggests that sedimentary provenance of the Hainan Delta was primarily derived from Hainan Island,with a accumulation rate of 3.3×10^-3 m/yr occupied almost half area of the delta and of greater than 7×10^-33 m/yr in the southeast edge of the delta.?3?The paleogeomorphic characteristics of R1 and R2 obtained from strata backstripping reflected the gradual migration trend of hainan delta from north to south and east,indicating that the terrigenous clasts from hainan island are the key factors controlling the development of delta and the evolution of geomorphic morphology.The backstripping method was used in this study to reconstruct the paleogeomorphic characters of the NSCS at the two critical stages of R1 and R2.The results showed that coastal line was also retracted landward at the South China and Vietnam continental margin with sea level rising,but this was not true for Hainan Island.Besides,the NE-SW trending trough in the Qiongdongnan Basin and NW-SE trending trough in the Yinggehai Basin was well developed at the time of R2,which were faded away at the time of R1,such trough of R1 stage only remained in a very small scale in the southmost of the Yinggehai.However,the NE-SW trending sea contours were still existed,suggesting that it was the Hainan Delta that influenced the paleogeomorphic characters of the NSCS during R2-R1 period.The southward and eastward migration of the Hainan Delta strongly suggest that the sedimentary provenance was from Hainan Island which high likely played a key role in shaping the paleogeomorphy of the NSCS.?4?The Morphodynamics model proposed that sediments eroded from the Hainan Island and unloaded at the northwest of the Hainan Island coastal region is the main mechanism for formation of the Hainan Delta by geomorphologic evolution,ocean currents,tides and river plume.During the evolution of the Hainan delta,the contribution from Red River is very minor,although a great number of rivers existed in the northwest of the NSCS.Morphological dynamic models used in this study showed that there are winter cyclone circulations and summer anticyclonic circulations by modeling 3D circulations along the NSCS margin,both of which have lower velocities.Such patterns of circulations are beneficial to the sediment accumulation in the Beibu Gulf,particular in the circulation centers.I proposed that the large circulations drived by monsoon,river plume drived by buoyancy force,and interaction of tides were the main factors that has controlled sand transportion along continental margin of the NSCS.If this is the case,then sedimentary provenance of the Hainan delta was proposed to be derived from the Hainan Island,with a small quantity of sediments were likely delivered by the Red River system.The Red River,if has made any contributions to the formation of the Hainan Delta,would deliver sediments through two channels,one is the West Channel in the winter season and another is the East Channel in the Summer.To validate this model,I use sediment runoff data of the large modern rivers flowing along the Vietnam and South China continental margin and Hainan Island to model the sediment distribution pattern in the next decade after the formation of the R2 unconformity.The result showed that there were two large sedimentary bodies in the NSCS continental margin,one located northern Beibu Gulf near the South China continental margin,and another located southwest of the Hainan Island.?5?Based on detail analyses of the high-frequency stratigraphic sequences,residual thickness,paleogeomorphology of R1 and R2,paleochannel and morphological modeling of R2,this study concluded that the Hainan Delta formed during the rapid sea level rise stage with sediments primarily being derived from the Hainan Island?minor sediments derived from the Red River system?because all evidences supporting river networks connected the island with the delta,including high accumulation ratio existed near the coastal region of south Hainan Island and lower ratio far away the island.Weak monsoon event,i.e.,summer monsoon strengthen and winter monsoon weaken,and analyses of the climate data?for example,precipitation of the Hainan Island increases in the winter season because of the strengthen of the winter monsoon?,suggest that weathering enhanced during the transition stage of MIS4 and MIS3,resulting in large amount of sediments produced and unloaded along NSCS margin,and finally formed the Hainan Delta in a very short time scale?10 Kyr?.Therefore,it is obvious that enhanced winter monsoon induced the strong weathering along the NSCS continental margin and would be the primary factor that influenced the paleogeomorphology and sedimentation during this period.Overall,this paper concluded that the sea level change and Asian monsoon evolution controlled Pleistocene sedimentation of the NSCS continental margin and the unique paleogeomorphology in this region.