Abstract:The Mesozoic-Cenozoic tectonic activity in eastern China was mainly controlled by the interaction of the Pacific plate and the Philippine plate with the Eurasian plate. In recent years, the concept of multisphere tectonics of earth system has provided a new perspective for understanding the interaction between various layers of the East Asian convergence system. Based on the 1:1 million marine regional geological survey, this paper comprehensively summarizes the geological structure of China's sea areas and their adjacent areas, initially forming the “multisphere interactions in East Asia ocean-continent convergence zone” with the core of “one margin, two-stage subduction, three-stage extension, and hierarchical control”. In addition, a new first-order tectonic unit division scheme was first proposed in the region between the oceanic plate and the continental margin, referred to as “East Asia continental margin convergence zone”. According to the differences of tectonic evolution, this tectonic unit could be roughly divided into the northern Japan-Ryukyu section and the southern Philippine section based on Taiwan Island. “East Asia continental margin convergence zone” has provided a powerful explanation for the geodynamic process of multiple geotectonic events in East Asia under the West Pacific subduction and Philippine Sea plate subduction system since the Meso-Cenozoic. It formed geomorphic features in China's sea areas which show zonal variation in East China's sea areas and cyclic variation in South China's sea areas. The geomorphic framework further caused the sedimentary differentiation model of “Great River-Great Delta-Terrigenous-Banded” and “Short source river-High angle slope-Multisource-Annular” in East and South China's sea areas, respectively.

Abstract:Lots of investigation and research have been carried out in the South China Sea (SCS), and many genetic models, such as extrusion model of Indochina Peninsular, back-arc extension model and subduction and dragging model of Proto-South China Sea (PSCS), have been proposed. However, none of these models is widely accepted because they don't fit with some geological phenomena. Due to its special tectonic location, the surrounding tectonic environment has undergone complex reorganization. Starting with the interaction of the three plates and in combination with the real data of SCS, the authors put forward a back-arc extension-sinistral shear model in this paper. It is considered that the SCS is a back-arc basin of PSCS northward subduction, and was triggered by the large-scale left-lateral strike-slip formed by the northward drift of the Philippine Sea Plate. The left-lateral strike-slip on the western margin of SCS caused by Indo-Eurasian collision changed the direction of spreading axis of the southwest sub-basin from nearly east-west to north-east direction. The evolution of the SCS and its adjacent area since the late Mesozoic can be summarized as the follows: 1) the Australian plate began to drift northward, and the Neotethys subducted northward in the early Cretaceous, leading to back-arc extension and the formation of PSCS; 2) From the end of Cretaceous to Eocene, the PSCS subducted northward, resulting in formation of continental margin rift; 3) In Early Oligocene, the large scale sinistral strike-slip on the western margin of the Philippine Sea plate triggered back-arc extension of SCS from east to west; 4) At the end of Oligocene, the rollback by subduction of PSCS resulted in the spreading axis jumping to the south; accordingly, it shifted from nearly E-W to NW direction by the influence of sinistral strike-slip on the west margin of SCS; 5) In late Early Miocene, SCS spreading was stopped by collision of Nansha-northern Palawan block with Cagayan Ridge.

Abstract:The tectonic activities of the continental margin of eastern China during the Late Cenozoic were mainly concentrated on the eastern margin of the East China Sea. Since the Miocene, a series of major tectonic processes, such as the subduction of the Philippine plate, the back-arc rifting of the Okinawa Trough and the Taiwan arc-continent collision, have shaped the present tectonic-geomorphological patterns of the Ryukyu trench-arc-back-arc basin system, the Taiwan collision orogenic belt and the northeastern South China Sea. Based on the interpretation of gravity, magnetism and multi-channel seismic data and combined with previous research results, the authors restored the tectonic evolution history of the Okinawa Trough and clarified the relationship between the back-arc tension of the Okinawa Trough and the Taiwan arc-land collision. On such a basis, the interaction processes between the Eurasian plate, the Philippine Sea plate and the South China Sea plate since Miocene were reconstructed, and a new model of plate interaction was proposed. This work is helpful to the further understanding of the constraints and effects of deep dynamic and thermal processes on shallow tectonic framework changes in the context of plate convergence on the East Asian continental margin.

Abstract:Discriminating the distribution of the Mesozoic strata on the southeastern South China Sea (SCS) continental margin is of great importance for understanding the SCS evolution as well as the oil and gas potential. Due to limited data constraints and varied stratigraphic system divisions, the distribution and tectonic features of the Mesozoic sequences in the southeastern SCS have remained controversial. Assisted by drilling and dredging data, the authors comprehensively identified the Mesozoic sequence using a combination of well-seismic comparison, seismic reflection characteristics, internal velocity analysis, contact relationship between igneous edifices and strata, and structural deformation characteristics. The results show that the Mesozoic sequence is characterized by parallel, continuous, and low-frequency reflections, with internal velocity of 3400-4200 m/s. As the burial depth or/and the degree of metamorphism increases, the internal velocity increases to 4500-5500 m/s. The seismic reflections of the Mesozoic strata in the depth is fuzzy, and most of them have no obvious bottom reflection. Despite the fact that the characteristics of seismic reflection are clear in a specific area, it is still difficult to make a comparative study and conduct a whole region seismic explanation because the range of the Mesozoic strata is limited. Among them, the Mesozoic strata in the Northwest, Southwest Palawan basin, Liyue and Andubei basin show low-angle tilting or nearly horizontal characteristics. By contrast, the Mesozoic strata in the northern part of Jiuzhang basin located in the southwest of Liyue have a velocity of 3500-4500 m/s, which is higher than the overlying Cenozoic velocity and coincides with the result constrained by drilling layer. In addition, the Mesozoic strata tilt or deflect at a high angle, which may be related to the emplacement of magmatic activity. Compared with the distribution of Mesozoic volcanic arc and identified igneous bodies, the authors infer that the first style Mesozoic strata are scattered in the forearc basin near volcanic arc and thus show a relatively weak tectonic activity, whereas the second style strata are distributed in the inter-arc basin and have a strong magmatic activity.

Abstract:China seas and their adjacent areas are the key areas for establishing the "21st Century Maritime Silk Road" and building a community of marine destiny and promoting sustainable development. They also constitute one of the most typical and representative research areas in earth science. The 1:1 million regional geological survey of China seas is a national basic public welfare marine geological work guided by national demand. Through the continuous investigation of nearly 20 years, China Geological Survey has realized the comprehensive coverage of 1:1 million regional geological survey of China seas and systematically obtained the basic data of marine geology and geophysics. Based on these numerous data, the comprehensive achievements of“one series of maps, one data set, one research report”have been recently raised to a new level. These results have vigorously improved the overall level of the marine geological work of China and filled in the blank of small-scale marine geological survey of China. The work has preliminarily found out the geological structure situation and the potential of resources and environment in China seas and also provided detailed and reliable geological data to serve the needs of national economy and social development.

Abstract:Based on the comprehensive analysis of 28 boreholes of late Quaternary sedimentation and the high-resolution seismic profiles, the authors studied the stratigraphic characteristics identification and sedimentary environment rebuilding by comparing the results of other 46 boreholes as references in China's sea shelves. Three distinct transgression processes were discovered in the shelves of China's sea areas, with 3 marine strata well preserved. The strata of MIS1 and MIS3 are relatively thin in the Bohai Sea and the East China Sea, but thick in the Yellow Sea. There exists little sedimentation of Last Glacial period in the Bohai Sea and the Yellow Sea, but there have been quite integrated sedimentation units without obvious hiatus in the East China Sea since 100 ka B.P.. There were coastal and continental deposits in the inner shelf and nearshore and neritic deposition in the outer shelf of the South China Sea during late Pleistocene, with neritic sedimentation in Holocene. As a whole, the sedimentary environment changes of China's sea shelves since MIS5 in turn primarily were neritic and offshore deposits in MIS5 (74-128 ka), continental sedimentation of lagoon-fuvial-coast in MIS4 (60-74 ka), neritic and tidal flat deposits in MIS3 (24-60 ka), terrestrial or fluvial sedimentation in MIS2 (12-24 ka), and tidal sand ridge and neritic deposition in Holocene. Furthermore; the neotectonic movement and regional subsidence also influenced the late Quaternary stratigraphic development and their preservation in China seas. The results will provide direct evidence for the study of China's Quaternary sedimentation and global climate changes

Abstract:A large turbidite fan system was found from southeastern South China Sea basin through the latest multi-channel seismic profiles in the regional geological and geophysical measurement data set. This fan system occurred in the water depth from 2000 m to 3800 m of the present sea level, with forward northwest direction to the central basin edge 150-260 km in length. The fan system was interpreted to have been formed from middle Miocene to Quaternary with series of turbidite fan sequences, mainly composed of sediment waves, channel filling, submarine fans, and mass flows. In time scale, from the early to late stage, the size and architecture of fan system were constantly changing, underlying valuable information of sea level change, depositional process and tectonic evolution. In spatial scale, the turbidite sand bodies were formed by the superimposed layers of turbidite flow sediments. The channels were well developed on the seabed within this fan system. The bottom of the channel sand bodies are distributed on "V" shape in panel, extending from south to north, revealing mainly sources from Liyue Bank and north Palawan Island area. The turbidite fan's formation was obviously controlled by tectonic movement, on the one hand related to the uplift of Liyue Bank-Palawan Island since the middle Miocene and, and on the other hand related to magmatic activity since the late Miocene. The turbidite current mechanism occurred in the transition of continental margin to abyssal plain in the southeast of the South China Sea, driving particles transportation from shallow sea to deep basin. This new interpreted large turbidite fan system presents an important linkage of “source-to-sink” sedimentary system, and benefits to revealing deep-sea sedimentary evolution process in the South China Sea.

Abstract:As the largest marginal sea in western Pacific, the South China Sea is rich in oil and gas resources, and the difference in hydrocarbon occurrence laws between the north and south is large. On the basis of the systematic analysis of the MesozoicCenozoic strata and sedimentary facies development characteristics of the South China Sea, the petroleum geological conditions of the typical petroliferous basins in the south and north of the South China Sea were compared and analyzed. The results show that the reconstruction of the sedimentary strata of the ancient South China Sea by the opening process of the South China Sea causes the existing residual strata to separate from north to south, mainly distributed in the Pearl River Mouth Basin and the Tainan Basin in the north, and in the Lile Basin in the south. It is inferred that there are Mesozoic-Cenozoic hydrocarbon accumulation models of “selfgeneration and self-storage”, “new-generation and paleo-storage” or “paleo-generation and new-storage”. The Cenozoic oil and gas reservoirs in the South China Sea are under the tectonic stress background of “north extension and south extrusion”. The distribution of large clastic reservoirs has been controlled by the paleo-water system since Paleogene. Regional tectonic movements and sea level change controlled the development of Miocene carbonate reservoirs. The deep-water canyons, deep-sea fans, and fan-delta sedimentary systems controlled by paleo-geomorphology since the early Cenozoic have spawned potential deep-water reservoirs. In a word, the south and north sedimentary strata of the South China Sea have their own characteristics, and the hydrocarbon accumulation conditions have their own advantages, but they all have good prospects for oil and gas exploration. This paper is expected to provide reference for future strategic deployment and selection of oil and gas exploration in the South China Sea.

Abstract:Coastal islands of northern Fujian are mainly composed of volcanic rocks, which were produced by the paleo-Pacific oceanic subduction. These volcanic rocks provide critical information about the geodynamic evolution of paleo-Pacific Ocean. In this paper, the authors report new zircon U-Pb ages and whole-rock major and trace element geochemistry of the felsic rocks from the Dongtaishan Island of northern Fujian. Two geochronological samples yielded zircon ages of ~92 Ma and ~84 Ma, indicating that the volcanic rocks in Dongtaishan Island were formed in Late Cretaceous. Geochemically, all these felsic samples are calcalkaline, enriched in Rb, Ba, and depleted in Nb, Ta, Sr and Eu, suggesting a geochemical affinity with arc volcanic rocks. It is thus concluded that these volcanic rocks were derived by partial melting of ancient crustal metasedimentary sources and followed by fractional crystallization in the shallow magma chamber. These new data, combined with the oceanward migration of late Mesozoic magmatic activity, have led the authors to propose that the coastal islands of northern Fujian were triggered by the slab rollback during oceanic subduction of paleo-Pacific Ocean.

Abstract:On the basis of the latest multi-channel seismic profiles and geophysical data such as gravity, magnetism and topography, the spatial distribution characteristics of faults, the development period of faults, the internal structural deformation feature of faults and deep crustal structure in the southern Zhongsha Bank (ZB) were revealed, and the formation mechanism of the faults was also discussed. The results show that the tectonic property of the southern continental margin of ZB is non-volcanic passive continental margin. From northwest to southeast, the crustal properties change from thinned continental crust to oceanic-continental transitional crust and then to normal oceanic crust. The depth of Moho discontinuity rises rapidly from 26 km beneath the ZB to 10-12 km beneath the ocean basin. The obvious negative gravity anomaly area from the steep slope of the ZB to the coast front is the oceaniccontinental transitional zone, and the boundary of gravity anomaly change from high value negative anomaly to low value positive and negative anomaly in the ocean basin is the oceanic-continental boundary. There are four sets of deep and large normal faults with stepped sea-dipping in the southern part of the ZB, which were mainly developed in the Late Oligocene to the Middle Miocene. The early extension of the fault development mechanism was related to the NS-trending extension of the sub-basin in the eastern South China Sea (SCS), and the later compressive stress was related to the NW-trending subduction of the Philippine plate towards the SCS. This study is helpful to the better understanding of the expansion history of the South China Sea basin and the types of passive continental margins in the SCS.

Abstract:China seas have broad continental shelf, continental slope and deep sea basin, with many rivers entering the seas and transporting a huge amount of terrigenous materials into these seas. The sediments record such environmental information as sealand interactions, sea circulation changes, sea level change, material transport and climate change. The grain size characteristics of sediments can reflect such information as sedimentary dynamics, material sources and transport distance, and the changes of sedimentary environment can be studied through the grain size composition, parameters and various diagrams. Previous researchers have carried out lots of studies concerning the grain size distribution characteristics of China seas surface sediments, and obtained many achievements. However, most of these studies were concentrated on a certain sea area or region, lacking a macroscopic and systematic understanding of the sedimentary types in the whole sea area of China. Based on "1:1 000 000 marine regional geological survey project" performed by China Geological Survey, the authors collected 4300 samples of surface sediments from China seas and, through the sediment grain size analysis in combination with data available, studied characteristics of sea surface sediment deposition type, material sources and migration patterns in China seas. In this paper, the shallow sea and bathyal sediments are mainly divided into 5 and 7 sediment types according to the gravel bearing and non-gravel bearing nature, and the deep-sea sediments are mainly divided into 9 sediment types. The results show that sea surface sediment deposition exhibits varied types and complicated sources, which is mainly controlled by the material sources, hydrodynamic condition and the change of landform. Generally the eastern sea areas exhibit "large rivers-broad shelf-residual slow deposition" banded sedimentary differentiation model, whereas in the sea areas of the South China Sea, there is "short source river-multi-type shelf-rapid gravity flow deposits" ring banded sedimentary differentiation model. The results obtained by the authors provide basic data for studying the macroscopic distribution of marine sediments in China seas and are of great significance for understanding the dynamic process of marine sedimentation. At the same time, the distribution characteristics of sediment grain size also have an indicative significance for sea placers distribution.

Abstract:In this paper, the authors analyzed over 1000 kilometers' multi-channel seismic line (the CFT profile) across the southwestern subbasin of the South China Sea, with identification of seismic interface characteristics and seismic sequence division, and analyzed the detachment effect of detachment faults on the thinning of continental crust in segments. On the basis of the previous research results, an analysis of the crustal structure characteristics of the South China Sea was carried out, and its structural characteristics were summarized. The southwest sub-basin can be divided into four stages of sedimentary response during the series of continental marginal rifting and sea basin spreading and post-expansion: Stage A was the pre-rift stage, showing that the small faults was generated under tensile stress; Stage B was the rifting stage, when the crust of continental margin was thinning in the tensile stress, the fault generated in Stage A was rotated, and the large tilting detachment fault appeared. The sediments were the synrift deposits, and this stage ended with the breakup unconformity; Stage C was the basin expansion stage, during which the sea basin began to expand and the tensile stress shifted from the continental margin to the ocean basin; Stage D was the post-spreading stage, after ending of the basin expansion, the sediments were a set of stable deposits.

Abstract:Based on the geophysical and geophysical data collected in recent years, the authors carried out the joint gravitymagnetic-seismic inversion in eastern China's sea areas and studied the characteristics of the basement and geological structure of regionalization and predicted the favorable oil-bearing prospective areas of Mesozoic. The basement of the East China Sea shelf basin is composed of Proterozoic and Paleozoic strata, and 5 sets of tectonic layers are developed in the caprock. The south Yellow Sea basin having dual basement and caprock feature can be divided into 8 tectonic layers. The East China Sea shelf basin is dominated by the late Mesozoic Yanshanian transformation. The lower tectonic layer of the Yellow Sea basin is mainly characterized by extrusion deformation while the middle tectonic layer is mainly characterized by multiple structural styles formed by extension. The depth of the Moho surface is stable in the study area, which is usually about 30 km. The upper mantle is slightly elevated in the Wunansha-middle uplift. The Moho depth in the East China Sea shelf basin is generally deeper than that in the South Yellow Sea Basin—Sulu orogenic belt. Eastern China's sea areas and their adjacent areas can be divided into 2 first level stratigraphic regions (Eurasian), 6 second level stratigraphic regions and 14 third level stratigraphic regions. The study of large cross-section stratigraphic structure reveals that the Mesozoic and Paleozoic strata are widely developed in south Yellow Sea and the East China Sea. The thick Meso-Paleozoic strata provide a rich material basis for oil and gas formation and accumulation.

Abstract:It was discovered that there existed two classic types of carbonate cycle, namely, Atlantic type and Pacific type, in the late Quaternary sediments in the South China Sea. In this paper, some characteristics of carbonate cycles in the South China Sea and the sea area southeast of Taiwan were discussed using the data of δ¹⁸O, CaCO₃ content, Al₂O₃ content, SiO₂ content, planktonic foraminireral abundance, and calcareous nannofossil abundance. These data were derived from thirteen cores. Generally, CaCO₃ content and SiO₂ content are good proxies for the representation of carbonate cycles, and these two variables often display inverse relationship. In the study area, the types of carbonate cycle include not only the classic Atlantic and Pacific types, but also irregular forms. The Atlantic-type carbonate cycle is intimately related to sea level fluctuation in the glacial-interglacial cycle, and the studied cores with this type cycle are mainly located in the region whose water depth is less than 3000 m in the South China Sea. The Atlantic-type carbonate cycle on the north slope of the South China Sea was likely accompanied by the enrichment process of coarser (finer) particles during the periods of sea level descent (rise). The cores featured by Pacific-type carbonate cycle in the South China Sea are distributed within a big depth range across the modern calcite lysocline, which suggests that carbonate dissolution cycle was not the primary cause of the Pacific-type carbonate cycle. The carbonate cycle type of the studied core derived from the sea area east of Taiwan was neither Atlantic-type nor Pacific-type. The sedimentation rates of sub-bottom strata in the study area had little relationship with the types of carbonate cycle, but were deeply influenced by water depth and the glacialinterglacial sea level fluctuations. The sedimentation rates were increased with increasing water depth, and the average sedimentation rates during MIS2 was more than twice the ones during MIS1.

Abstract:In this paper, the authors discussed the clay mineral provenances and their environmental significance since the last Deglaciation on the basis of clay minerals and AMS¹⁴C dating analysis carried out for Core CS2, which was located in the central Okinawa Trough. The results show that Core CS2 mainly consists of illite and lesser amounts of chlorite, with associated kaolinite and smectite. Downcore variability of clay mineral content allows Core CS2 to be divided into three units. The sediments were primarily derived from Changjiang with lesser amounts from Huanghe and Taiwan in unit 1 (19-12 ka BP) and unit 2 (12-8 ka BP), which was mainly controlled by sea level change. In unit 3 (8-0 ka BP), the sediments were primarily derived from Changjiang and Taiwan, with lesser amounts from Huanghe, which was mainly controlled by Kuroshio evolution. The ratios of (smectite+ kaolinite)/(illite+chlorite) at CS2 were adopted as proxies for East Asian monsoon evolution. The consistent variation of this clay proxy with those from Sanbao Cave δ¹⁸O and GISP2 δ¹⁸O shows that two profound shifts of the East Asian winter monsoon intensity and the intensity of winter monsoon relative to summer monsoon occurred at 16.4-14.8 ka BP and 12.8-11.6 ka BP.

Abstract:Based on the high-resolution shallow seismic profiles measured in the northern outer shelf of the East China Sea by the "1:1000，000 marine regional geological survey" project, the authors summarized the internal reflection structure, external morphology and other seismic reflection characteristics of the disaster geological bodies in the northern outer shelf of East China Sea, and identified various types of hazard geological bodies such as high angle active faults, submarine volcanoes, magmatic diapirs, submarine landslides, buried ancient channels and buried ancient delta and tidal sand ridges. The distribution, genetic mechanism of various disaster geological bodies were concluded, and the influence of disaster geological bodies on marine oil and gas exploration and marine engineering construction were discussed. The identification of these disaster geological bodies can provide valuable references for the marine production activities and engineering construction in the study area.

Abstract:The geochemical characteristics of rare earth elements in the surface sediments of the Bohai Sea, the Yellow Sea and the East China Sea have been studied systematically and macroscopically through the investigation, sampling, testing and analysis of eastern China's sea areas by China Geological Survey in the past 20 years. The results show that there are four REE enriched areas in the surface sediments of the eastern coast of China with the REE content generally higher than 180 μg/g. The distribution of highvalue areas is consistent with mud areas in eastern coast of China, such as mud area of western Bohai Sea, middle of south Yellow Sea, southeast of Jeju Island and inner shelf of the East China Sea. Chondrite-normalized REE patterns and UCC-normalized patterns in the surface sediments of the three seas are relatively homogeneous, with the distribution characteristics similar to features of China's mainland. It is found that δEu is most strongly negatively correlated with the content of rare earth elements in eastern China's sea areas, followed by the average grain size of sediments with significant positive correlation, as shown by the correlation analysis of the characteristic parameters of rare earth elements. The δEu-(La/Yb)n map was used to identify the material sources for the REE high-value areas, and the result indicates that the sediments in the mud area of west Bohai Sea and the middle of south Yellow Sea are mainly from the same source. It is generally considered that sediments from the mud area of the southwest of Jeju Island are mainly from Yangtze River and Yellow River, but the river input from the southeast of the Korean Peninsula can't be ignored, that the sediments from the north end of the mud area of the inner shelf of the East China Sea are basically from the Yangtze River, while the south end shows different REE fractionation characteristics which are related to the input of small and mediumsized rivers along the coast of Fujian, Zhejiang and Taiwan Island. The material source, sediment grain size brought by large and middle-small rivers and the flow system patterns of the sea areas jointly determine the geochemical characteristics of REE in the surface sediments of eastern China's sea areas.

Abstract:Based on the new high resolution 2D seismic data and drilling data obtained from the western Taiwan Strait and combined with the regional geological data and previous research result, the authors established the high resolution equitime stratigraphic framework of the western Taiwan strait and, on such a basis, determined the geological age attributes and sedimentary facies system characteristics. The results show that there are 6 third-order sequence boundaries in the Quaternary strata,which are correspondingly divided into 5 third-order sequences. The seismic facies in the study area are mainly sheet-like parallel-subparallel seismic facies, progradationa seismic facies, incised valley filling seismic facies, low-continuous filling seismic facies and filling chaotic seismic facies, with different seismic facies and their combinations representing specific sedimentary systems. According to the above analysis, it can be seen that the Cenozoic sediments are mainly developed in the sea-land transition zone, and alluvial/pluvial plain facies and coastal plain facies are mainly developed in the study area. The sedimentary evolution characteristics were hence analyzed to provide geological basis for fine exploration in the western Taiwan Strait, which has great guiding significance for future hydrocarbon exploration.

Abstract:Based on high-precision multi-beam, single-beam, side-scan sonar and other data， the authors found two submarine sand wave groups in the offshore areas of Fujian's Minjiang estuary and Sanshawan estuary, respectively. In this paper, the geomorphic characteristics of the development of the submarine sand wave were analyzed in detail. According to the shape of the development of the sand wave and the trend of the sand ridge line, it can be divided into three types: flexural sand wave, linear sand wave and transitional sand wave. Both sand wave groups show a slower slope on the tide and a steeper tide， which is characterized by branching at the end of the sand wave and is accompanied by a certain degree of bending deformation. Sources of sedimentary materials and hydrodynamic environment are two main factors that control the development of sand wave groups on the sea floor. The reduction of sources of near-shore bedrock erosion materials, river sediment materials and ocean current transport materials in the study area breaks the material supply that maintains the dynamic balance of sand waves. Underflow scouring provides the starting conditions for seabed sediments. Combined with the development characteristics of sand ripples, residual dunes and seabed sand waves, the authors hold that sand waves have certain activities and have a law of continuous migration to the steep side of the slope. Finally, through the analysis of the causes and evolution rules of sand wave activity, the authors discussed the influence of the existence of sand wave on the application of marine engineering in detail, and put forward the corresponding preventive measures, which provides a certain reference for the construction of offshore seabed engineering and disaster prevention.

Abstract:The shallow sea in the north of Hainan Island is rich in marine sand resources, which are mainly composed of sand and gravel deposits. The marine sand resources are concentrated on the tidal ridge area of the east mouth of Qiongzhou Strait, estuary delta and submarine coastal slope. Through the investigation of the bedform, strata and sediments of the seafloor, the distribution and the scale of marine sand resources in two regions were discussed separately together with the marine sand geological features, and the marine sand resource volumes were also estimated. On the basis of the different sediment environments, marine sand resources potential has been discussed systematically. The marine sand in the study area is widely distributed, but the characteristics and scale of the east region and the west region are quite different. The marine sand in the west region is mainly sandy gravel and muddy sand with a thickness of 5 to 10 meters, whereas the marine sand in the east region is dominated by sand, gravel sand and gravel with a thickness of 10 to 50 meters, suggesting a larger scale than the west region. There are totally 9 resource potential areas with a resource volume of 9.14 billion m³, and the rest of the area is a prospective area with a resource volume of 7.15 billion m³. According to such factors as topography, hydrodynamic and sedimentary environment, the marine sand sediment system can be divided into tidal sand ridge accumulation system, estuary underwater Delta accumulation system, submarine coastal slope accumulation system and erosion residue system. Among them, the tidal sand ridge accumulation system located in the shallow water area of the eastern part of the Qiongzhou Strait has a continuous distribution area of 328 km², an average thickness of more than 25 meters, and a resource volume of 8.39 billion m³, suggesting huge resource potential.

Abstract:The chemical alteration index method is an economic, practical and effective method for recovering the weathering history and sediment flux of the source area. The research on provenance feature of Liwan Sag, Pearl River Mouth Basin (PRMB), has been insufficient. In this paper, the weathering history and sediment flux of source area during late Oligocene to early Miocene were restored by analysis of chemical alteration index (CIA) based on the core data. In addition, their influence on depositional filling in the study area was also discussed. The results show that the source area during late Oligocene in the study area experienced a strong weathering stage, and the abundant sediment supply in this period constituted one of the important controlling factors for the rapid progradation and spectacular scale of shelf margin delta and gravity flow depositional systems in the northwest of the study area. On the other hand， during the early Miocene, the weathering of NW provenance area weakened and the sediment supply was reduced, but the sediment supply in the eastern area was more sufficient than that in the western part. It is shown that, in addition to the main provenance from the northwest, the local provenance had an important influence on the depositional infilling pattern during the early Miocene. It is inferred that the sudden resurrection of the eastern provenance, i.e., the Eastern Paleo-uplift and the Xingning Uplift, constituted one of the important control factors for the formation and development of the early Miocene grooves in the eastern area. In the study of sediment source area in the ultra-deep water area, the semi-quantitative method of CIA has certain indicative meaning for the determination of local provenance in basin.

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