Geochemical characteristics of Sr isotopes in the LS33 drill core from the Qiongdongnan Basin, South China Sea, and their response to the uplift of the Tibetan Plateau

Making full use of modern analytical and testing techniques to explore and establish new indexes or methods for extracting paleoseawater geochemical information from sediments will help to reconstruct the sedimentary paleoenvironment in different research areas.The connection between the subsidence of the South China Sea basin and the uplift of the Tibetan Plateau has been a scientific concern in recent decades.To explore the information on the sedimentary paleoenvironment,provenance changes and uplift of Tibetan Plateau contained in core sediments(debris),we selected core samples from Well LS33 in the Qiongdongnan Basin,South China Sea,and analyzed the contents of typical elements(Al,Th,and rare earth elements)that can indicate changes in provenance and the Sr isotopic compositions,which can reveal the geochemical characteristics of the paleoseawater depending on the type of material(authigenic carbonate and terrigenous detritus).The results show the following:(1)during the late Miocene,the Red River transported a large amount of detrital sediments from the ancient continental block(South China)to the Qiongdongnan Basin.(2)The authigenic carbonates accurately record changes in the 87Sr/86Sr ratios in the South China Sea since the Oligocene.These ratios reflect the semi-closed marginal sea environment of the South China Sea(relative to the ocean)and the sedimentary paleoenvironment evolution process of the deep-water area of the Qiongdongnan Basin from continental to transitional and then to bathyal.(3)Since the Neogene,the variations in the 87Sr/86Sr ratio in the authigenic carbonates have been consistent with the variations in the uplift rate of the Tibetan Plateau and the sediment accumulation rate in the Qiongdongnan Basin.These consistent changes indicate the complex geological process of the change in the rock weathering intensity and terrigenous Sr flux caused by changes in the uplift rate of the Tibetan Plateau,which influence the Sr isotope composition of seawater.

Evolution of Sedimentation and Tectonics of the Youjiang Composite Basin, South China

Located at the southern margin of the South China plate, the Youjiang basin is a closely related to the NW- and NE-trending syndepositional faults in respect to the configuration and structure of the basin. The evolution of the Youjiang basin progressed through two stages. In the Hercynian period, the opening of the Ailaoshan-Honghe ocean basin gave rise to a number of NW-trending rift belts in the Youjiang area. During this period, deep-water sediments were dominant and the basin was possesed of the characteristics of the rift system of passive continental margins. In the early Indosinian after the Dongwu movement, the circum - Pacific tectonism led to a major change in the configuration and structure of the basin. In the meantime, the Ailaoshan ocean basin began to be subducted towards the northeast, thus causing the basin to be split and expand again, and then the basin developed into the stage of the back -arc basin. At the end of the Indosinian period, the basin gradually closed from east to west, thus ending up the history of development of the Youjiang basin. In the various developmental stages, sediments in the basin, also limited by the Dongwu movement, were formed by superposition of sedimentation systems with different features, and a typical two-layered structure, which consisted of starved and compensated sediments, also occurred in the early stage. Likewise, volcanic activity in the basin involved two stages. Basin volcanic rocks of alkali and alkali-calcic series, which were controlled by NW-trending faults, occurred in the early stage, and basic and intermediate-acid volcanic rocks of calc-alkali and calcic series, which were controlled by NW- and NE-trending faults. From the above it follows that the Youjiang basin was formed by the combined action of the Palaeotethys and circum -Pacific tectonism.

Gravity Flow on Slope and Abyssal Systems in the Qiongdongnan Basin,Northern South China Sea

The study of new seismic data permits the identification of sediment gravity flows in terms of internal architecture and the distribution on shelf and abyssal setting in the Qiongdongnan Basin （QDNB）. Six gravity flow types are recognized： （1） turbidite channels with a truncational basal and concordant overburden relationship along the shelf edge and slope, comprising laterally-shifting and vertically-aggrading channel complexes; （2） slides with a spoon-shaped morphology slip steps on the shelf-break and generated from the deformation of poorly-consolidated and high water content sediments; （3） slumps are limited on the shelf slope, triggered either by an anomalous slope gradient or by fault activity; （4） turbidite sheet complexes （TSC） were ascribed to the basin-floor fan and slope fan origin, occasionally feeding the deep marine deposits by turbidity currents; （5） sediment waves occurring in the lower slope-basin floor, and covering an area of approximately 400 km2, were generated beneath currents flowing across the sea bed; and （6） the central canyon in the deep water area represents an exceptive type of gravity flow composed of an association of debris flow, turbidite channels, and TSC. It presents planar multisegment and vertical multiphase characteristics. Turbidite associated with good petrophysical property in the canyon could be treated as a potential exploration target in the QDNB.

Astronomical Dating of the Middle Miocene Hanjiang Formation in the Pearl River Mouth Basin,South China Sea

The Hanjiang Formation of Langhian age （middle Miocene） in the Pearl River Mouth Basin （PRMB）, South China Sea consists of deltaic siliciclastic and neritic shelf carbonate rhythmic alternations, which form one of the potential reservoirs of the basin. To improve stratigraphic resolutions for hydrocarbon prospecting and exploration in the basin, the present study undertakes spectral analysis of high-resolution natural gamma-ray （NGR） well-logging record to determine the dominant frequency components and test whether Milankovitch orbital signals are recorded in rhythmic successions. Analytical results indicate the orbital cycles of precession （~19 ka and ~23 ka）, obliquity （-41 ka）, and eccentricity （~100 ka and --405 ka）, which provide the strong evidence for astronomically driven climate changes in the rhythmic alternation successions. Within biochronological constraint, a high-resolution astronomical timescale was constructed through the astronomical tuning of the NGR record to recent astronomically calculated variation of Earth＇s orbit. The astronomically tuned timescale can be applied to calculate astronomical ages for the geological events and bioevents recognized throughout the period. The first downhole occurrences of foraminifers Globorotalia peripheroronda and Globigerinoides sicanus are dated at 14.546 Ma and 14.919 Ma, respectively, which are slightly different from earlier estimates in the South China Sea. When compared with the global sea-level change chart, the astronomical estimate for the sequences recognized based on microfossil distributions have the same end time but the different initiation time. This is probably due to the local or regional tectonic activities superimposed on eustatic rise which postponed the effect of global sea-level rising. Astronomical timescale also resolves the depositional evolution history for the Langhian Stage （middle Miocene） with a variation that strongly resembles that of Earth＇s orbital eccentricity predicted from 13.65 Ma to 15.97 Ma. We infer that the main factor controlling the variability of the sedimentation rate in the Hanjiang Formation is related to the ^-405-ka-period eccentricity.

Tectonic Evolution of the Wanan Basin,Southwestern South China Sea

Quantitative studies on the extension and subsidence of the Wanan Basin were carried out based on available seismic and borehole data together with regional geological data. Using balanced cross-section and backstripping techniques, we reconstructed the stratigraphic deposition and tectonic evolution histories of the basin. The basin formed from the Eocene and was generally in an extensional/transtensional state except for the Late Miocene local compressoin. The major basin extension ocurred in the Oligocene and Early Miocene （before -16.3 Ma） and thereafter uniform stretch in a smaller rate. The northern and middle basin extended intensely earlier during 38.6-23.3 Ma, while the southern basin was mainly stretched during 23.3-16.3 Ma. The basin formation and development are related to alternating sinistral to dextral strike-slip motions along the Wanan Fault Zone. The dominant dynamics may be caused by the seafloor spreading of the South China Sea and the its peripheral plate interaction. The basin tectonic evolution is divided into five phases： initial rifting, main rifting, rift-drift transition, structural inversion, and thermal subsidence.

Sediment Compaction and Pore Pressure Prediction in Deepwater Basin of the South China Sea: Estimation from ODP and IODP Drilling Well Data

Overpressure in deepwater basins not only causes serious soft sediment deformation, but also significantly affects the safety of drilling operations. Therefore, prediction of overpressure in sediments has become an important task in deepwater oil exploration and development. In this study, we analyze the drilling data from ODP Leg 184 Sites 1144, 1146, and 1148, and IODP Leg 349 Sites U1431, U1432, U1433, and U1435 to study the sediment compaction and controls in the northern South China Sea. Sedimentation rate, sediment content, distribution area, and buried depth are the factors that influence sediment compaction in the deepwater basin of the South China Sea. Among these factors, the sediment content is the most important. The fitted normal compacted coefficients and mudline porosity for an interval of 50 m shows disciplinary variation versus depth. The pore pressure predicted from different fitted results shows varying overpressure situations. The normal compaction trend from Site 1144 reflects the porosity variation trend in stable deposition basins in the northern South China Sea. The predicted pore pressure shows overpressure at Site 1144, which is attributed to compaction disequilibrium. Nevertheless, the mixed lithology column may influence the predicted overpressure at Site 1148, which is responsible for the confusing result. Above all, we find that sediment compaction should serve as a proxy for pore pressure in the deepwater basin of the South China Sea.

ON THE ORIGIN AGE OF THE SOUTHWEST BASIN IN THE SOUTH CHINA SEA

This paper propounds that the origin and evolution of the Southwet Basin (SWB) in the South China Sea (SCSh) are closely related with those of the SCS, reviews various viewpoints on its origin age with a large gap between the oldest age and the youngest age. offers some suggestions on the SWB’s origin age and gives some proposals to ascertain satisfactorily the origin and evolution, and multi-phasal and multiaxial spreading of the SCS and SWB.

Thermodynamic Modeling of Fluid-Bearing Natural Gas Inclusions for Geothermometer and Geobarometer of Overpressured Environments in Qiongdongnan Basin, South China Sea

It is a very difficult problem to directly determine fluid pressure duringhydrocarbon migration and accumulation in sedimentary basins. pVt modeling of coupling hydrocarbonfluid inclusion of its coeval aqueous fluid inclusion provides a powerful tool for establishing therelationship of formation pressure evolution with time. Homogenization temperature of fluidinclusion can routinely be measured under microthermometric microscopy. Crushing technique has beenemployed to obtain the composition of fluid inclusions, and the commercial software VTFLINC easilyand rapidly completes the construction of p-t phase diagram. The minimum trapping pressure ofhydrocarbon fluid inclusion would be then determined in the p-t space. In this paper, three samplesof YC21-1-1 and YC21-1-4 wells at YC21-1 structural closure, Qiongdongnan basin, South China Sea,were selected for the pVt modeling practice, and the formation pressure coefficient (equals to fluidpressure/hydrostatic pressure) changing trend with time has primarily been established. Themodeling results also indicate that the reservoirs of Ling-shui and Yacheng formations in YC21-1structure are within a very high potential system and would have undergone a discharging of thermalfluids through top seal rupture, which depicts that there is a very high risk for natural gasexploration in this area.

Geochemical and microbial characters of sediment from the gas hydrate area in the Taixinan Basin, South China Sea

The Taixinan Basin is one of the most potential gas hydrate bearing areas in the South China Sea and abundant gas hydrates have been discovered during expedition in 2013. In this study, geochemical and microbial methods are combinedly used to characterize the sediments from a shallow piston Core DH_CL_11（gas hydrate free） and a gas hydrate-bearing drilling Core GMGS2-16 in this basin. Geochemical analyses indicate that anaerobic oxidation of methane（AOM） which is speculated to be linked to the ongoing gas hydrate dissociation is taking place in Core DH_CL_11 at deep. For Core GMGS2-16, AOM related to past episodes of methane seepage are suggested to dominate during its diagenetic process; while the relatively enriched δ18O bulk-sediment values indicate that methane involved in AOM might be released from the ＂episodic dissociation＂ of gas hydrate.Microbial analyses indicate that the predominant phyla in the bacterial communities are Firmicutes and Proteobacteria（Gammaproteobacteria and Epsilonproteobacteria）, while the dominant taxa in the archaeal communities are Marine_Benthic_Group_B（MBGB）, Halobacteria, Thermoplasmata, Methanobacteria,Methanomicrobia, Group C3 and MCG. Under parallel experimental operations, comparable dominant members（Firmicutes and MBGB） are found in the piston Core DH_CL_11 and the near surface layer of the long drilling Core GMGS2-16. Moreover, these members have been found predominant in other known gas hydrate bearing cores, and the dominant of MBGB has even been found significantly related to gas hydrate occurrence. Therefore,a high possibility for the existing of gas hydrate underlying Core DH_CL_11 is inferred, which is consistent with the geochemical analyses. In all, combined geochemical and microbiological analyses are more informative in characterizing sediments from gas hydrate-associated areas in the South China Sea.

Leaf Cuticle Microstructure of Machilus maomingensis sp.nov.(Lauraceae) from the Eocene of the Maoming Basin,South China

Machilus maomingensis Jin et Tang,sp.nov.from the Eocene Youganwo Formation of the Maoming Basin,South China,is studied in detail from the perspective of the leaf architecture and the excellently preserved cuticle.The leaf margin is entire,the primary venation is pinnate;6 to 8secondary veins are present and the major secondary venation is eucamptodromous.Stomata are paracytic and occur on the lower epidermis.Trichome bases are unicellular.Oil cells are present in the upper epidermis.The new fossil species described has features of the Lauraceae,particularly of the extant genus Machilus Rumphius ex Nees,and it is most similar to Machilus chinensis（Bentham）Hemsley,an extant species distributed in southern China and Vietnam.Hence,Machilus maomingensis sp.nov.is inferred to live in a warm and humid climate.The discovery of the present fossil indicates that Machilus has existed in South China from at least the Eocene.

Geological Features of Devonian Sedimentary Basins in South China and Their Deposition and Mineralization

The Devonian succession in South China is well-known for its complete development, vari-fied sedimentary types, remarkable lithofacies variation and abundant mineral resources. The South China plate was formed by the collision and collage of the Yangtze plate and the Cathaysian plate. The collision began approximately at the Jinningian stage and the collage was not finalized until the Guangxian movement. It was on the South China plate with a somewhat different nature of the basement that the Devonian deposition formed.

The main controlling factors and developmental models of Oligocene source rocks in the Qiongdongnan Basin,northern South China Sea

Coals developed in the Oligocene Yacheng and Lingshui formations in the Qiongdongnan Basin have high organic matter abundance, and the dark mudstones in the two formations have reached a good source rock standard but with strong heterogeneity. Through the analysis of trace elements, organic macerals and biomarkers, it is indicated that plankton has made little contribution to Oligocene source rocks compared with the terrestrial higher plants. The organic matter preservation depends on hydrodynamics and the redox environment, and the former is the major factor in the study area. During the sedimentary period of the Yacheng Formation, tidal flats were developed in the central uplift zone, where the hydrodynamic conditions were weak and the input of terrestrial organic matter was abundant. So the Yacheng Salient of the central uplift zone is the most favorable area for the development of source rocks, followed by the central depression zone. During the sedimentary period of the Lingshui Formation, the organic matter input was sufficient in the central depression zone due to multiple sources of sediments. The semi-enclosed environment was favorable for organic matter accumulation, so high quality source rocks could be easily formed in this area, followed by the Yacheng salient of central uplift zone. Source rocks were less developed in the northern depression zone owing to poor preservation conditions,

Reserves of Natural Gas Hydrates Equivalent to 100–150 Billion m^3 Natural Gas Has Been Discovered in the Pearl Mouth Basin of the South China Sea

Scientists have suggested that combustible ice in global marginal seas,deep trough areas and ocean basins covers an area of 400million km2,and its total reserve is twice the amount of the global proven oil,coal and gas reserves,equivalent to 50 times the amount of conventional natural gas reserves.

Characteristics and Origin of Yacheng Gas Field in Qiongdongnan Basin,South China Sea

The Yacheng gas field lies in the foot wall of the No. 1 fault, the boundary fault between the Yinggehai and Qiongdongnan basins. An overpressured system developed in the Meishan Formation near the No. 1 fault in the gas field and in the adjacent Yinggehai basin. Away from this fault into the Qiongdongnan basin, the overpressure diminishes. Below 3 600 m in the gas field, an obvious thermal anomaly occurs. The gases show obvious compositional heterogeneities which reflect reservoir filling process and origin of the gas field. The gas field was charged from both the Qiongdongnan and the Yinggehai basins but mainly from the former. Hydrocarbons sourced from the Qiongdongnan basin have relatively low maturities while hydrocarbons from the Yinggehai basin have relatively high maturities.

The Earlier Spreading of South China Sea Basin due to the Late Mesozoic to Early Cenozoic Extension of South China Block: Structural Styles and Chronological Evidence from the Dulong-Song Chay Metamorphic Dome, Southwest China

To understand the forming and tectonic evolution of the South China Sea basin, new data of the structural styles and geochronology were obtained from the Dulong-Song Chay dome, southeastern Yunnan and northern Vietnam. The structural styles were acquired through field investigation and geo-chronological dating was carried out using zircon SHRIMP Ⅱ U-P and argon isotopic analyses. The South China Sea basin extension occurred firstly at Late Mesozoic to Early Cenozoic, and then at Late Oligocene to Middle Miocene （32-17 Ma）. The second stage of extension formed immediately after the first stage, and both extensions have a consistent forming mechanism. New structural analysis and geo-chronological data do not support the models of ＂backarc spreading＂ and ＂strike-slip faults producing the extension＂. Then what mechanism resulted in the extension of South China Sea basin？ The data indicate that at least two episodes of major extensional tectonics, i.e. , the D1 deformation at 237-228 Ma resulted in the rising and exhumation of the dome, and D2 deformation at 86-78 Ma overprinted and redeformed the dome. Of them, the D2 shows a consistent forming time, extensional direction and tectonic regime among Dulong-Song Chay dome, South China block and the northern margin of the South China Sea basin. Regional geology has proved that the northern margin of the South China Sea basin belongs to the South China block, therefore, we interpreted that the Late Mesozoic to Early Cenozoic extensional tectonics occurred in the northern margin of the South China Sea basin due to the intraplate deformation of the South China block, while the Ailaoshan-Red River sinistral slip strengthened the Cenozoic extension in the South China Sea basin.

Types of the Continental Slope System Since Miocene in Qiongdongnan Basin,Offshore Hainan,Northern South China Sea

Very high resolution seismic record were used to study the sedimentary processes and morphosedimentary features of the continental slope system since Miocene in Qiongdongnan Basin(QDNB),offshore Hainan,northern South China Sea.It can be divided into four types based on the sedimentary processes and morphology:wide and gentle slope,sigmoid-progradational slope,slumping slope and channeled slope.Different features of

Moho Structure of the Southwest Sub-Basin, South China Sea, from a Multichannel Seismic Reflection Profile NH973-1

Moho structure provides important clues for understanding crustal structure,isostatic state and magmatic flux from mantle to surface.Across-basin Moho structure of the South China Sea(SCS)is important for understanding crustal evolution mechanisms of both continental break-up and seafloor spreading processes.Southwest Sub-basin(SWSB)opened up the latest and has the closest continental margins,making it the best to study the across-basin structure.Multichannel seismic(MCS)reflection data of line NH973-1 that crosses SWSB in NW-SE direction were reprocessed in order to image Moho structure.In MCS data,Moho reflectors are observed in places,which were not revealed in prior researches.The Moho generally shows symmetric structure on both sides of the central rift valley(CRV)and with variations in crustal thickness.Around CRV,the Moho is 2 seconds depth in two-way travel time(TWTT)beneath the igneous basement,which corresponds to 7 km depth,indicating normal oceanic crustal accretion during the ending of seafloor spreading.Close to the continent-ocean boundary(COB),the Moho becomes shallow to 1 second depth in TWTT(3.5 km),implying strong crustal thinning towards the continent,probably because of poor magma supply at the beginning of seafloor spreading.At south COB,the Moho depth under the crust almost reaches zero,which could be explained as a result of exhumed mantle.In addition,two low-angle,deep-penetrating normal faults are observed at south COB.The faults cut across the Moho into the upper mantle,which may be attributed to lithospheric hyper-stretching at COB during continental break-up.

Layer-block tectonics of Cenozoic basements and formation of intra-plate basins in Nansha micro-plate, southern South China Sea

Layer-block tectonics （LBT） concept, with the core of pluralistic geodynamic outlook and multilayer-sliding tectonic outlook, is one of new keys to study 3-dimensional solid and its 4-dimensional evolution history of global tectonic system controlled by global geodynamics system. The LBT concept is applied to study the lithospheric tectonics of the southern South China Sea （SCS）. Based on the analysis of about 30 000 km of geophysical and geological data, some layer-blocks in the Nansha micro-plate can be divided as Nansha ultra-crustal layer-block, Zengmu crustal layer-block, Nanwei （Rifleman bank）-Andu （Ardasier bank） and Liyue （Reed bank） North Palawan crustal layer-blocks, Andu-Bisheng and Liyue-Banyue basemental layer-blocks. The basic characteristics of the basemental layer-blocks have been dicussed, and three intra-plate basin groups are identified. The intra-plate basins within Nansha micro-plate can be divided into three basin groups of Nanwei- Andu, Feixin-Nanhua, and Liyue-North Palawan based on the different geodynamics. In the light of pluralistic geodynamic concept, the upheaving force induced by the mid-crust plastic layer is proposed as the main dynamical force which causes the formation of the intra-plate basins within the Nansha micro-plate. Finally, models of a face-to-face dip-slip detachment of basemental layerblock and a unilateral dip-slip-detachment of basemental layer-block are put forward for the forming mechanisms of the Nanwei Andu and Liyue-North Palawan intra-plate basin groups, respectively.

The Gravity Flow on a Shelf-Edge-Slope-Basin-Floor Complex in Qiongdongnan Basin,Northern South China Sea

The Qiongdongnan Basin lies in the west part of the northern South China Sea.To the west the basin is bound by the Red River Fault and the Yinggehai Basin,to the east by the Pearl River Mouth Basin,and to the south by the Xisha Rise.In this study,the sequence stratigraphic framework was set up and 11 third-order sequences were distinguished.Based

Composition and Origin of Shallow Biogenetic Gases in the Baise Basin, South China

Based on the analytical data of over 30 gas samples, combined with geochemical and geological backgrounds, the composition and distribution characteristics of shallow biogenetic gases in the Baise Basin, a Tertiary residual basin in southern China, were extensively investigated, and the origin and formation mechanism tentatively approached. The shallow gases are primarily composed of gaseous hydrocarbons, generally accounting for over 90%. The abundances of methane and C2＋ homologues show a relatively wide range of variation, mainly 50%-100% and 0%-50%, respectively, depending on the mixing proportions between biogenetic and thermogenic gases. A highly negative carbon isotope is the significant signature for the shallow gases with δ^13C1 values of -55‰ to -75‰. According to molecular and isotopic compositions and light hydrocarbon parameters, the shallow gases in the basin can be classified into three types of origins： biogenetic gas, biogenetic/thermogenic mixed gas, and oii-biodegraded gas. They exhibit regular distribution both spatially and temporally, and are believed to be associated with the maturity of adjoining gas source rocks and biodegraded oil accumulation. The Baigang and Nadu source rocks can be considered to have experienced early and late gas generation during early burial and after basin uplift respectively. A late accumulation mechanism of multiple gas sources is put forward for the formation of the shallow gas reservoirs, which is responsible for the variations in chemical and isotopic composition of the gases in depth profile.