neogene stratigraphy and paleoenvironments of china abstract neogene deposits are widely distributed in north and west china whereas in southeast china they are confined to isolated basins the neogene in china is predominated by non marine facies and marine deposits are limited to the leizhou peninsula some islands and continental shelves although miocene brackish water deposits were encountered in xinjiang northwest china and a very brief marine transgression took place in north china near the end of the pliocene non marine neogene deposits in china are largely fluvial and lacustrine in origin and composed of mudstones sandstones and conglomerates with local occurrences of lignites and diatomites the maximum thickness may exceed 2000 m e g the qaidam basin a rich fauna and flora have been found in many localities and the most famous are the middle miocene diatomites at shanwang shandong province with unusually well preserved vertebrate and plant fossils a total of 7 mammal stages have been distinguished in china 5 in the miocene 2 in the pliocene and tentatively correlated with the european stages a major realignment of climatic zones occurred before the paleogene neogene boundary when the paleogene climatic zones with a nw trending broad arid zone across the whole of china was eliminated and the arid zone moved to northwest china presumably indicating the onset of the monsoon circulation system during the last few years the marine neogene has been intensively studied in the leizhou peninsula and the three major sedimentary basins off the south china coast the beibu gulf gulf of tonkin basin the zhujiangkou pearl river mouth basin and the yinggehai basin this sequence is over 3000 m thick ranges from the oligocene to the pleistocene and consists of grey mudstones sandstones and conglomerates interbedded with biogenic deposits such as reef limestones and sandy chalk at least 6 foraminiferal assemblages can be recognized in the three basins the psudohastigerina micra assemblage from the oligocene the globigerina ciperoensis cassigerinella chipolensis assemblage from the lower miocene the globoratalia siakensis orbulina suturalis assemblage from the middle miocene the globigerina nepenthes globorotalia acostaensis assemblage from the upper miocene and or lower plicoene the globorotalia tosaensis globigerinoides extremus assemblage from the upper pliocene and the globorotalia truncatulinoides assemblage from the quaternary the nannofossil succession and larger foraminiferal faunas confirm the ages the tropical nature of the neogene faunas is revealed by the occurrences of large foraminifers and warm water species of other groups the transgression cycles are controlled by tectonic processes during the oligocene and early miocene but later by eustatic movements

the red river fault system in the tonkin gulf vietnam abstract the red river fault system in the tonkin gulf offshore haiphong was studied using seismic profiles calibrated by deep wells well characterised left lateral strike slip occurred continuously within a narrow 30 km wide zone southwest of the vinh minh fault between 30 ma and 5 5 ma however the corresponding amount of offset probably does not exceed a few tens of kilometres no sign of post 5 5 ma right lateral motion can be detected prior to 30 ma there is a widespread extension in a wider 100 km wide zone which could be related to a significant amount of left lateral motion the motion of the fault splays to the northeast of the vinh minh fault prior to 30 ma was absorbed in the rifting of the gulf of beibu a 15 5 ma unconformity separates the transtensional regime from a later transpresional regime this 15 5 ma date coincides with the cessation of sea floor spreading in the south china sea

polygonal faults and their implications for hydrocarbon reservoirs in the southern qiongdongnan basin south china sea the polygonal faults were identified in qiongdongnan basin south china sea by using the technique of time coherent slice and horizon flattening of high resolution 3d seismic data these polygonal faults occur in three tiers of the upper meishan formation and the huangliu formation the faults have lengths of 150 1500 m spacings of 50 3000 m throws of 10 40 m and dips of 50 90 degrees tectonic evolution in the qiongdongnan basin can be divided into a rifted stage and a post rifted stage tectonic faults are widely distributed in the rifted sequences but are not well developed in the post rifted stage few faults in the post rifted sequences might suggest the absence of a migration pathway for hydrocarbon or other fluids however the existence of polygonal faults in the post rifted sequences can serve as the pathway and promote the hydrocarbon migration and accumulation in the qiongdongnan basin during the post rifted stage c 2010 elsevier ltd all rights reserved

upward flow of north pacific deep water in the northern south china sea as deduced from the occurrence of drift sediments 1 reflection seismic profiles show that drift sediments occur in the depth range of 1200 2800 m on the continental margin off southeast china these drift deposits were generated by upward flow near the dongsha islands of the north pacific deep water npdw which enters the south china sea via the bashi channel sill depth 2500 m this flow results in deposition of slope sediments resuspended off east and south taiwan on the slope southeast of the dongsha islands at 1200 m water depth this upward flow probably terminates and the npdw mixes with the surface water sediment ages from odp site 1144 southeast of the dongsha islands show that this process started at least 1 1 m y ago indications for mass wasting in the area of upslope water flow document the simultaneous occurrence of current and gravity controlled sedimentation processes however bottom current related drift sedimentation dominates

an integrated view of the methane system in the pockmarks at vestnesa ridge 79 n accepted manuscript version published version available at https doi org 10 1016 j margeo 2017 06 006 accepted manuscript version licensed cc by nc nd 4 0

three dimensional seismic characterization of a complex sediment drift in the south china sea evidence for unsteady flow regime this study describes a previously unobserved reflection seismic configuration comprising a honeycomb planform and a repeated erosion infill cross section based on high resolution three dimensional two dimensional seismic data and bathymetric data the honeycomb structures cover an area of more than 5000km 2 and are developed within the late miocene to recent deep water sediments of the north western south china sea linear erosional troughs up to 10km long and 1km wide are widely developed in this area are intimately related to the particular seismic configuration and interpreted to represent a new type of sediment drift that is caused by unsteady bottom current regimes operating since the late miocene the unsteady bottom current regimes are suggested to be triggered by irregular seabed morphologies considerable sea floor topography was generated as a direct result of tectonic movements in the area since the late miocene and this topography then influenced the pathways of strong bottom currents this study highlights that i an unsteady bottom current regime can be laterally extensive and persist for millions of years ii structurally controlled sea floor relief plays an important role in controlling the depositional pattern and iii the bottom currents were active since the late miocene flowing from the south east through the xisha guangle gateway and crossing the honeycomb structure zone this study documents a new style of drift and will help to improve current knowledge of palaeoceanography and understanding of the south china sea deep water circulation which is at present still poorly understood

carbonate platform growth and demise offshore central vietnam effects of early miocene transgression and subsequent onshore uplift abstract miocene carbonate platforms cover a large part of the central vietnamese south china sea margin early carbonate deposition took place on two regional platforms separated by a narrow depression developed along the trace of the east vietnam boundary fault zone west of the east vietnam boundary fault zone the tuy hoa carbonate platform fringes the continental margin between da nang and nha trang here platform growth initiated during the early miocene and continued until middle miocene time when regional uplift led to subaerial exposure termination of platform growth and karstification east of the fault zone the triton carbonate platform was also initiated during the early miocene carbonate growth thrived during early and part of middle miocene time and a thick clean lower and middle miocene carbonate succession cover the triton horst and the qui nhon ridge during the middle miocene partial drowning resulted in the split up of the triton carbonate platform repeated partial drowning events throughout the middle and late miocene resulted in westwards retreat of platform growth and eventual platform drowning and termination of carbonate deposition modern carbonate growth continues on isolated platforms hosting the paracel islands farther seawards the onset of widespread carbonate deposition largely reflects the early miocene transgression of the area linked with early post rift subsidence and the opening of the south china sea the mid neogene shift in carbonate deposition is interpreted as a consequence of regional uplift and denudation of central and south indochina starting during middle miocene time when the tuy hoa carbonate platform became subaerially exposed stressed carbonate growth conditions on the triton carbonate platform probably resulted from increased inorganic nutrient input derived from the uplifted mainland possibly enhanced by deteriorated climatic conditions and rapid sea level fluctuations promoting platform drowning

acoustic and biological characteristics of seafloor depressions in the north yellow sea basin of china active fluid seepage in shallow water seafloor abstract high resolution multibeam bathymetry revealed the occurrence of numerous craterlike depressions so called pockmarks in the sea floor of the north yellow sea basin nysb to investigate whether these pockmarks are related to any type of fluid flow or gas seepage from subsurface stratum acoustic survey and box samplings were conducted in a selected area containing several typical pockmarks this allowed for an integrated analysis of the morphology distribution and subsurface strata as well as a direct comparison of seafloor characteristics among different areas inside and outside of a pockmark additionally surface sediments were analyzed for particle structure and grain size based on the combination of multibeam bathymetry and backscatter intensity data 282 seafloor depressions were accurately identified moreover the formation of pockmark can be attributed to existence of gas chimneys and other shallow gases revealed by shallow seismic profile records in the subsurface strata sediments inside and outside the pockmark showed remarkable discrepancies in terms of grain size and benthic fauna additionally strong internal reflection area was observed inside the depressions where benthic habitats are abundant and sediments contain strawberry pyrite and carbonate cementation side scan sonar also recorded sediment plumes in the water column which indicates that the pockmarks are still active several pearl string like accumulations of pockmark were found in the study area which well agree with the distribution of buried channels and lakes these results thus provide new insights in interpreting shallow water fluid seepage with implications for our current knowledge of overall sedimentation process in offshore continental shelves

short distance variability in slope bed forms along the southwestern adriatic margin central mediterranean abstract the slope of sw adriatic margin sam is impinged today by two bottom water masses of thermohaline origin that interact constructively along a preferred path the levantine intermediate water liw generated in the eastern mediterranean through intense evaporation and flowing along the contour and the north adriatic dense water naddw formed on the shallow north adriatic and cascading episodically off the shelf these two southward flowing bottom water masses impact the upper slope which is characterized by an irregular physiographic setting and together create the bottom current deposits e g fine grained contourite drifts and sediment waves and related erosional features e g furrows scours comet marks this study is focussed on a small open slope area of the sam ca 35 25 km where high resolution seismic data allow observe elongated contour parallel scours widespread erosion surfaces accompanied by wide crescent shape scours and furrow fields and giant comet marks spatially associated and genetically linked along a common current path further downslope and away from this highly energetic current path a field of sediment waves migrates upcurrent and shows bifurcated crests in some cases the downcurrent limbs of the sediment waves display a well defined erosional character on seismic profiles and furrows on side scan sonar mosaics all bottom current features observed are in equilibrium with the present day oceanographic regime as documented by the available current metre data moreover sediment cores suggest that the sediment waves are actively migrating during the present interglacial possibly implying that bottom waters formation was enhanced after the shallow adriatic shelf was drowned during the late quaternary eustatic rise the sam slope represents therefore an ideal site to improve our understanding of the depositional impact of bottom currents along the mediterranean margins and may provide useful hints on the activity of bottom currents elsewhere on the mediterranean margins

sill related seafloor domes in the zhongjiannan basin western south china sea abstract magmatism can exert significant impact on sedimentary basins such as the zhongjiannan basin zjnb western south china sea we have evaluated multibeam bathymetric and multichannel seismic reflection data acquired by the guangzhou marine geological survey in recent years in order to investigate the distribution the characteristics and the subsurface structures related to seafloor domes found in the northeastern zjnb our data revealed forty two domes at water depths between 2312 m and 2870 m which are clustered around volcanic mounds large seamounts and along the edge of the central depression in the study area these domes are generally circular to elongate or irregular in plan view with large basal areas and they also have gentler flanks dips of 1 46 7 73 with vertical reliefs ranging from tens to hundreds of meters in seismic sections majority of the domes are underlain by variably shaped and complex magmatic sills which provide a cause effect relationship between domes formation and igneous intrusions these intrusions heat surrounding organic rich sediments release hydrocarbons fluidize sediment pore waters and form gas rich fluids which fill in sediment and uplift overlying strata immediately above the sills to form forced folds which are manifested as seafloor domes these sill folds dome structures have important implications for understanding geomorphologic features caused by sills emplaced at depth

gas seepage and assumed mud diapirism in the italian central adriatic sea abstract an assemblage of acoustic and topographical features recorded on high resolution 3 5 khz sub bottom profiler records are discussed a close relationship between locally elevated sea bed mounds or ridges and the several acoustic features is found in the adriatic sea off ortona italy these features include acoustically turbid or opaque zones assumed to represent gas charged sediments acoustically transparent sediments assumed to represent sediments with relatively high pressured pore water free of large gas bubbles acoustic fountains assumed to represent 1 2 m wide sediment columns of relatively high pressured pore water where the gas charged pore water may migrate vertically through the sediment acoustic voids assumed to be up to 50 m wide sediment zones near the sea bed containing high pressured gas charged pore water finely layered sediments assumed to represent sediments with sharp density contrasts caused by varying gas concentrations a function of grain size and stagnant pore water gas seepage to the water column originating at the acoustic voids the positive topographical sea bed features are assumed to represent sediment clay diapiric structures the mechanism of diapir formation and its association with the gas charged sediments and gas seepages is discussed diapirism is assumed to result when plastic clay near the sea bed is charged with gas migrating from below bubbles forming and concentrating in the plastic clay causes local density reversals whereby the clay flows upwards due to buoyancy thus forming the elevated topographical features and the associated gas seepages

seismic characteristics of a reef carbonate reservoir and implications for hydrocarbon exploration in deepwater of the qiongdongnan basin northern south china sea our analysis of approximately 40 000 km of multichannel 2 d seismic data reef oil field seismic data and data from several boreholes led to the identification of two areas of reef carbonate reservoirs in deepwater areas water depth 500 in of the qiongdongnan basin qdnb northern south china sea high resolution sequence stratigraphic analysis revealed that the transgressive and highstand system tracts of the mid miocene meishan formation in the beijiao and ledong lingshui depressions developed reef carbonates the seismic features of the reef carbonates in these two areas include chaotic bedding intermittent internal reflections chaotic or blank reflections mounded reflections and apparent amplitude anomalies similar to the seismic characteristics of the lh11 1 reef reservoir in the dongsha uplift and island reef of the salawati basin indonesia which house large oil fields the impedance values of reefs in the beijiao and ledong lingshui depressions are 8000 9000 g cc x m s impedance sections reveal that the impedance of the lh11 1 reef reservoir in the northern south china sea is 800010000 g cc x m s whereas that of pure limestone in bd23 1 1 is 10000 g cc x m s the mid miocene paleogeography of the beijiao depression was dominated by offshore and neritic environments with only part of the southern beijiao uplift emergent at that time the input of terrigenous sediments was relatively minor in this area meaning that terrigenous source areas were insignificant in terms of the beijiao depression reef carbonates were probably widely distributed throughout the depression as with the ledong lingshui depression the combined geological and geophysical data indicate that shelf margin atolls were well developed in the beijiao depression as in the ledong lingshui depression where small scale patch or pinnacle reefs developed these reef carbonates are promising reservoirs representing important targets for deepwater hydrocarbon exploration c 2008 elsevier ltd all rights reserved

gas hydrate distributions in sediments of pockmarks from the nigerian margin results and interpretation from shallow drilling a joint research expedition between the french ifremer and the german marum was conducted in 2011 using the r v pourquoi pas to study gas hydrate distributions in a pockmark field 1141 1199 meters below sea surface at the continental margin of nigeria the sea floor drill rig mebo of marum was used to recover sediments as deep as 56 74 meters below seafloor the presence of gas hydrates in specific core sections was deduced from temperature anomalies recorded during continuous records of infrared thermal scanning and anomalies in pore water chloride concentrations in situ sediment temperature measurements showed elevated geothermal gradients of up to 258 c km in the center of the so called pockmark a which is up to 4 6 times higher than that in the background sediment 72 c km the gas hydrate distribution and thermal regime in the pockmark are largely controlled by the intensity periodicity and direction of fluid flow the joint interaction between fluid flow gas hydrate formation and dissolution and the thermal regime governs pockmark formation and evolution on the nigerian continental margin

seismic scale funnel shaped collapse features from the paleocene eocene of the north west shelf of australia abstract a multi tiered cluster of funnel shaped seismic anomalies can be observed over 11 000 km2 in the lower tertiary of the carnarvon basin nw shelf of australia the lithology is dominated by deep water carbonates and marls individual funnel shaped depressions typically have a diameter exceeding 1 km and a depth of a few hundred meters the strata inside some of the anomalies can be correlated with surrounding layers in the best documented example the correlation shows that the infill has foundered into the funnel shaped depression the collapse appears to have removed about 1 km3 below sedimentary cover the anomalies were emplaced in deep water when gas was actively migrating in the basin and above a structural high based on this context and using recently described hydrate pockmarks as live analogs the following model is proposed for the emplacement of these collapsed features 1 thermogenic gas migrated up into the fine grained paleogene series and froze up into hydrates upon entering the gas hydrate stability zone 2 the correlative volume increase resulted in conical failure of the overburden and gas migrating along the failure impregnated the surrounding sediments with hydrates 3 as the hydrate stability zone moved up hydrate dissolution dissociation expelled gas and associated liquefied sediment from the cone while the non impregnated core foundered towards the bottom of the cone three such episodes are recorded in the study area during the paleogene two of them lie in close temporal vicinity to the main hyperthermal events of this period volume calculations indicate that the amount of gas released in our study area is two orders of magnitude lower than what would be needed to trigger the paleocene eocene thermal maximum event suggesting that the anomalies are rather developed as a consequence of global hyperthermal events

the physics of gas chimney and pockmark formation with implications for assessment of seafloor hazards and gas sequestration pockmarks form where fluids discharge through seafloor sediments rapidly enough to make them quick and are common where gas is present in near seafloor sediments this paper investigates how gas might lead to pockmark formation the process is envisioned as follows a capillary seal traps gas beneath a fine grained sediment layer or layers perhaps layers whose pores have been reduced in size by hydrate crystallization gas accumulates until its pressure is sufficient for gas to invade the seal the seal then fails completely a unique aspect of capillary seals releasing a large fraction of the accumulated gas into an upward propagating gas chimney which displaces water like a piston as it rises near the seafloor the water flow causes the sediments to become quick i e liquefied in the sense that grain to grain contact is lost and the grains are suspended dynamically by the upward flow the quickened sediment is removed by ocean bottom currents and a pockmark is formed equations that approximately describe this gas piston water drive show that deformation of the sediments above the chimney and water flow fast enough to quicken the sediments begins when the gas chimney reaches half way from the base of its source gas pocket to the seafloor for uniform near surface sediment permeability this is a buoyancy control not a permeability control the rate the gas chimney grows depends on sediment permeability and the ratio of the depth below seafloor of the top of the gas pocket to the thickness of the gas pocket at the time of seal failure plausible estimates of these parameters suggest gas chimneys at blake ridge could reach the seafloor in less than a decade or more than a century depending mainly on the permeability of the deforming near surface sediments since these become quick before gas is expelled gas venting will not provide a useful warning of the seafloor instabilities that are related to pockmark formation however detecting gas chimney growth might be a useful risk predictor any area underlain by a gas chimney that extends half way or more to the surface should be avoided c 2009 elsevier ltd all rights reserved

geological and oceanographic controls on seabed fluid escape structures in the northern zhongjiannan basin south china sea abstract mega pockmarks which can reach a few kilometers in width and a few hundred meters in depth and associated fluid escape features in the northern zhongjiannan basin south china sea have received much attention in the recent years due to their large variety of morphologies and sizes in this work an integrated interpretation of detailed multibeam bathymetry conventional multi channel seismic reflection and seismic oceanography sections covering the northern zhongjiannan basin south china sea are used to image the seafloor morphology the subsurface geology and the water column in order to better understand the geological and oceanographic controls on the formation and evolution of the widespread fluid escape features in this region regional uplift and volcanism took place in this area near the palaeogene neogene boundary and during the neogene quaternary post rift thermal subsidence period differential subsidence and diapirism deformed the overlying kilometer scale successions mainly after the miocene focused flow of fluids originated at depth occurred along gas pipes polygonal faults tapered reflectors and faults into the shallow sediments resulting in intense fluid blowouts and complex fluid escape structures on the seafloor our results show that elongated pockmarks pockmark gullies and gullies in the northern slope are generally controlled by underlying buried channels and gullies however pockmark gullies and gullies in the southern slope are controlled by gravity sliding slumping along with crescent pockmarks developing parallel to water depth contours to varying degrees intense fluid escape activities associated with mud diapirism and gas escape in the study area resulted in mud extrusion and the formation of mud volcanoes complex seabed morphologies and irregular pockmarks mega pockmarks collapse structures and blind valleys have formed at the apexes of tectonic uplifts related to volcanism and above basement highs which suggests that these processes played a role in creating pathways for fluid escape bottom currents associated with the south china sea western boundary current imaged here for the first time with the seismic oceanographic method mainly flow to the south and erode the seafloor making pockmark walls and slopes of gullies and channels in the north steeper than those in the south some fluid escape structures such as the mega pockmarks and pockmark gullies are buried by sediment transported by the bottom currents turbidity currents flowing downslope along the long pockmark gullies also erode the seafloor and make the nw slopes of the mud volcanoes less steep than those in the se eddies imaged by the seismic sections may also play a role in transporting sediment to the deep sea in this area some of the reported fluid escape structures are still active as indicated by the fluid pathways that reach the seafloor and plumes in the water column imaged by the seismic sections

pockmark like depressions near the goliat hydrocarbon field barents sea morphology and genesis abstract pockmarks are observed worldwide along the continental margins and are inferred to be indicators of fluid expulsion in the present study we have analysed multibeam bathymetry and 2d 3d seismic data from the south western barents sea in relation to gas hydrate stability field and sediment type to examine pockmark genesis seismic attributes of the sediments at and beneath the seafloor have been analysed to study the factors related to pockmark formation the seabed depths in the study area are just outside the methane hydrate stability field but the presence of higher order hydrocarbon gases such as ethane and or propane in the expelled fluids may cause localised gas hydrate formation the selective occurrence of pockmarks in regions of specific seabed sediment types indicates that their formation is more closely related to the type of seabed sediment than the source path of fluid venting such as faults the presence of high acoustic backscatter amplitudes at the centre of the pockmarks indicates harder coarser sediments likely linked to removal of soft material the pockmarks show high seismic reflection amplitudes along their fringes indicating deposition of carbonates precipitated from upwelling fluids high seismic amplitude gas anomalies underlying the region away from the pockmarks indicate active fluid flow from hydrocarbon source rocks beneath which is blocked by overlying less permeable formations in areas of consolidated sediments the upward flow is limited to open fault locations while soft sediment areas allow diffused flow of fluids and hence formation of pockmarks over a wider region through removal of fine grained material

estimating the time of pockmark formation in the sw xisha uplift south china sea using reaction transport modeling carbon cycling and fluid seepage in marine sediments over the late quaternary were investigated at a now extinct pockmark located in a mega pockmark field in the sw xisha uplift nw south china sea measured particulate organic carbon poc content and porewater sulfate so4 2 dissolved inorganic carbon dic concentrations and 34s so4 2 distributions were used to constrain a non steady state reaction transport model and quantify poc mineralization rates as well as estimate the time when fluid flow ceased at the investigated pockmark an increase in poc content and 34s so4 2 and a decrease in sulfate concentrations in the upper ca 2m at the pockmark and a reference core implied an increase in the flux and reactivity of organic matter during the early holocene around 10kyrb p caused by enhanced primary productivity during the strengthened southwestern summer monsoon these features were simulated with the model assuming a holocene increase in poc flux and reactivity subsequently starting from an initial condition reminiscent of a modern active cold seep hydrate ridge hindcast simulations showed that fluid seepage at the pockmark ceased ca 39kyr ago this corresponds to a relative sea level high stand which is believed to be associated with gas hydrate stabilization the non steady state model presented in this contribution can also be used to constrain the time when fluid seepage ceased at other presently extinct cold seeps when suitable sediment and porewater data are available

kuroshio intrusion and the circulation in the south china sea 1 the princeton ocean model is used to study the circulation in the south china sea scs and its seasonal transition kuroshio enters leaves the scs through the southern northern portion of the luzon strait the annually averaged net volume flux through the luzon strait is 2 sv into the scs with seasonal reversals the inflow season is from may to january with the maximum intrusion of kuroshio water reaching the western scs during fall in compensation of summertime surface offshore transport associated with coastal upwelling from february to april the net transport reverses from the scs to the pacific the intruded kuroshio often forms an anticyclonic current loop west of the luzon strait the current loop separates near the dongsha islands with the northward branch continuously feeding the south china sea warm current scswc near the shelf break and the westward branch becoming the south china sea branch of kuroshio on the slope which is most apparent in the fall the scswc appears from december to february on the seaward side of the shelf break flowing eastward against the prevailing wind diagnosis shows that the onshore ekman transport due to northeasterly monsoon generates upwelling when moving upslope and the particular distributions of the density and sea level associated with the cross shelf motion supports the scswc

in situ x ray diffraction study of the effect of water on the garnet perovskite transformation in morb and implications for the penetration of oceanic crust into the lower mantle abstract the phase boundary of the transformation of garnet to perovskite in hydrous mid ocean ridge basalt morb was determined by in situ high pressure and high temperature x ray diffraction xrd experiments the garnet bearing assemblage was formed in experiments at pressures below 21 gpa in contrast an assemblage of mg perovskite ca perovskite stishovite cf phase and or nal phase is stable at pressures above 21 gpa garnet and perovskite bearing assemblages were synthesized at a pressure range of 20 25 gpa and a temperature range of 1000 1600 c on the basis of our results we have located the garnet perovskite phase boundary and find that it can be described by the following equation p gpa 0 004t c 17 5 which positions it 2 gpa lower under dry conditions if the top 7 km of the slab is colder than 1200 c in the transition zone perovskite will appear at a depth shallower than that where ringwoodite decomposes this result indicates that the morb layer becomes denser than the peridotite portion of the slab throughout the transition zone and the lower mantle and thus no density crossover would exist between basalt and underlying peridotite along the cold subducting slab geotherm

fault intersections as critical hydrocarbon leakage zones integrated field study and numerical modelling of an example from the timor sea australia fault intersections are identified as important sites for hydrocarbon leakage from the skua oil field in the timor sea australia integrated structural and fluid history data sets suggest that these fault intersections may be efficient and long lived fluid conduits three dimensional 3d numerical modelling based on fault patterns observed in the skua field generated zones of high dilation in the vicinity of fault intersections during contraction even at low bulk strain values in nature these dilational zones are likely to be sites of high structural permeability containing concentrated open fracture networks ideal for high fluid flux the potential for fluid leakage from these zones may be further enhanced where low shear strain occurs due to mechanical locking at the fault intersection although not tested in the numerical experiments fault gouge development is likely to be less extensive in these zones of low shear strain reducing the probability of forming membrane seals the modelling results support previously published charge and leakage history studies of the skua field and highlight the potential for large volumes of hydrocarbons to be lost where fault intersection zones breach the top seal fault intersections may therefore play a significant role in influencing trap integrity conditions in other areas

cold seep systems in the south china sea an overview abstract three decades after the discovery of cold seep systems various sites of hydrocarbon seepage have been found in the south china sea scs over the past decade these sites have become model systems for understanding the variability of hydrocarbon seepage and associated biogeochemical processes in this review we describe the cold seep systems of the scs with an emphasis on seafloor manifestations fluid sources biogeochemical processes and macroecology seafloor features associated with seeps include mud volcanoes pockmarks and carbonate deposits a common characteristic of cold seeps is the occurrence of authigenic i e in situ precipitated carbonate minerals these carbonates commonly exhibit low 13c and high 18o values suggesting the incorporation of methane derived carbon and oxygen derived from gas hydrate water biogeochemical processes such as sulfate driven anaerobic oxidation of methane sd aom the key process at seeps have been studied in detail with the aim of establishing geochemical proxies to trace these processes into the geological past we also detail the features characterizing seep ecosystems understanding the impact of decomposing methane hydrate on the marine carbon budget remains challenging and requires additional seafloor observations as well as models predicting how gas hydrate responds to changing conditions such as temperature increase sea level rise and episodic mass wasting

submarine erosional troughs in the northern south china sea evidence for early miocene deepwater circulation and paleoceanographic change abstract the paleoceanography of the south china sea scs is poorly understood although recent cruises and an odp expedition leg 184 have provided a useful general framework here we report previously unrecognized multi km scale sedimentary structures consisting of coupled ridges and troughs within the early miocene strata using high resolution 3d seismic data from the northern scs margin the troughs are interpreted as linear and or arcuate erosional features which truncate the underlying strata they range in orientation from e w in the southern part of the study area to nearly ne sw in the northwestern part of the study area the spacing of erosional troughs varies from less than 0 1 km in the southern part to more than 3 km in the northern part and they are about 50 1500 m in width with a relief of 10 90 m the erosional troughs are parallel to the regional strike of the early miocene slope of the northern scs from their internal geometry context regional distribution and most importantly their orientation relative to the paleo slope gradient at the time of their formation it is argued that they formed due to the action of bottom currents linked to the broader paleo circulation regime in the basin we infer that the paleo bottom currents intruding from the pacific flowed along the northern scs slope resulting in the occurrence of localized erosional troughs as deep water masses impinged on and interacted with the sea floor topography this study provides new insights into the early miocene paleoceanography of the scs and demonstrates the utility of 3d seismic data to enhance our understanding of paleo bottom current circulation regimes

morphologies classification and genesis of pockmarks mud volcanoes and associated fluid escape features in the northern zhongjiannan basin south china sea abstract based on new high resolution multi beam bathymetry and multichannel seismic reflection data two new groups of numerous pockmarks and mud volcanoes were discovered in the northern zhongjiannan basin at water depths between 600 and 1400 m individual pockmarks are circular elliptical crescent shaped or elongated with diameters ranging from several hundreds to thousands of meters and tens or hundreds of meters in depth and they often form groups or strings crescent pockmarks approximately 500 1500 m wide in cross section and 50 150 m deep occur widely in the southern study area both as individual features and in groups or curvilinear chains and they are more widespread and unique in this area than anywhere else in the world conical mud volcanoes mostly with kilometer wide diameters and ca 100 m high mainly develop in the northern study area as individual features or in groups seismic data show that the observed pockmarks are associated with different kinds of fluid escape structures and conduits such as gas chimneys diapirs zones of acoustic blanking acoustic turbidity and enhanced reflections inclined faults small fractures and polygonal faults the mapped mud volcanoes appear to be fed from deep diapirs along two main conduit types the conventional conduits with downward tapering cones and another other conduit type with a narrow conduit in the lower half and emanative leakage passages in the upper half various types of pockmarks are found and a comprehensive pockmark classification scheme is proposed according to a their shape in plan view which includes circular elliptical crescent comet shape elongated and irregular b their magnitude which includes small normal giant and mega pockmarks and c their composite pattern which includes composite pockmarks pockmark strings and pockmark groups for the genesis of the crescent pockmark strings a 5 stage speculative formation model is proposed implying possible controlling factors of gravity sliding slumping fluid escape activity and sandbodies intrusion seismic data suggest that the mud volcanoes have likely undergone two episodes of evolution which include a widespread drastic formation of mud volcanoes with dome like structures a wide range of seepage occurrences and the formation of complex sediments in the first stage and the appearance of pockmarks conical mud volcanoes groups and the formation of deformed filled or broken buried complex sediments by continuous fluid flow processes during stage 2 in the pliocene and the quaternary pockmark gullies are extensively found along the slope due to the interaction of slope failure and fluid escape processes the cenozoic sedimentary cover is thin in the northern zhongjiannan basin which evolved from a late cretaceous or palaeogene oligocene rift to the neogene quaternary post rift thermal subsidence and therefore intense deep thermogenic fluid flow can easily affect the soft cenozoic sediments produce complex sediment deformation and form numerous fluid flow structures at the seafloor in particular pockmarks and mud volcanoes that dominantly emerged during the period of post rift thermal subsidence and neotectonic movement since ca 5 5 ma

evidence of a widespread paleo pockmarked field in the orange basin an indication of an early eocene massive fluid escape event offshore south africa abstract the detailed interpretation and seismic attribute analysis of 2d and 3d seismic reflection data sets revealed the presence of a distinct seismic horizon displaying circular and elongated depressions in early eocene sediments along the northwestern slope of the orange basin on the southwest african margin their occurrence within a single seismic horizon indicates a contemporaneous formation process we interpret the depressions as paleo pockmarks which occur as single pockmarks predominantly nne ssw aligned pockmark trains and coalescing elongated pockmarks the paleo pockmarked surface covers an area of 2800 km 2 and has been mapped by integrating the 3d dataset with industrial 2d seismic grids the spatial and temporal occurrence of the paleo pockmarks is associated to two episodes of rapid sedimentation during the late maastrichtian to paleocene comprising fine grained shelf eroded material we propose that they were formed during the early eocene by the seepage of both 1 overpressured pore fluids from disequilibrium compaction of upper maastrichtian sediments and 2 thermogenic gas from underlying cretaceous mature source rocks we show that the location of the pockmarks is controlled by the underlying faults system of the paleo slope the paleocene eocene transition is known for its series of hyperthermal events characterized by negative excursions of the carbon isotope curve the early eocene widespread event of fluid and thermogenic gas escape identified in this work can be an example of the type of carbon release processes occurring in hydrocarbon prone sedimentary basins during this time

effects of temperature and humidity upon the transport of sedimentary particles by wind aeolian transport of sedimentary particles is a well recognized phenomenon in cold regions but the effect of low temperature t and specific humidity q upon particle entrainment has not previously been investigated this study reports on wind tunnel experiments 12 c t 32 c and 1 g kg 1 q 10 g kg 1 that suggest the aerodynamic drag required to entrain sand sized particles can be 30 lower in cold or high latitude settings as compared to hot deserts for any given regional wind velocity this effect will increase the wind strength index and thereby the proportion of time a surface is active at low temperature

timing and configuration of the gongga ii glaciation in the hailuogou valley eastern tibetan plateau a glacier climate modeling method abstract the timing and extent of late quaternary glaciation in the hailuogou valley of eastern tibetan plateau have been extensively studied over past decades debate however exists in the timing of the gongga ii moraine that represents the maximum glaciation in the hailuogou valley furthermore the gongga ii glaciation is not quantitatively reconstructed and what climate conditions could support this maximum glaciation is unknown this study used a glacier climate model driven by data from local weather stations to simulate the gongga ii glacier extent by fitting modeled glaciers to field geological evidence running multiple simulations with different temperature precipitation combinations we inferred the climate scenarios sufficient to generate the gongga ii glacier extent the modeled gongga ii glaciers have an area of 119 km 2 and an average volume of 1 35 10 10 m 3 with an average equilibrium line altitude of 4050 m asl combined with other independent climate reconstructions the glacier climate model simulations suggested that the gongga ii glacial stage most likely occurred in the lgm

geological controls on focused fluid flow associated with seafloor seeps in the lower congo basin a synthesis of backscatter imagery coupled with a large 3d seismic dataset in the lower congo basin lcb reveals a patchy distribution of features interpreted to be associated with fluid seepage from 300 m to 2500 m water depth with the exception of one region of anomalous backscatter positive relief mounds all inferred seep sites occur in negative relief pockmarks the extensive 3d seismic dataset in the lcb offers a unique opportunity to study the plumbing system that is feeding surface cold seep systems and in general to reconstruct the relationship between tectonics and fluid flow in continental margins the fluid seeps in the lcb are associated with morphologically stratigraphically or tectonically controlled focused fluid flow the integration of the geophysical datasets backscatter imagery coupled to 3d seismic clearly indicates that fluid seeps are not randomly distributed but their seabed organization reflects 1 the location of the underlying structure reservoir or trap where the fluids are coming from 2 the geometry and morphology of the reservoir trap and 3 the discontinuities in the sedimentary column along which fluids have migrated in the lcb seafloor pockmarks are always associated with underlying tectonic structures fault zones salt diapirs polygonal faults or buried sedimentary bodies turbiditic channels erosional surfaces whereas they never occur above sub horizontal parallel stratified fine grained sediments even if triggering processes can not be clearly defined here we propose a model of seafloor fluid seep organization which represents a new tool for identifying the geometry of flow pathways and the underlying buried bodies where the fluids are originating from this qualitative 3d model provides insight into the geohydrologic processes of continental margins

a honeycomb seafloor morphology in carbonate sediment of the carnegie ridge offshore ecuador formation and potential geodynamic significance based on swath bathymetry two dimensional high resolution seismic reflection profiles and ocean drilling program deep sea drilling project odp dsdp data we describe a seafloor honeycomb pattern and propose a model for its formation in pliocene miocene carbonate deposited on the uneven oceanic basement of the carnegie ridge offshore ecuador hydrothermal fluids derived from the basement aquifer fractured and dissolved carbonate sediment creating seafloor pits above basements highs fluids expelled along polygonal faults may have assisted the nucleation of seafloor depressions at the pliocene pleistocene boundary strong bottom currents scoured previously damaged sediments enlarging the initial depressions and producing the seafloor honeycomb pattern this regional erosive episode was contemporaneous with the final closing of the isthmus of panama and the clogging of the ecuador trench by the subduction of the carnegie ridge so that the honeycomb pattern may be viewed as a regional marker of these two geodynamic events

hydrocarbon plumbing systems above the snohvit gas field structural control and implications for thermogenic methane leakage in the hammerfest basin sw barents sea abstract based on the analysis of the high resolution 3d seismic data from the sw barents sea we study the hydrocarbon plumbing system above the snohvit and albatross gas field to investigate the geo morphological manifestation and the dynamics of leakage from the reservoir fluid and gas escape to the seafloor is manifested in this area as mega pockmarks 1 2 km wide large pockmarks

tectonic and structural controls on neogene fluid release in the patagonian continental margin abstract analysis of high resolution multi beam bathymetry 2d multi channel seismic and high resolution seismic sub bottom profiles revealed the presence of widespread fluid escape features in the middle slope of the patagonian continental margin on the sea bottom these features correspond to pockmarks and mud volcanoes whereas in the sub surface they are represented in the seismic records by several acoustic anomalies such as chimneys acoustic blanking enhanced reflectors and reverse phase enhanced reflectors among others some of these acoustic signatures can be traced to syn rift deposits that fill a deep re activated and inverted graben analysis of pockmarks elongation and pockmarks alignments show a good correlation with inverted normal faults suggesting that faults and fractures have influenced the pockmark s shape and might have acted as pathways for upward fluid migration some of the acoustic anomalies associated with pockmarks are interpreted as evidence of gas the gas observed in the seismic data seems to be thermogenic and the seismic data suggests a deep origin associated with over pressured syn rift deposits two morphometrically different sets of pockmarks were identified an older set hosted by miocene aged rocks and a younger set hosted by quaternary deposits different potential triggers are discussed for the genesis of the miocene pockmarks in relation to the seismostratigraphy structural geology and regional tectonics it is concluded that tectonic activity associated with the neogene inversion of the graben faults due to andean compression is the most likely cause for the formation these pockmarks the presence of gas charged sediment and young pockmarks also suggest that after the middle miocene tectonic climax a more recent pulse of release of fluids occurred

seismic evidence and formation mechanism of gas hydrates in the zhongjiannan basin western margin of the south china sea abstract an analysis of 3d seismic data from the zhongjiannan basin in the western margin of the south china sea scs reveals seismic evidence of gas hydrates and associated gases including pockmarks a bottom simulating reflector bsr enhanced reflection er reverse polarity reflection rpr and a dim amplitude zone daz the bsr mainly surrounds zhongjian island covering an area of 350 km2 in this 3d survey area the bsr area and pockmark area do not match each other where there is a pockmark developed there is no bsr the gas hydrate layer builds upward from the base of the stability zone with a thickness of less than 100 m a mature pockmark usually consists of an outside trough a middle ridge and one or more central pits with a diameter of several kilometers and a depth of several hundreds of meters the process of pockmark creation entails methane consumption dense faults in the study area efficiently transport fluid from large depths to the shallow layer supporting the formation of gas hydrate and ultimately the pockmark

3d analogue modeling of the south china sea a discussion on breakup pattern the opening of the south china sea is one of the most important cenozoic events in se asia in order to investigate its tectonic evolution three analogue modeling experiments were compared the modeling results suggest that rifting pattern and orientation change of the rift zone were related to initial theological stratification affected by the ductile flow of silicone lower crust and honey asthenosphere the faults became flattened especially close to the breakup area we conjecture that the slope area may bear relatively hotter and thinner lithosphere than the shelf area from the beginning of rifting due to stretching and mantle upwelling associated with this regional extension which in turn lead to the change in initial rheological stratification and therefore the rifting pattern from shelf to slope in the experiments breakup developed first at isolated points which grew and coalesced to become a single spreading area the conjugate boundaries were either all concave or all convex where a rigid massif was located at the divergent boundary stretching history was different in this case the northern and southern areas thinned rapidly and developed into two deep troughs which may finally evolve into spreading centers the shape of the massif controls the orientation and boundary shape of the spreading area it is interpreted that crustal breakup was more viscous in style and the nw sub sea basin developed along the northern trough of the zhongsha xisha massif macclesfield bank c 2008 elsevier ltd all rights reserved

a tectonic model for the tertiary evolution of strike slip faults and rift basins in se asia abstract models for the tertiary evolution of se asia fall into two main types a pure escape tectonics model with no proto south china sea and subduction of proto south china sea oceanic crust beneath borneo a related problem is which if any of the main strike slip faults mae ping three pagodas and aliao shan red river asrr cross sundaland to the nw borneo margin to facilitate continental extrusion recent results investigating strike slip faults rift basins and metamorphic core complexes are reviewed and a revised tectonic model for se asia proposed key points of the new model include 1 the asrr shear zone was mainly active in the eocene oligocene in order to link with extension in the south china sea the asrr was less active during the miocene tens of kilometres of sinistral displacement with minor amounts of south china sea spreading centre extension transferred to the asrr shear zone 2 at least three important regions of metamorphic core complex development affected indochina from the oligocene miocene mogok gneiss belt doi inthanon and doi suthep around the asrr shear zone hence paleogene crustal thickening buoyancy driven crustal collapse and lower crustal flow are important elements of the tertiary evolution of indochina 3 subduction of a proto south china sea oceanic crust during the eocene early miocene is necessary to explain the geological evolution of nw borneo and must be built into any model for the region 4 the eocene oligocene collision of ne india with burma activated extrusion tectonics along the three pagodas mae ping ranong and klong marui faults and right lateral motion along the sumatran subduction zone 5 the only strike slip fault link to the nw borneo margin occurred along the trend of the asrr fault system which passes along strike into a right lateral transform system including the baram line

subsurface fluid flow at an active cold seep area in the qiongdongnan basin northern south china sea abstract the active haima cold seep sites were discovered by the remotely operated vehicle on the western part of the northern slope of south china sea in 2015 and 2016 however the subsurface structures in the seep area are not well studied which prevents the further understanding of the evolution of fluid venting system in the active cold seep area in 2017 a high resolution 2d seismic line transecting the two main seep sites was acquired and processed in this study the subsurface structures were well resolved on the seismic profile the new observations include the widespread subsurface fluid flow features i e chimney structures and pockmarks and the pervasively distributed magmatic activities seismic indicators for hydrothermal vents were also observed on the seismic section this new discovery confirms the contribution of magmatic activities to the evolution of subsurface fluid flow in the study area minor faults under the recognizable resolution of seismic profile which can act as conduits for the subsurface fluid flow were also identified in the fine grained sediment sequences overall these results provide new insights into the evolution of subsurface fluid flow in the study area

denudation history of onshore central vietnam constraints on the cenozoic evolution of the western margin of the south china sea abstract apatite fission track analysis is used to monitor the timing and rate of upper crustal cooling and denudation across central vietnam in response to rifting of the south china sea results show regional denudation in the syn and post rift phases of the south china sea occurred at a similar rate 40 10 m myr consistent with the sedimentation record in adjacent offshore basins this suggests that extension was not the dominant factor controlling cenozoic denudation probably because the study area was sufficiently distant from the actively rifted continental margin a significant change in regional cooling rate during the upper miocene inferred denudation rates also shift from 34 m myr to 390 500 m myr is linked to enhanced erosion and deposition of prograding sediments in adjacent offshore basins this shift in denudation is associated and contemporaneous with initiation of regional basaltic magmatism and development of topography

sedimentary environment of the faroe shetland and faroe bank channels north east atlantic and the use of bedforms as indicators of bottom current velocity in the deep ocean in the northeast atlantic much of the deep cold water flow between the norwegian sea and the main north atlantic basin passes through the faroe shetland and faroe bank channels generating strong persistent bottom currents capable of eroding and transporting sediment up to and including gravel a large variety of sedimentary bedforms including scours furrows comet marks barchan dunes sand sheets and sediment drifts is documented using sidescan sonar images seismic profiles seabed photographs and sediment cores from the floor of the channel published information on current velocities associated with the various bedforms has been used to reconstruct the pattern of bottom currents acting on the channel floor the results broadly reflect the current pattern predicted on the basis of regional oceanographic observations but add considerable detail the internal consistency of the results suggests that the methods used are robust giving confidence in the fine detail of the observed bottom current structure bottom current velocities in the range 1ae0 m s 1 are indicated by the range of observed bedforms with the strongest currents associated with south west transport of norwegian sea deep water nsdw at water depths of 800 1200 m the main nsdw flow forms a relatively narrow core that follows the base of the faroes slope this core follows the 90 change in trend of the faroes slope at the junction between the faroe shetland and faroe bank channels the strongest currents within the nsdw core are found over the shallowest sill in the faroe shetland channel and in the narrowest part of the channel immediately downstream of the sill and are generated by topographic constriction of the flow eastward flow of deep water along the northern flank of the wyville thomson ridge suggests a complex current pattern with some recirculation of deep water within the deep faroe bank channel basin the observations suggest that coriolis force is the main agent controlling the westward deflection of the nsdwinto the faroe bank channel contradicting a previous suggestion that this was controlled by the topography of the wyville thomson ridge

elongate depressions on the southern slope of the norwegian trench skagerrak morphology and evolution abstract multibeam echosounder surveys conducted on the southern slope of the norwegian trench between norway and denmark show that elongate depressions interpreted as current modified pockmarks are widespread in water depths of 100 500 m in the holocene recent succession the long axes of the depressions are oriented at an angle of about 15 downslope relative to the depth contours the long axes appear to almost exactly parallel the effective bottom currents in this part of the skagerrak the present current directions were probably established about 4000 years ago following a major shift in the oceanographic circulation pattern the most characteristic feature of the depressions is their large size with maximum depths of 45 m widths of 400 m and lengths of 2 km strong bottom currents continuously transport particles thrown into suspension by the seepage process away from the pockmarks and prevent further deposition continuous seepage of shallow gas at fixed pockmark sites over long time periods combined with generally high sedimentation rates are thought to be responsible for the exceptional size of many depressions the shallow gas probably has a mixed biogenic thermogenic origin however the thermogenic component is considered to be most important for the formation of pockmarks seepage from permeable layers in the jurassic cretaceous bedrock is evidenced by linear rows of depressions above these horizons while pockmarks above sub surface faults indicate gas seeps also from there in shallow water areas the shape and size of pockmarks are locally dependent on their location with respect to giant sand waves which trend sub parallel to the depth contours and are interpreted to be of late weichselian to early holocene age in deeper water areas the pockmarks have been further modified by gravity slides caused by collapse of steep pockmark walls

the morphologies and genesis of mega pockmarks near the xisha uplift south china sea pockmarks are normally regarded to be manifestations of fluids escape through the seabed kilometer wide depressions here called mega pockmarks occur as widespread features near the xisha uplift northern south china sea most of the pockmarks observed in this area are multi kilometers and much larger than normal pockmarks reported worldwide the maximum diameter observed is 3210 m and the maximum depth observed is 165 2 m based on multibeam data and 2d seismic data the pockmarks are circular elliptical and crescentic in plan view seismic profiles show that the genesis of pockmarks is related to fluid gas and or pore water escape according to the fluids pathways the pockmarks fall into four types 1 gas chimney related 2 depositional boundary related 3 gas chimney and inclined structure fault related 4 inclined structure related bottom currents are strong and complex in the study area the multibeam data and seismic profiles indicate that they may play an important role on extension maintenance and shaping of pockmarks the research of the study area is in its initial stage and the identification of these features as indicators of fluid flow is probably useful for hydrocarbon exploration c 2011 elsevier ltd all rights reserved

influence of intermediate water in the western okinawa trough by the outflow from the south china sea although the kuroshio and the south china sea scs waters flow in and out of the luzon strait near surface the scs water mainly flows out of the scs at mid depth between 350 and 1350 m in all seasons and years that we studied the eastward flowing scs intermediate water is blocked by and joins the northward flowing kuroshio at about 122 e thus the kuroshio intermediate water kiw is composed of the west philippine sea wps proper water on the east centered at 600 m depth because the ridge east of taiwan separating the okinawa trough from the wps is mostly shallower than 600 m the eastern portion of the kiw is largely blocked by the ridge and is forced to turn eastward this water does not enter the okinawa trough freely until it reaches a deep gap in the ryukyu ridge farther north subsequently forming the eastern portion of the intermediate water in the trough the western portion of the kiw is the mixed water of scs and wps the nutrient rich scs intermediate water centered at 500 m is less influenced by the ridge and is able to flow into the okinawa trough the scs intermediate water thus retains its signature in the southern and western okinawa trough upwells onto the east china sea shelf and becomes a major source of nutrients to the shelf water

focused hydrocarbon migration in shallow sediments of a pockmark cluster in the niger delta off nigeria the niger delta is one of the largest hydrocarbon basin offshore africa and it is well known for the presence of active pockmarks on the seabed during the guineco mebo cruise in 2011 long cores were taken from a pockmark cluster in order to investigate the state of its current activity gas hydrates oil and pore water were sampled for geochemical studies the resulting dataset combined with seismic data reveal that shallow hydrocarbon migration in the upper sedimentary section was focused exclusively within the pockmarks there is a clear tendency for gas migration within the hydrate bearing pockmarks and oil migration within the carbonate rich one this trend is interpreted as a consequence of hydrate dissolution followed by carbonate precipitation in the course of the evolution of these pockmarks we also demonstrate that anaerobic oxidation of methane aom is the main process responsible for the depletion of pore water sulfate with depths of the sulfate methane transition zone smtz ranging between 1 8 and 33 4 m in addition a numerical transport reaction model was used to estimate the age of hydrate layer formation from the present day sulfate profiles the results show that the sampled hydrate layers were formed between 21 and 3750 years before present overall this work shows the importance of fluid flow on the dynamics of pockmarks and the investigated cluster offers new opportunities for future cross site comparison studies our results imply that sudden discharges of gas can create hydrate layers within the upper sedimentary column which can affect the seafloor morphology over few decades this article is protected by copyright all rights reserved

anomalous depressions in the northern yellow sea basin evidences for their evolution processes abstract depressions of both known and unknown origin are observed in continental margin basins and ocean floors worldwide a field of anomalous depressions with dome structures within the centers are documented for the first time using sub bottom seismic profiles multibeam bathymetry data and well data in the northern yellow sea basin they have complex seabed morphologies circular elliptical polygonal linear and crescent shapes with kilometer scale diameters and shallow depths 10 s decimeters to meters focused fluid flow pathways such as gas chimneys faults and pipes and free gas enhanced amplitude anomalies are observed within the shallow strata the depressions are interpreted as immature pockmarks possibly caused by insufficient fluid supply compared to fully developed normal pockmarks they are suspected to still be in the middle stage of pockmark evolution their formation processes are suspected to be controlled by sub seabed plumbing fluid flux and bottom currents this study is important for geologic hazard assessments in the study area and it also contributes to the full understanding of pockmark evolution

geological evolution regional perspectives and hydrocarbon potential of the northwest phu khanh basin offshore central vietnam abstract seismic stratigraphic and structural analyses of the northwest phu khanh basin offshore central vietnam based on 2 d seismic data indicate that the initial rifting began during the latest cretaceous or palaeogene controlled by left lateral transtension along the east vietnam boundary fault zone evbfz and northwest southeast directed extension east of the evbfz rifting stopped due to transpression during middle oligocene times but resumed by left lateral transtension during the late oligocene thick sequences of lacustrine and alluvial sediments were deposited during the palaeogene rift periods the late oligocene rifting ended due to inversion triggered by right lateral wrenching near the palaeogene neogene boundary following the onset of this inversion regional uplift and volcanism took place in the southern half of the study area and contemporaneous subsidence and transgression took place farther north leading to widespread carbonate deposition as the right lateral wrenching decreased during the early neogene thermal subsidence and siliciclastic sedimentation became dominant resulting in the buildup and southward propagation of the shelf slope sediment accumulation and subsidence rates increased after the middle miocene times due to eastward tilting of central vietnam and the adjacent offshore area potential direct hydrocarbon indicators dhis in the phu khanh basin include common amplitude anomalies gas chimney like features and seafloor gas seeps in addition oil seeps are found at dam thi nai immediately landward of the basin geochemical analyses of the oil seeps indicate the existence of at least two early to peak mature source rocks maturation modelling combined with the seismic analysis suggests the likely presence of oil kitchens 40 50 km downdip in the basin and a fairly simple oil migration route the basin is probably charged from lacustrine syn rift mudstones humic coals and fore reef marls potential reservoirs are turbidite lowstand delta shelf and coastal sandstones as well as platform and reef carbonates and fractured basement various structural and stratigraphic traps formed during the palaeogene and early neogene prior to the main stage of petroleum generation

seismic detection of marine methane hydrate as offshore petroleum exploration and development move into deeper water industry must contend increasingly with gas hydrate a solid compound that binds water and a low molecular weight gas usually methane gas hydrate has been long studied in industry from an engineering viewpoint due to its tendency to clog gas pipelines however hydrate also occurs naturally wherever there are high pressures low temperatures and sufficient concentrations of gas and water these conditions prevail in two natural environments both of which are sites of active exploration permafrost regions and marine sediments on continental slopes in this article we discuss seismic detection of gas hydrate in marine sediments gas hydrate in deepwater sediments poses both new opportunities and new hazards an enormous quantity of natural gas likely far exceeding the global inventory of conventional fossil fuels is locked up worldwide in hydrates ex traction of this unconventional resource presents unique exploration engineering and economic challenges and several countries including the united states japan canada india and korea have initiated joint industry academic governmental programs to begin studying those challenges hydrates also constitute a potential drilling hazard because hydrates are only stable in a restricted range of pressure and temperature any activity that sufficiently raises temperature or lowers pressure could destabilize them releasing potentially large volumes of gas and decreasing the shear strength of the host sediments assessment of the opportunities and hazards associated with hydrates requires reliable methods of detecting hydrate and accurate maps of their distribution and concentration hydrate may occur only within the upper few hundred meters of deepwater sediment at any depth between the seafloor and the base of the stability zone which is controlled by local pressure and temperature hydrate is occasionally exposed at the seafloor where it can be detected either visually or acoustically by strong seismic reflection amplitudes or high backscatter