hydrocarbon plumbing systems of salt minibasins offshore angola revealed by three dimensional seismic analysis analysis of three dimensional seismic data from the lower congo basin offshore angola reveals numerous fluid flow features in the miocene to holocene succession and the potential for large shielded traps underneath basinward overhanging salt structures the fluid flow evidence includes present day sea floor pockmarks clustered above salt structures pliocene pleistocene stacked paleopockmarks and miocene pockmark fields other fluid flow features include high amplitude cylindrical pipe structures 60 to 300 m 197 984 ft wide and 25 to 300 m 82 984 ft high within lower and middle miocene strata thick 150 m 492 ft high reflectivity zones within the pliocene succession associated with bottom simulating reflections and subvertical low amplitude chimneys originating from the deeper section 1 km 0 6 mi beneath the sea floor the miocene pockmark fields occur at a specific horizon suggesting a regional fluid expulsion event at ca 12 ma and the miocene fluid flow regime is interpreted to be dominated by thermogenic fluids supplied via carrier beds and leaking vertically above structural highs the pliocene pleistocene fluid flow regime was dominated by short distance vertical fluid migration and expulsion related to early stage diagenetic processes involving biogenic methane and pore water the present day fluid flow regime is inferred to be dominated by thermogenic fluids primarily controlled by kilometer scale salt flank controlled migration the study emphasizes the use of seismically imaged fluid flow features in hydrocarbon systems analysis by documenting the evolution of an overburden plumbing system through time involving several fluid types and flow regimes depending on the spatiotemporal availability of thermogenic and diagenetic fluids and the tectonostratigraphic occurrence of aquifers traps and seals

methane leakage from evolving petroleum systems masses rates and inferences for climate feedback abstract the immense mass of organic carbon contained in sedimentary systems currently estimated at 1 56 10 10 tg des marais et al 1992 bears the potential of affecting global climate through the release of thermally or biologically generated methane to the atmosphere here we investigate the potential of naturally occurring gas leakage controlled by petroleum generation and degradation as a forcing mechanism for climate at geologic time scales we addressed the potential methane contributions to the atmosphere during the evolution of petroleum systems in two different petroliferous geological settings the western canada sedimentary basin wcsb and the central graben area of the north sea besides 3d numerical simulation different types of mass balance and theoretical approaches were applied depending on the data available and the processes taking place in each basin in the case of the wcsb we estimate maximum thermogenic methane leakage rates in the order of 10 2 10 3 tg yr and maximum biogenic methane generation rates of 10 2 tg yr in the case of the central graben maximum estimates for thermogenic methane leakage are in the order in 10 3 tg yr extrapolation of our results to a global scale suggests that at least as a single process thermal gas generation in hydrocarbon kitchen areas would not be able to influence climate although it may contribute to a positive feedback conversely only the sudden release of subsurface methane accumulations formed over geologic timescales can possibly allow for petroleum systems to exert an effect on climate

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

tertiary sea level movements around southern africa sedimentological micropaleontological and seismic profiling data elucidate the history of tertiary sea level movements around southern africa these new data show that landward movement of the sea began in early paleocene time and continued into the early eocene the sea probably reached its maximum paleocene height during the early eocene and is today represented by outcrops up to at least 204 m and probably as high as 360 m above modern sea level a brief regressive pulse occurred during the middle eocene with renewed transgression in the late eocene a major regression followed probably spanning all of oligocene and early miocene times this regression exposed much of the continental shelf and is clearly represented on seismic reflection profiles by a widespread unconformity the major neogene transgression began in the middle miocene and probably reached its greatest extent in the late miocene or early pliocene the overall middle miocene to early pliocene transgression was interrupted by a brie

a blast of gas in the latest paleocene simulating first order effects of massive dissociation of oceanic methane hydrate carbonate and organic matter deposited during the latest paleocene thermal maximum is characterized by a remarkable 2 5 excursion in delta 13c that occurred over approximately 10 4 yr and returned to near initial values in an exponential pattern over approximately 2 x 10 5 yr it has been hypothesized that this excursion signifies transfer of 1 4 to 2 8 x 10 18 g of ch4 from oceanic hydrates to the combined ocean atmosphere inorganic carbon reservoir a scenario with 1 12 x 10 18 g of ch4 is numerically simulated here within the framework of the present day global carbon cycle to test the plausibility of the hypothesis we find that 1 the delta 13c of the deep ocean shallow ocean and atmosphere decreases by 2 3 over 10 4 yr and returns to initial values in an exponential pattern over approximately 2 x 10 5 yr 2 the depth of the lysocline shoals by up to 400 m over 10 4 yr and this rise is most pronounced in one ocean region and 3 global surface temperature increases by approximately 2 degrees c over 10 4 yr and returns to initial values over approximately 2 x 10 6 yr the first effect is quantitatively consistent with the geologic record the latter two effects are qualitatively consistent with observations thus significant ch4 release from oceanic hydrates is a plausible explanation for observed carbon cycle perturbations during the thermal maximum this conclusion is of broad interest because the flux of ch4 invoked during the maximum is of similar magnitude to that released to the atmosphere from present day anthropogenic ch4 sources

tectonically induced adjustment of passive margin accommodation space influence on the hydrocarbon potential of the orange basin south africa a seismic stratigraphic investigation integrated with the structural modeling of the southern part of the orange basin passive margin south africa demonstrates that a single tectonic event resulted in a significant alteration to both the location and style of sediment accumulation during its postrift evolution this alteration to the margin has a significant effect on the hydrocarbon system of the area we predict that it increased the hydrocarbon potential of the area the evolution of the margin can be divided into two principal phases the first comprised an overall aggradational shelf margin with little or no deformation during the cretaceous deposition during the late cretaceous was punctuated by an episode of margin tilting that resulted in significant erosion of the inner margin and alteration of the margin architecture the second phase of deposition during the tertiary occurred to the west of the cretaceous shelf margin and was characterized by significant margin instability and the development of a coupled growth fault and toe thrust system this change in passive margin configuration and the associated switch in the location of overburden accumulation is likely to have increased the petroleum prospectivity of the deep water part of the margin we predict that the rapid western seaward migration of sediment accumulation resulted in the maturation of the high quality distal source interval whereas the resulting toe thrust geometry provides suitable structural traps for the hydrocarbons

gas bubble cavities in deltaic muds lake powell delta glen canyon national recreation area hite utah abstract cavities produced by migrating gas mainly methane emanating from the near subsurface are a rarely reported sedimentary structure from either modern or ancient clastic sediments the delta of the colorado river in lake powell glen canyon national recreation area provides an excellent recently accessible locality for examining soft sediment deformation structures generated by escaping gases and fluids sub meter scale to tens of meter scale structures including craters salses and sand and mud volcanoes are present on the modern lake powell delta abundant sub centimeter scale cavities are found in excavations through crater sediments and exposed along cut bank exposures of the colorado river in cross section the cavities are subvertical irregular to sigmoidal to lenticular shaped 1 3 mm short axes up to 3 0 cm long axes and 1 cm intermediate axes cavities are developed in cm scale laminated silty clay and clay graded beds the tops of the cavities are wider than their bases the cavities observed in the lake powell delta are interpreted as mode 1 fractures and probably represent a clastic morphologic analog to molar tooth carbonate structures the characteristics of the cavities and the geometries and deformation of the surrounding delta mud host are consistent with bubbles and fracturing generated by gas migration experiments and theoretical calculations but vary because of inherent heterogeneity of the sediments fractures developed as gas migrated into pre existing cavities causing overpressure in cavities and leading to subvertical mode 1 fracture propagation recent sudden decompression related to rapid lake level drops of lake powell may have enhanced the release of gas primarily bacterially produced leading to the development of these soft sediment deformation structures

complex pockmarks with carbonate ridges off mid norway products of sediment degassing abstract a new type of pockmark has been discovered off mid norway they contain up to 10 m high ridges of methane derived authigenic carbonate mdac rock the pockmarks were investigated using rovs remotely operated vehicles in 2003 and 2004 the complex pockmarks are located well within the gas hydrate pressure temperature stability field and are associated with a regional bsr bottom simulating reflector our high resolution seismic data demonstrate that the near surface sedimentary bedding dips into vertical pipe structures underlying the pockmarks and that the pipes are deeply rooted the pockmarks are circular in plan view and contain up to 190 m long and 40 m wide ridges formed by up to 4 m wide carbonate rocks piled on top of each other in irregular piles other key features of the complex pockmarks include 1 a distinct fauna with local bacterial mats small tubeworms stalked crinoids and pycnogonids sea spiders and 2 the presence of light hydrocarbon gases c 1 c 5 in clay rich sediments the methane 13 c values range from 54 to 69 pdb suggesting the presence of both bacterial and thermogenic gases during our two day observation periods only micro seepage occurred without ebullition or other visual fluid flow petrography and geochemistry of carbonate blocks from the pockmarks suggest precipitation of methane derived aragonite within the sediments 13 c 52 to 58 pdb we suggest that the complex pockmarks formed by sudden catastrophic fluid flow and that the large carbonate ridges represent dislodged mdacs originally formed subsurface prior to the event subsequent micro seepage is still active and may indicate that the pipes represent fluid flow conduits which transport both bacterial and thermogenic hydrocarbons to the water column our data do not suggest pressure buildup and do not predict future eruptions from the complex pockmarks thus our data rather suggest that the catastrophic formation of the complex pockmarks may be a one time event

hummock fields in the norway basin and eastern iceland plateau rayleigh taylor instabilities side scan imagery and 3 5 khz profiles in the norway basin reveal 3 4 10 4 km 2 of sea floor covered by regularly spaced 500 2000 m low relief hummocks previously attributed to bottom currents extrapolating from nearby borehole data i attribute these mound fields to gravitational rayleigh taylor instabilities resulting from superposition of 50 100 m of thick pliocene pleistocene glacigenic glacier derived sandy clays density 1800 kg m 3 on a several hundred metre thickness of miocene biosiliceous oozes having a density of 1300 kg m 3 this hypothesis predicts or explains similar hummock fields in many oceanic regions where such density inversions exist

seafloor facies related to upward methane flux within a giant pockmark of the lower congo basin the origin of the cold fluid venting from a giant pockmark within the lower congo basin has been elucidated based upon results of precise mapping submersible dives gravity coring and isotopic analyses realized under a total ifremer cooperative project zaiango and biozaire projects during four dives of the ifremer rov victor 6000 the bottom was filmed hard and soft samples were lifted from the sediment and water samples were collected with a ctd rosette system the detailed dip map shows that the 800 m wide giant pockmark is a composite feature due to the coalescence of multiple 100 m wide depressions that displays a broad range of biological mineralogical and chemical features on the seafloor leading to a seafloor anomaly recorded on the multibeam imagery methane rich fluids migrating through the sedimentary column from a buried palaeochannel clearly react with the sulphate rich circulating sea water to produce hydrogen sulphide and bicarbonate ions this situation leads to a concentrical arrangement of the sedimentary facies with methane related features in the centre and sulphide related features at the periphery this organization is correlated with high levels of methane up to 20 mol l measured in the centre of the giant pockmark responsible for the crystallization of gas hydrates at the bottom in this model the concentrical organization of mineralogical and biological features reflects a geochemical partitioning related to the peripheral progressive mixing of the methane flux

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

the chronology and tectonic style of landscape evolution along the elevated atlantic continental margin of south africa resolved by joint apatite fission track and u th sm he thermochronology atlantic type continental margins have long been considered passive tectonic settings throughout the entire postrift phase recent studies question the long term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin s evolution the namaqualand sector of the western continental margin of south africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity here we present the first systematic study using joint apatite fission track and apatite u th sm he thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region apatite fission track ages range from 58 3 2 6 to 132 2 3 6ma with mean track lengths between 10 9 0 19 and 14 35 0 22 m and mean u th sm he sample ages range from 55 8 31 3 to 120 6 31 4ma joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150 130ma and 110 90ma with limited cooling during the cenozoic estimates of denudation based on these thermal histories predict approximately 1 3 km of denudation coinciding with two major tectonic events the first event during the early cretaceous was driven by continental rifting and the development and removal of synrift topography the second event during the late cretaceous includes localized reactivation of basement structures as well as regional mantle driven uplift relative tectonic stability prevailed during the cenozoic and regional denudation over this time is constrained to be less than 1 km

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

3d seismic analysis of the ak fault orange basin south africa implications for hydrocarbon leakage and offshore neotectonics abstract a high resolution 3d reflection seismic dataset was used to achieve a detailed description of the half graben bounded ak fault in the ibhubesi gas field of the offshore orange basin seismic analysis of the fault involved the combination of both the conventional seismic interpretation and seismic attributes the main objective of the study was to provide insights into hydrocarbon leakage and neotectonic activity along the west coast shelf of south africa the ak fault is among the several half graben bounded faults that formed in the late jurassic to early cretaceous during the separation of the south american and the african continents the ak fault consists of five seismically detectable segments whose evolution reflects characteristics of both the coherent and the isolated end members model of segmented fault origin diapiric structures that are direct indicators of vertical fluid movement were also observed along the ak fault system providing evidence of significant roles that the fault plays in promoting the vertical migration of hydrocarbon in the orange basin recent reactivation of the fault has also occurred along an oblique extensional regime that is dominated by normal dip slip faulting with minor shear components along the tips

gas hydrate fluid flow and free gas formation of the bottom simulating reflector abstract gas hydrate in continental margins is commonly indicated by a prominent bottom simulating seismic reflector bsr that occurs a few hundred metres below the seabed the bsr marks the boundary between sediments containing gas hydrate above and free gas below most of the reflection amplitude is caused by the underlying free gas gas hydrate can occur without a bsr however and the controls on its formation are not well understood here we describe two complementary mechanisms for free gas accumulation beneath the gas hydrate stability zone ghsz the first is the well recognised hydrate recycling mechanism that generates gas from dissociating hydrate when the base of the ghsz moves upward relative to hydrate bearing sediment the second is a recently identified mechanism in which the relationship between the advection and diffusion of dissolved gas with the local solubility curve allows the liquid phase to become saturated in a thick layer beneath the ghsz when hydrate is present near its base this mechanism for gas production called the solubility curvature mechanism is possible in systems where the influence of diffusion becomes important relative to the influence of advection and where the gas water solubility decreases to a minimum several hundred metres below the ghsz we investigate a number of areas in which gas hydrate occurs to determine where gas formation is dominated by the solubility curvature mechanism and where it is dominated by hydrate recycling we show that the former is dominant in areas with low rates of upward fluid flow such as old rifted continental margins low rates of seafloor uplift and high geothermal gradient and or pressure conversely free gas formation is dominated by hydrate recycling where there are rapid rates of upward fluid flow and seabed uplift such as in subduction zone accretionary wedges using these two mechanisms to investigate the formation of free gas beneath gas hydrate in continental margins we are able to resolve a number of problems these include why sub bsr free gas zones are generally thin in convergent margins and thick in passive margins and why areas such as the gulf of mexico and mackenzie delta do not have regional bsrs finally we use our understanding of how bsrs are formed to show that regional gas hydrate is unlikely to exist in substantial quantities in passive continental margins outside areas with an observable bsr

bulls eye pockmarks and polygonal faulting in the lower congo basin relative timing and implications for fluid expulsion during shallow burial abstract this study describes a new type of pockmark association from the lower congo basin offshore west africa consisting of up to 8 stacked paleopockmarks separated by intervals of drape and onlap fill the stacked paleopockmarks occur within the depocentres of polygonally faulted plio pleistocene sediments and are distributed evenly in the downslope parts of two salt mini basins the majority of the stacked pockmarks initiated synchronously in the late pliocene 3 ma with a subordinate initiation phase in the mid pliocene 4 ma the primary agents in pockmark formation are interpreted to be pore water expelled during early stage compaction together with biogenic methane bottom simulating reflections bsrs associated with free gas overlain by gas hydrates are currently found in the area it is speculated that biogenic methane accumulated within and below a clathrate cap which was repeatedly breached forming pockmarks at discrete horizons separated by intervals of draping sedimentation the mid and late pliocene pockmark initiations appear to coincide with sea level falls following periods of relatively stable highstand conditions several subsequent pockmark horizons may similarly correlate with subsequent sea level falls during the late pliocene and early pleistocene the stacked paleopockmarks are completely surrounded by polygonal faults and consistently occur within polygonal fault cells that crosscut the succession containing the stacked pockmarks early stage compaction and dewatering of the pliocene sediments thus preceded polygonal faulting providing a constraint on the conditions leading to polygonal faulting of the fine grained host sediments the relationship documented here is interpreted as due to the presence of a hydrate cap in the plio pleistocene mini basins which may have retarded the normal compaction processes and facilitated pockmark formation by allowing the build up of gas hydrate and free gas in the basin centres the relative timing and spatial relationships implies that fluids expelled due to polygonal faulting were not implicated in pockmark formation in this area

mesozoic exhumation of the southern cape south africa quantified using apatite fission track thermochronology southern africa s topography is distinctive an inland plateau of low relief and high average elevation is separated from a coastal plane of high relief and low average elevation by a steeply dipping escarpment the origin and evolution of this topography is poorly understood because unlike high plateaus elsewhere its development cannot be easily linked to present day compressional plate boundary processes understanding the development of this regional landscape since the break up of gondwana is a first order step towards resolving regional epeirogenesis we present data that quantifies the timing and extent of exhumation across the southern cape escarpment and coastal plane using apatite fission track analysis afta of 25 outcrop samples and 31 samples from three deep boreholes kw1 67 sa1 66 cr1 68 outcrop fission track aft ages are cretaceous and are significantly younger than the stratigraphic ages of their host rocks indicating that the samples have experienced elevated paleotemperatures mean track lengths vary from 11 86 to 14 23 m the lack of cenozoic apatite ages suggests that major cooling was over by the end cretaceous the results for three boreholes situated seaward south of the escarpment indicate an episode of increased denudation in the mid late cretaceous 100 80 ma an earlier episode of increased denudation 140 120 ma is identified from a borehole north of the escarpment thermal modelling indicates a history involving 2 5 3 5 km of denudation in the mid late cretaceous 100 80 ma at a rate of 175 to 125 m ma the aft data suggest that less than 1 km of overburden has been eroded regionally since the late cretaceous 80 ma at a rate of 10 to 15 m ma but do not discount the possibility of minor in relative amplitude episodes of uplift and river incision through the cenozoic the reasons for rapid denudation in these early and mid cretaceous episodes are less clear but may be related to epeirogenic uplift associated with an increase in mantle buoyancy as reflected in two punctuated episodes of alkaline intrusions e g kimberlites across southern africa and contemporaneous formation of two large mafic igneous provinces 130 and 90 ma flanking its continental margins because cenozoic denudation rates are relatively minimal epeirogenic uplift of southern africa and its distinct topography cannot be primarily related to cenozoic mantle processes consistent with the lack of any significant igneous activity across this region during that time

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

tectonic subsidence history and thermal evolution of the orange basin the orange basin offshore southwest africa appears to represent a classical example of continental rifting and break up associated with large scale transient volcanism the presence of lower crustal bodies of high seismic velocities indicates that large volumes of igneous crust formed as a consequence of lithospheric extension we present results of a combined approach using subsidence analysis and basin history inversion models our results show that a classical uniform stretching model does not account for the observed tectonic subsidence moreover we find that the thermal and subsidence implications of underplating need to be considered another departure from the uniform stretching model is renewed sub crustal thinning and linked to that uplift in the cenozoic that is necessary to reproduce the observed phases of erosion and the present day depth of the basin the dimension of these events has been examined and quantified in terms of tectonic uplift and sub crustal thinning based on these forward models we predict the heat flow evolution not only for the available real wells but also for virtual wells over the entire study area finally the hydrocarbon potential and the temperature evolution is presented and shown in combination with inferred maturation of the sediments for depth intervals which comprise potential source rocks 2009 elsevier ltd all rights reserved

late mesozoic and tertiary sediment supply to the eastern cape basin se atlantic and palaeo drainage systems in southwestern africa isopach asymmetry and sediment component changes in dsdp cores from the se atlantic orange basin support the hypothesis of major drainage system changes in sw africa during late cretaceous cenozoic time this involved alternations in the use of the 28 s modern orange river and 31 s modern olifants river exit points across the western escarpment by rivers carrying run off from the upper orange vaal catchment areas as well as radical re organizations of internal drainage geometry it is postulated that during late cretaceous times the 28 s exit was used with the middle orange river following a course in the interior well to the south up to 150 km of its modern channel sediment discharge rates from this river were relatively high at least 10 106 m3 yr 1 and resulted in rapid advancement of the continental margin sediment prism west of the mouth by large scale slumping the palaeogene orange vaal river exit was via the 31 s escarpment crossing and during the later part of this period the cape canyon was cut across the continental shelf and slope a significant reduction in sediment discharge to 2 0 106 m3 yr 1 suggests that the lower tertiary climate for sw africa was drier than that of late cretaceous times however aridity did not commence until late miocene times when the orange vaal discharge had switched back to the 28 s exit modern sediment discharge rates 6 5 106 m3 yr 1 are relatively high and reflect soil erosion caused by agricultural activity the two major alterations in exit point of the orange vaal late cretaceous early tertiary and late oligocene early miocene are related to periods of low sea level which promoted river capture adjacent to the western escarpment an additional factor in the first course change may have been the disruption of the middle orange channel by late cretaceous igneous intrusions less important internal reorganizations of the drainage system are postulated in late miocene pleistocene times economic implications for offshore diamond distribution are briefly mentioned

giant irregular pockmark craters in the palaeogene of the outer moray firth basin uk north sea a cluster of irregular pockmark craters of palaeogene age in block 15 18 in the outer moray firth uk north sea have been mapped on 3d seismic data with well control the craters range from 0 5 to 4 km in diameter between 50 and 200 m in depth and have been buried by tertiary subsidence and sedimentation to present day depths of between 1000 and 1400 m sub sea they are circular to elliptical in plan view and are located in a delta front and upper pro deltaic slope position on the dornoch delta the structures are confined to the top of the lower eocene balder formation and regional mapping across the moray firth basin shows they are restricted to an area of ca 15 15 km2 well data show that the craters are sand filled and have excavated an argillaceous and tuffaceous unit the crater filling sands are sealed by overlying mudstone units planar wing like features emanate from the margins of the craters well control indicates that these seismically identified wings consist of sandstone suggesting that the craters have been modified by an episode of sand remobilisation and injection the proposed mechanism for the formation of these structures is large scale fluid eruption probably shallow gas at the early eocene seafloor these craters are significantly larger in size than those typically observed on contemporary sea floors we argue that the gas is more likely to have been sourced from deeper jurassic or carboniferous sediments within the witch ground graben than from shallow biogenic origins these structures rank in scale alongside the largest known pockmarks and demonstrate that natural fluid and hydrocarbon seeps have a long ancestry in this basin

passive margin evolution and its controls on natural gas leakage in the southern orange basin blocks 3 4 offshore south africa using a 2d seismic dataset that covers part of the southern orange basin offshore south africa we reconstructed the geological evolution of the basin this evolutionary model was then used to investigate the occurrence of natural gas within the sedimentary column and the distribution of gas leakage features in relation to the observed sedimentary and tectonic structures developed in the post rift succession since the early cretaceous the cretaceous succession has been subdivided into five seismic units the highest sedimentation rates occur within the barremian aptian unit c1 and the turonian coniacian unit c3 two cenozoic units t1 and t2 have been distinguished these show a sudden decrease in sedimentation rate for the whole of the cenozoic three phases of gravitational tectonics with two late cretaceous phases of mass movement in the northern study area and cenozoic slumping in the southern study area have been related to sedimentation rates sea level changes paleoenvironmental evolution and regional tectonics the occurrence of natural gas leakage follows a coast parallel distribution within the study area in the near shore part at water depths shallower than 400 m massive gas chimneys penetrate through the sediment layers and reach the near surface within an intermediate narrow band between 300 and 500 m water depth the gas migrates more diffusely through sub vertical faulted cretaceous sediments while in the outer part of the basin through the cretaceous and cenozoic gravitational wedges only very few signs of gas accumulation and migration can be seen along the faults a conceptual model has been established with the aptian source rock generating gas in the outer part of the basin this source rock underlies the cenozoic wedge in the south and the thick cretaceous wedge in the north and is a postulated source for the natural gas within the sedimentary column this thermogenically generated gas does not migrate directly through the gravitational faults and the above lying sediments but moves buoyancy driven up dip along stratigraphic layers to escape through the sediments to the sea floor in the inner shelf area

giant hummocks in deep water marine sediments evidence for large scale differential compaction and density inversion during early burial this paper describes large scale hummock structures from an oligocene miocene succession in the faeroe shetland trough off the northern coast of the united kingdom these hummocks have a wavelength of 1 2 km and an amplitude of about 50 m they are characterized by a polygonal planform geometry two successive depositional packages in this probably pelagic dominated interval have this form of structural expression but troughs of the younger set are directly superposed on crests of the older set they occur at a present day subsea bottom depth of about 1000 m and cover an area of 6000 km 2 this extraordinary structural configuration is attributed to a protracted depositional and deformational history in which a density inversion was established hummock formation was initiated in response to differential loading above an irregular polygonal fault system and then subsequent collapse produced syndepositional troughs in the overlying units these structures are similar in geometry to hummocks described from near surface sediments in the norway basin by p r vogt and are the largest type of density inversion deformation structure yet described from a clastic sedimentary succession

gas seepage as an indicator of deeper prospective reservoirs a study based on exploration 3d seismic data three periods of sustained gas seepage in geological time have been revealed in danish block 5604 26 in the north sea by the use of exploration 3d seismic data the most recent period is indicated by a cluster of seismic chimneys which ties in to buried craters near the seabed and possible present gas escape through the seabed along with amplitude anomalies indicating a shallow gas sand charged by gas migrating from a deeper level the cluster of seismic chimneys indicative of vertical gas migration is visible down to 1 5 s twt 1500 ms and therefore the gas is interpreted as migrating from a deeper stratigraphic horizon below this level it becomes difficult to see the chimneys due to complex faulting the faults may work as gas migration pathways the geometry of the cluster of seismic chimneys indicates that the gas has been migrating from one point the nearest possible source of the gas is an underlying prospect where an oil and gas discovery has been made two earlier periods of gas seepage are indicated by mounds possibly carbonate buildups over gas seepages in pliocene time and similarly buried craters formed by gas seepages in an earlier period in pliocene time the results of this study are that gas seepage is a periodical process in geological time and that its presence and associated features can be used as an indicator of deeper prospective reservoirs

linking source and sink evaluating the balance between onshore erosion and offshore sediment accumulation since gondwana break up south africa abstract the geomorphic origin and evolution of the tectonically unique interior highland of southern africa the kalahari plateau and its flanking low lying coastal planes remain largely unresolved because of a lack of regional quantitative analyses of its uplift and erosion history here we focus on the southern cape south africa and link onshore denudation based on new apatite fission track thermochronology results to offshore sediment accumulation using abundant well data and a seismic reflection profile we attempt to relate source and sink in order to resolve some first order issues concerning timing of the exhumation and development of the topographic features of southern africa the volume of sediment accumulated off south africa s south coast is calculated using 173 wells and a seismic reflection profile a total uncompacted sediment volume of 268 500 km3 accumulated off south africa s south coasts since 136 ma in the outeniqua and southern outeniqua basins accumulation volumes and rates were highest in the early cretaceous 48 800 104 km3 at 8150 km3 ma from 136 to 130 ma and 57 500 104 km3 at 5750 km3 ma from 130 to 120 ma and mid late cretaceous 83 700 104 km3 at 3200 km3 ma from 93 to 67 ma volumes and accumulation rates were lowest for the early mid cretaceous 47 400 104 km3 at 1750 km3 ma from 120 to 93 ma and the cenozoic 31 200 104 km3 at 450 km3 ma from 67 to 0 ma although our analysis shows that the accumulated volume of offshore sediments does not match the calculated volume of onshore erosion as quantified through apatite fission track thermochronology e g tinker j h de wit m j brown r 2008 mesozoic exhumation of the 439 southern cape south africa quantified using apatite fission track thermochronology tectonophysics doi 10 1016 j tecto 2007 10 009 the timing of increased sediment accumulation closely matches the timing of increased onshore denudation this suggests that the greatest volumes of material were transported from source to sink during two distinct cretaceous episodes and that the processes driving onshore denudation decreased by an order of magnitude during the cenozoic

sediment deposits in the cape basin indications for shifting ocean currents the benguela current system running off southwest africa is one of the world s largest upwelling regions the current has strongly influenced sedimentary features on the continental margin to unravel its development seismic stratigraphy tied to drilling results from ocean drilling program leg 175 sites 10851087 was established four units southern cape basin scb 1 to scb 4 were defined for the cenozoic sediments the upper unit scb 1 1 5 ma characterized by continuous high amplitude reflectors represents global cooling and glacial interglacial cycles unit scb 2 14 ma distinguished by low amplitude reflections is associated with the onset of the upwelling system and establishment of the modern circulation pattern in the cape basin slump scarps are concentrated along the middle and upper shelf slope suggesting they are caused by a combination of mass movements triggered by bottom currents and slope instabilities because of increased deposition associated with the upwelling a westward extension and or movement of upwelling filaments is interpreted from the observed seaward shift of scarp locations with time erosion associated with stronger currents probably thinned unit scb 2 in the south the two lower units scb 3 and scb 4 56 ma probably represent material eroded from the shelf break and deposited during a major oligoceneearly miocene regression that is consistent with a significant uplift of southern africa the basal reflector scb d of unit scb 4 is associated with the prominent reflector d or l described in previous publications

intra to post messinian deep water gas piping in the levant basin se mediterranean abstract this study presents evidences for fluid flow manifested as pipe structures in the post evaporitic overburden characterised from a 3d seismic data from the levant basin offshore central israel interestingly these pipes are genetically and spatially contextualized east and west of the study area all rooted in the messinian evaporite substratum pipes in the western group accounts for 83 of the pipe population are crudely cylindrical oval to elliptical in planform with diameter and height ranging 350 2000 m and 320 420 m respectively internal configurations within this group varies from chaotic to concave upward reflections diagnostic of fluid induced collapse pipes in the eastern group are seepage pipes appearing conical in shape with height of 350 510 m and diameter of 320 420 m the eastern group pipes are characterized by lack of internal reflectivity and or disrupted stratal reflections the western group indicates an episode of fluid flow till the mid pliocene compared to late pliocene in the eastern group a conceptual model for the pipes in the western group is proposed to have occurred from subjacent dissolution of the messinian evaporite under deep water marine conditions during the pliocene by vertically focused fluid flow from intra messinian realm dissolving the top evaporites and inducing systematic collapse in the overburden the onset of which may have been triggered by seismicity conversely pipes in the eastern group are proposed to have developed from breaching the top evaporite by pressurized fluids that developed from lateral pressure transfer due to differential loading of the overburden and salt tectonics these pipe structures may pose a potential risk to deep water exploration and development the fluid expulsion phenomena documented in this study has wider implications for the hydrodynamics of many basins where thick evaporite layers are widely developed

the late rifting phase and continental break up of the southern south atlantic the mode and timing of volcanic rifting and formation of earliest oceanic crust

structural evolution of the orange basin gravity driven system offshore namibia the orange basin records the development of the late jurassic to present day volcanic rifted passive margin of namibia regional extension is recorded by a late jurassic to lower cretaceous syn rift megasequence which is separated from a cretaceous to present day post rift megasequence by the late hauterivian ca 130 ma break up unconformity the late cretaceous post rift evolution of the basin is characterized by episodic gravitational collapse of the margin gravitational collapse is recorded as a series of shale detached gravity slide systems consisting of an up dip extensional domain that is linked to a down dip zone of contraction domain along a thin basal detachment of turonian age the extensional domain is characterized by basinward dipping listric faults that sole into the basal detachment the contractional domain consists of landward dipping listric faults and strongly asymmetric basinward verging thrust related folds growth stratal patterns suggest that the gravitational collapse of the margin was short lived spanning from the coniacian ca 90 ma to the santonian ca 83 ma structural restorations of the main gravity driven system show a lack of balance between up dip extension 24 km and down dip shortening 16 km gravity sliding in the namibian margin is interpreted to have occurred as a series of episodic short lived gravity sliding between the cenomanian ca 100 ma and the campanian ca 80 ma gravity sliding and spreading are interpreted to be the result of episodic cratonic uplift combined with differential thermal subsidence sliding may have also been favoured by the presence of an efficient detachment layer in turonian source rocks

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

seismic character of bottom simulating reflectors examples from the mid norwegian margin three classes of bottom simulating reflectors bsr cross cut the post breakup sediments of the mid norwegian margin the first class is caused by free gas at the base of the pressure and temperature dependent gas hydrate stability zone the second class of bsr is caused by the diagenetic transition from opal a to opal ct the third class of bsr is always observed underneath the opal a opal ct transition but heat flow data and the amplitude characteristics of this arrival exclude one of the known silicate diagenetic transitions or gas hydrates as the explanation for this reflector odp site 643 drilling results from the voring plateau suggest two possible processes as the reason for this third bsr a smectite illite conversion or b a sudden increase in the abundance of authigenic carbonates the genesis of both is pressure and temperature dependent and could potentially result in a cross cutting seismic reflector the data are not conclusive as to which process is causing the third class of observed bsr

1000 m long gas blow out pipes abstract this study presents seismic observation of pipe anomalies from offshore nigeria outcrops of blow out pipes from rhodes greece and geophysical modelling of an acoustic pipe the studies give insight into how pipes form their internal structure the seismic image and geophysical artefacts related to the pipes over one hundred seafloor craters 100 m 700 m wide and up to 30 m deep have been observed on the seafloor offshore nigeria they are underlain by interpreted cones and seismic pipe anomalies that can be traced down to reservoir zones at 1000 m 1300 m below the seafloor the seismic pipe anomalies are 50 m 150 m wide and almost vertical they are interpreted as up scaled pipes found in outcrops on rhodes greece the outcrops show pipe related structures at three levels lowest the reservoir rock contains metre sized cavities which are filled with a mixture of clay derived from the overlying cap rock in the middle several circular to oval structures in plane view of pipes are observed in the lowest part of the cap rock highest 15 m into the clay cap rock strongly sheared country rock forms circular structures with a core of structureless clay based on outcrop observation on rhodes we constructed an acoustic model of a 50 m wide and 1000 m long pipe seismic modelling proves that such pipes would be expressed in seismic data that they are similar to the seismic pipe anomalies offshore nigeria but this study also revealed that prominent intra pipe reflections are artefacts a formation model for the pipes is suggested high fluid overpressure in the reservoir generated hydro fractures from the reservoir to seafloor where a mixture of gas and fluid flowed at high speed to form pipes cones and seafloor craters after hours to weeks of gas and fluid flow through the pipe the pore pressure in the reservoir dropped and the blow out terminated muddy slurry fell back and plugged the cavity in the reservoir and the pipe

iceberg scours pits and pockmarks in the north falkland basin abstract glaciation in the southern hemisphere is limited by the availability of land from which to seed ice sheets the extents of the antarctic patagonian and new zealand ice sheets at the last glacial maximum lgm are relatively well known although the rates and styles of their retreat after the lgm are poorly constrained particularly in antarctica offshore records of glaciation are relatively sparse in the southern hemisphere despite the potential ocean climate insights that can be gained from records of glaciation that are preserved offshore in this study we document the occurrence of iceberg scours and accompanying pits within the north falkland basin c 50 s and discuss their origin the cross sectional shapes of scours are u to v shaped and occur in present day water depths of 280 to 460 m individual scours are up to 38 km long 1 km wide and up to 10 m deep the scours are observed as erosional linear to curvilinear depressions showing only one point of contact between the iceberg and seafloor often with raised berms composed of excavated material identified either side of the main depression undulating width of scours is interpreted as an effect of rotation of the iceberg keel during scour excavation the elongate morphology of the scours differentiates them from asymmetrical pits interpreted to represent iceberg impact pits and symmetrical pockmarks interpreted to form due to fluid expulsion in cross section the differentiation is highly interpretative but the 3d bathymetric expression is unequivocal the sinusoidal character of the scours suggests the interaction between local tidal currents and the east falkland malvinas currents in the north falkland basin at the time of formation offshore and onshore landscape analysis is used to determine potential sources of icebergs and suggests that they were most likely sourced from the antarctic peninsula these results inform our understanding of southern hemisphere ocean climate interactions during the last glacial cycle and suggest that the east falklands malvinas current a key current in the southern hemisphere bringing cold low salinity antarctic derived waters into the south atlantic was in operation during the last glacial cycle the accumulation of icebergs west of the falkland islands would also result in further cooling from fresh meltwater perturbations enhancing the development of a potential ice bridge along the argentinian coast

mesozoic break up of sw gondwana implications for regional hydrocarbon potential of the southern south atlantic abstract this work provides new palinspastic palaeofacies reconstructions of sw gondwana incorporating rotation of a falkland malvinas microplate we discuss the implications of this for the tectonic evolution of the southern south atlantic and hence for the regional hydrocarbon potential existing gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents in most continental reconstructions the falkland malvinas plateau was assumed to be a rigid fragment of pre permian south american crust however it has been suggested on the basis of palaeomagnetic data that the falkland malvinas islands were rotated by 180 after 190 ma this rotation hypothesis has been successfully tested on the basis of devonian stratigraphy and palaeontology permian stratigraphy and sedimentology and late palaeozoic and early mesozoic structure making it unlikely that the plateau behaved as a rigid structure during breakup we have explored the consequences of accepting this hypothesis for the tectonic evolution of sw gondwana by compiling new palaeogeographic maps for the permian cretaceous of the southern atlantic area to achieve a realistic close fit we have devised a pre rift proxy for the ocean continent boundary for the south atlantic in order to produce the best fit it is necessary to subdivide south america into four plates the consequences of this are far reaching our work suggests that although sedimentary basins were initiated at different times three major tectonic phases can be recognised in regional terms these can be thought of as pre syn and post rift during the pre rift time until the late triassic the area was dominated by compressional tectonism and formed part of the gondwana foreland the falkland malvinas islands lay east of africa the falkland malvinas plateau was 33 shorter and patagonia was displaced east with respect to the rest of south america in part along the line of the gastre fault system potential source facies are dominantly post glacial black shales of late permian age deposited in lacustrine or hyposaline marine environments these rocks would also be an effective regional seal sandstones deposited in the late permian would be dominantly volcaniclastic with poor reservoir qualities triassic sandstones tend to be more mature there was significant extension from about 210 ma end triassic until the south atlantic opened at about 130 ma early cretaceous in the early syn rift phase extension was accompanied by strike slip faulting and block rotation later extension was accompanied by extrusion of large volumes of lava early opening of the south atlantic was oblique which created basins at high angle to the trend of the ocean on the argentine margin and resulted in microplate rotation in ne brazil intermittent physical barriers controlled deposition of upper jurassic cretaceous anoxic sediments during breakup some of these mudrock units are effective seals with likely regional extent during crustal reorganisation clastic sediments changed from a uniform volcaniclastic provenance to local derivation with variable reservoir quality in the late rift and early post rift phase continental extension changed from oblique to normal and basins developed parallel to the continental margins of the south atlantic this change coincides with the main rifting in the equatorial basins of brazil and the early impact of the santa helena plume it resulted in widespread development of unconformities the abandonment of the reconcavo tucano jatoba rift and the end of ne brazil plate rotation which remained attached to south america there was extensive deposition of evaporites concentrated in but not restricted to the area north of the rio grande rise walvis ridge widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements our main conclusion is that the regional hydrocarbon potential of the southern south atlantic has been constrained by the tectonic evolution

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

reconstruction of the southwestern african continental margin by backward modeling abstract as the evolution of the southwestern african passive continental margin and in particular the vertical movements in this area are still a matter of debate this study aims to quantify the past vertical movements by reconstructing the paleobathymetric evolution of the southwestern african margin during the post rift phase to quantify the vertical movements inferred from paleobathymetries different subsidence components that have affected the passive continental margin are calculated and interpreted with respect to the generating processes our investigation takes advantage of information on the present day sedimentary thicknesses and the crustal configuration along the sw african margin by using this detailed information and modeling backward in time we attempt to separate different subsidence components such as the load induced subsidence and the thermal subsidence for different time intervals during the post rift phase calculating the amounts of each subsidence component and subtracting them from the present day configuration yields paleobathymetries which are interpreted with respect to the evolution of the southwestern african passive margin these restored paleobathymetries for different time steps during the post rift phase indicate spatial and temporal variations of vertical movements including phases of seafloor uplift as a result our investigation allows to give minimum estimates of the vertical movements that have occurred during the post rift phase in addition to predominant subsidence about 1000 m of seafloor uplift are deduced for the northern and southern parts of the sw african margin quantifying the amounts of vertical movements cannot finally clarify the generating processes behind these movements but the timing and magnitude of seafloor uplift indicate a deep mantle mechanism

rapid south atlantic spreading changes and coeval vertical motion in surrounding continents evidence for temporal changes of pressure driven upper mantle flow the south atlantic region displays 1 a topographic gradient across the basin with africa elevated relative to south america 2 a bimodal spreading history with fast spreading rates in late cretaceous and eo oligocene and 3 episodic regional uplift events in the adjacent continents concentrated in late cretaceous and oligocene here we show that these observations can be linked by dynamic processes within earth s mantle through temporal changes in asthenosphere flow beneath the region the topographic gradient implies westward pressure driven mantle flow beneath the basin while the rapid spreading rate changes on order 10 million years require significant decoupling of regional plate motion from the large scale mantle buoyancy distribution through a mechanically weak asthenosphere andean topographic growth in late miocene can explain the most recent south atlantic spreading velocity reduction arising from increased plate boundary forcing associated with the newly elevated topography but this mechanism is unlikely to explain the late cretaceous tertiary spreading variations as changes in andean paleoelevation at the time are small we propose an unsteady pressure driven flow component in the asthenosphere beneath the south atlantic region to explain the late cretaceous tertiary spreading rate variations temporal changes in mantle flow due to temporal changes in regional mantle pressure gradients imply a correlation of horizontal and vertical motions we find that this prediction from our models agrees with geologic and geophysical observations of the south atlantic region including episodes of passive margin uplift regional basin reactivation and magmatic activity

post rift stratigraphic evolution of the atlantic margin of namibia and south africa implications for the vertical movements of the margin and the uplift history of the south african plateau abstract the aim of this study is to constrain the post rift deformations of the atlantic passive margin of namibia and south africa using an extensive industrial 2d reflection seismic dataset calibrated by wells and onshore outcrops that have been revaluated in age biostratigraphy in order to discuss the evolution of the south african plateau uplift the first order evolution of the margin is tectonically driven and can be divided into three principal phases the first 131 93 5 ma comprises an overall retrogradational trend that results from a rate of accommodation created by the thermal flexure of the margin that is higher than the sediment supply the second 93 5 66 ma comprises an overall aggradational progradational trend that results from a relative increase in sediment supply due to an uplift and subsequent erosion of the margin and the inland domain the third 66 0 ma is retrogradational again in response to a decrease in sediment supply induced by a relative stability of the margin and climate conditions this study demonstrates that the present day configuration of the margin is acquired during the late cretaceous from 81 to 66 ma with a strike long wavelength and seaward tilt of the margin which could be related to the growth of the onshore main escarpment we also characterize a regional oligocene unconformity marked by a significant downward shift of the shoreline which suggests a moderate uplift of the inland domain and could be associated with a reactivation of the relic late cretaceous relief

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

major controlling factors on hydrocarbon generation and leakage in south atlantic conjugate margins a comparative study of colorado orange campos and lower congo basins abstract we present a supra regional comparative study of the major internal and external factors controlling source rock sr maturation and hydrocarbon hc generation and leakage in two pairs of conjugate margins across the south atlantic the brazil campos basin angola lower congo basin margins located in the central segment and the argentina colorado basin south africa orange basin in the southern segment our approach is based on the analysis and integration of borehole data 1d numerical modeling 2d seismic reflection data and published reports coupling of modeling results sedimentation rate calculation and seal bypass system analysis reveal that 1 oil window is reached by syn rift srs in the southern segment during the early to late cretaceous when thermal subsidence is still active while in the central segment they reach it in late cretaceous neogene during a salt remobilization phase and 2 early hc generation from post rift srs in the southern segment and from all srs in the central segment appears to be controlled mainly by episodes of increased sedimentation rates the latter seems to be associated with the andes uplift history for the western south atlantic basins campos and colorado and to a possibly climate driven response for the eastern south atlantic basins orange and lower congo additionally we observe that the effect of volcanism on sr maturation in the southern segment is very local the comparison of cretaceous mass transport deposit mtd episodes with hc peak of generation and paleo leakage indicators in the southern segment revealed the possible causal effect that hc generation and leakage have over mtd development interestingly paleogene leakage indicators which were identified in the argentina south africa conjugate margins occur contemporaneously to low sedimentation rate periods nonetheless present day leakage indicators which were also identified in both pairs of conjugate margins might be related to seal failure events linked to eustatic sea level drops

fluid flow features in hydrocarbon plumbing systems what do they tell us about the basin evolution abstract fluid flow features such as sandstone intrusions mud volcano systems and pockmarks represent well known geological structures generated due to flow of fluids in the subsurface however the integration of fluid flow features in quantitative and qualitative basin analysis is still in its infancy this study presents a review and a qualitative integration of fluid flow features in basin analysis of the danish north sea area and of salt mini basins offshore angola based on results from six case studies focusing on the detection mapping and evaluation of features related to focused fluid flow in these areas in the study the diversity of seismically imaged fluid flow features in hydrocarbon plumbing systems is reviewed and their utility is demonstrated this is done by focusing on the aspects of fluid flow related features that can be integrated into basin analysis studies and how these may help us increase the understanding of basin evolution and to achieve the major goals of basin analysis such as understanding the depositional and tectonic context of the basin and the presence and extent of hydrocarbons particularly concerning the determination of fluid flow timing triggers sources and routes fluid flow features represent a major but underused tool in basin analysis

segmentation and volcano tectonic characteristics along the sw african continental margin south atlantic as derived from multichannel seismic and potential field data abstract regional seismic reflection and potential field data document the south atlantic s break up history between 39 s and 19 s from the early cretaceous onwards previous maps of distribution of volcanics along the margin showed volcanics along the whole african margin based on extrapolation of data based on previously unpublished marine geophysical data we found the southernmost 460 km long margin segment to be lacking huge volumes of break up related volcanic effusives northwards break up was accompanied by the emplacement of huge volumes of volcanic material prominently featured in seismic sections as huge wedge shaped seaward dipping reflectors sdrs detailed mapping of offsets left and right stepping and variations in structural character of the volcanics reveal the segmentation along and the break up history of the margin several superimposed sdr sequences suggesting episodicity of volcanic emplacement divided by periods of erosion and sedimentation are distinct along southerly lines losing prominence northwards a main outcome of our study is that this passive margin is not continuously of the volcanic type and that the change from a non volcanic to a volcanic margin occurs abruptly we define four distinct first order segments along the 2400 km section of the southwestern african margin covered by our seismic data from south to north these first order segments are magma poor segment i segment ii with enormous sdrs volumes decreasing sdrs volumes in segment iii segment iv again with enormous volcanic output likely influenced by walvis ridge volcanism most important is that there is no systematic increase in the volumes of the effusives towards the tristan da cunha hot spot rather there is an alternating pattern in the sdrs volumes and widths the boundary between the volcanic and magma poor margin segments in the southernmost study area is sharp 10s of km which we propose is reflected in magnetic anomaly data as well we suggest that this variability along the margin is mainly due to a change in stretching rifting character from oblique during the early stages of breakup to conventional seafloor spreading from chron m4 130 ma onwards

mineralogy fluid inclusion petrography and stable isotope geochemistry of pb zn ag veins at the shizhuyuan deposit hunan province southeastern china the shizhuyuan polymetallic deposit is located in the central part of the nanling region southeastern china and consists of proximal w sn mo bi skarns and greisens and distal pb zn ag veins the sulfides and sulfosalts in the distal veins formed in three distinct stages 1 an early stage of pyrite and arsenopyrite 2 a middle stage of sphalerite and chalcopyrite and 3 a late stage of galena ag sn and bi bearing sulfides and sulfosalts and pyrrhotite combined sulfide and sulfosalt geothermometry and fluid inclusion analyses indicate that the early stage of mineralization occurred at a temperature of 400 c and involved boiling under hydrostatic pressure 200 bar with the temperature of the system dropping during the late stage to 200 c laser raman analysis indicates that the fluid inclusions within the studied minerals are dominated by h2o although some contain carbonate solids and ch4 gas vein hosted sulfides have 34s values of 3 8 6 3 that are interpreted as indicative of a magmatic source of sulfur the mineralization process can be summarized as follows an aqueous fluid exsolved on final crystallization of the qianlishan pluton ascended along fracture zones cooled to 400 c and boiled under hydrostatic conditions and with decreasing temperature and sulfur fugacity sulfide and sulfosalt minerals precipitated successively from the ag cu zn fe pb sb as s bearing fluid system

atmospheric methane from organic carbon mobilization in sedimentary basins the sleeping giant abstract the mass of organic carbon in sedimentary basins amounts to a staggering 1016 t dwarfing the mass contained in coal oil gas and all living systems by ten thousand fold the evolution of this giant mass during subsidence and uplift via chemical physical and biological processes not only controls fossil energy resource occurrence worldwide but also has the capacity for driving global climate only a tiny change in the degree of leakage particularly if focused through the hydrate cycle can result in globally significant greenhouse gas emissions to date neither climate models nor atmospheric co2 budget estimates have quantitatively included methane from thermal or microbial cracking of sedimentary organic matter deep in sedimentary basins recent estimates of average low latitude eocene surface temperatures beyond 30 c require extreme levels of atmospheric co2 methane degassing from sedimentary basins may be a mechanism to explain increases of atmospheric co2 to values as much as 20 times higher than pre industrial values increased natural gas emission could have been set in motion either by global tectonic processes such as pulses of activity in the global alpine fold belt leading to increased basin subsidence and maturation rates in the prolific jurassic and cretaceous organic rich sediments or by increased magmatic activity such as observed in the northern atlantic around the paleocene eocene boundary increased natural gas emission would have led to global warming that was accentuated by long lasting positive feedback effects through temperature transfer from the surface into sedimentary basins massive gas hydrate dissociation may have been an additional positive feedback factor during hyperthermals superimposed on long term warming such as the paleocene eocene thermal maximum petm as geologic sources may have contributed over one third of global atmospheric methane in pre industrial time variability in methane flux from sedimentary basins may have driven global climate not only at time scales of millions of years but also over geologically short periods of time earth system models linking atmospheric ocean and earth surface processes at different timescales with the sedimentary organic carbon cycle are the tools that need to be developed in order to investigate the role of methane from sedimentary basins in earth s climate

ages of epithermal deposits in mexico regional significance and links with the evolution of tertiary volcanism we present an analysis leading to a new space time classification of the mexican epithermal deposits based on new data and interpretations from seven deposits in central mexico and a new age for the la guitarra deposit temascaltepec district from the latter adularia from a barren stage of mineralization and from quartz bands of the main ore stage together with orthoclase phenocrysts from host monzogranites with a known late laramide age were analyzed by the 40ar 39ar method the ages obtained from adularia samples are 32 9 0 1 ma and 33 3 0 1 ma corresponding to the middle part of lower oligocene these ages are slightly younger than that of the ignimbrite succession exposed next to the study area and they suggest that the deposit is related to the crystallization of comagmatic intrusive rocks contrary to previous assumptions of a miocene age for several epithermal deposits in southern mexico the ages from la guitarra show that south of the trans mexican volcanic belt these deposits may be oligocene or older in addition the interpretation of a genetic link between intrusive rocks and epithermal mineralization is consistent with previous work on fluid geochemistry of the deposit to evaluate the obtained ages in a regional context we analyzed the geographic distribution and age of tertiary epithermal deposits in central mexico our study shows that mineral deposits coincide with the main volcanic pulses of the sierra madre occidental the distribution of epithermal deposits can be thus divided into three main groups 1 between 48 and 40 ma 2 between 36 and 27 ma and 3 between 23 and 18 ma within the first group are the batopilas and topia deposits that record the oldest ages found for epithermal deposits in mexico deposits of this age are hosted by the lower volcanic supergroup of the sierra madre occidental and are related to the laramide magmatism of the sierra madre occidental many of the epithermal deposits in mexico occur within the second time span in a northwest southeast belt from chihuahua to the mexico and guerrero states these deposits are related to the main episode of ignimbrite flare up of the sierra madre occidental the third group includes the bolanos san martin de bolanos and pachuca real del monte deposits plus possibly seven other epithermal deposits analyzed in this work these deposits are related to the last ignimbrite flare up of the sierra madre occidental with a general west northwest east southeast distribution recently documented in the southern sierra madre occidental

gas hydrate and mud volcanoes on the southwest african continental margin off south africa widespread occurrence of bottom simulating reflectors bsrs has been detected in multichannel seismic profiles on the upper continental slope in the southern periphery of the orange river delta probably indicating the presence of large quantities of gas hydrate in this area this report is the first to show the presence of bsrs on seismic records on the southwest african continental margin south of the walvis ridge another remarkable feature in the area is the occurrence of a large number of mud volcanoes the distribution of the bsrs and the location of the mud volcanoes are controlled by the locations of active faults the gas hydrate in this region may consist of a mixture of microbial and thermogenic gas whereas much of the gas flowing through the mud volcanoes probably originated from deep seated aptian source shales

offshore mud volcanoes and onland faulting in southwestern africa neotectonic implications and constraints on the regional stress field abstract recently discovered mud volcanoes in the orange basin offshore southwestern africa denote the existence of neotectonic faults in the submerged continental shelf interpretation of seismic lines perpendicular to the trend of the alignment of the mud volcanoes shows flower structures diagnostic of strike slip faulting along a n nnw direction analysis at the regional scale of onland neotectonic features in southwestern africa shows that recent faulting occurred both in central namibia and namaqualand south africa and that it created both n nnw and nw trending lineaments it is proposed that the newly discovered offshore neotectonic activity and the onland structures described in this paper represent the structural expression of the same stress field these structures form a set of conjugate transtensive faults which constrain the regional horizontal greatest compressive stress in a nw nnw direction such stress orientation also supported by in situ stress measurements defines the so called wegener stress anomaly the predominant present day stress field of southwest africa the wegener anomaly is incompatible with the stress orientation required by plate scale tectonic constraints mainly in the form of recently published gps motion values for the african plate

denudation along the atlantic passive margin new insights from apatite fission track analysis on the western coast of south africa apatite fission track aft data from two traverses across the great escarpment of the western coast of south africa are used to reconstruct the tectonic evolution and denudation history of this sector of the atlantic passive margin fission track ages range between 180 and 86 ma modelling of this data identifies two distinct cooling events the first event between 160 and 138 ma is recorded only by the rocks above the escarpment in the karoo area and is tentatively linked to post karoo magmatism 180 ma thermal relaxation the second between 115 and 90 ma results instead from a tectonically induced denudation episode responsible for the removal of up to 2 5 km of crust across the coastal zone in front of the escarpment and less than 1 km on the elevated interior plateau based on these results it is suggested that the cretaceous is the time when most of the elevated topography of southern africa was generated with only a minor cenozoic contribution

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

pockmark formation by porewater expulsion during rapid progradation in the offshore taranaki basin new zealand abstract after the first discovery of seabed pockmarks in the 1960s and 1970s many examples of both modern and ancient pockmarks have been reported from sedimentary basins around the world the exact mechanisms and fluids involved in pockmark formation are still subject to debate with many studies inferring that pockmark formation is a direct indication of hydrocarbon expulsion this study provides the first description of a buried pliocene paleo pockmark field in the offshore taranaki basin new zealand the paleo pockmarks are located approximately 1 2 1 3 km below the seabed and are well imaged by three dimensional seismic data their spatial density and size increases towards the distal portions of the fan and they vary in size between 45 and 580 m in diameter and 2 35 ms twt pockmark size is inversely correlated with the fan thickness in the study area detailed analysis rules out any links with faulting while the relationship with fan variations in the depositional thickness suggests porewater expulsion during overburden progradation as the most likely cause of the paleo pockmarks a model is proposed in which rapid sediment loading generated overpressure which was greatest on the proximal fan due to the lateral gradient in the sediment load imposed by clinoform progradation fluids were consequently forced towards the distal fan where eventually the pore pressure exceeded the fracture gradient of the seal the implications of this study are that not all pockmarks can be used as evidence for hydrocarbon expulsion in frontier regions and care should be taken to examine the subsurface context plumbing systems and any associated acoustic anomalies before concluding on the pockmark origin

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

late cenozoic evolution of the outer continental shelf at the head of the cape canyon south africa abstract cenozoic sedimentary successions have a restricted distribution both onshore and offshore along the passive western margin of southern africa and much controversy and uncertainty remains on the geological evolution of this margin siliciclastic authigenic and biogenic sediments varying in age from cretaceous to holocene recovered from a number of vibrocores collected between 200 and 500 m water depth at the head of the cape canyon provide the basis for an integrated bio sr isotopic and lithostratigraphic study the study area is underlain by unconsolidated middle cretaceous interbedded woody quartzose sand and kaolinitic mud a large regional erosional hiatus separates the upper oligocene to lower miocene carbonate sediments from these cretaceous sediments the upper oligocene to lower miocene 18 1 27 2 ma carbonate succession records third order sea level fluctuations superimposed on an overall marine transgression the interpreted late oligocene depositional environments imply increased bottom water activity over time and the peak in phosphogenesis at 23 6 26 1 ma indicates the onset of warm water upwelling calcareous deposition ended at around 18 ma when downwarping of the continental shelf associated with uplift of the continental interior resulted in the seaward tilt of the carbonate succession to greater water depths increased terrigenous input is recorded as inner shelf prograding quartzose delta front deposits following uplift of the interior uplift combined with a long term marine regression resulted in the widespread erosion of middle to upper miocene successions generally recovered only as reworked glauconite carbonate fluorapatite cfa mixed mineral pellets above the erosional hiatus glauconite rich sediments indicate overall low accumulation rates and multiple reworking events of organic rich clayey sediments during the middle to late miocene early pliocene onset of cold water upwelling at least intermittently by 15 ma established hydrographic conditions similar to the benguela upwelling system with a coast parallel mudbelt present on the middle shelf a middle pliocene marine regression reworked 15 8 9 1 ma inner shelf quartzose delta front sediments across the outer shelf late pliocene early pleistocene uplift of the continental interior resulted in the fault controlled incision of the head of the cape canyon along the seaward flank of the buoyant agulhas columbine basement block uplift increased quaternary terrigenous sedimentation on the shelf where the facies distribution was clearly controlled by canyon head bathymetry high frequency glacio eustatic sea level fluctuations resulted in late quaternary erosion and slumping across the outer shelf and the continued downcutting of the cape canyon lowstand shelly gravels are common including the last glacial maximum shell lag which grades into a faecal pellet rich holocene mud drape over much of the study area the late cenozoic evolution of the western margin recorded in the complex condensed sediment record of the study area is characterised by low sediment accumulation rates and multiple reworking events the integrated stratigraphic interpretation emphasises the importance of eustasy tectonic activity fluvial sediment input palaeoceanographic and climatic changes

characterization of microbial activity in pockmark fields of the sw barents sea abstract multibeam bathymetry revealed the occurrence of numerous craterlike depressions so called pockmarks on the sea floor of the hammerfest basin and the loppa high south western barents sea to investigate whether these pockmarks are related to ongoing gas seepage microbial processes associated with methane metabolism were analyzed using geochemical biogeochemical and microbiological techniques gravity cores were collected along transects crossing individual pockmarks allowing a direct comparison between different locations inside assumed activity center on the rim and outside of a pockmark reference sites concentrations of hydrocarbons in the sediment particularly methane were measured as headspace free gas and in the occluded and adsorbed gas fraction down to a depth of 2 6 m below sea floor mbsf sulfate reduction rates were quantified by radiotracer incubations concentrations of dissolved sulfate in the porewater were determined as well neither the sulfate profiles nor the gas measurements show any evidence of microbial activity or active fluid venting methane concentrations and sulfate reduction rates were extremely low or even below the detection limit the results show that the observed sediment structures are most likely paleo pockmarks their formation probably occurred during the last deglaciation

warm eocene climate enhanced petroleum generation from cretaceous source rocks a potential climate feedback mechanism 1 earth surface temperatures including in the deep sea increased by 5 10 c from the late paleocene ca 58 myr ago to the early eocene climatic optimum eeco centered at about 51 myr ago a large 2 5 drop in 13c of carbonate spans much of this interval this suggests a long term increase in the net flux of13c depleted carbon to the ocean and atmosphere that is difficult to explain by changes in surficial carbon cycling alone we reveal a relationship between surface temperature increase and increased petroleum generation in sedimentary basins operating on 100 kyr to myr time scales we propose that early eocene warming has led to a synchronization of periods of maximum petroleum generation and enhanced generation in otherwise unproductive basins through extension of the volume of source rock within the oil and gas window across hundreds of sedimentary basins globally modelling the thermal evolution of four sedimentary basins in the southwest pacific predicted an up to 50 increase in petroleum generation that would have significantly increased leakage of light hydrocarbons and oil degeneration products into the atmosphere extrapolating our modelling results to hundreds of sedimentary basins worldwide suggests that globally increased leakage could have caused a climate feedback effect driving or enhancing early eocene climate warming

distal relationship of the taihexian pb zn au deposit to the dengfuxian magmatic hydrothermal system south china constraints from mineralogy fluid inclusion h o pb and in situ s isotopes abstract the taihexian pb zn au vein style deposit is located in the well preserved dengfuxian magmatic hydrothermal w sn ore field south china the deposit can be divided into three mineralization stages i pyrite arsenopyrite chalcopyrite gold tourmaline quartz stage ii pyrite arsenopyrite chalcopyrite sphalerite galena quartz stage and iii pyrite galena sphalerite quartz calcite fluorite stage liquid rich nacl aqueous inclusions vapor rich nacl aqueous inclusions co2 rich inclusions and three phase co2 h2o inclusions occur in quartz and calcite of the ore veins at taihexian the ore veins of the three stages formed at around 310 c 210 c and 170 c respectively precipitated from h2o nacl co2 ch4 fluids with salinities averaging around 5 wt nacl eq the ore forming mechanisms include fluid immiscibility a potential trigger for metal precipitation from hydrothermal fluids during stage i fluid mixing during stage ii and cooling during stage iii oxygen 18ofluid 1 5 6 9 and hydrogen dfluid 61 to 53 isotopes suggest that the ore forming fluids originated from a mixture of magmatic water with ground water and or meteoric water sulfur isotopes 34s 7 8 3 1 suggest a magmatic origin for sulfur in stages i and ii and then possibly mixing with sulfur from sedimentary rocks in stage iii lead isotopes 206pb 204pb 18 05 18 65 207pb 204pb 15 53 15 75 208pb 204pb 37 91 38 88 indicate that ore metals originated mainly from the upper crust with minor mantle contributions comparing arsenopyrite sulfur isotope and fluid inclusion geothermometers it is estimated that the ore formation pressure was about 58 120 mpa corresponding to a depth of approximately 2 2 4 5 km taihexian appears to form part of a regional magmatic hydrothermal system centered on the dengfuxian pluton

backward modelling of the subsidence evolution of the colorado basin offshore argentina and its relation to the evolution of the conjugate orange basin offshore sw africa abstract in this study we focus on reconstructing the post rift subsidence evolution of the colorado basin offshore argentina we make use of detailed structural information about its present day configuration of the sedimentary infill and the crystalline crust this information is used as input in a backward modelling approach which relies on the assumption of local isostasy to reconstruct the amount of subsidence as induced by the sedimentary load through different time stages we also attempt a quantification of the thermal effects on the subsidence as induced by the rifting here included by following the uniform stretching model of lithosphere thinning and exponentially cooling through time based on the available information about the present day geological state of the system our modelling results indicate a rather continuous post rift subsidence for the colorado basin and give no significant evidence of any noticeable uplift phase in a second stage we compare the post rift evolution of the colorado basin with the subsidence evolution as constrained for its conjugate sw african passive margin the orange basin despite these two basins formed almost coevally and therefore in a similar large scale geodynamic context their post rift subsidence histories differ based on this result we discuss causative tectonic processes likely to provide an explanation to the observed differences we therefore conclude that it is most probable that additional tectonic components other than the ridge push from the spreading of the south atlantic ocean are required to explain the observed differences in the subsidence of the two basins along the conjugate passive margins such additional tectonic components might be related to a dynamic mantle component in the form of either plume activity africa or a subducting slab and the presence of an ongoing compressional stress system as revealed for different areas in south america