seabed pockmarks associated with deepwater corals off se brazilian continental slope santos basin several factors bear evidence of chemosynthetic ecosystem occurrence in the brazilian coast these motivated our investigation to sample three pockmarks located on the continental slope off se brazil 26js and 46jw at f700 m depth the crater like features were f1 km in diameter and 100 m in depth and were identified with an echo sounder ek 500 they were sampled along two transects using a rectangular dredge our main objective was to identify possible evidence of a seep related community the surrounding sediments of the pockmarks seem to be filled with coral rubble harboring a rich fauna of metazoans including epilithic and chasmolithic organisms a total of 9565 organisms were sampled the most abundant group was the ophiuroidea followed by cirripedia and polychaeta a number of new occurrences and new species were registered the polychaete worm amelinna was the first record for the atlantic ocean which has been found in reducing habitats such as wood islands the echo sounder records also showed the presence of big deepwater coral mounds associated with the edge of the pockmarks these features are f 20 m in height and from 180 to 360 m wide records of demersal fishes were also registered right above coral mounds the presence of deepwater coral mounds at the edge of pockmarks may be related to the flow of the cold oxygen rich antarctic intermediate water over the bottom or else may benefit from the fertilization of the water by the microseepage of hydrocarbons from the sediment we suggest a more thorough survey in order to map and recognize these environments furthermore our findings may shed some light on new research perspectives concerning the ecology biogeography and evolution of reducing habitats off the brazilian coast d 2004 elsevier b v all rights reserved

comparison of prestack stereotomography and nip wave tomography for velocity model building instances from the messinian evaporites velocity model determination during seismic data processing is crucial for any kind of depth imaging we compared two approaches of grid tomography prestack stereotomography and normal incidence point nip wave tomography whereas nip wave tomography is based on wavefield attributes obtained during the common reflection surface stack and thus on the underlying hyperbolic second order traveltime approximation prestack stereotomography describes traveltimes by local slopes i e linearly in the prestack data domain to analyze the impact of the different traveltime approximations and the different input data domains on velocity model building we applied two implementations of these techniques to two profiles of a field marine data set from the levante basin eastern mediterranean because ofthe presence of a thick tabular mobile unit of the messinian evaporites strong vertical and lateral velocity contrasts had been expected the velocity models revealed the reconstruction of high velocity contrasts

methane derived authigenic carbonates formed by subduction induced pore water expulsion along the oregon washington margin authigenic magnesian calcite dolomite and aragonite are precipitated in the uppermost terrigenous sediments of the washington oregon accretionary prism by subduction induced dewatering these distinctive carbonates are methane derived and occur at sites of concentrated pore water expulsion unique biologic communities that subsist at least indirectly on methane suess et al 1985 are also found at some of these sites the methane which is dominantly biogenic is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement carbonate precipitation occurs below the oxic layer probably no deeper than several centimetres to a few metres below the seabed cementation may be induced by three factors 1 increased carbonate alkalinity resulting from microbial sulfate reduction 2 decreased co 2 solubility resulting from a pressure decrease when the pore water escapes the prism and or 3 the addition of ca 2 and mg 2 ions from sea water near the sediment water interface the convergent margin setting engenders precipitation of authigenic carbonates in several ways compressive stresses induce anomalously rapid compaction and dewatering rates and they may cause overpressuring in migrating pore water thereby delaying precipitation of carbonates until pressure is released near the sediment water interface structural deformation of the accretionary prism creates pathways such as fault zones secondary fracture porosity and dipping permeable layers often exposed by mass movement for efficient advection and expulsion of methane enriched pore water these characteristic conditions which lead to the precipitation of methane derived carbonates may be found at other convergent margins

modeling the northern adriatic double gyre response to intense bora wind a revisit 1 a combination of recent intensive observations and simulations with two numerical models is used to revisit the issue of the northern adriatic response to strong bora episodes new observed and simulated data reinforce the view that an episode of strong bora wind provokes a double gyre cyclonic trieste and anticyclonic rovinj response north of the po delta pula line during an intense bora episode both measured and modeled statistics picture a downwind highly polarized and almost depth independent flow within the trieste gyre nw arm its ne arm maintains a sharp polarization and strong depth dependence while exhibiting lower speeds with models in good accord with observations the current statistics for rovinj gyre provide lower maximum and average speed values and less polarized but still rather depth independent flow while exhibiting clockwise rotation the north arm of the senj gyre positioned south of the po delta pula line enjoys more lateral freedom and exhibits less rectilinear flow our review reinforces the notion that modeling studies based on ecmwf wind forcing fail to properly take into account the orographic control of the dinaric alps and to produce correct bora induced gyral pattern the coamps model successfully simulated the onset duration and decay of the wind peaks but exhibited a tendency to overpredict the strength of the bora wind our simulations have identified the shallow nw coastal strip as an important source of colder water observed in a sequence of remotely sensed sst fields derived from avhrr data

fluid flow through carbonate platforms as evidence for deep seated reservoirs in northwest australia fluid flow features in carbonate platforms of the outer browse basin northwest australia are investigated using 3d seismic and borehole data during the cenozoic the study area evolved from a carbonate ramp to a rimmed platform with isolated carbonate build ups as such it played a key role in focusing fluid on their buoyant flow to the surface statistical analyses of direct hydrocarbon indicators show fluid flow to be focused in elevated areas i e the greater is the focusing of fluid the larger is the carbonate platform or isolated build up locally karst systems created regions of enhanced permeability through the evolving carbonate stratigraphy karstified horizons are located on the top of carbonate build ups and platform clinoforms as fluid buoyantly concentrates and diffuses up into topographic highs in turn the putative migration of gas and fluid generated hypogenic karst systems enhancing permeability in otherwise lower permeability rock based on the interpreted data we suggest a pivotal relationship between the presence of carbonate build ups and karsts in the browse basin and hydrocarbon accumulations at depth areas of elevated topography focus sub surface fluid enhancing the permeability of carbonate successions in turn focused fluid flow can lead to the generation of methanogenic carbonates promoting the growth of isolated build ups and leading to further generation of elevated features this grants the identification of similar features on seismic data from northwest australia and other equatorial margins in the world as a valid proxy for the recognition of hydrocarbon accumulations below thick carbonate successions

morphology of pockmarks along the western continental margin of india employing multibeam bathymetry and backscatter data this study addresses the morphology of pockmarks along the western continental margin of india using multibeam bathymetry and backscatter data here for the first time we have utilized the application of arcgis geographical information system for understanding the morphology of pockmarks from the western continental margin of india the pockmarks observed in water depths of 145 330 m are circular elliptical or elongated in plan view with an average length and width of 157 72 m and 83 19 m respectively the average pockmark relief and perimeter are 1 9 0 9 m and 412 181 m respectively the pockmarks have average areas and volumes of 10 759 m and 15 315 m respectively spatial separation that coincides with 210 m isobath divides the pockmarks into two groups with differing distributions and morphologies these pockmarks originated from seepages of biogenic or thermogenic gas or from pore fluids from deeper sources migrated vertically along the faults besides a possible structural control the pockmark morphologies are also affected by bottom currents and or by submarine slumping the average acoustic backscatter strength from pockmark centre is higher 35 db than the average backscatter of the total area 40 db which suggests their possible linkage to the precipitation of diagenitic minerals from biodegradation of seepage material

seismic stratigraphy and depositional signatures of the maldive carbonate system indian ocean abstract as documented by industrial seismic reflection profiles and well data the cenozoic stratigraphic evolution of the maldive carbonate system shows three distinct intervals characterized by their own specific depositional signatures late palaeocene volcanics generated by the reunion hotspot filled deeper pull apart structures and created a flat topography that promoted the onset of the maldive carbonate system in the early eocene some of the initial structural depressions remained as deep internal seaways surrounded by reefal margins until the late oligocene the eocene early oligocene stratigraphic signature is essentially aggradational and corresponds to a transgressive cycle associated with a second order sea level rise in the mid oligocene a major sea level fall exposed the neritic carbonate system and restricted sedimentation to the former deep internal basins during the next transgression a neritic carbonate system re established itself on the newly flooded platform and was finally drowned near the oligocene miocene boundary the main effect of this second order transgressive cycle was the transformation of the central troughs into one single deep basin in the neogene the palaeo maldive inner sea when a reef framework was established in the early miocene near the present location of the platform edge systematic bank margin progradation toward the central inner sea basin became enhanced this neogene progradational phase is explained as the response to a long term sea level fall associated with a second order regressive cycle finally the late pliocene pleistocene depositional signature is mainly aggradational and corresponds to the regional modification from flat topped carbonate banks to atoll physiographies the antecedent karst theory suggests that increased karstification was triggered by high frequency eustatic and climatic changes in the last 3 0 ma

marine pore water sulfate profiles indicate in situ methane flux from underlying gas hydrate marine pore water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the carolina rise and blake ridge u s atlantic continental margin here sulfate gradients are linear implying that sulfate depletion is driven by methane flux from below rather than by the flux of sedimentary organic matter from above thus these linear sulfate gradients can be used to quantify and assess in situ methane flux which is a function of the methane inventory below

monsoon induced partial carbonate platform drowning maldives indian ocean multibeam maps and high resolution seismic images from the maldives reveal that a late miocene to early pliocene partial drowning of the platform was linked to strong sea bottom currents in the upper miocene to holocene currents shaped the drowned banks the current moats along the bank edges and the submarine dune fields bottom currents in the maldives are driven by the monsoon it is proposed that the onset and the intensification of the monsoon during the neogene provoked platform drowning through injection of nutrients into surface waters since the late miocene topographically triggered nutrient upwelling and vigorous currents switched the maldives atolls into an aggradational to backstepping mode which is a growth pattern usually attributed to episodes of rising sea level

fields of multi kilometer scale sub circular depressions in the carnegie ridge sedimentary blanket effect of underwater carbonate dissolution offshore ecuador the carnegie ridge is a volcanic ridge with a carbonate sediment drape during the salieri cruise multibeam bathymetry was collected across carnegie ridge with the simrad em120 of the r v sonne the most conspicuous features discovered on the carnegie ridge are fields of circular closed depressions widely distributed along the mid slope of the northern and southern flanks of the ridge between 1500 and 2600 m water depth these circular depressions are 1 4 km wide and typically 100 400 m deep most are flat floored and some are so densely packed that they form a honeycomb pattern the depressions were carved into the ridge sedimentary blanket which consists of carbonate sediment and has been dated from upper miocene to upper pleistocene several hypotheses including pockmark origin sediment creeping paleo topography of the volcanic basement effects of subbottom currents and both marine and subaerial karstic origins are discussed we believe that underwater dissolution process merits the most serious consideration regarding the origin of the closed depression

giant pockmarks in a carbonate platform maldives indian ocean abstract circular structures and depressions in carbonate platforms are known to represent karst chimneys or sinkholes which form as a response to rock solution this formation mechanism is plausible for shallow water carbonates which lie in the reach of meteoric diagenesis or fresh water lenses circular structures which occur in deeper waters however need an alternative interpretation such an example of sea floor depressions in more than 300 m deep waters occurs in the inner sea of the maldives carbonate platform in the indian ocean the structures were mapped with multibeam and parasound multi channel seismics were used to link the depressions with structures at depth the circular depressions have diameters of up to 3000 m and depths of up to 180 m the craters are interpreted as pockmarks formed through the venting of gas and fluids gas and fluid lenses below the pockmarks are reflected by bright spots in the seismic sections as well as a reduction of the instantaneous frequency these areas at depth are linked to chimneys connected to faults and drowned oligocene carbonate banks a model is presented that relates the different forms and sizes of the structures to distinct development stages of sea floor deformation to one process early stages of gas and fluid migration into the shallow part of the sedimentary succession induce formation of dome shaped bodies initial gas and fluid escape to the sea floor is reflected by the formation of sand volcanoes and aligned small pockmarks active pockmarks are the deepest and have the shape of truncated cones in cross section mature pockmarks are characterized by erosion of the flanks of the structure by bottom currents late stage pockmarks are bowl shaped in cross section and are to different degrees filled by drift sediments packages of strata revealing high reflection amplitudes and high interval velocities interpreted as microbially mediated carbonate precipitates underlie some of the pockmarks the pockmarks in the maldives show that circular structures other than solution related features can be abundant in carbonate platform deposits and that such structures may be more abundant in the geological record of carbonate platforms as previously thought pockmarks in the maldives indicate that the archipelago is an example of a hydrocarbon system which consists of an isolated oceanic carbonate platform overlying a volcanic basement and lacustrine source rocks

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

aragonite cycles diagenesis caught in the act organic biomarker and nannofossil assemblages are used in combination with sedimentary petrology to identify the role of diagenesis for the formation of a rhythmic calcareous succession a record from the early pliocene of the maldives ocean drilling program odp leg 115 is presented containing precession and eccentricity cycles expressed as variable aragonite content additional sub milankovitch cycles are caused by rhythmic precipitation of calcite cement in the lower part of the interval comparison with palaeo productivity indicators nannofossils chlorin total organic carbon suggests that cementation occurs preferentially in intervals characterized by increasing or decreasing productivity the coupled variability in productivity and carbonate diagenesis is attributed to the effect of organic matter degradation in the sediment the observed combination of primary and diagenetic factors hampers the interpretation of the cyclicity on milankovitch and sub milankovitch scales diagenetically stable proxies for palaeo productivity such as nannofossil assemblage data were used to distinguish between palaeoclimate and diagenetic influences

genesis of a submarine sinkhole without subaerial exposure straits of florida a sinkhole has been identified on side scan sonar images and from near bottom echo sounder data in the southern straits of florida in 575 m of water sinkholes are often thought to form exclusively in subaerial environments and for this reason have been used as indicators of sea level this sinkhole exists within a quaternary sediment apron in water depths too great to have been subaerially exposed by neogene sea level lowstands thus indicating that sinkholes can develop within the submarine environment

imaging tertiary carbonate system the maldives indian ocean insights into carbonate sequence interpretation seismic imaging of carbonate systems is often more complicated than their siliciclastic counterparts carbonates sediments typically get cemented very early and their seismic velocities are normally faster than those of siliciclastic sediments this reduces the resolution of carbonates in comparison to the clastics when using identical acquisition design carbonates are also easily dissoved which may lead to karsting a phenomenon unique to carbonate rocks karst surfaces commonly form highly irregular topography that causes energy dispersion and makes it difficult to image the sediments or rocks below it

gas venting and late quaternary sedimentation in the persian arabian gulf abstract high resolution 3 5 khz echo sounding profiles and piston cores were used to reconstruct the microtopography and late quaternary depositional history of the persian gulf perversive throughout the seafloor of the gulf is an extensive network of pockmarks formed by seepages of thermogenic gas these gas seeps and bottom water exiting the gulf via the strait of hormuz are the most significant processes controlling present day sedimentation in the region erosion by these seeps has been so intense in the baiban shelf in the strait of hormuz as to create a hoodoo like terrain on the outer shelf the surfical geology of the gulf documents a short lived transgression 29 400 to 22 800 years ago during the wisconsin regression which began 125 000 years ago the wisconsin regressive maxima when sea level dropped to 120 130 m about 21 000 20 000 years ago and the climate was dry and eolian and paralic sedimentation characterized the gulf the holocene transgression 18 000 to 12 000 years ago when the climate was more humid than during the climax of the wisconsin regression a dry phase 12 000 to 9000 years ago when the persian gulf was a site of eolian and carbonate deposition and the present sediment cycle during the last 9000 years under a more humid regime it was during the present cycle that southeast trending marl lobes were deposited off iran arabia acquired its hyper arid climate about 3000 years ago and the gulf attained its present configuration about 1000 years ago as a result of the construction of the tigris euphrates delta at its head and tectonism and aggradation along its arabian and iranian flanks

carbonate platform flanks slope angle and sediment fabric more than 20 examples of fossil carbonate platform systems were compared for slope angle and sediment fabric plots of slope angle versus sediment fabric show that grainy non cohesive mud free sediments build steeper slopes than muddy cohesive sediments examples near the end members of grainy and muddy carbonate platform flanks are found in the triassic of the dolomites in northern italy and in the bahamas respectively they document the flank geometry and the processes readjusting the slope profile once the limiting slope angle is exceeded the grainy flank sediments in the dolomites modified by shearing and avalanching produce straight slope profiles with declivities up to 35 whereas the muddy bahamian flank sediments modified by large scale creep and rotational to translational sliding and slumping produce a concave upwards slope profile inclined at less than 4 the comparison between slope angle and sediment fabric indicates that the physical behaviour of sediments in the gravity field angle of shearing and mode of readjustment processes is linked to the composition of the slope sediment among the variables such as sea level subsidence climate plate motion and oceanographic setting windward leeward sediment fabric is suggested to be a major if not the major control on slope angle and slope curvature of carbonate platform flanks besides the recently documented tendency of carbonate sediments to build steeper slopes than siliciclastics this proposed relation sheds new light on the analysis and quantification of the variables influencing the geometry and depositional evolution of carbonate systems furthermore it provides an opportunity to deduce sediment composition from seismic lines and predict lithology prior to drilling

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

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

carbonate mounds in a mud volcano province off north west morocco key to processes and controls abstract this paper presents a new cluster of carbonate mounds discovered in 2002 in the gulf of cadiz off morocco r v belgica 2002 in water depths of 500 to 600 m amidst a field of giant mud volcanoes multibeam bathymetry side scan sonar imagery and 2d seismics are analyzed to present four mound provinces 1 the pen duick mound province on the pen duick escarpment 2 the renard mound province on the renard ridge 3 the vernadsky mound province on the vernadsky ridge and the al idrisi mound province on the gas blanked sediments above the buried al idrisi ridge video imagery and surface samples are described to ground truth the different mound areas the paradox is that nearly no live corals are presently being observed at the surface of the mounds while the mound cores display throughout a high number of reef forming cold water coral fragments scleractinians in association with numerous associated fauna formerly inhabiting the econiches provided by the coral framework environmental and oceanographic conditions during the recent past glacials stadials were probably more favourable for cold water coral growth pore water analyses in on mound cores at the south eastern edge of pen duick escarpment give evidence of focused higher methane fluxes and sulphate reduction rates on mounds than in the surrounding sediments cores from several mounds display horizons of strong corrosion and dissolution of the coral fragments a three phase model for carbonate mound evolution in these settings is proposed 1 in a first stage external controls positive oceanographic and environmental conditions the presence of an active planktonic food chain based on a high primary production and a suitable substrate are responsible for the initiation of cold water coral growth 2 once the cold water corals established an initial framework sedimentation becomes an important factor controlling mound growth the cold water corals baffle the sediments 3 throughout mound growth the mound may episodically be affected by diagenetic processes responsible for aragonite dissolution and probably carbonate precipitation

quantifying the origin and geometry of circular sag structures in northern fort worth basin texas paleocave collapse pull apart fault systems or hydrothermal alteration three dimensional seismic data reveal numerous subcircular sag structures in the northern fort worth basin the structures are defined by concentric faults that extend vertically upward 760 1060 m 2500 3500 ft from the lower ordovician ellenburger group the largest structures remained active into the lower desmoinesian strawn group criteria are outlined for defining seismically resolvable sag structures and a detailed quantitative analysis of the geometries of these circular features was undertaken results are compared and contrasted with reviews of subsurface collapse mechanisms and strike slip processes that are known to produce subsurface circular to subcircular sag geometries in plan view in this manner we develop several constraints for differentiating collapse related sag structures from strike slip related sag structures qualitative analyses indicate that the geometries observed are strongly analogous to subsurface collapse features where material is removed at depth to create a void into which the overburden subsequently sags and collapses quantitative analyses support the formation of these features by incremental collapse and suprastratal deformation above a linked system of coalesced collapsed paleocaves within the ellenburger group observations and criteria presented herein provide valuable information in defining seismically resolvable collapse features worldwide and help distinguish sag features of collapse affinity from those of other origins

complex local moho topography in the western carpathians indication of the alcapa and the european plate contact seismic data from deep refraction and wide angle reflection profiles intersecting the western carpathians show distinct upper mantle pn phases with anomalous apparent velocities identified in the first and later arrivals their systematic analysis indicates that such phases are present in numerous seismic sections both for in line and off line shots they are observed in data from profiles intersecting the carpathians in the west at the contact with the bohemian massif similar feature was also found in data at the northern edge of the carpathians at the contact with the north european platform modelling of these anomalous pn phases shows that they originate due to local structural anomalies of the moho discontinuity detected in several places along the western carpathian arc such anomalies are located in close lateral proximity of the pieniny klippen belt representing the contact between the stable european plate in the north and the alcapa alpine carpathian pannonian microplate in the south thus the complex local moho topography modelled from the pn phases suggests tectonic relation to the formation of the carpathian orogen the result is supported by correlation with the large scale carpathian conductivity anomaly modelled in the carpathians at a mid crustal level relative lateral position of these two structures together with the pieniny klippen belt at the surface delineates a zone affected by deformations at various depths along the whole western carpathian arc

seismic velocities from the yaquina forearc basin off peru evidence for free gas within the gas hydrate stability zone multichannel seismic mcs data from the yaquina forearc basin off peru reveal a complex distribution of gas and gas hydrate related reflections lateral variations of the reflection pattern at the assumed base of the gas hydrate stability zone in terms of continuity amplitude and signal attenuation underneath are observed as well as the possible occurrence of paleo bottom simulating reflectors bsrs phase reversed reflections above the bottom simulating reflector point to free gas within the gas hydrate stability zone ghsz to constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the mcs line from high resolution ocean bottom hydrophone recordings with two independent methods heat flux values estimated on the basis of the velocity depth functions increase with decreasing amplitude of the bsr and peak near chemoherms these results suggest a model of the yaquina basin where free gas is trapped under parts of the bsr and within the ghsz particularly under the seafloor and under an erosional unconformity the hypothesis of a paleo bsr that reflects the uplift of the base of the hydrate stability zone caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values

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

circular geological structures outcropping in the sedimentary basins of saudi arabia abstract a number of circular geological structures outcrop in the sedimentary basins of saudi arabia several are reviewed here to assess their mode of origin they are unrelated to each other are easily seen on aerial imagery and their origins are assessed here on the basis of new fieldwork and reflection seismic data the structures range in size from hundreds of meters to several kilometers in diameter jabal rayah 28 39 n 37 11 30 e is 5 km in diameter with ring faults and anomalously steep dips in siluro devonian strata reflection seismic data demonstrate that underlying strata are structureless and jabal rayah is interpreted as a probable impact structure ash shutbah 21 37 08 n 45 39 21 e is 2 3 km in diameter and consists of a zone of concentric sub horizontal folds in mesozoic carbonates surrounding a central area of disharmonic steeply plunging folds and a mass of stratigraphically anomalous sandstone there are dissolution structures in the vicinity and although this is a viable explanation the uniquely large and complex nature of ash shutbah suggests an alternative origin by impact reflection seismic data show that underlying strata in the vicinity of ash shutbah are structureless a province of at least 100 circular structures occurs in northeast saudi arabia aerial imagery and reflection seismic data shows that these structures are a mixture of bioherms and dissolution collapse features the kidan crater 22 06 53 n 53 26 38 e is a subcircular feature approximately 100 m in diameter exposed in quaternary sabkha between large sand dunes in the rub al khali field investigation reveals an unfaulted bowl with a slightly fractured floor in sabkha facies 3d reflection seismic data shows a small fault underlying the crater leading to a fluid escape interpretation for the depression

chemoherms on hydrate ridge unique microbially mediated carbonate build ups growing into the water column two active chemoherm build ups growing freely up into the oceanic water column the pinnacle and the south east knoll chemoherms have been discovered at hydrate ridge on the cascadia continental margin these microbially mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle shaped morphology with steep flanks the recovered rocks are pure carbonates dominated by aragonite based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates the fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates the strongly 13c depleted carbon isotope values of the samples as low as 48 1 pdb are characteristic of methane as the major carbon source for the carbonate formation the methane rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom simulating reflector bsr and rise through fault systems the 18o values of the aragonitic chemoherm carbonates are substantially higher as high as 5 0 pdb than the expected equilibrium value for an aragonite forming from ambient seawater 3 5 pdb as a first approximation this indicates formation from glacial ocean water but other factors are considered as well a conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates this model explains the function of the consortium of archaea and sulfate reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane aom and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate dependent anaerobic oxidation of methane within an oxic environment the contact between beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro environment where carbonate formation takes place