Search

Fluid migration in sedimentary basins has profound effects on a range of geological processes, including the methane cycle, tectonic and sedimentary geohazards, and microbial communities in the oceans. The Alboran Sea is a tectonically active basin characterized by contourite drifts that host migrating fluids, expressed in places by pockmarks and mud volcanoes, the latter associated with seafloor methane seepage. In this study, we examine the composition and origin of near-seafloor fluids in the Alboran Sea using sediment cores (up to 20 m long) from a pockmark field (site CL06), a nearby background area (site CL04) and a fault zone (site CL55).We use halogens (Cl, Br, and I) dissolved in interstitial water to understand the origin of fluids in the Alboran Sea. Chlorine is considered a conservative ion in interstitial water geochemistry, its concentration changing with pore water salinity. Iodine has a strong biophilic character and is incorporated in organic matter deposited with sediments, which during burial decomposes in response to geothermal heat or microbial activity to produce methane. Iodine and methane concentrations are strongly correlated and highly concentrated compared to seawater, so that iodine has been used as a methane tracer. Bromide also has a weak biophilic character and behaves similarly to iodine.Interstitial water was extracted aboard ship using Rhizon samplers. Chloride concentration was determined by ion chromatography (ICS-1600, DIONEX) at the Tokyo University of Marine Science and Technology; iodine and bromine concentrations were determined by Inductively coupled plasma mass spectrometry (ICP-MS Agilent 7500) at Micro Analysis Laboratory, Tandem accelerator (MALT), University of Tokyo.The results reveal halogen profiles that differ between the pockmark and fault sites, providing evidence of different modes of fluid migration within the contourite drifts of the Alboran Sea:(1)Pockmark and background sites: surprisingly, halogen profiles are similar at these two sites. Cl concentration decreases with depth from 610 to 590 mM over the 15 m length of the cores, a trend indicating fresher water is present in deeper sediments. I and Br concentrations increase with depth (I: 0 to 70 µM, Br: 760 to 820 µM). I and Br are strongly enriched (up to 8% and 60%, respectively) by a deep fluid source, which may relate to high TOC or evaporated seawater in deeper sediment.(2)Fault zone site: in contrast to the other two sites, Cl concentration increases with depth from 600 to 610 mM over the 16 m length of the core 55, a trend indicating saline water is dominant in deeper sediments. I and Br concentrations increase with depth (I: 35 to 70 µM, Br: 800 to 830 µM). I and Br concentrations in near-seafloor sediments are usually less strongly affected by organic decomposition, with concentrations as low as seawater; however, at site 55, I and Br are strongly enriched in near-seafloor sediments. This observation suggests vertical fluid migration is active and reaches the seafloor to maintain high I and Br concentrations.

International audience; The Amazon fan contains a gas hydrate province known from a bottom-simulating reflection (BSR) that lies within an upper slope compressional belt. In this study, the extent and character of the BSR and its relation to thrust-fold structures is examined using a grid of 2D and 3D seismic data. We show the BSR to comprise a series of elongate patches up to 16 km wide that are present along 300 km of the slope in water depths of 750-2250 m and extend over a total area of 6800 km2. The elongate BSR patches show a strong spatial correspondence with the arcuate crests of thrustfold anticlines. In profile, the BSR patches exhibit convex forms and/or locally irregular relief that rises toward the seafloor. In plan, 3D seismic horizon maps reveal columnar BSR elevations up to 1 km wide, which rise beneath seafloor mounds and depressions, up to 0.5 km wide and 30 m in relief, interpreted as small mud volcanoes and possible pockmarks. The elongate BSR patches are interpreted to record the structurally-controlled rise of warm, gas-rich fluids into the crests of thrust-folds and their leakage into the gas hydrate stability zone, and in places to seafloor, through a near-surface system of faults, hydrofractures and vents.; O leque submarino do Amazonas contém uma zona de hidratos de gás conhecida por um bottom-simulating reflection (BSR) presente nos cinturões compressivos do talude. Para reconhecimento doBSR e de suas feições, foi utilizada uma base de dados de sísmica 2D/3D. O BSR ocorre na forma deuma série de áreas isoladas de formas alongadas, até 16 km de largura, que se estendem por 300 kmda margem entre 750-2250 m de profondidade, e cobrem área total de 6800 km2. As áreas de BSR mostram forte correspondência espacial com as cristas arqueadas dos cinturões compressivos. Em perfis, as áreas de BSR exibe em certos locais morfologias convexas e/ou irregulares, que se elevam em direção ao fundo do mar. Em plano, mapas 3D dos horizontes sísmicos revelam elevações colunares do BSR de até 1 km de diâmetro, que ascendem em direção a feições batimétricas de elevações arredondadas e depressões de até 0,5 km de largura e 30 m de relevo, interpretadas como pequenos vulcões de lama e possíveis pockmarks. As ocorrências de formas alongadas do BSR são interpretadas como o registro de fluxo estruturalmente controlado de fluidos gasosos quentes nas cristas dos cinturões compressivos e de seu escape para a zona de estabilidade dos hidratos de gás, e em locais para o fundo do mar, através de un sistema de falhas, hidrofraturas e chaminés.

Funding information: This work was funded by the Natural Environment Research Council consortium grant BRITICE-CHRONO NE/J009768/1 and by the NERC Radio- carbon Facility and the NERC Cosmogenic Isotope Analysis Facility. The project benefited from the PalGlac team of researchers with funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme to CDC (Grant Agreement No. 787263) and supporting SB and AI. JCE acknowledges support from a NERC independent fellowship award (NE/R014574/1). We thank the PISM team of ice-sheet modellers and Evan Gowan for ICESHEET, and note that the development of PISM was supported by NSF grants PLR- 1603799 and PLR-1644277 and NASA grant NNX17AG65G. DP acknowledges funding from the Italian PNRA project IPY GLAMAR (grant number 2009/A2.15), and from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 656821 (project SEAGAS). The BRITICE‐CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British–Irish Ice Sheet between 31 000 and 15 000 years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500 days of field investigation yielding 18 000 km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet‐wide reconstructions consistent with these findings and using retreat patterns and dates for the inter‐transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand‐year timesteps are presented. The maximum ice volume of 1.8 m sea level equivalent occurred at 23 ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22 ka. The tipping point of deglaciation at 22 ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine‐influenced sectors collapsed rapidly. First order controls on ice‐sheet demise were glacio‐isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector‐scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British–Irish Ice Sheet is now the world’s most well‐constrained and a valuable data‐rich environment for improving ice‐sheet modelling. Publisher PDF

The continental shelf region West of Ireland contains a rich geomorphic and sedimentary record of glacially-influenced marine processes, including ice sheet extension from Ireland into the Atlantic during the last cold period (Late Midlandian glaciation in Ireland). Intricate sets of curvilinear ridges of varying form and scale across the outer shelf have been recently associated with grounded glacial ice withdrawal from maximum limits. The region lies at the southern limit of the glaciation of the Atlantic margin of northwest Europe, and thus occupies an important position in palaeoglaciological reconstructions. Studies of seabed morphology west of Ireland have been greatly enhanced by exhaustive multibeam bathymetric coverage of the Irish National Seabed Survey, associated sediment sampling and high-resolution shallow seismic profiles; in places, these have been complemented by the use of industry 3D seismic data cubes to visualise the internal structure and geomorphology of glacigenic ridges and large-scale sediment aprons. This paper presents observations from combined sediment ground truthing, high-resolution geophysical and bathymetric data, focusing on the Porcupine Bank, Saddle and Inner shelf areas of the Western Irish Shelf, in order to examine available models of shelf glaciation.

International audience; Large deep-sea fans are useful settings to study gas hydrate systems, the rapid burial of organic-rich sediment driving linked processes of gas generation, fluid expulsion and syn-sedimentary tectonism. The Nile deep-sea fan (100,000 km2) is a collapsing Late Cenozoic depocentre that is both a hydrocarbon province and an area of widespread seafloor fluid seepage. Evidence for gas hydrates has been reported in this area, but remains poorly documented. Available seismic and well data are used together with information on seafloor features to characterise a deep-water (1500– 2700 m) gas hydrate system in the central Nile fan. The system is in part expressed as a bottom simulating reflection (BSR) discontinuously observed across a relatively small area (6000 km2), both cross-cutting the stratified fill of fault-bound slope basins, and upslope of the basins within thick unstratified mass transport deposits. West of the BSR area, log data from two wells 45 km apart indicate the presence of gas hydrates in intervals up to 75 m thick near the base of the stability zone. Gas hydrates are also likely to be present near the seafloor within hundreds of pockmark-like mounds that record gas venting through the stability zone, most observed west of the BSR area. The central Nile fan thus contains a gas hydrate system expressed as two areas of comparable size, one with a discontinuous BSR but few seafloor gas vents, another lacking a BSR but with downhole evidence of gas hydrates and abundant gas venting. This heterogenous character is suggested to reflect spatial variations in fluid expulsion from the Nile fan, which can inhibit BSR formation while favouring gas hydrate accumulation over wide areas. This possibility has implications for other large deep-sea fans, many of which have restricted BSRs but may contain more extensive gas hydrate systems.

International audience; The existence of gas hydrate systems along Brazil's vast continental margin has been known since the 1980s, based on observations of bottom simulating reflectors (BSRs) in two large shelf-slope depocenters: (1) the Amazon deep-sea fan in the Foz do Amazonas Basin and (2) Rio Grande Cone in Pelotas Basin. These depocenters are both undergoing gravitational collapse above deep detachment surfaces, resulting in upslope extensional and downslope compressional domains. The BSR is discontinuous across the Amazon deep-sea fan, mainly observed at water depths of 600-2800 m and at anticlines within an upper slope thrust-fold belt related to the compressional domain of the fan. Conversely, a fairly continuous BSR extends across Rio Grande Cone at water depths of 520-3500 m, within both extensional and compressional domains. Interestingly, the well-defined BSR that spans Rio Grande Cone rises to meet the seafloor at water depths of 515-520 m, forming an unusual 'BSR outcrop'. This phenomenon has been observed previously in only a few locations worldwide. Gas hydrates have been recovered within piston cores taken from seafloor seeps in both depocenters, and analyses reveal that gas is dominated by methane of microbial origin.

International audience; The BRITICE-CHRONO Project has generated a suite of recently published radiocarbon ages from deglacial sequences offshore in the Celtic and Irish seas and terrestrial cosmogenic nuclide and optically stimulated luminescence ages from adjacent onshore sites. All published data are integrated here with new geochronological data from Wales in a revised Bayesian analysis that enables reconstruction of ice retreat dynamics across the basin. Patterns and changes in the pace of deglaciation are conditioned more by topographic constraints and internal ice dynamics than by external controls. The data indicate a major but rapid and very short-lived extensive thin ice advance of the Irish Sea Ice Stream (ISIS) more than 300 km south of St George's Channel to a marine calving margin at the shelf break at 25.5 ka; this may have been preceded by extensive ice accumulation plugging the constriction of St George's Channel. The release event between 25 and 26 ka is interpreted to have stimulated fast ice streaming and diverted ice to the west in the northern Irish Sea into the main axis of the marine ISIS away from terrestrial ice terminating in the English Midlands, a process initiating ice stagnation and the formation of an extensive dead ice landscape in the Midlands.

Ocean warming related to climate change has been proposed to cause the dissociation of gas hydrate deposits and methane leakage on the seafloor. This process occurs in places where the edge of the gas hydrate stability zone in sediments meets the overlying warmer oceans in upper slope settings. Here we present new evidence based on the analysis of a large multi-disciplinary and multi-scale dataset from such a location in the western South Atlantic, which records massive gas release to the ocean. The results provide a unique opportunity to examine ocean-hydrate interactions over millennial and decadal scales, and the first evidence from the southern hemisphere for the effects of contemporary ocean warming on gas hydrate stability. Widespread hydrate dissociation results in a highly focused advective methane flux that is not fully accessible to anaerobic oxidation, challenging the assumption that it is mostly consumed by sulfate reduction before reaching the seafloor., Ocean warming could enable the release of methane related to hydrate dissociation from the ocean floor, a process thought to have triggered abrupt climate changes in Earth history. Here the authors detect this process in action, observing a massive release of methane from a site in the South Atlantic Ocean.

International audience; The Amazon Fan provides a natural laboratory to study the generation of overpressure, due to rapid late Cenozoic burial that has resulted in gravitational collapse above shale detachments. Here we examine collapse systems for the first time using the techniques of petroleum systems analysis. We propose an integrated methodology based on numerical modeling constrained by the structural restoration of a seismic profile across the southwestern fan. The results provide information on the evolution of pore pressure and temperature and their implications for the operation of the detachment and overlying extensional and compressional faults during the deposition of up to 6 km of sediment over the last 8 Ma. The modelled thermal history implies that fluid release by smectite-to-illite transformation has taken place within the thickening sedimentary succession, but has not significantly contributed to pore pressures along the detachment. Modeling of hydrocarbon generation and migration from source rocks beneath the fan indicates gas accumulated in successions at depths of 102–103 m beneath the detachment without influencing pore pressures along it. In contrast, model results indicate that overpressures have varied in response to disequilibrium compaction. Fault activity within the collapse system took place during phases of higher sedimentation rates, and ceased from 5.5 to 3.7 Ma when sediment supply to the SE fan decreased. From 2 Ma, renewed sediment flux and shelf-slope progradation led to a basinward migration both of overpressure along the detachment and of fault activity above it. We conclude that gravity tectonics in the Amazon Fan over the last 8 Ma have been mainly controlled by overpressures due to disequilibrium compaction, with secondary contributions from clay mineral transformation. Present-day pressure conditions show that the southeastern Amazon Fan is not at equilibrium and gravity driven deformation could occur at any time.

International audience; The stability of methane hydrates on continental margins worldwide is sensitive to changes in temperature and pressure conditions. It has been shown how gradual increases in bottom water temperatures due to ocean warming over post-glacial timescales can destabilize shallow oceanic hydrate deposits, causing their dissociation and gas release into the ocean. However, bottom water temperatures (BWT) may also vary significantly over much shorter timescales, including due to seasonal temperature oscillations of the ocean bottom currents. In this study, we investigate how a shallow methane hydrate deposit responds to seasonal BWT oscillations with an amplitude of up to 1.5 °C. We use the TOUGH + HYDRATE code to model changes in the methane hydrate stability zone (MHSZ) using data from the Rio Grande Cone, in the South Atlantic Ocean off the Brazilian coast. In all the cases studied, BWT oscillations resulted in significant gaseous methane fluxes into the ocean for up to 10 years, followed by a short period of small fluxes of gaseous methane into the ocean, until they stopped completely. On the other hand, aqueous methane was released into the ocean during the 100 years simulated, for all the cases studied. During the temperature oscillations, the MHSZ recedes continuously both horizontally and, in a smaller scale, vertically, until a permanent and a seasonal region in MHSZ are defined. Sensitivity tests were carried out for parameters of porosity, thermal conductivity and initial hydrate saturation, which were shown to play an important role on the volume of methane released into the ocean and on the time interval in which such release occurs. Overall, the results indicate that in a system with no gas recharge from the bottom, seasonal temperature oscillations alone cannot account for long-term gas release into the ocean.

International audience; Deep-sea fans have been proposed to act as carbon sinks, rapid deposition driving shallow methanogenesis to favor net storage within the gas hydrate stability zone (GHSZ). Here, we present new evidence of widespread gas venting from the GHSZ on the upper Amazon deep-sea fan, together with analyses of the first samples of gas hydrates recovered offshore NE Brazil. Multibeam water column and seafloor imagery over an 18,000-km2 area of the upper Amazon fan reveal 53 water column gas plumes, rising from venting features in water depths of 650–2600 m. Most gas vents (60%) are located along seafloor faults that record the ongoing gravitational collapse of the fan above deep décollements, while others (40%) are located in water depths of 650–715 m within the upper edge of the GHSZ. Gas compositions from hydrates recovered in vents at three locations on and north of the fan indicate biogenic sources (dominantly methane with 2–15% of CO2; δ13C from − 81.1 to − 77.3‰), whereas samples from vents adjacent to the fan proper include possible thermogenic contributions (methane 95%, CO2 4%, and ethane 1%; δ13C – 59.2‰). These results concur with previous findings that the upper edge of the GHSZ may be sensitive to temporal changes in water temperatures, but further point to the importance of gas escape from within areas of gas hydrate stability. Our results suggest the role of fluid migration along pathways created by faulting within rapidly deposited passive margin depocenters, which are increasingly recognized to undergo gravitational collapse above décollements. Our findings add to evidence that gas can escape from sediments to the sea in areas where gas hydrates are stable on passive margins, and suggest the need of further studies of the dynamics of deep-sea depocenters in relation to carbon cycling.

International audience; Methane release from the seafloor is commonly associated with chemosynthesis-based cold seep ecosystems that facilitate the precipitation of authigenic carbonates. It has been proposed that carbonate growth results in self-sealing, but little is known regarding the evolution of cold seep structures in relation to fluid migration pathways. This study investigates structures resulting from gas seepage along ring faults peripheral to Venere mud volcano (1600 m water depth), based on multibeam bathymetry and seafloor backscatter data collected by an autonomous underwater vehicle, together with photomosaics, video observations, and samples obtained by a remotely operated vehicle. Sites of focused fluid flow are identified by gas bubble streams rising from the seafloor while anaerobic oxidation of methane (AOM) over wider areas is indicated by the occurrence of chemosynthesis-based organisms (microbial mats, vesicomyid clams, vestimentiferan tube worms). At some sites, flakes of gas hydrate were observed in the water column during sampling. A range of carbonate structures exists at these sites: 1) flat and extensive pavements; 2) mounds with disseminated nodules or centimeter-thick crusts; 3) fractured mounds with exposed, decimeter-thick crusts; and 4) seafloor depressions lined by decimeter-thick crusts. The mineralogy and stable carbon isotopic compositions of the carbonates are consistent with anaerobic oxidation of methane from thermogenic sources, and possible near-seabed influence of gas hydrate formation and dissolution. The seafloor expressions of seepage are inferred to be controlled by the interaction of fluid flow due to carbonate precipitation and gas hydrate formation. A conceptual model for mound development is proposed in the context of the known timescales of seep-colonization and rates of carbonate precipitation: (A) the onset of hydrocarbon-rich fluid seepage through hemipelagic sediments leads to (B) the establishment of microbial mats on flat seafloor over decadal timescales and is followed by (C) the growth of pavements cemented by carbonates that seal the seafloor; over longer timescales (centuries to millennia), carbonate growth and subsurface gas hydrate formation/dissolution lead to (D) upward doming and fracturing of carbonate mounds, re-sealing and stacking of carbonates and in some cases to (E) their collapse to form seafloor depressions. Gas migration through fractures in the carbonates allows re-sealing and fuels AOM to provide habitats for chemosynthesis-based fauna. This evolutionary scenario is argued to be broadly applicable to the development of ruptured mounds and collapse features described at other seepage sites in the Mediterranean Sea and elsewhere.

Glacigenic sediment fans recording shelf edge deposition from marine-terminating ice sheets have previously been recognised along the NW European continental margin from Svalbard to as far south as Donegal, off north-west Ireland. Here we present evidence of a previously unrecognised partially glacially-fed Plio-Pleistocene sediment wedge on the continental shelf west of central Ireland using 2D and 3D seismic reflection data correlated to a commercial borehole. The ‘Connemara Fan’ covers a shelf area of approximately 9000 km2 in water depths of 125–310 m, extending westwards into the Porcupine Seabight from the Irish Mainland Shelf. The wedge comprises up to 160 m of sediment that culminates in a prominent moraininc ridge at seabed and contains two discordant reflection surfaces (R1 and 2) that subdivide it into three seismic units (SU1–3). Stratigraphic boreholes 27/24-2 and 2A located on the inner shelf show that the lower unit (SU1) is composed of Pliocene marine sediments, while SU2 and 3 comprise glacially influenced facies of Quaternary age. Extracts from a 3D seismic data volume within the central part of the fan show channels within the Pliocene succession, while iceberg scours are observed on the R1 and R2 reflectors. The Connemara Fan is inferred to record sediment supply from central western Ireland, with Quaternary units probably recording at least two glacial advance-retreat cycles with ice sheets repeatedly grounding across the inner (Irish Mainland) shelf. Our findings extend the range of glacially-influenced grounding line depocentres southwards along the NW European continental margin.

Gas hydrate provinces occur in two sedimentary basins along Brazil’s continental margin: (1) The Rio Grande Cone in the southeast, and (2) the Amazon deep-sea fan in the equatorial region. The occurrence of gas hydrates in these depocenters was first detected geophysically and has recently been proven by seafloor sampling of gas vents, detected as water column acoustic anomalies rising from seafloor depressions (pockmarks) and/or mounds, many associated with seafloor faults formed by the gravitational collapse of both depocenters. The gas vents include typical features of cold seep systems, including shallow sulphate reduction depths (

International audience; Gas hydrate provinces occur in two sedimentary basins along Brazil’s continental margin: (1) The Rio Grande Cone in the southeast, and (2) the Amazon deep-sea fan in the equatorial region. The occurrence of gas hydrates in these depocenters was first detected geophysically and has recently been proven by seafloor sampling of gas vents, detected as water column acoustic anomalies rising from seafloor depressions (pockmarks) and/or mounds, many associated with seafloor faults formed by the gravitational collapse of both depocenters. The gas vents include typical features of cold seep systems, including shallow sulphate reduction depths (

International audience; An integrated geophysical, geological, and geochemical investigation of seabed fluid venting was carried out in upper Bantry Bay, a large marine inlet on the southwest coast of Ireland. The results provide evidence of the seafloor venting of gas rich fluids, resulting in the formation of a pockmark field identified here for the first time. The pockmarks occur in an area where sub-bottom profiles provide evidence of chimney-like features interpreted to record upward gas migration through Quaternary sediments to the seafloor. Three vibrocores up to 6 m long were acquired in water depths of 24-34 m, two from the pockmark field and one from outside. Methane of predominantly biogenic origin was quantified in all three cores by headspace analysis of sediment sub-samples. Well-defined sulfate methane transition zones (SMTZs) were observed in two of the cores, the shallowest (1.25 m) inside the pockmark field and the other (3.75 m) outside. It is likely that an SMTZ occurs at the location of the third core, also within the pockmark field, although beneath the samples obtained during this study. Gas release possibly from a combination of various faulting mechanisms and shallow methanogenesis appears to drive diffuse pore fluid migration across wide areas, while focused flow through the pockmarks may be related to gas originating from the Owenberg River Fault and methanogenesis of pre-glacial lacustrine sediments preserved in a bedrock basin. Analysis of phospholipid fatty acids (PLFAs) and archaeal isoprenoid hydrocarbons was used to investigate the microbial ecology of these sediments. Anaerobic oxidation of methane (AOM) may play a role in controlling release of CH4 to the water column and atmosphere in this shallow gas setting, potentially mediated by syntrophic sulfate reducing bacteria (SRB) and anaerobic methanotrophic archaea (ANME).

International audience; The Amazon continental shelf hosted one of the world's largest mixed carbonate-siliciclastic platforms from the late Paleocene onwards - the Amapá carbonates. The platform architecture, however, remains poorly understood and causes and timing of the cessation of carbonate deposition are still controversial. Here we present a stratigraphic analysis of the Neogene succession of the Amapá carbonates, based on a grid of 2D/3D seismic data correlated to revised micropaleontological data from exploration wells. The results provide improved constraints on the age of the transition from predominantly carbonate to siliciclastic sedimentation, which is shown to have varied through time across three different sectors of the shelf (NW, Central and SE). Four Neogene evolutionary stages of carbonate deposition could be defined and dated with reference to the new age model: (1) between ca. 24 and 8 Ma a predominantly aggrading mixed carbonate-siliciclastic shelf prevailed across the entire region carbonate production gave way to siliciclastic sedimentation across the Central and SE shelves; (2) between 8 and 5.5 Ma carbonate production continued to dominate the NW shelf, as deposition was able to keep up with base level oscillations; (3) between 5.5 and 3.7 Ma (early Pliocene), sediment supply from the paleo-Amazon River promoted the progressive burial of carbonates on the inner NW shelf, while carbonates production continued on the outer shelf (until 3.7 Ma). Longer-lasting carbonate sedimentation on the NW shelf can be explained by a lesser influx of siliciclastic sediments due to the paleo-geography of the Central shelf, characterized by a 150-km-wide embayment, which directed most terrigenous sediments sourced from the paleo-Amazon River to the continental slope and deep ocean; (4) from 3.7 Ma onwards, when the Central shelf embayment became completely filled, continuous sediment supply to the NW shelf resulted in the final transition from carbonate to siliciclastic-dominated environments on the entire Offshore Amazon Basin.

International audience; The Celtic Sea contains the world's largest continental shelf sediment ridges. These megaridges were initially interpreted as tidal features formed during post-glacial marine transgression, but glacigenic sediments have been recovered from their flanks. We examine the stratigraphy of the megaridges using new decimetric-resolution geophysical data correlated to sediment cores to test hypothetical tidal vs glacial modes of formation. The megaridges comprise three main units, 1) a superficial fining-upward drape that extends across the shelf above an unconformity. Underlying this drape is 2), the Melville Formation (MFm) which comprises the upper bulk of the megaridges, sometimes displaying dipping internal acoustic reflections and consisting of medium to coarse sand and shell fragments; characteristics consistent with either a tidal or glacifluvial origin. The MFm unconformably overlies 3), the Upper Little Sole Formation (ULSFm), previously interpreted to be of late Pliocene to early Pleistocene age, but here shown to correlate to Late Pleistocene glacigenic sediments forming a precursor topography. The superficial drape is interpreted as a product of prolonged wave energy as tidal currents diminished during the final stages of post-glacial marine transgression. We argue that the stratigraphy constrains the age of the MFm to between 24.3 and 14 ka BP, based on published dates, coeval with deglaciation and a modelled period of megatidal conditions during post-glacial marine transgression. Stratigraphically and sedimentologically, the megaridges could represent preserved glacifluvial features, but we suggest that they comprise post-glacial tidal deposits (MFm) mantling a partially-eroded glacial topography (ULSFm). The observed stratigraphy suggests that ice extended to the continental shelf-edge.

Three regional unconformity surfaces typify the Plio-Pleistocene fill of the onshore to offshore basins of the Calabrian Arc, an arcuate terrane migrating to the SE above the NW subducting Ionian lithosphere, between the southern Apennines and Sicily. These unconformity surfaces formed in the mid-Pliocene (late Zanclean to early Piacenzian), the early Pleistocene (intra-Gelasian) and the mid-Pleistocene (late Calabrian). Their expression varies significantly across the area, so that from one to three unconformities may be recognized within any given basin. Several lines of evidence suggest that the spatially variable development of the unconformities across the Arc records interruptions of subsidence by phases of uplift and deformation, related to the extent and nature of successive tectonic events linked to episodic subduction zone retreat and slab fragmentation during the Plio-Pleistocene interval. In particular, the mid-Pliocene unconformity is associated with uplift and deformation on the Ionian flank of the Calabrian Arc that interrupted a phase of basin subsidence coeval with opening of the Vavilov back-arc basin, and may record either interference of the Arc with the Apulian microplate, or an episode of out-of-sequence tectonic thickening within` the advancing accretionary wedge. The two Pleistocene unconformities bracket a phase of basin collapse in northern Calabria that coincides with the ultra-fast opening of the Marsili backarc basin, and are associated with uplift and contractional–transpressional deformation that record interference of the subducting slab with adjacent microplates. The recognition of a synchronous development of unconformities in the Calabrian basins provides key references for further studies of the geodynamic evolution of the central Mediterranean.

Mud volcanoes (MVs) are abundant along the eastern Mediterranean subduction zones, recording mud breccia extrusion over long timescales (106 years), but to date relatively few have been recognised in the northern Ionian Sea on the Calabrian accretionary prism (CAP). In the present study, the seafloor distribution and recent activity of MVs is investigated across a 35,600 km2 sector of the CAP using a regional acoustic dataset (multibeam bathymetric and backscatter imagery, integrated with subbottom profiles) locally ground-truthed by sediment cores. A total of 54 MVs are identified across water depths of 150–2,750 m using up to four geophysical criteria: distinctive morphology, high backscatter, unstratified subbottom facies and, in one case, a hydroacoustic flare. Fourteen MVs are identified from 3–4 criteria, of which five have been previously proven by cores containing mud breccia beneath up to 1.6 m of hemipelagic sediments (Madonna dello Ionio MVs 1–3, Pythagoras MV and the newly named Sartori MV), while nine others are identified for the first time (Athena, Catanzaro, Cerere, Diana, Giunone, Minerva, ‘right foot’, Venere 1 and 2). Forty other as yet unnamed MVs are inferred from 1–2 geophysical criteria (three from distinctive morphology alone). All but one possible MV lie on the inner plateau of the CAP, landwards of the Calabrian Escarpment in a zone up to 120 km wide that includes the inner pre-Messinian wedge and the fore-arc basins, where they are interpreted to record the ascent from depth of overpressured fluids that interacted with tectonic structures and with evaporitic or shale seals within the fore-arc basins. The rise of fluids may have been triggered by post-Messinian out-of-sequence tectonism that affected the entire pre-Messinian prism, but Plio-Quaternary sedimentation rates and depositional styles support the inference that significant mud volcanism has taken place only on the inner plateau. Sedimentation rates across the CAP applied to a 12 khz sonar detection depth of 225 cm imply that all MVs with backscatter signatures (50 of 54) have erupted mud breccias within the last 56 ka, and within the last 12.5 ka in the fore-arc basins. Ages of eruption estimated from the depth of cored mud breccias at five MVs, and a seismo-stratigraphic relationship at a sixth, indicate episodes at the last glacial maximum ca. 20 ka BP and during the postglacial period. Eruptive episodes within the Calabrian MV province constitute recurrent geohazards, separated by longer periods of quiescent (subdued) fluid seepage that are likely to support gas hydrate formation and chemosynthetic ecosystems.

We report the detection of mound-like landforms interpreted as mud volcanoes within Firsoff crater, in the Arabia Terra region of Mars. The mounds stratigraphically overlie layered deposits within the crater, which consist of high albedo well-bedded deposits disrupted in meter-sized polygons. Hundreds of mounds are observed on the crater floor, as isolated or composite subcircular cones 100–500 m in diameter and tens of meters high. More than one third of the mounds (35%) have subcircular depressions 5–39 m in diameter at their apices that we interpret as vents. The mounds consist of meter-sized boulders of high albedo, embedded in an apparently finer-grained unit; we interpret these deposits as mud volcanic breccia, containing boulders of reworked light-toned layered deposits carried upward from deeper portions of the succession. The mounds are mainly located near the rim-bordering faults and in places aligned with fractures, suggesting subsurface pathways for fluid migration. The mounds are interpreted as fossil mud volcanoes, recording the upward migration of overpressured fluids through the crater infill from deeper crustal sources, localized by impact-related faults. The geological conditions and processes that might have controlled sedimentary expulsion on Mars are discussed. Mud volcanism has most of its controls in the subsurface. As a consequence, subsurface fluid flow processes could be expected in other craters within Arabia Terra and are less likely in higher, adjacent highland terrains.

The regional Quaternary seismo-stratigraphy of NW Gulf of St. Lawrence, based on 5700 line km of high resolution seismic reflection profiles, is described. The Quaternary sequence can be locally missing or can exceed 1.3 km in thickness. Five major stratigraphic units are recognized, which vary in their character and distribution so that at any location a variety of bedrock types may be overlain by a distinctive Quaternary sequence. These units relate to the advance and retreat of the Late Wisconsinan Ice Sheet. We interpret these units as: Unit 1, recording the presence of grounded glacial ice, including ice-loaded and ice-deposited sediments. Unit 2, ice-proximal coarse-grained sediment deposited either as a thin, conformable layer during the rapid retreat of an ice terminus, or as a wedge-shaped fan marking the position of an ice front still stand. Unit 3, ice-distal fine-grained sediment deposited from meltwater plumes at times of elevated sea levels and rapidly ablating sea ice. Unit 4, paraglacial deltaic sediment marking the melting of terrestrially-based ice caps, and the concommittant growth of deltas, rapidly prograding into a seaway undergoing rapidly falling sea levels. Unit 5, postglacial sediment reflecting the winnowing of shallow areas and deposition of organic-rich mud in deep basins, under modern sea level and océanographie conditions. A conceptual model dealing with the deposition of sediment associated with the withdrawal of a continental ice sheet is developed. The model includes the dynamics associated with the initial ice advance, terminal ice dynamics, retreat of the ice terminus, stable ice-fronts during the recessional phase, ice sheets ablating on land, and postglacial sedimentation under conditions of fluctuating sea levels., Le présent article se fonde sur des profils effectués par sismique reflection sur 5700 kilomètres dans le nord-ouest du golfe du Saint-Laurent. La séquence quaternaire, parfois absente, peut ailleurs avoir plus de 1,3 kilomètre d'épaisseur. On a relevé cinq grandes unités stratigraphiques dont la nature et la répartition varient. Liées à l'évolution de l'inlandsis du Wisconsinien supérieur, elles se présentent comme suit: unité 1 : sédiments de contact renfermant des sédiments comprimés par le glacier et d'autres déposés par lui; unité 2: sédiments proximaux grossiers, déposés soit en une couche mince et concordante durant le retrait rapide d'un front, soit sous forme de cône marquant la position d'un front stable; unité 3: sédiments distaux fins, déposés par des langues d'eau de fusion dans une mer à niveaux élevés et dont la glace fondait rapidement; unité 4: sédiments deltaïques paraglaciaires révélant la fusion de glaces continentales et la formation rapide et concomitante de deltas dans une mer accusant de fortes baisses de niveau : unité 5: sédiments postglaciaires révélant le triage de zones peu profondes et le dépôt, en profondeur, de boues riches en matières organiques, dans des conditions rappelant celles d'aujourd'hui. Un modèle, exposant les étapes de sédimentation associées au retrait d'un inlandsis, tient compte des dynamiques associées à la première avancée glaciaire, à la position glaciaire finale, au retrait du front glaciaire, à la stabilité du front pendant le retrait, à la fusion de l'inlandsis et à la sédimentation postglaciaire dans une mer aux niveaux changeants., Die régionale Quaternâr Seismo-Stratigraphie des Nordwestens vom Sankt-Lorenz-Golf wird beschrieben, gestûtzt auf 5700 km lange seismische Reflexions-profile mit hoher Auflôsung. Die Quatemàr-Sequenz kann stellenweise fehlen oder auch ùber 1.3 km dick sein. Fùnf stratigraphische Haupt einheiten werden bestimmt. Einheit 1, welche das Vorhandensein von glazialem bodeneis einschliesslich durch Eis kompri-mierte und durch Eis abgelagerte Sedimente dokumentiert. Einheit 2, ein grobgekôrntes Eisproximal-Sediment, das entweder als eine dùnne, gleichgelagerte Schicht wâhrend des schnellen Rûckzugs eines Eisterminus abgelagert wurde, oder als ein keilfôrmiger Fâcher, der den Stillstand einer Eisfront markiert. Einheit 3, ein feinkôrniges Eis-Distal-Sediment, das von Schmelzwasserzungen in Zeiten erhôhter Meeresspiegel und rasch schmelzenden Meereises abgelagert wurde. Einheit 4. ein paraglaziales Delta-Sediment, welches das Schmelzen von auf dem Festland gelagerten Eiskappen markiert und das damit verbundene Anwachsen der Deltas, die schnell zu einem Seeweg vorrùckten und schnell fallenden Meeresspiegeln ausgesetzt waren. Einheit 5, ein postglaziales Sediment, das das Aussortieren seichter Gebiete spiegelt sowie Ablagerung von organisch reichem Schlamm in tiefen Becken und das unter modernen Meeresspiegel - und ozeanographischen Bedingungen. Es wird ein Begriffsmodell entwickelt, das sich mit der Ablagerung von Sediment im Zusammenhang mit dem Ruckzug einer kontinentalen Eisdecke befasst.

A buried bedrock trough 350 m deep extends 100 km above Saguenay Fjord beneath the North Channel of the middle estuary. Four of five regional seismostratigraphic units are recognized in and adjacent to the trough; unit 1 (glacial ice-contact) and older sediments might also be present beneath the largely unpenetrated trough axis. Units 2 and 3 represent thick glacial marine sediments deposited in the >550 m deep waters of the Goldthwait Sea after glacial withdrawal ca. 13 ka BP: lower draped muds 10-20 m thick (unit 2) suggest deposition proximal to a retreating ice margin, while upper onlapping muds > 290 m thick (unit 3) record distal basin-filling; lateral transition to a coarse-grained proximal wedge 5*260 m thick (unit 2) is indicated by unit 3 reflectors rising and strengthening towards the Saguenay entrance, where a stable ice-margin ca. 13-11 ka BP supplied sediment to the lower and middle estuary. Unit 4 corresponds to lobes over 30 m thick on both sides of the upper North Channel, recording marginal input from glacial fluvio-deltaic sources. Unit 5 (estuarine sands, gravels and muds =£30 m thick) unconformably overlies glacial units. A smooth unconformity surface records erosion (at least 15 m, to axial depths >150m) by strong currents; irregular relief above depths of 25-50 m might relate to relative sea levels below present ca. 7-6 ka BP. Sand bedforms (apparently inactive) occur at the estuary floor, and possibly buried beneath estuarine muds; buried bedforms would imply an early Holocene genesis. Greatest thicknesses of estuarine mud coincide with adjacent fluvial discharges. Sandy/gravelly veneers form the estuary floor in most places. Mass displacement has disturbed units 3 and 5 along the northern, and locally southern, walls of the North Channel., Une fosse enfouie à socle rocheux de plus de 350 m de profondeur s'étend sur 100 km en amont du fjord du Saguenay sous le chenal du Nord. Quatre des cinq unités sismostratigraphiques quaternaires ont été identifiées à l'intérieur ou à la périphérie de la fosse. L'unité n° 1 (de contact glaciaire) et des sédiments plus anciens sont peut-être aussi présents sous l'axe de cette fosse en grande partie insondée. Les unités nos 2 et 3 sont constituées d'une grande épaisseur de sédiments glacio-marins mis en place dans les eaux de la Mer de Goldthwait (> 550 m) après le retrait glaciaire, vers 13 ka BP; les boues de recouvrement de la couche inférieure, de 10 à 20 m d'épaisseur (unité n° 2), témoignent d'une mise en place à proximité de la marge en recul, alors que les boues sus-jacentes de 290 m d'épaisseur (unité n° 3) reflètent un remplissage distal du bassin. Le passage latéral à une accumulation proximale de matériel à grains grossiers de 5*260 m d'épaisseur (unité n° 2) est indiqué par l'élévation et le renforcement des réflecteurs vers l'entrée du Saguenay, où une marge glaciaire stable a fourni des sédiments à l'estuaire inférieur et moyen de 13 à 11 ka BP environ. L'unité n° 4 correspond à des lobes de plus de 30 m d'épaisseur de chaque côté du chenal du Nord, témoignant d'une accumulation marginale à partir de sources fluvio-deltaïques glaciaires. L'unité n° 5 (sables, graviers et boues estuariennes de =£30 m d'épaisseur) recouvre en discordance les unités glaciaires. Une surface de discordance unie témoigne d'une érosion (de 15 m, à des profondeurs de >150m dans l'axe) par de forts courants; la topographie irrégulière de 25-50 m de profondeur pourrait être liée à l'abaissement des niveaux marins, vers 7-6 ka BP., Vergrabenes anstehendes Gestein erstreckt sien ùber 100 km in > 350 m Tiefe oberhalb des Saguenay-Fjords unter dem Nord-Kanal des mittleren Mùndungsbeckens. Vier von fùnf regionalen seismostratigraphischen Einheiten sind in und neben dem Trog erkannt worden; Einheit 1 (glazialer Eiskontakt) und altère Sedimente kônnten auch unter der weitgehend nichtergrùndeten Trog-Achse vorhan-den sein. Die Einheiten 2 ind 3 bestehen aus dicken glazialen marinen Sedimenten, welche in dem 290 m Dicke (Einheit 3) distale Becken-Fùllung belegt; eine latérale Ùberleitung zu einem grobkôrnigen proximalen Keil von 3--260 m Dicke (Einheit 2) wird durch die Anhebung und Verstàrkung der Einheit 3 - Reflektoren zum Eingang des Saguenay hin angezeigt, wo ein stabiler Eissaum etwa 13-11 ka v.u.Z. Sediment zum unteren und mittleren Mùndungsbecken brachte. Einheit 4 entspricht ùber 30 m dicken Loben beiderseits des oberen Nord-Kanals, und belegt eine Anlagerung von glazialen FluB-Delta-Quellen am Rand. Einheit 5 (Mùndungsbecken-Sand, Kies und Schlamm =s30 m ûick) ùberlagert in Diskordanz die glazialen Einheiten. Eine glatte Diskoranz-Oberflâche belegt eine Erosion (mindestens 15 m, bis zu Tiefen von > 150 m in der Achse) durch starke Strômungen; das unregelmàBige Relief ùber den Tiefen von 25-50 m kônnte mit relativen Meeresniveaus, die etwa um 7-6 ka v.u.Z. unter den gegenwàrtigen lagen, zusammenhàngen.

Previous reconstructions of the British–Irish Ice Sheet (BIIS) envisage ice streaming from the Irish Sea to the Celtic Sea at the Last Glacial Maximum, to a limit on the mid-shelf of the Irish-UK sectors. We present evidence from sediment cores and geophysical profiles that the BIIS extended 150 km farther seaward to reach the continental shelf edge. Three cores recently acquired from the flank of outer Cockburn Bank, a shelf-crossing sediment ridge, terminated in an eroded glacigenic layer including two facies: overconsolidated stratified diamicts; and finely-bedded muddy sand containing micro- and macrofossil species of cold water affinities. We interpret these facies to result from subglacial deformation and glacimarine deposition from turbid meltwater plumes. A date of 24,265 ± 195 cal BP on a chipped but unabraded mollusc valve in the glacimarine sediments indicates withdrawal of a tidewater ice sheet margin from the shelf edge by this time, consistent with evidence from deep-sea cores for ice-rafted debris peaks of Celtic Sea provenance between 25.5 and 23.4 ka BP. Together with terrestrial evidence, this supports rapid (ca 2 ka) purging of the BIIS by an ice stream that advanced from the Irish Sea to the shelf edge and collapsed back during Heinrich event 2.

The sediment-undersupplied eastern margin of the Rockall Trough, west of Ireland, is incised by numerous canyons and slope failure features. Swath multibeam bathymetry has been integrated with 2D seismic profiles to constrain the Neogene evolution of the slope and its canyons. The morphology varies along the margin, with canyon heads located at mid-slope depths in the south but extending onto the shelf in the north. West of Porcupine Bank, slope gullies connect with a distributative channel system on the trough floor, while north of Porcupine Bank the basin floor is flat and featureless. Draped fault-blocks and deep structures exerted an important influence on slope gradients, canyon extent and geometry. A ‘bottom driven’ upslope–retrogressive slope failure mechanism is inferred for canyon formation. They were initiated by failure localisation following widespread slope rotation and instability linked to differential subsidence that produced a latest Eocene–early Oligocene (C30) regional unconformity. In the NE Rockall, where the greatest density of canyons occurs, a large mass failure wedge directly overlies the C30 surface and the seabed canyons have incised the upper part of the wedge. Axial profile data indicate that canyons in the NE Rockall Trough formed in Mid-Cenozoic times but were locally reutilised as sediment conduits during Plio-Pleistocene slope progradation.

In this paper new and previously published data on the Pleistocene glacial impact on the NW European margin from Ireland to Svalbard (between c. 48°N–80°N) are compiled. The morphology of the glaciated part of the European margin strongly reflects repeated occurrence of fast-moving ice streams, creating numerous glacial troughs/channels that are separated by shallow bank areas. End-moraines have been identified at several locations on the shelf, suggesting shelf-edge glaciation along the major part of the margin during the Last Glacial Maximum. Deposition of stacked units of glacigenic debris flows on the continental slope form fans at a number of locations from 55°N and northwards, whereas the margin to the south of this is characterised by the presence of submarine canyons. Glaciation curves, based primarily on information from the glacial fed fan systems, that depict the Pleistocene trends in extent of glaciations along the margin have been compiled. These curves suggest that extensive shelf glaciations started around Svalbard at 1.6–1.3 Ma, while repeated periods of shelf-edge glaciations on the UK margin started with MIS 12 (c. 0.45 Ma). The available evidence for MIS 2 suggest that shelf-edge glaciation for the whole margin was reached between c. 28 and 22 14 C ka BP and maximum positions after this were more limited in some regions (North Sea and Lofoten). The last glacial advance on the margin has been dated to 15–13.5 14 C ka BP, and by c. 13 14 C ka BP the shelf areas were completely deglaciated. The Younger Dryas (Loch Lomond) advance reached the coastal areas in only a few regions.

The present paper provides an overview of glacial related seabed features and sedimentary sequences found along the formerly glaciated NW European margin and compare it with those found on contemporary glaciated margins from both the Southern and Northern Hemispheres. A brief review of the seabed physiography and strata architecture of the margins under consideration is followed by comparison of the most relevant similarities and differences. Comparison of the present-day bathymetric setting of both former and contemporary glaciated margins reveals no clear link to the effect of neither ice sheet or sediment load. Three different types of glacially eroded shelf transverse troughs have been identified, while marginal troughs seem connected to similar geological settings everywhere. Beyond the shelf edge interaction between downslope and alongslope processes has occurred resulting, amongst others, in the formation of large sedimentary mounds on the rise. More frequent large-scale mass wasting occurs on the former glaciated NW European margin than the Greenland and Antarctic margins in the latest Neogene to recent times. A two-stage evolution of the shelf prograding wedges is observed on all margins under consideration, which may reflect a general development of an ice cover from an initial phase of non- to restricted glaciation, evolving to a mature stage of expansive glaciation.

The lower Neogene stratigraphy of the NW European Atlantic margin, from the Voring to the Porcupine basins, is interpreted to record a discrete phase of compressional tectonism that spanned at least 8 Ma from the earliest to the early mid-Miocene. This compressional tectonism may be coeval with a local reorganisation of the NE Atlantic plate system with the transfer of the Jan Mayen micro-plate from Greenland to Europe. The compressional tectonics has resulted in a number of stratigraphic sequences of complex character bounded by regional base Neogene and intra-Miocene unconformities. These are traceable across a range of depths and record distortion of the basin margins and changes in deep-water circulation patterns. This episode of compressional tectonics has also resulted in the creation of a number of anticlinal domes along the Norwegian, Faroese and UK Atlantic margins. The stratigraphic and structural evidence are interpreted to record two stages in the development of the margin: the first being characterised by a prolonged period of regional flexure in response to the build-up of compressive stresses; the second stage is the development of anticlinal structures that led to a rapid release of stress. In the Wyville–Thomson–Faroes region, compressional deformation influenced the creation of the present-day deep-water conduit of the Faroe Bank Channel, which is interpreted to be an early Neogene syncline. Together, the Faroe Bank and Faroe–Shetland channels represent the deepest water passageway across the Greenland–Scotland Ridge. The early Neogene development of this conduit is considered to mark the onset of deep-water exchange across this oceanic gateway.

The North Atlantic margins are archetypally passive, yet they have experienced post-rift vertical movements of up to kilometre scale. The Cenozoic history of such movements along the NW European margin, from Ireland to mid-Norway, is examined by integrating published analyses of uplift and subsidence with higher resolution tectono-stratigraphic indicators of relative movements (including results from the STRATAGEM project). Three episodes of epeirogenic movement are identified, in the early, mid- and late Cenozoic, distinct from at least one phase of compressive tectonism. Two forms of epeirogenic movement are recognised, referred to as tilting (coeval subsidence and uplift, rotations

The results of an integrated sedimentological and seismic stratigraphical analysis of three borehole sites on the eastern flank of the Rockall Basin, offshore western Ireland are reported. Two sites were drilled on the western slope of the Porcupine High, above the North and South Brona basins (boreholes 83/20-sb01, 83/24-sb01 and 83/24-sb02), and one on the northern flank of the Porcupine High (16/28-sb01), above the Macdara Basin. The cores establish that the half-graben basins marginal to the eastern Rockall Basin contain Jurassic deposits and that they were inverted sometime in the Late Jurassic or Early Cretaceous. An angular unconformity above the Brona basins is overlain by a condensed, tripartite Cretaceous succession (‘brownsand’, ‘greensand’, chalky micrite) that records stepwise deepening, with evidence for a Cenomanian-Turonian phase of normal faulting. Above the Macdara Basin, the unconformity is overlain by a basalt that was cored at the 16/28 site and is interpreted to represent a flow of Cretaceous age derived from the Drol Igneous Centre. At all three borehole sites, Cretaceous strata are onlapped (or downlapped) by Paleocene-Eocene strata that display evidence of a minor episode of fault reactivation above the Brona basins. Cored Eocene strata vary from clastic to carbonate-prone from north to south and smectitic clays are common at the 16/28 site. Post-Mid-Eocene westward tilting of the Rockall slope rotated the Eocene stratigraphy and the underlying Cretaceous deposits (including the lava flow in the 16/28 area) at least 3° down to the west. Slope development resulted in extensional sliding and the erosion of the C30 deep-water unconformity that is onlapped by Miocene slope deposits. C30 was cored in the 83/20 area where it cuts down into Cretaceous strata and is crusted with phosphates and the Cretaceous beneath Mn-impregnated.

A regional stratigraphic framework for the Neogene succession along and across the NW European margin is presented, based on a regional seismic and sample database. The stratigraphy provides constraints on the timing and nature of the mid- to late Cenozoic differential tectonic movements that have drivenmajor changes in sediment supply, oceanographic circulation and climate (culminating in continental glaciation). The overall context for Neogene deposition on the margin was established in the mid-Cenozoic, when rapid, km-scale differential subsidence (sagging) created the present-day deep-water basins. The Neogene is subdivided into lower (Miocene–lower Pliocene) and upper (lower Pliocene–Holocene) intervals. The lower Neogene contains evidence of early to mid-Miocene compressive tectonism, including inversion anticlines and multiple unconformities that record uplift and erosion of basin margins, as well as changes in deep-water currents. These movements culminated in a major expansion of contourite drifts in the mid-Miocene, argued to reflect enhanced deep-water exchange across the Wyville-Thomson Ridge Complex, via the Faroe Conduit. The distribution and amplitude of the intra-Miocene movements are consistent with deformation and basin margin flexure in response to enhanced intra-plate compressive stresses during a local plate reorganization (transfer of the Jan Mayen Ridge from Greenland to Europe). The upper Neogene records a seaward tilting (

Seismic reflection methods provide continuous access both to stratigraphy (vertical) and to subsurface morphology (horizontal), for which the scales of interest may differ by orders of magnitude. Seismic surveys of Quaternary successions have generally sought to optimise vertical resolution, through the use of higher source frequency content. Here, I show that low-frequency bandwidth is not necessarily a limiting factor for the seismic resolution of glacigenic morpho-sedimentary features. Observations are presented from a buried network of large mid-Pleistocene (Elsterian) tunnel-valleys in the southern North Sea Basin, across a 100×130 km study area with water depths less than 30 m. Low-frequency 2D and 3D seismic multi-channel data, acquired for deeper hydrocarbon exploration, are compared with previously available high-frequency single- and multi-channel profiles (5–15 km grid spacing). The low-frequency data contribute to a new understanding of the basal morphology and fill stratigraphy of the tunnel-valleys, in part due to higher data densities (≥1 km grid spacing), but also to improved imaging of reflectors at depth. The tunnel-valleys are seen to be overdeepened troughs, shallow (≤0.5 km) relative to their widths (≤6 km). The basal unconformity defines a series of arborescent elements, convergent to the south; erosional overlap by younger elements to the north has resulted in anastomosing patterns in places. The fill is dominated by axially downlapping clinoforms, descending to the north, onlapped and overlain by subhorizontal reflectors. Well data show that sand-dominated glaciofluvial sediments are overlain by glaciolacustrine to marine muds. Better definition of the clinoforms on low- versus high-frequency multi-channel data is suggested to reflect the coarse spatial scale of the backset glaciofluvial strata. The results support a simple interpretation of time-transgressive tunnel-valley formation by coeval glaciofluvial erosion and backfill beneath the outer tens of kilometres of the northward receding Elsterian ice sheet margin. Comparable submarginal interpretations have been proposed for drainage features (tunnel-valleys and eskers) of the last deglaciation of both northern Europe and North America using integrated geomorphologic and stratigraphic methods. Commercial 2D and 3D seismic data are widely available from exploration areas such as the North Sea and are argued to constitute an underexploited resource for Quaternary research.

International audience; Existing knowledge on the distribution of mud volcanoes (MVs) and other significant fluid/free gas-venting features (mud cones, mud pies, mud-brine pools, mud carbonate cones, gas chimneys and, in some cases, pockmark fields) discovered on the seafloor of the Mediterranean Sea and in the nearby Gulf of Cadiz has been compiled using regional geophysical information (including multibeam coverage of most deepwater areas). The resulting dataset comprises both features proven from geological sampling, or in situ observations, and many previously unrecognized MVs inferred from geophysical evidence. The synthesis reveals that MVs clearly have non-random distributions that correspond to two main geodynamic settings: (1) the vast majority occur along the various tectono-sedimentary accretionary wedges of the Africa-Eurasia subduction zone, particularly in the central and eastern Mediterranean basins (external Calabrian Arc, Mediterranean Ridge, Florence Rise) but also along its westernmost boundary in the Gulf of Cadiz; (2) other MVs characterize thick depocentres along parts of the Mesozoic passive continental margins that border Africa from eastern Tunisia to the Levantine coasts, particularly off Egypt and, locally, within some areas of the western Mediterranean back-arc basins. Meaningfully accounting for MV distribution necessitates evidence of overpressured fluids and mud-rich layers. In addition, cross-correlations between MVs and other GIS-based data, such as maps of the Messinian evaporite basins and/or active (or recently active) tectonic trends, stress the importance of assessing geological control in terms of the presence, or not, of thick seals and potential conduits. It is contended that new MV discoveries may be expected in the study region, particularly along the southern Ionian Sea continental margins.

International audience; The central Nile Deep-Sea Fan contains a broad area of seabed destabilisation in association with fluid seepage: slope-parallel sediment undulations are associated with multibeam high-backscatter patches (HBPs) related to authigenic carbonates. During the 2011 APINIL campaign, a deep-towed sidescan and profiling system (SAR) was used to acquire high-resolution data along three transects across water depths of 1,700-2,650 m. Three seabed domains are distinguished, all developed within stratified sediments overlying mass-transport deposits (MTDs). Upslope of the undulations (

The Ionian margins of Calabria are affected by repeated sediment failures, recorded by slide scars at seabed and stacked slide deposits. We present a reconstruction of the geometry and dynamics of one of the largest seabed features, the Assi failure on the relatively steep slope off southern Calabria, and use it as input to numerical modeling to evaluate the potential tsunamigenic hazard. The Assi failure is up to 6 km wide and at least 18 km long, and involved the displacement of ca. 2 km3 of sediment, inferred to have taken place within the last 4,000 years in two main phases. The first and larger phase is used as input to the tsunami modeling, on the assumption that the slide moved in a single step as a coherent mass of 1.85 km3, in order to evaluate the most disruptive possible consequences. The results indicate that within 8 minutes, waves just over 1 m in height affect the southern Calabrian coast between Monasterace and Roccella Jonica, where their capacity to cause damage could be amplified in small harbours. This shows that tsunamis represent a hazard for Ionian coastal areas, and calls for accurate monitoring and further study.

International audience; On the central Nile deep-sea fan, stratified sediments overlying mass-transport deposits (MTDs) are deformed into slope-parallel seabed undulations associated with fluid seepage. The western part of this system, in water depths of 1,950-2,250 m, is examined using multi-frequency data from hull-mounted and deep-towed swath/profiling systems. Sub-bottom profiles show sub-vertical fluid pipes that terminate both at and below seabed, and gas signatures along fault planes bounding the undulations. Fluid seepage is recorded by high- to intermediate-backscatter patches (HBPs, IBPs) that differ in appearance on multibeam imagery (30 kHz, ≤3 m penetration) and sidescan swaths (170/190 kHz

Voluminous mass-transport deposits (MTD) have been identified on seismic profiles across the central Nile Deep-Sea Fan (NDSF). The youngest MTDs are buried under 30–100 m of well-stratified slope deposits that, in water depths of 1,800–2,600 m, are characterized by undulating reflectors correlated with slope-parallel seabed ridges and troughs. Seabed imagery shows that, in the western part of the central NDSF, short, arcuate undulations are associated with fluid venting (carbonate pavements, gas flares), while to the east, long, linear undulations have erosional furrows on their downslope flanks and fluid seeps are less common. Sub-bottom profiles suggest that the western undulations correspond to rotated fault-blocks above the buried MTDs, while those in the east are sediment waves generated by gravity flows. We suggest that fluids coming from dewatering of MTDs and/or from deeper layers generate overpressures along the boundary between MTDs and overlying fine-grained sediment, resulting in a slow downslope movement of the sediment cover and formation of tilted blocks separated by faults. Fluids can migrate to the seafloor, leading to the construction of carbonate pavements. Where the sediment cover stabilizes, sediment deposition by gravity flows may continue building sediment waves. These results suggest that complex processes may follow the emplacement of large MTDs, significantly impacting continental-slope evolution.

The Ionian margins of Calabria and Apulia (IMCA) have been affected by mass movements of varying style, scale and age. Here we present examples of seabed and subsurface features identified along more than 400 km of the IMCA from multibeam seabed imagery and subbottom profiles acquired by OGS since 2005. Four different types of mass movement phenomena are recognized with expression at seabed and in the shallow subsurface: (1) mass transport complexes (MTCs) within intra-slope basins, (2) isolated slide scars (ISS) along open slopes, (3) slope-parallel sediment undulations (SPSU) recording block-rotations linked to fluid migration, and (4) headwall and sidewall scarps (HSC) in submarine canyons. Preliminary analyses of sedimentary processes suggest that both open-slope failures capable of triggering tsunamis and retrogression of canyon headwalls within 1–3 km of the Calabrian coast represent potential geohazards for coastal populations and offshore infrastructures.

16 pages, 11 figures, This paper presents a computational model for mapping the regional 3D distribution in which seafloor gas hydrates would be stable, that is carried out in a Geographical Information System (GIS) environment. The construction of the model is comprised of three primary steps, namely: (1) the construction of surfaces for the various variables based on available 3D data (seafloor temperature, geothermal gradient and depth-pressure); (2) the calculation of the gas function equilibrium functions for the various hydrocarbon compositions reported from hydrate and sediment samples; and (3) the calculation of the thickness of the hydrate stability zone. The solution is based on a transcendental function, which is solved iteratively in a GIS environment. The model has been applied in the northernmost continental slope of the Gulf of Cadiz, an area where an abundant supply for hydrate formation, such as extensive hydrocarbon seeps, diapirs and fault structures, is combined with deep undercurrents and a complex seafloor morphology. In the Gulf of Cadiz, the model depicts the distribution of the base of the gas hydrate stability zone for both biogenic and thermogenic gas compositions, and explains the geometry and distribution of geological structures derived from gas venting in the Tasyo Field (Gulf of Cadiz) and the generation of BSR levels on the upper continental slope. © 2009 Elsevier Ltd. All rights reserved, This work is part of the ESF Euromargins projects Mvseis (O1-LEC-EMA24F, REN2002-11669-E-MAR) and Moundforce (O1-LEC-EMA06F, REN2002-11668-E-MAR), and Consolider-Ingenio 2010 CSD2006-0041-“TopoIberia”. CJD is a member of UCM Research Group 910198 and CGL2005-01336/BTE

Mud volcanoes recently discovered on the offshore Calabrian Arc are investigated at two sites 60 km apart, in water depths of 1650--2300 m, using swath bathymetry, 2D&3D multichannel seismic and cores. The seabed and subsurface data provide information on their formation and functioning in relation to tectonic activity during the rapid Plio-Quaternary advance of the accretionary prism. Fore-arc extension and thrust-belt compression are seen to have involved two main phases of activity, separated by a regional unconformity recording a mid-Pliocene (3.5–3.0 Ma) tectonic reorganization. The two sites of mud volcanism lie in contrasting tectonic settings (inner fore-arc basin vs central fold-and-thrust belt) and record differing forms of seabed extrusive activity (twin mud cones and a caldera vs a broad mud pie). At both sites, subsurface data show that mud volcanism took place throughout the second tectonic phase, since the late Pliocene; differing forms of mud extrusion were accompanied by subsidence to form depressions beneath and within extrusive edifices up to 1.5 km thick. The basal subsidence depressions point to sources within the succession of thrusts underlying the inner to central Arc, consistent with microfossils within cored mud breccias from both sites that are derived from strata as old as Late Cretaceous. These results are argued to support a model of mud volcanism in which deeply-rooted fluid conduits drive a process of near-surface sediment mobilization. The onset of extrusive activity is inferred to have been triggered by the mid-Pliocene tectonic reorganization, the change in stress fields (which post-dates the migration of frontal compression across the area by >4 Ma) opening pathways for the release of overpressured fluids from deep within the accretionary prism. The rising fluids interacted with shallow (

We divide the Arctic continental shelf of Canada into three regions: (1) the Baffin Shelf, (2) the Arctic Channels and (3) the Polar Shelf. All are deep ice-dominated shelves largely floored by relict sediments. Iceberg scours occur to depths of 315 m on the Baffin Shelf but scours at depths deeper than this, and most in the Arctic Channels, are relict. All areas have been affected by late glacial fluctuations of the Laurentide Ice Sheet and local glaciers. Three principal seismic stratigraphic units are recognized and are interpreted as a glacial, glacial marine and postglacial stratigraphic sequence. Seismic Unit I is a transparent unit (mud) that lies in basins, Unit II is acoustically stratified with a mantling geometry and Unit III is acoustically incoherent with a constructional geometry in places. Units II and III interfinger in some areas. Core samples show that Unit I consists of bioturbated hemipelagic muds with ice-rafted sand and gravel, Unit II varies from a massive to laminated mud deposited from suspension and Unit III, rarely sampled, is a diamicton. In the Arctic Channels Unit II is noticeably thinner than it is on Baffin Shelf. Radiocarbon dates on foraminifera or bivalves indicate that the base of Unit II varies from 30,000 years to circa 8000 years, whereas postglacial Unit I was deposited over the last 5000–7000 years. During much of the Holocene the shelves have been starved of sediments as fiord basins were exposed during ice retreat and served as sediment sinks.

The geological record of glacial drainage includes both sub- and proglacial channel systems. The distinction of such types has been proposed as a guide to former ice margins in the central North Sea, based on interpretation of seismic profiles [Jansen, 1976]. However, channel systems may be spatially overlapped by ice-margin advance and retreat. 3D-seismic methods allow horizontal access to the subsurface and interpretation by geomorphological analogy [Brown, 1991]. Here, a single time-slice from the central North Sea (Fig. 1) reveals superimposed networks of buried channels over a 400 km2 area (Figs. 2, 3).

Subaqueous ice-scours are mainly known as surficial phenomena, formed by floating or glacier ice, acoustically imaged using sidescan sonar. Buried ice-scours have seldom been recognised in the geological record. Here, a reconstruction of the plan form of buried scours from seismic profiles [Hovland and Judd, 1988] is contrasted with the ready access to subsurface morphological information afforded by a 3D-seismic horizontal section [Gallagher et al., 1991]. The 2D and 3D datasets come from the central North Sea and the mid-Norwegian Shelf (Haltenbanken), respectively (Fig. 1).

Fluvial channels potentially occur within the Quaternary stratigraphy of continental shelves world-wide, as a consequence of repeated glacioeustatic lowerings of relative sea level. Buried channels are well-suited to imaging by 3D-seismic methods due to a combination of contrast with surrounding materials and spatial resolution of their distinctive plan form [e. g. Brown,1991; Davies et al.,1992]. This is illustrated using 3D-data from the southern North Sea (Fig. 1). Horizontal seismic sections across a 220 km2 area provide morphological evidence of a fluvial channel system that drained to the north (Figs. 2, 3).

Tunnel-valleys are characteristic elements of glaciated sedimentary basins that record meltwater drainage beneath ice sheets [e.g. Ehlers and Wingfield,1991]. They are not particular to marine settings, but do occur on and beneath the present continental shelves. The southern North Sea contains large examples, locally up to 500 m in relief, formed during the mid-Pleistocene Elsterian glaciation [Cameron et al., 1987]. A 3D-seismic volume across a 660 km2 portion of a wider study area (Fig. 1) provides information on their form and fill character (Figs. 2–4). The results illustrate the utility of high spatial resolution for Quaternary morpho-stratigraphical analyses.