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The lower part of the Mount Messenger Formation, which crops out along the North Taranaki coast (North Island, New Zealand), is widely regarded to display classic examples of exhumed basin floor fan strata. This interpretation is supported by numerous publications on the sedimentary facies and depositional environments inferred for these strata, which are exposed in coastal cliff exposures about 20 m high with limited lateral extent. Consequently, for almost three decades, this succession has served as an analogue and training ground for geoscientists investigating the features of basin floor fan deposits and their reservoir characteristics. Here, we bring together seismic stratigraphy and seismic geomorphology of 2D and 3D seismic reflection datasets, as well as photogrammetric models based on images collected using uncrewed aerial vehicle (UAV) drones from selected locations along the outcrop belt to challenge this paradigm. We analyzed seismic reflection data in an area immediately offshore and down-dip of the strata exposed in the coastal outcrop and found no objective criteria for submarine fan depositional elements based on accepted seismic stratigraphic criteria. Furthermore, mapping of seismic reflectors revealed large submarine channel systems (up to 3 km wide and up to 200 m deep) within the lower part of the Mount Messenger Formation. Submarine channel depositional elements (less than 250 m wide and 25 m thick) are also observed to incise through the outcrops, but they are generally below seismic resolution. In the context of the regional geology, we conclude that the lower Mount Messenger Formation accumulated as continental slope sediments cross-cut by channels rather than as basin floor fans. This study issues a cautionary tale about applying facies analysis to outcrop with limited 3D exposure to interpret sedimentary environments. The spatial distribution of facies and architectural elements viewed in seismic data are invaluable for testing the veracity of facies interpretations. • Facies interpretations of lower Mount Messenger Formation outcrops on the North Taranaki coastal section are challenged. • Combined seismic stratigraphy and seismic geomorphology of seismic reflection data with photogrammetric models of outcrop. • No objective seismic stratal patterns of submarine fans were observed. Instead, seismic mapping reveals channelform dominance. • Photogrammetric outcrop models are used to identify channel-element margins and bedset parameters. • Lower Mount Messenger Formation was deposited on a continental slope. [ABSTRACT FROM AUTHOR]

This paper presents a detailed description of the lithostratigraphy of mostly contouritic deposits from the Piedra Buena Terrace (PBT), collected at water depth of 2–2.5 km. It is based upon a multi-proxy investigation of five gravity cores, including visual description and scanning, grain size distributions, microfossils and sand fraction study, supported by the analysis of high-resolution sub-bottom profiles. Two older seismic units were identified in addition to those previously described in the area. The defined lithostratigraphic units were correlated to the upper seismic units, while the age of the younger units was confirmed using AMS-14C dating. In terms of the sedimentary facies, the dark quartz-glauconitic sands, which seem to be a fingerprint of the Piedra Buena Terrace, were investigated by applying the X-ray diffraction and performing grain SEM - SE images. The sandy facies correspond to sea-level low-stands during cold glacials (MIS 2 and MIS 4), when glauconite and terrigenous material were transported from the emerged shelf down and along the Patagonian continental slope by vigorous bottom currents. The latter is confirmed by an occurrence of parts of bi-gradational sequences reported from the grain-size distributions in lithostratigraphic units 2 and 4. Units 1 and 3, represented by mostly hemipelagic calcareous ooze, were formed during warm high sea-level stages (MIS 1 and 3). Unlike Unit 1, Unit 3 also demonstrates clear contourite features, similar to Units 2 and 4. They include common bioturbation, occurrence of mottles, lenses, irregular thin layers of sand, erosional boundaries and hiatuses between and within the units, good sorting of fine sands, and microfossil reworking. Together with our previous data from canyons on the Perito Moreno Terrace and the Ameghino segment, supported by available publications from the PBT and Mar del Plata areas, this study demonstrates that the Patagonian Continental Margin could be used as a natural laboratory to investigate and better understand contourite systems. • Massive quartz-glauconite sands discovered on the PBT. • Detailed litho- and chronostratigraphy provided for ∼70 ka. • Contourite sediments identified between ∼70 and ∼14ka. • More developped glauconitization phase during glacials. • New seismic units and unconformities identified in the PBT. [ABSTRACT FROM AUTHOR]

Magmatism and crustal structural characteristics are well-known phenomena that support understanding the marginal sea's tectonic evolution. The South China Sea (SCS) has become a natural laboratory for researchers to investigate the evolution of the marginal sea because of its complex tectonic background and multiple phases of magmatic activities. However, due to limitations in resolution and depth of observations, the evolutionary process of the SCS is still controversial. We use the latest processed multi-channel seismic reflection profile Line AB to evaluate the sedimentary sequences, crustal tectonics, and the Cenozoic magmatism in the southern SCS. The gravity modeling results indicate a reduction in crustal thickness from 18 km in the proximal domain to 7 km in the Continent-Ocean Transition (COT). The COT is roughly 35 km wide, representing a rapid break up process of the SCS. A high-velocity layer (HVL) of about 3 km thickness was formed in the continental slope crust, interpreted as post-spreading magmatic underplating. In addition, four sedimentary sequences, post-spreading magmatic intrusion, and the possible magma conduits were identified in the profile. Combining with previous ocean bottom seismometer (OBS) data, multi-channel seismic data, fault system data, and petrologic observations, we indicate the deep mantle magmatic material upwelled to the crust and further intruded into sediments along the hyper-extended crust or detachment faults during the post-spreading stage. We contend that cooling and heat shrinking within the SCS oceanic lithosphere have the potential to initiate decompression melt deformation in the continental slope. In summary, we construct a model of the origin and migration mechanism of extensive post-spreading magmatism in the SCS. Our research reveals the Cenozoic evolution and the HVL of the southern SCS, and provides a possible genesis for the anomalously young magmatism observed at the continental margin. • A high-velocity layer in lower crust is observed in southern South China Sea. • A narrow COT (∼35 km) was found in the southern continental margin. • The post-spreading magmatic underplating formed the HVL. • The extensional crust and detachment faults promote the post-spreading magmatism. • The cooling and heat shrinking of oceanic lithosphere reactivated the faults. [ABSTRACT FROM AUTHOR]

The frontal emplacement of the submarine and sublacustrine mass-transport complexes (MTCs) are subdivided into the frontally emergent and confined types. However, it remains unclear what factors control the variability of the frontal emplacement of submarine MTCs. High-resolution 2D and 3D seismic data are utilized to investigate the variability of four MTCs_1–4. The results show that (1) Four MTCs_1–4 herein in the lower slope are subdivided into frontally emergent MTCs_1 and 2 and frontally confined MTCs_3 and 4 (2) The magnitude (e.g., length, width, area and volume); of (frontally) emergent MTCs_1 and 2 is larger than that of confined MTCs_3 and 4; (3) Their flow directions and sources are variable; (4) Height drop (HD) and depth of the basal shear surface (DB) of emergent and confined MTCs_1–4 are greatly variable. Their HDs and DBs as controlling factors present/determine their gravitational potential energy, and HD/DB ratio controls their emplacement type (frontal emergence or confinement). High gravitational potential energy (mainly determined by high HD) of the sliding mass enables landslides to ramp out on the coeval seafloor, resulting in the creation of the gentle and/or moderate fontal ramps. Low potential energy (mainly presented by large DB) of the sliding mass prevents landslides from overrunning the undeformed downslope strata, forming the steep frontal ramps. Scales of sedimentation accumulation rate, slope gradient and earthquakes as possible triggering factors affect variability of MTCs_1–4. The sources of emergent MTCs_1 and 2 are from the continental margin slope. Confined MTC_3 comes from the upper slope of the northern flank of the Xisha uplift and confined MTC_4 derives from a point source (local lower slope). The variability of their gravitational potential energy, mainly depending on variable HDs and DBs, predominantly determines the emplacement variability of submarine MTCs and eventually affects their variable evolutions. This study provides new perspectives and enriches the repository of frontally confined and emergent (submarine) MTCs. • Frontally confined and emergent MTCs are studied using 2D and 3D seismic data. • Frontally emergent and confined MTCs are of difference in flow direction and sources. • HD and DB ratio can separate frontally emergent (high ratio) from confined (low ratio) MTCs. • Scales of sediment accumulation rate, slope gradient and earthquake can affect the scale-variability of MTCs. [ABSTRACT FROM AUTHOR]

Fault activities in sea areas always produce devastative marine geohazards. For marine geohazard assessment, it is important to know the location, geometric structure, and activity regularity of active faults. In this study, high-resolution multi-channel seismic and topographic data are used to investigate the exact location, geometric structure and latest activity of the Continental Slope Fault Zone (CSFZ) in the northern South China Sea. The results show that the Qiongdongnan (QDN) segment of the CSFZ is a 194 km long NEE-trending fault zone that developed near the transition of continental shelf and slope. In its geometric structure, the CSFZ (QDN segment) cuts through the Cenozoic basement and breaks upward to near the seafloor with high-angle dip. According to whether it ruptures to the seafloor, it can be divided into eastern and western segments. In the western segment, the fault scarps about 8∼9 m high can clearly be seen on the seafloor. Whereas in the eastern segment, the fault shows a blind characteristic below the seafloor. The analysis of fault throws and expansion indices show that CSFZ (QDN segment) have remained active since the late Pleistocene. Due to the CSFZ is an active fault and geographically adjacent steep slope, which may cause large interpolate landslides, and then leading to destructive tsunamis. As a result, the CSFZ has great seismogenic potential and may produce marine geohazard chain. Thus, it is of great significance to the assessment of marine geohazard chain posed by the CSFZ for improving hazard mitigation. • The exact location, geometric structure and latest activity of the Continental Slope Fault Zone are investigated. • The Continental Slope Fault Zone has been active since the Late Pleistocene. • The Continental Slope Fault Zone has great seismogenic potential and might trigger marine geohazard. [ABSTRACT FROM AUTHOR]

This work presents the first known application of a new approach to tsunami modelling, by linking a discrete cellular automata (CA) dynamic model of a submarine landslide to an off-the-shelf tsunami modelling package. This has the advantage of simulating the temporal evolution of the submarine landslide rather than as a single rigid sliding mass. We first tested that the coupled model was able to satisfactorily reproduce observed tsunami wave heights resulting from the flank collapse of the Anak Krakatau volcano in the Sunda Strait (Indonesia) in December 2018, before applying it to a speculative area of interest on the North Borneo (Continental) Shelf (NBS). The NBS has one of the thickest near-shore clastic sediment wedges known globally (12 km thickness) and a very long pre-history of submarine landsliding. We modelled a small slide (5 km3 volume) because this is far more likely to occur than a recurrence of the gigantic Brunei slide (1200 km3 volume). Results indicate that a 5 km3 submarine landslide generates multiple tsunami waves, the largest of which reached 8 m at the coast of nearby Balabac Island (The Philippines). Small tsunami waves also arrived on the central Vietnam coast, more than 900 km distant. A larger potential failure on the NBS therefore appears capable of causing a tsunami with greater runup heights, which could pose a risk to much of central and southern Vietnam, northern Borneo (Sabah, Brunei) and Palawan. Finally, we note that the South China Sea is fringed by a necklace of other thick offshore clastic sediment accumulations, most notably on the southern China continental margin, offshore southern Taiwan, and offshore central Vietnam. Given the proof-of-concept of our coupled CA–tsunami modelling approach described here, we recommend this also be applied in these areas to assess the risk posed by a wide range of submarine landslides to coastal populations and infrastructural assets (including nuclear power plants). • This work is a proof-of-concept that cellular automata (CA) can be used to simulate a tsunamigenic submarine landslide. • The CA method for landslide simulation can be coupled with a tsunami generation model. • A small (5 km3) simulated landslide on the Northwest Borneo Shelf produces locally dangerous tsunami waves. • Potential failures on continental slopes surrounding the South China Sea pose significant tsunami risk. [ABSTRACT FROM AUTHOR]

The purpose of this paper is (i) to re-assess the presence of submarine fan systems of Middle and Late Miocene age (Moki and Mount Messenger formations) previously mapped on the continental slope in offshore southern and central Taranaki Basin, New Zealand, and (ii), to demonstrate the occurrence of two newly mapped Late Miocene submarine fans in the deep-water part of offshore western Taranaki Basin, based on accepted seismic stratigraphic criteria for submarine fans. In our work we have found that the fans previously mapped amongst canyons and channels in the Moki and Mount Messenger formations on the upper and middle continental slope in southern and central Taranaki Basin do not fulfil established stratal criteria for submarine fan deposits. Moreover, detuning of 3D seismic reflection volumes for the Moki and Mount Messenger formation intervals ahead of amplitude attribute extraction, shows that the strata previously mapped as submarine fans based on amplitude alone, are merely artifacts of bed thickness. The two new fans systems of Late Miocene age that we map here, both attributed to the Mount Messenger Formation, principally comprise mudstone and represent a minor proportion of the Late Miocene succession that accumulated in southern and central parts of the basin. Our remapping of slope canyons and channels in the southern part of the basin and the identification of the two new fan systems in the western part of the basin are embodied in three new paloegeographic maps for the Middle and Late Miocene development of Taranaki Basin. A modern analogue for the two new Late Miocene fans in Taranaki Basin is the Hikurangi Fan at the terminus of the Hikurangi Channel, located outboard of the modern subduction zone of the Hikurangi margin along Eastern North Island. The new Taranaki Basin deep-water fans documented here, may have significance for hydrocarbon exploration in deep water parts of Taranaki Basin, which is largely unexplored. • New seismic geomorphologic mapping of the upper Moki and lower Mount Messenger formations in Taranaki Basin, New Zealand. • Critical reassessment of previously published submarine fan systems in southern and central Taranaki Basin and their dismissal on standard seismic stratigraphic criteria. • Seismic mapping of two new submarine fan systems in western Taranaki Basin. • Updated Taranaki Basin palaeogeographic maps for ∼14 Ma, ∼12 Ma and ∼6 Ma. [ABSTRACT FROM AUTHOR]

The continental shelf and slope, with upwelling systems, are usually accompanied by the formation of oxygen minimum zones, thereby stimulating marine paleoproductivity and promoting organic matter (OM) enrichment. However, due to a more oxygen-enriched environment, a distinct paleoecological characteristic, and the uncertainty of upwelling effects, it remains ambiguous whether a similar mechanism of OM preservation occurs in the adjacent carbonate platform. Here, limestone–marl alternations (LMAs) that developed in the Yangtze Carbonate Platform (YCP) during the Middle Permian were selected as the research object and compared with the chert–mudstone alternations developed on the Lower Yangtze continental shelf (LYCS). Both sets of rhythmites belong to the contemporaneous products of the Permian Chert Event in South China. Considering that the Middle Permian rhythmites in South China contain the Milankovitch record and that upwelling was controlled by astronomically forced monsoons, this study investigated the impact of upwelling on the carbonate platform by adopting astronomical cycle research. Based on GR data, a framework of astronomical cycles was established. The suboxic to anoxic conditions of the YCP are supported by evidence from syngenetic pyrite framboids, Ce/Ce* ranges, and U EF –Mo EF covariation. Upwelling geochemical indicators and the detection of a pivotal (∼210-kyr) astronomical cycle regarding upwelling indicate that the YCP was affected by upwelling. The paleoproductivity/OM/fossil-related data exhibit this ∼210-kyr astronomical cycle, and the correspondence between the converted AMs determines that upwelling enhanced marine primary paleoproductivity and OM preservation. However, the intensities of short eccentricity cycles exceed those of the ∼210 kyr cycle in the time series analyses of data from the nearshore YCP. Additionally, the covariations between the upwelling proxy and total fossil content confirm the impact of upwelling on biomass of benthic organisms. Therefore, the (monsoon-driven) upwelling system in the continental shelf can extend to the carbonate platform. After reaching the platform, under the control of a (an) (astronomically forced) climatic–oceanic system, limited upwelling can stimulate primary productivity and marine OM preservation; however, upwelling reduces biomass in the bottom water. In the nearshore side of the platform, detrital dilution likely replaces upwelling as the primary stimulus for productivity. Moreover, this study reveals and explains the ∼200-kyr cycles in the Permian strata of South China for the first time. [Display omitted] • The ∼200-kyr cycles are first revealed and explained in the Permian strata of South China. • Limited upwelling increases productivity and OM preservation in the carbonate platform. • Upwelling promotes productivity and thus depletes oxygen, reducing benthic biomass in the platform. • A model for the effect of the upwelling systems along continental shelves on carbonate platforms is proposed. • Milankovitch cycles are the basis of the Maokou LMAs forming a set of source rocks. [ABSTRACT FROM AUTHOR]

In 2018, Guangzhou Marine Geological Survey conducted the fifth gas hydrate drilling expedition (GMGS5) in the northwestern continental slope of South China Sea. We use pore water geochemistry, pressure core CT scanning, degassing quantification, seismic data and in situ temperature test at four drill sites (W01, W07, W08 and W09) to investigate the gas hydrate characteristics and discuss the dynamics of the gas hydrate system. Based on the CT scanning and pressure core degassing, we demonstrate the coexistence of pore-filling and vein-like gas hydrates. Gas hydrate saturation estimated based on the chloride concentration (0–92%) is higher than that based on the degassing quantification (0–52.9%). The saturation discrepancy is mainly resulted from the sampling discrimination. Based on the average gas hydrate saturation estimated using chloride concentration and degassing, the methane gas volume contained in the gas hydrate of one cubic meter sediment at drill sites W07, W08 and W09 are calculated to be 13.7 m3, 26.82 m3 and 22.24 m3 and 3.03 m3, 14.30 m3 and 16.25 m3. The geothermal gradients estimated based on the in situ temperature measurements at sites W07, W08 and W09 (102–111 °C/km) are more than 50% higher than that at the background site W01 (65 °C/km). It is inferred that the fractures and faults in the gas chimney at sites W07, W08 and W09 are the major contributors for the efficient fluid pathways and high geothermal gradients. The pore water sulfate profiles and inferred sulfate-reduction methane-oxidation interface (SMI) depths indicate that methane flux is lowest at site W01 and highest at site W08. The SMI depths and top of gas hydrate occurrence is linearly correlated which provides an experienced function for understanding the local gas hydrate distribution. Kick-type sulfate profiles at sites W07 and W09 indicate an ongoing increase of methane flux and corresponding shoaling SMI depths. In addition, chloride enrichment in the shallow sediment at site W08 indicates a recent fast gas hydrate formation process. The kick-type sulfate profiles, positive chloride anomalies and authigenic carbonates at multiple depths suggest that the system experienced dynamic changes in fluid flux and multi-stage gas hydrate evolution on time scale from months to thousands of years. Understanding the controlling mechanism and periodicity of the composite pressure system is important for evaluating the gas hydrate resource and carbon cycling. • Gas hydrates were drilled for the first time in the Qiongdongnan Basin. • Pore-filling and vein-like gas hydrates co-exist in the cores. • Kick type sulfate profiles indicate the transit state of methane flux. • Positive pore water chloride anomalies indicate the recent fast gas hydrate formation. • Dynamic fluid flux and multi-stage gas hydrate evolution happened on time scale from months to thousands of years. [ABSTRACT FROM AUTHOR]

Numerous normal faults, expressing thin-skinned salt tectonics, are detected along the Levant continental slope, 15–30 km west of the Israeli coast. Previous studies estimated that this faulting initiated during Late Pliocene to Early Pleistocene, which leaves an uncertainty of about 50% in the total duration of active faulting. The purpose of this study is to improve the dating of the onset of faulting, and to reconstruct displacement rates with time. This will allow better understanding of salt tectonics as well as better assessment of geohazards. To increase dating resolution, we use 2D and 3D seismic material integrated with well logs and cuttings to combine bio- and seismo-stratigraphic data and to divide the Plio-Quaternary section offshore Israel to 4 depositional units: Pliocene (5.33–2.6 Ma); Gelasian (2.6–1.8 Ma); Calabrian-Ionian (1.8–0.35 Ma); and Ionian-Holocene (<0.35 Ma). Then, we measure displacement of dated horizons, reconstruct displacement history along faults, and identify syn-tectonic depositional units that thicken towards growth faults. Our analysis shows that during the Pliocene (duration of 2.7 my) faulting activity was minor. In the Gelasian (duration of 0.8 my) faulting activity peaked alongside huge slumping. In the past 1.8 my, faulting and slumping had both decreased, but they are still active today. The observation that the onset of faulting significantly predates the progradation of the shelf edge to its present location indicates that differential loading was probably not the first trigger that initiated salt tectonics offshore Israel. The association of intense faulting and slumping in the Gelasian is interpreted as a result of basinwards tilting due to regional inland uplift and basin subsidence. Yet positive feedback between faulting and slumping cannot be ruled out. The presence of fault scars on the present seafloor, where current sedimentation rate is similar and even faster than the average displacement rate, apparently indicates that faulting is dominated by sudden seismic ruptures rather than by continued creeping. Hence, considering sedimentation rate of about 50 m/my, a 1 m high fault scarp should be roughly younger than about 20 ky. [ABSTRACT FROM AUTHOR]

Pointer Ridge is a gas hydrate prospect on the South China Sea continental slope offshore SW Taiwan. It is characterized by densely distributed bottom simulating reflections (BSRs), active gas seepage, and potential sandy gas hydrate reservoirs. To understand how the fluids have migrated toward the seafloor, and the role of geological processes in the gas hydrate system, we have collected and analyzed high-quality 2D and 3D reflection seismic data. We first mapped the spatial distribution of the BSRs, and interpreted a major normal fault, Pointer Ridge Fault (PR Fault). The NE-SW trending fault dips to the east, and separates the erosional regime to the west from the depositional regime to the east. One active vent site was identified directly above the PR Fault, while another is located on a topographic high to the west of the fault. On the hanging block of the fault we found at least one major unconformity. The seismic data indicate refilled channels with coarser-grained sediments in the hanging wall of the normal fault. Seismic attribute analysis shows subsurface fluid conduits and potential gas hydrate reservoirs. We propose two types of gas chimneys, which are separated by the fault. Gas plumes derived from hydroacoustic data are mostly from the footwall block of the fault. We infer that fluid flow is more active in the erosional environment compared to the depositional one, and that this is the result of reduced overburden. The methane-bearing fluids migrate upward along the PR Fault and chimneys and form hydrates above the base of the gas hydrate stability zone. Based on seismic interpretation and seismic attribute analysis, we postulate that the channel infill constitutes the most promising hydrate reservoirs in this geological setting. In the surveyed area of Pointer Ridge these channels occur mainly below the gas hydrate stability zone. • Present original 2D/3D seismic data to reveal the geologic setting of a potential gas hydrate prospect off SW Taiwan. • Active fluid flow processes are studied by analyzing water column and seismic data. • A conceptual model is proposed for the gas hydrate system of Pointer Ridge by detailed seismic attribute analysis. • Potential gas hydrate reservoirs that might be targets for future exploration are identified. [ABSTRACT FROM AUTHOR]

Combined seismic images and core data from shallow gas-charged, deep-water sands in the western Bay of Bengal provide a detailed record of channel formation and evolution. The KG-D6 Block, western Bay of Bengal, includes two main Pliocene deep-water channel complex sets, termed the A and B complex sets, currently under gas production by Reliance Industries. These represent sinuous channels developed at the base of the delta slope off of the Godavari River and at the head of a long continental slope into the deep Bay of Bengal. The A complex set includes two main channel complexes, the older A150 and younger A100 channel complexes. The A complex provides an excellent opportunity to explore the geometry, sedimentation, and evolution of a small submarine channel system because of shallow reservoir depth, excellent 3D seismic coverage, availability of cores, and the nearly complete system-wide gas charge of the sandstones. Relict sea-floor topography and the processes of avulsion and splay development figure importantly in the overall evolution of slope channel systems. In the KG-D6 area, a sand-rich avulsion splay derived from an older channel complex became the template upon which the A channel complex set evolved. The component A150 and A100 channel complexes developed along a single sinuous course, with the channel complexes building through coupled channel erosion and coeval levee construction. The Godavari slope channels are not confined by scalloped valley walls and did not migrate laterally during their evolution. Mass transport deposits (MTDs) occur in all parts of the channel-fill section. The A100 channel complex displays a well-developed fining-upward cycle in most sections apparently not reflecting shifts in channel position or meander cuts offs, as inferred for some other deep-water channel systems, but rather through changes in the activity within the channels and through advances and retreats of frontal splays up and down the channel system. The Godavari data demonstrate the importance of avulsion and splay development in the evolution of deep-water sedimentary deposits, even high on passive margin slopes, which has implications for understanding the evolution of sinuous submarine channels and the development and distribution of hydrocarbon reservoir compartments. • An integrated dataset from the Godavari channel system presents a rare opportunity to study the sedimentology of deep-water splays. • Relict seafloor topography and processes of avulsions and splays figure importantly in the evolution of slope channel systems. • Avulsion and splay development influences aggradation of deep-marine sediments even high on passive margin slopes. [ABSTRACT FROM AUTHOR]

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. • New age models allow to clarify the Neogene history of the Amazon shelfal carbonates. • Differential subsidence strongly controls shelf architecture between ca. 24–3,7Ma • Higher siliciclastic influx suppressed the Central and NW shelf carbonates at ca. 8Ma • Carbonate production locally persisted on the NW Amazon shelf until ca. 3.7Ma • Reduction of regional subsidence rates led to the death of shelfal carbonates. [ABSTRACT FROM AUTHOR]

A submarine channel network, named Abalama Channel System (ACS), has been recognised in the subsurface of the Niger Delta continental slope. It overlies a mass-transport complex (MTC) and consists of six channel segments, delimited by five avulsion points and one confluence point. High-resolution 3D seismic data are used to investigate the development of the ACS and to describe the interaction between the channels and the underlying MTC. The MTC mainly consists of highly disaggregated materials (MTC matrixes) and in plan-view has a very complex fingered geometry, characterised by the presence of erosional remnants (remnant blocks). The different character of the MTC matrixes compared to that of the remnant blocks likely resulted in a bathymetry characterised by negative and positive relief, which provided the initial confinement for the channels of the ACS. In areas where the MTC-induced confinement was weak or decreased abruptly, channels tended to develop higher sinuosity, increasing channels instability and ultimately causing avulsions. Three ideal categories of submarine channel avulsions are observed. Type 1 is characterised by parent and avulsion channel having similar size and maturity; Type 2 is characterised by a large, high-maturity parent channel and a small, low-maturity avulsion channel; Type 3 emphasizes the larger scale and higher maturity of the avulsion channel compared to the parent channel. In the distal part of the study area, topography related to mud diapirs provided lateral confinement that captured flows avulsed at different times resulting in a channel confluence phenomenon. Submarine channel network evolution recorded by avulsion and confluence points represents an important research theme in deep-water sedimentology, as it controls the final distribution of sediments and the extension of sands in the whole deep-water depositional system; hence this study can be used to guide hydrocarbon exploration in analogue systems. • A channel system consisting of six avulsion channels is analysed offshore Nigeria. • MTC-related confinement controls the development of avulsion channels. • Three ideal categories of submarine channel avulsions are observed. • Topography related to mud diapirs provided lateral confinement to capture channel flows. [ABSTRACT FROM AUTHOR]

Most sequence stratigraphy models have been derived from the observation of seismic profiles, which still retain the original form of the sedimentary stage. For folded and fractured formations, there is no way to establish a full view of the basin-wide continuous stratal configuration. This paper presents an approach to restore the sedimentary sequence original structural profile through a process of segmented line-drawing of seismic events, reassembly and modification. We use a case study from the Pearl River Mouth Basin, in the South China Sea, to demonstrate the principle and method of this technology. After restoration, we obtained a basin-wide, sedimentary stage form and continuous sequence stratigraphic structure profiles, and thus further investigated the sedimentary landforms, palaeo-water depth, sedimentary systems, stacking pattern, stratal configuration, parasequence sets and systems tract, which may be ambiguous in conventional seismic profiles. This approach may be applied to the study of basin and sequence stratigraphy and basin sedimentary process. • A new technique for restoring the sedimentary sequence original form seismic profile by the method of line-drawing. • Remove faults and folds, a complete sequence stratigraphic profile of the entire basin can be displayed on A4 paper. • The original form profile can be used to determine the genetic unit (systems tract) and sedimentary environment. • To estimate the water depth and determine the source rock (condensed section) buried in deep depth. [ABSTRACT FROM AUTHOR]

Abstract This work reassesses two contrasting Late Miocene-Holocene stratigraphic frameworks for the Pearl River Mouth Basin (PRMB), northern South China Sea. Two distinct stratigraphic frameworks based on seismic-stratigraphic (SFI) and biostratigraphic (SFII) data are compared in terms of their applicability and accuracy. In order to stress the differences between both frameworks, major stratigraphic boundaries (T1-base Quaternary and T2-base Pliocene) derived from SFI and SFII were revisited and described. Sedimentation rates estimated for the two frameworks, complemented with published data, indicate that average sedimentation rates and trends for SFI match the regional reference data for Quaternary strata. In terms of well-seismic ties, two allochthonous sedimentary units with similar lithology and grain sizes were identified as comprising fine-grained turbidites at the bottom, and fine-grained sediment failures at the top, revealing widespread mass wasting on the middle to lower continental slope. In such a setting, nannofossil assemblages accumulated on the continental slope were likely reworked because of their relatively minute sizes. Sediment cores with low recovery rates collected during a first hydrate expedition in 2007 have led to important misunderstandings when defining the first and last appearances for nannofossil species. Hence, stratigraphic framework I (SFI) - based on a combination of seismic and sequence stratigraphic data - is suggested as more suitable to attain stratigraphic correlations across the continental slope of the PRMB. Our findings can resolve the ambiguities and uncertainties arising from the two contrasting stratigraphic frameworks, providing a robust foundation to reassess the tectono-sedimentary evolution of the PRMB, South China Sea. Highlights • Two stratigraphic frameworks for the Pearl River Mouth Basin are reassessed. • Submarine channels and MTDs are valid Late Miocene-Holocene stratigraphic markers. • Stratigraphic frameworks need to combine seismic and sequence stratigraphic data. [ABSTRACT FROM AUTHOR]

The early Paleozoic witnessed two spectacular radiations of marine organisms—the Cambrian (Є) Explosion and the Great Ordovician (O) Biodiversification Event. However, the period between them was characterized by a marine biodiversity plateau attributed to recurring biocrises. In the current study, we present the I/Ca records from slope lime mudstones of the Green Point Formation in western Newfoundland, along with a three-sink U-isotope mass balance model and previously reported limestone δ238U carb signals of the same interval, to further explore oceanic redox conditions at the Є-O boundary. The I/Ca ratios of the lime mudstones, deposited along the eastern Laurentian (western Iapetan) continental slope, exhibit a narrow range between 0.02 and 0.33 μmol/mol. Poor correlations between the I/Ca values and their [Sr], δ18O, Mn/Sr, Fe/Sr, Mg/Ca, and δ13C org counterparts, together with near-micritic textures of the limestones, argue against significant influences of postdepositional alterations on the I/Ca signatures. The iodine-depleted lime mudstones, with I/Ca values well below the Proterozoic Eon baseline I/Ca ratios (∼0.5–1 μmol/mol), suggest the presence of shallow marine oxic-anoxic interfaces along the regional continental margin. Substantially low limestone I/Ca ratios (<0.5 μmol/mol) and dysoxic to anoxic depositional conditions have also been reported from several other age-equivalent sections deposited along the shelf and slope of ancient Iapetan and Laurentian continental margins. As a result, seawaters surrounding the Iapetan and Laurentian continental margins at the Є-O transition might have been commonly poorly oxygenated with shallow oxyclines or expanded oxygen minimum zones in the shelf and/or slope areas. This interpretation is further supported by our three-sink U-isotopemass balance modeling, which predicts widespread marine anoxia at the Є-O boundary with anoxic to euxinic water covering 1.0–21.1% of the ocean floor, significantly higher than the modern day (∼0.2%). Furthermore, the wide range of the oceanic δ238U OC (−1.22 to −0.25‰) values, estimated from the limestone δ238U carb signals, might reflect oceanic redox oscillations during this period. However, the δ238U OC fluctuations could also be attributed to variable accumulations of the 238U-enriched authigenic U phases during early diagenesis. Overall, evidence from the lime mudstone I/Ca ratios and the estimated extent of marine anoxia at the Є-O boundary in this study aligns with earlier viewpoints that the slowness of marine biodiversity accumulation during the late Cambrian and the Early Ordovician was linked to widespread oceanic anoxia. • Lime mudstone I/Ca signatures across the Є-O GSSP boundary. • Shallow marine oxyclines along the Iapetan and Laurentian continental margins. • Expanded marine anoxia predicted by a three-sink U-isotope mass balance model. [ABSTRACT FROM AUTHOR]

Multi-well pressure data from the Magnolia Field, located on a flank of the salt-bounded Titan passive mini-basin in the Garden Banks area of the continental slope of the Gulf of Mexico, indicate remarkably high overpressures that vary, at similar depths, by up to 10 MPa between sand bodies 1 km apart. In the present paper, we integrate geological and geophysical analysis with 2D forward hydro-mechanical evolutionary modelling to assess the contribution of both disequilibrium compaction and diapir-related tectonic loading to the observed overpressure and to understand controls on pressure compartmentalisation. The 2D finite element evolutionary model captured the sedimentation of isolated sand channels bounded by mud-dominated sediments close to a rising salt wall which led to tectonic loading on sediments. Comparison of results from the 2D and 1D models shows that disequilibrium compaction can explain most of the overpressure as a result of very rapid deposition of mainly mud-rich, low permeability sediments; tectonic loading contributes around 7% of the observed overpressure. The models also show that linked to the high sedimentation rates, small variations in the permeability and connectivity of the mud-rich sections that bound the channel sands result in highly compartmentalised pressure distributions in adjacent sand bodies. • Evolutionary hydromechanical simulation of a GoM mini-basin. • Prediction of structure, strain, stress, and overpressure histories. • Quantification of disequilibrium compaction vs tectonically induced overpressure. • Modelling of overpressure compartmentalisation in isolated sand channels. [ABSTRACT FROM AUTHOR]

Submarine fans are ubiquitous in modern deep-water environments. Such systems reported in the literature are largely fed by shelf-indented canyons receiving ample fluvial or longshore drift sediment inputs, whereas those nourished by slope-confined canyons far away from terrigenous sediment sources remain poorly understood. Herein, we report such a system, the Yitong submarine canyons – fan apron system (YCFS), from the northern South China Sea. This system was studied using multibeam bathymetric and two-dimensional multichannel seismic data, combined with geological constraints from nearby International Ocean Discovery Program drilling sites. YCFS consists of approximately 12 submarine canyons confined in the lower continental slope of >1200 m water depth, which feed a fan apron in the connecting abyssal plain. The canyons are erosion-dominated, as evidenced by their V-shaped cross-sections with up to 708 m deep incisions. Canyon infillings comprise mass transport deposits and turbidites, and interfluves exhibit features such as landslides, creep-typed sediment waves, and gullies, indicating the canyons and interfluves are dominated by mass transport and turbidity-current processes. The fan apron is generally depositional, in which channels and channel-levee complexes, turbidite lobes, and mass transport deposits comprise the main depositional elements. In the canyon-to-fan transitions, supercritical turbidity current features and bedforms, including plunge pools and connected sediment mounds, cyclic steps, and antidunes, are developed, which are linked to the deceleration and hydraulic-jump effects of the high-velocity turbidity currents shedding off the canyons. Seismic stratigraphic analysis revealed that YCFS was initiated in Late Miocene. The formation and maintenance of YCFS are linked to the long-term existence of an upslope margin-parallel basement high. The latter could have maintained a steep slope condition (up to 6.5°) that favors the development of canyons and supercritical turbidity currents. Because of the blocking effect of the basement high , YCFS receives limited supply of the shallow-water sediments. • A deep-sea fan apron fed by slope-confined canyons lacks ample land-based supply. • Developed supercritical-flow bedforms indicate high-energy fan apron environments. • Tectonism plays a critical role in the development of the system. [ABSTRACT FROM AUTHOR]

Submarine fans in tectonically active continental-slope basins are targets of petroleum exploration and production. These slope fans commonly comprise compensationally stacked sandy and muddy architectural elements, including mass-transport deposits, weakly confined to distributary channel-and-lobe deposits, and leveed-channel deposits. The lateral continuity and vertical connectivity of these architectural elements are important uncertainties in reservoir characterization that influence fluid-flow behavior during hydrocarbon production. Here, we use a simple forward stratigraphic model to simulate the stratigraphic patterns and illuminate the likely distribution of fine-scale, sub-seismic heterogeneity in a slope fan. We used published seismic-reflection horizons from the tectonically active Columbus basin, offshore Trinidad, to define the top and base of a Pleistocene submarine fan. We then simulated the stratigraphic evolution of the slope fan with a series of DionisosFlow™ forward stratigraphic models. All variables were kept constant during the simulations in order to test the hypothesis that the autogenic evolution of the surface topography alone, as a result of erosion and deposition, can produce compensational-stacking patterns common in submarine fans. A reference-case model is similar to the thickness trend of published isochron maps of the Trinidad slope fan. The reference-case model also produced patterns of compensational stacking. Varying the time step impacts the heterogeneity of the model. Shorter time steps are characterized by less sediment accumulation, which results in less sediment diversion during the subsequent time step, more gradual migration of channel deposits, shorter offset distances of depocenters, and shorter length-scale heterogeneity compared to longer time steps. Thus, a key characteristic of slope-fan deposits is autogenic compensational stacking, without any external forcing, which governs heterogeneity in these reservoirs. Furthermore, our results suggest that relatively simple diffusion-based models can produce realistic compensation patterns and future work will be focused on higher-resolution model calibration to seismic-reflection data and the influence of input variables on heterogeneity of channel-and-lobe deposits of slope fans. [ABSTRACT FROM AUTHOR]

Continental transform faults accommodate lateral motion between two adjacent plates. Often, evidence for such motion is hard to detect due to onland processes of erosion. Investigation of a submerged passive continental margin adjacent to a transform plate boundary might provide indications for the developmental stages of the transform. To examine this hypothesis, we analyzed the Messinian-to-recent stratigraphy and structure of the Levant continental margin that resides ca. 100 km from the Dead Sea fault (DSF) continental transform. The analysis is based on interpretation of seismic reflection data (2D and 3D time migrated, and pre-stack 3D depth migrated) and four boreholes. It includes construction of structural maps, tracking of stacking patterns and calculations of sedimentation rates. Data show that the Levant margin developed through three phases: (1) Late Miocene-Earliest Zanclean (∼6-∼5 Ma) NNE-SSW left lateral strike-slip faulting (2) Zanclean-Late Gelasian (∼5–1.8 Ma) syn-depositional folding striking in the same direction. The top Gelasian surface marks an abrupt change, which is expressed as (3) mass slumping that occurred due to regional basinward tilt. Continuation of differential subsidence is manifested by Calabrian-to-recent progradation (1.8–0 Ma). While Nile sediment flux peaked at 1.8 Ma and decreased during the Calabrian-to-recent, our data shows an increase in sedimentation on the Levant margin throughout the entire Plio-Pleistocene. This discrepancy is caused by the regional tilting of phase 3, which shifted the depocenter to the west. Correlations to onshore findings suggest that the faulting, folding and tilting observed on the continental margin represent localization of strain towards the DSF during the Pliocene-Gelasian (5.3–1.8 Ma). This is attributed to the consequent deepening of the DSF and the incipient collision of the Eratosthenes Seamount with the Cyprus Arc. The continental margin records the development of the nearby active transform through structural and sedimentary modifications. [ABSTRACT FROM AUTHOR]

The seismic characteristics of focused fluid-related features such as pockmarks and carbonate build-ups on the Nigerian continental slope have been investigated using complementary seismic data sets (reprocessed 3D exploration data and 2D Very High Resolution hull-mounted and near-bottom seismic data) coupled with the results of previous studies (sedimentological, geotechnical and geochemical analyses). The results show different types of fluid-related features within the hemipelagic phase of long duration (∼1 Myr) Turbidite/Hemipelagic cycles. They are the product of the disturbance of the hosting sediments following two main distinct processes: (1) no or low sediment deposition caused by fluid escape associated with seafloor settlement (benthic fauna, carbonate precipitation, gas hydrate formation) and (2) post-depositional erosion, caused either by fluid release (dewatering from underlying features such as channels and upward migration from a deep source) or by collapse (destabilisation of gas hydrates). The temporal evolution reveals a decrease in fluid flow during the hemipelagic deposition as witnessed by the decrease in methanogenic activity and by gas hydrate destabilisation. The spatial distribution of fluid-related features seems to be controlled by the type of fault (small fault or structural lineament). [ABSTRACT FROM AUTHOR]

This study describes and characterises the spatial distribution and evolution of Carboniferous deep-water sediment gravity flows and their associated deposits in the West Junggar region of north-western China. It uses a combination of outcrop sections, cores and seismic profiles to define nine lithofacies associated with a different sediment gravity flow types, including turbidity currents, debris flows, hyperpycnal flows and supercritical flows. The subduction and closure of the remnant West Junggar Ocean basin demonstrated variable palaeogeographical characteristics and configurations during different periods of the Carboniferous. Fossil assemblages and zircon dating data indicate that the target strata have completely recorded the evolution of the Western Junggar remnant ocean in the Carboniferous. The basin evolution is divided into the stable ocean basin stage, the slow subduction and filling stage and the fast subduction and filling stage. Active faulting and varying slope gradients affected the occurrence and types of sediment gravity flows during each stage. Tectonic and seismic activities associated with subduction caused continental slope instabilities and basinward transport of sediment by gravity flows with active faults on the continental slope triggering sediment gravity flows. At the beginning of subduction, the ability of gravity flows to transport coarse debris was limited owing to the shallow gradient of the continental slope. Sediment transport via gravity flows was dominated by sandy debris flows, muddy debris flows, turbidity currents and less frequent flood-controll3ed hyperpycnal flows. As the basin entered the fast subduction and filling stage, the slope gradient steepened and the frequency of active thrust faulting increased, which triggered high-density turbidity currents and supercritical flows. Additionally, the warm and humid paleoclimate led to the development of hyperpycnal flows during the fast-filling stage. This study demonstrates the importance of understanding the character and distribution of sediment gravity flows and associated deposits, especially, the implications for the hydrocarbon industry. • The evolution of West Junggar remnant ocean basin is divided into three stages. • Sediment gravity flow deposits formed during the subduction and closure of the remnant ocean basin. • Slope gradient and thrust faults were key controls on types of gravity flow type and resultant deposit character. • In china, the ancient deep water gravity flow reservoirs are discovered in Junggar Basin. [ABSTRACT FROM AUTHOR]

Free gas is an important trigger of instability on continental slopes, and resulting mass-wasting strata can potentially form competent seals to hydrocarbon accumulations. This work uses two high-quality 3D seismic volumes to investigate fluid accumulations at the base of mass-transport deposits in the Pearl River Mouth Basin, South China Sea. In parallel, IODP/ODP borehole data are used to document the petrophysical character of mass-transport deposits formed in similar continental-slope environments to the South China Sea. The interpreted data show gas accumulations as comprising enhanced seismic reflections that are discordant, or vertically stacked, below mass-transport deposits with chaotic seismic facies. Gas was accumulated in basal shear zones of mass-transport deposits in response to differences in capillary pressure and porosity. Free gas in Zone A covers an area of at least 18 km 2 . In Zone B, the free gas is sub-circular in plan view and covers an area of 30.58 km 2 for a volume of sediment approaching 1.5 km 3 . This work is important as it shows that vertical migration of gas is not significant in mass-transport deposits from the Pearl River Mouth Basin, but up-dip migration along their basal shear zones is suggested in multiple locations. As a result, free gas can pinch-out laterally to extend 1–2 km beyond these same basal shear zones. As a corollary, we show that free gas accumulations below mass-transport deposits comprise an important geohazard and should be taken into account when drilling continental-slope successions in both the South China Sea and continental margins recording important mass wasting. Strata charged with free gas form weak layers, hinting at a novel trigger of retrogressive slope failures on continental slopes worldwide. [ABSTRACT FROM AUTHOR]

Since early Pliocene, a counterclockwise surface gyre transported Nile derived silt and clay northeastwards along the Levant coast, where a basinward prograding plastered drift emerged. Based on high-resolution seismic reflection data we develop a middle to late Pleistocene sequence stratigraphic scheme for this plastered drift. For creating stacked sections of the seismic data we used the common reflection surface (CRS) stack technology which enhanced lateral reflection continuity and visibility of deep reflections. The shelf comprises stratigraphic sequences which show classical, systems tract like stacking patterns of sea level controlled sequences such as offlapping forced regression deposits or diachronous ravinement surfaces which formed during base level rise. On the slope, base level was periodically located well below the wave base and thus rather controlled by hydrodynamics, presumably by high-velocity contour currents. Hence, the term ‘ deep base level’ is introduced. The deep base level controlled especially down and backstepping slope deposits. This example shows that care has to be taken when interpreting subsurface data containing typical systems tract like seismic sequences, since such geometries do not necessarily imply shallow water deposition of the sediments. A chronostratigraphic analysis based on the seismic stratigraphy indicates that base level fluctuations were related to eccentricity driven glacio-eustatic fluctuations. Periodic mass wasting, facilitated by foreset over-steepening and possibly triggered by salt tectonics or erosion by the contour current occurred during late base level fall or early base level rise. [ABSTRACT FROM AUTHOR]

Geological Carbon Sequestration (GCS) needs to increase in scale by a hundred to thousand-fold to achieve the climate targets set out by the 2015 Paris Agreement. As such, assessment of storage potential and containment risk is required across many types of subsurface opportunities (including depleted fields and saline aquifers). In particular, the assessment of containment risk can be a daunting task, as it requires rock property calibration of heterogeneous intervals that are usually poorly sampled. The sequence-stratigraphic method is applied here using two case studies to illustrate how it can be utilized to articulate containment potential and risk for storage of immiscible supercritical Carbon dioxide at basin-to-prospect scales. In the onshore case studies presented here, facies dominated by (detrital) clay-mineral aggregates provide the most consistent excellent seals capable of retaining in excess of 200 m Carbon dioxide plume heights. Perhaps contrary to held paradigms, more distal basinal facies tend to be more inconsistent seals, due to a dominance of biogenic depositional components and complex diagenetic alterations. In the case studies, thick successions of argillaceous mudstones are typically associated with transgressive to early-highstand retreating to aggrading muddy clinoform deposits. These clinoform deposits can be shore (delta) attached or represent more distal offshore contourite mudbelts. In addition to the broad sequence-stratigraphic setting of appropriate lithofacies, the combination of basin type and sediment delivery will govern the geometry and lateral extend of favourable top seal deposits. Continental-crust foreland basins may develop favourable accumulations across thousands of square kilometres, whereas failed basins tend to exhibit much narrower facies belts. Finally, while perhaps logistically challenged, continental slope to basin-floor post-rift sediments of passive margins connected to large river input systems provide broad and thick favourable mud facies belts over distributive reservoir deposits. • The maximum CO 2 plume height that can be attained below siliciclastic top seals should be limited to two hundred meters. • The best sealing facies in marine clastic basins are associated with the accumulation of detrital clay mineral aggregates. • Mudstones dominated by biogenic components have inconsistent sealing attributes limited to plume heights of fifty meters. • Continental Foreland basins provide significant lateral extent of favourable facies belts compared to failed (rift) basins. • Post-rift clastic successions in passive margins provide another favourable setting for Geological Carbon Sequestration. [ABSTRACT FROM AUTHOR]

New insights into equatorial slope morphology were acquired through analysis of the continental margin of the Potiguar Basin (NE Brazil). In this paper, we present the first full data coverage of the seafloor between the upper and middle continental slopes (100–1300 m) adjacent to the Brazilian equatorial margin, developed using multibeam bathymetric data. Some of the submarine canyons mapped in this study have wall gradients greater than 35°. Wide (∼1700 km) and deep (∼250 m) incisions are present on the continental slope and can be linked to incised valleys that are underfilled or incised only on the outer shelf at depths up to 60 m. Two different types of canyons were identified. Canyons of one type are characterized by heads that indent the shelf edge, association with incised valleys and large fluvial systems, high sinuosity, ‘V’ shapes, and terraces along margins, in addition to erosive features such as landslides and gullies. These characteristics suggest that canyons of this type are associated with the deposition of submarine fan systems, which are considered permeable hydrocarbon reservoirs, on the base of the continental slope. The presence of gullies and sediment waves illustrates the role of bottom currents in the shaping of the slope. The enlargement of the canyons in the study area and the changes in their courses where they cross an important fault suggest that tectonic activity has probably also influenced the morphology of the deep-water environments of the Potiguar Basin. The results of this study constitute initial steps in describing and understanding submarine canyons as part of the equatorial continental Brazilian margin. [ABSTRACT FROM AUTHOR]

The Sinú Fold Belt, located in the southwestern part of the South Caribbean Deformed Belt, northwest Colombia, has been interpreted as the outer portion of an accretionary wedge resulting from the subduction of the Caribbean Plate under the South American Plate. Several geological sections, across and along the offshore portion of the Sinú Fold Belt, have been constructed through geological interpretation of 2D seismic sections constrained by a few exploration wells. Their analysis has allowed us to quantify the amount of horizontal motion (shortening responsible for the structures and basal detachment displacement) and vertical motion (tectonic subsidence/uplift and wedge vertical thickening) undergone by the belt over time; unravel its recent evolution consisting of almost no active structures in the south edge and a large amount of active structures northwards; quantify its pore pressure, coefficient of friction and strength using its wedge geometry consisting of high taper angles in the lower continental slope near the deformation front and lower taper angles in the upper continental slope away from it; and understand the aseismic and trench-lacking character of the belt. These observations have been crucial to identify the main features of the morphostructural zones across the belt (Colombian Basin, lower continental slope, upper continental slope and continental shelf) and establish a new along-strike zoning (north region influenced by the Magdalena Fan/Canoas Oblique Ramp, Sinú Fold Belt s.s., and south salient influenced by the Uramita Fault). • The Sinú Belt resulted from large detachment motion and minor internal shortening. • Vertical motion (tectonic uplift/thickening) is much lower than horizontal motion. • The Sinú Belt developed according to a forward propagation sequence. • Slow motion, gentle flexure and under compaction controlled seismicity/trench lack. • The Sinú Belt exhibits an along and across-strike morphostructural zoning. [ABSTRACT FROM AUTHOR]

An association of authigenic pyrite and gypsum has been found in deep-sea sediments from a core retrieved on the continental slope of the Santos Basin, South Atlantic. Pyrite occur mainly as tubular concretions formed by aggregates of framboids while gypsum show a rosette and acicular microcrystals habit. The concomitant presence of these minerals indicates the occurrence of biogeochemical reactions in marine environments that can be associated with methane seepage environments. Sulphur isotopic composition of authigenic minerals, magnetic susceptibility, and redox sensitive trace elements concentrations in bulk sediments are also used to characterize redox conditions and delimit a sulphate-methane transition zone (SMTZ) in this site. In this region, a pockmark field with methane release activity has already been recognized in sediments deposited during the Last Glacial Maximum. The conditions for the formation of these minerals, as well as their crystal habits, indicate the effect of biogeochemical processes consistent with methane seep environments. Therefore, our data provides new evidence about the occurrence of a past methane activity in this area. • Authigenic minerals in continental slope marine sediments demonstrate methane flux in the South Atlantic Ocean. • Framboidal texture in tubular pyrite indicates early diagenesis in dysoxic-suboxic conditions. • Authigenic character of gypsum designates formation by oxidation of pyrite. • Magnetic susceptibility data delimit the sulphate-methane transition zone in the sedimentary column. • The sulphur isotope signatures of pyrite and gypsum revealed a genetic relationship between these minerals. [ABSTRACT FROM AUTHOR]

High-resolution multi-beam bathymetry, 3D and 2D seismic reflection profiles from the South China Sea are used to investigate the morphology, characteristics, origin and implications to petroleum systems of recurrent slope failure in the Qiongdongnan Basin, northern South China Sea. Seven Late Miocene-Holocene mass-transport deposits (MTDs) and numerous submarine canyons were identified on the continental slope and rise, providing new insights on the evolution of an enigmatic region of the South China Sea. This paper defends that the interpreted MTDs were triggered by a combination of high sedimentation rates and local tectonic uplift. By comparing the stratigraphy of the study area with local sea-level curves, we show that a significant portion of shelf-edge deposits effectively bypassed most of the continental slope during the Miocene-Quaternary to accumulate as large MTDs on its lower part (i.e. on the 'basin-floor'), independently of sea-level changes. Our work has implications to the current sequence stratigraphic models for continental margins, and sheds new light on the reservoir potential of Miocene units in the South China Sea. Hence, regions where base-of-slope fans are expected to occur are, in the study area, occupied by large-scale recurrent MTDs sourced from the shelf edge. Stratigraphically, recurrent slope instability resulted: a) in abrupt episodes of accommodation space creation on the shelf-edge, b) on a moderate reduction in accommodation space at the base of the continental slope, and c) in a complete separation between shelf and distal slope depositional systems, with most stratigraphic markers on 3D seismic data being diachronous across the continental margin. As MTDs also comprise the bulk of channel-fill deposits in large submarine canyons, we propose that the reservoir potential of channel-fill deposits in the South China Sea is closely dependent on the nature of the sediment (i.e. net-to-gross ratio) eroded and transported by these same MTDs. [ABSTRACT FROM AUTHOR]

We integrate morphological, geological and geophysical observations with oceanographic measurements and modelling results to interpret the processes behind the origin and evolution of sandwaves on the upper continental slope of the southwestern Barents Sea. Seven sandwave fields covering c. 130 km 2 and with a volume of c. 72 × 10 6 m 3 occur in water depths of c. 475–800 m. Individual sandwaves reach heights up to 6 m. The sandwaves are sinusoidal with asymmetric stoss–lee side relationships, and time lapse multibeam bathymetry analysis indicates that some sandwaves have migrated up to 10 m towards the NNW over a 4-year time period. This contour current parallel migration is counteracted by currents in several other directions, and the sand unit is thus not a simple contourite. Measurements show prominent diurnal period oscillations and clear spring-neap variations of current direction and speed with along-slope and cross-slope current velocities up to 75 cm s −1 and 65 cm s −1 , respectively. Numerical ocean modelling results produce eddies travelling along the slope, positionally stable, daily recurring vortices and bottom current velocity up to 100 cm s −1 in the sandwave fields. Sandwave migration towards the SE is also observed, and the cross-slope currents are capable of transporting sand up and down the slope, as evidenced by ripples migrating normal to the sandwaves. Eroded sand accumulates in the sandwave fields along the boundary between North Atlantic Water and Norwegian Sea Arctic Intermediate Water. This is a consequence of the combined effect of the NNW directed along-slope Norwegian Atlantic Current and tidally induced topographic waves. The sandwave fields started to form 11 000–13 000 14 C yrs BP or possibly later by current erosion of glacial sediments on the continental shelf and slope. [ABSTRACT FROM AUTHOR]

In the deep subsurface sediments of the Andaman continental slope, in situ methane generation/oxidation could be coupled to the cycling of Mn, as the fluid flow characterized by high methane and Mn could occur in accretionary wedge sediments by diagenetic and tectonic processes. Laboratory studies on Mn cycling by subsurface sediment microbial communities were therefore undertaken 1) to study, further, possible in situ mechanisms of Mn cycling and 2) to examine how Mn redox reactions might be coupled to methane generation/oxidation. Biotic experiments were conducted with uniformly mixed subsamples amended with 100 μM Mn 2+ in the presence (G + ) and absence (G − ) of added glucose (55.5 μM). The corresponding abiotic controls included set-ups poisoned with 15 mM sodium azide. Further, to relate the results of in vitro experiments on Mn cycling, to the methane related processes occurring in the subsurface sediments, pore water concentration of Mn, total cell numbers and the abundance of methanogens, methanotrophs and fermenters were determined. Results of the experiment on Mn cycling showed the immobilization of Mn occurred under oxic conditions and mobilization under suboxic conditions in the absence of added glucose at P ≤ 0.001 with abiotic > biotic. Whereas, in the presence of added glucose, immobilization occurred under both oxic and suboxic conditions at P ≤ 0.001 with biotic > abiotic, oxic > suboxic. The biotic cycling of Mn at 360 mbsf coincided with the total cell numbers (1.53 × 10 8 cells g −1 ), increased methane levels (89,100 ppm-v) and the abundance of methanogens (1.0 × 10 3 MPN g −1 ). Besides, the distribution of aerobic methanotrophs decreased in abundance with depth. Also, the abundance of fermenters (3.5 × 10 3 cells g −1 ) at 626.7 mbsf coincided with the relatively high concentration of Mn (319.1 μM) in sediment pore water. The results indicate that subsurface sediments harbor microorganisms that partake significantly in the cycling of Mn wherein, the availability of organic carbon dictates the direction in which the reactions occur. Besides, aerobic oxidation of methane and Mn has been reported to occur under reducing conditions. Thus, the present findings suggest that Mn redox changes affect the methane oxidation/production rates by serving either as an electron donor and/or an electron acceptor. [ABSTRACT FROM AUTHOR]

In the deep subsurface sediments of the Andaman continental slope, in situ methane generation/oxidation could be coupled to the cycling of Mn, as the fluid flow characterized by high methane and Mn could occur in accretionary wedge sediments by diagenetic and tectonic processes. Laboratory studies on Mn cycling by subsurface sediment microbial communities were therefore undertaken 1) to study, further, possible in situ mechanisms of Mn cycling and 2) to examine how Mn redox reactions might be coupled to methane generation/oxidation. Biotic experiments were conducted with uniformly mixed subsamples amended with 100 μM Mn2+ in the presence (G+) and absence (G−) of added glucose (55.5 μM). The corresponding abiotic controls included set-ups poisoned with 15 mM sodium azide. Further, to relate the results of in vitro experiments on Mn cycling, to the methane related processes occurring in the subsurface sediments, pore water concentration of Mn, total cell numbers and the abundance of methanogens, methanotrophs and fermenters were determined. Results of the experiment on Mn cycling showed the immobilization of Mn occurred under oxic conditions and mobilization under suboxic conditions in the absence of added glucose at P ≤ 0.001 with abiotic > biotic. Whereas, in the presence of added glucose, immobilization occurred under both oxic and suboxic conditions at P ≤ 0.001 with biotic > abiotic, oxic > suboxic. The biotic cycling of Mn at 360 mbsf coincided with the total cell numbers (1.53 × 108 cells g−1), increased methane levels (89,100 ppm-v) and the abundance of methanogens (1.0 × 10³ MPN g−1). Besides, the distribution of aerobic methanotrophs decreased in abundance with depth. Also, the abundance of fermenters (3.5 × 10³ cells g−1) at 626.7 mbsf coincided with the relatively high concentration of Mn (319.1 μM) in sediment pore water. The results indicate that subsurface sediments harbor microorganisms that partake significantly in the cycling of Mn wherein, the availability of organic carbon dictates the direction in which the reactions occur. Besides, aerobic oxidation of methane and Mn has been reported to occur under reducing conditions. Thus, the present findings suggest that Mn redox changes affect the methane oxidation/production rates by serving either as an electron donor and/or an electron acceptor. [ABSTRACT FROM AUTHOR]

Mass transport deposits (MTDs) in the Krishna-Godavari basin from the eastern Indian margin are common sedimentary features over the modern continental slope. Quantitative understanding of the scale and age of these MTDs is relevant to understanding their triggering mechanisms and environmental feedbacks, as well as for interpreting pore water chemical profiles. Pore water profiles in sediments recovered during the 2006 Indian National Gas Hydrate Project (NGHP-01) expedition suggest that MTDs are present at seven of the ten sites cored in the Krishna-Godavari basin. Kinetic modeling of the S-shaped pore water sulfate and ammonium profiles from these sites provides quantitative estimates of the individual MTDs thickness, time elapsed after the event, rate of organic matter-fueled sulfate reduction, and time required to reach a new steady state. Model results suggest that the MTDs at the seven study sites are 8-25 m thick and are 300-1600 years old. Within the MTD sections, the organic matter-fueled sulfate reduction rates are 130-1200 mmol/m2/yr and the time needed to reach a new steady state ranges from 2000 to 3800 years. In comparison to depth-integrated sulfate reduction rates estimated in other regions, our estimates are relatively high reflecting the much thicker sulfate reduction zone that results from the MTDs. A positive correlation is observed between water depth and the MTD activity index defined as thickness of MTD package divided by its emplacement age. This correlation suggests the close relationship between MTD activity and water depth which agrees with previous studies of MTDs in this region and elsewhere. We caution against using the pore water data from sites experiencing significant MTDs to infer metabolic processes and to quantify steady-state reaction rates as our model results confirm the remarkable influence of transient MTDs on pore water profiles. [ABSTRACT FROM AUTHOR]

High-resolution and high-density 2-D multichannel seismic data, combined with high-precision multibeam bathymetric map, are utilized to investigate the characteristics and distribution of submarine landslides in the middle of the northern continental slope, South China Sea. In the region, a series of 19 downslope-extending submarine canyons are developed. The canyons are kilometers apart, and separated by inter-canyon sedimentary ridges. Numerous submarine landslides, bounded by headscarps and basal glide surfaces, are identified on the seismic profiles by their distorted to chaotic reflections. Listric faults and rotational blocks in head areas and compressional folds and inverse faults at the toes of the landslides are possibly developed. Three types of submarine landslides, i.e., creeps, slumps, and landslide complexes, are recognized. These landslides are mostly distributed in the head areas and on the flanks of the canyons. As the most widespread landslides in the region, creeps are usually composed of multiple laterally-coalesced creep bodies, in which the boundaries of singular component creep bodies are difficult to delineate. In addition, a total of 77 landslides are defined, including 61 singular slumps and 16 landslide complexes that consist of two or more component landslides. Statistics show that most landslides are of a small dimension (0.53–18.09 km² in area) and a short runout distance (less than 3.5 km). Regional and local slope gradients and rheological behavior of the displaced materials might play important roles in the generation and distribution of the submarine landslides. A conceptual model for the co-evolution of the canyons and the associated landslides in the study area is presented. In the model it is assumed that the canyons are initiated from gullies created by landslides on steeper sites of the continental slope. The nascent canyons would then experience successive retrogressive landsliding events to extend upslope; at the same time canyon downcutting or incision would steepen the canyon walls to induce more landslides. [ABSTRACT FROM AUTHOR]

The identification of several Cretaceous to Paleogene mixed turbidite-contourite systems along the upper to middle continental slope of Nova Scotia provides a unique opportunity to investigate the spatial and temporal variability of their morphological elements. The mixed systems were studied using 3D seismic reflection data and well-established chronostratigraphy from five exploration wells. Seismic interpretations and correlations suggest four main evolutionary stages for the Late Jurassic – Paleogene sedimentary record, characterized by distinctive, diagnostic features at large (>1–10 km) to small (<100 m) scales: 1) initial turbidite systems developed between 160 and 125 Ma with extensive tributary channel networks along the upper continental slope that fed submarine channels and channel-levees further down-slope; 2) onset and growth of mixed systems at 125–78 Ma, characterized by a seaward progradation and NE migration of down-slope elongated mounded drifts and wide, U-shaped channels formed under potentially synchronous interactions between a SW-flowing bottom current and SE-directed turbidity flows; 3) shift of mixed systems at 78–50 Ma with SW migration of the mounded drifts and submarine channels, associated with a potential switch of the bottom current direction at 78 Ma, from SW towards NE; and 4) burial stage post-50 Ma, characterized by several stages of hemipelagic deposition followed by gully erosion at approximately 50, 40, and 35 Ma. The gradual transition from turbidite systems to fully developed mixed systems is associated with the tectonic and sedimentary background of the Nova Scotian margin during the Early to Late Cretaceous, as well as the establishment of a Cretaceous to Eocene paleoceanographic circulation. Comparisons with other ancient and modern mixed systems reveal similar morphological features (e.g., mounded drifts and submarine channels) formed under synchronous and asynchronous interactions between along- and down-slope processes. Their lateral distribution and vertical variability reflect a gradual change between the most influential process, from down-slope turbidity currents to along-slope bottom currents. Secondary depositional features along the mounded drifts, such as N–S oriented sediment waves, result from along-slope sediment redistribution and remobilization near the seafloor. Mixed depositional systems form unconventional plays and may become future targets for energy geosciences in a search for hydrocarbons or carbon capture and storage. • Cretaceous to Eocene mixed depositional systems are identified off Nova Scotia. • Distinct morphological elements are recognized at different spatial scales. • A gradual transition occurred between turbidite, mixed and contourite settings. • Synchronous and asynchronous interactions between along- and down-slope processes. • Basins and gateways reconfigurations affected the North Atlantic paleo-circulation. [ABSTRACT FROM AUTHOR]

This article describes the results of a comprehensive analysis of the causes of intensive gas emission over a large zone (ca. 80 × 220 km) of the Central Laptev shelf area. The upper parts of 28 sections of common depth point (CDP) seismic lines acquired by the JSC MAGE, covering a total length of 5930 km over an area of 55,000 km2, were interpreted and 519 anomalous objects potentially connected with gas saturation were revealed. For the first time, the boundary between frozen and thawed sediments was defined in the Laptev Sea. The absence of subsea permafrost and gas hydrates in the intensive gas seepage zone was determined. The connection between gas seeps and deep-seated faults identified in seismic sections was substantiated. This study demonstrated that seismotectonic factors caused a large spatial window of long-term Earth degassing with direct intensive migration of deep thermogenic gas to the hydrosphere through a system of faults. A zone of potential gas hydrate distribution was observed on the continental slope. • Permafrost and gas hydrates are not predicted in the zone of gas seeps in the Central Laptev area. • Gas seeps in the Central Laptev area are connected with deep-seated faults. • Seismotectonic factors must contribute to gas seepage in the Central Laptev area. • Gas hydrates are predicted on the continental slope of the Laptev Sea. [ABSTRACT FROM AUTHOR]

Abstract: High resolution and multichannel seismic profiles coupled with multibeam echosounder (seafloor relief) data, acquired along the northern Sicily continental margin (southern Tyrrhenian Sea), document the occurrence of mound and pockmark features, revealing fluid escape processes. Along this margin, morphology of the high-gradient continental slope is irregular due to the presence of structural highs, slope failures and canyons, and is interrupted by flat areas at a mean depth of 1500 m. Seismostratigraphic analysis tools and methods were used to identify fluid escape structures and to work out a classification on the basis of their morpho-acoustic characteristics. The detailed 3D bathymetric chart was used to define the top view morphologic features and their areal distribution. With the aim to evaluate the geochemical content of fluids, we collected a 2.3 m long sediment core in correspondence of a pockmark at a depth of 414 m. Pore waters were sampled every 10 cm and analysed in relation to their conductivity (EC) and composition (δ18O, δD, Li, Na, K, Mg, F, Cl, Br, NO3, SO4). The new data show the occurrence of different types of structures with highly contrasting seismic and morphologic signatures, both dome-type and concave-upward structures. The latter have a characteristic circular shape and are known as pockmarks. Morphobathymetric, stratigraphic and structural data suggest that these structures occur along fault planes, mainly associated with diagenetic carbonates and fluid venting activity. Pockmarks could be the result of both fault and landslide structures, as they appear aligned along a straight direction and occur in proximity of the slope, and are associated with slope instabilities. The structural features are possibly associated with the recent tectonics mapped on-land as well as the widespread seismicity of the margin. Geochemical features reveal that pore water is slightly enriched in heavy isotopes with respect to Mediterranean seawater, while the distribution profiles of EC, ion concentration (Cl, SO4, Na, K, Mg, Ca), ion/Chloride ratios (Na/Cl, K/Cl, Ca/Cl, Mg/Cl and Alk/Cl) seem to indicate the existence of an external source of fluids and the occurrence of sediment-fluids interaction processes. A possible mechanism causing pore water freshening could be the destabilisation of gas hydrates. [Copyright &y& Elsevier]

Abstract: Sediments of the continental slope are commonly bioturbated by endo- and epibenthic organisms, particularly in and around submarine canyons and channels. This study reviews the architecture and depositional environments associated with canyons and channels on the continental slope, and assesses the key physical and chemical conditions encountered in and around these conduits. Hydrodynamic energy, concentration and quality of organic carbon, dissolved oxygen concentration and sedimentation rate are identified as key controls on the composition of benthic ecosystems in slope environments. Submarine canyons and channels focus a variety of turbid and clear-water currents, all of which serve to increase the concentration of oxygen, labile organic carbon and other nutrients, which tend to elevate the abundance and biodiversity in the seafloor sediments, compared with those of the surrounding slope. Ancient slope channel and canyon systems reflect some of the variation in ichnological assemblages that is seen in modern analogues, although processes of erosion and trace fossil preservation mean that the benthic environment is often incompletely preserved in the ancient record. By integrating current understanding of sedimentology, oceanography, biology and ichnology of slope environments it is possible to provide a first order summary of the inter-relationships between ichnology and depositional environments on the continental slope. The combination of these data has the potential to improve our understanding of changes in deep marine benthic ecosystems through geological time, and to further the use of ichnology in assessing hydrocarbon reservoir presence, quality and performance from bioturbated slope, canyon and channel-levee hydrocarbon reservoirs. [Copyright &y& Elsevier]

Gas hydrate-bearing sediments provide an extreme and unique habitat for bacteria and archaea, but the relationship between the microbial community and gas hydrate formation is poorly known. In the Shenhu area of the northern South China Sea, it was inferred that the gas source is mixed biogenic-thermogenic, and contradictions exist between relatively low total organic carbon content and high saturation of gas hydrates. The synthesized geophysical and geological evidence showed that deep thermogenic fluid could migrate upward through migrating pathways. In this study, samples were collected from Shenhu area from 224 to 271 mbsf (meters below the seafloor), which covered gas hydrate-bearing sediments (GHBS), free gas-bearing sediments (FGBS), and non-gas hydrate-bearing sediments (NGHBS), and 16S rRNA gene sequencing was used to characterize bacterial and archaeal community structure. The results revealed significant differences of microbial community structure among the three sediment types as indicated by alpha (richness and diversity indices) and beta (Bray-Curtis-based non-metric multidimensional scaling) diversity analyses as well as community composition analyses. In bacterial community, abundant genera included Pseudomonas , Halomonas , Alcanivorax , Thiobacillus , Marinobacter , Rhodococcus , and Dietzia , some of which were identified as putative hydrocarbon degraders. Most abundant archaeal genera were potential methanogenic archaea, and the dominant genera between GHBS and FGBS differed. Multiple types of methanogenesis were noted, and the hydrogenotrophic type was inferred to be predominant. Deviations of temperature of methanogens from the geothermal gradient of the study area again suggested the migration of organic matters. Based on the characterization of bacterial and archaeal community structure in the deep subsurface sediments, we proposed a geological model for the gas hydrate formation and accumulation of the gas hydrate reservoir, emphasizing the possibility of microbially-mediated secondary microbial gas formation from deep thermogenic fluid. This study characterized bacterial and archaeal community structure in deep subsurface sediments, which can provide insight into the potential roles of bacteria and archaea including hydrocarbon degradation and methanogenesis in gas hydrate formation and help reevaluate the contribution of deep thermogenic fluid to gas hydrate formation in the Shenhu area of the northern continental slope of the South China Sea. • 16S rRNA gene sequencing was employed to characterize microbial community of deep subsurface sediments in the Shenhu area. • Significant differences in microbial community structure were observed among sediment types (GHBS, FGBS, and NGHBS). • Putative hydrocarbon degraders and potential methanogenic archaea (with hydrogenotrophic type predominant) were detected. • The contribution of deep thermogenic fluid to gas hydrate formation in the Shenhu area should be reevaluated. [ABSTRACT FROM AUTHOR]

We discovered a large field of cylindrically-shaped sediment chimneys at the seafloor along the flank of Farnella Canyon (water depth ∼2775 m) at the base of the Sigsbee Escarpment in the northwestern Gulf of Mexico. The chimneys are about 10 cm in diameter with 1 cm wide axial cavities; they rise about 0.5–1 m above the seafloor and continue down into the seabed. Their composition and fabric were characterized using X-ray diffraction, carbonate content, stable oxygen and carbon isotopes, and scanning electron microscopy and microanalysis. Seismic profiles and bathymetric data from the location were analyzed to seek links between the chimneys and seabed geological structures. Results indicate that the vented liquids were warm seawater with brine and there is no evidence of hydrocarbons. These chimneys are residual sediment-filled fluid-escape conduits formed in the seabed from hemipelagic sediments that were locally consolidated by the pressure of fluid moving upward through the seabed in adjacent conduits and bound together by iron oxide and amorphous silica precipitates, as well as authigenic dolomite. The chimneys were subsequently exposed at the seafloor when bottom currents eroded the surrounding seabed. Concentric and crosscut bands evident in chimney cross sections indicate venting occurred episodically (venting is not currently active). The chimney field is situated at the toe of the Jurassic salt massif that extends beneath much of the continental slope. Salt movement created a pressure gradient within the seabed, directing porewater flow toward the base of the slope where the chimneys occur; thus, venting most likely occurred during times of salt mobility and shortly thereafter. • Deep-water sediment chimneys discovered at base of Gulf of Mexico's continental slope. • Unique chimneys formed without authigenic carbonate from hydrocarbon degradation. • Salt drives fluid flow up faults and then through chimneys in overlying sediments. • Fluid flow promotes tight-packing of sediment microfabric and the precipitation of Fe-oxides and minor amounts of dolomite. • Enhanced fluid venting is associated with salt movement, as when sea level is low and the sedimentation rate on the slope is high. [ABSTRACT FROM AUTHOR]

Abstract: The transfer of sediment from the upper continental slope to rise is poorly documented along the southeast African passive margin. New swath bathymetric and sub-bottom data collected in the Natal Valley, southwest Indian Ocean, provide insight into the evolution of the Tugela canyon and fan system. Several distinct downslope changes in canyon morphology are noted. The canyon increases in relief and widens with depth. Basement outcrop is restricted to the head of the canyon becoming less prominent with depth. Step-like terracing of the canyon walls and floor becomes prominent in the mid-slope portions of the canyon and is related to a marked increase in the cross sectional asymmetry of the canyon profile. The contemporary Tugela canyon rests within a depression of the last phase of infilling. The canyon is the product of downslope erosion, and incision, caused by several phases of hinterland uplift in the mid Oligocene, mid Miocene and late Pliocene. Each phase was followed by pelagic infilling of the palaeo-canyon form. Downslope, the uplift phases are preserved in the cut-terraces and axial incisions within the main canyon thalweg. The contemporary canyon is a moribund feature, sediment starvation of the shelf area by current sweeping of the Agulhas current has decreased the material available for canyon incision and fan development. Additional current sweeping by the North Atlantic Deep Water current has stunted the development of the associated fan complex. [Copyright &y& Elsevier]

Abstract: Seep carbonates represent excellent archives of seepage activities near seafloors. In order to provide insights on sources of seeping fluids, stable carbon and oxygen isotopes and 87Sr/86Sr ratios were applied to seep carbonates from Shenhu, SW Dongsha, and NE Dongsha on the northern continental slope of the South China Sea. U/Th dating was selected to constrain the time involved in carbonate formation and the potential driving force. The δ13C values of the Shenhu and NE Dongsha carbonates vary from −52.3‰ to −32.6‰ (V-PDB), indicating that biogenic methane is the primary carbon source at both locations. The δ13C values of the SW Dongsha carbonates are much higher, ranging from −18.8‰ to −11.4‰ (V-PDB), suggesting that thermogenic methane is the predominant carbon source in this region. A relatively deep origin of thermogenic methane for the SW Dongsha seeps is also supported by somewhat lower 87Sr/86Sr ratios of the samples (from 0.709025 to 0.709097). The δ18O values of the Shenhu and NE Dongsha carbonates vary from 1.4‰ to 5.1‰ (V-PDB), while the δ18O values of the SW Dongsha carbonates range from 2.5‰ to 3.4‰ (V-PDB). Many samples from Shenhu and NE Dongsha demonstrate an 18O-enriched fluid source that most likely originated from waters released from the dissociation of locally abundant gas hydrate. U/Th ages of the Shenhu carbonates show a large range from 330 ka to 152 ka BP and the NE Dongsha carbonates have ages from 77 ka to 63 ka BP. Interestingly, U/Th ages of most samples are either within times of sea-level lowstands or correspond to periods when the sea-level was falling. Taken together, our data suggest that in the South China Sea gas hydrate dissociation during sea-level lowstands or falling stages resulted from reduced hydrastastic pressures, which in turn enhanced the seep activities and promoted the formation of authigenic carbonates close to the seafloor. [Copyright &y& Elsevier]

Abstract: The accumulation of bathymetric and seismic data on many continental margins has provided valuable new insights into variability of seascape morphology. The present challenge is in finding appropriate manners to characterise that morphology so that variations between areas and between potential influences can be recorded. In this paper, we employ hypsometry (area-elevation analysis) and present parameters derived from 65 hypsometric curves which represent distinct slope-basin shapes, based on data from the central USA Atlantic continental slope. The distinction of morphologic signatures has derived from the statistical moments, such as skewness and kurtosis, in describing the hypsometric curves of the submarine basins. Moreover, basin geometry in planview and network structure, have shown to be remarkably well reflected in the shape of the hypsometric curve. Based on the combined analysis of morphology and submarine processes and scenarios, we propose that concave hypsometry is characteristic of trellis networks that probably initiated by sporadic erosion by sedimentary flows, spilling over the shelf edge. The convex hypsometry that characterises poor- to well- developed dendritic networks, heading at or below the shelf break, is attributed to retrograde erosion and sediment fallout events, respectively. These results present initial step in helping to infer submarine processes from morphology, and provide initial information about the environmental scenarios for areas where the erosional record is unknown. [Copyright &y& Elsevier]

Abstract: The hydrate deposits in Shenhu Area on northern continental slope of the South China Sea is regarded as one of the promising targets for gas hydrate exploration and exploitation in China. The hydrates concentrate at base of gas hydrate stability zone (GHSZ) with a peak saturation of 48%. However, the formation mechanism and controlling factors of the hydrates remain unclear. Previous studies suggested that sedimentation flux and water flux are two fundamental parameters controlling hydrate accumulation. Here, we have applied a one-dimensional dynamic model to characterize the hydrate evolution pattern at a drilling site of SH2 in Shenhu Area. The hydrate saturations modeled at current sedimentation rate and water flow rate were much different from the values approved. Some perturbations of the two parameters seemed to be useless to improve the modeling. The big finding was that the modeled result basically matched the saturation values at initial hydrate saturations of 20–22%. The results indicated that massive gas hydrates formed in the early fractured sediment in Shenhu Area, the sediment evolved by inheriting the “ancient” hydrate and lasted for ∼1.5 Myrs before present and produces the current hydrate feature. The modeling implies that resource potential of Shenhu hydrate has been decreasing. [Copyright &y& Elsevier]

Abstract: The north flowing Falkland/Malvinas Current has generated sediment drifts at a depth of 1200–1600 m in the Patagonian middle continental slope out of early Holocene hemipelagics, late Pleistocene ice rafted clastics, and Neogene fluvial sediments. Possibly there may be two generations of drifts, Pleistocene on the outer middle slope and Holocene on the inner shelf. The ice rafted debris originated in Antarctica, at a distance of 2000–4000 km south of Patagonia. Scattered over the upper and middle slopes, at depths ranging from 300 to 1400 m, are cold-water coral reefs of less than a meter to about ten of meters in relief. It is inferred that most of cold-water coral structures flourish as a consequence of the Falkland/Malvinas Current that concentrates the food supply at the reef sites. Growth of cold-water coral reefs, documented by digital submarine photographs on the upper slope, at a depth of 300/400 m, may be promoted by upwelling of nutrient-rich waters and associated plankton blooms created by the intrusion of Falkland/Malvinas Current onto the outer shelf. [Copyright &y& Elsevier]

Abstract: Drilling/coring activities onboard JOIDES Resolution for hydrate resource estimation have confirmed gas hydrate in the continental slope of Krishna-Godavari (KG) basin, Bay of Bengal and the expedition recovered fracture filled gas hydrate at the site NGHP-01-10. In this paper we analyze high resolution multi-channel seismic (MCS), high resolution sparker (HRS), bathymetry, and sub-bottom profiler data in the vicinity of site NGHP-01-10 to understand the fault system and thermal regime. We interpreted the large-scale fault system (>5 km) predominantly oriented in NNW-SSE direction near NGHP-01-10 site, which plays an important role in gas hydrate formation and its distribution. The increase in interval velocity from the baseline velocity of 1600 m/s to 1750–1800 m/s within the gas hydrate stability zone (GHSZ) is considered as a proxy for the gas hydrate occurrence, whereas the drop in interval velocity to 1400 m/s suggest the presence of free gas below the GHSZ. The analysis of interval velocity suggests that the high concentration of gas hydrate occurs close to the large-scale fault system. We conclude that the gas hydrate concentration near site NGHP-01-10, and likely in the entire KG Basin, is controlled primarily by the faults and therefore has high spatial variability. We also estimated the heat flow and geothermal gradient (GTG) in the vicinity of NGHP-01-10 site using depth and temperature of the seafloor and the BSR. We observed an abnormal GTG increase from 38 °C/km to 45 °C/km at the top of the mound, which remarkably agrees with the measured temperature gradient at the mound (NGHP-01-10) and away from the mound (NGHP-01-03). We analyze various geological scenarios such as topography, salinity, thermal non-equilibrium of BSR and fluid/gas advection along the fault system to explain the observed increase in GTG. The geophysical data along with the coring results suggest that the fluid advection along the fault system is the primary mechanism that explains the increase in GTG. The approximate advective fluid flux estimated based on the thermal measurement is of the order of few tenths of mm/yr (0.37–0.6 mm/yr). [Copyright &y& Elsevier]

Abstract: Multidisciplinary surveys were conducted to investigate gas seepage and gas hydrate accumulation on the northeastern Sakhalin continental slope (NESS), Sea of Okhotsk, during joint Korean–Russian–Japanese expeditions conducted from 2003 to 2007 (CHAOS and SSGH projects). One hundred sixty-one gas seeps were detected in a 2000 km2 area of the NESS (between 53°45′N and 54°45′N). Active gas seeps in a gas hydrate province on the NESS were evident from features in the water column, on the seafloor, and in the subsurface: well-defined hydroacoustic anomalies (gas flares), side-scan sonar structures with high backscatter intensity (seepage structures), bathymetric structures (pockmarks and mounds), gas- and gas-hydrate-related seismic features (bottom-simulating reflectors, gas chimneys, high-amplitude reflectors, and acoustic blanking), high methane concentrations in seawater, and gas hydrates in sediment near the seafloor. These expressions were generally spatially related; a gas flare would be associated with a seepage structure (mound), below which a gas chimney was present. The spatial distribution of gas seeps on the NESS is controlled by four types of geological structures: faults, the shelf break, seafloor canyons, and submarine slides. Gas chimneys that produced enhanced reflection on high-resolution seismic profiles are interpreted as active pathways for upward gas migration to the seafloor. The chimneys and gas flares are good indicators of active seepage. [Copyright &y& Elsevier]

Abstract: The study of many slope channel systems has led to the development of rules in the form of observations, measurements, and hypotheses. For example, we hypothesize that high abandonment relief can strongly influence the location of the subsequent channel element and will result in an organized channel stacking pattern in which the path of the younger channel element approximates the path of the former element. The rules were developed with the objective of constructing forward models of petroleum reservoirs that are internally consistent, reproducible, and quantifiable. Channelized turbidite deposits can be interpreted to be the product of multiple cycles of waxing–waning flow energy at multiple scales. Systematic changes in the volume and caliber of turbidity flows through time trigger a fall of the equilibrium profile, which drives erosion and sediment bypass across the slope, followed by a rise of the equilibrium profile, which allows deposition on the slope of increasingly mud-rich sediments through time. In most turbidite successions, at least three scales of waxing–waning cyclicity can be interpreted: element, complex set, and sequence. The stacking pattern of channel elements within a complex set-scale cycle tends to be sequential: (1) erosion and sediment bypass; (2) amalgamation of channel elements associated with a low rate of aggradation; (3) a disorganized stacking pattern of channel elements associated with a moderate rate of aggradation; and (4) an organized stacking pattern of channel elements associated with a high rate of aggradation. Stages 1 and 2 may be absent or minor in mud-rich systems but prominent in sand-rich systems. Conversely, stage 4 may be prominent in mud-rich systems but absent in sand-rich systems. Event-based forward modeling, utilizing rules, can produce realistic architectures, such as the four stages described above. Multiple realizations and multiple alternative models can be constructed to quantitatively examine the probability of specific parameters of interest such as pore volume and connectivity. [Copyright &y& Elsevier]

Abstract: Late Cenozoic seismic stratigraphy of the Vøring continental margin has been studied in detail, with emphasis on the geological development of the Naust Formation deposited during the last 3 million years. The Kai Formation (15–3 Ma) comprises mainly biogenic ooze deposited in the Møre and Vøring Basins. In Naust time, there was a marked increase in supply of sediments from the inner shelf areas and the western part of the Scandinavian mountain range, and glaciers expanded to the shelf and reached the shelf edge several times during the last 1.5–2 million years. During early to mid Naust time the shelf was widened by westerly prograding sediment units, but for a long period the shallowest part of the Helland-Hansen Arch (HHA) formed a barrier preventing glacially derived debris from being distributed farther west. West of the HHA, mainly stratified marine and glacimarine sediments were deposited. A substantial part of these sediments were transported by the north-flowing Norwegian Atlantic Current, which redistributed suspended particles from ice streams, rivers, coastal erosion and seabed winnowing. After burial of the crest of the HHA at c. 0.5 Ma, glacial debris and slide deposits were also deposited west of this high. In the north, massive units of glacial debris were distributed beyond the crest of the HHA, also in mid Naust time, thinning westwards and interfingering with fine-grained sediments on the lower slope. The Sklinnadjupet Slide, inferred to be c. 250,000 years old, corresponds in age with an earlier huge slide in the Storegga area. An elongated area of uneven seabed topography previously interpreted as diapirs (Vigrid diapirs) on the slope west of the HHA is shown to be formed by ooze eruption from the crest of the arch and submarine sliding. [ABSTRACT FROM AUTHOR]