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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]

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]

In a submarine canyon-incised continental slope, the observed bottom simulation reflection (BSR) frequently appears not to match with the predicted base of the gas hydrate stability zone (BGHSZ). However, the mechanism behind this discrepancy is often debated due to the lack of in-situ borehole measurements. In the Shenhu area, a submarine canyon region on the northern continental slope of the South China Sea, a suite of datasets, including 3D seismic data, well logs and in-situ borehole temperature measurements enable us to study the gas hydrate stability zone. In-situ borehole temperature measurements reveal a nonlinear geothermal gradient within shallow 200 m-thick marine sediments. The geothermal gradient fit with the uppermost sediment interval (<50 m) neglected produces a better match between the depths of predicted BGHSZ and observed BSR. The gas hydrate stability zone (GHSZ) modeling using in-situ borehole measurement information at Site W07 shows that the predicted BGHSZ agrees well with the imaged BSR located atop an inter-canyon ridge, demonstrating that the gas hydrate system is in a steady state. Two-dimensional GHSZ modeling along seismic profiles transecting the ridge shows that the observed BSRs along the western margin of the ridge are deeper than the predicted BGHSZ. We interpret this apparent discrepancy to be attributed to topographic effects. Two scales of topographic effects, i.e. an alternating distribution of canyons and inter-canyon ridges, and localized crests and troughs over the crest of inter-canyon ridges, are revealed to influence gas hydrate system in the study area. The curved BGHSZ resulted from the topographic effect could facilitate the concentration of gas hydrate by enlarging the accumulation space and sealing free gas immediately below BGHSZ. • Gas hydrate system in the vicinity of site W07 is in equilibrium. • Topographic effect accounts for the apparent discrepancy between the observed BSR and the predicted BGHSZ. • Two scales of topographic effect impact the gas hydrate system in the study area. • Curved BGHSZ facilitates the concentration of gas hydrate. [ABSTRACT FROM AUTHOR]

Isolated, detached sands provide opportunities for large-volume stratigraphic traps in many deepwater petroleum systems. Here we provide a review of the different types of sandbody detachments based on published data from the modern-day seafloor and recent (generally Quaternary-present), shallow-buried strata. Detachment mechanisms can be classified based on their timing of formation relative to deposition of the detached sandbody as well as their process of formation. Syndepositional detachment mechanisms include flow transformation associated with slope failure (Class 1), turbidity current erosion (Class 2), and contourite deposition (Class 3). Post-depositional detachment is related to subsequent erosive processes and truncation of the pre-existing sandbody, either by submarine channels (Class 4), mass-transport events (Class 5), post-depositional sliding or faulting (Class 6) or bottom currents (Class 7). Examples of each of these mechanisms are identified on the modern seafloor, and show that detached sandbodies can form at different locations along the continental slope and rise (from upper slope to basin floor), and between or within different architectural elements (i.e., canyon, channels and lobes). This variation in formation style results in detached sands of highly variable sizes (tens to hundreds of kilometres) and geometries across and along the depositional profile, which are dependent upon the erosive and/or depositional processes involved, as well as the seafloor topography of the area in question. Whilst modern seafloor systems may not always represent the final stratigraphic architecture in the subsurface, they provide important insights into the development of detached sandbodies and therefore serve as potential analogues for subsurface stratigraphic traps. • Numerous processes lead to detached sands on the modern seafloor. • A new classification scheme is proposed for deepwater sand detachment processes. • The effectiveness and location of sand detachment varies between processes. • An actualistic approach can help to de-risk hydrocarbon plays. [ABSTRACT FROM AUTHOR]

The study area is located at the southern part of the Cameroon Atlantic Margin (CAM) in the Gulf of Guinea. This work analyses 20 wells and 02 Cameroon Span seismic lines to re-examine the stratigraphical nature of the Top Albian surface of the CAM for a better understanding of the basin evolution. The results obtained by wells analysis show that, from the Douala sub-basin (Dehane) onshore to the Kribi-Campo continental slope, the Top Albian surface has an erosive shape. The seismic analysis reveals that this surface has two characters: erosive from the continental shelf to continental slope and concordant in the deep basin. This observation indicates that the Top Albian surface corresponds to an unconformity (u/c) on shallow waters with its correlative conformity (cc) in deep basin. The erosive form was amplified by erosion caused by the margin uplift. The combination of seismic and wells data reveals that in shallow waters, the Top Albian u/c erodes Aptian-Albian deposits. These sediments mainly consist of sandstones, shales and evaporitic unit. The occurrence of dark shales before late Albian epoch, suggests that the Douala/Kribi-Campo (DKC) Basin was already transgressed by marine sediments at this time. Hence, the Top Albian surface resulted probably from a relative sea-level fall, mainly caused by the tectonic uplift. The 3D modeling carried out provides a better resolution of its distribution in the study area. This surface boundary has regional extension on the DKC Basin. Furthermore, the Top Albian surface seals both good sandstones reservoir rocks and dark shales source-rocks in the studied basin. • The Top Albian surface of the South Sanaga area shows two erosive and concordant characters. • This surface resulted mainly by a relative sea-level fall induced by margin uplift. • Aptian-Albian stage corresponds to an aggradational/progradational phase. • This surface seals reservoir rocks and constitutes an indicator of source rocks. [ABSTRACT FROM AUTHOR]

The purpose of this study is to differentiate and characterise contouritic sands in two different locations with variable sediment compositions (siliciclastic and bioclastic) based on a multiproxy approach that includes the analysis of sedimentary texture, semi-quantitative geochemistry, microfacies and ichnological information, as well as a Principal Component Analysis (PCA) applied to geochemical and sedimentary data on sediment cores. The integration of sedimentological analyses and the PCA permits also to the differentiation between fine-grained deep-water deposits such as hemipelagites, muddy contourites and hyperpycnites. A depositional model is proposed here, based on geochemical and sedimentological characteristics of the Holocene-highstand Mozambique Channel upper slope sands, and glacial-lowstand Corsica Contourite Depositional System middle slope sands. The upper continental slope of the Mozambican margin is characterised by siliciclastic sandy contourites, muddy contourites and muddy hyperpycnites. Mozambique siliciclastic sandy contourites constitute large accumulations of well-sorted very fine to coarse sand with evidences of strong winnowing and reworking under high-energy conditions. The sedimentary facies represents highstand contourite sands and shows a reversely-graded trend. The contourite drift on the Pianosa ridge (eastern flank of the Corsica Trough) consists of bioclastic sandy and muddy contourites and hemipelagites. Bioclastic sandy contourites are made up of shallow-marine winnowed bioclasts with a reversely- and normally-graded trend and represent lowstand contourite sands. The PCA in the two environments —showing a distinctive geochemical signal— allows for differentiation of the contourite deposits. In siliciclastic sands, reworking is marked by an accumulation of Si, Zr, and Sr in fine- to medium-grained sands. In bioclastic sands, reworking is less evident but it is characterised by accumulations of Ca and Sr. The reworking and winnowing bottom current effects are also observed at the microfacies scale. Both types of contourite deposits show evidences of intermittent depositional conditions depending on the ichnodiversity, distribution and abundance of trace fossils. This work is useful to discriminate similar fine-grained deposits in different continental margins which would contribute to a better understanding of sedimentary deposits and processes in deep-marine environments. [Display omitted] • Reworking proxies are different for bioclastic and siliciclastic sands. • Winnowing and reworking proxies have a distinctive geochemical signal and they are supported at the microfacies scale. • Bi-gradational contourite sequence show evidences of intermittent depositional conditions. • Sediment input and distance to sediment source has a strong impact on the bi-gradational contourite sequence formation. • Contourite drifts are not only formed by contourite deposits. [ABSTRACT FROM AUTHOR]

Following the termination of the Messinian salinity crisis ~5.3 million years ago, massive sedimentation in the Eastern Mediterranean Sea formed the huge Nile Delta. Alongside delta propagation, a continental shelf was accreted along the Levant margin. For several decades it was assumed that these two sedimentary structures were closely connected. Levant shelf deposits are composed of Nile-derived sediments and present-day measurements show that sand is currently being transported alongshore from the Nile Delta to offshore Israel. This study reexamines the existing paradigm about sediment transport and shelf-delta connection. We show that longshore sand transport is just a small part of a much larger process termed here margin-parallel sediment transport (MPST). Sand is transported in a nearshore shallow-water belt where marine currents are highly energetic. At the same time, shale is transported at greater depths over the deeper shelf and the continental slope where marine currents are weaker. To model the accretion of the Levant shelf alongside the Nile Delta we use a 3D, diffusion-based, stratigraphic modeling tool (DionisosFlow) with a new module representing MPST. Our results show that margin-parallel transport of silt and clay in the deeper waters accounts for the bulk of deposition offshore Israel and is responsible for the development of the Levant shelf. Moreover, though MPST has begun coevally with delta formation, massive accretion of the Levant shelf was delayed by 2–3 My. Initially, a continental shelf formed offshore Sinai, then offshore Israel, and most recently along the Lebanon coast. Our model also demonstrates the significant lithological differences observed between sedimentation in front of the Nile River mouth and along adjacent continental margin. High energy down-slope sediment transport carries sand, silt, and clay, whereas margin-parallel currents are relatively weak and carry mainly silt and clay. One exception within the margin-parallel system is the highly energetic nearshore current that transports sand. Thus, we point out, MPST is an efficient separator between shale and sand. • The Levant continental shelf was accreted alongside Nile Delta progradation. • Margin Parallel Sediment Transport takes place both in shallow and deep waters. • Longshore sand transport in shallow waters is a small fraction of MPST. • MPST starts immediately, but accretion of the nearby shelf is delayed. • MPST is an efficient differentiator between shale and sand. [ABSTRACT FROM AUTHOR]

When biogenic methane produced in marine shallow sediments flows laterally into hydrate stable zones, there is likely to be formation of disperse stratigraphic-diffusive hydrate reservoirs that generally appear thin-bedded and disseminated. The stratigraphic-diffusive layers are also potential industrial exploitation targets because they have moderate abundance, wide distribution, and convenient mining maneuverability. After success of the second trial production in Shenhu area of South China Sea, it is urgent for us to find out the evolution dynamics of the stratigraphic-diffusive hydrate system which is widespread along oceanic continental slopes. Here, the Pearl River Mouth Basin in the northern slope area of South China Sea as a typical passive margin was selected to probe the principles of the accumulation mechanism and distribution pattern of stratigraphic-diffusive methane hydrates, determine the optimal degree of aggregation and spatial extension, and thus evaluate the potential recovery value. Based on the composite stratigraphic sequence, we theoretically described the occurrence characteristics of this type of stratigraphic-diffusive hydrate reservoirs. A two-dimensional model that coupled sedimentation process, fluid flow and reaction kinetics was designed to gauge the influence of engineering geological conditions and other controlling factors since the Pliocene (5.3 Ma) in this system. Several key parameters (including porosity, temperature, salt, and hydrate content) at three different moments, 5 Ma ago, 1.5 Ma ago and at present, were exhibited. The impacts of geothermal gradient and kinetic reaction coefficient on the evolution process were also discussed. Finally, the methane generated from local particulate organic carbons and different start time was used to verify the rationality of this theoretical approach. Our two-dimensional model clearly depicted the occurrence and accumulation characteristics of local stratigraphic-diffusive hydrates, disclosed the exploration value and exhibited the exploitation possibility. • Theoretically characterizes occurrence of slope stratigraphic-diffusive hydrates. • Comprehensively couples sedimentation, flow and reaction along slope surface. • Definitely demonstrates evolution process of MH reservoirs in Shenhu slope area. • Reasonably verifies hydrate geological applicability of this two-dimensional model. • Clearly discloses necessary exploration value and potential extraction possibility. [ABSTRACT FROM AUTHOR]

Among all the modern cool water carbonate margins in the world, the Great Australian Bight (GAB) presents several unique characteristics: a near complete absence of terrigenous input, highly energetic swell conditions producing vigorous off-shelf sedimentary transport to the upper continental slope, and overall high-sedimentation rates during the Pleistocene. All these conditions promoted biota and facies that are more typical of a highly productive carbonate margin. High-resolution bathymetric data collected from marine voyages between 2015 and 2017 revealed that the continental slopes of the central GAB are characterised by numerous channels and canyons as well as a large number of sedimentary mass-wasting features including kilometre scale submarine landslides and slumps. The morphometric and structural analysis of these seabed features across the continental slopes suggest that the three key controlling factors are: deep-rooted faults breaching the seabed, increasing slope gradient at the beginning of the lower continental slope, and channel wall erosion. Pulses of rapid sedimentary accumulation on the upper continental slopes, as well as differential uplift and subsidence combined with sea-level changes, appear to be the main trigger mechanisms for mass wasting. The GAB is the largest cool-water carbonate ramp system in the world. As such, the scale of sedimentary erosion and transport through channels and canyons caused by mass wasting identified in this work represents a major mechanism of carbon transfer to the deep oceans. Whilst high carbonate sedimentation during the Pleistocene in the GAB could be an important CO 2 sink in the global ocean, high sedimentation on the outer shelf and continental slope could also increase mass wasting which could act as an effective mechanism for carbon transfer to the deep oceans. However, continental slopes in the Great Australian Bight have, until recently, remained poorly studied. • Mass wasting features and submarine canyons are very common across the central GAB. • There are three main factors that appear to control the location of mass failures; deep faults, increased slope gradient and canyon wall erosions. • Trigger mechanisms include differential uplift/subsidence, high Pleistocene sedimentation and sea level changes. • Regional seismic studies most likely underestimated the Cenozoic sedimentary rates. • The Mass wasting identified in this study have transferred significant volumes of carbonate sediments to deep sea, representing a major source of carbon sink in the global ocean. [ABSTRACT FROM AUTHOR]

The Aptian-Albian boundary is a deeply investigated time interval on the basis of the deep oceanographic, environmental and climatic changes, globally recorded along the sedimentary rock successions. A regional latest Aptian extensional pulse in the Southern Tethyan margin is here envisaged as being responsible for different events of sedimentation and erosion in the Panormide carbonate platform as highlighted by the outcropping sections studied from the NW Sicilian fold and thrust belt. Integrating facies and stratal pattern analysis with backstripping methods has permitted to define the interaction between tectonics and sedimentary factors of the Lower Cretaceous shallow-water carbonates. Based on geometric configuration, lateral facies relationships, and thicknesses variations of the Lower Cretaceous Requienid limestone, is here envisaged a paleophysiography characterised by banks separated by deep channels. The margins of the isolated carbonate platform were oriented towards both the present-day west, where originally the Cala Rossa intraplatform basin developed driven by transtensional movements, and to the present-day east, where the continental slope was spreading. Including subsidence analysis and significance of the top-unconformity of the Requienid limestone a tectonic event at the end of the Aptian was detected as responsible for the fragmentation of the platform in various rotating fault-blocks and reworking of shallow-water carbonates towards the slope. The variable subsidence rates correspond to the different responses, both emersion and drowning, of the various platform-blocks to the extensional tectonics. Paleoceanographic changes, detected around the world during the Aptian-Albian, are well recorded also from the Sicilian sections, where continental clays – as the product of weathering of crystalline rocks – metallic crusts (hardgrounds) and condensed phosphatized pelagic deposits – as the product of sediment starvation, condensation and drowning – cover the Lower Cretaceous shallow-water carbonates respectively in the different faulted blocks of the segmented carbonate platform. Chemical and mineralogical analyses confirm for these deposits an influence in their formation caused by variations in oceanographic and environmental conditions, as increasing of metal ions in the surficial sea-water and high rates of groundwater precipitation. Paleoclimate evaluation reveals changes from the greenhouse arid conditions, developed during the lower Cretaceous and permitting the sedimentation of the shallow-water carbonates, to warm-humid climate condition during the latest Aptian-lower Albian boundary. • Paleoectonic setting has controlled the paleotopography and sedimentary evolution. • Lower Cretaceous paleophysiography characterised by isolated banks and deep channels. • Paleoclimatic changes from greenhouse arid conditions (Barremian-Aptian) to warm-humid climate condition (Aptian-Albian). [ABSTRACT FROM AUTHOR]

The Messinian Salinity Crisis (MSC) is a controversial geological event that influenced the Mediterranean Basin in the late Miocene leaving behind a widespread Salt Giant. Today, more than 90% of the Messinian evaporitic deposits are located offshore, buried below the Plio-Quaternary sediments and have thus been studied mainly by marine seismic reflection imaging. Onshore-offshore records' comparisons and correlations should be considered a key approach to progress in our understanding of the MSC. This approach has however not been widely explored so far. Indeed, because of the erosion on the Messinian continental shelves and slopes during the MSC, only few places in the Mediterranean domain offers the opportunity to compare onshore and offshore records that have been preserved from erosion. In this paper, we compare for the first time the MSC records from two basins that were lying at intermediate water depths during the MSC and in which salt layers emplaced in topographic lows: the Central Mallorca Depression (CMD) in the Balearic Promontory, and the Caltanissetta Basin (CB) in Sicily. The reduced tectonic movements in the CMD since the late Miocene (Messinian) till recent days, favored the conservation of most of the MSC records in a configuration relatively close to their original configuration, thus allowing a comparison with the reference records outcropping in Sicily. We perform seismic interpretation of a wide seismic reflection dataset in the study area with the aim of refining the mapping of the Messinian units covering the Balearic Promontory (BP) and restituting their depositional history based on a detailed comparison with the Messinian evaporitic units of the Sicilian Caltanissetta Basin. We discuss how this history matches with the existing 3-stages chrono-stratigraphic model. We show that the Messinian units of Central Mallorca Depression could be an undeformed analog of those outcropping on-land in the Sicilian Caltanissetta Basin, thus questioning the contemporaneous onset of the salt deposition on the Mediterranean scale. We show a change in seismic facies at a certain range of depth between stage 1 MSC units, and wonder if this could reflect the threshold/maximum depth of deposition of bottom growth PLG selenites passing more distally to pelagic snowfall cumulate gypsum. Moreover, we confirm that PLG could be deposited in water depths exceeding 200 m. • First Messinian salinity crisis seismic vs outcrop comparison and correlation. • Updated map of the Messinian Salinity Crisis units in the Balearic Promontory. • Central Mallorca Depression versus Central Caltanissetta Basin. • Evidence of a major erosional event on the promontory due to important base level drawdown. • New Messinian 3-stages depositional scenario in the Central Mallorca Depression. [ABSTRACT FROM AUTHOR]

The Late Cretaceous record offshore Argentina comprises an extensive mixed (turbidite-contourite) depositional system over 280,000 km2. This offers a key site to investigate complex assemblages of morphosedimentary features and their depositional processes during one of the major tectonic events in the southern hemisphere: the breakup of Gondwana (125 Ma) and the opening of the South Atlantic Ocean. The mixed depositional system was studied using a new 2D multichannel seismic reflection dataset and well data. This system developed along the continental slope and rise between 3500 and 6500 m SSL with nineteen 300–500 m thick, mounded drifts, separated by sixteen 2–5 km wide submarine channels. Seismic interpretation and correlations suggest four main evolutionary stages: a) the pre-drift stage (~125–89.8 Ma) from the Aptian to the Coniacian marks thermal subsidence of the margin followed by initiation of a turbidite depositional system; b) the onset stage (~89.8–81 Ma) from the Coniacian to the Campanian records the first exchange between SE turbidity flows and weak SW bottom currents; c) the growth stage (~81–66 Ma) records maximum growth from the Campanian to the Maastrichtian, characterized by the SW progradation and expansion of the mixed system due to more frequent interactions between turbidity and alongslope bottom currents; and d) the burial stage (~66 Ma) marks the cessation of the mixed system in the Paleocene due to bottom current intensification and transition to a pure contourite system, which persists until the present day. These four evolutionary phases register the Mesozoic to Cenozoic paleoceanographic fluctuations, associated with the northward opening of the South Atlantic Ocean and the establishment of a new deep-water circulation regime. Control factors for this mixed depositional system link inherited morphological structures, regional tectonic events, a mutable oceanic circulation, and recurrent gravity-driven processes. The present results were also compared with other mixed systems of similar or different geological age to contextualize the paleoceanographic and conceptual implications for deep-marine sedimentary environments. This comparison allowed us to identify two end members of drift and channel migration (upstream versus downstream), differentiated by the dominance of alongslope bottom currents vs. downslope turbidity currents. The distribution of the morphological elements and their lateral migration reflects the imprint of the most influential process and the changing energy, velocity, frequency and timing between the two processes. A future challenge will be to determine the full spectrum of mixed/hybrid systems, from turbidite-dominated to contourite-dominated settings, and how they differ vertically and spatially in the sedimentary record. • Hybrid turbidite-contourite depositional system along the Argentine Margin. • Coeval interaction between alongslope and downslope processes. • Hybrid deposits will reflect the imprint of the most influential and energetic process. • Long-term transition from a margin dominated by gravity flows to bottom-currents. • Establishment of new intermediate and deep-water circulation by the Late Cretaceous. [ABSTRACT FROM AUTHOR]

This study presents novel findings on the Pliocene and Quaternary evolution of the Gela Basin (Strait of Sicily, Mediterranean Sea), an area recording the interaction between tectonics, climate change at Milankovitch and sub-Milankovitch timescales, and dynamic water masses exchange between the eastern and western Mediterranean Sea. The calibration of seismic profiles with exploration boreholes allowed for the refining of the chronostratigraphic framework of the Gela Basin and highlighted the main phases of margin growth. Since the Pliocene, the margin has recorded the deposition of 100 m high shelf-edge clinothems, accompanied by sediment drifts on the slope and mass-transport deposits (MTDs), possibly triggered by seismic activity. Through the Plio-Quaternary the locus of deposition of sediment drifts migrated upslope due to a progressive shift of bottom currents. After the Middle Pleistocene Transition (MPT) the margin experienced an accelerated outbuilding with the deposition of a 700 m thick succession in only 0.8 Myr. At this time, a marked change in sedimentary architecture reflects the growth of shelf-edge clinothems and associated MTDs on the slope, and the spreading of contourite deposits over a broader and generally shallower area. Sediment flux to the basin and the intensity of bottom currents appear both paced at 100 kyr eccentricity orbital cycles. The growth of bottom current deposits on a large portion of the upper slope and outer shelf likely reflects constraints to the bottom current flow by the margin morphology, inherited from Miocene and Pliocene tectonics. Overall, a combination of long-term tectonic activity, climate change and shifts in oceanographic regime resulted in a complex along-strike variability of the margin morphology and stratigraphic architecture and affected where and when MTDs were emplaced. These conclusions show how climate cyclicity influence sediment supply which combined with margin morphology can promote slope instability in continental margins. • Evolution of the Gela Basin (Mediterranean Sea) during Plio-Quaternary investigated. • The outbuilding of the margin changed after the Middle Pleistocene Transition. • Contourite deposits play a key role in margin destabilization. • Clinothems and associated submarine landslides are paced at Milankovitch cyclicity. • Erosional unconformities act as glide planes for submarine landslides. [ABSTRACT FROM AUTHOR]