Continental Margin Areas Research Articles

Biogeochemistry and timing of methane-derived carbonate formation at Leirdjupet fault complex, SW Barents sea

The origin of modern seafloor methane emissions in the Barents Sea is tightly connected to the glacio-tectonic and oceanographic transformations following the last ice age. Those regional events induced geological structure re-activation and destabilization of gas hydrate reservoirs over large areas of the European continental margins, sustaining widespread fluid plumbing systems. Despite the increasing number of new active seep discoveries, their accurate geochronology and paleo-dynamic is still poorly resolved, thus hindering precise identification of triggering factors and mechanisms controlling past and future seafloor emissions. Here, we report the distribution, petrographic (thin section, electron backscatter diffraction), isotopic (δ13C, δ18O) and lipid biomarker composition of methane-derived carbonates collected from Leirdjupet Fault Complex, SW Barents Sea, at 300 m depth during an ROV survey in 2021. Carbonates are located inside a 120 x 220 m elongated pockmark and form <10 m2 bodies protruding for about 2 m above the adjacent seafloor. Microstructural analyses of vein-filling cements showed the occurrence of three–five generations of isopachous aragonitic cement separated by dissolution surfaces indicative of intermittent oxidizing conditions. The integration of phase-specific isotopic analysis and U/Th dating showed δ13C values between −28.6‰ to −10.1‰ and δ18O between 4.6‰ and 5.3‰, enabling us to track carbonate mineral precipitation over the last ∼8 ka. Lipid biomarkers and their compound-specific δ13C analysis in the bulk carbonate revealed the presence of anaerobic methanotrophic archaea of the ANME-2 clade associated with sulfate-reducing bacteria of the Seep-SRB1 clade, as well as traces of petroleum. Our results indicate that methane and petroleum seepage in this area followed a similar evolution as in other southernmost Barents Sea sites controlled by the asynchronous deglaciation of the Barents Sea shelf, and that methane-derived carbonate precipitation is still an active process at many Arctic locations.

Insights into deep metallogenic setting inSEChina based on geophysical data

SE China is regarded as one of the most important mineral resource bases in China. The spatio‐temporal distribution of the magmatism and mineralization in the area has a distinct pattern due to the uneven intensity of crust–mantle interactions. We analysed gravity, magnetic and magnetotelluric (MT) data from the study area. The lithosphere in the north‐eastern and coastal parts of South China features intense magnetic, low resistivity anomalies and low density of lithospheric mantle material components. Meanwhile, the crust in the Nanling region features high resistivity and low magnetic granitic pluton. As we view from the north‐east to the south‐west, the crust–mantle interaction in SE China weakens unevenly and the magmatic origins for ore‐forming are progressively shallow. In the continental margin area, the subducted plate partially melted and rose along deep faults, interacting with the lithospheric mantle, which created conditions for the formation of the large porphyry Cu deposits. In the inland Nanling region, the Palaeo‐Pacific Plate has subducted to a considerable depth, the influence of deep disturbances was diminished, and therefore the metallogenic magmas of W–Sn deposits are dominated by continental crust remelting granites. While the tectonic environment of the Pb–Zn–Ag polymetallic deposits is in a transitional zone between porphyry Cu deposits and W–Sn deposits, small amounts of femic magma mixed with large amounts of granitic magma to form the mineralized granites. The geophysical data provide corresponding evidence to support the mineralization dynamics model based on oblique Pacific subduction.

The mysterious oil spill in the northeastern coast of Brazil: tracking offshore seawater and the need for improved vessel facilities

A scientific cruise between November 15 and December 18, 2019 aboard R/V Vital de Oliveira collected offshore seawater samples at 59 stations between the states of Ceará and Bahia. The objective was to identify hydrocarbon levels potentially associated with the mysterious oil spill that reached the northeastern coast of Brazil. Median concentrations of aliphatic hydrocarbons (AHs, 0.79 µg L-1) and polycyclic aromatic hydrocarbons (PAHs, 5.39 ng L-1) do not indicate contamination by crude oil and are comparable to baseline levels previously found in other areas of the Brazilian continental margin. The detailed composition of both groups of hydrocarbons revealed that some samples were contaminated during sampling and/or handling on-board. The data set presented herein highlights the difficulty in tracking small oil patches spread over a huge ocean area and, more importantly, the need for improving protocols of national research vessels used to collect seawater samples for trace organic analyses.

A comparative study on natural gas hydrate accumulation models at active and passive continental margins

The comparative study on natural gas hydrate accumulation models between active and passive continental margins as well as their controlling factors is of great significance to the guidance of natural gas hydrate exploration. Based on the data and research results of international typical active continental margin hydrate accumulation areas such as the Cascadia margin of the Northeast Pacific, the Nankai trough, etc. and passive continental margin areas like the Blake Ridge, the models of the gas hydrate accumulation system are summarized and numerically simulated, and a preliminary comparison of active and passive continental margin reservoir accumulation models was also carried out. The following results were obtained. (1) The active continental margin provides a driving force and channel for vertical gas migration, which induces deep free gas and in-situ biogas to migrate along the fault. The migration channels are mainly faults, fractures and slumps produced by subduction–accretion. (2) Coarse-grained turbidity sediments such as silt and sandy silt have good porosity and permeability. Moreover, the sediment thickness on the accretionary wedge is large, which provides a good storage space for hydrate accumulation. (3) Numerical simulations of the Blake Ridge, and Niger Delta hydrate accumulation show that the passive continental margin lacks the lateral stress caused by the subduction zone compared with the active continental margin. However, due to the plastic materials in the thick sedimentary layer, high-pressure fluids and volcanic activities outside the continental margin, vertical accretion and tensile stress are generated and the accumulation rate of diffusion-type hydrates mainly depends on the methane supply rate. (4) Organic matter content, gas production rate, geothermal gradient and sedimentation rate at the passive continental margin have different effects on the spatial distribution of hydrate content. Mud volcanoes or diapir structures provide an ideal place for the formation and occurrence of hydrates.

Provenance, tectonic setting and source area-paleoweathering of sandstones of the Bahariya Formation in the Bahariya Oasis, Egypt: An implication to paleoclimate and paleogeography of the southern Neo-Tethys region during Early Cenomanian

The Lower Cenomanian Bahariya Formation forms the most prolific hydrocarbon reservoir in the Western Desert of Egypt, and represents a good example to illustrate the paleoclimatic conditions and paleogeographical evolution in the southern Neo-Tethys region during Early Cenomanian. It is also particularly important for its diverse vertebrate fauna including dipnoans, chelonians, crocodyliforms, squamates, plesiosaurs, decapod crustaceans and dinosaurs, which reflect sustained regional biostratigraphical and paleoenvironmental significance. In this study, a multidisciplinary approach of fieldwork, petrography and bulk-rock geochemistry of sandstones of the Bahariya Formation has been performed to constrain their provenance, tectonic setting, source area-paleoweathering and paleoclimatic conditions within the Neo-Tethyan realm. Petrographically, the studied sandstones are mainly of quartz-arenite type with subordinate sub-arkose and sublithic ones. Quartz types, trace and rare earth element concentrations and ratios indicate the sandstones were sourced from felsic granitic rocks with a rare contribution of felsic gneisses. The modal analysis reflects that the sandstones are mainly of cratonic interior and quartzose recycled tectonic provenance. The chondrite-normalized REE distribution pattern of the Bahariya sandstones are characterized by enriched LREE and flat HREE similar to those of UCC with negative Eu/Eu* anomalies. Major and trace elements of the sandstones suggest their deposition on a Neo-Tethys passive continental margin related to the Early Cretaceous rifting. The weathering indices (CIA, CIW, PIA and ICV) and the Al2O3-(CaO + Na2O)-K2O diagram suggest moderate to severe chemical weathering in the source area under semi-humid to humid climatic conditions. In conclusions, the Bahariya sandstones could be sourced by a fluvial reworking of moderately- to intensively-weathered Precambrian granites and gneisses of the surrounding Gebel Uweinat Massive (at the southwest of the study area), and were deposited in a low-lying basin (Bahariya area) of the Neo-Tethys passive continental margin. In a broad interest, results of this study could contribute in a reconstruction of the paleoclimatic conditions and paleogeographical evolution of the southern Neo-Tethys region during Early Cenomanian.

Habitat suitability and environmental niche comparison of cold-water coral species along the Brazilian continental margin

In face of increasing anthropogenic disturbance in the deep sea, it is a priority to better understand the regional distribution of cold-water corals (CWC). These organisms create some of the most species-rich habitats in the deep sea and, for this reason, they must be properly protected and managed. In this study, we aimed to identify suitable habitat for multiple CWC taxa off the Brazilian continental margin and compare their environmental niches. Habitat suitability models were developed using the Maxent approach, which allowed for the prediction of species distribution and for the identification of potential ‘hot spot’ areas that may be important for biodiversity conservation. Ecological niches were determined by a PCA-env approach, and niche similarity and equivalence were evaluated based on niche overlap using the Schoener's D metric. Potentially suitable habitat for Octocorallia covered a broad latitudinal range encompassing nearly the entire Brazilian continental margin, whereas Scleractinia had greater potentially suitable habitat in the Central and Southern areas. Scleractinian species were observed to slightly differ in their environmental niche, with non-reef-forming species being more tolerant to a wider range of environmental conditions in comparison with reef-forming species, inhabiting a wider area of the South American continental margin. Due to the high potential suitability for several CWC species, the Central and Southern parts of the Brazilian continental margin should be considered as potential areas high CWC diversity. Considering the current state of the art and strategic assessment tools, these areas are important targets for conservation, management, and environmental impact assessment. Most reef-forming species had similar but not directly equivalent ecological niches, indicating that mapping efforts and management planning should consider CWCs at the species level.

Mixgenetic concept of of oil and gas fields formation in basement and sedimentary cover on the shelf of South Vietnam

According to the geodynamic model of oil and gas formation, the most favorable conditions for the oil and gas fields are formed in the mobile zones of the earth’s crust, especially in areas of active continental margins, characterized by high seismicity, the presence of deep faults, the development of subduction and riftogenic processes. Therefore, it is logical that most of the world’s oil and gas deposits are concentrated in rifts or in the vicinity of paleo- and modern subduction zones. The study of the unique oil deposits in the granite basement of the White Tiger field, using data from other fields in the world, allows concluding that the formation of oil deposits in the basement can occur not only due to the resources of adjacent oil and gas deposits. Taking into account modern geodynamic ideas, in the context of the Earth’s internal geospheres, at least three oil generation zones can be distinguished: mantle-asthenospheric abiogenic synthesis; subduction-dissipative biomineral synthesis; stratospheric-biogenic synthesis. Obviously, all these three zones, as a single open system for the generation of hydrocarbons, will be interconnected only in conditions of deep faults, active continental margins and other parts of the Earth’s crust. This suggests that there are deep generation zones, which are currently fueling the developed fields.

Postrift stress field inversion in the Potiguar Basin, Brazil – Implications for petroleum systems and evolution of the equatorial margin of South America

Rifting and related normal stress regime in the equatorial continental margin of Brazil ceased during the Late Cretaceous, when the stress regimes in eastern South America and West Africa changed to induce strike-slip or reverse motion. In this study, we explore the postrift tectonic, geomorphic, magmatic, and sedimentary responses to stress changes using the Potiguar Basin, the easternmost basin in the equatorial margin of Brazil, as a case study. We use field and topographic data, 2D seismic reflection lines, vertical electric soundings, and geochronological and borehole data to constrain the stress evolution of the Potiguar Basin from the Late Cretaceous to the Quaternary, discussing the role of basin inversion on sedimentation and landforms. Our results indicate the presence of two strike-slip stress regimes after rifting. The first stress field (SF1) occurred from Late Cretaceous to the middle Miocene and consisted of a N-S-oriented maximum subhorizontal compression and an E-W-oriented extension. The second stress field (SF2) took place from the middle Miocene to the present day and included subhorizontal E-W to NW-SE compression combined with N-S and NE-SW subhorizontal extension. Emplacement of volcanic rocks occurred along transtensional faults, with a principal peak during SF1 at 20–30 Ma and a subordinate peak during SF2 at 5–10 Ma. In response to shortening during SF2, a 70-km-long and 50-km-wide dome formed, where marine Miocene strata were uplifted to ~250 m asl. This uplift induced the displacement of alluvial channels away from the dome. Anticlines formed by transpression along the main NE-SW-striking faults during both SF1 and ST2 acted as traps in the petroleum system. Similar shifts and stress field inversions documented in other areas of the Brazilian continental margin are consistent with the Neogene rise of the Andes and may have implications for reconstructing the tectonic history of the Equatorial Atlantic margin of South America.

Multi-chronometer thermochronological modelling of the Late Neoproterozoic to recent t-T-evolution of the SE coastal region of Brazil

South-eastern Brazil is as an important geological archive for understanding and reconstructing various plate tectonic stages of the Wilson Cycle. In the Neoproterozoic, the area of the today's South Atlantic passive continental margin (SAPCM: e.g. between São Paulo and Laguna) of south-eastern Brazil underwent subduction, followed by the collision of the contemporary plates of South America and Africa creating a Neoproterozoic orogeny within the supercontinent Gondwana. During the Palaeozoic and Lower Mesozoic (stage 1), the future SAPCM, as an intracratonic area, experienced erosion, denudation of the Neoproterozoic mobile belts (Pan African/Brasiliano orogeny), and large basin formation (Paraná Basin) (stage 2). Possibly plume-driven pre-to syn-rift (embryonic), ocean spreading (juvenile), and post-break up (mature) processes led to the recent evolution of the SAPCM since the Upper Mesozoic (stage 3).For the first time, this research aims to reconstruct the syn-to post-orogenic t-T-evolution of Neoproterozoic basement rocks of the SE coastal region of Brazil covering the entire geological evolution since the Late Neoproterozoic. Therefore, this study uses geochronological and thermochronological data combined with numerical modelling. This includes published geochronological data of Neoproterozoic basement samples such as U-Pb, Sm-Nd and Rb-Sr analyses, and low temperature thermochronology (LTT) data revealed by K/Ar, 40Ar/39Ar analyses. To this existing LTT data set, we report new apatite (AFT) and zircon (ZFT) fission-track, and (U-Th-Sm)/He (AHe, ZHe) data. Numerical modelling of that LTT data attached to the existing geochronological data indicates the following evolution:- Stage 1: In the central part of the future SAPCM, the Pan African/Brasiliano post-orogenic cooling and exhumation (uplift and erosion of Neoproterozoic rocks to the surface) history occurs in three phases: (i) rapid Late Neoproterozoic exhumation, (ii) a period of relative thermal stability (temperatures of about 200–300 °C) in which rocks reside at upper crust levels during the Early Cambrian to Devonian, and (iii) a second rapid exhumation phase moving the Neoproterozoic basement rocks to the surface during the Devonian. The northern and southern parts indicate a distinct post-orogenic exhumation suggesting faster cooling and exhumation from the Late Neoproterozoic to Devonian/Carboniferous than in the central section.- Stage 2: A phase of subsidence leading to the formation of the Paraná Basin followed by pre-to syn-rift processes and the emplacement of the Paraná-Etendeka flood basalts.- Stage 3: Post-South Atlantic break up processes, such as erosion and exhumation.

Deep-water sediment waves as a special gas hydrate reservoirs in the Northeastern South China Sea

The gas hydrate is attracting increasing attention in the sedimentary geology as an alternative energy source in the future. Recent exploration efforts have indicated that gas hydrate is commonly associated with sediment waves in slope area of quasi-passive continental margin. Based on the deep-water seismic data and core data of the northeastern part of the South China Sea (SCS), this research describes the seismic and geological characteristics of the sediment waves and gas hydrate reservoirs, and discusses the accumulation model of gas hydrate in sediment waves reservoir. The results demonstrate that: (1) the sediment waves widely appear in study area, and those have a “wave-like” bedform and are formed by deep-water contour or turbid current sedimentation. Meanwhile, the sediment waves are affected by gas leakage, diapirism, and neotectonics. (2) The stable zone of gas hydrate which simulated calculation by theory is consistent with core data. The complete and typical gas hydrate seismic model including a “bottom-simulating reflection (BSR) + blank zone (BZ) + enhanced reflector body (eHB)” structure can be observed in the sediment waves seismic profile of the study area. (3) The gas hydrates are stored in sediment waves, which are influenced by the original structure of the sediments and have different gas hydrate accumulation dynamics that are driven by diffusion and/or leakage. Thus, the sediment waves as a favorable reservoir of gas hydrate have research significance.

Hydrocarbon Generation by the Rocks of the Bremer Formation in Adjacent Areas of the Nonvolcanic Passive Margins of Australia and Antarctica

This paper analyzes differences in the history of hydrocarbon (HC) generation by the rocks of the Bremer 1–6 formations in adjacent areas of the nonvolcanic passive continental margins of Australia and Antarctica. The problem is examined by the example of the numerical reconstruction of the burial and thermal history of two sedimentary sequences of approximately equal thicknesses: the section of well 19–2012 in the Bremer sub-basin of the southwestern margin of Australia and the section of pseudowell 2 in the adjacent area of the passive margin of Antarctica on seismic profile 5909 across the Mawson Sea. The asymmetry of Gondwana rifting in the region of interest resulted in asymmetry in the tectonic structure and development of adjacent areas of passive margins and, as a consequence, significantly different histories of HC generation by the rocks of the Bremer 1–6 formations in these areas. Modeling indicates that the rocks of the Bremer 1 and 2 formations are mainly gas-prone in the Bremer basin and can become oil-prone in the Mawson Sea region of the Antarctic margin. In contrast, according to modeling, the rocks of the Bremer 4 and 5 formations generate a minor amount of HC in the well 19–2012 area of the Bremer sub-basin and considerable amounts of heavy and light oil in the adjacent Antarctic margin area at pseudowell 2 in the Mawson Sea.

Submarine Landslides on the Western Slope of the Kuril Basin, Sea of Okhotsk

Landslide processes on the western slope of the Kuril Basin were studied using bathymetry and seismic data obtained under the international KOMEX and SSGH projects. Slope areas containing landslides, landslide blocks and mass-transport deposits were distinguished. Large-scale landslides occupying an area of more than 100 km2 are located in such areas of open continental margins as the slopes of the North Hokkaido Marginal Plateau and Terpeniya Ridge. Landslide blocks up to 2 km in size and mass-transport deposits are located in submarine canyons and fans in Terpeniya Bay. The age of landslides has been estimated as Middle Pleistocene–Holocene. Landslides are most likely triggered by seismic activity and gas saturation of sediments. Subsequent slope failure seems quite probable within the study area, and landslides capable of generating tsunamis may occur.

Nitrogen isotopic composition of sediments from the eastern Tethys during Oceanic Anoxic Event 2

Low nitrogen isotopic compositions (δ15N) have been reported in deposits from the proto-North Atlantic and western Tethys during Oceanic Anoxic Event 2 (OAE2) at the Cenomanian-Turonian (C-T) transition. It has been proposed that the low δ15N values may have resulted from enhancement of N2 fixation and/or incomplete assimilation of the upwelled ammonium due to expansion of anoxic/euxinic conditions. However, few δ15N measurements are available for the sediments from the eastern Tethys and the Pacific and therefore the global significance of the N cycle and its environmental implications remains to be tested. Here, we present δ15N data from OAE2 deposits in the Gongzha section in the Tingri area of southern Tibet, which was located on the southern margin of the eastern Tethys during the mid-Cretaceous. The δ15N data reveal a large perturbation of the N cycle during OAE2. The pronounced negative shift in δ15N prior to OAE2, as well as the persistent low δ15N values during OAE2, imply the significant contribution of N2 fixation. The changes in the marine N cycle suggest that the expansion of an anoxic environment may also have occurred in the southern continental margin areas of eastern Tethys. The coincidence of the marine faunal turnover and the perturbation of the N cycle suggests that the expansion of anoxic water masses may have played an important role in the extinction of marine organisms across the C-T boundary in eastern Tethys.

3D Process-Oriented Gravity Modelling applied north of 49°S on the Argentine continental margin

The Process-Oriented Gravity Modelling (POGM) technique represents a useful way to distinguish the contribution that different geological processes make to the observed gravity in passive margins. The POGM is an innovative gravity modelling approach that can give us information about the role that processes such as sedimentation and magmatic underplating, together with their loading effects, may play in the evolution of a margin. In this work, the POGM methodology has been applied with in a 2D and 3D approach. 2D profiles spaced every one arc-minute in the area of the Argentine continental margin, between 38.5°S and 49°S latitude and 64°W and 50°W longitude, were used to generate the latter.The 3D POGM was also solved and the result was compared with that obtained from 2D profiles. The comparison with the observed anomaly, using the 3D approach from 2D profiles gave results with enhanced resolution.The best fit between the calculated and observed gravity anomaly is given by an effective elastic thickness of 15 km. A cortical thickness map obtained as a result of the POGM calculations shows basin areas characterized by a thinned crust and a structural variation where the continental-oceanic boundary (COB) could be indicated. Besides, results of POGM allow us to detect an alignment between the Valdés and Rawson basins and possibly a third basin as a probable aulacogen. A stretching factor analysis shows that in these basins a stretching period existed but it did not reach the stage of oceanic crust formation. A strong positive residue in the Colorado basin is shown by the flexural isostatic anomaly, suggesting that the basin may continue in subsidence.

Deep structure of the Lofoten‐Vesterålen segment of the Mid‐Norwegian continental margin and adjacent areas derived from 3‐D density modeling

AbstractTo understand the major structural features of the sedimentary cover and crystalline crust of the Lofoten‐Vesterålen margin and the northern part of the Vøring segment of the Mid‐Norwegian continental margin, a lithosphere‐scale 3‐D structural model has been constructed. This model extends from the exposed crystalline rocks of the Fennoscandian Shield in the east to the Cenozoic oceanic domain of the Norwegian‐Greenland Sea in the west, covering the Vestfjorden, Ribban, and Røst Basins and the northern parts of the Vøring Basin and Trøndelag Platform. All available published and/or released data have been used to set the initial 3‐D model which has been validated by means of 3‐D density forward modeling to obtain a gravity‐consistent 3‐D structural/density model. Results from the 3‐D density modeling reveal that relatively thick sedimentary rocks are present in the distal Røst Basin below the lava flows. The presence of a low‐density more than 20 km thick granitic body has been modeled within the middle‐upper crystalline crust beneath the eastern part of the Vestfjorden Basin and the adjacent mainland. Moreover, the results of the 3‐D density modeling indicate the presence of an atypical low‐density lithospheric mantle beneath a large part of the Lofoten‐Vesterålen margin which is required to fit the regional component of the modeled gravity with the observed one. The pronounced crustal feature within the model area is the Bivrost Lineament that appears to be the deeply seated lithosphere‐scale boundary that delineates clearly the Lofoten‐Vesterålen segment from the Vøring margin showing contrasting densities and crustal thicknesses.

Mid-Cretaceous carbon cycle perturbations and Oceanic Anoxic Events recorded in southern Tibet

The organic carbon isotope (δ13Corg) curve for ~1.7-km-thick mid-Cretaceous strata of the Chaqiela section in Gamba area, southern Tibet is presented in this study. C-isotopic chemostratigraphic correlation combined with biostratigraphic constraints show that the Chaqiela section spans early Aptian through early Campanian period, and that almost all of the carbon cycle perturbations and Oceanic Anoxic Events during the mid-Cretaceous period are well recorded in the continental margin area of the southeastern Tethys Ocean. Significantly, two levels of methane-derived authigenic carbonates were identified at the onset of OAE1b near the Aptian-Albian boundary. We suggest that an increase in methane release from gas hydrates, potentially driven by sea-level fall and bottom water temperature increase, may have contributed to the large negative δ13Corg excursions and global warming during OAE1b.

First laboratory insight on the behavioral rhythms of the bathyal crab Geryon longipes

The deep sea is the largest and at the same time least explored biome on Earth, but quantitative studies on the behavior of bathyal organisms are scarce because of the intrinsic difficulties related to in situ observations and maintaining animals in aquaria. In this study, we reported, for the first time, laboratory observations on locomotor rhythms and other behavioral observations (i.e. feeding, exploring and self-grooming) for the bathyal crab Geryon longipes. Crabs were collected on the middle-lower slope (720–1750m) off the coast of Blanes (Spain). Inertial (18h) water currents and monochromatic blue (i.e. 470nm) light-darkness (24h) cycles were simulated in two different experiments in flume tanks endowed with burrows. Both cycles were simulated in order to investigate activity rhythms regulation in Mediterranean deep-sea benthos. Crabs showed rhythmic locomotor activity synchronized to both water currents and light-darkness cycles. In general terms, feeding and exploring behaviors also followed the same pattern. Results presented here indicate the importance of local inertial (18h) periodicity of water currents at the seabed as a temporal cue regulating the behavior of bathyal benthic fauna in all continental margin areas where the effects of tides is negligible.

Magma emplacement: an important trigger leading to slope failures in deep‐water areas of northern continental margin of South China Sea

AbstractSubmarine slope failures are considered as a serious geo‐hazard because they can rupture undersea pipelines, and seafloor engineering facilities can be subverted by destroying the seabed stability. Submarine slope failures are widely developed in deep‐water areas of the northern continental margin of the South China Sea, yet the mechanisms involved in their generation remain poorly known. Observations have proposed that slope instability can be due either to decomposition of gas hydrate or to magma emplacement. Here, we present new high‐resolution 2D seismic data from the deep‐water areas of the northern continental margin of the South China Sea to reveal an extensive and intimate relationship between magma emplacement and slope failures. This paper shows four landslides on the Dongsha, Shenhu, Xisha and Qiongdongnan slopes that occurred at different locations of the northern continental slope of the South China Sea. There are seismic evidences that magmatic rocks commonly developed and migrated upwards to the overlying layers and they caused these layers to uplift as a dome and possibly decreased their strength and predisposition to failure. The data reveal in detail how magma emplacement influenced slope stability and the seismic reflection features of this type of submarine landslides. The findings illustrate the importance of magma emplacement resulting in slope instability. This paper proposes that magma emplacement is a basic and critical trigger leading to slope failures in the deep‐water areas of the northern continental margin of the South China Sea and establishes genetic models for this important trigger. Copyright © 2016 John Wiley & Sons, Ltd.

Contributions to the Petrology of Upper Cretaceous of El Duwi Formation at Gabal El Galala El Qibliya area, North Eastern Desert, Egypt

The Upper Cretaceous succession exposed at Saint Anthony , Saint Paul, and Wadi El Dakhl areas was measured and studied in detail. The studied sequence is subdivided lithostratigraphically, into the Galala Formation (Cenomanian-Early Turonian), Wata Fotrmation (Late Turonian), El Duwi Formation (Coniacian-Santonian), and Sudr Chalk (Campanian-Maastrichtian), Esna Shale (Paleocene-Early Eocene) and Thebes Formation (Early Eocene). The examined rock units in Gabal El Galala area include El Duwi Formation and Sudr Chalk were studied lithostratigraphically, petrography, mineralogy, geochemistry, radioactivity and tectonic setting. On the other hand, the major and trace elements were focused on the studied rock units. The Upper Cretaceous exposures in southeastern Galala represented by Carbonate – dominated successions inter-bedded with shale, a few sandstone beds, and marl horizons and capped by Sudr chalk. The average chemical compositions of the shale from the study area are (33.37-59.55%) for SiO2, (6.51-16.49%) for Al2O3, (1.07-2.33 %) for CaO, (1.16-2.09%) for MgO, (0.99-3.10%) for Na2O, (0.45-1.6%) for K2O, (1-2.86%) for P2O5, (5.37-8.865%) for Fe2O3, (31-791 ppm) for Rb, (291-1500 ppm) for Zr, (331-2834 ppm) Sr, (15-61 ppm) for Ga, (89-711 ppm) for V and (9-45 ppm) for U. Detailed environmental scanning-electron microscope (ESEM) study was carried out on the shale and Chalk. This study appears the characterization of the assembly as pyrite, gypsum and jarosite. The tectonic setting reveals that the shale and chalk of the upper Cretaceous in El Duwi Formation were originated from greywacke in active continental margin area.

Diversity of the benthic macrofauna off northern Namibia from the shelf to the deep sea

In late summer 2011, shortly after an upwelling event, 17 stations ranging from 30 to 2513m water depth have been sampled at 20° south in the northern part of the Benguela Current Large Marine Ecosystem (BCLME) for the investigation of the benthic macrofauna. Sediments of this area are dominated by silt. At the time of sampling, oxygen conditions on the shelf were poor (between 0.42 and 0.68ml l−1) but not hypoxic. Below 400m, however, concentrations rose steadily up to 5.28ml l−1. Macrozoobenthic communities along this depth gradient are described, revealing among others the community structure for the continental margin area and the deep sea off northern Namibia for the first time. Cluster analysis revealed 5 different communities along the depth gradient with three shelf communities, one continental margin community and one deep-sea community. All in all, 314 different taxa were found with polychaetes being the most abundant group. Diversity index (Shannon) was lowest for the shallow water community with 2.21 and highest for the deep-sea community with 4.79, showing a clear trend with increasing water depth. Species richness, however, reached its maximum with 187 taxa along the continental margin between 400 and 1300m water depth. Dominant species for each community are named with the two Cumacea, Iphinoeafricana and Upselaspis caparti, being characteristic for the shallow water community. On the shelf, we found surprisingly high biomass values (23–123g m−2), mainly caused by polychaetes, the bivalve Sinupharus galatheae and the gastropod Nassarius vinctus. In terms of composition, the remaining communities were dominated by polychaetes with members of the Paraonidae dominating along the continental margin where we also found surprisingly high abundances of the bivalves Pecten sp. and Dosinia sp. Spionid polychaetes and some representatives of the genus Paraonis were the most common organisms for the deep-sea community.