West African Continental Margin Research Articles

Low abundance and high patchiness of decapod fauna sampled with van Veen grab on the West African continental margin (Gulf of Guinea, Ghana)

Sixty morphospecies of Decapoda (Malacostraca: Crustacea) representing 34 families were recorded in the material collected in 2012 from 265 van Veen grab (0.1 m2) samples, from the nine transects distributed along the coast of Ghana in the 25–1000 m depth range. The examined material was dominated by the Diogenidae, Panopeidae, Leucosiidae, Pilumnidae and Xanthidae families. Species accumulation curves showed undersampling of the studied area and a large number of the morphospecies comprised singletons and doubletons. Panopeus africanus was the most frequent morphospecies in the analysed material (9.1% of all samples). We observed a substantial decrease of diversity (Shannon Index) and abundance along a depth gradient. Species richness also decreased with depth, starting from the highest number of morphospecies ‒ 38 at 25 m depth, then 33 at 50 m, 17 at 100 m, 11 at 250 m, 8 at 500 m and ending with 1 morphospecies at 1000 m bottom depth. Higher diversity was observed on the continental shelf (25–250 m – 57 morphospecies), while on the slope (500–1000 m) only eight morphospecies were recorded. Numerous factors of natural and anthropogenic origin may affect decapod communities on the coast of Ghana. Since our material was collected using a sampler collecting material at a very small scale, the observed patterns might be affected by the sampling method.

Euryalida (Echinodermata, Ophiuroidea) from Northwest Africa

From 2004 to 2012, ten multidisciplinary oceanographic surveys were conducted along the coast of Northwest Africa, between the Strait of Gibraltar and the northern border of Sierra Leone. A total of five species of Euryalida Lamarck, 1816 belonging to three families were captured at 29 of the 1298 stations sampled in the area. Among them, Astrodendrum juancarlosi sp. nov. is described and figured in this paper. Ophiocreas oedipus Lyman, 1879 is recorded for the first time on West African continental margin and Gorgonocephalus pustulatum (H.L. Clark, 1916), an Indo-Pacific species only known from South African coast in the Atlantic, is reported off Guinea-Bissau, greatly extending to the North its Atlantic distribution. In addition, Asteroschema inornatum Koehler, 1906, a northeast Atlantic species, is recorded for the first time in African waters, off Western Sahara, extending its range of distribution to the south. Our data also extend the bathymetric distribution of A. inornatum to shallower waters and of G. pustulatum to deeper waters. The association of some euryalids with certain species of pennatulaceans and gorgonians is also described.

High polychaete diversity in the Gulf of Guinea (West African continental margin): The influence of local and intermediate scale ecological factors on a background of regional patterns

The Tropical East Atlantic is one of the least studied areas in the world's oceans, and thus a blank spot on the map of marine studies. Shaped by dynamic currents and shifting water masses, it is a key region in discussions about marine ecology, biodiversity, and zoogeography, while facing numerous, poorly understood, and unmonitored threats associated with climate change, acidification, and pollution. Polychaete diversity was assessed along four transects along the Ghana coast, from shallow to deep bottoms and distributed along the whole upwelling marine ecoregion. Despite high sampling effort, steep species accumulation curves demonstrated the necessity of further sampling in the region. We observed zonation of fauna by depth, and a decrease in species richness from 25 m to 1000 m depth. Polychaete communities were influenced by sediment type, presence of oxygen minimum zones, and local disturbances caused by elevated barium concentrations. Similar evenness along the depth gradient reflected the importance of rare species in the community structure. Differences in phylogenetic diversity, as reflected by taxonomic distinctness, were small, which suggested high ecosystem stability. The highly variable species richness at small scale (meters) showed the importance of ecological factors giving rise to microhabitat diversity, although we also noticed intermediate scale (50–300 km) differences affecting community structure. About 44 % of the species were rare (i.e. recorded only in three or fewer samples), highlighting the level of patchiness, while one fifth was distributed on all transects, therefore along the whole upwelling ecoregion, demonstrating the influence of the regional species pool on local communities at particular stations. Our study yielded 253 species, increasing the number of polychaetes known from this region by at least 50 %. This casts doubt on previous findings regarding Atlantic bioregionalization, biodiversity estimates and endemism, which appear to have been more pronouncedly affected by sampling bias than previously thought.

Rock physics as a tool for seismic stratigraphy and structural interpretation of late Miocene southwestern Deep Offshore Niger Delta, West African Continental Margin

Rock physics as a tool for seismic stratigraphy and structural interpretation of late Miocene southwestern Deep Offshore Niger Delta, West African Continental Margin

Modelling of polychaete functional diversity: Large marine ecosystem response to multiple natural factors and human impacts on the West African continental margin

Polychaetes are one of the most diverse groups of marine organisms, characterized by high species richness, diversity of feeding guilds, life styles, and mobility types. Marine annelids are useful indicators of ecosystem responses to changes in environmental conditions. The aim of our study was to assess the influence of natural and anthropogenic factors on functional diversity of polychaete communities in the Gulf of Guinea, a large marine ecosystem (LME) located in West Africa. This area can be considered as a model marine ecosystem affected by various human influences, such as pollution associated with the oil industry. Material was collected in 2012 across the coast of Ghana. Samples were gathered along four transects, each with six sampling stations (25–1000 m depth range). Analyses of functional richness and evenness, based on generalized linear mixed-effect models and hierarchical partitioning, allowed for complex assessments of the interactions between polychaete communities and environmental factors (e.g., sediments, total organic matter, salinity, fluorescence, oxygen, concentration of toxic metals, total hydrocarbons). Overall species richness of polychaetes was outstandingly high, with 253 species recorded. Functional richness decreased along a depth gradient, while functional evenness increased with depth, and was positively correlated with Ba content, which reached the highest values in the upper bathyal. Gravel content was an important factor in shaping functional composition of shallow water communities. High values of functional richness observed in the shallows may be an expression of high stability of this ecosystem, at the same time indicating its high resilience. Elevated concentrations of lead also influenced community structure at a local scale. Our study demonstrated how a complex set of factors operating along a depth gradient can influence the functional composition of communities. These results are crucial for future management of industrial and environmental protection activities in this region.

A time-lapse case study in West Africa: Integrating disciplines for a complete reservoir study and field management

Development and management of deepwater oil fields requires material investment and must be supported by application of quality technological solutions and tools. To maximize the advantages that can derive from these, it is essential to integrate technologies and disciplines throughout the field cycle. This integration makes it possible to identify and fix issues early and seize business opportunities quickly. We describe the activity carried out on a field characterized by a high-quality oil-bearing clastic reservoir located in a deepwater belt along a West African continental passive margin. We highlight how geoscience specialists from both geologic and geophysical branches have contributed to construction of the static and dynamic reservoir model, field monitoring, model updating, and operations optimization. The wide geologic and geophysical experience in the area made it possible to build a petroelastic model and a sedimentological model that were integrated in the population of a geocellular model. Seismic data also were used to support reservoir monitoring of the producing field. A dedicated 4D seismic project helped our understanding of fluid movement and fault behaviors in the field. The 4D information was interpreted and shared within the integrated team with the goal of identifying further business opportunities and driving decision making over our area of interest.

Natural and anthropogenic factors influencing abundance of the benthic macrofauna along the shelf and slope of the Gulf of Guinea, a large marine ecosystem off West Africa

Summary The West African continental margin belongs to the least known areas in terms of the ecology of benthic ecosystems. At the same time, this region is influenced by various threats associated with human activities, including industrialisation and oil excavation. Here, we analyse the abundance and distribution patterns of macrozoobenthic communities along the coast of Ghana. The material was collected in 2012 on nine transects at depths ranging from 25 to 1000 m. Over 200 quantitative samples were collected using a 0.1-m2 van Veen grab. Generally, the mean density of macrozoobenthos decreased gradually from the shallow zone (25 m: 231.4 ± 262.2 ind./0.1 m2) down to bathyal depths (1000 m: 55.4 ± 51.4 ind./0.1 m2), but we observed intermediate scale variability in distribution patterns among the transects along the Ghanaian coast. Analysis of environmental factors showed no evidence of substantial pollution, although levels of hydrocarbons, barium and some other toxic metals show some local increases at particular stations, especially on the continental slope. Cluster analysis based on Bray–Curtis similarity and abundance of higher taxonomic groups of macrofauna yielded five groups of stations, while SIMPER analysis demonstrated that polychaetes and amphipods contributed most significantly to within-group similarity. Canonical Correspondence Analysis demonstrated that PAH, THC and toxic metal levels (Ba, Cd, Pb), as well as oxygen concentration, were the most important factors structuring benthic communities.

New constraints from detrital zircon ages on prolonged, multiphase transition from the Cadomian accretionary orogen to a passive margin of Gondwana

The Avalonian–Cadomian belt formed as a collage of accretionary complexes, island arcs, and sedimentary basins along the northern margin of Gondwana during the Neoproterozoic times. One of the enigmatic issues is how and when this active margin evolved to a passive margin in the southern realms of the Rheic Ocean during late Cambrian to early Ordovician. This issue is addressed here using U–Pb detrital zircon dating of key stratigraphic sections in the Teplá–Barrandian unit, Bohemian Massif. The new ages constrain the base of the Davle volcanic arc to 608 ± 6 Ma, 591 ± 6 Ma, and 590 ± 4 Ma and its termination to 563 ± 7 Ma. The lower part of the overlying Příbram–Jince continental basin was deposited between 512 ± 5 and 511 ± 3 Ma. These brackets define a time span of about 50 M.y. for the geodynamic transition from Cadomian subduction, accretion, and arc magmatism to a passive margin and lithospheric extension. In our model, a major role is assigned to a presumably curved geometry of the adjacent West African continental margin, which consisted of the outboard Armorican–Saxothuringian portion at an angle to the inboard Iberian–Barrandian portion. We suggest that during oblique subduction, ridge–trench collision and slab break-off occurred in the former at ca. 540 Ma, while subduction/accretion continued in the latter and was terminated by a major pulse of sediment accretion at around 527 Ma and ridge subduction at ca. 524–520 Ma. Finally, the tectonic regime in the Cadomian arc and accreted upper crust switched to extension associated with crustal remelting to produce intraplate magmatism and development of continental basins with short-lived marine transgressions between ca. 512 Ma and ca. 488 Ma. The continued extension led to rifting and basaltic volcanism, directly linked to the diachronous opening of the Rheic Ocean. We conclude that the prolonged transition from active to passive margin in the Cadomian belt was driven two principal geodynamic causes: first by oblique oceanic subduction and ridge–trench interactions, bearing many similarities to modern Cordilleran-type active margins, and then by the far-field slab-pull force of the subducting Iapetus Ocean.

STRUCTURES AND HYDROCARBON PROSPECTS IN EMI FIELD, OFFSHORE NIGER DELTA

The Niger Delta is ranked among the world’s major hprovinces in the world. Oil and gas in t he Niger Delta are mainly trapped in sandstones and unconsolidated sands in t he Agbada formation and the structural traps are no t easy to map precisely because they are very slight, not obvious and very complicated. The main aim was to determine the stru ctures and hydrocarbon prospects in Emi-field, off shore Niger Delta using seismic data integrated with well logs. Four (4) h orizons namely L1, L2, L3 and L4 were selected and structural maps drawn for each of the horizons. Fault closures of high quality hydro carbon prospects were identified and delineated. The integration of seism ic data with well logs greatly improved the extent of accuracy and exactness of structural maps in hydrocarbon prospects and its de velopment in Emi Field. --------------------------------------------------- --------------------***---------------------------- The Niger Delta is ranked among the world’s major hydrocarbon provinces in the world. It is the most important in the West African continental margin. Oil and gas in the Niger Delta are primarily produced from sandstones and unconsolidated sands mainly in the Agbada formation. The goal of oil and gas exploration is to identify and delineate structural traps suitable for profitably exploitabl e accumulations and delineate the extent of discoveri es in field appraisals and development. These traps are very sl ight, not obvious and very complicated and are therefore not easy to map precisely. Major improvements in seismic and well logs have made it possible to map such structural arrang ements with high degree of accuracy and exactness. Seismic profiles provide almost a continuous latera l view of the subsurface by defining its geometry and providi ng an estimate of the acoustic impedance which is related to the formation densities and velocities. However, vertical details are limited due to length y duration of the individual seismic wavelets and the occurren ce of overlapping wavelets from closely spaced reflectors . Well logs are limited in their definition of lateral variatio n of subsurface parameters. Thus, the extent of accuracy in mapping complicated structural plays would be improved greatly by combining seismic data with well logs ([1]; [2]; [3 ]). In addition, the risk associated with finding oil and gas in very slight, not obvious and complicated structural plac es will be reduced tremendously since such integration will he lp to discern differences between poor and rich reservoir s and also greatly improve the extent of accuracy and exactnes s of the structural maps in hydrocarbon prospects and its de velopment. In this research, 3-D seismic reflection data were integrated with well logs so as to characterize the subsurface geometry and also determine the structures and hydrocarbon p rospects of the field. 1.1 Location and Geological Setting of the Field

Magnetotelluric deep soundings, gravity and geoid in the south São Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating

In the south São Francisco craton a circular and 8-m amplitude geoid anomaly coincides with the outcropping terrain of an Archean–Paleoproterozoic basement. Broadband magnetotelluric (MT) data inversions of two radial profiles within the positive geoid and Bouguer gravity anomaly yield geo-electrical crustal sections, whereby the lower crust is locally more conductive (10 to 100 Ωm) in spatial coincidence with a denser lower crust modeled by the gravity data. This anomalous lower crust may have resulted from magmatic underplating, associated with Mesoarchean and Proterozoic episodes of tholeiitic dike intrusion. Long-period MT soundings reveal a low electrical resistivity mantle (20 to 200 Ωm) from depths beyond 120 km. Forward geoid modeling, using the scope of the low electrical resistivity region within the mantle as a constraint, entails a density increase (40 to 50 kg/m 3) possibly due to Fe enrichment of mantle minerals. However, this factor alone does not explain the observed resistivity. A supplemented presence of small amounts of percolated carbonatite melting (~ 0.005 vol.%), dissolved water and enhanced oxygen fugacity within the peridotitic mantle are viable agents that could explain the less resistive upper mantle. We propose that metasomatic processes confined in the sub-continental lithospheric mantle foster the conditions for a low degree melting with variable CO 2, H 2O and Fe content. Even though the precise age of this metasomatism is unknown it might be older than the Early Cretaceous based on the evidence that a high-degree of melting in a lithospheric mantle impregnated with carbonatites originated the tholeiitic dike intrusions dispersed from the southeastern border of the São Francisco craton, during the onset of the lithosphere extension and break-up of the western Gondwana. The proxies are the NE Paraná and Espinhaço (130 Ma, Ar/Ar ages) tholeiitic dikes, which contain (~ 3%) carbonatites in their composition. The occurrence of a positive geoid anomaly (+ 10 m) and pre-tholeiites (age > 138 Ma), carbonatites and kimberlites along the west African continental margin (Angola and Namibia) reinforces the presumed age of the São Francisco–Congo craton rejuvenation to be prior to its fragmentation in the Lower Cretaceous.

Thermal history of the Rio Muni (West Africa)–NE Brazil margins during continental breakup

We document the thermal record of breakup of the conjugate Rio Muni (West Africa) and NE Brazil margins using apatite fission track analysis, vitrinite reflectance data and stratigraphic observations from both margins. These results permit determination of the timing of four cooling episodes, and the temperature of samples at the onset of each episode. All samples are interpreted to have experienced higher temperatures in the geological past due to i) elevated basal heatflow (palaeogeothermal gradient in Rio Muni-1 well decaying from 58 °C/km during the Mid Cretaceous to 21.5 °C/km in the Late Cenozoic) and ii) progressive exhumation from formerly greater burial depth. A well constrained history of changing palaeogeothermal gradient allows for much more precise quantification of the thickness of eroded section (exhumation) than if a constant heatflow is assumed. Cooling episodes identified from the palaeotemperature data at 110–95 Ma (both margins) and 85–70 Ma (Rio Muni only) coincide with major unconformities signifying, respectively, the cessation of rifting (breakup) and compressional shortening that affected the African continent following the establishment of post-rift sedimentation (drift). The interval between these separate unconformities is occupied by allochthonous rafts of shallow-water carbonates recording gravitational collapse of a marginal platform. The rift shoulder uplift that triggered this collapse was enhanced by local transpression associated with the obliquely divergent Ascension Fracture Zone, and thermal doming due to the coeval St Helena and Ascension Plumes. The data also reveal a c.45–35 Ma cooling episode, attributed to deep sea erosion at the onset of Eo-Oligocene ice growth, and a c.15–10 Ma episode interpreted as the record of Miocene exhumation of the West African continental margin related to continent-wide plume development. Integration of thermal history methods with traditional seismic- and stratigraphy-based observations yields a dynamic picture of kilometre-scale fluctuations in base level through the breakup and early drift phases of development of these margins. Major unconformities at ocean margins are likely to represent composite surfaces recording not only eustasy, but also regional plate margin-generated deformation, local ‘intra-basinal’ reorganization, and the amplifying effect of negative feedbacks between these processes.

Mega-pockmarks and linear pockmark trains on the West African continental margin

Seabed pockmarks, the manifestation of the natural process of fluid escape at the seabed, are a widespread feature of the equatorial West African continental margin. Pockmarks occur singly, in small groups, in large random fields and in organized arrays or ‘pockmark trains’. Pockmark trains are associated with areas of steeper seabed gradient and evolve though time to form deep gullies. Pockmark gullies may exceed 1 km in width and extend for 10–20 km down slope, and form through the interaction of slope failure and fluid escape processes. Gullies maintain a rugose internal geometry throughout their development and do not represent sediment transport pathways to the deep basin. The geological processes that form seabed pockmarks and pockmark gullies are active today and these features may represent a hazard to subsea infrastructure.

Seismic stratigraphic and geomorphic analysis of deep-marine deposition along the West African continental margin

Abstract The West African continental margin evolution is preserved in a small source-distant setting (20 x 30 km area) by changes in lobe-channel-levee seismic geomorphological elements within a threefold seismic stratigraphic hierarchy. The C . 32 Ma depositional record of rift, drift and depositional outbuilding of the margin by gravity-driven adjustment, deformation and deposition produced a hierarchy of second- through fourth-order stratigraphic cycles bounded by laterally continuous fine-grained drapes inferred to record prolonged periods of sediment starvation. The margin outbuilding phase, the focus of this contribution, consists of three second-order adjustment bounded cycles (ABCs) that record major adjustment and/or modification of the deep-marine depositional system. Seven third-order cycles also show changes in depositional trend and seismic facies architecture. Ten fourth-order cycles, best resolved within the upper part of the succession, consist of multiple, wedge-shaped and compensating, lobe-channel-levee complexes up to 20 km wide. These complexes show an upward increase in channel-levee and decrease in lobe proportion. They also show an upward change from lobes incised by sinuous channels to channels deflected to lobe flanks. Outcrop and shallow core-calibrated analogues from the Permian Brushy Canyon Formation, and modern Amazon and Zaire Fans help constrain these patterns. Changes in the sediment composition and volume of subaqueous flows at their point of origin, and subsequent gravitational deformation, syn-sedimentary mass-wasting and large-scale fan avulsion punctuating deep-marine sedimentation, adjust deep-marine depositional pattern during basin margin outbuilding. Lobe-channel-levee distributions in this sediment source-distant setting record a progressive increase in local topographic relief and gradient related to the basinward migration of deformation during depositional outbuilding of the continental margin. Two important conclusions derived from this record include (1) the importance of local seabed topography and gradient on producing changes in depositional pattern, and (2) that repeated and cyclic changes in these patterns reflect adjustment/deformation within, and probably restricted to, the deep-marine record. Integrated seismic stratigraphic and geomorphic analysis delineates multiple scales of these adjustment-bounded cycles. The evolving map patterns record adjustment by shifts in geomorphic pattern and orientation. These local geomorphic changes can be used to predict longer-term and larger-scale changes in the depositional record of the continental margin evolution. This analytical approach should have general utility along high shelf-to-basin relief margins with similar gravity-driven deformation.

Soil organic matter as an important contributor to Late Quaternary sediments of the tropical West African continental margin

The contribution of soil organic matter (SOM) to continental margins is largely ignored in studies on the carbon budget of marine sediments. Detailed geochemical investigations of late Quaternary sediments (245–0 ka) from the Niger and Congo deep-sea fans, however, reveal that C org/N tot ratios and isotopic signatures of bulk organic matter (δ 13C org) in both fans are essentially determined by the supply of various types of SOM from the river catchments thus providing a fundamentally different interpretation of established proxies in marine sciences. On the Niger fan, increased C org/N tot and δ 13C org (up to −17‰) were driven by generally nitrogen-poor but 13C-enriched terrigenous plant debris and SOM from C 4/C 3 vegetation/Entisol domains (grass- and tree-savannah on young, sandy soils) supplied during arid climate conditions. Opposite, humid climates supported drainage of C 3/C 4 vegetation/Alfisol/Ultisol domains (forest and tree-savannah on older/developed, clay-bearing soils) that resulted in lower C org/N tot and δ 13C org (< −20‰) in the Niger fan record. Sediments from the Congo fan contain a thermally stable organic fraction that is absent on the Niger fan. This distinct organic fraction relates to strongly degraded SOM of old and highly developed, kaolinite-rich ferallitic soils (Oxisols) that cover large areas of the Congo River basin. Reduced supply of this nitrogen-rich and 12C-depleted SOM during arid climates is compensated by an elevated input of marine OM from the high-productive Congo up-welling area. This climate-driven interplay of marine productivity and fluvial SOM supply explains the significantly smaller variability and generally lower values of C org/N tot and δ 13C org for the Congo fan records. This study emphasizes that ignoring the presence of SOM results in a severe underestimation of the terrigenous organic fraction leading to erroneous paleoenvironmental interpretations at least for continental margin records. Furthermore, burial of SOM in marine sediments needs more systematic investigation combining marine and continental sciences to assess its global relevance for long-term sequestration of atmospheric CO 2.

Constraining uplift and denudation of west African continental margin by inversion of stacking velocity data

An inverse model has been developed to determine the magnitude of denudation at seabed and subsurface unconformities by using root‐mean‐square (RMS) stacking velocity data derived from processing a set of seismic reflection profiles. This approach provides superior spatial coverage in comparison to other methods, such as vitrinite reflectance, apatite fission track, and sonic velocity modeling, which are restricted to borehole locations. The model assumes exponential porosity decay with depth and a standard velocity‐porosity relationship in order to compute a synthetic RMS velocity profile. Denudation values at two levels in the stratigraphy are then adjusted until the fit between the model and the data is optimized. Successful modeling is dependent upon independent estimates of the initial porosity of sediment since significant trade‐off occurs between initial porosity and denudation. Application to the west African shelf shows that 0.5–1 km of denudation occurred along the entire margin, probably during late Neogene times. The amount of denudation decreases oceanward and was probably triggered by regional tilting associated with initiation and/or regeneration of continent‐wide mantle convective upwelling, which is thought to have affected much of subequatorial Africa. A subsurface Oligocene unconformity represents as much as 2.5 km of denudation and was probably produced by initiation of an oceanic current.

Deep structure of the West African continental margin (Congo, Zaïre, Angola), between 5°S and 8°S, from reflection/refraction seismics and gravity data

The deep structure of the West African continental margin between 5°S and 8°S was investigated using vertical reflection and wide-angle reflection/refraction techniques, during the ZaïAngo project, a joint programme conducted in 2000 April by Ifremer and TotalFinaElf. To penetrate below the salt layer, a non-conventional, low-frequency seismic source was used in the ‘single-bubble’ mode, together with ocean bottom instruments (hydrophones and seismometers) and a 4.5 km long streamer that recorded multichannel seismic reflection (MCS). The data show that the continental crust thins abruptly over a lateral distance of less than 50 km, from 30 km thick below the continental platform (based on gravity data), to less than 4 km thick below the Lower Congo Basin that formed prior to the Aptian salt deposition. This subsalt sedimentary basin (180 km wide, 4 km thick, with velocities varying from 4.7 km s-1 to 5.8 km s-1 at the bottom) is located between the foot of the continental slope and the oceanic domain. It is underlain by crust of an intermediary or transitional type, between continental crust and what can be recognized as oceanic crust. In the transitional zone, a crustal upper layer is present below the pre-salt sedimentary basin, 3 to 7 km thick, with velocities increasing from 5.8 km s-1 at the top to 6.8 km s-1 at the bottom of the layer. This layer appears to thin regularly, from 6-7 km thick below the depocentre of the pre-salt sedimentary basin to 3-4 km thick below the western termination of the basin. Below this upper crustal layer, an anomalous velocity layer (7.2 to 7.8 km s-1), is documented, below the eastern side of the basin, where the crustal thinning is at a maximum. The origin of this layer is unknown. Several arguments, like rifting duration (between 15 Ma and 30 Ma) or the absence of seaward-dipping reflectors, precludes the hypothesis of underplated mantle material, but other hypotheses (such as serpentinized material or high-grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks) are plausible. Near the ocean termination of the basin, the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles (‘7+11’ and 14) having a dense ocean bottom seismometer/hydrophone (OBS/OBH) spacing. On these profiles, an anomalous velocity layer is present in the westernmost part of the transitional zone (below the basement ridge) and in the oceanic domain. This layer, absent on profile 3, may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process.

Syn-rift regional subsidence across the West African continental margin: the role of lower plate ductile extension

Abstract New ostracode data from the West African margin indicate that the Outer Basin Sediment Wedge (also termed the ‘pre-salt wedge’ and the ‘pre-salt sag basin’) is Neocomian to Aptian in age and is contemporaneous with syn-rift deposits developed inboard of the Atlantic hinge zone. Despite the fact that the Outer Basin Sediment Wedge is clearly a syn-rift deposit, it does not exhibit any of the diagnostic characteristics of brittle deformation, such as the existence of normal faults and the faulting and rotation of crustal blocks. Such features are common between the Atlantic and Eastern hinges for the early stages of rifting between West Africa and Brazil, which occurred as a series of extensional phases commencing in the Berriasian and culminating in the Late Aptian. To reconcile the concomitant development of fault-controlled subsidence between the hinges and across the Atlantic hinge zone and sag-basin development seaward of the Atlantic hinge zone requires that: (1) extension seaward of the Atlantic hinge is the result of strain-partitioning between a relatively non-deforming upper crust (i.e. the upper plate) and a ductile-deforming lower crust and lithospheric mantle (i.e. the lower plate) during the second and third rift phases, while (2) between the hinges, early brittle deformation (normal faulting) progresses to ductile deformation in the third rift phase. During the third rift phase, lower plate ductile deformation across the entire region generated regional subsidence both seaward of the Atlantic hinge and between the hinges with little attendant brittle deformation. This extension style produced, directly or indirectly, a sequence of crucial events across the West African margin: (1) the development of the pre-Chela unconformity as lake level dropped in the Early Aptian, exposing the prograding deltas of the Argilles Vertes Formation; (2) the regional development of the Chela unconformity and transgressive lag deposits of the Chela Formation in the Mid-Aptian; (3) the development of regionally extensive, shallow-water, restricted marine conditions across the entire margin (between West Africa and Brazil) immediately prior to evaporite precipitation; and (4) the development of significant post-rift accommodation (deposition of the Late Cretaceous, Paleogene and Neogene formations) in the same region previously characterized by minor syn-rift faulting, repeated dessication cycles (allowing the precipitation of thick evaporites) and negligible erosional truncation of earlier syn-rift units. Previous workers have suggested that the Loeme evaporites were formed as part of the rapid, early post-rift phase of basin subsidence as the region became inundated by sea water across the Walvis Ridge. In this model, it is difficult to develop the restrictive environments required to deposit the thick (&gt;1 km) evaporites of the Loeme Formation (and the equivalent Ezanga and Ibura evaporites of Gabon and Brazil, respectively) across the entire West African-Brazilian rift system. The existence of shallow-water environments across the entire region is not consistent with water depths determined from the relief of clinoform foresets existing immediately prior to evaporite deposition thus requiring tectonic uplift of the deep-water regions. These evaporites, therefore, appear to be part of the late-stage syn-rift sediment package and the break-up unconformity, if it exists, separates the Loeme evaporites below from the overlying Albian carbonates. A direct consequence of ductile extension is one of increased heat input accompanying the rift stage in those areas dominated by syn-rift sag-basin development. The distribution and amplitude of the heat pulse is governed by the geometry of the mid-crustal weak zone and the distribution and amplitude of the lower plate extension. Seaward of the Atlantic hinge zone, the maximum heat flow is predicted to be in excess of 200 mW m −2 , whereas between the hinge zones, the heat flow is significantly less and ranges between 20mW/m 2 and 100 mW/m 2 . Because sediment temperature is a function of thermal conductivity and thickness of sediment overburden, the viability of syn-rift sources and prospectivity of the deep-water West African margin will, to a large degree, depend on the delicate interplay between the cooling of the extended lithosphere and subsequent burial of source rocks as a function of time.

Late Cretaceous to early Quaternary organic sedimentation in the eastern Equatorial Atlantic

Reconstructing the long-term evolution of organic sedimentation in the eastern Equatorial Atlantic (ODP Leg 159) provides information about the history of the climate/ocean system, sediment accumulation, and deposition of hydrocarbon-prone rocks. The recovery of a continuous, 1200 m long sequence at ODP Site 959 covering sediments from Albian (?) to the present day (about 120 Ma) makes this position a key location to study these aspects in a tropical oceanic setting. New high resolution carbon and pyrolysis records identify three main periods of enhanced organic carbon accumulation in the eastern tropical Atlantic, i.e. the late Cretaceous, the Eocene–Oligocene, and the Pliocene–Pleistocene. Formation of Upper Cretaceous black shales off West Africa was closely related to the tectono-sedimentary evolution of the semi-isolated Deep Ivorian Basin north of the Côte d’Ivoire–Ghana Transform Margin. Their deposition was confined to certain intervals of the last two Cretaceous anoxic events, the early Turonian OAE2 and the Coniacian–Santonian OAE3. Organic geochemical characteristics of laminated Coniacian–Santonian shales reveal peak organic carbon concentrations of up to 17% and kerogen type I/II organic matter, which qualify them as excellent hydrocarbon source rocks, similar to those reported from other marginal and deep sea basins. A middle to late Eocene high productivity period occurred off equatorial West Africa. Porcellanites deposited during that interval show enhanced total organic carbon (TOC) accumulation and a good hydrocarbon potential associated with oil-prone kerogen. Deposition of these TOC-rich beds was likely related to a reversal in the deep-water circulation in the adjacent Sierra Leone Basin. Accordingly, outflow of old deep waters of Southern Ocean origin from the Sierra Leone Basin into the northern Gulf of Guinea favored upwelling of nutrient-enriched waters and simultaneously enhanced the preservation potential of sedimentary organic matter along the West African continental margin. A pronounced cyclicity in the carbon record of Oligocene–lower Miocene diatomite–chalk interbeds indicates orbital forcing of paleoceanographic conditions in the eastern Equatorial Atlantic since the Oligocene–Miocene transition. A similar control may date back to the early Oligocene but has to be confirmed by further studies. Latest Miocene–early Pliocene organic carbon deposition was closely linked to the evolution of the African trade winds, continental upwelling in the eastern Equatorial Atlantic, ocean chemistry and eustatic sea level fluctuations. Reduction in carbonate carbon preservation associated with enhanced carbon dissolution is recorded in the uppermost Miocene (5.82–5.2 Ma) section and suggests that the latest Miocene carbon record of Site 959 documents the influence of corrosive deep waters which formed in response to the Messinian Salinity Crisis. Furthermore, sea level-related displacement of higher productive areas towards the West African shelf edge is indicated at 5.65, 5.6, 5.55, 5.2, 4.8 Ma. In view of humid conditions in tropical Africa and a strong West African monsoonal system around the Miocene–Pliocene transition, the onset of pronounced TOC cycles at about 5.6 Ma marks the first establishment of upwelling cycles in the northern Gulf of Guinea. An amplification in organic carbon deposition at 3.3 Ma and 2.45 Ma links organic sedimentation in the tropical eastern Equatorial Atlantic to the main steps of northern hemisphere glaciation and testifies to the late Pliocene transition from humid to arid conditions in central and western African climate. Aridification of central Africa around 2.8 Ma is not clearly recorded at Site 959. However, decreased and highly fluctuating carbonate carbon concentrations are observed from 2.85 Ma on that may relate to enhanced terrigenous (eolian) dilution from Africa.

Megaregional rift‐drift structural controls on hydrocarbon accumulations offshore West Africa

Oil and gas accumulations on the West African continental margin appear to be geographically constrained by geologic and tectonic processes associated with the formation of the present‐day South Atlantic Ocean.

The Canary Islands swell: a coherence analysis of bathymetry and gravity

SUMMARY The Canary Archipelago is an intraplate volcanic chain, located near the West African continental margin, emplaced on old oceanic lithosphere of Jurassic age, with an extended volcanic activity since Middle Miocene. The adjacent seafloor does not show the broad oceanic swell usually observed in hotspot-generated oceanic islands. However, the observation of a noticeable depth anomaly in the basement west of the Canaries might indicate that the swell is masked by a thick sedimentary cover and the influence of the Canarian volcanism. We use a spectral approach, based on coherence analysis, to determine the swell and its compensation mechanism. The coherence between gravity and topography indicates that the swell is caused by a subsurface load correlated with the surface volcanic load. The residual gravity/geoid anomaly indicates that the subsurface load extends 600 km SSW and 800 km N and NNE of the islands. We used computed depth anomalies from available deep seismic profiles to constrain the extent and amplitude of the basement uplift caused by a relatively low-density anomaly within the lithospheric mantle, and coherence analysis to constrain the elastic thickness of the lithosphere (T e ) and the compensation depth of the swell. Depth anomalies and coherence are well simulated with T e =28‐36 km, compensation depth of 40‐65 km, and a negative density contrast within the lithosphere of ~33 kg m’3. The density contrast corresponds to a temperature increment of ~325°C, which we interpret to be partially maintained by a low-viscosity convective layer in the lowermost lithosphere, and which probably involves the shallower parts of the asthenosphere. This interpretation does not require a significant rejuvenation of the mechanical properties of the lithosphere.