African Margin Research Articles

Source Rock Presence and Maturity Along the Northwest African (MSGBC) Margin: Results from a Seep Sample and Basin Modelling Study

Source Rock Presence and Maturity Along the Northwest African (MSGBC) Margin: Results from a Seep Sample and Basin Modelling Study

Enhanced erosion and silicate weathering of the West African craton during the late Cretaceous cooling evidenced by mineralogical and Hf[sbnd]Nd isotope proxies

The evolution of oceanic temperatures between the Turonian and the K/T boundary indicates a long-term cooling coincident with a decrease of atmospheric CO2 levels, yet the cause of climate cooling at that time still remains debated. In this study, we evaluated the possible implication of enhanced silicate weathering as a sink for atmospheric CO2 by applying paired NdHf isotope measurements to detrital clay records from the West African margin. The use of this novel proxy for chemical weathering intensity (ΔɛHf(t)clay) was complemented by additional mineralogical and major-trace element analyses in order to investigate the variability of mechanical erosion patterns and further explore potential linkages between tectonics, weathering and climate during the late Cretaceous.Our ΔɛHf(t)clay data suggest more intense silicate weathering on the West African Craton during the Santonian to the middle Campanian period, coincident with enhanced physical erosional inputs as inferred from higher Quartz/Clays and Feldspar/Clays ratios. This observation suggests that the shift towards intensified chemical weathering at that time was driven by enhanced mechanical erosion, possibly related to a moderate tectonic event on the West African craton. Evidence for increasing kaolinite contents and higher ΔɛHf(t)clay values during the Maastrichtian point towards more hydrolysing conditions, inducing either destabilization of older Mesozoic lateritic material or favouring the development of kaolinite-rich soils.Overall, this study was compared with several new data of chemical weathering evolution along the south Atlantic margins, adding new insights on tectonic-weathering-climate interactions during the late Cretaceous, suggesting a possible link between silicate weathering feedbacks and global cooling at that time.

Palaeogeography and tectono‐stratigraphic evolution of the Aptian Ezanga‐Loémé evaporites along the proximal domain of the south Gabon‐Congo‐Cabinda margin

AbstractDuring the Early Cretaceous, massive evaporite accumulations formed in the opening South Atlantic. However, the depositional model of these salts is still poorly constrained at the scale of the West African margin. The present study focuses along the proximal domain of the south Gabon‐Congo‐Cabinda margin and is based on (i) log interpretations of 246 wells crossing undeformed to weakly deformed evaporite intervals and (ii) a structural characterization of the basement. The evaporites show 11 regional evaporite depositional cycles (CI–CXI) bounded by meter‐thick shale beds. The cycles display alternating meter‐scale carnallite‐halite beds that can be correlated over several hundred kilometres, and CVI, CVII, CVIIIa and CX culminate in localized tachyhydrite accumulations. Cross section correlations and isopach maps help to understand the palaeogeographical evolution of each cycle and depositional environments that evolved from relatively deep at the base of cycles, to very shallow at their top. CI formed a mosaic of halite‐prone depocenters deposited in pre‐salt topographic relief. CII and CIII were deposited uniformly over a flattened basin in a highly extended brine pond. From CIV to CVIIIa, the stratigraphic architecture of the salts was shaped by freshwater inflow sourced from the north and possible basement movements. This setting, together with an increased confinement of the proximal domain from the distal one with basin drawdown, favoured the development of depocenters with perennial subaqueous conditions and extreme salinities, in which more than 70 m of tachyhydrite accumulation could locally be preserved. From CVIIIb to CXI, the basin returned to a flat depositional setting without well‐developed depocenters and with increasing subsidence westwards. Marine influx increased starting from CX, allowing the deposition of sulphate beds. The salt section is capped by anhydrite deposits interbedded with clastic and dolomite, before the final marine invasion of the basin. For the first time, this study provides a large‐scale depositional tectonostratigraphic setting of the Aptian salts in the proximal domain of the West African margin. The results are of interest for K‐Mg salts exploration resources in the Aptian and pave the way for further investigation of the salt depositional environment in the distal domain of the margin.

Analogue modelling of a salt ridge growth in M'Rhila-Labaied-Trozza fault relay zone, central Tunisia

Tunisia, located in the Northern African margin, was subjected to Late Permian-Early Cretaceous N–S extension subsequent to Pangea breakup and leading to the Tethyan opening and widening. During the Mid- and Late Cretaceous, the NE–SW crustal extension that established the Pantelleria-Malta-Sirt grabens system underlining the eastern margin stretched also the Tunisian Atlassic domain, creating and/or reactivating many NW-SE extensional structures as much as the grabens in central Tunisia and leading to volcanism and halokinesis along several weak zones. Neogene compressional tectonics inverted the earlier extensional structures and impacted most of the halokinetic features established within the post-Triassic cover. The M'Rhila-Labaied-Trozza fault relay zone, located in the middle of central Tunisia, shows both extensional and compressional structures with Triassic extrusive features and seems to be a key feature for understanding the geodynamic development of the area. In this work, we combined field and geophysical data together with analogue models to decipher the structure and the kinematic evolution of the study area. The results showed that the studied structures are associated with reactive Mesozoic salt ridges established along an extensional fault relay zone that utilized NW-SE, E-W and NE-SW inherited fractures. These initially created extensional corridors allowing Triassic evaporite extrusion and accumulation, then influenced the deformation of the area in transpressional and local pure compressional regimes depending on the fractures and stress orientations during tectonic inversion. Analogue guided interpretation of the successive deformational stages of the study area from salt ridge growth to its tectonic inversion.

An integrated stratigraphic re-evaluation of key Central Atlantic DSDP sites

Re-sampling and new analysis of DSDP core from key Central Atlantic sites, drilled over 30 years ago, has allowed an updated stratigraphic framework to be developed. This study refines the timing and nature of key unconformities that have previously been documented to occur elsewhere in the oceanic domain of the Central Atlantic during the drilling of many DSDP legs. Biostratigraphic dating using calcareous nannofossils and palynology has been performed on 168 samples. This higher biostratigraphic resolution improves regional correlation and has allowed identification of major hiati previously unrecognised within the oceanic domain stratigraphy.This work builds on the pioneering work of the DSDP cruises, and utilises the valuable data collected for new age dating, supplemented with new organic geochemistry analysis and sedimentological characterisation to improve documentation of the Mesozoic strata. Results are extrapolated along the continental margin of northwest Africa and the conjugate in South America, using regional two-dimensional seismic sections, providing a correlation between the oceanic domain stratigraphy and shelfal depositional systems penetrated by hydrocarbon exploration wells.At DSDP Site 367, a middle Berriasian unconformity is recognised for the first time in Core 33, where middle Berriasian white pelagic limestones (Blake-Bahama Formation) sit unconformably on Early Tithonian red argillaceous limestones (Cat Gap Formation). Faunal dating indicates a ca. 5 Myr hiatus at the Jurassic-Cretaceous boundary. Marl interbeds within the pelagic limestones increase in occurrence through the Barremian, reflecting increased terrigenous input, with associated total organic content (TOC) averaging 5%.A second major lithological break is identified at DSDP Site 367, interpreted as a base Albian unconformity, between the Blake-Bahama and overlying Hatteras Formations. Sediments above are intra late Albian variegated claystones with abundant terrestrial detritus and distal turbidites. Seismic data that intersects the well location show onlapping reflections of early-middle Albian strata landward of DSDP Site 367, indicating paleo-topography prior to Albian deposition. These sediments are contemporaneous with a major progradational phase on the shelf and a sedimentary wedge observed at the base of the carbonate escarpment. Early Albian sediments sit above the interpreted base Albian unconformity at DSDP Site 534 A on the conjugate US Atlantic margin, suggesting the super-regional nature of this event. The refined dating provides age calibration of a basin-wide unconformity, previously known from seismo-stratigraphic studies as Reflector β.In addition to the revised stratigraphic framework, evidence is presented for the occurrence of seaward dipping reflectors (SDRs) northwest of the Guinea Plateau. SDRs have not been reported along this section of the African margin, demonstrating volcanic addition during early seafloor spreading and extending the occurrence of volcanics associated with the African Blake Spur magnetic anomaly further south beyond the Cap Vert fracture zone previously mapped. At DSDP Site 368 we also note that the late Albian to late Cenomanian strata (Hatteras Formation) is composed of a 164 m thick organic-rich interval, containing up to 37% TOC, with facies and organic geochemical heterogeneity. This transgressive sequence is associated with a flooding of the shelf.This revised stratigraphic framework provides a solid foundation for future Central Atlantic studies intending to use these sites as calibration of the oceanic domain stratigraphy.

Interplay of positive structural inversion and salt tectonics: The case study of the central Algerian margin, Western Mediterranean

Convergence between African and European plates generates compressional strain, primarily concentrated along the northern African margin. This is testified on the Algerian margin by numerous earthquakes (e.g. Bougrine et al., 2019) and by the presence of active folds and thrusts. Multi-channel seismic reflection profiles from the MARADJA I survey reveal north-verging thrusts rooted below the Messinian units, and the geometries of the Messinian salt structures. This study examines the characteristics of salt tectonics offshore Algiers and Dellys, focusing on the effect of the positive structural inversion of the former passive margin on geometries, timing, and mechanisms of salt deformation. The interpretation of seismic reflection and multi-beam bathymetric data of the MARADJA I survey, along with its comparison with analogue models, allowed us to reconstruct the salt tectonics processes on the margin and to identify the predominant role of a plateau uplift on salt deformation. Early and ubiquitous salt deformation by downbuilding was followed by a major phase of plateau uplift (end of Messinian Crisis), leading to westward gravity gliding and a slowdown of the salt deformation above the plateau. Km-tall salt structures were developed and thick minibasins deposited. Salt tectonics is nowadays active only where the relationship between salt and overburden thickness is favorable.

Origin, evolution and significance of giant buried sediment mounds near the Sahara Slide Complex, North‐west African margin

AbstractMixed turbidite–contourite depositional systems result from interactions between down‐slope turbidity currents and along‐slope bottom currents, comprising excellent records of past oceanographic currents. Modern and ancient systems have been widely documented along the continental margins of the Atlantic Ocean. Yet, few examples have so far been identified on the North‐west African continental margin, limiting understanding of the sedimentary and palaeoceanographic evolution in this area. This work uses two‐dimensional seismic reflection profiles to report, for the first time, the presence of three giant sediment mounds beneath the headwall region of the Sahara Slide Complex. The sediment mounds are elongated and separated by two broad canyons, showing a north‐west/south‐east orientation that is roughly perpendicular to the continental margin. These mounds are 24 to 37 km long and 12 to 17 km wide, reaching a maximum height of ca 1000 m. Numerous slide scarps are observed within and along the flanks of the mounds, hinting at the occurrence of submarine landslides during their development. Based on their geometries, external shapes, internal seismic architecture and stratigraphic stacking patterns, it is proposed that these sediment mounds comprise down‐slope elongated mounded drifts formed in a mixed turbidite–contourite system during four evolutionary stages: onset, growth, maintenance and burial. The significance of this work is that it demonstrates the gradual transition from a turbidite system to a full mixed turbidite–contourite system to be associated, in the study area, with the establishment of strong ocean currents along north‐west Africa.

Tectono-stratigraphic architecture, depositional systems and salt tectonics to strike-slip faulting in Kribi-Campo-Cameroon Atlantic margin with an unsupervised machine learning approach (West African margin)

Located in the Gulf of Guinea, the Kribi-Campo sub-basin belongs to the Aptian salt basins along the West African Margin. In this paper, we investigated the tectono-stratigraphic architecture of the basin, focusing on the role of salt tectonics and strike-slip faults along the Kribi Fracture Zone with implications for reservoir prediction.Using 2D seismic data and well data interpreted through sequence stratigraphy with integrated seismic attributes analysis with Python Programming and unsupervised Machine Learning, at least 6 s-order sequences, indicating three main stages of tectono-stratigraphic evolution were determined: pre-salt syn-rift, post-salt rift climax and post-rift stages. The pre-salt syn-rift stage with KTS1 tectonosequence (Barremian-Aptian) reveals a transform rifting along NE-SW transfer faults associated with N–S to NNE-SSW syn-rift longitudinal faults bounding a NW-SE half-graben filled with alluvial to lacustrine-fan delta deposits. The post-salt rift-climax stage (Lower to Upper Cretaceous) includes 2 s-order tectonosequences (KTS2 and KTS3) associated to the salt tectonics and Campo High uplift. During the rift-climax stage, the growth of salt diapirs developed syncline withdrawal basins filled by early forced regression, mid transgressive and late normal regressive systems tracts. The early rift climax underline some fine grained hangingwall fan or delta deposits and coarse grained fans from the footwall of fault scarps. The post-rift stage (Paleogene to Neogene) contains at least three main tectonosequences KTS4, KTS5 and KTS6-7. The first one developed some turbiditic lobe complexes considered as mass transport complexes and feeder channel-lobe complexes cutting the unstable shelf edge of the Campo High. The last two developed submarine Channel Complexes associated to lobes towards the southern part and braided delta to tidal channels towards the northern part of the Kribi-Campo sub-basin.The reservoir distribution in Kribi-Campo sub-basin reveals some channels, fan lobes reservoirs and stacked channels reaching up to the polygonal fault systems.

Tectonics and evolution of the Palmeiras and Xai-Xai Grabens, offshore southern Mozambique

The tectonic evolution of the Mozambique Basin was controlled by the break-up of Gondwana which led to the development of extensional domains along the East African margin. As a result, multiple grabens systems were formed both onshore and offshore southern Mozambique. Two of them are the Palmeiras and Xai-Xai grabens, which are discussed in greater details in this study using an extensive seismic dataset, together with regional gravity and magnetic data. The data show that the Palmeiras Graben evolved from a single extensional phase in the Middle to Late Jurassic (Kimmeridgian?). In contrast, the Xai-Xai Graben records three extensional phases. The first extensional phase occurred in the Late Jurassic (Tithonian?) to Early Cretaceous (Berriasian), the second during the Aptian-Albian and the third in the Late Miocene. The tectonic evolution of these grabens strongly controlled their sedimentary infill during Late Jurassic and Early Cretaceous rifting.Volcanism associated to these tectonic events is documented by onshore and offshore boreholes, as well as by the presence of buried volcanic bodies, lava extruded along fault planes and intrusions of Mesozoic age. Furthermore, a volcano-sedimentary sequence and dome type volcanoes were imaged in seismic data.Differences in tectonic activity impacted the oil and gas potential of both the Palmeiras and Xai-Xai grabens. The sedimentary sequences deposited in the Palmeiras Graben were preserved due to a period of relative tectonic stability predominating from the Aptian to the present day. The burial of the Palmeiras Graben during this period of tectonic stability under a thermal subsidence favoured the generation and preservation of hydrocarbons. In contrast, the Xai-Xai Graben was impacted by intense tectonics, which led to a limited or restricted deposition of strata since the Late Cretaceous to the Paleocene – Early Eocene. As a consequence of a long lasting tectonic activity in the Xai-Xai Graben, uplifting and erosion of potential reservoirs and destruction of existing traps, impacted on the prospectivity for hydrocarbons.

High connectivity among Vesicomyid bivalves from cold seeps and deep-sea fans of Congo

Chemosynthetic ecosystems are scattered in the deep ocean, harbouring highly specialized communities, among which the degree of connectivity and dispersal is scarcely studied. This gap is largely due to limited range distribution, either real or due to highly partial exploration, preventing the availability of a sufficient number of samples for population genetic analysis. For the few species that escaped this gap, large-scale panmixia was often reported, raising wonders as to the evolutionary mechanisms involved in the first steps of speciation. Vesicomyid bivalves are one of the most abundant groups of chemosynthetic fauna, for which depth was proposed as an essential driver of differentiation. Early stages of speciation are thus expected to involve genetic differentiation along depth gradients. The vesicomyid bivalve Christineconcha regab was studied across widely separated localities along the Western African margin, from the cold-seeps of Regab pockmarks located at 3150 m depth on the Congo margin to the turbiditic lobes of the Congo deep-sea fan located at 5000 m depth, using mitochondrial (COI) sequences and eight microsatellite loci. Despite rather high density in relation to high organic matter availability, results obtained showed rather low levels of genetic diversity at both mtDNA and microsatellites. The main consistent pattern of differentiation was observed across depths from Regab pockmark (South-Western part) and lobe areas (Lobe B and C). This is likely due to the largest sample sizes characterizing Regab_SW and Lobe C allowing the detection of faint genetic differentiation, and possibly to a stronger signature in the demographically declining (thus acknowledgedly under sampled) Lobe B. Other significant results were not congruent among markers, suggesting low statistical power due to limited sample size or the occurrence of chaotic genetic patchiness. Altogether, the results suggest the occurrence of effective gene flow at regional scale, and departure from equilibrium in the recently discovered lobes of the Congo River, possibly resulting from unstable environmental conditions and recurrent events of extinction recolonization.

Geochemistry of metasedimentary rocks from the Eastern Pyrenees (Iberian Peninsula): Implications for correlation of Ediacaran terranes along the Gondwanan margin

The North African margin of Gondwana formed by subduction and accretion during Ediacaran to Cambrian times. Sections of this active margin were incorporated during the late Paleozoic to the Variscan and Appalachian orogens. In the Variscan realm, these sections define in some regions a Cadomian basement comprising thick metasedimentary series associated with coeval subduction-related magmatic rock dated in the range 650–500 Ma. In the Cap de Creus Massif (Eastern Pyrenean domain), the Cadomian metasedimentary rocks formed from sediments derived from mixed felsic and intermediate/mafic sources, intruded by granitoid and minor mafic igneous rocks. These series were formed in a peri-Gondwanan volcanic arc setting, like other Cadomian series described in the SW of the Iberian Massif, such as Serie Negra Group of the Ossa Morena Complex. However, the Nd model ages of the Cap de Creus metasedimentary samples range from 1431 to 1620 Ma and are younger than those of their SW Iberia equivalents. This result can be interpreted as an indication of an eastern location in the Ediacaran - Cambrian peri-Gondwanan magmatic arc. Furthermore, the Cadomian series in the Central Iberian Zone have even younger Nd model ages than those of SW and NE Iberia. We present two end-member paleogeographic models consistent with reconstruction of the initial location of the Cap de Creus Massif and its relations with other domains (Ossa Morena Complex and Central Iberian Zone) in this sector of the North African Gondwanan margin.

Upper Cenomanian caprinulid-radiolitid rudists (Bivalvia) from the Gattar Member of Jebel el Kebar (central Tunisia): Stratigraphical implications and palaeobiogeographical relationships with coeval rudist-assemblages from carbonate platforms of the southern Tethyan margin

The Gattar Member (upper part of the Zebbag Formation) is a prominent dolomitic limestone unit in Tunisia, the age of which was ascribed either to the Turonian, the Cenomanian, or the Cenomanian−Turonian. Here, we present the discovery of a Caprinulidae-Radiolitidae rudist asssemblage in the Gattar Member of Jebel el Kebar (central Tunisia). This consists of Caprinula cf. boissyi, Sauvagesia cf. sharpei, Durania arnaudi, Neocaprina sp. and Eoradiolites sp. The detailed description of two sections of the Gattar Member in Jebel el Kebar allows us to define the sedimentological and palaeoecological attributes of the rudist-bearing carbonates and their sequential relationships with the Zebbag and Rouana stratigraphical units respectively. The caprinulid-radiolitid assemblage is age-calibrated, based on the ammonite standard zonation of the Cenomanian stage from Portugal, South-Eastern France, and Oman, where rudist, benthic foraminifera and ammonite Zones have been correlated. Its age is late Cenomanian. The correlations of the Gattar Member between central Tunisia and the Gafsa-Chotts area are established on the base of ammonite and rudist biostratigraphical markers. The age of the Gattar Member of southern Tunisia and of its stratigraphical counterpart in western Libya is discussed. At last, we analyze the palaeobiogeographical distribution of the upper Cenomanian caprinulid-radiolitid assemblages from coeval carbonate platforms of the North African margin (Algeria, Morocco) and of the North Arabian margin (Levant region: Syria, Jordan, Lebanon, Israel, North Sinai).

Reconstructing redox variations in a young, expanding ocean basin (Cretaceous Central Atlantic)

The redox sensitive elements (RSE) are indicators of organic matter (OM)-rich rocks and provide valuable information on paleoenvironmental parameters, including organic productivity, chemical evolution of water masses, ocean residence times and sea-bottom redox conditions. Yet, their potential to understand global environmental changes is hindered by the scarcity of continuous geological records, often highly fragmented in space and time. In this contribution, a large dataset from five closely spaced sedimentary sections in the Western African margin is used to reconstruct a 40 myr-long record of the redox conditions in a time spanning from the early Aptian to early–middle Campanian. Studied intervals were originally located in different positions of the embryonic Atlantic Ocean, and intercept four separated oceanic anoxic events (OAE1c, OAE1d, OAE2 and OAE3). The complete geochemical characterization revealed the changes of Mn, V, Ni, Cu, Zn, Mo, U and Se during the deposition of OM-rich sediments, as a function of their depositional environments, redox conditions, and time. It is shown that, although enrichments of RSE are triggered by increasingly reducing seafloor conditions, the extent to which these elements are retained within the sedimentary record depend on both local variations in detrital inputs and global perturbations of the marine RSE inventories. The analyses show that RSE are characteristically enriched with respect to global shale values, having geochemical characteristics (i.e., high Mo/TOC and high Mo/U) suggestive of suboxic to sulfidic conditions from the late Albian to Campanian.

Paleostress evolution of the West Africa passive margin: New insights from calcite twinning paleopiezometry in the deeply buried syn-rift TOCA formation (Lower Congo basin)

Accurate characterization of hydrocarbon reservoirs requires a good understanding of the paleostress history of their host sedimentary basin. This study aims at reconstructing the paleoburial and paleostress history of the syn-rift Barremian TOCA Fm. in the Lower Congo basin (West African margin). Two oriented borehole cores drilled offshore Congo were investigated using stress inversion techniques applied to calcite mechanical twins to constrain paleostress orientations and magnitudes. The inversion of calcite twins was performed on a widespread early diagenetic cement (~129 to ~113 Ma) and revealed that the TOCA Fm. has undergone a complex polyphase stress history, including (1) extensional stress regimes with vertical σ1 and horizontal σ3 trending NE-SW and WNW-ESE (N100) associated with the rifting episode preceding the opening of the South Atlantic (130 to 112 Ma), and with mean differential stresses of ~47 MPa for (σ1-σ3) and ~ 20 MPa for (σ2-σ3). The early N100 extension is associated with the development of normal faults striking N-S that likely reactivated inherited basement structures. The direction of extension evolved during the Barremian into the main NE-SW extension marked by the dominant normal faults striking NW-SE. (2) Compressional and strike-slip stress regimes with horizontal σ1 oriented ~N-S possibly related to the far-field intraplate transmission of orogenic stresses generated at the distant Africa-Eurasia plate boundary at ~67–60 Ma, and with mean differential stresses of 40 MPa for (σ1-σ3) and 28 MPa for (σ2-σ3). (3) Compressional and strike-slip stress regimes with horizontal σ1 oriented ENE-WSW to ~E-W that we tentatively relate to the mid-Atlantic ridge push and prevailing since ~15 Ma onwards, associated with mean differential stresses of ~45 MPa for (σ1-σ3) and ~ 18 MPa for (σ2-σ3). Our results reveal that the TOCA Fm. recorded mainly far-field stress effects that could be related to interactions between the African plate and surrounding plates, and that divergent passive margins may experience a complex tectonic history including both extensional and compressional events.

Stagnant North Atlantic Deep Water Heat Uptake With Reduced Atlantic Meridional Overturning Circulation During the Last Deglaciation

AbstractAtlantic Meridional Overturning Circulation (AMOC) plays a major role in the climate system by modulating the depth and rate of oceanic heat storage. Some climate simulations suggest that reduced AMOC decreases bottom water ventilation and that the heat absorbed by the ocean starts to mix downwards, warming Atlantic intermediate waters. This has been corroborated for the western North Atlantic by benthic foraminifera geochemical records from periods of reduced AMOC during the last deglaciation. However, the deep‐water response remains poorly constrained, and the lack of direct paleotemperature reconstructions limits our understanding about the effects of reduced circulation on ocean heat uptake. We present a new reconstruction of bottom water temperatures from core GeoB9508‐5 (2,384 m water depth, 15°29.90°N/17°56.88°W) off the northwestern African Margin. Our paleotemperature record, based on Uvigerina spp. Mg/Ca, shows two episodes of intense transient deep water warming in times of decreasing overturning circulation, followed by long periods of heat uptake stagnation. First, during AMOC slowdown in the Heinrich stadial 1, when paleotemperatures of ∼2°C persisted for ∼5.4 Kyr coincident with the weakest stage of AMOC; and second in the Younger Dryas, when bottom water temperatures >4°C lasted ∼2.5 Kyr during a less intense AMOC decline. This suggests a stagnation of deep‐water heat uptake in the deep NE Atlantic possibly linked to a reduced downward advection of heat during times of a reduced AMOC, supporting the hypothesis that AMOC strength sets the depth of oceanic heat storage in the North Atlantic.

Structural and tectonostratigraphic evolution of Matruh Basin, northern Western Desert, Egypt: An example of an inverted rift basin

The Matruh Basin is located in the northwestern part of the Western Desert of Egypt. Being part of the structurally complex northeast African margin, the basin underwent multiple phases of Mesozoic-Cenozoic deformation. Detailed structural analysis of the subsurface structures of this basin, using high-quality 3D seismic and well datasets, has identified two main fault sets: 1) Jurassic- Early Cretaceous (Barremian) NNE-striking normal faults; and 2) Late Cretaceous NW-striking normal faults. The older NNE-striking normal faults were later inverted due to Late Cretaceous compression forming fault-propagation folds (inversion anticlines) in their upper tips. This positive inversion initiated with the “Santonian Event” and continued into the Early Cenozoic. The NW-striking normal faults were not inverted due to their unfavorable orientations.Most inversion structures in the greater Western Desert trend E-W or ENE-WSW and reflect the reactivation of similarly oriented pre-existing normal faults. The NNE orientation of the old, major extensional structures in the Matruh Basin and the resulting control on the later inversion fabric reflects a very different fault architecture. This has dramatically impacted sedimentation patterns and potential hydrocarbon accumulations. The Matruh Basin illustrates the complexities that can be present along the North African Neotethyan margin and underlines the necessity to avoid regional generalizations where gaps in data may be present.

The provenance of Avalonia and its tectonic implications: a critical reappraisal

Abstract The late Neoproterozoic–Cambrian interval is characterized by global-scale orogenesis, rapid continental growth and profound changes in Earth systems. Orogenic activity involved continental collisions spanning more than 100 myr, culminating in Gondwana amalgamation. Avalonia is an example of arc magmatism and accretionary tectonics as subduction zones re-located to Gondwana's periphery in the aftermath of those collisions, and its evolution provides significant constraints for global reconstructions. Comprising late Neoproterozoic ( c. 650–570 Ma) arc-related magmatic and metasedimentary rocks, Avalonia is defined as a composite terrane by its latest Ediacaran–Ordovician overstep sequence: a distinctive, siliciclastic-dominated cover bearing ‘Acado-Baltic’ fauna. This definition implies that Neoproterozoic Avalonia may consist of several terranes, and so precise palaeomagnetic or provenance determination in one locality need not apply to all. On the basis of detrital zircon and Nd isotopic data, Avalonia and other lithotectonically related terranes, such as Cadomia, have long been thought to have resided along the Amazonian–West African margin of Gondwana between c. 650 and 500 Ma – Avalonia connected to Amazonia, and Cadomia to West Africa. These interpretations have constrained Paleozoic reconstructions, many of which imply that the departure of several peri-Gondwanan terranes led to the Early Paleozoic development of the Rheic Ocean whose subsequent demise in the Late Paleozoic led to Pangaea's amalgamation. Since these ideas were proposed, several new lines of evidence have challenged the Amazonian affinity of Avalonia. First, there is evidence that some Avalonian terranes may have been ‘peri-Baltican’ during the Neoproterozoic. Baltica was originally excluded as a potential source for Avalonia because, unlike Amazonia, arc-related Neoproterozoic rocks were not documented. However, subsequent recognition of Ediacaran arc-related sequences in the Timanides of northeastern Baltica invalidates this assumption. Second, detailed palaeontological and lithostratigraphic studies have been interpreted to reflect an insular Avalonia, well removed from either Gondwana or Baltica during the Ediacaran and early Cambrian. Third, recent palaeomagnetic data have raised the possibility of an ocean (Clymene Ocean) between Amazonia and West Africa in the late Neoproterozoic, thereby challenging conventional reconstructions that show the ‘peri-Gondwanan’ terranes as a contiguous belt straddling the suture zone between these cratons. In this contribution, we critically re-evaluate the provenance of the so-called ‘peri-Gondwanan’ terranes, the contiguity of the so-called ‘Avalonian–Cadomian’ belt and the validity of the various plate tectonic models based on the traditional interpretation of these terranes. In addition, we draw attention to critical uncertainties and the challenges that lie ahead.

Palaeoenvironment of the Albian‐Cenomanian Mundeck Formation in the Douala Basin (SW, Cameroon): Evidence from facies analysis and geochemistry

The Lower Cretaceous syn‐rift deposits of the Douala Basin (Mundeck Formation), located in the equatorial West African margin were investigated in order to determine the palaeoenvironment and organic matter source, redox conditions, palaeoclimate, and tectonic settings, based on facies, organic and inorganic geochemical analyses. The sediments consist of pebble, coarse, medium‐ to fine‐grained sandstone, siltstone, and grey to black fossiliferous shales. The shales are characterized by an average total organic carbon (TOC) of 1.02%, fair to poor source generative potential with present‐day hydrogen index values, which range from 31 to 350 mg HC/g TOC suggesting that the shale contains Type III kerogens with mainly gas generative potential. The organic particles are dominated by phytoclasts (40% on average) and non‐fluorescent amorphous organic matter (34% on average). The palynomorphs represent about 26% on average with a large amount of pollen grain and a minor amount of dinoflagellate cysts and microforaminifera test linings. The inferred depositional environment of the studied sediments is an alluvial fan to fluvio‐lacustrine with shallow marine incursion. This depositional environment received the contribution of aquatic algae and microorganisms with a large amount of terrestrial organic matter, evidenced by the presence of some dinoflagellate cysts and microforaminifera test linings, and relatively higher amounts of SiO2, Al2O3, and phytoclasts. The presence of trace elements such as Ba, Sr, V, Ni, Co and Cr in the analysed samples and their ratio, suggest suboxic to anoxic conditions of deposition with low salinity. The warm to cold and dry climatic conditions prevailed during the deposition of the Mundeck deposits and sediments from active continental margin setting during the lower Cretaceous periods in the Douala Basin as confirmed by inorganic geochemical data. This study sheds light on the early stage of the evolution of the Douala Basin which could be integrated into the context of a mid‐Jurassic to lower Cretaceous regional evolution of the opening of a passive margin. This evolution is characterized by an alluvial fan, fluviatile, lacustrine, lagoon and marine depositional system. This depositional system is recognized as part of the petroleum system recognized within the syn‐rift deposits in the others basins along the West African margin, where giant hydrocarbons have been discovered.

Upper Mantle Anisotropy beneath Northern Algeria from Shear-Wave Splitting: Anisotropy beneath Northern Algeria

Northern Algeria is a large region in the north-western of Africa, lying on the collision boundary between the African and Eurasian plates. Few studies on Lithosphere and Mantle deformation have been conducted in this region. To better understand the seismic anisotropy patterns beneath this area, we used data from 17 broadband stations installed in Tellian and the Saharan atlases using the SKS shear wave splitting method by processing hundreds of teleseismic events. To estimate the seismic anisotropy, two parameters were calculated, the fast polarization direction and the delay time for each station-event pair. The results show that the seismic anisotropy can be described by two main orientations, ENE-WSW follows the general trend of the Saharan Atlas, particularly in the central and western parts, and ESE-WNW follows the Hodna Mountains in south-eastern Algeria. Our results show that the anisotropy can be explained by single and heterogeneity in the anisotropic structure, where the measurements are very scattered, and the delay time and fast direction changed with the events backazimuth. In the ABSD, CBBR and CDCN stations, which lie in the arc between the Sahara Atlas and the Aurès Mountains, the origin hypothesis of the seismic anisotropy can be linked to the existence of a detached slab. Moreover, in ABZH, OKGL and EARB stations located in the Kabylide and Western regions, it can be associated with the Gibraltar slab. Both slabs sinking in the African margin mantle were previously imaged by seismic tomography. For the remaining stations, the single-layer best explains the observed seismic anisotropy from their regular fast polarization direction. The comparison of the obtained fast directions with GPS measurements shows that anisotropy fast axes are nearly perpendicular to the convergence direction between the African and Eurasia plates.

Paleoenvironmental and diagenetic evolution of the Aptian Pre-Salt succession in Namibe Basin (Onshore Angola)

The Aptian Pre-Salt sedimentary succession cropping out in Cangulo palaeovalley onshore Namibe Basin (Angola) was studied by a combination of field and analytical techniques to constrain the sedimentary and diagenetic evolution of the uppermost sag sequence of the South Atlantic passive margin. Field observations allows definition of four transgressive-regressive cycles characterised by fluvial to tidal-influenced mixed clastic-carbonate and carbonate-dominated deposits, that locally show evidence of evaporite dissolution; highlighting that evaporite deposition started earlier than deposition of the regional South Atlantic Loeme-Bambata evaporite formations. Two separate pre-salt carbonate units have been differentiated within the Cangulo Fm; i) a lower transitional to marginal marine, and ii) a younger upper non-marine freshwater travertine system, that is documented for the first time in the west African margin. Transgressive-regressive cycles control the early diagenesis of the tidal carbonates that include dolomitization due to mixing fluids during transgressions, and karstification due to evaporite dissolution by meteoric water circulation during regressive events. Clastic supply appears to have been completely shut down during carbonate deposition, suggesting major climatic change associated with carbonate deposition. During the lowstand between the two carbonate units, fluid flow through Cangulo palaeovalley was re-established resulting in extensive karstification and formation of a large-scale erosional unconformity that is interpreted to be time equivalent to an intra Chela-Cuvo Fm. event. The top of the studied succession corresponds to the transgressive deposits of the Bambata evaporites that are not preserved in the Cangulo palaeovalley due to its erosion but are regionally developed. The results of this study can be directly linked to along strike age equivalent Pre-Salt successions cropping out in the Namibe, Benguela and Kwanza basins, and directly offshore Angola and Brazil using well and seismic data. These new data shed important new light and constraints on the depositional and diagenetic evolution of the complex Pre-Salt reservoir systems of the South Atlantic, and the depositional and bathymetric setting at the time of onset of the main south Atlantic evaporite deposition.