West African Margin Research Articles

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.

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.

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.

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.

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.

Lower Miocene quantitative calcareous nannofossil biostratigraphy from the Tropical Atlantic

Quantitative analysis of the lower Miocene of Ocean Drilling Program Hole 959A from the West African margin was performed to document all the calcareous nannofossil biostratigraphic events present. Combined with data from previous investigations of the lower Miocene from the tropical Atlantic, this research identifies and tests the viability of markers used in current zonation schemes, recognizes alternative markers for age boundaries, and examines statistically the most probable order of events in the lower Miocene using the Ranking and Scaling method (RASC). In Hole 959A, all major zonal and subzonal boundaries from CN1 to CN4 were identified, except for the boundary between Subzones CN1a and CN1b, using primary and secondary markers from the Okada and Bukry (1980) zonation. In addition, all age boundaries from the Chattian to Langhian stages were recognized or closely estimated using the calcareous nannoplankton markers. The resultant list of events extracted from Hole 959Aalong with events from other seven sites were examined biostratigraphically using RASC. A well threshold of four was selected as an appropriate control parameter, resulting in 22 events in the optimum sequence, 13 of which had a low standard deviation. Furthermore, interpolation of ages of events using Age/Depth model for Hole 959A was examined. The extracted ages provided a reasonable preliminary age estimate for the secondary events.

The zinc isotope composition of late Holocene open-ocean marine sediments

The controls on Zn burial fluxes into marine sediments are not well constrained by existing datasets. To address this problem, new Zn-isotope data have been generated from a globally distributed array of late-Holocene age sediments that accumulated in open ocean settings underneath a diverse range of depositional conditions (Namibian margin, West African margin, Arabian margin, Californian margin, Peruvian margin, Southern Ocean, Indian Ocean, North and South Atlantic Oceans, South China Sea). The mean non-detrital isotopic composition of Zn in the whole dataset (δ66/64Zn = 0.37 ± 0.25 ‰, 2. S.D., n = 33) is only slightly lower than the modern deep-ocean composition (0.45 ± 0.14 ‰). Locations with total organic carbon >1 wt% have greater non-detrital enrichments of Zn and higher Zn isotope compositions than organic-poor sites, although the uncertainties overlap. Zinc associated with organic matter burial (using Zn/C ratios) and/or oxyhydroxide burial (using Zn/Fe and Zn/Mn ratios) account for a combined total of ~16–73% of the total non-detrital Zn content of the investigated sediments, therefore requiring an additional burial flux of Zn to balance the sedimentary budget. This flux is probably related to ZnS formation although Zn incorporation into authigenic clays is also possible. The new data indicate that open-ocean marine sediments track the deep-ocean Zn isotope composition to within 0.1–0.2 ‰, except in areas where isotopically distinct Zn fractions dominate the non-detrital Zn budget. The fraction of Zn removed into different sedimentary components is used to calculate Zn residence times ranging from ~2.7 kyr to 13.5 kyr that are within the range of previous estimates based on input fluxes to the ocean.

The Western Cameroon Highland margin: Is there any geomorphometric evidence of active tectonics?

The Western Cameroon Highland margin (WCHM) within the West African margin displays an uneven high relief. Its evolutionary model in vogue suggests the tectonic activity of the Cameroon Volcanic Line (CVL). An evident question in this regard is whether there is any geomorphometric evidence in support of what is this widespread view? We evaluate this question using field controls and geomorphometric indices of active tectonics derived for 13 main river basins with the help of digital elevation model (DEM)‐30 m data. The WCHM is developed on Proterozoic granite‐gneissic basement and the Cretaceous to Recent volcano‐sedimentary cover and was affected by regional active tectonic features. It is marked by transient signs of different landscape evolutionary stages recorded by the relief and drainage network; shown by gradual adjustment of rivers to equilibrium stages; and cautioned by a greatest variation of DEM‐based geomorphic indices. These indices, hypsometric integral (HI = 0.10–0.35), topography relief (TR = 595–2,710), topography dissection (TD = 114–829), valley floor width to valley height ratio (Vf arg = 0.61–14.91), transverse topographic symmetric factor (T‐index = 0.18–0.49), and basin asymmetric factor (AF = 1.08–24.37) highlight elevated and roughness character of that margin, from the Benue Trough and culminating to the CVL. River longitudinal profiles reveal variably and deeply incised WCHM by the drainage network, reinforced by ongoing tectonic activities. Knickpoints (Kps), stream length‐gradient (SL), uplift (U = 1.17–2.0), and a variety of normalized steepness (Ksn = 11.63–635.16) evidence the diversity and preeminent tectonic controls. The above results show that the WCHM is widely considered to be the consequence of the mantle dynamisms and regional tectonics since Proterozoic, the CVL activities, and the erosional processes.

Orbital‐ and Millennial‐Scale Variability in Northwest African Dust Emissions Over the Past 67,000 years

AbstractReconstructions of aeolian dust flux to West African margin sediments can be used to explore changing atmospheric circulation and hydroclimate over North Africa on millennial to orbital timescales. Here, we extend West African margin dust flux records back to 37 ka in a transect of sites from 19° to 27°N, and back to 67 ka at Ocean Drilling Program (ODP) Hole 658C, in order to explore the interplay of orbital and high‐latitude forcings on North African climate and make quantitative estimates of dust flux during the core of the Last Glacial Maximum (LGM). The ODP 658C record shows a Green Sahara interval from 60 to 50 ka during a time of high Northern Hemisphere summer insolation, with dust fluxes similar to levels during the early Holocene African Humid Period, and an abrupt peak in flux during Heinrich event 5a (H5a). Dust fluxes increase from 50 to 35 ka while the high‐latitude Northern Hemisphere cools, with peaks in dust flux associated with North Atlantic cool events. From 35 ka through the LGM dust deposition decreases in all cores, and little response is observed to low‐latitude insolation changes. Dust fluxes at sites from 21° to 27°N were near late Holocene levels during the LGM time slice, suggesting a more muted LGM response than observed from mid‐latitude dust sources. Records along the northwest African margin suggest important differences in wind responses during different stadials, with maximum dust flux anomalies centered south of 20°N during H1 and north of 20°N during the Younger Dryas.

Major Element Signatures of Silicate Dust Deposited on the West African Margin: Links With Transport Patterns and Provenance Regions

AbstractMineral dust deposition characteristics are poorly constrained, even in the Northeastern Tropical Atlantic Ocean, which is immediately downwind of the Saharan desert and the largest marine repository of aeolian dust in the world. Here, we report on a 2‐year (March 2013–February 2015) time series of deposited dust on the Senegalese margin. This record enables us to document the chemical variability (major elements) of the settling Saharan dust (<30 µm silicate fraction, i.e., carbonate‐free) at a resolution varying from one week to one day, along with the deposition flux. This continuous time series reveals a greater geochemical diversity than previously reported, particularly during the dry winter‐spring season when continental trade winds sweep across vast regions of West Africa at low atmospheric levels before reaching the sampling site. By contrast, during major deposition events, which make up for most of the yearly flux, our record shows that Saharan dust chemical composition displays much narrower ranges. Trajectory analyses indicate that these relatively well‐defined chemical signatures are due to the limited number of provenance sectors involved during major deposition occurrences. The chemical characterization of the dust deposited during these events, hence, allows identifying the major element fingerprint of the related source regions, the most important one being a sizable area at the border of Algeria and Mali including the Tanezrouft desert north of the Taoudeni basin. Also, since these major events are associated with major Saharan outbreaks, they provide estimations of the prevailing elemental signatures for Saharan dust impacting the Northeastern Tropical Atlantic Ocean.

From Pan-African Transpression to Cadomian Transtension at the West African Margin: New U–Pb zircon Ages from the Eastern Saghro Inlier (Anti-Atlas, Morocco)

Abstract New U–Pb zircon ages from the Eastern Saghro massif in the Anti-Atlas of Morocco demonstrate a link between Pan-African transpressive collision at c. 600 Ma and transtension caused by the onset of Cadomian subduction and arc development from c. 570 Ma onwards. We present new U–Pb laser ablation inductively coupled plasma mass spectrometry ages of detrital and magmatic zircon from the Saghro, M'Gouna, and Ouarzazate Groups. The siliciclastic deposits of the Saghro Group were deposited in a back-arc setting developed on stretched continental crust of the West African margin. Collision with the Atlas–Meseta domain led to the closure of the back-arc basin before 600 Ma. Time of exhumation and surface exposure of the newly formed Pan-African basement is bracketed to c. 30 Ma owing to the maximum depositional age of 571 ± 4 Ma of the overlying M'Gouna Group. The U–Pb age of 567 ± 4 Ma for the lowermost ignimbrite of the Ouarzazate Group limits the time for the deposition of the M'Gouna Group to less than 4 Ma. The Pan-African orogeny was finished at c. 600 Ma whereas the onset of transtension related to Cadomian back-arc formation was very much younger from c. 570 Ma onwards.

Well Log Petrophysical Analysis and Fluid Characterization of Reservoirs, Rio Del Rey Basin, Cameroon (West African Margin, Gulf of Guinea)

The quick-look and gas chromatography analyses were used for formation evaluation of four depth intervals in a well (well A) located within the offshore of the Rio Del Rey basin. The results show 3 water reservoirs (R1 to R3) and 1 hydrocarbon reservoir (R4). The quick-look reveals that the hydrocarbon (oil and gas) reservoir is a shaley sandstone or a radioactive sandstone located between 4898-4932 Mmd which is filled by oil and gas and with a good porosity. The chromatographic gas ratio analysis reveals that the hydrocarbon reservoir is filled by a productive gas which may be a wet gas. The result provided by the gas chromatography is a false result probably due to its limitation which is that the hydrocarbon component must exist at the gaseous phase (C1-C5) to be detected and analyzed. The gas chromatography based its analysis only in the C1 to C5 range, in oil we have from C1 to C8.

Evidence for seasonal cycles in deep-sea fish abundances: A great migration in the deep SE Atlantic?

Animal migrations are of global ecological significance, providing mechanisms for the transport of nutrients and energy between distant locations. In much of the deep sea (>200m water depth), the export of nutrients from the surface ocean provides a crucial but seasonally variable energy source to seafloor ecosystems. Seasonal faunal migrations have been hypothesized to occur on the deep seafloor as a result, but have not been documented. Here, we analyse a 7.5-year record of photographic data from the Deep-ocean Environmental Long-term Observatory Systems seafloor observatories to determine whether there was evidence of seasonal (intra-annual) migratory behaviours in a deep-sea fish assemblage on the West African margin and, if so, identify potential cues for the behaviour. Our findings demonstrate a correlation between intra-annual changes in demersal fish abundance at 1,400m depth and satellite-derived estimates of primary production off the coast of Angola. Highest fish abundances were observed in late November with a smaller peak in June, occurring approximately 4months after corresponding peaks in primary production. Observed changes in fish abundance occurred too rapidly to be explained by recruitment or mortality, and must therefore have a behavioural driver. Given the recurrent patterns observed, and the established importance of bottom-up trophic structuring in deep-sea ecosystems, we hypothesize that a large fraction of the fish assemblage may conduct seasonal migrations in this region, and propose seasonal variability in surface ocean primary production as a plausible cause. Such trophic control could lead to changes in the abundance of fishes across the seafloor by affecting secondary production of prey species and/or carrion availability for example. In summary, we present the first evidence for seasonally recurring patterns in deep-sea demersal fish abundances over a 7-year period, and demonstrate a previously unobserved level of dynamism in the deep sea, potentially mirroring the great migrations so well characterized in terrestrial systems.

Assessment of relative active tectonics in Edea – Eseka region (SW Cameroon, Central Africa)

The Edea-Eseka region (EER), a portion of West African Margin located in SW Cameroon between 3°30′—4°00′ N and 10°00′—10°55′ E and submitted to equatorial climate since early Miocene is used here as a proxy for climatic conditions. Geomorphological analysis based on morphometric parameters computed from DEM (30 m) combined with geological maps and field observations were used to constrain geomorphic evolution and landscape adjustment to tectonic processes in this humid zone. Three types of geomorphic indices reflecting specially patterns of differential uplift, those efficient in detecting tilting, and those revealing interactions between erosion and tectonics.-The obtained results show that lithological boundaries and base level controls on landscape are very limited. Tectonic processes, peculiarly uplift induced by faults or tilting at local scale and by mantle dynamics at large scale coupled with climate condition, represent the main factors that drove the geomorphic evolution of this area discriminating three main morphotectonic provinces different by their geomorphic characteristics, uplift and degree of incision. There are, from West to East: (1) the coastal province (CP), deeply incised and built on Douala-Kribi/Campo basin (<65 Ma) set up during the South Atlantic opening, recorded the very low uplift rate; (2) the intermediate province (IP), developed on Nyong (2400–1800 Ma) and Oubanguide (650–540 Ma) crystallophyllian complexes and marked by high differential erosion, includes middle to high residual reliefs; and (3) the upper province (UP), developed like the pervious on Nyong and Oubanguide crystallophyllian complexes is characterized exclusively by continental high reliefs, representing high post-Cambrian continental uplifts.-Several geomorphic indices used to evaluate tectonic activities and their average the index of relative active tectonics (IAT) show that tectonic activity differs from one province to another and within each province. Generally, it increases from coast (coastal province) to hinterland (UP). EER are represented by classes 2 and 3 of IAT indicating moderate to high active tectonics. Class 1 revealing the highest tectonic activity, occurs exclusively in the UP. Class 2 corresponding to high tectonic activity happens largely in the south of the IP along the Nyong Fault and the UP where relief are particularly high. Class 3 indicates the moderate relative tectonic in the IP. Class 4 mainly takes place in tectonically low to inactive Douala-Kribi/Campo basin, except along the South part of Sanaga Fault.The development and the evolution of Edea-Eseka region landscape is then acted by a dominant high-uplift rate in SW Cameroon. This tectonic activity apparently incompatible with the passive margin context is however realistic in this area, on one hand justified by the numerous reactivations over time of faults crossing the region that generate earthquakes in some places and on the other hand, by the activity of the Cameroon Volcanic Line, and particularly that of the Mount Cameroon located near the study area. It appears at the end that, due to mantle dynamics, the EER has been rejuvenated under the tectonics and climate forcings, returning to a transient stage of its geomorphic evolution.

Gradient based first- and second-order filters for the demarcation of continental–oceanic boundaries using satellite gravity data

SUMMARY Demarcating the continental–oceanic boundary (COB) is extremely important as it contributes to an understanding of the tectonic environments of marginal basins through plate reconstructions. In general, the COB is estimated by jointly interpreting the features of the basement geomorphology, bathymetry and geophysical datasets, including gravity, magnetic, seismic and geochemical data. We propose a novel methodology named the tilt-theta angles correlation (TTAC), a second-order filtering approach to estimate the COB using high-resolution satellite gravity data. This method computes the angular difference or correlation angle between the vectors computed from the gradients of the tilt and theta derivative-based first-order filters. Oceanic regions generally exhibit small correlation angles (∼0°), thereby indicating a direct correlation; in contrast, continental regions depict large correlation angles (∼180°) and thus display an inverse correlation. The TTAC method marks the COB at the spatial position where the correlation angles abruptly shift from inverse to direct. Tests using synthetic data and the larger spectral energy content of TTAC for long wavelengths over tilt, theta and minus of the sign of vertical derivative (-SiVD) filters attest the effectiveness of the new methodology over first-order derivative based filters. Additionally, tests utilizing real data over both the West African Transform Margin (WATM) and across the South Atlantic Conjugate Margin (SACM) provide favourable results compared with the previous methodologies used to demarcate the COB. The COB estimates derived using the new methodology are consistent with the COB estimates obtained during recent plate-reconstruction studies. We also compare our results with an earlier interpretation along a conjugate margin encompassing magma-poor and magma-rich regions. In the complex rifted environments of the SACM, in which volcanic rocks are dominant, the TTAC methodology substantially improves the previous COB estimates, thereby establishing it as an efficient edge marker by simultaneously reducing small-scale geological noise and enhancing regional-scale geological contributions.

Mesozoic biochronostratigraphy and paleoenvironment of the South Atlantic: A revised framework based on 20 DSDP and ODP deep-water sites

The South American and West African margins of the South Atlantic and the Southern Ocean are characterized by Mesozoic black shales that serve as source rocks of known and unknown petroleum systems. The knowledge of regional chronostratigraphy and paleoenvironment enables the prediction of the stratigraphic and geographic distribution of black shales with precision, as well as the understanding of local heterogeneity in regional- or global-scale oceanographic changes, such as oceanic anoxic events. Through sample re-analyses, this study provides a high-resolution Mesozoic chronostratigraphic and paleoenvironmental framework with the help of new micropaleontological data generated from 17 DSDP and ODP sites (out of 20 study sites), ranging from the Equatorial South Atlantic to the Southern South Atlantic. A series of eight regional Mesozoic unconformities are proposed and correlated to previously known unconformities or seismic reflectors. The regional paleoenvironmental interpretation, which utilizes the standardized chronostratigraphic framework, is herein accomplished for four time-horizons, namely, Middle Jurassic–Valanginian, Hauterivian–Albian, Cenomanian–Turonian, and Coniacian–Maastrichtian. Chronostratigraphic cross-sections and reconstruction maps for the time-horizons show that significant changes in paleoenvironmental parameters, such as bottom-water oxygenation and allochthonous sedimentation, were governed by the tectonic evolution of the South Atlantic margins.

The Brunswick magnetic anomaly: Geophysical signature and geologic source

Research Article| February 28, 2019 The Brunswick magnetic anomaly: Geophysical signature and geologic source Patrick D. Duff; Patrick D. Duff 1School of Earth, Ocean, and Environment, University of South Carolina, Columbia, South Carolina 29208, USA Search for other works by this author on: GSW Google Scholar James N. Kellogg James N. Kellogg 1School of Earth, Ocean, and Environment, University of South Carolina, Columbia, South Carolina 29208, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Patrick D. Duff 1School of Earth, Ocean, and Environment, University of South Carolina, Columbia, South Carolina 29208, USA James N. Kellogg 1School of Earth, Ocean, and Environment, University of South Carolina, Columbia, South Carolina 29208, USA Publisher: Geological Society of America Received: 08 Aug 2018 Revision Received: 27 Jan 2019 Accepted: 29 Jan 2019 First Online: 28 Feb 2019 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2019 Geological Society of America Geology (2019) 47 (4): 355–358. https://doi.org/10.1130/G45462.1 Article history Received: 08 Aug 2018 Revision Received: 27 Jan 2019 Accepted: 29 Jan 2019 First Online: 28 Feb 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Patrick D. Duff, James N. Kellogg; The Brunswick magnetic anomaly: Geophysical signature and geologic source. Geology 2019;; 47 (4): 355–358. doi: https://doi.org/10.1130/G45462.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We used integrated analyses and forward and inverse modeling of potential field data to interpret the enigmatic Brunswick magnetic anomaly (BMA), a prominent negative magnetic lineation that is developed within the ocean-continent transition of the Southeast Georgia Embayment. From magnetic profiles extracted from gridded data, residual magnetic maps, and simultaneous 2.5-dimensional forward and 3-dimensional Euler inverse modeling of the gravity and magnetic fields, we suggest that the source of the BMA is a series of semicontinuous to discrete, late-stage rift-related mafic intrusions of Mesozoic age. This interpretation is supported by the observation that the BMA is independent of the East Coast magnetic anomaly, implying a predrift source, and that the amplitude and frequency of the anomaly change nearshore across a fracture zone that divides the offshore BMA, where continental breakup occurred, from the onshore BMA. Modeling demonstrates for the first time that a Mesozoic rift-related mafic body can explain the anomaly onshore. The lack of a BMA equivalent on the West African margin may indicate that Atlantic rifting began with a single lithospheric dislocation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Diversity and origin of quartz cements in continental carbonates: Example from the Lower Cretaceous rift deposits of the South Atlantic margin

Silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis. The Lower Cretaceous rift deposits of the South Atlantic equatorial margin, which are intensively explored by petroleum companies, provide good examples of such silicifications in carbonates, exhibiting a wide diversity of petrographic habit of early to late quartz cements. In order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity, we integrated detailed petrography of diagenetic sequences and quartz habit with δ18Oquartz measurements (by SIMS) of individual cements observed in samples from the offshore and onshore basins of the West African margin. The petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length-fast or length-slow habit, in addition to laminated microquartz and micro- or mega-quartz forms. Amongst the isotopic analysis, the δ18Oquartz data show that length-slow cements are generally strongly enriched in 18O (δ18Oquartz ranging from 31 to 37‰ SMOW), whereas length-fast forms show less elevated values (<32‰ SMOW). The highest δ18Oquartz values for fibrous microquartz are interpreted to reflect precipitation from evaporated meteoric fluids at temperatures >25 °C and <100 °C. The alkalinity required to favor the precipitation of length-slow fibrous microquartz cements is probably related to fluid/rock interactions with underlying mantel-related or basic volcanic rocks. Such interactions would be in agreement with the recent geodynamic models of the South Atlantic passive margin. The length-fast fibrous microquartz associated with δ18Oquartz values ranging from 27 to 32‰ SMOW, probably reflect precipitation from moderately to non evaporated, fairly neutral to acid, fluids. The partial dissolution of carbonate cements prior to quartz cementations represents the signature of those acidic conditions. We therefore suggest that acidic pH was obtained through fluid/rock interactions with the intermediate to acid volcanic rocks encountered along the palaeohydrological pathway. Other quartz phases, such as the megaquartz cement, exhibit highly variable δ18Oquartz values ranging from 20 to 40‰. This variation may reflect significant variation in temperature conditions (between 100 and 200 °C) or changes in fluid δ18O at the small scale. For these populations of non-fibrous quartz cement, the very high δ18Oquartz values may reflect a contribution of fluids that have either suffered strong evaporation or strong water/rock interaction.