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NE-dipping normal fault (eastern edge of the Lambare ́ ne ́ Horst) contemporaneous with deposition of the Agoula series and reactivated during the syn-rift phase (location in Fig. 6).
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The North Gabon coastal rift basins consist of a set of 130–150 long-segment asymmetrically tilted half grabens (Interior Basin) and 000–020 short-segment en échelon half grabens (N'Komi Basin) separated by 040–060 major transverse faults. Tectono-sedimentary analysis of field and subsurface data reveals the control exerted by extensional tectonism...
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FTG data were acquired over an onshore Gabon Block. The objectives of this survey were to delineate accurately the salt structures; to derive a base salt structure map and to map basement structures associated with rifting. The survey area is situated along a clear trend of oil & gas subsalt fields that runs roughly north-south from offshore fields...
The Early Cretaceous northeast Brazilian Sergipe microplate, formed at the northern end of the South Atlantic rift between South America and Africa, closely resembles the modern Sinai microplate at the northern end of the Red Sea in size, shape, and relative motion. Both formed where east-northeast trending transverse shear zones arrested northward...
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... Different rift segments were generated under an overall NE-SW-oriented extension during the opening of the South Atlantic (Rabinovich and LaBrecque, 1979;Moulin et al., 2010;Chaboureau et al., 2013): the equatorial segment, the central segment, the southern segment and the Falkland segment, the central segment being the focus of this study. These segments are characterized by distinct extensional and subsidence histories and are bounded by NE-SW-striking transform structures (Mbina Mounguengui and Guiraud, 2009). ...
... (Bidiet et al., 1998). The so-called "basement" is composed of both the crystalline part of the Congo craton and the overlying Precambrian pre-rift sediments (Delpomdor et al., 2008;Mbina Mounguengui and Guiraud, 2009;Préat et al., 2010). Presalt basins are mainly filled by fluvial and lacustrine deposits (Guiraud et al., 2010), and Fm., from bottom to top: (i) TOCA 1 which is composed of microbialites; (ii) TOCA 2 showing microbial encrustations of shells, microbial mats and thromboliths; and (iii) TOCA 3 made of the accumulation of granular and coquina facies, i.e., mainly freshwater lamellibranch tests belonging to the Unionidae group (Grosdidier et al., 1996). ...
... showing the classification of kerogens in the studied samples and thermal maturity (modified after Espitalié et al., 1985). that suggested by Kribi/Campo, North Gabon, Rio Muni Basin (Brownfield & Charpentier, 2006;Mbina Mounguengui et al., 2003;Mbina Mounguengui & Guiraud, 2009;Ntamak-Nida et al., 2008). ...
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. Based on facies analysis, geochemical and palynofacies data, the palaeoenvironment of the lower Cretaceous syn‐rift Mundeck Formation of the Douala Basin has been proposed. The shales contain Type III kerogens with mainly gas‐generative potential. The result highlights petroleum prospectivity and the early‐stage evolution of the Douala Basin.
... three main intervals from rifting to the present-day (Fig. 1b): (1) the presalt section , which consists of continental syn-rift sequence, (2) the salt section , which marks the major incursion of the South Atlantic Ocean into the rift (transition phase), and (3) the post-salt section (112 to present-day), which essentially consists of marine sequences deposited on the subsiding passive margin of West Africa (McHargue, 1990). Because Jurassic, Triassic and Palaeozoic series are missing (Bidiet et al., 1998), the Cretaceous-Cenozoic sedimentary series rests directly on the so-called Precambrian 'basement' (Fig. 1b & c), which comprises both the crystalline part of the Congo craton and the overlying Precambrian pre-rift sediments (Delpomdor et al., 2008;Mbina Mounguengui and Guiraud, 2009;Préat et al., 2010). Fluvial and lacustrine deposits dominate the syn-rift filling of the pre-salt basins (Guiraud et al., 2010), and were dated using ostracods, pollens and spores biozones (Grosdidier, 1967;Chaboureau et al., 2013). ...
The timing of porosity destruction in reservoir rock is key for hydrocarbon exploration. This study constrains the porosity destruction of the syn-rift Barremian (130-125 Ma) TOCA Fm. From one drill core located in the Lower Congo basin (West African passive margin). We apply an unprecedented multi proxy approach to reconstruct the absolute timing of mechanical compaction, pressure-solution and cementation as well as their relative contribution to porosity destruction. LA-ICP-MS U–Pb geochronology combined with petrological observations suggest that the main interstitial diagenetic cement C1 filling the porosity of the packstones and dated 127.4 ± 2.2 to 119.4 ± 6.4 Ma predates pressure-solution. The quantification of the cement distribution by image analysis indicates that C1 clogged ∼30% of the initial porosity before the strata were buried at 400 m. We assume that the early cementation limited the effect of mechanical compaction on porosity reduction which is estimated to ∼2% in the first 100 m of burial. In contrast, a first order estimate of the chemical compaction-reprecipitation can account for up to ∼6% of the initial porosity destruction, as estimated by the inversion of sedimentary stylolite roughness. This technique also documents that the burial-related pressure-solution in the TOCA Fm. Occurred in the 550–1750 m burial depth range corresponding to a time spanning from 117 to 95 Ma considering the burial-time model of the TOCA Fm. Our study reveals that the initial porosity of carbonates was reduced down to its current value of 4–8% within the first 35 Ma of its burial history, reaching ∼10% after only 10 Ma, i.e. in the first 400–500 m of burial. This case study indicates that the fate of reservoir properties in bioclastic carbonate formations such as the TOCA Fm. May be largely controlled by early, very shallow diagenetic processes rather than subsequent deep-burial mesogenetic reactions.
... The results of the present study indicate high primary productivity and dysoxic to anoxic conditions in the deepest part of the palaeo-lake during deposition; the deep-water lacustrine conditions passed laterally into a more oxic shallow-water shelf characterized by significant terrigenous input including woody phytoclast material. Similar Early Cretaceous depositional environments have been interpreted in the Benue Trough (Babangida et al., 2015;Sarki Yandoka et al., 2017); in the Doba, Bongor, Termit and Muglad Basins (locations in Fig. 1; Dou et al., 2018aDou et al., , 2020Liu et al., 2015); in the Kribi/Campo, Douala and Rio-Del-Rey Basins of southern coastal Cameroon (Ntamak-Nida et al., 2008;Agyingi et al., 2019); in the North Gabon margin (N'Komi Basin: Mbina et al., 2009); and in the northern part of the Sergipe -Alagoas Basin along the conjugate margin of Brazil (Mello and Katz, 2000). The results therefore suggest that organic matter accumulated during the Neocomian -Barremian in the Mayo Oulo-Léré Basin in a lacustrine environment with variable depositional conditions. ...
The synrift Mayo Oulo‐Léré Basin in Northern Cameroon is located in the transition zone between the West and Central African Rift Systems. Structural and stratigraphic elements of the basin resemble those of the Yola Basin in NE Nigeria, an extension of the Upper Benue Trough. The Lower Cretaceous lacustrine shales with source rock potential which occur in nearby rift basins are also present in the Mayo Oulo‐Léré Basin. These shales were investigated at two outcrop locations (Badesi and Tchontchi), and samples collected (n = 60) were subjected to palynofacies and bulk geochemical analyses to evaluate their petroleum generation potential and to interpret their depositional environment. At the studied locations, shales were divided into two lithofacies: grey to black laminated shales containing algal‐bacterial OM together with common woody (phytoclast) debris (“facies Fml”); and cm‐bedded shales which had a higher content of algal‐bacterial OM but a lower phytoclast content (“facies Fmlc”). Palynological and bulk geochemical data indicate that the shales contain well‐preserved organic matter (OM) and locally display good to excellent oil generation potential. Average TOC contents are 2.7% and 1.4% for samples of the Fmlc and Fml facies shales respectively. HI values (94‐889 mg HC/g TOC and 131‐638 mg HC/g TOC respectively) suggest that the shales contain Types I to III kerogen. Organic material in the Fmlc facies shales is dominated by amorphous organic matter (AOM: 90% on average) with a low phytoclast content (6% on average); whereas samples of the Fml facies shales contain less AOM (74% on average) and have a higher phytoclast content (23% on average). AOM in the Fmlc shales is highly fluorescent; these shales are interpreted to have been deposited in dysoxic to anoxic conditions. The AOM in the Fml shales is weakly fluorescent and the shales were deposited under more oxic conditions. The kerogen in the shales ranges from immature to early oil window mature. Average values of the pyrolysis S2 yield are 15.5 mg HC/g of rock and 7 mg HC/g of rock for samples from Fmlc and Fml facies shales respectively. The shales increase in thickness northwards towards the Logone Birni Basin where they may have reached the oil window, as in neighbouring basins. The results of this study of lacustrine shales from the Mayo Oulo‐Léré Basin suggests that there may be potential for oil exploration in northern Cameroon.
... • The Phanerozoic domain outcrops in the West at the location of the coastal basin linked to the opening event of the South Atlantic Ocean during the Lower Cretaceous [24] and in the East at the location of the Paleogene to Quaternary sandstones and the aeolian sands of the Batéké plateau [8] [25]. ...
The Gabon geology offered favorable and natural environments for the formation of various types of mineralization. The Etéké gold district, aim of this study, is located in the Ngounié province (southern Gabon) on the western edge of the Chaillu massif. Geologically, the gold mineralization is associated with the Eburnean orogeny and hosted in the Archean greenstone belts. Also, this deposit is covered by a significant vegetation cover and a very extensive lateritic weathering profile, which hinders the most accurate study. Through this paper, we aim to propose a genesis pattern of this mineralization via a multidisciplinary approach. To do this, a petrographic, metallogenic, and geo-chemical characterization has been established in the different sectors of the Etéké deposit. The studied deposits display varied facies which are encased in granitoid. They are essentially formed of abundant granitoid, and amphibolite compared to the volcano-sedimentary formations. These rocks display mag-matic textures, affected by metamorphism, and not sufficiently preserved. Based on our multidisciplinary approach, the studied samples collected from the core's boreholes allowed us to decipher a volcanogenic and metamorphosed origin of the gold genesis.
... The South Atlantic can be subdivided into the Equatorial, Central and Austral segments (Moulin et al., 2005). These segments formed during diachronous, northward propagating rifting and resulted in the breakup of Gondwana from late Jurassic-Neocomian (Austral segment) to the Albian (Equatorial segment) (Mounguengui and Guiraud, 2009;Davison, 1999). The detailed temporal and spatial evolution of breakup is, however, debated as indicated by conflicting plate kinematic reconstructions (e.g. ...
... Dental Fm., Fig. 1; Dupré et al., 2007). This thick pre-salt sequence formed during crustal thinning (~127-118 Ma; Mounguengui and Guiraud, 2009) and early hyperextension and is overlain by thick evaporites deposited during the latest Aptian (113-112 Ma). The exact time and duration of evaporite deposition has not been established with confidence since the more distal parts of the margin have not been drilled. ...
In this study, we present an interpretation of a 3D depth-migrated dataset imaging the Ocean Continent-Transition (OCT) of the southern Gabon margin. Located in an area where salt is not strongly overflowing the OCT, it allows the geometrical relationships of salt deposition and its relative timing to breakup to be determined. The OCT is formed by extensional propagators floored by exhumed mantle limited by either fault-bounded upper-crustal or pre-salt sediments and/or magmatic additions. Based on our interpretations, we conclude that the pre-salt sequence was deposited over a wide area spanning crustal thinning to the onset of hyperextension and exhumation. The relative timing of salt deposition is constrained by the observations that detachment faults break away in the pre-salt sequence and that some of these faults also strongly offset the base salt and are linked to the creation of accommodation space in the salt. This is direct proof that salt was deposited during mantle exhumation. Salt deposition is interpreted to have occurred at variable depths. Evaporites formed approximately 1 km below sea level in the north, above a 4 to 6 km thick pre-salt sequence, and as deep as 2 km in the south. In this area, salt is interpreted to directly overlie exhumed mantle, but whether it was deposited on mantle or juxtaposed during exhumation remains more debatable. Space created in the faulted outer trough was filled partly by autochthonous salt and possibly also by downslope flow. Rifting continued after the cessation of salt deposition, followed by magmatic additions in the outer high and ultimately the onset of seafloor spreading and the formation of Penrose oceanic crust.
... Le soulèvement concerne des reliefs préexistants généralement non montagneux (Peulvast et Vanney, 2002 ;Flageollet, 2008). Birot, 1982 ;Thomas, 1995 ;Ufimtsev, 2009 ;Gunnell et Harbor, 2010 ;Japsen et al., 2012 ; Afrique du Sud Continu depuis le Crétacé (moins de 150 millions d'années) avec épisodes tectoniques datés dans les périodes récentes voire actuelles Burke et Gunnell, 2008 ;Tinker et al., 2008 ;Braun et al., 2014 ;Richardson et al., 2017 ;Baby et al., 2018 Angola Pigeon, 1984 ;Braun, 2018 Écosse depuis le mésozoïque (< 150 millions d'années) Godard, 1982 ;Le Coeur, 1991 Gabon continu depuis début mésozoïque avec activation nette au Mio-Pliocène (< 15 millions d'années) Dupré et al., 2007 ;Mounguengui et Guiraud, 2009 ;Dupré et al., 2011 ;Decker et al., 2013 ;Wildman et al., 2015 Ghats indiens depuis le mésozoïque (< 250 millions d'années Widdowson et Cox, 1996 ;Gunnell, 1998 ;Catherine et al., 2007 ;Ajayakumar et al., 2013 ;Sahu et al., 2013 ;Catherine et al., 2015 Groenland Borges et al., 2001 ;Terrinha et al., 2003 ;Cabral et al., 2004 ;Jimenez-Munt et al., 2010 ;Duarte et al., 2013 ;Sallares et al., 2013 ;Dean et al., 2015 ;Somoza et al., 2019 Scandinavie depuis le mésozoïque (< 250 millions d'années Battiau, 1982 ;Godard, 1982 ;Bonow et al., 2014 ;Japsen et al., 2019 Vosges Cénozoïque (< 65 millions d'années) Flageollet, 2008 D'après la littérature scientifique, l'ensemble géographique constitué d'une plaine abyssale, actuellement vers 4 000 m de profondeur, le talus continental, la plateforme continentale, la plaine littorale et la montagne du bourrelet est un modèle dont les dynamiques évolutives sont variées. ...
What is a “realistic” approach to mountains in geography: the example of passive margin mountains
The mountain is an essential object of geography but, like other objects, it opposes an approach offering elements of description and explanation based on a realistic, and above all geological representation, to another one more recent taking counts the representations of the populations. For the geography taught but also for the use which is made of it from a legislative point of view in France with the Mountain Law, the realistic approach dominates. The question is to know if this one, which bases a normative approach of the mountain is still valid in the light of the most recent scientific advances. The passive margin mountains, emblematic mountains of physical geography, are used here as an example of a type of mountain. From an in-depth analysis of the scientific literature, two points stand out: the recent genesis of these mountains, like all mountains on the surface of the earth, and the variety of their evolution. Putting geographic science into debate here does not consist in destroying its status in its function of cognitive action, in particular in the service of uses requested by society, but in recognizing the biases and limits of certain knowledges.
... The first sediments related to opening of the South Atlantic were deposited in the Central Atlantic and southernmost Southern Atlantic during the Late Jurassic (Davison, 1999;Guiraud et al., 2010), and possibly earlier (Mouguengui and Guiraud, 2009;Dupre et al., 2011;Gladzcenko et al., 1998). This stage is recorded by the deposition of a relatively thin sequence of continental red beds (Davison, 1999;Dupre et al., 2011). ...
... Volcanics of this age are scarce in Gabon but more abundant in the Kwanza basin (Marzoli et al., 1999). Syn-rift sedimentation of the Angola and Gabon margins was initially dominated by continental clastics and evolved progressively to shales and carbonates deposited in alkali lake environments (Mouguengui and Guiraud, 2009;Guiraud et al., 2010;Dupre et al., 2011;Saller et al., 2016). The main syn-rift sequence is capped by lacustrine to shallow marine Late Barremian to Early Aptian siliciclastics and carbonates (known as the "sag"), that are in turn overlain by a thick evaporite unit, simply called the "salt", of middle to late Aptian age: the Ezanga Fm in Gabon, and the Loeme Salt in Angola (Mouguengui and Guiraud, 2009;Saller et al., 2016). ...
... Syn-rift sedimentation of the Angola and Gabon margins was initially dominated by continental clastics and evolved progressively to shales and carbonates deposited in alkali lake environments (Mouguengui and Guiraud, 2009;Guiraud et al., 2010;Dupre et al., 2011;Saller et al., 2016). The main syn-rift sequence is capped by lacustrine to shallow marine Late Barremian to Early Aptian siliciclastics and carbonates (known as the "sag"), that are in turn overlain by a thick evaporite unit, simply called the "salt", of middle to late Aptian age: the Ezanga Fm in Gabon, and the Loeme Salt in Angola (Mouguengui and Guiraud, 2009;Saller et al., 2016). Volcanic dykes of late Barremian to early Aptian age (around 125 Ma, Marzoli et al., 1999) were intruded roughly synchronous with the initiation of evaporite precipitation. ...
Serial 2D reflection seismic profiles spanning the continent-ocean transition in the offshore Kwanza and southern Gabon basins have been interpreted and integrated with 2¹/2D gravity and magnetic anomaly modelling. These continental margins are characterized by the presence of a distal domain in which continental crust has been highly stretched and faulted, overlain by thick sequences of Early Cretaceous syn-rift sediments, that are in turn overlain by a thick but highly deformed Aptian evaporite unit. The evaporites are overlain by post-breakup sediments (Cretaceous to Recent in age). Oceanwards, syn-rift sediments are overlain by a volcanic unit that thickens basinward. This volcanic unit forms the base and the lateral-equivalent of the distal evaporite basin, developing into positive-relief volcanic edifices that acted as the oceanward margin of the evaporite basin. These volcanic units extend from the distal domain of the passive margin to the oceanic crust and are interpreted to represent volcanism that immediately predates lithospheric breakup and formation of oceanic crust, as indicated by the progressive transition into a constant-thickness oceanic crust. Volcanism synchronous with the late stages of passive margin development, and related hydrothermalism, can account for the high alkalinity that dominated the pre-salt lacustrine environments and could also have contributed to the modification of marine waters that led to the deposition of the thick Aptian evaporites in the South Atlantic Ocean. Breakup volcanism and its associated thermal impact can also help explain the apparently anomalous shallow water depth of these margins preserved during the latest rift and earliest post-rift stages.
... This exploration proved the hydrocarbon potential of Lower Cretaceous syn-rift units. Several reservoir levels are present in sands from the Neocomian to the Aptian, laterally and vertically juxtaposed with a number of source rocks which also act as seals (Teisserenc and Villemin, 1990;Mounguengui and Guiraud, 2009). ...
... 3) Oblique rifting occurs [13] [40]- [45]) when the relative plate motion is not parallel to the directions of the principal strain, e.g., [46] [47], and the plates spread obliquely compared to the ridge/rift trend [18]. Rift obliquity can be moderate (15˚ -45˚) [48] to high (45˚ -75˚) [49], but extension is accommodated by both the normal and the strike-slip faults [18], and characterised by en échelon segments that are surface expression of deeper structures [41] [48]- [53]. ...
