West And Central African Rift System Research Articles

Pyrite Dissolution in the Cretaceous Yogou Formation of the Niger (Chad) Basin: Implications for Basin Evolution under a Rift Tectonic Setting.

This is the first study of pyrite minerals in the entire West and Central African Rift System (WCARS). Several polished organic-rich core samples from the Cretaceous Yogou Formation of the Niger (Chad) Basin located in the WCARS were investigated for their pyrite content using FE-SEM and SEM-EDS imaging techniques. An attempt was made to classify the types and provenance of the pyrites and to highlight the control of rift fractures on the oxidation and dissolution of pyrites in the region. Three major types of pyrites are present in the studied formation, including euhedral pyrite (EPy) crystals, pyrite framboids (FPy), and sunflower pyrites (SPy). A statistical analysis of 307 FPy shows that the framboids are diagenetically formed with an average diameter of 6.61 μm. SPy is present in a relatively low amount compared to framboids. The pyrites underwent a variety of diagenetic modifications, from mechanical compaction to oxidation, dissolution, and recrystallization. Unoxidized pyrites primarily contain Fe, S, and C, but oxidized pyrites also contain O, Al, and Si. There is a strong correlation between the fractures and the spatial distribution of the physicochemical alteration of the pyrite in the study. Dissolution in relatively deep-buried samples occurs mainly along fracture planes. The fractures provide a pathway for oxidants and other metal elements to reach the pyrites. The pattern of pyrite dissolution reflects the timing of fracture formation and fracture activities as a purveyor or drainage for fluids in the organic-rich samples investigated. The pyrites are associated intimately with organic matter (OM); thus, the relationship between the fracture and the pyrites' transformation is significant in the assessment of organic matter preservation at deep-burial depth.

Delineating structural features of North Cameroon using the EIGEN6C4 high‐resolution global gravitational model

The Cretaceous basins of the West and Central African Rift System (WCARS) are characterized by sinistral strike–slip movements observed between the Benue and Arabian–Nubian microplates. These basins share a similar basin‐filling structure, history, and tectonic processes. They have attracted a significant interest in the study of their geodynamics and subsurface structure through geophysical exploration methods. Nevertheless, North Cameroon located in this microplate confluence has not yet been investigated in terms of identifying geological structures, such as lineaments and faults. To contribute to a better understanding of the northern Cameroon structural framework and its tectonic context, we investigate the lineaments in northern Cameroon by using the EIGEN‐6C4 gravitational model and a variety of recent processing techniques (Tilt derivative [TDR], Theta Map [TM], Horizontal tilt angle [TDX], and Edge Detector [ED]). Our results show that the lineaments are trending in ENE–WSW, NW–SE, NE–SW, and WNW–ESE directions. These directions are consistent with the Cretaceous Garoua Rift and granitoids of the Cameroon‐Chad line. The presented result clearly delineates continuous architectonic features of the structural edges of the Saharan Metacraton, the Cretaceous Garoua Rift, the Bibemi‐Léré granitoids, and the Adamawa faults.

The crustal configuration of the West and Central African Rift System from gravity and seismic data analysis

SUMMARYThe West and Central African Rift System (WCARS) is the only stable continental geological structure on Earth that is formed by large-scale topographic massifs (swells). However, knowledge of the origin and evolution of the WCARS remains limited mainly due to the scarcity of high-resolution geophysical observations. To better understand the crustal structure beneath the entire WCARS, we use the XGM2016 global gravitational model and the ETOPO1 global topographic-bathymetric model to determine a gravimetric Moho (crust-mantle boundary) model constrained by seismic Moho depth estimates at 41 seismological stations distributed irregularly within the study area. The result reveals a regional Moho deepening to ∼40 km beneath the Hoggar, Aïr and Tibesti Massifs. The largest Moho deepening to ∼46 km is detected beneath the Archean Congo Craton, while the Moho depth under the Adamawa Plateau reaches 42 km. The Moho geometry beneath the Chad Basin, the Chad Lineament and the Termit Basin is relatively even with the Moho depth mostly within 24–26 km. A significant Moho deepening as well as large Moho depth variations within 32–45 km beneath the Saharan Metacraton and the Congo Craton (especially under its northern margin) reflect the metacratonization processes that occurred during the Neoproterozoic. The Niger Delta and the Benue Trough are characterized by a very thin continental crust with the Moho depth varying from ∼20 km in the south along the Atlantic coastline to ∼24 km in the northeastern branch of the Cretaceous Benue Trough around the Garoua-Yola Rift.

Geochemical characteristics and origins of biodegraded oils in the Bongor Basin (Chad) and their implications for petroleum exploration

The Bongor Basin is a typical lacustrine passive-rifted basin situated in the West and Central African Rift System (WCARS). It has experienced two phases of tectonic inversion and features a complex process of petroleum generation and accumulation. A total of 41 crude oil samples from the basin were geochemically analyzed to investigate their compositions of molecular markers. The results show that the oils have similar origins and are likely to belong to the same oil population. However, there are significant differences in geochemical characteristics and physical properties, caused by the secondary alteration. The relative contents and distribution patterns of normal alkanes and acyclic isoprenoids indicate that some of the oils have suffered biodegradation to varying degrees. The samples can be divided into three categories according to their relative degrees of degradation: normal oil, slightly biodegraded oil (PM 1–3), and severely biodegraded oil (PM 5–7). The burial depth of oil reservoirs in this area is the predominant factor impacting on the level of biodegradation. Crude oils in reservoirs with burial depths of less than 800 m are all severely biodegraded, while oils in reservoirs with burial depths greater than 1300 m have experienced no evident biodegradation. In reservoirs with burial depths between 800 m and 1300 m, the biodegradation degrees vary from normal to severely biodegraded. Oil reservoirs with burial depths less than 1300 m and adjacent to major faults are readily subject to biodegradation, while reservoirs with similar burial depths, but a certain distance away from major faults, have suffered no evident biodegradation. Moreover, if primary reservoirs have been modified by tectonic activity after accumulation, the crude oils are more likely to be biodegraded. Faulted anticline traps may create more favorable geological conditions for preservation of crude oil than reverse extrusion anticline reservoirs. This study may provide practical guidance for the assessment and prediction of oil quality in future oil exploration.

The release of petroleum from Central Africa rift basins over geological time as deduced from petroleum systems modelling

The Cretaceous basins within the Central Africa Rift sub-system (CAS) share similar tectonic processes, histories, and styles of basin fill, raising the question of the extent to which oil and gas generation is synchronised across the West and Central African Rift System (WCARS). Basin modelling was used to calculate the amount of hydrocarbons generated, the development of sealing capacity and losses of petroleum to the surface for the Melut, Fula, Sufyan, Bagarra, Doseo, and Doba rift basins. It was found that critical periods of hydrocarbon generation within the CAS occurred in the Lower Cretaceous, Upper Cretaceous and Paleocene-Eocene. During the Lower Cretaceous, regional seals had yet to be deposited, while during the Upper Cretaceous and Paleocene-Eocene, seals were poorly compacted and likely still did not constitute a good seal. During these intervals of time, high volumes of petroleum left the petroleum system (e.g. seeped to the surface). The resultant rates of methane seepage for the CAS rift-basins was 101–105 tons/year and is both quantitatively significant, and comparable to similar basins and rift systems. This rate of seepage is sufficiently high as to imply that micro-seepage alone would not have been capable of releasing the methane and that at least some seepage would have been catastrophic.

The lithofacies and depositional paleoenvironment of the Bima Sandstone in Girei and Environs, Yola Arm, Upper Benue Trough, Northeastern Nigeria

The Benue Trough is a contiguous intracontinental basin that formed part of the mega west and central African rift system (WCARS). The Cretaceous sedimentary succession preserved up to 6 km thick. The Aptian-Albian Bima Sandstone non-conformably overlies the Precambrian basement rock and the sandstone is the oldest Cretaceous succession in the Upper Benue Trough. This paper aimed at understanding its lithofacies and depositional environment in Girei and environs within Yola arm of the Upper Benue Trough. From the field studies, 13 lithofacies were identified; massive matrix supported and gravel dominated facies (Gmm), matrix supported gravel (Gmg), clast supported gravel (Gci), gravel stratified (Gh), and gravel stratified trough cross-bedded sandstone (Gt), sand, fine to very coarse grained trough crossbedded sandstone (St), sand, fine, medium, coarse to very coarse grained group planar cross-bedded sandstone (Sp), low angle planar crossbedded sandstone (Sl), sand, fine to coarse grained ripple cross laminated sandstone (Sr), sand, very fine, pebbly coarse grained horizontal bedded sandstone(Sh), Sand, fine to coarse grained massive sandstone (Sm), Convuluted sandstone (Sc), silt and mud (Fsm), and mudstone (Fm). The Bima Sandstone in the study area is sub-divided into proximal alluvial fan facies are composed of Gmm, Gmg, Gci, and Fsm, lithofacies, while the mid and distal fan facies are sequentially characterised by Gh, Gt, St, Sp, Sl, Sr, Sm, Sc, and Fm lithofacies. The alluvial fan facies are matrix supported and are restricted to the basin margin, and it may represent the basal part of the outcrop lithosections, the lack of internal organization of these lithofacies reflects high-strength viscous debris flow and it speaks volume of emergence of stream near the upland, however, the Gci facies probably indicate advancement of stream influence and reduction of gravity, the associated Fsm facies could be a backswamp and/or abandoned channel deposits. The mid fan facies have basal surface that is concave-upward in transverse crossection, well-developed thick amalgamated trough crossbedded sandstone (St) facies, theses attribute represent deep channel deposits, high flow regime, and continuous creation of accommodation space, it also points towards deep perennial sand braided river system. The distal fan facies is composed of amalgamated planar cross-bedded sandstone (Sp) associated with horizontal bedded sandstone (Sh), ripple cross lamination (Sr) and mudstone (Fm) facies with better developed floodplain deposit, pointing towards the influence of steady periodic high flow with topographic difference and it reflects shallow perennial sand-bed braided river. The lithofacies of the Bima Sandstone in Girei and environs, within the Yola arm generally show architectural stacking pattern that is generally thinning and fining upward indicating inactivity and/or fan retrogradation, and the depositional paleoenvironment varies from alluvial to braided river setting. The mid to distal fan facies of the Bima Sandstone can serve as good reservoir for hydrocarbon while the mudstone which is the floodplain deposit can serve as the seals and/or traps for the accumulated hydrocarbon.

Application of paleostress analysis for the identification of potential instability precursors within the Benue Trough Nigeria

Structures such as faults, joints and fractures of diverse patterns have acted as precursors of several slope instability cases within the Benue Trough Nigeria. In some cases, the structures by their nature weakened and also created avenues that streams took advantage to further destabilize the rock slopes. In other cases, structure orientation played significant roles in the mobility and eventual runout distance of debris flow and avalanches in the region. Detailed field-based structural, fracture and paleostress analyses were therefore carried out to determine the fractural patterns that correlate to reported instability and landslide cases in the region; and to produce models that reveal areas with heightened risk. Three fracture sets were isolated from analysis of fracture orientations and field relationships: Pre-folding (JT), Syn-Folding (JS) and Post Folding (JC) fracture systems. Paleostress analysis carried out on these fracture systems using the TENSOR™ software tool yielded three paleostress tensors corresponding to transtensional stress tensor with ENE-WSW direction of maximum extension (SHMIN), oblique compressive (transpressional) tensor with NW-SE direction of maximum shortening (SHMAX), and transtensional tensor with WNW-ESE direction of maximum extension (SHMIN). These tensors are related to the prevailing plate tectonic stress regimes affecting the entire Benue trough and the West and Central African Rift System (WCARS). Our pre- and post-tectonic models have revealed the reasons for instability and the likely places where future failures may be located. This is the first such analyses in the region and it is hoped that the results can broaden the use and applicability of paleostresses in failure-prone terrains for future risk and disaster reduction/assessment within the Trough and in other areas prone to structure-controlled landslides disaster.

LITHOSPHERIC STRUCTURE OF THE WEST AND CENTRAL AFRICAN RIFT SYSTEM FROM REGIONAL THREE-DIMENSIONAL GRAVITY MODELLING

A three-dimensional interpretation of the central part of the West and Central African Rift System (WCARS) is presented using the Earth Gravity Model 2008 (EGM2008). The 3-D model presented here provides new insights into the regional lithospheric structure of the central sector of the WCARS. The 3-D model reveals a possible crustal thickness and density distribution beneath the rift system, and the depth extent of magmatic activity in the Benue Trough is revealed for the first time. Because of asthenospheric uplift, the crust under the Benue Trough is thinned. The zone of crustal thinning (<30 km) coincides with the trace of an intrusion beneath the trough, and the maximum of which is attained in the Yola Basin. The Adamawa and Kapsiki Plateaus, on the other hand, show by far the largest crustal thickness in the region (34 to 36 km). The inferred zone of intrusion coincides with the maximum gravity anomaly of the rift zone. The great depth (ca. 14 km) to the top of the intrusion along the rift may explain the absence of magmatism in the Benue Trough compared to the Cameroon Volcanic Line (CVL) or the East African Rift System (EARS). The relatively small nature of the intrusion leads to the conclusion that small-scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the Benue Trough.

Geochemical characterisation of Early Cretaceous lacustrine sediments of Bima Formation, Yola Sub-basin, Northern Benue Trough, NE Nigeria: Organic matter input, preservation, paleoenvironment and palaeoclimatic conditions

The Bima Formation is the oldest lithogenetic unit occupying the base of the Cretaceous successions in the Yola Sub-basin of the Northern Benue Trough. It is differentiated into Lower (B1), Middle (B2) and Upper (B3) Bima Members. Integrated organic and inorganic geochemical studies were applied on the lacustrine sediments of the Bima Formation with the aim of reconstructing the palaeodepositional environment and organic matter input in response to climate and tectonism. The analysed sediments were deposited in a freshwater lacustrine environment with a low-salinity stratified water column and suboxic to relatively anoxic conditions, as indicated by the bulk geochemical parameters. The biomarkers provide evidence for a contribution of aquatic algae and microorganisms, with a significant amount of terrigenous organic matter input. The preservation of the organic matter is possibly attributed to the stratified water column with low salinity and suboxic to relatively anoxic conditions. Based on the geochemistry of major and trace elements, the sediments were deposited during semi-arid climatic conditions within passive continental margin setting. This agrees with the tectonic events in the West and Central African Rift System (WCARS) during the Early Cretaceous period.

Geophysical evidence of Cretaceous volcanics in Logone Birni Basin (Northern Cameroon), Central Africa, and consequences for the West and Central African Rift System

Detailed analyses and interpretation realized by combining existing 2D reflection seismic and Gravity/Magnetic data of the Logone Birni Basin (LBB) in the West and Central African Rift System (WCAS) have revealed the distribution of the main buried volcanic bodies as well as their relationships with the structural and tectonic evolution of this basin. The volcanic activity in the LBB is restricted to the Cretaceous period. Three main volcanic episodes are identified and are associated to the Neocomian, Late Albian and Cenomanian–Turonian rifting phases respectively. The volcanic bodies within the Lower Cretaceous are either lying directly on basement or are mainly interbedded with the contemporaneous sediments whereas the Upper Cretaceous bodies are morphologically expressed in the forms of dykes and sills. The volcanic activity was more intense in the western region of the central LBB (Zina sub-basin) along the Cameroon–Nigeria border whereas it was scanty and scattered in the other parts of the basin. The main volcanic dykes are found on the flanks of the major faults bounding basement horsts or in crestal positions in association with syndepositional faults.Although WCAS is associated with large amount of crustal extension and minor volcanism, the intense volcanic activity observed in LBB during the Cretaceous suggests that the intrusive zone during this period was confined to the basement beneath the study area flanked respectively to the north, south and southwest by the Lake Chad, Poli and Chum triple junctions. Tensional stresses generated by this localized domal uplift accounts for most of the observed tectonic structures where major faults transected the entire lithosphere, thus providing conduits for magma migration.

On the use of global potential field models for regional interpretation of the West and Central African Rift System

The use in regional interpretations of the Earth Gravity Model (EGM08) and Earth magnetic model (EMAG2) is evaluated by comparison to ground gravity and aeromagnetic data in the central sector of the West and Central African Rift System (WCARS). The comparison includes upward continuation, spectral analysis and pseudogravity calculation and statistical evaluation. A correlation between EMAG2 (which contains roughly 25 km resolution aeromagnetic data in the region) and near-surface aeromagnetic data over WCARS is only true for the very low wavelength part but a strong similarity between EGM08 and ground gravity data can be confirmed. Interpretation of the EGM08 data allows identifying and confirming the position of major structural trends, and provides new information on the crustal architecture. The lineaments limiting different grabens forming the Logone Birni basin and in the southern Chad basin are identified. The results presented here show the use of EGM08 data for regional interpretations over Cameroon and adjacent countries, which can overcome the absence and sparseness of data in developing countries and remote areas.

Basement control in the development of the early cretaceous West and Central African rift system

Abstract The structural framework of the Precambrian basement of the West and Central African Rift System (WCARS) is described in order to examine the role of ancient structures in the development of this Early Cretaceous rift system. Basement structures are represented in the region by large Pan-African mobile belts (built at ca. 600 Ma) surrounding the > 2 Ga West African, Congo and Sao Francisco cratons. Except for the small Gao trough (eastern Mali) located near the contact nappe of the Pan-African Iforas suture zone along the edge of the West African craton, the entire WCARS is located within the internal domains of the Pan-African mobile belts. Within these domains, two main structural features occur as the main basement control of the WCARS: (1) an extensive network of near vertical shear zones which trend north-south through the Congo, Brazil, Nigeria, Niger and Algeria, and roughly east-west through northeastern Brazil and Central Africa. The shear zones correspond to intra-continental strike-slip faults which accompanied the oblique collision between the West African, Congo, and Sao Francisco cratons during the Late Proterozoic; (2) a steep metamorphic NW-SE-trending belt which corresponds to a pre-Pan-African (ca. 730 Ma) ophiolitic suture zone along the eastern edge of the Trans-Saharian mobile belt. The post-Pan-African magmatic and tectonic evolution of the basement is also described in order to examine the state of the lithosphere prior to the break-up which occurred in the earliest Cretaceous. After the Pan-African thermo-tectonic event, the basement of the WCARS experienced a long period of intra-plate magmatic activity. This widespread magmatism in part relates to the activity of intra-plate hotspots which have controlled relative uplift, subsidence and occasionally block faulting. During the Paleozoic and the early Mesozoic, this tectonic activity was restricted to west of the Hoggar, west of Air and northern Cameroon. During the Late Jurassic-Early Cretaceous the St. Helena mantle plume may have had an important effect in weakening the lithosphere across a broad zone from Brazil to Sudan. However, causes of extension and stretching are likely to be found in a plate tectonic framework. At that time, the opening of both the Indian and Atlantic oceans were the dominant forces which favoured the development of the WCARS and its propagation by strike-slip movements and extensional displacements along pre-existing Pan-African lithospheric zones of weakness.

Petroleum Geology of Cretaceous-Tertiary Rift Basins in Niger, Chad, and Central African Republic

This overview of the petroleum geology of rift basins in Niger, Chad, and Central African Republic (CAR) is based on exploration work by Exxon and partners in the years 1969-1991. The work included 50,000 km of modern reflection seismic, 53 exploration wells, 1,000,000 km2 of aeromagnetic coverage, and about 10,500 km of gravity profiles. The results outline ten Cretaceous and Tertiary rift basins, which constitute a major part of the West and Central African rift system (WCARS). The rift basins derive from a multiphased geologic history dating from the Pan-African (approximately 750-550 Ma) to the Holocene. Main rifting was probably caused by the stretching and subsidence of African crustal blocks during the breakup of Gondwana in the Early Cretaceous and by r activated plate movements in the early Tertiary. WCARS in the study area is divided into the West African rift subsystem (WAS) and the Central African rift subsystem (CAS). WAS basins in Niger and Chad are chiefly extensional, and are filled by up to 13,000 m of Lower Cretaceous to Holocene continental and marine clastics. The basins contain five oil (19-43°API) and two oil and gas accumulations in Upper Cretaceous and Eocene sandstone reservoirs. The hydrocarbons are sourced and sealed by Upper Cretaceous and Eocene marine and lacustrine shales. The most common structural styles and hydrocarbon traps usually are associated with normal fault blocks. CAS rift basins in Chad and CAR are extensional and transtensional, and are filled by up to 7500 m of chiefly Lower Cretaceous continental clastics. The basins contain eight oil (15-39°API) and one oil and gas discovery in Lower and Upper Cretaceous sandstone reservoirs. The hydrocarbons are sourced by Lower Cretaceous shales and sealed by interbedded lacustrine and flood-plain shales. Structural styles range from simple fault blocks through complex flower structures. The main hydrocarbon traps are in contractional anticlines. Geological conditions favor the discovery of potentially commercial volumes of oil in WCARS basins of Niger, Chad and CAR.