Margin Of Africa Research Articles

Coeval development of extensional and contractional features along transform margins: insights from the Diaz Marginal Ridge

Abstract The Diaz Marginal Ridge (DMR), on the southern transform margin of South Africa, is a bathymetric feature parallel to the Agulhas Falkland Fracture Zone (AFFZ) that has long been considered an archetype marginal ridge; and yet its origin and evolution remains unconstrained. Using recently acquired seismic data we present a new structural interpretation of the DMR and its association with the evolution of both the AFFZ and the Southern Outeniqua Basin. In contrast to previous scenarios invoking thermo-mechanical explanations for its evolution, we observe a more straightforward structural model in which the genesis of the DMR results from the structural inversion of a Jurassic rift basin. This inversion resulted in the progressive onlap of latest Valanginian–Hauterivian-aged stratigraphic units, important for the formation of stratigraphic plays of the recent Brulpadda discovery. Paradoxically, this contraction is contemporaneous with renewed extension observed in the inboard normal faults. The orientation of the DMR and inboard structures have been demonstrated to be controlled by the underlying Cape Fold Belt (CFB) fabric. The onset of motion across the AFFZ shear system led to east–west-orientated maximum stress and north–south-orientated minimum stress. We propose this stress re-orientation resulted in strain partitioning across existing structures whereby in addition to strike-slip on the AFFZ there was coeval extension and contraction, the nature of which was determined by fault orientation. The fault orientation in turn was controlled by a change in orientation of the underlying CFB. Our model provides new insights into the interplay of changes in regional stress orientation with basement fabric and localized magmatism along an evolving transform. The application of horizontal strain partitioning can provide an explanation of similar features observed on other transform margins.

Overview on GPlates: focus on plate reconstruction

Plate tectonic reconstructions have been employed in geosciences since 1970s, in the context of hydrocarbon exploration, regional geology and paleobiology. Such studies have given valuable inputs for climate and geodynamic computations, present-day mantle structure, models of plate motion, and the interpretation of the drift of hotspots, true polar wander (TPW), sea level and stratigraphic signals. However, geodynamic models generated in the past by incorporating global plate tectonic reconstructions have limitations. To overcome this, GPlates software brings forward a new era of interactive plate tectonic reconstruction software integrated with GIS databases that incorporates a wide variety of geological and geophysical data. Besides modelling tectonic and crustal evolution, GPlates has also been used in visualizing paleogeography and paleobathymetry, in understanding deep carbon cycle, subduction zone initiation, mantle evolution, investigating earthquakes and predicting future supercontinents. the software has been widely used in hydrocarbon exploration along the passive conjugate margins such as from the margins of South America and Africa and has provided promising results in acquiring new and more reliable prospecting criteria for petroleum systems. Additionally, it has become an integral tool for paleolatitude calculations, modelling of paleoclimate and paleoenvironment. This article reviews key plate reconstructions that have been carried out using GPlates, the typical constraints and the set of input parameters.

Rare and widespread: integrating Bayesian MCMC approaches, Sanger sequencing and Hyb-Seq phylogenomics to reconstruct the origin of the enigmatic Rand Flora genus Camptoloma.

Genera that are widespread, with geographically discontinuous distributions and represented by few species, are intriguing. Is their achieved disjunct distribution recent or ancient in origin? Why are they species-poor? The Rand Flora is a continental-scale pattern in which closely related species appear codistributed in isolated regions over the continental margins of Africa. Genus Camptoloma (Scrophulariaceae) is the most notable example, comprising three species isolated from each other on the northwest, eastern, and southwest Africa. We employed Sanger sequencing of nuclear and plastid markers, together with genomic target sequencing of 2190 low-copy nuclear genes, to infer interspecies relationships and the position of Camptoloma within Scrophulariaceae by using supermatrix and multispecies-coalescent approaches. Lineage divergence times and ancestral ranges were inferred with Bayesian Markov chain Monte Carlo (MCMC) approaches. The population history was estimated with phylogeographic coalescent methods. Camptoloma rotundifolium, restricted to Southern Africa, was shown to be a sister species to the disjunct clade formed by C. canariense, endemic to the Canary Islands, and C. lyperiiflorum, distributed in the Horn of Africa-Southern Arabia. Camptoloma was inferred to be sister to the mostly South African tribes Teedieae and Buddlejeae. Stem divergence was dated in the Late Miocene, while the origin of the extant disjunction was inferred as Early Pliocene. The current disjunct distribution of Camptoloma across Africa was likely the result of fragmentation and extinction and/or population bottlenecking events associated with historical aridification cycles during the Neogene; the pattern of species divergence, from south to north, is consistent with the "climatic refugia" Rand Flora hypothesis.

Paleogene granite from offshore of Morocco (DSDP Leg 79): crustal recycling at a passive continental margin of NW Africa

Paleogene granite from offshore of Morocco (DSDP Leg 79): crustal recycling at a passive continental margin of NW Africa

Exploratory plays of the Foz do Amazonas Basin, NW portion, in deep and ultra-deep waters, Brazilian Equatorial Margin

A petroleum geology study was carried out at Foz do Amazonas Basin northwest portion, a new exploratory frontier basin on the Brazilian Equatorial Margin. Seismic and well data were interpreted to search similar play of some oil discoveries in correlated basins. The study area is in deep/ultra-deep waters, where the influence of the Amazon Cone is less expressive and 90 km east of Zaedyus oil discovery on Guiana-Suriname Basin. There were interpreted four seismic horizons, four chronostratigraphic intervals, two hypothetical petroleum systems, two exploratory Cretaceous plays, and an estimative of the source rock maturity. The petroleum systems are: Cassiporé/Codó–Limoeiro (late Aptian-Late Cretaceous) and Limoeiro–Limoeiro (Cenomanian/Turonian-Late Cretaceous). The top seismic horizons interpreted are Basement, Albian, Cretaceous, and Middle Miocene, determining four chronostratigraphic intervals: Early Cretaceous, Late Cretaceous, Paleocene to Middle Miocene, and Late Miocene to Recent. There were interpreted two Cretaceous plays: Slope to Plain and Deep/Ultra-Deep Waters. The main source rocks are the shales of the Codó Formation (late Aptian) and Limoeiro Formation (Cenomanian-Turonian). The Codó Formation reaches 10% TOC, has Type I kerogen, excellent potential for oil generation, and thermal maturity. The Limoeiro Formation may have 4,4% TOC, Type II kerogen, and very good petroleum potential. The reservoir rocks are Late Cretaceous Limoeiro Formation turbiditic sandstones. The seal rocks are marine shales from Late Cretaceous Limoeiro Formation. The traps are mainly stratigraphic (pinch-out). The migration routes are mainly lateral or, exceptionally, vertical, where the faults are present. The estimated source rock maturity indicates that at least one of them is below the oil window top. It was concluded that there is a great hydrocarbon exploratory potential to this part of the basin, similar to the discoveries that occurred in the correlated basins of the equatorial margins of Africa and South America.

Opposite Symmetry in the Lithospheric Structure of the Alboran and Algerian Basins and Their Margins (Western Mediterranean): Geodynamic Implications

AbstractThe geodynamic evolution of the Western Mediterranean for the past 35 My is a matter of debate. Present‐day structure and composition of the lithosphere and sublithospheric mantle may help in constraining the geodynamic evolution of the region. We use an integrated geophysical‐petrological modeling to derive and compare the present‐day thermal, density and compositional structure of the lithosphere and sublithospheric mantle along two NNW‐SSE oriented transects crossing the back‐arc Alboran and Algerian basins, from onshore Iberia to the northern Africa margin. The crust is constrained using available seismic data and geological cross‐sections, whereas seismic tomography and mantle xenoliths constrain the upper mantle structure and composition. Results show a thick crust (37 and 30 km) and a relative deep LAB (130 and 150 km) underneath the HP/LT metamorphic units of the Internal Betics and Greater Kabylies, respectively, which contrast with the 16 km thick magmatic crust of the Alboran Basin and the 10 km thick oceanic crust of the Algerian Basin. The sharp change in lithosphere thickness, from the orogenic wedge to the back‐arc basins, contrasts with the gentler lithosphere thickening toward the respective opposed margins. Our results confirm the presence of detached slabs ∼400 °C colder than upper mantle and a fertile composition than the continental lithospheric mantle beneath the External Betics and Saharan Atlas. Presence of detached quasi‐vertical sublithospheric slabs dipping toward the SSE in the Betics and toward the NNW in the Kabylies and the opposed symmetric lithospheric structure support an opposite dipping subduction and retreat of two adjacent segments of the Jurassic Ligurian‐Tethys realm.

Pleistocene Sediments of the Submarine Continental Margin of Southwestern Africa

Pleistocene Sediments of the Submarine Continental Margin of Southwestern Africa

New petrographic and U–Pb geochronology data from the Mazagan Escarpment, offshore Morocco: Support for an African origin

Two samples of a granodiorite and a hypersthene granodiorite provided a unique opportunity to investigate the nature of the basement of the Mazagan Escarpment, the northwestern margin of continental Africa (Morocco) on a steep part of the lower continental slope in offshore Morocco. We conducted U–Pb LA-ICPMS geochronology on zircon from the granodiorite, which was acquired from a deep sea drilling program core DSDP544, and on zircon and monazite from a hypersthene granodiorite collected previously during the Cyamaz submersible campaign, to determine the ages of the rocks of the Mazagan Escarpment and interpret their origins. Zircon from the granodiorite yielded a 556 ± 10 Ma crystallization age, abundant inherited zircon cores up to 620 Ma and three ~2.68, ~1.76 and ~1.20 Ga cores. The hypersthene granodiorite yielded ~1950–1750 Ma zircon and ~1820–1640 Ma monazite ages. Some rocks of the closest onshore continental platform have recently been interpreted as belonging to an exotic Avalonian terrane, and correlated with the Mazagan Escarpment. Our data do not provide evidence that the Mazagan Escarpment is part of an exotic terrane. It is more likely that the hypersthene granodiorite formed in an extensional setting after the 2.25–2.07 Ga Eburnean orogeny, at a time of otherwise predominantly mafic magmatism that may have caused magmatic underplating and heating from below. The granodiorite formed during the late Ediacaran, which is characterized by widespread magmatism in Northwest Africa. Therefore, it is likely that the rocks of the Mazagan Escarpment have a Northwest African origin, which implies that the Pangean suture zone lies west of it.

Rural–urban connectivity strengthens agrarian peace: Evidence from a study of gender and motorcycle taxis in Sierra Leone

AbstractRural‐based insurgencies disrupted the forest margins of Upper West Africa in the 1990s. A subsequent return to peace was accompanied by strong growth in small‐scale trade in foodstuffs and other agrarian produce in high demand in towns. Motor cycle taxis are a feature of this increased rural–urban market integration. It was a mode of transport pioneered by ex‐combatants. Where rural women were once attacked by rural young men without job prospects press ganged into fighting for the rebels, bike taxi riders now carry them to rural periodic markets, many of which are new since the end of conflict. The study provides an analytical account of these developments, drawing on discussions with villagers in three heavily war‐affected localities of Sierra Leone. The evidence indicates that communities divided by conflict have quietly built new cooperative links conducive to peace based on local agricultural production and petty trade.

Long-lived stable shelf deposition along Gondwana's southern margin during the Ordovician-Silurian: Inferences from U[sbnd]Pb detrital zircon ages of the Table Mountain Group (South Africa) and correlatives in Argentina and the Falklands/Malvinas Islands

The Ordovician-Silurian siliciclastic units of the Table Mountain Group (TMG; Cape Supergroup) of the Cape Fold Belt (CFB) are classically considered to signify a period of long-lived sedimentation along the southern margin of southern Africa. Despite its vast, prominent outcrop, meaningful interpretations regarding the provenance of the TMG, based on a representative number of UPb detrital zircon age fractions, remains lacking. Within southern Gondwana, the TMG succession is considered broadly time-correlative to the Sierra del Volcan and Balcarce formations of the Sierras Tandilia and lowermost successions of the Ventana Group of the Sierras Australes in Argentina and the Port Stephens Formation of the West Falkland Group (WFG) in the Falkland/Malvinas Islands and Natal Group (South Africa). UPb detrital zircon ages have previously been reported for the Balcarce Formation and Ventana and West Falkland groups. However, their age fractions have not necessarily been compared to the broadly time –equivalent units of the TMG. In addition to the overall scarcity of detrital zircon age data on the TMG, the validity of using conventional, qualitative probability density diagrams to display and interpret such data sets have come into question, with some authors arguing for a quantitative approach for comparing detrital zircon age data sets. A total of 1521 concordant UPb detrital zircon ages and some 330 LuHf isotope analysis for the Peninsula- and Skurweberg formations (TMG) are therefore contributed, from samples obtained throughout the CFB, along with detrital zircon age fractions for the Balcarce, Providencia and Port Stephens formations. A qualitative as well as quantitative approach to compare the UPb detrital zircon age fractions measured for these units were adapted. Noticeable similarities among detrital zircon age fractions of the Peninsula- and Skurweberg formations throughout the western to south-eastern reaches of the CFB could be confirmed by quantitative pairwise comparisons. These point towards very little change in sediment source during the deposition of the lower to upper formations of the TMG during the Ordovician-Silurian. Pairwise comparisons of the TMG zircon age fractions to those of the Balcarce-, Providencia and Port Stephens formations revealed that they all have major late Neoproterozoic to Cambrian and a late Mesoproterozoic to early Neoproterozoic age fractions in common, with some minor contributions from Ordovician-Silurian sources. Due to the overlap in prevalent detrital zircon age fractions as well as Hf-isotope signature of these units, it is difficult to assign a specific provenance area for them with a fair amount of confidence, even if dominant palaeocurrent directions are considered. A quantitative pairwise comparison among data sets of the same or time-equivalent formation did not necessarily allow for age fractions from different source areas to be sufficiently distinguished from each other. In contrast to the classically inferred Namaqua-Natal Metamorphic Complex (NNMC) as source major proto-source for the TMG, it is argued that the sediment sources were located much further to the north such as the Damara and Mozambique belts, given that parts of the NNMC were likely denudated and covered by the sediments such as those of the Nama and Vanrhynsdorp Groups during the Ordovician-Silurian. Older sedimentary successions such as the Nama Group, were likely recycled during the deposition of the TMG: a scenario which also provides a feasible explanation for the noticeable lack of Archean ages among the detrital zircon age fractions. The ages of the youngest detrital zircon fraction conforms to the maximum age of deposition as proposed in literature, although the newly reported ages does not allow for more stringent cross-basin correlations. Regions within Patagonia, considered to have been in relatively close proximity to the relevant depositories, and possibly the Famatinian Belt (Argentina), are considered the source of the youngest Ordovician-Silurian detrital zircons.

Education marginalization in sub-Saharan Africa: Policies, politics, and marginality

Education marginalization in sub-Saharan Africa: Policies, politics, and marginality

Shoreline Changes and Coastal Erosion: The Case Study of the Coast of Togo (Bight of Benin, West Africa Margin)

The coastal strip between the Volta River delta and the westernmost portion of Benin (West Africa Margin of Atlantic Basin) is highly populated (e.g., Lomé) due to migrations from inland areas. The coastal zone has proved to be very vulnerable because of the potential development of sometimes catastrophic events related to different and interacting causes, resulting in negative effects on natural ecosystems and socio-economic conditions. The main problem is the marked erosion of large coastal sectors with maximum retreat rates of the order of 5 m/year. The continuous loss of territory leads to a progressive impoverishment of activities and human resources and to the increase of geological risk factors. The coastal erosion is induced both by natural and anthropic causes and can be controlled only by means of prevention programs, detailed scientific studies and targeted technical interventions. The main erosional processes observed in the study area are triggered by the presence of the Lomé port and other human activities on the coastal strip, including the water extraction from the subsoil, which induces subsidence and the use of sediments as inert material. These elements, together with the reduction of the solid supply from Volta River (caused by the realization of the Akosombo dam) are among the main factors that control the medium and long-term evolution of the area. Also relative sea level changes, which take into account also tectonic and/or isostatic components, can contribute to the process. In order to have a real understanding of the coastal dynamics and evolution, it would be necessary to develop a scientific structure through the collaboration of all countries of the Bight of Benin affected by coastal erosion. The aim should be primarily to collect the interdisciplinary quantitative data necessary to develop a scientific knowledge background of the Bight of Benin coastal/ocean system. In conclusion, some proposals are presented to reduce the vulnerability of the coastal area as for example to plan surveys for the realization of appropriate coastal protection works, such as walls, revetments, groins, etc. A possible expansion of the port of Lomè is also considered. Proposals comprise the constitution of a Supranational Scientific Committee as a coordinating structure on erosion for both the study of phenomena and planning interventions.

Petrographic and diagenetic study of siliciclastic Jurassic sediments from the northeastern margin of Africa: Implication for reservoir quality

Northeastern Egypt is one of the most important exposures in the Middle East and North Africa to study Jurassic facies. The present study analyzes 130 thin sections from a subsurface well (Well X) in the northern Gulf of Suez, and from two surface sections at Gebel Maghara (north Sinai) and Khashm Elgalala (North Eastern Desert), Egypt for petrographic and lithofacies analyses. In the present study, detailed petrographic analysis is used as a tool to better understand the diagenetic history of the Early to Late Jurassic siliciclastic sediments; the identified diagenetic elements are inferred concerning the reservoir quality. For this, sidewall cores, scanning electron microscopy and thin sections are used to detail detrital and authigenic mineralogy; these are then used to infer the depositional framework, factors controlling reservoir characteristics, and the operating diagenetic processes. The inferred depositional paleoenvironment is a prograding and retrograding linear siliciclastic shoreline within a shallow marine carbonate platform with coal swamps and occasional cross-cutting rivers. Diagenesis, petrographical characteristics and depositional conditions are the main factors controlling continental and marine reservoir architecture. The diagenetic processes affecting siliciclastic sedimentation are near the surface, shallow to intermediate burial, and deep burial cycles with different pore fluid filling at each stage. The siliciclastic sediments have been categorized into seven depositional lithofacies - calcareous claystone (S1a), carbonaceous claystone (S1b), siltstone (S2), planar and trough cross-bedded sandstone (S3), coarse, well rounded, large scale trough cross-bedded sandstone (S4), ooid sandstone (S5) and coal (C). Grain compaction, various phases of syntaxial quartz overgrowth, carbonate cementation and replacement, creation of dissolution porosity, and clay authigenesis are the most important diagenetic processes that have affected the siliciclastic continental and marine sediments. Many different diagenetic events, both destructive and constructive, have modified porosity. The destructive events include compaction and cementation (of silica, kaolinite, ferroan dolomite, Illite, and anhydride). The constructive events include silica cementation, feldspar dissolution, and ferroan dolomite dissolution. Additionally, present data also suggest that the continental sandstones have excellent reservoir quality whereas the marine sandstones have good to very good reservoir potential. Despite the excellent reservoir quality of the continental sandstone lithofacies, the effective stratigraphic seal is leaking. The interbedded calcareous and carbonaceous claystone lithofacies may provide an excellent stratigraphic seal for these siliciclastic reservoirs. Based on these analyses, a model is proposed that can be used as a template for subsurface Jurassic reservoir characterization and reservoir discrimination.

Application of seismic stratigraphy in reservoir characterisation: a case study of the passive margin deposits of the northern Orange Basin, South Africa

The Barremian-Cenozoic depositional sequences in the northern Orange Basin, SW, South Africa, were investigated using the principles of seismic stratigraphy to understand the interplay of tectonics and sedimentary processes in the distribution of potential hydrocarbon reservoirs. A seismic stratigraphic workflow (seismic sequence, seismic facies and lithofacies analysis) was completed by utilising three seismic lines (L1, L2 and L3) tied to Wireline data (gamma, checkshots and sonic) in two exploration wells (A1 and A2). Seven depositional sequences were mapped followed by the creation of lithofacies log interpreted from the gamma-ray log (GR) by setting maximum GR value at 60 API for Sandstone, 60–100 API for Siltstone and above 100 API for Shale. Six seismic facies units are recognised based on internal geometry and configurations of the seismic reflectors; Tangential-Oblique (SF1), Hummocky (SF2), Wavy-Parallel (SF3), Chaotic (SF4), Sub-parallel/parallel (SF5) and Divergent (SF6). SF4 is dominant within the Barremian-Aptian sequence and expressed in an incised valley fill, suggesting mass transport deposition accompanied by strong hydrodynamic conditions. Evidence of sedimentary basins progradation is seen within the Late-Albian-Turonian sequences, because of the occurrences of SF2, SF6 and SF 4 facies. SF5 facies is prominent in the Maastrichtian/Campanian sequence, indicating that the deposition of sediments may have been accompanied by uniform margin subsidence after the Late-Cretaceous uplift of the Africa margin. The occurrence of SF1 and SF4 facies within the Cenozoic sequence indicates terrigenous pro-deltaic deposits and mass transport deposits, respectively. Further results from seismic-lithofacies modelling reveal that sand deposits of Barremian-Aptian (SF4 facies unit) and Albian sequences (SF2 and SF6 facies units) are potential stratigraphic reservoirs in this part of the basin.

Variations in deep water masses along the western margin of South Africa, spanning the last two glacial terminations

The neodymium isotope composition (εNd) of planktic foraminifera and the relative abundances and δ13C of the benthic foraminiferal species C. wuellerstorfi from two gravity cores, retrieved from the western margin of South Africa, provide a late Quaternary (MIS 6 to MIS 1) record of the variations in North Atlantic Deep Water (NADW) and Southern Component Water (SCW). The εNd records from cores at water depths of 3522 m and 3631 m reveal a strong influence of SCW (Lower Circumpolar Deep Water (LCDW) and Antarctic Bottom Water (AABW)) during the peak glacial periods of MIS 6 (εNd = −8.75 to −7.35) and MIS 2 (εNd = −8.44 to −7.27). The strength of the SCW influence decreases while that of the NADW increases during glacial terminations (GT) II (εNd = −10.61 to −9.99) and GT I (εNd = −10.75 to −9.67). More radiogenic εNd values along the flow path of NADW are interpreted to indicate the weakening of NADW penetration southward, as a result of decreasing deep water formation in the North Atlantic during peak glacial periods. The relative abundance and δ13C of C. wuellerstorfi are greater during glacial terminations and interglacials than during peak glacial periods, which is consistent with the less radiogenic εNd values following GT II and GT I. These results suggest that C. wuellerstorfi abundances can be a useful indicator of deep water masses when combined with εNd and δ13C records.

The 2 Ga eclogites of Central Tanzania: Directly linking age and metamorphism

The ca. 2 Ga retrogressed eclogites in the Usagaran Belt in central Tanzania are among the oldest documented eclogites in the world. As such, they have been used to pinpoint the thermal conditions of the onset of modern style subduction on Earth. Two samples of retrogressed mafic eclogite have been interrogated to reconstruct the metamorphic history of the subducted crust. The samples are several kilometres apart, separated by a mylonitic upper amphibolite-transitional granulite foliation. Lu--Hf garnet geochronology pinpoints eclogite metamorphism at 1994 ± 9 Ma, supported by rutile U--Pb geochronology. Zircon U--Pb geochronology supports subduction metamorphism at ca. 2000 Ma. Mineral equilibria modelling and Zr in rutile thermometry suggests the retrogressed eclogites reached a minimum pressure of 18 kbar at approximately 800 °C, consistent with a warm subduction thermal gradient. These conditions were followed by amphibolite-granulite-facies overprinting. Proposed tectonic models for the formation of the eclogites involve either east-dipping subduction of the Tanzanian Craton margin, or west-dipping subduction of oceanic crust under the Tanzanian Craton. The retrogressed eclogites were likely exhumed via slab breakoff driven buoyancy of the oceanic crust and transferred to shallow crustal levels in a back arc position. This data, combined with data from eclogites from the Eburnian-Transamazonian and Ubende orogens, demonstrate the possible existence of a large subduction system on the margin of west Africa from ca. 2.1–1.9 Ga. This system has led to the preservation of several eclogitic rocks, which suggests that modern style subduction was operating on significant geographic and temporal scales at this time.

Co-publishing with Africa North–South–North

The decolonization of African studies extends beyond content to ethical partnerships between the North and the African continent. One key component of realizing partnership is through publishing. African studies research published by Northern publishers is not often even minimally available in Africa; and this is despite scholars on the continent often being partners or facilitators in research undertaken by Northern scholars. Northern publishers have perceived no commercial gain, given small African markets, lack of purchasing power, and lack of distribution systems. Conversely, African publishers have efficient distribution into the North through African Books Collective, owned and governed by them. But in suitable rare cases the African publisher can broker co-publications with Northern publishers who want the originating rights. In the light of these issues, African Books Collective launched an initiative to seek to break the deadlock. In partnership with the International African Institute, and with the active support of the African Studies Associations of the UK and the US, work is proceeding with publishers in the North and the South to broker co-publishing or co-editions to address this historic marginalization of Africa.

A late Mesozoic – early Cenozoic sedimentary recycling system on the Gondwana rifted margin of southeast Africa

Abstract Detrital zircons in late Cretaceous – Palaeogene, calcareous sandstone and conglomerate deposited in continental basins on the southeastern African margin after the breakup of Gondwana have characteristic combinations of age and epsilon-Hf that indicate an origin by recycling of Palaeoproterozoic (Waterberg, Soutpansberg and Pretoria groups) and Phanerozoic (Karoo Supergroup) cover successions. The latter is dominant in the south and east (Boane, Mahosi, Chilojo Cliffs), and the Palaeoproterozoic sources in the northwest (Pafuri, Wright’s Tower, Masisi). This recycling and mixing regime was restricted to late Mesozoic and Palaeogene time in northeastern South Africa and adjoining parts of Mozambique and Zimbabwe. Detrital zircon distribution patterns in these deposits reflect the denudation history of the southern African continental surface after breakup of the Gondwana supercontinent.

3D simulations of salt tectonics in the Kwanza Basin: Insights from analogue and Discrete-Element numerical modeling

The Kwanza Basin is located at the passive margin of West Africa in the Central Segment of the South Atlantic, and is an important area for oil and gas exploration. The distribution and deformation characteristics of thick Lower Cretaceous (Aptian) salt deposits in the basin have significant impacts on hydrocarbon accumulation. However, reports on their 3D interpretation, together with an understanding of the genesis and evolution of salt tectonics in this area, are scarce in the literature. In this paper, 3D analogue experiments and 3D Discrete-Element numerical simulations are carried out to analyze the zonal characteristics (extension-translation-contraction), the main control factors, and to develop a kinematic model of salt tectonics in the basin. According to our modeling, we make the following conclusions. (1) The zonal characteristics of salt tectonics formed in the basin reflect the superposition of gravity gliding and gravity spreading. The main control factor of basin-scale salt deformation is basement structural activity, which can be summarized into three points: early post-salt basement extension, initial presence of local variation of base of salt gradient and more recent differential subsidence between landward side and seaward side. (2) Salt deformation is episodic, having experienced one stage of regional deformation (Late Aptian to Albian) and two stages of inherited developmental deformation (Campanian, and Eocene to Miocene), mainly resulting in forward expansion towards the ocean. (3) During the early stage of salt deformation, faults developed in suprasalt layers in the extensional and translational zones of the basin, resulting in local high-velocity areas and a large vertical upward velocity component in the salt layer. Later, due to blocking by the outer high region and increasing overburden, salt in contact with the outer high developed a vertical upward velocity component, gradually forming thrust structures. (4) 3D analysis highlights the pervasive lateral deformation of the salt. The main factors governing it are the distribution of accommodation in the salt basin and the initial morphology of the salt layer. In the early stages, salt layer had a velocity component to the salt-scarce side (from north to south), whereas in the later stages, it developed a velocity component to the wide side of the basin (from south to north), leading to the gradual accumulation of salt in the north of the basin. We have shown that 3D simulations can complete the analysis of salt deformation both parallel and perpendicular to the slope in a passive-margin basin. This may provide guidance for the exploration of oil and gas reservoirs in the context of salt tectonics, especially in asymmetrical basins where seismic imaging is poor and has to be supplemented by strong geological concepts including understanding 3D movement of salt.

An Integration of Magnetic, Gravity and Seismic Data in Evaluating the Algoa Basin in the Eastern Cape Province of South Africa for Stratigraphic and Structural Geodynamics

The Algoa Basin is a hydrocarbon potential basin which lies off the southern continental margin of South Africa, along the coastal region of Port Elizabeth area with both onshore and offshore components. Despite the continued consensus on the basin’s hydrocarbon potential, both its structural and stratigraphic geodynamics are still not well documented. In this study, gravity data constrained by borehole data was modelled to reveal onshore subsurface geology whereas the seismic sections were used for offshore. In addition, magnetic data was used to delineate magnetic zones and lineaments. The onshore magnetic results show the localised occurrence of large-scale geologic bodies, with high and positive anomalies forming bounding rims coincidental with the edges of the basin. The average depths to the top of the shallowest and deepest magnetic sources were estimated from the power spectrum to be approximately 0.4–0.6 km and 9.5 km, respectively. The onshore Bouguer gravity map is characterised by a gravity low zone over the basin’s sediment where it attains its lowest value over the depocentre. 2¾ D gravity modelling results along five profiles show that the basin comprises of two to three ‘“V” shaped’ sub-basins of shallow crustal depth with sediment thickness of up to 4.6 km. The models also revealed discontinuity in sediment units, confirming the diachronous nature of the Uitenhage Group. Analyses of offshore seven seismic sections show that the stratigraphic sequence consists of diachronous geological units, which are characterised by displacements, forming a series of minor and major faults. Complementary to onshore gravity models, seismic sections also shows relatively shallow vertical extension to the Paleozoic crustal basement between 1.3 and 5.3 km.