South African Continental Margin Research Articles

Late Quaternary responses to glacial-interglacial climate cycles of offshore Benguela region and associated foraminifera in the southeast Atlantic along Namibia and western South Africa

The Benguela Upwelling System (BUS) exerts a major oceanographic and sedimentary influence on the Namibian and western South African continental margin. Previous literature has distinguished between the Northern Benguela Region (NBR) and Southern Benguela Region (SBR) due to the varying sedimentary, oceanographic, upwelling and faunal nature of the two areas. A comprehensive understanding of how upwelling and palaeoceanography between these two subsystems relate to, or differ between each other, is limited. Late Quaternary foraminifera are extremely useful in recording and tracking past oceanographic and climatic changes in the region. Planktic foraminifera from two cores recovered from the western continental slope of South Africa, at a water depth of ∼3500 m, were analysed and compared to previous studies from the region to determine any variations in the water mass, oxygenation, upwelling and productivity history between the NBR and SBR, and during glacial-interglacial periods. Results from this study indicate that influence from subpolar, subtropical and upwelled surface waters play a major role in the faunal distributions and oceanographic history of the area. Abundances of subpolar species are lowest off western South Africa during interglacial periods, indicating obstruction from the subtropical convergence and restriction of the polar fronts during those periods. Benthic and planktic foraminifera indicate strengthened upwelling and the extension of upwelling cells further slopeward during glacial periods. Increased productivity associated with stronger upwelling during glacial periods led to increased nutrient and organic matter delivery to the seafloor, resulting in eutrophic conditions. These environments are further amplified by nutrient-rich bottom water masses that are strengthened during these periods. Upwelling intensity and productivity also exert controls on the benthic foraminifera, where epifaunal taxa abundances and epifaunal-infaunal ratios increase from the NBR to SBR, and glacial epifaunal-infaunal ratios decrease compared to interglacial periods. Higher abundances of tropical-subtropical species were recorded in the NBR and Agulhas retroflection relative to western South Africa, indicative of the influence of the Angola-Benguela Front to the north of upwelling zones and the inflow of warm Agulhas Current waters from the South Indian Ocean to the southwest of South Africa. This study therefore reveals that the NBR and SBR vary in their palaeoceanographic record as a result of their position to important southeast Atlantic oceanographic features such as the Angola-Benguela Front, the Agulhas Current, the Benguela Current and polar fronts.

Deep circulation changes in the South Atlantic since the Last Glacial Maximum from Nd isotope and multi-proxy records

We report down-core sedimentary Nd isotope (εNd) records from two South Atlantic sediment cores, MD02-2594 and GeoB3603-2, located on the western South African continental margin. The core sites are positioned downstream of the present-day flow path of North Atlantic Deep Water (NADW) and close to the Southern Ocean, which makes them suitable for reconstructing past variability in NADW circulation over the last glacial cycle. The Fe–Mn leachates εNd records show a coherent decreasing trend from glacial radiogenic values towards less radiogenic values during the Holocene. This trend is confirmed by εNd in fish debris and mixed planktonic foraminifera, albeit with an offset during the Holocene to lower values relative to the leachates, matching the present-day composition of NADW in the Cape Basin. We interpret the εNd changes as reflecting the glacial shoaling of Southern Ocean waters to shallower depths combined with the admixing of southward flowing Northern Component Water (NCW). A compilation of Atlantic εNd records reveals increasing radiogenic isotope signatures towards the south and with increasing depth. This signal is most prominent during the Last Glacial Maximum (LGM) and of similar amplitude across the Atlantic basin, suggesting continuous deep water production in the North Atlantic and export to the South Atlantic and the Southern Ocean. The amplitude of the εNd change from the LGM to Holocene is largest in the southernmost cores, implying a greater sensitivity to the deglacial strengthening of NADW at these sites. This signal impacted most prominently the South Atlantic deep and bottom water layers that were particularly deprived of NCW during the LGM. The εNd variations correlate with changes in 231Pa/230Th ratios and benthic δ13C across the deglacial transition. Together with the contrasting 231Pa/230Th: εNd pattern of the North and South Atlantic, this indicates a progressive reorganization of the AMOC to full strength during the Holocene.

Crust‐scale 3D model of the Western Bredasdorp Basin (Southern South Africa): data‐based insights from combined isostatic and 3D gravity modelling

AbstractThe southern South African continental margin documents a complex margin system that has undergone both continental rifting and transform processes in a manner that its present‐day architecture and geodynamic evolution can only be better understood through the application of a multidisciplinary and multi‐scale geo‐modelling procedure. In this study, we focus on the proximal section of the larger Bredasdorp sub‐basin (the westernmost of the five southern South African offshore Mesozoic sub‐basins), which is hereto referred as the Western Bredasdorp Basin. Integration of 1200 km of 2D seismic‐reflection profiles, well‐logs and cores yields a consistent 3D structural model of the Upper Jurassic‐Cenozoic sedimentary megasequence comprising six stratigraphic layers that represent the syn‐rift to post‐rift successions with geometric information and lithology‐depth‐dependent properties (porosities and densities). We subsequently applied a combined approach based on Airy's isostatic concept and 3D gravity modelling to predict the depth to the crust‐mantle boundary (Moho) as well as the density structure of the deep crust. The best‐fit 3D model with the measured gravity field is only achievable by considering a heterogeneous deep crustal domain, consisting of an uppermost less dense prerift meta‐sedimentary layer [ρ = 2600 kg m−3] with a series of structural domains. To reproduce the observed density variations for the Upper Cenomanian–Cenozoic sequence, our model predicts a cumulative eroded thickness of ca. 800–1200 m of Tertiary sediments, which may be related to the Late Miocene margin uplift. Analyses of the key features of the first crust‐scale 3D model of the basin, ranging from thickness distribution pattern, Moho shallowing trend, sub‐crustal thinning to shallow and deep crustal extensional regimes, suggest that basin initiation is typical of a mantle involvement deep‐seated pull‐apart setting that is associated with the development of the Agulhas‐Falkland dextral shear zone, and that the system is not in isostatic equilibrium at present day due to a mass excess in the eastern domain of the basin that may be linked to a compensating rise of the asthenospheric mantle during crustal extension. Further corroborating the strike‐slip setting is the variations of sedimentation rates through time. The estimated syn‐rift sedimentation rates are three to four times higher than the post‐rift sedimentation, thereby indicating that a rather fast and short‐lived subsidence during the syn‐rift phase is succeeded by a significantly poor passive margin development in the post‐rift phase. Moreover, the derived lithospheric stretching factors [β = 1.5–1.75] for the main basin axis do not conform to the weak post‐rift subsidence. This therefore suggests that a differential thinning of the crust and the mantle‐lithosphere typical for strike‐slip basins, rather than the classical uniform stretching model, may be applicable to the Western Bredasdorp Basin.

PASSIVE MARGIN EVOLUTION AND CONTROLS ON NATURAL GAS LEAKAGE IN THE ORANGE BASIN, SOUTH AFRICA

Throughout exploration Block 2 of the Orange Basin offshore the South African continental margin, different natural gas leakage features and the relationship between natural gas leakage with structural and stratigraphic elements were studied. This study also quantifies liquid/gas hydrocarbon generation, migration and seepage dynamics through the post-rift history of the basin. The interpretation of seismic data reveals two mega-sequences: Cretaceous and Cenozoic that are subdivided by major stratigraphic unconformities into 5 and 2 sub-units, respectively. The basin is also divided into 2 structural domains: 1. an extensional domain characterized by basinward dipping listric normal faults rooted at the Cenomanian/Turonian level identified between 500 to 1500 m of present-day depth, 2. a compressional domain that accommodates the up-dip extension on the lower slope, and which is characterized by landward dipping thrust faults. One hundred and thirteen observed gas chimneys are identified and classified into stratigraphically-controlled (sa-c) and structurally-controlled (s-c) chimneys. The ratio of s-c versus s-ac chimneys is estimated as 2:5, which suggest a strong stratigraphic control on natural gas leakage. The chimneys either terminate at the seafloor where active leaking gas is manifested by pockmarks, or are sealed within the Miocene (14 Ma) sequence as paleo-pockmarks. The s-c chimneys are located along the normal faults in the extensional domain, and terminate as seafloor mounds up to 1500 m in diameter and with heights between 10 to 50 m. The sa-c pockmarks range between 100 to 400 m in diameter, and are linked to stratigraphic onlaps and pinch-outs within the Aptian sequence. Several giant chimneys, with diameters of more than 7 km, are also identified. At least one of these displays apparent internal gravitational collapse structures. Bright spots indicative of gas presence within these large chimneys were identified, but there is no evidence of acoustic turbidity or seismic pull-downs within these large structures. This suggests the giant chimneys are inactive paleo-gas-escape structures. Modelling suggests that gas from the lower Aptian and the Barremian source rocks migrates laterally-updip to the proximal parts of the basin where it accumulates beneath the Cenomanian/Turonian sequence that acts as a regional seal. Across the shelf-break and the upper slope, chimneys and pockmarks are fed from younger Cenomanian/Turonian source rocks. The migration model also indicates that fluids are about 24 times more likely to flow out of the study area than to be preserved within it. Since methane gas escaping across the sea floor into the exosphere (combined hydrosphere and atmosphere) may contribute to Earth’s climate fluctuations, and because escaping gas must have been cut off when at least half of identified s-c chimneys were sealed within the Miocene sequence, decrease of gas escape along the southern African continental margin may have to be factored into global Neogene cooling models.

Sedimentary deposits on the southern South African continental margin: Slumping versus non-deposition or erosion by oceanic currents?

Seismic profiles extending from the southern South African shelf into the deep sea reveal a strong erosional activity, which affects large parts of the continental margin. Quaternary to Oligocene units and, in places, the complete sedimentary column appear to have been removed. Mass movements were previously considered as the origin of this erosion. However, structures indicating slumping can be identified in only a few places. The erosional activity is confined to specific water depths, which correlate well with the activity levels of water masses observed here. We thus suggest that the Agulhas Current, Antarctic Intermediate Water, North Atlantic Deep Water, and Antarctic Bottom Water have intensively shaped the sedimentary units over a long period. This indicates a stable circulation comparable to that at present day since the Neogene.

Cenozoic oceanic circulation within the South African gateway: indications from seismic stratigraphy

The gateway south of South Africa allows the exchange of water masses between the Indian/Pacific Oceans (warm surface water masses) and the Atlantic (cold deep and bottom water masses). This heat transfers maintains the global conveyor belt. Hence the South African continental margin represents an important gateway, whose detailed evolution is not yet fully understood. The sedimentary structures within this gateway represent an archive of the onset and modifications of paths and intensities of water masses related to the development of both Northern and Southern Hemisphere ice shields. A compilation of the evolution of the oceanic circulation within the South African gateway since the Early Eocene on the basis of seismic reflection data is put forward and discussed. The effect of a proto-AABW can be found in the southern Cape Basin and south of South Africa as early as Early Eocene-Early Oligocene. In the period Early Oligocene-Middle Miocene a current equivalent to AAIW/Agulhas Retroflection leaves its oldest traces identifiable on the eastern Agulhas Plateau. Indications for the Benguela Current can be found in the Cape Basin in Middle Miocene times. With the onset of NADW in the period Middle Miocene-Early Pliocene the branch of AABW flowing through the Agulhas Passage is weakened and finally deflected to the south in Early Pliocene-Holocene times.

Deep crustal structure of the sheared South African continental margin: first results of the Agulhas-Karoo Geoscience Transect

The southern margin of South Africa developed as a consequence of shear motion between the African and South American plates along the Agulhas-Falkland Transform Fault during the Early Cretaceous break-up of Gondwana. The Agulhas-Karoo Geoscience Transect crosses this continental margin, and includes two combined offshore-onshore seismic reflection/refraction profiles. We present results from the western offshore profile. Using ocean-bottom seismometers and a dense airgun shot pattern, a detailed image of the velocity – depth structure of the margin from the Agulhas Passage to the Agulhas Bank was derived. Modelling reveals crustal thicknesses of between 7 km and 30 km along the profile. The upper crust has P-wave velocities of between 5.6 and 6.6 km/s and the lower crust has velocities that lie between 6.4 and 7.1 km/s. Uppermost mantle velocities range from 7.8 to 8.0 km/s. The 52 km wide continent-ocean-transition zone, where the Moho rises steeply, occurs at the Agulhas-Falkland Fracture Zone. Beneath the Southern Outeniqua Basin and the Diaz Marginal Ridge, a zone of relatively low velocities (~5 km/s) with a thickness of up to 3 km, can be discerned in the upper crust. We interpret this zone as an old basin filled with pre-break-up metasediments, which may be related to the Cape Fold Belt. We suggest that the origin of the Diaz Marginal Ridge is bound up with the tectonic history of this basin as it exhibits a similar velocity structure. Almost no stratified sediment cover exists in the Agulhas Passage because of strong erosion due to ocean currents. The velocity structure and seismic reflection results indicate the presence of alternating layers of volcanic flows and sediments, with a mean velocity of about 4 km/s. We suggest these volcanic flows were an accompaniment of possible re-activation events of parts of the Agulhas-Falkland Fracture Zone. Tectonic motion seems to be sub vertical instead of strike-slip along the re-activated part of the fracture zone. Therefore, a relation to the uplift of the South African crust may be possible.

Quaternary continental margin sedimentation off the southeast coast of South Africa

A sedimentological study of Quarternary deposits in a deep-sea channel off the South African continental margin reveals that terrigenous and shelf (T and S) components from the sand fraction are concentrated on the upper continental slope and the channel floor. In comparison, sediments from the lower flanks of the channel contain relatively low amounts of these components. Based on these and seismic studies, four major controls on this distribution pattern have been established: (1) a marginal fracture ridge; (2) fluctuations in sea level; (3) dilution of the T and S components by biogenic carbonate; and (4) redistribution of the T and S components and dissolution of the carbonate by turbulent, corrosive bottom currents. Variability in the T and S component distribution patterns is related to changes in the influence of the last three controls due to climatic fluctuations. During interglacial periods, higher sea levels lead to the trapping of coarse-grained T and S components on the shelf; the shallow carbonate compensation depth (CCD) coupled with turbulent bottom currents leads to the dissolution of the (mainly planktonic) carbonate component on the floor of the Agulhas Passage; and redistribution of the T and S components by strong bottom currents is effected. Glacial periods are characterized by lower sea levels enabling T and S components to be transported directly offshelf into the passage; the deep CCD and weak bottom water turbulence leads to the dilution of the T and S component on the passage floor by carbonate; bottom currents are too weak to effectively redistribute this component.

Age of phosphorites on the South African continental margin

The first phosphorite rocks recovered from the sea floor were dredged from the South African continental margin in 1873. Notwithstanding the more than 100 years which have since passed, the age of these rocks has not previously been established. Phosphorites from 33 stations on the South African margin have been dated using planktic foraminifera and calcareous nannoplankton. Foraminifera proved the most useful of the two groups; most age assignments are based on various species belonging to the genera Globorotalia and Orbulina. Phosphatization probably started during the Middle Miocene and continued into Pliocene times, but appears to have ceased prior to the Quaternary. The conglomeratic variety of phosphorites may be slightly younger than the majority of non-conglomeratic phosphorites, based on a comparison of planktic foraminifera in conglomerate clasts with those in the interclastic matrix. Phosphate on most of the Agulhas Bank was derived from deep, sub-surface dynamic upwelling, rather than surface upwelling, during the Neogene.

The Petrology and Origin of Some Glauconitic and Glauco-Conglomeratic Phosphorites from the South African Continental Margin

ABSTRACT Glauconitic and glauco-conglomeratic phosphorite rocks are areally widespread on the South African continental margin. The glauconitic phosphorites ( 18% P2O5) have packstone or wackestone textures and are composed of sand-sized grains of glauconite, with lesser amounts of microfossils and terrigenous grains set in a matrix of collophane-francolite and micrite. The glauco-conglomeratic phosphorites ( 17% P2O5) are composed of variably phosphatized limestone pebbles set in a matrix very similar in texture and composition to the glauconitic phosphorites. Fifteen representative samples were selected for major element analysis and the relatively low P2O5 concentrations (15-21%) determined for these rocks reflect the diluent effects of allogenic grains. Substitution of CO32- for PO43- is indicated by the relatively high F/P2O5 ratios and the high apatite CO2 levels determined. Coupled substitution of Na+ and S6+ for Ca2+ and P5+ is also indicated, and high Fe2O3, K2O and MgO levels reflect the presence of glauconite. Many of the sedimentary features exhibited by these texturally heterogeneous rocks are incompatible under normal hydrodynamic conditions, suggesting an unusual depositional environment. It is proposed that the matrix cement was originally lime mud and that under the influence of a regressive coastline/shallowing sea, tidal and storm wave currents were responsible for the accumulation of the proto-phosphorite sediments in file relatively shallow and flat continental shelf areas. In deeper and steeper continental slope areas it is suggested that sediment accumulation resulted from the action of turbidity currents and mudflows. Lithification of the sediments is considered to have resulted from the phosphatization of lime mud by interstitial waters rich in phosphates. A shallow water lago nal/estuarine environment is indicated for the diagenesis of those rocks found in shallow continental shelf areas, whereas an open ocean environment is indicated for those rocks found in deeper water on the continental slope.

Post-Palaeozoic stratigraphy of the eastern Agulhas Bank, South African continental margin

The Agulhas Bank, which forms the continental margin on the southern tip of Africa, consists of a thick Meso-Cainozoic sedimentary sequence (up to 6.2 km) resting on and behind pre-Mesozoic continental acoustic basement. The stratigraphy of this sequence is outlined and its history and facies variations mentioned where they are known. Refraction seismic velocity and bottom sample data indicate a basic three-fold subdivision of the Mesozoic sequence, which can be correlated with the onshore succession in the Algoa Basin. It is separated by a major hiatus from the Cainozoic sediments, which consist of a Palaeogene and Neogene sequence subdivided by another well-defined level of erosion. Various formations within the Cainozoic are defined and named. An outline of the bathymetry of the eastern Agulhas Bank is also given.

Limestone lithofacies from the South African continental margin

Carbonate sedimentation has played an important part in the Cainozoic history of the South African offshore areas. The Agulhas Bank sector of the continental shelf and upper slope is covered by a patchy veneer of relict and Recent sediment. Under this thin cover of sediment lies a succession of Cainozoic carbonate rocks very different from those exposed onshore. The Agulhas Bank rocks have been divided into the following lithofacies: ( 1) lagoonal-quartzose lime wackestones; ( 2) pelagicforaminiferal lime packstones; and ( 3) infralittoral-skeletal lime packstones. The rocks within each lithofacies are not necessarily time-equivalents; indeed any one lithofacies may have a considerable time range. The oldest rocks found (Palaeocene-Eocene) are lagoonal-quartzose packstones. This facies represents accumulation of terrigenous grains, glauconite, and sparse skeletal fragments in quiet water, probably under lagoonal conditions. Micritic carbonate, the original sediment matrix, has been mostly altered to microspar. The carbonate mineralogy of this lithofacies is wholly low-Mg calcite. The pelagic-foraminiferal packstones (Miocene-Pliocene) were deposited in an outer shelfupper slope environment. These packstones are composed predominantly of planktonic foraminifers and bryozoans embedded in micrite and are a mixture of high- and low-Mg calcite. Lithification of this facies was at least partly submarine. The infralittoral-skeletal packstones represent coarse nearshore sediments (chiefly terrigenous grains, glauconite, and broken shells) most of which were left behind as sea level rose during the RissWürm Interglacial. The facies was lithified by micrite and fibrous aragonite under submarine conditions, and is still composed of a mixture of aragonite, high-Mg calcite, and low-Mg calcite.

Petrology and Origin of Some Phosphorites from the South African Continental Margin

ABSTRACT Phosphorites (phosphatized limestones) are areally widespread on the South African continental margin. These rocks have packstone or wackestone textures and are typically composed of microfossils (chiefly planktonic foraminifers), with lesser amounts of macrofossil fragments and terrigenous grains set in a collophane/micrite matrix. The average P2O5 content of these rocks is about fifteen percent. Some varieties are exceedingly ferruginous, owing to the intimately mixed goethite in their matrices. The faunal and textural characteristics of these phosphorites are strikingly similar to those of certain unphosphatized Miocene limestones dredged from the same area. Mineralogical and geochemical evidence indicates that the lime mud matrix of some of these limestones was replaced by francolite. Phosphatization was accomplished under submarine conditions by permeating, phosphate-rich solutions.