Bastien Linol

New Regional Correlations Between the Congo, Paraná and Cape-Karoo Basins of Southwest Gondwana

Pioneering stratigraphic correlations by J. Keidel and A. du Toit, in the first half of the twentieth century, first highlighted significant similarities between the age-rock sequences in southern Africa and eastern South America, supporting A. Wegener’s concept of a united Gondwana supercontinent. Based on subsequent field investigations and modern sedimentary basin analysis of the Congo Basin of central Africa and the Parana Basin of southeastern Brazil, we revisit these early correlations and derive new paleogeographic reconstructions of the interior of southwest Gondwana. Following late Neoproterozoic-Early Cambrian amalgamation of Gondwana, earliest Paleozoic continental red-bed sediments were deposited regionally southward across the peneplained Central African and Kalahari Shields. Thereafter, Ordovician-Devonian subsidence along a vast shallow marine platform bordering the southwestern margin of Gondwana linked the Parana Basin with the Cape-Karoo Basin of South Africa. Equivalent sequences are absent in the Congo Basin. In contrast, succeeding Carboniferous-Permian and Triassic successions are similar in all the Congo, Parana and Cape-Karoo Basins, including thick transgressive glacial and deglaciation sequences overlain by progressively terrestrial and arid sediments, which suggest a single Central West Gondwana Basin (CWGB) complex. This late Paleozoic-early Mesozoic cycle of subsidence of the CWGB can possibly be linked to long wavelength flexure of Gondwana continental lithosphere related to the Mauritanian-Variscan and Cape-de la Ventana orogens along the northwestern and southern margins of the supercontinent, around ca. 300 Ma and 250 Ma, respectively. Following Jurassic-Cretaceous hot and arid sedimentation across southwest Gondwana culminated in widespread deposition of northerly-derived aeolian dunes, episodically interrupted by successive eruption of Large Igneous Provinces during the initial phases of Gondwana break-up (ca. 183 Ma and 132 Ma). This shared sedimentation and climatic history of the Congo, Parana and Cape-Karoo Basins was then disrupted by the Early Cretaceous opening of the South Atlantic Ocean and the Kalahari epeirogeny, after which the Congo Basin survived and recorded intermittent phases of lacustrine and fluvial deposition.

From source to sink in central Gondwana: Exhumation of the Precambrian basement rocks of Tanzania and sediment accumulation in the adjacent Congo basin

Apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated (>1000 m a.s.l.), but low relief Tanzanian Craton. Coeval episodes of sedimentation documented within adjacent Paleozoic to Mesozoic basins of southern Tanzania and the Congo basin of the Democratic Republic of Congo (DRC) indicate that most of the cooling in the basement rocks in Tanzania was linked to erosion. Basement samples were from an exploration borehole located within the craton, and up to 2200 m below surface. Surface samples were also analysed. AFT dates range between 317 ± 33 Ma and 188 ± 44 Ma. Alpha (Ft)-corrected AHe dates are between 433 ± 24 Ma and 154 ± 20 Ma. Modelling of the data reveals two important periods of cooling within the craton; one during the Carboniferous-Triassic (340 -220 Ma) and a later, less well constrained episode, during the late Cretaceous. The later exhumation is well detected proximal to the East African Rift (70 Ma). Thermal histories combined with the estimated geothermal gradient of 9 °C/km constrained by the AFT and AHe data from the craton and a mean surface temperature of 20 °C, indicate removal of up to 9 ± 2 km of overburden since the end-Paleozoic. The correlation of erosion of the craton and sedimentation and subsidence within the Congo basin in the Paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far-field compressional tectonic processes, and thereafter through deep mantle upwelling and epeirogeny tectonic processes.

Precambrian Basement of the Congo Basin and Its Flanking Terrains

Africa’s four major Proterozoic shields were tectonically separated from one-another within the late Meso- to early Neo-Proterozoic Rodinia supercontinent along Rodinian mobile belts (‘Grenvillian’ in North America; ‘Kibaran’ in Africa; although, ironically, the type Kibaran rocks of Africa, now Karagwe-Ankole Belt in Rwanda and Burundi, have been found to be older and no longer define a Rodinian-age mobile belt. Following subsequent break-up of Rodinia, the four disparate African Shields welded together during the late Neoproterozoic to Early Cambrian amalgamation of Gondwana (ca. 500–800 Ma). The geology, geochronology and evolution of one of these Shields, the Central African Shield (CAS) is described here in some detail because it forms the basement to the Congo Basin (CB) that is the focus of this book. The CAS hosts both the world’s oldest (2.8 Ga) and youngest diamondiferous kimberlites and a number of complex Archean Cratons that outcrop along the flanks of the CB. The CB of central Africa is underlain and completely surrounded by peneplained Precambrian basement that spans a history more than 3.6 Ga, including rocks along its northeastern edge possibly as old as 4.0 Ga, separated and surrounded by numerous Paleo- to Neo-Proterozoic mobile belts. This Precambrian complex assemblage includes the peripheral (outcropping) Kasai, Cuango, Ntem, Bouca and Mboumou-Uganda cratonic blocks that can be assembled into three larger Congolese Cratons unexposed beneath the CB: the SouthWest-, the Cuvette- (or Central-), and the NorthEast- Congo Cratons (SW-, C- and NE-CC) and that in turn, together, constitute the Congo Shield (CS). Subsequently these three aggregated along the Central Angola Mobile Belt (CAMB; ca. 2.0–2.3 Ga) and the West Central African Mobile Belt [WCAMB; ca. 2.0–2.5 Ga to form the SouthWest Congo Shield (SWCS). In the east, the CS enlarged farther to form the CAS during the Proterozoic along Eburnian, Kibaran and Pan-African orogenic belts, but the details of the accretion processes of these continental domains remain uncertain.

Cenozoic Landscape Evolution in and Around the Congo Basin: Constraints from Sediments and Planation Surfaces

The Congo Cuvette (or Basin) is a large depression with an average elevation of 350 m, surrounded by topographic highs: the Central African Atlantic Swell (west), the Cameroon Highlands and the Ubangian Rise (north), the western flank of the East African Dome (east) and the Kalahari and Angolese Plateau (south). Our objective is to perform a geomorphological and stratigraphical study of the Congo Cuvette and its flanking relief to characterize the Cenozoic landscape evolution of Central Africa and associated vertical movements.

Multiphase Phanerozoic Subsidence and Uplift History Recorded in the Congo Basin: A Complex Successor Basin

The Congo Basin of central Africa is a large iconic Phanerozoic sedimentary basin whose origin and tectonic evolution are poorly understood, mostly because of a lack of modern stratigraphic data, reflecting a long hiatus in field investigations during the past five decades. It is usually assumed that the Congo Basin experienced a long and continuous history of slow subsidence since the late Precambrian (e.g. 2–4 m/Ma), linked to steady-state mantle processes. Here, we used revised sedimentological and stratigraphic data of the four historic deep boreholes drilled in the center of the basin to calculate a new first-order model for its subsidence and uplift history. Because the sedimentary sequences of this basin are largely terrestrial, we apply a new backstripping method especially designed for continental domain. The results reveal two main episodes of subsidence: initially rapid subsidence during the Carboniferous-Triassic (10–20 m/Ma), and then slower subsidence during the Jurassic-Cretaceous (5–10 m/Ma), punctuated by several uplifts at 160–180 Ma (e.g. ‘Karoo’), 120–140 Ma (e.g. ‘Parana-Etendeka’), and again in the Cenozoic, ca. 30–50 Ma (e.g. ‘Ethiopian’). This complex, multiphase subsidence and uplift history of the Congo Basin can be linked to evolving far-field geodynamic processes that first led the formation of Pangea (large-scale compression) during the late Paleozoic, and then to its break-up associated with successive outpourings of Large Igneous Provinces (or hotspot plumes) and the opening of the Indian and South Atlantic Oceans around Africa.

Facies Analyses, Chronostratigraphy and Paleo-Environmental Reconstructions of Jurassic to Cretaceous Sequences of the Congo Basin

The Congo Basin is characterized by an extensive and relatively thick (ca. 1 km) succession of Jurassic-Cretaceous sedimentary sequences that preserves a unique record of the tectonic and climatic evolution of central Africa during the main period of break-up of Gondwana and the emergence of the Indian and South Atlantic Oceans. New facies analysis and detailed correlations of these ‘Congo’ sequences are described from field observations in the southwestern Congo Basin and by re-logging cores and well logs from four deep boreholes drilled in the center of the basin in the 1950s and 1970s. The lowermost Upper Jurassic sequence (the Stanleyville Group) records a short marine incursion of the proto-Indian Ocean into the northern Congo Basin, and is in turn overlain to the south by widespread aeolian dune deposits (the Lower Kwango Group), which correlate well with other Upper Jurassic to Lower Cretaceous aeolian sequences in Namibia and eastern Brazil, attesting to a giant ‘Sahara-like’ paleo-desert across central West Gondwana, just before the separation of Africa from South America. U-Pb detrital zircons geochronology from this aeolian sequence in the Congo Basin dates mid-Silurian (ca. 430 Ma), Permian-Triassic (ca. 240 and 290 Ma) and Jurassic (ca. 190 Ma) magmatic zircons, here proposed to have been sourced from abundant volcanic activity along the proto-Andes, in southernmost Gondwana. Two successive middle Cretaceous lacustrine sequences in the center of the Congo Basin (the Loia and Bokungu Groups), first analcime-rich and episodically anoxic, and then more carbonated, are interpreted to record an episode of basin stagnation following the eruption of the Parana-Etendeka Large Igneous Province and a subsequent hot/humid climate maximum during the opening of the South Atlantic Ocean. Late Cretaceous sedimentation in the Congo Basin terminated with fluvial sediments (the Upper Kwango Group) suggesting marginal uplifts during the Kalahari epeirogeny. The top of these sequences is truncated by a regional Cenozoic peneplanation surface.

Paleogeography and Tectono-Stratigraphy of Carboniferous-Permian and Triassic ‘Karoo-Like’ Sequences of the Congo Basin

The Congo Basin is a large Phanerozoic sedimentary basin with up to 3–6 km of Carboniferous to Triassic sequences, comparable to those of the Karoo Basins of southern Gondwana. Here, we present a substantially revised stratigraphy for the Congo Basin, based on new field observations, seismic and borehole data, together with paleontology and new geochronology. In the center of the basin, the deepest boreholes intercept 3 to 4 km thick successions of conglomerates and red sandstones that overlie carbonate rocks, which correlate to deformed upper Neoproterozoic (Pan African) platform sequences extending beyond the Congo Basin into the Pan African orogenic zones (e.g. the West Congolian Group). The overlying sequences are dated biostratigraphically to be Carboniferous-Permian (the Lukuga Group) and Triassic (the Haute Lueki Group) in age. A regional erosion surface separates these two groups, possibly related to late Paleozoic intracontinental deformation associated with the Mauritanian-Variscan and Cape-de la Ventana orogens flanking the northwestern and southern margins of Gondwana, respectively. This change in basin paleogeography is consistent with detrital zircons dated from these sequences that suggest the ca. 1.4 Ga Kibaran Belt along the eastern margin of the Congo Basin stopped acting as a major source during the early Mesozoic.

Formation and Collapse of the Kalahari Duricrust [‘African Surface’] Across the Congo Basin, with Implications for Changes in Rates of Cenozoic Off-Shore Sedimentation

Following Gondwana break-up and the separation of Africa from Antarctica, India and South America, significant climate fluctuations and epeirogenic uplift have influenced the formation of the Kalahari Plateau (also known as the ‘African Surface’) and the Congo Basin (also known as the ‘Cuvette centrale’), two iconic landscapes of central sub-Saharan Africa. Both landscapes are covered by an extensive Upper Cretaceous-Cenozoic sedimentary sequence with hard-caps (duricrusts) of calcrete and silcrete of the Kalahari Group. These terrestrial sediments preserve a record over more than 80 million years of the climatic and tectonic history of south-central Africa, yet they represent a major challenge to accurately date, and to correlate precisely, because of lack of suitable materials for radiometric dating techniques, and because episodes of bioturbation, weathering, and erosion have left a condensed sequence of less than 500 m. Based on recent drilling and field investigations across the Kalahari Plateau in Botswana, and eroded surfaces exposed along the southern margin of the Congo Basin, in southwest Democratic Republic of Congo, new regional correlations of the Kalahari Group reveal that a large part of the carapace of silcrete and calcrete, here named the ‘Kalahari Surface’, across the Congo Basin disintegrated during the Cenozoic, exposing a vast area with up to 500–600 m of underlying poorly consolidated Jurassic-Cretaceousred-beds to fluvial erosion. When linked to the off-shore sedimentation history of the Congo Fan along the Atlantic margin, the terrestrial observations suggest that accelerated erosion of more than 0.5 × 106 km3 of the red-beds and collapse of the Kalahari Surface across the Congo Basin closely match the sudden increase of sedimentation and the estimated accumulation of ca. 0.7 × 106 km3 of Oligocene to Recent marine sediments. Discharge of the proto-Congo river systems draining the Congo Basin through the Congo delta was possibly linked to increasingly humid conditions during the mid-Cenozoic and may, in turn, have had significant effect on the global ocean water chemistry.

Origin and Evolution of the Cape Mountains and Karoo Basin

The lithostratigraphy and structure of the southern Karoo Basin is analyzed based on detailed re-logging of 11 deep boreholes drilled by SOEKOR in the 1960s. The Karoo Supergroup here is between 750 and 5540 m thick. The sequence starts with an extensive cover of glaciomarine and/or glaciolacustrine bedded diamictites with black shales of the mid-Carboniferous to Lower Permian Dwyka Group (353–744 m thick), capped by 79– 569 m black shales of the lowermost Ecca Group deposited during rapid deglaciation that mark a relatively short-lived time horizon. The black shales are overlain by middleto outer-fan turbidites in the southernmost deeper part of the basin that grade over a distance of 50 km northward into alternating dark gray silty shales with fine sandstones and carbonated mudstones characteristic of shallower shelf-slope deposits. The successions become regionally sandier and thicker bedded upward, implying a first-order regression that is marked by at least two second-order regressive surfaces, but the transition to the fluvial, Middle Permian to Triassic Beaufort Group cannot at this stage be mapped with confidence. In addition, along the southern margin, flanking the Cape Fold Belt, the Karoo successions are variably deformed, whilst farther north increasingly abundant dolerite sills intrude progressively to greater depths, all of which complicate unraveling the basin-wide stratigraphy.

New Structural Data and U/Pb Dates from the Gamtoos Complex and Lowermost Cape Supergroup of the Eastern Cape Fold Belt, in Support of a Southward Paleo-Subduction Polarity

Recent seismic profiling, together with new field mapping and U-Pb zircon geochronology, suggest south-dipping plate subduction polarity linked to the Permo-Triassic Cape Fold Belt of South Africa, during the Gondwanide Orogen. Here, we present further evidence that rocks of the Gamtoos Complex, predating the deposition of the Lower Paleozoic Cape Supergroup, west of Port Elizabeth, form part of an accretionary prism, or lower detachment, of decollement horizons in the “pre-Cape” basement sequences that extend into the overlying quartzites and phyllites of the lower Table Mountain Group (Cape Supergroup) as splay faults penetrating into the upper sequences of the eastern section of the Cape Fold Belt.