Rapid erosion of the Southern African Plateau as it climbs over a mantle superswell

International audienceWe present new sedimentary flux data confirming that a large pulse of erosion affected the Southern African Plateau in the Late Cretaceous and is likely to be related to a major uplift episode of the plateau. This short phase of erosion (i.e., less than 30 Myr in duration) has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift: given its size, the rise of the African superplume is likely to have lasted much longer. Here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent-wide drainage reorganization, leading to substantial denudation in a relatively short amount of time. The amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the Karoo sedimentary and volcanic rocks and the underlying basement. We also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse, suggesting that climate may have also played a role in triggering the denudation. The diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are, however, better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling

Inverting Passive Margin Stratigraphy for Marine Sediment Transport Dynamics over Geologic Time

Passive margin stratigraphy contains time-integrated records of landscapes that have long since vanished. Quantitatively reading the stratigraphic record using coupled landscape evolution and stratigraphic forward models (SFMs) is a promising approach to extracting information about landscape history. However, there is no consensus about the optimal form of simple SFMs because there has been a lack of direct tests against observed stratigraphy in well-constrained test cases. Specifically, the extent to which SFM behaviour over geologic space and timescales should be governed by local (downslope sediment flux depends only on local slope) versus nonlocal (sediment flux depends on factors other than local slope, such as the history of slopes experienced along a transport pathway) processes is currently unclear. Here, we develop a nonlocal, nonlinear SFM that incorporates slope bypass and long-distance sediment transport, both of which have been previously identified as important model components but not thoroughly tested. Our model collapses to the local, linear model under certain parameterizations such that best-fit parameter values can indicate optimal model structure. Comparing 2-D implementations of both models against seven detailed seismic sections from the Southeast Atlantic Margin, we invert the stratigraphic data for best-fit model parameter values and demonstrate that best-fit parameterizations are not compatible with the local, linear diffusion model. Fitting observed stratigraphy requires parameter values consistent with important contributions from slope bypass and long-distance transport processes. The nonlocal, nonlinear model yields improved fits to the data regardless of whether the model is compared against only the modern bathymetric surface or the full set of seismic reflectors identified in the data. Results suggest that processes of sediment bypass and long-distance transport are required to model realistic passive margin stratigraphy and are therefore important to consider when inverting the stratigraphic record to infer past perturbations to source regions

A 60-million-year Cenozoic history of western Amazonian ecosystems in Contamana, eastern Peru

Weprovide a synopsis of ~60million years of life history in Neotropical lowlands, based on a comprehensive survey of the Cenozoic deposits along the Quebrada Cachiyacu near Contamana in PeruvianAmazonia. The 34 fossilbearing localities identified have yielded a diversity of fossil remains, including vertebrates,mollusks, arthropods, plant fossils, and microorganisms, ranging from the early Paleocene to the lateMiocene–?Pliocene (N20 successive levels). This Cenozoic series includes the base of the Huchpayacu Formation (Fm.; early Paleocene; lacustrine/ fluvial environments; charophyte-dominated assemblage), the Pozo Fm. (middle + ?late Eocene; marine then freshwater environments; most diversified biomes), and complete sections for the Chambira Fm. (late Oligocene–late early Miocene; freshwater environments; vertebrate-dominated faunas), the Pebas Fm. (late early to early late Miocene; freshwater environments with an increasing marine influence; excellent fossil record), and Ipururo Fm. (late Miocene–?Pliocene; fully fluvial environments; virtually no fossils preserved). At least 485 fossil species are recognized in the Contamana area (~250 ‘plants’, ~212 animals, and 23 foraminifera). Based on taxonomic lists from each stratigraphic interval, high-level taxonomic diversity remained fairly constant throughout themiddle Eocene–Miocene interval (8-12 classes), ordinal diversity fluctuated to a greater degree, and family/species diversity generally declined, with a drastic drop in the early Miocene. The Paleocene–?Pliocene fossil assemblages from Contamana attest at least to four biogeographic histories inherited from (i) Mesozoic Gondwanan times, (ii) the Panamerican realm prior to (iii) the time of South America’s Cenozoic “splendid isolation”, and (iv) Neotropical ecosystems in the Americas. No direct evidence of any North American terrestrial immigrant has yet been recognized in the Miocene record at Contamana.Facultad de Ciencias Naturales y Muse

Post-rift vertical movements of the southern African passive margins since 130 Ma : a combined approach : basin analysis, landforms study

Le plateau sud-africain (ou Kalahari) est le plateau anorogénique le plus grand au monde. Sa très grande longueur d’onde (×1000 km) et son altitude moyenne élevée (1000-1500 m) impliquent des processus mantelliques. La cinétique et l’origine de ce relief sont mal comprises. D’un côté, les études géomorphologiques le considèrent comme un relief mis en place à la fin de l’intervalle Cénozoïque ( 1000 m. Most mechanisms proposed to explain its elevation gain imply mantle processes. The age of the uplift and the different steps of relief growth are still debated. On one hand, a Late Cretaceous uplift is supported both by thermochronological studies and sedimentary flux quantifications. On the other hand, geomorphological studies suggest a Late Cenozoic uplift scenario (<30 Ma). However few attentions were paid to the evolution of the overall geomorphic system, from the upstream erosional system to the downstream depositional system. This study is based on two different approaches : onshore, on the mapping and chronology of all the macroforms (weathering surfaces and associated alterites, pediments and pediplains, incised rivers, wave-cut platforms) dated by intersection with the few preserved sediments and the volcanics (mainly kimberlites pipes) ; offshore, on a more classical dataset of seismic lines and petroleum wells, coupled with biostratigraphic revaluations (characterization and dating of vertical movements of the margins - sediment volume measurement). The main result of this study is that the South African Plateau is an old Upper Cretaceous relief (90-70 Ma) reactivated during Oligocene (30-15 Ma) times. Its evolution can be summarized as follows : 100-70 Ma (Cenomanian to Campanian): low elevation plateau (0-500 m) with older and higher reliefs located along the Indian side, acting as a main divide between the Atlantic and the Indian Oceans. First uplift occurred in the east at ~92 Ma, with a fast flexuration of the Indian margins. This initiates a paroxysm of the erosion (90-80 Ma) with the growth of a large delta along the Atlantic margin (Orange delta). Deformation migrated progressively westward and resulted on the growth of the Atlantic marginal bulge between 81 and 70 Ma. Most of the present-day relief was probably created at this time. This is supported by the decrease of the sedimentary flux which suggests a reorganisation of the interior drainage pattern ; 70-30 Ma (Uppermost Cretaceous-Paleogene): most of the relief is fossilized and weathered - relative tectonic quiescence ; 0-15 Ma (Oligocene-Early Miocene): second period of the South African Plateau uplift. Most of the deformation took place along the Indian side of the Plateau (strike flexure) feeding the Zambezi, Limpopo and Tugela deltas ; since at least Middle Miocene times, all those reliefs have been fossilized, with very low erosion rates (x1m/Ma), in response to the major aridification of southern Africa

Mouvements verticaux des marges passives d’Afrique australe depuis 130 Ma, étude couplée : stratigraphie de bassin : analyse des formes du relief

The South African (Kalahari) Plateau is the world's largest non-orogenic plateau. It forms a large-scale topographic anomaly (×1000 km) which rises from sea level to > 1000 m. Most mechanisms proposed to explain its elevation gain imply mantle processes. The age of the uplift and the different steps of relief growth are still debated. On one hand, a Late Cretaceous uplift is supported both by thermochronological studies and sedimentary flux quantifications. On the other hand, geomorphological studies suggest a Late Cenozoic uplift scenario (<30 Ma). However few attentions were paid to the evolution of the overall geomorphic system, from the upstream erosional system to the downstream depositional system. This study is based on two different approaches : onshore, on the mapping and chronology of all the macroforms (weathering surfaces and associated alterites, pediments and pediplains, incised rivers, wave-cut platforms) dated by intersection with the few preserved sediments and the volcanics (mainly kimberlites pipes) ; offshore, on a more classical dataset of seismic lines and petroleum wells, coupled with biostratigraphic revaluations (characterization and dating of vertical movements of the margins - sediment volume measurement). The main result of this study is that the South African Plateau is an old Upper Cretaceous relief (90-70 Ma) reactivated during Oligocene (30-15 Ma) times. Its evolution can be summarized as follows : 100-70 Ma (Cenomanian to Campanian): low elevation plateau (0-500 m) with older and higher reliefs located along the Indian side, acting as a main divide between the Atlantic and the Indian Oceans. First uplift occurred in the east at ~92 Ma, with a fast flexuration of the Indian margins. This initiates a paroxysm of the erosion (90-80 Ma) with the growth of a large delta along the Atlantic margin (Orange delta). Deformation migrated progressively westward and resulted on the growth of the Atlantic marginal bulge between 81 and 70 Ma. Most of the present-day relief was probably created at this time. This is supported by the decrease of the sedimentary flux which suggests a reorganisation of the interior drainage pattern ; 70-30 Ma (Uppermost Cretaceous-Paleogene): most of the relief is fossilized and weathered - relative tectonic quiescence ; 0-15 Ma (Oligocene-Early Miocene): second period of the South African Plateau uplift. Most of the deformation took place along the Indian side of the Plateau (strike flexure) feeding the Zambezi, Limpopo and Tugela deltas ; since at least Middle Miocene times, all those reliefs have been fossilized, with very low erosion rates (x1m/Ma), in response to the major aridification of southern Africa.Le plateau sud-africain (ou Kalahari) est le plateau anorogénique le plus grand au monde. Sa très grande longueur d’onde (×1000 km) et son altitude moyenne élevée (1000-1500 m) impliquent des processus mantelliques. La cinétique et l’origine de ce relief sont mal comprises. D’un côté, les études géomorphologiques le considèrent comme un relief mis en place à la fin de l’intervalle Cénozoïque (<30 Ma). A l’inverse, les données thermochronologiques montrent deux phases de dénudation pendant l’intervalle crétacé, corrélées à des phases d’accélération du flux silicoclastique sur les marges, qui suggèrent qu’il s’agirait d’un relief plus ancien hérité du Crétacé supérieur. Peu d’études ont porté sur l’évolution du système terre-mer depuis le bassin versant en érosion aux marges en sédimentation. Ce travail de thèse repose donc sur une double approche : une analyse géomorphologique des formes du relief (surfaces d’aplanissement) à terre, basée sur leur (i) cartographie, (ii) chronologie relative, (iii) relation avec les profils d’altération et (iv) datation au moyen des placages sédimentaires et du volcanisme datés qui les fossilisent ; une analyse stratigraphique de l’intervalle post-rift des marges, basée sur l’interprétation de données de sub-surface (lignes sismiques et puits), réévaluées en âge (biostratigraphie), pour (i) identifier, dater et mesurer les déformations des marges et de leur relief amont , (ii) mesurer les flux silicoclastiques, produits de l’érosion continentale. Un calendrier et une cartographie des déformations ont été obtenus sur les marges et mis en relation avec les différentes générations de surfaces d’aplanissement étagées qui caractérisent le relief du plateau sud-africain. Au moins deux périodes de déformation ont été identifiées au Crétacé supérieur (92-70 Ma) et à l’Oligocène (30-15 Ma). L’évolution est la suivante : 100 - 70 Ma (Cénomanien à Campanien) : plateau à très grande longueur d’onde, peu élevé (0-500 m), bordé à l’est par des reliefs plus hauts et plus anciens le long des marges indiennes, qui agissent comme une ligne de partage des eaux entre l’océan Atlantique et l’océan Indien. La déformation est initiée à l’est avec une flexuration brève, à grande longueur d’onde, des marges indiennes aux alentours de ~92Ma. Cette première surrection marque un paroxysme d’érosion enregistré par la mise en place d’un delta géant sur la marge atlantique (delta de l’Orange). La déformation migre ensuite vers l’ouest avec la croissance du bourrelet marginal atlantique entre 81 et 70 Ma. Le relief acquiert sa configuration actuelle comme l’indique une diminution du flux silicoclastique sur la marge atlantique qui traduit un changement majeur du système de drainage ; 70-30 Ma (Crétacé terminal-Paléogène) : période d’apparente non déformation. Le relief est fossilisé et intensément altéré (latérites) ; 30-15 Ma (Oligocène - Miocène inférieur) : deuxième surrection du plateau sud-africain qui acquière sa topographie actuelle. La déformation semble plus importante à l’est du plateau - flexure des marges nord indiennes initiée à ~25 Ma qui alimente les grands deltas de l’océan Indien (Zambèze, Limpopo, Tugela) ; le relief est fossilisé à partir du Miocène moyen, synchrone d’une aridification majeure de l’Afrique australe

EVOLUTION TERRE-MER DES MARGES INDIENNES DU PLATEAUSUD-AFRICAIN - ÉTUDE COUPLEE STRATIGRAPHIQUE ETGEOMORPHOLOGIQUE

National audienceLe plateau Sud-Africain (ou Kalahari) s’étend depuis l’Afrique du Sud jusqu’à l’Angolasur plus de 2000 km. Son extension de très grande longueur d’onde, impliquant des processus mantelliquesencore mal compris, en fait le plus grand plateau anorogénique du monde. Il est séparé d’unefrange côtière dégradée et étroite (100 km) par un escarpement qui culmine à une topogra- phiemoyenne comprise entre 1 000 et 1 500 m d’altitude.L’origine de cet escarpement qui caractérise l’amont du relief des marges sud-africaines esttrès débattue. D’un côté, les géomorphologues l’interprètent comme un relief hérité de surrectionssuccessives qui alternent avec des phases d’érosion par retrait d’escarpement, pendant la fin del’intervalle Cénozoïque (King, 1982 ; Partridge & Maud, 1987). A l’inverse, les donnéesthermochronologiques et les modèles numériques d’érosion (Gallagher & Brown, 1999; Van der Beeket al., 2002) suggèrent qu’il s’agirait d’un relief ancien, hérité de la phase de rift au Crétacéinférieur.L’objectif de cette étude est de discuter l’origine et l’évolution de l’escarpement duPlateau Sud-Africain à partir d’une double approche, basée sur : (1) l’analyse stratigraphique delignes sismiques marines le long de la marge Indienne d’Afrique australe (depuis la plaine duLimpopo, jusqu’au cône de la Tugela) ; (2) l’analyse des formes du relief en amont de la marge(chronologie relative des formes du relief, datation par intersection avec le volcanisme et les placagessédimentaires préservés).Les premières conclusions de notre étude sont les suivantes :• La frange côtière en amont de la marge Indienne d’Afrique australe est formée d’un emboitementde 5 générations de surfaces d’aplanissement (pédiments).• L’essentiel du relief préservé en amont de la marge est d’âge Crétacé supérieur. Il est à rattacherà deux grandes phases de surrection, associées à la mise en place de deux prismes de régression forcée dudeuxième ordre : un à la transition Cénomanien-Turonien (90-95 Ma) et un à la transitionMaastrichtien-Paléocène (~66Ma).• L’escarpement du plateau aurait donc une histoire polyphasée mais serait probablement un reliefhérité du Crétacé supérieur.• La topographie actuelle de l’amont de la marge serait acquise pendant le Cénozoïque, avec unparoxysme de déformation à la transition Lutétien-Bartonien (~40 Ma)

GROWTH OF THE GREAT ESCARPMENT ACROSS THE INDIAN MARGIN OF SOUTH AFRICA: a couple stratigraphic-geomorphologicstudy

International audienceThe South African Plateau is formed by marginal bulges clustered around an intracontinental basin (the KalahariBasin) with a mean elevation between 1000 and 1400 m. On seaward side, marginal bulges form major escarpmentsthat can reach an elevation up to 3500 m in the Drakensberg area, boundering the high elevation continentfrom a dissected coastal region.The factors controlling escarpment evolution of those high-elevation passive margins are highly debated. On theone hand, geomorphic studies interpret escarpments in term of pulses of uplift and scarp retreat (King, The NatalMonocline, 1982; Partridge & Maud, S.Afr.J.Geol., 1987). On the other hand, thermochronological data andnumerical models of escarpment erosion (Gallagher & Brown, Phil.Trans.R.Soc.Lon., 1999; Van der Beek et al.,J.Geophys.Res., 2002) suggest that escarpments predate the breakup with a minimal escarpment retreat duringpost-rift margin evolution.To answer this question, we studied the Indian margin of South Africa (from Bushveld area to Port-Elizabeth)using sequence stratigraphy analysis of industrial seismic lines and wells. This study is coupled with an analysisof the adjacent landforms, constrained by dated sediments and weathering deposits.The first outcomes of our study are:1. A first uplift during Late Cenomanian (95-90 Ma) created an initial escarpment along the Indian coast.2. A second uplift occurred during the latest Cretaceous to earliest Cenozoïc with a sequential tilting andtruncations of the inner part of the margin followed by the incision of pediments on the seaward side of the initialescarpment,3. A third uplift that occurred during Late Eocene – Early Oligocene and Miocene with the incision of two newgenerations of pediments.These preliminary results suggest that the “Great Escarpment” along the Indian coast of South Africa resultsfrom the stepping of at least four generations of pediments which record the polyphasic uplift history of theSouth African Plateau during the last 100 My

THE AFRICAN SURFACE (85-45 Ma): A RECORD OF MANTLE DEFORMATIONS SINCE 35 Ma

International audienceAfrica is characterized by a bimodal topography with long (x100 km) to very long (x1000 km) wavelengthplateaus and domes. The 300-400 m topographic mode corresponds to the Sahara on which is superimposed swells(Hoggar, Tibesti..) and the Congo Interior Basin. The 900-1100 m mode corresponds to the Southern African(Kalahari) Plateau and the East African and Ethiopian Domes.The landforms responsible of the African topography are of three types (1) etchplains (mantled or stripped), (2)pediments and pediplains and (3) incised valleys. Those different landforms are stepped with mantled etchplainsat higher elevation and pediments/stripped etchplains are lower elevation. Some of those landforms can be datedusing either direct geochronological evidences on lateritic weathering profiles or geological evidences such as therelationship between landforms and dated magmatism or sediments.We used the stepping of successive pedimentsas a proxy of deformation, making sure that they record successive base level fall.We mapped at Africa-scale, a major widespread etchplain known as the African Surface (King, 1949; Burke &Gunnel, 2008). This surface was dated both by geochronology (e.g. Beauvais et al., 2008 in Burkina, Deller, 2012in North Ethiopia) and on geological evidences (interfingering or reworking of laterites in sedimentary basins suchas Iullemmeden Basin or the Tanzanian Margin). The paroxysm of weathering was during Early Eocene times(EOCM) but started earlier in Late Cretaceous with more or less younger ages according to its location in Africa.Geometrical restorations of pediments indicate that this surface was (1) at sea level in northern and central Africawith unknown upstream gradients and (2) superimposed on a Late Cretaceous plateau in southern Africa.The main period of very long wavelenghth deformation occurred around the Oligocene-Eocene boundary withthe uplift of northern Africa or the beginning of the growth the East African dome. Some other long wavelengthreliefs are younger, Early Miocene for the Central Africa Atlantic Swell and the uplift of the Congo Basin at 300m or Pliocene for the Angola Mountains.The implications in term of mantle dynamics are discussed

Tilting of continental interiors over asource of dynamic topography : Late Cretaceouserosion of the South African plateau

International audienceNew sedimentary flux data confirm the observation that a large pulseof erosion affected the South African Plateau in the late Cretaceous.This episode of rapid erosion (less than 30 Myr) is likely to be relatedto a major uplift phase that is apparently difficult to reconcile witha possible mantle origin, namely the presence of low density body inthe underlying mantle causing flow and dynamical uplift of the continent.Given its size, the growth and rise of this so-called "African superswell"is likely to have taken one to several hundred million years.Here we demonstrate by using a simple model for fluvial erosion thattilting of the continent as it rides over a wide source of dynamic topographycan not only cause uplift of the plateau but also lead to substantialerosion of large surface areas in a relatively short amount of time, becausethe tilting produces a continental-scale drainage re-organization.We show that embedding a lithological contrast, such as the one thatmight have existed between a thick layer of Karoo volcano-clastic sedimentsoverlying basement, greatly amplifies the rate of erosion (and sedimentation)during the tilting episode.We demonstrate that our scenariois consistent with paleogeographic reconstructions of the position of theAfrican continent with respect to the African superswell, the temporaland spatial evolution of kimberlite eruptions across southern Africa andthe past and present-day highly asymmetrical drainage geometry