Depositional model and stratigraphic architecture of rift climax Gilbert-type fan deltas (Gulf of Corinth, Greece)

International audienceFacies, depositional model and stratigraphic architecture of Pleistocene giant Gilbert-type fan deltas are presented, based on outcrop data from the Derveni­Akrata region along the southern coast of the Gulf of Corinth, Greece. The common tripartite consisting of topset, foreset and bottomset [Gilbert, G.K., 1885. The topographic features of lake shores: Washington, D.C., United States Geol. Survey, 5th Annual Report, 69­123.] has been identified, as well as the most distal environment consisting of turbidites, and is organised in a repetitive pattern of four main systems tracts showing a clear facies and volumetric partitioning. The first systems tract (ST1) is characterised by the lack of topset beds and the development of a by-pass surface instead, thick foresets and bottomset beds, and thick well-developed turbiditic systems. This systems tract (ST1) is organised in an overall progradational pattern. The second systems tract (ST2) is characterised by a thin topset and almost no foreset equivalent. This systems tract is not always well-preserved and is organised in an overall retrograding trend with a landward shift in the position of the offlap break. The offshore is characterised by massive sandy turbidites. The third systems tract (ST3) is characterised by small-scale deltas prograding above the staked topsets of the giant Gilbert-type fan delta. Those small Gilbert-type fan deltas are generally organised in a pure progradation evolving to an aggradational­progradational pattern. In the distal setting of those small Gilbert-type fan deltas, almost no deposits are preserved on the remaining topography of the previous Gilbert-type fan delta. The fourth systems tract (ST4) is characterised by continuous vertically aggrading topsets that laterally pass into aggrading and prograding foresets. Bottomsets and distal turbiditic systems are starved. This fourth systems tract (ST4) is organised in an overall aggrading trend. These giant Gilbert-type fan deltas correspond to the Middle Group of the Corinth Rift infill and their stratigraphic development was strongly influenced by evolving rift structure. They record the migration of the depocenter from the rift shoulder to the rift axis in four main sequences from ca. 1.5 to 0.7 Ma, related to the migration of fault activity. It is worth noting that the maximum paleobathymetry was recorded during the final stage of the progradation of the Middle Group, suggesting that the rift climax was diachronous at the scale of the entire basin. The rapid (< 1 Ma) structural and sedimentological evolution, the migration of fault activity as well as the youth of the Corinth Rift, are probably exceptional factors allowing the characterisation of such diachronism

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

Le Cénozoïque du bassin de Paris : un enregistrement sédimentaire haute résolution des déformations lithosphériques en régime de faible subsidence

International audienceLe bassin de Paris est considéré comme un exemple typique de bassin intracratonique affecté par une subsidence thermique long terme. Le Cénozoïque correspond à une période de faible subsidence (épaisseurs inférieures à 350m) et marque la fin du fonctionnement de ce bassin. C’est en outre une période de forte déformation de la plaque européenne, dans un contexte de convergence Afrique-Eurasie et d’ouverture de l'Atlantique Nord caractérisée par des inversions de grabens dans le Nord et l’Est de l’Europe. Si de nombreux hiatus ont été identifiés, les déformations cénozoïques du bassin de Paris, situées sur une croute à l’équilibre et leur relation aux contraintes en limite de plaque restent méconnues. Cette thèse a pour objectif de recomposer à haute résolution spatiale et temporelle (de l’ordre du million d’années) les géométries sédimentaires 3D et les paléogéographies successives du Paléocène au début de l’Oligocène. Ce travail qui s’appuie sur de nombreuses données biostratigraphiques consiste en une approche couplée de sédimentologie de faciès et de corrélations diagraphiques (500 puits) selon les principes de la stratigraphie séquentielle. Trois ordres de séquences sont définis. Les cycles d’ordre supérieur (4ème et 3ème ordre) enregistrent les variations climato-eustatiques. Cinq séquences de dépôt (2e ordre), limitées par des discontinuités et/ou des réorganisations paléogéographiques sont identifiées : (1) Maastrichtien-Danien ; (2) Thanétien-Yprésien ; (3) Lutétien-Bartonien ; (4) Bartonien-Priabonien terminal et (5) Priabonien terminal-Chattien. Les architectures des séquences (1) à (4) sont contrôlées par des phases de flexures. Après des émersions lors des paroxysmes de flexuration, la relaxation progressive des flexures se traduit tout d’abord par la mise en place de profils pentés et ouverts, puis par des profils de plus en plus plats et confinés associés à une transgression généralisée. Du Thanétien au Bartonien s’observent des flexures d’axe E-W, dont les âges sont compatibles avec les différentes phases de la convergence Ibérie-Eurasie. En outre, une déformation de courte durée à l’Yprésien basal est rattachée à l'ouverture de l'Atlantique Nord. Enfin, une réorientation majeure du bassin possiblement liée au début de la collision Apulie-Eurasie est observée au Priabonien. Ce travail fournit un calage à haute résolution pour la compréhension et la modélisation des déformations intraplaques. Différentes tailles de flexure, de l’ordre de 150 à plus 300 km sont observées traduisant une implication d’épaisseurs plus ou moins importantes de la lithosphère

Effect of the thermal gradient variation through geological time on basin modeling; a case study: The Paris basin.

Many studies investigated the thermal modeling of the Paris basin for petroleum interests during the 1970s. Most of the softwares developed by oil companies or research centers were based on the assumption of a constant thermal gradient. In order to take into consideration the variation of the thermal gradient during basin evolution, we developed the TherMO's Visual Basic 1D program. We applied our model to twenty boreholes located along a cross-section roughly running EW over 150 km in the center of the Paris basin. The numerical results were calibrated with organic matter maturity data. TherMO's simulates the amount of heat provided to the sedimentary organic matter. The heat parameter simulated shows lateral variation along the cross-section. It decreases from Rambouillet to Trou Aux Loups boreholes (87–66 mW/m2) at about 100 km more to the east whereas the heat flux value simulated is 73 mW/m2 in St. Loup borehole. The mean thermal gradient calculated for liassic horizons at 87 My for the Rambouillet well is 50.4 °C/km. This value is similar to previously published results. By integrating the calculation of the thermal gradients and conductivities related to the burial of each stratigraphic sequence, our approach points out variations in the thermal regimes the sedimentary organic matter (SOM) has been subjected to through geological time

Implications of spatial and temporal evolutions of thermal parameters in basin modelling

International audienceThis paper presents the Paris Basin numerical modelling at a high sequential resolution scale (1–5 my). Simulations were carried out from the computation of thermal gradients and conductivities varying with the burial of genetic units. Geologic heating rates are also calculated throughout the burial of the stratigraphic sequences. Thermal energies are then deduced. The Paris Basin is well known for its hydrocarbon potential in Liassic sediments. This study is focused on an east–west cross-section through the basin. The results show spatial and temporal variations of thermal parameters from the western to the eastern part of the profile. The reactivation of Hercynian fracture systems during the Mesozoic may be responsible for the computed variations in thermal conductivities and thermal gradients. Major geodynamic events also played a role in the simulated thermal history. Variations of the thermal energy are observed and are well correlated with the burial history of the basin. We suggest linking the simulated thermal energies to the thermal cracking of the organic matter. Our results are consistent with the prediction of hydrocarbon potential in the Cretaceous period. Consequently, this approach provides new insights to improve petroleum generation modelling issues

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

Rapid South Atlantic spreading changes and coeval vertical motion in surrounding continents: Evidence for temporal changes of pressure-driven upper mantle flow

International audienceThe South Atlantic region displays (1) a topographic gradient across the basin, with Africa elevated relative to South America, (2) a bimodal spreading history with fast spreading rates in Late Cretaceous and Eo-Oligocene, and (3) episodic regional uplift events in the adjacent continents concentrated in Late Cretaceous and Oligocene. Here we show that these observations can be linked by dynamic processes within Earth's mantle, through temporal changes in asthenosphere flow beneath the region. The topographic gradient implies westward, pressure-driven mantle flow beneath the basin, while the rapid spreading rate changes, on order 10 million years, require significant decoupling of regional plate motion from the large-scale mantle buoyancy distribution through a mechanically weak asthenosphere. Andean topographic growth in late Miocene can explain the most recent South Atlantic spreading velocity reduction, arising from increased plate boundary forcing associated with the newly elevated topography. But this mechanism is unlikely to explain the Late Cretaceous/Tertiary spreading variations, as changes in Andean paleoelevation at the time are small. We propose an unsteady pressure-driven flow component in the asthenosphere beneath the South Atlantic region to explain the Late Cretaceous/Tertiary spreading rate variations. Temporal changes in mantle flow due to temporal changes in regional mantle pressure gradients imply a correlation of horizontal and vertical motions: we find that this prediction from our models agrees with geologic and geophysical observations of the South Atlantic region, including episodes of passive margin uplift, regional basin reactivation, and magmatic activity

Seismic stratigraphic framework and depositional history for Cretaceous and Cenozoic contourite depositional systems of the Mozambique Channel, SW Indian Ocean

International audienceThis study describes previously unrecognized contourite depositional systems (CDSs) in the Mozambique Channel which constrain palaeoceanographic models for this area. The stratigraphic stacking patterns record nine seismic units (SU1 to SU9) separated by eight major discontinuities (a to h, oldest to youngest). Key seismic markers in CDS evolutionary history occur during Aptian-Albian (~122 Ma), late Cenomanian (94 Ma), early (38.2–36.2 Ma) and late (25–23 Ma) Oligocene, and early-middle Miocene (~17–15 Ma) epochs. These record onset (~122 to 94 Ma), growth (94 to 25–23 Ma), maintenance (25–23 to 17–15 Ma), and burial (17–15 Ma to the actual time) stages for CDSs. CDSs first develop during the onset stage which coincides with the opening and deepening of the African-Southern Ocean gateway (at 122 and 100 Ma, respectively). The growth stage, beginning in the late Cenomanian (94 Ma), correlates with the opening and deepening of the Equatorial Atlantic gateway. During the growth stage, two major shifts in sedimentary stacking pattern occur which coincide with palaeoceanographic changes during the early (38.2–36.2 Ma) and late (25–23 Ma) Oligocene. These in turn coincide with the onset and local enhancement of Antarctic water masses. CDS growth continued until the early-middle Miocene during the maintenance stage (~17–15 Ma). Most CDS growth ceased at the end of the maintenance stage. Circulation of the North Atlantic water mass into the Southern Hemisphere led to a deepening of Antarctic water masses in the area

Palaeoenvironments and palaeoclimate records in lacustrine deposits at the Eocene-Oligocene Transition. New insight from de CDB1 borehole (Rennes Basin, France)

On behalf the CINERGY project, a 675m-long borehole was cored through the sedimentary column (405 m) and the basement (270 m). The studied interval encompasses 3 lithostratigraphic units, from 66m to 405m depth. The chronostratigraphic framework of the sedimentary series relies on benthic foraminifera and palynology. Palaeoenvironmental reconstruction is based on sedimentology, pollen analysis and clay mineralogy. 95% of the series deposited continuously from the Early-Middle Bartonian to the Rupelian, including 290 m-thick lacustrine shaly organic deposits, alternating open and closed system depending on the floatting vegetal mats extension. Climate evolution is illustrated by palynological content. The Eocene climate evolves from a from warm and humid condition to a more contrasted seasons. the EOT is marked by a short drier and fresher period, the Rupelian is characterized by disappearance of megathermic taxa but back to more humid condition. Clay mineralogy changes more abruptly at the E/O boundary (~200 m depth), from an almost pure kaolinitic content to a kaolinite-smectite dominated assemblage. Whether the sudden mineralogical change across de E/O boundary is related to a change in source material or to a change in the hydrolysis in soil clay production, a climatic origin is very likely

Paléoenvironnements et paléoclimats enregistrés dans le bassin éo-oligocène de Rennes. Apports du forage CDB1 (projet CINERGY).

Dans le cadre du projet CINERGY, un forage carotté de 675 m a recoupé l'ensemble de la série sédimentaire remplissant ce bassin, puis la partie altérée du socle sur plus de 100 m et le socle sain sur 150 m. La série sédimentaire carottée a fait l'objet d'une première étude intégrant biostratigraphie, minéralogie, sédimentologie de faciès et analyse séquentielle, visant à établir le cadre chronostratigraphique, l'évolution des paléoenvironnements et du climat. L'intervalle étudié (66-405 m) comprend 4 unités lithostratigraphiques : - les Marnes à Natica (66-85 m) correspondent à des dépôts laguno-marins argilo-calcaires. La malacofaune et les associations de dinokystes sont typiquement rupéliennes ; - les Sapropèles inférieurs (85-206 m) sont caractérisés par des alternances d'argiles laminées et massives à traces de racines. Le Rupélien inférieur est attesté par la présence du pollen B. hohli. La limite E/O est localisée dans l'intervalle 189-206 m ; - les Argiles à Mohria (206-375 m) ont des faciès similaires, mais particulièrement riches en matière organique. Ces deux formations témoignent d'un environnement lacustre à palustre. Les associations polliniques sont d'âge Priabonien ; - la Formation basale argilo-sableuse (375-405 m) correspond à l'apparition des premiers sables. Les environnements de dépôts fluctuent entre fluvio-lacustre et fluvio-marin. Ces dépôts ont pu être datés par les pollens et les foraminifères du Lutétien supérieur au Bartonien. L'analyse séquentielle de cette succession montre deux MFS majeures, l'une dans les dépôts côtiers vers 380 m, l'autre dans les dépôts très organiques lacustres vers 240 m. A l'échelle de la séquence élémentaire, deux types d'unités génétiques sont définies selon l'environnement lacustre ou lagunaire. La comparaison du bassin de Rennes avec celui de Saffré indique une subsidence nettement plus importante pendant le Priabonien à Rennes. La comparaison avec d'autres bassins ouest-européens place les bassins de Rennes et Saffré dans une dynamique intermédiaire entre les bassins d'effondrement très subsidents comme le fossé rhénan et les bassins intracratoniques parisien, belge ou anglais. L'étude palynologique montre un changement important de la végétation à la fin de l'Eocène avec l'augmentation des Pinacées, puis un changement plus profond du climat à l'Oligocène avec le développement des Herbacées, attestant d'une saison sèche plus marquée, et l'apparition de la chlorite-smectite. A l'échelle ouest-européenne, on constate la simultanéité de ce refroidissement fini-éocène. Une étude à plus haute résolution de cet intervalle clé, particulièrement bien enregistré à Rennes, fournira des réponses sur ce changement global