La Seine fluviatile plio-quaternaire en baie de Seine : évolution morphologique et sédimentaire (rôle du substratum géologique et des cycles climato-eustatiques)

The bay of Seine is a large shallow marine depression of about 5 000 km², located between the central English Channel to the North, the Cotentin peninsula to the West, and the Paris basin to the South and the East. VHR seismic and surface rock coring data acquired between 2007 and 2009 for the present study allowed revising the geological map of the bay of Seine, drawing the relationships between the rocky substrate and the periglacial plio-quaternary Seine river, and finally proposing for the latter a new depositional and evolution model. The substratum of the bay of Seine features various stratigraphic formations and geological structures similar to the one described inland in the Seine watershed. In terms of main structure two areas are distinguished: a monoclinal structure throughout the bay, and a syncline structure in the northern border, both affected by faults. Some new features have been pointed out during the substrate study, including the recognition of major stratigraphic surfaces, well known onshore but that were not identified until now offshore, the existence of tertiary gravitational sliding blocks in the syncline area. A drowned plio-quaternary palaeo-valley network is incised in this bedrock substratum. The plio-quaternary climatic cycles and the nature of the substratum (lithology, paleo-relief, slope, structural faults and folds) play a major role in the evolution of the fluvial system of the palaeo-Seine. This evolution was studied through the description of the strath and nested fill terraces observed on VHR seismic profiles. The palaeo-Seine shows three main strath terraces (the "ancient" Seine, of earlymiddle Pleistocene age , the Saalian Seine, and the Weichselian Seine). The Pleistocene terraces in the palaeo- Seine allow to distinguish at least one significant episode of uplift. This middle Pleistocene uplift, occuring between the end of Saalian and the beginning of Weichselien, was estimated between 0.07 and 0.1 mm/year, and appears in the continuity of the general uplift observed in Northern Europe since the Neogene, associated to compressive displacements related to the convergence of Africa and Eurasia.La baie de Seine forme aujourd'hui une vaste dépression marine d'environ 5 000 km 2 , située entre la Manche centrale au Nord, le Cotentin à l'Ouest et le Bassin parisien au Sud et à l'Est. Les nouvelles données sismiques THR (2007-2009) couplées aux données de dragages (2009) acquises en baie de Seine pour la thèse, ont permis de réviser la carte géologique du substratum de la Baie de Seine, d'établir les liens entre ce substratum et la Seine périglaciaire plio-quaternaire et de proposer pour cette dernière un nouveau modèle de dépôt et d'évolution. Le substratum géologique de la baie de Seine présente des formations stratigraphiques et des structures géologiques variées, similaires à celles décrites dans le bassin versant de la baie de Seine. Ce substratum consiste en une structure monoclinale dans l'ensemble de la baie et une structure synclinale au Nord de la baie, toutes deux affectées par des failles. Plusieurs faits nouveaux ont pu être mis en évidence au cours de l'étude du substratum, notamment la reconnaissance de surfaces stratigraphiques majeures connues à terre mais non identifiées jusqu'à présent en baie de Seine, l'existence dans la zone du synclinal de glissements gravitaires tertiaires. Un réseau de paléovallées submergé plio-quaternaire est incisé dans le substratum. Les cycles " glaciaire-interglaciaire " plio-quaternaires et la nature du substratum (lithologie, cuesta, pente, failles, déformations) jouent un rôle majeur dans l'évolution du système fluviatile de la paléoSeine. Cette évolution a été étudiée au travers de la description des terrasses étagées et des nappes emboîtées identifiées en sismique. La paléoSeine montre ainsi l'étagement de trois terrasses alluviales (la Seine ancienne du début du Pléistocène moyen ; la Seine saalienne ; la Seine weichsélienne). Cette reconnaissance des terrasses pléistocènes de la paléoSeine permet de distinguer au moins un épisode important de surrection. Cette surrection, datée du Pléistocène moyen au début du Weichsélien, est estimée à environ 0,07 à 0,1 mm/an, et s'inscrit dans le prolongement du soulèvement connu en Europe du Nord depuis le Néogène avec des mouvements compressifs liés à la convergence Afrique-Eurasie

The Late Holocene sediment infilling and beach barrier dynamics of the Thau lagoon (Gulf of Lions, Mediterranean sea, SE France)

A study combining very high resolution seismic and sediment core data has been carried out on the Thau lagoon (Mediterranean coast, microtidal setting, SE France) in order to understand more clearly the dynamics and Holocene chronology of its closure through the different stages of its filling. One main seismic unit (U2) has been defined into the infill, above the rocky basement (U0) and a composite unit U1, which is interpreted as remnants of Pleistocene fluvial terraces or/and to early marine Holocene deposits. Unit U2, that reaches locally 9 m in thickness, rests conformably on U1 in the central part of the lagoon and onlaps U0 or U1 close to the edge of the lagoon. It is divided in two sub-units, U2-1 and U2-2. U2-2 rests paraconformably on U2-1 in the central part of the lagoon where the infill is the thickest, while a marked erosional unconformity is observed between U2-1 and U2-2 on topographic highs of the basement and on the seaward edge of the lagoon. A total of seven elementary sequences have been observed in U2-1 and U2-2.;According to core data, U2 consists in a series of mud-dominated sequences, with shell fragments dispersed at the base. The vertical distribution of the fauna into U2-1 and U2-2 reveals a lagoonal environment. However in U2-1, marine species are more abundant in the south of the lagoon. C-14 AMS dating provides three ages: similar to 6000 cal yr B.P. in the lower part of U2-1 on CAL1, 5400 cal yr B.P. just above the boundary between U2-1 and U2-2 on CAL4, similar to 3000 cal yr B.P. in the middle of U2-2 on CAL4.;A scenario to explain the lagoon infill stratigraphy and geometry is proposed. The beginning of the lagoon infill occurred with the initiation of the barrier construction, as soon as the sea-level rise slowed down significantly, i.e. between 7000-6000 yr B.P. The sediment-fill began into the back-barrier system, with a high rate of sedimentation for U2-1, according to the radiocarbon data. At 5400 yr B.P., the barrier is assumed to be totally closed leading to the deposition of the fully lagoonal U2-2 succession. The unconformity between U2-1 and U2-2 is interpreted as the result of a rapid landward retreat of the barrier. This severe retreat could be related to climate forcing and/or brutal change in sediment discharge driven by the Rhone River, which is the main sediment source of the longshore drift. In this scenario, the last sub-unit, U2-2, represents most of the late Holocene infill. In this framework, the elementary sequences observed in U2-2 could be related to high frequency climate changes of about 1000-1500 years periodicity according to C-14 dates

The Central English Channel troughs: major source-to-sink remnants or giant tidal scours?

International audienceThe Central English Channel troughs correspond to elongated incisions up to 250 m-deep, at several locations at the bottom of this sea corridor. Depending on their location, they are usually interpreted as part of the submerged quaternary paleovalley network or as resulting from megaflood events. Shedding light on these features, their age, and the processes underlying their development is key for understanding their significance in terms of event geology. The interpretation of a dense grid of high-resolution marine seismic data acquired in the Bay of Seine area reveals that the extensive Quaternary paleovalley and trough network commonly as associated to the "Channel River" system is actually subdivided into at least two superimposed and unrelated incised networks. The overlying network corresponds to fluvial incisions developing during low sea-level conditions of Pleistocene time and connects to the present day fluvial network. The underlying network corresponds to the troughs and appears as a complex, deeper, relatively discontinuous and isolated network. This older network shows unexpected local incision depth up to c.350-400 m-deep and complex sedimentary infill involving several sedimentary processes and environments from fluvial to tidal and shallow-marine. We discuss these observations and their implications for understanding the origin, age and development of the troughs all over the English Channel, from the Dangeard Troughs in the Dover Strait to the Hurd Deep at the western end. We also raise questions about the significance of these large incised features in terms of source-to-sink system of northwestern Europe

Evolution des connaissances du bassin Anglo-Parisien - Apports de la géologie marine : Exemple du Mésozoïque et des géométries sédimentaires de la craie du pays de Caux

International audienceLe " Bassin de Paris " fait partie intégrante du bassin sédimentaire intracratonique Anglo-Parisien, qui couvre une majeure partie du Nord de la France et du Sud-Est de l'Angleterre. A plusieurs reprises, les phases de déformation tectonique affectent l'évolution du bassin en modifiant la sédimentation, la répartition des aires de subsidence, et l'orientation des paléogéographies, notamment sur sa bordure ouest à la transition Jurassique-Crétacé et lors de l'inversion en Manche au Cénozoïque. Cette évolution du remplissage sédimentaire et de la tectonique de cette zone reste encore peu détaillée en raison d'un manque de données de subsurface (puits, sismique), généralement localisées au centre du Bassin de Paris. De 2007 à 2009, les missions d'acquisition sismique (BaiSeine07, SeineHR08 et SeineTHR08) et de prélèvements au carottier à roche (Carobseine09) ont été effectuées en baie de Seine dans le cadre d'une collaboration Université de Caen-Basse Normandie / BRGM. La qualité et la densité des données acquises a permis de détailler le substratum mésozoïque, principalement jurassique, de part et d'autre de la Paléo-Seine plio-quaternaire, et de proposer une cartographie géologique révisée de baie de Seine (thèse de M. Benabdellouahed, 2011). Quelques profils acquis au large du Pays de Caux permettent d'étendre les études aux séries du Crétacé à l'est. L'interprétation de ces profils, couplés aux données bathymétriques (SHOM), topographiques Lidar et photographiques (IGN), et de sondage en mer (BRGM), ont permis (i) d'effectuer les corrélations terre-mer fines, (ii) d'établir un atlas sismique des formations crétacées, (iii) d'imager les structures et les géométries principales du remplissage cénozoïque, et (iv) d'étendre la cartographie géologique de baie de Seine vers le nord et l'est (Paquet et al., en prep.). Un exemple d'apport de connaissance est l'imagerie fine des géométries sédimentaires en ondulation de la craie, visibles sur les falaises et le platier entre Etretat et Fécamp. Ces géométries trouvent leur origine dans des courants marins affectant le fond. Une meilleure connaissance de leurs dimensions, orientation et extension géographique fournira des éléments pour mieux comprendre la paléogéographie de la zone et son évolution au Crétacé supérieur. La campagne MERCAUX d'acquisition sismique " sparker " très haute résolution (aout-septembre 2013) s'intègre dans cette logique d'amélioration des connaissances de la bordure ouest du Bassin anglo-parisien, et de son évolution géologique

Recent uplift of the Atlantic Atlas (offshore West Morocco): Tectonic arch and submarine terraces

Re-examination of marine geophysical data from the continental margin of West Morocco reveals a broad zone characterized by deformation, active faults and updoming offshore the High Atlas (Morocco margin), situated next to the Tafelney Plateau. Both seismic reflection and swath-bathymetric data, acquired during Mirror marine geophysical survey in 2011, indicate recent uplift of the margin including uplift of the basement. This deformation, which we propose to name the Atlantic Atlas tectonic arch, is interpreted to result largely through uplift of the basement, which originated during the Central Atlantic rifting stage - or even during phases of Hercynian deformation. This has produced a large number of closely spaced normal and reverse faults, “piano key faults”, originating from the basement and affecting the entire sedimentary sequence, as well as the seafloor. The presence of four terraces in the Essaouira canyon system at about 3500 meters water depth and “piano key faults” and the fact that these also affect the seafloor, indicate that the Atlantic Atlas is still active north of Agadir canyon. We propose that recent uplift is causing morphogenesis of four terraces in the Essaouira canyon system. In this paper the role of both Canary plume migration and ongoing convergence between the African and Eurasian plates in the formation of the Atlantic Atlas are discussed as possibilities to explain the presence of a tectonic arch in the region. The process of reactivation of passive margins is still not well understood. The region north of Agadir canyon represents a key area to better understand this process

Opening of the central Atlantic Ocean: Implications for geometric rifting and asymmetric initial seafloor spreading after continental breakup

Study of the deep structure of conjugate passive continental margins combined with detailed plate kinematic reconstructions can provide constraints on the mechanisms of rifting and formation of initial oceanic crust. In this study the central Atlantic conjugate margins are compared based on compilation of wide-angle seismic profiles from NW Africa Nova Scotian and U.S. passive margins. The patterns of volcanism, crustal thickness, geometry, and seismic velocities in the transition zone suggest symmetric rifting followed by asymmetric oceanic crustal accretion. Conjugate profiles in the southern central Atlantic image differences in the continental crustal thickness. While profiles on the eastern U.S. margin are characterized by thick layers of magmatic underplating, no such underplate was imaged along the African continental margin. In the north, two wide-angle seismic profiles acquired in exactly conjugate positions show that the crustal geometry of the unthinned continental crust and the necking zone are nearly symmetric. A region including seismic velocities too high to be explained by either continental or oceanic crust is imaged along the Canadian side, corresponding on the African side to an oceanic crust with slightly elevated velocities. These might result from asymmetric spreading creating seafloor by faulting the existing lithosphere on the Canadian side and the emplacement of magmatic oceanic crust including pockets of serpentinite on the Moroccan margin. After isochron M25, a large-scale plate reorganization might then have led to an increase in spreading velocity and the production of thin magmatic crust on both sides

Origin of step-like and lobate seafloor features along the continental shelf off Rio de Janeiro State, Santos basin-Brazil

A combined analysis of seismic and morphological features identified in a set of high-resolution seismic reflection and bathymetric data, shows a systematic relationship between major modern seafloor morphological traces and the basinward migration of Late Pleistocene coastlines along the continental shelf of the Santos basin (Rio de Janeiro State, SE Brazil). Observed fairly continuous and sinuous mid-outer shelf escarpments are related to the sea-level variations and shelf exposure during the Last Glacial cycle. A bathymetric step at -110 m is an erosional remnant of offlapping detached forced-regressive wedges that spread over 50 km in the shelf-dip direction, probably developed during periods of falling sea level between MIS 3 and 2. A second major escarpment at -130 m was interpreted as the shoreline during the LGM, at the time of most extensive subaerial exposure of the continental shelf. However, a distal escarpment at -150 m is expressed as a straight contour feature along the two main shelf-edge embayments that characterize the shelf break. This escarpment is coupled with a basal seaward-inclined and highly eroded ramp, and was interpreted as the erosional action of bottom currents during the last transgression due to the displacement of the southward flowing Brazil Current towards the present-day outer shelf. Previously published articles have regarded the morphological features observed on the modern shelf as indicators of stillstands during the post-LGM transgression. We conclude that, on the contrary, most of these features are actually from earlier parts of the Late Pleistocene and were formed in a regressive scenario under oscillating and relative slow sea-level fall

Updating geological knowledge of the Anglo-Parisian Basin from offshore mapping and investigations using very-high resolution seismic data

International audienceAs the French Geological Survey, one of the objectives of BRGM is the continuous updating of the geological knowledge of the French territory. Whereas the onshore area has been completely mapped and will be progressively rendered in 3D (RGF program), the geology of the continental shelf is mostly summarized at the 1:1,000,000 scale from the synthesis of various documents including 1:250,000 maps from the 70s.Since the late 90s, the use of recent very high resolution (VHR) marine seismic and bathymetric acquisition systems, together with precise positioning from GPS allow (i) to update the geological mapping and (ii) to reveal details that can only be provided with such kind of data. Recent data acquisition cruises, collaborations, and data sharing with other research institutes and companies (Universities, Ifremer, SHOM, Total,…) set up a framework to undertake modern geological mapping over the shelf and to answer scientific questions related to sedimentary basin evolution.The English Channel is one key area of study where sedimentary series of the Anglo-Parisian Basin outcrop directly at sea bed or lies under a thin plio-quaternary cover. This configuration provides a unique access to the meso-cenozoic series using VHR data and eases mapping.Besides mapping aspects, data acquired from 2013 to 2016 reveal details in the structure of contourites-like undulating geometries within the upper cretaceous chalk, as well as tectonic features along regional faults that help to better constrain their kinematics. Those data also provide new insights into the understanding of the “recent” paleovalley networks that are preserved over the English Channel

Recent uplift of the Atlantic Atlas (offshore west Morocco) : tectonic arch and submarine terraces

(IF 2.69; Q1)International audienceRe-examination of marine geophysical data from the continental margin of West Morocco reveals a broad zone characterized by deformation, active faults and updoming offshore the High Atlas (Morocco margin), situated next to the Tafelney Plateau. Both seismic reflection and swath-bathymetric data, acquired during Mirror marine geophysical survey in 2011, indicate recent uplift of the margin including uplift of the basement.This deformation, which we propose to name the Atlantic Atlas tectonic arch, is interpreted to result largely through uplift of the basement, which originated during the Central Atlantic rifting stage - or even during phases of Hercynian deformation. This has produced a large number of closely spaced normal and reverse faults, “piano key faults”, originating from the basement and affecting the entire sedimentary sequence, as well as the seafloor.The presence of four terraces in the Essaouira canyon system at about 3500 meters water depth and “piano key faults” and the fact that these also affect the seafloor, indicate that the Atlantic Atlas is still active north of Agadir canyon.We propose that recent uplift is causing morphogenesis of four terraces in the Essaouira canyon system. In this paper the role of both Canary plume migration and ongoing convergence between the African and Eurasian plates in the formation of the Atlantic Atlas are discussed as possibilities to explain the presence of a tectonic arch in the region. The process of reactivation of passive margins is still not well understood. The region north of Agadir canyon represents a key area to better understand this process

臺灣幾種柑橘砧木之研究

International audienceS U M M A R Y During the Sanba (Santos basin seismic transect) experiment in 2010–2011, a 380-km-long combined wide-angle and reflection seismic profile has been acquired using 30 ocean-bottom seismometers, a 4.5 km seismic streamer and a 8900 in. 3 airgun array. The Sanba 3 profile crosses the southern flank of the Sao Paulo Plateau, the Sao Paulo Ridge and the easternmost Santos Basin in an east–west direction. Its eastern end is located on undisturbed oceanic crust. Tomographic and forward modelling of the wide-angle seismic data reveals that the sedimentary thickness is variable with only 1–2 km on top of the ridge and thickening to 4–5 km in the basin. Crustal thickness at the ridge is about 18 km and the relative layer thickness and velocity gradients indicate a continental origin of this ridge. The eastern Santos Basin is underlain by crust of only 5 km thickness, characterized by high seismic velocities between 6.20 km s –1 in the upper crust and 7.40 km s –1 in the lower crust. Three hypotheses for the nature of the crust in this region are tested here: (i) thinned continental crust, (ii) serpentinized upper mantle material, (iii) thin oceanic crust. As seismic velocity gradients seem to rule out a continental origin of this region, and clear Moho reflections argue against serpentinized upper mantle, we propose that the crust underlying the easternmost Santos Basin is of oceanic origin. Deviations from normal oceanic crustal velocities in the lower crust (6.70–7.00 km s –1) can be explained by accretion at slow spreading rates leading to the inclusion of serpentinite into the lower crust at the onset of organized seafloor spreading