The last glacial maximum Balearic Abyssal Plain megabed revisited

Megabeds are thick sedimentary layers extending over thousands square kilometres in deep sea basins and are thought to result from large slope failures triggered by major external events. Such deposits have been found in at least three areas of the Mediterranean Sea. Although their discovery dates back to the early 1980s, many questions remain, concerning their initiation, source area, extent, and the nature of their emplacement. One of the largest previously documented megabeds was emplaced during the Last Glacial Maximum across the Balearic Abyssal Plain with a thickness of 8-10 m in water depths of up to 2800 m. New 3.5 kHz sub-bottom profiles and sediment cores provide greater constraint on the lateral variability of the megabed and allow to map it beyond previous estimates, with a revised areal extent up to 90,000-100,000 km2. Megabed terminations show gradual pinch-out to the West and an abrupt eastward termination against the Sardinia steep margin. The megabed presents both in seismic profiles and in sediment cores a tripartite subdivision likely corresponding to changes in flow regimes across the basin with a central area of sandy facies and erosional base oriented NNE-SSW allowing renewed discussions about sources and trigger of the megabed

Étude de la transition chenal-levées-lobe dans les systèmes turbiditiques récents (application à l'éventail turbiditique de l'Amazone et au Néofan du Petit-Rhône)

La zone de transition chenal-levées/lobes a été étudiée dans 3 systèmes turbiditiques situés dans des contextes régionaux différents: un système intra-pente du Delta du Niger, le complexe de lobes à la terminaison des 8 derniers chenaux de l éventail de l Amazone et la terminaison du Néochenal, dernier chenal de l éventail du Petit-Rhône. - Dans les trois cas, l agencement des dépôts et leur évolution dans l espace et le temps révèlent un fort contrôle de la configuration des réceptacles (taille et forme), de la morphologie des fonds (pentes et taux de changement des pentes) et du degré de confinement. - Les systèmes de l Amazone et du Petit-Rhône, dont les lobes se sont construits à partir du Dernier Maximum Glaciaire et lors de la remontée du niveau marin qui a suivi, ont en outre permis de souligner l impact majeur, sur la mise en place des lobes terminaux, de certains forçages externes (événements climatiques et variations eustatiques), qui déterminent les variations des flux sédimentaires arrivant au bassin. Ainsi, bien que situés à des distances considérables des sources fluviatiles et dans des zones distales où les variations de relief sont très ténues, les lobes terminaux de ces deux grands systèmes turbiditiques se sont révélés contraints à la fois par des processus internes (processus de compensation) et par des processus externes (déconnexion post-glaciaire du fleuve et du bassin).The channel-lobe transition zone has been studied in three turbiditic systems located in distinct regional settings: an intra-slope system in the Niger delta slope (offshore Nigeria), the Amazon channel-mouth lobe complex at the termination of the last eight channel-levee systems and the Neochannel termination, the last active channel of the Petit-Rhone turbidite system. - In the three cases, the internaI geometry and stacking pattern of the deposits show a significant control of the configuration (size and shape) of the receiving basins, their morphology (gradients and subtle gradient changes), and their degree of confinement. - The Amazon and Petit-Rhone systems, with lobes built after the Last Glacial Maximum and the ensuing sea-level rise, evidence the major impact of external forcings (climatic events and eustatic variations) that induce variations in sediment fluxes delivered to the deep basin and therefore exert a control on channel-mouth lobes construction. The channel-mouth lobes of the Amazon and Petit-Rhone systems, even if they are located at great distances from the river mouths, at the termination of the turbidite systems and in areas characterised by subtle morphological variations, are both constrained by combined internai (accommodation and compensation) and external factors (post-glacial deconnection between the fluvial system and the deep basin).BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Avulsion process in turbidite systems (stratigraphic and lithologic records)

Grâce aux données de forages ODP du Leg 155, corrélées aux coupes sismiques haute résolution de la campagne Lobestory d Ifremer, l éventail géant de l Amazone est devenu une zone privilégiée pour étudier l évolution des systémes chenaux-levées et le processus d avulsion qui régit l architecture des systémes turbiditiques. Les travaux antérieurs avaient montré que le processus d avulsion est un des processus fondamentaux de la construction des édifices turbiditiques. Il aboutit au changement brutal du cours d un chenal et à son abandon en aval du point d avulsion. Ce processus et les phénoménes qui en découlent (courants non-confinés, rupture de profil d équilibre du chenal, etc.) sont à l origine d une distribution sédimentaire particulière, avec des dépôts grossiers (HARPs) en semelle des chenaux. Dans le travail présenté, les paquets de réflecteurs de fortes amplitudes (HARPs) situés à la base de ces systèmes fortement aggradant, ont pu étre caractérisés en sismique comme la superposition de 4 corps sismiques élémentaires correspondant à des processus de mise en place distincts. Ils peuvent étre décrits comme une amalgamation de petites unités sableuses, chenalisées ou non, mises en place pa une succession de courants de turbiditié très sableux et de haute densité. L étude sismique et bathymétrique de corps particuliers repérés dans l éventail du Zaïre a permis de compléter notre connaissance des premiers (effondrement de la levée) et derniers stades (abandon du chenal père et début d aggradation du chenal fils aprés dépôts des HARPs). Ces résultats ont permis de préciser les modèles d avulsion proposés jusqu ici.Our study focuses on the avulsion process, the key process for the depocenter migration of stacked channel-levee systems in the deepsea. To present, the avulsïon process has been explained as the abandonment of a channel (parent channel) and the birth of a new one (new channel). A true indice of avulsion has been given by the detection of the HARPs, a seismic term known since the 80s for highamplitude reflection packets at the base of new channel-levee systems. The HARPs are suggested, in analogy to the studies from ODP Leg 155 of the Amazon Fan, to be sandy deposits in lows adjacent to breached parent channels. Our present work, by re-visiting the Amazon Fan with multichannel high-resolution seismic data, highlights the great complexity of the HARPs architecture and their lithological heterogeneity. The study of some specific zones within the Zaïre Fan allowed revealing both the earliest stages of an avulsion process related to levee breaches and first splays linked to unconfined flows on the levee, and the latest stage of this process, with the first levee deposition and evolution following the HARPs deposition. These results provide new insights for the understanding of the processes involved during an avulsion event (levee breach, HARPs accumulation, levee aggradation). By describing the inter-relationship of the different architectural sub-bodies of the HARPs and giving details on both the earliest and latest stages of the avulsion, our results also provide clues to refine the previous conceptual models and reach a higher resolution knowledge of this process.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Geological setting and evolution of the Valencia and Rhone Deep Sea Fans

4 pagesThe area of study is located at the junction between two of the major turbiditic systems of the northern part of the Western Mediterranean sea, the Valencia Fan to the south and the Rhone Fan to the north. These fans originate from very different areas of the basi

Diachronous salt tectonics along the Gulf of Lions margin, Western Mediterranean

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Channel-mouth lobe complex of the recent Amazon Fan: The missing piece

International audienceThe Amazon Fan is one of the largest river-fed submarine fan systems in the world. It is located in the Western Equatorial Atlantic and extends 1100 km northward from the Amazon canyon head located at the shelf break off northern Brazil. Numerous previous investigations on the Amazon Fan mostly focused on the upper and middle parts of the fan. Two recent cruises have provided a new dataset based on bathymetric, acoustic imagery and 3.5 kHz sub-bottom profiler data. It is the first detailed exploration of the youngest Amazon channel-mouth lobe complex that remained the “missing piece” of one of the best studied modern deep-sea fans. The stratigraphic framework of the surveyed channel-mouth lobe complex of the Amazon Fan consists of eight channel-levee systems and their associated channel-mouth lobe systems (from Brown to Amazon). These depositional systems record a complex depositional history in a short time period between the Last Glacial Maximum (not, vert, similar 20,000 cal yr BP) and the rise in sea-level until 10,100 cal yr BP, when sediment supply to the fan ceased. The main body of the lobes is characterised by a general low to average backscatter facies (from white to light grey) scattered with higher backscatter fingers (dark grey). Two types of lobes are distinguished based on the planform shape of their backscatter facies: (1) elongated and narrow lobes with branching patterns like the veins of a leaf and (2) radial lobes with crescent-shaped features opened in the downstream direction, that do not have a clear organisation within the lobe. The eight channel-levee systems and associated lobes were built in a time span of not, vert, similar 10 ka and with an average time between successive avulsions of 1250 years. The life span of lobes in the Amazon system is around 600 years which is very short compared to the life span of the fan as a whole. The observed wide range in channel lengths and lobe sizes suggests that each system had a variable time duration and/or sediment supply rate. The variability of the channel lengths could be controlled by internal and/or external forcings linked to processes occurring at different stages in the system in the Amazon drainage basin, on the Amazon shelf, at the canyon head or at the channels bifurcation point. Numerous channel-levee systems and lobes are concentrated in a relatively narrow area. Locally, the pre-existing morphology of the distal depositional area influences the shape and the location of lobes, providing an argument for strong topographical control where local gradient and degree of topographic confinement are key control parameters

CYCLES IN THE ARCHITECTURE OF THE QUATERNARY ZAIRE TURBIDITE SYSTEM: A POSSIBLE LINK WITH CLIMATE

The presently active Zaire (Congo) turbidite system reveals a well-organized Quaternary architecture, with depocenters that partly overlap each other in response to avulsion. Based on previous work, more than 76 channels are organized into three individual fans (Northern, Southern, and Axial Fan, from the oldest to the youngest). A statistical analysis of both longitudinal and lateral migration of depocenters was conducted. The longitudinal shifts were studied through the temporal evolution of the channel lengths and the distances to the bifurcation points from a common reference point arbitrarily positioned on the canyon course, up-dip from the most proximal bifurcation point. The number of bifurcation points on a channel was also calculated. These three architectural parameters show a cyclic organization through time, better expressed in the Axial Fan, with cycles of down-fan and up-fan movements reflecting prograding– retrograding cycles. Based on a previous study of the kaolinite/smectite (K/S) ratio in the hemipelagic drape covering the Southern Fan, i.e., contemporaneous with the building of the Axial Fan, the prograding peaks of the Axial cycles are correlated to peaks in K/S, which reflect phases of intense Zaire River water discharge, during warm and humid interglacial periods. These correlations suggest that both the channel lengths and the avulsion process are controlled by climate changes that appear as a major forcing factor throughout the Quaternary. The effects of climate control can be modified by the interplay of other internal and/or external factors. Study of the lateral migration revealed that topographic compensation is the major autogenic control, and that external factors such as tectonic evolution in the drainage basin of the Zaire River or halokinesis at the Angola base of slope can locally play a significant role in the location of depocenters

Diachronous salt tectonics along the Gulf of Lions margin, Western Mediterranean

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