Processus sédimentaires actuels et évolution morphologique associée du canyon de Capbreton : description, observation et modélisation.

The submarine canyon of Capbreton, South East of the Bay of Biscay, forms a deep incision through the continental shelf and slope. Its sedimentary activity is abundantly described and documented; however, many questions remain concerning the details of the processes affecting the transfer of sediment from upstream to downstream. This study is based on the analysis and comparison of repeated bathymetric surveys, between 1998 and 2018, at depths ranging from 10 to 1500 m. This rare time series highlights a rapid morphological evolution, responding to its current activity. It is marked by alternating periods of filling and erosion at the axis of the canyon, and a continuous deposition of sediments in the head of the canyon over the last 20 years. The deepening of the narrow thalweg is induced by erosion according to the presence of upstream-migrating knickpoints. It has also been shown that the partial obstruction of the channels, in the tight meanders, precedes the initiation of new knickpoints. The study, supplemented by the analysis of ADCP current meter data, meteorological time series and sediment cores, highlights the link between meteorological conditions and the hydrodynamics of the canyon. For the first time in the Capbreton canyon, several strong turbidity currents were recorded (1 m/s local velocity and 6.5 m/s estimated front velocity), inertial waves were also highlighted. It is shown that storms are one of the main factors triggering these currents, both strongly controlling the sedimentary dynamics of the canyon. Numerical modelling allows us to discuss the alimentation of the canyon from the flows generated at its head. The simulated currents indicate that a fraction of fine sediment is necessary to generate flows along the canyon. Coarse sediments (sands) and their deposits are rapidly deposited and progress through the recurrence of events.Le canyon sous-marin de Capbreton, au sud-est du golfe de Gascogne, forme une profonde incision à travers le plateau et le talus continental. Son activité sédimentaire est abondamment décrite et documentée, cependant, de nombreuses questions subsistent concernant le détail des processus affectant le transfert des sédiments de l’amont vers l’aval. Cette étude est basée sur l'analyse et la comparaison de relevés bathymétriques répétés, entre 1998 et 2018, à des profondeurs allant de 10 à 1500 m. Cette rare série temporelle met en évidence une évolution morphologique rapide, répondant à son activité actuelle. Elle est marquée par l'alternance entre des périodes de remplissage et d'érosion à l'axe du canyon, et un dépôt continu de sédiments dans la tête du canyon au cours des 20 dernières années. Le creusement du talweg est induit par l'érosion liée au recul de knickpoints migrant régulièrement vers l'amont du système. Il est également démontré que l'obstruction partielle des chenaux, dans les méandres serrés, précède l'initiation de nouveaux knickpoints. L'étude, complétée par l'analyse de données de courantomètres ADCP, de séries temporelles météorologiques et de prélèvements sédimentaires, met en évidence le lien entre les conditions météorologiques et l’hydrodynamique du canyon. Pour la première fois dans le canyon de Capbreton, plusieurs forts courants de turbidités ont été enregistrés (1 m/s en vitesse locale et 6,5 m/s en vitesse de front estimée), des ondes quasi-inertielles ont été mises en évidence. Il est démontré que les tempêtes sont l'un des principaux facteurs déclenchant ces courants, qui tous deux contrôlent fortement la dynamique sédimentaire du canyon. La modélisation numérique nous permet de discuter de l'alimentation du canyon à partir des écoulements générés à sa tête. Les courant simulés indiquent qu'un apport de sédiments fins est nécessaire pour générer des écoulements le long du canyon. Les sédiments grossiers (sables) et leurs dépôts se déposent rapidement et ne progressent que par la récurrence des évènements

Les sinkholes géants abyssaux du Blake Bahama escarpement : relations entre structures, fluides et physiographie des reliefs carbonatés.

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The abyssal giant sinkholes of the Blake Bahama Escarpment: evidence of focused deep-ocean carbonate dissolution

International audienceThis study reports the discovery of abyssal giant depressions located at the toe of the Bahamian carbonate platform, along the Blake Bahama structurally-controlled Escarpment (BBE) that exhibits up to 4 km of submarine elevation above the San Salvador Abyssal Plain (SSAP). Analysis of seismic reflection and bathymetric data collected during the CARAMBAR 2 cruise revealed the presence of 29 submarine depressions; their water depths range from 4584 m to 4967 m whereas their negative reliefs are elliptical in shape, range in diameter from 255 m to 1819 m, and in depth from 30 m to 185 m. The depression alignment trends are parallel to the BBE as well as to structural lineaments of the area, exclusively between 2200 and 5000 m from its toe, and overlies a buried carbonate bench in which a high-amplitude seismic anomaly has been detected. The depression density interestingly increases where the recognized structural lineaments intersect the BBE. Based on their physical attributes (i.e. location, jagged morphologies, water depths), we interpret these depressions as collapse sinkholes rather than pockmarks or plunge pools. The aforementioned observations suggest an atypical relationship between the spatial occurrence of the giant abyssal sinkholes, the carbonate platform tectonic structures, the buried carbonate bench that underlies the hemipelagites in the SSAP and the geomorphology of the area. According to the wider literature that reports fluid seepages along submarine carbonate escarpments, we propose that the ground water entrance during low sea-level stands, the dissolution of evaporites by meteoric water, the platform-scale thermal convection and the seawater entrance at the platform edge most probably collectively act in concert to favor the circulation of brines and therefore the corrosion within the Bahamian carbonate platform. These mechanisms are particularly efficient along the structural heterogeneities (e.g. the Sunniland Fracture Zone, SFZ) which act as fluid conduits localizing the dissolution and control the physiography of the area by maintaining the location of the sedimentary pathways. The dense fluids would migrate along the faults towards the BBE free edge and are subsequently trapped into the buried carbonate bench that laterally disappears below the low-permeability deep-sea hemipelagites of the SSAP. In consequence, the trapped corrosive fluids dissolve the carbonates preferentially along the tectonic structures such as the SFZ. They are this way at the origin of the BBE curvature and generate collapse-structures in the overlying fine-grained deposits finally resulting in the formation of giant abyssal sinkholes. This structurally-directed process of dissolution seems efficient to provide a brines density head to move out down to >4.5 km of water depth and is believed to have played a major role in the BBE 5-6 km erosional retreat

Upstream migrating knickpoints and related sedimentary processes in a submarine canyon from a rare 20-year morphobathymetric time-lapse (Capbreton submarine canyon, Bay of Biscay, France)

The Capbreton submarine canyon is a striking feature of the south-east of the Bay of Biscay. This canyon forms a deep incision through the continental shelf and slope, and displays remarkable structures linked to its present-day hydrosedimentary activity. Its head has been disconnected from the Adour River since 1310 CE, but remains close enough to the coast to be supplied with sediment by longshore drift. Gravity processes in the canyon body are abundantly described and documented, but activity in the head and the fan of the canyon is poorly constrained. Furthermore, many questions remain regarding the details of processes affecting the head, the body and the fan of the Capbreton canyon. In this work, we address the paucity of documentation concerning (1) the temporal evolution of sediment transfer between the head and the deep reaches of the canyon, and (2) the interaction between gravity processes and the morphology of the canyon floor, including both shaping and feedback mechanisms. This study is based on the analysis and comparison of eight multibeam bathymetric surveys acquired in the upper part of the Capbreton canyon between 1998 and 2018, in depths ranging 10–320 m below sea level. This rare time series exposes the morphological evolution of this outstanding dynamic area over the last 20 years. Our work shows that much of the changes are located in the canyon floor and head. Following a period characterized by a unique flat floor thalweg, the canyon was affected by an incision with low lateral terraces which resulted in a narrow axial thalweg. The deepening of the narrow thalweg was induced by retrogressive erosion according to the presence of upstream-migrating knickpoints, while low elevation residual terraces formed as the canyon reached a local equilibrium profile. The flat thalweg observed in 1998 is likely a result of a partial filling of the canyon thalweg by a substantial emptying of the canyon head and significant mass transfer to the proximal part of the canyon. A flat floor thalweg was not observed again in the remaining of our time series (since 2010), suggesting a possible quieter working mode of the canyon. We also propose the first accurate volume quantification of sediment displacement on the canyon floor. Our findings underline the alternation of filling and erosive periods in the canyon axis and an unexpected continuous sediment deposition in the canyon head during the last 20 years