Volcaniclastic sedimentation on the submarine slopes of a basaltic hotspot volcano: Piton de la Fournaise volcano (La Réunion Island, Indian Ocean)

International audienceVolcaniclastic successions are well-described in volcanic arc setting but rare in hotspot environments. The present work proposes a facies model of volcaniclastic sedimentation related to basaltic hotspot volcanoes as exemplified by the Piton de la Fournaise volcano (La Réunion Island). The facies model is based on a multi-scale approach combining high-resolution multibeam and backscatter data, deep-water photographs, side scan imagery and Kullenberg piston cores. Data show that a wide range of gravity flows and erosional features develop in the active volcaniclastic sedimentary system. Coastal and submarine instabilities are the main processes shaping the volcano's submarine morphology. Meanwhile, the evolution and dynamics of the sedimentary system are strongly linked with the morpho-structural evolution of the subaerial part of the volcano. The proposed model is characterized by a cyclic succession of stages: (1) a growing stage during which sedimentary activity is mainly restricted to proximal and mid-slope areas; (2) a collapse stage that entirely reshapes the morphology of the submarine slopes; and (3) an erosional stage related to a slow down of volcanic activity, enabling the development of large deep-sea fans

Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island

International audienceRecent oceanographic surveys revealed the existence of five volcaniclastic deep-sea fans off La Réunion Island. The Cilaos fan is a large volcaniclastic submarine fan, connected to rivers that episodically experience torrential floods through a narrow and steep shelf-slope system. New piston cores presented in this study together with echosounder profiles give new insight into the evolution of this extensive and sand-rich turbidite system. The Cilaos fan extends over 15,000 km2 on an abyssal plain and is compartmentalized by topographic highs. Located southwest of the island, the sedimentary system consists of a canyon area and a deep sea fan divided into a proximal and a distal fan. The proximal fan is characterized by its wide extent and coarse-grained turbidites. The distal fan is characterized by elongated structures and fine-grained turbidites. A detailed morphological study of the fan which includes the analysis of swath bathymetry, backscatter, echosounder, and piston core data shows that the Cilaos fan is a complex volcaniclastic deep-sea fan, highly influenced by preexisting seafloor irregularities. The canyons and the slope area show a complex and evolving sediment feeding system with a direct sediment input by the river and irregular sediment supply by submarine landslide. Three main construction stages are identified for this system: (1) an old incision phase of the channels forming wide turbidites extending over the entire distal fan; (2) a period of no or low activity characterized by a thick layer of hemipelagic mud; and (3) a local reactivation of the channel in the proximal fan. Each stage seems to be linked to a different sediment source with a progressively increasing contribution of hemipelagic sediment and mud in younger stages

Direct sediment transfer from land to deep-sea: Insights into shallow multibeam bathymetry at La Réunion Island

International audienceSubmarine canyon heads are key areas for understanding the triggering factors of gravity currents responsible for the transfer of detrital sediment to the deep basins. This contribution offers a detailed picture of canyon heads off La Réunion Island, with high-resolution multibeam bathymetry in the water depth range of 4-220 m. The present feeding of the Cilaos turbidite system, one of the largest modern volcaniclastic systems in the world, is deduced from morphological and sedimentological interpretations of newly acquired data. The study highlights small-scale sedimentary features indicating hydrodynamic and sedimentary processes. A direct connexion between the Saint-Etienne river mouth and submarine canyons is evidenced by the complete incision of the shelf and the presence of canyon heads connected to the modern deltaic bar. This direct connection, supplied by river torrential floods (cyclonic floods every two or three years), suggests the continuity of high-density fluvial flows to submarine gravity flows, forming hyperpycnal flows in the canyon. The initiation of secondary submarine gravity flows by storm waves (large austral waves and cyclonic waves) is also proposed for submarine canyons with large canyon heads developed in the surf zone from a sandy coastal bar. Bedforms in active canyon axis are considered as an indicator of the frequent activity of high-density turbidity currents. Moreover, a morphological record of last glacial and deglacial sea level variations is preserved, and particularly the Last Glacial Maximum sea level with the presence of small vertical cliffs, observed in this bathymetric data, which likely corresponds to a paleo-shoreline or paleo-reefs

Deep crustal faults and the origin and long-term flank stability of Mt. Etna : first results from the CIRCEE cruise (Oct. 2013)

The relation between deep crustal faults and the origin of Mount Etna, the largest and most active volcano in Europe has long been suspected due to its unusual geodynamic location. Results from a new marine geophysical survey offshore Eastern Sicily reveal the detailed geometry (location, length, dip and orientation) of a two-branched 200-km long, lithospheric scale fault system, long sought for as being the cause of Mount Etna. Using high-resolution bathymetry and seismic profiling, we image a 60-km long, previously unidentified, NW trending fault with evidence of recent displacement at the seafloor, offsetting Holocene sediments. This newly identified fault connects NE of Catania, to a known 40-km long, offshore-onshore fault system dissecting the southeastern flank of Mount Etna, generally interpreted as purely gravitational collapse structures.peer-reviewe

Full reciprocal-space mapping up to 2000 K under controlled atmosphere: the multipurpose QMAX furnace

We acknowledge the ESRF and the French Collaborating Research Group (F-CRG) for provision of synchrotron radiation facility beamtime. H. Song is thanked for his work on the preliminary thermomechanical calculations and the general sizing of the furnace. The building of the furnace has been realized under the technical supervision of D. de Barros. The experiments on the sapphire vicinal surfaces and on the IUCr zirconia powder were realized with the valuable help of C. Matringe (IRCER) and J. B. Marijon (PIMM), respectively. The authors are thankful to I. Cabodi and O. Bories (Saint-Gobain CREE) for the supply of the bulk-zirconia-based materials. M. Huger and F. Gouraud from the IRCER laboratory and T. O¨ rs and V. Michel from the PIMM labora-tory are strongly thanked for their involvement during these experiments. The oxidation experiments were realized in the frame of a research program funded by the CEA Paris–Saclay Centre in collaboration with R. Guillou, M. Lesaux, D. Menut and J. L. Bechade, who are also gratefully acknowledged.A furnace that covers the temperature range from room temperature up to 2000 K has been designed, built and implemented on the D2AM beamline at the ESRF. The QMAX furnace is devoted to the full exploration of the reciprocal hemispace located above the sample surface. It is well suited for symmetric and asymmetric 3D reciprocal space mapping. Owing to the hemispherical design of the furnace, 3D grazing-incidence small- and wide-angle scattering and diffraction measurements are possible. Inert and reactive experiments can be performed at atmospheric pressure under controlled gas flux. It is demonstrated that the QMAX furnace allows monitoring of structural phase transitions as well as microstructural evolution at the nanoscale, such as self-organization processes, crystal growth and strain relaxation. A time-resolved in situ oxidation experiment illustrates the capability to probe the high-temperature reactivity of materials.The QMAX furnace has been designed and built as part of the QMAX Project No. ANR-09-NANO-031 funded by the French National Research Agency (ANR). Experiments on the bulk-zirconia-based samples were done in the frame of the ASZTECH research program funded by the ANR (ANR-12-RMNP-0007)

Morphological analysis of the upper reaches of the Kukuy Canyon derived from shallow bathymetry

We present preliminary results on the morphology of the upper reaches of the Kukuy Canyon and Selenga shelf in front of Proval Bay (Lake Baikal), derived from newly acquired, high-resolution bathymetry. Numerous and varied erosional and transport features provide an interpretation framework for source to sink transfer and gravity flow processes in this shallow and active tectonic environment, suggesting on-going gravity instabilities and sediment-laden flows. Scarps in the canyon head are likely signatures of retrogressive incision of the western tributary and eastward lateral migration of the western tributary, the latter coming within about 1 km of the shoreline. Immature gullies incising the upper-slope feedings of the Kukuy Canyon indicate gravity flows with low erosional power. Large arcuate scarps on the break of the narrow shelf east of Proval Bay reveal gravity instabilities. The morphological connection between the Selenga Delta and the Kukuy Canyon suggests a direct pathway for fluvial sediment focused through breaches in the Sakhalin sand shoal, with likely occurrence of hyperpycnal flows into canyons heads during high sediment discharges. The neotectonic activity affects both the accommodation space around the prograding delta via earthquake-induced subsidence of coastal areas, and the location of incisions through slope instability triggering. Subsequent surveys allowing diachronic analysis would help determining the influence of tectonic and climatic factors controlling sediment transfer across the land-lake continuum and interpreting the morphological signature of the associated gravity processes shaping the delta and surrounding shelf and canyons

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

THE KRAMIS DEEP-SEA FAN OFF WESTERN ALGERIA: ROLE OF SEDIMENT WAVES IN TURBIDITIC LEVEE GROWTH

International audienceThe Kramis deep-sea fan extends over 45 km at the base of the western Algerian continental slope between 2000 and 2550 m water depth and covers an area of approximately 1200 km2. The Kramis Fan was initiated after Messinian time, evolved during the Plio- Quaternary, and, is still active, as proved by submarine cable breaks during the 1954 Orléansville earthquake. The Kramis Fan is fed by two perpendicular canyons: the Kramis Canyon and the Khadra Canyon, merging in a single E-W-oriented channel confined at the foot of the slope. It is strongly asymmetric with a super-developed levee on the right-hand side of the channel, the Kramis Ridge. Based on recent multibeam, side-scan sonar, and sediment core data (Maradja, 2003 and 2005, Prisma, 2004, and Prisme, 2007 cruises), we describe the morphology and internal structure of the fan and particularly the sediment ridge, showing marked lateral changes in the sediment-wave morphology and their association with a series of large scours in the intermediate part of the ridge aligned in the continuity of the Khadra Canyon direction. Overall, the Kramis Ridge is formed by turbidity currents overspilling the ridge crest, which is 100 m above the channel floor, with two exceptions. In the distal part of the ridge the subdued ridge-crest height probably causes continuous overspill, testified by sediment waves migrating parallel to the channel. The scours occur in the intermediate part of the ridge where the ridge height is only 50-60 m; scours are interpreted as the result of cyclic steps due to flow stripping of currents provided by the intersection of the Khadra Canyon with the Kramis Canyon and Channel system. The scours probably postdate the main growth of the Kramis Ridge and induce the local erosion of the ridge, which could correspond to a new channel initiation cutting the ridge. The superposition or the interaction of flows with different directions is responsible of the amplification of the size of the sediment waves with erosional downside flanks and their transformation in scours. The Kramis Fan provides a clear example of flow interaction to explain the presence of large sediment waves and scours on modern submarine fans

Sedimentary Architecture in Meanders of a Submarine Channel: Detailed Study of the Present Congo Turbidite Channel (Zaiango Project)

International audienceSinuous deep-water channels are recognized in most large deep-sea fans in the world. They present a particular interest to oil companies, since they are significant hydrocarbon reservoirs in deep offshore environments. The understanding of their geometries and their internal sedimentary architecture is necessary to better characterize reservoir heterogeneity of sinuous submarine channels. Therefore, numerous studies have been undertaken recently to better understand the behavior and sedimentary architecture of deep-water channels. The aim of this paper is to present our results concerning the development of the meandering channel of the present Congo turbidite system (or Zaire turbidite system). The study is based on high-resolution data including multibeam bathymetry, seismic lines, echosounder profiles, high-resolution side-scan sonar images, and gravity cores, collected by IFREMER along the submarine Congo channel between 1994 and 2000, during Guiness and ZaiAngo surveys. The present Congo turbidite channel is a long incised turbidite channel. It is presently active. It has been built gradually by progradation of the distal depositional area. The most distal part of the channel is the youngest part and shows an immature morphology: the channel presents a low incision and a low sinuosity. In contrast, the upper part of the channel has undergone a long evolutionary history. Its pathway is mature and complex, with numerous abandoned meanders visible in the morphology. This paper presents evidence of progressive channel migration and meander development of the Congo channel. It describes and explains the presence of terraces inside the channel. The detailed characterization of channel morphology and migration geometry shows that the evolution of the channel path is very similar to fluvial meandering systems with (1) lateral meander extension or growing, (2) downstream translation of the thalweg, and (3) meander cutoff. Seismic and 3.5 kHz echosounder profiles show that the terraces, which are visible in the seafloor morphology, are not the imprints of incisional processes. Terraces are true depositional units infilling the channel. They are built during and after the lateral migration of the channel. They are composed of (1) point-bar deposits and (2) inner-levee deposits aggrading above the point bar deposits. Point-bar deposits are characterized by low-angle oblique reflectors forming deposits with a sigmoidal shape. They seem very similar to those observed in fluvial systems. The similarity between fluvial and turbidite point bars suggests that the basal part of the turbidity currents flowing in this channel can be considered as very similar to river flow. With the high-resolution dataset collected in a present Congo turbidite channel, we provide a new description of the channel morphology and evolution, at a "reservoir" scale, intermediate between outcrop observations and 2D and 3D seismic data. The detailed interpretation of intrachannel sedimentation, associated with lateral channel migration, also provides new data for interpretation of flow dynamics in submarine meandering channels