Roger Griboulard

Multi-process generated sediment waves on the Landes Plateau (Bay of Biscay, North Atlantic)

Detailed analyses of recently collected bathymetric and sparker seismic data, support a new interpretation of the Landes Plateau field of sediment waves located on the Aquitaine upper continental slope (Bay of Biscay). The wave geometry, previously described as the result of a major sediment failure, is interpreted as a structure with a complex origin including the interaction of depositional and gravity deformation processes. Depositional processes are mainly recorded by the upslope migrating pattern of the waves resulting from oblique or sigmoid downlap reflections on the upslope flank of the waves and by toplapping and truncated reflections on the downslope flank. Hemipelagic and turbiditic sedimentation may be involved in the wave building as well as contouritic processes that could be related to the existing northward polar current and internal waves. Gravity deformations are syndepositional, discontinuous and of low amplitude, affecting thick layers which alternate with undeformed layers. They seem to correspond to gentle sediment creeping or stretching associated with minor listric or compaction-like faults, and possible limited back rotation of sediment blocks. These multi-process generated sediment waves could be rather common on the continental margins as they could have been mistaken with either depositional or deformational structures.

The origin and processes of mud volcanism: new insights from Trinidad

Abstract The mobilized sediments expelled by the mud volcanoes in Trinidad correspond to liquefied argillaceous and sandy material in which the solid fraction is systematically polygenic and originating from several formations (Cretaceous to Pliocene). The mud is notably rich in thingrained quartz that is angular and frequently mechanically damaged related to shearing at great depth, during the sedimentary burial, and/or hydraulic fracturing processes. The exotic clasts are mostly fractured fragments from various formations of the tectonic wedge (mostly Palaeocene to Miocene). The origin of the solid particles of the mud is polygenic, including deep Cretaceous-Palaeogene horizons close to the décollement, and various materials from the stratigraphic pile pierced by the mud conduits. Moreover, the fluids expelled by the mud volcanoes have a deep origin and notably the gas phase is thermogenic methane generated probably below a depth of 5000 m. The effusions occur either during cycles of moderate effusion of mud and fluids (quiescence regime), or during catastrophic events responsible for the expulsion of huge volumes of mud, clasts and fluids (transient regime). Available subsurface data suggest that the deep structure of the mud volcanoes includes: (1) a focused deep conduit at depth in the zone of overpressure; (2) a mud chamber intruding the surrounding formations around and above the top of the abnormal pressure zone; and (3) a superficial outlet leading to the surface vents.

Composition and spatial organization of a cold seep community on the South Barbados accretionary prism: Tectonic, geochemical and sedimentary context

Abstract Deep faunal assemblages were observed on cold seeps, between 1000 and 2000m depth, along the southern termination of the Barbados accretionary prism, during the Caracolante II and Diapicar cruises. Faunal composition and microdistribution of a cold seep community were analyzed at one site using deep-sea photographs and seismic (3.5 Kz) profiles. The community is dominated by large mussels up to 20cm long (possibly related to the genus Bathymodiolus ), tiny vesicomyid clams, and vestimentiferan clusters in which animals are up to 2m long. There is a high density of sponges (three species), bryozoans and gorgonians. Other taxa occasionally associated with this assemblage include galatheid crabs, anemones, holothurians and possibly pennatulaceans which are numerous at boundaries of the site. A typical deep-sea fauna of fish, asteroids and ophiuroids is also scattered about the vicinity. This seep community is distributed as a band of discontinuous colonies, 40–50m in width and more than 10km in length. Living colonies alternate with accumulations shells and bare areas. The substratum is hard mud covered by a ferrugineous crust and cemented by carbonates. The early diagenetic character of the bottom and the occurrence of biological assemblages indicate that seepages of sulphur and/or methane, which have been revealed along the decollement zone of the Barbados prism, must diffuse through the sea floor. These biological and geological features are located on the crests of NE-SW anticlinal ridges and mud volcanoes which have been built by diaprism.

Understanding continent‐ocean sediment transfer

Submarine canyons are narrow but deep submarine valleys that extend for hundreds of meters. They represent the most impressive structures that shape the present morphology of passive continental margins. They can occur off the mouth of rivers: the Tagus, Zaire, Amazon, and Orinoco in the Atlantic; the Indus in the Indian Ocean; and the Var, Rhone, and Ebro in the Mediterranean. Some are at times disconnected from any stream mouth such as the Nazare canyon, off Portugal, despite the fact that it is close to the coast. Some were connected to a river mouth during lowstands of sea level, such as the Wilmington canyon in the northwest Atlantic, or the Blackmud canyon in the northeast Atlantic.

Quaternary sandy deposits and canyons on the Venezuelan margin and south Barbados accretionary prism

Abstract The geomorphological and tectonic setting, distribution and origin of Late Quaternary sandy deposits on the southern termination of the Barbados accretionary prism and the adjacent abyssal plain have been investigated using Seabeam and 3.5 kHz echosounding data, and lithological and mineralogical analyses of 28 Kullenberg cores. Quaternary sands on this prism are deposited in piggyback basins. They are transported along three major canyons, the morphology and trend of which are highly variable and which are tectonically controlled. The narrow and relatively shallow canyon sections are in some ways like the major distributary channels of deep sea fans, but they are true canyons in terms of the sedimentary processes taking place, namely active erosion associated with the continuous uplift of tectonic structures and deposition of only slightly elevated sediment levees. The sand is partly trapped on basin floors as flat sediment sheets composed of thinly and thickly bedded fine grained sandy turbidites. High deposition rates occurred during the last period of low sea levels. The sands originated from the Orinoco-Trinidad shelf, as inferred from geographical considerations and clay and heavy mineral provenance data (predominently kaolinite, illite, hornblende and epidote). Sands of the abyssal plain settled on a deep sea fan system that trends north-south, parallel to the toe of the prism. Most of the sediments are introduced at the southern end of the system where they are delivered by the three major canyons which merge, on the fan, in a northward braided channel system. This material consists of sandy turbidites that originated from the Orinoco-Trinidad margin. The grain size of the sands is similar to or coarser than the grain size of the prism sands. Sand components are identical and preclude supply from the Amazon river. The sedimentation rate, during periods of low sea level, seems to be lower. A significant percentage of smectite in the clay fraction suggests that a minor part of the material may be derived from the Demerara margin, located closer to the main smectite source, i.e. the Amazon delta. These results emphasize the difference between the sand distribution on active and passive margins.

Le canyon de Capbreton : nouvelles approches morphostructurales et morphosédimentaires. Premiers résultats de la campagne Itsas

A recent cruise of the Suroit using new technologies for acquisition and processing of acoustic data (very high-resolution seismic and swath mapping) associated to an accurate satellite positioning, allowed producing a new bathymetric map of the upper part of the canyon. It shows the meandering shape of the canyon and superposed polyphase terrace. Such a morphology is due to activity of gravity flows that occurred as soon as the Upper Eocene. Very high-resolution seismic data allowed to confirm the influence of old tectonic structures on the location and the general shape of the canyon and to show that neotectonics that affects Plio-Quaternary deposits corresponds to recurrent activity of old tectonic structures.

The Orinoco turbidite system: Tectonic controls on sea-floor morphology and sedimentation

Because of its location in an active margin context, the sand-rich Orinoco turbidite system is controlled morphologically and tectonically by the compressional structures of the Barbados prism, and as a consequence, the sedimentation system does not exhibit a classic fan geometry. The sea-floor geometry between the slope of the front of the Barbados prism and the slope of the Guyana margin induces the convergence of the turbidite channels toward the abyssal plain at the front of the Barbados accretionary prism. Also, whereas in most passive margins the turbidite systems are commonly organized upstream to downstream as canyon, then channel levee, then lobes, here, because of the control by active tectonics, the sedimentary system is organized as channel levee, then canyons, then channelized lobes. In shallow water, landward of the prism, the system has multiple sources with several distributaries, and progressively downward, the channel courses are more complex with frequent convergences or divergences that are emphasized by the effects of the undulating sea-floor morphologies. Erosional processes are almost absent in the upper part of the turbidite system shallower than 1500 m (4921 ft). Erosion along channels develops mostly between 2000 and 4000 m (6562 and 13,123 ft) of water depth, above the compressional structures of the Barbados prism. Incisions show irregular meandering and sinuous courses in the low-relief segments and less sinuous courses where channels incise the structures. Larger incisions (canyons) are 3 km (1.9 mi) wide and 300 m (984 ft) deep. The occurrence of different phases of successive incisions is responsible for the development of morphologically correlative terraces in both flanks of the canyons. This might be the consequence of two mechanisms: the tectonic activity of the deformation front characterized by progressive uplift and thrusting of recent sediments, and the superimposition of the cycles of the Orinoco turbidite system. Piston-core surveys have demonstrated that turbidite sediments moving through the channel and canyon system and deposited in the abyssal plain are mostly coarse sandy deposits covered by recent pelagic planktonic-rich sedimentation, which indicates that sand deposition slowed down during the postglacial sea level rise.

Clay diapiric structures within the strike-slip margin of the southern leg of the Barbados prism

Abstract The study area is situated 200 km to the east of Trinidad; in the southern sector of the Barbados accretionary wedge. Analysis of seismic profiles and Seabeam bathymetric data, permit the identification of an important dextral wrench fault system oriented N95°E which induced morphostructural elements, characteristic of this type of kinematics in a thick sedimentary cover, separated from its basement by a decollement level. This decollement level is due to the presence of Miocene clay beds, supersaturated with water which under the effect of stresses, trigger clay diapiric processes in this transtensional sector. We shall demonstrate that the boundary between the Caribbean Plate and the South American plate continues as far as 250 km to the east of Trinidad.

Tectonic control on sedimentary processes in the southern termination of the Barbados Prism

Abstract The purpose of this paper is to evaluate the tectonic control on the origin, nature, distribution and reworking of sediments on an active margin. The study is based on Kullenberg cores, 3.5-kHz seismic profiles and Side-scan Sonar imagery collected on the South Barbados Accretionary Prism, together with observations from submersibles. The most abundant surficial sediment in this area was derived from the South American continent and from hemipelagic particles. This sediment is transported in great quantity through canyons that exemplify the regional tectonic trend and is trapped in basins located between the active accretionary ridges and the other elevated structures, as shown by on-lap geometry and progressive discordances. A second type of sediment consists of clayey diapiric material that originates from Miocene beds. This material is extruded to the surface through mud volcanoes, mud domes and diapiric ridges. The vertical migration of clays was partly initiated by tectonic deformation and guided by major faults. The supply of this diapiric material to the sedimentation in adjacent basins is quite substantial. The last type of sediment consists of diagenetic crusts formed on the sea-floor along faults, where fluids are expelled from the prism. Patterns of reworked sediments are strongly controlled by recent tectonic deformation. Sedimentary features such as bulges, sliding masses, continuous or discontinuous scarps and other erosional lineations are directed parallel to the active tectonic trends (N10–30 °, N50 °, N100–110 °, N140 °). Sliding and other gravity flows are triggered by tectonic activity, mainly where accretion and shear structures interplay and where the ascent of diapiric structures is responsible for steep slopes. The breakup of diagenetic crusts is caused by faulting. Erosive cascading deep currents are enhanced by the high relief afforded by an active ridge.

Chapter 14 Review of the tectonic controls and sedimentary patterns in late neogene piggyback basins on the barbados ridge complex

Abstract A review of the tectonic control and sedimentary patterns of the late Neogene piggyback basins of the Barbados Ridge Complex is proposed, mainly based on the analysis of seismic reflection profiles and sidescan sonar images. The geometrical relationships between the thrust faults and the syntectonic deposits show that the frontal piggyback basins are controlled by active thrusting. Deformations, with distinct scales, control their evolution: (1) a rapid tilting, strictly localized at the back limb of anticlines, and attributed to migration of active axial surfaces during fault-bend fold propagation; (2) a complex activation of the major thrust system, at a scale of a few kilometers: blind thrusts corresponding to frontal propagation develop seaward whereas motion along thrusts occurs backward. At about 50 km back of the deformation front, abundant muddy material raises up through pre-existing faults and disturbs arcward piggyback basins. The superimposition of the diapiric structures upon deformations linked to tectonic accretion (development of backthrusts and reactivating of forward verging thrusts) and inherited oceanic basement ridges leads to the individualized development of sub-basins bounded by steep topographic features. Most of the sediments of the South Barbados piggyback basins originate from the South American continent and are massively transported to the abyssal plain through canyons. Their course is driven by the main regional structures and their morphology reflects the tectonic activity of the features where they run. Oceanic ridges, by damming and collecting turbidite material control the thickness of sediments added to the complex and then the depth of the decollement and size and filling of the piggyback basins. Tectonics, by generating routes along the faults and excess fluid pressures control the main location and importance of mud diapirs and authigenic deposits. These stiff carbonate crusts preferentially develop on diapiric domes or on the back limb of anticlines up to the edge of supra-prism basins. Clayey diapiric material may be found within the basins where they form important mud flows as well as sliding masses. Tectonic and diapiric structures control gravity reworking processes, whereas structural relieves locally disturb bottom currents and hence control some erosion processes. The development of the piggyback basins of the BRC is closely linked to the evolution of a thrust wedge. The formation of frontal basins is mainly controlled by a forward-verging thrust system that forms a brittle wedge, whereas the development of the arcward basins is mainly controlled by subcretion of deep muds that induces mud diapirism, ductile deformation in the lowermost part of the wedge, a regional gentle slope, and the occurrence of both backward and forward verging thrusts. The evolution of the piggyback basins of the BRC also reflects the north to south changes in width and thickness of the wedge which are mainly related to variations of the increase of the sediment supply. Piggyback basins then evolve from minor depressions filled with very poor sediments in the north to about 10 km long overfilled basins in the south of the complex where abundant sedimentation occurs.