E. Gonthier

Contourite drift molded by deep Mediterranean outflow

Since its inception in early Pliocene time, the deep Mediterranean outflow has eroded and sculptured the southern Iberian margin. The 50-km-long, 300-m-thick Faro Drift, constructed by this powerful bottom current, has been the subject of detailed study. Sandy, silty, and muddy contourite facies can be clearly characterized, and a distinctive vertical arrangement or “sequence” of these facies can be related to hydrodynamic fluctuation. These results are important for the identification of ancient contourites, the reconstruction of paleocirculation patterns, and documentation of the effects of Quaternary climates on the Mediterranean outflow.

Facies distribution and textural variation in Faro Drift contourites: Velocity fluctuation and drift growth

Abstract Detailed study of seismic profiles, piston cores and bottom photographs from the Faro Drift on the southern margin of Portugal has led to a better understanding of drift development and its relationship to bottom current circulation. Data on the contourite facies characteristics and the surface microphysiography have been published elsewhere; here, we concentrate on sediment distribution and geometry. Longitudinal trends in facies types, mean grain size, sedimentary structures and composition can be interpreted in terms of relative intensity of currents over different parts of the Drift. These are generally more intense in the marginal channels and at the upstream or eastern end of the drift. Three different scales of vertical variation of facies can be identified. At the large scale, 300–500 m of sediment has accumulated over 4–6 Ma in a regular vertical succession due to the northward progradation of the Drift. At the medium scale, the upper 20–30 m of sediment shows alternating phases of active lateral progradation and uniform vertical accumulation that may correlate with episodes of more and less current activity related to high and low sea-level stands respectively over the past 0.3 Ma. At the small scale, the topmost 2–3 m of sediment deposited in approximately 0.03 Ma shows three zones of coarser-grained sediments separated by finer-grained contourites. This sequence can also be interpreted in terms of long-term fluctuation in bottom current activity. Although the signal is clearly complex, this kind of analysis of sedimentary drifts can lead to more accurate reconstruction of paleocirculation patterns.

Submersible study of mud volcanoes seaward of the Barbados accretionary wedge: sedimentology, structure and rheology

Abstract In 1992, the Nautile went to a mud volcano field located east of the Barbados accretionary wedge near 13 ° 50N. Using nannofossil analysis on cores, we determined the sedimentation rate, and provided a new estimation of the age of the mud volcanoes (750,000 years for the oldest one). Six structures have been explored with the submersible Nautile, and manifestations of fluid venting (chimneys, carbonate cementation and chemosynthetic communities) were observed on all. Sedimentological analysis identifies two sources of diapiric mud. Most mud volcanoes expel mud containing Late Miocene to Quaternary faunae that have the same composition as sediments drilled above the Barbados wedge decollement. One volcano also contains older Oligocene taxa, with a mud composition corresponding to the sedimentary sequence below the decollement. We use diving observations to map the fine-scale morphology, the distribution of chemosynthetic fauna and define two end-member types of structures: mud-pies (flat topped mud volcanoes) and conical mounds. Mud-pies (Atalante and Cyclops) are characterised by the presence of a lake of highporosity mud (70% to 75%) in their central parts. Chemosynthetic benthic communities ( Calyptogena colonies and sponge bushes) are concentrated in the outer parts. Contrasting morphologies of the two mud-pies indicate different stages of activity: Cyclops is growing whereas Atalante is collapsing. Expulsion of water and methane occurs mostly through the mud lake and may be stronger during the collapse phase. On conical mounds there are no mud lakes, fluid venting concentrates near the summit and occurs through carbonate cemented chimneys which form within the sediment. Viscosity measurements have been carried out on mud samples from the two mud-pies and one conical mound. All mud samples have a plastic fluid behaviour, the plastic threshold decreases with porosity, and thixotropy is observed for a porosity of more than 70%. An analogue experiment shows that for this thixotropic mud, shearing in the feeding conduit liquefies the mud which then spreads to form a mud-pie. Conical mounds form when the mud remains plastic. We show that the dissociation of methane hydrate is the cause of the high porosity in mud-pies and confirm that these structures are a consequence of large-scale dissociation of methane hydrate at the base of its stability field. Dissociation of hydrates before and during ascent is only slightly contributing to the pore fluid in conical mounds, but solid hydrates still present in the mud may contribute to its buoyancy.

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.

Sediment flux patterns in the northeastern Atlantic: variability since the last interglacial

Abstract The distribution of terrigenous and carbonate absolute accumulation rates in the northeastern Atlantic during the last climatic cycle shows that climate and sediment accumulation are closely coupled, either directly or through oceanic circulation and eustatic variation. The strong contrast between the glacial and Holocene flux distribution is related to oceanic circulation changes: either surface circulation which governs both the ice-rafting supply and the planktonic biogenic production, or, more particularly, deep circulations via the reinitiation of the Iceland-Scotland overflow. Glacial erosion and the low sea-level stand enhance the terrigenous supply, but the Holocene deep circulation reinforcement leads to important material reworking. As a consequence, an almost constant average sediment flux is obtained (4g· −2 ·ky −1 during the Holocene, compared to 4.7 g·cm −2 ·ky −1 during the glacial period). During the Holocene period, the notable increase in mean carbonate flux (from 1.5 to 3 g·cm −2 ·ky −1 ) is associated with carbonate biogenic production, but the carbonate flux distribution shows the influence of redistributive agents. On margins, fluxes do not simply decrease from continental sources to the oceanic basin. Eustatic drops leads to a shift in the areas of preferential accumulation from the upper to the lower slope. During the deglacial warming period, the fluxes are respectively an average of two and three times higher than during the Holocene and the glacial periods. Their importance highlights the major impact of rapid climatic changes. Finally, local conditions — the location of terrigenous sources and the presence of contour currents — are among the many factors which will modify the trends observed on a global scale.

Faro-Albufeira drift complex, northern Gulf of Cadiz

Abstract The northern margin of the Gulf of Cadiz is swept by Mediterranean Outflow Water between about 500 and 1000 m water depth. This warm, saline, thermohaline, bottom current attains velocities in excess of 1 m s-1 through the narrow and relatively shallow Gibraltar gateway, and then descends and slows as it moves towards the north and west around the Iberian margin. It was established in its present form in the latest Miocene, following tectonic re-opening of the Gibraltar gateway, and has since helped to sculpt the slope region in conjunction with downslope processes and diapiric intrusion. The principal area of contourite deposition, up to 600 m in thickness, is the Faro-Albufeira drift complex in a mid-slope setting some 30 km south of Faro. This comprises an elongate low-mounded drift (Faro-Albufeira) and adjacent broad sheeted drifts (Faro and Bartolomeu Dias Planaltos), flanked and partly dissected by deep, erosional, bottom-current channels and buried channels. The seismic character is one of progradational-aggradational depositional units with laterally extensive sub-parallel reflectors, widespread discontinuities and a large-scale cyclicity in seismic facies. The upper 10 m of cored section comprises muddy, silty and sandy contourites of mixed terrigenous and biogenic composition, that show small-scale cyclicity in grain size and associated sedimentary features. Rates of accumulation varied from < 1 to 14.5 cm ka-1 (cores), and 3.5 to 29.5 cm ka-1 (seismics). The large and small-scale cyclicity noted can be related to fluctuation in bottom current velocity related to climate and sea-level changes, although the precise correlation between these events remains uncertain.

Mediterranean outflow through the Strait of Gibraltar since 18,000 Years B.P.: Mineralogical and geochemical arguments

Data obtained from mineralogical and geochemical analyses of piston and gravity cores, recovered from an area off Lisbon (Portugal) to the Alboran Sea (Mediterranean), serve as a basis for better understanding the past 18,000 years of hydrological exchanges at Gibraltar. Tracers used in this study are smectite, kaolinite, Ta, Th, La. One of the primary sources of particles both into and out of the Mediterranean is the Guadalquivir River. These particles are transported back into the Atlantic in the Mediterranean outflow water, and deposited along the Iberian slope. No evidence for reversal of this outflow current was found in those cores, since 18,000 years B.P.

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.

The Sao Tomé deep-sea turbidite system (southern Brazil Basin): Cenozoic seismic stratigraphy and sedimentary processes

The Sao Tom deep-sea turbidite system, elongated parallel to the rise of the south Brazilian continental margin, was first interpreted as a channel-levee system resulting from contour-current activity. Study of new seismic data permits the proposal of a stratigraphy for the system and a new interpretation of depositional processes. Three major depositional units have been recognized that are separated by major erosive discontinuities. The basal unit seems to be Paleocene to lower or middle Eocene, and the second one, subdivided into two subunits, is probably upper Oligocene to middle Miocene. Both units show superimposed north-to-southchannelized turbidite systems, with supply provided directly from a channel network that crosses the upper margin in the north. The third unit is upper Miocene(?) to Pliocene or Quaternary and is still under predominantly gravity processes: turbidite processes in the lower and upper subunits, and major mass-flow processes in the median subunit. The sediment sources are located either in the north or in the south, with sediment provided by major deep-sea channels. The base of the upper subunit is well marked by an erosive discontinuity (late Pliocene or PlioceneQuaternary boundary). Impact of the contour currents is mainly recorded as widespread erosive surfaces (seismic discontinuities) correlated to global hydrological events and transparent or wavy deposits. Because this system contains a significant amount of upper Quaternary sands, it suggests the occurrence of petroleum reservoirs along the rise and the Sao Paulo Plateau in the lower continental slope.

Canyon morphology on a modern carbonate slope of the Bahamas: Evidence of regional tectonic tilting

New high-quality multibeam data presented here depict the northern slope of the Little Bahama Bank (Bahamas). The survey reveals the details of large- and small-scale morphologies that look like siliciclastic systems at a smaller scale, including large-scale slope failure scars and canyon morphologies, previously interpreted as gullies and creep lobes. The slope exhibits mature turbidite systems built by mass-fl ow events and turbidity currents. The sediment transport processes are probably more complex than expected. Slope failures show sinuous head scarps with various sizes, and most of the scars are fi lled with recent sediment. Canyons have amphitheater-shaped heads resulting from coalescing slump scars, and are fl oored by terraces that are interpreted as slump deposits. Canyons rapidly open on a short channel and a depositional fan-shaped lobe. The entire system extends for ~40 km. The development of these small turbidite systems, similar to siliciclastic systems, is due to the lack of cementation related to alongshore current energy forcing the transport of fi ne particles and fl ow differentiation. Detailed analyses of bathymetric data show that the canyon and failurescar morphology and geometry vary following a west-east trend along the bank slope. The changing parameters are canyon length and width, depth of incision, and canyon and channel sinuosity. Accordingly, failure scars are larger and deeper eastward. These observations are consistent with a westward tectonic tilt of the bank during the Cenozoic.