Bruno Savoye

Morphology and architecture of the present canyon and channel system of the Zaire deep-sea fan

Abstract The Zaire deep-sea fan is one of the largest fans in the world still affected by turbidite sedimentation along a unique active meandering turbidite channel. This active channel is fed by turbidity currents, which are generated at the Zaire River mouth, and flow via a deeply entrenched canyon across the shelf and the continental slope. Based on a detailed study of the morphology and architecture of the present Zaire Canyon/Channel, several main zones can be defined (the canyon, the upper-fan valley, the upper and the lower channel-levee system leading into distal lobes). They are characterised by different behaviours in terms of erosion, transport and sedimentation within the canyon/channel. An important characteristic of the Zaire Canyon/Channel is the deep incision of the thalweg, well below the regional sea floor along its whole path. The origin of this entrenchment may be linked to the incision of the Zaire Canyon back across the shelf during the last sea-level rise. This incision of the canyon has allowed the continuity of turbidite activity during the Holocene, in maintaining the connection between the canyon head and the river mouth (in contrast to most of other large deep-sea fans, which are generally inactive during highstands). The entrenchment of the Zaire Canyon/Channel limits the overflow of turbidity currents and the turbidite sedimentation over levees, and prevents avulsion along the upper part of the Zaire Channel. Most of the sediment transported in turbidity currents are probably led down to the lower channel-levee system and the distal lobes.

Direct observation of intense turbidity current activity in the Zaire submarine valley at 4000 m water depth

A large turbidity current was detected in the Zaire submarine valley at 4000 m water depth. Current meters, turbidimeter and sediment trap deployed on a mooring located in the channel axis, although they were damaged, recorded the signature of a very high energy event. An average velocity of more than 121 cm s−1 was measured 150 m above the channel floor. Coarse sand and plant debris were collected at 40 m height. The turbidity current clearly overflowed the edges of the valley as demonstrated by the large quantity of turbiditic material (464 mg organic carbon m−2 d−1) found in the sediment trap moored 13 km south from the channel axis.

Detailed seismic-reflection and sedimentary study of turbidite sediment waves on the Var Sedimentary Ridge (SE France): significance for sediment transport and deposition and for the mechanisms of sediment-wave construction

Abstract Sediment waves have been observed on the backside of levees in deep-sea turbidite systems where they are built by turbidity currents that spill out of channels and spread sediment over the levees. In an attempt to understand the way in which, sediment waves are initiated, prograde and/or aggrade, two sediment waves were selected within the present Var turbidite system (Ligurian Sea, north-western Mediterranean) for a detailed sedimentary study. The data include high-resolution seismic-reflection profiles, 3.5-kHz echo-sounder profile, high-resolution side-scan imagery and cores collected from the upstream flank, crest, and downstream flank of both sediment waves. Core-to-core correlations allow interpretation of the dynamic of gravity-flows across the sediment waves. The asymmetrical internal structure of the waves results from higher rates of sediment deposition on the upstream flank and lower rates of deposition and erosion on the downstream flank and further results from active progradation stages during periods of greater gravity-flow activity. The strong differences of depositional processes across the sediment waves result through time in the individualisation of large and interconnected sand bodies in the distal part of the levee.

Architecture of an active mud-rich turbidite system: The Zaire Fan (Congo-Angola margin southeast Atlantic): Results from ZaïAngo 1 and 2 cruises

Multichannel seismic data newly acquired during two ZaAngo surveys now provide an almost complete view of the Quaternary architecture of the Zaire Fan. Extending laterally from the southern Gabon margin to the Angola margin and longitudinally more than 800 km, the overall fan consists of three main individual fans that were deposited successively as overlapping depocenters. The individual fans are composed of channel/levee systems exhibiting similar seismic facies, external configurations, and organization to those described in other large mud-rich systems (e.g., the Amazon Fan). In particular, high-amplitude reflection units with a high oil-reservoir potential are recognized almost systematically as a basal sole for channel/levee systems. They possibly include true high-amplitude reflection packets related to avulsion processes below the avulsion points and coarse-grained basal levees related to the initial stages of levee aggradation subsequent to the avulsion. Correlations with Ocean Drilling Program Leg 175 Site 1077 indicate that the studied part of the Zaire Fan began to build in the late Pleistocene (780 ka). During the upper Quaternary, a great number of channel/levee systems (more than 80) were developed, possibly explained either by its permanent activity even during high sea level conditions or by the low Zaire River inputs. The frequent occurrence of channel entrenchment of either old or recent channels is another characteristic specific to the fan. Overdeepening of channels is probably partly caused by regressive erosion inside the parent channel in response to an avulsion and also in part because of other causes that are not fully understood.

Quaternary development of migrating sediment waves in the Var deep-sea fan: distribution, growth pattern, and implication for levee evolution

Abstract A field of sediment waves built by turbidity currents on the Var deep-sea fan has been studied using a large amount of seismic-reflection data and cores. To understand the spatial organisation and evolution of the sediment waves, maps of both surficial and an older subsurface wave field were prepared. Three different sediment wave geometries (symmetrical to asymmetrical cross-section) were recognised and can be linked to particular styles of evolution through time (from simple vertical aggradation to upslope and upcurrent progradation). Each geometry appears to be linked with: (i) a particular location on the Var Sedimentary Ridge; (ii) the type of turbidite deposits; (iii) the local gradient slope; and (iv) the height of the Ridge. Several factors control the nature and distribution of sediment waves and these factors do not change significantly through time, as suggested by the common geometries, evolution and distribution of the sediment waves between the present and fossil fields. Supercritical flow conditions and high sediment supply allow the development of well-developed and prograding sediment waves, while subcritical flow conditions and low sediment supply allow the formation of smaller and aggrading sediment waves. The impact of these two factors is also influenced by the morphology of the fan valley, the height of the Ridge, and the type of transported sediment. The evolution of sediment wave amplitude is also strongly influenced by the distance of sediment waves from the Ridge crest, while wavelength evolution is more influenced by slope gradient. The building of the Var Sedimentary Ridge and particularly the asymmetry and the overall morphology of the levee are directly controlled by the growth pattern of the sediment waves.

Twentieth century floods recorded in the deep Mediterranean sediments

Flood-generated turbidity currents represent an important process of marine sedimentation. However, no deposit related to this process has yet been described at sea. Turbiditic sequences cored at 2000 m water depth in the Mediterranean show tractive structures and superposition of reverse and normal grading. They are related to floods triggered during the twentieth century. Deposits from flood-generated turbidity currents have a great impact on the interpretation of deep-ocean paleoclimate records obtained near continental margins by linking deep-marine sedimentary records to continental climatic changes through flood frequency and magnitude. Implications of this study should help redefine the reservoir character of oil fields in fine-grained turbidites.

The Var submarine sedimentary system: understanding Holocene sediment delivery processes and their importance to the geological record

Abstract The Var system extends off Nice in the Western Mediterranean. It comprises a river, a delta and a submarine valley leading to a deep-sea fan that together have been in operation since the Early Pliocene. The Var system is an area experiencing active sediment transport, where at least three major types of sediment transfer process are identified: hyperpycnal turbid plumes, surge-like turbidity currents generated by shallow failures induced by excess pore pressure during river flood periods, and by large earthquake-triggered slides. The last two processes might generate higher-density turbidity currents, but at different return intervals. Hydrological data, direct observations of the sea floor, geotechnical testing and numerical modelling confirm the very high frequency of these sediment transfer events. Some of the processes have catastrophic surge behaviour, others are continuous during periods of river flooding. In the latter case, all the sediment supplied to the vicinity of the river mouth is transferred seaward without or with only brief periods of deposition. The geological record of such continuous activity remains difficult to identify. The palaeo-events identified in sedimentary series are often widespread, high-magnitude events with return periods close to a millennium, i.e. usually beyond historical records. Normal ‘background’ processes provide only thin deposits that are not interpretable in the geological record.

Processes of Sediment-Wave Construction Along the Present Zaire Deep-Sea Meandering Channel: Role of Meanders and Flow Stripping

ABSTRACT Detailed study of fine-grained migrating sediment waves and processes of sediment deposition along the modern, active, meandering channel of the Zaire turbidite system was made on recent cruises. The data used in this study include EM12 bathymetry and backscatter imagery, 3.5 kHz profiles, high-resolution seismic-reflection profiles and Kullenberg piston cores. Migrating sediment waves are built under several different flow energies and hydrodynamic conditions (subcritical or supercritical condition) according to their location along the modern Zaire channel. The process of sediment-wave initiation is inferred from cores and high-resolution seismic-reflection profiles. Overflow density and sediment supply delivered to levees appear as critical parameters for wave initiation. In other cases, a preexisting wavy topography is observed that acts as a template favoring sediment-wave initiation. Core-to-core correlation between the two flanks of a single sediment wave reveals no major differences in the lithofacies, suggesting no change of the dynamics of overflows during sediment deposition on both flanks of the waves. Comparison of number, thickness, and type of individual beds in cores collected on both flanks of the wave suggests that most lower-velocity and density overflows depositing sediments on the upstream flank bypass on the downstream flank. This process is probably related to a strong decrease then increase of the bed shear stress on the upstream and downstream flank of the wave, respectively. Interpretation of lithofacies suggests that construction of migrating sediment waves is closely linked to repeated successive spillovers from the head and body of a single channelized turbidity current. Construction of a single wave is rapid, less than 5000 years.

Multiple terraces within the deep incised Zaire Valley (ZaïAngo Project): are they confined levees?

Abstract Terraces have been frequently observed and described along turbidite valleys. Many interpretations have been aimed at determining the origin of these structures, including a tectonic origin, succession of infilling and incision processes, channel-wall slumps, or inner levee aggradation. The Zaire submarine valley presents a complex structure with multiple terraces bordering a deep incised meandering thalweg. The detailed analysis of the morphology, the seismic structure and the recent sedimentation (in cores) along the Zaire upper-fan valley show that terraces are inner levees confined within the incised valley. Many terraces correspond to the infilling of abandoned meanders, and aggrade by deposition of turbidite sequences due to current overflows. The major process affecting the initiation and the development of terraces inside the valley is the vertical incision of the thalweg, simultaneously with meander migration.

Geotechnical characteristics and instability of submarine slope sediments, the nice slope (N‐W Mediterranean Sea)

Abstract The slope off Nice (French Riviera) is characterized by the progradation of a large deltaic sedimentary body fed by the Var River. It principally consists of fine‐grained plio‐quaternary deposits interbedded with conglomerates. These deposits are cut by deep valleys and canyons and have been submitted to degradation through active mass‐wasting processes at water depth of 20–500 m. Several sediment facies are differentiated and classified by laboratory‐determined mass physical and geotechnical properties of nineteen gravity core samples. This classification is used in order to produce a geotechnical map of the surficial sediment of the area. The study of consolidation states of these sediments reveals a sedimentary hiatus appearing under Holocene deposits on the eastern wall of the major canyon (Var Canyon) between 900 and 1500 m water depth. This sedimentary hiatus, which extends for several kilometers, corresponds to an erosion surface rather than an omission surface according to the overconsoli...