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Patterns and processes of sediment dispersal on the continental slope off Nice, SE France
Marine Geology 162 Ž2000. 405–422 www.elsevier.nlrlocatermargeo
Patterns and processes of sediment dispersal on the continental slope off Nice, SE France Ingo Klaucke ) , Bruno Savoye, Pierre Cochonat IFREMER, DRO-GM, Laboratoire EnÕironnements Sedimentaires B.P. 70, 29280 Plouzane, ´ ´ France Received 30 September 1998; accepted 11 May 1999
Abstract The distribution of surficial sediments and sediment dispersion patterns on the steep continental slope off Nice ŽSE France. have been studied using side-scan sonar, 3.5 kHz profiles, short piston cores and bottom photographs. The input of terrigenous material to the Baie des Anges, a submarine embayment bounded by two prominent ridges, is dominated by fluvial input from the Var River, the Paillon River being only a minor source. The Var River provides very coarse bedload material Žgravel and cobble. directly to the head of the Var Canyon. Gravel and cobble deposits are found all along the Var Canyon and the Upper Fan Valley of the Var submarine fan and have been shaped into gravel waves. The fine particles Žsuspension load. are separated from the coarse bedload upon entering the sea and form up to 60-m thick deposits on the uppermost continental slope of the Baie des Anges. These deposits are formed by settling out of sediment plumes. The presence of silt and fine sand laminae that decrease in thickness and frequency away from the Var River mouth indicate the influence of meso- and hyperpycnal flows on these plume deposits. Areas outside the Baie des Anges are not connected to major fluvial input and receive only hemipelagic sediments. These primary deposits are highly unstable and sediment failure due to seismic loading, sedimentary loading or undercutting is frequent. Sediment failure produces secondary sediment gravity flows that export most of the material to the basin, but also produce turbidity-current over-spill deposits on ridges bounding the slope canyons and on terraces within the Var Canyon. Slump and debris-flow deposits are also observed. At least some of these secondary flows erode the continental slope as cross-cutting chutes on the upper continental slope and erosional scours in the Upper Fan Valley demonstrate. Modern sediment dispersal patterns on the continental slope off Nice are proposed as a modern analogue to lowstand conditions on continental margins. In fact, the absence of a continental shelf together with a steep slope strongly reduces the influence of sealevel on the physiography of the margin. q 2000 Elsevier Science B.V. All rights reserved. Keywords: continental slope; sediment dispersion; fluvial input; sediment plumes; slope failure; sediment-gravity flows
1. Introduction
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Corresponding author. Southampton Oceanography Centre, Challenger Division for Seafloor Processes, Empress Dock, Southampton, SO14 3TA UK. fax: q44-1703-596554; E-mail: [email protected]
Our knowledge of the processes, morphology, structure and evolution of deep-sea clastic deposits advanced considerably since Kuenen Ž1950. first suggested turbidity currents at continental margins as
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the possible process for their deposition. Most of the studies of deep-sea clastic systems have, however, concentrated on the marine environment and looked only at what happens within the ocean. As a consequence, the mechanisms and patterns of sediment transfer from the continent to the deep-sea are still rather poorly understood. For a long time, stepwise sediment transfer with intermediate storage of sediment in deltas and on the continental shelf has been favoured ŽNormark and Piper, 1991.. In contrast to lacustrine environments, fluvial input Žexcept for exceptionally high sediment loads. was considered not dense enough to form underflows Žhyperpycnal flows. that could transport material directly onto the continental slope and into the deep-sea Že.g., Gilbert, 1983; Syvitski et al., 1987.. Instead, fluvial sediments had first to be deposited in deltas or on prodelta slopes before they could be remobilised by sediment failure and transferred to the deep-sea ŽEmbley and Jacobi, 1986.. Recent quantitative studies of the sediment and fluid discharge of many rivers world-wide has shown that a variety of them, although not the largest rivers, are capable of forming hyperpycnal flows ŽMulder and Syvitski, 1995.. Direct transfer of sediment from the continent to the deep-sea is therefore possible in a number of circumstances. Concerning the patterns of sediment transfer, only few studies exist because modern conditions of high sealevel resulted in completely different settings with respect to lowstand conditions when the shoreline was close to the shelf-edge and fluvial sediments were delivered directly to the continental slope. Recent turbidity-current activity that could be related directly to fluvial input has been inferred for the Zaire submarine fan ŽHeezen et al., 1964; Droz et al., 1996. and observed on the Yellow River delta ŽWright et al., 1988., but remain isolated cases. The Var submarine fan in the Ligurian Basin, NW Mediterranean ŽFig. 1. also shows signs of recent mass wasting activity ŽGennesseaux et al., 1971, 1980; Auffret et al., 1982; Mulder et al., 1996..
Stepwise and continuous mass transfers have been identified. Stepwise transfer of sediment is evidenced by the observation of mass wasting processes on the continental slope ŽAuffret et al., 1982; Klaucke and Cochonat, 1999., while large flooding events of the Var River should be capable of forming frequent hyperpycnal flows ŽMulder et al., 1998.. In addition, the Var submarine fan may be an appropriate analogue for continental margins active at times of lowered sealevel, because the absence of a continental shelf ŽPautot, 1981. allows a direct link between the continent and the deep-sea. A whole range of acoustic data of the seafloor and near sub-bottom complemented by ground-truthing has been used to study the patterns of sediment dispersion and transfer to the deep-sea on the continental shelf off Nice. This paper presents the results of the detailed investigation of seafloor morphology and surficial sediment distribution on the continental slope off Nice, and attempts to relate these to the interplay between sediment deposition and erosion.
2. Datasets and methods A large range of acoustic data exist from the study area including acoustic imagery of multibeam bathymetry systems ŽSimrad EM12d and EM1000., deep-towed side-scan sonar and 3.5 kHz profiles ŽSAR, collected in 1986., as well as conventional airgun seismic. All these instruments use different frequencies resulting in different resolutions and subbottom penetration. The Simrad EM12d uses 12–13 kHz and serves as a regional reconnaissance tool. EMl2d-data from the continental slope only distinguish between high backscatter reflectivity underlining the canyon floors and low backscatter reflectivity for all other areas ŽSavoye et al., 1997.. The EMl000 works at very high frequency Ž95 kHz. which restricts its use to less than 1000 m water depth.
Fig. 1. ŽA. Overview map of the Var submarine fan system in the Ligurian Sea. ŽB. Close-up of the continental slope off Nice showing the location of profiles mentioned in the text. P s Paillon River.
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Acoustic backscatter of the EM1000 did not provide information on the surficial sediments due to the extremely dissected nature of the upper continental slope, i.e., the response of the tool is dominated by seafloor topography. As a consequence, the detailed geomorphologic and sedimentologic analysis of the seafloor off Nice is based essentially on SAR ŽSysteme Acoustique ` Remorque´. side-scan sonar imagery that covers a total of about 1300 km2 of which the portion covering the continental slope is presented in Fig. 2, and on deep-towed 3.5 kHz profiles. The SAR works at a frequency of 200 kHz and is towed at a depth of 100–150 m above the seafloor. For this reason it is operationally necessary to follow topographic depressions ŽFig. 2.. The side-scan sonar data have been compiled into a mosaic despite numerous uncertainties concerning the position of the tow-fish ŽFig. 2.. The mosaic has then been interpreted together with 3.5 kHz profiles, numerous short piston cores and submersible observations. Extrapolation of depositional facies from 3.5 kHz profiles has been favoured, as direct calibration of side-scan sonar data is generally not possible, while interpretation of 3.5 kHz echo-facies is a well established concept Že.g., Damuth, 1980.. In order to produce a geomorphic– sedimentologic map ŽFig. 3. from the interpretation of the side-scan mosaic, bathymetric information from several data-sets of multibeam bathymetry ŽSeaBeam, EM12d, EM1000. has been used to correct and readjust the location of the different deposits and features.
3. Seafloor morphology The continental margin off Nice is characterised by an absent or very narrow continental shelf ŽFig. 1.. Multibeam bathymetry shows that the continental slope is very steep with average gradients of 118 that may reach 278 along the side-walls of the canyons ŽMonti and Carre, ´ 1979; Pautot, 1981.. The part of the continental slope studied here comprises the Baie des Anges and the Cap Ferrat Slope to the east of it. The Baie des Anges is a large submarine embayment bound by the Cap d’Antibes Ridge in the west and the Cap Ferrat Ridge in the east ŽFig. 1.
3.1. The Baie des Anges The Baie des Anges comprises two major submarine canyons that are highly erosive and coalesce in 1650 m water depth: the Var Canyon in the west and the Paillon Canyon in the east. These canyons are erosive, as several km of shoreline retreat ŽFig. 1. and the formation of a deep thalweg in the upper Paillon Canyon ŽFig. 4. indicate. The Var Canyon is connected directly to the Var River and is slightly sinuous in its upper reaches. Flat-floored, it has a relief height between 150 and 500 m, while channel width ranges between 300 and 1250 m. Channel width is locally restricted by the formation of terraces on the inner side of meander bends ŽFigs. 1 and 3.. These terraces are highly irregular in the upper canyon reaches above 1000 m water depth, but they become more regular and flat-topped down-canyon ŽFig. 5., where they reach heights of up to 50 m above the canyon floor. The eastern flank of the Cap d’Antibes Ridge also shows evidence for raised terraces of the Var Canyon that now lie at mid-height of the Cap d’Antibes Ridge, i.e., 250 m above the present canyon floor ŽFigs. 1 and 3.. The lower reaches of the Var Canyon are covered with gravel waves having wavelengths of 30–40 m and heights of a few metres ŽFig. 6; Malinverno et al., 1988.. Further downchannel these gravel waves give way to regularly spaced, pear-shaped scours that have a sharp upper boundary and a diffuse lower end ŽFig. 7.. These scours are up to 20 m deep and may cover surfaces of a few square kilometres. Spacing and depth of the erosional scours gradually decrease downchannel. The Paillon Canyon on the eastern side of the Baie des Anges has direct connection to the Paillon River and a V-shaped cross-section in its upper reaches ŽFig. 4. that becomes U-shaped down-channel ŽFig. 1.. The connection between the Paillon Canyon and the Paillon River, however, is not as clear as in the case of the Var. Width of the Paillon Canyon increases gradually along the channel to reach 1500 m while channel depth increases from 150 m in 800 m water depth to 500 m at the confluence with the south-western canyon ŽFig. 1.. Gravel waves similar to those in the Var Canyon are present near the confluence with the latter ŽFig. 6..
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Fig. 2. SAR side-scan sonar mosaic of the Baie des Anges and adjacent areas. The mosaic is displayed in normal polarity, i.e., high backscatter areas are dark. Note the orientation of the profiles along the canyons and a slight discrepancy between the imagery and underlain bathymetry. Contour lines shown are at 200 m interval.
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Fig. 3. Synthetic geomorphologic and sedimentologic map of the continental slope off Nice. Black dots indicate the location of piston cores.
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Fig. 4. SAR side-scan sonar image and 3.5 kHz profile of the upper Paillon Canyon showing highly gullied canyon walls and a deeply entrenched thalweg. Profile location is given in Fig. 1.
Fig. 5. SAR side-scan sonar image and 3.5 kHz profile showing a flat-topped terrace on the floor of the Var Canyon. The apparent inclination of the terrace is due to oblique orientation of the profile with respect to channel axis. Note the continuity of the gravel-wave surface below the terrace. For profile location see Fig. 1.
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confluence with the Var and Paillon Canyons. The south-western canyon is separated from the Var Canyon by the sharp-crested Central Ridge that is highly gullied, as are the Cap d’Antibes and Cap Ferrat Ridges ŽFigs. 1, 2 and 8.. This contrasts with the ridge separating the Central Valley from the north-eastern valley ŽFig. 1.. This ridge is much broader and presents some larger chutes that are not perpendicular to the ridge crest, but more or less parallel to it. 3.2. The Cap Ferrat Slope The continental slope east of the Cap Ferrat Ridge shows two broad valleys stretching NW–SE, i.e., slightly obliquely to the slope ŽFig. 1.. Relief on this part of the continental slope is not as strong as in the Baie des Anges with gradients generally less than 78. At many locations there is evidence for amphitheatre-like slump scars ŽKlaucke and Cochonat, 1999; shown as deep-seated failures on Fig. 3.. Unfortunately, current bathymetric data do not allow estimates of the depth of these features.
Fig. 6. Blow-up of side-scan sonar image showing the wavelength of the gravel waves and indicating different transport directions at the confluence of the Var, Paillon and south-western canyons. For location see Fig. 1.
The continental slope between the two canyons is composed of a multitude of up to 60 m deep chutes that form a radial pattern with the apex pointing to the airport ŽFig. 1.. Many of the chutes cross-cut each other. Below about 1000 m water depth Žless in the north-east., the chutes converge into two broad, U-shaped valleys with a general NW–SE orientation ŽFig. 1.. The north-eastern canyon is formed at the junction of three parallel chutes in 800 m water depth and thereafter broadens from 1000 to 1750 m. It terminates as a hanging valley in 1100 m water depth, lying about 100 m above the floor of the Paillon Canyon. The south-western canyon Žpreviously called median valley; Malinverno et al., 1988; Mulder et al., 1994. is also formed at the junction of several parallel chutes in about 1100 m water depth. It decreases in width from 2250 to 900 m near the
4. Sediment distribution and processes 4.1. Sediment deposition Sediment deposition is divided here into primary sediment input Žin the case of the Baie des Anges, this will be fluvial input. and secondary sedimentation that follows remobilization of previously deposited material. 4.1.1. Primary sediment input The bulk of terrigenous sediment enters the study area via the Var River that has a drainage basin of about 2820 km2 and a marked seasonal regime with important flash-floods in autumn and spring ŽMulder et al., 1998.. The Paillon River also enters the Baie des Anges, but is a negligible contributor of terrigenous sediment nowadays. Modes of sediment input from the Var River include bedload transport of the coarse fraction and suspension transport for the fine fraction. The bedload material is injected directly into the Var Canyon head and may be transported by
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Fig. 7. SAR side-scan sonar images and 3.5 kHz profiles of the Upper Fan Valley showing the presence of erosional scours on the channel floor. For location see Fig. 1.
Fig. 8. SAR side-scan sonar image and 3.5 kHz profile showing the strongly gullied canyon walls of the Central Ridge. Location is shown on Fig. 1.
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hyperpycnal flows ŽMulder et al., 1998.. Gravel to cobble-size clasts shaped into sediment waves cover most of the canyon floors of the Var Canyon, the south-western canyon and the lower Paillon Canyon ŽFigs. 6 and 9.. In less than 1000 m water depth the gravel is covered by a thin layer of fine-grained sediment ŽFig. 9B.. Suspended sediments on the other hand, form sediment plumes upon entering the sea. These sediment plumes will spread at the surface Žhypopycnal., near the seafloor Žhyperpycnal., or at intermediate levels Žmesopycnal. depending on their sediment charge and resulting density, and are diverted mainly to the east following major westerly winds and currents ŽSage and Chamley, 1977.. The extent of these
deposits is much greater towards the east of the Var River mouth than to the west of it ŽFig. 3.. Mulder et al. Ž1998. argue for the existence of hyperpycnal plumes at the Var River mouth during normally strong discharges that may have return frequencies of 2–200 years. Analysis of several short piston cores from the uppermost continental slope ŽFig. 10. shows possible evidence for meso- or hyperpycnal plumes in the form of centimetre-thick, well-sorted silty layers containing occasional high amounts of plant debris. The number, thickness and mean grain size of these silty intervals decrease away from the Var River mouth. In addition, the silty–clayey deposits that are well laminated and variably coloured Žred, brown, olive, grey, beige. become increasingly
Fig. 9. Bottom photographs showing gravel deposits on the floor of the Var Canyon. ŽA. Cobble dunes with amplitudes of a few metres. ŽB. Gravel material covered by a thin layer of fine-grained sediments. ŽC. Erosional cut along the side-wall of the Var Canyon. ŽD. Erosional scar on terrace deposits in the Var Canyon.
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Fig. 10. Lithology and grain-size logs for selected sediment cores from the continental slope off Nice. For location of the cores refer to Fig. 3. 417
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homogeneous and uniform in colour Žbrown. away from the Var River mouth. Plume deposits in general are found on the uppermost continental slope and stretch slightly further downslope in the south-western and north-eastern canyons ŽFig. 3.. The thickness of the plume deposits varies strongly between the ridges and chutes on the upper slope. Thickness is greatest on the ridges, sometimes showing a marked asymmetry between the western Žthick. and the eastern ridge flank Žthin., and reaches 60 m close to the Var River mouth. Along the ridges it decreases gradually away from the Var River mouth, while most of the chutes are devoid of this type of deposit. Plume deposits are characterised by acoustically well-stratified sediments overlying highly reflective deposits of great extent that represent gravel ŽFig. 12.. This reflector has been cored in the south-western canyon and gravelly material with a layer of well-rounded cobbles at their base has been retrieved. Areas outside the Baie des Anges are not connected to major river input and do not seem to receive much coarse material as can be judged from the few sediment cores taken in that area. The sediments are generally composed of well-bioturbated, homogeneous, fine-grained hemipelagic deposits ŽFig. 10.. 4.1.2. Secondary sedimentation The continental slope off Nice is subject to various processes of slope degradation that result in sediment gravity flows ŽKlaucke and Cochonat, 1999.. These flows not only export material to the basin, but also deposit part of their material on the slope where they form what should be called secondary deposits. Some of these secondary deposits, like turbidity-current spill-over deposits, are difficult to differentiate from similar deposits of hyperpycnal flows in the Var Canyon. On 3.5 kHz profiles turbidity-current deposits are identified by well-stratified, continuous and parallel reflections, an echo-character that is very similar to plume deposits ŽFig. 12.. In consequence, depositional setting and piston cores have been used to distinguish between the two types: turbidites and hyperpycnal deposits. Turbidites are found on the ridges bordering the Var and south-western canyons and on terraces in the Var Canyon ŽFig. 3.. The terraces are composed
of acoustically well-stratified sediments, and generally overlie highly reflective deposits forming the modern canyon floor ŽFig. 5.. Acoustically more disorganised gravity flow deposits, such as debris flow and slump deposits are also present on the upper continental slope of the Baie des Anges just south of the Paillon Canyon, on the sedimentary ridge separating the south-western from the northeastern valley, and at several locations on the floor of the Var Canyon ŽFig. 3.. Most of the features interpreted as slump deposits are found in excess of 600 m water depth. 4.2. Erosion The morphology of the Baie des Anges ŽFig. 1. and bottom observations ŽFig. 9C, D. strongly suggest that erosive processes operate on the continental slope off Nice. Erosion generally is of two types: erosion due to failure of slope sediments and erosion by sediment gravity flows in the canyons and channels, with the latter generally being mostly the result of the former. 4.2.1. Sediment failure Many deposits on the continental slope off Nice are underconsolidated ŽCochonat et al., 1993., unstable ŽMulder et al., 1994., and subject to widespread sediment failure ŽKlaucke and Cochonat, 1999.. Based on depositional environment, failure style and triggering mechanism, six main cases of sediment failure can be distinguished ŽKlaucke and Cochonat, 1999.. The volumetrically most important failure events occur in deposits of thick accumulations of fine-grained sediments such as plume deposits, turbidity-current spill-over deposits or hemipelagic deposits. Triggering mechanisms for sediment failure are variable and include undercutting, sedimentary loading and seismic loading. Seismic loading may well be the most important triggering mechanism, as the study area is subject to significant seismic activity ŽRehault and Bethoux, 1984.. Whatever the triggering mechanism, SAR side-scan sonar imagery shows numerous evidence for past failure events with approximate initial volumes of displaced sediment being as high as 40 times the volume of the 1979 event ŽFigs. 2 and 3; Klaucke and Cochonat, 1999..
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4.2.2. Erosion by sediment graÕity flow Following failure initiation, mass-wasting processes may experience many flow transformations and may turn into erosive sediment gravity flows ŽHampton, 1972; Parker, 1982.. In principle, primary sediment gravity flows Žhyperpycnal plumes. may erode the seafloor and increase their sediment load ŽMulder et al., 1998.. Considerable erosion along the flow path has been inferred from volume considerations for the latest submarine landslide in 1979 ŽPiper and Savoye, 1993; Mulder et al., 1997.. In that particular case, the initial sediment volume increased more than 10-fold. Evidence for erosive sediment flow can be found with a possible trace of the 1979 event on the upper continental slope ŽFig. 2. and erosional scours on the floor of lower Var Canyon that are cut into the mud layer covering the seafloor ŽFig. 7.. The erosional scours diminish in size down-channel while their spacing increases. Other evidence for erosive sediment flow is provided by cross-cutting of chutes on the uppermost continental slope. The cross-cutting indicates erosive flows that are insensitive to previous slope morphology. Each major failure event generates a flow that will erode a new path into underconsolidated plume deposits. Finally, the formation of a thalweg on the upper reaches of the Paillon Canyon also suggests erosive sediment flow. Erosive sediment flow within the major canyons is also evidenced by older, already consolidated layers outcropping at the base of the canyon walls ŽFig. 2..
5. Discussion The morphology of the continental slope off Nice has been suggested to be the result of tectonic movements triggering large-scale destabilisation ŽPautot, 1981; Guillocheau et al., 1983.. More recent work based on seismic stratigraphy already pointed towards more complex interactions between the Messinian paleomorphology and subsequent tectonic and sedimentary processes ŽSavoye et al., 1993.. Important neotectonic movements directly influencing seafloor morphology are not detectable in the data-set although other studies indicate their presence in the region ŽDubar et al., 1992; Chaumillon et
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al., 1994.. Sedimentary processes, therefore, seem to be mainly responsible for the observed slope morphology. As our study shows, sedimentary processes are mainly fluvial input and sediment failure. Among the questions that need further discussion are the evolution of these processes with time, and the interaction of the processes with existing morphology and substrate. Most of the coarse bedload of the Var River is nowadays trapped in the lowermost reaches of the river. It is likely that this kind of material has been transferred to the continental slope during the Pleistocene and formed the gravel deposits underlying the plume deposits on the upper slope ŽFig. 11.. The extent of the seismic reflector associated with these gravel deposits suggests that the continental slope of the Baie des Anges received large amounts of coarse-grained fluvial input in the past. The grain size of the deposits precluded transport in suspension and requires that slope canyons like the south-western and north-eastern canyons were once connected to the river mouth. Previous studies argued that coarse material in the Var system results from reworking of the Messinian erosional surface ŽMalinverno et al., 1988.. Conglomerate deposits of probably Messinian age outcrop in the Lower Paillon Canyon ŽFig. 3.. However, these outcrops are located downslope of many of the coarse-grained deposits on the continental slope and cannot be at the origin of the latter. Reworked Messinian conglomerates may, on the other hand, contribute to gravelly deposits in the Upper Fan Valley. In contrast to the input of bedload material in the Pleistocene, the Holocene sedimentation pattern is dominated by the input of suspended material. Differences in thickness of the plume deposits between the troughs and ridges of the chutes on the upper slope are mainly related to erosion in the chutes. Settling out of sediment plumes being the main mechanism, preferential accumulation should be observed in the troughs not on the ridges. Tongues of plume deposits in the sout-western and north-eastern canyons illustrate this effect. Differences in thickness between a thicker western flank and a thinner eastern flank, on the other hand, are probably related to differences in accumulation rates due to easterly currents ŽSage and Chamley, 1977.. Currents, how-
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Fig. 11. SAR side-scan sonar image and 3.5 kHz profile from the upper continental slope showing well-stratified plume deposits overlying strongly reflective gravel. For location see Fig. 1.
ever, seem not to be strong enough to erode sediments on the upper slope. These sediments are underconsolidated ŽCochonat et al., 1993., which facilitates failure and erosion of the deposits by secondary sediment-gravity flows, and explains cross-cutting of the chutes on the upper slope. All deposits have weak shear strengths and no existing chute is favoured by a subsequent flow. A different type of interaction of flows with existing substrate is shown in the Var Canyon. The presence of terraces overlying the Pleistocene gravel ŽFig. 5. indicates that Holocene flows are no longer competent enough to flush the whole canyon. Incipient stages of terrace formation can be seen in the uppermost reaches of the Var Canyon, where slump deposits resulting from failure of the canyon walls are located on the outer side of canyon bends. Subsequent currents in the canyon may either erode these deposits or cover them with spill-over deposits, de-
pending on the strength of the flows. Their presence in the upper reaches of the Var Canyon indicates that hyperpycnal flows in the Var Canyon were either non-erosive or did not reach as far down-channel. Mulder et al. Ž1998. give 600 m as the maximum water depth of erosion by high-frequency hyperpycnal plumes. A final type of interaction between sediment gravity flows and substrate is present in the lower Var Canyon and the Upper Fan Valley. Large cobbles like those forming many of the gravel waves ŽFig. 9A. could not have been transported by recent sediment gravity flows in the Var Canyon ŽMulder et al., 1997.. As a consequence, the observed gravel waves must be either remnants of Pleistocene flows, or shaped into sediment waves by Holocene flows. The former is unlikely since the gravel waves are not at all covered by Holocene sediments. The latter seems possible, especially since recent flows were capable
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of eroding deep scours into mud-capped gravel deposits further downslope.
6. Conclusions The distribution of surficial sediments on the continental slope of the Baie des Anges is largely dominated by fluvial input from the Var River. This fluvial input is responsible for widespread deposition of plume sediments on the upper continental slope, as well as coarse-grained canyon-fill deposits. The coarse material in the canyons, however, is most likely of Pleistocene age, as it is covered in many places by fine-grained plume deposits. Nowadays the coarsest constituents of fluvial input are trapped in the lowermost reaches of the Var River. Although only the Var Canyon is now connected to the Var River, it can be speculated that other submarine canyons of the Baie des Anges were also connected to fluvial input in the past. The sediments on the continental slope are potentially unstable and subject to failure with earthquake loading, undercutting and sediment loading as the principal triggering mechanisms. Secondary sediment gravity flows generated by these failures interact differently with the slope environment depending on local morphology and substrate. Part of their load is deposited on the slope Žmainly as turbidity current spill-over deposits on ridges and terraces.. Their sediment load increases by eroding chutes on the upper slope and scours in the Upper Fan Valley. They probably shape older gravel deposits into gravel waves, and finally, they exported most of their sediment load to the basin. Our analysis shows that the overall physiography of the continental slope off Nice did not change dramatically over time. What changed are the quantities and the nature of the material delivered to the slope environment. The Pleistocene was dominated by the input of large quantities of coarse bedload material, while the Holocene sees mainly input of suspended sediment. The Holocene therefore shows conditions of sediment delivery that are comparable to those active at passive margins during times of lowered sealevel. As a result, present day sediment dispersion patterns on the continental slope off Nice seem to be a good modern equivalent to study
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sediment dispersal patterns on continental slopes during lowered sea-level. However, future studies will have to quantify the importance of the different sediment transport mechanisms and to evaluate the sediment fluxes on the slope.
Acknowledgements This study has been possible through a Marie Curie Postdoctoral Fellowship from the European Union TMR programme Žcontract ERBFMBICT96l144. to the first author. We are grateful to all the scientists, captains, officers and crews of the various cruises to the Baie des Anges, conducted by IFREMER and the Laboratoire de Geodynamique sous´ marine, Villefranche sur Mer. E. Le Drezen ŽIFREMER. helped processing the SAR data. Constructive criticism by journal reviewers Dan Evans and Neil Kenyon greatly improved the manuscript.
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Gennesseaux, M., Guibout, M., Lacombe, H., 1971. Enregistrement de courants de turbidite´ dans la vallee ´ sous-marine du Var ŽAlpes–Maritimes.. C.R. Acad. Sci. Paris 273D, 2456–2459. Gennesseaux, M., Mauffret, A., Pautot, G., 1980. Les glissements sous-marins de la pente continentale nic¸oise et la rupture de cables en mer Ligure ŽMediterranee ˆ ´ ´ occidentale.. C.R. Acad. Sci. Paris 290, 959–962. Gilbert, R., 1983. Sedimentary processes of Canadian Arctic fjords. Sediment Geol. 36, 147–175. Guillocheau, P., Pautot, G., Auzende, J.M., 1983. Les canyons du Var et du Paillon Žmarge des Alpes meridionales — Mediter´ ´ ranee ´ occidentale.: une origine quaternaire par glissement. C.R. Acad. Sci. Paris 296, 91–96. Hampton, M.A., 1972. The role of subaqueous debris flows in generating turbidity currents. J. Sediment Res. 42, 775–793. Heezen, B.C., Ewing, M., Granelli, N.C.L., Menzies, R.J., Schneider, E.D., 1964. Congo submarine canyon. AAPG Bull. 48, 1126–1149. Klaucke, I., Cochonat, P., 1999. Analysis of past seafloor failures on the continental slope off Nice, SE France. Geo-Marine Lett., accepted. Kuenen, P.R., 1950. Turbidity currents of high density, 18th Int. Geol. Congress 1948, London, pp. 44–52. Malinverno, A., Ryan, W.B.F., Auffret, G.A., Pautot, G., 1988. Sonar images of recent failure events on the continental margin off Nice, France. Geol. Soc. Am. Special Paper 229, 59–75. Monti, S., Carre, de la Baie des ´ D., 1979. Carte bathymetrique ´ ´ Anges, Nice, Cote CNEXO, Brest. ˆ d’Azur. 1:25 000, Editions Mulder, T., Syvitski, J.P.M., 1995. Turbidity currents generated at river mouths during exceptional discharges to the world oceans. J. Geol. 103, 285–299. Mulder, T., Tisot, J.-P., Cochonat, P., Bourillet, J.-F., 1994. Regional assessment of mass failure events in the Baie des Anges, Mediterranean Sea. Mar. Geol. 122, 29–45. Mulder, T., Savoye, B., Syvitski, J.P.M., Cochonat, P., 1996. Origine des courants de turbidite´ enregistres ´ a` l’embouchure du Var en 1971. C.R. Acad. Sci. Paris 322, 301–307. Mulder, T., Savoye, B., Syvitski, J.M.P., 1997. Numerical model-
ing a mid-size gravity flow: the 1979 Nice turbidity current Ždynamics, processes, sediment budget and seafloor impact.. Sedimentology 44, 305–326. Mulder, T., Savoye, B., Syvitski, J.P.M., Piper, D.J.W., 1998. The Var Submarine System: understanding Holocene sediment delivery processes and their importance to the geological record. In: Stoker, M.S., Evans, D., Cramp, A. ŽEds.., Geological Processes on Continental Margins: Sedimentation, Mass Wasting and Stability. Spec. Publ. Geol. Soc. London, pp. 146–166. Normark, W.R., Piper, D.J.W., 1991. Initiation processes and flow evolution of turbidity currents: implications for the depositional record. SEPM Spec. Publ. 46, 207–230. Parker, G., 1982. Conditions for the ignition of catastrophically erosive turbidity currents. Mar. Geol. 46, 307–327. Pautot, G., 1981. Cadre morphologique de la Baie des Anges. Modele ` d’instabilite´ de pente continentale. Oceanol. Acta 4, 203–212. Piper, D.J.W., Savoye, B., 1993. Processes of Late Quaternary turbidity-current flow and deposition on the Var deep-sea fan, north-west Mediterranean Sea. Sedimentology 40, 557–582. Rehault, J.-P., Bethoux, N., 1984. Earthquake relocation in the Ligurian Sea ŽWestern Mediterranean.: geological interpretation. Mar. Geol. 55, 429–445. Sage, L., Chamley, H., 1977. Mineraux argileux et dynamique ´ sedimentaire au large du Var ŽS.-E. France.. Ann. Soc. Sci. ´ Nat. Archeologie Toulon Var, pp. 133–139. ´ Savoye, B., Piper, D.J.W., Droz, L., 1993. Plio-Pleistocene evolution of the Var deep-sea fan off the French Riviera. Mar. Petrol. Geol. 10, 550–571. Savoye, B., Rehault, J.-P., Le Suave, ´ R., Monti, S., Le Drezen, E., 1997. Synthese et imagerie acoustique ŽCorse et ` bathymetrique ´ mer Ligure.. Notice des cartes bathymetriques et imagerie ´ acoustique de la mer Ligure au 1r500 000e. Editions IFREMER, Plouzane, ´ 22 pp. Syvitski, J.P.M., Burell, D.C., Skei, J., 1987. Fjords — Processes and Products. Springer Verlag, New York, 379 pp. Wright, L.D., Wiseman, W.J., Bornhold, B.D., Prior, D.B., Suhayda, J.N., Keller, G.H., Yang, Z.-S., Fan, Y.B., 1988. Marine dispersal and deposition of Yellow River silts by gravity-driven underflows. Nature 332, 629–632.
Patterns and processes of sediment dispersal on the continental slope off Nice, SE France Ingo Klaucke ) , Bruno Savoye, Pierre Cochonat IFREMER, DRO-GM, Laboratoire EnÕironnements Sedimentaires B.P. 70, 29280 Plouzane, ´ ´ France Received 30 September 1998; accepted 11 May 1999
Abstract The distribution of surficial sediments and sediment dispersion patterns on the steep continental slope off Nice ŽSE France. have been studied using side-scan sonar, 3.5 kHz profiles, short piston cores and bottom photographs. The input of terrigenous material to the Baie des Anges, a submarine embayment bounded by two prominent ridges, is dominated by fluvial input from the Var River, the Paillon River being only a minor source. The Var River provides very coarse bedload material Žgravel and cobble. directly to the head of the Var Canyon. Gravel and cobble deposits are found all along the Var Canyon and the Upper Fan Valley of the Var submarine fan and have been shaped into gravel waves. The fine particles Žsuspension load. are separated from the coarse bedload upon entering the sea and form up to 60-m thick deposits on the uppermost continental slope of the Baie des Anges. These deposits are formed by settling out of sediment plumes. The presence of silt and fine sand laminae that decrease in thickness and frequency away from the Var River mouth indicate the influence of meso- and hyperpycnal flows on these plume deposits. Areas outside the Baie des Anges are not connected to major fluvial input and receive only hemipelagic sediments. These primary deposits are highly unstable and sediment failure due to seismic loading, sedimentary loading or undercutting is frequent. Sediment failure produces secondary sediment gravity flows that export most of the material to the basin, but also produce turbidity-current over-spill deposits on ridges bounding the slope canyons and on terraces within the Var Canyon. Slump and debris-flow deposits are also observed. At least some of these secondary flows erode the continental slope as cross-cutting chutes on the upper continental slope and erosional scours in the Upper Fan Valley demonstrate. Modern sediment dispersal patterns on the continental slope off Nice are proposed as a modern analogue to lowstand conditions on continental margins. In fact, the absence of a continental shelf together with a steep slope strongly reduces the influence of sealevel on the physiography of the margin. q 2000 Elsevier Science B.V. All rights reserved. Keywords: continental slope; sediment dispersion; fluvial input; sediment plumes; slope failure; sediment-gravity flows
1. Introduction
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Corresponding author. Southampton Oceanography Centre, Challenger Division for Seafloor Processes, Empress Dock, Southampton, SO14 3TA UK. fax: q44-1703-596554; E-mail: [email protected]
Our knowledge of the processes, morphology, structure and evolution of deep-sea clastic deposits advanced considerably since Kuenen Ž1950. first suggested turbidity currents at continental margins as
0025-3227r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 Ž 9 9 . 0 0 0 6 3 - 8
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the possible process for their deposition. Most of the studies of deep-sea clastic systems have, however, concentrated on the marine environment and looked only at what happens within the ocean. As a consequence, the mechanisms and patterns of sediment transfer from the continent to the deep-sea are still rather poorly understood. For a long time, stepwise sediment transfer with intermediate storage of sediment in deltas and on the continental shelf has been favoured ŽNormark and Piper, 1991.. In contrast to lacustrine environments, fluvial input Žexcept for exceptionally high sediment loads. was considered not dense enough to form underflows Žhyperpycnal flows. that could transport material directly onto the continental slope and into the deep-sea Že.g., Gilbert, 1983; Syvitski et al., 1987.. Instead, fluvial sediments had first to be deposited in deltas or on prodelta slopes before they could be remobilised by sediment failure and transferred to the deep-sea ŽEmbley and Jacobi, 1986.. Recent quantitative studies of the sediment and fluid discharge of many rivers world-wide has shown that a variety of them, although not the largest rivers, are capable of forming hyperpycnal flows ŽMulder and Syvitski, 1995.. Direct transfer of sediment from the continent to the deep-sea is therefore possible in a number of circumstances. Concerning the patterns of sediment transfer, only few studies exist because modern conditions of high sealevel resulted in completely different settings with respect to lowstand conditions when the shoreline was close to the shelf-edge and fluvial sediments were delivered directly to the continental slope. Recent turbidity-current activity that could be related directly to fluvial input has been inferred for the Zaire submarine fan ŽHeezen et al., 1964; Droz et al., 1996. and observed on the Yellow River delta ŽWright et al., 1988., but remain isolated cases. The Var submarine fan in the Ligurian Basin, NW Mediterranean ŽFig. 1. also shows signs of recent mass wasting activity ŽGennesseaux et al., 1971, 1980; Auffret et al., 1982; Mulder et al., 1996..
Stepwise and continuous mass transfers have been identified. Stepwise transfer of sediment is evidenced by the observation of mass wasting processes on the continental slope ŽAuffret et al., 1982; Klaucke and Cochonat, 1999., while large flooding events of the Var River should be capable of forming frequent hyperpycnal flows ŽMulder et al., 1998.. In addition, the Var submarine fan may be an appropriate analogue for continental margins active at times of lowered sealevel, because the absence of a continental shelf ŽPautot, 1981. allows a direct link between the continent and the deep-sea. A whole range of acoustic data of the seafloor and near sub-bottom complemented by ground-truthing has been used to study the patterns of sediment dispersion and transfer to the deep-sea on the continental shelf off Nice. This paper presents the results of the detailed investigation of seafloor morphology and surficial sediment distribution on the continental slope off Nice, and attempts to relate these to the interplay between sediment deposition and erosion.
2. Datasets and methods A large range of acoustic data exist from the study area including acoustic imagery of multibeam bathymetry systems ŽSimrad EM12d and EM1000., deep-towed side-scan sonar and 3.5 kHz profiles ŽSAR, collected in 1986., as well as conventional airgun seismic. All these instruments use different frequencies resulting in different resolutions and subbottom penetration. The Simrad EM12d uses 12–13 kHz and serves as a regional reconnaissance tool. EMl2d-data from the continental slope only distinguish between high backscatter reflectivity underlining the canyon floors and low backscatter reflectivity for all other areas ŽSavoye et al., 1997.. The EMl000 works at very high frequency Ž95 kHz. which restricts its use to less than 1000 m water depth.
Fig. 1. ŽA. Overview map of the Var submarine fan system in the Ligurian Sea. ŽB. Close-up of the continental slope off Nice showing the location of profiles mentioned in the text. P s Paillon River.
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Acoustic backscatter of the EM1000 did not provide information on the surficial sediments due to the extremely dissected nature of the upper continental slope, i.e., the response of the tool is dominated by seafloor topography. As a consequence, the detailed geomorphologic and sedimentologic analysis of the seafloor off Nice is based essentially on SAR ŽSysteme Acoustique ` Remorque´. side-scan sonar imagery that covers a total of about 1300 km2 of which the portion covering the continental slope is presented in Fig. 2, and on deep-towed 3.5 kHz profiles. The SAR works at a frequency of 200 kHz and is towed at a depth of 100–150 m above the seafloor. For this reason it is operationally necessary to follow topographic depressions ŽFig. 2.. The side-scan sonar data have been compiled into a mosaic despite numerous uncertainties concerning the position of the tow-fish ŽFig. 2.. The mosaic has then been interpreted together with 3.5 kHz profiles, numerous short piston cores and submersible observations. Extrapolation of depositional facies from 3.5 kHz profiles has been favoured, as direct calibration of side-scan sonar data is generally not possible, while interpretation of 3.5 kHz echo-facies is a well established concept Že.g., Damuth, 1980.. In order to produce a geomorphic– sedimentologic map ŽFig. 3. from the interpretation of the side-scan mosaic, bathymetric information from several data-sets of multibeam bathymetry ŽSeaBeam, EM12d, EM1000. has been used to correct and readjust the location of the different deposits and features.
3. Seafloor morphology The continental margin off Nice is characterised by an absent or very narrow continental shelf ŽFig. 1.. Multibeam bathymetry shows that the continental slope is very steep with average gradients of 118 that may reach 278 along the side-walls of the canyons ŽMonti and Carre, ´ 1979; Pautot, 1981.. The part of the continental slope studied here comprises the Baie des Anges and the Cap Ferrat Slope to the east of it. The Baie des Anges is a large submarine embayment bound by the Cap d’Antibes Ridge in the west and the Cap Ferrat Ridge in the east ŽFig. 1.
3.1. The Baie des Anges The Baie des Anges comprises two major submarine canyons that are highly erosive and coalesce in 1650 m water depth: the Var Canyon in the west and the Paillon Canyon in the east. These canyons are erosive, as several km of shoreline retreat ŽFig. 1. and the formation of a deep thalweg in the upper Paillon Canyon ŽFig. 4. indicate. The Var Canyon is connected directly to the Var River and is slightly sinuous in its upper reaches. Flat-floored, it has a relief height between 150 and 500 m, while channel width ranges between 300 and 1250 m. Channel width is locally restricted by the formation of terraces on the inner side of meander bends ŽFigs. 1 and 3.. These terraces are highly irregular in the upper canyon reaches above 1000 m water depth, but they become more regular and flat-topped down-canyon ŽFig. 5., where they reach heights of up to 50 m above the canyon floor. The eastern flank of the Cap d’Antibes Ridge also shows evidence for raised terraces of the Var Canyon that now lie at mid-height of the Cap d’Antibes Ridge, i.e., 250 m above the present canyon floor ŽFigs. 1 and 3.. The lower reaches of the Var Canyon are covered with gravel waves having wavelengths of 30–40 m and heights of a few metres ŽFig. 6; Malinverno et al., 1988.. Further downchannel these gravel waves give way to regularly spaced, pear-shaped scours that have a sharp upper boundary and a diffuse lower end ŽFig. 7.. These scours are up to 20 m deep and may cover surfaces of a few square kilometres. Spacing and depth of the erosional scours gradually decrease downchannel. The Paillon Canyon on the eastern side of the Baie des Anges has direct connection to the Paillon River and a V-shaped cross-section in its upper reaches ŽFig. 4. that becomes U-shaped down-channel ŽFig. 1.. The connection between the Paillon Canyon and the Paillon River, however, is not as clear as in the case of the Var. Width of the Paillon Canyon increases gradually along the channel to reach 1500 m while channel depth increases from 150 m in 800 m water depth to 500 m at the confluence with the south-western canyon ŽFig. 1.. Gravel waves similar to those in the Var Canyon are present near the confluence with the latter ŽFig. 6..
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Fig. 2. SAR side-scan sonar mosaic of the Baie des Anges and adjacent areas. The mosaic is displayed in normal polarity, i.e., high backscatter areas are dark. Note the orientation of the profiles along the canyons and a slight discrepancy between the imagery and underlain bathymetry. Contour lines shown are at 200 m interval.
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pp. 411–412
Fig. 3. Synthetic geomorphologic and sedimentologic map of the continental slope off Nice. Black dots indicate the location of piston cores.
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Fig. 4. SAR side-scan sonar image and 3.5 kHz profile of the upper Paillon Canyon showing highly gullied canyon walls and a deeply entrenched thalweg. Profile location is given in Fig. 1.
Fig. 5. SAR side-scan sonar image and 3.5 kHz profile showing a flat-topped terrace on the floor of the Var Canyon. The apparent inclination of the terrace is due to oblique orientation of the profile with respect to channel axis. Note the continuity of the gravel-wave surface below the terrace. For profile location see Fig. 1.
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confluence with the Var and Paillon Canyons. The south-western canyon is separated from the Var Canyon by the sharp-crested Central Ridge that is highly gullied, as are the Cap d’Antibes and Cap Ferrat Ridges ŽFigs. 1, 2 and 8.. This contrasts with the ridge separating the Central Valley from the north-eastern valley ŽFig. 1.. This ridge is much broader and presents some larger chutes that are not perpendicular to the ridge crest, but more or less parallel to it. 3.2. The Cap Ferrat Slope The continental slope east of the Cap Ferrat Ridge shows two broad valleys stretching NW–SE, i.e., slightly obliquely to the slope ŽFig. 1.. Relief on this part of the continental slope is not as strong as in the Baie des Anges with gradients generally less than 78. At many locations there is evidence for amphitheatre-like slump scars ŽKlaucke and Cochonat, 1999; shown as deep-seated failures on Fig. 3.. Unfortunately, current bathymetric data do not allow estimates of the depth of these features.
Fig. 6. Blow-up of side-scan sonar image showing the wavelength of the gravel waves and indicating different transport directions at the confluence of the Var, Paillon and south-western canyons. For location see Fig. 1.
The continental slope between the two canyons is composed of a multitude of up to 60 m deep chutes that form a radial pattern with the apex pointing to the airport ŽFig. 1.. Many of the chutes cross-cut each other. Below about 1000 m water depth Žless in the north-east., the chutes converge into two broad, U-shaped valleys with a general NW–SE orientation ŽFig. 1.. The north-eastern canyon is formed at the junction of three parallel chutes in 800 m water depth and thereafter broadens from 1000 to 1750 m. It terminates as a hanging valley in 1100 m water depth, lying about 100 m above the floor of the Paillon Canyon. The south-western canyon Žpreviously called median valley; Malinverno et al., 1988; Mulder et al., 1994. is also formed at the junction of several parallel chutes in about 1100 m water depth. It decreases in width from 2250 to 900 m near the
4. Sediment distribution and processes 4.1. Sediment deposition Sediment deposition is divided here into primary sediment input Žin the case of the Baie des Anges, this will be fluvial input. and secondary sedimentation that follows remobilization of previously deposited material. 4.1.1. Primary sediment input The bulk of terrigenous sediment enters the study area via the Var River that has a drainage basin of about 2820 km2 and a marked seasonal regime with important flash-floods in autumn and spring ŽMulder et al., 1998.. The Paillon River also enters the Baie des Anges, but is a negligible contributor of terrigenous sediment nowadays. Modes of sediment input from the Var River include bedload transport of the coarse fraction and suspension transport for the fine fraction. The bedload material is injected directly into the Var Canyon head and may be transported by
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Fig. 7. SAR side-scan sonar images and 3.5 kHz profiles of the Upper Fan Valley showing the presence of erosional scours on the channel floor. For location see Fig. 1.
Fig. 8. SAR side-scan sonar image and 3.5 kHz profile showing the strongly gullied canyon walls of the Central Ridge. Location is shown on Fig. 1.
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hyperpycnal flows ŽMulder et al., 1998.. Gravel to cobble-size clasts shaped into sediment waves cover most of the canyon floors of the Var Canyon, the south-western canyon and the lower Paillon Canyon ŽFigs. 6 and 9.. In less than 1000 m water depth the gravel is covered by a thin layer of fine-grained sediment ŽFig. 9B.. Suspended sediments on the other hand, form sediment plumes upon entering the sea. These sediment plumes will spread at the surface Žhypopycnal., near the seafloor Žhyperpycnal., or at intermediate levels Žmesopycnal. depending on their sediment charge and resulting density, and are diverted mainly to the east following major westerly winds and currents ŽSage and Chamley, 1977.. The extent of these
deposits is much greater towards the east of the Var River mouth than to the west of it ŽFig. 3.. Mulder et al. Ž1998. argue for the existence of hyperpycnal plumes at the Var River mouth during normally strong discharges that may have return frequencies of 2–200 years. Analysis of several short piston cores from the uppermost continental slope ŽFig. 10. shows possible evidence for meso- or hyperpycnal plumes in the form of centimetre-thick, well-sorted silty layers containing occasional high amounts of plant debris. The number, thickness and mean grain size of these silty intervals decrease away from the Var River mouth. In addition, the silty–clayey deposits that are well laminated and variably coloured Žred, brown, olive, grey, beige. become increasingly
Fig. 9. Bottom photographs showing gravel deposits on the floor of the Var Canyon. ŽA. Cobble dunes with amplitudes of a few metres. ŽB. Gravel material covered by a thin layer of fine-grained sediments. ŽC. Erosional cut along the side-wall of the Var Canyon. ŽD. Erosional scar on terrace deposits in the Var Canyon.
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Fig. 10. Lithology and grain-size logs for selected sediment cores from the continental slope off Nice. For location of the cores refer to Fig. 3. 417
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homogeneous and uniform in colour Žbrown. away from the Var River mouth. Plume deposits in general are found on the uppermost continental slope and stretch slightly further downslope in the south-western and north-eastern canyons ŽFig. 3.. The thickness of the plume deposits varies strongly between the ridges and chutes on the upper slope. Thickness is greatest on the ridges, sometimes showing a marked asymmetry between the western Žthick. and the eastern ridge flank Žthin., and reaches 60 m close to the Var River mouth. Along the ridges it decreases gradually away from the Var River mouth, while most of the chutes are devoid of this type of deposit. Plume deposits are characterised by acoustically well-stratified sediments overlying highly reflective deposits of great extent that represent gravel ŽFig. 12.. This reflector has been cored in the south-western canyon and gravelly material with a layer of well-rounded cobbles at their base has been retrieved. Areas outside the Baie des Anges are not connected to major river input and do not seem to receive much coarse material as can be judged from the few sediment cores taken in that area. The sediments are generally composed of well-bioturbated, homogeneous, fine-grained hemipelagic deposits ŽFig. 10.. 4.1.2. Secondary sedimentation The continental slope off Nice is subject to various processes of slope degradation that result in sediment gravity flows ŽKlaucke and Cochonat, 1999.. These flows not only export material to the basin, but also deposit part of their material on the slope where they form what should be called secondary deposits. Some of these secondary deposits, like turbidity-current spill-over deposits, are difficult to differentiate from similar deposits of hyperpycnal flows in the Var Canyon. On 3.5 kHz profiles turbidity-current deposits are identified by well-stratified, continuous and parallel reflections, an echo-character that is very similar to plume deposits ŽFig. 12.. In consequence, depositional setting and piston cores have been used to distinguish between the two types: turbidites and hyperpycnal deposits. Turbidites are found on the ridges bordering the Var and south-western canyons and on terraces in the Var Canyon ŽFig. 3.. The terraces are composed
of acoustically well-stratified sediments, and generally overlie highly reflective deposits forming the modern canyon floor ŽFig. 5.. Acoustically more disorganised gravity flow deposits, such as debris flow and slump deposits are also present on the upper continental slope of the Baie des Anges just south of the Paillon Canyon, on the sedimentary ridge separating the south-western from the northeastern valley, and at several locations on the floor of the Var Canyon ŽFig. 3.. Most of the features interpreted as slump deposits are found in excess of 600 m water depth. 4.2. Erosion The morphology of the Baie des Anges ŽFig. 1. and bottom observations ŽFig. 9C, D. strongly suggest that erosive processes operate on the continental slope off Nice. Erosion generally is of two types: erosion due to failure of slope sediments and erosion by sediment gravity flows in the canyons and channels, with the latter generally being mostly the result of the former. 4.2.1. Sediment failure Many deposits on the continental slope off Nice are underconsolidated ŽCochonat et al., 1993., unstable ŽMulder et al., 1994., and subject to widespread sediment failure ŽKlaucke and Cochonat, 1999.. Based on depositional environment, failure style and triggering mechanism, six main cases of sediment failure can be distinguished ŽKlaucke and Cochonat, 1999.. The volumetrically most important failure events occur in deposits of thick accumulations of fine-grained sediments such as plume deposits, turbidity-current spill-over deposits or hemipelagic deposits. Triggering mechanisms for sediment failure are variable and include undercutting, sedimentary loading and seismic loading. Seismic loading may well be the most important triggering mechanism, as the study area is subject to significant seismic activity ŽRehault and Bethoux, 1984.. Whatever the triggering mechanism, SAR side-scan sonar imagery shows numerous evidence for past failure events with approximate initial volumes of displaced sediment being as high as 40 times the volume of the 1979 event ŽFigs. 2 and 3; Klaucke and Cochonat, 1999..
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4.2.2. Erosion by sediment graÕity flow Following failure initiation, mass-wasting processes may experience many flow transformations and may turn into erosive sediment gravity flows ŽHampton, 1972; Parker, 1982.. In principle, primary sediment gravity flows Žhyperpycnal plumes. may erode the seafloor and increase their sediment load ŽMulder et al., 1998.. Considerable erosion along the flow path has been inferred from volume considerations for the latest submarine landslide in 1979 ŽPiper and Savoye, 1993; Mulder et al., 1997.. In that particular case, the initial sediment volume increased more than 10-fold. Evidence for erosive sediment flow can be found with a possible trace of the 1979 event on the upper continental slope ŽFig. 2. and erosional scours on the floor of lower Var Canyon that are cut into the mud layer covering the seafloor ŽFig. 7.. The erosional scours diminish in size down-channel while their spacing increases. Other evidence for erosive sediment flow is provided by cross-cutting of chutes on the uppermost continental slope. The cross-cutting indicates erosive flows that are insensitive to previous slope morphology. Each major failure event generates a flow that will erode a new path into underconsolidated plume deposits. Finally, the formation of a thalweg on the upper reaches of the Paillon Canyon also suggests erosive sediment flow. Erosive sediment flow within the major canyons is also evidenced by older, already consolidated layers outcropping at the base of the canyon walls ŽFig. 2..
5. Discussion The morphology of the continental slope off Nice has been suggested to be the result of tectonic movements triggering large-scale destabilisation ŽPautot, 1981; Guillocheau et al., 1983.. More recent work based on seismic stratigraphy already pointed towards more complex interactions between the Messinian paleomorphology and subsequent tectonic and sedimentary processes ŽSavoye et al., 1993.. Important neotectonic movements directly influencing seafloor morphology are not detectable in the data-set although other studies indicate their presence in the region ŽDubar et al., 1992; Chaumillon et
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al., 1994.. Sedimentary processes, therefore, seem to be mainly responsible for the observed slope morphology. As our study shows, sedimentary processes are mainly fluvial input and sediment failure. Among the questions that need further discussion are the evolution of these processes with time, and the interaction of the processes with existing morphology and substrate. Most of the coarse bedload of the Var River is nowadays trapped in the lowermost reaches of the river. It is likely that this kind of material has been transferred to the continental slope during the Pleistocene and formed the gravel deposits underlying the plume deposits on the upper slope ŽFig. 11.. The extent of the seismic reflector associated with these gravel deposits suggests that the continental slope of the Baie des Anges received large amounts of coarse-grained fluvial input in the past. The grain size of the deposits precluded transport in suspension and requires that slope canyons like the south-western and north-eastern canyons were once connected to the river mouth. Previous studies argued that coarse material in the Var system results from reworking of the Messinian erosional surface ŽMalinverno et al., 1988.. Conglomerate deposits of probably Messinian age outcrop in the Lower Paillon Canyon ŽFig. 3.. However, these outcrops are located downslope of many of the coarse-grained deposits on the continental slope and cannot be at the origin of the latter. Reworked Messinian conglomerates may, on the other hand, contribute to gravelly deposits in the Upper Fan Valley. In contrast to the input of bedload material in the Pleistocene, the Holocene sedimentation pattern is dominated by the input of suspended material. Differences in thickness of the plume deposits between the troughs and ridges of the chutes on the upper slope are mainly related to erosion in the chutes. Settling out of sediment plumes being the main mechanism, preferential accumulation should be observed in the troughs not on the ridges. Tongues of plume deposits in the sout-western and north-eastern canyons illustrate this effect. Differences in thickness between a thicker western flank and a thinner eastern flank, on the other hand, are probably related to differences in accumulation rates due to easterly currents ŽSage and Chamley, 1977.. Currents, how-
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Fig. 11. SAR side-scan sonar image and 3.5 kHz profile from the upper continental slope showing well-stratified plume deposits overlying strongly reflective gravel. For location see Fig. 1.
ever, seem not to be strong enough to erode sediments on the upper slope. These sediments are underconsolidated ŽCochonat et al., 1993., which facilitates failure and erosion of the deposits by secondary sediment-gravity flows, and explains cross-cutting of the chutes on the upper slope. All deposits have weak shear strengths and no existing chute is favoured by a subsequent flow. A different type of interaction of flows with existing substrate is shown in the Var Canyon. The presence of terraces overlying the Pleistocene gravel ŽFig. 5. indicates that Holocene flows are no longer competent enough to flush the whole canyon. Incipient stages of terrace formation can be seen in the uppermost reaches of the Var Canyon, where slump deposits resulting from failure of the canyon walls are located on the outer side of canyon bends. Subsequent currents in the canyon may either erode these deposits or cover them with spill-over deposits, de-
pending on the strength of the flows. Their presence in the upper reaches of the Var Canyon indicates that hyperpycnal flows in the Var Canyon were either non-erosive or did not reach as far down-channel. Mulder et al. Ž1998. give 600 m as the maximum water depth of erosion by high-frequency hyperpycnal plumes. A final type of interaction between sediment gravity flows and substrate is present in the lower Var Canyon and the Upper Fan Valley. Large cobbles like those forming many of the gravel waves ŽFig. 9A. could not have been transported by recent sediment gravity flows in the Var Canyon ŽMulder et al., 1997.. As a consequence, the observed gravel waves must be either remnants of Pleistocene flows, or shaped into sediment waves by Holocene flows. The former is unlikely since the gravel waves are not at all covered by Holocene sediments. The latter seems possible, especially since recent flows were capable
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of eroding deep scours into mud-capped gravel deposits further downslope.
6. Conclusions The distribution of surficial sediments on the continental slope of the Baie des Anges is largely dominated by fluvial input from the Var River. This fluvial input is responsible for widespread deposition of plume sediments on the upper continental slope, as well as coarse-grained canyon-fill deposits. The coarse material in the canyons, however, is most likely of Pleistocene age, as it is covered in many places by fine-grained plume deposits. Nowadays the coarsest constituents of fluvial input are trapped in the lowermost reaches of the Var River. Although only the Var Canyon is now connected to the Var River, it can be speculated that other submarine canyons of the Baie des Anges were also connected to fluvial input in the past. The sediments on the continental slope are potentially unstable and subject to failure with earthquake loading, undercutting and sediment loading as the principal triggering mechanisms. Secondary sediment gravity flows generated by these failures interact differently with the slope environment depending on local morphology and substrate. Part of their load is deposited on the slope Žmainly as turbidity current spill-over deposits on ridges and terraces.. Their sediment load increases by eroding chutes on the upper slope and scours in the Upper Fan Valley. They probably shape older gravel deposits into gravel waves, and finally, they exported most of their sediment load to the basin. Our analysis shows that the overall physiography of the continental slope off Nice did not change dramatically over time. What changed are the quantities and the nature of the material delivered to the slope environment. The Pleistocene was dominated by the input of large quantities of coarse bedload material, while the Holocene sees mainly input of suspended sediment. The Holocene therefore shows conditions of sediment delivery that are comparable to those active at passive margins during times of lowered sealevel. As a result, present day sediment dispersion patterns on the continental slope off Nice seem to be a good modern equivalent to study
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sediment dispersal patterns on continental slopes during lowered sea-level. However, future studies will have to quantify the importance of the different sediment transport mechanisms and to evaluate the sediment fluxes on the slope.
Acknowledgements This study has been possible through a Marie Curie Postdoctoral Fellowship from the European Union TMR programme Žcontract ERBFMBICT96l144. to the first author. We are grateful to all the scientists, captains, officers and crews of the various cruises to the Baie des Anges, conducted by IFREMER and the Laboratoire de Geodynamique sous´ marine, Villefranche sur Mer. E. Le Drezen ŽIFREMER. helped processing the SAR data. Constructive criticism by journal reviewers Dan Evans and Neil Kenyon greatly improved the manuscript.
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