Recent sedimentary events in the western Gulf of Lions (Western Mediterranean)

Marine Geology 176 (2001) 23±37

www.elsevier.nl/locate/margeo

Recent sedimentary events in the western Gulf of Lions (Western Mediterranean) L. Droz a,*, R. Kergoat b, P. Cochonat c,1, S. Berne c,1 a

CNRS, UMR 6538 ªDomaines Oceaniquesº, Institut Universitaire EuropeÂen de la Mer, Universite de Bretagne Occidentale, Place Nicolas Copernic, 29280 PlouzaneÂ, France b Universite de Lille 1, 59655 Villeneuve d'Asq Cedex, France c IFREMER, Laboratoire Environnements SeÂdimentaires, BP70, 29280 PlouzaneÂ, France Received 4 August 2000; accepted 6 March 2001

Analysis of very high-resolution data (3.5 kHz pro®les and EM12D acoustic imagery) collected during the CALMAR (Catalono-Languedocian Margin cruise) (1997) highlighted the occurrence of two very recent, but previously unknown erosional sedimentary events on the continental rise, between the RhoÃne Fan and the Pyreneo-Languedocian Ridge. These events, preserved as unconformable, very thin lobes observable on acoustic imagery but not on 3.5 kHz echo-soundings, relate to erosional processes linked to near-bottom currents that seem to characterize the Holocene hydrodynamical conditions in the deep Gulf of Lions. They follow a Pleistocene regime when gravity sedimentation dominated on the Gulf of Lions margin either through aggradational turbidite deposition (RhoÃne Fan and Pyreneo-Languedocian Ridge), or through remobilization of sediments by regional mass-movements processes. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Western Mediterranean; Sedimentary processes; Sediment movement; Turbidites; Debris ¯ow; Continental slope and rise; Seismic architecture; Quaternary; Holocene

The Gulf of Lions has been the site of numerous geophysical and geological investigations for a long time. Most recent studies were devoted either to the continental shelf where a signi®cant database is now available (i.e. Tesson et al., 1993; Berne et al., 1998b; Rabineau et al., 1998) or to the continental slope and rise off the RhoÃne Delta (Droz, 1983; Droz and Bellaiche, 1985; Torres, 1995; Torres et al., 1997). * Corresponding author. Fax: 133-298-498760. E-mail addresses: [email protected] (L. Droz), [email protected] (P. Cochonat), [email protected] (S. BerneÂ). 1 Fax: 133-298-224570.

On the contrary, the Pyreneo-Languedocian slope and rise in the western part of the Gulf of Lions remained poorly investigated since the work reported in Canals (1985), Canals and Got (1986) and Alonso et al. (1991). This area was investigated on the R/V L'Atalante during the CAtalono-Languedocian MARgin (CALMAR) cruise of IFREMER conducted in 1997 (Berne et al., 1999) (Fig. 1). We present here the results of the detailed analysis of a small area of the rise, located between the PyreneoLanguedocian Ridge and the RhoÃne Fan, where very recent, but up to now unknown, sedimentary events have occurred, and perhaps may be occurring at present.

0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(01)00147-5

24

L. Drozet et al. / Marine Geology 176 (2001) 23±37

Fig. 1. Location of the study area in the framework of the western Gulf of Lions, including the Pyreneo-Languedocian Ridge (PL Ridge), the Petit-RhoÃne Fan, and the outlets of the Pyreneo-Languedocian and La Fonera canyons. Track-lines and core locations are indicated. Data types of track-lines and core notations are given in Fig. 6.

The present study synthesizes results from the analysis of several types of geophysical data obtained during the CALMAR cruise including EM12 D multibeam bathymetry and acoustic imagery, very highresolution seismic data (3.5 kHz pro®les), and sixchannel high-resolution seismic pro®les (Fig. 1). Some previously collected data were also used; they include a set of nine piston cores (MeÂar, 1984; Limonov et al., 1993), three very high-resolution 5 kHz

pro®les and Mak I sonograms from TREDMAR 92 cruise of Moscow State University (Kenyon et al., 1995; Torres et al., 1997), and older seismic lines on the RhoÃne neofan (Droz, 1983, 1991).

3.1. Morphological and architectural contexts The morphology of the western margin of the Gulf

L. Drozet et al. / Marine Geology 176 (2001) 23±37

25

Fig. 2. EM12D bathymetric map of the western Gulf of Lions, and preliminary morpho-sedimentary interpretation of the CALMAR area (from Berne et al., 1999). PLC: Pyreneo-Languedocian canyons, PL Ridge: Pyreneo-Languedocian Ridge, R: Petit-RhoÃne Canyon, M: Marti Canyon, Cl: Catherine-Laurence Canyon, S: SeÁte Canyon, H: HeÂrault Canyon, A: Aude Canyon, P: Pruvost Canyon, L: Lacaze-Duthiers Canyon, Cc: Cap Creus Canyon, F: La Fonera Canyon. Thick dashed lines indicate linear escarpments, probably structurally-controlled. The ®ve main sedimentary bodies are numbered except unit 3, because of unknown precise areal extension. Shadings indicate their lateral extension as known prior to CALMAR cruise.

of Lions has been described by Berne et al. (1999) (Fig. 2). The studied area is located at the base of slope, in a depression between three major morphologic features, the continental slope to the north, the Pyreneo-Languedocian Ridge (PL Ridge) to the west and the RhoÃne Fan to the east. These features constitute multiple sources for the material that has accumulated in the depression. 3.2. Continental slope and canyons To the west, the PL canyons (Cap Creus, LacazeDuthiers, Pruvost, Aude, HeÂrault canyons from west to east) form a converging network of relatively straight to sinuous canyons whose depth of incisions reaches up to 800 m deep. The easternmost and main canyon of this converging system, the SeÁte Canyon,

which is the most deeply incised, ultimately collects these canyons. Most of the PL canyons are PlioQuaternary features and show a complex evolution with several incision phases and westward migration that led to recent formation of the converging pattern (Droz et al., 1997). Some of them, however, such as the Lacaze-Duthiers Canyon, occupy the same position as Messinian canyons. In the easternmost part of the study area, the PetitRhoÃne Canyon appears strongly different with a tightly meandering course and lacking tributaries (Bellaiche et al., 1983). Between the SeÁte and Petit-RhoÃne canyons, two shorter and straighter canyons (Catherine-Laurence and Marti canyons) join each other at the base of slope and are ultimately captured by the SeÁte Canyon near its mouth.

26

L. Drozet et al. / Marine Geology 176 (2001) 23±37

Fig. 3. Six-channel seismic line crossing the PL Ridge at the mouth of SeÁte Canyon. The inset shows the location of the pro®le, with respect to the approximate extension of the PL Ridge (2), the RhoÃne Fan (1) and neofan (5) and Marti Canyon (M). See also Fig. 1 for location. Pro®le CASR59, CALMAR cruise.

The south-western corner of the area is characterized by the basinward extension of the Spanish La Fonera Canyon oriented nearly W±E. 3.3. Sedimentary features of the continental rise In the studied area, several sedimentary bodies originating from various sources are stacked, most of them without, or with only faint, morphological expression, except the PL Ridge and RhoÃne Fan. At the seismic scale, at least ®ve main depositional bodies (or their distal extensions) deposited during distinct sedimentary events, were previously known to be partially stacked (Fig. 2): (1) The western fringe of the Petit-RhoÃne Fan. To the east of the studied area, the Petit-RhoÃne Canyon is linked to a thick symmetrical pile of turbidites, the RhoÃne Fan (unit 1, event 1), which accumulated at the base of slope during the Plio-Quaternary (Fig. 4). The main Petit-RhoÃne Valley extending at the

mouth of the canyon can be followed far to the south where it branches into several distributaries (Coutellier, 1985). (2) The eastern part of the PL Ridge. The PL Ridge (unit 2, event 2) is a high, 0.8 s twtt thick sedimentary ridge deposited at the termination of the converging system of the PL canyons (Figs. 2 and 3). It is located at the base of slope, on the south-western side of the submarine valley that forms the mouth of SeÁte Canyon. Seismic facies of the PL Ridge are similar to those classically encountered in turbidite systems (i.e. association of high amplitude re¯ectors in channel and moderate amplitude continuous re¯ectors on levees and overbanks) (Fig. 3). The architecture of the PL Ridge shows numerous similarities with the Var sedimentary Ridge (Savoye et al., 1993) in that it is characterized by the over-development of one levee (the western one), and by the occurrence of oblique asymmetrical sediment waves (Fig. 2) and continuous eastward migration of the valley (Droz et al., 1997).

L. Drozet et al. / Marine Geology 176 (2001) 23±37

27

Fig. 4. Sparker seismic line crossing the western levee of the RhoÃne Fan, showing the debris ¯ow and the neofan resting unconformably on the RhoÃne Fan (from Torres et al., 1997). The circled numbers refer to the seismic units that are superposed in this area: 1, RhoÃne Fan; 3, ªlower interlobe unitº; 4, western debris ¯ow, 5, RhoÃne neofan. Unit 2 (PL Ridge) is not present. Horizon Dp is an erosional surface separating two major complexes of the RhoÃne Fan. See inset and Fig. 1 for location. Pro®le Cor79-80, CORNIDE cruise.

The combined effect of Coriolis Force and LiguroProvencËal bottom circulation, ¯owing from east to west along the margin, could be the cause of the general asymmetry of the PL Ridge. Units 1 and 2 could have been deposited synchronously. Whatever their relative chronology, they were deposited as clearly separated depocenters. Later units successively accumulated, in®lling the trough between the former depocenters. These later deposits lie on an erosional unconformity (Fig. 4), the origin of which is unclear, truncating the upper RhoÃne Fan strata to the west. (3) Theªlower inter-lobe unitº (MeÂar, 1984). This unit (unit 3, event 3), the origin of which is not yet fully understood, is composed of chaotic facies very similar to the neofan (see 5 below; Fig. 4). Its precise areal extent is unknown. (4) The western debris ¯ow. A sediment body, characterized by a transparent acoustic facies that is interpreted as a debris-¯ow (unit 4, event 4) (Droz and Bellaiche, 1985; Bellaiche et al., 1986), rests unconformably on the western levee of the RhoÃne Fan (Fig. 4). A comparable acoustically transparent mass of

possible similar age, is also present on the eastern side of the fan (Bellaiche et al., 1986), attesting to major sedimentary instabilities at the end of the main aggrading activity of the RhoÃne Fan that was shown to occur around 21 ka (MeÂar, 1984). Both debris-¯ows are linked to morphologic scars, some of which being clearly linked to listric faults created by halokinetic movements of the underlying Messinian salt layer. (5) The chaotic neofan. The neofan (unit 5, event 5) was deposited unconformably at the termination of a short incized channel (the neochannel) on the western fringe of the main body of the RhoÃne Fan (Figs. 2 and 4). This channel resulted from a recent westward avulsion of the Petit-RhoÃne Valley relatively upfan and led to the deposition of a small partly sandy lobe. The neofan is supposed to be Holocene in age (MeÂar, 1984; Torres et al., 1997) and was previously presumed to be possibly still active, since an age as recent as 130 a bp (Table 2) has been reported (MeÂar, 1984). In addition to these already known ®ve main sedimentary events, CALMAR data have revealed two more very recent events that occurred in the study area (unit/event 6 and event 7: see below, and Fig. 7).

28

L. Drozet et al. / Marine Geology 176 (2001) 23±37

Fig. 5. Main groups of echo facies encountered in the study area. See Table 1 for description of facies.

Very high-resolution seismic data and cores permitted to characterize these very recent events and the chronology of all the gravity transport events

during the Late Quaternary. Because of very highresolution required to study such recent sedimentary events, the data used (3.5 and 5 kHz pro®les, acoustic imagery) cannot image the too deeply seated unit 3 as they have poor penetration. Units 1 and 2 are recorded

L. Drozet et al. / Marine Geology 176 (2001) 23±37

29

Table 1 Description of main 3.5 and 5 kHz echo facies, associated morphologies, and interpretation

Strati®ed echo-facies

Transparent echo-facies

3.5 kHz echo-facies

5 kHz echo-facies

Description

Main physiographical occurrence

S1

MS1

Parallel and continuous re¯ections Undulated parallel and continuous re¯ectors

RhoÃne Fan western levee

S2

MS2

Discontinous re¯ectors prolonged surface echo

T1

MT1

Absence of re¯ectors thin surface echo

T2

MT2

Presence of a basal re¯ector, prolonged surface echo Similar to MT2 but with an Distal southern extension of T1/ hummocky top surface MT1, outlet of La Fonera Canyon

Thin remobilized sediments (distal debris ¯ow)

Strong and prolonged surface echo, with discontinuous re¯ectors at the top Strong and prolonged surface echo, absence of re¯ectors

Base of slope, in the vicinity of Marti and Catherine-Laurence canyons, small depressions on the PL Ridge Bottom of SeÁte, Cap Creus and La Fonera canyons

Very coarse-grained sediments

Large irregular hyperbolae, thin surface echo Small regular hyperbolae

Base of PL slope, canyons ¯anks, Artifacts related to abruptness scars of reliefs, masking underlying true echo-facies Isolated ¯at units at the outlet of Erosion by strong currents SeÁte Canyon and along the RhoÃne Neochannel

S1a

MT2a

Nonpenetrative echo-facies

F1

F2

Hyperbolic echo-facies

H1 H2

in their sur®cial parts and are included in this description, since they constitute the ªbasementº of units 4 and 5. 4.1. Very high-resolution echo facies and interpretations Combined analysis of very high-resolution seismic data (3.5 kHz pro®les), associated morphological framework and sediment lithologies when available, resulted in the classi®cation of echo facies into four main groups (Fig. 5). These groups are similar to those reported in earlier works from adjacent sectors of the Western Mediterranean, either the RhoÃne Fan

Interpretation

Overbank deposition by over¯ow of turbidity currents Pyreneo-Languedocian Ridge Overbank deposition by over¯ow of turbidity currents, with migrating sediment-waves RhoÃne Neofan and a smaller lobe Coarse-grained turbidites north of the Neofan deposited by unchannelized turbidity currents Base of slope depression between the RhoÃne Fan and PL Ridge, under the Neofan Distal southern extension of T1/ MT1

Thick remobilized sediments (debris ¯ow)

Thin remobilized sediments (distal debris ¯ow), eroded by currents ¯owing through La Fonera Canyon

Very coarse-grained sediments

(Coutellier, 1985) or farther eastwards, the LiguroProvencËal margin (Gaullier, 1993). Other data used (5 kHz pro®les) offer a better penetration below the sea ¯oor, but the facies themselves are not drastically different. The four main groups are (in order of decreasing areal extent): (1) the transparent facies, (2) the strati®ed facies, (3) the non-penetrative facies and (4) the hyperbolic facies. Each of these groups can be subdivided into two to three main sub-groups. Table 1 synthesizes the main characteristics, associated morphologies, and interpretations about the possible origin and signi®cance of the main groups of facies. The distribution of these facies is shown in Fig. 6:

30

L. Drozet et al. / Marine Geology 176 (2001) 23±37

Fig. 6. Distribution of the very high-resolution echo facies and location of cores available in the area. See text and Table 1 for description of echo facies, and Fig. 5 for examples; see Fig. 2 for canyon legend.

The strati®ed facies (S) are typically encountered on turbidite levees (S1), either on the RhoÃne Fan (unit 1) or on the undulated PL Ridge (unit 2), or on lobes (S2) including the neofan (unit 5) and a smaller lobate body located northwards of the neofan (see below). More discontinuous re¯ectors in the neofan and the

northern small lobe are interpreted to be due to higher sand content, as con®rmed by the lithology of cores 81CO50 and 81CO52 (Table 2). The transparent facies (T), imaging the debris-¯ow (unit 4), occupy a broad region under the neofan between the PL Ridge and the RhoÃne Fan (T1), and

L. Drozet et al. / Marine Geology 176 (2001) 23±37

31

Table 2 Lithological description of piston cores in the study area, with corresponding high-resolution pro®le echo facies, cored sedimentary units, and measured ( p , source: MeÂar, 1984) or estimated ( p p , source: Limonov et al., 1993) ages of sediments. Note that, due to the coring technique, the very top of the cores are most likely to be missing, and ages indicated at the top of cores 80COR01 and 81CO49 are probably not representative of the surface sediment Cores

Echo-facies

Units

Lithological description (from top to bottom)

Age

80COR01 p

T2/MT2

6

top: 3.45 ^ 0.1 ka bp a 50 cm bsf: 5.2 ^ 0.1 ka bp a

81CO49 p

T1

4

S2 S2 S2

6 6 5

10±20 cm of post-glacial mud Alternating centimetric-scale sandy layers and greyey silty mud 10±20 cm of post-glacial mud alternating centimetric-scale sandy layers and greyey silty mud grey mud with numerous diagenetic aggregates of monosulfates ungraded sands (300 cm) ungraded sands with Holocene mud clasts turbidites (50 cm) silty mud with mud or sand clasts (150 cm) sandy mud (1 cm) foraminifer-rich mud (29 cm) carbonaceaous mud (7 cm) silty mud (1 cm) mud (4 cm) mud with cross laminations (4 cm) carbonaceous mud (2 cm) clayey mud (1 cm) bioturbated mud with numerous foraminifers (27.5 cm) carbonaceous mud (2 cm) turbidites (83 cm) compacted brown mud

81CO50 p 81CO52 p 81CA04 p

Length (cm)

310 258 200

65 p p

37

S2/MS2

5

66 p p

10

T1/MS2a

5

67 p p

122

T1/MT2

4 or 5

68 p p a b

10

MS2a

5 or 6

top: 4.9 ^ 0.1 ka bp a

top: 130 ^ 50 a bp a (pteropods) 50 cm bsf: 23.6 ^ 0.7 ka bp a (shelly bed) Holocene b Holocene b

Holocene b WuÈrm b Holocene b

Radiocarbon dates. Biostratigraphy.

on the lower slope between Marti and Petit-RhoÃne canyons (T2). Upslope, several morphologic scars on the continental slope between SeÁte and Marti canyons, and scars on the western levee of the RhoÃne Fan could represent the sources of the debris¯ow (see Figs. 2 and 8). The non-penetrative facies (F) are characteristic of ªroughº sea¯oor, either on slope sector between SeÁte and Marti canyons (F1), or on canyons ¯oor (F2) where it is probably indicative of coarse-grained surface material. The hyperbolic facies (H) are indicative of irregular topography diffracting acoustic waves. They are located on slope sectors with abrupt gradients (H1) or on relatively ¯at areas, in the neochannel or at the mouth of SeÁte Canyon (H2). In the later case,

facies H2 is interpreted as imaging sea¯oor irregularities created by erosion by strong currents. The 10 m high morphologic scar at the outlet of SeÁte Canyon (Fig. 6, H2) is thought to represent the minimum amount of sediments that have been removed from the SeÁte Canyon and Valley ¯oors. 4.2. Acoustic imagery and interpretation The EM12 D swath mapping system produces a planar acoustic image of the sea¯oor at a shallow and inconstant depth (0±8 m), due to the variation of penetration which depends strongly on physical properties of the sediments and incident angle of acoustic waves (Unterseh, 1999). The acoustic mosaic of the entire continental slope

Fig. 7. Backscattering acoustic image of CALMAR study area (a) and interpretation (b) (slighlty modi®ed from Kergoat, 1998, and Berne et al., 1999). Light tones: low backscattering; dark tones: high backscattering. See Fig. 2 for canyon legend.

32 L. Drozet et al. / Marine Geology 176 (2001) 23±37

L. Drozet et al. / Marine Geology 176 (2001) 23±37

and rise (Berne et al., 1999) (Fig. 7) appears relatively highly backscattering (dark tones), except in some restricted areas, including the study area, between the RhoÃne Fan and PL Ridge, where two sectors with lower backscattering levels (light tones) are present. 1-A generally low backscatter area occurs as small patches of alternating low and moderate backscatter arranged in a large area covering both the debris ¯ow and the neofan. This facies (unit 6) occupies about 1200 km 2 (Kergoat, 1998). Since this low backscattering area covers both units 4 and 5, that were, prior to this study, the most recent units known in this area of the rise, it is proposed that this backscattering facies corresponds to very recent deposits. These deposits, nearly not visible on 3.5 kHz data, are probably very thin (,1 m). Distribution of this facies, downstream from the SeÁte erosional area (facies H2, Fig. 6), suggests that these sediments could originate from material eroded inside the SeÁte Canyon and at its mouth and redeposited by non-channelized currents. 2-A lobate zone of moderate backscattering facies occurs at the outlet of La Fonera Canyon. On the north-eastern side of this `lobe', straight and long NW±SE alignments are interpreted as erosional dunes. On its south-western side, E±W wavy and short lines, very similar to, although smaller than, those observed on the PL Ridge, are supposed to represent the crest of sediment waves. These lineations are interpreted as evidence for circulation inside the canyon, causing erosion at the left side of its mouth, and possibly depositing small sediment waves on its right side (event 7). The deposited product of this erosion is probably very thin, and was not identi®ed on the 3.5 kHz data, on this lobate moderate backscattering zone, nor downslope from it. The moderate backscattering zone seems to cover the previous low backscatter area (Fig. 7). Because no other sedimentary event, neither depositional nor erosional, is evidenced in this area, we propose that erosion at the mouth of La Fonera Canyon (event 7) is the latest sedimentary event that occurred on the rise of the PL margin. 4.3. Recent sedimentary evolution 4.3.1. Depositional history The study of the interfan area between the PL Ridge and the RhoÃne Fan has highlighted several erosional/

33

depositional events since the end of major deposition on the turbidite systems of the Gulf of Lions. This part of the rise, which was not an area of intense activity earlier, became a preferred site of erosion and accumulation of sediments, while the PL Ridge and RhoÃne Fan became chie¯y inactive, indicating a shift of activity and a change of sedimentation processes from aggradational turbidite deposition to mainly erosional, sheet-like unchannelized deposition. Four main sedimentary events have been documented from the new CALMAR data (the previous events not being visible on data used). They followed one another until the present-day. The two latest events were unknown before CALMAR cruise. Following the deposition of turbidite systems (units 1 and 2) and of unit 3, the area was the site of the following successive events (Fig. 8): ÐA period of mass-movements (event 4) resulted in the deposition of a giant debris-¯ow (unit 4). This debris-¯ow is 40±50 km wide and extends at least 100 km from the northern base of slope to the southernmost side of the investigated area, its areal extent being estimated to at least 25 £ 10 6 km 2 (Kergoat, 1998). Its thickness is more than 80 m (maximum penetration on 3.5 kHz data). There are several possible source-areas for this debris¯ow, including failure of the western levee of the RhoÃne Fan, in relation with halokinetic movements of listric faults (Droz, 1983), the lower slope between Marti and SeÁte canyons, which is marked by numerous morphological scars, and the ¯oor of SeÁte Canyon itself. Another possible source is the erosion of the shelf-edge around the CatalanoLanguedocian canyons as proposed by Berne et al. (1999). ÐA period of reactivation of turbiditic activity (event 5) on the RhoÃne Fan, that resulted in the deposition of the neofan (unit 5). Another small lobe of similar facies (unit ª5?º, Fig. 8) is evident north of the neofan. This faces an area of the fan where the levee crest is lowered due to fault scars. This small lobe could be contemporaneous with the neofan but no data demonstrates this relationship. ÐA period of strong current activity (event 6) inside SeÁte Canyon (and probably also in its tributaries, the PL canyons) resulted in an erosional/depositional body at the termination of the canyon (unit 6).

Fig. 8. Main sedimentary events that occurred during the Late Pleistocene and the Holocene in the western Gulf of Lions. (a) Present-day superposition of units resulting from seven main sedimentary events (numbered). (b) Chronological deposition with proposed respective ages, as deduced from dating of cores (see Table 2).

34 L. Drozet et al. / Marine Geology 176 (2001) 23±37

L. Drozet et al. / Marine Geology 176 (2001) 23±37

ÐA very recent, or even still active, period of highenergy circulation inside La Fonera Canyon caused mainly erosion at its termination (event 7). Events 6 and 7 are very recent since they are only visible on acoustic imagery, and this erosional/depositional regime seems to characterize the Holocene high sea level stand. However, we cannot exclude the possibility that similar events could occur at any time during sedimentation of the margin, and thus during the deposition of previous units 4 and 5. 4.3.2. Possible chronostratigraphy Among the available cores (Table 2 and Figs. 1 and 6) some have provided biostratigraphic and radiocarbon dates suitable to document the chronostratigraphy of the area (Fig. 8b). This area became the main depocenter of this portion of the margin after the cessation of active deposition on the RhoÃne Fan (not including the neofan) that is thought to have occurred by 21 ka bp (MeÂar, 1984). Ages obtained in core 81COR01 (MeÂar, 1984) between 3.45 ka bp at the top to 5.2 ka bp at 50 cm below sea-¯oor, and that were earlier considered to date the topmost sediments of the debris-¯ow (unit 4), are, from our interpretation, more probably representative of the very recent debris ¯ow sediment cover, and similar to the thin deposits of unit 6. In the same way, the very recent (130 a bp, MeÂar, 1984) radicarbon age obtained at the top of core 81CO52, if valid, is also probably indicative of recent sediments linked to SeÁte depositional lobe (unit 6).

The occurrence of several sedimentary units of comparable acoustic facies and probably similar period of accumulation as the western debris ¯ow (event 4), in other parts of the Western Mediterranean, seems to indicate that mass-movement processes have been particularly active during the Late Quaternary. On the eastern levee of the RhoÃne Fan, Droz and Bellaiche (1985) described an extended, up to 100 m thick debris ¯ow, lying unconformably on overbank deposits. This 170 km 3 (Coutellier, 1985) eastern debris ¯ow was interpreted to originate mainly from the Petit-RhoÃne±Grand-RhoÃne intercanyon area

35

where numerous erosional scars occur on the slope, and partly from slidings inside the Grand-RhoÃne Canyon. Off the Ebro shelf, the ªBig 95º slide represents an estimated total volume of 26 km 3 (Berne et al., 1998a; Canals et al., 2000). The ªmegaturbiditeº identi®ed in the Balearic abyssal plain (Rothwell et al., 1998), is a giant unit of 500 km 3 of volume and about 10 m thick. Except the Balearic abyssal plain megaturbidite, that was shown to have accumulated during the Upper Pleistocene (22 ka bp, Rothwell et al., 1998), the age of emplacement of these gravity deposits is not well constrained. Event 4, on the PyreneoLanguedocian rise, is inferred to have occurred after 21 ka bp (age of the stop of activity of the RhoÃne Fan), that is some times close to the end of isotopic stage 2. The eastern debris ¯ow is not dated, but with respect to its position on the RhoÃne Fan, could be of the same age. The morphologically very fresh ªBig 95º slide is supposed to be the latest major sedimentary event in the area (Berne et al., 1998a). Considered all together, these mass-movement deposits represent an estimated total volume of more than 900 km 3 of remobilized sediments, most probably contemporaneously emplaced during a major period of instability in different parts of the Western Mediterranean. This instability period is, therefore, assumed to mark a major regional event.

The new CALMAR data show that, during the Late Quaternary, the Gulf of Lions rise has been the site of two main sedimentary periods characterized by very different sedimentological processes. During the isotopic stage 2 (around 21 ka bp) sedimentation conditions changed drastically from a mainly aggradational ªregularº turbidite activity that had been more or less continuous since the Pliocene, related to the feeding activity of the Pyreneo-Languedocian and Rhodanian canyons, to a period essentially characterized by remobilization by either mass-movement or erosional processes of formerly deposited sediments. The western debris ¯ow (event 4), inferred to have been emplaced in Late Pleistocene, attests to a period of instability that probably also generated the other

36

L. Drozet et al. / Marine Geology 176 (2001) 23±37

mass-movement deposits identi®ed in various parts of the Western Mediterranean (e.g. Eastern debris ¯ow of the RhoÃne Fan: Droz and Bellaiche, 1985; ªBig 95º slide of the Ebro slope: Berne et al., 1998a; ªmegaturbiditeº of the Balearic abyssal plain: Rothwell et al., 1998). This instability period appears as a major regional event. Following this episode of mass-movement, ªfreshº ¯uvial inputs are trapped in estuaries because of the post-glacial rise of sea-level (Posamentier et al., 1998) and canyons and channels seem to be mainly undergoing erosion. Deposition in the basin appears volumetrically and geographically restricted and originates from recycling of older sediments: (a) the neofan sands (event 5) result from the reshaping of the channel ¯oor by upstream retrogressive erosion in order to restore a new equilibrium pro®le consecutively to avulsion (Pirmez and Flood, 1995; Pirmez et al. 1997; Lopez, 2001); (b) shelf-edge erosion during the early transgression reworks shelf-perched lowstand shorefaces and provides sands to the slope/ rise (Berne et al., 1998b); (c) strong erosional currents inside and at the outlet of SeÁte Canyon remove sediments that are redeposited downstream as a lobate unchannelized body (event 6); lastly, currents ¯owing inside La Fonera Canyon erode or reshape older sediments (event 7), with probably minor redeposition. At the present stage, the question of the origin of these erosional currents is not solved. Several hypotheses will have to be tested in the future. They include (a) currents generated by mass-wasting on the continental slope and rise, (b) circulation of Deep Mediterrean Water, (c) winter downwellings, or a combination of these different processes.

New data used and published in this paper were collected onboard the R/V L'Atalante. We thank the Captain and crew members of L'Atalante and the EM12D and seismic operators from GENAVIR for technical support during CALMAR 97 cruise. Processing of EM12D bathymetry, EM12D acoustic imagery and seismic pro®les was realized by, respectively, B. Loubrieu, E. Le Drezen and J.-P. Le Formal from IFREMER. Dr. Rothwell and an anonymous reviewer greatly improved the manuscript.

Alonso, B., Canals, M., Got, H., Maldonado, A., 1991. Sea valleys and related depositional systems in the Gulf of Lions and Ebro continental margin. Am. Assoc. Petrol. Geol. Bull. 75, 1195± 1214. Bellaiche, G., Orsolini, P., Petit-Perrin, B., Berthon, J.L., Ravenne, C., Coutellier, V., Droz, L., Aloisi, J.C., Got, H., MeÂar, Y., Monaco, A., Auzende, J.M., Beuzart, P., Monti, S., 1983. Morphologie au sea-beam de l'Eventail sous-marin profond du RhoÃne (RhoÃne deep-sea fan) et de son canyon affeÂrent. C. R. Acad. Sci. 296, 579±583 (seÂrie II). Bellaiche, G., Coutellier, V., Droz, L., 1986. Seismic evidence of widespread mass transport deposits in the RhoÃne deep-sea fan: their role in the fan construction. Mar. Geol. 71, 327±340. BerneÂ, S., Canals, M., Alonso, B., Loubrieu, B., Cochonat, P., the ªBIG 95º, ªCALMAR97º shipboard parties, 1998. Recent slope failures and mass-movements in the NW Mediterranean Sea. Sea¯oor characterisation/mapping including swath bathymetry, side-scan sonar and geophysical surveys. Third European Marine Science and Technology Conference. Lisbon, May 23±27 (Session report) pp. 111±126. BerneÂ, S., Lericolais, G., Marsset, T., Bourillet, J.-F., De Batist, M., 1998b. Erosional offshore sand ridges and lowstand shorefaces: examples from tide- and wave-dominated environments of France. J. Sediment. Res. 68 (4), 540±555. BerneÂ, S., Loubrieu, B., et l'Equipe Calmar embarqueÂe, 1999. Canyons et processus seÂdimentaires reÂcents sur la marge occidentale du Golfe du Lion. Premiers reÂsultats de la campagne Calmar. C. R. Acad. Sci. Paris, Sciences de la Terre et des PlaneÁtes 328, 471±477. Canals, M., 1985. Estructura sedimentaria y evolucion morfologica del talud y el glacis continentales del Golfo de Leon: fenomenos de destabilizacion de la coberta sedimentaria plio-cuaternaria. Thesis, Barcelona University, 618 pp. Canals, M., Got, H., 1986. La morphologie de la pente continentale du Golfe du Lion: une reÂsultante structuro-seÂdimentaire. Vie et Milieu 36, 153±163. Canals, M., Casamor, J.L., Urgeles, R., Lastras, G., Calafat, A.M., De Batist, M., Masson, D.G., BerneÂ, S., Alonso, B., 2000. The Ebro continental margin, Western Mediterranean Sea: interplay between canyon-channel systems and mass wasting processes. Gulf Coast Section-SEPM Foundation 20th Annual Bob. F. Perkins Research Conference on Deep-water Reservoirs of the World. Houston, Texas, December 2000, CD Edition, 20 pp. Coutellier, V., 1985. Mise en eÂvidence et roÃle des mouvements gravitaires dans l'eÂvolution de la marge continentale: exemple des marges du Golfe du Lion et de la Provence occidentale. 3rd cycle Thesis, Paris VI University, 189 pp. Droz, L., 1983. L'eÂventail sous-marin profond du RhoÃne (Golfe du Lion): grands traits morphologiques et structure semi-profonde. 3rd cycle Thesis, Paris VI University, 195 pp. Droz, L., 1991. Les eÂventail sous-marins profonds: structure et eÂvolution seÂdimentaire, aÁ partir de l'eÂtude comparative de trois eÂdi®ces, l'Eventail du RhoÃne, la Ride du Var, le CoÃne de l'Indus. Habilitation aÁ Diriger des Recherches, Universite Paris VI, 254 pp.

L. Drozet et al. / Marine Geology 176 (2001) 23±37 Droz, L., Bellaiche, G., 1985. Rhone deep-sea fan: morphostructure and growth pattern. Am. Assoc. Petrol. Geol. Bull. 69, 460±479. Droz, L., Loubrieu, B., BerneÂ, S., Cochonat, P., CALMAR shipboard scienti®c party, 1997. Turbidites of the Western Golfe du Lion: relationships between Pyreneo-Languedocian and Rhone inputs. European Geophysical Society, 23rd General Assembly, Nice, avril 20±24 1998. Annales Geophysicae, part I, Society Symposia. Solid Earth Geophysics and Geodesy. Supplement I to vol. 16, p. C307. Gaullier, V., 1993. Diapirisme salifeÁre et dynamique seÂdimentaire dans le bassin liguro-provencËal: donneÂes sismiques et modeÁles analogiques. 3rd cycle Thesis, Paris VI University. Kenyon, N.H., Millington, J., Droz, L., Ivanov, M.K., 1995. Scour holes in a channel-lobe transition zone on the RhoÃne Cone. In: Pickering, K.T., Hiscott, R.N., Kenyon, N.H., Lucchi, F.R., Smith, R.D.A. (Eds.), Atlas of Deep-water Environments: Architectural Style in Turbidite Systems. Chapman and Hall, London, pp. 212±215. Kergoat, R., 1998. Mise en place d'un debris-¯ow sur le glacis catalano-languedocien. MeÂmoire de DEA, Universite des Sciences et Techniques de Lille, Lille, 56 pp. Limonov, A.F., Woodside, J.M., Ivanov, M.K., 1993. Initial results of the UNESCO-ESF `Training Through Research' Cruise of R/ V Gelendzhik in the Western Mediterranean, June±July 1992. Geological and Geophysical Investigations of Western Mediterranean Deep Sea Fans. UNESCO Reports in Marine Science, 154 pp. Lopez, M., 2001. Architecture and depositional pattern of the Quaternary deep-sea fan of the Amazon. Mar. Petrol. Geol. (in press). MeÂar, Y., 1984. SeÂquences et uniteÂs seÂdimentaires du glacis rhodanien. 3rd cycle Thesis, Perpignan University, 223 pp. Pirmez, C., Flood, R.D., 1995. Morphology and structure of Amazon Channel. In: Flood, R.D., Piper, D.J.W., Klaus, A. (Eds.), Proceedings of the Ocean Drilling Program, Initial Reports 155, 23±45. Pirmez, C., Hiscott, R.N., Kronen Jr., J.D., 1997. Sandy turbidite

37

successions at the base of channel-levee systems of the Amazon Fan revealed by FMS and cores: unraveling the facies architecture of large submarine fans. In: Flood, R.D., Piper, D.J.W., Klaus, A., Peterson, L.C. (Eds.), Proceedings of the Ocean Drilling Program, Scienti®c Results 155, 7±33. Posamentier, H.W., Jervey, M.T., Vail, P.R., 1998. Eustatic controls on clastic deposition. II. Sequences and system tract models. Sea-level changes: an integrated approach, Wilgus, C.K., Hastings, B.S., Kendall, C.G.S.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), SEPM, Spec. Publ. 42, 125±154. Rabineau, M., BerneÂ, S., Le Drezen, E., Lericolais, G., Rotunno, M., 1998. 3D architecture of lowstand and transgressive Quaternary sand bodies on the outer shelf of the Gulf of Lions, France. Mar. Petrol. Geol. 15, 439±452. Rothwell, R.G., Thomson, J., KaÈhler, G., 1998. Low sea-level emplacement of a very large Late Pleistocene megaturbidite in the western Mediterranean Sea. Nature 392, 377±380. Savoye, B., Piper, D., Droz, L., 1993. Plio-Pleistocene evolution of the Var deep-sea fan off the French riviera. Mar. Petrol. Geol. 10, 550±571. Tesson, M., Allen, G.P., Ravenne, C., 1993. Late Pleistocene shelfperched lowstand wedges on the RhoÃne continental shelf. Sequence stratigraphy and facies associations. In: Posamentier, H.W., Summerhayes, C.P., Haq, B.A., Allen, G.P. (Eds.), Int. Assoc. Sedimentol., Spec. Publ. 18, 183±196. Torres, J., 1995. Analyse deÂtailleÂe du transfert de seÂdiment du continent vers le bassin: le Quaternaire terminal au large du delta du RhoÃne (MeÂditerraneÂe nord-occidentale). Thesis, Universite de Bretagne Occidentale, Brest, 353 pp. Torres, J., Droz, L., Savoye, B., Terentieva, E., Cochonat, P., Kenyon, N.H., Canals, M., 1997. Deep-sea avulsion and morphosedimentary evolution of the RhoÃne Fan Valley and Neofan during the Late Quaternary (northwestern Mediterranean Sea). Sedimentology 44, 457±477. Unterseh, S., 1999. Cartographie et caracteÂrisation du fond marin par sondeur multifaisceaux. University thesis, University of Nancy, Institut National Polytechnique de Lorraine, 234 pp.