Sedimentary Geology 229 (2010) 110–123

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Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o

The sandy channel–lobe depositional systems in the Gulf of Cadiz: Gravity processes forced by contour current processes V. Hanquiez a,⁎, T. Mulder a, S. Toucanne a, P. Lecroart a, C. Bonnel b, E. Marchès a, E. Gonthier a a b

UMR/CNRS 5805-EPOC, Université Bordeaux 1, avenue des facultés, 33405 Talence cedex, France Laboratoire de Modélisation et d'Imagerie en Géosciences UMR 5212, Université de Pau et des Pays de l'Adour, Bât IPRA, Avenue de l'Université, BP 1155, 64013 Pau Cedex, France

a r t i c l e

i n f o

Available online 18 May 2009 Keywords: Gulf of Cadiz Mediterranean Outflow Water Contourite channels Sandy lobes Climate forcing

a b s t r a c t The sedimentation in the Gulf of Cadiz (NE Atlantic Ocean) is significantly controlled by the Mediterranean Outflow Water (MOW). Along its pathway onto the continental slope, the MOW is canalized by contourite channels, some of them feeding gravity sandy channel–lobe depositional systems firstly recognized in previous study [Habgood et al., 2003. Deep-water sediment wave fields, bottom current sand channels and gravity flow channel–lobe systems: Gulf of Cadiz, NE Atlantic. Sedimentology 50(3), 483–510.]. Using very high resolution acoustic data and cores, a detailed characterization and a new evolution pattern of these channel–lobe depositional systems is established. Complex internal geometry of the lobes shows several depositional units revealing a polyphase evolution of these systems, with a general progradation punctuated by retrogradation and avulsion phases. A gravity origin controlled by contouritic processes and climatic changes is demonstrated for the feeding and the evolution of these sandy channel–lobe depositional systems. Climate oscillations, via the MOW variations, act as a major forcing of the activity of the channel– lobe depositional systems during the Late Quaternary. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The Gulf of Cadiz (Eastern part of the Central Atlantic Ocean) extends from the Straits of Gibraltar (Spain) to Cape San Vicente (Portugal) (Fig. 1A). The oceanography and the sedimentation of the Gulf of Cadiz are influenced by a permanent current of warm and saline intermediate water mass (Mediterranean Outflow Water, MOW, Fig. 1A) flowing out the Mediterranean Sea along the upper and middle slopes between around 500–1400 m water depth towards the Atlantic Ocean (Madelain, 1970). Due to the predominance of contour currents in the upper and middle slopes of the Gulf of Cadiz, many studies focused on the contourite deposits (drifts), especially on their morphology and architecture (Faugères et al., 1985, 1994, 1984; Gonthier et al., 1984; Kenyon and Belderson, 1973; Stow et al., 1986, among others). Here, quality of very high resolution acoustic data set allows a new description of three channel–lobe depositional systems (Tasyo, Aveiro and Lolita systems, Fig. 1B), previously identified by Habgood et al. (2003), showing many similarities with the channel–levee–lobe complexes found in the deep sea turbidite systems (e.g., Deptuck et al., 2008; Normark, 1978; Normark et al., 1993; Walker, 1978). Originality of the Gulf of Cadiz lies in the absence of canyon upstream of the channel–lobe depositional systems (CLS). Based on the spatial organization and internal architecture of these lobe complexes, our

⁎ Corresponding author. Tel.: +33 5 40 00 34 35; fax: +33 5 56 84 08 48. E-mail address: [email protected] (V. Hanquiez). 0037-0738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2009.05.008

study suggests a new evolution pattern for these depositional systems and shows a major role of the MOW on their evolution. 2. Regional background The study area is located within the overflow sedimentary lobe sector of the Gulf of Cadiz Contourite Depositional System (HernándezMolina et al., 2003), which has been generated by the MOW, then controlled both by Pliocene and Quaternary environmental and paleoceanographic changes (Hernández-Molina et al., 2003, 2006). The lobe sector is characterized by the circulation of the Mediterranean Lower Water (southern branch of the MOW) which currently interacts with the sea floor down to 1400 m water depth (Hanquiez, 2006; Madelain, 1970). The Mediterranean Lower Water is channelled westward by the Main MOW Channel, (Mulder et al., 2003) and northward by the Cadiz Contourite Channel (Hernández-Molina et al., 2003) (Fig. 1). South of the Cadiz Contourite Channel and the Cadiz Valley (Hernández-Molina et al., 2003, 2006; Llave, 2004) (Fig. 1), and west of the Main MOW Channel, the Giant Contouritic Levee built by the overflow of the Main MOW Branch is observed (Mulder et al., 2003) (Fig. 1). Active secondary channels, such as the Gil Eanes Channel (Kenyon and Belderson, 1973), are present on the Giant Contouritic Levee and drain downslope a part of the MOW (Habgood et al., 2003; Hanquiez et al., 2007; Mulder et al., 2003) (Fig. 1). The Tasyo, Aveiro and Lolita CLS on which we focused in this study are described from the distal boundary of the Giant Contouritic Levee, and are consequently out of the present-day influence of the MOW. Their path developed in areas

V. Hanquiez et al. / Sedimentary Geology 229 (2010) 110–123 Fig. 1. A. Map of the Gulf of Cadiz showing the general MOW pathway (black dotted arrows, modified from Madelain, 1970). B. Illuminated, grey-shaded view of the study area mapped during the CADISAR and CADISAR2 cruises based on EM300 multibeam echosounder (black rectangles are channel–lobe depositional system–CLS-location); white stars are core location. AMV: Aveiro Mud Volcano; CC: Cadiz Contourite Channel; CL: Giant Contouritic Levee; CV: Cadiz Valley; GEC: Gil Eanes Channel; LMV: Lolita Mud Volcano; MMC: Main MOW Channel; PB: ponded basins; SC: active secondary channels; SPMV: St. Petersburg Mud Volcano; TMV: Tasyo Mud Volcano.

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112 V. Hanquiez et al. / Sedimentary Geology 229 (2010) 110–123 Fig. 2. A. High resolution (30 × 30 m grid) illuminated, grey-shaded EM300 bathymetric map of the Tasyo CLS (see location on Fig. 1); white stars are core location. B. High resolution (12.5 × 12.5 m grid) EM300 acoustic imagery of the Tasyo CLS. C. Interpretative map of the spatial signature of the Tasyo CLS; black dotted lines are sediment wave crests.

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characterized by small ponded basins (Mulder et al., 2003) and mud volcanoes (e.g. St. Petersburg, Tasyo, and Aveiro mud volcanoes; Kenyon et al., 2000; Pinheiro et al., 2003; Somoza et al., 2003) (Fig. 1). 3. Material and methods The geophysical data presented in this paper were collected on the R.V. “Le Suroît” during the CADISAR (2001) and CADISAR2 (2004) cruises. Bathymetric and acoustic imagery data were acquired with a SIMRAD EM300 multibeam echosounder (32 kHz) and processed with CARAIBES software. The spatial and vertical bathymetry resolution is 30 m × 30 m and 2 m, respectively, and the imagery spatial resolution is 12.5 m. The acoustic data were corrected for salinity and density effects using 3 CTD (SBE19 probes) and 85 thermoprobes (Sippican). The multibeam data set has been locally supported by the superficial information of the cores (Hanquiez et al., 2007). EM300 imagery was completed by SAR (Système Acoustique Remorqué) imagery (180 kHz, 0.25 m spatial resolution; Farcy and Voisset, 1985). The TRITON ELICS sub-bottom echosounder was used to characterize upper sediment layers. This equipment provides a very high resolution seismic reflection using the Chirp mode (2–5 kHz) and allows up to 75 m and 0.75 m signal penetration and vertical resolution, respectively. Sedimentological analysis (grain size, facies, components, X-ray imagery) of six Küllenberg cores retrieved in the Tasyo, Aveiro and Lolita CLS during the CADISAR2 and the CADIZ-GORRINGE (R.V. “Urania”, 2004) cruises were performed in order to characterize their composition. Two other corings attempted in the distal part of the Lolita CLS failed due to the coarse-grained nature of the sea floor (Mulder, 2004). 4. Results 4.1. Tasyo channel–lobe depositional system 4.1.1. Morphological features The Tasyo CLS is located south-east of the Giant Contouritic Levee, south of the St Petersburg Mud Volcano and east of the Tasyo Mud

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Volcano (Fig. 2A). It is a NNE/SSW directed channel stretching between 1 048 and 1 222 m water depth with a regional slope b1°. This 19 km long straight channel (sinuosity = 1.21) is 550 m wide and 30 m deep in this upstream part, narrowing to 250 m and shoaling to 3 m thereafter. Along its path, the Tasyo Channel develops through a wavy topography showing wave crests which are mainly directed NNE/SSW and particularly well-developed on the left channel levee. South of the Tasyo Mud Volcano, the morphology gradually smoothed. The EM300 resolution precludes the observation of a sedimentary lobe off the channel. 4.1.2. Backscatter analysis The Tasyo CLS shows a low backscatter compared to the surrounding sea floor (Fig. 2B). The talweg and levees show a very low backscatter, especially south of 35°48′N. In the lower part of the Tasyo Channel, a N/S oriented and elongated low backscatter patch (13 km2) is observed. In agreement with Habgood et al. (2003), we interpret this sedimentary feature as a gravity flow lobe. Backscatter variations allow the identification of a first lobe (lobe B in Fig. 2C) partly covered by a lower backscatter lobe (lobe A in Fig. 2C) crossed in this upstream part by the Tasyo Channel. In details, N/S to NW/SE directed lineaments are observed over the lobe A. We discuss their origin in the last section of this paper. Chirp seismic data reveal high amplitude reflections in the Tasyo Channel (Fig. 3). The channel levees (b5 m) are symmetrical and are defined by hyperbolic or semi prolonged bottom echoes. Westward of the Tasyo CLS, the sea floor shows a prolonged bottom echo and a stairs-like morphology. We interpret this morphology as the imprint of sedimentary lobes: lobes A and B, previously described from the EM300 imagery, and lobe C, non-visible on the multibeam reflectivity map (Fig. 3). 4.1.3. Lithology Core CADI2KS15 retrieved in the lobe A of the Tasyo CLS is composed of an unimodal homogeneous, massive and structureless medium sand (120 µm b D50 b180 µm; D90 = 320 µm) (Fig. 4A). This sandy facies is also characterized by typical temperate to tropical

Fig. 3. Chirp seismic profile with interpretation across the Tasyo CLS (see location on Fig. 2).

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Fig. 4. Synthetic descriptions of cores acquired in the Tasyo (frame A. and B., see location on Figs. 1 and 2) and Aveiro (frame C. to E., see location on Figs. 1 and 5) CLS. Black points on the D50/D90 plots are locations of the illustrative frequency distributions.

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Fig. 4 (continued ).

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Fig. 6. Very high resolution (2.5 × 2.5 m grid) SAR imagery section (A) with interpretation (B) in the western part of the Aveiro CLS (see location on Fig. 5).

Fig. 7. Chirp seismic profile with interpretation across the Aveiro CLS (see location on Fig. 5).

planktic foraminifera such as Globigerinoides ruber, Globigerinoides conglobatus, Globigerinoides sacculifer, Globigerinoides trilobus and Globorotalia hirsuta (right coiling) (Duprat, comm. pers.).

Core SWIM04-41 retrieved directly east of the lobe A (ca. 500 m of core CADI2KS15), reveals a significant different lithology. An unimodal bioturbated clayey facies (D50 = 5 µm; D90 = 32 µm) composes the

Fig. 5. A. High resolution (30 × 30 m grid) illuminated, grey-shaded EM300 bathymetric map of the Aveiro CLS (see location on Fig. 1); white stars are core location. B. High resolution (12.5 × 12.5 m grid) EM300 acoustic imagery of the Aveiro CLS. C. Interpretative map of the spatial signature of the Aveiro CLS.

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upper part of the core. Rare centimetric sandy-rich layers are observed. A bimodal silty clayey facies (D50 = 8 µm; D90 = 160 µm), that we describe as mottled (Gonthier et al., 1984), composes the base of the core (Fig. 4B).

25 µm b D90 b 50 µm). At the base of core, intense bioturbation dominated by Zoophycos is observed (Fig. 4E).

4.2. Aveiro channel–lobe depositional system

4.3.1. Morphological features The Lolita CLS is located to the end of the Cadiz Contourite Channel, south of the Lolita Mud Volcano, in the shallow part of the Cadiz Valley (Fig. 8A). The Lolita Channel is a ENE/WSW directed channel stretching between 1525 and 1930 m water depth with a 1.5° regional slope. This 18 km long straight channel (S = 1.14) is 500 m wide and 30 m deep upstream, narrowing to 300 m and shoaling to 6 m thereafter. Three sections compose the channel at its head, and southward, two N/S directed tributaries joined it. In its central part, the Lolita Channel cut through an irregular mounded morphology, as a result of a mass deposit originating from the south Lolita Mud Volcano slope, where the failure scar is thoroughly visible. In the more distal part of the Lolita CLS, EM300 resolution precludes the observation of a sedimentary lobe off the channel.

4.2.1. Morphological features The Aveiro CLS is located in the eastern part of the ponded basin area, west of the Giant Contouritic Levee (Fig. 5A). The Aveiro Channel stretches between 1 308 and 1 465 m water depth with a regional slope b1°. This 12 km long straight channel (sinuosity = 1.16) is 450 m wide and 12 m deep, narrowing to 250 m and shoaling to 2 m thereafter. The Aveiro Channel is E/W oriented upstream and is lined by the end of the bottom-current Gil Eanes Channel lobe (Habgood et al., 2003). Westward, it cuts across a smooth topography and shows a NE/SW direction. The EM300 resolution precludes the observation of a sedimentary lobe off the channel. 4.2.2. Backscatter analysis The Aveiro CLS shows a low backscatter similar to the Gil Eanes Channel outlet, (Fig. 5B). In detail, the medium backscatter of the channel is lined on both sides by a lower backscatter edge. West of 7°35′W, a NNE/SSW oriented and elongated low backscattered patch (16 km2) is observed. In agreement with Habgood et al. (2003), we interpret this sediment feature as a gravity flow lobe. Backscatter variations allow the identification of a first lobe (lobe C in Fig. 5C) covered by a lower backscatter lobe (lobe B in Fig. 5C). Southward, a very low backscatter lobe (lobe A in Fig. 5C) covers the B and C lobes. In the western part of the Aveiro CLS, lobe A and lobe B are crossed by the Aveiro Channel. SAR imagery reveals the irregular surface of the Aveiro CLS lobes (Fig. 6). These unevenness result from N/S to E/W oriented shallow linear structures, of whom the Aveiro channel is the most pronounced. Chirp seismic data reveal high amplitude reflections in the Aveiro Channel (Fig. 7). This channel is bordered by 5 m thick levees with a right levee more developed. Levees are defined by a hyperbolic or semi prolonged bottom echo, which also characterizes the sedimentary lobe complex. In this lobe complex, discontinuous subbottom reflectors are observed. Outcrops of these reflectors correspond to the limits between the three lobes previously describe from the EM300 imagery. 4.2.3. Lithology Core CADI2KS16 retrieved in the Gil Eanes Channel lobe is composed of a unimodal medium sandy facies (D50 = 190 µm; D90 = 330 µm) (Fig. 4C). The massive feature of this facies is underlined by the lack of sedimentary structures on the X-ray imagery, except for two laminated centimetric levels around 20 and 35 cm depth. Core CADI2KS17 retrieved in the Aveiro Channel shows two sedimentary sequences separated by erosive contacts (S1 and S2 in Fig. 4D). S1 is composed of a bimodal clayey sandy mottled facies (D50 = 55 µm; D90 = 160 µm). S2 is composed of numerous unimodal sandy sub-sequences (85 µm b D50 b140 µm; 160 µm b D90 b 290 µm) separated by erosive contacts and showing cross beddings. From the base to the top, core CADI2KS17 is characterized by typical temperate to tropical planktic foraminifera, similar to those previously identified in the Tasyo CLS (core CADI2KS15). Core CADI2KS18 retrieved just outside of the Aveiro CLS shows alternation of a unimodal clayey facies (D50 = 5 µm; D90 = 20 µm) and a bimodal silty clayey mottled facies (5 µm b D50 b 8 µm;

4.3. Lolita channel–lobe depositional system

4.3.2. Backscatter analysis The Lolita CLS shows a low backscatter compared with the surrounding sea floor (Fig. 8B). In detail, the high backscatter of the Lolita Channel is lined on both sides by a low backscatter edge. Downstream of the Lolita Channel, an E/W oriented and elongated low backscattered patch (19 km2) is observed. In agreement with Habgood et al. (2003), we interpret this sediment feature as a gravity flow lobe. Backscatter variations allow the identification of a first lobe (lobe C in Fig. 8C) covered on the east by a lower backscatter lobe (lobe B in Fig. 8C). Lobe B is covered southward by a lower backscatter lobe (lobe A in Fig. 8C). In the western part of the Lolita CLS, lobe A and lobe B are crossed by the Lolita Channel. In detail, numerous E/W oriented irregular lineations are observed over the A and B lobes. We discuss their origin in the discussion. Chirp seismic data reveal a prolonged bottom echo in the Lolita CLS (Fig. 9A and B). In the proximal part of the lobe complex, the hyperbolic bottom echo highlights the presence of another channel (Fig. 9B). Two, more or less marked, outcropping reflectors are observed on Fig. 9A. We interpret these surfaces as limits of the lobes. 4.3.3. Lithology Core CADI2KS02 retrieved in the lobe C of the Lolita CLS is only 10 cm long and composed of a bimodal silty clayey mottled facies (D50 = 20 µm; D90 = 250 µm). 5. Discussion 5.1. Morphology and sedimentary evolution of the channel–lobe depositional systems The Tasyo, Aveiro and Lolita CLS show strong morphology similarities. These depositional systems present a 10 m thick lobe complex supplied by a short straight feeder channel. Acoustically, CLS are characterized by a low backscatter from the channel head to the distal part of the lobe complex, similar to those observed along the sandy major contouritic channels (i.e., Main MOW and Cadiz Contourite channels by Hanquiez et al., 2007). This comparison suggests the coarse-grained nature of the surface of the Tasyo, Aveiro and Lolita CLS. This is confirmed by the sandy sediments described on the Tasyo and Aveiro lobes with cores CADI2KS15 and CADI2KS17, respectively. By analogy with numerous studies in turbiditic environments (Bonnel et al., 2005; Deptuck et al., 2008; Gervais

Fig. 8. A. High resolution (30 × 30 m grid) illuminated, grey-shaded EM300 bathymetric map of the Lolita CLS (see location on Fig. 1); white stars are core location. B. High resolution (12.5 × 12.5 m grid) EM300 acoustic imagery of the Lolita CLS (this area is surveyed twice: mosaic 1 and 2 corresponds to CADISAR2 and CADISAR data, respectively). C. Interpretative map of the spatial signature of the Lolita CLS.

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Fig. 9. A. B. Chirp seismic profiles with interpretation across the Lolita CLS (see location on Fig. 8).

et al., 2006; Piper and Savoye, 1993; Twichell et al., 1991; among others), we interpret these sedimentary bodies as gravity driving forced deposits, i.e. the product of turbidity currents and/or debris flows. This affirmation is consistent with the progressive downward decrease of the channel width and depth, classically described in the deep sea turbidite systems and clearly different from current-controlled channels (see Habgood et al., 2003 for a thorough review). As a result, we consider the Tasyo, Aveiro and Lolita channels as preferential conduits for downslope transport of coarse-grained sediments. The gravity processes generating this transport are likely to explain the low backscatter fringes observed on both sides of the channels that may result from the overflowing of the sand-rich currents. Numerous lineations are observed at the surface of the studied lobes. These lineations have been previously recognized in the Lolita CLS by Habgood et al. (2003). In agreement with the “hairy deposits” recognized in the Mississippi (Twichell et al., 1991) and Zaire (Bonnel, 2005) lobe complexes, Habgood et al. (2003) interpret these features as a dendritic and bifurcating channel system, thus suggesting that channelling developed over the lobes. The very high resolution of acoustic data presented herein (i.e. SAR imagery) allows to describe, for the first time, such lineations at the surface of the Tasyo, Aveiro and Lolita CLS. Although we consider the above hypothesis as relevant, we proposed an alternative hypothesis for their formation suggesting that these superficial lineations could also likely represent the boundary of superimposed sedimentary lobes. Such assumption is supported by recent results from the complex turbiditic lobes of the east Corsica margin (Deptuck et al., 2008; Gervais et al., 2006, 2004). The hypothesis we propose here implies that lobes consist of the accumulation of several sedimentary units, each of them being related to a single deposit event. The succession of positively-graded sandy sequences and numerous erosive surfaces in core CADI2KS17 (Fig. 4D) confirm this assumption. According to the spatial and vertical structure of the Aveiro, Tasyo and Lolita CLS, we propose a conceptual model describing their complex, i.e. polyphased, evolution (Fig. 10). Based on the very high resolution data we acquired, numerous stages were recognized. The first stage corresponds to the deposit of a distal lobe, i.e. lobe C in the Aveiro and Lolita CLS and lobe B in the Tasyo CLS (Fig. 10A). This stage is

followed by a retrogradation/aggradation phase leading to the lobe B deposit in the Aveiro and Lolita CLS (Fig. 10B). Finally, the deposit of the most recent lobe, i.e. lobe A in the Aveiro, Tasyo and Lolita CLS, corresponds to a progradation/aggradation combined with a lateral migration of the CLS (Fig. 10C). Such pattern is coherent with the evolution of depositional gravity lobes in turbiditic environments, for instance on the eastern Corsica margin (Deptuck et al., 2008; Gervais et al., 2004) and in the Var and Zaire turbidite systems (Bonnel, 2005). The sea floor morphology appears of primary importance in the evolution of the Aveiro, Tasyo and Lolita CLS. Firstly, we assume that the retrogradation of the lobes is controlled by the presence of previous depositional lobes, the latter constituting an obstacle to the following sandy gravity flows. In agreement with Gervais et al. (2006), we thus define lobes B of the Aveiro and Lolita CLS as forced retrograding deposits, reflecting an autocyclic process. Also, the sudden southward diversion of the Aveiro and Lolita channels around 7°35′W (Fig. 5) and 8°05′W (Fig. 8), respectively, suggests possible avulsion processes in the evolution of the studied CLS. This assumption is reinforced by the presence of a non-active channel, regarding the backscatter response (Fig. 8B), north of the Lolita Channel (Fig. 9B). This observation finally suggests that the Lolita lobe B is composed by two distinct units. Secondly, external features to the CLS impact on their evolution (i.e. allocyclic). If the pathway of the Tasyo CSL appears influenced by the hummocky topography (i.e. the mud wave field in Fig. 2), the distal part of this system is strongly controlled by the presence of the Tasyo Mud Volcano. We assume that this tectonic-controlled feature (Somoza et al., 2003) likely generates the eastward migration of the southern part of the Tasyo CLS. More generally, Late Quaternary neotectonic (Llave et al., 2006; Rodero et al., 1999) could also produced local subsidence and hence available space for depositional lobes. 5.2. Climatic-controlled channel–lobe depositional systems The trigger mechanisms generating gravity flows in the Gulf of Cadiz could have various origins (e.g. Mulder et al., 2009). Although no mechanism can be rejected, particularly the seismicity which is very important in this area (Gutscher et al., 2002; Zitellini et al., 1999,

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Fig. 10. Inferred evolution for the CLS in the Gulf of Cadiz.

among others), we assume that the direct connection of the Aveiro, Tasyo and Lolita CLS with MOW-controlled channels implies that both feeding and activity of the studied CLS are strongly controlled by the MOW. Variations in the MOW intensity and its spatial fluctuations are related to climate-eustatic oscillations (Llave, 2004; Mulder et al., 2002; Nelson et al., 1999; Rohling and Bryden, 1994; Toucanne et al., 2007; Vergnaud-Grazzini et al., 1989; Voelker et al., 2006; among others). In details, numerous studies reveal the presence of a enhanced Mediterranean Lower Water velocity, as well as denser, during cold climatic periods of the last 50,000 years (Llave et al., 2006; Rogerson et al., 2005; Schönfeld, 1997; Schönfeld and Zahn, 2000; Voelker et al., 2006). Due to the deeper bathymetric position of the

Mediterranean Lower Water during these intervals (e.g. Schönfeld and Zahn, 2000), the hydrologic and sedimentologic conditions in each CLS likely changed. We assume that the increased flow of the Mediterranean Lower Water favoured sediment supply at the CLS head, potentially triggering gravity flows, due to the enhanced winnowing and sediment transport along the contouritic channels as supported by recent sedimentologic studies (e.g., Llave et al., 2007; Rogerson et al., 2005). We also suggest that the increased shear stress over the distal part of the Giant Contouritic Levee, where sediment accumulate due to the MOW split off, could generate sediment destabilisation and hence gravity flows (Fig. 5). As a result, a more intense activity in the Aveiro, Tasyo and Lolita CLS is expected during cold climatic periods. Long well-dated sedimentary records within

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lobes are required to test this assumption. The cores presented herein do not permit to sample glacial sediments (see discussion below). Nevertheless, the biostratigraphic analysis of these cores coupled to reconstructions of MOW oscillations since the last glacial period (e.g. Voelker et al., 2006) confirm the strong relationship between the evolution of the channel–lobe depositional systems and climateforced MOW intensity. The planktic foraminifera association recognized in the whole cores CADI2KS15 and CADI2KS17 demonstrate a post-glacial deposition of the most recent lobes (i.e. lobe A) in the Tasyo and Aveiro CLS. Indeed, the described temperate to tropical foraminifera association strongly suggests a Late Holocene age (Duprat, comm. pers.) for the coring lobes and a concomitant activity of the Tasyo and Aveiro CLS. Recent studies show the Holocene as a complex period punctuated by submillenial scale climatic events (Bond et al., 2001, 1997; Campbell et al., 1998; O'Brien et al., 1995; Viau et al., 2002), which likely forced MOW velocity changes as supported by recent high resolution sedimentological studies (Llave et al., 2006; Rogerson et al., 2005; Toucanne et al., 2007; Voelker et al., 2006). In detail, Rogerson et al. (2005) and Voelker et al. (2006) show a very active Mediterranean Lower Water during the Late Holocene. By analogy with the expected glacial pattern and in agreement with the planktic foraminifera association recognized in cores CADI2KS15 and CADI2KS17, we assume that the Mediterranean Lower Water velocity was important enough during the Late Holocene to trigger gravitational activity in the Tasyo and Aveiro CLS. As a result, we consider the climate oscillations, via the MOW variations, as a major forcing of the activity of the channel–lobe depositional systems located at the end of the Giant contouritic Levee, at least during the Late Quaternary. 6. Conclusion Very high resolution acoustic data and cores collected during the CADISAR (2001) and CADISAR2 (2004) oceanographic cruises allowed a detailed characterisation of complex channel–lobe depositional systems (CLS) in the Gulf of Cadiz, i.e. the Tasyo, Aveiro and Lolita CLS. Our study reveals that the Tasyo, Aveiro and Lolita CLS have an evolution similar to classic turbidite channel–lobe systems. In detail we show: (1) some distinct periods of activity including general progradation and aggradation alternating with retrogradation, migration and avulsion phases; (2) the strong influence of the sea floor topography on the evolution and morphology of the channel–lobe depositional systems; (3) the presence of numerous lobe units and development of a channelling in the lobe complexes. Although the Aveiro, Tasyo and Lolita CLS present strong similarities with classical turbiditic lobes, these systems do not show direct continental connection, classically fed via canyons, but are directly positioned at the end of MOW-controlled channels. As a result, we assume that the activity in the Aveiro, Tasyo and Lolita CLS is strongly dependant from MOW variations. In the light of our sedimentological and biostratigraphical data, the last deposit event in the studied CLS occurred during the Late Holocene, i.e. the last period of strong activity of the MOW. As revealed by numerous studies over the last decades, climate thus appears as the major sedimentary forcing in the Gulf of Cadiz. Acknowledgments Authors thank GENAVIR, crew of the CADISAR, CADIZ-GORRINGE (Nevio Zitellini chief-scientist) and CADISAR2 cruises. We thank J. St Paul, D. Poirier and G. Chabeau for their technical assistance and J. Duprat for the biostratigraphic study. J. Hernández-Molina is

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