The Le Danois Contourite Depositional System: Interactions between the Mediterranean Outflow Water and the upper Cantabrian slope (North Iberian margin)

Marine Geology 274 (2010) 1–20

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The Le Danois Contourite Depositional System: Interactions between the Mediterranean Outflow Water and the upper Cantabrian slope (North Iberian margin) D. Van Rooij a,⁎, J. Iglesias b, F.J. Hernández-Molina c, G. Ercilla b, M. Gomez-Ballesteros d, D. Casas b, E. Llave e, A. De Hauwere a, S. Garcia-Gil c, J. Acosta d, J.-P. Henriet a a

Renard Centre of Marine Geology, Dept. Geology & Soil Science, Ghent University, Krijgslaan 281 S8, B-9000 Gent, Belgium Dpto. de Geologia Marina y Oceanografie Física, Instituto de Ciencias del Mar, CMIMA-CSIC, Paseo Maritimo de la Barceloneta, E-08003 Barcelona, Spain c Facultad de Ciencias del Mar, Dpto. de Geociencias Marinas, Universidad de Vigo, E-36200 Vigo (Pontevedra), Spain d Instituto Español de Oceanografia, c/ Corazon de Maria 8, E-28002 Madrid, Spain e Insituto Geologico y Minero de Espana, Geologia Marina, Rios Rosas 23, E-28003 Madrid, Spain b

a r t i c l e

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Article history: Received 29 April 2009 Received in revised form 26 January 2010 Accepted 3 March 2010 Available online 19 March 2010 Communicated by D.J.W. Piper Keywords: contourite depositional system sediment drift seismic stratigraphy Mediterranean Outflow Water North Iberian margin Le Danois Bank

a b s t r a c t The Le Danois Contourite Depositional System (CDS), located in an intraslope basin along the Cantabrian margin, is unique with respect to the known sedimentary systems along the upper slope of the Biscay margin. Whereas the steep Biscay slopes are dominated by downslope processes, the Le Danois CDS has been generated by alongslope processes and has a strong potential to contain a record of the Neogene palaeoceanography. This paper will focus on the onset, development and present-day functioning of this system with respect to its unique morphological control and the responsible local oceanographic processes. New bathymetric and seismic reflection data show that the past and present Le Danois CDS is shaped by the Mediterranean Outflow Water, conditioned by seafloor irregularities and two topographic highs; the large Le Danois Bank and the smaller Vizco High. The seismic stratigraphic analysis carried out on the contourite deposits has allowed to identify 3 seismic sequences, separated by 3 major regional discontinuities. Changes in depositional styles, the vertical stacking of seismic units and the nature of the discontinuities suggest a correlation with the development of the Cadiz CDS and well-known palaeoceanographic events along the NE Atlantic margin. The first clues for bottom-current deposits are identified in the Lower Sequence, which is developed after tentatively the Lower Pliocene. The drift deposits of both the Lower and Middle Sequences were confined into two palaeobasins within the intraslope basin. However, from the Middle Pleistocene Revolution (0.9 Ma) onwards, the contouritic deposition is intensified due to the switch to a “full glacial” mode with 100 ka cyclicity. This has allowed the development of the present-day depositional and erosive features, such as respectively elongated mounded and separated drifts, plastered drifts, moats and slide scars. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Contouritic bottom-current processes play an essential role in the deep-marine environment where they are responsible in shaping the seafloor (Heezen et al., 1966; Kenyon and Belderson, 1973; Tucholke and Ewing, 1974; Stow et al., 2002; Laberg et al., 2005), providing records of palaeoceanographic changes (Ellwood and Ledbetter, 1979; McCave et al., 1995; Hernández-Molina et al., 2003) and generating large depositional systems, as important as turbidite bodies (Faugères and Stow, 1993; Faugères et al., 1999; Viana et al., 2007; Rebesco and Camerlenghi, 2008). Whereas contourite drifts refer to depositional

⁎ Corresponding author. Tel.: + 32 9 2644583; fax: + 32 9 2644967. E-mail address: [email protected] (D. Van Rooij). 0025-3227/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2010.03.001

features (Faugères et al., 1999; Rebesco, 2005), a contourite depositional system (CDS) refers to genetically linked erosive and depositional features, associated with one or more contourite drifts (Hernández-Molina et al., 2006, 2009; Rebesco and Camerlenghi, 2008). The occurrence of contourite depositional systems along the ocean margins is driven by tectonic, environmental and oceanographic factors. Tectonic activity represents a key factor in producing morphological changes on the seafloor and thereby controlling the development of new pathways for the core and branches of the impinging current at each evolutionary stage of the slope (Reed et al., 1987; Cunningham et al., 2002; Reeder et al., 2002; MacLachlan et al., 2008). However, such morphological steering might also be induced by erosive features created by mass-wasting (Laberg et al., 2001; Bryn et al., 2005) or even by biogenic build-up such as cold-water coral mounds (Van Rooij et al., 2007a, 2009; Huvenne et al., 2009). In the

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long term (2nd to 3rd order cycles), this is a driving force behind drift stratigraphy, architectural changes and the location of large-scale erosive features. On the other hand, environmental (whether palaeoclimatologic or sea-level) and palaeoceanographic changes are other essential factors steering slope contourite generation (Laberg et al., 2005; Duarte and Viana, 2007). On a short-term level (4th order cycles or higher), they even control the vertical contourite stacking pattern, sequences and facies (Llave et al., 2001, 2006; Hernández-Molina et al., 2008). One of the best documented contourite depositional systems along the Iberian Atlantic margin was generated during the Pliocene and Quaternary by the highly saline and warm Mediterranean Outflow Water (MOW) on the middle slope of the Gulf of Cadiz, extending along the west Iberian margin (Kenyon and Belderson, 1973; Gonthier et al., 1984; Stow et al., 1986; Nelson et al., 1999; Mulder et al., 2003; Hernández-Molina et al., 2006; Llave et al., 2007). This depositional system is conditioned by the strong MOW outflow velocities, reaching nearly 300 cm/s close to the Strait of Gibraltar, slowing down to approximately 80 cm/s at Cape St. Vincent (Ambar and Howe, 1979; Cherubin et al., 2000). Once the MOW has left the Gulf of Cadiz, it flows northward along NE Atlantic margin and locally creates contourite deposits, such as, along the Western Iberian margin (Alves et al., 2003) at the foot of the Galicia Bank (Ercilla et al., 2008b), the Le Danois Bank on the Cantabrian margin (Ercilla et al., 2008a; Iglesias, 2009) and within Porcupine Seabight (Van Rooij et al., 2007a). These drift deposits occur near the upper or lower boundary of the MOW, predominantly due to enhancing of the bottom-current velocity by internal tides and waves or through morphologic control. In this paper, we will zoom on the Le Danois Contourite Depositional System (Fig. 1). The presence of this CDS along the upper slope of the southern continental margin of the Bay of Biscay is unique, whereas its steep and incised slopes are generally dominated by gravitational processes (Bourillet et al., 2006; Ercilla et al., 2008a; Iglesias, 2009). Here, we will focus on the onset and evolution of this CDS with respect to the unique morphological control and the responsible local oceanographic processes. As such, we will establish the possible sources of sediment supply, discuss the interaction between the local oceanographic and sedimentary processes, as well as its potential to record the variability of the Neogene MOW palaeoceanography. 2. Regional setting 2.1. Geology The Bay of Biscay is a large W–E wedge-shaped re-entrant of the northeastern Atlantic Ocean, bound by the Cantabrian continental margin at its southern side, by the NW–SE trending Celtic and Armorican margin in the north, and by the Aquitanian margin in the east (Fig. 1A). The tectonic history started with the opening of the Bay of Biscay through seafloor spreading, underwent successive tectonic phases of rifting (from Triassic to Lower Cretaceous times), continued with passive margin development (during the Upper Cretaceous), and ended with its partial closure throughout a phase of compression during the Cenozoic (Boillot et al., 1979; Derégnaucourt and Boillot, 1982; Thinon et al., 2001; Gallastegui et al., 2002). The compressive deformation occurred mainly along its southern border, the Cantabrian margin, from the Palaeocene up to Oligocene, when the Iberian and European plates converged (Boillot and Capdevila, 1977; Srivastava et al., 1990; Alvarez-Marrón et al., 1997). This motion resulted in the partial closure of the Bay of Biscay, the shortening and deformation of the Cantabrian margin and the uplift and deformation of the Cantabrian Mountains (Olivet, 1978; Grimaud et al., 1982; Pérez-Estaún et al., 1995; Pulgar et al., 1996). This final tectonic phase resulted in the present-day morphostructural configuration, which is characterized by inversed faults, thrusts and folds of different scales

and orientations, outcropping or buried basement ridges and tectonically-controlled submarine canyons and valleys (Gallastegui et al., 2002). The Mesozoic to Quaternary deposits have a very irregular distribution, and their stratigraphy reflects the structural evolution of the Cantabrian margin. In fact, three general tectonosedimentary sequences may be defined; syn-rift, syn-orogenic (during the Pyrenean compression) and post-tectonic. During the latter phase, the margin reflects lower tectonic activity whereas the Neogene and Quaternary deposits (up to 4 s TWT) smoothened several tectonic landscapes (Alvarez-Marrón et al., 1997; Gallastegui et al., 2002; Iglesias, 2009). This last sedimentation phase is characterized by a strong canyon development that homogenised the seafloor morphology of the continental rise with the emplacement of several deep-sea clastic systems (Cremer, 1983; Faugères et al., 1998; Bourillet et al., 2006; Ercilla et al., 2008b; Iglesias, 2009). The morphology of the Cantabrian continental margin reflects the above mentioned structural trends and is characterized by a narrow continental shelf which passes abruptly into a continental slope with a variable relief (Ercilla et al., 2008a). The continental slope is affected by large tectonically-controlled submarine canyons running down to the continental rise (Belderson and Kenyon, 1976; Cremer, 1981; Kenyon, 1987). Likewise, the slope is affected by structural highs, such as the Le Danois Bank, which favours the presence of an intraslope basin (Boillot et al., 1979; Ercilla et al., 2008a). 2.2. Oceanography In the Bay of Biscay, most of the water masses are of North Atlantic origin (Pollard et al., 1996). The uppermost water mass is the Eastern North Atlantic Central Water (ENACW), which extends to depths of about 400 to 600 m (Fig. 2). The ENACW is characterized by a cyclonic gyre in the Bay of Biscay with average velocity of 1 cm/s (GonzálezPola, 2006). Between 400 to 500 and 1500 m water depth, the Mediterranean Outflow Water (MOW) follows the continental slope as a contour current (Fig. 2). Its circulation is conditioned by seafloor irregularities and the Coriolis effect. MOW velocities have been measured at 8°W and 6°W in the Bay of Biscay with minimum values of 2–3 cm/s (Pingree and Le Cann, 1990; Diaz del Rio et al., 1998). Although there is a lack of detailed information of the MOW circulation in the Bay of Biscay, it seems that the MOW splits into two branches around Galicia Bank, of which one branch continues towards the north, while the other one flows eastward along the Cantabrian margin slope (Iorga and Lozier, 1999; González-Pola, 2006). From Ortegal Spur to Santander, MOW propagates along the slope, although with reduced velocities. However, the MOW is likely to be influenced by first the Aviles Canyon and later by the Le Danois Bank, which could introduce isopycnal doming. Further to the east, in the Santander Promontory, after crossing the Torrelavega Canyon, the MOW has a lower salinity and temperature (Valencia et al., 2004; González-Pola, 2006). Between 1500 and 3000 m water depth, the North Atlantic Deep Water (NADW) is recognized. It includes a core of Labrador Sea Water (LSW) at a depth of about 1800 m, observed as a new salinity minimum down to 2000 m (Vangriesheim and Khripounoff, 1990; McCartney, 1992). Below the NADW, the Lower Deep Water is identified, which mainly seems to result from the mixing of the deep Antarctic Bottom Water and the Labrador Deep Water (Botas et al., 1989; Haynes and Barton, 1990; McCartney, 1992). A cyclonic recirculation cell over the Biscay Abyssal plain is recognized with a characteristic poleward velocity near the continental margin of 1.2 (±1.0) cm/s (Dickson et al., 1985; Paillet and Mercier, 1997). 2.3. MOW palaeoceanography The complex pathways of the MOW in the Gulf of Cadiz have been studied extensively during the last three decades (Madelain, 1970;

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Fig. 1. (A) Location of the study area (red box) within the Bay of Biscay, with indication of main physiographic elements; (1) Galicia Bank, (2) Ortegal Spur, (3) Aviles Canyon, (4) Cantabrian Margin, (5) Capbreton canyon, (6) Aquitanian Margin, (7) Cap Ferret Canyon, (8) Armorican Margin, (9) Celtic Margin, (10) Goban Spur, (11) Porcupine Seabight and (12) Biscay Abyssal Plain. (B) Regional map of the study area with indication of the available geophysical dataset (GEBCO contour lines every 250 m). (C) Zoom on the intraslope basin along the Cantabrian margin, with the location of the most important seismic profiles (GEBCO contour lines every 100 m).

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Fig. 2. (A) General circulation of the water masses in the Bay of Biscay, modified from Pingree and Le Cann (1990), with indication of the T–S profiles and bathymetric profiles. (B) Temperature–Salinity diagram from the water masses in the Bay of Biscay, modified from González-Pola (2006). (C) General circulation pattern of the MOW in the North Atlantic, modified from Iorga and Lozier (1999). (D), (E) S–N bathymetric profiles from the Cantabrian continental shelf towards the Le Danois Bank, respectively at its eastern and western extremity.

Thorpe, 1976; Baringer and Price, 1999; Iorga and Lozier, 1999; Nelson et al., 1999; Hernández-Molina et al., 2006). Here, we present an overview of the main evolutionary phases of the MOW and its temporal variability along the NE Atlantic margin. The water mass exchange between the Atlantic and Mediterranean basins initiated after 5.23 Ma, due to the opening of the Gibraltar Strait (Duggen et al., 2003; Hernández-Molina et al., 2006). The subsequent modifications to this exchange, influenced by eustatic sea-level and palaeoclimatic changes, have been studied in detail from the complex stratigraphy and stacking pattern of the Cadiz CDS (Llave et al., 2001, 2007; Hernández-Molina et al., 2006). According to Hernández-Molina et al. (2006), the first evidence of MOW generation has been recognized in seismic profiles from 4.2 Ma,

associated to the Lower Pliocene Revolution (LPR). Due to enhanced seasonal aridity in the Mediterranean area from 3.5 to 3.3 Ma, a longterm rise in bottom-water salinity and temperatures was observed in the Alboran Sea and on Goban Spur (Khelifi et al., 2009). These changes culminated towards the Upper Pliocene Revolution (UPR) at 2.4 Ma, when the present-day circulation pattern was established after a global cooling event (Loubere, 1987; Hernández-Molina et al., 2006). Although the LPR marks the onset of contourite deposition within the Cadiz CDS, the UPR has lead to the formation of the distinctly mounded shape of the present drifts due to an enhanced MOW circulation (Hernández-Molina et al., 2006). The distal effects of an Upper Pliocene MOW have been observed in the Porcupine Seabight, where it has established a suitable environment for the

Fig. 3. Shaded relief multibeam bathymetric map of the study area (contour interval is 50 m) with indication of the inferred present-day local oceanographic behaviour of the Mediterranean Outflow Water (yellow dashed line).

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initiation of cold-water coral mound growth (Kano et al., 2007; Huvenne et al., 2009). An important change in the climatic trend occurred around 900– 920 ka, during the Middle Pleistocene Revolution (MPR). This was related to an important shift in glacial/interglacial periodicity from 41 to 100 ka and an increase in the cycle amplitude (Head and Gibbard, 2005; Lisiecki and Raymo, 2005, 2007). During this period, the palaeoceanographic changes became more related to climatic and sea-level changes, and are markedly visible within the stacking pattern of the depositional sequences, with an enhanced mounded morphology (Hernández-Molina et al., 2006; Llave et al., 2006, 2007). In the Porcupine Seabight, this had lead to the development of a complex contourite depositional system (Van Rooij et al., 2007a, 2009). Within the course of the Late Pleistocene, glacially lowered sealevels reduced the MOW outflow volume. However, a dryer Mediterranean climate induced a higher MOW salinity and density, causing more intense and deeper bottom currents in the Gulf of Cadiz (Cacho et al., 2000; Schönfeld and Zahn, 2000; Hernández-Molina et al., 2006; Voelker et al., 2006). Identical MOW velocity increases were observed during Dansgaard–Oeschger stadials, Heinrich Events and the Younger Dryas (Llave et al., 2006; Toucanne et al., 2007). This was responsible for a reduced MOW advection towards the north, as documented along the southern Portuguese margin (Schönfeld and Zahn, 2000). Conversely, the impact of glaciations on the MOW is reversed in the rest of the NE Atlantic margin. During interglacial periods, it is the presence of the MOW that locally causes enhanced bottom-current flow through internal waves and tides, while the reduced occurrence of the MOW during glacial or stadial episodes has lead to more sluggish bottom currents (Dorschel et al., 2005; Rüggeberg et al., 2007; Van Rooij et al., 2007b; White, 2007). 3. Methods Several datasets have been used for the present study, which were acquired within the framework of different projects and/or cruises (ECOMARG, MARCONI and GALIPOR). It comprises multibeam bathymetry and single channel seismic records of medium and high resolutions (Fig. 1). A high-resolution bathymetric map (Fig. 3) was obtained with the SIMRAD EM 300 multibeam system. The acquired data was processed using the SIMRAD NEPTUNE package, obtaining a bathymetry grid of 15 m resolution at 500 to 600 m and at 25–50 m at depths down to about 1000 m. Seismic records were collected using airgun and sparker systems. The latter was used for surveying a more detailed sector of the study area, south of Le Danois Bank. The airgun records were obtained using a 140 in.3 sleeve gun array, located at a depth of 3.5 m, with a shot frequency of 8 s. The airguns were fired with four Hamworthy air compressors, producing a 140 bar firing pressure. The receptor system was a SIG streamer with an active section 150 m long. The penetration of the acoustic signal is about 1 s (TWT). The source of the sparker seismic profiles was a 120 electrode SIG sparker, triggered every 3 s attaining 500 J energy at discharge. The sample frequency was 6 kHz with a recording time of 2.5 s TWT. The standard vertical resolution of this method varies from 0.4 to 1 m with a penetration between 200 and 500 ms TWT. The processing of these data involved basic bandpass-filtering and swell-filtering.

break is located at about 150 to 200 m water depth. The upper slope has two major domains: a) a proximal domain with an average slope gradient of 6°, approximately 10 km wide, 29 km long, located between 200 and 400–500 m water depth; and b) a distal domain with a complex physiography between 400 and 1600 m water depth. It represents the morphologic expression of an inner slope basin, and comprises two highs; the large Le Danois Bank and the smaller Vizco High to the west. The presence of Le Danois Bank creates the W–E intraslope basin, which is 17 to 25 km wide and approximately 50 km long. The Vizco High (1400–1050 m) displays a sub-rounded shape in plain view of 7 km wide, and is about 200 m high. The Le Danois Bank has an elongated convex southward shape of about 72 km long and 15 km wide. Its top is flat, representing a large W–E platform between 550 and 600 m water depth. Its gentle southwestern slope, facing the intraslope basin, deepens from 500 to 850 m water depth with a gradient of about 2° and an irregular seafloor. On the other hand, the southeastern flank is rougher, with an average slope gradient of 12° from 600 to 800 m. At its easternmost extremity, there is a noticeable small high (named here as the ECOMARG High) of about 100 m high, with a 3° slope, between 800 and 1800 m water depths. The northern flank of the Le Danois Bank represents the upper part of the lower slope and has a regionally variable gradient between 16.5° in the upper part and about 18° in the middle part. The base of the lower slope is at about 4400 m water depth where the continental rise starts. Within the intraslope domain, the Le Danois Contourite Depositional System (CDS) is recognized, as well as three associated sedimentary systems: the Lastres Canyon system, the Le Danois Leveed Channel System and the Le Danois mass-wasting system. 4.2. Le Danois Contourite Depositional System The Le Danois CDS is constrained between the proximal domain of the upper slope and the Le Danois Bank, between 400 and 1500 m water depth (Figs. 3 and 4). This system is composed of several depositional and erosive features. 4.2.1. Depositional features

4.1. General physiography

4.2.1.1. Elongated mounded and separated drifts. Two elongated mounded and separated drifts have been identified within the Le Danois CDS: the Gijón and Le Danois Drifts. The Gijón Drift is a SE trending elongated mounded and separated contourite deposit located on the upper slope at about 400–850 m water depth (Fig. 4). It is bound southward by the Gijón Moat, northward by the Le Danois Drift, and westward by the Lastres Canyon system. The drift shows a positive relief of about 100 m in the NW part, becoming smaller (b50 m) to the SE. It has a maximum width of about 10 km and is approximately 31 km long. The Le Danois Drift is an ESE trending elongated mounded and separated drift, developed between 800 and 1500 to 1600 m water depth. It is located at the foot of the southern face of Le Danois Bank, and is separated from it by the Le Danois Moat (Fig. 4). It is bound southward by the Gijón Drift and the Lastres Canyon system, and eastward by the start of the Le Danois Leveed Channel system. The drift is about 45 km long, 50 m high and has a variable width, with a maximum of 10 km in the central part of the drift, 3.5 to 4 km in the W and 4.7 km towards the E. Between the Gijón and the Le Danois Drifts, there is a 35 km long sub-horizontal transitional zone with a SE trend (Fig. 4). It is 9.5 km wide in the NW, 2.3 km in the central zone and 7.1 km in the SW.

Detailed analyses of the multibeam bathymetry allowed to identify three main physiographic domains in the studied sector of the Cantabrian margin: a narrow continental shelf, a complex upper continental slope and an abrupt lower continental slope (Fig. 4). The shelf has a variable width (between 17 and 32 km), and the shelf

4.2.1.2. Plastered drifts. Along the southern slope of the Le Danois Bank, three plastered drifts have been identified (Fig. 4). Plastered Drift 1 is located on the western edge between 600 and 750 m water depth. The other two are located on the eastern edge; Plastered Drift 2 between 750 m and 1100 m, and Plastered Drift 3 between

4. Morphologic characteristics

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Fig. 4. Morphosedimentary map of the study area, based upon the multibeam bathymetry and available seismic profiles.

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1100 and 1550 m. They are all characterized by a mounded relief of about 40 m. Plastered Drift 1 is 12 km long and 5.5 km wide. It displays a SE trend and ends southward against the Le Danois Moat, while it is bound eastward by a remarkable downslope valley (Fig. 4). Plastered Drift 2 is 22 km long and 6 km wide with ENE trend. It ends sharply against the ECOMARG High toward the NW, and is bound by the Plastered Drift 3 and the Le Danois Moat towards the south. Plastered Drift 3 is about 20 km long and 6 km wide, showing a NE trend and extends downslope until the Le Danois Moat and Le Danois Leveed Channel. 4.2.1.3. Sediment waves. Two sediment wave fields have been identified (Fig. 4). One area (4 km²) is located in the western area, south of the Vizco High, from 850 to 1300 m of water depth. Here, these seafloor undulations seem to migrate towards the SE, with wavelengths between 750 and 1200 m and amplitudes between 30 and 75 m. The second field (6.5 km²) is located on the eastern area, at about 1500 m water depth. They display wavelengths between 800 and 1650 m and amplitudes between 35 and 130 m. These waves seem to be migrating toward the SW. 4.2.2. Erosive features 4.2.2.1. Moats. Two moats have been identified: Gijón and Le Danois. The Gijón Moat is bound to the north by the Gijón Drift and to the south by the proximal domain of the upper slope. It is the upslope continuation of the Gijón Canyon, it has a NW–SE trend and is about 45 km long and 1 to 4 km wide. It starts from 1100 m water depth in the west, having an incision of 200 m, up to 400 m water depth, where its expression narrows and disappears (Fig. 4). The Le Danois Moat separates the Le Danois Bank from the Le Danois Drift. It displays a WNW–ESE trend and deepens from 800 to 1500 m water depth towards the east. The moat merges eastward into the Le Danois Leveed Channel. It is approximately 48 km long and has a variable width between 2.8 and 0.8 km. The relief is also variable; in the proximal reaches, the incision is 75 m, increasing to 105 m in the centre to decrease again to 98 m in the distal reaches. 4.2.2.2. Slide scars. Slides are locally affecting the Gijón and Le Danois Moats and the Le Danois Drift (Fig. 4). They are recognizable due to a rough seafloor, characterized by arcuate and semicircular slide scars associated to deformed sediment. These scars have the tendency to coalesce forming multiple slides, creating large areas of erosion displaying a downslope oriented amphitheatre-like failure surface, and sediment masses with a slightly to highly deformed seafloor. Their size is variable, ranging from 10 to 100 km. The removed sediment encompasses areas of tens of m2 at the Gijón and Le Danois Moat walls, and at the southern part of the Le Danois Drift. 4.2.2.3. Scours. Scour alignments are observed over the Plastered Drift 3 (Fig. 4) with a NE and ENE trend. They are between 28 and 5.5 km in length and show a maximal vertical incision of about 5 m deep over width of 1250 m. 4.3. Sedimentary systems associated to Le Danois CDS The Le Danois CDS is laterally connected with the Le Danois Leveed Channel system (Fig. 4). At about 1500 m water depth, the Le Danois Moat transforms downslope into the 42 km long Le Danois Leveed Channel. It is bordered by an overbank area on its right hand side, where a well-defined levee is recognized. This probably is a small turbiditic channel–levee system, comparable to the Var or Cap Ferret system (Faugères et al., 1999; Ercilla et al., 2008a) The Le Danois Leveed Channel displays and arc-shape plan-view that runs parallel to the Le Danois Bank. It has a downslope trend that varies from NE to N and finally towards the NW. The cross-section changes from V-shaped

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in the proximal part to U-shaped in the middle part and again to Vshaped in the distal part. The inner face of the left channel wall is affected by gullies of 1–10 m length. The Lastres Canyon system is located to the southeast of the Le Danois CDS (Fig. 4). The Lastres Canyon heads initially show a NE–SW trend, whereas it abruptly changes to N–S in the central part and finally ends with an E–W orientation near the distal domain of the upper slope. This canyon is 4.5–5 km wide with a floor of 0.5–1 km wide. At least two tributaries running down to the canyon floor are identified on its right side. The sharp pathway changes indicate that the location of the canyon system is structurally-controlled (Boillot et al., 1974; Derégnaucourt and Boillot, 1982). The canyon walls and those of their tributaries are affected by a well-developed drainage features that form a network of gullies. Arcuate to circular slide scars are observed on the W–E border of the left canyon wall. Ercilla et al. (2008a) suggest that these are retrogressive slides of which their occurrence seems to be associated with the steep slope gradients of the canyon wall. Additionally, a mass-wasting system is identified along the steep walls of the Le Danois Bank. It is formed by isolated and multiple scars, deformed sediments defining an irregular seafloor and gully drainage systems. The slide scars located on the northern flank of the Le Danois Bank evolve downslope to gullies perpendicular and oblique to the regional slope. The gullies are from tens of metres to several kilometres long, and are separated by narrow and sharp ridges. 5. Seismic stratigraphy of the Le Danois CDS The seismic stratigraphic study of the Le Danois CDS has allowed to define three seismic sequences; lower (L), middle (M) and upper (U), bound by three major discontinuities: B/L, L/M and M/U, respectively (Fig. 5). The middle sequence M comprises three seismic units, from older to younger: Ma, Mb and Mc (Figs. 5 and 6). The upper sequence U is composed of three seismic units, which are, from older to younger Ua, Ub and Uc (Figs. 5 and 6). All units show cyclic changes in seismic amplitudes; a transparent/weak zone that passes to smooth, parallel reflections of moderate-to-high amplitude in the upper part; and a high-amplitude erosive continuous surface at the top. The sedimentary package defined by these three major sequences overlies mostly Miocene deposits confined between basement palaeohighs (Ercilla et al., 2008a). These palaeohighs create two W–E trending subbasins in the deeper area of the intraslope basin (Figs. 5 and 7). They are at least 60 km long and parallel to the Le Danois Bank. The northern palaeobasin (NPB) runs downslope widening towards east from about 4 to 9 km and extends at least down to 1600 m water depth. The southern palaeobasin (SPB) is 8 km wide and narrows towards the Lastres Canyon where it finally disappears before the northern palaeobasin. 5.1. Lower seismic sequence This sequence is identified as the filling deposits of the NPB and SPB (Figs. 5, 7 and 8). They rest on the B/L discontinuity that is defined by an irregular reflection. Its erosive character is more pronounced in the SPB. Here, the B/L discontinuity defines a U-shape valley incision up to 8 km wide and 250 ms TWT deep, whereas in the NPB it is a dipping planar surface. The upper boundary of the L sequence, the L/M discontinuity, also displays an erosive character which is more pronounced in the SPB. Here, it affects the B/L discontinuity making both boundaries undifferentiated (Fig. 5). This erosion also produces a U-shape valley of 8 km wide and 250 ms TWT deep, characterized by an irregular seafloor. In the NPB, this discontinuity is a southward dipping planar surface. The geometry of the L sequence is different in both palaeobasins. In the NPB, it abuts the sides of its surrounding highs, displaying a subtabular geometry with a thickness of about 80 ms TWT. In the SPB

12 D. Van Rooij et al. / Marine Geology 274 (2010) 1–20 Fig. 5. N–S Airgun profile L4, illustrating the emplacement of the Le Danois Drift and the Gijón Drift in the intraslope basin. Insets (A), (B) and (C) respectively show detailed interpretations of the Le Danois Drift in the northern palaeobasin (NPB), the drift transition zone in the southern palaeobasin (SPB) and the Gijón Drift and moat. (AB: acoustic basement; SB: subbasins).

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Fig. 6. N–S high-resolution sparker profile RCMG05 over the Le Danois Drift indicating the transition from confined drifts (Middle sequence) towards mounded elongated drifts (Upper sequence). (AB: acoustic basement).

it shows a concave-upward lenticular cross-section of about 70 ms TWT thick that pinches out toward the palaeobasin sides. From a seismic point of view, both palaeobasins display remarkable differences in facies type. In the NPB, the lower sequence comprises a high-amplitude sub-parallel reflection configuration. Towards the Le Danois Bank, it laterally passes to chaotic and wavy short reflections of high to low amplitude (Figs. 5 and 8). Their vertical arrangement suggests an apparent upslope migration that progressively onlaps the flank of Le Danois Bank. In the SPB, the lower sequence is characterized by an undulated stratified facies of high lateral continuity, high to medium amplitude and sigmoidal geometry. Although only one seismic profile (Fig. 5) allows to describe this particular deposit, we suggest it shares characteristics with a confined drift (Reed et al., 1987; Rebesco and Stow, 2001). This drift is asymmetric and associated with a moat located on the northern side of the palaeobasin. The moat is characterized by discontinuous stratified and chaotic facies with acoustic amplitude higher than that of the drift. The stratal pattern reflects a southward upslope migration that progressively onlaps and fills the SPB. This pattern suggests the presence of a (second) drift-moat association, which probably has been eroded in the southern part of the SPB.

5.2. Middle seismic sequence Sequence M also contributes to the filling of both palaeobasins, gradually burying their palaeotopography (Fig. 5). Both palaeobasins continue displaying different seismic facies. Within each of them, the three units Ma, Mb and Mc show a similar depositional stratal growth pattern. The lower L/M discontinuity is an onlap surface, while the upper M/U discontinuity is a prominent irregular erosive reflection where at least three relatively small depressions/subbasins (SB) are identified: a northern (5 km wide) over the NPB and a central (2.5 km wide) and southern (8 km wide) over the SPB (Fig. 5). 5.2.1. Northern Palaeobasin (NPB) In the NPB, sequence M is up to 150 ms TWT thick, subdivided into units Ma (45 ms TWT), Mb (50 ms TWT) and Mc (55 ms TWT). Their internal boundaries locally display downlap and truncation terminations. Within this sequence, mostly confined drift deposits, and locally sediment waves, are recognized (Figs. 5 and 8). From units Ma to Mc, the confined drifts progressively evolve from subtabular to mounded geometries, bound by a moat north of the drift (Figs. 6 and 8). They are characterized by sub-parallel stratified facies with different acoustic

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Fig. 7. S–N Airgun profile L13 illustrating the Le Danois Drift (over the Northern Palaeo Basin), Le Danois Moat and Plastered Drift 1. (AB: acoustic basement).

amplitude. The moats are characterized by discontinuous and chaotic stratified reflections of higher acoustic amplitude. The vertical stacking of these deposits shows a northward upslope migration with a progressive onlap over the L/M discontinuity, gradually reaching the flank of Le Danois Bank (Figs. 5, 6 and 8).

from 20 to less than 10 ms TWT. These kilometric-scaled Ma and Mb sediment waves display an aggrading pattern without evidence of migration. Unit Mc contains reflections of short lateral continuity, resting unconformably over unit Mb (Fig. 5). They possibly resemble irregular, metric scale sediment waves.

5.2.2. Southern Palaeobasin (SPB) In the SPB, sequence M is up to 140 ms TWT thick (Fig. 5). As within the NPB, units Ma (50 ms TWT), Mb (45 ms TWT) and Mc (45 ms TWT) are separated by unconformities. These units are largely composed of sediment waves characterized by wavy stratified reflections with a variable acoustic amplitude and wave scale change from units Ma to Mc. The acoustic character varies vertically, increasing towards unit Mc (Fig. 5). The sediment waves in units Ma and Mb are stratified reflections with lateral continuity between them. They have an unconformable configuration with convergent reflections approaching the wave crests. Their wavelength increases vertically from 1 to 1.5 km, whilst their amplitude decreases vertically

5.3. Upper seismic sequence 5.3.1. General characteristics Sequence U occupies a larger area than the previous ones, extending from the uppermost slope (400 m water depth) down to the foot of Le Danois Bank (Fig. 5). Its thickness is variable, ranging from 180 to 250 ms TWT. It is characterized by a large variety of contouritic deposits which are interpreted as mounded elongated and separated drifts, mounded elongated drifts, confined drifts and plastered drifts. This sequence is bound by the M/U discontinuity whose palaeotopography roughly reflects the present-day morphology of the intraslope basin. Locally, some differences are found due to

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Fig. 8. N–S high-resolution sparker profile RCMG03 over the Le Danois Drift within the northern palaeobasin. (AB: acoustic basement).

its irregular palaeotopography where the three small subbasins (SB) are identified (900 to 1800 m water depth). These irregularities are progressively filled by the seismic units Ua, Ub and Uc (Figs. 5, 7 and 8). The discontinuities between these units mark changes in the internal architecture and/or contourite deposits. They are mostly concordant reflections that laterally change to erosive in the deeper part of the intraslope basin. This infers that the drifts underwent several relocations and modifications of depocenter location. The mounded elongated and separated drifts are defined by an unconformable and roughly conformable stratified facies of high lateral continuity displaying mounded and sigmoidal geometries. The acoustic facies show a cyclic vertical pattern from low to high amplitude in each seismic unit, including a general upward increase throughout the sequence (Figs. 6 and 8). The drifts generally are asymmetric, with a short steep flank towards the moat and a smooth and large backside flank (Figs. 5 and 8). Some drifts display more than one crest due to the presence of a subsidiary drift associated to a small moat on its backside (Fig. 7). The elongated mounded drifts are associated with one moat close to the palaeohighs (Fig. 8). The moat is characterized by discontinuous stratified and chaotic facies with locally higher acoustic amplitude than those observed in the drift. Some of the moat reflections laterally continue into the drifts (Figs. 6 and 8), while others onlap on U-shaped surfaces of high-amplitude resembling cut-and-fill features (Fig. 7). The general stratal pattern of the drift-moat association reflects for the first time a clear an upslope migration, showing onlap on the palaeohighs and the seafloor of the upper continental slope (Fig. 9). 5.3.2. Seismic unit Ua Generally, seismic unit Ua (average thickness 99 ms TWT) contains three different mounded elongated drifts, located within the inherited northern, central and southern subbasins (Fig. 5). However, in the westernmost part, the drift in the northern subbasin still seems to be confined (Fig. 8). These deposits show a smooth mounded shape but, they have a patchy distribution, as they are

interrupted by the acoustic basement (Fig. 5). The three drifts display upslope migration, gradually filling and moving outside the subbasins. 5.3.3. Seismic unit Ub The drift deposits of seismic unit Ub (average thickness 68 ms TWT) completely mask the M/U palaeotopography and only gently reminders of the northern and southern subbasins remain. In this unit, both the Le Danois Drift and (especially) the Gijón Drift show an increased lateral development, respectively fully becoming an elongated and separated or elongated mounded drift (Fig. 5). This new area (275 km2) is mainly located on the proximal upper slope and extends from 400 to 1000 water depth between the Gijón and Lastres canyons (Figs. 5 and 9). In the northern subbasin, the Le Danois Moat gradually becomes better developed compared to the opposite (inherited) one which progressively disappears throughout unit Ub (Figs. 5 and 8). Both the northern and southern subbasin deposits pinch out in opposite directions, progressively extending toward the axis of the intraslope basin. The resolution of the seismic profiles does not allow to determine interfingering between both southern and northern deposits. 5.3.4. Seismic unit Uc The presence of the mounded and separated elongated Le Danois and Gijón Drifts appears to be enhanced during the deposition of seismic unit Uc (Figs. 5 and 8). They are built by vertical stacking of upslope prograding stratified sediment packages (total average thickness 50 ms TWT). The total upslope migration (Ub and Uc) of the Le Danois Drift amounts to about 2.8 km on a horizontal scale and 130 ms TWT in average relief. The internal architecture of the Gijón drift-moat association shows more than 5 km of (southward) upslope migration (Fig. 5). This migration produces a lateral accretionary fill of the Gijón Moat with drift deposits. By consequence, the valley/moat is asymmetric with a depositional northeastern wall and an erosive southwestern one. Towards the transition zone between Le Danois and Gijón Drift, an aggrading facies is observed, characterized by

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Fig. 9. E–W Airgun profile L5 illustrating the Gijón Drift and Moat. (AB: acoustic basement).

parallel-laminated, laterally continuous reflections with an onlap reflection configuration. 5.3.5. Plastered drifts Locally, sequence U is present on the southeastern flank of the Le Danois Bank, where it shows an isolated patched distribution (8 km wide) with a comparatively lower thickness (below 50 ms TWT) (Figs. 4 and 10). These deposits are recognized as plastered drifts, characterized by a conformable stratified seismic facies of high lateral continuity and high to very high acoustic amplitude (Figs. 7 and 10). This facies shows an aggrading pattern that rests unconformably over folded and faulted deposits of Le Danois Bank. Although Plastered Drift 2 was interpreted by Ercilla et al. (2008a) as a slide (Fig. 7), a neighbouring high-resolution seismic profile (Fig. 10) and the multibeam bathymetry (Fig. 4) allow to identify this deposit as a plastered drift. It typically displays a mounded and laminar geometry in crosssections and an irregular elongated shape in plan-view (Faugères et al., 1999; Hernández-Molina et al., 2006). 6. Discussion 6.1. Present-day local oceanographic model of the MOW Although hydrographic measurements are limited in the area, we deduced a local hydrodynamic model from the distribution of

morphosedimentary features. Additionally, the local physiography of the margin and the structural features that could interact with MOW water mass have also been taken into account. The Le Danois Bank represents a large obstacle within the eastward MOW flow. Its presence could introduce isopycnal doming in the upper part of the MOW and separation of its flow into two major branches. Similar processes (involving the MOW) have been reported in other locations such as the Galicia Bank (Iorga and Lozier, 1999) and the Guadalquivir Bank (Hernández-Molina et al., 2003; Llave et al., 2007). In a similar way, but on a smaller scale, the occurrence of the Vizco High (Fig. 4) also represents an obstacle, funnelling MOW through the Gijón canyon. Its influence will remain restricted to the Gijón canyon and drift area. As such, it is proposed both the Le Danois Bank and Vizco High influence the MOW flow to separate into three different branches: a northern, central and southern branch (Fig. 3). A northern branch of the MOW flows along the northern flank of Le Danois Bank. However, based on the present available data, suggesting dominantly gravitational processes, we cannot substantiate any depositional or erosive evidence of this branch. The southern branch, located between the Vizco High and the proximal domain of the upper slope, flows along the Gijón Drift and Moat, while the central branch flows between the Vizco High and the Le Danois Bank, creating the Le Danois Drift and Moat. The southern branch of the MOW is forced to flow upslope, loosing activity over the transition towards the Lastres Canyon System. Similarly, the central branch flows along the southern

Fig. 10. N–S high-resolution sparker profile RCMG09 on the Le Danois Bank flank of Plastered Drift 2. (AB: acoustic basement).

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flank of Le Danois Bank, remains restricted along the intraslope basin and is disconnected from the seafloor at the transition from the Le Danois Moat to the Le Danois Leveed Channel. 6.2. Present-day control of MOW on the Le Danois CDS Here, we will discuss the present-day interaction between hydrodynamic and sedimentary processes and sources, along respectively the southern and central MOW branch. Every branch has a “tabular” behaviour but when interacting with the seafloor, a more turbulent core generates non-depositional or erosional processes which produce the moats. 6.2.1. Southern MOW branch South of the Vizco High, a significant bottom current flow can be inferred where the southern branch of the MOW impinges on the seafloor. Afterwards, this branch flows right confined into the possibly tectonically-controlled Gijón Canyon that runs WNW–ESE along the proximal domain of the upper slope. In this situation, the current is accelerated and generates enough turbulence as a helicoidal flow to progressively and locally erode the bedrock and gravitational deposits that constitute the southern flank creating the Gijón Moat (Fig. 3). The eroded sediment is incorporated into the southern branch and deposited northward building up the Gijón mounded elongated and separated drift at its left-hand side (Figs. 5 and 9). The expression of the Gijón Moat and mounded drift ends at the same upper limit of the MOW at 400 m. Alternatively, if the Gijón Canyon is active during glacial lowstands, it could deliver sediment from upslope as a turbidite system. In this case, an eastward flowing MOW would progressively lose its intensity upslope, countered by turbidity currents. However, no gullies are observed along the upper slope and the general morphology is relatively smooth slope compared to Lastres canyon. 6.2.2. Central MOW branch The interaction between the central branch and Le Danois Bank starts on the southwesternmost flank of Le Danois Bank where Plastered Drift 1 is deposited in water depths between 600 and 750 m. By definition, plastered drifts are created by a broad non-focused current on a gentle slope under low current velocities (Faugères et al., 1999; Rebesco, 2005). A possible causal mechanism for this deposit could thus be proposed through the interaction of a broad upslope flowing part of the central branch, eroding the upslope part of Le Danois Bank. Additionally, fine-grained suspended matter pirated from the Aviles Canyon could also be deposited on the plastered drifts. Unfortunately, the absence of a detailed seismic architecture disables to gain an insight into the construction of this deposit. The main influence of the central MOW branch is observed within the intraslope basin, where it has a W–E flow with enough turbulence to erode the nearsurface sediment, create the Le Danois Moat and deposit the sediment constructing the Le Danois Drift (Fig. 3). Additional sources to feed the drift build-up may be also considered. An important sediment source could thus be provided from the Aviles Canyon, which is located west of the CDS. Suspended sediment (nepheloid layers) escaping from the upper canyon system could be pirated eastward by the MOW and deposited within the intraslope basin. Likewise, gravitational processes affecting the southern flank of the Le Danois Bank (Figs. 4 and 7) may contribute to supply the material building up the mounded drift. Another possible and more direct sediment supply could be expected from the shelf during regressive and lowstand (glacial) periods. With the increasing water depths towards the east, the influence of the central branch of the MOW on the Le Danois drift progressively disappears. Plastered Drifts 2 and 3 are located respectively between 750– 1100 m and 1100–1550 m along the southern Le Danois Bank flank. Consequently, they are most probably created due to the tabular

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behaviour of the detached central MOW branch (Figs. 2E and 3). Moreover, the lower occurrence of Plastered Drift 3 coincides with the interface between the MOW and the NADW (LSW) at 1500–1550 m, as well as with the transition between the Le Danois Moat toward the Le Danois Leveed Channel. Fine-grained suspended material escaping from the Le Danois CDS could be transported to the plastered drifts. Another possible sediment source, mainly for Plastered Drift 3, are the overflowing plumes of fine sediment from turbidity currents along the Le Danois Leveed Channel. The water depth of the MOW/NADW interface also corresponds with the eastern, SW migrating, sediment wave field located at eastern boundary of the Le Danois CDS (Fig. 4). Although no seismic profiles are available to investigate whether these waves are due to bottom currents or gravity deformations (Gardner et al., 1999; Faugères et al., 2002), we suggest that they could be related to internal wave movements created along the MOW/NADW interface (Valencia et al., 2004; González-Pola, 2006). 6.3. Interaction of the Le Danois CDS with the Le Danois Leveed Channel System The Le Danois CDS evolves downslope to the Le Danois Leveed Channel System at the interface between MOW and NADW (LSW), and thus where the MOW loses contact with the seafloor (Figs. 2E and 3). The evolution from moat to leveed channel occurs in a transition zone from 1200 m to 1500 m where the regional slope gradient gradually increases from about 0.9° to 1.7° and locally up to 4°. This increase in gradient could favour a change in the sedimentary process from contouritic, associated to the central MOW branch, to gravitational (turbiditic). The sediment supply feeding this leveed channel can be coming from a combination of various sources which might act together. These possible sources are: (1) sediment transported along the Le Danois moat towards to leveed channel system; and (2) episodic slope instabilities generating turbidity currents from instable drift levees (Fig. 7) or from the southern flank of the Le Danois Bank (including Plastered Drift 3). The latter might also be driven through the influence of internal waves associated to the MOW/NADW interface. The contribution of these sources could vary with time depending on MOW flow competence and the depth of the MOW/NADW interface, (e.g. glacial versus interglacial periods). Depending on their interplay, both moat and channel may be working simultaneously or have an unrelated activity. 6.4. Evolution of Le Danois Contourite Depositional System The spatial and temporal distribution of the contourite deposits within the Le Danois intraslope basin reflects that the depositional history involved several relocations and changes in drift type (Fig. 5). The three major sequences of the Le Danois CDS invoke three main evolutionary stages. The subsequent changes in morphology, stacking pattern, as well as the timing and extent of their sequence boundaries will be discussed with respect to the MOW palaeoceanography. 6.4.1. Initial stage This stage is coincident with the onset of the Lower seismic sequence. The irregular B/L discontinuity represents the start of the bottom current flow within the intraslope basin. As such, it marks the beginning of MOW interaction with this sector of the Cantabrian margin. During this initial stage, the influence and vigour of the MOW might still be temperate. This could coincide with the onset of the Cadiz CDS during the Intra Lower Pliocene at about 4.2–4.0 Ma (Hernández-Molina et al., 2006; Llave et al., 2007). An equivalent of such an LPR discontinuity is also present along the NE Atlantic margin (Stoker et al., 2005). The deposition, localization and geometry of the Le Danois CDS are predominantly controlled by the presence and interaction of such an

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“immature” MOW with the palaeotopography created by the palaeohighs and the LPR erosive action upon the Miocene sedimentary basement. Upon entry within the intraslope basin, the Lower Pliocene MOW is split by the palaeohighs and confined within the palaeobasins, creating vortices. Despite the limited number of seismic profiles and the eroded upper part, the inferred confined drift in the SPB suggests a constrained circulation with increased velocities at the sidewall (Faugères et al., 1999; Rebesco, 2005). This, however, is not the case in the NPB, where the chaotic and wavy reflections deposited on the northern flank suggest the influence of enhanced hydrodynamic environment. The sediment input is still relatively moderate due to the lower Pliocene low-amplitude sea-level variability (Zachos et al., 2001; Lisiecki and Raymo, 2007). 6.4.2. Second stage This stage is coincident with the Middle seismic sequence which has the L/M discontinuity as its lower boundary. Similar to the initial stage, the seismic facies and the depositional geometries observed within the M sequence indicate that the depositional dynamics within both subbasins remain different. In the NPB, the progressive evolution from subtabular (Ma) to mounded (Mb and Mc) confined drifts with moat features, illustrates the gradual increase of bottom-current flow. The mere presence of these confined drifts indicates that the MOW is still confined by the Miocene palaeotopography. Adjacent moats indicate local flow cores close to the sidewalls of those irregularities, such as observed in other confined drifts (Reed et al., 1987; Van Rooij et al., 2007a). With respect to the SPB, a confined and wavy behaviour of the MOW allowed the deposition of vertically stacked stationary sediment waves (Fig. 5). Toward the end of this stage, the M sequence deposits will have filled most of the previous subbasins formed by the B/L discontinuity and the predefined tectonic setting. The changes in drift geometry and depositional styles may be directly related to the palaeoclimatic processes initiated during the Upper Pliocene Revolution (UPR) at about 2.4 Ma. Within the Cadiz CDS, this margin-wide event marked the start of a present-day oceanographic exchange model with a more intense MOW and the further development of elongated mounded and separate contourite deposition in the Faro-Albufeira drift (Hernández-Molina et al., 2006). Further along the NW European margin, it started to play a pronounced role in the margin stratigraphy (Laberg et al., 2005; Stoker et al., 2005) and has set the stage for cold-water coral mound growth in the Porcupine Seabight (Huvenne et al., 2009). As such, the early Pleistocene effects of climatic control on the presence and flow intensity of the bottom-water masses, and on the sedimentary input, becomes more visible within the sedimentary record of the Le Danois CDS. This is mainly expressed through the introduction of the stacking of medium-resolution seismic subunits and a more pronounced (interglacial) current-controlled shaping of these subunits into confined sediment drifts. 6.4.3. Third stage This stage is coincident with Upper seismic sequence. The upper (M/U) boundary of the Middle sequence was created by a new drastic event which produced intense erosion, creating three minor subbasins (Fig. 6). This truncation is the base of the Upper sequence and the start of the third evolutionary stage. Nevertheless, a complete obliteration of the entire palaeotopography was already achieved after the deposition of the youngest unit Ua. Both an apparently more vigorous MOW behaviour and the palaeotopographic changes enabled a progressive adjustment in the architectural pattern and growing style of the contourite depositional system towards an elongate drift type. The contouritic deposits will occupy more and more space; within the course of the U sequence deposition, plastered drift bodies appeared on the southern flank of the Le Danois Bank and the Gijón Drift started quickly growing upslope towards the Cantabrian shelf. During this third stage, there is also a significant

change in medium-resolution subunit thickness. Hence, seen the significant changes introduced after the M/U discontinuity, we suggest it can be correlated with the Middle Pleistocene Revolution (MPR), which has induced similar changes in the Cadiz CDS (Hernández-Molina et al., 2006; Llave et al., 2007) and within the Porcupine CDS (Van Rooij et al., 2007a, 2009; Huvenne et al., 2009). Due to the absence of the topographic irregularities, the MOW behaviour evolves to the present-day situation, with fewer cores and a more extensive (or broader) influence. After the MPR erosion, one large, extended basin is created over the entire intraslope domain. This has lead to a unification of seismic facies and enabled a more extensive effect of depositional processes related to the MOW, especially in the proximal domain of the upper slope, where the Gijón Drift is going to be formed. As such, the previously deposited confined drifts change to the present morphosedimentary features, where more elongated, mounded and separated drifts are dominant. Unit Ua plays a transition role in this change. Although Ua still seems to be locally confined (Figs. 5 and 8), it grows out of the three subbasins as an elongated drift (Fig. 7). From the deposition of unit Ub onwards, all drifts show an elongated mounded upslope prograding style. Ub also completely masks the M/U palaeotopography and ends the influence of the palaeo- and subbasins. In the northern part of the intraslope basin, it grows further as the Le Danois Drift. From the base of Ub, there is also a drastic increase in growth of the Gijón Drift towards the southern part of the basin. The progressive occupation of this space is initially explained by the more vigorous behaviour of MOW that now is affected by the Vizco High and Gijón Canyon. In general, after the MPR there is not only a marked change in geometry from confined to mounded elongated drifts, but also a pronounced sedimentation increase and a more clear upslope migration of the drift-moat association. Likewise, the deposition of plastered drifts starts to build on the southern flank of the Le Danois Bank. Similar to the Cadiz CDS, the middle slope has been influenced by a high-velocity lower core of MOW during lowstand periods. Additionally, the increased activity of submarine canyons and of slope gravitational processes during lowstands may have provided an additional supply of sediment. By contrast, during the highstands, much of the terrigenic sediments were trapped within the shelf (Llave et al., 2006, Hernández-Molina et al., 2006). 7. Conclusions The intraslope basin created in between the Asturias continental shelf and the Le Danois Bank on the Cantabrian continental margin has hosted since the Lower Pliocene an impressive contourite depositional system. The major driving forces for the Le Danois CDS are predominantly the pre-Pliocene structurally-controlled palaeotopography and the Plio-Pleistocene climate and sea-level variability. The palaeotopography has governed the circulation model of the MOW and the type of contouritic deposits, while the climate and sealevel changes influenced their sediment types (acoustic facies), stratigraphic pattern and thickness. Indirectly, the climate and sealevel changes also drive the oceanography and general behaviour of the MOW. Similarities in the seismic stratigraphic framework and growth patterns have led to a tentative correlation with the Cadiz CDS and palaeoceanographic events along the NW European Margin. The three major sequences of the Le Danois CDS invoke three main evolutionary stages. During the first two stages, the CDS was controlled by the palaeotopography. Whereas in the Lower sequence (tentatively Lower to Upper Pliocene), the influence of bottom-current deposits could be inferred, the Upper Pliocene deposits of the Middle sequence are confined drifts. The Upper sequence was influenced by the palaeoceanographic changes during the Middle Pleistocene, boosting the formation of elongated mounded and separated drifts, plastered drifts and associated erosive features. The present-day Le Danois CDS also interacts with the adjacent sedimentary systems. It

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is fed partly by mass-wasting from the Le Danois Bank and receives sediment input from the Lastres Canyon System. The Le Danois Moat continues towards the lower slope as the Le Danois Leveed Channel System. The present-day key factors that are controlling the Le Danois CDS are: the local morphology of the margin, sediment supply and the local oceanographic behaviour of the MOW. The latter is predominantly influenced by the Le Danois Bank and Vizco High, which represent large obstacles for the eastward MOW circulation. Through probably isopycnal doming, the main MOW flow is split into three different branches. A northern branch flows north of the Le Danois Bank, without any evidence of drift deposition. The central branch flows within the intraslope basin, through the Le Danois Moat. It has shaped the Le Danois Drift and Plastered Drifts 1, 2 and 3 on the flank of the Le Danois Bank. A southern branch flows upslope the Asturias continental slope, creating the Gijón Moat and the Gijón Drift, with minor episodical influence of the Gijón Canyon. As such, it is the first time that a contourite depositional system along the Cantabrian (or even Biscay) margin is described in detail. The sedimentary record of this system allows to decode important palaeoceanographic events related to MOW bottom-current fluctuations, which are essential to the understanding of the presence and behaviour of intermediate water masses such as the MOW within the Bay of Biscay. Acknowledgements The Spanish “Comisión Interministerial de Ciencia y Tecnología” (CYCIT) supported this research through MARCONI Project (REN2001-1734 C03-01/M); ECOMARG project (REN2002-00916/ MAR) and CONTOURIBER (CTM 2008-06399-C04/MAR). This study also framed within ESF Euromargins MOUNDFORCE, EC FP5 RTN EURODOM and EC FP6 HERMES (contract GOCE-CT-2005-511234-1). We would like to acknowledge the efforts of the captains and crews of the involved research campaigns. The comments and suggestions of the editor, J.A. Howe and J.C. Faugères were highly appreciated and significantly improved the manuscript. The contribution of J. Iglesias was possible thanks to the CSIC grant UAC-2005-0044. This work has been partially carried out during a research stages of F.J. HernándezMolina at NOCS and Heriot-Watt University (UK) funded by the ‘Mobility Award’ from the Spanish Ministry of Education and Science (PR2006-0275 and PR2009-0343). D. Van Rooij is a post-doctoral fellow of the FWO Flanders. References Alvarez-Marrón, J., Rubio, E., Torne, M., 1997. Subduction-related structures of North Iberian Margin. Journal of Geophysical Research 102 (10), 22245–22511. Alves, T.M., Gawthorpe, R.L., Hunt, D.W., Monteiro, J.H., 2003. Cenozoic tectonosedimentary evolution of the western Iberian margin. Marine Geology 195 (1–4), 75–108. Ambar, I., Howe, M.R., 1979. Observations of the Mediterranean Outflow. 2. Deep circulation in the vicinity of the Gulf of Cadiz. Deep-Sea Research Part A— Oceanographic Research Papers 26 (5), 555–568. Baringer, M.O., Price, J.F., 1999. A review of the physical oceanography of the Mediterranean outflow. Marine Geology 155 (1–2), 63–82. Belderson, R.H., Kenyon, N.H., 1976. Long-range sonar views of submarine canyons. Marine Geology 22 (69–74). Boillot, G., Capdevila, R., 1977. The Pyrenees: subduction and collision? Earth and Planetary Science Letters 35, 151–160. Boillot, G., Dupeuble, P.A., Hennequin-Marchand, I., Lamboy, M., Lepetre, J.P., Musellec, P., 1974. Le role des décrochements "tardi-hercyniens" dans l'évolution structurale de la marge continentale et dans la localisation des grands canyons sous-marins a l'ouest et au nord de la péninsule Ibérique. Revue de Géologie Dynamique et de Géographie Physique 16 (1), 75–86. Boillot, G., Dupeuble, P.A., Malod, J., 1979. Subduction and tectonics on the continentalmargin off northern Spain. Marine Geology 32 (1–2), 53–70. Botas, J.A., Férnandez, E., Bode, A., Anadón, R., 1989. Water masses off central Cantabrian coast. Scientia Marina 53, 755–761. Bourillet, J.F., Zaragosi, S., Mulder, T., 2006. The French Atlantic margin and deep-sea submarine systems. Geo-Marine Letters 26 (6), 311–315.

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