the le danois contourite depositional system: … · ﬂows northward along ne atlantic margin and...

Marine Geology 274 (2010) 1–20

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The Le Danois Contourite Depositional System: Interactions between theMediterranean 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, Belgiumb Dpto. de Geologia Marina y Oceanografie Física, Instituto de Ciencias del Mar, CMIMA-CSIC, Paseo Maritimo de la Barceloneta, E-08003 Barcelona, Spainc Facultad de Ciencias del Mar, Dpto. de Geociencias Marinas, Universidad de Vigo, E-36200 Vigo (Pontevedra), Spaind Instituto Español de Oceanografia, c/ Corazon de Maria 8, E-28002 Madrid, Spaine Insituto Geologico y Minero de Espana, Geologia Marina, Rios Rosas 23, E-28003 Madrid, Spain

⁎ Corresponding author. Tel.: +32 9 2644583; fax: +E-mail address: [email protected] (D. Van Ro

0025-3227/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.margeo.2010.03.001

a b s t r a c t
a r t i c l e i n f o
Article history:Received 29 April 2009Received in revised form 26 January 2010Accepted 3 March 2010Available online 19 March 2010

Communicated by D.J.W. Piper

Keywords:contourite depositional systemsediment driftseismic stratigraphyMediterranean Outflow WaterNorth Iberian marginLe Danois Bank

The Le Danois Contourite Depositional System (CDS), located in an intraslope basin along the Cantabrianmargin, is unique with respect to the known sedimentary systems along the upper slope of the Biscaymargin. Whereas the steep Biscay slopes are dominated by downslope processes, the Le Danois CDS has beengenerated by alongslope processes and has a strong potential to contain a record of the Neogenepalaeoceanography. This paper will focus on the onset, development and present-day functioning of thissystem 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 theMediterranean Outflow Water, conditioned by seafloor irregularities and two topographic highs; the large LeDanois Bank and the smaller Vizco High. The seismic stratigraphic analysis carried out on the contouritedeposits has allowed to identify 3 seismic sequences, separated by 3 major regional discontinuities. Changesin depositional styles, the vertical stacking of seismic units and the nature of the discontinuities suggest acorrelation with the development of the Cadiz CDS and well-known palaeoceanographic events along the NEAtlantic margin. The first clues for bottom-current deposits are identified in the Lower Sequence, which isdeveloped after tentatively the Lower Pliocene. The drift deposits of both the Lower and Middle Sequenceswere confined into two palaeobasins within the intraslope basin. However, from the Middle PleistoceneRevolution (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 erosivefeatures, such as respectively elongated mounded and separated drifts, plastered drifts, moats and slidescars.

32 9 2644967.oij).

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1. Introduction

Contouritic bottom-current processes play an essential role in thedeep-marine environment where they are responsible in shaping theseafloor (Heezen et al., 1966; Kenyon and Belderson, 1973; Tucholkeand Ewing, 1974; Stow et al., 2002; Laberg et al., 2005), providingrecords of palaeoceanographic changes (Ellwood and Ledbetter, 1979;McCave et al., 1995; Hernández-Molina et al., 2003) and generatinglarge depositional systems, as important as turbidite bodies (Faugèresand Stow, 1993; Faugères et al., 1999; Viana et al., 2007; Rebesco andCamerlenghi, 2008). Whereas contourite drifts refer to depositional

features (Faugères et al., 1999; Rebesco, 2005), a contouritedepositional system (CDS) refers to genetically linked erosive anddepositional 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 theocean margins is driven by tectonic, environmental and oceano-graphic factors. Tectonic activity represents a key factor in producingmorphological changes on the seafloor and thereby controlling thedevelopment of new pathways for the core and branches of theimpinging 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 inducedby erosive features created by mass-wasting (Laberg et al., 2001; Brynet al., 2005) or even by biogenic build-up such as cold-water coralmounds (Van Rooij et al., 2007a, 2009; Huvenne et al., 2009). In the

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2 D. Van Rooij et al. / Marine Geology 274 (2010) 1–20

long term (2nd to 3rd order cycles), this is a driving force behind driftstratigraphy, architectural changes and the location of large-scaleerosive features. On the other hand, environmental (whetherpalaeoclimatologic or sea-level) and palaeoceanographic changesare 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 contouritestacking pattern, sequences and facies (Llave et al., 2001, 2006;Hernández-Molina et al., 2008).

One of the best documented contourite depositional systems alongthe Iberian Atlantic margin was generated during the Pliocene andQuaternary by the highly saline and warm Mediterranean OutflowWater (MOW) on the middle slope of the Gulf of Cadiz, extendingalong thewest Iberianmargin (Kenyon and Belderson, 1973; Gonthieret al., 1984; Stow et al., 1986; Nelson et al., 1999; Mulder et al., 2003;Hernández-Molina et al., 2006; Llave et al., 2007). This depositionalsystem is conditioned by the strongMOWoutflow velocities, reachingnearly 300 cm/s close to the Strait of Gibraltar, slowing down toapproximately 80 cm/s at Cape St. Vincent (Ambar and Howe, 1979;Cherubin et al., 2000). Once the MOW has left the Gulf of Cadiz, itflows northward along NE Atlantic margin and locally createscontourite 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) andwithin Porcupine Seabight (Van Rooij et al.,2007a). These drift deposits occur near the upper or lower boundaryof the MOW, predominantly due to enhancing of the bottom-currentvelocity by internal tides and waves or through morphologic control.

In this paper, we will zoom on the Le Danois ContouriteDepositional System (Fig. 1). The presence of this CDS along theupper slope of the southern continental margin of the Bay of Biscay isunique, whereas its steep and incised slopes are generally dominatedby gravitational processes (Bourillet et al., 2006; Ercilla et al., 2008a;Iglesias, 2009). Here, we will focus on the onset and evolution of thisCDS with respect to the unique morphological control and theresponsible local oceanographic processes. As such, we will establishthe possible sources of sediment supply, discuss the interactionbetween the local oceanographic and sedimentary processes, as wellas its potential to record the variability of the Neogene MOWpalaeoceanography.

2. Regional setting

2.1. Geology

The Bay of Biscay is a large W–E wedge-shaped re-entrant of thenortheastern Atlantic Ocean, bound by the Cantabrian continentalmargin at its southern side, by the NW–SE trending Celtic andArmorican margin in the north, and by the Aquitanian margin in theeast (Fig. 1A). The tectonic history started with the opening of the Bayof Biscay through seafloor spreading, underwent successive tectonicphases of rifting (from Triassic to Lower Cretaceous times), continuedwith passivemargin development (during the Upper Cretaceous), andended with its partial closure throughout a phase of compressionduring the Cenozoic (Boillot et al., 1979; Derégnaucourt and Boillot,1982; Thinon et al., 2001; Gallastegui et al., 2002). The compressivedeformation occurred mainly along its southern border, the Cantab-rian margin, from the Palaeocene up to Oligocene, when the Iberianand European plates converged (Boillot and Capdevila, 1977;Srivastava et al., 1990; Alvarez-Marrón et al., 1997). This motionresulted in the partial closure of the Bay of Biscay, the shortening anddeformation of the Cantabrian margin and the uplift and deformationof the Cantabrian Mountains (Olivet, 1978; Grimaud et al., 1982;Pérez-Estaún et al., 1995; Pulgar et al., 1996). This final tectonic phaseresulted in the present-day morphostructural configuration, which ischaracterized by inversed faults, thrusts and folds of different scales

and orientations, outcropping or buried basement ridges andtectonically-controlled submarine canyons and valleys (Gallasteguiet al., 2002). The Mesozoic to Quaternary deposits have a veryirregular distribution, and their stratigraphy reflects the structuralevolution of the Cantabrian margin. In fact, three general tectono-sedimentary sequences may be defined; syn-rift, syn-orogenic(during the Pyrenean compression) and post-tectonic. During thelatter phase, the margin reflects lower tectonic activity whereas theNeogene and Quaternary deposits (up to 4 s TWT) smoothenedseveral tectonic landscapes (Alvarez-Marrón et al., 1997; Gallasteguiet al., 2002; Iglesias, 2009). This last sedimentation phase ischaracterized by a strong canyon development that homogenisedthe seafloormorphology of the continental rise with the emplacementof 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 theabove mentioned structural trends and is characterized by a narrowcontinental shelf which passes abruptly into a continental slopewith avariable relief (Ercilla et al., 2008a). The continental slope is affectedby large tectonically-controlled submarine canyons running down tothe continental rise (Belderson and Kenyon, 1976; Cremer, 1981;Kenyon, 1987). Likewise, the slope is affected by structural highs, suchas the Le Danois Bank, which favours the presence of an intraslopebasin (Boillot et al., 1979; Ercilla et al., 2008a).

2.2. Oceanography

In the Bay of Biscay, most of the water masses are of North Atlanticorigin (Pollard et al., 1996). The uppermost water mass is the EasternNorth Atlantic Central Water (ENACW), which extends to depths ofabout 400 to 600 m (Fig. 2). The ENACW is characterized by a cyclonicgyre in the Bay of Biscay with average velocity of 1 cm/s (González-Pola, 2006).

Between 400 to 500 and 1500 m water depth, the MediterraneanOutflow Water (MOW) follows the continental slope as a contourcurrent (Fig. 2). Its circulation is conditioned by seafloor irregularitiesand the Coriolis effect. MOW velocities have been measured at 8°Wand 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 thereis a lack of detailed information of the MOW circulation in the Bay ofBiscay, it seems that the MOW splits into two branches around GaliciaBank, of which one branch continues towards the north, while theother one flows eastward along the Cantabrian margin slope (Iorgaand Lozier, 1999; González-Pola, 2006). From Ortegal Spur toSantander, MOW propagates along the slope, although with reducedvelocities. However, the MOW is likely to be influenced by first theAviles Canyon and later by the Le Danois Bank, which could introduceisopycnal doming. Further to the east, in the Santander Promontory,after crossing the Torrelavega Canyon, the MOW has a lower salinityand temperature (Valencia et al., 2004; González-Pola, 2006).

Between 1500 and 3000 m water depth, the North Atlantic DeepWater (NADW) is recognized. It includes a core of Labrador SeaWater(LSW) at a depth of about 1800 m, observed as a new salinityminimum down to 2000 m (Vangriesheim and Khripounoff, 1990;McCartney, 1992). Below the NADW, the Lower Deep Water isidentified, which mainly seems to result from the mixing of the deepAntarctic Bottom Water and the Labrador Deep Water (Botas et al.,1989; Haynes and Barton, 1990; McCartney, 1992). A cyclonicrecirculation cell over the Biscay Abyssal plain is recognized with acharacteristic 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 beenstudied extensively during the last three decades (Madelain, 1970;

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 intraslopebasin along the Cantabrian margin, with the location of the most important seismic profiles (GEBCO contour lines every 100 m).

3D. Van Rooij et al. / Marine Geology 274 (2010) 1–20

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 thewater masses in the Bay of Biscay, modified fromGonzález-Pola (2006). (C) General circulation pattern of theMOW 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 westernextremity.


Thorpe, 1976; Baringer and Price, 1999; Iorga and Lozier, 1999;Nelson et al., 1999; Hernández-Molina et al., 2006). Here, we presentan overview of the main evolutionary phases of the MOW and itstemporal variability along the NE Atlantic margin. The water massexchange between the Atlantic and Mediterranean basins initiatedafter 5.23 Ma, due to the opening of the Gibraltar Strait (Duggen et al.,2003; Hernández-Molina et al., 2006). The subsequent modificationsto this exchange, influenced by eustatic sea-level and palaeoclimaticchanges, have been studied in detail from the complex stratigraphyand 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 ofMOW generation has been recognized in seismic profiles from 4.2 Ma,

Fig. 3. Shaded relief multibeam bathymetric map of the study area (contour interval is 50Mediterranean Outflow Water (yellow dashed line).

associated to the Lower Pliocene Revolution (LPR). Due to enhancedseasonal aridity in the Mediterranean area from 3.5 to 3.3 Ma, a long-term rise in bottom-water salinity and temperatures was observed inthe Alboran Sea and on Goban Spur (Khelifi et al., 2009). Thesechanges culminated towards the Upper Pliocene Revolution (UPR) at2.4 Ma, when the present-day circulation pattern was establishedafter a global cooling event (Loubere, 1987; Hernández-Molina et al.,2006). Although the LPR marks the onset of contourite depositionwithin the Cadiz CDS, the UPR has lead to the formation of thedistinctly mounded shape of the present drifts due to an enhancedMOW circulation (Hernández-Molina et al., 2006). The distal effects ofan Upper Pliocene MOW have been observed in the PorcupineSeabight, where it has established a suitable environment for the

m) with indication of the inferred present-day local oceanographic behaviour of the


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 wasrelated to an important shift in glacial/interglacial periodicity from 41to 100 ka and an increase in the cycle amplitude (Head and Gibbard,2005; Lisiecki and Raymo, 2005, 2007). During this period, thepalaeoceanographic changes became more related to climatic andsea-level changes, and are markedly visible within the stackingpattern of the depositional sequences, with an enhanced moundedmorphology (Hernández-Molina et al., 2006; Llave et al., 2006, 2007).In the Porcupine Seabight, this had lead to the development of acomplex contourite depositional system (Van Rooij et al., 2007a,2009).

Within the course of the Late Pleistocene, glacially lowered sea-levels reduced the MOW outflow volume. However, a dryerMediterranean 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 etal., 2006; Voelker et al., 2006). Identical MOW velocity increases wereobserved during Dansgaard–Oeschger stadials, Heinrich Events andthe Younger Dryas (Llave et al., 2006; Toucanne et al., 2007). This wasresponsible for a reduced MOW advection towards the north, asdocumented along the southern Portuguese margin (Schönfeld andZahn, 2000). Conversely, the impact of glaciations on the MOW isreversed in the rest of the NE Atlantic margin. During interglacialperiods, it is the presence of the MOW that locally causes enhancedbottom-current flow through internal waves and tides, while thereduced occurrence of the MOW during glacial or stadial episodes haslead 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 wereacquired within the framework of different projects and/or cruises(ECOMARG, MARCONI and GALIPOR). It comprises multibeambathymetry and single channel seismic records of medium and highresolutions (Fig. 1). A high-resolution bathymetric map (Fig. 3) wasobtained with the SIMRAD EM 300 multibeam system. The acquireddata was processed using the SIMRAD NEPTUNE package, obtaining abathymetry grid of 15 m resolution at 500 to 600 m and at 25–50 m atdepths down to about 1000 m. Seismic records were collected usingairgun and sparker systems. The latter was used for surveying a moredetailed sector of the study area, south of Le Danois Bank. The airgunrecords were obtained using a 140 in.3 sleeve gun array, located at adepth of 3.5 m, with a shot frequency of 8 s. The airguns were firedwith four Hamworthy air compressors, producing a 140 bar firingpressure. The receptor system was a SIG streamer with an activesection 150 m long. The penetration of the acoustic signal is about 1 s(TWT). The source of the sparker seismic profiles was a 120 electrodeSIG 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 mwith a penetration between 200 and 500 ms TWT. The processing ofthese data involved basic bandpass-filtering and swell-filtering.

4. Morphologic characteristics

4.1. General physiography

Detailed analyses of themultibeam bathymetry allowed to identifythree main physiographic domains in the studied sector of theCantabrian margin: a narrow continental shelf, a complex uppercontinental slope and an abrupt lower continental slope (Fig. 4). Theshelf has a variable width (between 17 and 32 km), and the shelf

break is located at about 150 to 200 m water depth. The upper slopehas two major domains: a) a proximal domain with an average slopegradient of 6°, approximately 10 km wide, 29 km long, locatedbetween 200 and 400–500 m water depth; and b) a distal domainwith a complex physiography between 400 and 1600 m water depth.It represents the morphologic expression of an inner slope basin, andcomprises two highs; the large Le Danois Bank and the smaller VizcoHigh to the west. The presence of Le Danois Bank creates the W–Eintraslope basin, which is 17 to 25 kmwide and approximately 50 kmlong. The Vizco High (1400–1050 m) displays a sub-rounded shape inplain view of 7 km wide, and is about 200 m high. The Le Danois Bankhas an elongated convex southward shape of about 72 km long and15 kmwide. Its top is flat, representing a largeW–E platform between550 and 600 m water depth. Its gentle southwestern slope, facing theintraslope basin, deepens from 500 to 850 m water depth with agradient of about 2° and an irregular seafloor. On the other hand, thesoutheastern flank is rougher, with an average slope gradient of 12°from 600 to 800 m. At its easternmost extremity, there is a noticeablesmall high (named here as the ECOMARG High) of about 100 m high,with a 3° slope, between 800 and 1800 m water depths. The northernflank of the Le Danois Bank represents the upper part of the lowerslope and has a regionally variable gradient between 16.5° in theupper part and about 18° in the middle part. The base of the lowerslope is at about 4400 m water depth where the continental risestarts. Within the intraslope domain, the Le Danois ContouriteDepositional System (CDS) is recognized, as well as three associatedsedimentary systems: the Lastres Canyon system, the Le DanoisLeveed 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 ofthe upper slope and the Le Danois Bank, between 400 and 1500 mwater depth (Figs. 3 and 4). This system is composed of severaldepositional and erosive features.

4.2.1. Depositional features

4.2.1.1. Elongated mounded and separated drifts. Two elongatedmounded and separated drifts have been identified within the LeDanois CDS: the Gijón and Le Danois Drifts. The Gijón Drift is a SEtrending elongated mounded and separated contourite depositlocated on the upper slope at about 400–850 m water depth(Fig. 4). It is bound southward by the Gijón Moat, northward by theLe Danois Drift, and westward by the Lastres Canyon system. The driftshows a positive relief of about 100 m in the NW part, becomingsmaller (b50 m) to the SE. It has a maximum width of about 10 kmand is approximately 31 km long.

The Le Danois Drift is an ESE trending elongated mounded andseparated drift, developed between 800 and 1500 to 1600 m waterdepth. 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 boundsouthward by the Gijón Drift and the Lastres Canyon system, andeastward by the start of the Le Danois Leveed Channel system. Thedrift is about 45 km long, 50 m high and has a variable width, with amaximumof 10 km in the central part of the drift, 3.5 to 4 km in theWand 4.7 km towards the E.

Between the Gijón and the Le Danois Drifts, there is a 35 km longsub-horizontal transitional zone with a SE trend (Fig. 4). It is 9.5 kmwide in the NW, 2.3 km in the central zone and 7.1 km in the SW.

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

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

pp. 7–10D. Van Rooij et al. / Marine Geology 274 (2010) 1–20


1100 and 1550 m. They are all characterized by a mounded relief ofabout 40 m. Plastered Drift 1 is 12 km long and 5.5 km wide. Itdisplays 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 ENEtrend. It ends sharply against the ECOMARG High toward the NW,and is bound by the Plastered Drift 3 and the Le Danois Moattowards the south. Plastered Drift 3 is about 20 km long and 6 kmwide, showing a NE trend and extends downslope until the LeDanois Moat and Le Danois Leveed Channel.

4.2.1.3. Sediment waves. Two sediment wave fields have beenidentified (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, withwavelengths between 750 and 1200 m and amplitudes between 30and 75 m. The second field (6.5 km²) is located on the eastern area, atabout 1500 m water depth. They display wavelengths between 800and 1650 m and amplitudes between 35 and 130 m. These wavesseem 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 thesouth by the proximal domain of the upper slope. It is the upslopecontinuation of the Gijón Canyon, it has a NW–SE trend and is about45 km long and 1 to 4 km wide. It starts from 1100 m water depth inthewest, having an incision of 200 m, up to 400 mwater depth, whereits expression narrows and disappears (Fig. 4). The Le Danois Moatseparates the Le Danois Bank from the Le Danois Drift. It displays aWNW–ESE trend and deepens from 800 to 1500 m water depthtowards the east. The moat merges eastward into the Le DanoisLeveed Channel. It is approximately 48 km long and has a variablewidth between 2.8 and 0.8 km. The relief is also variable; in theproximal reaches, the incision is 75 m, increasing to 105 m in thecentre to decrease again to 98 m in the distal reaches.

4.2.2.2. Slide scars. Slides are locally affecting the Gijón and Le DanoisMoats and the Le Danois Drift (Fig. 4). They are recognizable due to arough seafloor, characterized by arcuate and semicircular slide scarsassociated to deformed sediment. These scars have the tendency tocoalesce forming multiple slides, creating large areas of erosiondisplaying 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 removedsediment encompasses areas of tens of m2 at the Gijón and Le DanoisMoat walls, and at the southern part of the Le Danois Drift.

4.2.2.3. Scours. Scour alignments are observed over the Plastered Drift3 (Fig. 4) with a NE and ENE trend. They are between 28 and 5.5 km inlength and show a maximal vertical incision of about 5 m deep overwidth of 1250 m.

4.3. Sedimentary systems associated to Le Danois CDS

The Le Danois CDS is laterally connected with the Le Danois LeveedChannel system (Fig. 4). At about 1500 m water depth, the Le DanoisMoat transforms downslope into the 42 km long Le Danois LeveedChannel. It is bordered by an overbank area on its right hand side,where a well-defined levee is recognized. This probably is a smallturbiditic channel–levee system, comparable to the Var or Cap Ferretsystem (Faugères et al., 1999; Ercilla et al., 2008a) The Le DanoisLeveed Channel displays and arc-shape plan-view that runs parallel tothe Le Danois Bank. It has a downslope trend that varies from NE to Nand finally towards the NW. The cross-section changes from V-shaped

in the proximal part to U-shaped in the middle part and again to V-shaped in the distal part. The inner face of the left channel wall isaffected by gullies of 1–10 m length.

The Lastres Canyon system is located to the southeast of the LeDanois CDS (Fig. 4). The Lastres Canyon heads initially show a NE–SWtrend, whereas it abruptly changes to N–S in the central part andfinally ends with an E–W orientation near the distal domain of theupper slope. This canyon is 4.5–5 km wide with a floor of 0.5–1 kmwide. At least two tributaries running down to the canyon floor areidentified on its right side. The sharp pathway changes indicate thatthe location of the canyon system is structurally-controlled (Boillotet al., 1974; Derégnaucourt and Boillot, 1982). The canyon walls andthose of their tributaries are affected by a well-developed drainagefeatures that form a network of gullies. Arcuate to circular slide scarsare observed on the W–E border of the left canyon wall. Ercilla et al.(2008a) suggest that these are retrogressive slides of which theiroccurrence seems to be associated with the steep slope gradients ofthe canyon wall.

Additionally, a mass-wasting system is identified along the steepwalls of the Le Danois Bank. It is formed by isolated andmultiple scars,deformed sediments defining an irregular seafloor and gully drainagesystems. The slide scars located on the northern flank of the Le DanoisBank evolve downslope to gullies perpendicular and oblique to theregional slope. The gullies are from tens of metres to severalkilometres 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 todefine 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, fromolder to younger: Ma, Mb and Mc (Figs. 5 and 6). The upper sequenceU is composed of three seismic units, which are, from older to youngerUa, Ub and Uc (Figs. 5 and 6). All units show cyclic changes in seismicamplitudes; a transparent/weak zone that passes to smooth, parallelreflections of moderate-to-high amplitude in the upper part; and ahigh-amplitude erosive continuous surface at the top. The sedimen-tary package defined by these three major sequences overlies mostlyMiocene deposits confined between basement palaeohighs (Ercillaet al., 2008a). These palaeohighs create two W–E trending subbasinsin the deeper area of the intraslope basin (Figs. 5 and 7). They are atleast 60 km long and parallel to the Le Danois Bank. The northernpalaeobasin (NPB) runs downslopewidening towards east from about4 to 9 km and extends at least down to 1600 m water depth. Thesouthern palaeobasin (SPB) is 8 km wide and narrows towards theLastres Canyon where it finally disappears before the northernpalaeobasin.

5.1. Lower seismic sequence

This sequence is identified as the filling deposits of the NPB andSPB (Figs. 5, 7 and 8). They rest on the B/L discontinuity that is definedby an irregular reflection. Its erosive character is more pronounced inthe SPB. Here, the B/L discontinuity defines a U-shape valley incisionup to 8 km wide and 250 ms TWT deep, whereas in the NPB it is adipping planar surface. The upper boundary of the L sequence, the L/Mdiscontinuity, also displays an erosive character which is morepronounced in the SPB. Here, it affects the B/L discontinuity makingboth boundaries undifferentiated (Fig. 5). This erosion also produces aU-shape valley of 8 km wide and 250 ms TWT deep, characterized byan irregular seafloor. In the NPB, this discontinuity is a southwarddipping planar surface.

The geometry of the L sequence is different in both palaeobasins. Inthe NPB, it abuts the sides of its surrounding highs, displaying asubtabular geometry with a thickness of about 80 ms TWT. In the SPB

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 msTWT thick that pinches out toward the palaeobasin sides.

From a seismic point of view, both palaeobasins display remark-able differences in facies type. In the NPB, the lower sequencecomprises a high-amplitude sub-parallel reflection configuration.Towards the Le Danois Bank, it laterally passes to chaotic and wavyshort reflections of high to low amplitude (Figs. 5 and 8). Their verticalarrangement suggests an apparent upslope migration that progres-sively onlaps the flank of Le Danois Bank. In the SPB, the lowersequence is characterized by an undulated stratified facies of highlateral continuity, high to medium amplitude and sigmoidal geom-etry. Although only one seismic profile (Fig. 5) allows to describe thisparticular deposit, we suggest it shares characteristics with a confineddrift (Reed et al., 1987; Rebesco and Stow, 2001). This drift isasymmetric and associated with a moat located on the northern sideof the palaeobasin. The moat is characterized by discontinuousstratified and chaotic facies with acoustic amplitude higher thanthat of the drift. The stratal pattern reflects a southward upslopemigration that progressively onlaps and fills the SPB. This patternsuggests the presence of a (second) drift-moat association, whichprobably 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 palaeobasinscontinue displaying different seismic facies. Within each of them, thethree units Ma, Mb and Mc show a similar depositional stratal growthpattern. The lower L/M discontinuity is an onlap surface, while theupper M/U discontinuity is a prominent irregular erosive reflectionwhere at least three relatively small depressions/subbasins (SB) areidentified: a northern (5 kmwide) over the NPB and a central (2.5 kmwide) 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). Theirinternal boundaries locally display downlap and truncation termina-tions.Within this sequence, mostly confined drift deposits, and locallysediment waves, are recognized (Figs. 5 and 8). From units Ma to Mc,the confined drifts progressively evolve from subtabular to moundedgeometries, bound by amoat north of the drift (Figs. 6 and 8). They arecharacterized by sub-parallel stratified facies with different acoustic

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 chaoticstratified reflections of higher acoustic amplitude. The verticalstacking of these deposits shows a northward upslope migrationwith a progressive onlap over the L/M discontinuity, graduallyreaching the flank of Le Danois Bank (Figs. 5, 6 and 8).

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 largelycomposed of sediment waves characterized by wavy stratifiedreflections with a variable acoustic amplitude and wave scale changefrom units Ma to Mc. The acoustic character varies vertically,increasing towards unit Mc (Fig. 5). The sediment waves in unitsMa and Mb are stratified reflections with lateral continuity betweenthem. They have an unconformable configuration with convergentreflections approaching the wave crests. Their wavelength increasesvertically from 1 to 1.5 km, whilst their amplitude decreases vertically

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

5.3. Upper seismic sequence

5.3.1. General characteristicsSequence U occupies a larger area than the previous ones,

extending from the uppermost slope (400 m water depth) down tothe foot of Le Danois Bank (Fig. 5). Its thickness is variable, rangingfrom 180 to 250 ms TWT. It is characterized by a large variety ofcontouritic deposits which are interpreted as mounded elongated andseparated drifts, mounded elongated drifts, confined drifts andplastered drifts. This sequence is bound by the M/U discontinuitywhose palaeotopography roughly reflects the present-day morphol-ogy of the intraslope basin. Locally, some differences are found due to

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 areprogressively filled by the seismic units Ua, Ub andUc (Figs. 5, 7 and8).The discontinuities between these units mark changes in the internalarchitecture and/or contourite deposits. They are mostly concordantreflections that laterally change to erosive in the deeper part of theintraslope basin. This infers that the drifts underwent severalrelocations and modifications of depocenter location.

The mounded elongated and separated drifts are defined by anunconformable and roughly conformable stratified facies of highlateral continuity displaying mounded and sigmoidal geometries. Theacoustic facies show a cyclic vertical pattern from low to highamplitude in each seismic unit, including a general upward increasethroughout the sequence (Figs. 6 and 8). The drifts generally areasymmetric, with a short steep flank towards the moat and a smoothand large backside flank (Figs. 5 and 8). Some drifts display more thanone crest due to the presence of a subsidiary drift associated to a smallmoat on its backside (Fig. 7). The elongated mounded drifts areassociatedwith onemoat close to the palaeohighs (Fig. 8). Themoat ischaracterized by discontinuous stratified and chaotic facies withlocally higher acoustic amplitude than those observed in the drift.Some of the moat reflections laterally continue into the drifts (Figs. 6and 8), while others onlap on U-shaped surfaces of high-amplituderesembling cut-and-fill features (Fig. 7). The general stratal pattern ofthe drift-moat association reflects for the first time a clear an upslopemigration, showing onlap on the palaeohighs and the seafloor of theupper continental slope (Fig. 9).

5.3.2. Seismic unit UaGenerally, seismic unit Ua (average thickness 99 ms TWT)

contains three different mounded elongated drifts, located withinthe inherited northern, central and southern subbasins (Fig. 5).However, in the westernmost part, the drift in the northern subbasinstill seems to be confined (Fig. 8). These deposits show a smoothmounded shape but, they have a patchy distribution, as they are

interrupted by the acoustic basement (Fig. 5). The three drifts displayupslopemigration, gradually filling andmoving outside the subbasins.

5.3.3. Seismic unit UbThe drift deposits of seismic unit Ub (average thickness 68 ms

TWT) completely mask the M/U palaeotopography and only gentlyreminders of the northern and southern subbasins remain. In this unit,both the Le Danois Drift and (especially) the Gijón Drift show anincreased lateral development, respectively fully becoming anelongated and separated or elongated mounded drift (Fig. 5). Thisnew area (275 km2) is mainly located on the proximal upper slopeand extends from 400 to 1000 water depth between the Gijón andLastres canyons (Figs. 5 and 9). In the northern subbasin, the LeDanois Moat gradually becomes better developed compared to theopposite (inherited) one which progressively disappears throughoutunit Ub (Figs. 5 and 8). Both the northern and southern subbasindeposits pinch out in opposite directions, progressively extendingtoward the axis of the intraslope basin. The resolution of the seismicprofiles does not allow to determine interfingering between bothsouthern and northern deposits.

5.3.4. Seismic unit UcThe presence of the mounded and separated elongated Le Danois

and Gijón Drifts appears to be enhanced during the deposition ofseismic unit Uc (Figs. 5 and 8). They are built by vertical stacking ofupslope prograding stratified sediment packages (total averagethickness 50 ms TWT). The total upslope migration (Ub and Uc) ofthe Le Danois Drift amounts to about 2.8 km on a horizontal scale and130 ms TWT in average relief. The internal architecture of the Gijóndrift-moat association shows more than 5 km of (southward) upslopemigration (Fig. 5). This migration produces a lateral accretionary fill ofthe Gijón Moat with drift deposits. By consequence, the valley/moat isasymmetric with a depositional northeastern wall and an erosivesouthwestern one. Towards the transition zone between Le Danoisand Gijón Drift, an aggrading facies is observed, characterized by

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


parallel-laminated, laterally continuous reflections with an onlapreflection configuration.

5.3.5. Plastered driftsLocally, sequence U is present on the southeastern flank of the Le

Danois Bank, where it shows an isolated patched distribution (8 kmwide) 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 lateralcontinuity and high to very high acoustic amplitude (Figs. 7 and 10).This facies shows an aggrading pattern that rests unconformably overfolded and faulted deposits of Le Danois Bank. Although PlasteredDrift 2 was interpreted by Ercilla et al. (2008a) as a slide (Fig. 7), aneighbouring high-resolution seismic profile (Fig. 10) and the multi-beam bathymetry (Fig. 4) allow to identify this deposit as a plastereddrift. It typically displays a mounded and laminar geometry in cross-sections and an irregular elongated shape in plan-view (Faugèreset 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, wededuced a local hydrodynamic model from the distribution of

Fig. 10. N–S high-resolution sparker profile RCMG09 on the Le Da

morphosedimentary features. Additionally, the local physiographyof the margin and the structural features that could interact withMOW water mass have also been taken into account.

The Le Danois Bank represents a large obstaclewithin the eastwardMOW flow. Its presence could introduce isopycnal doming in theupper part of the MOW and separation of its flow into two majorbranches. Similar processes (involving the MOW) have been reportedin other locations such as the Galicia Bank (Iorga and Lozier, 1999) andthe Guadalquivir Bank (Hernández-Molina et al., 2003; Llave et al.,2007). In a similar way, but on a smaller scale, the occurrence of theVizco High (Fig. 4) also represents an obstacle, funnelling MOWthrough the Gijón canyon. Its influence will remain restricted to theGijón canyon and drift area. As such, it is proposed both the Le DanoisBank and Vizco High influence the MOW flow to separate into threedifferent branches: a northern, central and southern branch (Fig. 3).

A northern branch of theMOW flows along the northern flank of LeDanois Bank. However, based on the present available data, suggest-ing dominantly gravitational processes, we cannot substantiate anydepositional or erosive evidence of this branch. The southern branch,located between the Vizco High and the proximal domain of the upperslope, flows along the Gijón Drift and Moat, while the central branchflows between the Vizco High and the Le Danois Bank, creating the LeDanois Drift and Moat. The southern branch of the MOW is forced toflow upslope, loosing activity over the transition towards the LastresCanyon System. Similarly, the central branch flows along the southern

nois Bank flank of Plastered Drift 2. (AB: acoustic basement).


flank of Le Danois Bank, remains restricted along the intraslope basinand is disconnected from the seafloor at the transition from the LeDanois 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 betweenhydrodynamic and sedimentary processes and sources, along respec-tively the southern and central MOW branch. Every branch has a“tabular” behaviour but when interacting with the seafloor, a moreturbulent core generates non-depositional or erosional processeswhich produce the moats.

6.2.1. Southern MOW branchSouth of the Vizco High, a significant bottom current flow can be

inferred where the southern branch of the MOW impinges on theseafloor. Afterwards, this branch flows right confined into the possiblytectonically-controlled Gijón Canyon that runs WNW–ESE along theproximal domain of the upper slope. In this situation, the current isaccelerated and generates enough turbulence as a helicoidal flow toprogressively and locally erode the bedrock and gravitational depositsthat constitute the southern flank creating the GijónMoat (Fig. 3). Theeroded sediment is incorporated into the southern branch anddeposited northward building up the Gijón mounded elongated andseparated drift at its left-hand side (Figs. 5 and 9). The expression ofthe Gijón Moat and mounded drift ends at the same upper limit of theMOW at 400 m. Alternatively, if the Gijón Canyon is active duringglacial lowstands, it could deliver sediment from upslope as aturbidite system. In this case, an eastward flowing MOW wouldprogressively lose its intensity upslope, countered by turbiditycurrents. However, no gullies are observed along the upper slopeand the general morphology is relatively smooth slope compared toLastres canyon.

6.2.2. Central MOW branchThe interaction between the central branch and Le Danois Bank

starts on the southwesternmost flank of Le Danois Bank wherePlastered Drift 1 is deposited in water depths between 600 and 750 m.By definition, plastered drifts are created by a broad non-focusedcurrent on a gentle slope under low current velocities (Faugères et al.,1999; Rebesco, 2005). A possible causal mechanism for this depositcould thus be proposed through the interaction of a broad upslopeflowing part of the central branch, eroding the upslope part of LeDanois Bank. Additionally, fine-grained suspended matter piratedfrom the Aviles Canyon could also be deposited on the plastered drifts.Unfortunately, the absence of a detailed seismic architecture disablesto gain an insight into the construction of this deposit.

The main influence of the central MOW branch is observed withinthe intraslope basin, where it has a W–E flowwith enough turbulenceto erode the nearsurface sediment, create the Le Danois Moat anddeposit 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 AvilesCanyon, which is located west of the CDS. Suspended sediment(nepheloid layers) escaping from the upper canyon system could bepirated eastward by the MOW and deposited within the intraslopebasin. Likewise, gravitational processes affecting the southern flank ofthe Le Danois Bank (Figs. 4 and 7) may contribute to supply thematerial building up the mounded drift. Another possible and moredirect sediment supply could be expected from the shelf duringregressive and lowstand (glacial) periods. With the increasing waterdepths towards the east, the influence of the central branch of theMOW 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

behaviour of the detached central MOW branch (Figs. 2E and 3).Moreover, the lower occurrence of Plastered Drift 3 coincides with theinterface between the MOW and the NADW (LSW) at 1500–1550 m,as well as with the transition between the Le Danois Moat toward theLe Danois Leveed Channel. Fine-grained suspended material escapingfrom the Le Danois CDS could be transported to the plastered drifts.Another possible sediment source, mainly for Plastered Drift 3, are theoverflowing plumes of fine sediment from turbidity currents along theLe Danois Leveed Channel.

The water depth of the MOW/NADW interface also correspondswith the eastern, SW migrating, sediment wave field located ateastern boundary of the Le Danois CDS (Fig. 4). Although no seismicprofiles are available to investigate whether these waves are due tobottom currents or gravity deformations (Gardner et al., 1999;Faugères et al., 2002), we suggest that they could be related tointernal 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 ChannelSystem

The Le Danois CDS evolves downslope to the Le Danois LeveedChannel System at the interface between MOW and NADW (LSW), andthus where the MOW loses contact with the seafloor (Figs. 2E and 3).The evolution from moat to leveed channel occurs in a transition zonefrom 1200m to 1500 m where the regional slope gradient graduallyincreases from about 0.9° to 1.7° and locally up to 4°. This increase ingradient could favour a change in the sedimentary process fromcontouritic, associated to the central MOW branch, to gravitational(turbiditic). The sediment supply feeding this leveed channel can becoming froma combinationof various sourceswhichmight act together.These possible sources are: (1) sediment transported along the LeDanois moat towards to leveed channel system; and (2) episodic slopeinstabilities generating turbidity currents from instable drift levees(Fig. 7) or from the southern flank of the Le Danois Bank (includingPlastered Drift 3). The latter might also be driven through the influenceof internal waves associated to the MOW/NADW interface. Thecontribution of these sources could vary with time depending onMOW 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 anunrelated activity.

6.4. Evolution of Le Danois Contourite Depositional System

The spatial and temporal distribution of the contourite depositswithin the Le Danois intraslope basin reflects that the depositionalhistory involved several relocations and changes in drift type (Fig. 5).The three major sequences of the Le Danois CDS invoke three mainevolutionary stages. The subsequent changes in morphology, stackingpattern, as well as the timing and extent of their sequence boundarieswill be discussed with respect to the MOW palaeoceanography.

6.4.1. Initial stageThis stage is coincident with the onset of the Lower seismic

sequence. The irregular B/L discontinuity represents the start of thebottom current flow within the intraslope basin. As such, it marks thebeginning of MOW interaction with this sector of the Cantabrianmargin. During this initial stage, the influence and vigour of the MOWmight still be temperate. This could coincide with the onset of theCadiz CDS during the Intra Lower Pliocene at about 4.2–4.0 Ma(Hernández-Molina et al., 2006; Llave et al., 2007). An equivalent ofsuch 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 arepredominantly controlled by the presence and interaction of such an


“immature” MOW with the palaeotopography created by thepalaeohighs and the LPR erosive action upon the Miocene sedimen-tary basement. Upon entry within the intraslope basin, the LowerPliocene MOW is split by the palaeohighs and confined within thepalaeobasins, creating vortices. Despite the limited number of seismicprofiles and the eroded upper part, the inferred confined drift in theSPB suggests a constrained circulation with increased velocities at thesidewall (Faugères et al., 1999; Rebesco, 2005). This, however, is notthe case in the NPB, where the chaotic and wavy reflections depositedon the northern flank suggest the influence of enhanced hydrody-namic environment. The sediment input is still relatively moderatedue to the lower Pliocene low-amplitude sea-level variability (Zachoset al., 2001; Lisiecki and Raymo, 2007).

6.4.2. Second stageThis stage is coincident with the Middle seismic sequence which

has the L/M discontinuity as its lower boundary. Similar to the initialstage, the seismic facies and the depositional geometries observedwithin theM sequence indicate that the depositional dynamics withinboth subbasins remain different. In the NPB, the progressive evolutionfrom subtabular (Ma) to mounded (Mb and Mc) confined drifts withmoat features, illustrates the gradual increase of bottom-current flow.The mere presence of these confined drifts indicates that the MOW isstill confined by the Miocene palaeotopography. Adjacent moatsindicate local flow cores close to the sidewalls of those irregularities,such as observed in other confined drifts (Reed et al., 1987; Van Rooijet al., 2007a). With respect to the SPB, a confined and wavy behaviourof the MOW allowed the deposition of vertically stacked stationarysediment waves (Fig. 5). Toward the end of this stage, the M sequencedeposits will have filled most of the previous subbasins formed by theB/L discontinuity and the predefined tectonic setting.

The changes in drift geometry and depositional styles may bedirectly related to the palaeoclimatic processes initiated during theUpper Pliocene Revolution (UPR) at about 2.4 Ma. Within the CadizCDS, this margin-wide event marked the start of a present-dayoceanographic exchange model with a more intense MOW and thefurther development of elongated mounded and separate contouritedeposition in the Faro-Albufeira drift (Hernández-Molina et al., 2006).Further along the NW European margin, it started to play apronounced role in the margin stratigraphy (Laberg et al., 2005;Stoker et al., 2005) and has set the stage for cold-water coral moundgrowth in the Porcupine Seabight (Huvenne et al., 2009). As such, theearly Pleistocene effects of climatic control on the presence and flowintensity of the bottom-water masses, and on the sedimentary input,becomes more visible within the sedimentary record of the Le DanoisCDS. This is mainly expressed through the introduction of the stackingof medium-resolution seismic subunits and a more pronounced(interglacial) current-controlled shaping of these subunits intoconfined sediment drifts.

6.4.3. Third stageThis stage is coincident with Upper seismic sequence. The upper

(M/U) boundary of the Middle sequence was created by a new drasticevent which produced intense erosion, creating three minor sub-basins (Fig. 6). This truncation is the base of the Upper sequence andthe start of the third evolutionary stage. Nevertheless, a completeobliteration of the entire palaeotopography was already achievedafter the deposition of the youngest unit Ua. Both an apparently morevigorous MOW behaviour and the palaeotopographic changesenabled a progressive adjustment in the architectural pattern andgrowing style of the contourite depositional system towards anelongate drift type. The contouritic deposits will occupy more andmore space; within the course of the U sequence deposition, plastereddrift bodies appeared on the southern flank of the Le Danois Bank andthe Gijón Drift started quickly growing upslope towards theCantabrian shelf. During this third stage, there is also a significant

change in medium-resolution subunit thickness. Hence, seen thesignificant changes introduced after the M/U discontinuity, wesuggest 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 thePorcupine CDS (Van Rooij et al., 2007a, 2009; Huvenne et al., 2009).

Due to the absence of the topographic irregularities, the MOWbehaviour evolves to the present-day situation, with fewer cores and amore extensive (or broader) influence. After the MPR erosion, onelarge, extended basin is created over the entire intraslope domain.This has lead to a unification of seismic facies and enabled a moreextensive effect of depositional processes related to the MOW,especially in the proximal domain of the upper slope, where theGijón Drift is going to be formed. As such, the previously depositedconfined 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 seemsto be locally confined (Figs. 5 and 8), it grows out of the threesubbasins as an elongated drift (Fig. 7). From the deposition of unit Ubonwards, all drifts show an elongated mounded upslope progradingstyle. Ub also completely masks the M/U palaeotopography and endsthe influence of the palaeo- and subbasins. In the northern part of theintraslope basin, it grows further as the Le Danois Drift. From the baseof Ub, there is also a drastic increase in growth of the Gijón Drifttowards the southern part of the basin. The progressive occupation ofthis space is initially explained by the more vigorous behaviour ofMOW that now is affected by the Vizco High and Gijón Canyon. Ingeneral, after the MPR there is not only a marked change in geometryfrom confined to mounded elongated drifts, but also a pronouncedsedimentation increase and a more clear upslope migration of thedrift-moat association. Likewise, the deposition of plastered driftsstarts to build on the southern flank of the Le Danois Bank. Similar tothe Cadiz CDS, themiddle slope has been influenced by a high-velocitylower core of MOW during lowstand periods. Additionally, theincreased activity of submarine canyons and of slope gravitationalprocesses during lowstands may have provided an additional supplyof sediment. By contrast, during the highstands, much of the terrigenicsediments were trapped within the shelf (Llave et al., 2006,Hernández-Molina et al., 2006).

7. Conclusions

The intraslope basin created in between the Asturias continentalshelf and the Le Danois Bank on the Cantabrian continental marginhas hosted since the Lower Pliocene an impressive contouritedepositional system. The major driving forces for the Le Danois CDSare predominantly the pre-Pliocene structurally-controlled palaeoto-pography and the Plio-Pleistocene climate and sea-level variability.The palaeotopography has governed the circulation model of theMOW and the type of contouritic deposits, while the climate and sea-level changes influenced their sediment types (acoustic facies),stratigraphic pattern and thickness. Indirectly, the climate and sea-level changes also drive the oceanography and general behaviour ofthe MOW. Similarities in the seismic stratigraphic framework andgrowth patterns have led to a tentative correlation with the Cadiz CDSand palaeoceanographic events along the NW European Margin.

The three major sequences of the Le Danois CDS invoke three mainevolutionary stages. During the first two stages, the CDS wascontrolled by the palaeotopography. Whereas in the Lower sequence(tentatively Lower toUpper Pliocene), the influence of bottom-currentdeposits could be inferred, the Upper Pliocene deposits of the Middlesequence are confined drifts. The Upper sequence was influenced bythe palaeoceanographic changes during the Middle Pleistocene,boosting the formation of elongated mounded and separated drifts,plastered drifts and associated erosive features. The present-day LeDanois CDS also interacts with the adjacent sedimentary systems. It


is fed partly by mass-wasting from the Le Danois Bank and receivessediment input from the Lastres Canyon System. The Le Danois Moatcontinues towards the lower slope as the Le Danois Leveed ChannelSystem.

The present-day key factors that are controlling the Le Danois CDSare: the local morphology of the margin, sediment supply and thelocal oceanographic behaviour of the MOW. The latter is predomi-nantly influenced by the Le Danois Bank and Vizco High, whichrepresent large obstacles for the eastward MOW circulation. Throughprobably isopycnal doming, the main MOW flow is split into threedifferent branches. A northern branch flows north of the Le DanoisBank, without any evidence of drift deposition. The central branchflows within the intraslope basin, through the Le Danois Moat. It hasshaped the Le Danois Drift and Plastered Drifts 1, 2 and 3 on the flankof the Le Danois Bank. A southern branch flows upslope the Asturiascontinental slope, creating the Gijón Moat and the Gijón Drift, withminor episodical influence of the Gijón Canyon.

As such, it is the first time that a contourite depositional systemalong the Cantabrian (or even Biscay) margin is described in detail.The sedimentary record of this system allows to decode importantpalaeoceanographic events related to MOW bottom-current fluctua-tions, which are essential to the understanding of the presence andbehaviour of intermediate water masses such as the MOWwithin theBay 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 studyalso framed within ESF Euromargins MOUNDFORCE, EC FP5 RTNEURODOM and EC FP6 HERMES (contract GOCE-CT-2005-511234-1).Wewould like to acknowledge the efforts of the captains and crews ofthe involved research campaigns. The comments and suggestions ofthe editor, J.A. Howe and J.C. Faugères were highly appreciated andsignificantly improved the manuscript. The contribution of J. Iglesiaswas possible thanks to the CSIC grant UAC-2005-0044. This work hasbeen partially carried out during a research stages of F.J. Hernández-Molina 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-doctoralfellow of the FWO Flanders.

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