Stratigraphy and paleoceanography of a topography-controlled contourite drift in the Pen Duick area, southern Gulf of Cádiz

Marine Geology 349 (2014) 136–151

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Marine Geology

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Stratigraphy and paleoceanography of a topography-controlledcontourite drift in the Pen Duick area, southern Gulf of Cádiz

T. Vandorpe a,⁎, D. Van Rooij a, H. de Haas b

a Ghent University, Department of Geology & Soil Science, Renard Centre of Marine Geology, Krijgslaan 281 S8, 9000 Ghent, Belgiumb NIOZ Royal Netherlands Institute for Sea Research, Department of Marine Geology, P.O. Box 59, 1790 AB Den Burg, The Netherlands

⁎ Corresponding author at: Krijgslaan 281, S8 9000 GheE-mail address: [email protected] (T. Vando

0025-3227/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.margeo.2014.01.007

a b s t r a c t
a r t i c l e i n f o
Article history:Received 9 September 2013Received in revised form 13 January 2014Accepted 15 January 2014Available online 23 January 2014

Communicated by D.J.W. Piper

Keywords:Gulf of Cádizcontourite driftPen Duick Escarpmentmud volcanoseismic stratigraphyAntarctic Intermediate Water

The northernpart of theGulf of Cádiz has and still is receiving a lot of attention from the scientific community dueto (among others) the recent IODP Expedition 339. In contrast, its southern part, or theMoroccanmargin has re-ceived far less attention, althoughmud volcanoes, diapiric ridges and cold-water corals are present in this region.The El Arraiche mud volcano field is characterized by a compressive regime creating several ridges and assistingthe migration of hydrocarbon fluids towards the seabed surface. This study presents seismic and multibeam ev-idence for the existence of a contourite drift at water depths between 550 and 650 m along the southwesternflank of the Pen Duick Escarpment and Gemini Mud Volcano, within the El Arraiche Mud Volcano field. Fromthe onset of the Quaternary, when the escarpment started to lift and the local mud volcanism initiated,contouritic deposition was initiated as well at the foot of both topographic obstacles. Initially, fairly low-velocity bottom currents gave rise to sheeted drift deposits, affected by the uplift of the escarpment or mud ex-trusion. From theMiddle Pleistocene onwards, separatedmounded drift deposits were formed due to intensifiedbottom currents. An Antarctic Intermediate Water origin is inferred as driving mechanism for the drift develop-ment, although glacial conditions are not yet well constrained. The influence of Mediterranean Outflow Water(MOW) cannot be substantiated here. Moreover, the changes recorded within this contourite drift differ fromthe MOW-dominated contourite depositional system in the northern Gulf of Cádiz, as drift deposits only occuras early as the base of the Quaternary (compared to the Early Pliocene for the north) andmounded drift depositsonly occur from the Middle Pleistocene onwards (compared to the Early Pleistocene). Cold-water coral moundshave been observedwithin and on top of the sedimentary sequence at the foot of the PenDuick Escarpment. Thisimplies that environmental conditions in which cold-water corals thrive were not necessarily restricted to thetop of the Pen Duick Escarpment.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Contourite deposits have first been recognized in the 1960s(Heezen et al., 1966; Rebesco et al., 2014) and were initially exclu-sively associated with thermohaline currents. In particular the de-posits along the eastern North American margin, shaped by thewestern boundary undercurrent, received the dominant attention(Schneider et al., 1967; McCave and Tucholke, 1986). From then on-wards, gradually a larger variety of contourite expressions, both insmall and large scale, have been discovered (Faugères and Stow,1993; Hernández-Molina et al., 2011). Other factors, such as obsta-cles and internal waves, are now also acknowledged for being ableto create contourite deposits (Hernández-Molina et al., 2006a;Rebesco et al., 2014). As a consequence, the definition of contouriteshas been adapted to sediments deposited or significantly affected by

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bottom currents (Stow et al., 2002; Rebesco et al., 2005). Not onlydepositional, but also erosional features (furrows, moats, scours, etc.)are generated as well by bottom currents (Hernández-Molina et al.,2006b). Due to their enhanced sedimentation rates, contourites areideal recorders for paleoclimatological and paleoceanographic changes,due to the well-established association of bottom currents (intensity)and climate (Cacho et al., 2000; Llave et al., 2007b; Frigola et al.,2008). Due to the effect of bottom currents in sorting grain size classes,the economic potential of these deposits as reservoir systems may notbe underestimated (Viana et al., 2007; Brackenridge et al., 2013;Rebesco et al., 2014). Besides their ability to deposit laterally continuousmuddy contourite sheets, operating as an impermeable seal, long-lasting bottom currents may also create extensive and “clean” sandsheets; e.g. the Campos basin sands, Brazil (Viana et al., 1998a) or theGrand Bank sands, NW Atlantic, offshore Newfoundland (Dalrympleet al., 1992). These could serve as potential reservoir systems (Vianaet al., 1998b; Viana et al., 2007).

In the Gulf of Cádiz, a large contourite depositional system (CDS) ispresent along the Iberianmargin due to its interactionwith the vigorous

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137T. Vandorpe et al. / Marine Geology 349 (2014) 136–151

Mediterranean Outflow Water (MOW) (Llave et al., 2001; Hernández-Molina et al., 2006b; García et al., 2009). A CDS is the association of var-ious drifts and related erosional features, able to vary laterally aswell asvertically (Hernàndez-Molina et al., 2008). This Cádiz CDS has beenstudied extensively over the past decades as it is very large (over10,000 km2), documents the major MOW changes and has a significantsand content in its proximal domain (Hernández-Molina et al., 2006b;Toucanne et al., 2007; Roque et al., 2012). The global interest in this sys-tem resulted in IODP Expedition 339 (November 2011–January 2012),which aimed to perform a detailed study of the regional climatic andoceanographic changes, as well as its effects on a global scale and onthe distribution of clean and well-sorted sands (Expedition 339Scientists, 2012). In contrast to the already long-lasting attention forthe northern part, the southern Gulf of Cádiz, along the Moroccan mar-gin, remained largely unstudied up till recently, although a southernbranch of the MOW is reported and meddies (Mediterranean eddies)are observed here (Richardson et al., 2000; Ambar et al., 2008). Few au-thors reported the presence of contouritic deposits in this area (Casaset al., 2010; Van Rooij et al., 2011).

The El Arraiche Mud Volcano field has been studied extensively inthe last decade, due to several reasons. Firstly, an extensional tectonicregime within a compressional area is present (Flinch, 1993), resultingin the presence of extensional ridges (of which the Pen Duick Escarp-ment is one, see Fig. 2), characterized by rotated blocks, bound by lystricfaults (Van Rensbergen et al., 2005). Secondly, many mud volcanoes(e.g. Gemini MV, see Fig. 2) have been recognized and investigated inthis region (Van Rensbergen et al., 2005; Perez-Garcia et al., 2011) andare interpreted to be resulting from subsurface diapirism (Perez-Garcia et al., 2011; Haffert et al., 2013). Themud volcano activity is esti-mated to have started 2.4Ma ago (Van Rensbergen et al., 2005) and oc-curred in several phases (Perez-Garcia et al., 2011). Thirdly, thediscovery of cold-water corals (CWC) in the Gulf of Cádiz initiatedmore research on their spatial and temporal variability (among othersFoubert et al. (2008), Wienberg et al. (2010) and Van Rooij et al.(2011)). However, only a few living CWC have been observed (De Molet al., 2011), whereas mostly fossil corals or coral rubble are presenton the seabed or on mounds (Wienberg et al., 2009; Wienberg et al.,2010). This may be attributed to changing environmental conditions(paleo-productivity, paleoceanography and change in food supply)which were more favorable during glacial periods in the Gulf of Cádiz(Wienberg et al., 2010; Van Rooij et al., 2011). Along the Northeast At-lantic margin (Porcupine Seabight, Rockall Trough, Norwegianmargin),CWC are thriving nowadays (Frank et al., 2009). However, along theMorocco margin, past environmental conditions are difficult to assessasmany of the CWCmounds show signs of early diagenesis and are sub-ject to erosion (Foubert et al., 2008; Frank et al., 2009; Pirlet et al., 2010).As a consequence, the paleo-environmental conditions have to be in-ferred from different parameters. The off-mound history, recordedwithin drift deposits, may provide a good recorder of this variability.

A small-scale contourite drift is known from the foot of the PenDuick Escarpment (Fig. 2) (Van Rooij et al., 2011), proposed here to benamed the Pen Duick drift. The position along two obstacles (the PenDuick Escarpment and the Gemini MV), as well as the nearby presenceof cold-water corals, enables the study of the effect of topographicheights on contourite drift growth. Also, since the CWC moundson top of the PDE have been affected by diagenesis, the paleo-environmental conditions favorable for their growth have to be inferr-ed from different depositional environments. Therefore, a nearbycontourite drift with elevated sedimentation rates is ideal to inferthese conditions. This paper attempts to unravel the spatial and tempo-ral evolution of the Pen Duick drift, the involved (paleo)oceanographicprocesses and the interplay and influence of mud extrusion activity oncontouritic deposition. Associated to these questions, an assessmentwill bemade if this contourite driftmay be able to unveil more informa-tion on the paleoceanographic variability of the region and its influenceon CWC mound growth.

2. Regional setting

2.1. Geology & geomorphology

The present-day structure and geomorphology of the Gulf of Cádiz isthe result of several episodes of rifting, extension and compression fromthe Triassic onwards (Maldonado et al., 1999; Medialdea et al., 2004).Large allochtonouswedges have been emplaced due to ongoing obliqueEuropean–African convergence and thewestwardmotion of the Gibral-tar Arc (during theMiocene), creating the Alboran domain (Maldonadoet al., 1999). The sedimentary cover on top of this allochtonouswedge isNeogene in age and is pierced by numerousmud volcanoes, salt diapirs,diapiric ridges and other fluid escape features, such as pockmarks(Somoza et al., 2003; Medialdea et al., 2009). Most mud volcanoes arelocated within the offshore Betic–Rifian domain of the accretionarywedge (Medialdea et al., 2004) and are grouped into fields: the Guadal-quivir ridge field, the TAYSO field, the deep Portuguesemargin field andthe Spanish–Moroccanmargin field (Fig. 1). The migration of hydrocar-bon fluids towards the surface is facilitated by the presence of manyfaults and, in turn, they fuel the mud volcanoes (Pinheiro et al., 2003;Van Rensbergen et al., 2005).

The Pen Duick Escarpment (PDE) is located in the southern Gulf ofCádiz, between 35°10′N to 35°30′N and 6°30′W to 6°55′W (Figs. 1, 2).It is a part of the Renard Ridge that originated due to the compressiveregime in the area, which is in contrast to the extensional regime inthe main part of the Gulf of Cádiz (Van Rensbergen et al., 2005). Theridges are bounded by lystric faults (Flinch, 1993) and the compressionis estimated to have started 2.4 Ma ago (Van Rensbergen et al., 2005).Sevenmudvolcanoes are observed in this area, known as the El ArraicheMud Volcano province (Van Rensbergen et al., 2005) being part of theSpanish–Moroccan margin field (Fig. 1).

So-called “juvenile” (with respect to size) CWCmounds occur on topof Renard Ridge (Foubert et al., 2008), such as Alpha, Beta and Gammamound on the PDE (Foubert et al., 2008; Frank et al., 2009; De Molet al., 2011; Van Rooij et al., 2011). Their occurrence on top of theridge is the result of both hydrodynamic and seepage factors.

A three-stage model for CWC mound growth was proposed byFoubert et al. (2008). First of all, oceanographic, environmental andfood-supply conditions need to be optimal. Wienberg et al. (2010)proved that CWC mound growth prevailed during glacial periods dueto enhanced productivity conditions (more aeolian dust and increasedupwelling). Secondly, also the prevailing sediment dynamics are con-sidered to be important for sustained mound growth. The supply offood particles and prevention of burial occurs due to increased bottomcurrents (Wienberg et al., 2009; Wienberg et al., 2010; Van Rooij et al.,2011). Accumulation rates for mound growth as high as 220 cm/kahave been reported during glacials, while during times of reducedmound development (mostly interglacials), only growth rates of 0 to5 cm/kahave been reported (Frank et al., 2009). Thirdly, diagenetic pro-cesses will be responsible for internal geochemical changes (aragonitedissolution and carbonate precipitation) within CWC mounds, addingto its stability, but also to the “resetting” of the paleoenvironmental re-cording capability of CWC records (Pirlet et al., 2010).

2.2. Oceanography

The oceanography of the northern Gulf of Cádiz iswell knowndue tothe presence of the extensively studied CDS originating from the MOW(Iorga and Lozier, 1999; Serra et al., 2010; Hernández-Molina et al.,2011). This warm and salinewatermass flows out of theMediterraneanSea via the Gibraltar Strait and continues as an intermediatewater mass(500 to 1400 m water depth) along the Southern Iberian slope due tothe Coriolis deflection. After its exit out of the Strait of Gibraltar, theMOW is split up into twomain branches: the upper and lowerMediter-ranean waters (García, 2002; Hernández-Molina et al., 2006b). Theupper core flows along the shelf edge at depths of 500 to 800 m and

Fig. 1. Overview of the oceanic circulation and main geomorphological features within the Gulf of Cádiz. The white dot with a black center indicates the position of the LAR-A1 well. Thetransparent white box highlights the studied area (Fig. 2) and the double arrow shows the transect of the CTD profile (Fig. 3). The dashed yellow line indicates the extent of grid Q2,discussed by Maad et al. (2010). GDR: Guadalquivir mud volcano field, DPM: Deep Portuguese mud volcano field, NACW: North Atlantic Central Water, AAIW: Antarctic IntermediateWater, MOW:MediterraneanOutflowWater, NADW:North Atlantic DeepWater, UC:MOWupper current, IB:MOW intermediate branch, PB:MOWprincipal branch, SB:MOWsouthernbranch.

138 T. Vandorpe et al. / Marine Geology 349 (2014) 136–151

the lower core at depths of 750 to 1400 m to the west-northwest. Thislast branch splits up into three major branches (from north to south):the intermediate, the principal and the southern branch (Fig. 1)(Louarn and Morin, 2011).

By contrast, the oceanography of the southern Gulf of Cádiz is lessstudied. In the PDE area (Fig. 2), 4 water masses are known to occur:the North Atlantic Surface Water (NASW, 0–100 m), the North AtlanticCentral Water (NACW, 100–600 m), the Antarctic Intermediate Water(AAIW, 600–1500 m) and the North Atlantic Deep Water (NADW,beneath 1500 m) (Machín et al., 2006a; Ambar et al., 2008; Louarnand Morin, 2011). NASW represents the upper 100 m of the water col-umn and consists of modified NACW. The NACW is characterized by alinear decrease in temperature (16°–12 °C) and salinity (36.25–35.5)(Criado-Aldeanueva et al., 2006; Louarn and Morin, 2011) and flowsfrom west to east in the Gulf of Cádiz. The NACW recirculates alongthe African coast due to deflection of a coastal upwelling zone and splitsup along the Iberian margin, where one branch directly flows into theMediterranean Sea (Fig. 1) (Machín et al., 2006b). The general circula-tion pattern of these upper two water masses is anticyclonic, as theyare part of the Azores current, which is in turn part of the northeasternAtlantic circulation (Machín et al., 2006a). The AAIW is characterized bylowoxygen and high silicate values (Louarn andMorin, 2011). Althoughintensive mixing with under- and overlying water masses makes itsdirect recognition problematic, AAIW is known to flow cyclonicallyalong the African coast (entering the Gulf of Cádiz in the southwestand exiting it in the northwest) (Louarn and Morin, 2011). Below1500 m, NADW is present, and flows from south to north in the

Gulf of Cádiz (Fig. 1), along the Atlantic margin. It is characterized bylow salinities (b35.5) and temperatures (b8 °C) (Louarn and Morin,2011).

The PDE experiences the influence of NACW at its top (550 mwaterdepth) and AAIWat its foot (650mwater depth, Fig. 3) (Van Rooij et al.,2011). At present, MOW does not occur along the PDE, as Mediterra-nean waters are not observed above 700 m water depth (Fig. 3)(Mienis et al., 2012) and CTD data do not indicate their presence (VanRooij et al., 2011). However, meddies are known to transport MOWsouth of the Strait of Gibraltar (Fig. 3) (Richardson et al., 2000; Ambaret al., 2008) and the MOW is strongly influenced by glacial–interglacialalternations, with a stronger MOW during glacial periods (Toucanneet al., 2007). So, the influence of the MOW in the region cannot be ex-cluded, certainly as Van Rooij et al. (2011) and Foubert et al. (2008) in-ferred the possibility of an intensified glacial MOW, being able to reachthe PDE through enhanced meddy activity.

3. Material and methods

Within the framework of the R/V Belgica “CADIPOR” cruises (2001,2005 and 2007) and the “Pen Duick” campaign (2009) in the southernGulf of Cádiz, a total of 520 km high-resolution single channel sparkerseismic profiles have been acquired at the foot of the Pen Duick Escarp-ment and Gemini Mud Volcano. A SIG sparker (80 electrodes in 2001,120 electrodes in 2005, 2007 and 2009) has been used, with a shot in-terval of 2 s (3 s in 2009). The energies reached 500 J and a 8 kHz sam-pling frequency has been used. A record length of 1.6 s TWT (in 2001),

Fig. 2. Topographic features in the PenDuick area and location of the discussed seismic profiles. The thin black line in the south of the region shows thewesternmost part of the connectionprofile to the region described by Maad et al. (2010). This profile (although not presented in this paper) has been used to correlate unit Q2 (Maad et al., 2010) with the Pen Duick region.The inset shows the topography perpendicular to the Gemini mud volcano. The thick white arrows indicate the position of the moats and the inferred direction of the prevailing current.The full white dots indicate the six presumedmounds at the base of the PDE, while thewhite circles indicate alpha (southernmost), beta (central) and gamma (northernmost) cold-watercoral mound (Van Rooij et al., 2011). The thin white dashed line indicates the Renard Ridge, off which the PDE is a part. LdT: Lazarillo del Torres (mud volcano).Multibeam bathymetry from Van Rensbergen et al. (2005).


1.8 s TWT (in 2005 and 2007) and 2.5 s TWT (in 2009) was obtained.The profiles were acquired with acquisition velocities within the rangeof 3 to 4 knots.

Fig. 3. A) Salinity (colors) and temperature (contours, °C) transect, 10–15 km south of the Gemiocean database (World Ocean Database, 05/02/2013). B) Salinity-temperature diagram of all a

All seismic profiles (Figs. 5, 6 and 7) have been processed using theDECO Geophysical RadexPro processing software. A swell filter,bandpass filter (Butterworth type, low cut at 200 Hz, low-cut slope of

ni Mud Volcano and Pen Duick Escarpment (PDE). The data were obtained from theworldvailable CTD stations along the profile.

image of Fig.�2

image of Fig.�3

Fig. 4.Multichannel airgun seismic profile (and interpretation) displaying 5 seismic stratigraphic units. The interpretations of unit 5 and the upper part of unit 4 are shown indetail,where-as the detail of the upper units is displayed in Figs. 5, 6 and 7. A large fault is present in the center of the profile and smaller ones to the southwest. The lower part of the large fault isconcealed by a chaotic reflection expression. The small arrow in the ENE indicates the infill progradation of the depression (within the black box).


image of Fig.�4


24 dB/s and high cut at 1500 Hz, high-cut slope of 36 dB/s), predictivedeconvolution, 2D spike removal and amplitude corrections have beenapplied.

Themultibeam data (700 km2 in total), recorded during the CADIPORI cruise (2001) has been obtained using the SIMRAD EM1002 system, ex-tended with a deep water module, permanently installed on R/V Belgica.The swathwidthwas 500m above 500mwater depth and 750m below.The data have been corrected and cleaned using Kongsberg's Merlin andNeptune packages. The footprint at 400mwater depth is 15 × 15m. Thisdataset was already extensively described with respect to the main geo-morphological features and mud volcanoes in Van Rensbergen et al.(2005).

Profile M2005_105 (Fig. 4) has been acquired by the R/V Pelagia in2006 within the framework of the ESF EuroDIVERSITY MiCROSYSTEMSproject. Three airguns (10, 20 and 40 in.3 volume)were used and towedin a frame at respectively 1.3, 1.8 and 2.0mdepth. The gunswere towed37 m behind the stern of the ship and fired every 5 s at a pressure of100 bars, resulting in an average distance between the shots of 10.5 m(4.2 knots sailing speed). The streamer (towed at a depth of 1 mbelow surface) consists of four 63 m long sections with 6 channelseach. Each channel has 10 Teledyne T2 hydrophones (interval of 1 m).The data were recorded by the Geo-Resources Geo-Trace 24 hard- andsoftware system, consisting of a 24 channel digital pre-amplificationsystem. The record length was 2.0 s TWT and the sampling interval0.5 ms. When recording, a bandpass filter (30 Hz high pass and 700low pass) was applied. On board, the lines were stacked and a prelimi-nary migration has been performed.

4. Results

4.1. Geomorphology

The study area extends from 35° 22′N to 35° 14′N and 6° 52′Wto 6°45′W (Fig. 2). Within the investigated area, the multibeam data showsthe presence of the PDE (about 80–100 m above the seafloor), threemud volcanoes (e.g. Gemini MV, about 150 m high), CWC mounds (ontop of PDE, previously discussed by among others De Mol et al. (2011)and Van Rooij et al. (2011)) and 6 mounded features along the footof the PDE. They have a diameter between 200 and 300 m and are be-tween 5 and 10 m high, and previously did not yet receive too muchattention.

A semi-continuous channel, with widths varying between 200 and300 m, is present along the foot of the PDE (Fig. 2). The channel isabout 10 km long from its most southeastern (along Gemini MV) toits most northwestern expression (northern edge PDE). The first 2km of this channel have an east–west orientation, following thesouthern border of the Gemini MV. Here, the most pronounced ex-pression of the channel is observed with depth differences up to 15m (see inset Fig. 2). Then, it changes to a south-southeast to north-northwest orientation for 8 km, following the base of the Gemini MVand PDE, respectively. When the channel passes the boundary be-tween the Gemini MV and the PDE, the depth decreases to about 5m, after which it increases again along the PDE. Along the PDE, thedepth of this channel varies between 5 and 15 m, with a gradual de-crease in expression towards the first of the six mounded features.Along these mounded features, two channels are observed: onewhich continues along the foot of the PDE and increases in depth, toformer values of about 15 m and a second one WSW of the moundedfeatures (depths of about 15 m). Both gradually lose their expressiontowards the northwest.

Immediately south (along the Gemini MV) or southwest (alongthe PDE) of the channel, aligned (sediment) mounded ridges arepresent. They rise about 3 to 5m above the smoothly southwest dippingseafloor (about 1°) and 5 to 20 m above the base of the channel (seeinset Fig. 2).

4.2. Seismic stratigraphy

Based on the sparker seismic profiles and the multichannel airgunprofile, 5 seismic stratigraphic units can be discerned, separated by 4discontinuities D1 to D4. Due to the lower penetration of the sparkersource (about 400 ms TWT, while at least 800 ms TWT for the airgun),only 4 of the 5 units can be observed on the higher-frequency sparkerprofiles. The PDE and Gemini MV may be considered the acoustic base-ment in the ENE, but in the WSW, no actual acoustic basement is ob-served. This is due to the attenuation of the signal in the rather thicksedimentary package. Reflections can be distinguished in the sparkerprofiles down to depths of about 1200 ms TWT (Figs. 5, 6, 7) anddown to 1700 ms TWT in the airgun profile (Fig. 4). Below this depth,the multiple inhibits any further observations. Fig. 4 shows that(semi-) continuous deposits of units 1 to 5 are bounded by the PDE orGemini MV in the northeast and a paleohigh in the southwest. In thesouthwest, this paleohigh rises to 150 to 200 ms beneath the seafloor(Fig. 4). Its seismic facies consists of very chaotic, discontinuous reflec-tions of varying intensity. This facies differs from the seismic facieswithin the mud volcanoes in the fact that the paleohigh still containssparse reflections, while the mud volcanoes and tectonic ridges displayan almost acoustically transparent facies (Figs. 5, 6 and 7).

4.2.1. Unit 5Unit 5 (only visible in Fig. 4) consists of low-amplitude semi-

continuous reflections at the base and more continuous, slightly higheramplitude reflections at the top. In the southwest, the unit is intersectedby many normal faults (see below). Unit 5 displays a more low-angleonlap onto the basement in the east, while in the west a higher angleof onlap onto the paleohigh is observed (Fig. 4). In Fig. 4, the top ofthe unit is incised deeply (±60 ms TWT) along Gemini MV. The thick-nesses of unit 5 vary between 150 ms TWT in the west-southwest andup to 550 ms TWT in the center of the basin. The maximum thicknesscannot be calculated, as the multiple inhibits the observation of thelower boundary (Fig. 4).

Unit 5 and a small part of unit 4 are affected by 2 distinct fault pat-terns: one major fault is located in the center of the basin and at leastnine smaller ones to the west-southwest (Fig. 4). The large fault(about 750m long and a dip of about 50–55°) is a west-dipping normalfaultwith offsets of 10msTWT at its top (about 1250msTWT), evolvingto zero offsets at 1450 ms TWT. Deeper down, the offset cannotbe determined anymore due to a chaotic seismic expression. Thesmaller faults are west-dipping normal faults with offsets inferior to 5ms TWT. The activity along the faults stops within the lower part ofunit 4.

4.2.2. Unit 4The boundary between units 5 and 4 is erosive, evidenced by the in-

cision into unit 5 along Gemini MV (Fig. 4). Unit 4 consists of low tomedium-amplitude, continuous reflections at the base and medium-amplitude, continuous reflections at the top (Figs. 5, 6). Overall, slightlymounded deposits are present at about 500 m SSW of PDE and GeminiMV (Figs. 5, 6). Unit 4fills a small incision at the intersection alongGem-ini MV (only visible in Fig. 4). The base of the incision contains discon-tinuous, slightly chaotic deposits, while more continuous depositsarise on top. Unit 4 contains eight small subunits, each one about 25to 35 ms TWT thick (Figs. 5, 6), all occurring within a cyclic pattern.All subunits display very high amplitude reflections at their base andlower amplitudes on top. Subunit c has slightly higher amplitude de-posits, compared to the other 7 subunits. Generally, all subunits are con-formable, but they pinch out near the PDE (Fig. 6), evidenced by theuplift of individual reflections and a decrease in thickness of the sub-units towards the PDE. The pinch-out is most pronounced at the baseof unit 4 and gradually diminishes upwards. Along Gemini MV, a differ-ent pattern is observed: the boundaries between the subunits displaysmall erosional features. At the base of unit 4, first evidence of a

Fig. 5. Single channel sparker seismic profile (and interpretation) perpendicular to theGeminimud volcano, displaying4 out of 5 units. The lower boundary of unit 4 is based on correlationwith other profiles. Relative bottom current intensities are indicated by the size of the bottom current symbol, whereas the green arrow shows the increasing convex nature of the sed-iments of unit 3. The green-shaded surfaces near the Gemini MV outflow lobes indicate when mud eruptions did not occur and when sediments were deposited on top of the extrudedmud. The table at the left side of the figure summarizes the units, subunits and interpreted marine isotopic stages (MIS).


image of Fig.�5

Fig. 6. Single channel sparker seismic profile (and interpretation) perpendicular to the Pen Duick Escarpment (PDE), displaying 4 out of 5 units. The green arrow indicates the increase inmounded nature of the sediments in unit 3. The table at the left side of thefigure summarizes the units, subunits and interpretedmarine isotopic stages (MIS). Relative currents intensitiesare indicated by the size of the current symbol.


image of Fig.�6

Fig. 7. Single channel sparker seismic profile (and interpretation)perpendicular to the PenDuick Escarpment, across one of themounded features at its foot (Fig. 2). The green shaded areaswithin units 3 and 1 indicate respectively buried and surfacing cold-water coral (CWC)mounds. The yellowpart of unit 1 illustrates the location of themounded sediment driftWSWof theCWC mound.


Christmas-tree structure appears (Fig. 5). This pattern of mud extru-sions disrupts the sedimentation at the foot of Gemini MV and can beobserved in all other units as well. The reflections on top of these extru-sions display a convex pattern (Fig. 5). The faults affecting unit 5 alsodisrupt the basal sediments of unit 4, but the small depression, caused

by the central large normal fault, is gradually flattened throughoutthis unit (Figs. 4, 6). The thicknesses generally vary between 200 and250 ms TWT with a gradual decrease towards the northwest (Fig. 8).In the west-southwest, the thicknesses are reduced due to the presenceof the paleohigh (Figs. 4, 8).

image of Fig.�7

Fig. 8. Isopachmaps of units 4 to 1. The thicknesses are displayed inms TWT (two-way travel time). In unit 4, the uplift of the PDE can be observed by the reduced thickness at its foot. Inunit 2, the location of the two large mud extrusions are recognizable due to very thin or absent deposits near the Gemini MV and PDE.


4.2.3. Unit 3Units 4 and 3 are separated by a subtle angular unconformity. Small-

scale erosion affects the deposits near both PDE and Gemini MV, butthese small depressions are filled within unit 3 (Figs. 5, 6). Near thePDE, unit 3 has low-amplitude reflections at its base and moderate-amplitude reflections at its top (Fig. 6). Near the Gemini MV, unit 3consists of mostly high-amplitude reflections with a low-amplitudepart in the middle of the unit (Fig. 5). Four subunits are present inthis unit. The variability in amplitudes strength of the differentsubunits is rather low (although some high-amplitude reflectionsare present) and they can be distinguished by small angular uncon-formities (visible along both topographies, Figs. 5, 6). A channel ispresent along Gemini MV and a large part of PDE, with mounded sedi-ments on its southwestern side. The channel is about 150 m wide andre-incises itself continuously into underlying sediments, causing an in-crease in channel depth upwards (5 to 20 ms TWT). Deeper incisionsare present along the PDE (Figs. 5, 6).

Along the northernmost part of PDE, unit 3 has a different appear-ance: very high amplitudes at the base, moderate in the middle andhigh again at the top (Fig. 7). Also, no evidence for a channel along thePDE is observed here; the sedimentation pattern is obscured due tothe presence of one of the mounded features and the associateddiffraction hyperbola. But what really sets this area apart is the oc-currence of 3 large (20–30 ms TWT high and up to 200 m wide)and 2 small (b10 ms TWT high and less than 50 m wide) moundedfeatures within this unit (Fig. 7). They have an acoustically almost

transparent appearance. The two small ones are located deepercompared to the larger mounds (Fig. 7). The three large moundedstructures originate at the same stratigraphic level, which is 15 to25 ms TWT above the base of unit 3 (Fig. 7). The deposits on top ofthese mounded features are therefore characterized by a concavegeometry.

The thicknesses are fairly constant and vary around 100 ms TWT.Only along the mounded features at the foot of PDE, 70 ms TWT of sed-iment is present (Fig. 8). Slightly reduced values (a decrease of about 20ms TWT) are observed as well within the vicinity of PDE or Gemini MV(Fig. 8).

4.2.4. Unit 2The boundary between units 3 and 2 (D2) is the most erosive of the

entire sedimentary sequence. Along PDE and Gemini MV, up to 25 msTWT of unit 3 sediments are eroded, creating a channel-like morpholo-gy along the discontinuity. The channel, which is present throughoutthe entire unit, is filled differently along PDE and Gemini MV (Figs. 5,6). Along Gemini MV, continuous high-amplitude reflections areencountered within the channel. Just outside the channel, the reflec-tions have a concave appearance, while in the rest of the unit, high-amplitude, nearly horizontal, continuous deposits are present (Figs. 5,6). Along PDE, a nearly reflection-free triangular shaped block is en-countered within the channel. This block has a small run-out up to 50ms TWT thick and 300 m long. Fig. 8 shows the position of thesedeposits.

image of Fig.�8


Based on small erosional surfaces and changes in acoustic appear-ance, 3 subunits are discerned within unit 2. The lowermost subunit(15–20 ms TWT thick) contains lower amplitudes compared to theupper two subunits (Figs. 5, 6). Within the upper subunit, near thePDE, a channel (about 100 mwide and only a fewms TWT deep) is ob-served about 750m southwest of the PDE (Fig. 6). Due to the thinning ofthis unit towards the north (Fig. 8), the subunits are no longer discern-able in Fig. 7. Unit 2 is the thinnest of the sedimentary sequence withvalues varying around 50 ms TWT (Fig. 8). At the position of the trian-gular shaped block, thicknesses are reduced to zero.

4.2.5. Unit 1A partly erosive boundary separates units 2 and 1, although it is less

erosive compared to the previous discontinuity. Along Gemini MV, atthe boundary between units 2 and 1, a large concave extrusion (up to800mwide and 40msTWTdeep) is observed (Fig. 5). A channel is pres-ent along the PDE andGeminiMV, although its respective geometry dif-fers. Along Gemini MV, the channel is rather wide (about 800 m and 20ms TWT deep) at the base. Subunit 1c is incised along the Gemini MV(Fig. 5). Along the PDE, a smaller channel (200 m wide and 10 msTWT deep) is present SSW of the triangular shaped deposits withinthe lower two subunits (1a and b). However, within the upper subunit(1c), near PDE, this channel widens to 1 km width and is just about 25ms TWT deep (Fig. 6). Along the northern part of the PDE, a secondchannel occurs just southwest of the mounded features at the base ofthe PDE. This channel is 150mwide and up to 15ms TWT deep. South-west of this second channel, mounded sediments are observed whichpass into conformable sheet-like deposits. In total, 3 subunits are pres-ent, discernable through small angular unconformities. All subunitsconsist of high-amplitude, continuous reflections. In Fig. 7, the subunitsare again not discernable due to the decreases in thickness (Fig. 8). Thethicknesses are slightly larger than unit 2: on average 50 to 55 ms TWT(Fig. 8). The mounded features near the base of the PDE displays re-markable seismic features: its WSW side contains chaotic, very low-amplitude reflections, while its ENE side displays short horizontal, con-tinuous reflections which seem to be a prolongation of the underlyingsedimentation.

5. Discussion

5.1. Sedimentary processes

5.1.1. Initiation (unit 5)The deposits of unit 5 drape the acoustic basement and gently flatten

the deepest parts. The settling of sedimentary particles in absence ofstrong currents, resulting in hemipelagic deposits (depending on theamount of biogenic material), is proposed as the main depositionalmechanism. Unit 5 and the base of unit 4 are affected by faults: severalsmaller faults at the flanks of the paleohigh and a large normal fault inthe middle of the basin. These faults may be due to the compressionalregime in the region (Van Rensbergen et al., 2005). However, as thereis only one profile showing these features, they cannot be mappedand as a consequence, the actual orientation and direction cannot be de-rived. The paleohigh resembles the compressional tectonic ridges(Vernadsky, Renard) and may also be a compressional ridge that didnot reach to the present-day seafloor and is covered by sediments as aconsequence.

5.1.2. Sheeted drift (unit 4)From unit 4 onwards, very gradually, a sheeted and detached

mounded contourite drift develops parallel to the local relief.Unit 4 mostly consists of horizontal, continuous, slightly mounded

deposits. Only along the northern part of the PDE, the mounded natureis absent. This geometry increases towards the top of the unit (Figs. 5, 6)and indicates together with the fairly uniform thickness along the PDEand Gemini MV (Fig. 8), the aggradational stacking pattern and the

location along a steep slope towards slope sheeted drift deposits(Faugères and Stow, 2008). The presence of out-pinching strata, in anonlap configuration, onto the PDE within the entire unit suggests asyn-lift sedimentation, affected by the uplift of the PDE, as suggestedby Van Rooij et al. (2011). Similarly, syntectonic sedimentation hasbeen observed to the east of the study area by Maad et al. (2010). Theuplift gradually reduces towards the top of unit 4 and is not noticeableanymore in younger deposits. This means that the uplift of the PDEceased at D3. The Renard Ridge is an extension of the Larache Ridge,which originates due to anticlinal folds along which fluid expulsiontakes place (Maad et al., 2010). The uplift of the ridges (such as theLarache Ridge) has been attributed to compressional stresses (Somozaet al., 2003; Maad et al., 2010), originating from the NW–SE conver-gence of the African and Eurasian plates (Maldonado et al., 1999;Medialdea et al., 2009). As a consequence, the compressional forces,uplifting the PDE, must have diminished throughout and stopped atthe top of unit 4 in this area. This must be a local effect, as theAfrican–Eurasian convergence is still ongoing.

5.1.3. Mounded drift: units 3–1Within the sedimentary sequence, the most striking change is the

evolution from horizontal, slightly mounded (and along the PDEpinched-out) deposits (unit 4) into upslope prograding, mounded de-posits with a moat (the observed channel) along the PDE and GeminiMV (units 3 to 1, Figs. 5, 6). Mounded drifts are associated with higherbottom current velocities (between 10 and 30 cm/s (Stow et al.,2008)), compared to sheeted drifts (10 cm/s), which were describedto be present within unit 4. The bottom current intensity also increasesduring the deposition of unit 3, especially along the southern part of thePDEas thedepth of themoat (5 to 20msTWT) and the expression of theassociated sediment drift mound increases (Figs. 5, 6). This can beinterpreted as the evolution from the initiation of mounded drifts to-wards a continuous presence of stronger bottom currents within themoat, resulting into a gradual change from non-depositional (lowerparts of unit 3) into erosional action (upper parts of unit 3). Along thenorthern part of the PDE, a typical moat and associated drift mound isabsent and hemipelagic sedimentation is present (Fig. 7). This can beinterpreted as a lateral decline in bottom current strength. The declinein thickness of unit 3 (Fig. 8) from southeast to northwest illustratesthe lateral change in sedimentation pattern: mounded drift depositsare thicker and occur in the southeastern part, whereas the hemipelagicsediments are thinner and occur in the northwest.

Three mound-like structures, originating at the same stratigraphiclevel, have been observed within unit 3 in Fig. 7. They closely resembleburied cold-water coral (CWC) mounds discussed by Huvenne et al.(2003) and Iacono et al. (2014) and are classified as such as a conse-quence. The occurrence of CWC mounds in this part of the study areaimplies that environmental conditions (food supply, prevention to buri-al) were right during at least a certain amount of time for CWC to flour-ish (Foubert et al., 2008; Wienberg et al., 2009; Wienberg et al., 2010).

An erosive boundary separates units 3 and 2 (Figs. 6, 7). Unit 2 ischaracterized by a larger (and along Gemini MV wider) moat, exceptfor the northern part of the PDE. In this area, the moat is still absent, in-dicating the continuing lateral decline in bottom current intensity. Theerosive nature of the moat indicates stronger bottom currents, capableof eroding more sediment. A large mud extrusion is observed at thebase of subunit 2c (Fig. 5). This has the same seismic characteristics as,and is positioned at the same stratigraphic level as, the triangular de-posits in Fig. 6. These similarities aswell as the position of the triangulardeposits near Gemini MV indicate a muddy origin for the triangular de-posits aswell, extruded at the sameperiod. The separate patches ofmud(Fig. 2) may be the consequence of two different pathways, certainly asGemini MV actually consists of two mud volcanoes within one mudcone (Van Rensbergen et al., 2005). The thickness maps (Fig. 8) clearlyshow the position of the mud extrusion. After the large mud extrusion,the depocenter shifted SSW-wards, indicated by the relocation of the


moat and mounded sediments for about 500 m (Figs. 5, 6). Along Gem-ini MV, this relocation only occurs at the base of unit 1, whereas alongthe PDE this already occurred within subunit 2c.

Unit 1 consists of elongatemounded drift deposits along both topog-raphies, even along the northern part of the PDE (Figs. 5, 6 and 7). Thisindicates focused bottom currents along the entire PDE, strong enoughto create drift deposits. A broad and deepmoat is present during the de-position of unit 1. AlongGeminiMV, themoat is at its widest (about 500m) of the entire Pen Duick drift (Fig. 5). Only on the present-day sea-floor, a narrow, deep moat occurs directly SSW of Gemini MV againand is accompanied by a shift in the drift depocenter (Fig. 5). Alongthe PDE, a narrow but deepmoat is present about 500m SSW of the es-carpment (Fig. 6). Also here, on the present-day seafloor, the position ofthe moat shifted to the foot of the PDE.

The occurrence of semi-buried CWCmounds, recognized at the baseof the northern part of the PDE, is a most peculiar feature (Fig. 7). Inmorphology (width, height, shape) they resemble the CWCmounds ob-served on top of the PDE (Foubert et al., 2008; Van Rooij et al., 2011).However, the presence of continuous reflections facing the PDE side of

Fig. 9. Summarizing sketches of the contourite drift evolution along both Pen Duick Escarpmentof the current symbol. Dark orange zones indicate activemud flowextrusions from theGeminiMthe foot of the PDE.

these mounded features, resembling the sedimentation below, contra-dicts a full CWC origin. Given the fact that the WSW part of themound contains a seismic facies resembling the buried mounds of unit3 (Fig. 7) and the ENE part contains continuous, parallel, horizontal re-flections, a dual origin for the mounded features is proposed. CWCstarted to settle at the base of unit 1, near the PDE, where gradually amound was built against which sediment was deposited, provided bythe bottom current along the PDE (inset Fig. 9). This created these “hy-brid”mounds, consisting of both a sedimentary (ENE) and CWCmound(WSW) part. This implies that conditions for CWC to thrive were favor-able at the foot of the PDE in this region. However, further investigationsare required to further reveal and understand the exact nature and ori-gin of these features. A small moat along the WSW part of the moundsimplies a bottom current flowing along them. A bottom current flowson the WSW flank of the mounds (probably due to their presence)and separated mounded drift deposits are present (Fig. 7). Whetherthe bottom current flowing alongGeminiMV and PDE splits or a secondbottom current, unrelated to the first one, is present, cannot be derivedfrom the present dataset (Fig. 2).

and Gemini Mud Volcano. The relative bottom current intensities are indicated by the sizeV. The inset sketchwithin unit 1 along the PDE shows the build-up of the CWCmounds at

image of Fig.�9

Fig. 10. Comparison of the evolution of the Pen Duick drift with clear MOW-controlleddrifts in the northern Gulf of Cádiz (Roque et al., 2012; Brackenridge et al., 2013) andthe Bay of Biscay (Van Rooij et al., 2010). Red boxes represent pre-contourite deposits, yel-low boxes signify sheeted drifts and green is used for mounded drift deposits. BQD: BaseQuaternary Discontinuity, LPR: Lower Pliocene Revolution, MIS: Marine Isotopic Stage,MPR: Middle Pleistocene Revolution, UPD: Upper Pliocene Discontinuity.


5.2. Chronostratigraphy

The spatial and temporal distribution of the Pen Duick drift indicatesa depositional history including several changes in sedimentation pat-tern. Two major (D4, D3) and two minor transitions (D2, D1) are re-corded as unconformities separating the depositional sequences. D4marks the initiation of the drift deposition, while from D3 the transitionfrom sheeted to elongated mounded drifts is initiated. D2 and D1 indi-cate changes within the elongated mounded drift deposits. These alter-ations have been compared to surrounding regions in order to constructa possible chronostratigraphy. Maad et al. (2010) discussed the seismicstratigraphy of the northwestern Moroccan Atlantic continental shelfbased on sparker seismic data. Their unit Q2 is considered to have aMiddle to Upper Pleistocene age (correlation with well LAR-A1, 25 kmeast-southeast of the study area from this paper). The correlation(through connecting seismic profiles) of this unit to the investigatedregion reveals that at least units 3, 2 and 1 are within that age range.

Petrographic studies of mud breccia clasts show that theAl Idrissi mud volcano field is situated on an Upper Miocene–Pliocenesedimentary basin (Akhmanov et al., 2003; Pinheiro et al., 2003),implying that the sedimentary deposits described in this paper are ofPlio-Pleistocene age. The mud volcanoes appeared 2.4 Ma ago (VanRensbergen et al., 2005) or 2.6 Ma according to Perez-Garcia et al.(2011), implying that the entire contourite drift (units 4 to 1) is post-Pliocene in age and only unit 5 has a possible Pliocene age. Moreover,the basal unconformity, discussed by Van Rensbergen et al. (2005)and Perez-Garcia et al. (2011), is set at an age of 2.6 Ma, which couldplausibly correlate with D4. This implies that discontinuity D4 could co-incidentally be associated with the BQD (base Quaternary discontinu-ity), set at 2.588 Ma (Gibbard et al., 2010).

During the Quaternary, the Middle Pleistocene Revolution is themost important paleoceanographic change in the entire North-AtlanticOcean, coevalwith the switch to a “full” glacialmodewith 100 ka eccen-tricity cyclicity (Shackleton et al., 1990; Paillard, 1998; Marino et al.,2009). In many contourite systems, this is accompanied by an increasein sedimentation rates (Van Rooij et al., 2010; Roque et al., 2012;Müller-Michaelis et al., 2013), mostly evidenced by higher amplitudereflections (Hernández-Molina et al., 2006a; Llave et al., 2007b; VanRooij et al., 2007). A similar observation has been made here; a transi-tion from sheeted to elongate mounded drift deposits and a gradual in-crease in amplitudes of the reflections throughout units 3, 2 and 1(Figs. 5, 6, 7). Therefore, discontinuity D3 may tentatively be correlatedto the MPR (0.920 Ma).

The continental shelves and upper slopes are more prone to eustaticsea-level variations compared to deep-water environments. Hence, inresponse, bottom currents fluctuate more in these shallow water re-gions (Hernández-Molina et al., 2002; Verdicchio and Trincardi, 2008;Ridente et al., 2009). The upper three units display features which canbe attributed to climatic variations: a cyclic pattern of progradationalsubunits (onto the PDE or Gemini MV) with reflections of varying am-plitudes. In nearby regions (Alboran Sea, northern Gulf of Cádiz), themajority (65–80%) of the sediment is deposited during regressive andlowstand periods (Hernández-Molina et al., 2002). Except for theupper parts of the sedimentary column (Van Rooij et al., 2011), no over-all sedimentation rates are known yet for this region. Hence, the sub-units have been tentatively linked to glacial marine isotopic stages(MIS). This correlation has been applied before along the Adriatic mar-gin by Ridente et al. (2009) and along the southwestern Mallorca shelfby Vandorpe et al. (2011). The ten discerned subunits from the MPRto Recent can be correlated to the ten (even) MIS (Lisiecki and Raymo,2005) (Figs. 5, 6). A periodicity of 80 to 120 ka is obtained due to thiscorrelation, which is in agreement with the obliquity pacing hypothesisproposed by Huybers (2007), stating that glacial cycles vary by 80 or120 ka in the late-Pleistocene by skipping one or two obliquity beats.As a result, D2 and D1 could be linked to MIS 15 (about 575 ka) and 9(about 325 ka) respectively.

Based on this tentative chronostratigraphy and the averagemeasured thicknesses (the conversion into metric scale is based on atheoretical seismic velocity of 1650 m/s within the sediment), the sed-imentation rate for the period between BQD and MPR (unit 4) is10.8 cm/ka and between MPR and Recent (units 3 to 1) 17.0 cm/ka.These rates are close to or within the range of theoretical values for sed-iment drifts: 3–10 cm/ka for sheeted drifts (unit 4) and 5–30 cm/ka formounded drifts (units 3 to 1) (Stow et al., 2008). Van Rooij et al. (2011)obtained a sedimentation rate of 8–14 cm/s for MIS3, which is compat-ible with the calculated values. Also, the increase in sedimentation rateafter the MPR is consistent with the aforementioned enhanced sedi-mentation rates. This tentative chronostratigraphy also points outsome differences in uplift of the ridges throughout the entire Gulf ofCádiz. Whereas the Cádiz and Guadalquivir ridges are reactivated dur-ing theMPR andMIS 6, changing the sedimentation pattern in the Gua-dalquivir and Huelva drifts (Llave et al., 2007a), the PDE does not showsigns of uplift during these periods (Fig. 6). Indeed, uplift would havestopped in this region at the MPR, implying that timing of compressionmay vary locally within the Gulf of Cádiz.

5.3. Comparison to the northern Gulf of Cádiz paleoceanography

When comparing the depositional history of the Pen Duick drift toother systems along upper continental slopes of the Northeast AtlanticOcean (Cádiz and Le Danois CDS), several resemblances and differencescan be identified (Fig. 10). While in the Cádiz and Le Danois CDS, driftdeposits are present from the Pliocene onwards, the PDE area only con-tains Quaternary drift deposits. After theMPR, an intensification of bot-tom currents and an accompanying growth of the CDS occurs in all driftsystems, expressed by a severe growth phase of the mounded drift de-posits in the Cádiz and Le Danois drifts (Van Rooij et al., 2010; Llaveet al., 2011; Roque et al., 2012; Brackenridge et al., 2013) and by the evo-lution from sheeted to mounded drift deposits in the PDE area (Figs. 5,6). A final intensification stage is observed in the Cádiz CDS at aboutMIS 12 (Llave et al., 2007a; Marchès et al., 2010; Llave et al., 2011;Roque et al., 2012), while in the Pen Duick drift, this intensification istentatively set at MIS 15. The boundary at MIS 9 observed in the PenDuick drift is not encountered in the other systems. The vast above-mentioned differences in evolution between MOW-controlled CDS's

image of Fig.�10


and the Pen Duick drift suggest that the MOW is not likely to be themain driving force behind the formation or shaping of the Pen Duickdrift. Although the MOW is present within the area as meddies(Ambar et al., 2008), along the PDE it is not observed (only in deeperwater settings, Fig. 3). During glacial periods, MOW flows at even great-er depths (García et al., 2009) and therefore, its presence at the foot ofthe PDE during glacials is not likely as well.

A more suitable and plausible candidate may be the Antarctic Inter-mediateWater (AAIW). It enters theGulf of Cádiz in the south andflowsalong the African coast towards the north, before being outcompeted bythe MOW (Machín et al., 2006a; Louarn and Morin, 2011). CTD datafrom Van Rooij et al. (2011) and Mienis et al. (2012) indicate the pres-ence of AAIW at the foot of the escarpment, while NACW is present ontop. The absence of MOW, the proved presence of AAIW and thedepth range in which the Pen Duick drift occurs, favor a recent AAIW-influence.Most of the sediment is inferred to be depositedduring glacialperiods and the production of AAIW is known to increase during colderperiods (e.g. during the last glacialmaximum(Makou et al., 2010;Waineret al., 2012) and during Heinrich events (Jung et al., 2011)), leading tothicker deposits along the Argentinian margin, for example Voigtet al. (2013). These arguments support the idea of an AAIW-originof the Pen Duick drift, even during glacial periods. However, thenorthern extent of the glacial AAIW is not yet fully known (Oppoand Curry, 2005, 2012); its presence north of 27°S is not yet proven.This implies that glacial north Atlantic intermediate water, known tooccur in this region and present down till depths of about 2200 m(Marchitto and Broecker, 2006), might well be involved in the originof the Pen Duick drift.

Mienis et al. (2012) showed the presence of northeast-directed cur-rents at the foot of the PDE. This is in agreementwith the observed seis-mic characteristics and geomorphology of the Pen Duick drift. A moat ispresent along the PDE and Gemini MV, indicating a bottom currentalong both topographies. This bottom current has been active through-out the entire Quaternary (Figs. 5, 6 and 7), depositing sediment SSWofGemini MV and PDE. Taking into account the pathway of the bottomcurrent and the Coriolis deflection to the right in the northern hemi-sphere, the bottom current is inferred to have a northeastward di-rection. The CTD data and the flow direction at the foot of thetopographies together hint towards a bottom current coming from thesouth, being deflected by the Gemini MV and PDE and which continuesto flow along the bases of the topographies due to the Coriolis deflec-tion. The start of drift formation (unit 4, Figs. 5, 6) 2.588 Ma ago coin-cides with the first signs of mud eruption and the initial uplift of thePDE (Van Rensbergen et al., 2005; Perez-Garcia et al., 2011). So, onlywhenGeminiMV and PDE rose high enough to alter the contemporane-ous bottom current circulation pattern and enhance its intensity(Faugères et al., 1999), the PenDuick drift started to form. Higher veloc-ities due to small-scale topographies (such as seamounts, mud volca-noes, and escarpments) are commonly described (Stow et al., 2009;Hernández-Molina et al., 2011; Rebesco et al., 2014). Taking into ac-count the fact that bottom currents are on average 8 cm/s on the plainat the foot of the PDE (Mienis et al., 2012) and that drift deposits solelyoccur along the topographies, the Pen Duick drift is an excellent exam-ple of an obstacle-related contourite system.

6. Conclusions

Based on high-resolution sparker single-channel seismic profiles, anairgun multi-channel seismic profile and multibeam data, a contouritedrift along the southwestern border of the Pen Duick Escarpment andGeminiMudVolcanohas been described in terms of sedimentary evolu-tion and paleoceanography. Five main conclusions may be drawn fromthis study:

1. The Pen Duick drift is an excellent example of an obstacle-relateddrift as it occurs in an area with generally low-intensity bottom

currents. Bottom currents are deflected against the topographies(PDE and GeminiMV) and build a contourite drift along them, paral-lel to the local relief.

2. The Pen Duick drift originates at the base of the Quaternary, creatingsheeted drift deposits (unit 4). The bottom currents intensified afterthe MPR, leading to the deposition of separated mounded driftdeposits (units 3 to 1). A general intensification of bottom currentsis inferred from the MPR to present.

3. The presence of the Gemini MV interferes with the drift deposits:several extrusions are recorded within the sedimentary sequenceleading to a Christmas-tree structure along the mud volcano. Thepresence of a large block of mud along both PDE and Gemini MVwithin unit 2 indicates a large eruption in that period.

4. The Pen Duick drift has a possible AAIW-origin whichwould make itthe most northern expression of AAIW within the Atlantic Ocean.Evidence (both drift architecture and CTD data) for the presence ofMOW at the foot of the PDE is not present.

5. CWC mounds have been found within and on top of the northernpart of the PenDuick drift. Thismeans that environmental conditionswere right for CWC to flourish not only on top of the PDE, but also atits base. The “hybrid” mounds were active from the base of unit 1(about 71 ka). The presence of the buriedmoundswithin unit 3 indi-cates that also in the past (between about 900 and 600 ka), periodswith conditions favorable for CWC mound growth were present.

Acknowledgments

The authors would like to acknowledge the captains and crews ofR/V Belgica and R/V Pelagia for the many successful cruises to thestudy area. The helpful and stimulating discussions with J.-P. Henriet(UGent) are greatly appreciated. The data was acquired within theframework of the following past projects: ESF EuroDIVERSITYMiCROSYSTEMS, ESF EuroMARGINS MoundForce, EC FP6 HERMES(GOCE-CT-2005-511234) and EC FP7 IP HERMIONE (Grant agreementno. 226354). This research was supported by the Spanish project CTM(2012)-39599-C03 (MOWER). The aims and strategies of this paperwere elaborated within the framework of the FWO-Flanders project“CONTOURITE-3D”, as well as IGCP project 619 and INQUA project1204. Finally, we wish to thank F.J. Hernández-Molina (Royal HollowayUniversity of London) and an anonymous reviewer for their helpfulcomments.

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Stratigraphy and paleoceanography of a topography-controlled contourite drift in the Pen Duick area, southern Gulf of Cádiz