Contourite Depositional Systems in the El Arraiche Area, Moroccan Atlantic Margin

Home , Hemipelagic sediment, Porcupine Seabight, Till

FACULTEIT WETENSCHAPPEN Opleiding Master of Science in de geologie

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

Freek Vancraeynest

Academiejaar 2014–2015

Scriptie voorgelegd tot het behalen van de graad Van Master of Science in de geologie

Promotor: Prof. Dr. D. Van Rooij Begeleider: T. Vandorpe Leescommissie: Prof. Dr. S. Bertrand, Dr. M. Garcia

ACKNOWLEDGMENTS

This master dissertation is the final hurdle in my career as a geology student and is the highlight of my journey in becoming a solid scientist. The happy ending of my studies and this dissertation would not have been possible without the support of a whole bunch of people.

Therefore, I would like to thank prof. David Van Rooij for the opportunity he gave me to start my thesis at the RCMG and immerse myself in the interesting study of contourites. I appreciate his advice and support during the writing of this thesis, and not the least the knowledge I gathered from him in the past five years. Secondly I would like to thank Thomas Vandorpe for the excellent guidance and never- ending support throughout this study. He was always stand-by for questions and remarks and put a lot of effort in reading and correcting this master thesis. I do not exaggerate by saying I would not have succeeded without their permanent support.

I would like to thank my fellow students for the fun we had during our studentship, in the courses and especially during the time we spent on our days (and nights) out. Not to forget the great times we had on our field trips. Particularly this semester their presence was a huge support, for the joint smart and to put into perspective the struggle we had to go through the past months.

Of course I would thank my parents for giving me the chance to begin my studies and for the support they gave me during the past five years. I am grateful for how they trusted me and let me do my thing, whether it was a great plan or not.

Thank you! Freek

1 TABLE OF CONTENTS

ACKNOWLEDGMENTS 1

TABLE OF CONTENTS 2

TABLE OF FIGURES 3

INTRODUCTION 4

REGIONAL SETTING 8 Geology & geomorphology 8 Oceanography 9

MATERIAL & METHODS 12

RESULTS 14 Geomorphology 14 Seismic stratigraphy 15 Unit 6 22 Unit 5 22 Unit 4 23 Unit 3 25 Unit 2 26 Unit 1 27

DISCUSSION 30 Sedimentary processes 30 Uplift and initiation 30 drift development 31 Tidal currents 33 Chronostratigraphy and palaeoceanographic comparison 34

CONCLUSIONS 43

DUTCH SUMMARY 44

REFERENCES 48

2 TABLE OF FIGURES

Figure 1: Morphosedimentary map of the Contourite depositional system on the middle slope of Gulf of Cádiz. (Hernandez-Molina et al., 2003) ...... 4 Figure 2: Oceanic circulation in the Gulf of Cádiz. (figure adapted from Vandorpe et al. (2014)) ..... 10 Figure 3: Overview of seismic lines...... 12 Figure 4: Overview of topographic features in the El Arraiche area...... 15 Figure 5: Seismic profile of the northern part of the area perpendicular to the Renard Ridge ...... 16 Figure 6: Seismic profile near the Western Channel, perpendicular to the Renard Ridge...... 17 Figure 7: Seismic profile of the narrow part of the Renard Ridge and the two mounded features along the Vernadsky Ridge ...... 18 Figure 8: Seismic profile of the Eastern Channel of the Renard Ridge and the two mounded features along the Vernadsky Ridge ...... 19 Figure 9: Seismic profile of the Eastern Channel of the Renard Ridge and only one mounded structure along the Vernadsky Ridge ...... 20 Figure 10: Schematic overview of the (sub-)units in the east of the area as presented in figure 9...... 21 Figure 11: Isopach map of unit 5...... 23 Figure 12: Seismic profile displaying the two mounded parted features along the Vernadsky Ridge..24 Figure 13: Isopach map of unit 4...... 25 Figure 14: isopach map of unit 3...... 26 Figure 15: Isopach map of unit 2...... 27 Figure 16: Isopach map of unit 1...... 28 Figure 17: Depthmaps of boundaries B5 to B1...... 29 Figure 18: transverse profile of the study area, parallel and centered between both ridges...... 38 Figure 19: schematic overview of figure 18: transverse profile of the study area, parallel and centered between both ridges...... 39 Figure 20: comparison of the evolution of the RND and VSD with the PDE drift (Vandorpe et al., 2014) and with clear MOW-controlled drifts in the northern Gulf of Cádiz (Roque et al., 2002; Brackenride et al., 2013), the Bay of Biscay (Van Rooij et al., 2010) and the Alboran Sea (Juan et al., 2012; Somoza et al., 2012). Figure adapted from Vandorpe et al., 2014...... 41 Figure 21: Overview of the currents in the El Arraiche mud volcano field (modified from Vandorpe et al. (subm.))...... 42

3 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

INTRODUCTION

Contourites were first identified nearly 50 years ago, but little attention was given to the subject (Faugères et al., 1993). Only the last two decades the field is advancing. The ambiguity in its definition and the non-straightforward characteristics of contourite facies are two reasons why contourite research has not been advanced as fast as it should. The definition of a contourite is ambiguous and they form only an end-member in a continuum of deep-sea sedimentary facies (Rebesco et al., 2014). Some facies models were proposed (Gonthier et al., 1984; Stow and Faugeres, 2008) but diagnostic criteria are still not robust. As such, further research is still needed to find an international accepted definition and diagnostic criteria have to be identified. New techniques have great potential for further research. A triple-stage approach was recommended by Nielsen et al. (2008) and Rebesco and Stow (2001) to identify sediment deposited by bottom currents. The analysis must include the overall architecture of the deposit (gross geometry and large-scale depositional units), the internal architecture (structure and subunits) and the seismic attributes and facies in every subunit. Contourites are sediments deposited or affected by bottom currents (Stow et al., 2002b; Rebesco et al., 2005; Stow and Faugeres, 2008). Bottom currents play an important role in the transport, deposition and erosion of deep-sea sediments. Strictly, contourites are formed by along-slope bottom currents, but not all bottom currents which create contourites follow the contours. Contourites can originate from many possible physical drivers like salinity, heat and atmospheric forcing and from different current types such as density driven currents (Johnson et al., 2002), deep-water tidal currents (Stow et al., 2013) and eddies (Serra et al., 2010). Contourites are widespread and cover large areas of the present ocean floors and continental margins. They occur in a broad range of environments, from the deep ocean (Uenzelmann-Neben and Gohl, 2012) over continental slopes (Li et al., 2013) to shallow margins (Vandorpe et al., 2014) and even in lakes (Heirman et al., 2012). Contourite drifts are sediment bodies build up and maintained by bottom currents (Rebesco et al., 2007). The drifts can be thick and extensive and occur in a broad range of environments, if a significant sediment input is available (Stow and Faugeres, 2008). Contourite drifts have different morphologies which are significantly controlled by the physiographic and geological setting in which they develop and by the different water masses involved (both their velocities and directions). The drift may thus be a palaeocurrent indicator. Drifts have mostly a mounded and elongated geometry accompanied by a concave moat but other depositional and erosional structures are possible. Different classification systems were set up based on drift morphology or location (Rebesco and Stow, 2001; Stow et al., 2002a; Rebesco et al., 2005; Hernandez-Molina et al., 2008b). The contourite drifts are characterized by a degree of mounding and elongation (Rebesco et al., 2014). Although there is some overlap amongst the different drift types forming a continuous spectrum. Elongated, mounded drifts and sheeted drifts can be considered as two end members of the spectrum (see figure 16 in Rebesco et al. (2014)). Elongated mounded drifts can be subdivided into separated and detached drifts. Separated drifts are divided from the steep slope by an erosional or non-depositional concave moat. The flow is focused along this moat (e.g. North Iberian Margin, (Van Rooij et al., 2010)). Detached drifts deviate away from the slope against which it first began to form. Mounded drifts are associated with high bottom current velocities (10-30 cm/s) (Stow and Faugeres, 2008). Sheeted drifts have a broad and faintly mounded geometry, slightly thinning towards the margins (e.g. Gulf of Cádiz (Llave et al., 2001; Llave et al., 2007; Hernandez-Molina et al., 2008b)) and are associated with lower velocities (10 cm/s). Confined drifts are mounded but have distinct moats on both flanks and are elongated along the basin (e.g. Lake Baikal (Ceramicola et al., 2001)). Plastered drifts are smaller than elongated, mounded drifts, but more mounded than sheeted drifts. They are typically located along a gentle slope and swept by low-velocity currents. Channel-related drifts lie in gateways with high-velocity currents. Other categories are patch drifts with an irregular morphology, infill drifts formed at the head of a scar and fault-controlled drifts. Mixed drifts involve a significant interaction of other depositional processes with along-slope currents. Besides deposition, also erosion or non-deposition occurs within a contourite. The erosional features are classified by Hernandez-Molina et al. (2008b) and Garcia et al. (2009) (see fig. 9 in Garcia et al. (2009)). A subdivision is made between areal and linear features. This last category is split up into

4 1. Introduction contourite channels, moats and marginal valleys. Marginal valleys occur against and around topographic obstacles, such as ridges and are the result of flow instabilities by the interaction of the current with the obstacles (Garcia et al., 2009). The currents are destabilized and their speed increases. As a result eddies and vortices are formed leeward of the obstacle (Guo et al., 2000; MacCready and Pawlak, 2001). Contourite moats originate when erosion or non-deposition occurs beneath the core of the current. Sediments are deposited away from the slope, where current velocities decrease (Hernandez-Molina et al., 2006). As a result of the upslope migration of the core, truncated reflections occur on the flank of the ridge and prograding, layered reflections occur on the other flank. (Garcia et al., 2009). Contourite channels are similar to contourite moats, but lack depositional features. Contourite channels are mainly formed by the erosive action of bottom currents (Hernandez-Molina et al., 2006). The study of contourites is crucial for at least three fields of fundamental and applied research (Rebesco et al., 2014). Slope-stability and geological hazard assessment is important because of the variations in the sedimentary succession on the submarine slopes, often related to the composition and physical properties of contourites (Solheim et al., 2005). Overpressured glide planes may be provided when fine contouritic sediment is loaded, preventing the escape of the water. The second research field is the hydrocarbon exploration. Contourites can affect the reservoir geometry and quality or distribute sealing rocks. Robust flows can for example form clean, sandy contourites forming an excellent source rock (Viana, 2001; Viana et al., 2007). The contouritic sediments yield temporal records of relatively high resolution, as the accumulation rates are usually high and continuous. Therefore the sediment may contain information on the palaeoceanography and palaeoclimatology. Especially changes in circulation pattern and current velocity may be preserved (Grutzner et al., 2003; Hernandez-Molina et al., 2003). The timescales range from tens to millions of years (Rebesco et al., 2014). A Contourite Depositional System (CDS) is an association of several connected contourite drifts and the erosional features as a consequence of the currents interaction with the seabed (Hernandez-Molina et al., 2003; Hernandez-Molina et al., 2006; Hernandez-Molina et al., 2008a; Hernandez-Molina et al., 2009a). A large contourite depositional system is present in the northern part of the Gulf of Cádiz due to the interaction of the Mediterranean Outflow Water (MOW) with the complex morphology of the Southern Iberian margin (Llave et al., 2001; Hernandez-Molina et al., 2006; Garcia et al., 2009). The Cádiz CDS documents changes in the MOW which has built up hundreds of meters thick and kilometres long sediment drifts over the past 4.2 My, deposited since the Lower Pliocene Revolution (Hernandez- Molina et al., 2006; Van Rooij et al., 2011). The CDS of the northern part started to develop in the Early Pliocene when the Strait of Gibraltar opened (Maldonado et al., 1999; Llave et al., 2006). From the Pleistocene on, the deposition of the CDS was influenced by the glacial-interglacial cycles (Llave et al., 2006; Toucanne et al., 2007). Glacial periods are characterized by low sedimentation rates indicating enhanced currents (Schonfeld and Zahn, 2000; Llave et al., 2006). Whereas interglacials are characterized by high sedimentation rates and slows currents. This is due to an intensified and deeper flowing glacial MOW. According to Faugères et al. (1999) along-slope processes were more common during interglacials and generated a CDS. In glacial, lowstand periods, downslope process dominated. The Cádiz CDS is located on the middle slope and comprises five morphosedimentary sectors (figure 1): Proximal Scour and Sand-Ribbons; Overflow Sedimentary Lobe; Channels and Ridges; Contourite drifts and Submarine canyons sectors. These sectors are directly related to the deceleration of the MOW branches and the Coriolis force (Hernandez-Molina et al., 2003).

5 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

Figure 1: Morphosedimentary map of the Contourite depositional system on the middle slope of the Gulf of Cádiz. Sedimentary deposit types and bed forms are shown for five morphosedimentary sectors (Hernandez-Molina et al., 2003).

The Proximal Scour and Sand-Ribbons sector is closest to the Strait of Gibraltar where erosive features are dominant but also depositional features such as sand dunes occur. Erosion occurs because of the MOW’s high velocity after its entrance in the Gulf of Cádiz. Deposition occurs when the velocity decreases. The Overflow Sedimentary Lobe sector is a complex combination of depositional, gravitational and erosive processes and has a fanlike shape. Several furrows occur in the area and were defined as free-standing bottom current channels (Habgood et al., 2003). Seaward of the furrows, several sedimentary lobes are identified with small gravitational collapses along their margins. In the Channels and Ridges sector five main contourite channels have been distinguished with an along-slope and downslope zone. In a first phase the MOW interacts with the margin and drift deposits are created. Later on, the MOW becomes channelized and sinuous moats are formed. The contourite channels change abruptly their direction in a downslope trend when they reach the Cádiz and Guadalquivir diapiric ridges and an increase in velocity is measured (Nelson et al., 1993). As a consequence the tectonic activity in the area played an important role in the distribution of sediment (Hernandez-Molina et al., 2003). Contourite channels are located on the southeast flank of the diapiric ridges while many marginal valleys have been detected on the north western flank. In the Contourite deposition sector (the central and northwest areas) sedimentary processes dominate, which build up both elongated mounded drifts and sheeted drifts. The submarine canyons sector is located in the western area. The southern Gulf of Cádiz is less studied and the presence of a CDS is less likely because of the absence of MOW in the largest part of this area. Although some authors report the presence of one branch and some meddies (Richardson et al., 2000; Ambar et al., 2008). During glacial periods a MOW influence in the El Arraiche area is even less plausible. Although there is an enhanced meddy activity during glacial periods, the MOW flows deeper (Garcia et al., 2009) and is as such unlikely to be present in the area (Vandorpe et al., 2014). Multiple contourites occur in in the El Arraiche field around the ridges and mud volcanoes (figure 4). The El Arraiche field is one of the many mud volcano fields along the Moroccan Atlantic margin (Somoza et al., 2003; Van Rensbergen et al., 2005) and was discovered in 2000 during the TTR10 cruise.

6 1. Introduction

The discovery of juvenile carbonate mound structures and mud volcanoes arouse interest for the area. The area was first fully mapped in 2002 during the R/V Belgica CADIPOR campaign. It lies within the accretionary realm but outside the active olistostrome units. Eight mud volcanoes with different dimensions are present in water depths ranging from 200 to 700 meter. The largest, Al Idrissi mud volcano, is 255 m high and 5.4 km wide (Van Rensbergen et al., 2005). The mud volcanoes cluster around two, sub-parallel diapiric ridges (the Renard and Vernadsky Ridge, both with steep fault escarpments) and are associated with active extensional, normal faults (Van Rensbergen et al., 2005). The mud volcanoes have their root in the Upper Cretaceous deposits (Pinheiro et al., 2003). Their activity started 2.4 Ma ago (Van Rensbergen et al., 2005) and occurred in several phases (Perez-Garcia et al., 2011). Their presence is an indication for over-pressured and gas-rich sediments. Mud volcanoes are often found along strike-slip faults and thrusts (Pinheiro et al., 2003). The mud volcanoes were formed due to the convergence of Eurasia and Africa, causing an increase in the pore pressure (Medialdea et al., 2004). Overpressured fluids migrate towards the surface and this process is facilitated by the faults which fuel the mud volcanoes (Van Rensbergen et al., 2005). Mud volcanoes are often associated with contourite depositional systems (Somoza et al., 2012), the erosion or deposition of hundreds of meters of contourite sediments produces cyclic changes in the sedimentary loading, favouring lateral and vertical fluid migration (Stow and Faugeres, 2008; Stow et al., 2008). Mud volcanoes also interact with contourite systems as obstacles, affecting the flow path and flow velocity (Hernandez-Molina et al., 2006; Stow et al., 2008). The Pliocene-Quaternary units in the basin between the two ridges are locally deformed due to the ascending fluids (Van Rensbergen et al., 2005). Several small-scale contourite drifts occur around the two ridges in the northern part of the El Arraiche field (figure 4) (Vandorpe et al., subm.). One occurs at the southern foot of the Pen Duick Escarpment and is studied in great detail by Vandorpe et al. (2014). Van Rooij et al. (2011) proposed to name this drift the Pen Duick Drift. The Pen Duick Drift was identified as a topography-controlled contourite drift with a separated, mounded contourite morphology. The Pen Duick Drift originates at the base of the Quaternary by the uplift of the Renard Ridge and the bottom currents have intensified ever since. According to Vandorpe et al. (sumb.) the Pen Duick Drift may have a mixed Antarctic Intermediate Water (AAIW) and North Atlantic Central Water (NACW) origin. A small drift occurs along the southern flank of the Renard Ridge and is called the Renard South drift (Vandorpe et al., subm.). The drift along the northern flank of the Renard Ridge is studied in this paper together with the southern flank of the Vernadsky Ridge (figure 4). Vandorpe et al. (subm.) proposed to name these drifts respectively the Renard North Drift (RND) and the Vernadsky South Drift (VSD). Small and erosive features occur along the north and south flank of the mud volcanoes with the southern flank eroded deepest (Vandorpe et al., subm.). This MSc thesis mainly focuses on the palaeoclimatology and palaeoceanography of the northern area between both ridges. By studying the depositional and erosional features present in the study area, a reconstruction of the processes responsible for the creation of these deposits is done. This may lead to a local palaeoceanographic model. A better understanding of the palaeocirculation can help to understand the behaviour of obstacle related drifts, the origin of the cold-water coral mounds in the area, and the relation between the bottom currents and the deep-water ecosystems. This can be achieved by comparing the results of this study with the results of the Pen Duick drift (Vandorpe et al., 2014) and similar environments in the Northern Gulf of Cádiz (Llave et al., 2011; Roque et al., 2012; Brackenridge et al., 2013), on the eastern side of the Gibraltar strait in the Ceuta drift (Ercilla et al., 2002; Juan et al., 2012; Somoza et al., 2012 ) and the Le Danois drift (Van Rooij et al., 2010).

7 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

REGIONAL SETTING

Geology & geomorphology

The Gulf of Cádiz is part of the Atlantic Ocean and is bound by the Moroccan margin in the south and the Iberian peninsula in the north. It is connected to the Mediterranean Sea in the east through the Strait of Gibraltar. The Gulf of Cádiz is situated between 9°W to 6° 45’ W and 34°N to 37° 15’N. From the shelf onwards the bathymetry increases from 200 m up to 4 km in the Horseshoe and Seine abyssal plains (Van Rensbergen et al., 2005) (figure 2). The Gulf of Cádiz is divided in a northern and southern part. The northern part is well studied, while less attention has been paid to the southern part. The Gulf of Cádiz is situated at the boundary between the Eurasian and African plate. Complex tectonic forces act on the area since the Triassic, causing several periods of rifting, compression and strike-slip motion. (Srivastava et al., 1990; Sartori et al., 1994; Maldonado and Nelson, 1999; Gutscher, 2002; Medialdea et al., 2004). The tectonic history of the Gulf of Cádiz started with a sinistral, oblique extension. The extension opened a small seaway connecting the Atlantic ocean with the Neothethys. The extensional phase lasted until the Middle Cretaceous (Dewey et al., 1989) forming the Gulf of Cádiz and the Gibraltar passage. The southern margin was formed in the Jurassic during the break-up of Gondwana and Laurussia (Ziegler, 1999). The Middle Cretaceous to recent evolution of the Gulf of Cádiz can be split up in three distinct phases. In the first phase the northern margins of the Gulf of Cádiz were initiated out of a passive margin associated to a half-graben extensional system during the Mesozoic and Early Cenozoic (Maldonado et al., 1999). From the Late Eocene to the Early Miocene a north-south convergence of Iberia and Africa started the second phase, leading to a transpressive regime (Maldonado et al., 1999). A third phase started in the Early-Middle Miocene with an accelerated subsidence causing the emplacement of a olistostrome unit. The evolution of the subsiding basin caused the Betic-Rifian orogeny and the opening of the Western Mediterranean basins (Maldonado et al., 1999; Hernandez-Molina et al., 2006). The north-central Rif (Alboran domain) moved several hundreds of kilometres to the west resulting in the Rif-Betic-Alboran block present in the Iberian-Moroccan region in between the two major plates (Gutscher et al., 2009a). The olistostrome unit was formed by gravitational thin-skinned tectonics gliding over a Triassic salt décollement (Somoza et al., 2003). The allochtonous nappes were emplaced during the Tortonian (Late Miocene) forming an accretionary wedge-type environment (Maldonado et al., 1999; Medialdea et al., 2004). The main part of the olistostrome unit occupies the central part of the Gulf of Cádiz as a lobe-shaped structure (Maldonado et al., 1999). It extends from the Spanish and Moroccan coastal margin until the Horseshoe and Seine abyssal plains with a maximum thickness reaching about 2.75 km (Gutscher et al., 2009b). The former processes induced very high rates of basin subsidence and strong diapiric activity (Maldonado and Nelson, 1999; Maldonado et al., 1999; Somoza et al., 1999; Medialdea et al., 2004) The north-south convergence, active since the Oligocene, gave origin to high-angle reversed faults in the Gulf of Cádiz that evolved eastwards into right lateral strike- slip faults and ended in the Late-Miocene. Next, an oblique and slower northwest-southeast convergence initiated (Rosenbaum et al., 2002; Medialdea et al., 2004). The olistostrome unit is covered by a 0.2 to 2 km thick Neogene sedimentary cover (Pinheiro et al., 2003; Medialdea et al., 2004). Neotectonic reactivation of the accretionary wedge by strike-slip and compressional deformation is given by the presence of mud volcanoes, salt diapirs and fluid escape features piercing the sedimentary cover (Pinheiro et al., 2003; Medialdea et al., 2004; Van Rooij et al., 2011). Most of the mud volcanoes are located within the offshore Betic-Rifian domain and are grouped into fields (Medialdea et al., 2004). The mud volcanoes are rooted in the Upper Cretaceous deposits and their activity started 2.4 Ma ago (Van Rensbergen et al., 2003). The El Arraiche mud volcano field is located in the southern part of the Gulf of Cádiz, on top of the accretionary wedge but south of the olistostrome complex and is bordered by the Renard Ridge in the west and Vernadsky Ridge in the east. Both ridges are sub-parallel and have steep fault escarpments.

8 2. Regional setting

The escarpment south of the Renard Ridge is called the Pen Duick Escarpment (PDE). A topography- controlled contourite drift occurs at the foot of this escarpment and is intensively studied by Vandorpe et al. (2014). The ridges are situated in water depths of about 700 m. The ridges have their origin in the extensional tectonics with compressive ridges as a result. The extensional force created large, rotated blocks bound by lystric faults that causing the formation of Plio-Pleistocene depocentres including the El Arraiche field (Flinch, 1993). The rotated blocks served as pathways for fluid migration, fuelling the mud volcanoes. Multiple mud volcanoes are present in the area (figure 4) around both ridges and in the same water depths. The Adamastor and Al Idrissi mud volcano are situated in between both ridges. Numerous cold-water corals are clustered on top of the ridges and around the mud volcanoes (Wienberg et al., 2010). The cold-water corals (CWC) are both present in the subsurface and on the seabed. Five cold-water coral provinces can be distinguished in the El Arraiche mud volcano field (Foubert et al., 2008; Van den Berghe, 2015) (figure 4) : the Pen Duick Mound Province, the Renard Mound Province, the Vernadsky Mound province, the Mercator Mound Province and the Al Idrissi Mound Province. The occurrence of CWC’s is the result of hydrodynamic and seepage factors (Van Rooij et al., 2011). Cold- water corals do not rely on photosynthesis but on the supply of organic matter in the water column. (Maignien et al., 2010). Therefore, ridges are a good substrate for juvenile CWC growth, because of their elevated topography and hard substrate. More food passes through at higher levels and a hard substrate is necessary for the initial settling (Foubert et al., 2008). CWC’s are also dominantly present in areas with strong currents. The currents reduce the deposition of fine sediment and transport large food quantities (Roberts et al., 2006). There is an important interplay between current and sediment regime in the mound development. Mounds are often present in sedimentary drifts, often with accelerated currents (Huvenne et al., 2003). The CWC’s in the Gulf of Cádiz only occur in a juvenile growth stage (Foubert et al., 2008). Therefore they are of great interest for the CWC research. CWC’s are present in the Gulf of Cádiz for more than 300 ka but the overall growth started at 48 ka. Although CWC’s are abundant in both glacial and interglacial periods, CWC have a preference for glacial periods (Foubert et al., 2008; Wienberg et al., 2010). The growth of most of the CWC’s in the Gulf of Cádiz ended at the beginning of the Holocene. The study of contourites helps to reconstruct the palaeoceanography in the area which is necessary to reconstruct the environmental conditions that favour cold-water coral growth.

Oceanography

Several surface and deep water currents play an important role in the oceanography of the Gulf of Cádiz (figure 2). Their flow path is mainly influenced by the exchange between the Mediterranean and Atlantic Ocean and the topography in the Gulf of Cádiz. The general surface circulation is an anticyclonic gyre (Pelegri et al., 2005). It can be interpreted as the last meander of the Azores current. Near-surface shoreward currents also exist in the southern Gulf of Cádiz with a southward recirculation (Machin et al., 2006). As the geology, the oceanography of the southern part is less constrained compared to the northern part. Four different water masses can be distinguished in the southern Gulf of Cádiz and are also measured in the Pen Duick Escarpment (PDE) area (Vandorpe et al., 2014). The North Atlantic Surface Water (NASW, 0-100 m), North Atlantic Central Water (NACW, 100-600 m), the Antarctic Intermediate Water (AAIW, 600-1500m) and the North Atlantic Deep Water (NADW, beneath 1500m). The MOW is not observed above 700 and thus not present along the PDE (Van Rooij et al., 2011). However, the presence of meddies, transporting MOW south of Gibraltar near the Moroccan shelf is known in the southern part (Richardson et al., 2000; Ambar et al., 2008).

9 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

Figure 2: Oceanic circulation in the Gulf of Cádiz. The white box indicates the position of the study area. The yellow arrows point at tidal currents active along the Moroccan margin. NACW: North Atlantic Central Water, AAIW: Antarctic Intermediate Water, MOW: Mediterranean Outflow Water, NADW: North Atlantic Deep Water, UC: MOW upper current, IB: MOW intermediate branch, PB: MOW principal branch, SB: MOW southern branch (figure adapted from Vandorpe et al. (2014)) The North Atlantic Surface Water is the water mass of the upper 100 meter in the Gulf of Cádiz. The water mass is considered as modified North Atlantic Intermediate Water because of its interaction with the atmosphere. The temperature ranges between 16°C and 22°C and the salinity varies between 36.3 and 36.5 (Louarn and Morin, 2011). The North Atlantic Central Water is the water mass flowing just below the surface waters at a depth of 100 to 600 m. It stretches down to the isohaline of 35.6 (McCave and Hall, 2002). The temperature typically decreases and ranges from 16.45 to 10.63°C. Nutrients are low in the upper layer but increase towards the bottom (Criado-Aldeanueva et al., 2006; Louarn and Morin, 2011). The NACW is characterized by the flow of the current in three directions (Machin et al., 2006). Towards the coast into the Strait of Gibraltar, along the African coast in a southward direction and in a westward direction due to the incorporation into Mediterranean Water. In the southern part there is possibly low-saline Antarctic Intermediate Water (AAIW) underneath the NACW (Iorga and Lozier, 1999; Hernandez-Guerra et al., 2001) or high-saline Mediterranean Water in the north, closer to the Strait of Gibraltar. These intermediate water masses flow between 600 and 1500 m water depths. There is often mixing of these two water masses with an intermediate salinity as result (Iorga and Lozier, 1999; Pelegri et al., 2005; Machin et al., 2006).

10 2. Regional setting

AAIW has a temperature of 9.67-10.25°C and a salinity of 35.63-35.79 (Louarn and Morin, 2011). The AAIW is low on oxygen and high on silicates. Several studies already described the presence of the AAIW in the eastern North Atlantic Ocean along the African coast at the latitudes of the Canaries Archipelago (Roemmich and Wunsch, 1985; Castro et al., 1998; Perez et al., 2001; Ambar et al., 2002; Llinas et al., 2002; Machin et al., 2006). Two lenses of the AAIW are present in the southern Gulf of Cádiz. The water mass is situated at a depth of 900m (Louarn and Morin, 2011). The AAIW enters the Gulf of Cádiz in the southwestern part of the basin, along the continental shelf. The AAIW spreads cyclonically in the gulf and is pushed away by the MOW. The AAIW is difficult to discern due to the intense mixing of the water mass with under- and overlying water masses (Louarn and Morin, 2011). The Mediterranean Outflow Water (MOW) flows out of the Strait of Gibraltar where it sinks due to its -3 high salinity and density (θ>28.8 kg m ), the initial temperature is 13.50°C (Louarn and Morin, 2011). Entering the Gulf of Cádiz, the MOW follows the northern topography and sinks further and mixes with the surrounding waters (madelain, 1970). The MOW is general observed between 500 and 1400 m depth in the northern Gulf (Hernandez-Molina et al., 2011). As it flows further away from the Strait of Gibraltar, MOW mixes with NACW and the salinity and temperature decreases with depth. A salinity equilibrium is reached about 50 km seaward of the Strait of Gibraltar (at 8°W in the northwestern part of the Gulf) (Howe et al., 1974; Zenk et al., 1975; Ambar and Howe, 1979; Zenk and Armi, 1990). After its outflow of the Strait of Gibraltar, the MOW is split up into two main branches: the upper and lower Mediterranean waters (Hernandez-Molina et al., 2006), respectively flowing along the shelf at depths of 500 to 800m and more west-northwestward at depths of 750 to 1400 m. The latter splits up in three more branches (Louarn and Morin, 2011). The MOW does not flow in the southern Gulf of Cádiz but meddies are known to transport MOW southwards (Richardson et al., 2000; Ambar et al., 2008). According to the CTD data of Vandorpe et al. (2014), the MOW is present at depths of 1000-1500 m in the Gulf of Cádiz. During glacials, a stronger, saltier and denser MOW sinks to depths of about 700 m deeper than today (Schneider et al., 2000; Llave et al., 2006; Toucanne et al., 2007; Garcia et al., 2009). Therefore the presence of the MOW in the El Arraiche field, located at a depth of about 700 m is not likely (Vandorpe et al., 2014).The re-establishment of water mass exchange between the Mediterranean Sea and Atlantic ocean began after the end of the Messian Salinity Crisis (Cita, 2001). A change to a colder and arid climate (Fauquette et al., 1998) triggered two periods of enhanced production of cooler and saltier MOW and an more vigorous circulation. The first period was at the onset of the Northern Hemisphere glaciations (3.1-2.7 Ma) (Khélifi et al., 2009). The second period coincided with the Mid- Pleistocene Revolution (1.2-0.55 Ma) (Hayward et al., 2009). North Atlantic Deep Water is present in the deeper parts of the basin (Mauritzen et al., 2001; Ambar et al., 2002; Louarn and Morin, 2011). The North Atlantic Deep Water (NADW) is the deepest water mass and only occurs at depths below 1500m. The NADW is characterised by dense and cold water with a high oxygen saturation (70.0-70.5%). The NADW has typically low temperatures (3-8°C) and high salinities (34.95 – 35.20).

11 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

MATERIAL & METHODS

Since the discovery of juvenile carbonate mound structures and mud volcanoes in the Gulf of Cádiz, several research campaigns were carried out in the Southern Gulf of Cádiz: the R/V Belgica “CADIPOR” campaigns (2002, 2005 & 2007), “Pen Duick” campaign (2009) and “COMIC” campaign (2013). The aim of this last campaign was to acquire seismic profiles across both the drift deposits and the mounded structures southwest of the Pen Duick escarpment in order to unravel their palaeoceanographic and sedimentary history. In this study, seismic lines of the CADIPOR II cruise (2005) and the “COMIC” cruise are used (figure 3). In total almost 1000 km of high resolution single channel Sparker seismic data was acquired during the “COMIC” campaign. Of these, about 126 km covered the El Arraiche mud volcano field. About 200 km of seismic data was used from the CADIPOR II campaign. In 2005 a SIG sparker (120 electrodes) was used with a shot interval of 2 s and energies reaching 400-500J. The sampling frequency was set at 8 kHz and a record length of 1.8 s TWT was obtained. In 2013 the shot interval was 2 s and energies reached 500 or 600 J. The sampling frequency was 10 kHz and record length of 1.8 s TWT was used. In both campaigns the ship’s velocity was maintained at about 3 -3.5 knots. All of the lines of the 2013 campaign were shot during calm sea state (1-3). The multibeam data were recorded during the CADIPOR I cruise (2001) and cover an area of 700 km² in total. It has been obtained using the SIMRAD EM1002 system extended with a deep water module. The swath width was 500 or 750 m depending on the water depth (500 m water depth as transition). The data was corrected and cleaned using Kongsberg’s Merlin and Neptune packages. At a water depth of 400 m the footprint was 15x15 m.

Figure 3: Overview of seismic lines. Red: lines shot during R/V Belgica “COMIC” campaign (2013). White: lines shot during R/V Belgica “CADIPOR II” campaign (2005). MV: mud volcano, dQ MV: don Quichote mud volcano. The seismic profiles were processed using the DECO Geophysical RadexPro software. First a bandpass filter was applied (Ormsby type, with a gradual low-cut mostly between 100 and 180 Hz and a high cut between 1200 and 1500 Hz). Next, a swell filter was used followed by a burst noise removal, spherical

12 3. Material & methods amplitude correction and finally a top cut. All data were saved as SEG-Y data. Fifteen profiles from the “COMIC” cruise and 20 profiles from the CADIPOR II campaign were processed. The seismic lines were all interpreted using the Kingdom suite software. Grid maps were created with the same software to create depth and isopach maps. Triangulation was used to grid the data in Global Mapper 15. The Surfer 11 software was used to create the contour maps of the isopach maps and depth maps created in the Kingdom suite software. All maps were edited with Corel draw.

13 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

RESULTS

Geomorphology

The investigated area extends from 35°20’N to 35°28’N and 6°54’ W to 6°45’W (figure 4) and is delineated by two ridges: the Renard Ridge in the southwest and the Vernadsky Ridge in the northeast. The Adamastor mud volcano is centred in the south between both ridges. The mud volcano is 2 km wide at the base and is about 160 meter high (Gardner and Shashkin, 2000). A large CWC province occurs on the most northern tip of the Vernadsky Ridge, the Vernadsky Mound Province (figure 4) (Foubert et al., 2008). A discontinuous channel is present at the northern foot of the Renard Ridge, split into two deep parts. The westernmost part of the channel is located at 35°23’ N and 6°51’ W. This part will be referred to as the Western Channel (figure 4 and 6). The second part is located in between the Renard Ridge and the Adamastor mud volcano. This channel will be referred to as the Eastern Channel (figure 4, 8 and 9). The Western Channel has a northwest-southeast orientation and is about 5 km long. It is bended around the western promontory of the Renard Ridge (figure 4). The Eastern Channel is about 6 km long and has the same orientation. Both channels along the Renard Ridge have a maximum width of 2 km and their depths are respectively 15 m in the west and 90 m in the east. The broadest and deepest part of the each channel occurs at the northern promontories of the Renard Ridge (figure 4). A small mounded feature with prograding reflectors is present in the Eastern Channel. The channel and mounded part has a width of more than 400 m (figure 9) and is only present in the eastern end of the Eastern Channel (figure 4). In between both channels a less erosive channel occurs where the Renard Ridge narrows (figure 7). The concatenated channels along the Renard Ridge follow the topography. Two parallel elongated mounded parts occur along the Vernadsky Ridge located at 35°24’N and 6°48’ W and are only present for a short length along the ridge (figure 4). The mounded parts are about 5 km long and have a northwest-southeast orientation. Only the outer edges are slightly bended, following the topography (figure 4 and 8). The northern channel between the ridge and the north-easternmost mounded part is about 3 m deep and is up to 300 m wide (figure 9). The southern channel between the two mounded parted features is 5 m deep and has a maximum width of about 500 meters (figure 7). This last channel is deeper but smaller towards the northwest. The area can be divided into four parts with a lateral varying geometry of the seismic facies (figure 4). Their location is described on the basis of the two deep channels visible on the multibeam map (figure 4) and are perpendicular to both ridges (NE-SW direction). The first part is located north of the northwestern extent of the Renard Ridge. The second area is located north of the Western Channel. The third area is located between both channels and the last area lies north of the Eastern Channel.

14 4. Results

Figure 4: Overview of topographic features in the El Arraiche mud volcano field. The black window shows the limits of the study area. The blue lines show the contourite channels. VSD: Vernadsky South Drift, RND: Renard North Drift, RSD: Renard South drift; PDD: Pen Duick Drift (Vandorpe et al., subm.). The northern promontories of the Renard Ridge are indicated by the orange lines. The dark opaque fields mark the four mound provinces discerned by Van Rensbergen et al. (2005). The Mercator MP was discerned by Van den Berghe (2015) .MP: mound province . The discerned mounds in the northern edge of the Vernadsky MP are given by the yellow circles. The full circles mark the mounded parts visible in the profiles. The red lines mark the mud volcanoes. dQ: don Quichote MV, LdT: Lazarillo de Tormes MV. The white dotted lines separate the five different areas with a varying geometry of the seismic facies discerned along the Renard Ridge.

Seismic stratigraphy

Six units are discerned (U6-U1), separated by 5 stratigraphic boundaries (B5-B1). The Renard Ridge is a part of the acoustic basement in the southeast and the Vernadsky Ridge in the northwest. The Vernadsky Ridge is only visible in some profiles of the “COMIC” campaign and only in the deeper parts of the profile because the lines did not extend long enough northwards to record the full ridge. Profiles across the Vernadsky Ridge were shot during the CADIPOR II campaign but these do not display the sediments in the basin between both ridges. The acoustic basement in between both ridges is not entirely visible because of the attenuation of the signal by the thick sedimentary packages (penetration depth is only about 400 ms TWT). Numerous normal faults are visible in the profiles (figure 5 and 18). The faults reach the top of unit 4 and some even go higher up in the stratigraphic sequence (subunit 3.1). One large fault is present north of the Western Channel and reaches the top of subunit 3.1 (figure 18). All faults have an angle between 75 and 90°.

15 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

Figure 5: Seismic profile of the northern part of the study area, perpendicular to the Renard Ridge (figure 3). Faults are indicated with black dashed lines. The boundaries are indicated with full red lines. Subunits are indicated with black lines and the cyclic subunits with black dotted lines. CWC’s are indicated with blue lines. The direction of the bottom current is given by the bottom current symbol. The yellow frame marks the wavy deposits.

16 4. Results

Figure 6: Seismic profile near the Western Channel, perpendicular to the Renard Ridge (figure 3). Faults are indicated with black dashed lines. The boundaries are indicated by full red lines. Subunits are indicated by black lines and cyclic subunits by black dotted lines. CWC’s are indicated by blue lines. The direction of the bottom current is given by the bottom current symbol. The Vernadsky Ridge is visible in the northeast.

17 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

Figure 7: Seismic profile of the narrow part of the Renard Ridge and the two mounded features along the Vernadsky Ridge (figure 3). Faults are indicated by black dashed lines. The boundaries are indicated by full red lines. Subunits are indicated by black lines and cyclic subunits by black dotted lines. CWC’s are indicated by blue lines. The direction of the bottom current is given by the bottom current symbol.

18 4. Results

Figure 8: Seismic profile of the Eastern Channel of the Renard Ridge and the two mounded features along the Vernadsky Ridge (figure 3). Faults are indicated by black dashed lines. The boundaries are indicated by full red lines. Subunits are indicated by black lines and cyclic subunits by black dotted lines. CWC’s are indicated by blue lines. The direction of the bottom current is given by the bottom current symbol

19 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

Figure 9: Seismic profile of the Eastern Channel of the Renard Ridge and the northern mounded part along the Vernadsky Ridge (figure 3). Faults are indicated by black dashed lines. Subunits are indicated by black lines and cyclic subunits by black dotted lines. The boundaries are indicated by full red lines. CWC’s are indicated by blue lines. The direction of the bottom current is given by the bottom current symbol. A small elongated mounded part is visible in the Eastern Channel .

20 4. Results

Figure 10: Schematic overview of the (sub-)units in the east of the area as presented in figure 9. Onlapping and prograding reflectors are marked with a black arrow. Dashed lines point at erosional surfaces. The (paleo-)bottom currents are marked by the bottom current symbols. CWC’s are visualized by the light blue spots.

21 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

Unit 6 Unit 6 is the lowest unit that fills up the basin. The unit is bounded by the basement and boundary B5. Unit 6 is characterized by low amplitude reflectors interbedded with two layers of high amplitude, continuous reflectors (figure 19). The upper part of the unit has higher amplitude reflectors with one very high amplitude reflector interbedded. The characteristics of the upper part of the unit are only visible in the western part of the area (figures 5, 6, 7 and 11). The thickness of the unit is unknown because of the attenuation of the signal but has a thickness of at least 200 ms TWT in the centre of the basin. Sediments onlap with a high angle onto the Renard Ridge. The angle of onlap is decreasing upwards in the stratigraphy. The reflectors show a wave-like pattern along the northwestern edge of the Renard Ridge (figure 5). The deposits on the Renard Ridge are steepest southeast of the Western Channel (figure 7) and less steep along the Eastern Channel (figure 8 and 9). In figure 6, situated below the Western Channel, the angle of onlap decreases at the top of the unit. Where the Renard Ridge narrows, sediment are deposited further west upon the ridge (figure 7). The reflectors onlap on the Vernadsky Ridge and have approximately the same angle over the whole length of the Vernadsky Ridge (figures 7, 8 and 9). The angle of onlap of the reflectors onto the Vernadsky Ridge is higher than those on the Renard Ridge. A high is present in the north, halfway between the ridges (figure 5) where the reflectors suddenly go up. The reflectors here have an extremely low amplitude and are disrupted by six faults. This high is not present anymore north of the Western Channel.

Unit 5 Unit 5 is bounded by boundaries B5 and B4 . Both boundaries are not erosive. B5 marks the transition to a cyclic depositional pattern in unit 5. The unit is characterized by a sequence of cyclic subunits with high amplitude, continuous reflectors at the base followed by a thicker part of low amplitude, continuous reflectors. Eight subunits are discerned. Unit 5 has a maximum thickness of about 150 ms TWT (figure 11) and each subunit has a maximum thickness of about 10 to 28 ms TWT. The lowest (5.1) and third (5.3) subunit are the thickest deposits. But their high amplitude reflectors are less clear expressed. The two upper subunits (subunit 5.7 and 5.8) are the thinnest deposits. Subunit 5.7 has more continuous high amplitude reflectors. Some profiles show higher amplitudes in one or more subunits, but this cannot be allocated to one specific subunit or location. The high amplitude reflectors are clearest in the southeastern part of the area (figure 8 and 9). After the deposition of unit 6, a low was still present in between the ridges with its deepest part in the southeastern part of the area (figure 17, boundary 5). Unit 5 itself fills up this basin partly causing thicker deposits in this low (figure 11), but the low remains present in the southeastern part of the basin at the top of unit 5 (figure 17, boundary 4). The deposits pinch out towards the ridges and thin towards the northwest. Sediment is deposited upon the narrow part of the Renard Ridge as well. Along the Renard Ridge the sediment is deformed in the lower part of the unit. The angle of onlap decreases upwards in all profiles. Along the northwestern edge of the Renard Ridge, the wavy deformation of the reflectors becomes more expressed (figure 5). At the top of unit 5, the reflectors in the curve are subhorizontal. Two mounded parts with a channel on their southwest side originate in unit 5 along the promontories. The Western Channel starts developing in subunit 5.4 (figure 6) and becomes more elongated during the upper subunits, although the full length of the current channel is not reached. The channel is about 400 m wide and is maximum 20 ms TWT deep. The reflectors show a mounded geometry, with increasing expression upwards in the stratigraphy. In the eastern part a channel starts to develop in subunit 5.8, eroding partly subunit 5.7. The channel development is only present below the centre of the current channel (figure 9). The channel is 500 m broad and only 15 ms TWT deep. In the narrow part of the Renard Ridge, sediment is deposited on top of the ridge and the reflectors pinch out against the Renard Ridge (figure 7).

4. Results

Along the Vernadsky Ridge the reflectors show pinch out against the ridge and are deformed to a wavy configuration (figure 7 and 9). The reflectors display a high onlap onto the Vernadsky Ridge.

Figure 11: Isopach map of unit 5. Unit 5 thins towards the ridges. The thickest package is deposited in the east of the basin. A theoretical seismic velocity of 1700 m/s is used to calculate the thickness of the unit.

Unit 4 Unit 4 is bounded by boundary B3 and B4 and is characterized by the presence of numerous CWC’s. A CWC can be recognized as a nearly acoustically transparent spot in the sedimentary sequence. Boundary B4 is marked by the end of the cyclic pattern present in unit 5 and by the origin of several CWC’s. B4 is slightly erosive along the Eastern Channel. Boundary B3 is marked by the end of the last CWC’s present in unit 4 and is erosive along the Renard Ridge (figure 6 and 9). Unit 4 is divided into four subunits. The lowest subunit (4.1) has continuous, high amplitude and thin reflectors. Some small CWC’s are present within this unit and have their top at the upper boundary of subunit 4.1 (figure 5 and 6). Subunits 4.2 and 4.3 are characterized by a thick package of low amplitude reflectors. multiple CWC’s are present that have their origin at the base and their end at the top of the subunit. The largest (up to 40 ms TWT) CWC’s occur in both subunits. The third subunit has few high amplitude reflectors below. The upper subunit (subunit 4.4) has higher amplitudes, below with thin reflectors and with thick reflectors above. A low amplitude reflector is interbedded in subunit 4.4 and less CWC’s are present. Unit 4 has a maximum thickness of about 134 ms TWT (figure 17) and fills up most of the topographic differences in the basin (figure 17, boundary B3). The reflectors are subhorizontal in the centre of the basin by the end of unit 4 but are still rising towards the Vernadsky Ridge. The unit thins towards the northwest and towards both ridges. A mounded feature develops along the northwestern edge of the Renard Ridge during subunit 4.2 (figure 5) with slightly prograding reflectors thinning towards the ridge. The channel and mounded part that started forming in unit 5 below the centre of the present Western Channel are developing and elongating along the Renard Ridge (figure 6). The channel evolves into a broader (880 m) and deeper (40 ms TWT)

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

erosional structure. Half of subunit 4.4 (25 ms TWT) is eroded below the channel. Sediments are deposited onto the narrow and lower part of the Renard Ridge, thinning higher on the ridge (figure 7). Northeast of the narrow part of the ridge, shallow mounded sediments are deposited starting at the base of subunit 4.2 (figure 7). Along the eastern part of the Renard Ridge (figure 9), the mounded feature develops towards the west in subunit 4.2 (figure 8), no distinct channel is visible. The channel in the centre (figure 9), becomes wider (800 m) and deeper (80 ms TWT) and slightly eroded the upper reflectors of unit 5. The reflectors below the channel are chaotic, especially in the upper subunit (4.4). Subunit 4.1 and 4.2 are slightly eroded. Half of subunit 4.4 (20 ms TWT) is eroded. Along the southeast of the Vernadsky Ridge, subunit 4.1 follows the same trend as unit 5 and is deposited on top of the Vernadsky Ridge. A chaotic structure starts developing at the base of subunit 4.2 (figure 7 and 8). The chaotic structure is present in the southeastern half of the area and is visible on the multibeam map as the northernmost mounded part along the Vernadsky Ridge (figure 4). Only the easternmost part of northern mounded part does not show the chaotic structure (figure 9). The structure is small and reaches until the seabed. Two mounded parts flank the structure in the south and in the north. figure 12 shows a SWW-NEE transect over the two elongated mounded parts along the Vernadsky Ridge. A mounded configuration is present along the south of the Vernadsky Ridge from subunit 4.3 on (figure 9 and 12). The onset of a small mounded part south of the chaotic structure is present at the base of subunit 4.4 (figure 7 and 8). A very shallow channel is present in the north of eacht mounded feature. In the northwestern part of the area, the reflectors rise and thin towards the Vernadsky Ridge (figure 6).

Figure 12: Seismic profile displaying the two mounded part features along the Vernadsky Ridge. The boundaries are given by the full red lines. The subunits are indicated by the black lines and the cylic subunits by the dotted black lines. CWC are given by the blue lines. The flow direction of the bottom currents are given by the bottom current indicators.

4. Results

Figure 13: Isopach map of unit 4. The unit thins towards the ridges. The thickest package fills up the deep in the south of the basin. A theoretical seismic velocity of 1700 m/s is used to calculate the thickness of the unit.

Unit 3 Unit 3 is bounded by boundaries B2 and B3. Boundary B3 is characterized by the sudden end of the presence of CWC’s in unit 4 and a more expressed development of a mounded part along the Vernadsky Ridge. Boundary 3 is erosive along the Renard Ridge near the two channels (figure 6 and 9). The CWC’s visible on the multibeam map in the Vernadsky mound province have their origin during unit 3 (figure 6). Unit 3 is characterized by uniform, low amplitude and thin reflectors interbedded with two high amplitude reflectors. A sequence of very high amplitude reflectors lays on top but is only visible in the centre of the basin. These characteristics of unit 3 are only clear in the eastern part of the basin (figure 7 and 9). This unit is characterized by similar, medium amplitude reflectors (figure 5 and 6) in the western part. Unit 3 is divided in two subunits. Unit 3.2 contains high amplitude reflectors, while the lower one contains low amplitude reflectors. The division can only be observed in the western part by correlating the reflectors using the transverse profiles (figure 18). Unit 3 has a maximum thickness of about 54 ms TWT. The thickest part of unit 3 is situated where the basin has its maximum depth (figure 14). Unit 3 thins towards the northwest and towards both ridges. One large fault interrupts the unit and ends at boundary B2. This is the only fault crossing unit 3. The fault is situated halfway between both ridges north of the Western Channel along the RND (figure 18). The mounded part that started forming along the northwestern edge of the Renard Ridge is developing in unit 3 and becomes more expressed (figure 5). The Western Channel (figure 6) eroded 15 ms TWT of unit 4. Subunit 3.2 is eroded by unit 2 on the flank of the channel. In the narrow part, sediment is still deposited upon the Renard Ridge. And a shallow mounded parted feature is further developing northeast of the flank. In the Eastern Channel (figure 9), erosion occurred below unit 3. In the western part of the Eastern Channel, only a slightly mounded feature is present along the Renard Ridge (figure 8). The mounded

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

part did not change its geometry since unit 4. The reflectors of unit 3 in the western part of the Eastern Channel are abruptly eroded for about 28 ms TWT until almost the lower subunit. Both channels along the Renard Ridge are clearly expressed on the depth map of boundary 3 (figure 17). Along the northwestern part of the Vernadsky Ridge the reflectors rise upwards and unit 3 reaches the seabed in the northeast (figure 6). The southern mounded part along the Vernadsky Ridge becomes more expressed in unit 3. A channel is forming south of the chaotic structure (figure 7 and 8). The northern mounded part along the Vernadsky Ridge was already present in the former unit and is developing into a broader mounded part in subunit 3.3. A channel is present just south of the Vernadsky Ridge (figure 9 and 12).

Figure 14: isopach map of unit 3. The thinning along the Renard Ridge is due to the erosion within the channel and the pinching out upon the Renard Ridge in the narrow part. Hemipelagic sediment is deposited along the Vernadsky Mound Province and indicated by the red ellipse. A theoretical seismic velocity of 1700 m/s is used to calculate the thickness of the unit.

Unit 2 Unit 2 is bounded by boundary B2 and boundary B1. Boundary 2 is erosive along the Western Channel (figure 6) and is characterized by a sudden broader southern mounded part and wider channel along the Vernadsky Ridge (figure 7 and 8). The unit has continuous but lower amplitude reflectors (compared to unit 3) in the lowest subunit (2.1). Subunit 2.2 has thick, high amplitude reflectors. The CWC’s that had their origin in unit 3 are still present in unit 2. The unit has a maximum thickness of about 56 ms TWT (figure 13). The thickest part is situated on top of the deepest part of the basin. The unit thins towards the northwest and towards both ridges. Along the northwestern edge of the Renard Ridge, the mounded part becomes broader and forms the current seabed southwest of the top of the mounded part (figure 5). In the Western Channel, unit 2 eroded the subunit 3.2. Towards the Vernadsky Ridge the reflectors are thinning and rising (figure 6). Sediment pinches out upon the narrow part of the Renard Ridge, gradually closer to the channel.

4. Results

Northeast of the narrow part of the Renard Ridge, the mounded part becomes more expressed, with thickening deposits towards the centre of the basin (figure 7). In the Eastern Channel the whole unit is fully eroded in the western part (figure 8). Less erosion is present towards the east (figure 9). The reflectors on the flank have the same angle as in unit 3 and only the bottom of the channel is eroded. Both channels along the Renard Ridge are visible on the depth map of boundary 2 (figure 17)

Figure 15: Isopach map of unit 2. The thinning along the Renard and Vernadsky Ridges is due to the pinching out and the erosion of the reflectors in the drift. Less sediment is deposited in the north of the area. Mainly hemipelagic sediment is deposited along the Vernadsky Mound Province, indicated by the red ellipse. A theoretical seismic velocity of 1700 m/s is used to calculate the thickness of the unit. Along the western part of the Vernadsky Ridge, the unit thins and rises towards the ridge (figure 6). The unit ends south of the CWC’s (figure 6) and is a part of the current seabed southwest of the Vernadsky Ridge and the Vernadsky MP. In the southeast of the area (figure 6 and 7), the chaotic structure suddenly shifts northeast towards the ridge and the sediments in the southern mounded part are deposited upon the structure and pinch out against the structure. The southern mounded part becomes broader and higher upwards (figure 7 and 8). The northern mounded part is only formed by subunit 2.1 and half of subunit 2.2. The rest of subunit 2 shows toplap in front of the mounded part (figure 9 and 12).

Unit 1 Unit 1 is separated from unit 2 by an erosive discontinuity, only visible in the outer east along the Vernadsky Ridge where the upper reflectors display toplap against the base of unit 1 (figure 9). In the rest of the area a non-erosive discontinuity occurs along the Vernadsky Ridge. The reflectors of unit 1 show onlap onto unit 2 along the Vernadsky Ridge with a very low angle progradational towards the

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

ridge. Along the Renard Ridge the reflectors are retrograding stepwise from the ridge. Unit 1 is characterized by thin and high amplitude reflectors. The unit has a maximum thickness of about 16 ms TWT(figure 16) and thins towards the ridges. The unit has its maximum thickness where the deepest part of the basin is located although the difference is rather small. In the northwest of the area (figure 5), sediment is only deposited in between the mounded part along the Renard Ridge and the Vernadsky MP . Further southeast (figure 6) sediment is partly deposited on the eastern flank of the channel along the Renard Ridge and shifts away from the channel upward in the stratigraphy (figure 6 and 7). Along the Vernadsky Ridge sediment is deposited just south of the southern mounded part. Only the uppermost sediments are deposited on top of the southern mounded part. The reflectors onlap on unit 2 and are deposited stepwise closer to the Vernadsky Ridge. The small mounded part in the Eastern Channel is deposited on top of unit 2 (figure 9). The boundary between unit 2 and the small mounded part is erosive. The lower reflectors of the small mounded part are deposited horizontally in the channel. A mounded part is stepwise formed in the upper part of the small mounded part (figure 9).

Figure 16: Isopach map of unit 1. Unit 1 is thinner along the ridges due to the prograding nature of the reflectors towards the Vernadsky Ridge and away from the Renard Ridge. Unit 1 is only slightly thicker in the east of the basin. Mainly hemipelagic sediment is deposited during unit 1. A theoretical seismic velocity of 1700 m/s is used to calculate the thickness of the unit.

4. Results

Figure 17: Depthmaps of boundaries B5 to B1. The depth is displayed in ms TWT. On the first four maps the infill of the deep basin is visible in the south of the area. The formation of the deep channels of the Renard North Drift is visible on the maps of boundary 3 and 2 (red ellipses). Sediment is deposited on the narrow and low part of the Renard Ridge, thinning upward. The height difference in the southeast of the basin is filled up at the end unit 2 (Boundary 1).

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

DISCUSSION

Sedimentary processes

Uplift and initiation The depocentre created by the rotated blocks, is filled by unit 6 with the deepest part of the basin situated in the south of the area (figure 17, boundary 5). During deposition the basin gradually flattens. The parallel reflectors suggest a hemipelagic sedimentation pattern, by the absence of a strong bottom current. Sediment particles settle and are syntectonically deformed by the two uplifted ridges evidenced by the pinching-out configuration and onlap of the reflectors against the steep slope of both ridges (figure 5 and 9). Several normal faults affect the area, mostly in the northwest of the area (figure 5, 6 and 18). A northwestward strike can be supposed with a steep dip (75°). A high is present in the outer northwest of the area (figure 5) and six faults are located above this high. The high is visible in the northeastern corner of the multibeam map (figure 4) and may be a compressional ridge such as the Renard and Vernadsky Ridges. This ridge may only have reached the seabed in a small area and stands out for a few tens of meters. The southern part of the ridge is covered with sediment of unit 6 (figure 5). A similar palaeohigh was discerned byVandorpe et al. (2014) along the PDE but is not present above the seabed. Vandorpe et al. (2014) states that the palaeohigh might be a rotated block. The pinching out configuration and onlap of the reflectors in unit 5 indicate the continuation of syn-lift deposition along the Renard and Vernadsky Ridge. The angle of onlap decreases, thus the rate of uplift decreased and ended at the top of the unit. The reflectors along the Vernadsky Ridge are steeper and pinch-out higher than along the Renard Ridge, indicating a greater uplift of the Vernadsky Ridge. Most faults end at the top of unit 5 (figure 18), coinciding with the end of the uplift of the two ridges in the El Arraiche area. The three faults close to the Renard Ridge mark the uplift of the ridge until unit 5 (figure 5). Van Rooij et al. (2011) and Vandorpe et al. (2014) suggest a similar syntectonic uplift of the PDE, gradually decreasing upwards in the stratigraphy. Vandorpe et al. (2014) observed the end of the uplift at the top of their unit 4 (figure 20). The Renard and Vernadsky Ridges are rotated blocks originating from compressional stress caused by the convergence of Africa and Eurasia in a northwest-southeast direction (Maldonado et al., 1999; Medialdea et al., 2009). Although the African-Eurasian convergence is still on-going, the compressional force uplifting the two ridges must have diminished throughout unit 5. The end of the uplift of the two ridges must have been a local effect in the El Arraiche field. Similar syntectonic sedimentation has been observed by Maad et al. (2010) east of the study area along the Larache ridge of which the Renard Ridge is an extension. In unit 5 a channel accompanied by a mounded part in the northeast develops along the steep promontories of the Renard Ridge (figure 6 and 9). Along the western promontory (figure 6), the reflectors start with an aggradational stacking pattern and develops to a more progradational stacking with an upward migration of the mounded part towards the ridge. The mounded part gets a more convex geometry upward in the stratigraphy. Along the eastern promontory (figure 9), a channel is formed in subunit 5.8, eroding the lower subunit. a mounded part is present in the north with prograding reflectors (figure 9). The reflectors are sub-parallel, but not parallel to the erosional surface below (Faugères et al., 1999). These characteristic lead to an interpretation of the deposits as an elongated, mounded drift (Faugères et al., 1999; Rebesco and Stow, 2001). The presence of a distinct erosional or non-depositional moat in the southwest marks a separated drift (Rebesco et al., 2014) such as the Faro drift along the northern margin of the Gulf of Cádiz (Brackenridge et al., 2013) and the Gijón and Le Danois drifts along the North Iberian margin (Van Rooij et al., 2010). Unit 5 is marked by several discontinuities, indicating the alternation of sedimentation periods and erosional or non-depositional periods, typical for contourite drift deposits (Faugères et al., 1999). The

5. Discussion discontinuities are marked by continuous, high amplitude reflector present in the whole accumulation (Faugères et al., 1999). An increase in bottom current velocity occurred during unit 5 indicated by the origin of a small mounded drift. Only along the two promontories (figure 4) of the Renard Ridge the bottom current velocity increased enough to create a mounded drift, as obstacles can locally accelerate the bottom current velocity (Viana, 2008; Garcia et al., 2009). Along the narrow part (figure 7), the reflectors are parallel and no indications for a contourite drift are present. Hemipelagic sediment was deposited upon the ridge in between both channels. The bottom current broadens in between both promontories and its velocity decreased. The presence of a stronger bottom current along the northwestern extent of the Renard Ridge (figure 5) is evidenced by the wavy configuration of the reflectors, becoming more horizontal during unit 5. The uplift of the diapiric ridges during unit 5 and 6 has an important influence on the bottom current flow and the geometry of the contourite drift (Rebesco and Stow, 2001). The formation of a contourite drift could only be formed when the Renard Ridge was sufficiently lifted as evidenced by Vandorpe et al. (2014) along the PDE.

drift development During unit 4 a mounded part develops along the whole length of the Renard Ridge with a channel in the southwest. The channel and mounded part are most expressed in the Western and Eastern Channel (figure 6 and 9). The mounded part is characterized by an upward migration and prograding reflectors and has an elongated geometry along the ridge. Elongated, mounded drifts are associated with bottom current velocities between 10 and 30 cm/s (Stow et al., 2008). A mounded drift thus develops (Faugères et al., 1999) along the whole length of the Renard Ridge due to the increasing bottom current velocity.

The development of the Renard North Drift (RND) (figure 4), the elongated, mounded drift along the Renard Ridge occurred in several steps. During subunit 4.1 the mounded drift becomes more expressed along the promontories (figure 6 and 9) and erosion occurs in the centre of the Eastern Valley (figure 9). Marking a first stage of increasing bottom current velocity. A mounded drift develops along the northwestern extent in subunit 4.2 (figure 5), evidenced by the transition of the wavy reflectors into a mounded and upward migrating part (Faugères et al., 1999). During subunit 4.2 and 4.3 the Eastern Channel elongates to the west (Figure 8) and an erosional surface occurs in the deepest part (figure 9). Two stages of increased bottom current velocity occur in subunit 4.2 and 4.3 and the velocity is sufficient along the northwestern extent to create a mounded drift.

In the upper subunit (4.4), the reflectors on the narrow part of the Renard Ridge thin towards the ridge and are deposited upon the slope (figure 7). Northwest of the flank a shallow mounded part is formed with slightly prograding reflectors and the mounded part is migrating towards the ridge. A shallow channel is present southwest of the mounded part. The sediment is deposited upon a gentle slope. These characteristics indicate a drift type between a mounded drift (Faugères et al., 1999) and a plastered drift (Rebesco et al., 2014). A plastered drift is formed on a gentle slope by relatively low velocity currents (Rebesco et al., 2014). Rebesco et al. (2014) point at ambiguous characteristics of a plastered drift, as is the case in this part. Some can be considered as sheeted drifts (Rebesco et al., 2014) whereas others are considered as mounded, elongated drifts (Stow and Faugeres, 2008). A plastered to mounded drift was identified in the Ortegal Spur depositional system (Hernandez-Molina et al., 2011). An erosional surface is present below subunit 4.4 in the Western Channel. During subunit 4.4 the bottom current is sufficient to create a drift along the whole Renard Ridge (figure 21). The drift takes off in the northwestern edge of the ridge (figure 5), accelerates around the western promontory (figure 6) and slows down when the ridge narrows (figure 7). The current accelerates again flowing along the eastern promontory (figure 8 and 9). Along the whole length of the ridge the bottom current velocity is strong enough to create a drift. The geometry of the Renard Ridge plays an important role in creating high energetic bottom currents (Vandorpe et al., subm.). The steep slopes (15 - >20°) along the promontories of the Renard Ridge create

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

narrower and centred currents. The curvature of the ridge on these places is higher as well, creating more turbulent flows. These two factors create together with the presence of a promontory (Rebesco and Stow, 2001) highly energetic bottom currents, able to erode deeply incised channels as is the case along the Renard Ridge. These factors caused the first appearance of the mounded drift along the promontories and affected the contourite drift since the Renard Ridge was sufficiently uplifted (Rebesco and Stow, 2001). Two deep channels are created since the third unit along the northern promontories of the Renard Ridge (figure 6, 9 and 17, boundary 3 and boundary 2). The channels are repeatedly incised at the boundary between the (sub-)units during unit 3 and 2 and again refilled. The reflectors steeply dip on the flank of the channel. The strong erosion may be induced by two processes. The first process is the presence of flow instabilities creating marginal valleys (Garcia et al., 2009). The bottom current along the Pen Duick escarpment interacts with the Renard Ridge. AAIW flows in a northeastward direction against the PDE where the bottom current is mainly deflected towards the northwest, creating the PDE drift (Vandorpe et al., 2014). The AAIW is also forced to flow over the Renard Ridge and rolls of the northeastern flank with high velocities and eddies and vortices are formed (figure 21). The vortices roll off the ridges on the northern flank of the ridge, eroding the sediment and creating marginal valleys (figure 6 and 9). Marginal valleys occur in the Northern Gulf of Cádiz, west of the Guadalquivir and Donana diapiric ridges in the Channels and Ridges sector (figure 1), due to the interaction with the MOW (Garcia et al., 2009). Marginal valleys are up to 28.4 km long and reach widths of up to 5 km in the Northern Gulf of Cádiz (Garcia et al., 2009). They can reach incision depths of more than 250 m with an U- or V shape. Although shorter and narrower valleys occur as is the case along the Renard Ridge (lengths of about 5 km and widths of 1.5-2 km with incision depths of 75-90 m). The height of the ridges in the Northern Gulf of Cádiz is up to 150 m (Garcia et al., 2009) whilst the Renard Ridge is about 100 m. In the Northern Gulf of Cádiz, truncated reflections occur on the lee side of the diapiric ridge (Garcia et al., 2009). This is not the case along the Renard Ridge where sediment is still deposited on the mound, albeit very steeply (figure 6 and 9). The production of AAIW is known to increase during glacial periods, such as the Last Glacial Maximum (Makou et al., 2010; Wainer et al., 2012) and is able to flow over the Renard Ridge (figure 21). A repetitive occurrence of these flow instabilities during strong glacial periods created deeply incised valleys along the northern flank of the Renard Ridge. The eroded valleys were again partly refilled during the interglacials. The Renard North Drift becomes more expressed during unit 3 and 2, indicating higher bottom current velocities. Besides the occurrence of two deep channels along the promontories, also the drift along the northwestern part becomes more expressed (figure 5). The plastered-mounded drift in the narrow part is still present and has a more expressed mound in the northeast of the ridge (figure 7) indicating a more vigorous bottom current. An increase in bottom current velocity (Wainer et al., 2012) causes besides the formation of marginal valleys also a more expressed contourite drift. Along the Vernadsky Ridge, a chaotic structure occurs in the southwest during subunit 4.2 (figure 7 and 8). The presence of sediment deposited upon the chaotic structure indicates the firm property of the structure. The semi-transparent reflection indicates the presence of a large CWC. A mounded feature is present on both sides of the CWC, each with a shallow channel in the northeast (figure 12). During unit 4, the reflectors show an aggradational stacking pattern in the southern mounded part (figure 7 and 12) and a slightly prograding, migrating upward reflections in the northern mounded part (figure 9 and 12). The northern mounded part is present since subunit 4.2, the southern mounded part since subunit 4.4. This marks the development of a smaller, mounded drift (Faugères et al., 1999) with two flow paths separated by the CWC, the Vernadsky South Drift (figure 4). The bottom current originally flowed along the Vernadsky Ridge forming the northern mound and split up in subunit 4.4 due to the growth of a CWC. Vandorpe et al. (2014) reported the presence of the bottom current along the PDE with two flow paths that split due to the presence of seven mounds. The Vernadsky South Drift has only small dimensions during unit 4, indicating a broad and slow bottom current. The bottom current has a sufficient velocity to create a contourite drift during subunit 4.2 with a northwestward direction, evidenced by the northeastern location of the moat along the mound. The

5. Discussion current is deflected towards the ridge by the Coriolis force causing a lateral migration of the reflectors, typical for drifts (Rebesco and Stow, 2001). Along the Vernadsky Ridge, the reflectors of the northern mounded part migrate upward in the third unit (figure 9). The northern mound becomes more expressed with a shallow moat in the northeast (figure 9). Subunit 2.1 forms the top of the northern mound with the same geometry as during unit 3 (figure 9 and 12) whilst subunit 2.2 toplaps in front of the northern mound (figure 9). In the southern mound (figure 7 and 8), The reflectors are aggradational in unit 3 and onlap on the CWC during unit 2. The southern mound and moat become more expressed during unit 3 and suddenly enlarge in unit 2. Sediment is still deposited until the top of unit 2. The CWC shifts towards the northeast in unit 2 (figure 7 and 8). The mounded drift along the Vernadsky Ridge becomes thus more expressed during unit 3, indicating an increase in bottom current velocity. Although the moat along the northern mound is small, indicating a relative slow bottom current velocity. During unit 2, gradually less sediment is deposited on the northern mound, whilst the southern mound enlarges. This indicates a shift of the bottom current to one stronger flow path south of the CWC and the deceleration or absence of a current north of the CWC. In the northwestern part of study area, sediment is gradually deposited further away from the Vernadsky Ridge during the third and second unit (figure 6). A mounded part and channel lacks, illustrating a mainly hemipelagic deposit along the northwestern Vernadsky Ridge by the absence of strong bottom currents (figure 11 and 13-16) The development the Renard North Drift and Vernadsky South Drift coincides with CWC growth in the basin. Both phenomena’s depend on an increase in bottom current velocity (Wienberg et al., 2010; Rebesco et al., 2014). The subunits of unit 4 accord with four phases of CWC growth and a second phase of CWC growth starts in unit 3, forming the Vernadsky Mound Province (figure 4 and 6).

Tidal currents The reflectors in unit 2 are retrograding away from the channel along the Renard Ridge (figure 6 and 7) and prograding along the Vernadsky Ridge (figure 6 and 7). The upper reflectors of the unit are deposited on the southern mound along the Vernadsky Ridge. A different sedimentary situation occurs along both ridges with a decreasing sedimentation close to the Renard Ridge and an increasing sedimentation towards the Vernadsky Ridge. The reflectors are subparallel, mainly due to hemipelagic sedimentation.

The retrograding nature of the reflectors along the Renard Ridge marks an increasing bottom current velocity, preventing the hemipelagic sediment to settle gradually more northward (figure 6). An increasing bottom current velocity along the Vernadsky Ridge eroded the reflectors of unit 2 south of the CWC (figure 7) and prevents sediment to settle northeast of the southern mound. The Onlapping reflectors along the Vernadsky Ridge are created by a small flow path, slightly eroding the hemipelagic deposits. Chen et al. (2014) observed several small erosional features along a seamount, created by the differentiation of the bottom current into different flow paths. Although the features in Chen et al. (2014) are more deeply eroded, a similar Onlapping configuration is present. The flow path gradually becomes more focused towards the Vernadsky Ridge, enabling the hemipelagic sediment to settle closer to the ridge (figure 6 and 7). In the northwest of the area sediment is only deposited in between the two mounded parts along both ridges (figure 5). The reflectors are parallel. This indicates a hemipelagic deposit in the northwest. The bottom current along the Renard Ridge prevents the sediment to settle. In the Eastern Channel a very strong erosion occurred during unit 1. In the west of the Eastern Channel (figure 8) the reflectors of unit 2 were not steeply dipping and thus not affected by the high velocity currents whilst the reflectors in the east did (figure 9). The reflectors in the west of the upper three units are sharply truncated (figure 8). In the east, only subunit 3.2 and unit 2 are truncated (figure 9). The channel has a V-shape in the west, typical for a fast and deeply incising current. The change towards a very strong bottom current was abrupt in the west of the Eastern Channel. The abrupt change to a strong erosive regime can be attributed to semi diurnal internal tides that are

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

currently present in the area (figure 2) and flow in a southeast-northwest direction, perpendicular to the Moroccan coast (Mienis et al., 2012). The internal tides are reported down till 500 m. Average current speeds are around 10 cm/s along the PDE in an east-west direction with peak currents up to 25 cm/s (Van Rooij et al., 2011). The current is forced to flow between the Renard Ridge and Adamastor MV causing a large acceleration of the bottom current velocity between the features (Vandorpe et al., subm.). According to Vandorpe et al. (subm.) the tidal currents are stronger in a southeast direction, evidenced by a deeper channel along the north flank of the MV in the east of the area (figure 4 and 21). These MV are as well infected by the tidal currents (Vandorpe et al., subm.). A hypothesis for the presence of a small mounded drift in the eastern channel (figure 9) is the combination of a stronger southeastward current with the bottom current along the Renard Ridge, causing the small mounded drift in the channel. The mounded drift is build op by the bottom current and is not fully eroded by the weaker northwestward tidal current. Although both channels along the Renard Ridge have similar dimensions, they have different origins. The Western Channel has gradually developed towards a deeply incised marginal valley during the upper 3 units as was the eastern part of the Eastern Channel, preceded by an elongated, mounded drift development. The western part of the Eastern Channel on the other was rapidly incised and prolonged towards the west by tidal currents in the upper unit, also affecting the eastern part of the Channel.

Chronostratigraphy and palaeoceanographic comparison

The depositional history of the El Arraiche mud volcano field has known several changes in its sedimentation pattern. The first initiation of a contourite drift along the Renard Ridge started in unit 5 but the major transition to a bottom current-controlled deposition along the whole length of the Renard Ridge started in unit 4. The transition is marked by discontinuity B4. Several episodes of acceleration and deceleration of the bottom current occurred in the next three units (unit 4-2), indicated by the erosional surfaces in the deep channels along the Renard Ridge and the refill of the channels. The contourite drift along the Vernadsky Ridge only develops during unit 4 and has its major growth in the 3rd unit. The southern mound along the Vernadsky Ridge expands in the 2nd unit whilst the northern mound, closest to the ridge diminishes and has its end in subunit 2.2, when the bottom current velocity was not sufficient to retain the drift deposits of the northern mound. A chronostratigraphy is constructed (figure 19) by comparing the results of this study with the (chrono- )stratigraphy of the Pen Duick Drift (Vandorpe et al., 2014) and nearby regions (Hernandez-Molina et al., 2002) (figure 20). The marine isotopic stages (MIS) discerned by Lisiecki and Raymo (2005) are correlated with the subunits (figure 19). Eight rhythmic subunits are discerned in unit 5 (figure 19). Vandorpe et al. (2014) distinguished eight subunits with the same characteristics (few high amplitudes below and low amplitudes above) in unit 4 (figure 20). The discontinuity below the cyclic unit has been associated to the base Quaternary Discontinuity (BQD) by Vandorpe et al. (2014) with an age of 2.588 Ma (Gibbard et al., 2010). This age was correlated using the pattern of mud extrusions at the foot of the Gemini MV starting at the base of unit 4 in Vandorpe et al. (2014). Boundary B5 corresponds thus with the BQD (figure 19). This implies that unit 6 in this study has a Pliocene age while the upper 5 units have a Quaternary age. Before the BQD hemipelagic sediment covers the basement on both sides of the Renard Ridge (Vandorpe et al., 2014) and was syntectonically uplifted and deformed during the Pliocene (figure 20). Since the BQD, a sheeted drift occurs along the PDE (Vandorpe et al., 2014) whilst a mounded drift is present along the northern promontories of the Renard Ridge since subunit 5.5 (figure 20). This marks an increase of the bottom current velocity along the Renard Ridge corresponding with the onset of glaciations producing more vigorous bottom currents, as well corresponding with the uplift of the PDE, sufficient to create topography-controlled contour currents (Vandorpe et al., 2014). A sheeted drift occurred along the PDE before the Renard North Drift developed. This is due to the bottom current immediately deflected against the whole length of the PDE but not sufficient to form a drift after its deflection to the northern flank of the Renard Ridge. A mounded drift was only formed along the Renard

5. Discussion

Ridge due to the geometry of the promontories when the Renard was sufficiently uplifted (Rebesco and Stow, 2001; Vandorpe et al., subm.). The end of the cyclic unit coincides with the end of the uplift of the Renard Ridge. According to Vandorpe et al. (subm.) the uplift of the Renard Ridge has ended at the Middle Pleistocene Revolution (MPR) (0.920 Ma). As the deposits of the Renard North Drift are no longer affected by uplift from unit 4 onwards, B4 accords to the MPR (figure 20). The MPR forms the transition between 41 ky and 100 ky dominated glacial-interglacial cycles, with a more intensified glaciation and a dramatic sharpening of the contrast between warm and cold periods (Maslin and Ridgwell, 2005). Before the MPR the glacial-interglacial cycles responded to the obliquity periodicity of 41 ka (Imbrie et al., 1992). Maslin and Ridgwell (2005) state that eccentricity is not the primary driver of the 100 ky climate cycles, but that they are more closely linked to precession cycles. The periodicity is dominated by four or five cycles. The MPR is the last ‘major’ event in a trend towards a more intense global glaciation during the last few tens of millions of years and the most important palaeoceanographic change in the North- Atlantic Ocean (Marino et al., 2009) since the Quaternary. Eight cycles are distinguished between the MPR (0.920 Ma) and the BQD (2.6 Ma) in the study area and along the PDE by Vandorpe et al. (2014). Hernandez-Molina et al. (2002) discerned a 200 ka cyclicity in the Alboran Sea and the Northern Gulf of Cádiz. The internal stacking pattern in these sequences is dominated by 41 ka obliquity cycles generating small sea-level fluctuations. Four 200 ka cycles between 1.8 Ma and the MPR have been discerned in that region (Hernandez-Molina et al., 2002). This cyclicity can be extended until the most recent BQD of 2.6 Ma, which yields eight 200 ka cycles. This is consistent with the eight cycles distinguished in unit 5. The high amplitudes at the start of each cycle (figure 18 and 19) may indicate an initiation during glacial periods without deposition (Faugères et al., 1999). Non-deposition occurred during the glacials due to more vigorous bottom currents (Makou et al., 2010; Wainer et al., 2012) preventing high sedimentation rates. Although Hernandez-Molina et al. (2002) states that the majority of the sediment in the Alboran Sea and the northern Gulf of Cádiz is deposited during lowstand periods. Vandorpe et al. (2014) assumes an increased sedimentation during glacial periods along the PDE, based upon the observations of Hernandez-Molina et al. (2002). In this study, an increased sedimentation is proposed during interglacials, concomitant with lower current velocities. During glacial periods, the production of AAIW is higher (Makou et al., 2010; Wainer et al., 2012) and causes a more vigorous bottom current along the Renard and Vernadsky Ridge. Few sediment is deposited due to the higher velocities and erosion occurs during the most severe glacials. The same changes in deposition occur along the PDE and thus the dating of the discontinuities discerned in the stratigraphy by Vandorpe et al. (2014) are shifted with one MIS more towards an glacial stage to compare both stratigraphies along the Renard Ridge (figure 20). The MPR is accompanied by an increase in sedimentation rates in many contourite systems (Van Rooij et al., 2011; Roque et al., 2012; Müller-Michaelis et al., 2013). High sedimentation rates are often evidenced by high amplitude reflections (Hernandez-Molina et al., 2006; Llave et al., 2007; Van Rooij et al., 2007).Vandorpe et al. (2014) did observe higher amplitudes from the MPR on along the PDE, coinciding with the development of a sheeted drift. In this study, this observation is true in the northwestern part of the El Arriache field (figure 5 and 6) and the outer south (figure 9), but is less clear in the lower three subunits (4.1-4.3) of the central part (figure 7 and 8). Although the thickest packages are deposited in the southeast during unit 4 (figure 17). This corresponds with the absence of a contourite drift in the centre, where the deposits are mainly hemipelagic and filling up the low in the basin. The strong reflectors correspond with the presence of a contourite drift in the east and west along the Renard Ridge. Subunit 4.4 has higher reflectors along the whole length of the Renard Ridge (figure 18) coinciding with the presence of a mounded drift along the whole length of the ridge. The sedimentation and erosional patterns in the upper four units are influenced by climatic variations controlled by a 100 ky glacial-interglacial cycle since the MPR (Maslin and Ridgwell, 2005). In unit 4 the subunits are each separated by an erosional boundary below the Eastern Channel (figure 9) and coincide with four stages of CWC growth and decline. The subunits have high amplitude reflectors below, marking discontinuities (Faugères et al., 1999). Glacials are known to induce strong bottom

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

currents (Makou et al., 2010; Wainer et al., 2012), preventing high sedimentation rates, and able to erode the deposits. This strengthening of the bottom current is also indicated by the development of a mounded drift along the Vernadksy Ridge. CWC’s have their growth during glacials (Wienberg et al., 2009), and the growth start at the boundary between the subunits. This points at a glacial cycle in between the subunits and the deposition of the subunits during interglacials. Each subunit of unit 4 corresponds with one glacial-interglacial cycle, starting at the MPR (MIS 22) (figure 20). Unit 4 thus comprises MIS 22-15 . Vandorpe et al. (2014) discerned unit 3 with the same age constraint and a similar mounded drift development. CWC’s occur since the MPR along both sides of the ridge. The buried mounds along the PDE occurred between about 900 and 600 ka, the same age range of the CWC in this study (920-575 ka). Although the CWC’s in this study occur mostly along the Vernadsky Ridge and occurred in 4 stages of growth and decline. The same glacial-interglacial cycle is discerned during unit 3 and 2. The erosional phases in the marginal valleys along the Renard Ridge occurred during the glacial periods. The AAIW production is higher in glacials periods (Makou et al., 2010; Wainer et al., 2012), causing more vigorous bottom currents, able to flow over the Renard Ridge (figure 21). Unit 3 and 2 each include two glacial-interglacial cycles (MIS 14-11 and MIS 10-7) corresponding to one subunit per cycle. This coincides with the age constraint of unit 2 and the lower units of unit 1 along the PDE (Vandorpe et al., 2014) (figure 20). Subunit 3.2 has thick and very high amplitude reflectors, corresponding with MIS 11 and 12. This glacial-interglacial cycle is the most severe warming and cooling of the Late Pleistocene (Hernandez-Molina et al., 2002). MIS 12 is marked by a large glaciation, important changes in oceanic circulation and sea-level fall (Chaisson et al., 2002; Thunell et al., 2002; Head and Gibbard, 2005). However MIS 12 is not present as a strong erosional boundary in the area (figure 18 and 19) but the reflectors have very high amplitudes. Boundary B2 shows a strong erosion in the Western Channel and corresponds with MIS 10. Subunit 2.1 corresponds with MIS 9 and is comparable to the present interglacial conditions (Roque et al., 2012). Boundary B1 is marked by a change in depositional environment with toplap along the Vernadsky Ridge (figure 8) and non-deposition in the moat along the Renard Ridge. Non-deposition is typical for glacial periods. This boundary corresponds with MIS 6, known as the second most important glaciation during the Quaternary after MIS 12 (Ehlers and Gibbard, 2007). MIS 6 is present as an erosional surface along the Vernadsky Ridge but not in the PDE drift (Vandorpe et al., 2014). After B1, the bottom current velocity gradually strengthened. The deposition in unit 1 starts with MIS 5, comparable with the present interglacial conditions (Roque et al., 2012). The increasing bottom current velocity accords with a gradual cooling, during MIS 5-2 (figure 19) with a sudden change to an interglacial stadium in the Holocene (MIS 1) depositing the upper layers upon the southern mound of the Vernadsky South Drift. Mainly hemipelagic sediment settles in unit 1 due to an gradual change to a strong glacial in MIS 2 without an abrupt change towards a strong interglacial period in MIS 3 (figure 19). The similarities in the depositional history of the Renard North Drift, Vernadsky South Drift and Pen Duick Drift (Vandorpe et al., 2014) lead to the conclusion that the three contourite drifts are influence by the same water mass(-es), supposedly the AAIW (Vandorpe et al., 2014). The sedimentation rate for every discerned unit is calculated (table 1), based on the tentative chronostratigraphy and the measured thickness of the unit (figure 11, and 13-16). A theoretical seismic velocity of 1700 m/s is used for the conversion into metric scale. In the period between the BQD and MPR, hemipelagic sediment was deposited on an average rate of 4.8 cm/ka in the area, this is close to the range of a theoretic value for hemipelagic deposition of 5-15 cm/ka (Stow and Tabrez, 1998). For the three units above the average sedimentation rate is about 16 cm/ka. This rate is within the range of the theoretic values for mounded sediment drifts (5-30 cm/ka) (Stow et al., 2008) and similar as the average sedimentation rate obtained by Vandorpe et al. (2014) of 17 cm/ka in the upper 3 units from the MPR. A slower sedimentation rate of 4.4 cm/ka is obtained for the upper unit comprising the last 6 MIS due to the domination of hemipelagic sedimentation.

5. Discussion

Table 1: sedimentation rate in the El Arraiche mud volcano field during each unit. The conversion into metric scale is based on a theoretical seismic velocity of 1700 m/s within the sediment. The maximum sedimentation rate occurred in the southeastern centre of the area.

Max. sed rate Average sed. rate Unit Age (cm/ka) (cm/ka) Unit 1 MIS 6-1 7.5 4.4 Unit 2 MIS 10-7 28.2 15.4 Unit 3 MIS 14-11 26.0 15.6 Unit 4 MIS 22-15 32.8 17.9 Unit 5 BQD- MPR 7.7 4.8

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

Figure 18: transverse profile of the study area, parallel and centered between both ridges (figure 3). Faults are indicated by the dashed black lines. boundaries are given by the red lines. Subunits are marked by the full black lines and cyclic subunits by the dotted lines. CWC’s are indicated by the blue lines.

38 5. Discussion

Figure 19: schematic overview of figure 18: transverse profile of the study area, parallel and centered between both ridges (figure 3). faults are indicated by the dashed black lines. boundaries are given by the red lines. subunits are marked by the full black lines and cyclic subunits by the dotted lines. cwc’s are indicated by the blue lines.. MIS: Marine Isotopic Stage. The δ18O stack and correlated MIS labels from Lisecki and Raymo, (2005).

39 Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

A comparative study between the depositional history of the Pen Duick Drift and other contourite depositional systems along the upper continental slopes of the Northeast Atlantic ocean (the Cádiz CDS and Le Danois CDS) was done by Vandorpe et al. (2014). This comparison is extended with the Ceuta drift in the western Alboran Sea (figure 20) (Juan et al., 2012; Somoza et al., 2012). The Cádiz CDS and Le Danois CDS are MOW influenced (Van Rooij et al., 2011; Roque et al., 2012) while the Ceuta drift is influenced by the Western Mediterranean Deep Water (WMDW), flowing below 300 m along the Moroccan margin in the Alboran Sea (Somoza et al., 2012). The boundaries are as well compared to the stratigraphy of the Cádiz CDS and the Alboran Sea set up by Hernandez-Molina et al. (2002). A comparison with an AAIW influenced drift is however not possible due to a lack of palaeoceanographic knowledge of the AAIW and the location of the drifts in the southern hemisphere (Martinez-Méndez et al., 2013). The drifts along the continental slopes of the Northeast Atlantic ocean and in the Alboran Sea are present since the Late Pliocene Revolution (LPR) while the drifts in the El Arraiche field are only present since the Quaternary (figure 20). The sheeted drifts in the Cádiz CDS are an initial phase of deposition under bottom current activity, correlated with the first stages of enhanced MOW activity at about 3.5 Ma. The BQD is marked with a great outbuilding of the plastered drifts in the Ceuta drift (Ercilla et al., 2002) and by the buildup of mounded drifts in the Cádiz CDS and Le Danois CDS (Van Rooij et al., 2011; Roque et al., 2012) due to the strengthening of the MOW. The MPR is a strong boundary with an intense growth phase of the mounded drift deposits in the Cádiz and Le Danois drift (Van Rooij et al., 2010; Llave et al., 2011; Roque et al., 2012; Brackenridge et al., 2013) and a continuation of the contourite deposits dominating the sedimentation in the Alboran Sea (Juan et al., 2012) (figure 20). In the El Arraiche field, the MPR marks a transition to a general appearance of mounded drifts (figure 20). Both the MPR and BQD strongly changed the climate worldwide and influenced thus both the AAIW and MOW Although the main discontinuities (BQD and MPR) overlap between the El Arraiche field drifts and the MOW and WMDW influenced drifts, a distinct depositional history is present in both systems (figure 20). Since the MPR, two intensification stages are observed in the El Arraiche field at MIS 14 and MIS 10 whilst only one intensification stage occurs in the Cádiz CDS and the Ceuta drift (figure 20) at MIS 12 (Llave et al., 2007; Llave et al., 2011; Roque et al., 2012; Somoza et al., 2012). A last intensification stage is only observed in the Renard North Drift and in the Ceuta Drift (Somoza et al., 2012) at MIS 6 (figure 20). MIS 6 was as well observed along the Algarve margin by Marchès et al. (2010). All of the stages from both systems are observed in the stratigraphy of the Alboran Sea and northern Gulf of Cádiz (Hernandez-Molina et al., 2002).

5. Discussion

Figure 20: comparison of the evolution of the RND and VSD with the PDE drift (Vandorpe et al., 2014) and with clear MOW- controlled drifts in the northern Gulf of Cádiz (Roque et al., 2002; Brackenride et al., 2013), the Bay of Biscay (Van Rooij et al., 2010) and the Alboran Sea (Juan et al., 2012; Somoza et al., 2012). The outer right column shows a stratigraphy constructed by Hernández-Molina et al. (2002) in the Alboran Sea and Northern Gulf of Cádiz. Table adapted from Vandorpe et al., 2014. The three strong erosive events are indicated by the wave symbol. BQD: Base Quaternary Discontinuity, MPR: Middle Pleistocene revolution, LPR: Lower Pliocene Revolution, MIS: Marine Isotopic Stage. Figure adapted from Vandorpe et al., 2014. Red boxes indicate hemipelagic sedimentation. The yellow, green and blue boxes indicate respectively sheeted, mounded and plastered drifts. Based upon the absence of MOW in the PDE area and the differences in evolution with MOW-controlled drifts, The MOW is not likely to be the driving force in the formation of the PDE drift (Vandorpe et al., 2014). Certainly not during glacial periods as the MOW flows even deeper (Garcia et al., 2009). The same arguments are valid for the El Arriache field, indicating the absence of MOW as driving force for the drift formation. Vandorpe et al. (2014) point at the AAIW as a more plausible candidate for the formation of contourite drifts in the area. CTD data from Van Rooij et al. (2011) and Mienis et al. (2012) indicated the presence of AAIW at the foot of the Renard Ridge and the presence of NACW at the top. NACW is present at the foot of the mud volcanoes in the southeast, closer to the Moroccan margin and located in a shallower water depths (<600 m). The bottom current in the El Arraiche field is high on nutrients (Si, PO4, NH3 and NOx), indicating the presence of AAIW in the study area as well (Vandorpe et al., subm.). The mud volcanoes in the El Arraiche field towards the Moroccan margin are influenced by NACW, containing less nutrients (Vandorpe et al., subm.). Both water masses flow in a northward direction towards the El Arraiche field. Vandorpe et al. (subm.) suggests a mixing zone of both water masses located along the Renard Ridge at a depth of about 600 m. The AAIW is an important player in the global ocean circulation, although its past variability is poorly understood (Martinez-Méndez et al., 2013). Although the influence of AAIW during glacials is assumed, its the northern extent of the glacial AAIW is not yet fully known (Oppo and Curry, 2005, 2012) and its presence north of 27° is not yet proven. Strong semi-diurnal tides flow in a northwest- southeast direction since MIS 6 and are the dominant bottom water currents on the Renard Ridge (Mienis et al., 2012) and in the El Arraiche field. Such bottom currents could be generated by the interaction of the interface between AAIW and NACW with the Renard Ridge (Van Rooij et al., 2011). Internal tides are generated at the interface between the MOW and the Eastern North Atlantic Water (ENAW) in the Belgica mound province (Porcupine Seabight)

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

(Van Rooij et al., 2007; White, 2007). The semi-diurnal tides are responsible for the abrupt erosion between the Adamastor MV and the Renard Ridge. The observations in the previous sections combined with the oceanographic measurements of Vandorpe et al. (subm.) have some (palae-)oceanographic implications for the El Arraiche mud volcano field. The flow directions of the bottom currents and internal tides are summarized in figure 21. The bottom currents creating the RND and VSD are influenced by the Renard and Vernadsky Ridge. Both contourites are thus good examples of topography-controlled drifts and have a similar sedimentary history as the Pen Duick Drift, influenced by the AAIW and NACW.

Figure 21: Overview of the currents in the El Arraiche mud volcano field (modified from Vandorpe et al. (subm.)). The AAIW (Antarctic Intermediate Water) and NACW (North Atlantic Central Water) flow in a northward direction (Vandorpe et al., subm.) and are deflected by the Renard Ridge. A mixing zone is present at a depth of about 600 m. Internal tides are indicated by the yellow lines (Mienis et al., 2012). The contour currents are indicated by the red arrow. The relative flow velocity is indicated by the opacity (the strongest current is most opaque). The dotted blue lines are only present during glacial periods. The red dashed line are only present during certain periods. The green arrows indicated tidal induced contour currents.

6. Conclusion

CONCLUSION

The El Arraiche mud volcano field comprises several contourite drift systems (Vandorpe et al., subm.) whereof the Renard North Drift and the Vernadsky South Drift are studied in this master thesis. Both drifts were described based upon several high-resolution Sparker single channel seismic profiles and multibeam data. A sedimentary evolution and palaeoceanography of the drifts was drafted. Six conclusions may be drawn from this study. 1. The Renard North and Vernadsky South Drifts are excellent examples of topography-controlled drifts where slow bottom currents are accelerated due to the interaction with an obstacle. A contourite drift is build up along the foot of the diapiric ridges. The appearance and expression of the mounded drift is strongly influenced by the geometry of the ridge. 2. The first appearance of a contourite drift occurred in the El Arraiche mud volcano field since subunit 5.5 (1.8 Ma). An elongated mounded drift was deposited along the whole length of the Renard and Vernadsky Ridge since the Middle Pleistocene Revolution (U4). Since the MPR a gradual increase in bottom current velocity is inferred. 3. A transition to a severe erosion took place in unit 3 with the creation of two deep valleys. The origin of these valleys can be contributed to two processes. The first is the erosion of marginal valleys during glacial periods. The second is the creation of a more vigorous bottom current due to the geometry of the ridge. 4. The presence of internal tides is proven in the area (Mienis et al., 2012) and creates a deep eroded channel along the eastern promontory of the Renard Ridge where the internal tide is forced between the Adamastor mud volcano and the Renard Ridge. 5. Deposition occurred mainly during interglacials when the bottom current velocity decreased. During glacial periods erosion or non-deposition dominates. 6. The sedimentary history of the study area and the PDE drift is parallel and influenced by the same water masses. The contour currents in the El Arraiche mud volcano field have a mixed AAIW and NACW-origin. Evidence for the presence of both water masses is given by Vandorpe et al. (2014 and subm.). The presence of MOW in the area is not evidenced.

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

DUTCH SUMMARY

Het doel van deze master thesis is het achterhalen van de sedimentaire geschiedenis van het El Arraiche mud volcano gebied tussen de Renard en Vernadsky Rug door het bestuderen van de contourietafzettingen langsheen beide ruggen. Aan de hand van de afzettingsgeschiedenis wordt vervolgens getracht een (paleo-)oceanografische reconstructie van het gebied op te stellen en te vergelijken met de Pen Duick Drift. Deze kennis zal bijdragen bij het beter begrijpen van obstakel- gerelateerde contouriet drifts en hun relatie met diepwater ecosystemen. Contourieten zijn sedimenten die afgezet of verplaatst zijn door bodemstromingen die voornamelijk langsheen de continentale helling stromen (Stow et al., 2002b; Rebesco et al., 2005; Stow and Faugeres, 2008). Contourieten komen abundant en in een breed gamma van mariene milieus voor. Een contouriet drift is het sedimentaire deel van een contouriet, opgebouwd door de bodemstromingen (Rebesco et al., 2007). Contouriet drifts kunnen uiteenlopende morfologieën vertonen, voornamelijk beïnvloed door geologische en oceanografische setting. Behalve afzetting vindt er ook erosie of non-depositie met een uiteenlopende morfologie plaats binnen een contouriet (Garcia et al., 2009). Het onderzoek naar contourieten is op verschillende vlakken van belang. Variaties in de sedimentaire afzettingen, vaak gelieerd aan de eigenschappen van contourieten (Solheim et al., 2005), kunnen een invloed hebben op de stabiliteit van de helling. De afzetting van dikke, uniforme zandafzettingen zijn van belang als reservoir gesteente in de petroleum industrie. Fijnkorrelige contouriet afzettingen kunnen als afsluitingsgesteente dienen (Viana et al., 2007). Als laatste bevatten contouriet afzettingen gedetailleerde paleoceanografische en paleoklimatologische informatie dankzij hun continue en snelle accumulatiesnelheid (Hernandez-Molina et al., 2003). Een groep van verschillende contouriet driften en hun erosiestructuren vormt een Contourite Depositional System (CDS) (Herandez-Molina et al., 2009). In de noordelijke Golf van Cádiz komt een CDS voor door de interactie van de Mediterranean Outflow Water (MOW) met de complexe zuidelijke continentale rand van het Iberische schiereiland (figure 1). Sedimentaire drifts komen voor in de Cádiz CDS sinds de Lower Pliocene Revolution, 4.2 Ma ( Hernandez-Molina et al., 2006) en zijn sinds het Pleistoceen sterk beïnvloed door de glaciaal-interglaciaal cyclus. De zuidelijke Golf van Cádiz is minder bestudeerd dan het noordelijke deel. De aanwezigheid van een CDS is daar minder plausibel door de afwezigheid van MOW in het grootste deel van het gebied. De Golf van Cádiz maakt deel uit van de Atlantische oceaan en is begrensd door de Marokkaanse continentale rand in het zuiden en het Iberische schiereiland in het noorden. De Golf van Cádiz is gesitueerd op de grens van de Afrikaanse en Euraziatische tektonische plaat. Deze tektoniek heeft de geologische geschiedenis van het gebied sterk beïnvloed sinds het Trias (Medialdea et al., 2004), beginnend met de opening van een smalle zeegang tussen de Atlantische oceaan en de Neothethys. De zuidelijke continentale rand werd gevormd in het Jura door de splitsing van Gondwana en Laurussia (Ziegler, 1999). Drie opeenvolgende fasen worden onderscheiden in de tektonische geschiedenis sinds het Midden-Krijt (Maldonado et al., 1999). Achtereenvolgens wordt eerst de noordelijke continentale rand gevormd tot in het Vroeg-Cenozoïcum, vervolgens vindt van het Laat-Eoceen tot het Vroeg- Mioceen een noord-zuid convergentie plaats tussen het Iberische schiereiland en Afrika. Een derde fase startte na het Vroeg-Mioceen wanneer de subsidentie versnelde en een accretie wig en een olistostroom eenheid ontstond ten westen van de Straat van Gibraltar. Ten slotte startte een schuine en trage noordwest-zuidoost convergentie in het Laat-Mioceen. Neogene sedimenten van 0,2 tot 2 km dik bedekken de olistostroom en zijn doorboord door moddervulkanen, zoutpilaren en andere stroompaden voor vloeistoffen (Pinheiro et al. 2003; Medialdea et al., 2004). De meeste moddervulkanen komen voor in het Betic-Rifian domein en zijn gegroepeerd. Moddervulkanen worden aangedreven door koolwaterstoffen die naar het oppervlak migreren, vaak geholpen door de aanwezigheid van breuken (Pinheiro et al., 2003; Van Rensbergen et al., 2005). Uitbarstingen van moddervulkanen zijn meestal episodisch. Het El Arraiche mud volcano gebied is gesitueerd in het zuidelijke deel van de Golf van Cádiz op de accretie wig maar ten zuiden van de olistostroom eenheid (figuur 2). Het gebied is begrensd door twee

7. Dutch summary parallelle ruggen, de Renard Rug in het westen en de Vernadsky Rug in het oosten. De ruggen zijn geflankeerd door zeer steile breukhellingen. De Pen Duick Escarpment (PDE) is de westelijke helling van de Renard Ridge. Een contouriet drift komt voor langsheen de PDE en werd uitvoerig beschreven door Vandorpe et al. (2014). Extensionele krachten creëerden grote, gedraaide blokken verbonden door lystrische breuken. De gedraaide blokken vormen de ruggen zoals Renard en Vernadsky Rug. De bekkens tussen deze ruggen werden opgevuld met Plio-Pleistocene afzettingen (Flinch, 1993). Verschillende kleinschalige contouriet driften komen voor in het noordelijke deel van het El Arraiche mud volcano gebied (Vandorpe et al., in prep) (figuur 4). De Pen Duick Drift langsheen de PDE, de Renard South drift langs de zuidwestelijke flank van de Renard rug, ten noorden van de PDE drift. De Renard North Drift ligt langs de noordoostelijke flank en aan de zuidelijke flank van de Vernadsky rug is de Vernadsky South Drift gelegen. Veel koud-water koralen zijn gelegen op de ruggen en rondom de moddervulkanen. Vijf gebieden met koud-waterkoralen worden onderscheiden (Foubert et al., 2008, Van den Berghe, 2015). De groei van koud-water koralen hangt sterk af van de aanvoer van sediment in suspensie in de waterkolom en komen bijgevolg voornamelijk voor in gebieden met sterke bodem stromingen waar vaak contouriet drifts ontstaan (Huvenne et al., 2003). Koud-water koralen komen zowel tijdens glacialen als interglacialen voor, maar hebben een voorkeur voor de glaciale periodes (Foubert et al. 2008, wienberg et al., 2010). De oceanografie in de Golf van Cádiz (figuur 2) is sterk bepaald door de uitwisseling tussen de Middellandse zee en de Atlantische oceaan en de topografie van het gebied. Het oppervlaktewater stroomt wijzerzin (Pelegri et al., 2005). Daarnaast komen ook kustwaarts gerichte stromingen voor net onder het zee oppervlak die zuidwaarts weg stromen langsheen de kust (Machin et al., 2006). De oceanografie van het zuidelijke deel van de golf is net zoals de geologie minder bestudeerd. Vier verschillende watermassa’s komen voor in het zuidelijke gebied (Vandorpe et al., 2014). Het North Atlantic Surface Water bevindt zich bovenaan (NACW, 0-100m) en kan beschouwd worden als gemodificeerd North Atlantic Intermediate Water door de interactie met de atmosfeer. Het North Atlantic Central Water (NACW) bevindt zich op een diepte van 100 tot 600 m. De temperatuur daalt naar onder toe. Het NACW bevat weinig nutriënten bovenaan maar het neemt naar onder toe (Louarn and Morin, 2011). NACW stroomt in drie richtingen (Machin et al., 2006): langsheen de kust in de Straat van Gibraltar, Afgebogen naar het zuiden langsheen de Afrikaanse kust en westwaarts door de incorporatie in het Mediterrane water. Antarctic Intermediate Water (AAIW) bevindt zich onder de NACW op een diepte van 600 tot 1500 m. AAIW heeft een lage saliniteit en temperatuur (Louarn and Morin, 2011). De AAIW stroomt langsheen de Afrikaanse kust in de Golf van Cádiz waarna ze westwaarts wordt gestuwd door het Mediterrane water (Louarn and Morin, 2011). Het Mediterranean Outflow Water (MOW) stroomt vanuit de straat van Gibraltar waarna het zinkt door zijn hoge saliniteit, ondanks de warme temperatuur. De MOW volgt de noordelijke topografie in de Golf van Cádiz (Madelain, 1970). Het MOW bevindt zicht tussen de 500 en 1400 meter (Hernandez-Molina et al., 2011). De MOW stroomt niet in het zuidelijk deel van de Golf van Cádiz, al is het bestaan van meddies gekend die MOW zuidwaarts vervoeren (Ambar et al., 2008). De MOW was sterker en stroomde op grotere diepte tijdens glacialen. In de diepere delen (> 1500 m) bevindt zich het North Atlantic Deep Water (Louarn and Morin, 2011), gekenmerkt door zeer lage temperaturen en een hoge saliniteit. Het El Arraiche mud volcano field komt voor van een diepte van 700 meter tot aan de shelf (Vandorpe et al., 2014). Aan de voet van de PDE komt het AAIW voor en NACW stroomt langs de top van de PDE en aan de voet van de moddervulkanen dichter bij de shelf. In deze studie worden seismische profielen gebruikt die werden verzameld tijdens de CADIPOR II campagne in 2005 en de COMIC campagne in 2013. Al de seismische profielen werden bewerkt in RadexPro en geïnterpreteerd in Kingdom Suite. Drie verschillende fases worden onderscheiden in de afzettingsgeschiedenis van het studiegebied die gelijkaardig verloopt als de PDE drift. In een eerste fase wordt enkel hemipelagische sediment afgezet die syntectonisch werd vervormd door de opheffing van de ruggen (unit 5 en 6). Door de opheffing ontstaan ook normale breuken met een helling van ongeveer 75° (figuur 18) en een noord-westwaartse

Contourite depositional systems in the El Arraiche area, Moroccan Atlantic margin

strekking. De opheffing loopt door tot na de Quartair grens (2,588 Ma) en eindigt samen met de Mid Pleistocene Revolution (MPR) (0,920 Ma). Het einde van de opheffing is een lokaal effect aangezien de compressionele krachten die het opheffen van de ruggen heeft veroorzaakt nog steeds doorgaat met Afrikaans-Euraziatische convergentie (Medialdea et al., 2009). De Quartair grens duidt op de versterking van de glacialen. De productie van AAIW stijgt tijdens de glacialen en ook het MOW versterkt tijdens de glaciale periodes. Dit gaat samen met het ontstaan van een sheeted drift langsheen de PDE (Vandorpe et al., 2014) en een onderbroken mounded drift langsheen de noordelijke flank van de Renard rug (figuur 6 en 9) (unit 5). Twee mounded drifts zijn aanwezig ten noorden van de noordelijke uitlopers die de bodem stroming versterken en de vorming van een contouriet mogelijk maken. Daarnaast is ook de aanwezigheid van een bodemstroming te herkennen langsheen het noordelijkwestelijke einde van de Renard rug, al is een contourite drift afwezig (figuur 5). De bodemstroming gaat in een noordwestelijke richting langsheen de PDE waarna ze door de corioliskracht afbuigt omheen de Renard rug en in een zuidoostelijke richting langs de noordflank stroomt (figuur 21). De MPR gaat samen met een nieuwe versterking van het contrast tussen de glacialen en interglacialen en bijgevolg een krachtigere bodemstroming. Na de MPR ontwikkelt zich een mounded drift langsheen de PDE (Vandorpe et al., 2014) en de langsheen de volledige lengte van de Renard rug (unit 4), enkel ten noorden van het smalle gedeelte tussen de twee diepe kanalen ontstaat pas na MIS 16 een plastered drift. Tijdens de glacialen is de sedimentatie traag door de krachtigere bodemstromingen die meer sediment in suspensie houden. Tussen de MPR en MIS 14 komen vier fases van koud-water koraal groei en afname voor die wijzen op gunstige omstandigheden voor koud-water koraal groei. Langsheen de Vernadsky Rug ontstaat een mounded drift na de MPR die door een bodemstroming wordt veroorzaakt met twee stroompaden en in een noordwestelijke richting stroomt. De bodemstroming wordt gesplitst door de aanwezigheid van een grote koud-water koraal die aanwezig is sinds subunit 4.2. Na MIS 14 versterkt de bodemstroming langsheen de Renard Rug verder en worden twee diepe kanalen gecreëerd langsheen de twee noordelijke uitlopers van de Renard rug (figuur 4) (unit 2 en 3). Erosie vindt voornamelijk plaats tijdens de glacialen en gebeurt op tweeërlei manieren. Als eerste is de rug op deze plaats steiler wat samen met het uitsteken van de rug zorgt voor een versmalling van de stroming. Dit gaat gepaard met een onregelmatige geometrie van de helling die zorgt voor turbulente stromingen. Deze drie factoren zorgen er voor dat een sterke erosie mogelijk is. Daarnaast stroomt een krachtigere AAIW tijdens glacialen over de Renard rug. De stroming wordt instabiel en vortices rollen met een hoge snelheid naar beneden waar ze een diepe erosie mogelijk maken en zodoende een marginal valley vormen. De twee drifts langsheen de Vernadsky Ridge versterken eveneens na MIS 14. De zuidwestelijke mound groeit sterk na MIS 9/10 waardoor de CWC zich terugtrekt naar het noordoosten. De noordoostelijke mound eindigt tijdens MIS 7/8 door een te zwakke bodemstroming. Een tweede fase van koud-water koralen groei begint na MIS 14 en vormt de huidige mound provinces herkenbaar op de multibeam kaarten zoals de Vernadsky mound province. Een laatste fase start na MIS 6 (unit 1). Tijdens de laatste fase komen dagelijkse getijden stromingen opzetten die een belangrijke rol spelen in het gebied en in een noordwest-zuidoostelijke richting stromen, waarbij de stroming in een zuidoostelijke richting het sterkst is. De getijden stromingen worden langsheen het zuidoostelijke deel van Renard Rug en de Adamastor moddervulkaan gedwongen waardoor een zeer sterke erosie mogelijk is. Tijdens unit 1 wordt voornamelijk hemipelagisch sediment afgezet. De bodemstroming langsheen de Renard Rug wordt geleidelijk aan sterker. Langsheen de Vernadsky rug ontstaan de derde kleiner stroompad ten Zuidwesten van de zuidelijke mound (figuur 21). De afzettingsgeschiedenis (figuur 20) in het studiegebied is gelijkaardig aan de afzettingen langsheen de PDE (Vandorpe et al., 2014). Door het feit dat de MOW niet aanwezig is in het gebied en enkel de grote klimatologische veranderingen (BQD en MPR) overeenkomen met de CDS beïnvloedt door de MOW (Cádiz CDS, Le Danois CDS en de Ceuta drift) ligt een AAIW of NACW invloed waarschijnlijk aan de basis van de afzettingsgeschiedenis in het gebied. CTD data van Van Rooij et al. (2011) en Mienis et al. (2012) wijzen op de aanwezigheid van AAIW aan de voet van de PDE alsook de hoge nutrient concentratie (Vandorpe et al., in prep). De NACW stroomt over de zeebodem dichter naar de

7. Dutch summary continentale rand. Mogelijks is Een mixing zone van deze twee watermassa’s is verantwoordelijk voor de contouriet drifts in het gebied (Vandorpe et al., subm.). De Renard North Drift en Vernadsky South Drift zijn topografie-gecontroleerde contouriet drifts die een gelijkaardige afzettingsgeschiedenis vertonen met de Pen Duick Drift, beïnvloedt door de mixing zone van AAIW en NACW die in een noordelijke richting stromen en tegen de PDE afbuigen.

8. References

REFERENCES

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