Contourite depositional system after the exit of a strait: Case study from the late Miocene South Rifian Corridor, Morocco

Contourite depositional system after the exit of a strait: Case study from the late Miocene South Rifian Corridor, Morocco WOUTER DE WEGER* , FRANCISCO JAVIER HERN  ANDEZ-MOLINA*, OLMO MIGUEZ-SALAS†, SANDRA DE CASTRO* , MIGUEL BRUNO‡, DOMENICO CHIARELLA* , FRANCISCO JAVIER SIERRO§, GRAHAM BLACKBOURN¶and MOHAMED AMINE MANAR** *Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK (E-mail: Wouter[email protected]l.ac.uk) †Departamento de Estratigraf ıa y Paleontolog ıa, Universidad de Granada, Avd. Fuentenueva s/n, Granada, 18002, Spain ‡CACYTMAR, Universidad de C adiz, Campus de Puerto Real S/N, Puerto Real, C adiz, 11510, Spain §Departamento de Geolog ıa, Universidad de Salamanca, Plaza de los Ca ıdos, Salamanca, 37008, Spain ¶Blackbourn Geoconsulting, 26 East Pier Street, West Lothian, Bo’ness, EH51 9AB, UK **Office National des Hydrocarbures et de Mines (ONHYM), 34, Avenue Al Fadila, Rabat, BP 99, Morocco Associate Editor – Christian Betzler ABSTRACT Idealized facies of bottom current deposits (contourites) have been established for fine-grained contourite drifts in modern deep-marine sedimentary environments. Their equivalent facies in the ancient record however are only scarcely recognized due to the weathered nature of most fine-grained deposits in outcrop. Facies related to the erosional elements (i.e. contourite channels) of contourite depositional systems have not yet been properly established and related deposits in outcrop appear non-existent. To better understand the sedimentary facies and facies sequences of contourites, the upper Miocene contourite depositional systems of the South Rifian Corridor (Morocco) is investigated. This contourite depositional system formed by the dense palaeo- Mediterranean Outflow Water. Foraminifera assemblages were used for ageconstraints (7.51 to 7.35 Ma) and to determine the continental slope depositional domains. Nine sedimentary facies have been recognized based on lithology, grain-size, sedimentary structures and biogenic structures. These facies were subsequently grouped into five facies associations related to the main interpreted depositional processes (hemipelagic settling, contour currents and gravity flows). The vertical sedimentary facies succession records the tectonically induced, southward migration of the contourite depositional systems and the intermittent behaviour of the palaeo-Mediterranean Outflow Water, which is mainly driven by precession and millennial-scale climate variations. Tides substantially modulated the palaeo-Mediterranean Outflow Water on a sub-annual scale. This work shows exceptional examples of muddy and sandy contourite deposits in outcrop by which a facies distribution model from the proximal continental slope, the contourite channel to its adjacent contourite drift, is proposed. This model serves as a reference for contourite recognition both in modern environments and the ancient record. Furthermore, by establishing the hydrodynamics of overflow behaviour a framework is provided that improves process-based interpretation of deep-water bottom current deposits. 1 ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Sedimentology (2021) doi: 10.1111/sed.12882 Keywords Bottom currents, channels, contourites, deep-water sedimentation, Late Miocene, Morocco, Rifian corridors, tides. INTRODUCTION Over the last decade there has been a significant increase in published evidence of deep-marine bottom current processes and associated deposits, contourites. Most of these works however are based on two-dimensional and three-dimensional seismic profiles (Faug eres et al., 1999; Paulat et al., 2019), some from wells and cores (Gonthier et al., 1984; de Castro et al., 2020a,b; Hovikoski et al., 2020), but there is still very little information from the exposed ancient record. Sedimentation in deep-marine environments is predominantly controlled by pelagic, gravitational and contouritic processes, or their mixed occurrence (Fonnesu et al., 2020; Shanmugam, 2020; Stow & Smillie, 2020). When bottom currents represent the dominant depositional process, a contourite depositional system (CDS) develops (Hern andez-Molina et al., 2008). Currently, there is no widely accepted model for such systems, but they are generally composed of a combination of depositional (related to low current velocities) and erosional (related to high current velocities) elements. Both elements result from the hydrodynamic behaviour of water masses that reach and interact with the seafloor and are conditioned by bathymetry (Hern andez-Molina et al., 2006). As a result of the differentiation between areas prone to low current and high current velocities, contourite deposits are generally divided into muddy contourites (fine-grained) or sandy contourites (coarse-grained) (Stow & Faug eres, 2008). Most of the evidence used to identify contourites is based on the large morphological features of depositional elements (Faug eres et al., 1999; Hern andez-Molina et al., 2008), particularly on the large depositional contourite drifts, with less attention on the possible erosional contourite elements. Accordingly, the only generally accepted contourite facies model is the bi-gradational model (Faug eres et al., 1984; Gonthier et al., 1984; Stow & Faug eres, 2008) defined for finegrained contourite drifts. Therefore, it is often wrongly assumed that bottom currents only generate muddy contourite deposits, disregarding the fact that some authors highlighted the occurrence of sandier sediments deposited or reworked by the action of bottom currents. These sandier sediments are mainly found within contourite erosional elements, such as, channels (Hern andez-Molina et al., 2014a,b; Capella et al., 2017a; Brackenridge et al., 2018; de Weger et al., 2020), contourite terraces (Viana et al., 1998; Hern andez-Molina et al., 2009, 2016a,b, 2018; Mutti et al., 2014; de Castro et al., 2021) or in mixed turbidite and contourite depositional systems (Rebesco et al., 2002; Creaser et al., 2017; Sansom, 2018; Fonnesu et al., 2020; Fuhrmann et al., 2020; Rodrigues et al., 2021). The natural evolution of a CDS comprises the lateral migration of both depositional and erosional elements. This implies that, for example, drifts develop on top of channels (Chen et al., 2020) or channels erode into drifts (Llave et al., 2001; Hern andez-Molina et al., 2008, 2014b; Chen et al., 2020). This migration has been identified using seismic data, but the sedimentary facies and facies sequences related to these migrating environments are currently not established. This is a partial consequence of the lack of recognized CDS in outcrop which show lateral and vertical facies variability. Contourite outcrops, as mentioned earlier, have however only been scarcely recognized. This likely results from the lack of distinct diagnostic contourite features, hindering the scientific community to identify these deposits in outcrop (H€ uneke & Stow, 2008; Rebesco et al., 2014; Shanmugam, 2017). Furthermore, the problematic identification of muddy contourites in outcrop, for which most diagnostic criteria (the bi-gradational model and morphological features) exists, results from their relatively homogeneous and severely bioturbated nature (Gonthier et al., 1984), but also because most fine-grained outcrops are usually severely weathered. The recognition of sandy contourites, for which no true diagnostic criteria exist, is mainly hindered by the process-based interpretation of deep-marine sediments. There is a general idea that sandy deposits only enter the deep-marine realm by gravitational processes, and it is regularly wrongly assumed that deepmarine bottom currents are only related to thermohaline circulation, not reaching current velocities capable of reworking, transporting and ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology 2W. de Weger et al. depositing sandy sediment. The poorly understood hydrodynamic properties of deep-marine water masses that reach and interact with the seafloor stand in the way of such process-based interpretation of bottom current deposits. This paper investigates the sedimentary record of a well-exposed late Miocene CDS in the Saiss Basin, associated with the South Rifian Corridor of Morocco (Fig. 1). The main objectives of this study are to: (i) determine the geometries, sedimentary facies and facies sequences related to the lateral migration of the CDS; (ii) interpret its evolution; (iii) investigate the dynamics of overflow behaviour in a confined basin; and (iv) provide the much-needed clarification on the role of bottom currents to improve process-based interpretation for deep-marine contourites. The results presented herein will thus also serve as a reference for the recognition of contourite deposits. STUDY AREA AND GEOLOGICAL SETTING The studied sections are exposed in the Saiss Basin in northern Morocco (Fig. 1). This basin is part of the external zone of the Rif-Betic Arc, or Gibraltar Arc which forms an arc-shaped orogenic belt surrounding the Alboran Sea in the westernmost Mediterranean region. The Betic and Rifian corridors were late Miocene marine gateways that allowed Mediterranean–Atlantic water exchange (Fig. 1). The Rifian corridors evolved during the latest stage of Africa–Iberia collision in the late Tortonian (ca 8 Ma) as south-westward migrating foreland basins (Feinberg, 1986; Wernli, 1988; Sani et al., 2007). These foreland basins were limited northward by the earlier exhumed Rif orogenic wedge (Iribarren et al., 2009) and southward by the Atlas Mountains (Barbero et al., 2011). The corridor is generally divided into two strands (Fig. 1A); the North Rifian Corridor (NRC) related to the Rifian Intramontane basins, and the South Rifian Corridor (SRC) related to the Taza-Guercif and Saiss basins (Wernli, 1988). Both strands were separated by the chaotic complex of the accretionary wedge (Fig. 2), the emplacement of which over the African foreland started during the early Tortonian (Feinberg, 1986; Flinch, 1993; Chalouan & Michard, 2004; Michard et al., 2008). Since the accretionary wedge is locally overlain by Upper Miocene marine sediments, the Rifian corridors were at times a single wide gateway westward of the Taza Strait (Fig. 1). The central portion of the Taza Strait was characterized by a sill (the Taza Sill) (Flecker et al., 2015; Capella et al., 2017a; de Weger et al., 2020), which formed a submerged topographic high related to the east–west oriented thrust-front overlying the north-east/south-west oriented Middle Atlas. The Taza Strait separated the Taza–Guercif and the Saiss basins and the Taza Sill controlled the water mass exchange between the Mediterranean and the Atlantic during the late Miocene (Capella et al., 2017a; de Weger et al., 2020). The westernmost part of the Rifian corridors was in the Gharb Basin where both the Rifian Intramontane and Saiss basins merged (Sani et al., 2007). The Gharb Basin was located just west of the Prerifian Ridges (Figs 1A and 2) which form the arcuate shaped southernmost leading edge of the Rif Chain. The development of the Prerifian Ridges mainly took place during two phases (Rold an et al., 2014). The initial development occurred during the middle to late Miocene, accompanied by the southwestward gravitational emplacement of the accretionary wedge (Capella et al., 2017b). The second phase of compressional deformation happened during the late Tortonian to early Messinian during which the Prerifian Ridges were formed. These ridges represented a tectonically uplifting, likely subaqueous relief on the northern margin of the South Rifian Corridor during the Tortonian (Rold an et al., 2014; de Weger et al., 2020). Lithostratigraphy of the Saiss Basin The middle Miocene–late Pliocene Saiss Basin fill stratigraphy (Fig. 3) overlies a major angular unconformity. The basal foredeep sediments are mainly divided into two formations: (i) the Benni Ammar White Marl Formation, which pre-dates the emplacement of the accretionary wedge (Fig. 3); and (ii) the Blue Marl Formation, mainly post-dating its emplacement. Within the Blue Marl Formation up to 120 m thick sands were locally deposited (Capella et al., 2017a; de Weger et al., 2020). The accretionary wedge, consisting of Triassic to late Miocene tectonostratigraphic units, developed in the study area during the late Tortonian to Messinian (Sani et al., 2007) coinciding with the onset of Rifian Corridor sedimentation that started around 8 Ma (Wernli, 1988; Krijgsman et al., 1999; Gelati et al., 2000; Hilgen et al., 2000; Barhoun & Taoufio, 2008; Achalhi et al., 2016). On top of the Tortonian Blue Marls, mainly limited to the south Saiss Basin but also locally along the ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Tidal modulated laterally migrating contourite depositional system 3 northern margin of the Saiss basin, early Messinian to middle Pliocene shelfal and nearshore marine marls and sandstones are deposited (Capella et al., 2018). These shallow-marine deposits are in turn locally truncated by an erosional unconformity and overlain by lacustrine limestones (Taltasse, 1953; Capella et al., 2018). PALAEOCEANOGRAPHIC SETTING During the late Miocene, an Atlantic –Mediterranean connection existed through the Betic Corridor in southern Spain and the Rifian corridors in northern Morocco (Fig. 1A). After subduction of the accretionary wedge, as well as of the external Betic and Rif fold–thrust belts largely came to a halt in the late Tortonian (van Hinsbergen et al., 2014), tectonic uplift first took over in the Betic at ca 7.8 Ma (Betzler et al., 2006; Krijgsman et al., 2006) and slightly later, by ca 7 Ma, in the Rif orogeny (Capella et al., 2017b; Tulbure et al., 2017). All the Betic – Atlantic –Mediterranean connections were closed due to Africa–Iberia convergence by the early Messinian (Spakman et al., 2018). By ca 7.2 Ma the North Rifian Corridor was closed (Tulbure et al., 2017) but the South Rifian Corridor was still open during the earliest Messinian recording a transition to continental and lacustrine deposits by ca 6.9 Ma (Capella et al., 2017a). Since there is no evidence of a Fig. 1. (A) Satellite terrain image of the westernmost Mediterranean region, southern Spain and north-west Africa. The globe on the right-hand corner shows the location of the study area and depicts the thermohaline circulation pattern (red –shallow; blue –deep), the orange arrow indicates the pathway of the Mediterranean Outflow Water (MOW). In transparent blue a late Miocene palaeogeographic overlay of the Betic and Rifian corridors, NRC =North Rifian Corridor and SRC =South Rifian Corridor, after de Weger et al. (2020). The location of the main geological features, such as the Gharb, Saiss and Taza–Guercif basins, the Prerifian Ridges and the Taza Strait are indicated. (B) Satellite image showing the location of the study areas. The abbreviations stand for the section names; FN =Fes-north and EA =El Adergha. (C) Late Miocene reconstruction through the SRC, crosssection A–A’, of which the location is indicated in (A). The SRC accommodated Atlantic–Mediterranean exchange of surficial Atlantic and Mediterranean Deep Water. ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology 4W. de Weger et al. Mediterranean–Atlantic gateway through the Betic and Rifian corridors during the Messinian, the Gibraltar Corridor arguably became the sole Atlantic gateway during this period (Krijgsman et al., 2018). Between ca 5.97 Ma, the time of the onset of the Mediterranean Salinity Crisis (Hs€ uet al., 1973; Ryan & Hs€ u, 1973), and ca 5.6 Ma, the Gibraltar Corridor accommodated a two-way connection (Simon & Meijer, 2017) after which it only facilitated Mediterranean inflow until ca 5.33 Ma (Krijgsman et al., 2018). After the closure of all but one of the Betic corridors in the late Miocene, the North Rifian Corridor maintained the inflow of Atlantic water in the Mediterranean (Tulbure et al., 2017) whereas the South Rifian Corridor maintained Atlantic–Mediterranean water exchange similar to what is currently occurring in the Strait of Gibraltar (de Weger et al., 2020). The South Rifian Corridor likely accommodated inflow of the North Atlantic Surficial Water (NASW) and the Eastern North Atlantic Central Water (ENACW) into the Mediterranean, overriding a warm and highly saline water mass associated with the palaeo-Mediterranean Outflow Water (MOW). The water mass of the palaeo-MOW was formed due to net evaporation and cooling in the eastern Mediterranean (Fig. 1B). This net evaporation and cooling increased the density of Mediterranean surface water (Straume et al., Fig. 2. Regional geological map of the study area including the locations of the studied Upper Miocene outcrops; El Adergha (EA), Fes-north (FN) and Ain Kansera (EA) (modified after Saadi et al., 1980). Carb. Fact =carbonate factory, L =late, M =middle and E =early. Satellite images are derived from Google Earth©. Fig. 3. Middle Miocene to late Pliocene lithostratigraphy of the Saiss Basin. The intercalated sandstone bodies in the late Tortonian to Messinian Blue Marl Formation belong to the sections studied herein. ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Tidal modulated laterally migrating contourite depositional system 5 2020) which was ‘continuously’ replenished by cold and less saline Atlantic water. Subsequently, this increase in density forced this water to sink and ventilate the water column, a process known as intermediate and deep-water formation (Millot, 1999; Candela, 2001). The formation of Mediterranean Deep Water (MDW) (and thus the formation of water masses related to the palaeo-MOW) resulted in a significant density gradient, or water mass stratification, between the Mediterranean and the Atlantic. This density gradient drove water mass exchange by two-way flow (Rohling et al., 2015; Simon et al., 2017). During the late Miocene a dense palaeo-MOW flowed over the Taza Sill through the Taza Strait (Fig. 1) downward into and through the South Rifian Corridor towards the Atlantic (de Weger et al., 2020). METHODOLOGY Two well-exposed late Miocene outcrops, El Adergha and Fes-north (Figs 1, 2, 4A and 4B), from the South Rifian Corridor are described in detail in this paper to unravel the facies successions and their vertical and lateral changes. The El Adergha section has previously partly been described by Capella et al. (2017a) and their results have been taken into consideration. The El Adergha section has been however reinterpreted and has been put into a new palaeogeographic framework based on the results of this study. A third section, Ain Kansera (Fig. 2), previously studied by Capella et al. (2017a), has also been reviewed and considered for discussion. Because present observations and interpretations do not significantly divert from those made previously, the Ain Kansera section is not described in detail in this work. The sedimentary successions were studied by standard field techniques which include bedscale characterization of sedimentological and stratigraphic elements. Two stratigraphic sections were measured at centimetre to metrescale to document the key features such as lithology, grain-size and sorting, sedimentary structures, bedding thickness, nature of bed contacts and palaeocurrent indicators. These features form the basis for the facies analysis. Sixty-four palaeoflow indicators (crossstratification, ripple lamination and sole marks) were recorded across both outcrops. A bed-by-bed ichnological analysis was conducted. The distribution, types and abundance of trace fossils were characterized to describe stratigraphic trends throughout the sections. Ichnological observations focused on orientation, shape, length and diameter of individual burrow segments, configuration of burrow systems and taphonomy, allowing for ichnotaxonomical assignation. Seven samples for petrographic analysis (Table S1; Figs S1 and S2) were derived from indurated sand beds. Two sets of thin sections were prepared for each sample, one of which was impregnated with dyed resin to highlight porosity. The samples were examined under a Nikon Optiphot-pol petrographic microscope (Nikon, Tokyo, Japan) with integral Canon EOS- 50D camera system (Canon Inc., Tokyo, Japan). Modal analysis was carried out on three samples (EA3, FN1 and EA5) by determining the composition at 300 points using a stepping stage and associated PETROG TM software (Table S1). Eight samples for biostratigraphy, derived from marls that were more than 50 cm below the rock’s exposed surface, were analyzed. Three samples for the El Adergha section were compared to what has been published by Capella et al. (2017a) to integrate their results. Five new samples from the Fes-north section were compared to the biostratigraphic framework published in Capella et al. (2017a) and Tulbure et al. (2017) to date these samples based on quantitative changes in abundance of keeled and unkeeled Globorotalids. The palaeowater-depth was inferred from the benthic foraminifera assemblages of the biostratigraphic samples. The specific assemblages and relative abundance of benthic species were associated with depth ranges identified in the existing literature (P erez-Asensio et al., 2012). The mixed occurrence of shallow-water and deep-water species is considered the result of downslope transport and, hence, the deeperwater species are deemed most reliable in depositional depth estimates. RESULTS Studied sections The sections, El Adergha and Fes-north, comprise sandstone-rich intervals intercalated within the Blue Marl Formation (Figs 4 and 5). The sections are located 9.6 km apart (Figs 2 and 5). The El Adergha outcrop (34.0764024.860522) is located on the northern flank of the Saiss Basin ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology 6W. de Weger et al. (Figs 1 and 2B), 10 km ENE of the city of Fes and 7.5 km south-east of the westernmost expression of the Prerifian Ridges Jbel Zalagh. The section forms a topographic high with an up to 15 m thick sandstone body at its peak (Fig. 4A). Below the sandstone an up to 1000 m thick fine-grained succession related to the Blue Marl Formation occurs. The south-western flank is steeply inclined to near vertical whereas the northeastern flank shows a much shallower inclination. The upper sandy part of the outcrop forms a gently sloping concave geometry which measures roughly 330 m (west–east) by 100 m (north– south) in a horizontal plane (Fig. 4A). The Fes-north section (34.0763374.964840) is located north of the city of Fes, 3 km south of Jbel Zalagh and ca 7 km west of the El Adergha section (Figs 2B and 4B). The Fes-north section forms a north-east/south-west striking topographic sandy ridge that is well-exposed for roughly 100 m horizontally and 7 m vertically (Fig. 4B). The present-day shape of this outcrop is due to the soil and vegetation cover and does not represent the primary lithosome profile of the outcrop in cross-sectional view. The sandstone ridge overlies a succession of marlstone pertaining to the Blue Marl Formation. Age and depositional domain The studied outcrops show a range of facies related to different depositional environments, as such, the age and depositional domain estimates will be treated individually for each section. The biostratigraphic results from the El Adergha section indicate a depositional period between 7.51 Ma and 7.25 Ma for the uppermost 100 m of the section (Fig. 5). The sand-rich interval at the top, consisting of the uppermost 28 m (Figs 5 and 6) has been dated between 7.35 Ma and 7.25 Ma based on the common occurrence of Globorotalia menardii 5. This Fig. 4. Panoramic view of the El Adergha (A) and Fes-north (B) outcrops. These pictures highlight the general exposed geometries and the scale of the outcrops. Due to the location of the Fes-north section being far away from the nearest vantage point no better pictures are currently available. ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Tidal modulated laterally migrating contourite depositional system 7 species appears frequently between its first common occurrence at 7.35 Ma and the replacement of Globorotalia menardii by Globorotalia miotumida at 7.25 Ma (Sierro, 1985; Sierro et al., 1993, 2001; Hilgen et al., 2000; Capella et al., 2017a). Benthic foraminifer assemblages indicate a depositional domain in the slope for the blue marl package (250 to 400 m water depth) and an upper slope depositional domain (300 to 140 m water depth) for the sand package. These results are supported by the findings of Capella et al. (2017a). Most of the collected samples in the Fes-north section contain abundant planktonic foraminifera. In general, keeled globorotalid forms are scarce, but Globorotalia menardii 5 is common in some samples and Globorotalia menardii 4 is usually rarer. Based on this, this section is dated as being between the last common occurrence of Globorotalia menardii 4 at 7.51 Ma and the replacement of Globorotalia menardii by Globorotalia miotumida at 7.25 Ma (Sierro, 1985; Sierro et al., 1993, 2001; Hilgen et al., 2000; Capella et al., 2017a; Tulbure et al., 2017). The continuous presence of dominant sinistral specimen of Globorotalia scitula, including Globorotalia suterae led to a determination of an age older than 7.28 Ma, which is the age at which coiling in this group changed from sinistral to dextral (Sierro et al., 1993). The benthic foraminifer assemblages indicate a depositional domain in the slope with abundant deeper water taxa such as Lagena,Syphonina,Gyroidina,Melonis, etc., mixed with shallow water benthic species such as Elphidium,Ammonia,Nonion,Lobatula, etc. The percentage of planktonic foraminifera relative to benthic foraminifera is on average higher than 60%. Some samples contain reworked specimens from the Eocene to middle Miocene. Palaeogeographic reconstruction based on obtained ages and depositional domains de Weger et al. (2020) recently described a contourite channel system and the intermittent behaviour of palaeo-Mediterranean Outflow Water (palaeo-MOW). The system described therein is located north of the Prerifian Ridges, compared to the outcrops described here (Figs 1 and 2). Furthermore, the deposits on which their interpretation is based are dated between 7.8 Ma and 7.51 Ma and are thus older than the deposits described in this study (7.51 to 7.25 Ma). Although attributed to a different stratigraphic interval, they are thought to have likely been formed by similar deep-marine processes controlled by the overflow of the late Miocene palaeo-MOW. Fig. 5. Sedimentary logs for the Ain Kansera (modified after Capella et al., 2017a), Fes-north and El Adergha sections. Their location is provided in Fig. 2, and their relative distance, and distance to the Prerifian Ridges is indicated. Palaeocurrents are divided in two major components, along-slope Pc 1 (red) and down-slope Pc 2 (green). The main sedimentary features are indicated next to the log. HCS =hummocky cross-stratification, CS =cross-stratification, BG =bi-gradational and TD = turbidite (see text for interpretation). ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology 8W. de Weger et al. The geographical offset between the contourite depositional system north of the Prerifian Ridges (de Weger et al., 2020) and the El Adergha and Fes-north sections (Figs 1 and 2) has been related to the tectonic emplacement of these ridges. As described by Rold an et al. (2014), the Prerifian Ridges evolved since the late Miocene to the present. The northern channels were located on the frontal part of the accretionary wedge and recorded sedimentation until 7.51 Ma; however, once the Prerifian Ridges became more prominent during the late Tortonian, around 7.51 Ma, the main palaeo- MOW pathway was forced south of these ridges (Fig. 5), abandoning the northern channels. Further evidence for these changes is found in the Ain Kansera section described by Capella et al. (2017a). The Ain Kansera section, dated between 7.51 Ma and 7.35 Ma, consists of shallow-marine infralittoral deposits, and is located near the easternmost physical expression of the Prerifian Ridges, closer compared to the El Adergha and Fes-north sections (Fig. 5). The Ain Kansera section has previously been interpreted as a north-eastward prograding linear clastic coast with depositional water-depths ranging between 15 m and 100 m (Capella et al., 2017a). Based on the palaeogeographic reconstruction performed by Capella et al. (2017a), the thickness of underlying deposits for each section to the nearest expression of the orogenic/accretionary wedge and the African margin (Fig. 5), and the depositional domain obtained from benthic foraminifer assemblages, the Prerifian Ridges most likely formed a north–south oriented slope on the northern margin of the South Rifian Corridor (Fig. 6). The southern boundary of the South Rifian Corridor was located near the city of Sefrou (Capella et al., 2018), roughly 30 km south of the Ain Kansera section, indicating that between 7.51 Ma and 7.25 Ma the corridor was approximately 30 km wide. The Taza Sill was located 90 km eastward of the studied sections and the westernmost expression of the Prerifian Ridges is located roughly 85 km towards the west. This implies that the steeply flanked South Rifian corridor measured roughly 30 km in width and 175 km in length, westward of Taza Strait (Fig. 6). Sedimentary facies Nine different sedimentary facies (F1 to F9) and associated trace fossils have been distinguished in the two studied sections (Table 1; Figs 7 to 9). The sedimentary facies include: (i) F1 – fossil-rich blue marlstone; (ii) F2 –sandy marlstone; (iii) F3 –bi-gradational sandstone; (iv) F4 –heterolithic mudstone and sandstone; (v) F5 – heterolithic, cross-stratified and rippled sandstone; (vi) F6 –cross-stratified sandstone; (vii) F7 –sigmoidal, mud-draped sandstone; (viii) F8 –amalgamated, normal graded mudstone and sandstone; and (ix) F9 –deformed, heterolithic mudstone and sandstone. Fig. 6. Palaeogeographic reconstruction of the late Miocene South Rifian Corridor (7.51 to 7.25 Ma). The Prerifian Ridges (grey) might still have been submerged at this time. Red dots indicate sections studied herein. Yellow dots indicate sections previously studied, publications of these sections studied previously are denoted by superscript numbers ( 1 Tulbure et al., 2017, 2 de Weger et al., 2020, 3 Capella et al., 2018). Green squares indicate the relative location of major cities in the area. ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Tidal modulated laterally migrating contourite depositional system 9 Facies F1 –Fossil-rich blue marlstone Description Facies F1 consists of blueish marls, the sediments after which the Blue Marl Formation is named (Table 1; Fig. 7A to E). The intervals containing these dominantly structureless, fossil-rich marlstones (containing bivalves up to a centimetre in size) range in thickness from decimetres to hundreds of metres forming a continuous succession. Despite being dominantly blueish in colour, modest gradational changes ranging on average from 0.5 to 2.0 m in thickness from dark blueish-grey to more brownishgrey are common. This colour change results from slight increases in the biogenic and siliciclastic fraction consisting of up to fine-grained and very fine-grained sand, respectively. F1 only shows scarce discrete trace fossils. Interpretation The late Miocene blue marls of F1, are widely recognized in northern Morocco and other parts of the Mediterranean region where they are generally associated with open deep-marine conditions (DiGeronimo et al., 1981; Bernini et al., 1992; Krijgsman et al., 1999; Mansour & Saint- Martin, 1999; Barbieri & Ori, 2000; Gelati et al., 2000; van Assen et al., 2006; Capella et al., 2017a). F1 represents a low energy depositional environment where sedimentation is dominated by the vertical settling and lateral advection of both fine-grained biogenic and terrigenous particles through the water column, or hemipelagic sedimentation (Hesse, 1975; Stow & Piper, 1984; O’Brien et al., 1991; Einsele, 2000). The common increases in silt and sand content suggest fluctuations in carbonate productivity, terrigenous sediment supply and/or in the hydrodynamic regime. These alterations in silt and sand content might reflect the presence of low-density turbidity currents (Lowe, 1982), in which case they reflect Bouma divisions Td and Te (Bouma, 1962), diluted gravity flows and/or the activity of weak bottom currents (Stow & Faug eres, 2008; Rebesco et al., 2014; Stow & Smillie, 2020). Facies F2 –Sandy marlstone Description Facies F2 consists of fossil-rich sandy marlstone and very fine-grained muddy sandstone (Table 1; Fig. 7C, H and I). This facies occurs over intervals with thicknesses ranging from centimetres to tens of metres. The texture is homogeneous for the finest intervals that are more blueish-grey in colour. The coarser and more light-brownish grey intervals are regularly banded or laminated. Starved ripples of finegrained sand are scarce. Changes in grain-size distribution are generally gradational, but sharp basal contacts have been observed between muddy and more sandy deposits. Laminae with sharp basal bounding surfaces and ripples coincide with an increased abundance of finegrained, imbricated shell fragments (Fig. 7I) indicating palaeo-flow directions towards the west. The muddier intervals regularly contain well-preserved bivalves up to 1.5 cm in diameter. The trace fossil assemblage consists of abundant undifferentiated structures and scarce Planolites-like and Thalassinoides-like traces (Fig. 9A and C). Interpretation Facies F2 represents a sand-enriched equivalent of facies F1, reflecting deposition under higherenergy conditions and/or a change in sediment supply. This facies generally lacks sharp bounding surfaces and traction structures, although the presence of the occasional bioclast enriched laminae may represent the remnants of traction carpets. Furthermore, the occasionally observed sharp lower bounding surfaces and sand lenses, which are likely remnants of starved ripples, rule out a pure hemipelagic origin. Since sedimentary structures and other features related to turbiditic processes are lacking, it is likely that the change in sediment supply primarily resulted from hemipelagic settling of a turbidite suspension cloud also known as a hemiturbidite (Stow & Wetzel, 1990). However, the gradual (consecutive inverse and normalgraded) trends in grain-size distribution might reflect the presence of –and subtle changes in – bottom current velocities (Gonthier et al., 1984; H€ uneke et al., 2020). If hemiturbiditic processes were active, bottom currents, induced by the palaeo-MOW, likely deflected the turbidite suspension cloud downcurrent, changing the orientation from downslope to along-slope. This deflection might be inferred from occasionally occurring imbrication patterns of shells and subtle ripple laminae. Furthermore, bottom currents might have been able to winnow and rework the sediment, preventing the settling of the finest particles in the coarsest intervals of facies F2. Winnowing and reworking can also be used to explain the occasional sharp bounding surfaces that are regularly interpreted to result from peak current velocities in most ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology 16 W. de Weger et al. Fig. 9. Overview of recognized ichnospecies: Macaronichnus (Mn),Thalassinoides (Th),Planolites (Pl),Rosselia (Rs),Ophiomorpha (Op) and undifferentiated (Undif.) traces. The addition of (l) stands for ‘-like’. Facies are indicated with the coloured and labelled circles. (A) to (F) are taken from the El Adergha sections and (G) to (J) from the Fes-north section. ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Tidal modulated laterally migrating contourite depositional system 17 environments affected by bottom currents (Lucchi & Rebesco, 2007; Mart ın-Chivelet et al., 2008; Rebesco et al., 2014; de Castro et al., 2020a,b). Based on available data, it is hypothesized that the subtle compositional changes in facies F2 are the result of the interaction between hemipelagites, low-density turbidites and weak bottom currents. Fluctuations in the bottom current activity related to intensification and weakening of the palaeo-MOW might have reworked initial turbidite deposits and have caused fluctuations between contouritic and hemipelagic dominated periods. Based on grain-size, depositional texture and according to the bedformvelocity matrix proposed by Stow et al. (2009), bottom currents never exceeded 20 to 25 cm s 1 . Similar facies have recently been identified in the proximal and central sectors of the Gulf of Cadiz CDS, where muddy contourites are usually interbedded with hemipelagites and turbidites, forming metre-scale sedimentary deposits (de Castro et al., 2020b). Facies F3 –Bi-gradational sandstone Description Facies F3 consists of an inverse to normalgraded sandstone with a thickness ranging from 10 to 60 cm (Table 1; Figs 7B, 7J and 8J). This bi-gradational pattern consists of very finegrained, fine-grained and up to medium-grained sand. The sand is of mixed bioclastic–siliciclastic composition (Chiarella et al., 2017). Between the El Adergha and Fes-north section, differences in this facies have been observed. In the El Adergha section, facies F3 (Fig. 7B) is more bioclastic, contains glauconite and abundant bioturbation of Macaronichnus (Fig. 9A and B), rare Planolites,Thalassinoides and Rosselia (Fig. 9E), leaving a severely bioturbated appearance (Fig. 7J). However, despite the intense bioturbation, planar erosive surfaces between the inverse-graded and normal-graded division have been recognized in places. In the Fes-north section, F3 consists of distinct planar laminae with the thickest laminae (up to 1.5 cm) coinciding with the coarsest (medium-grained) sand fraction (Fig. 8J). Biogenic structures are limited to a low to moderate number of undifferentiated burrows, and scarce Planolites and Thalassinoides-like traces (Fig. 9H). Interpretation The bi-gradational sandstone of facies F3 in the El Adergha section very closely resembles the C3 division of the bi-gradational contourite facies model proposed by Faug eres et al. (1984), Gonthier et al. (1984) and Stow & Faug eres (2008). The very close resemblance of this bigradational facies to the C3-division of the ‘diagnostic contourite facies model’ favours the interpretation of a contouritic drift origin. This interpretation is supported by the findings of Capella et al. (2017a). Bi-gradational sequences, however, might also be formed in different types of currentinfluenced depositional settings where waning/ waxing currents are common, such as deltas with fluctuating river discharge. There is however no evidence of such current-influenced depositional systems in the surrounding area for the depositional period between 7.51 Ma and 7.25 Ma. Furthermore, the lack of distinct erosional surfaces both below and above facies F3 in the El Adergha section (Fig. 7I) indicates subtle changes in the depositional setting and thus the depositional domain did not significantly change to that inferred from the benthic foraminifer assemblage for facies F1 (i.e. the continental slope). This thus suggests that facies F3 was formed in the slope depositional domain, a common area for the formation of contourite drift deposits (Faug eres et al., 1999; Hern andez- Molina et al., 2008; Rebesco et al., 2014). Despite the slight differences in facies F3 observed between both outcrops, subtle breaks in sedimentation, indicated by the style of bioturbation and the relatively sharp bounding surfaces in the El Adergha and Fes-north sections, respectively (Figs 7I and 8J), show that the longterm flow fluctuation responsible for the bigradational sequence, was also affected by shorter scale flow variations. These shorter scale variations indicate that, at times, flow velocities were sufficiently high to cause a break in sedimentation, and/or there was a break in sediment supply. Omission surfaces in the central part of the bi-gradational sequence have previously been reported in similar sequences, interpreted as contourite drift deposits, from the ancient record (Rodr ıguez-Tovar et al., 2019; H€ uneke et al., 2020). Facies F4 –Heterolithic, thin-bedded mudstone and sandstone Description Facies F4 consists of heterolithic orange to reddish-grey clay and marlstone and brownish to orangish-grey very fine-grained up to mediumgrained sand (Table 1; Fig. 7J). The finely laminatedtothinlybeddedsands(<10 cm) are ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology 18 W. de Weger et al. tabular (Fig. 7D) and occasionally show ripples. The base of the sandstone beds is sharp or gradational and occasionally shows an undulatory geometry. The sand component is of mixed siliciclastic–bioclastic composition, enriched in glauconite (<10%). Muddy laminae that are interbedded with the sands, are often wavy, lenticular or discontinuous parallel. Other features include silty mud flasers and muddy rip-up clasts (Fig. 7F and J). A mottled appearance, predominantly concentrated in the finest sediments, results from intense, often undifferentiated bioturbation. The trace fossil assemblage consists of abundant Macaronichnus (Fig. 9D), common Parahentzschelinia and rare Planolites, Rosselia (Fig. 8E) and Thalassinoides. Interpretation Facies F4, in comparison to facies F1 and F2, due to its coarser medium grain-size and the presence of occasional traction structures, is related to higher peak current velocities. As this facies unconformably overlies facies F1 and F2 (Fig. 7D and B, respectively), it indicates a change in hydrodynamic conditions related to an increase in maximum flow velocity. Like facies F3, the depositional domain was likely in the slope, ruling out a shallow-marine currentdominated setting. The rhythmical variations in grain-sizes and co-occurrence of fine-grained sand and mud laminae suggest deposition through alternating periods of bedload and suspension transport. Since facies F4 closely resembles the stacked sand sheets described by Mart ın- Chivelet et al. (2008), Rebesco et al. (2014), de Castro et al. (2020a) and Hovikoski et al. (2020) it might have a similar origin related to bottom currents with alternating flow conditions. During peak current velocities, the fine-grained sand can form ripples whilst the finest fraction of the sediment is winnowed or re-incorporated as aggregates and rip-up clasts (de Castro et al., 2020a). H€ uneke et al. (2020) suggest that fluctuating bottom currents characterize deposition for all contourite divisions even though the controlling mechanism behind the short-term fluctuation of flow strength is unknown. Shanmugam (2008), however indicated that short-term oscillating energy conditions have been described from thermohaline and wind-driven bottom currents. Furthermore, climatic and tidal induced changes on the characteristics of the palaeo-MOW also affect variability in flow-conditions on shorter, millennial to sub-annual timescales (de Castro et al., 2020b; de Weger et al., 2020). These processes explain the development of heterolithic alternations as a consistent indicator of bottom current fluctuations. The preservation of primary sedimentary structures within the sandstone beds indicates that bottom currents remained sufficiently strong to winnow away fine-grained sediment and to prevent disruption due to bioturbation (de Castro et al., 2020a). Facies F5 –Heterolithic, cross-stratified mudstone and sandstone Description Facies F5 typically consists of heterolithic, dominantly thin-bedded (<10 cm) to mediumbedded (<20 cm) sandstones draped by mud (Table 1; Fig. 7G and H) that are part of large westward migrating foresets (Fig. 7B). These beds, which thus likely present foresets, show unidirectionally westward verging cross-strata (Fig. 7C) with bundles of thickening and thinning foreset-laminae and angular to tangential toe-set geometries (Figs 7G and 10A; more clearly visible in 10A). Beds form 1.0 to 1.5 m thick sets, associated with the previously mentioned westward migrating foresets. Cross-sets have a planar parallel to undulatory erosive base and regularly show traction carpets of muddy rip-up clasts and small (centimetre-scale) scours at their base (Fig. 7G). The beds occasionally contain ripples showing opposing current directions (Fig. 7G). The sand consists of fine-grained up to coarse-grained sand of orangish-brown colour. Bed boundaries regularly show Fe-Mn crusting. Biogenic structures are dominantly vertically oriented and consist of an assemblage of abundant Parahaentzschelinia (Fig. 9F) and Macaronichnus, common undifferentiated vertical structures, and rare traces such as Ophiomorpha-like and Skolithos vertical traces. Interpretation The cross-stratified nature and general tabular bedding of these westward migrating deposits suggests them to be relics of deep-marine twodimensional (2D) dunes. Based on the presence of thickening and thinning foreset-bundles (Allen, 1982; Longhitano & Nemec, 2005), gradual changes between angular to tangential toe-set geometries (Chiarella, 2016; Fig. 10) and sand– mud couplets (Nio & Yang, 1991; Longhitano et al., 2012), the formation of these deposits was influenced by oscillatory flow. As such, this facies can be related to a tidally modulated current. Facies F5, based on dominant palaeocurrent directions indicating westward flow, is ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Tidal modulated laterally migrating contourite depositional system 19 interpreted as a relic of westward migrating dunes. This interpretation is supported by the findings of Capella et al. (2017a). These dunes migrated under current velocities reaching more than 1 m s 1 (Stow et al., 2009) in a late Tortonian channel in front of a strait. These conditions infer a depositional environment where energy is more concentrated than in other sectors, such as the contourite channel. Similar sandy deposits have also been identified in modern contouritic channels (Nelson et al., 1993, 1999; Hern andez-Molina et al., 2006, 2014b; Stow et al., 2013; Brackenridge et al., 2018; Lozano et al., 2020). The dominance of vertical biogenic structures in F5 supports the occurrence of energetic environmental conditions. Facies F6 –Cross-stratified sandstone Description Facies F6 consists of sandstones with unidirectionally westward verging cross-strata which include bundles of thickening and thinning foresets that show alternations between angular to tangential toe-set geometries (Fig. 8D, F and K). Bedding is generally 10 to 100 cm thick and has sharp basal boundaries with common muddy rip-up clasts. The thickest beds have a subtle concave-up geometry. Sediment is composed of moderately to well-sorted, medium-grained to coarse-grained sand represented by mixed bioclastic–siliciclastic grains. This facies is moderately bioturbated with Ophiomorpha (Fig. 9G), occasional Skolithos and rare Planolites, Rosselia-like and Thalassinoides trace fossils. Sometimes burrow linings as well as bed boundaries show Fe-Mn crusting. Interpretation Like facies F5, the cross-stratified nature and the presence of tidal signatures of these westward migrating deposits (Fig. 5) suggests them to be relics of deep-marine 2D dunes the formation of which relies on a tidally modulated current near the seafloor. The lack of mud drapes indicates that, despite the tidally modulated alternating flow conditions, flow velocities never dropped below the threshold value to deposit and/or preserve muddy sediment, indicating that deposition took place under flow velocities ranging from ca 50 cm s 1 to over 1 m s 1 (Stow et al., 2009). These conditions also infer a depositional environment in the slope domain where energy is more concentrated than in other sectors, such as the contourite channel. Like facies F5, similar sandy deposits have also been identified in modern contouritic channels (Nelson et al., 1993, 1999; Hern andez-Molina et al., 2006, 2014b; Stow et al., 2013; Brackenridge et al., 2018; Lozano et al., 2020). The dominance of vertical biogenic structures in facies F6 also supports energetic environmental conditions. Facies F7 –Sigmoidal, mud-draped sandstone Description Facies F7 consists of sigmoidal cross-stratified sandstone alternating with mudstone (Table 1). The sandstone foresets are encapsulated in and draped by up to 1 cm thick, scarcely bioturbated mud forming distinct up to 30 cm thick bundles of thickening and thinning foresets (Fig. 8J). Muddy to marly rip-up clasts are common and mainly occur at the basal bounding surfaces. The sand fraction consists of up to granule-sized mixed siliciclastic–bioclastic sand. Biogenic structures are limited to scarce discrete trace fossils (for example, Thalassinoides-like and Planolites-like). Interpretation The sand–mud couplets recognized in the crossstrata of facies F7 suggest a fluctuation in the energy of the flow to form heterolithic bundles. The mud drapes and ripped-up muddy clasts are thought to represent fine particles originating from high suspended mud concentrations (Faas, 1991). Accordingly, the sandy intervals represent the record of current-dominated processes while the draping mud reflects a moment of reduced energy, favouring the decantation and drapes of the fine-grained material previously kept in suspension (Visser, 1980). According to Nio & Yang (1991), a bimodal grain-size with the muds interlayering medium-grained or coarse-grained sandstones is an indication of tide-modulated currents. The lack of sedimentary structures referable to wave action and high-energy environments suggest a depositional environment below the fair-weather wave base in a current-dominated environment. Facies F8 –Amalgamated, normal graded mudstone and sandstone Description Facies F8 consists of heterolithic mudstone and sandstone intervals (Table 1; Figs 7K and 8G). The tabular, generally thin-bedded, sandstones (up to 20 cm) are normal graded, and regularly ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology 20 W. de Weger et al. have small (centimetre-deep) basal scours. Although a semi-gradational transition from sand to mud occurs, boundaries can be easily distinguished. This facies in El Adergha, consists of thin, amalgamated beds presenting basal rip-up clasts of up to pebble sized marls, plane-parallel lamination and ripples, capped by a very thin marlstone. The sediment is of mixed siliciclastic– bioclastic composition. The sand is up to medium-grained and well-sorted to very wellsorted. The facies is bioturbated with scarce undifferentiated discrete trace fossils (Fig. 9I). Interpretation Facies F8 generally shows basal scours and clear bounding surfaces. The well-sorted to very wellsorted, normal graded beds and internal structures typically represent a decelerating turbulent flow from the upper flow regime to suspension fallout (Bouma, 1962) and are, as such, interpreted as turbidites. The sediment composition of the sand fraction in this facies varies between both the El Adergha and Fes-north sections (Fig. 10B to D). At the El Adergha section, facies F8 consists of very fine-grained, relatively mature siliciclastic sand whereas at the Fesnorth section, it consists of medium-grained, immature siliciclastic sand (Table S1; Fig. S2). This implies that facies F8 at the El Adergha section represents a more distal sector in respect to the Fes-north section. The composition and stacking pattern of facies F8 in El Adergha (Fig. 7K) is most like those observed in base-of- slope or basin wedge settings (Stow, 1985). The tabular thin bedded nature of facies F8 recognized in the Fes-north section (Fig. 8C, G and H) might also suggest a basinal turbidite (silty– sandy distal lobe) depositional setting. However, here facies F8 more likely represents the tail of turbidity current deposits on the slope (Mutti, 1992; Mutti et al., 2009; Mulder, 2011; Talling et al., 2012) or very low-density turbiditic currents on the slope as recently discussed by de Castro et al. (2020a,b) and H€ uneke et al. (2020). Facies F9 –Deformed, amalgamated mudstone and sandstone Description Facies F9 consists of amalgamated, heterolithic, thin-bedded (up to 10 cm) strata of marlstone and Fig. 10. Examples of cyclic thickening and thinning foreset intervals and cyclic alterations between angular and tangential toe-sets. These alternations are associated with cross-stratification and interpreted as the record of sandy bedforms migrating under unidirectional, tidal-modulated accelerating/decelerating currents. N =neap tide; S=spring tides. ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Tidal modulated laterally migrating contourite depositional system 21 sandstone up to medium-grained (Table S1; Fig. S2A). Strata are convolute and fluid escape structures are common (Fig. 8B, C and I). The primary sedimentary structure is like facies F8, consisting of amalgamated normally graded tabular, thin-bedded mudstone and sandstone. The base is sharp, erosive and in many cases seems to have deformed the underlying facies (Fig. 8H). This facies appears wedge shaped, thinning out northwestward. The preserved primary bedding is folded, indicating south to south-eastward deformational migration. Inclination of the modified but preserved primary strata varies laterally from high-angle (40°, westward dipping) to opposing angles (20°, eastward dipping) (Fig. 8D and I). Biogenic structures are scarce and discrete trace fossils are difficult to identify, a few Planoliteslike traces were observed (Fig. 9J). Interpretation The convolute and/or contorted nature, accompanied by the erosive basal surface recognized in facies F9 suggest that it represents the product of a slump (Leeder, 2009; Reading, 2009; Shanmugam, 2010). Additional evidence of the sudden displacement of large volumes of sediment is found in the fluid-escape structures locally present in the underlying sediment. Although slumps may occur over a wide range of depositional environments, their formation relies on slopes. Since the primary bedding of facies F9 is like F8, F9 represents down-slope, plastically deformed fine-grained turbidites. Ichnofacies Ichnological analysis from the El Adergha and Fes-north sections reveals, in general, low ichnodiversity in assemblages. El Adergha has a higher abundance of traces which are dominantly horizontal whereas the structures in Fesnorth are dominantly vertical. This difference is related to variable palaeoenvironmental conditions, especially with respect to hydrodynamic energy. The trace fossil assemblages at the El Adergha section are characterized by low ichnodiversity and a high abundance of traces produced by deposit feeders. The two main ichnogenera (Macaronichnus and Parahaentzschelinia) are typical of proximal, shallow marine environments related to the Skolithos ichnofacies (MacEachern et al., 2007, 2012; Buatois & M angano, 2011; Knaust, 2017). However, being typified by dominantly horizontal traces allows assignation to the Cruziana ichnofacies which is typical of deeper and more distal environments. In this context, the presence of vertical structures could be related to periods with stronger bottom currents creating higher energetic palaecological conditions like those in shallow marine environments (i.e. upper slope) at more distal and deeper settings (Miguez-Salas et al., 2020). In the Fes-north section, trace fossil assemblages record low ichnodiversity and a moderate to low abundance of traces of dominantly vertical forms. The main ichnogenus is Ophiomorpha.Ophiomorpha is generally, but not exclusively, characteristic of high-energy environments (i.e. shoreface) with well-sorted shifting sandy substrates, constituting a typical element of the Skolithos ichnofacies (MacEachern et al., 2007, 2012). However, the appearance of Ophiomorpha in deep-sea environments with gravity flow sediment supplies confirms the Ophiomorpha rudis ichnosubfacies (Uchman, 2009, and references therein). Thus, the presence of Ophiomorpha should not necessarily be related to the Skolithos ichnofacies, but to higher energetic environments at Fes-north than that corresponding to El Adergha. DEPOSITIONAL SUB-ENVIRONMENTS Depositional elements and facies associations The sedimentary facies are linked to form distinct facies associations (FA) and depositional elements (Table 2). The fine-grained sediments of facies F1 and F2 are grouped into FA1. FA1 occurs over intervals of one to hundreds of metres thick where both facies regularly grade into one another. FA1 is generally coarsening-upward grading from facies F1 to F2. FA2 consists of the distinct bi-gradational stacking of facies F1, F2, F3, F2 and F1 (Fig. 7J) or F2, F3 and F2 (Fig. 8J). FA3 consists of facies F4, which, albeit heterolithic, is considered an individual facies based on the regular lack of bounding surfaces. However, the heterolithic occurrence of sandstone and marlstone could be considered the alternation between fine-grained facies (F1 and F2) and sandrich facies somewhat similar in sedimentary composition as facies F3. FA4, consisting of facies F5, F6 and F7, typified by the presence of traction structures (mainly cross-stratification). FA5 is distinguished based on depositional features with palaeo-transport indicators perpendicular (downslope) to those observed in FA4. ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology 22 W. de Weger et al. Based on the: (i) palaeogeographic reconstruction of the South Rifian Corridor; (ii) the topography of the Prerifian Ridges on the northern margin of this corridor (Fig. 6); (iii) the interpretation that the studied sections have been deposited in a slope depositional domain; and (iv) evidence suggesting the presence of a late Miocene unidirectionally westward migrating, tidal influenced bottom current (Fig. 10), it is suggested that the studied sedimentary successions were formed predominantly by the action of the late Miocene palaeo-MOW. As such, the facies associations and depositional domains are interpreted within this framework (Fig. 11). Fine-grained marine sediment (FA1) Based on the available data, the fine-grained marine sediment of facies association FA1 and its subtle changes in composition is interpreted to result from the interaction between hemipelagic, low-density turbiditic and bottom current induced sedimentation. Fluctuations in the bottom current activity, related to the intensification and deceleration of the palaeo-MOW have primarily caused fluctuations between contouritic and hemipelagic dominated periods. Similar facies have recently been identified in the proximal and central sectors of the Gulf of Cadiz CDS where muddy contourites, associated with the drift, are usually interbedded with hemipelagites forming metre-scale sedimentary deposits (de Castro et al., 2020b). As such, facies association FA1 has been interpreted to represent the contourite drift or a zone that is located even more distally, further down the slope (Fig. 12). Contourites: drift-channel transitional deposits (FA2 and FA3) Both facies associations FA2 and FA3, due to their relative increase in grain-size and the presence of subtle indicators of bedload transport, are indicative of higher bottom current velocities compared to facies association FA1. This suggests an increase in the palaeo-MOW activity. However, the heterolithic occurrence of sand and mud indicates fluctuating energy conditions. The strength or velocity of bottom currents associated with overflow water decreases both laterally and down current of the core of the bottom current which is generally confined to a contourite channel (Fig. 11) (McCave & Tucholke, 1986; Faug eres et al., 1999; Llave et al., 2001; Rebesco et al., 2014; de Castro et al., 2020b). The core of the current is influenced by the Coriolis force. In the case of this study, the westward flowing palaeo-MOW was forced towards the right (north) against the north–south oriented palaeo-slope of the northern margin of the South Rifian Corridor (Fig. 11). Since the highest bottom current velocities are associated with the contourite channel and the weakest bottom currents to the drift (Fig. 12), both facies associations FA2 and FA3 represent a transitional depositional environment with weaker currents compared to the core and stronger currents compared to the drift. Facies like those described in facies associations FA2 and FA3 have recently been ascribed to a driftchannel transitional domain where energy conditions are slightly higher than in the drift due to the increased influence of the more closely Table 2. Sedimentary facies associations (FA) established for sedimentary deposits in the El Adergha and Fesnorth outcrops. Sediment size Orientation Facies association FA Dominant facies Thickness (m) Fine-grained sediment Vertical settling, along-slope and down-slope Contourite drift FA1 F1 –Hemipelagites F2 –Fine-grained turbidites or contourites up to 100s Coarse-grained sediment Along-slope Drift/channel transition FA2 F3 –Fine-grained sandy contourites 0.6 to 1.5 Channel/drift transition FA3 F4 –Bottom current reworked sands 0.1 to 5.0 Contourite channel FA4 F5, F6 and F7 –Coarse-grained sandy contourites 0.2 to 5.0 Down-slope Upper slope FA5 F8 –Turbidites F9 –Slumped deposits 0.1 to 2.0 ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Tidal modulated laterally migrating contourite depositional system 23 located core of the bottom current (de Castro et al., 2020b). Also, areas closer to the core of the bottom current are more strongly receptive to changes in the palaeo-MOW activity, explaining the fluctuating energy conditions. Contourites: channel fill deposits (FA4) Facies pertaining to facies association FA4 (F5, F6 and F7) are characterized by a unidirectional trend of palaeo-current indicators showing a direction perpendicular to the north–south oriented palaeo-slope (Figs 11 and 12), indicating a roughly 90°offset compared to those measured in facies association FA5 (Fig. 5). As facies association FA4 shows a slope-parallel trend of dune migration related to high energetic conditions, this facies association is ascribed to the contourite channel. The differences in sedimentary composition and structures between F5, F6 and F7 can be explained by their relative position within the contourite channel. As mentioned previously, the current velocity decreases away from the core of the bottom current which is also the case within the contourite channel. The palaeo-MOW core was forced northward by the Coriolis force, against the palaeo-slope and therefore towards the slope side of the contourite channel (Figs 11 and 12). This implies that the strength of the bottom current weakens within the channel towards the distally/deeper located drift, but also towards the slope because of increased shear stress. In this case the coarsest sediment related to facies F6 and F7 would be expected in the section closest to the Prerifian Ridges, Fes-north (Fig. 11) if both of the channel facies are related to the same channel. Furthermore, as mentioned in de Weger et al. (2020) the palaeo- MOW was intermittent on tectonic, orbital, climatic and tidal timescales. The explanation of processes behind changes in the modern MOW have recently been described by Sierro et al. (2020) who found that the MOW strength is mainly driven by precession cycles and the associated freshwater input in the Mediterranean. A decrease in Mediterranean freshwater input during precession maxima results in an increase in Mediterranean salinity and thus Mediterranean deep-water formation. Enhanced, dense Mediterranean deep-water formation results in higher density gradients with Atlantic water and an enhanced, or stronger, palaeo- MOW. On a millennial scale, the study found that Greenland stadials have a positive effect on Fig. 11. Palaeogeographic reconstruction of the late Miocene Rifian corridors between 7.51 Ma and 7.25 Ma. The palaeo-Mediterranean Outflow Water (palaeo-MOW) cascaded over the Taza Sill, through the Taza Strait into the South Rifian Corridor where it was forced against the northern margin by the Coriolis force. The Atlantic surficial water flowed through the South Rifian Corridor and Taza Strait into the Mediterranean. The studied sections, Fesnorth (FN) and El Adergha (EA) were located closely to or within the contourite channel changing over time. The red arrow, indicating the core of the palaeo-MOW depicts the trajectory of highest current-velocities confined within the channel, whereas the palaeo-MOW water mass was less confined. ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology 24 W. de Weger et al. buoyancy loss in the eastern Mediterranean which enhances the density gradient with Atlantic water. This results in a more vigorous palaeo-MOW. Similar controlling factors resulted in palaeo-MOW expansion (intensification) and collapse (deceleration) but also in its longer period (>seasonal) oscillatory flow behaviour. Expansion and collapse of the palaeo- MOW likely played an important role in the facies differences of FA4, where, during the deposition of facies F6 and F7, the palaeo-MOW was stronger compared to F5. However, during Fig. 12. Sketch showing the relation between the facies (F1 to F9), facies associations and the different depositional and erosional elements from the proximal continental slope to the contourite drift. The facies associations are related to their dominant depositional process and associated current velocities. The panels in the bottom left corner show examples of observed facies stacking patterns. Soft sediment deformation structures (SSD) are regularly observed below facies F9. ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Tidal modulated laterally migrating contourite depositional system 25 changes in the intensity of the palaeo- Mediterranean Outflow Water (palaeo-MOW) which decreases away from its core. The contourites are interstratified with hemipelagites, turbidites and slump deposits where their preservation is related to a decrease in palaeo- MOW intensity. The vertical sedimentary facies stacking pattern records the tectonically induced, southward migration of the contourite depositional system (CDS) and the intermittent behaviour of the palaeo-MOW which is mainly driven by precessional-scale and millennial-scale climate variations. The tidal signature in the sandy contourite deposits shows that tides played a key factor in modulating the palaeo-MOW on a subannual timescale. The results of this study indicate that bottom currents, their hydrodynamic conditions and their effect on deep-marine depositional processes and environments are controlled by: (i) the intermittent behaviour of flow (on–off); (ii) flow acceleration and deceleration; and (iii) tidal modulation. All of these factors should be considered in the process-based interpretation of all deep marine deposits possibly affected by bottom currents. Keeping in mind, the well-known expression of ‘the past is the key to the future’, contourite research, such as this study, could significantly contribute to the understanding of processes leading to changes in global ocean circulation and associated climate change. As this manuscript provides information regarding mechanisms that affect the overflow of dense Mediterranean water and gateway evolution, which impacts global ocean circulation, this work provides information on processes that contribute to climate change. ACKNOWLEDGEMENTS We are very appreciative of the help and support given by the Office National des Hydrocarbures et des Mines (ONHYM), Morocco. This project was funded by the Joint Industry Project supported by TOTAL, BP, ENI, ExxonMobil, Wintershal DEA, and TGS, executed in the framework of “The Drifters Research Group” at Royal Holloway University of London (RHUL), related to projects CTM 2012039599-C03, CGL2016-80445-R and CTM2016-75129-C3-1-R. The research contribution of O. Salas-Miguez was funded through a pre-doctoral grant from the Ministerio de Educacion, Cultura y Deporte (Gobierno Espana). Journal reviews by A.R. Viana and S.G. Longhitano are greatly appreciated, and their comments helped us to improve the clarity of the manuscript. DATA AVAILABILITY STATEMENT The data that support the findings of this study are available from the corresponding author upon reasonable request. 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Available at: https://pascal-francis.inist.fr/vibad/index.php?action=ge tRecordDetail&idt=19259060 Manuscript received 8 October 2020; revision 25 March 2021; revision accepted 5 April 2021 Supporting Information Additional information may be found in the online version of this article: Fig. S1. Microphotographs for facies F4 and F5. Fig. S2. Microphotographs for facies F6 and F8. Table S1. Modal analysis data based on 300-point counts in thin section. ©2021 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology Tidal modulated laterally migrating contourite depositional system 37