Kelvin Ikenna Chima
email: kelvin.chima@sorbonne-universite.fr

Christian Gorini

Marina Rabineau

Didier Granjeon

Damien Do Couto

Estelle Leroux

Nick Hoggmascall

Pliocene and Pleistocene stratigraphic evolution of the western Niger Delta intraslope basins: A

Keywords: Niger Delta, Intraslope basins, Pliocene, Pleistocene, Seismic stratigraphy, Allocyclic forcing, Eccentricity cycles. 1

published or not. The documents may come

Introduction

Deep-water sediments are important archives of the Earth"s history not only because they record past climatic, geodynamic, environmental and hydrodynamic changes (e.g. Burbank,1992;[START_REF] Molnar | Late Cenozoic increase in accumulation rates of terrestrial sediment: how Might Climate Change Have Affected Erosion Rates?[END_REF][START_REF] Catuneanu | Principles of sequence stratigraphy[END_REF][START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF][START_REF] Clift | Enhanced global continental erosion and exhumation driven by Oligo-Miocene climate change[END_REF]Zecchin and Catuneanu, 2010), but also because they host viable hydrocarbon fields (see review in [START_REF] Morley | [END_REF]. Understanding the complex interplay between allocyclic (e.g. eustasy, tectonics, climate and indirectly sediment supply), and autocyclic (e.g. channel avulsion) or regional forcings (e.g. delta lobe switching, shale tectonics etc.), is critical for interpretation of the evolution of deep-water sedimentary systems (e.g. Elliot, 1975;[START_REF] Jervey | Quantitative geological modelling of siliclastic rock sequences and their seismic expressions[END_REF][START_REF] Posamentier | Eustatic controls on clastic deposition I. Conceptual framework[END_REF][START_REF] Pulham | Controls on internal structure and architecture of sandstone bodies within Upper Carboniferous fluvial-dominated deltas[END_REF]Einselle, 1991;[START_REF] Wiener | Mobile shale in the Niger Delta: Characteristics, structure, and evolution[END_REF][START_REF] Miall | The Geology of Stratigraphic Sequences[END_REF][START_REF] Morley | Deep-water fold and thrust belt classification, tectonics, structure and hydrocarbon prospectivity[END_REF]Catuneanu and Zecchin, 2013).

The Pliocene and Pleistocene are ideal time intervals for this kind of study because eustasy and climate proxies (δ 18 O) are well constrained and show significant variations both in amplitude and frequency (e.g. [START_REF] Ruddiman | Pleistocene evolution: northern hemisphere ice sheets and North Atlantic Ocean[END_REF][START_REF] Imbrie | On the structure and origin of major glaciation cycles 2. The 100,000-year cycle[END_REF][START_REF] Bassinot | The astronomical theory of climate and the age of the Brunhes-Matuyama magnetic reversal[END_REF][START_REF] Leroy | Global setting to comparative charts of regional events[END_REF][START_REF] Shackleton | The 100,000-year Ice-Age cycle identified and found to lag temperature, carbon dioxide, and orbital eccentricity[END_REF][START_REF] Lisiecki | A Pliocene-Pleistocene stack of 57 globally distributed benthic δ 18 O records[END_REF][START_REF] Miller | The Phanerozoic Record of Global Sea Level Change[END_REF][START_REF] Gibbard | Formal ratification of the Quaternary System/Period and the Pleistocene Series/Epoch with a base at 2.58 Ma[END_REF][START_REF] Gibbard | Partitioning the Quaternary[END_REF][START_REF] Raymo | The accuracy of mid-Pliocene δ 18 O based ice volume and sea level reconstructions[END_REF]. Published climate proxies across the worlds" ocean basins show that the Pliocene and Pleistocene were controlled by orbital forcing (Milankovitch cycles) of three major periods namely: (i) precession (23 ka), (ii) obliquity (41 ka), and (iii) eccentricity (400 ka and 100 ka) [START_REF] Lisiecki | Plio-Pleistocene climate evolution: trends and transitions in glacial cycle dynamics[END_REF]. The early Pliocene from (5.3 Ma) to the early Pleistocene was dominated by obliquity cycles (41 ka), while the middle Pleistocene (circa 1.4 Ma) to the present was dominated by eccentricity (400 to 100) (Head and Gibbard, 2015a, b;[START_REF] Gibbard | Partitioning the Quaternary[END_REF]. Although the switch to the eccentricity cycles is poorly understood and a transition period from 1.4 to 0.4 Ma was suggested by Head and Gibbard (2015a, b), and [START_REF] Gibbard | Partitioning the Quaternary[END_REF], a marked change to increasingly severe glacial cycles, the so-called "0.8 Ma event" [START_REF] Lisiecki | Plio-Pleistocene climate evolution: trends and transitions in glacial cycle dynamics[END_REF]Head and Gibbard, 2015a, b) or "0.9 Ma" [START_REF] Miller | The Phanerozoic Record of Global Sea Level Change[END_REF]Elderfield et al., 2012), is usually interpreted as the Middle Pleistocene Transition (MPT).

Sedimentary records all over the world show the imprint of tectonic, climatic and glacioeustatic cycles of different orders [START_REF] Haq | Chronology of fluctuating sea levels since the triassic[END_REF]. Fourth-order climate cycles lasting 400 ka have been reported during the Pliocene e.g. on the continental shelf offshore Foz do Amazonas Basin (Brazil), with incised valleys, slope canyons and mass wasting [START_REF] Gorini | Late Neogene sequence stratigraphic evolution of the Foz do Amazonas Basin, Brazil[END_REF].

During the Pleistocene, fifth-order glacio-eustatic sea-level changes with 100 ka periodicity are also well documented with marked shoreline progradation on the shelf and increased deep-water sedimentation during glacials e.g. in the Mediterranean Sea over the last 500 ka [START_REF] Rabineau | Paleosea levels reconsidered from direct observation of paleo-shoreline position during Glacial Maxima (for the last 500 000 yr)[END_REF][START_REF] Ridente | Sedimentary response to\ climate and sea level change during the past ∼400 kyr from borehole PRAD1-2 (Adriatic Margin)[END_REF][START_REF] Lafosse | Late Pleistocene-Holocene history of a tectonically active segment of the continental margin (Nekor basin, Western Mediterranean, Morocco[END_REF]. During the Pliocene, sediment architecture in the Mediterranean Sea does not appear to be much imprinted by the 400 ka cycles as shown in seismic [START_REF] Rabineau | Quantifying subsidence and Isostasy using paleobathymetric markers : example from the Gulf of Lion[END_REF] and numerical simulations [START_REF] Leroux | Stratigraphic simulation on the shelf of the Gulf of Lions: testing subsidence rates and sea-level curves during Pliocene and Quaternary[END_REF], probably J o u r n a l P r e -p r o o f because of an anomaly in accommodation created by the erosion of the entire margin after the Messinian Salinity Crisis. 100-ka glacio-eustatic cycles have also been described worldwide e.g. offshore Alaska (Gulick et al., 2012), in the Gulf of Mexico [START_REF] Galloway | Cenozoic evolution of sediment accumulation in deltaic and shore-zone J o u r n a l P r e -p r o o f depositional systems, northern Gulf of Mexico Basin[END_REF] and the Bengal fan (Werber et al.,1997).

In the eastern Niger Delta, [START_REF] Jermannaud | Plio-Pleistocene sequence stratigraphic architecture of the eastern Niger Delta: A record of eustasy and aridification of Africa[END_REF] and [START_REF] Rouby | Gravity driven deformation controlled by the migration of the delta front: The Plio-Pleistocene of the Eastern Niger Delta[END_REF], documented a transition from global climate-controlled sequences from circa 4-2. [START_REF]Depositional model Despite the dissimilar seismic facies association in MSU 1 and MSU 2 (e.g. Fig. 2B, C), the overall sedimentary architecture observed in the two units (Figs. 3-10), suggests that allogenic forcings at different scales and basin tectonics controlled the stratigraphic evolution of the western Niger Delta intraslope basins over the last 5.5 Ma. Comparing our observations in MSU 1 and MSU 2 (Figs. 2-10) with published depositional models[END_REF] Ma to sediment supplycontrolled sequences from 2. [START_REF]Depositional model Despite the dissimilar seismic facies association in MSU 1 and MSU 2 (e.g. Fig. 2B, C), the overall sedimentary architecture observed in the two units (Figs. 3-10), suggests that allogenic forcings at different scales and basin tectonics controlled the stratigraphic evolution of the western Niger Delta intraslope basins over the last 5.5 Ma. Comparing our observations in MSU 1 and MSU 2 (Figs. 2-10) with published depositional models[END_REF] Ma to the present. These authors also demonstrated a general progradation and an increase in sedimentation rates from 4-2.5 Ma, followed by a retrogradation and a decrease in sedimentation rates over the last 2. 5 Ma. Additionally, Riboulot et al. (2012), showed that eccentricity forcing of 100 ka periodicities has controlled the stratigraphic evolution of the eastern Niger Delta continental shelf over the last 0.5 Ma.

In the western Niger Delta slope, where the present study was conducted (Fig. 1), [START_REF] Jobe | Rapid adjustment of submarine channel architecture to changes in sediment supply[END_REF][START_REF] Jobe | High resolution, millennial-scale patterns of bed compensation on a sand-rich intraslope submarine fan, western Niger Delta slope[END_REF]; red box in Fig. 1A,D), linked changes in sedimentation rates during the last 130 ka (MIS 5e to the present) to glacial/interglacial cycles with increased sedimentation rates during the lowering of the sea level (MIS3 and MIS2) and an overall reduction in sedimentation during a rapid post-glacial sea-level rise associated with the Meltwater pulse 1A at 14 ka. Earlier studies e.g. Lézine et al. (2005); [START_REF] Weldeab | Spatio-temporal evolution of the West African monsoon during the last deglaciation[END_REF]; [START_REF] Collins | Insolation and glacialinterglacial control on southwestern African Hydroclimate over the last 140,000 years[END_REF]; [START_REF] Govin | Astronomically forced variations in western African rainfall (21°N-20°S) during the Last Interglacial period[END_REF] and [START_REF] Armitage | West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad[END_REF], linked the increase in sedimentation rates in the Gulf of Guinea during the Quaternary to the West African Monsoon. A recent high-resolution, 3D seismic and sequence stratigraphic study of the western Niger Delta slope (yellow box in Fig. 1A,C,D,E) by Chima et al. (2019, dated the Neogene stratigraphic record from the Burdigalian (18. 5 Ma) to the late Miocene (5.5 Ma), and described interactions between mobile shale and erosional submarine channel over the last 5.5 Ma. However, the lack of biostratigraphic data for the Pliocene and J o u r n a l P r e -p r o o f Pleistocene poses a challenge to constraining the timing of major changes in sedimentary records.

Despite the above-mentioned studies of the Pliocene and Pleistocene sedimentary records in the Niger Delta, no study has investigated its control by orbital forcings from the late Messinian (5.5 Ma) to the present. Hence, the objective of this paper is to perform a detailed analysis of the stratigraphic evolution of the western Niger Delta intraslope basins over the last 5. [START_REF]Depositional model Despite the dissimilar seismic facies association in MSU 1 and MSU 2 (e.g. Fig. 2B, C), the overall sedimentary architecture observed in the two units (Figs. 3-10), suggests that allogenic forcings at different scales and basin tectonics controlled the stratigraphic evolution of the western Niger Delta intraslope basins over the last 5.5 Ma. Comparing our observations in MSU 1 and MSU 2 (Figs. 2-10) with published depositional models[END_REF] Ma with a view to (i) proposing a sequence stratigraphic framework for the Pliocene and Pleistocene intervals; and (ii) investigating their control by orbital forcings.

The Niger Delta is an ideal laboratory to study climate stratigraphy and slope depositional processes because it is located on a passive margin where the imprints of regional/global climate dynamics, sea-level variation, gravity-tectonics, drainage evolution and sediment supply are well imaged on high-resolution 3D seismic reflection data.

Regional Setting

The Cenozoic Niger Delta is located in the Gulf of Guinea on the equatorial Atlantic margin of West Africa (Fig. 1A). The delta is the twelfth largest petroleum province in the world [START_REF] Tuttle | The Niger Delta Petroleum System Chapter A: Tertiary Niger Delta (Akata-Agbada) Petroleum System, Niger Delta Province, Nigeria, Cameroon and Equatorial Guinea, Africa[END_REF]. It covers an area of ∼140,000 km 2 with a maximum sediment thickness of ∼ 12 km (Allen, 1965;[START_REF] Evamy | Hydrocarbon habitat of Tertiary Niger Delta[END_REF][START_REF] Doust | Divergent passive margin basins[END_REF]. The Niger Delta is divided into the Eastern and the Western lobes by a Cretaceous basement high [the Charcot Fracture Zone (CFZ)] [START_REF] Corredor | Structural styles in the deep-water fold and J o u r n a l P r e -p r o o f[END_REF]; Fig. 1A). The development of the delta since the late Eocene (Fig. 1B,C), has led to three major structural zones:(i) an extensional zone (EZ) below the continental shelf characterised by normal faults; (ii) a translational zone (TZ) below the upper slope dominated by mobile shales and intraslope basins, and (iii) a contractional zone (CZ) J o u r n a l P r e -p r o o f located at the toe of the slope characterised by reverse faults and detachment folds [START_REF] Doust | Divergent passive margin basins[END_REF][START_REF] Damuth | Neogene gravity tectonics and depositional processes on the deep Niger Delta continental margin[END_REF][START_REF] Cohen | Sedimentation and shale tectonics of the northwestern Niger Delta front[END_REF][START_REF] Connors | Compressive anticlines of the mid-outer slope, central Niger Delta (abs[END_REF][START_REF] Morgan | Structural controls on the positioning of submarine channels on the lower slopes of the Niger Delta[END_REF][START_REF] Bilotti | Detachment fold, Niger Delta[END_REF][START_REF] Corredor | Structural styles in the deep-water fold and J o u r n a l P r e -p r o o f[END_REF]Fig. 1A,C).

The classification scheme of [START_REF] Prather | Controls on reservoir distribution, architecture ad stratigraphic trapping in slope settings[END_REF] describes the western Niger Delta slope profile as a "graded slope" characterised by a seafloor whose downslope profile is elevated above the level of theoretical concave-upward, smoothed graded profile. Shale tectonics on the Niger Delta slope give rise to shale/thrust-cored structural highs flanked by mobile shale withdrawal intraslope basins that interact with sediment-gravity flows [START_REF] Doust | Divergent passive margin basins[END_REF][START_REF] Damuth | Neogene gravity tectonics and depositional processes on the deep Niger Delta continental margin[END_REF][START_REF] Cohen | Sedimentation and shale tectonics of the northwestern Niger Delta front[END_REF][START_REF] Connors | Compressive anticlines of the mid-outer slope, central Niger Delta (abs[END_REF]Graue, 2000;[START_REF] Morgan | Structural controls on the positioning of submarine channels on the lower slopes of the Niger Delta[END_REF][START_REF] Bilotti | Detachment fold, Niger Delta[END_REF][START_REF] Corredor | Structural styles in the deep-water fold and J o u r n a l P r e -p r o o f[END_REF][START_REF] Wiener | Mobile shale in the Niger Delta: Characteristics, structure, and evolution[END_REF][START_REF] Prather | Stratigraphic Response to Evolving Geomorphology in a Submarine Apron Perched on the Upper Niger Delta Slope[END_REF]; Fig. 1D,E). The western Niger Delta slope is in constant dynamic equilibrium as the movements of mobile shale create structural highs which, in turn, are smoothed by sediment-gravity flows that preferentially accumulate within adjacent intraslope basins [START_REF] Deptuck | Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea[END_REF][START_REF] Adeogba | Transient fan architecture and depositional controls from near-surface 3D seismic data, Niger Delta continental slope[END_REF][START_REF] Bakare | Effect of Growing Structures on Stratigraphic Evolution, Channel Architecture, and Submarine Fan Distribution, Niger Delta, West Africa[END_REF][START_REF] Prather | Stratigraphic Response to Evolving Geomorphology in a Submarine Apron Perched on the Upper Niger Delta Slope[END_REF][START_REF] Clark | Interactions between coeval sedimentations and deformation from the Niger Delta deep-water fold belt[END_REF][START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF] Fig. 1D,E). The distribution of shale/thrust-cored structures on the western Niger Delta slope significantly controls the morphology of submarine channels, depositional patterns, reservoir architecture and distribution (e.g. [START_REF] Adeogba | Transient fan architecture and depositional controls from near-surface 3D seismic data, Niger Delta continental slope[END_REF][START_REF] Bakare | Effect of Growing Structures on Stratigraphic Evolution, Channel Architecture, and Submarine Fan Distribution, Niger Delta, West Africa[END_REF][START_REF] Jobe | Rapid adjustment of submarine channel architecture to changes in sediment supply[END_REF][START_REF] Jobe | High resolution, millennial-scale patterns of bed compensation on a sand-rich intraslope submarine fan, western Niger Delta slope[END_REF][START_REF] Clark | Interactions between coeval sedimentations and deformation from the Niger Delta deep-water fold belt[END_REF][START_REF] Deptuck | Pleistocene seascape evolution above a "simple" stepped slope-western Niger Delta[END_REF][START_REF] Prather | Stratigraphic Response to Evolving Geomorphology in a Submarine Apron Perched on the Upper Niger Delta Slope[END_REF][START_REF] Jolly | Growth history of fault-related folds and interaction with seabed channels in the toe-thrust region of the deep-water Niger Delta[END_REF][START_REF] Hansen | Submarine channel evolution, terrace development, and preservation of intra-channel thin-bedded turbidites: Mahin and Avon channels, offshore Nigeria[END_REF][START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]. The present study was conducted in the translational zone of the western Niger Delta slope at the transition between the extensional and the contractional domains (Fig. 1A,C). The study area covers 638 km 2 at ~120 km from the presentday coastline and lies at water depths ranging from 900 to 1,755 m (Fig. 1E).

J o u r n a l P r e -p r o o f

Dataset and Methods

Dataset

We used high-resolution 3D seismic data and lithologic logs from five boreholes (see Fig. 1A,D,E), to study the Pliocene and the Pleistocene stratigraphic evolution of the western Niger Delta.

Seismic data

The 3D seismic data was processed as a zero-phase source wavelet in the American reverse standard polarity such that an increase in acoustic impedance corresponds to a trough (blue loop) in the wavelet, while a decrease in acoustic impedance is represented by a peak (red loop) in the wavelet. The horizontal resolution of the data is characterised by a stacking bin spacing of 25 m x 25 m (inline and crossline spacing), and a seismic recording sampling interval of 4 milliseconds two-way travel time (TWT). The dominant frequency of the data is 30 Hz over the first 3 seconds (TWT), which yields a vertical resolution of ~13.7 meters within the interval of interest.

Well data

The well data comprise gamma-ray logs from five wells (labelled W1, W2, W3, W4 and W5 in Fig. 1A,D,E). Wells W2-W5 penetrate only the lower part of the Pliocene whereas W1 covers the Neogene to the late Pleistocene (see Figs. 2;7 and 9B of Chima et al., 2019). The most recent available biostratigraphic dating corresponds to the late Messinian (circa 5.5 Ma), based on the last recorded occurrence of Discoaster quinqueramus (see [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF].

J o u r n a l P r e -p r o o f

Biostratigraphic data were not available for the interval of interest (the Pliocene and the Pleistocene).

Methods

Seismic stratigraphy

We applied the concept of seismic stratigraphy (after [START_REF] Mitchum | Seismic Stratigraphy and Global Changes of Sea Level, Part 7: Seismic Stratigraphic Interpretation Procedure: Seismic Stratigraphy: Applications to Hydrocarbon Exploration[END_REF] to analyse the sedimentary architecture of the Pliocene and Pleistocene sedimentary records of the western Niger Delta intraslope basins. Our seismic stratigraphic approach relied on the recognition of reflection amplitude, continuity, internal architecture, external geometry, the nature of bounding surfaces and erosional truncations to delineate key seismic surfaces as well as their association with seismic facies.

3D seismic geomorphology

We applied the 3D seismic geomorphological analysis technique described by [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF] and [START_REF] Catuneanu | Principles of sequence stratigraphy[END_REF] for our detailed study of the depositional environments corresponding to the seismic facies described above. This was achieved by detailed analysis of root mean square (RMS) seismic amplitudes, that makes it possible to distinguish lithological and depositional environments (see [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]and references therein). Analyses of the RMS amplitudes were combined with variance seismic attributes to guide the geological interpretation of the seismic data (Chopra and Mafort, 2007;[START_REF] Taner | Seismic attributes revisited[END_REF][START_REF] Damuth | Neogene gravity tectonics and depositional processes on the deep Niger Delta continental margin[END_REF][START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]. Three (3D) seismic geomorphological analyses enabled the detailed study of the space-time evolution of the Pliocene and the Pleistocene sedimentary records.

J o u r n a l P r e -p r o o f 3.2.3 Allostratigraphic partitioning and dating of the Pliocene and the Pleistocene stratigraphy Detailed analysis of RMS maps combined with variance analysis of seismic attributes and 3D geomorphological interpretation of the seismic data made it possible to define 21 mappable seismic surfaces, which bound the fourth-and fifth-order allostratigraphic sequences as defined by [START_REF] Catuneanu | Principles of sequence stratigraphy[END_REF] and [START_REF] Catuneanu | Sequence stratigraphy: Methodology and Nomenclature[END_REF]. These allostratigraphic units were grouped in two mega seismic units (MSU 1 and MSU 2). As no biostratigraphic data were available for the Pliocene or the Pleistocene, we used the most recent biostratigraphically constrained interval (the late Messinian) dated at ∼5.5 Ma based on the last recorded occurrence of Discoaster quinqueramus (see [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF], to define the age of the lower part of MSU 1. Then, we matched the key erosional and conformable seismic surfaces that show significant changes in the sedimentary architecture both in seismic lines and on 3D geomorphological maps with the highresolution Pliocene and the Pleistocene sea-level and oxygen isotope ( 18 O) curves derived from [START_REF] Lisiecki | A Pliocene-Pleistocene stack of 57 globally distributed benthic δ 18 O records[END_REF][START_REF] Raymo | The accuracy of mid-Pliocene δ 18 O based ice volume and sea level reconstructions[END_REF].

Results and Interpretations

Seismic facies analysis

The Pliocene and Pleistocene stratigraphy of the western Niger Delta intraslope basins were subdivided into two mega seismic units named MSU 1 and MSU 2, based on the observed changes in seismic geometries and depositional environments. MSU 1 and MSU 2 correspond to Unit 6 and Unit 7 respectively in [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]. Although a detailed seismic facies analysis of the study area was presented in [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF] (Fig. 1A,C,D,E; 2A), the present study sheds more light on facies association and improves our understanding of depositional processes J o u r n a l P r e -p r o o f and sequence stratigraphy in a deep-water (slope) setting. The six seismic facies previously described in the study area by [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF] include: (1) chaotic, discontinuous lowamplitude (B l ) seismic facies, characterised by elongate/tabular geometry, irregular tops and erosional bases, interpreted as mass-transport deposits (MTDs; Fig. 2A 1 ,B,C); (2) chaotic, discontinuous to moderately continuous, high-amplitude (B h ) seismic facies, characterised by irregular base, blocky external form and sinuous planform geometry, interpreted as turbidite feeder channels/aggrading or meandering channel levee complexes (CLCs; Fig. 2A 2 ,B); (3) coherent/slightly inclined, moderate to high-amplitude seismic facies that generally border the margins of concave-upward negative relief and display tortuous planform geometry, interpreted as MTDs (slides/slump blocks?; Fig. 2A 3 ); (4) parallel continuous to slightly convergent, highamplitude, (C bh ) seismic facies that display fan-shaped geometry with high RMS amplitude on the map, interpreted as distributary channels/lobes (2A 4 , C); ( 5) convergent/thinning, high-to moderate-amplitude, (C th ) seismic facies, characterised by wedge geometry, which borders negative relief, marked by sinuous planform geometry, interpreted as channel levee deposits (2A 5 , B, C); (6) transparent, parallel, continuous, draping seismic facies (D) that locally display small-scale chaotic internal architecture, bounded by conformable surfaces interpreted as hemipelagic drapes and/or muddy turbidites (2A 6 , B, C). The foregoing seismic facies and their interpretations were confirmed by 3D seismic geomorphology (Figs. 345678910) and are consistent with previous studies of the Niger Delta intraslope basins (e.g. [START_REF] Deptuck | Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea[END_REF]2012;[START_REF] Adeogba | Transient fan architecture and depositional controls from near-surface 3D seismic data, Niger Delta continental slope[END_REF][START_REF] Bakare | Effect of Growing Structures on Stratigraphic Evolution, Channel Architecture, and Submarine Fan Distribution, Niger Delta, West Africa[END_REF][START_REF] Hansen | Submarine channel evolution, terrace development, and preservation of intra-channel thin-bedded turbidites: Mahin and Avon channels, offshore Nigeria[END_REF].

Facies association

J o u r n a l P r e -p r o o f Seismic facies on the western Niger Delta intraslope basins display complex lateral and vertical arrangements inside and outside erosional submarine channels MSU 1 and MSU 2 (e.g. Fig. 2A,B,C). 4.2.1 Association of seismic facies in mega seismic unit 1 (MSU 1)

In MSU 1, we observed a 2.5 km wide concave-upward negative topography (185 m deep) that obstructs the lateral continuity of seismic reflections (e.g. Fig. 2B). Within this topographic feature, a narrow (1 km wide, 90 m deep), vertically stacked, concave-upward negative topographies were also observed (e.g. Fig. 2B). The 2.5 km and the 1 km wide negative topographies were interpreted as, respectively, erosional submarine channels and channel levee systems (CLSs) or channel forms (after [START_REF] Deptuck | Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea[END_REF]; e.g. Fig. 2B).

Association of seismic facies within the erosional submarine channels in MSU 1

The association of facies within the erosional submarine channels in MSU 1 includes: (i) B l seismic facies (MTDs), named basal MTDs; (ii) B h seismic facies, which correspond to turbidite feeder channels/aggrading or meandering channel levee complexes, and (iii) C th seismic facies, which correspond to channel levee deposits (e.g. Fig. 2A,B). Levee deposits were further subdivided into inner and outer levee (IL and OL) deposits, respectively, depending on whether sedimentation occurred inside or outside the erosional submarine channel (e.g. Fig. 2B). These B l, C th and the B h seismic facies generally infilled erosional submarine channels in MSU 1 (e.g. 2B). Each erosional submarine channel and associated infilling was generally abruptly overlain by another negative topography (erosional submarine channel) with or without complete J o u r n a l P r e -p r o o f preservation of the B l, C th and the B h seismic facies (Fig. 2B). The entire system is locally overlain by another B l seismic facies (MTD; e.g. Fig. 2B). 4.2.1.2 Association of seismic facies outside the erosional submarine channels in MSU 1

Outside the erosional submarine channels in MSU 1, seismic facies generally stack vertically from (i) B l facies (basal MTDs), (ii) the C th facies (channel levee deposits), (iii) the B l facies (MTDs), (iv) the C bh facies (distributary channels/lobe deposits) and the D seismic facies (hemipelagic drape) (e.g. Fig. 2C). The B l seismic facies (MTDs) generally disrupt the lateral continuity of the underlying C th seismic facies (e.g. 2C).

Association of seismic facies in MSU 2

In MSU 2, similar concave upward, negative topographies previously described and interpreted as erosional submarine channels and CLSs (Fig. 2B), were also observed in MSU 2 (e.g. Fig. 3A,B). How the seismic facies are associated inside and outside the erosional submarine channels is described below.

Association of seismic facies within the erosional submarine channels in MSU 2

In MSU 2, the seismic facies in the erosional submarine channels generally stack from (i) the B h facies (turbidite feeder channels/aggrading or meandering channel levee complexes); (ii) coherent/slightly inclined, moderate to high amplitude seismic facies that generally border the margins of concave-upward negative relief and display tortuous planform geometry (slide/slump blocks) (Fig. 2A; 3A, B) to (iii) the D facies (hemipelagic drape), where preserved (e.g. horizons S and X in Fig. 3A,B). The coherent/slightly inclined, moderate to high amplitude seismic facies J o u r n a l P r e -p r o o f (slide blocks/slump deposits) locally disrupt the lateral continuity of the underlying B h seismic facies (e.g. Fig. 3A,B).

Association of seismic facies outside the erosional submarine channels in MSU 2

In MSU 2, the association of seismic facies outside erosional submarine channels includes: (i) the C th facies (aggrading channel levee/overbank deposits); (ii) the B l facies (MTDs), and (iii) the D facies (hemipelagic drape) (see the stratigraphic interval between horizons T and V at the extreme left in Fig. 3A,B). As observed in MSU 1, the B l facies (MTDs) generally disrupt the lateral continuity of the C th seismic facies (e.g. Fig. 3A,B).

3D seismic geomorphology and sedimentary architecture

A total of 21 seismic surfaces (13 in MSU 1, labelled M-P) and (8 in MSU 2, labelled Q-X in Figs. 3-10), were analysed in detail in this study. Only a few representative erosional surfaces in the lower, middle, and upper parts of MSU 1 and MSU 2 which illustrate the main changes in the sedimentary architecture of the Pliocene and the Pleistocene are presented here.

Base of MSU 1 (horizon M)

Description

The base of MSU 1 (horizon M in Fig. 3B) is unerlain by > 80 m thick transparent, parallel continuous, D seismic facies (highlighted in blue in Fig. 3B). The M horizon is traceable through a significant part of the study area except across the overlying 185 m negative relief (horizon A in Fig. 3A) and the mounded seismic wipe out zone in the northern part of the study area (Figs. 1D,3A,B). The 185 m deep relief associated with horizon A marks the onset of the J o u r n a l P r e -p r o o f chaotic, discontinuous low amplitude (B l seismic facies), the convergent/thinning, high to moderate amplitude (C th seismic facies) and the parallel continuous to slightly convergent, high amplitude (C bh seismic facies) (Fig. 3A,B). An RMS amplitude map draped on a variance attribute of the 185 m deep relief associated with horizon A, shows the presence of a 1.5 km wide, east-west trending, relatively straight planform feature (labelled 1 in Fig. 3C), and two narrow winding features (labelled 2, 3 in Fig. 3C), in the southern part of the study area. The narrow winding feature (labelled 2 in Fig. 3C), is connected to a fan-shaped feature to the southwest. Linear scars with respectively, northeast-southwest, radial, and east-west orientations, are observed in the northern, central, and southern parts of the study area (Fig. 3C). A grey area (dashed red line) is observed in the north-eastern part of the study area characterised by radial features at its southern tip (Fig. 3C). The narrow winding feature (labelled 3 in Fig. 3C), crosses an oblique, elongated positive relief to the south. A north-south seismic line (CC I ), (located in Fig. 3C), illustrates the sedimentary architecture of horizon A (Fig. 3A,B). Northeast-southwest trending linear scars generally offset stratigraphy normally except in the north, where one of the scars (labelled Z in Fig. 3B) offsets stratigraphy normally at its lower part but inversely at its upper part.

Interpretation

The >80 m thick transparent, parallel continuous, draping seismic unit, characterised at the base by a conformable seismic surface (labelled horizon M in Fig. 3B), corresponds to the hemipelagic drape (HD)/muddy turbidites, dated at 5.5 Ma (see [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]. The conformable nature of this hemipelagic drape suggests that it passively draped the slope during the episode of maximum flooding of the continental shelf and sediment starvation in the deep J o u r n a l P r e -p r o o f basin (see [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]and references therein). This interpretation is supported by the fact that the D. quinqueramus (calcareous nannofossil) we used in dating the hemipelagic drape at 5.5

Ma is globally recognised as an important marker of the high sea level in the late Miocene (e.g. [START_REF] Backman | Biozonation and biochronology of Miocene through Pleistocene calcareous nannofossils from low and middle latitudes[END_REF]. The 1.5 km wide and 185 m deep negative relief associated with horizon A (Fig. 3B), which displays straight planform geometry (Fig. 3C), corresponds to the erosional submarine channel described in section 4.2.1. The lateral discontinuity of horizon M (the base of the hemipelagic drape dated at 5.5 Ma) caused by the overlying horizon A, further supports the interpretation of the latter as an erosional surface (Fig. 3B). The B l , C th and the C bh seismic facies that generally overlie horizon A correspond to, respectively, the MTDs, channel levee deposits/CLSs and the turbidite feeder channels/aggrading, or meandering channel levee complexes described above (Figs. 2A, B; 3A, B). The narrow, winding features (labelled 2, 3) south of the main erosional submarine channel in Fig. 3C, were interpreted as sinuous channel levee systems (CLSs) or channel forms (after [START_REF] Deptuck | Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea[END_REF]Fig. 3C). The fan-shaped feature associated with CLS 2 was interpreted as a transient fan/lobe deposit (after [START_REF] Adeogba | Transient fan architecture and depositional controls from near-surface 3D seismic data, Niger Delta continental slope[END_REF]; Fig. 3C). The seismic wipe out/grey area in the north-eastern part of the study area was interpreted as a mobile shale, while the linear scars and oblique/elongated feature, crossed by a CLS to the south, were respectively interpreted as normal faults and buried folds (see [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]Fig. 3C). The change in the stratal offset patterns along fault Z (Fig. 3B), from normal at the lower stratigraphic level to inverse at the upper stratigraphic level, was interpreted as fault inversion linked to shale tectonics.

Middle part of MSU 1 (horizons G and I)

Description

J o u r n a l P r e -p r o o f

An RMS amplitude map of horizon G draped on the variance attribute extraction in the middle part of MSU 1 (horizon G), reveals the presence of four narrow, winding features (labelled 1a, 2, 3 and 4 in Fig. 4A) located in the north-western and south-eastern parts of the study area. Compared to winding features 1a and 4, features 2 and 3 display elongated borders of moderate RMS amplitudes (Fig. 4A). Linear scars display trends that vary with the northeastsouthwest and east-west orientations, dominate the northern part of the study area, while the radial patterns dominate the central part of the study area (Fig. 4A). A NE-SW grey area subdivides the study area into northern and southern halves (Fig. 4A). An irregular, rounded, chaotic area is visible in the SE corner of the study area (dashed black line in Fig. 4A). A seismic line (DD I ) through the winding features 1a and 2, reveals the presence of a 1 km wide, 120 m deep relief, infilled with the chaotic, discontinuous to moderately continuous, high amplitude, B h and convergent/thinning, high to moderate amplitude, C th seismic facies that also infill erosional channels 1a and 2 (Fig. 4B).

Furthermore, a variance attribute extraction on the horizon I, in the middle of MSU 1 (above horizon G), reveals the presence of a 2.5 km wide, sinuous feature (labelled 1b) to the north, characterised by the presence of narrower, winding features in its central part (Fig. 5A). A chaotic area is visible on the outer side of the main sinuous feature to the northeast, which is characterised by smaller, linear or slightly curved features (Fig. 5A). A seismic line (BB I ) crossing the main sinuous feature (Fig. 5A) reveals the presence of a 176 m deep relief corresponding to horizon I (Fig. 5B). The 2.5 km wide negative relief associated with horizon I and the enclosed narrower sinuous features are infilled with the chaotic, discontinuous low amplitude, B l seismic facies, the convergent/thinning, high to moderate amplitude, C th seismic facies and the parallel continuous to slightly convergent, high amplitude, C bh seismic facies that J o u r n a l P r e -p r o o f generally disrupt the lateral continuity of the underlying B l and the C th seismic facies (Fig. 5B).

These seismic facies are unconformably overlain by a negative relief corresponding to horizon J (Fig. 5B). In turn, horizon J is overlain by a 166 m deep narrow relief, infilled with the chaotic, discontinuous to moderately continuous, high amplitude B h seismic facies, bordered by the convergent/thinning, high to moderate amplitude C th seismic facies (Fig. 5B). The entire system is abruptly overlain by the chaotic, discontinuous low amplitude, B l seismic facies, covered by hemipelagic drape and display an overall convex-upward geometry (Fig. 5B).

Interpretation

The four narrow winding features associated with horizon G in (Fig. 4A) were interpreted as CLSs or channels forms, while the moderate amplitude zones that parallel the borders of the CLSs 2 and 3 were interpreted as channel belts (Fig. 4A). The NE-SW oriented grey area/seismic wipe-out zone corresponds to the area affected by mobile shale and/or erosion (Fig. 4A). The irregular chaotic area in the SE corner of the study area was interpreted as MTD (Fig. 4A). The 1 km wide, 120 m deep relief associated with horizon G was interpreted as erosional CLS (Fig. 4B). The chaotic, discontinuous to moderately continuous, high amplitude, B h seismic facies and the convergent/thinning, high to moderate amplitude, C th seismic facies that respectively infill and border the erosional CLS associated with horizon G, were interpreted as turbidite feeder channels/aggrading or meandering channel levee complexes and outer levee deposits. The presence of the C th seismic facies (OL) deposits at nearly the same stratigraphic level as the infill of the erosional CLSs (labelled 1a in Fig. 4B), suggests that sediment-gravity flows within the channel were unconfined (see [START_REF] Deptuck | Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea[END_REF]Fig. 4B).

J o u r n a l P r e -p r o o f

The 2.5 km wide, sinuous feature associated with horizon I (Fig. 5A) was interpreted as a sinuous, erosional submarine channel/erosional surface, while the smaller winding features within the erosional channel were interpreted as CLSs (Fig. 5B). The chaotic, discontinuous low amplitude, B l seismic facies, the convergent/thinning, high to moderate amplitude, C th seismic facies and the parallel continuous to slightly convergent, high amplitude, C bh seismic facies that infill or border the erosional submarine channel and the channel forms were interpreted, respectively, as MTDs, channel levee deposits, and distributary channels/lobes (Figs. 2B,5B).

While the convergent/thinning, high to moderate amplitude, C th seismic facies, confined to the main erosional submarine channel (horizon I) was interpreted as inner levee (IL), the C th seismic facies on the outer bank of the erosional channels were interpreted as outer levee (OL) deposits (Figs. 2B,5B). The presence of the IL deposit to the right of the main erosional submarine channel, coupled with the narrowing and lateral shifts of the enclosed CLSs/channel forms (red arrow in Fig. 5B), were interpreted as recording different stages of meandering/lateral aggradation of the system. The presence of OLs at every stage of the erosional submarine channel (Fig. 5B), suggests that sediment-gravity flows were not confined during the development of the system. The convex-upward geometry, displayed by the entire system, suggests that it was overfilled.

Upper part of MSU 1 (horizon K) Description

An RMS amplitude map draped on a variance attribute extraction in the upper part of MSU 1, (labelled horizon K in Fig. 6A), reveals the presence of a 1.8 km wide, bifurcating planform feature in the northern part of the study area (labelled 1c in Fig. 6A). The feature J o u r n a l P r e -p r o o f displays an overall moderate RMS amplitude and is characterised by smaller, winding features with higher RMS amplitude (Fig. 6A). A linear relief is visible on the outer bend of the main system (Fig. 6A). A north-south seismic line (FF I ) through the centre of the system reveals a 1.8 km wide, 90 m deep negative relief, infilled with the chaotic, discontinuous to moderately continuous, high amplitude, B h seismic facies (Fig. 6B). The B h seismic facies is unconformably overlain by the chaotic, discontinuous low amplitude, B l seismic facies, covered by hemipelagic drape and display an overall convex-upward geometry (Fig. 6B).

Interpretation

The 1.8 km wide, 90 m deep relief, characterised by moderate RMS amplitudes and the enclosed, narrow winding features that display higher RMS amplitudes were interpreted as an erosional submarine channel and CLSs, respectively (Fig. 6A,B). The chaotic, discontinuous to moderately continuous, high amplitude, B h seismic facies which infill the erosional submarine channel were interpreted as turbidite feeder channels/aggrading or meandering channel levee complexes, while the overlying, unconformable, B l seismic facies were interpreted as MTDs (Fig. 5B). The sharp relief at the outer bend of the erosional submarine channel was interpreted as a terrace (Fig. 6A,B). The fact this feature matches the earlier erosional submarine channels (labelled 1a-c in Figs. 4A, 5A and 6A), suggests that the feature has been active since the time equivalent to horizon E (Fig. 4A). Between the early stage (labelled 1a in Fig. 4A) and the late stage (labelled 1c in Fig. 6B), this erosional channel migrated more than 10 km NW-SE, with an overall increase in width and erosional efficiency until the age equivalent to that of horizon K, when it was truncated by the overlying, B l seismic facies (MTDs; Fig. 6B). The convex-upward geometry, displayed by the entire system, suggests that it was overfilled.

J o u r n a l P r e -p r o o f 4.3.4 Base of MSU 2 (horizon Q) Description An RMS amplitude map at the base of MSU 2 draped on a variance attribute map (the horizon labelled Q in Fig. 3B), reveals the presence of a 2.5-km wide, sinuous NE-SW trending feature and smaller, NE-SW winding features (labelled 1, 2, 3 and 4, respectively, in Fig. 3D), all in the southern part of the study area. In the northern part of the study area, linear scars display east-west, northeast-southwest and radial patterns from the distal, central to proximal parts, respectively (Fig. 3D). Towards the northeast, two large dark rounded features (>1.5 km wide), characterised by low RMS amplitudes are flanked by smaller, dark rounded features (Fig. 3D).

The large, dark rounded feature to the extreme north-east is marked at its centre by a 0.3 km wide, rounded feature (Fig. 3D). In the north-western part of the study area, a 1 km wide sinuous feature highlighted in grey, marks the northern limit of the small, dark rounded features (Fig. 3D). A north-south seismic line (CC I ) crossing these features, illustrates their stratigraphic architecture (Fig. 3A,B).

Interpretation

The 2.5 km wide, NE-SW trending feature (labelled 1 in the southern part of Fig. 3B,D) and the sinuous feature highlighted in grey to the northwest, correspond to the eroded portion of the stratigraphy, while the narrow winding features (labelled 2, 3 and 4 in Fig. 3D) were interpreted as CLSs. The east-west and northeast-southwest trending as well as radial scars (Fig. 3D) that offset stratigraphy on seismic line (Fig. 3B), were interpreted as normal faults. The large and smaller, dark rounded features in the northern part of the study area were interpreted as mobile shales and fluid escape features, respectively (Fig. 3D). The 0.3 km wide, rounded 7C) reveals the presence of a NE-SW trending, 2.5 km wide sinuous feature, with low RMS amplitude (Fig. 7A,B). This sinuous feature increases in width downstream to the southwest and is characterised by many smaller winding features within the main system (Fig. 7A,B). Rounded relief features > 300 m wide are present on the middle outer bend of the system (Fig. 7A,B). Three narrow winding features are observed south of this sinuous feature, (Fig. 7A,B). Radial, NE-SW and E-W trending linear scars occur in the north, central and southern parts of the study area, respectively (Fig. 7A,B). To the north, two rounded, dark features with low RMS amplitudes are observed associated with the lower feature, flanked by smaller rounded, dark features which are confined within the NE-SW and E-W trending linear scars (Fig. 7A,B). A north-south seismic line (GG I ) through the main sinuous feature, reveals the presence of a 185-m deep, negative relief, characterised by elevated margins and irregular scars to the right of the feature (Fig. 7C,D,E). The 185-m deep relief is infilled with chaotic, discontinuous to moderately continuous, high amplitude, B h seismic facies, which in turn, are unconformably overlain by the coherent/slightly inclined, moderate to high amplitude seismic facies (Fig. 7C,D,E). The infill of the relief associated with horizon R is unconformably overlain by a 180 m relief and a 100 m deep relief that correspond, respectively, to horizons V and W (Figs. 7C,D,E;8D,E). These topographies are in turn infilled with chaotic, J o u r n a l P r e -p r o o f discontinuous to moderately continuous, high amplitude, B h seismic facies and coherent/slightly inclined, moderate to high amplitude seismic facies (Fig. 7C,D,E). The entire system is overlain by an irregular seismic reflection (horizon X), which, in turn, is covered by transparent, parallel continuous, D seismic facies (Fig. 7C,D,E).

Interpretation

The 2.5 km wide, 185 m deep sinuous feature, associated with horizon R was interpreted as a sinuous erosional submarine channel (Fig. 7A-E). The chaotic, discontinuous to moderately continuous, high amplitude, B h seismic facies and the coherent/slightly inclined, moderate to high amplitude seismic facies that infill the erosional channels/erosional surfaces associated with horizons R, V and W (Fig. 7A-E) were interpreted as turbidite feeder channels/aggrading or meandering channel levee complexes and MTDs (slides/slump blocks?), respectively. These turbidite feeder channels/aggrading or meandering channel levee complexes resemble the highly incisional, high amplitude reflections (HARS) previously described on the western Niger Delta slope (e.g. [START_REF] Deptuck | Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea[END_REF][START_REF] Heiniö | Knickpoint migration in submarine channels in response to fold growth, western Niger Delta[END_REF][START_REF] Hansen | Submarine channel evolution, terrace development, and preservation of intra-channel thin-bedded turbidites: Mahin and Avon channels, offshore Nigeria[END_REF] and elsewhere on the adjacent side of the equatorial Atlantic, the Rio Muni Basin (Equatorial Guinea) (see [START_REF] Jobe | Two fundamentally different types of submarine canyons along the continental margin of Equatorial Guinea[END_REF]. The transparent, parallel continuous, D seismic facies, which drape the entire system was interpreted as a hemipelagic drape (Fig. 7C-E). The > 300 m wide, rounded features on the middle outer bend of the erosional submarine channel were interpreted as slides/slump scars (Fig. 2A,7A,B). The narrow, winding features to the south of the main erosional channel were interpreted as sinuous CLSs or channel forms (see [START_REF] Deptuck | Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea[END_REF]; Fig. 7A,B). The lateral discontinuity of seismic reflections (e.g. the horizons P and Q) on the adjacent side of the erosional channel (Fig. 7C-E), demonstrates the erosional character of the sediment-gravity J o u r n a l P r e -p r o o f flows that sculpted the channel. This interpretation is supported by the channel"s deeply incised nature (185-m deep) and the presence of terraces (multiple slide/slump scars) along the elevated margins of the channel (Fig. 7C-E). The relatively confined nature of the erosional submarine channels, the dominance of channel infills by turbidite feeder channels/aggrading or meandering channel levee complexes within the channels and the presence of MTDs (slides/slumps?), distinguish the submarine channels observed in MSU 2 from those observed in MSU 1, where the channels are generally unconfined (e.g. Figs. 4B,5B). The vertical superposition of erosional submarine channels with minor lateral migration (Fig. 7C-E), demonstrates the complex cut and fill architecture of the system with overall vertical aggradation (red arrow in Fig. 7E). The radial, E-W and NE-SW trending linear scars distributed in the north, central and southern part of the study area, were interpreted as normal faults (Fig. 7A-C). The rounded, dark features with low RMS amplitudes were interpreted as mobile shales, while the smaller, dark rounded features that flank the mobile shales were interpreted as fluid escape features (Fig. 7A,B). These mobile shales, mud volcanoes, fluid escape features, BSR and pockmarks (Figs. 1E, 3A, 8A, 9A), support fluid overpressure in the study area (see also [START_REF] Riboulot | Morphological Signature of Fluid Flow Seepage in the Eastern Niger Submarine Delta (ENSD)[END_REF].

Upper part of the MSU 2 (horizons V and W) Description

An RMS amplitude map of horizon V draped on a variance attribute extraction at the upper part of MSU 2 reveals the presence of a 2.1 km wide, sinuous planform feature, characterised by moderate RMS amplitudes and smaller winding features within the main system (Fig. 8A,B). Smaller winding features also occur in the northern and southern parts of the study area (Fig. 8A,B). The borders of the main sinuous system are irregular and contain some J o u r n a l P r e -p r o o f detached curved loops at its outer bends (Fig. 8A,B). North of the system, a NE-SW trending, wedge-shaped chaotic area of low RMS amplitude is observed (dashed black lines in Fig. 8A,B).

Further to the north of the study area, two rounded, dark features with low RMS amplitudes are associated with smaller, dark rounded features (Fig. 8A,B). Some of the smaller, dark rounded features are confined within the NE-SW trending short and long linear scars (Fig. 7A,B). A NW-SE seismic line (HH I ) illustrates a 105 m deep negative relief associated with the 2.1 km wide sinuous feature (Fig. 8C-E). This feature is infilled with chaotic, discontinuous to moderately continuous, high amplitude, B h seismic facies which, in turn, are unconformably overlain by the coherent/slightly inclined, moderate to high amplitude seismic facies (Fig. 8C-E).

The infill of the erosional channel/surface associated with horizon V is abruptly overlain by another 100 m deep relief associated with the overlying horizon W (Figs. 7C-E; 8C, 9A). The topographies of horizons V and W respectively disrupt the lateral continuity of the relatively thick, convergent/thinning, high to moderate amplitude, C th seismic facies, the parallel continuous to slightly convergent, high amplitude, C bh seismic facies, and the draping, D seismic facies (Fig. 7C,8C). Towards the right of Fig. 8C, thin, laterally confined, somewhat coherent, low to moderate reflectivity seismic reflections overlie horizon V (above the V-shaped, linear scars, that offset seismic reflections in Fig. 8A-C).

An RMS amplitude map superimposed on a variance attribute map (the level of the horizon W), just above (the horizon V), reveals the presence of a 2.5 km wide, sinuous planform feature with low RMS amplitudes and characterised by narrow winding features (Fig. 9A). South of the main sinuous system, two elongate features, characterised by high RMS amplitudes in the eastern and western parts of the study area are observed, with narrow, winding features crossing the eastern elongate feature (Fig. 9A). To the north, two rounded, dark features are flanked by J o u r n a l P r e -p r o o f short and long linear scars that are in turn, flanked by smaller, dark rounded features (Fig. 9A).

An irregular chaotic area with an overall low RMS amplitude to the northeast (dashed black line in Fig. 9A), occurs adjacent to the rounded, dark features (Fig. 9A). An E-W oriented seismic line (JJ I ) which crosses the elongated features with high RMS amplitudes in the southern part of the study area, shows their high reflectivity and onlapping geometry at the margin of the intraslope basin (yellow arrow in the top left-hand corner of Fig. 9B). Although our observations of the previous seismic lines show that the infills of negative topographies are generally chaotic and difficult to trace (e.g. Figs. 3A,B,7C,D), the erosional surfaces (labelled horizons X and W in Fig. 8C-E), were locally correlated inside the negative relief.

Interpretation

The wide sinuous features associated with horizons V and W (Figs. 8A, B; 9A) were interpreted as meandering, erosional submarine channels, while the narrow winding features to the north and south of these features, were interpreted as sinuous CLSs or channel forms. (Figs.

8A, B; 9A, B). The chaotic, discontinuous to moderately continuous, high amplitude, B h seismic facies and the coherent/slightly inclined, moderate to high amplitude seismic facies that infill the erosional submarine channels associated with horizons V and W were respectively interpreted as turbidite feeder channels/aggrading or meandering channel levee complexes and MTDs (?slide blocks/slump deposits) (Fig. 8A-E). The occurrence of the slide/slump deposits at the same stratigraphic level as the laterally extensive MTDs that overlie the erosional surface (labelled horizon V; e.g. Fig. 8C-E), suggests that the former may derive from failures of the latter during major sediment-gravity flows. The D seismic facies interpreted as hemipelagic drape/muddy turbidites, where preserved, generally drape the erosional submarine channels and their infilling J o u r n a l P r e -p r o o f (Fig. 7C-E, 8C-E). The detached, curved loops on the outer bends of the erosional channel associated with horizon V were interpreted as abandoned meander loops (Fig. 8A,B). The chaotic areas with overall low RMS amplitudes north and northeast of the study area (Fig. 8A,B) were interpreted as MTDs. The moderate RMS amplitudes associated with the MTDs to the northeast of the rounded, dark features (mobile shales; Fig. 7A), suggest the presence of sands within the MTDs that were trapped within local topography associated with the mobile shales (Fig. 8A,B). The short, east-west trending linear scars, adjacent to the mobile shales and the associated small, dark rounded features, were interpreted as fault scars and fluid escape features, respectively (Fig. 8A). The longer linear scars confined within the shorter (fault scars), characterised by coherent/slightly deformed internal architecture with moderate to high reflectivity on the seismic line (e.g. Fig. 2B,5A) were interpreted as mobile shale-derived MTDs (see [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]. The lateral confinement of these MTDs adjacent to the mobile shales (Figs. 8A,9A), coupled with their somewhat coherent internal architecture and moderate to high RMS amplitudes, distinguish them from the regional MTDs (see [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]. The elongated features, characterised by high RMS amplitude in the southern part of the study area (Fig. 9A), were interpreted as transient fan/lobe deposits (after Adeobga et al., 2005). The winding features (CLSs) that cross the eastern transient fan/lobe suggest that sediment was bypassed on the eastern transient fan to a more distal setting beyond the limit of the 3D block we studied (Fig. 9A). The upward decrease in the thickness of slide/slump deposits from horizons R to W (Figs. 7C-E, 9C-E) suggests an overall decrease in the incisional ability of the sedimentgravity flows that sculpted the systems, or the gradual establishment of a local slope equilibrium profile in response to an overall decrease in shale tectonics. However, the upward decrease in the thickness of outer levees from the horizons R-W (Figs. 8C-E, 9C-E), suggests increased J o u r n a l P r e -p r o o f between MSU 1 and MSU 2 (P/Q) boundary could have occurred around 0.8-1.0 Ma during the Middle Pleistocene Transition (MPT) as described in the literature (0.8-0.9 Ma;[START_REF] Miller | The Phanerozoic Record of Global Sea Level Change[END_REF]Lisiecki and Raymo;[START_REF] Zecchin | The architectural variability of small-scale cycles in shelf and ramp clastic[END_REF]Fig. 10B,C). The eight seismic reflections identified in MSU 2 , could represent eight fifth-order erosional surfaces (sequence boundaries). The hemipelagic drape, which marks the transition between MSU 1 and MSU 2 (horizon P), correlates with the moderate oxygen isotope (3.5%) and moderate sea-level variations (~12.5 m), before the more pronounced sea-level cycles with amplitudes reaching 100 m (Fig. 10B,C). This hemipelagic drape (horizon P), like horizon M at the lower part of MSU 1, was inferred to have been deposited during an overall high relative-sea level (see Chima et al., 2019 and references therein). The assigned age (0.8-1.0 Ma) agrees with that attributed by [START_REF] Benjamin | Canyon-confined pockmarks on the western Niger Delta slope[END_REF], who suggested that this conformable seismic surface belonged to the middle Pleistocene. The repetitive motifs consisting of vertically stacked erosional submarine channels, infilled with aggrading/meandering channel levee complexes (CLCs) and MTDs (slides/slumps?), and hemipelagic drapes (where preserved), e.g. (Figs. 3A,B;8D,E), appear to record the regressive-transgressive cycles previously documented on the eastern Niger Delta continental shelf (e.g. [START_REF] Fatoke | Controls on Depositional Architecture and Sequence Stratigraphy of the Pliocene-Pleistocene Strata of Eastern Niger Delta, Nigeria[END_REF][START_REF] Jermannaud | Plio-Pleistocene sequence stratigraphic architecture of the eastern Niger Delta: A record of eustasy and aridification of Africa[END_REF][START_REF] Riboulot | Morphological Signature of Fluid Flow Seepage in the Eastern Niger Submarine Delta (ENSD)[END_REF]. Similar architectural elements have been documented in the deep-water setting, offshore the Rio Muni Basin (Equatorial Guinea), where the development of submarine canyons and their infilling turbidite systems were linked to a combination of allocyclic (relative sea level, tropical storms and tectonics), and autocyclic forcing, linked to submarine channel avulsions (see [START_REF] Jobe | Two fundamentally different types of submarine canyons along the continental margin of Equatorial Guinea[END_REF]. What is more, the submarine channels with similar architectural features described in this study , have been recorded in the deep-water Bowser Basin,

J o u r n a l P r e -p r o o f

Northern British Columbia (Canada), where slope gullies and their infilling turbidite systems were linked to relative sea-level changes and local slope failures [START_REF] Ricketts | Shelfbreak Gullies; Products of sea-level lowstands and J o u r n a l P r e -p r o o f sediment failure: Examples from Bowser Basin, Northern British Columbia[END_REF].

From the foregoing observations, it is reasonable to infer that the erosional surfaces observed in MSU 2 (horizons Q-X), notably, the repetitive, highly incisional submarine channels associated with horizons R, S, V, W and X (e.g. Figs. 3A,B;), could represent what we call "the falling stage erosional surfaces" (FSESs; Fig. 12), equivalent to the basal surface of forced regression (BSFR) that are generally observed on the continental shelf (see [START_REF] Jermannaud | Plio-Pleistocene sequence stratigraphic architecture of the eastern Niger Delta: A record of eustasy and aridification of Africa[END_REF]Roby et al., 2011;De Gasperi and Catuneanu, 2014;[START_REF] Catuneanu | Seqquence stratigraphy of deepwater systems[END_REF]. Where deeply incised, the FSESs are locally overlain by the previously described basal MTDs or the bypass facies (e.g. Fig. 5B), which we interpreted as the falling stage systems tracts (see [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF]De Gasperi and Catuneanu, 2014;[START_REF] Catuneanu | Seqquence stratigraphy of deepwater systems[END_REF]. The infilling, aggrading/meandering channel levee complexes (CLCs) and MTDs (slide/slumps?) (Figs. 3A,B; 7C-E; 8C-E), were interpreted as lowstand to early transgressive systems tracts, while the hemipelagic drapes that generally cover the entire system (where preserved) (e.g. Figs. 3A,B; 7C-E; 8C-E), were interpreted as late transgressive to highstand systems tracts (see [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF]De Gasperi and Catuneanu, 2014;[START_REF] Catuneanu | Seqquence stratigraphy of deepwater systems[END_REF]. These architectural elements are rarely completely preserved either inside or outside the erosional submarine channels due to incessant mass wasting on the western Niger Delta slope (e.g. Figs. 2345678910).

The estimated duration of the sequence in MSU 2 (see section 4.6 below), suggests that these architectural elements may have been controlled by glacio-eustatic sea-level oscillations over the last circa 0.8-1.0 Ma (see [START_REF] Miller | The Phanerozoic Record of Global Sea Level Change[END_REF]Lisiecki and Raymo;[START_REF] Zecchin | The architectural variability of small-scale cycles in shelf and ramp clastic[END_REF]Fig. 10B,C).

The estimated age of the last erosional surface in MSU 2 (horizon X; Figs. 3A,10B,C) was interpreted as the LGM, at around ~20 ka, as also suggested by [START_REF] Jobe | Rapid adjustment of submarine channel architecture to changes in sediment supply[END_REF]2016).

J o u r n a l P r e -p r o o f

Sequence stratigraphic interpretation of MSU 1

Based on the above interpretation of MSU 2, MSU 1 could correspond to a deposition between 5.5 Ma and 0.8 Ma (horizons M-P), with the 11 erosional surfaces (labelled A to K), representing 11 fourth-order FSESs (sequence boundaries; Fig. 10B,C). Although, we included the hemipelagic drape (horizon M, dated at 5.5 Ma) in MSU 1 for calibration of the lower part of the MSU 1, the overlying erosional surface (labelled horizon A), which marks the onset of MTDs and channel levee deposits (Fig. 3B), was interpreted as the actual basal limit of MSU 1.

The erosional surface associated with horizon A correlates with a high oxygen isotope value (> 4.5%), and one of the highest falls in the relative sea-level (> 25 m) during the early Pliocene (Fig. 10B,C). We suggest an age of 4.9 Ma for this 185 m deep erosional surface, reasoning that it could be correlated with the first major fall in sea level just after the late Miocene (5.5 Ma) high sea level (see [START_REF] Miller | The Phanerozoic Record of Global Sea Level Change[END_REF][START_REF] Lisiecki | Plio-Pleistocene climate evolution: trends and transitions in glacial cycle dynamics[END_REF][START_REF] Raymo | The accuracy of mid-Pliocene δ 18 O based ice volume and sea level reconstructions[END_REF]; Fig. 10B,C). The 4.9 Ma we propose for this erosional surface (horizon A in Fig. 10B,C), disagrees with the late Miocene proposed by [START_REF] Benjamin | Canyon-confined pockmarks on the western Niger Delta slope[END_REF], who lacked biostratigraphic control. The occurrence of the A horizon just above the hemipelagic drape dated at 5.5 Ma, its erosional character (Fig. 3A-C, 7C-E, 8C-E), and association with MTDs and CLSs, further support its nomination as a possible candidate for the fall in sea level in the early Pliocene around 4.9 Ma (see [START_REF] Miller | The Phanerozoic Record of Global Sea Level Change[END_REF][START_REF] Lisiecki | Plio-Pleistocene climate evolution: trends and transitions in glacial cycle dynamics[END_REF][START_REF] Raymo | The accuracy of mid-Pliocene δ 18 O based ice volume and sea level reconstructions[END_REF]; Fig. 10B,C).

The transition at the upper part of MSU 1 (the G/I interface), is an ~120-m deep erosional surface (e.g. Fig. 4B), which tentatively correlates with a (> 30) m fall in the relative sea level and relatively high oxygen isotope (> 3.5%) (Fig. 10B,C). The erosional character of this surface coupled with its association with multiple CLSs (e.g. Fig. 4A,B), suggest that it could J o u r n a l P r e -p r o o f have developed during a lowering of the relative sea level. The 176 m deep relief associated with horizon I (just above horizon G in Fig. 5B) and its complex cut and fill architecture, resemble the erosional submarine channels observed in MSU 2 (e.g. horizons R, V, W in Figs. 7,8,9A). The 176 m deep erosional submarine channel associated with horizon I and the overlying basal MTDs (bypass facies; e.g. Fig. 5B), could be interpreted as the falling stage erosional surface (FSES) and falling stage systems tracts, respectively (see [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF]De Gasperi and Catuneanu, 2014;[START_REF] Catuneanu | Seqquence stratigraphy of deepwater systems[END_REF] Similarly, the infilling turbidite feeder channels/aggrading or meandering CLCs (e.g. Fig. 5B) could be interpreted as lowstand to early transgressive systems tracts, while the overlying hemipelagic drape could represent the late transgressive to highstand systems tracts (e.g. 5B, 6B).

The transition from CLS dominated intervals to repetitive, highly incisional submarine channels and mass-wasting dominated intervals is visible around horizons G and I (e.g. Figs. 3,4,[START_REF]Depositional model Despite the dissimilar seismic facies association in MSU 1 and MSU 2 (e.g. Fig. 2B, C), the overall sedimentary architecture observed in the two units (Figs. 3-10), suggests that allogenic forcings at different scales and basin tectonics controlled the stratigraphic evolution of the western Niger Delta intraslope basins over the last 5.5 Ma. Comparing our observations in MSU 1 and MSU 2 (Figs. 2-10) with published depositional models[END_REF]. This change in the sedimentary architecture could be associated with a fall in the relative sea level probably during the early Pleistocene, following the Northern Hemisphere Glaciation (NHG), which, in the literature, is documented to have occurred around 2.6 Ma (see [START_REF] Miller | The Phanerozoic Record of Global Sea Level Change[END_REF][START_REF] Lisiecki | Plio-Pleistocene climate evolution: trends and transitions in glacial cycle dynamics[END_REF][START_REF] Gibbard | Formal ratification of the Quaternary System/Period and the Pleistocene Series/Epoch with a base at 2.58 Ma[END_REF]Head and Gibbard, 2015a, b;[START_REF] Gibbard | Partitioning the Quaternary[END_REF][START_REF] Raymo | The accuracy of mid-Pliocene δ 18 O based ice volume and sea level reconstructions[END_REF]; Fig. 10B,C). Hence, we propose erosional surfaces G and I as possible candidates for the early Pleistocene but favour the latter due to its repetitive, incisional and aggrading/meandering patterns that resemble those described in the deep-water western Niger Delta over the last circa 120 ka as controlled by glacio-eustatic sea-level oscillations and sediment supply (see [START_REF] Jobe | Rapid adjustment of submarine channel architecture to changes in sediment supply[END_REF]2016). 4.6 Sequence duration and sedimentation rates in MSU 1 and MSU 2

J o u r n a l P r e -p r o o f

Sequence duration measures the ratio of the time interval for seismic unit development to the number of enclosed seismic surfaces (see Zhang et al., 2019). Using the estimated ages above, the sequence duration for MSU 1 yields (5.5-0.8) Ma/13= ~0.4 Ma (~400 ka) and (0.8-0)

Ma/8 = ~0.1 Ma (~100 ka) for MSU 2. Using an average velocity of 1253.2 m/s obtained from Well-1 (W1) (Fig. 1E), the estimated sedimentation rates in non-decompacted MSU 1 and 2 are132 m/Ma and 370 m/Ma, respectively.

Discussion

We discuss the Pliocene and Pleistocene stratigraphic evolution of the western Niger Delta intraslope basins over the last 5.5 Ma in relation to allocyclic (glacio-eustatic) sea-level and basin tectonic forcings, and propose a deep-water depositional model.

Allocyclic (glacio-eustatic) sea-level forcing

Allocyclic factors refer to controls that are unrelated to the depositional system; these include climate, eustasy, tectonics and sediment supply [START_REF] Einsele | Cycles and Events in Stratigraphy[END_REF]. Orbital parameters, such as obliquity and eccentricity forcings control global sea level, climate, drainage basin erosion, sediment supply, and more locally, environmental energy, and consequently influence the architecture of sedimentary units and their bounding surfaces [START_REF] Zecchin | The architectural variability of small-scale cycles in shelf and ramp clastic[END_REF]2010;2012;Catuneanu and Zecchin, 2013).

High subsidence rates (350 m/Ma) offset by high sedimentation rates (36.5 cm/ka) have been shown to facilitate the preservation of eccentricity sequences of 100 ka on the eastern Niger Delta continental shelf over the last circa 0.5 Ma [START_REF] Riboulot | Geometry and chronology of late Quaternary depositional sequences in the Eastern Niger Submarine Delta[END_REF], in agreement with observations of the Mediterranean margin over the last circa 0.5 Ma, where subsidence rates as J o u r n a l P r e -p r o o f high as 255 m/Ma, accompanied by high sedimentation rates, enabled preservation of 100 ka eccentricity sequences, linked to glacio-eustatic sea-level oscillations during the Quaternary (see [START_REF] Rabineau | Paleosea levels reconsidered from direct observation of paleo-shoreline position during Glacial Maxima (for the last 500 000 yr)[END_REF][START_REF] Leroux | Stratigraphic simulation on the shelf of the Gulf of Lions: testing subsidence rates and sea-level curves during Pliocene and Quaternary[END_REF]. Although we did not estimate subsidence rates over the last circa 5.5 Ma, the overall increase in the sedimentation rates (x3) at the MPT (0.8-1.0 Ma), possibly accompanied by high subsidence rates (see [START_REF] Goudie | the drainage of Africa since the Cretaceous[END_REF], is inferred to have facilitated the preservation of the 100 ka sequences we observed in MSU 2. This interpretation is also in agreement with that of Gulick et al. (2012), who documented an overall increase in the sedimentation rates at a source-to-sink-scale on the New Jersey margin (USA), linked to glacioeustatic changes in sea level over the last 0.7-1.2 Ma.

Although the 400 ka eccentricity sequences we propose for MSU 1 are recognized to have strongly influenced sedimentation in ocean basins (e.g. [START_REF] Pälike | Extended orbitally forced palaeoclimatic records from the equatorial Atlantic Ceara Rise[END_REF][START_REF] Boulila | On the origin of Cenozoic and Mesozoic "third-order" eustatic sequences[END_REF], eccentricity has rarely been described in the deep basin architecture. MSU 1 displays a 400 ka sequence duration and is characterised by the ubiquity of discontinuous seismic reflections, the preponderance of regional MTDs and erosional surfaces, characterised by complex cut and fill architecture in the upper part of the unit (Fig. 3A,B,5A,B). These architectural features resemble the fourth-order incised valleys/slope canyons on the Foz do Amazonas continental shelf that were biostratigraphically constrained and, based on seismic records, calibrated as 400 ka eccentricity sequences in the Pliocene (see [START_REF] Gorini | Late Neogene sequence stratigraphic evolution of the Foz do Amazonas Basin, Brazil[END_REF].

Based on the repetitive motifs of erosional surfaces, the infilling aggrading/meandering channel levee complexes, hemipelagic draping and the sequence durations estimated in MSU 1 and MSU 2, our interpretation is that depositional sequences in the two units were organised in 400 ka and 100 ka sequences, respectively (Fig. 10B,C).

J o u r n a l P r e -p r o o f

The Pliocene and the Pleistocene stratigraphy of the western Niger Delta intraslope basins, respectively characterised by mass-wasting and turbidite sequences, and by hemipelagic drapes and turbidite sequences , is also comparable with the Pliocene and the Pleistocene stratigraphy of the Ulleung Basin, East Sea (Japan), which is characterised by, respectively, debris-flows and turbidite sequences, and hemipelagic drapes and turbidites sequences (see [START_REF] Yoo | Plio-Quaternary seismic stratigraphy and depositional history of the Ulleung Basin, East Sea: Association with debris-flow activity[END_REF]. In the Ulleung Basin, the Pliocene stratigraphy is mainly linked to tectonic deformation, while the Pleistocene is linked to changes in the relative sea level [START_REF] Yoo | Plio-Quaternary seismic stratigraphy and depositional history of the Ulleung Basin, East Sea: Association with debris-flow activity[END_REF]. However, the Pliocene and Pleistocene stratigraphy described in the present study contrasts with that of the northern Gulf of Mexico (GOM), where tectonic subsidence is reported to have facilitated thick hemipelagic sedimentation/muddy turbidites and thin MTDs during the Pliocene compared to the Pleistocene, which is dominated by thick MTDs and thin hemipelagic sediments [START_REF] Madof | Implications for sequence stratigraphy in muddy settings devoid of depositional sequences[END_REF][START_REF] Wu | Masstransport complexes (MTCs) document subsidence patterns in 1 a northern Gulf of Mexico salt minibasin[END_REF][START_REF] Catuneanu | Seqquence stratigraphy of deepwater systems[END_REF]. [START_REF] Madof | Implications for sequence stratigraphy in muddy settings devoid of depositional sequences[END_REF] and [START_REF] Wu | Masstransport complexes (MTCs) document subsidence patterns in 1 a northern Gulf of Mexico salt minibasin[END_REF], showed that the stratigraphy of the northern Gulf of Mexico over last 3.7 Ma was mainly controlled by differential sedimentation and associated salt-related subsidence, while the overall increase in sedimentation rates over the Pleistocene was largely controlled by the capture of the Ohio and Missouri Rivers by the Mississippi in the late Pleistocene (see also [START_REF] Prather | Classification, Lithologic Calibration and Stratigraphic Succession of Seismic Facies of Intraslope Basins, Deep-Water Gulf of Mexico[END_REF].

Pliocene and Pleistocene stratigraphy in the western Niger Delta intraslope basins was controlled by a combination of glacio-eustasy and shale tectonics (see discussion below). The observed increase in the sedimentation rates at the MPT (0.8-1.0 Ma), could be linked to (i) glacio-eustatic sea-level forcing linked to the Milankovitch cycles; (ii) regional climate forcing associated with the West African Monsoon, which has been shown to have interacted with drainage basins, erosion, and the supply of sediment to the Niger Delta since the Plio-Quaternary J o u r n a l P r e -p r o o f (see Lézine et al. (2005;[START_REF] Weldeab | Spatio-temporal evolution of the West African monsoon during the last deglaciation[END_REF][START_REF] Collins | Insolation and glacialinterglacial control on southwestern African Hydroclimate over the last 140,000 years[END_REF][START_REF] Govin | Astronomically forced variations in western African rainfall (21°N-20°S) during the Last Interglacial period[END_REF][START_REF] Armitage | West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad[END_REF]; (iii) a possible shift in sediment fluxes from the eastern to the western Niger Delta over the Pleistocene due either to delta lobe switching/regional climate evolution (see Jermanaud et al., 2010;and references therein), or tectonic reorganisation of the catchment (e.g. [START_REF] Goudie | the drainage of Africa since the Cretaceous[END_REF]. A recent study by [START_REF] Picot | Monsoon control on channel avulsions in the Late Quaternary Congo Fan[END_REF] in the adjacent Congo Basin demonstrated an overall architectural submarine channels avulsion that can be applied to most progradation/retrogradation patterns in response to climate forcing.

Basin tectonics

In terms of basin tectonics, control of the Pliocene and Pleistocene stratigraphy of the western Niger Delta intraslope basins, [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF], showed that the main erosional submarine channel in the study area (labelled 1, 2, 3; Fig. 11A,B), has been active since the Pliocene, and was generally deflected to the southern part of the study area with east-west orientation during the early and late active periods of mobile shales (stages 1 and 3 in Fig. 11A, B), and northwards, with a northeast-southwest orientation during quiescent shale tectonics (stage 2 in Fig. 11A,B). In the present study, we investigated the entire submarine channel systems over the last 5.5 Ma, and provide a better understanding of the control of basin tectonics and deep-water depositional systems (Fig. 11A,B).

The progressive shift of erosional submarine channel levee systems from the NW to the SE in MSU 1 (stages 1a-c; Fig. 11A) could be linked to avulsion events in response to (i) shale tectonics or (ii) changes in sediment supply. Similarly, the absence of submarine channels in the northern part of the study area in MSU 2 (Fig. 11B) could be due to (i) up-dip channel avulsions in response to the movement of mobile shales (see also [START_REF] Clark | Interactions between coeval sedimentations and deformation from the Niger Delta deep-water fold belt[END_REF][START_REF] Jolly | Growth history of fault-related folds and interaction with seabed channels in the toe-thrust region of the deep-water Niger Delta[END_REF] J o u r n a l P r e -p r o o f

Falling stage

Falling is controlled solely by episodes of relative sea-level falls and is the key criterion for the definition of stratigraphic cyclicity in deep-water settings [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF]De Gasperi and Catuneanu, 2014;[START_REF] Catuneanu | Seqquence stratigraphy of deepwater systems[END_REF]. The highly incisional erosional surfaces we observed in MSU 1 and MSU 2 (e.g. Figs. 3,[START_REF]Depositional model Despite the dissimilar seismic facies association in MSU 1 and MSU 2 (e.g. Fig. 2B, C), the overall sedimentary architecture observed in the two units (Figs. 3-10), suggests that allogenic forcings at different scales and basin tectonics controlled the stratigraphic evolution of the western Niger Delta intraslope basins over the last 5.5 Ma. Comparing our observations in MSU 1 and MSU 2 (Figs. 2-10) with published depositional models[END_REF]7,8,9A), which we interpreted as the falling stage erosional surfaces (FSESs; Fig. 12), were inferred to have developed respectively during the fourth-and fifth-order falling stage sea-level, erosion and bypass processes on the slope (Fig. 13A). While the basal MTDs, which locally overlie the FSESs (e.g. Fig. 5B), could represent the falling stage system tracts (Fig. 13A), high-density grain/mud flows are inferred to have delivered sandy to muddy submarine fans to the basin floor [START_REF] Catuneanu | Seqquence stratigraphy of deepwater systems[END_REF]. The occurrence of basal MTDs at the bases of the erosional submarine channels associated with horizons A and I (e.g. Figs. 3B, 4B, 5B, 7C, 8C), suggest that sediment-gravity flows were rich in mud and were partly sequestered in erosional submarine channels on the slope on their way to the deep basin (Fig. 13A). Similar observations have been reported in the deep-water eastern Niger Delta, where

MTDs were calibrated with core data and shown to be mud-rich (debris flows), linked to fourthorder relative sea-level fall (see Zhang et al., 2019). The overall absence of MTDs at the bases of the erosional submarine channels in MSU 2, suggests that the former was eroded or not deposited in erosional submarine channels in this unit unlike in MSU 1 (compare Figs. 3A,B, 5B, 7C-E; 7C-E). Similarly, the paucity of MTDs in MSU 2 (e.g. Figs. 3A, B; 10A-C), could be related to overall changes in the sediment supply/calibre, the flow dynamics, or the relatively quiescent shale tectonics over the Pleistocene.

J o u r n a l P r e -p r o o f

Lowstand

The turbidite feeder channels/aggrading or meandering channel levee complexes (CLCs) are the most dominant seismic facies described in the two FSESs (Figs. 3-10). In the complex infilling of erosional submarine channels, the ubiquity of successive erosions and mass-flow deposits render interpretation more complex (e.g. Figs. 12B,C). Similarly, the regional MTDs previously interpreted to have been sourced from the upper slope/shelfmargin (see [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]e.g. Figs. 3A, B; 5B 8C; 9B; 10A, B), were inferred here to be lowstand/early transgressive deposits (Fig. 13B,C). The high amplitude reflectivity (HARS) of the turbidite feeder channels/aggrading or meandering channel levee complexes character, suggests that they were deposited from sand-rich sediment-gravity flows (see also [START_REF] Chapin | Integrated seismic and subsurface characterization of Bonga Field, offshore Nigeria[END_REF][START_REF] Deptuck | Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea[END_REF][START_REF] Heiniö | Knickpoint migration in submarine channels in response to fold growth, western Niger Delta[END_REF][START_REF] Jobe | Two fundamentally different types of submarine canyons along the continental margin of Equatorial Guinea[END_REF]2015). Although slope overstepping by normal and reverse faulting are known to trigger mass failures on the slope (e.g. [START_REF] Ricketts | Shelfbreak Gullies; Products of sea-level lowstands and J o u r n a l P r e -p r o o f sediment failure: Examples from Bowser Basin, Northern British Columbia[END_REF][START_REF] Fatoke | Controls on Depositional Architecture and Sequence Stratigraphy of the Pliocene-Pleistocene Strata of Eastern Niger Delta, Nigeria[END_REF][START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF], the overall association of the regional MTDs with erosional events that correlate with relative sea-level lowering (e.g. horizon A in Fig. 3A,B; 10B, C), supports the interpretation of the former as lowstand deposits (see [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF]De Gasperi and Catuneanu, 2014;[START_REF] Catuneanu | Seqquence stratigraphy of deepwater systems[END_REF]. Low density turbidity currents and fine grain flows have been described as the main processes during the lowstand/early transgression [START_REF] Catuneanu | Seqquence stratigraphy of deepwater systems[END_REF]. Slope depositional J o u r n a l P r e -p r o o f patterns during the lowstand may have been locally influenced by the movement of mobile shales and associated fluid flows (see [START_REF] Riboulot | Geometry and chronology of late Quaternary depositional sequences in the Eastern Niger Submarine Delta[END_REF]Figs. 1D; 3A, B; 8A, 9A, 13C).

Transgression to Highstand

The thick hemipelagic sediments that generally drape the entire system, where preserved (e.g. Figs. 3B, 7C-E, 8C-E), were inferred to have been deposited during the late transgressions/highstands, when the deep-water settings were starved of clastic sediment (see [START_REF] Posamentier | Seismic geomorphology and stratigraphy of depositional elements in deep-water settings[END_REF]De Gasperi and Catuneanu, 2014;[START_REF] Catuneanu | Seqquence stratigraphy of deepwater systems[END_REF]; Fig. 13D).

The conformable nature of the hemipelagic drape (e.g. Figs. 3B, 7C-E, 8C-E), their mud-rich nature inferred from moderate gamma-ray log patterns (e.g. Fig. 13D) and their association with

Discoaster qinqueramus as recorded in Well 3 (W3) (Fig. 13D), support their interpretation as transgressive/highstand deposits (see also [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]. Hemipelagic sedimentation is inferred to have punctuated deep-water deposition by shutting down sediment-gravity flows (see Burgess et al., 2008) The foregoing processes were inferred to have occurred thirteen times in MSU 1 and eight times in MSU 2 (e.g. Fig. 10B,C). A simplified schematic diagram illustrating the longterm (4/5 th -order sea-level controlled) incision, infilling and hemipelagic draping of submarine channels on the western Niger Delta slope is shown in Fig. 13E. Although the proposed model mainly hinges on long-term changes in relative sea level, our observations suggest that shale tectonics locally influenced depositional patterns on the western Niger Delta intraslope basins over the last 5.5 Ma.

Conclusions

J o u r n a l P r e -p r o o f with biostratigraphic data in future studies, could serve as a useful analogue for further studies in the Gulf of Guinea, the equatorial Atlantic and other margins fed by large deltas.

An ongoing study is investigating the control of delta top extensional tectonics over sediment sequestration/bypass on the shelf and its implication for gravity tectonics in the western Niger Delta. J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f

  J o u r n a l P r e -p r o o f feature in the centre of the mobile shale to the northeast was interpreted as a giant pockmark (after Ribolout et al., 2012). 4.3.5 Middle part of MSU 2 (horizon R) Description An RMS amplitude map in the middle of MSU 2 superimposed on a variance attribute extraction (the level of horizon R in Fig.

Fig. 1 .

 1 Fig. 1. Location of the study area. A: superimposed relief and bathymetric map showing the

Fig. 3 :

 3 Fig. 3: The Pliocene and Pleistocene stratigraphic architecture of the western Niger Delta intraslope basins. A, B: Uninterpreted north-south seismic line and line drawing (CC I ), showing

Fig. 4 .

 4 Fig. 4. The sedimentary architecture of the middle part of

Fig. 9 .

 9 Fig. 9. Sedimentary architecture of the upper part of MSU 2. A: An interpreted RMS amplitude

Fig. 10 :

 10 Fig. 10: Sequence stratigraphy. A: An uninterpreted and B: an interpreted E-W seismic and line drawing (KK I ) shows the Pliocene and

Fig. 12 .

 12 Fig. 12. Schematic illustration of the facies association and terminology used in the western

Fig. 13 .

 13 Fig. 13. Proposed depositional model illustrating the long-term Pliocene and Pleistocene

Fig. 13 E

 13 Fig. 13 E: Schematic illustration of facies association and systems tracts in the erosional

J o u r n a l P r e -p r o o f

Acknowledgments

We thank the Federal Republic of Nigeria for sponsoring the PhD of Kelvin Ikenna Chima through the Petroleum Technology Development Fund (PTDF) Scholarship Programme.

We are grateful to Shell Nigeria not only for providing the data used in this study but also for permission to publish it. PTDF provided the computer workstation used in this study. Many thanks to Schlumberger for providing Petrel (TM) software, which we used to interpret the data.

The first author would like to thank the management and staff of Alex Ekwueme Federal University, Ndufu-Alike Ikwo, Ebonyi State, Nigeria for their support and encouragement during this research. This study was carried out at the Institut des Sciences de la Terre (ISTeP), Paris.

An insightful discussion with Bilal U. Haq helped improve the quality of this paper. Zane R.

Jobe is thanked for providing useful review comments on the first draft of the manuscript.

Miguel-Mora Glukstad assisted in velocity modelling for time-depth conversion. Special thanks to Olusegun Obilaja, Adekemi Afolayan, Babatope Atitebi, Richard Streatfield and Timi Tralagba (Shell Nigeria) for their invaluable support with data. This paper benefited from the comments and suggestions of three anonymous reviewers. We thank the editor-in-chief (Liviu Matenco), for granting us sufficient time to revise this paper.

J o u r n a l P r e -p r o o f

confinement of sediment-gravity flows, or an overall decrease in the mud fraction of the former.

The overall concave upward topography associated with erosional submarine channels in MSU 2, suggests that, prior to hemipelagic draping (e.g. Figs. 7C-E, 8C-E), they were underfilled compared to MSU 1, where erosional submarine channels were generally overfilled (e.g. Fig. 5B).

Based on the foregoing observations, three major changes occurred in the sedimentary records of the western Niger Delta intraslope basins over the last 5. [START_REF]Depositional model Despite the dissimilar seismic facies association in MSU 1 and MSU 2 (e.g. Fig. 2B, C), the overall sedimentary architecture observed in the two units (Figs. 3-10), suggests that allogenic forcings at different scales and basin tectonics controlled the stratigraphic evolution of the western Niger Delta intraslope basins over the last 5.5 Ma. Comparing our observations in MSU 1 and MSU 2 (Figs. 2-10) with published depositional models[END_REF][6][7][8][9][10]. These changes include [START_REF] Ruddiman | Pleistocene evolution: northern hemisphere ice sheets and North Atlantic Ocean[END_REF] the lower part of MSU 1 at the transition between the late Messinian thick hemipelagic drape dated at 5.5 Ma (horizon M) to the overlying erosional surface (horizon A), characterised by relatively confined, erosional submarine channels, thin MTDs and channel levee deposits (e.g. Fig. 3A,B); (2) the upper part of MSU 1 at the transition between horizons G and I, dominated by thick, laterally extensive MTDs and partially confined, repetitive incisional submarine channels/erosional surfaces, with complex cut and architecture (e.g. Figs. 2A, 3A-C, 5A, B, 6A, B); (3) the transition between MSU 1 and MSU 2 (horizons P/Q), which marks the onset of repetitive, highly incisional, submarine channels/erosional surfaces that are infilled with aggrading/meandering channel levee complexes, thin MTDs and hemipelagic drapes (where preserved; see for e.g. Figs. 3A, B, 7C-E 8C-E).

Sequence stratigraphic interpretation of MSU 2

The preceding 3D geomorphological analysis reveals that MSU 1 and MSU 2 present different seismic facies and morphology . Using the biostratigraphically constrained late Messinian (5.5 Ma), the modern seafloor (0 Ma), and taking the observed changes in the sedimentary architecture into account , we suggest that the change at the transition

J o u r n a l P r e -p r o o f

Detailed seismic stratigraphy and 3D geomorphological analysis of high-resolution 3D seismic data over the Pliocene and Pleistocene stratigraphic record of the western Niger Delta intraslope basins, showed that:

1. Glacio-eustatic sea-level changes since the last 5.5 Ma are recorded at regional scale in the intraslope basins of the western Niger Delta.

2. The depositional sequence in the intraslope basins of the western Niger Delta is generally defined by the falling stage erosional surface (FSES) at its base and top (sequence boundary), and is characterised by (i) basal MTDs/bypass facies (where preserved), (ii) turbidite feeder channels/aggrading or meandering channel levee complexes and/or MTDs (slides/slumps) and (iii) hemipelagic drape that successively represent the falling stage, lowstand to early transgressive, and late transgressive to highstand systems tracts. Earth-Sci. Rev., 84, 21-55. Zecchin, M., Caffau, M., Tosi, L., Civile, D., Brancolini, G., Rizzetto, F., and Roda, C., 2010, The impact of Late Quaternary glacio-eustasy and tectonics on sequence development: evidence from uplifting and subsiding settings in Italy. Terra Nova 22, 324-329. doi: 10.1111/j.1365-3121.2010.00953. Zecchin, M., Catuneanu, O. 2012, High-resolution J o u r n a l P r e -p r o o f Note the space-time variation in the orientation of erosional channels in panels A and B (labelled 1, 2 and 3). A, B (modified after [START_REF] Chima | Seismic stratigraphy and depositional architecture of Neogene intraslope basins, offshore western Niger Delta[END_REF]. See text for a more detailed explanation.