M Pitel-Roudaut

P Dupont

Tectonic-sedimentary architecture of surficial deposits along the continental slope offshore Romania (North of the Viteaz Canyon, Western Black Sea): Impact on sediment instabilities

Keywords: Romanian continental slope, gas/gas hydrates, turbidite systems, sedimentary instabilities, gravitational faults. 1. Introduction

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Sediment instabilities and gravity processes strongly affect continental slopes [START_REF] Masson | Submarine landslides: processes, triggers, and hazard prediction[END_REF][START_REF] Urlaub | Timing and frequency of large submarine landslides: implications for understanding triggers and future geohazard[END_REF]. Submarine landslides can involve large volumes of sediment [START_REF] Tailling | Large submarine landslides on continental slopes: Geohazards, methane release, and climate change[END_REF] and cause damage to telecommunication cables (Pope et al., 2017 and references therein) and seafloor structures (pipelines). They can also trigger tsunamis [START_REF] Piper | The sequence of events around the epicentre of the 1929 Grand Banks earthquake: initiation of debris flows and turbidity current inferred from sidescan sonar[END_REF][START_REF] Løvholt | On the landslide tsunami uncertainty and hazard[END_REF]. Their role in sudden gas release and the related potential climate changes is also open to debate [START_REF] Kennett | Methane hydrates in Quaternary climate change: The clathrate gun hypothesis[END_REF][START_REF] Maslin | Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis[END_REF]. Numerous features influence the generation or triggering of submarine landslides [START_REF] Masson | Submarine landslides: processes, triggers, and hazard prediction[END_REF].

Rapid sediment deposition and high sedimentation rates are well-known instability triggering mechanisms [START_REF] Lee | Undersea landslides: extent and significance in the Pacific Ocean, an update[END_REF]. The presence of gas in the sediments [START_REF] Field | A submarine landslide associated with shallow seafloor gas and gas hydrates off northern California[END_REF][START_REF] Garziglia | Mass-transport deposits on the Rosetta province (NW Nile deep-sea turbidite system, Egyptian margin): characteristics, distribution, and potential causal processes[END_REF][START_REF] Riboulot | Sea-level change and free gas occurrence influencing a submarine landslide and pockmark formation and distribution in deepwater Nigeria[END_REF], melting of gas hydrates (Sultan et al., 2004a, b;[START_REF] Pecher | Erosion of the seafloor at the top of the gas hydrate stability zone on the Hikurangi Margin, New Zealand[END_REF]Kim et al., 2013), and earthquakes can also lead to slope failure where the presence of a weak layer constitutes favourable conditions [START_REF] Locat | Weak Layers: Their definition and classification from a geotechnical perspective[END_REF]. The relative impacts of these factors are often inadequately constrained, mainly due to the difficulty in evaluating the role of each of these factors on the stacking pattern of sedimentary bodies.

Accordingly, this study aims to provide the detailed spatio-temporal tectonic and stratigraphic framework of an area located offshore Romania (NW Black Sea) where numerous sediment instabilities exist within surficial sediment deposits (Fig. 1; Fig. 2). Previous studies of this area have shown that the Mass Transport Deposits (MTDs) are located along the slope where numerous gas seeps and evidence of gas hydrates have been recorded [START_REF] Popescu | Seismic expression of gas and gas hydrates across the western Black Sea[END_REF][START_REF] Riboulot | Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania[END_REF][START_REF] Hillman | Gas migration pathways and slope failures in the Danube Fan, Black Sea[END_REF][START_REF] Ker | Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System[END_REF]. The work presented here is based on a multi-scale seismic database (mainly 2D-high-resolution and near-bottom very high-resolution). The data provide detailed stacking patterns of turbidite systems and MTDs, fault patterns, and their relationship with BSR (Bottom Simulating Reflector) and free gas extents. The main objective is to enhance the identification and definition of the relative importance of factors controlling sediment instability in this particular context of gas/gashydrate dynamics, multi-phased tectonics and sea-level fluctuations. [START_REF] Finetti | Geophysical study of the Black Sea[END_REF][START_REF] Yilmaz | Regional and Petroleum Geology of the Black Sea and Surrounding Region[END_REF][START_REF] Nikishin | Mesozoic and Cainozoic evolution of the Scythian Platform-Black Sea-Caucasus domain[END_REF][START_REF] Dinu | Stratigraphic and structural characteristics of the Romanian Black Sea shelf[END_REF][START_REF] Dinu | Gas hydrates, Flares Seeps Offshore Romania, A Review[END_REF][START_REF] Munteanu | Kinematics of back-arc inversion of the Western Black Sea Basin[END_REF]. Note the North Dobrogea orogenic belt (NDO), the Histria basin/trough well known in literature as Histria depression (HD over NDO) and the Eastern Black Sea (EBS), Western Black Sea (WBS), Mid Black Sea High (MBSH). The red line indicates the cross-section presented below. The red box indicates the location of the study area. B) Cross-section of the Romanian margin (from [START_REF] Matenco | The interplay between tectonics, sediment dynamics and gateways evolution in the Danube system from the Pannonian Basin to the western Black Sea[END_REF] showing the horsts and grabens resulting from the Mesozoic extensional phase and Cenozoic inversion. Note the gravitational deformation of prograding sediments overlying late Oligocene layers and the associated faults. Note the listric growth faults below the present shelf break. The red box indicates the location of the study area on the upper continental slope.

The Western Black Sea is part of a back-arc domain that has experienced complex evolution with transition from an extensional setting (Cretaceous-early Eocene) to a contractional setting (late Eocene-Middle Miocene) during the northward Neotethyan subduction and collision (Letouzay et al., 1977;[START_REF] Zonenshain | Deep basins of the Black Sea and Caspian Sea as remnants of Mesozoic back-arc basins[END_REF][START_REF] Finetti | Geophysical study of the Black Sea[END_REF][START_REF] Görür | Timing of opening of the Black Sea basin[END_REF][START_REF] Artyushkov | Role of crustal stretching on subsidence of continental crust[END_REF][START_REF] Robinson | Petroleum geology of the Black Sea[END_REF][START_REF] Nikishin | Mesozoic and Cainozoic evolution of the Scythian Platform-Black Sea-Caucasus domain[END_REF][START_REF] Georgiev | Geology and hydrocarbon Systems in the Western Black Sea[END_REF] (Fig. 1A). The rifting began during the Aptian and continued intermittently until the mid-Turonian [START_REF] Krezsek | The Messinian sea-level fall in the western Black Sea: small or large? Insights from offshore Romania[END_REF]. Continental break-up followed in the mid-Turonian caused by the regional uplift and erosion of the basin margin. Late Cretaceous-Paleogene times are marked by rift enlargement and eastward expansion and the development of the deep-sea basin [START_REF] Munteanu | Effects of large sea-level variations in connected basins: the Dacian-Black Sea system of the Eastern Paratethys[END_REF]. These two extensional stages led, in the northwestern area, to the formation of complex interplay between isolated blocks organized in horsts and grabens (Fig. 1B and red lines on Fig. 2), such as the Histria Basin (Fig. 1 and Fig. 2) which is located over the North Dobrogea Triassic-Jurassic orogenic belt [START_REF] Dinu | Stratigraphic and structural characteristics of the Romanian Black Sea shelf[END_REF]. Repeated periods of inversion from orogenic deformations (Balkans, Pontides, Crimean-Caucasus), are marked by the (re)activation of faults and associated folding during the late Cretaceous and Eocene-Miocene times [START_REF] Hippolyte | Geodynamics of Dobrogea (Romania): new constraints on the evolution of the Tornquist-Teysseyre Line, the Black Sea and the Carpathians[END_REF][START_REF] Munteanu | Kinematics of back-arc inversion of the Western Black Sea Basin[END_REF] (Fig. 1A).

Large thicknesses of sediments accumulated during the Pontian, i.e. Miocene-Pliocene transition from 6.04 to around 5 Ma [START_REF] Dinu | Transition from extensional to compressional structures in the Upper Miocene Shally deposits, Western Black Sea[END_REF][START_REF] Dinu | Stratigraphic and structural characteristics of the Romanian Black Sea shelf[END_REF][START_REF] Konerding | Seismic sequence stratigraphy, structure and subsidence history of the Romanian Black Sea shelf[END_REF]Tambrea, 2007) (Fig. 1B). At that time, which corresponds to the Messinian event, sea-level falls of debated amplitude (100-2300 m) led to large-scale erosion and significant progradation during the subsequent highstand [START_REF] Hsü | Messinian event in the black sea[END_REF][START_REF] Gillet | Messinian event in the black sea: Evidence of a Messinian erosional surface[END_REF][START_REF] Munteanu | Effects of large sea-level variations in connected basins: the Dacian-Black Sea system of the Eastern Paratethys[END_REF][START_REF] Krezsek | The Messinian sea-level fall in the western Black Sea: small or large? Insights from offshore Romania[END_REF]. The Dacian i.e. around 5 to 4 Ma, and the Romanian-Quaternary sediments reached thicknesses of up to 1200 m and 600 m respectively on the continental slope [START_REF] Konerding | Seismic sequence stratigraphy, structure and subsidence history of the Romanian Black Sea shelf[END_REF] (Fig. 1B).

The high sediment load and high subsidence led to the development of gravitational faults [START_REF] Konerding | Seismic sequence stratigraphy, structure and subsidence history of the Romanian Black Sea shelf[END_REF] (Fig. 1B) and overpressure of the Oligocene-early Miocene? sediments [START_REF] Dinu | Transition from extensional to compressional structures in the Upper Miocene Shally deposits, Western Black Sea[END_REF][START_REF] Bega | Neogene structural styles of the NW Black Sea region, offshore Romania[END_REF], which provided the detachment level for shelf collapse during the Messinian large-scale sea level drop [START_REF] Tambrea | Characteristics of the Tectonics and Lithostratigraphy of The Black Sea Shelf, Offshore Romania[END_REF][START_REF] Dinu | Transition from extensional to compressional structures in the Upper Miocene Shally deposits, Western Black Sea[END_REF][START_REF] Munteanu | Effects of large sea-level variations in connected basins: the Dacian-Black Sea system of the Eastern Paratethys[END_REF][START_REF] Schleder | Structural style in a Messinian (intra-Pontian) gravity-driven deformation system, western Black Sea, offshore Romania[END_REF].

During the Plio-Quaternary, thick successions of MTDs and turbidite deposits took place in front of modern rivers [START_REF] Munteanu | Effects of large sea-level variations in connected basins: the Dacian-Black Sea system of the Eastern Paratethys[END_REF][START_REF] Matenco | The interplay between tectonics, sediment dynamics and gateways evolution in the Danube system from the Pannonian Basin to the western Black Sea[END_REF]. The north-western Black Sea is currently dominated by a rather wide shelf (60-200 km) with a shelf break at 120 to 170 m water depth and canyon systems with large deep-sea fan complexes that mainly developed during sea-level Quaternary lowstands [START_REF] Winguth | Upper quaternary water level history and sedimentation in the northwestern Black Sea[END_REF][START_REF] Popescu | Analyse des processus sédimentaires récents dans l'éventail profond du Danube (Mer Noire)[END_REF]. The Danube delta system advanced into the Black Sea in the Romanian-Pleistocene in response to the increased sediment supply due to renewed uplift of the Carpathians [START_REF] Matoshko | The Plio-Pleistocene demise of the East Carpathians foreland fluvial system and arrival of the Paleo-Danube to the Black Sea[END_REF][START_REF] Krézsek | Filling of sedimentary basins and the birth of large rivers: The lower Danube network in the Dacian Basin, Romania[END_REF]. The Danube river reached the Black Sea for the first time about 900 ka ago and subsequently built up the Danube fan [START_REF] Winguth | Upper quaternary water level history and sedimentation in the northwestern Black Sea[END_REF].

Note that several earthquakes were recorded in this area, especially from deeply rooted faults (Fig. 2, Table 1). The north-western Black Sea is also rich in hydrocarbon fields (oil, gas) [START_REF] Kruglyakova | Natural oil and gas seeps on the Black Sea floor[END_REF][START_REF] Georgiev | Geology and hydrocarbon Systems in the Western Black Sea[END_REF], gas hydrates [START_REF] Popescu | Seismic expression of gas and gas hydrates across the western Black Sea[END_REF][START_REF] Merey | Investigation of gas hydrate potential of the Black Sea and modelling of gas production from a hypothetical Class 1 methane hydrate reservoir in the Black Sea conditions[END_REF], and surficial gas systems with abundant gas seeps, pockmarks, and shallow gas fronts (e.g. [START_REF] Polikarpov | Methane gas seeps in the Black Sea-a new object of molismology[END_REF][START_REF] Egorov | Gas seeps in the submarine Dnieper Canyon, Black Sea: acoustic, video and trawl data[END_REF][START_REF] Egorov | Methane seeps in the Black Sea: discovery, quantification and environmental assessment[END_REF][START_REF] Artemov | Methane emission to the hydro -and atmosphere by gas bubble streams in the Dnieper paleo-delta, the Black Sea[END_REF][START_REF] Naudts | Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea[END_REF][START_REF] Nikolovska | Hydroacoustic methodology for detection, localization, and quantification of gas bubbles rising from the seafloor at gas seeps from the eastern Black Sea[END_REF][START_REF] Greinert | Atmospheric methane flux from bubbling seeps: spatially extrapolated quantification from a Black Sea shelf area[END_REF][START_REF] Diaconu | Gas escape features on the Romanian Black Sea shelf -Examples from 2D seismic interpretation[END_REF][START_REF] Römer | Methane gas emissions of the Black Sea-mapping from the Crimean continental margin to the Kerch Peninsula slope[END_REF].. [START_REF] Dinu | Stratigraphic and structural characteristics of the Romanian Black Sea shelf[END_REF]Morosanu, 2007 and[START_REF] Munteanu | Kinematics of back-arc inversion of the Western Black Sea Basin[END_REF]in light blue, gravitational faults from Tambrea et al., 2002 and[START_REF] Dinu | Transition from extensional to compressional structures in the Upper Miocene Shally deposits, Western Black Sea[END_REF] in dark blue gravitational faults from [START_REF] Konerding | Seismic sequence stratigraphy, structure and subsidence history of the Romanian Black Sea shelf[END_REF] Note that the sets of faults of different colors have been mapped from distinct and incomplete grids of seismic profiles thus leading to an incomplete map. In grey dotted line, the border of the Histria Basin (from [START_REF] Pătruț | The Geological constitution of the Danube Delta[END_REF][START_REF] Anton | Litho-and biostratigraphy of the Eocene deposits from Istria Basin northern edge (Western Black Sea)[END_REF]. The earthquake epicenters (time period 1900-2019) 2 for references), the upper limit of the model-predicted steady-state GHSZ (red line) (from [START_REF] Ker | Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System[END_REF] and the observed BSR (yellow line) (Colin et al., 2020b) and, the gas flares (black dots, [START_REF] Riboulot | Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania[END_REF] (colored map). The black box corresponds to the maps presented in Fig. 7,15,19,20. The study area lies to the northeast of the Danube paleo-delta basinward of the shelf-break from around 180 to 1300 m water depth (Fig. 3A). The continental slope varies around 2°-4° between 200 and 500 m water depth respectively and up to 35° locally (canyon side walls).

The outer shelf and upper slope are incised by several canyons formed by erosion prolonged on the middle and lower slope to channel-levees (e.g., [START_REF] Popescu | Analyse des processus sédimentaires récents dans l'éventail profond du Danube (Mer Noire)[END_REF][START_REF] Riboulot | Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania[END_REF]. Around 20% of the seafloor surface is disturbed by scarps linked to instabilities. The scarps mainly occur in the canyons and paleo-valleys. Scarps also occur on the open upper slope between 200 and 500 m water depth. MTDs have been observed between 200 and 900 m water depth [START_REF] Riboulot | Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania[END_REF][START_REF] Hillman | Gas migration pathways and slope failures in the Danube Fan, Black Sea[END_REF]. Seaward of the study area, the deep-sea thick deposits of mostly clayey sediments were mainly supplied by the Danube and the Dnepr Rivers [START_REF] Soulet | Abrupt drainage cycles of the Fennoscandian Ice Sheet[END_REF][START_REF] Matenco | The interplay between tectonics, sediment dynamics and gateways evolution in the Danube system from the Pannonian Basin to the western Black Sea[END_REF]. Faults of interest in our study area are faults related to the Messinian gravitational collapse and younger reactivations mainly during middle Pliocene (Dacian, in local stage) [START_REF] Tambrea | Characteristics of the Tectonics and Lithostratigraphy of The Black Sea Shelf, Offshore Romania[END_REF][START_REF] Dinu | Transition from extensional to compressional structures in the Upper Miocene Shally deposits, Western Black Sea[END_REF][START_REF] Munteanu | Effects of large sea-level variations in connected basins: the Dacian-Black Sea system of the Eastern Paratethys[END_REF][START_REF] Schleder | Structural style in a Messinian (intra-Pontian) gravity-driven deformation system, western Black Sea, offshore Romania[END_REF]. The first generation (Middle Pontian) gravitational faults are related to the so-called "eastern gravity-driven deformation system" of [START_REF] Schleder | Structural style in a Messinian (intra-Pontian) gravity-driven deformation system, western Black Sea, offshore Romania[END_REF] characterized by an updip extensional domain with SW-NE oriented normal faults partially accommodated by a downdip contractional domain marked by a mainly W-E oriented thrust [START_REF] Tambrea | Characteristics of the Tectonics and Lithostratigraphy of The Black Sea Shelf, Offshore Romania[END_REF][START_REF] Dinu | Transition from extensional to compressional structures in the Upper Miocene Shally deposits, Western Black Sea[END_REF][START_REF] Munteanu | Effects of large sea-level variations in connected basins: the Dacian-Black Sea system of the Eastern Paratethys[END_REF][START_REF] Schleder | Structural style in a Messinian (intra-Pontian) gravity-driven deformation system, western Black Sea, offshore Romania[END_REF] (in light blue on Fig. 2, Fig. 3A and, Fig. 19B). Second generation (Dacian) gravitational faults have been observed in a grid of seismic profiles located near the shelf edge. These faults form mostly NE-SW trending grabens and horsts separated by NW-SE oriented transfer faults [START_REF] Bega | Neogene structural styles of the NW Black Sea region, offshore Romania[END_REF][START_REF] Konerding | Seismic sequence stratigraphy, structure and subsidence history of the Romanian Black Sea shelf[END_REF] (in dark blue on Fig. 2, Fig. 3A and Fig. 19B).

The presence of gas hydrates is inferred from the identification of the Bottom Simulating Reflector (BSR), which marks the base of the Gas Hydrate Stability Zone (GHSZ). The GHSZ corresponds to the zone where gas hydrates [START_REF] Sloan | Clathrate Hydrates of Natural Gases[END_REF][START_REF] Sloan | Clathrate Hydrates of Natural Gases[END_REF][START_REF] Kvenvolden | Gas hydrates-Geological perspective and global change[END_REF],

i.e. the ice-like solid form of gas, develop under conditions of low temperature, high pressure, and sufficient gas concentrations (Kvenvolden and McMenanim, 1980;[START_REF] Sloan | Clathrate Hydrates of Natural Gases[END_REF][START_REF] Sloan | Clathrate Hydrates of Natural Gases[END_REF][START_REF] Dickens | Methane hydrate in pore water: a simple theorical approach for geophysical applications[END_REF]. The BSR was first mapped by [START_REF] Ion | Seismo-acoustics evidence of gases in sedimentary edifices of the paleo-Danube realm, in: Turbidite systems and deep-sea fans of the Mediterranean and the Black seas[END_REF], then [START_REF] Popescu | Multiple bottom-simulating reflections in the Black Sea: potential proxies of past climate conditions[END_REF]. Pockmarks, gas flares, and seismic facies related to the presence of gas have been also reported. The gas flares were generally acoustically detected in the water column upward the landward limit of the GHSZ at 660 m water depth [START_REF] Popescu | The Danube submarine canyon (Black Sea): morphology and sedimentary processes[END_REF][START_REF] Riboulot | Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania[END_REF]. They mainly consist of gas emissions released to the ocean from the seafloor from underlying sources. Finally, the study area has undergone major changes in environmental conditions relative to sea-level changes and induced connectiondisconnection of the Marmara-Mediterranean Sea during last interglacial-glacial cycles [START_REF] Soulet | Black Sea "Lake" reservoir age evolution since the last glacial -hydrologic and climatic implications[END_REF][START_REF] Constantinescu | Evolution of the Danube Deep-Sea fan since the last glacial maximum: new insights into Black Sea water-level fluctuations[END_REF][START_REF] Wegwerth | Black Sea temperature response to glacial millennial-scale climate variability[END_REF][START_REF] Ballas | Influence of early diagenesis on geotechnical properties of clay sediments (Romania, Black Sea)[END_REF].

These cycles cause alternating marine and freshwater lake conditions [START_REF] Badertscher | Pleistocene water intrusions from the Mediterranean and Caspian seas into the Black Sea[END_REF] and connection-disconnection between the Danube River, its canyon, and consequently the deep-sea deposit systems [START_REF] Lericolais | High frequency sea level fluctuations recorded in the Black Sea since the LGM[END_REF][START_REF] Constantinescu | Evolution of the Danube Deep-Sea fan since the last glacial maximum: new insights into Black Sea water-level fluctuations[END_REF].

Methodology

The results presented in this paper originate from the interpretation of several data sets including (Fig. 3B):

(1) Multi-beam echo-sounder data acquired during the GHASS ( 2015 (2) 2D-high and very-high-resolution (VHR) seismic reflection profiles acquired during the 1998 BlaSON surveys of Ifremer and GeoEcoMar. High-resolution seismic profiling was implemented using a mini-GI air-gun seismic source (frequency range 150 Hz and vertical resolution of 2.5 m) and a 24-channel streamer. Sub-bottom profiles (Chirp) were acquired simultaneously to investigate surficial sediments.

(3) 2D-high-resolution (HR) multi-channel seismic profiles acquired during the GHASS survey using a single 24/24 mini-GI air-gun seismic source (central frequency at 115

Hz and vertical resolution of 3.2 m) for profiles GAS031, GAS033 to GAS039, two 24/24 for profile GAS030, a single 13/13 for profiles GAS040 and GAS041, and a 96-channel solid streamer (inter-trace of 6.25 m and maximum source-receiver offset of 650 m). These profiles are named HRxxx in Fig. 3B. The seismic dataset was processed using a conventional post-stack sequence with constant velocity (1500m/s) [START_REF] Ker | GHASS cruise Team[END_REF].

(4) 2D-very high-resolution hull-mounted Sub-Bottom Profiler (SBP) data acquired during the GHASS survey. The SBP operates at the frequency range 1800-5300 Hz, giving a vertical resolution close to 20 cm. The quality control of SBP data was performed using the Ifremer QC Subop software [START_REF] Dupont | Compte rendu d'acquisition sismique[END_REF][START_REF] Ker | GHASS cruise Team[END_REF].

(5) 2D-very high-resolution deep-towed seismic profiles were acquired with the Ifremer SYSIF system [START_REF] Marsset | High and very resolution deep-towed seismic system: performance and examples from deep water geohazard studies[END_REF][START_REF] Marsset | Deeptowed high resolution multichannel seismic imaging[END_REF][START_REF] Marsset | Deep-towed high resolution seismic imaging II: Determination of P-wave velocity distribution[END_REF] during the GHASS survey. SYSIF is a deep-towed seismic device hosting a Janus-Helmholtz transducer emitting a linear chirp signal in the frequency bandwidth 220-1050 Hz and a streamer, both towed at 2 knots, 100 meters above the seafloor. Processing includes correction of altitude, and seismic signature deconvolution and provides 15-m-lateral resolution at the seafloor and 1-m-vertical resolution.

All seismic data were integrated into IHS Kingdom software for interpretation. Measurements in meters were calculated from Two-Way Traveltime (TWT) values using v= 1500 m/s.

Seismic interpretation is based on: (1) analysis of the reflectors' lateral terminations (onlap, down lap, erosional truncation) according to the general principles of seismic stratigraphy (Mitchum et al., 1977); and (2) characterization of seismic facies and correlation of distinctive reflectors between profiles. The cross-correlation between complementary seismic datasets provided mapping of unconformities, sedimentary bodies, faults, and gas systems. SBP profiles are not presented in this paper due to the poor quality of data caused by the presence of shallow gas. Only one Blason profile is presented. SYSIF was mainly used to identify sedimentary discontinuities such as scarps related to MTDs, channel sidewalls, and faults. It also allowed to identify facies and seismic anomalies related to fluid/gas features and consequently permeable and seal layers. All seismic profiles are presented with overlapped interpretative line drawing in this paper.

Results

Seismic units and facies.

Architecture of the study area consists of the stacking of different types of seismic units classically encountered in similar slope and turbidite environments (e.g. [START_REF] Twichell | Depositional patterns of the Mississippi Fan surface: Evidence from GLORIA II and high-resolution seismic profiles[END_REF][START_REF] Piper | Mass-transport deposits of the Amazon Fan[END_REF][START_REF] Droz | Architecture of an active mud-rich turbidite system: the Zaire Fan (Congo-Angola margin southeast Atlantic) results from ZaïAngo 1 and 2 cruises[END_REF][START_REF] Madof | Stratigraphic controls on a salt-withdrawal intraslope minibasin, north-central Green Canyon, Gulf of Mexico: Implications for misinterpreting sea level change[END_REF] and already identified in the Danube Fan [START_REF] Zander | On the origin of multiple BSRs in the Danube deep-sea fan, Black Sea[END_REF][START_REF] Hillman | Gas migration pathways and slope failures in the Danube Fan, Black Sea[END_REF][START_REF] Winguth | Upper quaternary water level history and sedimentation in the northwestern Black Sea[END_REF][START_REF] Popescu | The Danube submarine canyon (Black Sea): morphology and sedimentary processes[END_REF]).

The majority of these units are the channel/levee systems and MTDs. For the remaining units, deposits correspond to infilling and draping near seafloor units. A detailed description of these units and their relationship with faults, gas signatures and key surfaces, in terms of sequential stratigraphy, is proposed in sections 4.2 to 4.6.

Mass Transport Deposits

MTDs are of two types and two generations, either 1) irregular masses of wide lateral extent (up to 300 km 2 in size and up to 80 m in thickness) characterized by a transparent to chaotic high amplitude facies, a very irregular top surface with hyperbolic reflections and an erosive basal surface, truncating underlying reflections. These MTDs are localized over a basal major erosional surface (e.g. MTD12 in Fig. 4); or 2) small masses up to around 10 km 2 in size and 35 m thickness characterized by hummocky, transparent to chaotic, and irregular seismic facies, with continuous, low to high amplitude reflections and an erosive basal surface. These MTDs are near the seafloor, localized at the upper slope and channel walls, where they rework superficial deposits (e.g. MTD2 in Fig. 8B).

Channel-levee systems

The channel is generally V-shaped with an erosional base. The levees are wedge-shaped and characterized by low amplitude, high frequency, remarkable continuity of reflectors. The lateral extent and thickness of channel-levees are variable. Some channels form a channel complex (e.g. channel complex B in Fig. 8A).

Filling sedimentation

The channel-levees abandonment is marked by channel infilling with transparent facies interpreted as distal overbank deposits from younger channels (e.g. Filling unit in Fig. 8A).

Draping sedimentation

The study area is covered by a draping unit of low amplitude, low frequency and continuous parallel reflectors interpreted as very fine-grained sediments. The thickness of this unit ranges from 20 to 40 m (e.g. Draping unit in Fig. 8A).

4.2 Basal regional unconformity, slide scarps, and associated MTDs A basal and regional unconformity, ranging between 300 m below the seafloor (mbsf) upstream to locally 10 mbsf downstream, has been identified despite the scarcity of HR seismic profiles (Fig. 4A). This unconformity corresponds to an erosional surface that truncates reflections of underlying seismic units (Fig. 4C). It is generally well expressed in the middle slope but poorly visible in the upper slope due to the presence of gas (Fig. 4B), except in the western part of the study area (see section 4.3). This erosional surface is characterized by several scarps (Fig. 4C; Fig. 5). The scarps were manually picked and mapped and vary in dimension, i.e. length (L) and height (H). Scarp alignments called SA (SA1 to SA13) have been identified (see for example Fig. 5 and Fig. 6) partly based on their dimensions and/or dip changes of underlying reflectors and their mapping shows a general NE-SW orientation (Fig. 7A). Dip changes in stratifications are characteristic of this area and are discussed in section 5.1. The basal unconformity displays two major erosional troughs up to around 300 m below paleo-valleys A and B (see the detailed description of paleo-valleys in section 4.3) (Fig. 8A). These erosional troughs are Wide MTDs lie directly over the erosional surface below the slope (see MTD12 on Fig. 4A and Fig. 4C) and at the bottom of paleo-valleys A and B (see section 4.3, Fig. 10; Fig. 16).

The map of MTDs associated with the erosional surface show two MTDs of wide extent i.e.

MTD12 and MTD13 (over MTD12) of at least respectively 300 and 75 km 2 in size and between 10 to 80 m in thickness (total volume > 50 km 3 ) (Fig. 5 and Fig. 7B). The upstream limit of MTD12 correlates with scarp alignments (SAs) SA1 and SA7 (Fig. 7B). MTD12 extends over several SAs basinward. It is characterized by an accordion-like external shape with generally upper extensive parts at the scarp heads and compressive bulges several kilometers from the scarp heads (Fig. 4C; Fig. 6). This implies that the SAs correspond to slide scars and that a polyphase slide event disturbed the slope deposits.

We underline the presence of a deeper (older) erosional surface covered by MTDs (see section 4.3, Fig. 13A, erosional surface 2). This surface is cut by the basal regional surface described above and the erosive axis of paleo-valley B. No further description is possible as only a few seismic lines were available to image such deep structures. (Viteaz) canyon -characterize the region (Fig. 2, Fig. 3). The Danube canyon/valley has already been described by [START_REF] Popescu | The Danube submarine canyon (Black Sea): morphology and sedimentary processes[END_REF] as an erosional major trough with steep flanks which incises the shelf at around 200 m water depth. Partial infilling of the canyon followed this initial erosional phase. The flat bottom of this through corresponds to the bottom of the main valley and is incised by a V-shaped thalweg that continues beyond the shelfbreak as an entrenched thalweg with well-developed lateral levees in the youngest Danube channel [START_REF] Popescu | The Danube submarine canyon (Black Sea): morphology and sedimentary processes[END_REF]. For consistency, we refer in this study to this description to define the terms of canyon, valley and channel-levees (see section 4.1 for the detailed description of channels and levees).

The geomorphology of paleo-valleys B and D has already been described by [START_REF] Riboulot | Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania[END_REF]. Paleo-valley A is not clearly expressed on the seafloor. Paleo-valley C shows a wide thalweg but its head is not clearly identified. The present seismic analysis has provided the geometry and internal structure of paleo-valleys and associated channel-levee systems, as well as their stacking pattern and the relative chronology as follows:

-Paleo-valley A appears over a U-shaped erosional trough of 1 to 4 km wide and ~150 m (head) to ~90 m (distal part) deep possibly connected to canyon A. As mentioned previously, the base of the trough is shaped by the erosional surface which is characterized by numerous scarps. More specifically, the lateral scarps define the walls (Fig. 8; Fig. 7 scarps in green) of the trough, and the basal cascading scarps SA7 to SA9 characterize the staircase topography of the bottom along the dip profile (Fig. 9). Three main MTDs lie on the bottom of its downstream part (Fig. 10). It is worth noting that valley A is filled by the stacked western levees of channel complex B i.e. B0 to B5. In the valley axis, the deposits of these levees are locally incised by small migrating channels (Fig. 8; Fig. 10). Downstream, over the basal MTDs, the levee deposits are shaped like a field of sediment waves (Fig. 10). A buried channel-levee system is observed off paleo-valley A and could correspond to the distal part of a channel belonging to channel complex A which seems to have disappeared (Fig. 11, channel A). -Canyon B (2.2 km wide and ~150 m deep) and its associated paleo-valley B are characterized by an axial thalweg (Fig. 3A) which is connected basinward to the youngest channel-levee system i.e. B5 (channel width of 800 m at ~ 1500 m water depth) of channel complex B (Fig. 8). As mentioned previously, an erosional trough, shaped by the basal unconformity and possibly connected to canyon B appears below this channel complex. (Fig. 13A). The singularity of channel complex B is the asymmetry of its walls (Fig. 8 and Fig. 13C). Indeed, the western wall is semi-linear whereas the eastern wall presents a staircase structure with two steps. The first channel B0 is located southwest of the present channel then channel B1 migrated northeastward of the present channel (Fig. 8). Channel B2 migrated to the position of the present channel. Channel B3 has remained in the B2 position; however, a collapse of its eastern levee has led to the formation of an elongated MTD along the eastern flank of paleo-valley B (Fig. 12). This explains the complex stair shape of the eastern side of channel complex B (Fig. 13A). An enigmatic mass lies eastward of levee B3

and at the same stratigraphic level. This enigmatic mass shows chaotic seismic facies suggesting deformed sediments but its external shape is regular (Fig. 13A andC). It has been interpreted as belonging to levee B3. It is worth noting that the morphology of the socalled Crest [START_REF] Riboulot | Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania[END_REF][START_REF] Ker | Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System[END_REF] results from the topography of levee B3.

Channels B4 and B5 have remained in the B3 position and have built their eastern levee directly on the elongated MTD. This has led to the formation of an internal levee in the eastern side of channel complex B. This internal levee reaches a length of 2.5 km and a thickness of 50 m and shows very clear continuous wedging reflections (Fig. 13B, and Fig. 8B). The map of the successive channel-levee systems B0 to the youngest shows their extent within paleo-valley B and on the slope of the study area (Fig. 15A-F). However, B0 to B2

were difficult to map due to the scarcity of HR profiles. The two paleo-valleys C and D have developed northeast of paleo-valley B and have fed channel complexes (Fig. 3A). Only two channels of these complexes, called channel C and channel D, were studied. These channels lie directly over the widespread MTD12 and sometimes over-incise it. (Fig. 15A).

The levees of channels C and D have a wide extent (12 km) and well-stratified deposits and their distal parts overlap the internal levee coeval to channels B4 and B5 (Fig. 8). These levees have been mapped using all seismic datasets (Fig. 15G-H).

• Paleo-valley Y is located at the seafloor between the Danube canyon and paleovalley A (Fig. 2; Fig. 3A). Channel Y appears over channel A and over channel complex B (Fig. 11). However, the seismic data are insufficient to map channel Y and to determine its location either between channel complex B and channel C or between channel D and the youngest Danube channel.

• Paleo-valley Danube is located southwest of paleo-valley A. The most recent Danube channel-levee system is observed from the slope to the basin (unit 4 from [START_REF] Popescu | Late Quaternary channel avulsions on the Danube deep-sea fan[END_REF] and crosscuts the distal parts of channel complexes B to D (Fig. 2; Fig. 17 The chronology of paleo-valleys and their connected channel complexes is based on the principle of superposition. Channel complex B crosscuts paleo-valley A and is partially covered by the distal part of the levees of channels C and D (Fig. 8). The activity of paleovalley A is continuous during the construction of channel complex B as revealed by the small channels affecting levee deposits to the seafloor where channel A crosscuts channel complex B (Fig. 3A). Channel D crosscuts channel C (Fig. 15H). The resulting chronology of paleo-valleys is, from the oldest to the youngest, A, B, C, D, and the most recent, the Danube paleo-valley which is connected to the youngest Danube channel.

The end of activity of paleo-valleys A, B, C, and D is marked by a unit of up to 30 m thickness showing low amplitude infilling seismic facies (Fig. 18). This infilling is attributed to late-stage distal overbank deposits from younger channels, such as possibly channels Y and Danube. The study area is covered by an almost draping unit (Fig. 18). This unit has been described from the core GAS-CS01 (GHASS survey) by Martinez-Lamas et al. ( 2020

3). A) Map of the superficial MTDs observed on SBP, SYSIF and/or HR seismic (red polygons). MTD1 appears within paleo-valley Y, MTD2, MTD3 and MTD4 within paleo-valley A with a retrogressive pattern, MTD5 is located between paleo-valleys A and B, MTD6 to MTD10 lie on the upper slope, and MTD11 on the middle slope. Note the MTDs on the walls of paleo-valley B

As mentioned previously (section 2.3), the seafloor surface is disturbed by scarps and associated MTDs linked to instabilities. Numerous MTDs disturb the superficial sediments of the headwalls and lateral walls of canyons/paleo-valleys A and B. The walls of channels along their entire length are also disturbed. Eleven MTDs located on the upper slope have been identified on seismic profiles and mapped (Fig. 8B, Fig. 19). In paleo-valley A, three distinct MTDs (MTD2 to MTD4) and their associated head scarps have been identified.

MTD2 is at around 10 mbsf, reaches 35 m in thickness, and 2 km 2 in extent (Fig. 10 and Fig. 8B). MTD3 is at around 2 mbsf, reaches 25 m in thickness, and 3 km 2 in extent (Fig. 8B).

MTD4 is at the seafloor and extends over a 9 km 2 area (Fig. 9). The three distinct MTDs suggest three main episodes of instability in a retrogressive pattern. It is worth noting that MTD5 could have occurred between MTD3 and MTD4 but this has not been identified in the seismic data (Fig. 19). Superficial MTDs can appear at the top of the field of the sediment wave revealing a decollement surface at the base of the draping unit (Fig. 10B).

Gas signatures and hydrates in upper sediments

As mentioned previously (section 2.3), the BSR occurs between ~700 and 1830 m water depth and its edge, i.e. its most landward termination (see Fig. 10 for an example), ranges in the predicted GHSZ [START_REF] Popescu | The Danube submarine canyon (Black Sea): morphology and sedimentary processes[END_REF][START_REF] Ker | GHASS cruise Team[END_REF][START_REF] Riboulot | Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania[END_REF]) (Fig. 3A). Gas signature is generally absent within the sediments located above the BSR (e.g., Fig. 5C), as confirmed by the velocity analysis of SYSIF data (Colin et al., 2020a) (Fig. 4D), and the absence of gas flares at the seafloor (Fig. 3A). However, there is one exception along the so-called Crest area where faults cross the BSR and, outcrop at the seafloor and feed gas flares [START_REF] Ker | Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System[END_REF]. Velocity analysis of SYSIF data (Colin et al., 2020a) shows the presence of gas below the BSR under the Crest (Fig. 13B). These observations highlight that gas hydrates play the role of an efficient seal layer in controlling gas-flare distribution.

Under the BSR, rare observable vertical columns of seismic anomalies with a pull-down effect, are interpreted as a localized accumulation of gas implying potential gas breaks through the sediments (Fig. 6 below SA12; Fig. 12). Others seem to show a slight vertical offset of reflectors suggesting potential faults. Acoustic chimneys also occur below the BSR and mainly westwards of channels C and D (Fig. 6).

In the free gas area at the landward termination of the BSR, most of the columns of seismic anomalies generally associated with underlying acoustic chimneys occur in levees associated with channels B to D (Fig. 4B). They do not reach the seafloor but stop at the level of the infilling unit and overlying draping lacustrine and marine sediments (Fig. 18).

Velocity analysis revealed the presence of free gas in the crests of sedimentary undulations (Colin et al., 2020a). In our study, these crests have been correlated to levees B and C. (Fig.

14B

. This confirms that the draping unit, and possibly the infill unit, bounds the upper limit of the gas front and consequently plays the role of a seal layer. Most of the gas flares are in association with sedimentary discontinuities such as the edge of canyon B and the paleovalleys and channel sidewalls (B, C and D), and slide scars (Fig. 3A). This is reminiscent of [START_REF] Skarke | Widespread methane leakage from the sea floor on the northern US Atlantic margin[END_REF].

Fault patterns and distribution

A couple of faults have been identified from seismic profiles (Fig. 19B). They appear as single faults or sets of faults. The single faults are sometimes listric (Fig. 5B), with some of them showing a normal vertical displacement of seismic reflectors (Fig 6,below SA13). The sets of faults include small-offset and high-angle to vertical faults (Fig. 6, below SA1; Fig. 9, below SA9). They are sometimes associated with acoustic chimneys which suggest the presence of gas (Fig. 13A, below the so-called Crest). Some faults coincide with a major dip change in stratification on both sides of the vertical columns of seismic anomalies (Fig. 6, below SA13; Fig. 10, below SA8).

These faults are not randomly distributed in the study area or the sediment stratigraphy (Fig. 19). Almost all these faults affect sediments below the basal regional unconformity. Only the fault system of the Crest seems to have propagated to the seafloor within the surficial sediments (Fig. 13). This fault zone feeds gas flares and seems deep-seated even if its termination was not identified due to lack of penetration of the seismic signal [START_REF] Ker | Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System[END_REF]. The eastern area is characterized by deeply rooted normal faults below the scarps of the basal unconformity. These faults sometimes become listric toward the BSR zone (Fig. 5B; Fig. 6). Sets of vertical faults occur below the BSR and at the BSR edge (Fig. 13A; Fig. 9, below SA9 and Fig. 10, below SA8), but few examples are observed cutting it (Fig. 12, below SA10). We note that this set of vertical faults seems to limit a free-gas zone below a marked BSR from a diffusegas zone without a BSR from either side (Fig. 13A). Upstream the BSR edge, the presence of gas facies and acoustic chimneys on seismic profiles does not allow us to identify faults directly (Fig. 4C). However, the abundance of faults reported on the shelf (see Fig. 3A) infers the presence of faults within the free-gas area (Fig. 19B).

Precise characterization of fault orientation is not possible in the study area with such a low coverage of HR seismic acquisition (Fig. 3). In the western zone, only a set of small offset faults with a NW-SE orientation can be determined from the correlation of closely spaced and parallel seismic lines along the east side of valley B (Fig. 19). We note that most of the vertical structures are observed from NE-SW seismic lines which could be consistent with transfer structures having a rough NW-SE orientation. In the eastern zone, the high-angle normal faults have NE-SW-oriented alignments.

Discussion

On the Dnepr paleo-delta in the NW Black Sea, it has been demonstrated that morphology, underlying stratigraphy, sediment properties, and the presence of gas hydrates control the distribution of gas flares [START_REF] Naudts | Geological and morphological setting of 2778 methane seeps in the Dnepr paleo delta, northwestern Black Sea[END_REF]. Our results are similar. We show that the extent of the BSR, faulting patterns, and arrangement of architectural elements such as canyons, channel-levees, slide scars, and MTDs control the distribution of seal layers and sedimentary discontinuities and consequently gas-flare distribution. However, we cannot exclude that seepage may be primarily associated with tectonically driven flows through regional faults [START_REF] Kruglyakova | Natural oil and gas seeps on the Black Sea floor[END_REF]. Moreover, the release of seismic energy during earthquakes could cause slope failures and subsequent gas escape along slide scarps.

Further investigation of the deep fluid plumbing system and sediment properties remains essential to discuss the factors controlling gas/fluid migration pathways and consequently gas-flare distribution.

This work highlighted the evolution of the tectonic-sedimentary architecture of surficial deposits and its impact on sediment instabilities, which is discussed below.

Origin of the Basal erosional surface and sediment instabilities

The basal erosional surface with the associated layer of MTDs is observable from the upper continental slope down to the deep basin [START_REF] Popescu | The Danube submarine canyon (Black Sea): morphology and sedimentary processes[END_REF][START_REF] Winguth | Upper quaternary water level history and sedimentation in the northwestern Black Sea[END_REF]Hillmann et al., 2018). This erosional surface corresponds to the Base Neoeuxinian Sequence Boundary of [START_REF] Hillman | Gas migration pathways and slope failures in the Danube Fan, Black Sea[END_REF] attributed to the Würmian with a minimum lake/sea level drop (about -130 m) at around 18000 BP [START_REF] Panin | Danube River Sediment Input and its Interaction with the Northwestern Black Sea Estuarine[END_REF]. The Base Neoeuxinian Sequence Boundary has been recently dated using 14C dating, at a minimum of ~22 ka [START_REF] Lericolais | Late Quaternary deep-sea sedimentation in the western Black Sea: new insights from recent coring and seismic data in the deep basin[END_REF] and was considered to mark the onset of activity in the Danube Fan during the last sea-level lowstand in the Black Sea [START_REF] Popescu | Late Quaternary channel avulsions on the Danube deep-sea fan[END_REF].

However, the correlation with core Gas-CS01 (base of the draping unit) indicates the age at a minimum of 32 ka [START_REF] Martinez-Lamas | Linking Danube River activity to Alpine Ice-Sheet fluctuations during the last glacial (ca. 33-17 ka BP): Insights into the continental signature of Heinrich Stadials[END_REF]. The presence of several built and abandoned channel-levee systems between this draping unit and the basal erosional surface suggests that this unconformity is significantly older.

It is generally considered that during sea-level falls, migration of the river-mouth to the shelf edge and rapid deltaic sedimentation leads to an increase in sedimentation load, overstepping and overpressuring in the prodelta and slope sediments, resulting in sediment failure, MTD formation, and canyon development [START_REF] Weimer | Sequence stratigraphy, facies geometries, and depositional history of the Mississippi fan, Gulf of Mexico[END_REF]Posamentier andKolla, 2003, Madof et al., 2009). A link between the MTDs, the scarp alignments of the basal erosional surface, and the faults has been observed (Fig. 7B). The presence of normal faults under SA3, SA4, and SA6 suggests that these sediment instabilities are related to normal faults underneath, potentially of gravitational origin. Major dip changes in stratification at SA7, SA8, and SA13 are also likely attributed to faults since these scarps are almost parallel to the mapped SW-NE oriented faults from [START_REF] Konerding | Seismic sequence stratigraphy, structure and subsidence history of the Romanian Black Sea shelf[END_REF] (in dark blue on Fig. 19B).

We highlight that the observed faults are mainly located below the basal unconformity and are mainly absent in the surficial deposits. This observation suggests that the faulting was predominantly active before or during basal erosional surface formation. We also note that the upper extensive parts of the MTDs lie over the scarp alignments whereas the compressive bulges are downstream (e.g. Fig. 4C). The contribution of fault activity is therefore identified as significant on slope failures and distribution of MTDs.

Sea-level falls may cause an additional effect, such as a reduction in hydrostatic pore pressure in the fluid reservoir that lowers gas solubility and results in gas exsolution. The subsequent accumulation of gas in porous layers interbedded within less permeable layers prevents fluid escape [START_REF] Magara | Compaction and fluid migration. Practical petroleum geology[END_REF][START_REF] Lafuerza | Overpressure within upper continental slope sediments from CPTU data, Gulf of Lion, NW Mediterranean Sea[END_REF] and facilitates overpressure build-up, which drives fluids along permeable migration pathways (e.g. faults), and may cause a drop in the effective stress of the sediment and favour its failure [START_REF] Bolton | The importance of overpressure timing and permeability evolution in fine-grained sediments undergoing shear[END_REF][START_REF] Dugan | Consolidation, effective stress, and fluid pressure of sediments from ODP Site 1073, US mid-Atlantic continental slope[END_REF][START_REF] Canals | Slope failure dynamics and impacts from seafloor and shallow sub-seafloor geophysical data: case studies from the COSTA Project[END_REF]. Reactivation of inherited faults is also possible and the subsequent fluid migration along them may contribute to the initiation and location of surficial sediment instabilities. This potential fault reactivation during sea-level fall does not exclude deep movements of over pressured layers at the regional scale. Indeed, examples of gravity-driven deformations have been demonstrated on the Plio-Pleistocene time scale in several deltaic settings characterized by underlying over pressured layers [START_REF] Dos Reis | Implications of salt-sediment interactions on the architecture of the Gulf of Lions deep-water sedimentary systems-western Mediterranean Sea[END_REF] for the Gulf of Lion; [START_REF] Knox | Development of the Cenozoic Niger delta in terms of the "escalator regression" model and impact on hydrocarbon distribution[END_REF][START_REF] Knox | Development of the Cenozoic Niger delta in terms of the "escalator regression" model and impact on hydrocarbon distribution[END_REF][START_REF] Rouby | Gravity driven deformation controlled by the migration of the delta front: The Plio-Pleistocene of the Eastern Niger Delta[END_REF] for the Niger delta). We note that regional-scale tectonic influence, such as the flexural uplift of the Carpathians foreland due to the Wallachian deformations, cannot be excluded from this fault activity along the Romanian margin [START_REF] Cloetingh | Thermo-mechanical controls on the mode of continental collision in the SE Carpathians (Romania)[END_REF].

It is worth noting that the presence of an older erosional surface (erosional surface 2 in Fig. 13A) with associated MTDs, suggests a recurrence of significant instabilities possibly during successive sea-level falls. Glacial and interglacial conditions control the alternating turbidite/hemipelagic deposits and may explain the recurrence of slope failures. This has been demonstrated for the Gulf of Lion where repeated slope failures occurred during glacial periods at specific stratigraphic levels which correspond to weak layers between clay-rich and turbidite sediments [START_REF] Badhani | Morphology of retrogressive failures in the Eastern Rhone Interfluve during the Last Glacial Maximum (Gulf of Lions, Western Mediterranean)[END_REF][START_REF] Badhani | Integrated geophysical, sedimentological and geotechnical investigation of submarine landslides in the Gulf of Lions (Western Mediterranean)[END_REF].

5.2

The channel-levee systems: Impact of fault systems and instabilities

The data highlight the build-up of channel-levee systems above the basal layer of MTDs and the Neoeuxinian sequence boundary (Fig. 15). This suggests that fluvially derived sediments were transported directly into canyon heads and then as channelized turbidity flows thanks to the connection between fluvial valleys incised in the shelf and canyons. The vertical succession from MTDs to channelized deposits is reminiscent of the "deep-water sequence"

of [START_REF] Weimer | Sequence stratigraphy, facies geometries, and depositional history of the Mississippi fan, Gulf of Mexico[END_REF]. The model of [START_REF] Winguth | Upper quaternary water level history and sedimentation in the northwestern Black Sea[END_REF] for the Danube and Dniepr fans can be applied to the study because it is based on the depositional model of [START_REF] Weimer | Sequence stratigraphy, facies geometries, and depositional history of the Mississippi fan, Gulf of Mexico[END_REF] for deep-sea fan systems, in which sea-level fluctuation controls fan development. We have integrated the tectonics and gas/gas-hydrate dynamics which contribute to sediment instabilities, into the model of [START_REF] Winguth | Upper quaternary water level history and sedimentation in the northwestern Black Sea[END_REF] and propose a diagram of the evolution of the study area since the last sea level fall (Fig. 20). During the sea-level fall, gravity-driven tectonic and/or fault reactivation with subsequent gas expulsion are invoked to trigger major sedimentary instabilities of the slope leading to the formation of MTDs (Fig. 20A). Then during lowstands, the fan system becomes the main site of deposition, and coarse material is confined to the channels, whereas finer material is swept onto the levees and beyond to form overbank deposits (Fig. 20B).

The relative chronology of paleo-valleys (see section 4.3) shows that the turbidite activity migrated northward from paleo-valleys A to D and then southward from paleo-valley D to the Viteaz canyon. We suppose the low sedimentary load on the shelf to be insufficient to induce gravity-deformation and subsequent fluid/gas expulsion.

Sediment instability during canyon activity is related to both erosive action within the canyon (sediment flow dynamics) and the presence of gas in surficial deposits (high sedimentation rate or migration from a fault system). Sediment instabilities were observed during canyon activity and formation of the erosional surface (Fig. 7B) but also during the formation of channel-levee systems (Fig. 8). According to Hillmann et al. ( 2018), the slope failure located on the western wall of paleo-valley B and labeled slump S2 by these authors shows no evidence for deposits downslope or in the canyon that could be related to this event. This suggests that the canyon was active at the time of slope failure. Our results show the same phenomenon for paleo-valley A, for which the complete removal of sediments within this valley suggests strong turbiditic activity during slope failure. Undercutting at the toe of a slope due to erosive action of turbidity currents in canyons may result in the axial incision of the canyon floor, triggering slope failure. Overpressure generation due to rapid deposition of slope sediments may have also contributed to slope instability [START_REF] Hillman | Gas migration pathways and slope failures in the Danube Fan, Black Sea[END_REF]. It is likely that, in the Viteaz (Danube) turbidite system, the location of gassy facies within the canyon and the upper part of the associated channel favored instability and possibly triggered sediment failures [START_REF] Popescu | Analyse des processus sédimentaires récents dans l'éventail profond du Danube (Mer Noire)[END_REF], e.g. Egorov et al., 1998 for the Dnepr Canyon). Gas migration from faults could also contribute to this fluid overpressure and related sediment instability in canyons. The collapse of the eastern levee of channel B3 (Fig. 12) and the sedimentary deformation of the levee, i.e. the enigmatic mass (Fig. 13), are suspected to be attributed to the gas flow observed from the Crest fault system. We, therefore, hypothesize that the combined effect of significant turbidite activity (at least in paleo-valleys A and B) and the presence of gas at the walls of canyons and valleys influences sediment stability and the concurrent development of canyons and/or paleo-valleys A to D (Fig. 20B).

Surficial sediment deposits and instabilities

The surficial infilling and draping units mark the abandonment of canyon-channel systems in the study area and migration of the turbidite activity to the south, i.e. within canyon Y and/or Danube (Viteaz) canyon/valley. These units record the last evolution of the Danube river activity linked to the Alpine ice-sheet fluctuations during the end of the last glacial 33-17 ka BP (lacustrine deposits, Martinez-Lamas et al., 2020), the last connection to the Marmara-Mediterranean Sea with sapropel and carbonates [START_REF] Constantinescu | Evolution of the Danube Deep-Sea fan since the last glacial maximum: new insights into Black Sea water-level fluctuations[END_REF] and marine sedimentation. This evolution from channel filling to marine sedimentation suggests a retrogradation of the deltaic system towards the coast during the sea-level rise and is consistent with the conventional sequence stratigraphic models (Mitchum and Vail, 1977;[START_REF] Vail | Seismic stratigraphy and global changes of sea level, part 4, global cycles of relative changes of sea level[END_REF][START_REF] Van Wagoner | An overview of the fundamentals of sequence stratigraphy and key definitions[END_REF]) (Fig. 20C).

Some MTDs are recorded within the draping unit along the upper slope, within the free-gas area, and in zones favorable to failure initiation such as the flanks of paleo-canyons and the sediment-wave area (Fig. 19; Fig. 10). According to [START_REF] Lericolais | High frequency sea level fluctuations recorded in the Black Sea since the LGM[END_REF], the Melt Water Pulse 1A at 12.5 ka 14 C BP is the source of water level rise between -40 to -20 m below the present-day sea level. Then, during the low water level stage from 11 to 8 ka 14 C BP (i.e., younger Dryas), a lacustrine sedimentary wedge system prograded on the Romanian shelf to the Danube canyon head. It is likely that this enhanced sediment supply onto the shelf led to abrupt overloading generating a new and short episode of gravity-deformation and subsequent fluid/gas expulsion underlying slope instabilities.

Sea level has been rising and the Danubian sediment supply decreasing since around 8 kya [START_REF] Panin | Danube River Sediment Input and its Interaction with the Northwestern Black Sea Estuarine[END_REF]. The present-day depocenters in the region are the Danube delta front and prodelta areas on the internal shelf i.e. to 50-60 m depth. According to [START_REF] Winguth | Upper quaternary water level history and sedimentation in the northwestern Black Sea[END_REF], during highstands, the deltaic region is far removed from the shelf edge and is the depocenter for much of the fluvial sediment input. The slope and basin are sediment-starved so that a condensed section is laid down as a result of hemipelagic deposition (Fig. 20D).

Sea level is still rising and outpaces subsidence within the coastal area of the Danube delta [START_REF] Panin | Radiocarbon dating of Danube delta deposits[END_REF]. This suggests that sedimentary load on the shelf is too low to induce present-day gravity-driven deformation. However, rapid sea-level rise is well known to induce bending stress on the continental slope, which can lead to elevated seismicity and potential fault reactivation that can consequentially trigger landslides [START_REF] Brothers | Sea-level-induced seismicity and submarine landslide occurrence[END_REF]. This could explain the origin of sub-surface MTDs, i.e., those buried under marine sediments (around 1 m thick from GAS-CS01, see section 4.3).

Potential role of seismicity and gas-hydrate dynamic on sediment instability

Of the mechanisms which can trigger sediment instabilities throughout geological recording are the seismic activity linked to the regional faulting pattern, the presence of gas in the sediment and the dissociation of gas hydrate (see [START_REF] Garziglia | Mass-transport deposits on the Rosetta province (NW Nile deep-sea turbidite system, Egyptian margin): characteristics, distribution, and potential causal processes[END_REF] for similar investigation for the Nile delta).

In the study area, earthquakes of magnitude > 7 have occurred along the Romania-Bulgaria coast and could have caused submarine slides and consequential tsunamis. For example, two earthquakes of magnitude = 7.1 and 7.2 were reported offshore NE Bulgaria [START_REF] Papadopoulos | Tsunami hazard in the Black Sea and the Azov Sea: a new tsunami catalogue[END_REF], and near the Sugar canyon [START_REF] Matova | Recent geological activity along the northeastern Bulgarian Black Sea coast[END_REF], respectively (Fig. 2 and Table 1). In the study area, a magnitude 3.9 earthquake occurred in 2007 at the location of the W-E oriented fault (Fig. 2 and Table 1,). Two other earthquakes of magnitudes 4.6 and 4 occurred further offshore the study area in 1986 and 2009 respectively. Despite their low magnitude, these earthquakes could have triggered surficial sediment instabilities, especially along the valley flanks such as for paleo-valleys A and B and also the deformation of levee B3, i.e. the enigmatic mass. Earthquakes of higher magnitude could occur in the study area.

The role of paleo-seismic activity in the initiation of MTDs is speculative but cannot be excluded, especially as scarp alignments and faults are spatially linked.

Another mechanism that can trigger sediment instabilities is gas hydrate dissociation and the subsequent free gas release [START_REF] Bouriak | Inferred gas hydrates and clay diapirs near the Storegga Slide on the southern edge of the Vøring Plateau, offshore Norway[END_REF][START_REF] Bünz | Fluid flow impact on slope failure from 3D seismic data: a case study in the Storegga Slide[END_REF][START_REF] Kwon | Submarine slope failure primed and triggered by bottom water warming in oceanic hydrate-bearing deposits[END_REF].

Several factors lead to gas-hydrate dissociation such as sea level fall [START_REF] Haq | Natural gas hydrates: Searching the long-term climatic ans slope stability records[END_REF][START_REF] Maslin | Sea-level-and gas-hydratecontrolled catastrophic sediment failures of the Amazon Fan[END_REF][START_REF] Maslin | Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis[END_REF][START_REF] Kennett | Methane hydrates in Quaternary climate change: The clathrate gun hypothesis[END_REF], penetration in the Black Sea of warmer Mediterranean water during the rapid transgression after the Last Glacial Maximum [START_REF] Zander | On the origin of multiple BSRs in the Danube deep-sea fan, Black Sea[END_REF][START_REF] Matenco | The interplay between tectonics, sediment dynamics and gateways evolution in the Danube system from the Pannonian Basin to the western Black Sea[END_REF], or salt diffusion within the sediment [START_REF] Riboulot | Freshwater lake to salt-water sea causing widespread hydrate dissociation in the Black Sea[END_REF]. Potentially, the superficial sedimentary instabilities close to the BSR edge such as MTD1 are provoked by gas hydrate dissociation (Fig. 19A). We also note the presence of normal faults which seem to propagate listrically toward the BSR. They are then potentially related to free gas release following the upward migration of gas hydrate since the last glacial maximum (Fig. 5, below SA5) (Dillon et al., 1998). Listric faults can also induce slope instabilities, a process that shaped these basin margins during Miocene-Pliocene times (Fig. 1B, [START_REF] Munteanu | Effects of large sea-level variations in connected basins: the Dacian-Black Sea system of the Eastern Paratethys[END_REF]. The field of sediment waves could also be interpreted as sediment destabilization by dissociation of gas hydrate (Fig. 10). Indeed, the substratum of sediment waves corresponds to the BSR. Moreover, the sets of vertical faults which coincide with the BSR edge (e.g. Fig. 9, below SA9) are more likely to be fractures linked to free-gas overpressure at the landward termination of the GHSZ than tectonic structures. We add that, conversely, these fractures can increase free-gas migration during BSR evolution and help gas-hydrate dissociation along the landward termination of the GHSZ (Yang and Davies, 2013). In the Dniepr paleodelta, northward of the study area offshore Crimea, the role of gas hydrate dynamics on slope failures remains unclear (Zandler et al., 2018). Nevertheless, because mass-wasting processes occur on channel-levees upstream the GHSZ, slope failure could have been facilitated by gas-hydrate destabilization [START_REF] Naudts | Geological and morphological setting of 2778 methane seeps in the Dnepr paleo delta, northwestern Black Sea[END_REF]. As gas hydrate dissociation is still in progress, the slope studied here is gradually weakening.

51 The evolution from MTDs to turbidite and then marine deposits is consistent with the conventional sequence stratigraphy models that consider sea-level changes as the main control factor of margin sedimentation. However, gravity-driven deformation with possible seismicity and gas/gas-hydrate dynamics are considered as significant additional parameters impacting slope instabilities offshore Romania.

Fig. 1 .

 1 Fig. 1. Geological context of the Black Sea. A) Tectonic map (compilation fromFinetti et al.,

Fig. 2 .

 2 Fig. 2. Morpho-bathymetric map based on EMODNET2016 and GEBCO2014 bathymetric data with superposed structural map, offshore Romania. Note the distribution of canyons and paleo-valleys. The distribution of faults is indicated with: in red, compilation from Diaconu et al., 2020 from the data of[START_REF] Dinu | Stratigraphic and structural characteristics of the Romanian Black Sea shelf[END_REF] Morosanu, 2007 and[START_REF] Munteanu | Kinematics of back-arc inversion of the Western Black Sea Basin[END_REF] in

Fig. 3 .

 3 Fig. 3. Presentation of the study area (see location on Fig. 2). A) Geological setting showingthe shelf break (dark yellow line)[START_REF] Riboulot | Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania[END_REF], the canyons/paleo-valleys and associated channel complexes (A, B, C, D, Y and Z), the Danube channel and Fan, the morphology lines i.e. sedimentary ridges (black barbed wires) (Assemblage project), the faulting pattern (see Fig.2for references), the upper limit of the model-predicted steady-state

  ) campaign (Reson SeaBat 7111 for shallow water 5-500 m, and 7150 for deeper water 200-2000 m) with a bathymetric resolution of 20 m (already published). They have provided the geomorphology of the seafloor from a dense grid of profiles. Water column acoustic data processed on board with SonarScope and GLOBE software have provided the distribution of gas flares. These profiles have not been mapped on Fig. 3B.

Fig. 4 .

 4 Fig.4. Seismic characteristics of the basal regional unconformity, scarp alignments (SA), MTDs, BSR and gas/GH. A) The high-resolution seismic dip Line HR31a-b (for location see Fig.3B) shows the extent of the basal unconformity. B) Zoom of Line HR31b showing the shelf break and SA1 (see Fig.7). The upstream part of the basal unconformity and SA1 are not very clear due to the presence of gas. Note the location of MTD12 over the unconformity.The upper colored reflectors correspond respectively to the top of the levees of channel complex B (yellow, brown, orange), channels C (turquoise blue) and D (green) and upper units (light yellow and seafloor). The crossing of the Line with channel C is indicated (Fig.3B). C) Zoom of Line HR31a showing the lower part of the basal regional unconformity. Note: the truncation of reflectors of underlying layers of this unconformity interpreted as an erosional surface; the presence of SA10 to SA12 on the erosional surface (see Fig.78); the extensive part (low deformation) of MTD12 downstream SA10 (Fig.4C) and the compressive part (high deformation) just upstream SA11; the chaotic facies of MTD12 and the hummocky top surface of the compressive bulge; the BSR (light green color) and seismic anomalies related to gas beneath; the acoustic mask and columns of seismic anomalies upstream the BSR edge. D) P-wave velocity analysis of Line HR31a. Note the velocity range m/s) which characterize the GH just above the BSR and the velocity range within MTD12 (1750-1900 m/s). The low velocity values (1200-1500 m/s) indicate the presence of free gasbearing sediments (from[START_REF] Ker | Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System[END_REF].

Fig. 5 .

 5 Fig. 5. Example of normal faults below scarp alignments in the eastern part of the study area.A) The high-resolution seismic Line HR36b (for location see Fig.3B) shows the shelf break, SA1 and SA2 (see Fig.7) and overlying MTD12 and MTD13. B) Line HR36a showing SA3 to SA6 and normal faults below these scarp alignments within the GHSZ. Note the listric shape below SA5. Arrows to the hanging wall/foot wall define the sense of motion.

Fig. 6 .

 6 Fig. 6. Relationship between scarp alignments of the erosional surface, MTDs, BSR, and faults. The high-resolution seismic Line HR30d (for location see Fig. 3B) shows the stacked small-offsets of reflectors (faults?) and pull down effects (gas?) below the BSR (in light green). Note the faults and the major dip change in stratification below SA13 at the erosional surface and the accordion-like shape of MTD12 with alternating extensive and compressive parts. Note at SA12, the location of the head of the extensive part which is generally the case on profiles.

Fig. 8 .

 8 Fig. 8. Cross profile of Paleo-valley A and channel complex B. A) The high-resolution seismicLine HR40 (for location see Fig.3B) shows the cross-section of Paleo-valley A and channels B0 to B5 and the basal erosional surface below. Note: the erosional trough and the lateral scarps marking the base of paleo-valley A and its filling by the western levees of channels of B0 to B5; the stacked migrating small channels within this filling. The channel complex B is marked by an asymmetric shape with a staircase structure of the eastern side. Note the migration of channels B0 to B2. The double arrows indicate the channels axes of B0 and B1.On the eastern side of channel complex B, the internal levee is considered as coeval to the levees B4 and B5 of the western side. Note the presence of distal levees of channels C and D and the filling and draping upper units. B) The SYSIF PL01PR06 profile (for location see Fig.3B) shows an example of superficial MTDs i.e. MTD2 and MTD3 within paleo-valley A over the western levee of channel complex B.

Fig. 9 .

 9 Fig. 9. Dip profile of Paleo-valley A and channel complex B. The high-resolution seismic Lines HR33b and HR33c (for location see Fig. 3B) show the cascading scarp alignments SA7 to SA10. Note the location of MTD4 and MTD3 near the sea floor and over respectively

Fig. 10 .

 10 Fig. 10. MTDs and sediment waves field within paleo-valley A. A) The high-resolution seismic Line HR72 (for location see Fig. 3B) shows the MTDs overlying the erosional surface. Note the reworking/deformation of levees deposits of channels B0 to B5. The upstream reworking is characterized by stacked small channels and levees deposits and the downstream reworking corresponds to the sediment waves field. Note also the vertical faults at the BSR edge and below SA8. B) SYSIF profile PL01PR03 showing the sub-surface MTD2 and the superficial MTD associated to the slide scarp within the draping unit at the top of the filling unit.

Fig. 11 .

 11 Fig. 11. Possible channels A and Y. The high-resolution seismic Line Blason B048 (for location see Fig. 3B) shows two superposed channels that could correspond from bottom to top to the channels A and Y. Note the stratigraphic location of channel complex B between these two channels.

Fig. 12 .Fig. 14 .

 1214 Fig. 12. Dip profile of channel complex B. The high-resolution seismic Line HR41 (for location see Fig. 3B) shows the truncated levees B2 and B3 and the Internal levee coeval to channels B4 and B5 overlying the MTD resulting from the collapse. Basal MTDs on the erosional surface similar to those observed on Line HR72 (see Fig. 10A) are not excluded but the presence of gas does not allow distinguishing these MTDs.

  ). This observation implies that the most recent Danube channel is younger than these channel complexes, in agreement with[START_REF] Hillman | Gas migration pathways and slope failures in the Danube Fan, Black Sea[END_REF]. Westward, the most recent Danube channel appears over possibly channel A and channel Z. According to[START_REF] Hillman | Gas migration pathways and slope failures in the Danube Fan, Black Sea[END_REF], Z would be below the erosional surface and very old (> 1 Ma) (Fig.17

Fig. 15 .

 15 Fig. 15. Extent of channel-levees systems of Paleo-valley A, channel complex B, C and D (see location of the map on Fig. 3). MTDs on the erosional surface (blue polygon) and lateral scarps of erosional troughs A and B have been reported (green color). The axes of channels are marked by a line of the same color as the color of the levees. The levee extent is indicated by a colored polygon. The channel-levees are successively stacked on the different maps. A) Valley A and possible channel A (purple color) Channel-levees B0 (pink color); B) Channellevees B1 (yellow color) over B0; C) B1 and B2 and Enigmatic mass interpreted as belonging to the levee B3; E) elongated MTD resulting from the collapse of B3 eastern levee; F) Internal levee on the elongated MTD and channel-levees B0 to B3. G) Channel-levees C (turquoise blue color); H) Channel-levees D (green color). The maps are based on

  ) as laminated clayey sequences that resemble the lacustrine varves ofDegens et al. (1978). These lacustrine sequences have been interpreted as hyperpycnites i.e. hyperpycnal turbidites produced by discharges of the Danube river at the end of the glacial period (ca.33-17 ka)[START_REF] Martinez-Lamas | Linking Danube River activity to Alpine Ice-Sheet fluctuations during the last glacial (ca. 33-17 ka BP): Insights into the continental signature of Heinrich Stadials[END_REF]. We note that only 1 m of marine sediments has formed since the last connection to the Mediterranean Sea.

Fig. 18 .

 18 Fig. 18. Filling and draping upper sedimentary layers. The SYSIF PL01PR09 profile (for location see Fig.3B) shows the unit (light yellow color) filling the underlying topography of the top of levee D (green color). The overlying draping unit outcrops at the seafloor. These two upper units act as a seal for upward gas migration as gas flares do not occur in this area (see Fig.3A).

Fig. 20 .

 20 Fig. 20. Evolution of the tectonic-sedimentary architecture of surficial deposits since the last sea level fall. A) Sea level fall, formation of the basal erosional surface boundary and the basal layer of MTDs. B) Lowstand and the development of the canyons and channel-levee systems. Some sediment instabilities are also recorded within the canyon during this stage.C) Sea-level rise and abandonment of the channel. Formation of the infilling and draping unit.Development of the surficial MTDs during a short stage of gravity-driven instability. D)Highstand with deltaic sedimentation and gas hydrate dissociation along the slope. The present-day gas flare location is mainly controlled by the presence of gas hydrates, the location of canyons and slide walls, the fault systems and the sealing draping unit.

Table 1 .

 1 The study area is indicated by the black box (see Fig.

	are from the ISC

3).

Table 1 :

 1 Earthquake Data extracted from the International Seismological Centre (2020).

Errors are not reported in the catalog. Distance in km from the closest station was calculated with 1° = ~ 111 km.

2.2 Previous work on the study area

only HR lines for channel-levees B0 to B2 and on all seismic profiles (HR, SYSIF, SDS) for channel-levees B3, C, D, the elongated MTD and the internal levee.

Acknowledgments

53

The officers and crew during the GHASS cruise on board R/V Pourquoi Pas? and the Genavir and Ifremer technical staff during and after the cruise are greatly thanked. S. Ker, V. Riboulot, B. Marsset, F. Colin, Y. Thomas are greatly thanked for their scientific and technical contribution. Andrew S. Madof and an anonymous reviewer are thanked for their comments that helped to improve this manuscript. We also thank the editor Liviu Matenco for his general comments. We sincerely thank Alison Chalm for revision of the English language. in the Western Black Sea. Geochemistry, Geophysics, Geosystems. 20,[442][443][444][445][446][447][448][449][450][451][452][453][454][455][456][457][458][459] https://doi. org/10.1029/2018GC007861. Kim, Y-G., Lee, S-M., Jin, Y.K., Baranov, B., Obzhirov, A., Salomatin, A., Shoji, H., 2013. The stability of gas hydrate field in the northeastern continental slope of Sakhalin Island, Sea of Okhotsk, as inferred from analysis of heat flow data and its implications for slope failures. Marine and Petroleum Geology. 45,

Data availability

Data sets used in the current study were acquired during the GHASS expedition (10.17600/15000500) and data set information are available in the GHASS cruise report (http://archimer.ifremer.fr/doc/00300/41141).