François Bache
email: francois.bache@upmc.fr

Jean Louis Olivet

Christian Gorini

Marina Rabineau

Juan Baztan

Daniel Aslanian

Jean-Pierre Suc

Messinian erosional and salinity crises: View from the Provence Basin (Gulf of Lions, Western Mediterranean)

Keywords: Messinian, Salinity Crisis, Gulf of Lions, Mediterranean, Provence Basin, Miocene, detrital deposits, erosion

Though the late Miocene "Messinian Salinity Crisis" has been intensely researched along the circum-Mediterranean basins, few studies have focused on the central part of the Mediterranean Basin and, especially, the pre-salt deposits. To improve our knowledge of the Messinian events, it is imperative to better understand this domain. In this study, we provide a more complete understanding of this central domain in the Provence Basin. We were able to recognize: a) thick marine detrital series (up to 1000 m) derived from the Messinian subaerial erosion which is partly prolongated in the distal part by b) a thick unit of deep marine deposits (up to 800 m) prior to the evaporites; c) a thick presumed alternation of detritals and evaporites (1500 m) below the mobile halite; and d) a two-step transgression at the end of the Messinian. Spatially, we document the eroded shelf to the deep basin (and from the western to the eastern parts of the Gulf of Lions), and temporally, we extend the interpretations from the early deposition of detritic sediments to the final sea-level rise. The results provide a new basis for discussion not only for the development of the Messinian Salinity Crisis but also for the reconstruction of the subsidence history of the Provence Basin.

Introduction

The reduced inflow of Atlantic Ocean water through the Betic and Rifian corridors (Fig. 1) at the end of the Miocene, together with a high evaporation rate, led to a significant lowering of the Mediterranean Sea's base level and gave rise to one of the most prominent episodes of the Sea's history, known as the "Messinian Salinity Crisis". This Salinity Crisis continues to raise questions and arouse interest. First, because of the wide geographical extent of the extreme environment, the Messinian gave rise to one of the largest evaporite basins known (2.5 millions km 2 ), comparable in size to the North Sea Permian basins [START_REF] Ziegler | Geological atlas of western and central Europe[END_REF]. Its comparatively younger (Neogene) age also makes it much more accessible to analysis and modelling than older and deeper large known basins. Second, the volume of the Messinian evaporite series is greater than 1 millions km 3 in the Mediterranean Basin [START_REF] Ryan | Geodynamic implications of the Messinian crisis of salinity[END_REF]. The Messinian (evaporitic and erosional) events are also distinctive in that they occurred in a relatively brief period of ~ 0.63 My [START_REF] Hilgen | Astronomical tuning as the basis for high resolution chronostratigraphy: the intricate history of the Messinian Salinity Crisis[END_REF] and during the history of an oceanictype basin which is at least 15 millions years old.

A supply of oceanic water to the basin is necessary to explain the thickness of the evaporite layer. In view of the absence of connections with the Indian Ocean, the history of the eastern Mediterranian Basins (e.g. Tyrrhenian, Ionian) is linked intimately to the western basin.

Within the western Mediterranean Sea, the Gulf of Lions is exceptional in that its sedimentary strata have not been deformed. In addition, the Gulf of Lions is characterized by relatively constant subsidence with continuous accommodation space for sediment accumulation. This margin is also characterized by a gentle slope, which prevents major remobilization and gravitational movements. This configuration, together with the availability of a vast data base, enables us to describe full geometries of the stratal patterns of Miocene series (from the intensely eroded geomorphologies on the shelf to the well preserved successions in the basin).

Previous studies have focused on "marginal" or "peripheral" basins (mainly present-day onshore areas) rather than on the "central" basins (present-day offshore areas). The central basins are relatively of wide extent and contain thick evaporitic sequences, while marginal basins are much smaller with reduced evaporitic sequences (Fig. 1). These basins have also been studied with two very different approaches due to their accessibility: outcrop studies, and some mines and boreholes in marginal basins and remote geophysical techniques in the central basins. So far the central Mediterranean Basin has been poorly known, due to its relative inaccessibility and lack of integration of available data.

Overview of previous works

Pioneer works based on field studies described a huge incision in the Rhône River valley at the end of Miocene [START_REF] Fontannes | Note sur l'extension de la faune de la mer pliocène dans le Sud-Est de la France[END_REF][START_REF] Depéret | Note sur le Pliocène et la position stratigraphique des couches à Congéries de Théziers (Gard)[END_REF][START_REF] Depéret | Sur la classification et le parallélisme du système Miocène[END_REF][START_REF] Denizot | Le Pliocène dans la vallée du Rhône[END_REF]. The isolation of the Mediterranean at that time, a drop in sea level, the subsequent invasion of the sea in the fluvial network in earliest Pliocene and the idea that a salinity crisis could have occurred were proposed very early [START_REF] Denizot | Le Pliocène dans la vallée du Rhône[END_REF][START_REF] Ruggieri | The Miocene and latter evolution of the Mediterranean Sea[END_REF]. The development of reflection profiling techniques and increasing exploration established the existence of a mobile layer capable of generating diapirs beneath the floor of most of the central basins of the Mediterranean Sea [START_REF] Alinat | Accidents de terrain en Mer de Ligurie[END_REF][START_REF] Hersey | Sedimentary basins of the Mediterranean sea[END_REF][START_REF] Menard | The Rhone deep-sea Fan[END_REF][START_REF] Glangeaud | Grandes structures de la mer Ligure, leur évolution et leurs relations avec les chaînes continentales[END_REF][START_REF] Ryan | Diapirism in the sedimentary basins of the Mediterranean Sea[END_REF][START_REF] Leenhardt | Le problème des dômes de la Méditerranée occidentale; étude géophysique de la structure A[END_REF][START_REF] Mauffret | Les dômes et les structures "anticlinales" de la Méditerranée occidentale au Nord-Est des baléares[END_REF][START_REF] Montadert | De l'âge tertiaire de la série salifère responsable des structures diapiriques en Méditerranée Occidentale (Nord-Est des Baléares)[END_REF][START_REF] Auzende | Upper Miocene salt layer in the western Mediterranean[END_REF][START_REF] Ryan | The tectonics and geology of the Mediterranean Sea[END_REF]. The origin of this layer was largely interpreted as related to salt deposition. However, different interpretations were proposed for the age of salt deposition and its disposition [START_REF] Glangeaud | Grandes structures de la mer Ligure, leur évolution et leurs relations avec les chaînes continentales[END_REF][START_REF] Cornet | Le graben Médian (Zone A) de la Méditerranée occidentale pourrait être Pontien[END_REF][START_REF] Ryan | The floor of the Medterranean sea. Part 1: The structure and evolution of the sedimentary basins[END_REF][START_REF] Mauffret | Les dômes et les structures "anticlinales" de la Méditerranée occidentale au Nord-Est des baléares[END_REF][START_REF] Montadert | De l'âge tertiaire de la série salifère responsable des structures diapiriques en Méditerranée Occidentale (Nord-Est des Baléares)[END_REF]. Using new and high quality seismic data acquired in the Mediterranean Basin in 1970, [START_REF] Auzende | Upper Miocene salt layer in the western Mediterranean[END_REF] proposed that the salt was late Miocene in age, following earlier suggestions from [START_REF] Denizot | Le Pliocène dans la vallée du Rhône[END_REF] and [START_REF] Ruggieri | The Miocene and latter evolution of the Mediterranean Sea[END_REF]. At the same time, the salt was cored during Leg 13 of the Deep Sea Drilling Project in 1970 along with its cover of gypsum, anhydrite, lacustrine mud and marls with clastics reworked from the margin. This layer was dubbed the "Upper Evaporites" by the Leg scientists. All these deposits were indisputably dated and interpreted for the first time as deep-basin products of the Messinian Salinity Crisis [START_REF] Ryan | [END_REF]Hsü, 1972b;Hsü et al., 1973b). Two models, both based on the deposition of evaporites in shallow water depth were proposed and initiated a heated debate in the scientific community: the "shallow water, shallow-basin desiccation model" [START_REF] Nesteroff | The sedimentary history of the Mediterranean area during the Neogene[END_REF]; and the "desiccated, deep basin model" (Hsü, 1972b;[START_REF] Cita | Mediterranean evaporite: paleontological arguments for a deep-basin desiccation model[END_REF][START_REF] Cita | Time scale and general synthesis[END_REF][START_REF] Hsü | The desiccated deep-basin model for the Messinian events[END_REF]Hsü et al., 1973a;[START_REF] Ryan | Geodynamic implications of the Messinian crisis of salinity[END_REF].

The first model suggests the existence of a shallow basin (several hundred meters deep) before the Salinity Crisis. This model envisioned vertical tectonic movement during the Pliocene that would have deepened the basin after the crisis [START_REF] Bourcart | La Méditerranée et la révolution du Pliocène[END_REF][START_REF] Pautot | La marge continental au large de l'Esterel (France) et les mouvements verticaux Pliocene[END_REF][START_REF] Auzende | Upper Miocene salt layer in the western Mediterranean[END_REF][START_REF] Burollet | Evolution géodynamique néogène de la Méditerranée occidentale[END_REF][START_REF] Stanley | Subsidence of the Western Mediterranean Basin in Pliocene-Quaternary Time: Further Evidence[END_REF][START_REF] Rouchy | La genèse des évaporites messiniennes de Méditerranée: un bilan[END_REF][START_REF] Rouchy | Commentaires sur une interprétation paléogéographique du domaine méditerranéen au cours du dépôt des évaporites messiniennes proposée à partir de l'étude des surfaces d'érosion périméditerranéennes[END_REF]. But considering that different basins that make up the Mediterranean are of different ages -some much older (such as the Ionian Sea), others much younger (such as the Tyrrhenian Sea) -this Alpine tectonic model soon became obsolete. The second model suggests the existence of a deep basin (over 1500 meters deep) before the Messinian crisis [START_REF] Argand | La tectonique de l'Asie[END_REF][START_REF] Cita | Mediterranean evaporite: paleontological arguments for a deep-basin desiccation model[END_REF][START_REF] Hsü | The desiccated deep-basin model for the Messinian events[END_REF]Hsü et al., 1973b;[START_REF] Hsü | Genesis of the Thethys and the Mediterranean[END_REF][START_REF] Montadert | Messinian event: seismic evidence[END_REF][START_REF] Stampfli | Messinian palaeorelief from a 3-D seismic survey in the Tarraco concession area (Spanish Mediterranean Sea)[END_REF] and a sea-level drop of around 1500 m. Three arguments were used to strengthen this theory: the tidal nature of the evaporites recovered in all the major basins (Hsü, 1972a(Hsü, , 1972b)); the pan-Mediterranean distribution of seismic reflector M, that was calibrated with the abrupt contact between the evaporites and the overlying Early Pliocene marls [START_REF] Ryan | Geodynamic implications of the Messinian crisis of salinity[END_REF], and the open marine, deep bathyal nature of the pelagic sediments immediately superposed on the evaporites [START_REF] Cita | Mediterranean evaporite: paleontological arguments for a deep-basin desiccation model[END_REF].

The deep basin model could also be defended by kinematic and geodynamic considerations: such a basin, opened by the rotation of a microcontinent during the Oligocene time (at around 30 My) in the general framework of African-European convergence [START_REF] Smith | Alpine deformation and the oceanic areas of the Tethys, Mediterranean and Atlantic[END_REF][START_REF] Dewey | Plate tectonics and the evolution of the Alpine system[END_REF] can at the time of the Messinian only have been deep. A final decisive argument in favour of this spectacular hypothesis came from studies on the marginal erosion coeval with the central basin evaporites all around the Mediterranean [START_REF] Barr | Late Tertiary channel system in Northern Lybia and its implications on Mediterranean sea level changes[END_REF][START_REF] Chumakov | Pliocene and Pleistocene deposits of the Nile Valley in Nubia and Upper Egypt[END_REF]Clauzon, 1973[START_REF] Clauzon | L'hypothèse eustatique et le creusement prépliocène de la vallée du Rhône[END_REF][START_REF] Ryan | The nature and distribution of Messinian erosional surface-indication of a several kilometer-deep Mediterranean in the Miocene[END_REF][START_REF] Clauzon | The Messinian Var canyon (Provence, Southern France) -Paleogeographic implications[END_REF][START_REF] Rizzini | Stratigraphy and sedimentation of a Neogen-Quaternary section in the Nile delta aera (A.R.E.)[END_REF][START_REF] Ryan | The nature and distribution of Messinian erosional surface-indication of a several kilometer-deep Mediterranean in the Miocene[END_REF][START_REF] Clauzon | Le canyon messinien de la Durance (Provence, France): Une preuve paléogéographique du bassin profond de dessiccation[END_REF][START_REF] Barber | Messinian subaerial erosion of the Proto-Nile delta[END_REF][START_REF] Clauzon | Le canyon messinien du Rhône: une preuve décisive du "dessicated deepbasin model[END_REF]. The convergence of observations has made it possible to exclude regional tectonic factors and confirm that the eustatic fall of more than 1500 m sculpted the Mediterranean river systems during the Messinian Crisis. This result was obtained mainly from onshore observations but it has also been supported by seismic reflection surveys over a width of some hundred kilometres on the Gulf of Lions shelf [START_REF] Burollet | The Neogene series drilled by the Mistral No. 1 well in the Gulf of Lion[END_REF][START_REF] Biju-Duval | Geology of the Mediterranean Sea basins[END_REF][START_REF] Burollet | Evolution géodynamique néogène de la Méditerranée occidentale[END_REF][START_REF] Gennesseaux | Le Golfe du Lion et le Paléo-Rhône messinien[END_REF][START_REF] Lefebvre | Evolution morphologique et structurale du Golfe du Lion. Essai de traitement statistique des données[END_REF]. The "Desiccated, deep basin model" (Hsü, 1972b;[START_REF] Cita | Mediterranean evaporite: paleontological arguments for a deep-basin desiccation model[END_REF][START_REF] Hsü | The desiccated deep-basin model for the Messinian events[END_REF]Hsü et al., 1973a) was therefore widely accepted at that time. Some years later, [START_REF] Gorini | Géodynamique d'une marge passive: le Golfe du Lion (Méditerranée Occidentale)[END_REF] and [START_REF] Guennoc | Histoire géologique du Golfe du Lion et cartographie du rift oligo-aquitanien et de la surface messinienne[END_REF] compiled a map of the subaerial erosion surface over some 15,000 km² in the shelf of the Gulf of Lions. This confirmed, over a distance of some 100 km, the existence of a major Languedocian paleoriver. In the eastern part of the shelf they also mapped the channel of a paleo-Rhône (Fig. 1). These observations although likely to provide us information on the paleoshorelines of the Messinian basin, were, unfortunately only mapped down to the upper continental slope.

Messinian evaporites have been described as three different sub-units from the top to the base:

1) The "Upper Evaporites" sequence with high amplitude reflectors (M reflectors) at its top, it has only been sampled in its upper part in the deep basin [START_REF] Ryan | Initial Reports of the Deep Sea Drilling Project[END_REF]; 2) The massive salt layer which has never been cored, its limits have long been recognized thanks to seismic interpretations [START_REF] Mauffret | Northwestern Mediterranean sedimentary basin from seismic reflection profile[END_REF][START_REF] Ryan | Quantitative evaluation of the depth of the Western Mediterranean before, during and after the Messinian salinity crisis[END_REF]; 3) A lower unit with high amplitude, well stratified reflections was first interpreted as a velocity artefact and then named "Lower Evaporites" using a simple analogy with the two evaporitic units observed in Sicily which are accessible for outcrop studies [START_REF] Decima | Osservazioni sulle evaporiti messiniane della Sicilia centro-meridionale[END_REF]. A thickness on the order of 500 m has been proposed [START_REF] Montadert | Messinian event: seismic evidence[END_REF]. Some major questions remain concerning the beginning of the crisis in the central Mediterranean Basin. The geometric physical link between the evaporitic series identified in marginal basins accessible for field studies and the evaporitic series of the central basins has never been made. The many interpretations concerning the marginal and central Messinian deposits are well summarized in a review article by [START_REF] Rouchy | The Messinian salinity crisis in the Mediterranean basin: A reassessment of the data and an integrated scenario[END_REF]. Two major groupings are evident: one that favours a synchronous deposition of the first evaporites in all the basins before the major phase of erosion [START_REF] Krijgsman | Chronology, causes and progression of the Messinian salinity crisis[END_REF]; and the other that favours a diachronous deposition of the evaporites through more than one phases of desiccation which would first have affected the marginal basins and later the central basins [START_REF] Clauzon | Alternate interpretation of the Messinian salinity crisis: controversy resolved?[END_REF][START_REF] Riding | Mediterranean Messinian salinity crisis: constraints from a coeval marginal basin, Sorbas, southeastern Spain[END_REF][START_REF] Butler | Calibrating the duration and timing of the Messinian salinity crisis in the Mediterranean: linked tectonoclimatic signals in thrust-top basins of Sicily[END_REF]. In spite of conflicting interpretations, most workers agree with a three-phase progression: 1) a period of partial confinement leading to a limited regression (onset of evaporite deposition in the marginal basins at 5.96 Ma [START_REF] Gautier | Age and duration of the messinian salinity crisis[END_REF][START_REF] Krijgsman | Chronology, causes and progression of the Messinian salinity crisis[END_REF][START_REF] Sierro | Messinian pre-evaporite sapropels and precession-induced oscillations in western Mediterranean climate[END_REF]; 2) a period of near desiccation (major regression); 3) followed by the Pliocene reflooding. Estimates differ on the age and duration of phase 2: beginning at 5.6 Ma [START_REF] Clauzon | Alternate interpretation of the Messinian salinity crisis: controversy resolved?[END_REF][START_REF] Krijgsman | Chronology, causes and progression of the Messinian salinity crisis[END_REF][START_REF] Rouchy | The Messinian salinity crisis in the Mediterranean basin: A reassessment of the data and an integrated scenario[END_REF], or slightly earlier [START_REF] Butler | Calibrating the duration and timing of the Messinian salinity crisis in the Mediterranean: linked tectonoclimatic signals in thrust-top basins of Sicily[END_REF]. The reflooding of the Mediterranean Basin is considered to have been sudden during the earliest Pliocene (Hsü et al., 1973a;[START_REF] Clauzon | Révision chronostratigraphique de la série pliocène traversée par le sondage Canet-1 (Pyrénées-Orientales): apports à la connaissance du Néogène du Roussillon[END_REF][START_REF] Pierre | Sedimentological and stable isotope changes at the Messinian/Pliocene boundary in the Eastern Mediterranean (Holes 968A, 969A, and 969B)[END_REF][START_REF] Blanc | The opening of the Plio-Quaternary Gilbraltar Strait: assessing the size of a cataclysm[END_REF][START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF] and a precise age has been proposed at 5.33 Ma [START_REF] Hilgen | A critical re-evaluation of the Miocene-Pliocene boundary as defined in Mediterranean[END_REF][START_REF] Van Couvering | The base of the Zanclean Stage and of the Pliocene Series[END_REF][START_REF] Lourens | The Neogene Period[END_REF]. Surprisingly, detritic deposits in the Gulf of Lions that must have originated during the huge erosional event were not described until 2002. [START_REF] Savoye | The Messinian event on the margin of the Mediterranean Sea in the Nice area, southern France[END_REF] identified some deposits in the Var region, but [START_REF] Lofi | La crise de salinité messinienne: conséquences directes et différées sur l'évolution sédimentaire de la marge du golfe du Lion[END_REF] first identified detrital sediments in the Provence Basin at the outlet of the Languedoc paleoriver. The small volume of the detrital products (1500 km 3 ) compared to the high volume of estimated erosional sediments (3000 km 3 ) was explained by the deposition of a part of detritus in the basin (intercalated with gypsum and anhydrite in the "Lower Evaporites" below the salt) [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF]. Recently, [START_REF] Lofi | Evidence for pre-messinian submarine canyons on the Gulf of Lions slope (Western Mediterranean)[END_REF] described pre-Messinian submarine paleo-canyons just below the detritals. We will refer to this proposition later in the Discussion Section. [START_REF] Sage | Seismic evidence for Messinian detrital deposits at the western Sardinia margin, northwestern Mediterranean[END_REF] and [START_REF] Maillard | Offshore evidence of polyphase erosion in the Valencia Basin (Northwestern Mediterranean): Scenario for the Messinian Salinity Crisis[END_REF] have also described detritals on the Sardinian and Valencia margins.

Data and method

One of the major assets of this study has been the large amount of data collected in the area for both industrial and academic purposes. A partnership with Total gave us access to an exceptional set of conventional and high-resolution seismic reflection data from the coast to the deep domain (Fig. 2). Seismic interpretations have been performed using the principles of seismic stratigraphy [START_REF] Vail | Seismic stratigraphy and global changes of sea-level. Seismic Stratigraphy -Applications to hydrocarbon exploration[END_REF]. We identified seismic units based on stratal terminations and configurations of seismic reflections. The large coverage of seismic data enabled us to map the units in 3D throughout the Gulf of Lions from Cap Creus to Provence and from the present day coast to the basin area (~ 2500 m water depth).

Additional data were obtained from the e-logs of nine industrial boreholes that sampled the sedimentary cover down to the substratum (Fig. 2). A detailed micropaleontological study [START_REF] Cravatte | Les sondages du Golfe du Lion: Stratigraphie, Sédimentologie[END_REF] provided information on the biostratigraphy and depositional environments of the Miocene, Pliocene and Quaternary successions in four of the wells (Mistral1, Sirocco1, Autan1 and Tramontane1). The data from these wells were synthesized in a compilation of all the drilling reports [START_REF] Guennoc | Histoire géologique du Golfe du Lion et cartographie du rift oligo-aquitanien et de la surface messinienne[END_REF].

The Ecors programme [START_REF] De Voogd | First deep seismic reflection transect from the Gulf of Lions to Sardinia (ECORS-CROP profiles in Western Mediterranean)[END_REF] provided three general seismic sections across the entire margin, completed by a series of ESP (Expanding Spread Profiles) [START_REF] Pascal | The ocean-continent boundary in the Gulf of Lion from analysis of expanding spread profiles and gravity modelling[END_REF]. ESP data and average velocities in wells were used to obtain propagation velocities from which it was possible to estimate the thickness of the series from the seismic data (timedepth conversion), thus giving access to volume estimates of the units involved.

5 Results: from the eroded Gulf of Lions shelf and slope domain to the evaporite domain Here, we will describe the depositional geometries of the Gulf of Lions from its eroded margin to the evaporite domain. Although these two domains have been known for many years, they were studied separately and the direct geometrical link between them was not established for all of the sedimentary series. We categorize three characteristic domains from the shoreline to the centre of the basin (Figs. 3 and4):

 The eroded domain, characterized by a single discordant surface between the Miocene deposits and the Plio-Pleistocene deposits (without any Messinian deposits).

 A complex intermediate domain, at the bottom of the continental slope, corresponding to the area in which the Messinian erosion products were deposited [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF].

 The evaporite domain characterized by a continuity of the succession throughout the Messinian period and by the presence of evaporites.

The eroded domain

A pervasive erosional surface (dark blue lines on Fig. 3) has long been identified in the Rhône Valley [START_REF] Denizot | Le Pliocène dans la vallée du Rhône[END_REF]Clauzon, 1973[START_REF] Clauzon | Le canyon messinien du Rhône: une preuve décisive du "dessicated deepbasin model[END_REF] and on the Gulf of Lions shelf where it is very clearly discernable in the seismic reflection profiles [START_REF] Ryan | The nature and distribution of Messinian erosional surface-indication of a several kilometer-deep Mediterranean in the Miocene[END_REF][START_REF] Gennesseaux | Le Golfe du Lion et le Paléo-Rhône messinien[END_REF][START_REF] Lefebvre | Evolution morphologique et structurale du Golfe du Lion. Essai de traitement statistique des données[END_REF][START_REF] Gorini | Géodynamique d'une marge passive: le Golfe du Lion (Méditerranée Occidentale)[END_REF][START_REF] Guennoc | Histoire géologique du Golfe du Lion et cartographie du rift oligo-aquitanien et de la surface messinienne[END_REF][START_REF] Lofi | La crise de salinité messinienne: conséquences directes et différées sur l'évolution sédimentaire de la marge du golfe du Lion[END_REF][START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF]. This erosion surface, i.e. the discordant contact between the Miocene deposits and the overlying prograding Plio-Pleistocene sequence beneath the shelf and slope, was named "Margin Erosion Surface" (MES) by [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF] and [START_REF] Lofi | Evidence for pre-messinian submarine canyons on the Gulf of Lions slope (Western Mediterranean)[END_REF].

The Miocene eroded series

The cross sections in Figure 3 (c,d,e) show that a large part of the Gulf of Lions is buried beneath a pre-Messinian sedimentary cover. Reflections are planar and parallel and show good continuity with few thickness variations. Landward, in the direction of Provence and the Pyrenees, the reflections terminate as onlaps on rises of pre-rift substratum (Fig. 3d); basinward, they prograde or lap out approximately up to the present-day slope (Fig. 3c). The pre-Messinian succession is eroded and slightly deformed, except close to the Pyrenees in the West where faults and roll-over tilting are observed [START_REF] Mauffret | Structural geometry in the eastern Pyrenees and western Gulf of Lion (Western Mediterranean)[END_REF][START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF].

Boreholes show that the erosion surface of the shelf truncates sediments of the Miocene age and is covered by sediments of the earliest stage of the Pliocene [START_REF] Cravatte | Les sondages du Golfe du Lion: Stratigraphie, Sédimentologie[END_REF]. Up to 7 My of the Upper Miocene sediment record are missing in Autan borehole at the shelf edge where youngest deposits are dated at ~12 My (post last occurrence of Globorotalia peripheroronda), having been removed by erosion during the Messinian Salinity Crisis.

However, the youngest Miocene sediments were found in the Tramontane well and were dated as Tortonian [START_REF] Cravatte | Les sondages du Golfe du Lion: Stratigraphie, Sédimentologie[END_REF]. In the Cicindelle borehole we found that the entire Miocene was removed so that the Pliocene lies directly on the substratum (Fig. 3d). The Gulf of Lions can be sub-divided into two main areas (Fig. 3d): a Languedoc area in the southwest where substratum was highly subsident so that an accommodation of 2000 to 3000 m was available for the Miocene sediments, and a Provence area where the substratum is in a much higher position and lack of accommodation prevented deposition and/or preservation of thick Miocene strata. It is also deeply incised.

Morphology of the Margin Erosion Surface

A large part of the MES had already been mapped and interpreted in the past. The mapping revealed a pattern of up to 5 th order dendritic drainage [START_REF] Gennesseaux | Le Golfe du Lion et le Paléo-Rhône messinien[END_REF][START_REF] Gorini | Contribution to the structural and sedimentary history of the Gulf of Lions (Western Mediterranean) from the ECORS profiles, industrial seismic profiles and well data[END_REF][START_REF] Guennoc | Histoire géologique du Golfe du Lion et cartographie du rift oligo-aquitanien et de la surface messinienne[END_REF][START_REF] Gorini | The Late Messinian salinity crisis and Late Miocene tectonism: interaction and consequences on the physiography and post-rift evolution of the Gulf of Lions margin[END_REF][START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF] with two main systems (Fig. 4). One to the East, corresponding to the Rhône (which was located East of present day Rhône River) together with a network from the region of Montpellier, both join up downstream into a single valley. The other to the West, with headwards extending from the Languedoc and Roussillon region. The Rhône largely incised the Mesozoic limestone substratum, whereas the Languedoc cuts mainly into the Miocene marls. In both cases, several hundred metres depth can be observed between the thalweg and the interfluves. This height however does not represent the total amount of erosion by the rivers, as interfluves themselves are eroded, so the total amount of erosion could be much greater (see next section).

The drainage networks (MES) have sculpted a "rough" or "badland" morphology [START_REF] Ryan | Messinian badlands on the southeastern margin of the Mediterranean sea[END_REF]. In this study we also observed that this morphology gives way basinward to a planar and "smooth" surface that is locally conformable with the underlying Miocene series but that is also locally erosional as it truncates the underlying succession of the intermediate domain (unit Dm on Fig. 3). This smooth surface slightly deepens seaward and extends over 60-70 km. The transition between the two morphologies (rough and smooth) is very clear and lies at a constant two-way traveltime depth of 1.6 seconds over most of the shelf (Fig. 6), albeit slightly less at the edges of the basin (1.4 seconds two-way traveltime in Provence and Catalonia). An interpretation of this change in morphology will be proposed later in the Discussion Section.

Volume eroded by the Margin Erosion Surface

It is possible to obtain a minimum volumetric estimate of the Miocene sediments that have been removed by erosion in the western part of the Gulf of Lions. Figure 7 shows the measurement method and the estimated values. The Miocene deposits, wherever they are observed, are extremely regular over a large part of the continental shelf and the first signs of a progradation only occur at approximately 90 km from the coast (Fig. 3c). Consequently, up to this point, one can simply extrapolate the intervals removed by erosion. This technique was used earlier by [START_REF] Mauffret | Structural geometry in the eastern Pyrenees and western Gulf of Lion (Western Mediterranean)[END_REF] and [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF] but only in the Languedoc and Roussillon areas which led to a minimum estimate of about 3000 km 3 of eroded sediments.

An average velocity of 2000 m/s [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF] was used for the evaluation of thicknesses within the Miocene and Messinian series. Here, we extended this technique to the East, to the Rhône area as far as the regional reference marker exists. Figure 7a gives a perspective view of three selected profile segments from the seismic coverage. LRM 08 on Figure 7 intersects the Miocene succession where it is best preserved. We extended the youngest observed horizon (Late Miocene) parallel to a regional marker horizon preserved within the series over the entire area. The minimum eroded thickness through extrapolation is shown in yellow on Figure 7. This new evaluation provides an estimated volume of 4000 km 3 of eroded sediments (Fig. 7b). Note that this amount of sediments does not take into account the entire eroded area.

If we consider the whole Rhône Valley and shelf of the Gulf of Lions where the erosion surface has been observed (> 20 000 Km 2 ), we can assume the eroded volume to be much higher (~10 000 Km 3 ). Note also that this volume does not take into account the direct input from the Rhône River. This volume of eroded sediment must have been transported downstream and deposited into the deep basin.

The intermediate domain (between the eroded shelf and the evaporite domain)

The intermediate domain is characterized by a seismic unit (unit Dm) sandwitched between the prograding Miocene deposits below and the Pliocene deposits above and bounded both at its base and top by discontinuities (Figs. 3a, b, c ande). One thus passes from an eroded domain, characterized by a single "rough" (MES) then "smooth" erosion surface occurring between Miocene and Pliocene sediments, to a more complex, intermediate domain where the Miocene and Pliocene sediments are separated by the unit Dm.

Description of unit D-geometries

The edge of the Miocene shelf is truncated by a surface inclined (~2.5°) towards the basin (surface in red in Figures 3a,b,c,e). This surface characterizes the base of unit Dm that shows a major incision (up to 1500 m) at the outlets of the Rhône and of the Pyrenees-Languedoc drainage networks (Fig. 5). The incision is less marked between these two areas.

Three subunits can be recognized in unit D whose extension has been mapped (Figs. 3 and4).

 Subunit Dm0 is the lower member of unit Dm and can be seen at the outlet of the Rhône. It is characterized by clinoforms that dip steeply basinward and extend deep beneath the salt. The clinoforms are up to 1 km in height, they are truncated upstream by the smoother surface described earlier (Fig. 3a,b).

 Subunit Dm1, lying unconformably on subunit Dm0, is present over the entire margin at the outlet of the Roussillon-Languedoc valleys and the Rhône valleys one. Like subunit Dm0, it is characterized by basinward dipping clinoforms (also up to 1 km in height) and also truncated upstream. Basinward, down-dip from the strata, we observe two distinct seismic facies (Fig. 3e): a chaotic facies located mainly on the outlet of the erosional valleys (on the western side); and a facies characterized by more or less continuous reflections (on the eastern side). This facies difference is probably due to whether or not the area had a direct connection with the drainage systems. On Figure 4 we can see rises of the substratum that most likely isolated the eastern side from a direct input of the Rhône and Languedoc sediments, so that sediments are more homogenous and probably more shalier. In both cases, the upper part of subunit Dm1 extends beneath the salt and becomes imbricated in a continuous high-amplitude reflector (LU1) present in the evaporite domain.

 The upper subunit, Dm2, is characterized by a chaotic high-amplitude seismic facies (called "CU" in [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF] located at the immediate outlet of the Languedoc drainage network. A direct connection with the Rhône system and a deposition of coarse deposits can be assumed. This subunit is also truncated in its upstream part.

The base of subunit Dm2 ties in basinward with the base of the mobile salt unit (MU).

Description of unit D in the boreholes

Two boreholes cross the unit Dm (Fig. 4). Autan1 is localized on the edge of shelf and GLP2 on the slope, at the limit of the salt deposit.

 Autan1 [START_REF] Cravatte | Les sondages du Golfe du Lion: Stratigraphie, Sédimentologie[END_REF] indicates, for the interval corresponding to the unit Dm (2424-2997 m), sandy carbonated clay with rare foraminiferas which are often broken and of small size. The lack of significant planktonic foraminiferas prevents precise dating for this interval, however an Upper to Middle Miocene age with marine environment is suggested [START_REF] Cravatte | Les sondages du Golfe du Lion: Stratigraphie, Sédimentologie[END_REF]. A gap of Messinian and Tortonian is also assumed. [START_REF] Cravatte | Les sondages du Golfe du Lion: Stratigraphie, Sédimentologie[END_REF] added that the cuttings of drilling are often not representative because of the significant contamination and the conditions of drilling.

The only representative samples are the slabs (one side core drillings) but they were few in number.

 GLP2 presents many reworkings at all levels of the borehole which made interpretation very tricky [START_REF] Brun | Sondage Golfe du Lion profond n°2[END_REF]. Under salt and anhydrite deposits related to Messinian, carbonated clays (sometimes with silt) are described. This interval, corresponding to unit Dm (3703-4856 m), provides limited information. An uncertain Burdigalian to Tortonian age is suggested.

Autan1 and GLP2 boreholes therefore provide poor fossil associations for the interval corresponding to the unit Dm. On top of that, reworkings described in GLP2 and Autan1 (broken forams) lead us to remain cautious on ages (undifferentiated Burdigalian to Tortonian, see 5.2.2). Samples in regressive seals, which are made of reworked and mixed material are known to be poor intervals for age credibility (B. Haq, personal communication).

Both ages given by these two boreholes are doubtful, and have not been used by us. A Messinian age for the deposits (reworking previous sediments) can not be rejected.

Volume of Unit Dm

Figure 8 shows the isopach map of Unit Dm. The maximum observed thickness is more than 1000 m, with the depocentre located downstream of the outlet of the Roussillon-Languedoc rivers and the Rhône River. The corresponding volume can be estimated at ~4700 km 3 if we consider the average velocity of 2000 m/s used by [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF]. In fact, a velocity of 3000-4000 m/s is probably more appropriate [START_REF] Fahlquist | Seismic refraction measurements in the western Mediterranean Sea[END_REF][START_REF] Leenhardt | Sondages sismiques continus en Méditerranée occidentale -Enregistrement, analyse, interprétation[END_REF], so that the volume of unit Dm could even reach values of 9400 km 3 . This does not include the most distal deposits located in the very deep basin area nor the lateral equivalent of the shelfedge prisms Dm0, Dm1, Dm2 towards the East.

The evaporite domain

Directly below the Pliocene and Quaternary sediments (Fig. 3c,f), the upstream extension of the "Upper Evaporites" is marked by onlaps onto the top of unit Dm. These "Upper Evaporites" made-up of intercalated beds of anhydrite and clay [START_REF] Ryan | Initial Reports of the Deep Sea Drilling Project[END_REF] and also named "Upper Unit" [START_REF] Lofi | Evidence for pre-messinian submarine canyons on the Gulf of Lions slope (Western Mediterranean)[END_REF], have been deformed by creeping and sliding of the underlying salt and by listric faults.

The massive salt underlying the "Upper Evaporites" is the most representative facies of the Messinian in the basin. It is characterized by a transparent seismic facies forming salt domes, formed as the salt flows since the early Pliocene and during the deposition of the Pliocene and Quaternary turbidites (Dos [START_REF] Reis | Implications of salt-sediment interactions on the architecture of the Gulf of Lions deep-water sedimentary systems--western Mediterranean Sea[END_REF]. Its original upstream extension (before movement) can be considered as the limit between the listric faults (which sole out at the base of the salt) and subunit Dm2 (see on Figure 3). This unit is named the "Mobile Unit" (MU) by [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF].

Below the mobile salt (MU) we found a unit characterized by continuous parallel highamplitude reflections (LU1). The upper part of this unit was described and interpreted as "Lower Evaporites" by analogy to the seismic facies of the "Upper Evaporites" and by analogy to the evaporite trilogy in Sicily [START_REF] Montadert | Messinian event: seismic evidence[END_REF]. The reflections clearly onlap the lower part of unit Dm (Dm0 and Dm1, Fig. 3c). The facies is thick in the basin (it reaches 0.6 seconds two-way traveltime) and thins over unit Dm in the intermediate domain.

The upper part of LU1 is imbricated with the upper part of subunit Dm1 (lateral facies transition).

Beneath the LU1 unit, we found a facies with average-amplitude reflections that are more or less continuous. This facies (LU0) is the lateral distal equivalent of the lower part of Unit Dm (subunits Dm0 and Dm1). The base of this distal unit is marked by a high-amplitude reflector that becomes erosive toward the intermediate domain and which corresponds to the base of unit Dm. The lowermost sediments (below LU0) rest directly on the basement and represent the deep deposits of the Miocene post-rift margin.

To summarize, we have described and correlated three major seismic domains. The first is characterized by intense erosion (MES), the second by deposition at the outlet of the river valleys (unit Dm), and the third by an evaporitic deposition. It should be noted that the base of unit Dm, characterized by major erosion in the intermediate domain, extends conformably and widely into the basin below LU0 unit (Fig. 3c).

Discussion

The results that we discuss here include the recognition of thick marine detritic deposits that provides the evidence of a huge detritic phase prior to the evaporite deposition in the central basins; the presence of presumed evaporites, with a thickness of up to 1500 m, located below the thick halite; and finally the evidence of a two-step transgression at the end of the Messinian.

The detrital succession derived from Messinian subaerial erosion

The analysis of depositional geometries provides evidences of a huge phase of subaerial erosion in the Rhône Valley and on the continental shelf of the Gulf of Lions (MES). A major drawdown was thus necessary to deeply incise these domains and particularly the Miocene shelf. We assume that only the major Messinian drawdown was able to produce this huge phase of erosion. This major drawdown (~ 1500 m) has been strongly argued in the past [START_REF] Ryan | The nature and distribution of Messinian erosional surface-indication of a several kilometer-deep Mediterranean in the Miocene[END_REF][START_REF] Gennesseaux | Le Golfe du Lion et le Paléo-Rhône messinien[END_REF][START_REF] Lefebvre | Evolution morphologique et structurale du Golfe du Lion. Essai de traitement statistique des données[END_REF][START_REF] Clauzon | Le canyon messinien du Rhône: une preuve décisive du "dessicated deepbasin model[END_REF][START_REF] Gorini | Géodynamique d'une marge passive: le Golfe du Lion (Méditerranée Occidentale)[END_REF][START_REF] Guennoc | Histoire géologique du Golfe du Lion et cartographie du rift oligo-aquitanien et de la surface messinienne[END_REF][START_REF] Lofi | La crise de salinité messinienne: conséquences directes et différées sur l'évolution sédimentaire de la marge du golfe du Lion[END_REF][START_REF] Gorini | The Late Messinian salinity crisis and Late Miocene tectonism: interaction and consequences on the physiography and post-rift evolution of the Gulf of Lions margin[END_REF][START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF]. This estimate mainly results from observations done during dives realized by Savoye and Piper [START_REF] Savoye | The Messinian event on the margin of the Mediterranean Sea in the Nice area, southern France[END_REF] and is now largely accepted as shown by the recent published "Consensus" about the MSC scenario (CIESM, 2008). However, no evidence had been produced of corresponding detrital deposits before 2002. Several studies have since proved [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF][START_REF] Sage | Seismic evidence for Messinian detrital deposits at the western Sardinia margin, northwestern Mediterranean[END_REF][START_REF] Maillard | Offshore evidence of polyphase erosion in the Valencia Basin (Northwestern Mediterranean): Scenario for the Messinian Salinity Crisis[END_REF] its existence between the evaporite domain and the foot of the continental slope. Nevertheless, the limit of its lower boundary (due to lack of seismic penetration) or its lateral correlation to the deep basin succession (due to the lack of lateral seismic data) have remained undetermined.

Unit Dm that we described is sandwitched between the Miocene shelf deposits and the Pliocene and Quaternary cover (Fig. 3). A major unconformity characterizes the base of unit Dm and other minor surfaces can also be observed within this unit (Fig. 3e). Two conflicting interpretations (depending on the position of the "Basal Erosion Surface" [START_REF] Maillard | Offshore evidence of polyphase erosion in the Valencia Basin (Northwestern Mediterranean): Scenario for the Messinian Salinity Crisis[END_REF], i.e., the discordant contact between the pre-salinity crisis deposits and the syn-crisis deposits) can be proposed and will be discussed here about the age of unit Dm.

 [START_REF] Lofi | Evidence for pre-messinian submarine canyons on the Gulf of Lions slope (Western Mediterranean)[END_REF] interpreted these discontinuities as paleo-submarine canyons that pre-date the initiation of the Messinian drawdown phase. Only the upper part of unit Dm (characterized by a chaotic high-amplitude seismic facies) is attributed to Messinian detritital deposits. Nevertheless, the volume of these chaotic deposits, estimated at around 1500 km 3 (with an average velocity of 2000 m/s) or 3000 km 3 (with 4000 m/s) by the same authors [START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF] is far less than the estimated volume of eroded material in the entire Rhône Valley and Gulf of Lions shelf (~10 000 km 3 ).

 On the contrary, we suggest that all of unit Dm is Messinian and that the major unconformity observed at its base should be linked to the beginning of the main Messinian drawdown of the Mediterranean Sea [START_REF] Bache | Evolution Oligo-Miocène des marges du micro océan Liguro Provençal[END_REF]. The full unit Dm, which has a volume of the same order of magnitude as the estimated volume of eroded material, is a probable candidate for the detrital deposits from the Messinian erosion.

Several other considerations support our interpretation:

Pre-Messinian vs Messinian fluctuations of sea level

The main Messinian drawdown is the most prominent such event to occur in the Mediterranean and probably in the world. The consequences of this drawdown had dramatic effect leading to abnormal amounts of erosion in the Rhône Valley and sediment transfer into the basin. Numerous sea-level fluctuations occurred before the period of the Messinian drawdown [START_REF] Haq | Chronology of fluctuating sea levels since the Triassic (250 million years ago to present)[END_REF] but none of them are comparable (100-200 m at the maximum).

The lower part of Unit Dm (Dm0 and the base of Dm1, the greatest in volume) correlates with LU0 (Fig. 3c). The Dm0-LU0 depositional sequences are genetically related sediments bounded by unconformity (base Dm0) and their correlative conformity (base of LU0). This phase therefore corresponds to a major sediment transfer, which built detrital wedges of thickness as much as 1000 m at the outlet of the Messinian rivers, and in the order of 800 m in the basin. A Messinian origin for only the upper part of unit Dm (characterized by a chaotic high-amplitude seismic facies) would mean that the erosive base of unit Dm (which is a regional major erosional surface that truncates the Miocene shelf) is not connected to the all important Messinian event but to a previous event. In this scenario the Messinian event would thus have produced less prominent unconformities (within the unit Dm) whereas the major regional erosional surface would have been produced by a previous event of lesser severity.

To us this scenario seems unlikely. Instead, the most likely interpretation in the context of the regional distribution of unit Dm and its erosive base is that it is a product of the major Messinian drawdown. The surface resulting from this major drawdown would have overshadowed all previous events. In the case in the Provence Basin this is certainly true where the MES sometimes erodes up to the substratum.

Position of the unit Dm

The mapping of unit Dm and its basal erosional surface identified three subunits at the outlet of Rhône and Roussillon-Languedoc Messinian paleo-rivers (Fig. 4). The MES represents this preserved subearial landscape just before the Zanclean refilling of the basin, i.e., the terminal Messinian exposed landscape. The first unit (Dm0) is principally located at the outlet of the Rhône network. The others (Dm1, Dm2) are also located at the outlet of Roussillon-Languedoc network. These locations can be explained by a drawdown so extensive that he first impacted the Rhône Valley (Dm0) and then the Gulf of Lions shelf (Dm1-Dm2) with the Roussillon-Languedoc rivers that became a major source of sediment supply (Figs. 9 and 10). Thus, seismic sequence geometries are consistent with a Messinian age for unit Dm and therefore we favor to attribute the major unconformity at its base to the onset of the major Messinian drawdown. Nevertheless, we do not rule out the occurrence of smaller erosional events (prior to the main Messinian drawdown) which may not have been preserved on the Messinian shelf edge; i.e., in the transitional domain. This interpretation have strong implications on the Messinian Salinity Crisis scenario.

6.2 The Messinian scenario as viewed from the "central" basin We must emphasize that the two-step scenario of the MSC proposed by [START_REF] Clauzon | Alternate interpretation of the Messinian salinity crisis: controversy resolved?[END_REF] is now widely recognized as the valid one by the respective authors of the Mediterraneanscale MSC scenarios mostly discussed during the last years, as illustrated by the "Consensus report" recently published (CIESM, 2008). We illustrate our interpretation of the Messinian evolution of the Provence Basin in Figures 9 and10. Following an initial and limited Messinian regression [START_REF] Clauzon | Alternate interpretation of the Messinian salinity crisis: controversy resolved?[END_REF] (Figs 9a and10a), we recognize four major phases as described below.

The first phase is marked by a major detrital event, underlying the lowermost evaporite (LU1), and related to the major Messinian drop in the Mediterranean sea level (yellow areas in Figures 9b and10b). This pre-evaporite step implies that thick evaporites in the central basin (visible at the seismic resolution) deposited after the subaerial exposition of the Gulf of Lions, certainly under low bathymetry. [START_REF] Loget | How did the Messinian Salinity Crisis end?[END_REF] have shown that consecutive intense regressive erosion developed inevitably in the Gibraltar area. It should be a likely process to explain a continuous input of marine waters necessary to precipitate enough evaporites in the desiccated Mediterranean Basin. The assumption that central basin evaporites partly deposited under a high bathymetry and before the major phase of erosion [START_REF] Krijgsman | Chronology, causes and progression of the Messinian salinity crisis[END_REF][START_REF] Meijer | A quantitative analysis of the desiccation and refilling of the Mediterranean during the Messinian Salinity Crisis[END_REF][START_REF] Krijgsman | Depositional environments of the Mediterranean "Lower Evaporites" of the Messinian salinity crisis: Constraints from quantitative analyses[END_REF][START_REF] Govers | Choking the Mediterranean to dehydration: The Messinian salinity crisis[END_REF][START_REF] Govers | Regional isostatic response to Messinian Salinity Crisis events[END_REF] should imply the observation of a major detritic event above evaporites in the basin. Such a depositional geometry has not been observed.

The second phase (Figs. 9c and10c) corresponds to a strong change in the sedimentary regime as shown by the onlaps of the sediments during this phase (LU1) onto the underlying detritic layer. Sedimentation evolves from the first detrital event (phase 1) to a massive salt deposition (at the top of LU1 unit) resulting from an increase of salt concentration and continuous input of marine waters within the almost desiccated basin. The corresponding seismic facies is comparable to that of the Upper Evaporites facies comprising of halites, gypsum, anhydrite, lacustrine mud and marls with clastics reworked from the margin.

Therefore, we attributed LU1 to the onset of evaporite/detrital deposition in the central Provence Basin. These "Lower Evaporites" present a thickness of ~1500 m, much higher than what was assumed previously (500 to 600 m) [START_REF] Montadert | Messinian event: seismic evidence[END_REF][START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF]. Such a thickness of Lower Evaporites must be tested in future quantitative studies of the Messinian Salinity Crisis.

The third and the fourth phases correspond to a two-step trangression at the end of the crisis.

An initial relatively slow sea level rise (Figs 9d and10d) permitted the development of a transgressive surface with smooth topography (light blue line) identified previously on seismic profiles. These flatten the top of regressive prisms (Dm0, Dm1, Dm2) and represent the limit between Messinian and Pliocene deposits (Fig. 6). During this relatively slow landward migration of the Messinian shoreline, the continuous action of waves and tides smoothed the reliefs of the Messinian erosional surface. This interpretation is supported by the presence of 50 m of azoic sand at the top of the evaporites in the GLP2. This unit, described by [START_REF] Gorini | Géodynamique d'une marge passive: le Golfe du Lion (Méditerranée Occidentale)[END_REF], could correspond to the transgressive sand from the upstream marine abrasion by wave ravinement. The fourth phase corresponds to the Zanclean rapid reflooding (Hsü et al., 1973a;[START_REF] Clauzon | Révision chronostratigraphique de la série pliocène traversée par le sondage Canet-1 (Pyrénées-Orientales): apports à la connaissance du Néogène du Roussillon[END_REF][START_REF] Pierre | Sedimentological and stable isotope changes at the Messinian/Pliocene boundary in the Eastern Mediterranean (Holes 968A, 969A, and 969B)[END_REF][START_REF] Blanc | The opening of the Plio-Quaternary Gilbraltar Strait: assessing the size of a cataclysm[END_REF][START_REF] Lofi | Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis[END_REF] and has been precisely dated at 5.332 Ma [START_REF] Hilgen | A critical re-evaluation of the Miocene-Pliocene boundary as defined in Mediterranean[END_REF][START_REF] Van Couvering | The base of the Zanclean Stage and of the Pliocene Series[END_REF][START_REF] Lourens | The Neogene Period[END_REF]. It is clearly marked by the transition between two morphologies (rough and smooth), at a constant two-way traveltime/depth of 1.6 seconds over the entire shelf (Fig. 6). Up to this two-way traveltime depth, the irregular 'rough' or badland topography (of MES) illustrates the Messinian paleogeography as it was at the end of the Messinian erosional period (Figs. 9d and10d, in dark blue). This rapid reflooding implies a cessation of the action of waves, which has preserved badland morphologies (Fig. 10e). The change in morphology corresponds therefore to the transition between a subaerial erosion (rough morphology) and a submarine erosion (smooth morphology). In this scenario, the 1.6 second limit corresponds to the position of the paleoshoreline at 5.332 Ma and is an appropriate marker for subsidence studies.

Conclusion

Our results support the deep-desiccated evaporite basin hypothesis (Hsü et al., 1973a): thick detrital deposits at the outlet of the Messinian Rhône and Messinian Languedocian and Pyrenean rivers are, as would be expected [START_REF] Ryan | The nature and distribution of Messinian erosional surface-indication of a several kilometer-deep Mediterranean in the Miocene[END_REF][START_REF] Clauzon | Le canyon messinien du Rhône: une preuve décisive du "dessicated deepbasin model[END_REF], present at the transition between the Miocene shelf and basin. On the basis of depositional geometries, studied for the first time over the entire margin and down to the central basin of the Western Mediterranean, we are able to underscore the following points:

 the evidence of a pre-evaporite phase corresponding to a prominent erosional crisis responding to a major drawdown of the Mediterranean seawater. Assuming than this major drawdown corresponds to the major Messinian drawdown, we can conclude that the Mediterranean bathymetry significantly decreased before the precipitation of central basins evaporites. A deep water formation seem unlikely.

 the presence of a thick probable "Lower Evaporites" series (with a thickness up to 1500 m) located below the salt sequence. This implies that the total thickness of Messinian deposits in the basin should as much as 3500 m (including the pre-evaporite phase and the salt). This thickness also implies that the relief from shelf to basin floor was already significant at the time of their deposition. The basin was gradually filled during the Messinian Salinity Crisis. This infilling would have had a significant effect on the vertical movements of the basin.

 the characteristics of the final discontinuity surface and of two types of morphology (rough and smooth) provides evidence of the basin being resubmerged at the end of the Messinian Crisis. This refilling was first moderate accompanied by transgressive ravinement and later rapid so as to "preserve" the paleoshoreline at 5.332 My and the Margin Erosion Surface. These markers of a two-step reflooding observed in the Gulf of Lions provide remarkable points of reference for subsidence studies. It will be necessary to correlate them at the scale of the whole Western Mediterranean, as well as within the Eastern basin.

Several authors have tried to study the subsidence in the Provence Basin and the isostatic readjustments related to the Messinian Crisis [START_REF] Ryan | Quantitative evaluation of the depth of the Western Mediterranean before, during and after the Messinian salinity crisis[END_REF][START_REF] Steckler | The Gulf of Lion: subsidence of a young continental margin[END_REF][START_REF] Burrus | Thermal and compaction processes in a young rifted basin containing evaporite: Gulf of Lions, France[END_REF][START_REF] Meijer | A quantitative analysis of the desiccation and refilling of the Mediterranean during the Messinian Salinity Crisis[END_REF][START_REF] Krijgsman | Depositional environments of the Mediterranean "Lower Evaporites" of the Messinian salinity crisis: Constraints from quantitative analyses[END_REF][START_REF] Govers | Choking the Mediterranean to dehydration: The Messinian salinity crisis[END_REF][START_REF] Govers | Regional isostatic response to Messinian Salinity Crisis events[END_REF]. The view that we outline provides new fodder for the study of subsidence of the Provence Basin and better understanding its structural evolution. An other interesting perspective of this work could be the study of the lithospheric response to strong and rapid variations of weight during the Messinian Erosional and Salinity crises.

Acknowledgements

This study was carried in the context of the GDR "Marges" French research program whose members we would like to thank for all the fruitful discussions that we have had over the years. We should also like to thank Total for making their data available and in particular Jean-Loup Rubino and Jacqueline Camy-Peyret for their precious help in working with the database. We would also like to warmly thank Georges Clauzon, Bilal Haq and Jean-Loup Rubino for fruitful discussions and advices on the paper. We acknowledge the EPSL editor, Peggy Delaney and journal reviewers (Bill Ryan and anonymous reviewers) for advice and comments on the manuscript that have greatly improved its quality. We also thank Alison Chalm for her language corrections.

Figure Captions

Figure 1: Location of the Messinian evaporite series (halite and other evaporites) in the Western Mediterranean (modified for the Gulf of Lions from [START_REF] Montadert | Messinian event: seismic evidence[END_REF]Rouchy and[START_REF] Rouchy | The Messinian salinity crisis in the Mediterranean basin: A reassessment of the data and an integrated scenario[END_REF] and the area drained by the Messinian rivers in Southeastern France (hatchured). The Late Miocene Betic and Rifian corridors (dotted line) are taken from [START_REF] Martin | The Messinian Guadalhorce corridor: the last northern, Atlantic-Mediterranean gateway[END_REF]. The study area is outlined in black. B: Isopach map of the eroded thickness. This thickness could only be estimated in the area where the reference Miocene reflector was still visible. The thickness of sediments eroded in the areas where the substratum is directly affected (Rhône side) is not taken into account. The significant, but minimum, estimated volume (more than 4000 km 3 and probably around 10 000 km 3 ) is to be compared with the volume of unit Dm located downstream in the intermediate domain.

Figure 2 :

 2 Figure 2: Seismic data and boreholes used for this study. The bold lines represent the location

Figure 3 :

 3 Figure 3: Line drawings perpendicular and parallel to the margin of the Gulf of Lions

Figure 4 :

 4 Figure 4: Map showing the sedimentary units and the erosion located just below the Pliocene.

Figure 5 :

 5 Figure 5: Detail of the transition from the eroded domain (a, b) to the intermediate domain (c,

Figure 6 :

 6 Figure 6: Detail of the transition from the 'rough' erosion surface (Margin Erosion Surface) to

Figure 7 :

 7 Figure 7: Estimated thickness of Miocene sediments eroded during the Messinian Event.

Figure 8 :

 8 Figure 8: Estimate of the volume of unit Dm deposited in the intermediate domain. Oneshould note that this volume (9400 km 3 ) is of the same order of magnitude as the eroded volume (around 10000 km 3 ). Unit Dm is thus the only unit that corresponds to the volume

Figure 9 :

 9 Figure 9: Paleogeographic synthesis of the observations made over the entire Gulf of Lions

Figure 10 :

 10 Figure 10: Synthetic cross section of the observations made over the entire Gulf of Lions

  Figure1Click here to download Figure: Fig1.eps

Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site