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Bull. Soc. géol. Fr., 2011, t. 182, no 2, pp. 163-180

Refining our knowledge of the Messinian salinity crisis records in the offshore domain through multi-site seismic analysis JOHANNA LOFI1, FRANÇOISE SAGE2, JACQUES DÉVERCHÈRE3, LIES LONCKE4, AGNÈS MAILLARD 5, VIRGINIE GAULLIER4, ISABELLE THINON6, HERVÉ GILLET7, POL GUENNOC6 and CHRISTIAN GORINI8 Key-words. – Messinian salinity crisis, Mediterranean, Seismic profiles, Evaporites, Erosion, Clastics.

Abstract. – The Messinian salinity crisis (MSC) [Hsü et al., 1973] has deeply shaped the Mediterranean landscape and triggered large sedimentary deposits (evaporites and clastics) in the deep basins within a short time span. Until recently, the MSC has mainly been analyzed independently, either through outcrops located onshore (e.g. Morocco, Cyprus, Spain, Sardinia, Italy) or through marine seismic profiles in the deep offshore. Each approach bears its own limitations: (1) on the one hand, land outcrops refer to incomplete Messinian successions that are geometrically disconnected from the offshore Messinian deposits owing to tectonics (e.g. Apennines) and/or because they accumulated at an early stage of the crisis in shallow marginal basins (e.g. Spain); (2) on the other hand, seismic profiles from the upper margins down to the deep basins allow to image and explore the entire MSC event as a continuous process, but with a lower resolution and with a lack of stratigraphical and lithological control, in the absence of full recovery of scientific boreholes.

We present here a synthesis of a set of modern geophysical data over the Mediterranean and Black seas allowing to image the Messinian markers (erosion surfaces, depositional units and their bounding surfaces) much better than previously and to study the spatio-temporal organisation of these markers from the inner-shelves down to the bathyal plains. The results from thirteen areas located offshore are compared, with common charts and nomenclatures. The comparative and multi-site approach developed here allows to analyse the record of the MSC on margin segments and basins that depict various structural, geodynamical and geological settings, to fix a number of local influencing factors (tectonics, subsidence, inherited topography, sedimentary fluxes...) and to partly assess their influence in facies and geometrical variations of the MSC units. We are thus able to extract from our analysis some recurrent signals related to the MSC ss., allowing us to discuss: (1) the amplitude and modalities of base-level changes during the MSC; (2) the depositional modalities of the MSC units in the deep basins; (3) the location of the erosion product of the margins and to emphasise (4) the major differences between the eastern and western Mediterranean basins.

Une meilleure connaissance des enregistrements de la crise de salinité messinienne en domaine marin grâce à l’analyse sismique multi-sites Mots-clés. – Crise de salinité messinienne, Méditerranée, Profils sismiques, Evaporites, Erosion, Clastiques.

Résumé. – La crise de salinité messinienne (CSM) [Hsü et al., 1973] a profondément modelé les paysages méditerranéens et généré d’épaisses accumulations sédimentaires (évaporites et dépôts clastiques) dans les bassins profonds sur une brève période de temps à l’échelle géologique. Jusqu’à présent, la CSM a principalement été étudiée distinctement, à terre, grâce aux affleurements (ex. Maroc, Chypre, Espagne, Sardaigne, Italie…), et en domaine marin, par l’intermédiaire de profils sismiques. Chacune de ces approches a ses propres avantages et limites : (1) les affleurements à terre sont relativement facile d’accès mais concernent des successions messiniennes incomplètes et déconnectées géométriquement des séries en mer, du fait de la tectonique post-crise (ex. Apennins) et/ou car elles se sont accumulées dans un premier temps de la crise seulement, dans des basins marginaux peu profonds (ex. Espagne) ; (2) les profils sismiques acquis de la plate-forme au bassin profond permettent quant à eux d’imager et d’étudier l’intégralité de la crise, mais avec une résolution bien inférieure à celle du terrain, et sans contraintes stratigraphiques et lithologiques en l’absence de forages scientifiques profonds.

Nous présentons ici une synthèse d’un jeu de données géophysiques modernes acquises en Méditerranée et en mer Noire, permettant d’imager les marqueurs messiniens (surfaces d’érosion, unités de dépôts et leurs limites) bien plus nettement que par le passé, et d’étudier l’organisation spatio-temporelle de ces marqueurs depuis les domaines de plates-formes internes jusqu’aux plaines bathyales. Les résultats issus de 13 sites d’études localisés en mer sont comparés, avec des chartes et nomenclatures communes. L’approche comparative multi-sites développée ici permet d’analyser les enregistrements de la CSM sur des segments de marges et de bassins localisés dans des contextes structuraux, géody-

1. Géosciences Montpellier. cc 60 Bât. 22. Univ. Montpellier 2. 34095 Montpellier cedex 05, France. johanna.lofi@gm.univ-montp2.fr 2. Géoazur – Université Pierre et Marie Curie-Paris 6, UNS-CNRS-IRD-OCA, B.P. 48, 06235 Villefranche-sur-Mer cedex, France 3. Université Européenne de Bretagne; Domaines Océaniques UMR 6538 CNRS – Université de Brest (UBO) – IUEM, Place N. Copernic 29280 Plouzané, France 4. Laboratoire IMAGES – E.A. 4218, Université de Perpignan Via Domitia, 52 av. Paul Alduy, 66860 Perpignan cedex, France 5. LMTG, Université Toulouse-CNRS-IRD-OMP, 14 av. E. Belin, 31400 Toulouse, France 6. BRGM, GEO-GBS, 3 avenue Claude Guillemin, BP 6009, 45060 Orléans cedex 02, France 7. UMR 5805-EPOC, Université Bordeaux 1, F-33405 Talence, France 8. ISTeP, Université Pierre et Marie Curie, Place Jussieu, 75252 Paris cedex 05, France Manuscrit déposé le 21 janvier 2009 ; accepté après révision le 29 avril 2010; Bull. Soc. géol. Fr., 2011, no 2

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namiques et tectoniques variés, de s’affranchir de certains facteurs de contrôle locaux (tectonique, subsidence, topographie héritée, flux sédimentaires…) et d’estimer l’influence de ces derniers sur les variations géométriques et de facies observées au sein des unités messiniennes. D’un site à l’autre, nous sommes capables d’extraire de notre analyse des signaux récurrents, liés à la CSM ss et nous permettant de discuter : (1) l’amplitude et les modalités des variations du niveau de base durant la CSM ; (2) les modalités de dépôt des unités messiniennes dans les bassins profonds ; (3) la localisation des produits issus de l’érosion des marges. (4) Nous mettons également l’accent sur les différences majeures qui existent entre les enregistrements sédimentaires messiniens observés dans les bassins méditerranéens orientaux et occidentaux.

INTRODUCTION AND KEY OBJECTIVES Less than 6 Ma ago, the Pan-Mediterranean region underwent rapid and dramatic paleo-environmental changes known as the Messinian salinity crisis (MSC) [Hsü et al., 1973]. This short-term event at the geological scale (~5.96-5.32 Ma [Gautier et al., 1994; Krijgsman et al., 1999]) results from the progressive closure of the connection between the Atlantic ocean and the Mediterranean sea [e.g. Benson et al., 1991; Ryan, 2011]. Among the most important characteristics of this event, we note: (1) a major sea-level fall exceeding 1500 m [Ryan and Cita, 1978] resulting in the massive erosion of the margins and the development of deep subaerial canyons onshore [Chumakov, 1973; Clauzon, 1973]; (2) the accumulation of the products of the erosion in the downslope domain of the margins at river mouths [Rizzini et al., 1978; Barber, 1981; Savoye and Piper, 1991; Lofi et al., 2005; Sage et al., 2005; Maillard et al., 2006; Obone Zue Obame, 2009]; (3) the deposition of thick evaporitic sequences in the deep Mediterranean abyssal plains [Montadert et al., 1970; Hsü et al., 1973; Lofi et al., 2005; Gaullier et al., 2010] and (4) a very rapid refilling of the Mediterranean basin during the Latest Miocene/Lower Pliocene, following the re-opening of the communication with the Atlantic ocean at the Gibraltar strait [Blanc, 2002; Loget and Van Den Driessche, 2006].

As summarized by Rouchy and Caruso [2006], the numerous recent MSC scenarii enhance difficulties in understanding the exact modalities of the crisis, essentially concerning the timing and numbers of the drawdown phases (and related erosions) and the chronology of some evaporite deposition. A near-consensus (that still needs to be tested) has recently been found [CIESM, 2008] around an adaptation of the deep-desiccated basin model [Hsü et al., 1973] and of the two-step model of Clauzon et al. [1996]. Several points are however still under debate and the detailed modalities of the crisis are not fully established. Disagreements are mainly linked to the fact that most of the works dealing with the MSC are based on outcrops actually located onshore and isolated from the deep basins (e.g. Morocco, Cyprus, Spain, Italy…). Some of those outcrops contain MSC evaporites that accumulated at an early stage of the crisis, in shallow water marginal/peripheral basins (e.g. Spain). These deposits, predating the drawdown phase [Clauzon et al., 1996a; CIESM, 2008], are incomplete and are not coeval with the Messinian successions accumulated in the deep basins (mostly present-day deepest areas of the Mediterranean). For these reasons, the indications given by these outcrops cannot help constraining the whole timing of the Messinian events during the drawdown. Some other MSC deposits, outcropping in the northern Apennines, are interpreted as uplifted deep-water Messinian successions Bull. Soc. géol. Fr., 2011, no 2

[Roveri et al., 2001]. The status of the MSC deposits of the central Sicilian basin (containing halite), is still subject to debate, being understood either as part of the deep basin deposits [Decima and Wezel, 1973; Ryan and Cita, 1978; Roveri et al., 2008; Ryan, 2009; Rouchy and Caruso, 2006] or as deeper, but still marginal, deposits [Clauzon et al., 1996a]. In any case, those outcrops are today fully disconnected from the deep basin Messinian sequence, and no stratigraphic, geographic, geometric or sedimentary correspondences can be directly established with offshore MSC deposits. Correlations with the deep basin are thus complex, preventing from building an integrated scenario of the MSC that would provide a time link between the deposition of marginal evaporites, the erosion of the margins, and the deposition of clastics and deep evaporites in the abyssal plains.

Seismic profiling shot from the upper margins down to the deep basins allows to image and explore nearly continuously the entire MSC sedimentary record. However, in the absence of fully recovered deep boreholes, offshore studies of the Messinian markers (depositional units and surfaces) are limited by the lack of lithologic and stratigraphic calibrations. Considerable progress will be done when the deep basin sequence will be drilled integrally. Waiting this, only the seismic approach can be envisaged. The complexity and the spatial, temporal and geometrical variability of the MSC signal as recovered offshore [eg. Hsu et al., 1973; Ryan, 1978; Ryan and Cita, 1978; Barber, 1981; Stampfli and Höcker, 1989; Savoye and Piper, 1991; Escutia and Maldonado, 1992; Guennoc et al., 2000; Lofi, 2002; Thinon et al., 2004; Lofi et al., 2005, 2008; Sage et al., 2005; Maillard et al., 2006; Bertoni and Cartwright, 2006, 2007; Gillet et al., 2007; Obone Zue Obame et al., 2007 and 2008; Gaullier et al., 2008] results from the interaction between local controlling factors that are specific to each study area, and the superimposed “regional” signal related to the MSC ss. As a matter of fact, the response of the margin/basin to the MSC drawdown appears closely related to local conditions such as : morphology of the margin, dimensions and initial bathymetry of the area, slope angle, lithology, dimension of the watersheds and of the continental shelf, proximity and height of aerial reliefs, tectonic context, subsidence, marine circulation... These factors play a key role on the spatial and temporal organisation of the Messinian erosions, on the location, amount and nature of the sediment eroded and on the modalities of sediment erosion/transfer/deposition toward/in the basins [eg. Lofi et al., 2005].

In this context, the objectives of our work is to gather and compare different data sets from several areas in Mediterranean basin in order to: 1) synthesise our knowledge of the MSC seismic markers offshore, and particularly to better define their characteristics and specify the organisation of

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these surfaces and/or deposits from shelf areas to deep-basin settings and 2) refine the modalities of the MSC event in the offshore domain.

METHODS We developed a new approach based on a multi-site comparative analysis at the scale of the Mediterranean and Black seas, with thirteen sites located in various tectonic and sedimentary contexts (fig. 1). These areas have been explored by multi-channel seismic reflection profiles with diverse signal resolution and/or penetration that have been analysed and compared [see Lofi et al., 2011 for a detailed description of the datasets]. This method allows, thanks to a careful comparison of the different margins and basins, to eliminate the local controlling factors and to extract from our analysis the recurrent signals related to the MSC ss (number of erosional surfaces, depth of markers, etc...).

The study areas are shown in figure 1. In the western Mediterranean, seismic data have been analysed off Algeria [Déverchère et al., 2005; Mauffret, 2007; Gaullier et al., 2008; Yelles et al., 2009], in the Valencia through [Maillard et al., 2006], in the Gulf of Lions [Guennoc et al., 2000; Dos Reis et al., 2005; Lofi et al., 2005; Bache et al., 2009a], in the Liguro-Provençal basin [Savoye and Piper, 1991; Obone Zue Obame et al., 2007, 2008, 2009], on the Corsica margins and basin [Guennoc et al., 2004; Thinon et al., 2004] and on the Sardinia margin [Sage et al., 2005; Cornée et al ., 2008]. In the Eastern Basin, the dataset

FIG. 1. – Digital terrain model of the Mediterranean and Black seas [Smith and Sandwell, 1997] showing the location of the thirteen areas used for the comparative study of the MSC seismic markers at the regional scale. The multi-site approach allows analysing the impact of the MSC on margin segments and basins that have various structural, geodynamical and geological backgrounds: intermediate depth basins: (2) Valencia through and (6) eastern Corsica; relatively narrow and steep margins: (4) Provençal and (5) Ligurian margins, (7) western Corsica, (8) western Sardinia and (10) Levant area; large thick gently sloping margins: (3) gulf of Lions, (9) Nile deep-sea fan, (13) Romanian margin; tectonically active areas: (1) Algerian margin, (12) Florence ridge and Antalya basin, (11) Cyprus arc. Modified from Lofi et al. [2011].

FIG. 1. – Modèle digital de terrain de la Méditerranée et de la mer Noire [Smith and Sandwell, 1997] montrant la localisation des treize sites utilisés dans l’analyse comparative des marqueurs sismiques de la CSM à l’échelle régionale. L’approche multi-sites permet d’analyser l’impact de la crise sur des segments de marges et de bassins localisés en contextes structuraux, géodynamiques et géologiques variés : bassins de profondeurs intermédiaires : (2) Valence et (6) Est-Corse ; marges relativement étroites et raides : (4) Provence, (5) Ligure, (7) Ouest-Corse, (8) Ouest-Sarde, (10) bassin du Levant ; marges larges, épaisses et peu pentues : (3) golfe du Lion, (9) éventail profond du Nile, (13) marge roumaine ; zones tectoniquement actives : (1) marge algérienne, (12) ride de Florence et bassin d’Antalya, (11) arc de Chypre. Modifié de Lofi et al., [2011].

covers the Egyptian margin [Loncke et al., 2004, 2006], the Levantine basin [Netzeband et al., 2006; Bertoni and Cartwright, 2006, 2007], the Cyprus arc [Maillard et al., 2010] and the Florence ridge [Sellier et al., 2007]. MCS markers have also been analysed in the Black sea [Gillet et al., 2007].

Erosion surfaces, depositional units and associated bounding surfaces observed on the seismic profiles have been defined and labelled based on a new global terminology for MSC markers recently proposed by Lofi et al.

[2008; 2011]. The terminology of the depositional units is based on their seismic facies and/or the geometrical relationship of the units with respect to the Mobile Unit MU (Messinian salt), which is generally the easiest to evidence on the seismic profiles thanks to its transparent facies and associated viscous deformation creating listric faults and salt diapirs in the overlying brittle sediments [eg. Gaullier and Bellaiche, 1996; Sage and Letouzey, 1990; Dos Reis et al., 2005; Loncke et al., 2006; Gaullier et al., 2000]. The MSC surfaces have been defined based on their relationship with the Pre-MSC units, the Messinian units downslope, and the Plio-Quaternary cover. Table I presents the Messinian seismic markers and their main characteristics in the Mediterranean sea. Figure 2 presents their schematic spatio-temporal organisation at the end of the crisis from a conceptual point of view.

RESULTS : MAIN CHARACTERISTICS OF THE MSC SEISMIC MARKERS During the crisis, the drawdown gives a new configuration to the Mediterranean basin and creates a succession of morphological and sedimentological changes. A major contrast exists between the margins and the deep basins: the former have been deeply eroded whereas the latter have accumulated sediments as thick evaporite and clastic units. The seismic markers of the MSC in the offshore domain thus correspond to erosion surfaces, depositional units and associated bounding surfaces (tabl. I and fig. 2). Several MSC erosion surfaces (MES, BES, IES, TES) are observed, some of which in association with the Messinian depositional units (CU, BU, UU, MU, LU in fig. 2). In this study, these MSC markers have been recognised over the thirteen study areas and are presented in figures 3, 4 and 5. The main seismic characteristics of those seismic markers are described briefly hereafter. For more details and seismic illustrations, one can refer to the work by Lofi et al. [2011].

Messinian surfaces Evidence for a substantial drop of the sea level during the MSC has been collected from numerous records of deep erosional features in offshore areas [Ryan et al., 1978].

When several erosion surfaces (BES, IES, TES) are observed in the intermediate-depth basins (i.e. basins that were located offshore at the beginning of the crisis but at a slightly shallower depth compared to the deepest areas) and at the lower margins of the western and eastern Mediterranean basins, only one major erosion surface (MES) is observed on the upper margins and in the Black sea (figs. 3 to 5).

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TABLE I. – Main characteristics of the seismic markers of the Messinian salinity crisis in the offshore domain: erosion surfaces, depositional units and associated bounding surfaces. Modified from Lofi et al. [2011].

TABL. I. – Principales caractéristiques en domaine marin des marqueurs sismiques de la crise de salinité messinienne : surfaces d’érosion, unités de dépôt et leurs limites. Modifié de Lofi et al.

[2011].

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Margin Erosion Surface (MES) The MES is the most striking feature, consisting of a widespread erosion surface generally well identified on the upper margins (fig. 5). Onshore, the MES is characterised by the presence of deep narrow incisions or “canyons” [Chumakov, 1973; Clauzon, 1973]. Offshore, numerous seismic investigations revealed the existence of Messinian paleo-fluvial networks (fig. 6) connected to the canyons onshore: e.g. Egyptian margin [Barber, 1981; Aal et al., 2000; Loncke et al., 2006], Gulf of Lions shelf [Guennoc et al., 2000], Ebro margin and Valencia trough [Stampfli and Höcker, 1989], western Sardinia margin [Cornée et al., 2008]. Although its depth extent is not easy to characterize everywhere and depends on the post-Messinian amount of subsidence (or uplift) undergone by each margin, the present-day deepest part of the MES is generally found between ~2 s two-way travel time (twtt) (Ligurian margin, Levant margin) and ~3.6 s twtt (Gulf of Lions, Valencia trough, West Sardinia margin), and coincides with the pinch-out of UU when present (fig. 2).

Bottom Erosion surface (BES) It is located at the base of the MSC deposits (tabl. I; figs. 2, 5). As shown in figure 7, it shows evidence for erosion (gullied morphology and/or pre-MSC reflection truncations) and/or for angular discordances (onlap of MSC

reflections). When identified, the BES extends locally beneath the onlaps of UU and MU [Ryan, 1978; Ryan and Cita, 1978; Loncke et al., 2004; Maillard et al., 2006; Bertoni and Cartwright, 2006, 2007]. In the Gulf of Lions, the BES even extends beneath the onlap of LU and progressively passes basinward to a correlative conformity at the base of LU (fig. 5-A; see also Fig. 10 in Lofi et al. [2005]).

This conformable surface is labelled BS, i.e. Bottom Surface, in such a case. The BES, as an erosion surface, extends down to depths generally ranging from 3.5 and 4.5 s twtt.

Intermediate Erosion Surfaces (IES) They correspond to some intermediate unconformities contained in the MSC depositional units and evidenced by reflection truncations (fig. 7). From a stratigraphical point of view, they were created after the BES and before the TES (see below). The IES show locally gullied morphologies, such as in the Valencia and East-Corsica basins (fig. 3-areas 2 and 6).

Top Erosion Surface (TES) The TES is bounding above the MSC deposits (tabl. I and fig. 2). In some places it shows evidence for erosion (e.g. gullied morphology and/or evidences for truncation of underlying MSC reflections). In some other places and basinward, it is a conformable surface (labelled TS, i.e. Top Surface, in such a case). In the Valencia and East-Corsica basins (fig. 3-areas 2 and 6) the erosional character of the TES is very clear at the top of UU, with sinuous central paleo-valleys and tributaries extending toward deepest areas [Ryan and Cita, 1978; Escutia and Maldonado 1992; Thinon et al., 2004; Maillard et al., 2006]. The TES is also observed in the Levant basin [Ryan, 1978] (fig. 3-area 10) at the top the salt unit (MU) and is interpreted in that area as a phase of subaerial erosion during the last stage of the crisis [Bertoni and Cartwright, 2007].

Depositional units

FIG. 2. – Schematic conceptual sketches (not to scale) across the western and eastern Mediterranean basins, illustrating the organisation of the MSC markers from the margins down to the deep basins at the end of the MSC. Marginal basins accumulated evaporites at an early stage of the crisis before the drawdown phase [Clauzon et al., 1996]. Intermediate-depth basins are located offshore but are shallower compared to the deepest areas of the basins. See table I for surfaces and depositional units nomenclature.

FIG. 2. – Sketches conceptuels schématiques (sans échelle) à travers les bassins méditerranéens occidentaux et orientaux illustrant l’organisation des marqueurs messiniens depuis la plate-forme jusqu’au domaine profond à la fin de la crise. Les bassins marginaux contiennent des évaporites accumulées à un stade précoce de la crise, avant la phase d’abaissement du niveau marin [Clauzon et al., 1996]. Les bassins de profondeur intermédiaire sont localisés en mer mais à des profondeurs moindres que les zones les plus profondes. Se reporter au tableau I pour la nomenclature des surfaces et unités de dépôt.

In the western Mediterranean deep basin, three distinct seismic units (also previously called Messinian trilogy) have been identified [Montadert et al., 1970]. This “trilogy” (composed of LU, MU, UU, from the oldest to the most recent units) is an aggrading sequence infilling the topographic lows. When a clear deep basin trilogy is locally observed in the Western Basin (figs. 5-A and 8-A), only one unit (MU) is clearly recovered on seismic data in the Eastern Basin (figs. 5-C and 8-B) and no seismic depositional Messinian units have been observed in the Black sea (fig. 5-D).

Lower Unit (LU) LU is the oldest unit identified in the Western Basin and consists of a group of continuous high amplitude reflections (figs. 5-A & 8-A). The two-way travel time (twtt) thickness of this unit is up to 0.25-0.35 s (i.e. ~450 to ~600 m thick using an internal velocity of 3.5 km/s). Recent works suggest that MSC pre-MU deposits would reach up to ~1500 m in the Gulf of Lions [Bache et al., 2009a] but this interpretation is debated [Lofi and Berné, 2008]. Lofi et al. [2005] Bull. Soc. géol. Fr., 2011, no 2

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proposed that in the absence of clastics (CU, see below) accumulated at margin feet, the Lower Unit (LU) onlaps the Miocene margins at a present day depth of ~4.5-5 s twtt in the Gulf of Lions. This geometry is however generally poorly imaged on other margins, either because this reflective unit does not exist everywhere or because salt prevents acoustic penetration below it. Its extension and thickness throughout the Western Basin is still thus poorly known.

Mobile Unit (MU) MU corresponds to the Messinian salt onlapping the Miocene margins. MU is evidenced on the seismic profiles by a characteristic transparent acoustic facies (interpreted as consisting dominantly of halite [Nely, 1994]) and by an associated viscous deformation (fig. 5) [Gaullier et al., 2000; Loncke et al., 2006]. In the Western Basin (figs. 3 and 8-A), MU is < 0.5 s twtt thick when non-deformed (i.e. <~1.2 km thick, using a 4.5 km/s velocity). In the Eastern Basin (fig. 4 and 8-B), MU is thicker (up to 1 s twtt thick, i.e. ~2.1 km using a 4.2 km/s velocity). However, the measured thickness may be overestimated as a result of inflation process by salt tectonics. MU contains several discontinuous reflection packages [Netzeband et al., 2006; Bertoni and Cartwright, 2006]. Those internal reflections apparently separate six depositional sequences.

Upper Unit (UU) UU is the uppermost and most recent deep basin unit of the Western Basin accumulated during the low-stand phase.

The thickness of this unit spans is up to 0.3-0.5 s twtt (i.e. ~500 to ~900 m thick using an internal velocity of 3.5 km/s). UU is mostly aggrading and onlaps the margin feet at a depth of ~2 to 3.2 s twtt (fig. 3). Such geometry evidences the shoaling of the basin floor during the accumulation of UU, when the sea level was lowered by > 1000 m.

UU is indicated by a group of parallel and relatively continuous reflections of relatively high amplitude (figs. 7 and 8 [Maillard et al., 2006]). A lateral change in seismic facies is often observed when approaching the margin feet, especially at the canyon mouths, where internal layering becomes rougher (eg. Provençal, Ligurian and Sardinian margins [Sage et al., 2005; Sage et al., 2011; Obone Zue Obame et al., 2011]).

Complex units (CU) Several units CU display a complex and variable internal seismic facies. They are situated above the BES. CU correspond either to chaotic or to roughly bedded seismic facies, more or less transparent (fig. 9). Their thickness can locally reach 0.7 s twtt (i.e. 1.1 km ± 150 m using a 3.2 ± 0.3 km/s velocity). CU are absent on the margin shelves, rarely observed on the upper slopes, and mainly recovered at the margin feet, either as fan shaped deposits at the Messinian

FIG. 3. – Digital terrain model of the western Mediterranean [Smith and Sandwell, 1997] with the present-day extent of the MSC seismic markers. The spatial extent of the Mobile Unit (MU) is coloured in yellow-greenish because it is located below the Upper Unit (UU). Sketches 1, 2 and 4 to 7 illustrate, for each area, the representative organisation of the markers along a landward-seaward axis. Depths are in second two-way travel time (s twtt). Sketches 3 and 8 are shown in figure 5. Modified from Maillard et al. [2011].

FIG. 3. – Modèle digital de terrain de la Méditerranée occidentale [Smith and Sandwell, 1997] montrant la localisation actuelle des marqueurs messiniens en domaine marin. L’extension spatiale de l’unité mobile (MU) est représentée en jaune/vert car elle est localisée sous l’unité supérieure (UU). Les sketches 1, 2 et 4 à 7 illustrent, pour chaque site, l’organisation représentative des marqueurs selon un axe terre-mer. Les profondeurs sont exprimées en secondes temps double (s twtt). Les sketches 3 et 8 sont présentés dans la figure 5. Modifié d’après Maillard et al., [2011].

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river mouths or as poorly organised bodies elsewhere (fig. 3-map). Because CU often makes the transition between the eroded slopes and the deep basin deposits, the fine stratigraphic relationships between these markers and the other Messinian units are complex. Timing and involved processes may differ significantly from one clastic fan to another at the scale of the basin.

DISCUSSION A careful comparison of the MSC markers on the studied margins and basins allows us to provide new constraints on the MSC impacts and to discuss the modalities of the crisis in the offshore domain.

Erosional processes associated to the drawdown phase.

The MSC sea-level drawdown is testified onshore by deep subaerial canyons [Clauzon, 1973; Clauzon et al., 1996b] and offshore by several erosion surfaces observed all over the Mediterranean and Black seas [Ryan, 1976; Ryan and Cita., 1978; Gillet et al., 2007]. The MES represents the entire time interval of the drawdown phase. It appears as a diachronic and polygenic surface essentially resulting from the combined action of: – early subaqueous large-scale mass-wasting processes, occurring at the beginning of the MSC drawdown, prior to the significant accumulation of the halite in the deep basins.

As discussed later on in the following section, those processes may be responsible for part of the volume of the clastic fans edified at some margin feet below MU, as well as for part of the LU accumulated above the abyssal plain [Lofi et al., 2005]. Such erosional processes are expected on other margins, the amount of material transferred to the basin depending of the initial morphology and thickness of the Miocene shelf and adjacent slope. For instance, evidence of small landslides in Morocco [Cornée et al., 2006] suggests that the amount of sediment provided by the destabilization of narrow shelves with steep slopes (such as the Ligurian margin) occurs at a smaller scale. Thick Messinian mass-movement deposits, probably largely controlled by active tectonics, have also been evidenced in the Apennine foredeep [Roveri et al., 2001; Manzi et al., 2007]; – subaerial rivers downcutting by retrogressive action to adjust to their new base level [eg. Clauzon, 1973; Loget and Van Den Driessche, 2006]. Fluvial erosion is testified by the Messinian drainage paleo-networks observed beneath the shelves and extending downslope [eg. Guennoc et al., 2000; Barber, 1981]. In addition, as pointed by Mitchell and Lofi [2008], the continuous character of the MES unconformity along the Mediterranean margins seems to support subaerial erosion as its origin. Indeed, submarine erosion occurs only where sedimentary mass flows or other causes of erosion occur, leaving some areas without channeled erosion, in contrast to subaerial erosion driven by ubiquitous rainfall [Pratson and Ryan, 1996]; – marine abrasion has also been recently proposed as a possible agent for late erosion. Based on morphological analysis of the MES, Bache et al. [2009a] suggested that beneath the slope, the relatively smooth morphology of MES may result from wave-ravinement during the re-flooding phase at the end of the crisis;

– dissolution of carbonated rock also contributed to erosion during the crisis. Subsurface dissolution is evidenced by numerous deep karst conduits observed both onshore and offshore around the Mediterranean basin that are classically related to the MSC [eg. Aunay and Le Strat, 2002; Arfib and Ganoulis, 2004; Gilli and Audra, 2004; Mocochain et al., 2006; Fleury et al., 2007]. It is also expected that where carbonated rocks were outcropping at the beginning of the MSC, they have been incised by rivers but also partly dissolved, in a proportion difficult to quantify. It has for instance been proved that the flat top of the Eratosthenes seamount, made of carbonates, has been eroded during the Messinian event [Robertson, 1998; Major and Ryan, 1999].

Products of the erosion of the margins The products of erosion of the margins have for long been an enigma since clastics have rarely been identified until now and appear not to correspond to the amount of erosion observed on land [e.g. Clauzon, 1973; Clauzon et al., 2008, and references therein]. Although our data set does not allow for a complete investigation of Messinian deposits at depth, we are now able from the comparative analysis made in this study to identify important deposits on some lower slopes and deep basins. They have accumulated under several possible forms: – one part is clearly recovered in CU interpreted as clastic deposits accumulated downslope during the MSC. According to its internal facies and geometry, CU is either interpreted as organised clastic fans deposited at the Messinian river mouths or, in other places, as poorly structured lower slope deposits. The spatio-temporal organisation of those fans varies from one margin to another. For instance, Lofi et al. [2005] proposed that the clastic fans of the gulf of Lions (fig. 5-B) accumulated in several phases, requiring both early subaqueous gravity and later fluvial depositional processes. These authors proposed that one part of the clastic fan, extending at the margin foot beneath MU, accumulated in a subaqueous environment at an early stage of the crisis.

Margin sediments may have been destabilized as a result of sea-level lowering [Lofi et al., 2005], margin steepening due to the weight of brines in the basin [Govers et al., 2009] and/or increasing density inversion between heavier overlying brines and lighter sediment pore waters [Ryan, 2009, 2011]. Younger clastics, coeval with MU and UU, also exist but are recovered more landward, on the rise and slope, in areas restricted to the Messinian paleo-valleys. In some places, these clastics are not directly connected to the basin units, preventing time calibration. In some smaller systems (ex. Ligurian and western Sardinian margins, fig. 3-area 5 and fig. 5-B) clastics are mostly imaged above MU, coeval with UU, and extend along the margin toe, whereas on the Provençal margin (fig. 3 – area 4) CU is coeval with both MU and UU [Obone Zue Obame et al., 2011]. On the Algerian margin (fig. 3-area 1), characterized by high reliefs on land, steep slope and long-lasting tectonic activity, clastic sediments are abundant and spread over the margin foot at least during the deposition of MU and UU [Déverchère et al., 2005; Capron et al., 2011]. These examples illustrate the wide variability of the spatial and temporal distribution of the clastics that probably reflects the combination of several factors, including dimension of the drainage basin, Bull. Soc. géol. Fr., 2011, no 2

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importance of reliefs and of tectonic activity, nature and thickness of the sediment cover and substratum. In large systems (eg. gulf of Lions and Nile deep-see fan), large amounts of clastics may pre-date salt deposition, probably because thick Miocene clastic shelves with abundant soft material were propitious to early large-scale submarine instabilities and clastic deposition downslope before significant accumulation of MU. On narrow margins with shallow substratum and much smaller drainage slopes, such phenomena are also expected, but in lower proportion. They have probably been massively eroded later on, in a subaerial context, once the maximum drawdown has been reached and river power was maximum. On such margins, most of

the Messinian margins clastic fans seem thus coeval with the basin evaporites UU and related to the low-stand phase; – one part of the eroded sediments may also be contained within LU [Lofi et al., 2005; Bache et al., 2009a]. As discussed above, early subaqueous large-scale mass-wasting processes may be at the origin of massive transfer of sediment down to the basin. If the coarser fraction possibly accumulated in the clastic fans recovered at the margin foot, a substantial part of sediments may have been exported more basinward, and be incorporated in turbidites and debris flows on the basin floor. Those deposits may partly account for the parallel reflectors of the LU observed above the abyssal plain;

FIG. 4. – Digital terrain model of the Eastern Mediterranean and Black sea areas [Smith and Sandwell, 1997] with the present-day extent of the MSC seismic units in the deep basin. Sketches 10 to 12 illustrate, for each area, the representative organisation of the markers along a landward-seaward axis.

Depths are in second two way travel time (s twtt). Sketches for areas 9 and 13 are shown in figure 5. Modified from Maillard et al. [2011]. Figures 3 and 4 illustrate that the response of the margins and basins to the MSC shows at the same time some similarities and differences: (1) the MES is observed everywhere testifying generalised drawdown at the scale of the Mediterranean and Black seas; (2) MSC seismic depositional units are observed in the basins and on the margin slopes, with a more or less complete association: the trilogy UU, MU, LU is visible in the Western Basin, locally together with clastics (CU); MU is the only unit clearly recovered in the Eastern Basin, locally together with clastics; no seismic depositional units have been observed in the Black sea; (3) Location of CU is related to Messinian fluvial systems (excepted for active tectonic areas). Their geometrical relationship with other MSC markers varies from one margin to another; (4) In the intermediate basins of the Western Mediterranean, LU and MU are absent and only a bedded unit (UU or BU) is observed, bracketed by very well expressed TES and BES. FIG. 4. – Modèle digital de terrain de la Méditerranée orientale et de la mer Noire [Smith and Sandwell, 1997] montrant la localisation actuelle des marqueurs messiniens en domaine marin. Les sketches 10 à 12 illustrent, pour chaque site, une organisation représentative des marqueurs selon un axe terre-mer. Les profondeurs sont exprimées en secondes temps double (s twtt). Modifié d’après Maillard et al., [2011]. Les figures 3 et 4 montrent que la réponse des marges et des bassins à la CSM présente à la fois des points communs et des différences : (1) la MES est observée partout, attestant de l’abaissement généralisé du niveau marin en Méditerranée et en mer Noire ; (2) les unités messiniennes sont observées, en association plus ou moins complète, dans les bassins et sur les pentes continentales : la trilogie UU,MU,LU est visible dans le bassin occidental, associée localement aux dépôts clastiques (CU) ; MU est l’unique unité clairement observée dans le bassin oriental, associée localement aux dépôts clastiques (CU) ; aucune unité de dépôt n’est observée en sismique en mer Noire ; (3) la localisation des dépôts clastiques (CU) est à mettre en relation avec les systèmes fluviatiles messiniens (sauf zones tectoniquement actives). Leur relation géométrique avec les autres marqueurs messiniens varie d’une marge à une autre ; (4) dans les bassins intermédiaires de Méditerranée occidentale, LU et MU sont absents et seule une unité litée est visible (UU ou BU), encadrée par des surfaces érosives (TES et BES) très bien exprimées.

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– since river erosion started as soon as the base-level fall began, and persisted throughout the MSC, MU also probably contains some clastics, although in much lower proportion compared to other MSC units. MU usually displays a transparent facies (fig. 8-A) and a ductile deformation style suggesting a high content of halite. This suggests that the sedimentation rate was very high when MU deposited or/and that salt accumulation occurred at an early stage, prior to the low-stand phase and associated drastic margin erosion. In the western Sardinia margin, an increased upward reflectivity within MU is indeed interpreted as an increase in clastics, gradually emplaced within the salt layer [Sage et al., 2005]. In the eastern Mediterranean basin, the amount of clastics in MU may be even more important as suggested by the internal discontinuous reflection packages observed within this unit (fig. 8-B), characterized by velocities below 4 km/s, i.e. lower than the velocities of the reflection-free seismic facies (halite) reaching 4.5 km/s [eg. Huebscher et al., 2008]. In the Florence ridge (fig. 4-area 12), MU is locally more chaotic close to the margins, also suggesting a mix with clastics [Loncke et al., 2011a,b]; – since erosion by rivers persisted throughout the whole drawdown phase, an appreciable amount of clastics are also contained in the UU, as evidenced by the marly levels sampled during DSDP [Hsü et al., 1973]. However, because UU has not been integrally drilled, the amount of clastics contained within this unit is difficult to estimate. In addition,

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because several different lithologies can produce the same seismic facies, the depositional environment equivalence of two a-priori identical seismic objects must be carefully discussed. We thus suspect that UU may display an important internal variability in terms of lithology and depositional environment since it is recovered at river mouths (increase of clastic fraction), in the centre of the deep basins (increase of evaporitic fraction) or in intermediate basins (possible increase of lacustrine fraction). A careful examination of the profiles allows to evidence slight lateral variations in UU seismic facies. In the Provençal, Ligurian and western Sardinia margins (fig. 3-areas 4, 5 and fig. 5-B), the seismic facies of UU differs at the outlet of the Messinian thalwegs from the one observed more distally. It here becomes either chaotic or roughly bedded, suggesting a higher amount of clastic content. Diving operations on the Ligurian margin supported this interpretation. UU sampled in this area consists of alternating levels of sandstones and conglomerates [Savoye and Piper, 1991].

The network drainage pattern at the top of UU in the Valencia basin (fig. 3 – area 2) seems also associated to a less well-bedded seismic facies (increased amount of clastics?).

Some sub-units can thus be distinguished locally within UU. These sub-units may reflect either a higher content of clastic material supplied by the Messinian river [Sage et al., 2005; Obone et al., 2011] and/or a reduced content of evaporitic fraction as a result of freshwater river runoff. The paleo-topography may also have significantly impacted the

FIG. 5. – Line drawings of the Gulf of Lions (A, area 3), western Sardinia (B, area 8), Nile deep-sea fan (C, area 9) and western Black sea (D, area 13) illustrating the organisation of the MSC seismic markers at the same vertical and horizontal scales. See figures 3 and 4 for line location. Modified from Gillet et al. [2007]; Lofi et al. [2008]; Sage et al. [2011]; Loncke et al. [2011].

FIG. 5. – Coupes sismiques interprétées du golfe du Lion (A, site 3), de la marge Ouest Sarde (B, site 8), de l’éventail profond du Nile (C, site 9) et de l’ouest de la mer Noire (D, site 13) illustrant l’organisation des marqueurs messiniens à une même échelle verticale et horizontale. Se reporter aux figures 3 et 4 pour la localisation. Modifié d’après Gillet et al. [2007]; Lofi et al. [2008]; Sage et al. [2011]; Loncke et al. [2011].

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FIG. 6. – Seismic profile LRM96 illustrating strike sections of two large Messinian paleo-valleys buried beneath the gulf of Lions inner shelf. The MES (margin erosion surface) deeply incises the pre-MSC deposits (truncated underlying reflections) and is overlain discordantly by the Plio-Quaternary deposits (PQ).

FIG. 6. – Profil sismique LRM96 illustrant en coupe la présence de deux larges paléo-vallées messiniennes enfouies sous la plate-forme interne du golfe du Lion. La MES (surface d’erosion marginale) incise profondément les séries anté-crises (réflecteurs sous-jacents tronqués) et est recouverte en discordance par les dépôts plio-quaternaires (PQ).

sedimentary records. In areas more or less disconnected from the deep basin, either because of volcanic intrusions (eg. East-Corsica basin, fig. 3-area 6) or topographic-sills, the depositional environments, circulation of water masses, salinity, amount of clastics trapped in the basin were probably different from those of the deep basin, resulting in different seismic facies. Thus in the East-Corsica basin, which was located at an intermediate-depth during the crisis, the MSC deposits consist of a bedded unit (BU), with a seismic facies slightly different from the one of UU in the deep basin; – despite the fact that carbonate dissolution during the crisis is difficult to quantify, we suggest that solutions derived from such processes may have contributed to MSC units composition. In the eastern Mediterranean, such processes could account for some of the internal reflectors observed within MU. For example, Huebscher et al. [2008] and Loncke et al. [2011a,b] described numerous internal reflectors within the MU in the vicinity of the carbonated Eratosthenes seamount area.

Nature of the deep basin trilogy in the Western Basin, and of MU in the Eastern Basin In the absence of fully recovered academic deep boreholes in the offshore areas, the age, lithology and depositional environment of the MU and LU can only be derived from indirect observations based on seismic analysis (configuration of the units, internal facies and type of deformation).

– The age, lithology and depositional environment of LU are probably the most speculative of the deep basin Messinian units. Firstly, this unit has never been drilled, and secondly it is generally poorly imaged. LU has been clearly imaged in the Gulf of Lions (fig. 5-A), and with caution possibly on the Algerian margin (fig. 3-area 1), but its extension at the scale of the basin is still questionable.

From a lithological point of view, it is not excluded that LU may contain interbedded hemipelagites or, if not dissolved, primary evaporite layers accumulated at the basin floor as a result of increased salt concentration in the basin. As mentioned above, interbedded sands and muds accumulated as giant deep-water turbidites may also partly account for the parallel reflectors of LU [Lofi et al., 2005]. Sea-level lowering Bull. Soc. géol. Fr., 2011, no 2

may have also triggered simultaneously the Messinian re-sedimented evaporites described in the Apennine foredeep and in Sicily [Roveri et al., 2001; 2008] and a possible chronostratigraphic correlation may thus exist between those deposits. However, due to tectonics, outcrops are today fully disconnected from the deep basin Messinian sequence, avoiding any direct geometrical correlation with the offshore MSC deposits. Depending of the initial lithology of the eroded margin, internal composition of LU may varies from one site to another in the Mediterranean. In the Gulf of Lions, it is thus not excluded that reworked gypsum may also partly account for the parallel reflections of LU, if some evaporites accumulated on the margin before the drawdown, at the first stage of the crisis. One problem however remains, related to the fact that if LU is partly of gravity origin, we should expect the presence of this unit also in the Eastern Basin, which is not the case until now. This point still needs to be clarified, although the sill-controlled two-stepped drawdown model proposed by Blanc [2000] and Ryan [2008] allows to reconcile our observations by accumulating LU in the Western Basin while MU is deposited in the Eastern Basin.

– The reflection-free seismic facies of the Mobile Unit (MU) is classically interpreted as consisting dominantly of halite [Nely, 1994], and corresponds to the Messinian Salt s.s. This interpretation is confirmed by the deformations observed in this unit and in its overlying overburden. This ductile layer allows to create a vigorous post-MSC salt tectonics, mainly with upslope listric normal growth faults in the brittle sedimentary cover (including UU, the Plio-Quaternary and locally CU) and downslope salt anticlines and diapirs [e.g. Loncke et al., 2006; Gaullier et al., 2000 and 2006]. In the Eastern Mediterranean, the internal discontinuous reflection packages that are observed within the MU are possibly related to lithological differences, such as intercalated layers of anhydrite, limestone or clastics and/or trapped fluids [eg. Bertoni and Cartwright, 2006; Huebscher et al., 2008].

– Concerning the most recent unit UU, it is the best known of the deep basin trilogy depositional units because its top has been sampled in several places during DSDP Leg XIII [Hsü et al., 1973]. An internal variability in terms of lithology and depositional environment is expected from the slight lateral seismic facies changes observed in UU. This interpretation is supported by the sampling operations.

At the margin feet, away from the Messinian outlets, UU contains arid and shallow water laminated anhydrite layers interbedded with marls containing micro-fauna and flora indicative of brackish environments. Some marine fauna have also been found in these marls. They consist of reworked material from diverse environment with different paleodepths. They are interpreted as essentially erosion products delivered to intermittent shallow to deep lakes flooding the basin floors [Cita, 1973]. These marls were possibly washed down from the margins in the wet portion of the MSC precession cycles. Submersible dives also allowed to sampled UU at the outlet of Messinian valleys [Savoye and Piper, 1991]. There, this unit consists of predominantly fluvial conglomerates thus differing from the one drilled more distally. At the basin scale, unit UU is mostly aggrading and onlaps the margin feet. Such geometry reflects the shoaling of the basin floor during the accumulation of UU. Above UU, biogenic ooze of early Pliocene age containing fauna

THE MESSINIAN SALINITY CRISIS RECORDS IN THE OFFSHORE DOMAIN

FIG. 7. – Seismic profile illustrating the presence of the MSC Upper Unit (UU) in the Valencia intermediate-depth basin. Here, this unit is bounded above and below by two erosion surfaces (TES and BES respectively) and contains internal erosions labelled IES. UU lies discordantly above the pre-MSC deposits and is overlain by the Plio-Quaternary sequence (PQ).

FIG. 7. – Profil sismique illustrant la présence de l’unité messinienne supérieure (UU) dans le bassin intermédiaire de Valence. UU est limitée au-dessus et au-dessous par deux surfaces d’érosion (TES et BES, respectivement) et contient plusieurs érosions internes appelées IES. UU est discordante sur les dépôts anté-crise et surmontée par la séquence plio-quaternaire (PQ).

assemblage living in normal seawater and in a bathyal setting [Cita, 1973] accumulated after the end of the crisis.

Major differences in the Messinian sedimentary successions at the Mediterranean regional scale.

Some major differences exist between the western and eastern Mediterranean basins in terms of thickness/facies of the seismic markers or of presence/absence of some of them.

– When a clear deep basin trilogy (UU, MU, LU) is locally observed in the western Mediterranean, only the Mobile Unit (MU) is clearly seismically recovered in the eastern Mediterranean (figs. 3, 4, 5 and 8).

– UU has been shown all around the western Mediterranean (up to 900 m thick) whereas its existence has never been demonstrated in the Eastern Basin. In the Levantine domain and Cyprus arc for example (fig. 4-areas 10 and 11), MU is tilted and its top corresponds to an erosion surface (TES, [Bertoni and Cartwright, 2007; Maillard et al., 2010]). Either UU did not accumulate during the crisis in the Eastern Basin, or it is too thin to be detected from the seismic profiles (indeed less than 50 m thick evaporitic dolomitic marls have been sampled locally on the Florence ridge during the DSDP Leg 42A [Hsü et al., 1978]) or it has been subsequently eroded before the end of the crisis. In such a case however, the product of erosion of this unit should be seismically recovered somewhere in the basin, which is not obvious at this time. In the Antalya basin (fig. 4-area 12), a bedded reflective unit locally observed at the top of the MSC deposits may correspond to UU. However, without borehole data, no final conclusion can be drawn regarding its age and origin, and a Pliocene age is not excluded.

– LU has been observed at least locally in the western Mediterranean whereas its existence is not clear in the Eastern Basin. LU is clearly absent in the Levant basin (fig. 4-area 10 and fig. 8-B) where MU is directly overlying the pre-MSC deposits [Bertoni and Cartwright, 2007]. In the Antalya basin, MU lies sometimes above a basinward thickening sequence of high amplitude reflectors that may correspond to LU. However, because of the low resolution

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of the seismic profiles and the weak data density, those seismic reflections could also reflect pre-MSC deposits or MSC clastics, by analogy with the Complex Unit (CU) observed locally in the Levant basin at the bottom of MU. – In the Eastern Basin, MU seems also thicker (up to 1 s twtt) compared to the Western Basin (0.5 s twtt) and contains internal reflection packages, suggesting that it is made of six depositional sub-sequences, whereas the MU of the Western Basin is roughly homogeneous even using petroleum seismic data [Ianev et al., 2007]. Because the two Mediterranean basins are now disconnected by tectonic structures off Sicilia [Jolivet et al., 2006], the lateral correlation between the observed MU is not possible. Moreover, a synchronicity is not expected, as numerical models pointed out the role of intermediate Mediterranean sills in creating diachronous halite deposits in the Eastern Basin (first) and in the Western Basin (later) [Blanc, 2000; Ryan, 2008].

– In the Black sea, the MSC event is only marked by a single erosion surface on the seismic lines (fig. 5-D) [Gillet et al., 2007; Tari et al., 2009]. Up to now, no MSC deposits have been observed on the seismic data, although thin clastic deposits is described in the DSDP drilling [Hsü and Giovanoli, 1979].

These discrepancies clearly suggest that controlling factors or depositional environments were different in the two Mediterranean sub-basins during de MSC. Dissimilarity may be explained by the combined action of several factors, among which: (1) the presence of some topographic sills in the Mediterranean basins, impacting on the water budgets

FIG. 8. – Seismic profiles from the western (A, gulf of Lions, modified from Lofi et al. [2005]) and eastern (B, Levant basin, modified from Bertoni and Cartwright [2005]) Mediterranean deep basins illustrating the main differences in the MSC sedimentary successions. In the Western Basin, the classical MSC seismic sequence consists of 3 distinct seismic units: UU, MU (~1000 m of halite) and LU. In the Eastern Basin, the seismic sequence consists of 1 seismic unit (MU, >2000 m) that can be divided in 6 sub-units based on the internal reflections it contains.

FIG. 8. – Profils sismiques acquis dans les bassins profonds de Méditerranée occidentale (A, golfe du Lion, modifié d’après Lofi et al. [2005]) et orientale (B, bassin du Levant, modifié d’après Bertoni et Cartwright [2005]) illustrant les principales différences entre les enregistrements messiniens. Dans le bassin occidental, la séquence sismique messinienne consiste classiquement en 3 unités distinctes : UU, MU (~1000 m de halite) et LU. Dans le bassin oriental, la séquence sismique messinienne consiste en 1 unité (MU, >2000 m) qui peut être subdivisée en 6 sous-unités sur la base des réflecteurs internes qu’elle contient.

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and base-level dynamics; and (2) climatic differences at the scale of the Mediterranean sub-basins. It has been proposed that the desiccation of the Black sea (fig. 5-D) during the crisis marked a sudden change from positive to negative hydraulic budget possibly related to a drastic climatic change as a result of the desiccation of the Mediterranean sea, or to a sudden eastern Paratethys drainage pattern reorganization [eg. Hsü and Giovanoli, 1979; Popescu 2006; Gillet et al., 2007]. At the present time, the possible impact of the desiccation in terms of local/regional atmospheric circulation is not clearly established.

Amplitude of the drawdown and base-level changes during the low-stand In the deepest part of the western Mediterranean basin, the Messinian trilogy (LU, MU, UU) is seismically concordant at the base and at the top respectively with the pre-MSC and the Plio-Quaternary sequences. However, in the absence of full recovery boreholes, few criteria allow to state if these areas ever encountered full desiccation(s), non-depositional phases or permanent immersion, mainly because of limited vertical resolution of seismic data. The Messinian sequence of the deepest basins appears however as the most complete at the scale of the Mediterranean basin, and possibly constitutes, if never emerged, a continuous recorder of the entire MSC. In more proximal areas, the trilogy is observed as a lateral onlap on the margin feet. This onlap reflects the progressive infilling of the abyssal plain by the Messinian deposits, as the subsidence did not compensate the extremely high sedimentation rate in the basin (up to 2100 m of halite in the Eastern Basin in less than < 0.3 m.y. if we consider that the measured thickness on the seismic profiles (~1 s twtt thick) is not overestimated). This peripheral onlap also shows that the registration of the MSC is clearly incomplete at the margin foot.

The top of UU sampled at the Balearic margin foot provides evidence for shallow-water depositional environments [Hsü et al., 1973]. Therefore this unit was partly deposited under shallow water conditions. The pinch-out of UU thus roughly marks the location of one paleo-shoreline before the re-filling of the basin at the end of the crisis. It can thus be used as a paleo-marker to quantify the vertical

deformation encountered at the scale of the Western Basin, because the present-day depth of UU pinch-out depends on the amount of post-Messinian subsidence or uplift undergone by each margin. In the Gulf of Lions (fig. 5-A), dominated by subsidence, this pinch-out is located at a present-day depth of ~ 3.5 s twtt (i.e. ~3500 m b.s.l.).

Two-D backstripping performed in this area provided an estimate of its paleo-depth of ~2000 m b.s.l. before basin refilling at the end of the crisis [Ryan, 1976; Lofi, 2002; Steckler et al., 2003]. This value is consistent with the mean depth of central part of the Valencia basin estimated to 1400 to 1600 m [Maillard et al., 2006]. This pinch-out is definitely the most reliable and consistent estimate of the Messinian sea level change made at the scale of the western Mediterranean basin until now, and testifies for the importance of the drawdown during the crisis. However, this pinch-out (and paleo-depth associated) does not mark the maximum sea-level drop in the western Mediterranean sea, since we found numerous seismic evidence for small amplitude base-level variations during the low-stand phase (during deposition of UU) and possibly for significantly deeper erosional surfaces (BES, fig. 2) that could be (at least partly) subaerial too (see discussion below).

In the Valencia basin (fig. 3-area 2), Maillard et al.

[2006] proposed that before and/or during the deposition of MU in the Provençal basin (deeper than the Valencia basin located more westward), the maximum sea-level drop resulted in the subaerial exposure of the whole Valencia basin and the creation of an erosion surface (the BES) over the study area, at the paroxysm of the crisis. Then, UU accumulated, progressively filling the basin and showing an aggrading trend. As the basin was shoaling, and the relative base-level consequently rising, UU deposited in onlap on the margin foot, above the BES. The morphology of the TES, at the top of UU, displays a drainage paleo-network extending toward the Provencal basin (deeper than the Valencia basin located more westward), suggesting a subaerial origin for this surface resulting from a sea-level lowering just before the refilling phase [Maillard et al., 2006]. This exposure phase is supported by thick gravels recovered from a channel drilled in the Valencia trough during the DSDP Leg XIII [Hsü et al., 1973]. Hiatuses, corresponding

FIG. 9. – Seismic profiles illustrating the presence of Complex Units (CU) in the Valencia basin (A, dip section, modified from Maillard et al. [2011]), Algerian basin (B, dip section, modified from Obone Zué Obame et al. [2011]) and gulf of Lions middle slope (C, strike section, modified from Lofi et al.

[2005]). The seismic facies of CU ranges from chaotic to roughly bedded, more or less transparent. Their spatio-temporal relationship with the other MSC markers varies from one area to another. CU are interpreted as clastic deposits resulting from the erosion of the margin.

FIG. 9. – Profils sismiques illustrant la présence d’unités complexes (CU) dans le bassin de Valence (A, coupe longitudinale, modifiée d’après Maillard et al. [2011]), le Bassin algérien (B, coupe longitudinale, modifiée d’après Obone Zué Obame et al., [2011]) et la pente moyenne du golfe du Lion (C, coupe transversale, modifiée d’après Lofi et al. [2005]). Le faciès sismique de CU varie d’un faciès chaotique à un faciès grossièrement lité, plus ou moins transparent. Sa relation spatio-temporelle avec les autres marqueurs sismiques messiniens varie d’une zone à l’autre. CU est interprété comme des dépôts clastiques résultants de l’érosion des marges.

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respectively to our BES and TES on the seismic profiles, have also been detected at the base and at the top of the pinch-out of UU at the edge of the Balearic abyssal plain (DSDP drilling site 372 [Cita et al., 1978]).

Well-expressed Messinian erosion surfaces (BES, IES, TES) with incisions are also observed in the East-Corsica intermediate basin at the top, within and at the bottom of what could be the equivalent of UU [Thinon et al., 2010].

However, at this time, this area cannot be correlated with the rest of the Western Mediterranean because of the presence of a volcanic intrusion at the outlet of the Messinian East-Corsica basin. The basin may thus have functioned as an independent lacustrine basin during the MSC, although a drainage pattern toward the Tyrrhenian sea suggests some connection, at least at the end of the crisis. Strong geometric and morphologic equivalences exist between eastern Corsica and Valencia basins. Their lateral equivalence however cannot be fully demonstrated since no dating exists.

Because of their intermediate-depth, such basins could be key areas for constraining the precise timing of the Messinian events in the future.

The existence of several Messinian erosion surfaces (BES, IES, TES) beneath, within and at the top of UU, is a strong evidence for base-level variations during the low-stand phase (during UU deposition). They may result from a combination of several factors among which the shallowing of the deep basin infilled by UU and sea-level oscillations possibly reflecting either (precession driven?) regional climate that influence the runoff in the watersheds and/or alternating episodes of the Atlantic advancing into and retreating from the Mediterranean [Escutia and Maldonado, 1992], although this latter hypothesis is hardly compatible with the lack of evidence for non-reworked marine fauna in UU. The erosional character of the BES, IES and TES is much more visible in the Valencia basin compared to, for example, the Gulf of Lions (fig. 3-area 2 and fig. 5-A). This illustrates the importance of basin paleodepth and morphology in the recording of the erosion offshore. Indeed, in the Valencia basin, erosion is enhanced by the very low gradient of the basin floor that favoured the recording of slight variations during the low-stand. In other words, a sea-level fall of a ten of meters on a gentle slope will result in a shift of the shoreline by several tens of km toward the basin and in the development of an associated fluvial drainage network (eg. Valencia basin [Maillard et al., 2006]). On the other hand, if the slope is steep, a sea-level fall of same amplitude will only shift the shoreline by few kilometres. As a consequence, the TES, IES and BES are rarely observed (or distinguishable) on steep paleo-margins, and only clastic fans that contain sometimes internal discordances and sub-facies (eg. western Sardinia margin [Sage et al., 2005]; Algerian margin [Capron, 2006; Lofi et al., 2011]; Provençal margin [Obone Zue Obame, 2009]) may be correlated with such base-level changes.

These erosion surfaces are also clearly observed in the Alboran Sea (Maillard, pers. comm.) and in the intermediate East-Corsica basin [Thinon et al., 2011] and northwestern Corsica margin [Guennoc et al., 2004], which both had gently sloping basin floors during the MSC. If several lowerings in sea-level are clearly imaged in the Western Basin during the lowstand phase, one matter of discussion concerns the oldest erosion surface (BES): how deep does it extend toward the centre of the basin? Is it

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subaerial or subaqueous in origin in this area? Because the BES can be traced locally beneath the onlap of MU, some authors suggested that there was a possible temporary subaerial exposure of the Miocene lower slopes prior to salt accumulation [eg. Ryan and Cita, 1978; Maillard and Mauffret, 2006]. In other words, the fall in sea-level would have reached a maximum depth (paroxysm of the drawdown) prior to halite (MU) deposition completion in the deep basin. More recently, Ryan [2009] suggested that the BES may have been initiated by the scouring action of high-velocity subaqueous flows.

Some authors evidenced the existence of major embayments of MU that are related to pre-salt erosion topographic lows located on some lower slopes [Dos Reis et al., 2005; Sage et al., 2005; Bertoni and Cartwright, 2006].

Some of those topographies may be related to pre-existing submarine canyons [Sage et al., 2005] possibly acting as preferential sites of submarine erosion in the earliest stages of the MSC [Bertoni and Cartwright, 2006] and as preferential sites of reactivation as subaerial channels during the lowstand phase [Lofi et al., 2008]. Few criteria allow to state if these areas have ever been subaerially reactivated beneath the pinch-out of the salt. MU started forming in subaqueous conditions from residual brine bodies whose depth cannot be estimated precisely. Here we lack crucial information concerning the thickness of the water column before, during and after the MU deposition. Few criteria also allow to state if MU accumulated in one or several phases and if short episodes of complete drying out down to the deepest basins occurred.

Ryan [2008, 2009] proposes that in a first stage, density stratification allowed the Mediterranean to concentrate towards halite saturation, prior to drawdown. Evaporative drawdown began once the Atlantic inflow became too small to balance evaporative loss, possibly resulting in the rapid precipitation of the halite in thicknesses inversely proportional to elevation above the basin floor. This would explain the geometry of MU (aggrading, in onlap on the Miocene margins, and with a flat sub-horizontal top surface before post-Messinian salt tectonics). This also suggests that MU deposition was possibly completed under shallow water conditions, which seems confirmed by brine composition analysis showing that the eastern Mediterranean must have been evaporated to near dryness [Wallmann et al., 1997].

This interpretation is also supported by the strong erosion surface (TES) at the top of MU in the Cyprus arc [Maillard et al., 2010] and the Levant basin, where it is considered as of subaerial origin [Bertoni and Cartwright, 2007]. In the Western Basin, we do not have evidence of erosion at the top of MU from seismic profile analysis, although the extent of the BES beneath the pinch-out of MU suggests that the deposition of at least the top of MU also probably occurred under shallow water conditions.

Because MU accumulated sub-horizontally, this unit can be used as a paleo-marker to quantify the vertical deformation encountered at the scale of the Mediterranean basins. The most landward listric normal growth fault observed at the margin feet indeed marks the initial pinch-out of MU, before basinward salt flow. Its present-day depth depends on the amount of post-Messinian subsidence or uplift undergone by each margin. In the Western Basin, the initial pinch-out of MU is located at present day depths of ranging from ~3.5 to 4.5 s twtt. This range reflects the post Bull. Soc. gĂŠol. Fr., 2011, no 2

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Messinian vertical movements that are different from one margin to another owing to local factors including subsidence (crustal or salt-related), tectonics, sediment load and/or pre-MSC units compaction. However, we can consider that this depth is rather similar at the scale of this basin (fig. 3), illustrating the absence of important deep-seated tectonic phases during the Plio-Quaternary. In the Eastern Basin, the present-day depth of MU initial pinch-out is much more variable from one area to another.

As an example, it is located at 1.8 s twtt east of Cyprus and 2 s twtt in the Levant margin, whereas it is observed at 5 s twtt in the Nile area. As in the Western Basin, such differences are related to regional factors (subsidence, tectonics, sediment load and compaction) amplified in this basin by the active convergent geodynamical context affecting the northern part of the basin since the end of the MSC. It is to be noted that in the western Mediterranean basin the pinch-out of UU at the margin feet constitutes a second paleo-marker that can also be used to quantify the vertical deformation encountered at the scale of the basin.

In short, in the western Mediterranean basin, the observation of erosion surfaces from seismic profile interpretation offshore thus supports the hypothesis that sea level most likely dropped at least 1500 m (and probably up to ~1900 m) during the Messinian drawdown in the Western Basin [Ryan, 1976; Steckler and Watts, 1980; Lofi, 2002; Steckler et al., 2003; Maillard et al., 2006]. Concerning the eastern Mediterranean basin, 2D stratigraphic simulation in the Levant basin allowed some authors to propose that during the crisis, the sea level dropped at least 800 m below its present-day level and may have been as low as 1500 m [Ben-Gai et al., 2005; Tibor and Ben-avraham, 2005].

Modalities of the drawdown It has recently been proposed that the final closure of the connection with the Atlantic ocean has been triggered by an isostatic peripheral uplift of the margins as a result of basin loading by brines and halite [Govers, 2009; Ryan, 2011].

Once the drawdown started (following first stage evaporite deposition in the peripheral basins [Clauzon et al., 1996]), Blanc [2000] proposed based on budget modeling and hydraulic calculations, that the base-level fall was two-stepped, as a consequence of the water body of the western Mediterranean dropping down to an intermediate sill (siculo-tunisian?) during the sea-level lowering [see also Ryan and Pitman, 2000; Ryan, 2008]. Some observations of the MSC seismic markers in the offshore area support the above interpretation. In the gulf of Lions, it clearly appears that a major change occurs in both the morphology and extent of the MSC markers at a present-day depth of 1700-1800 m. Lofi et al. [2005] evidenced that at this depth, one knick-point is visible in the longitudinal Messinian river profiles. This knick-point roughly correlates with the upstream limit of the clastic fan (CU) and with a change in the MES morphology passing from a rough topography to a smooth one. Following the model of Blanc [2000], these authors suggested that the observed geometries may result from a two-stepped drawdown triggered by an intermediate sill between the western and eastern Mediterranean basins. These authors also suggest that this sill, possibly located at an intermediate depth of ~400-800 m bsl [Lofi, 2002; Steckler et al., 2003], may Bull. Soc. gĂŠol. Fr., 2011, no 2

also have played a role at the end of the crisis leading to a two-stepped refilling of the western Mediterranean. Based on a 3D analysis of the morphology of the MES, Bache et al. [2009a,b] support this hypothesis for a two-stepped refilling, giving additional strong arguments. They furthermore propose that the refilling started with a moderate rise in base-level accompanied by the development of transgressive ravinement surface at the origin of the smooth Messinian relief observed on the slope, and was followed by a rapid rise in sea-level, fossilizing the MES [see also Savoye and Piper, 1991]. Some factors and processes possibly controlling this reflooding have been discussed by Bache et al. [2009b].

Offshore observations suggest that base-level changes during the MSC were complex and cannot be considered as a single continuous lowering and refilling event. This complexity may result from the presence of several topographic sills disconnecting the different Mediterranean basins and sub-basins during the MSC. Ryan [2009] proposed a scenario of the crisis stressing the importance of some Mediterranean sills (Sicilian, Apennine and Suez) in the control of temporary base-levels and in the timing of precipitation of the halite in the Mediterranean. Using numerical modeling involving the uplift of the Gibraltar and Sicily straits, Gargani and Rigollet [2007] proposed that numerous sea-level falls of short duration occurred before the final drawdown. At this point, the number, location, paleo-depth and role of the Mediterranean Messinian sills that probably played critical roles in water mass budgets and base-level variations, still need to be clearly established. The knowledge of the paleo-geography of the Mediterranean and Paratethys during the MSC is also essential for restoring the paleo-connections among the basins during higher sealevels [Clauzon et al., 2005, 2008; Krijgsman et al., 2010].

CONCLUSION The organisation of the MSC markers results from the superimposition of regional controlling factors related to the MCS s.s. and local controlling factors related to the structural and geodynamical evolution of the margins and basins.

Thanks to the comparative and multi-site approach developed here, the key seismic markers of the MSC in the offshore domain, namely the erosion surfaces and sedimentary units (and their limits), are described and discussed at the basin scale. Whatever the study area considered, we observe a more or less full association of these characteristic seismic markers showing, at a given time, similarities and differences. â&#x20AC;&#x201C; In the basin, while approaching the Messinian rivers mouths, the deep basin Messinian units (LU, MU and UU) gradually pass to clastic fans. The amount of clastics, their distribution, and their temporal relation with the halite layer differ from one margin to another. This supports that the clastic geometry is partly controlled by local controlling factors such as the dimension of the drainage basin, the absence/presence of reliefs, the regional tectonic activity/quiescence, the occurrence of a soft/hard pre-Messinian sediment cover. In particular, large shelves with thick sedimentary Miocene cover may have been submitted to large scale submarine mass-wasting processes and substantial erosion at an early stage of the crisis, while along narrowest

THE MESSINIAN SALINITY CRISIS RECORDS IN THE OFFSHORE DOMAIN

margins, most of the observed clastics are coeval with the low-stand sea-level, suggesting that most of the erosion occurred at a later stage during evaporite deposition.

– The onlaps of the evaporites and/or of their lateral clastic equivalent on the margin foot, provide important information on the Messinian low-stand phase: in the Western Basin, the pinch-out of the Upper Unit UU in the deep basin marks a paleo-coastline, indicating that the amplitude of the sea-level drop exceeded 1500 meters during the MSC. At the paroxysm of the drawdown, the sea level may have been even deeper since the erosion surface observed beneath the pinch-out of the salt can be either subaerial or submarine.

The lateral onlap of the Messinian Upper Unit on the margin feet supports a gradual infill of the deep basin by the sediments during the low-stand phase characterised by base-level oscillations (before the basin reflooding at the end of the crisis) as attested by erosion surfaces at the top of this unit.

– Major differences exist between the Messinian sedimentary successions in the Mediterranean area. In the western Mediterranean deep basin, the Messinian sequence is characterized by a trilogy (LU, MU, UU) whereas it is mostly composed of one unique unit (MU) in the Eastern Basin, and by a single erosion surface (MES) in the Black sea. Such discrepancies evidence that these main sub-basins evolved in different ways during the MSC. Sedimentation was thus probably controlled by several factors (climate, base-level dynamics, hydraulic budgets, sills, connexions with the global ocean...) that were dissimilar at the scale of the basin. If these

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discrepancies are taken into account in recent models [Ryan, 2008; Blanc, 2001], they should be integrated in any scenario dealing with the MSC at the Mediterranean scale.

Based on the multi-site approach, we propose some elements for a coherent scenario of the MSC in the offshore domain and at the Mediterranean scale. Moreover, the present-day depth of some of the MSC markers in the offshore area reflects the amount of post-Messinian subsidence or uplift undergone by each margin and basin. These markers could thus be used in the future as paleo-markers and constitute a powerful tool to quantify the Plio-Quaternary vertical motions encountered from one area to another. The efficiency of the MSC markers for the study of the post-Messinian deformation has already be proven onshore [Clauzon et al., 1987; 1996b; Rubino and Clauzon, 2008].

This approach will be extended to the offshore domain, at the scale of the Mediterranean basin.

Acknowledgements. – Thanks are due to the INSU (Institut National des Sciences de l’Univers, CNRS) ECLIPSE 2 (2004-2007) French research program, which is at the origin of this project. Some of the studies presented here have also been partly supported by the French Programs “GDR Marges” and “Actions Marges”, we therefore thank the consortium for this founding (CNRS-Institut National des Sciences de l’Univers, TOTAL, IFP, Ifremer, BRGM and IRD). The authors would like to acknowledge J.-P. Suc, C. Bertoni and C. Huebscher for discussions throughout the MSC event. We thank the journal reviewers, W.B.F. Ryan and L. Jolivet who improved the manuscript through very constructive comments. This paper is dedicated to Georges Clauzon who shared with our geophysicist community his knowledge and passion of the Messinian markers onshore, in order to provide an onshore-offshore scheme of the MSC as consistent as possible.

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