Land and sea study of the northeastern golfe du Lion rifted margin: The Oligocene - Miocene of...

Land and sea study of the northeastern golfe du Lion rifted margin: theOligocene – Miocene of southern Provence (Nerthe area, SE France)

JULIEN OUDET1, PHILIPPE MÜNCH*1,5, JEAN BORGOMANO1, FRÉDÉRIC QUILLEVERE2, MIHAELA C.MELINTE-DOBRINESCU3, FRANÇOIS DEMORY4, SOPHIE VISEUR1 and JEAN-JACQUES CORNEE5

Key-words. – Oligocene – Miocene, Mediterranean, Golfe du Lion, Bio-magnetostratigraphy, Land and sea approach.

Abstract. – In the western Mediterranean Sea, the Liguro-Provençal Basin (LPB) is a key area for studying passive mar-gins because of its recent formation and abundance of onshore and offshore data. The Nerthe area located in the north-ern margin of LPB provides the unique continuous Oligo-Miocene deposits contemporaneous of the transition rifting todrifting. However, the age of the deposits remains debated and the link between outcrops and offshore seismic data ispoorly constrained. The purpose of this paper is double. First, we intend to propose a new chronostratigraphic framebased on bio- (planktonic foraminifera, calcareous nannofossils) and magneto-stratigraphy. Second, we aim to make,through the integration of new highly time-resolved seismic data and field works, a coherent onshore-offshore linkconcretized by a 3D geological model. The new temporal and spatial data presented in this paper allow correlating theOligo-Miocene sequences, defining their geometry and specifying precisely the timing of syn- and post-rift stages. Thefirst marine transgression is now precisely dated latest Chattian within the syn-rift deposits and appears to be synchro-nous with the first marine deposits in the offshore wells and other marginal basins. The transition from syn-rift topost-rift appears to last 3.3 Ma at maximum, between 21.8 and 18.5 Ma (late Aquitanian to early Burdigalian). It is un-derlined by two major erosional unconformities bearing a hiatus of around 1 Ma. The post-rift started with a major ma-rine transgression that is now dated from middle Burdigalian, at around 18.5 Ma, as elsewhere in the LPB. Contrarily torecent proposals, the post-rift deposits are widely represented on the northeastern margin of the “Golfe du Lion”. There,the subsidence of the margin was low during the syn-rift and the transitional periods and high during the post-rift. Theonset of this high post-rift subsidence appears to be synchronous with the slowdown of the Corsica-Sardinia block(CSb) motion.

Etude terre-mer de la partie orientale de la marge du golfe du Lion : l’Oligo-Miocène de laProvence méridionale (massif de la Nerthe, France)

Mots-clés. – Oligocène – Miocène, Méditerranée, Golfe du Lion, Bio-magnétostratigraphie, Approche terre-mer.

Résumé. – Au sein de la mer Méditerranée occidentale, le bassin Liguro-Provençal (BLP) est une région clef pour étu-dier les marges passives du fait de sa formation récente et de l’abondance des données onshore et offshore. La région dela Nerthe située sur la marge nord du BLP présente les seuls dépôts oligo-miocènes contemporains de la transition rif-ting/drifting. Cependant, l’âge de ces dépôts reste débattu et le lien entre les affleurements et les données sismiques off-shore est peu contraint. L’objectif de ce travail est double. Dans un premier temps, nous proposerons un nouveau cadrechronostratigraphique basé sur la bio- (foraminifère planctoniques et nannofossiles calcaires) et magnéto-stratigraphie.Puis, nous bâtirons, grâce à l’intégration de données de terrain et de sismiques très haute résolution, un lien terre-mercohérent concrétisé par un modèle géologique 3D. La première transgression marine est maintenant précisément datéedu Chattien terminal au sein des dépôts syn-rift de la marge. Elle apparaît synchrone de celle observée dans les puitsoffshore et dans les autres secteurs de la marge. La transition du syn-rift au post-rift semble avoir duré 3,3 Ma au plus,entre 21,8 and 18.5 Ma (Aquitanien supérieur à Burdigalien inférieur). Cette transition est caractérisée par l’existencede deux surfaces d’érosion majeures portant chacune un hiatus d’environ 1 Ma. Le post-rift a débuté avec une transgres-sion majeure désormais datée d’environ 18,5 Ma (Burdigalien moyen) comme ailleurs dans le bassin Liguro-provençal.Contrairement à des propositions récentes, les dépôts du post-rift sont bien représentés sur la partie nord-est du golfe duLion. Dans cette région, la subsidence de la marge a été réduite pendant le rift et la période de transitionsyn-rift/post-rift et elle a rapidement augmenté pendant le post-rift. Le début de cette forte subsidence coïncide avec leralentissement de la vitesse de rotation du bloc corso-sarde.

Bull. Soc. géol. Fr., 2010, t. 181, no 6, pp. 591-607

Bull. Soc. géol. Fr., 2010, no 6

1. EA 4234-GSRC, Univ. Provence, 3 pl. V. Hugo, 13331 Marseille, France. [email protected]. UMR CNRS 5125 Paléoenvironnements et Paléobiosphère, Univ. Claude Bernard – Lyon 1, 27-43 Bd 11 Novembre 1918, 69622 Villeurbanne cedex,France3. National Institute of Marine Geology and Geo-ecology, 23-25 Dimitrie Onciul Street, RO-024053 Bucharest, Romania4. CEREGE, Europôle de l’Arbois, BP 80, 13545 Aix-en-Provence cedex 04, France5. UMR 5243 CNRS, Géosciences Montpellier, Univ. Montpellier II, CC 60, pl. Eugène Bataillon, 34095 Montpellier cedex 05 – FranceManuscrit déposé le 5 août 2009 ; accepté après révision le 15 juin 2010.

INTRODUCTION

In the northwestern Mediterranean Sea, the northern marginof the Liguro-Provençal Basin (LPB) is a key area forstudying passive margins because of its recent formationand abundance of onshore and offshore data (workinggroup “Golfe du Lion” in the French CNRS-INSU GdRMarges program; fig. 1). The formation of the LPB is re-lated to an eastward retreat and roll-back of the Neotethysoceanic plate subduction beneath the European plate duringthe Africa-Europe convergence [Carminati et al., 1998a and1998b; Gueguen et al., 1998; Séranne, 1999]. This roll-back is responsible for the back-arc oceanic opening of theLPB and for the contemporaneous counterclockwise rota-tion of the Corsica-Sardinia block (CSb) [Carminati et al.,1998a, 1998b; Gueguen et al., 1998; Séranne, 1999]. Theonset of the rifting of the LPB is not very well constrained.It has been proposed to be Rupelian on the basis of the ageof the alkaline magmatism in France and Sardinia [Séranne,1999; Cherchi et al., 2008]. It has also been proposed to beChattian on the basis of the age of syn-extensional depositsin the onshore and offshore domains of the “Golfe du Lion”[Cravatte et al., 1974 ; Hippolyte et al., 1993 ; Guennoc etal., 2000]. The end of the rifting has been correlated withthe beginning of the drifting within the proto-LPB dated ataround 20.5 Ma from palaeomagnetic studies [Montigny etal., 1981; Edel et al., 2001; Gattacceca et al., 2007]. Drift-ing and associated ~45o CSb rotation may have lasted until15-16 Ma [Gattacceca et al., 2007] or 18-17.5 Ma [Edelet al., 2001].

Most of previous studies on the Golfe du Lion margin[synthesis in Séranne, 1999 and Guennoc et al., 2000] werefocused on its southwestern and central parts because ahuge database compiling oil company multichannel seismicreflection profiles and nine offshore oil wells, is availablethere (fig. 1b). A recent interpretation of this huge databaseproposed that the northeastern part of the Golfe du Lionwas a shoal subjected to erosion since the end of the syn-riftperiod and, consequently, that there are almost no post-riftMiocene deposits in this part of the Golfe du Lion [Bache etal., 2010]. According to this work, Oligocene syn-rift de-posits should have only been preserved in the two major on-shore Camargue and Marseille basins [fig. 4 in Bache et al.,2010]. However, Chattian? to Miocene marine deposits areknown, since XIXo century, SW of Marseille in the Nerthearea [e.g. Matheron, 1832; Fontannes and Depéret, 1889-1992; Combaluzier, 1932; Andreieff et al., 1972; Nury,1988]. This discrepancy between onshore and offshorestudies questions both some biostratigraphic determinationsand some seismic interpretations. This study is dedicated tothe onshore and offshore geology of the Nerthe area(fig. 1b), key-area to understand the formation of the Golfedu Lion passive margin. Indeed, it is the only area whereOligocene? – Miocene marine deposits are outcroppingcontinuously. Moreover, it is also the only area where it ispossible to integrate onshore and neighbouring offshoredata. However, the chronostratigraphy of these deposits isstill not enough constrained and the link between outcropsand offshore seismic data is hampered by the lack of off-shore wells in the eastern part of the Golfe du Lion. Thus,the present study has two main objectives. The first one is


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FIG. 1. – a) Main geotectonic units and major structures of the Liguro-Provençal basin (LPB) margins. Dashed lines correspond to the ante-rift position ofthe Corsica-Sardinia block and of the Balearic islands: Co, Corsica; Ma, Majorca; Mi, Menorca; Sa, Sardinia. b) Seismic database used in this study.1) Main Oligocene-Miocene basins, 2) Mesozoic substratum (onshore), 3) Palaeozoic substratum (onshore), 4) normal faults, 5) Tertiary thrusts, 6) boreho-les, 7) VHR seismic lines (Eosys, 2006), 8) VHR seismic lines (IFREMER, 1999), 9) industrial seismic lines (TOTAL).FIG. 1. – a) Principales unités géotectoniques et structures principales des marges du bassin Liguro-Provençal (LPB). Les lignes en pointillés correspon-dent à la position anté-rift du bloc Corso-Sarde et des îles Baléares : Co, Corse ; Ma, Majorque ; Mi, Minorque ; Sa, Sardaigne. b) Ensemble des lignessismiques utilisées dans ce travail. 1) Principaux bassins Oligo-Miocènes, 2) substratum Mésozoïque (à terre), 3) substratum paleozoïque (à terre), 4) fail-les normales, 5) chevauchement tertiaires, 6) puits, 7) lignes sismique THR (Eosys, 2006), 8) lignes sismique THR (IFREMER, 1999), 9) lignes sismiquesindustrielles (TOTAL).

to establish a precise integrated chronostratigraphic frame-work of outcropping marine Oligocene? – Miocene depositsbased on bio- (calcareous nannofossils and planktonicforaminifera) and magneto-stratigraphy. The second one isto propose a correlation of onshore field data with offshoreseismic data thanks to new highly time-resolved seismiclines that we conducted in the coastal offshore domain.These new seismic data fill the gap between outcrops andoffshore conventional multichannel seismic data. The landand sea approach developped in this study allows to refinethe tectono-sedimentary evolution of the northeastern sectorof the Golfe du Lion passive margin and gives someinsights in the syn-rift/post-rift transition.

GEOLOGICAL SETTING

General setting, lithostratigraphy

The Nerthe massif is an E-W trending thrust structure char-acterised by a N vergence resulting from the Eocene Pyre-nean compression [e.g. Guieu, 1968 and 1973; Tempier,1987]. Oligocene and Miocene deposits are located on the

southern flank of the Nerthe massif, back from the thrustfront (fig. 2). The cumulated thickness of these outcroppingdeposits reaches 110 m (fig. 3). The succession lies uncon-formably on early Cretaceous limestones (Urgonian Facies)and gently dips SW (fig. 4a). In this paper, we follow thenomenclature of previous studies by Andreieff et al. [1972]and Maurel-Ferrandini [1976] who defined and mappedseven lithostratigraphic units attributed to the Chattian –Burdigalian interval, between the Rouet beach and theTamaris beach (figs 2 and 3). From base to top, the unitsare:

– continental Unit: fluviatile sandstones and conglom-erates unconformably overlain by red to yellow, silty-clayinterbeds. This unit exhibits syn-sedimentary normal fault-ing indicating a N015o extension [Hippolyte et al., 1993];

– parareefal Cap de Nautes Unit: silty marls, sand-stones and bioclastic limestones including two coral-reeflevels that can be traced over 2.5 km along the present-daycoast [R1 and R2 in Galloni et al., 2001];

– brackish Rousset Unit: silty marls and bioclastic (gas-tropods and bivalves) sandstones overlying a stromatoliticlayer;


THE OLIGOCENE – MIOCENE OF SOUTHERN PROVENCE (NERTHE AREA, SE FRANCE) 593

FIG. 2. – Geological map of Tertiary formations outcropping in the coastal Nerthe area. Synthesis of this work and previous works [Arthru, 1962;Andreieff et al., 1972; Geological map of Martigues, 1975; Borgovini, 2003]. The geological map is draped on a Digital Elevation Model (1/25 000 IGN),coordinate system : Lambert III Sud. Vertical exageration x1.5. The location of the VHR seismic profiles (L1-L5; Eosys, 2006) and of the termination ofindustrial seismic profiles (84 RM 01, 40, 42 and 44; 80 GL 22, 23 and 25; TOTAL) is shown in the coastal offshore domain.FIG. 2. – Carte géologique des formations tertiaires affleurantes dans la région côtière de la Nerthe. Synthèse de ce travail et des études antérieures [Arthru,1962; Andreieff et al., 1972; Carte géologique de Martigues, 1975; Borgovini, 2003]. La carte géologique est drapée sur un Modèle Digital d’Elévation (1/25000 IGN), système de coordonnées : Lambert III Sud. Exagération verticale x1.5. La position des profils sismiques THR (L1-L5; Eosys, 2006) et de la termi-naison des profils sismique industriels (84 RM 01, 40, 42 and 44; 80 GL 22, 23 and 25; TOTAL) est reportée dans le domaine offshore côtier.

– bioclastic Carry Unit: bioclastic limestones withabundant fauna (molluscs, bryozoans...) interbedded withsilty marls and fine-grained sands. In the uppermost part ofthe unit, is an index coral-reef [R3 in Galloni et al., 2001].The top of R3 displays early lithification features,synsedimentary fracturation, and karsts [Andreieff et al.,1972; Monleau et al., 1988, 1994; Galloni et al., 2001].A monogenic breccia with heterometric Mesozoic elements(up to 1 m), then bioclastic limestones or argillaceous sedi-ments unconformably overly the R3 reef [Andreieff et al.,1972; Galloni et al., 2001; Borgovini, 2003];

– biodetrital Sausset-les-Pins Unit: marls, shelly andbioclastic limestones and sandstones. This unit is character-ized by syn-sedimentary deformations, particularly up to2 m thick slumping-balls [Conesa et al., 2005];

– Plan de Sausset Unit: it unconformably overlies theprevious deposits through a weak angular unconformity(few degrees only). The base of this unit consists in apolygenic conglomerate rich in green-stained, quartz- grav-els. Above are pelagic marls and bioclastic limestones;

– La Couronne limestones Unit: rhodolithic limestonesand then coarse-grained sandstones organized in tidal dunesoverlying a slightly erosional surface.

Biostratigraphy and chronostratigraphicinterpretations

Despite a great number of biostratigraphic studies, the loca-tion of the Oligocene-Miocene boundary is still controver-sial (synthesis in fig. 3). Andreieff et al. [1972] located it atthe base of the Bioclastic Carry Unit (fig. 3). Andreieff etal. [1972] defined the lower part of the unit as Aquitanianantestratotypic deposits whereas the same succession hasbeen proposed as a parastratotype of the Aquitanian[Alvinerie et al., 1977]. Based on mammals, Hugueney etal. [1987] have suggested that this boundary may occur inthe uppermost part of the Bioclastic Carry Unit (fig. 3). Onthe basis of gastropods biostratigraphy, Nury [1988] placedit at the top of R2 reef within the Brackish Rousset Unit(fig. 3). Based on the grade-dating method, Monleau et al.[1988] proposed that Aquitanian deposits should lack in thearea and that deposits should be Oligocene in age up to thetop of the Biodetrital Sausset-les-Pins Unit (fig. 3).

The unconformity between the Biodetrital Sausset-les-Pins and Plan de Sausset units is commonly attributed to theAquitanian-Burdigalian boundary (fig. 3) [Andreieff et al.,1972; Maurel-Ferrandini, 1976; Borgovini, 2003; Demory etal., 2005]. Within the Plan de Sausset Unit, Maurel-Ferrandini[1976] identified the early and middle Burdigalian intervals(fig. 3). The Couronne Limestones Unit is either attributed tothe late Burdigalian [Maurel-Ferrandini, 1976] or to theLanghian [Anglada et al., 1986; Parize et al., 2007].

MATERIALS AND METHODS

Field data

The main objective of the field work was to propose a set ofgeo-referenced data (map traces, bedding attitude measure-ments, sedimentologic field logs, fault traces and dips mea-surements) and define geometrical constraints (crosssections) that allow us to provide a 3D geological recon-struction. A geological map (1/25,000) of Cenozoic

deposits was drawn (fig. 2). This map was built using exist-ing geological maps [Arthru et al., 1962; Andreieff et al.,1972; Martigues and Istres Geological map, BRGM, 1975;Borgovini, 2003] and additional field mapping.

Calcareous nannoplankton and planktonic foraminifera

To perform biostratigraphical analyses, seven selected sec-tions representing the complete outcropping Oligo-Miocenesuccession have been logged and sampled (coordinates infig. 5). For calcareous nannoplankton and planktonicforaminiferal biostratigraphic analyses, 23 and 15 sampleswere studied respectively. The biostratigraphic study coversthe succession from the Parareefal Unit to the CouronneLimestones Unit. Calcareous nannoplankton qualitativeanalyses were performed with a light polarizing Nikon


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FIG. 3. – Synthesis of main biostratigraphic and sequence stratigraphy stu-dies and comparison with this study. 1 : Andreieff et al. [1972]; 2 : Hugue-ney et al. [1987]; 3: Nury [1988]; 4: Monleau et al. [1988]; 5: Anglada[1971]; 6: Maurel-Ferrandini [1976]; 7: Parize et al. [2007], A: Arnaud etal. [1988]; B: Anglada et al. [1986]; C: Parize et Rubino [2007]; D: Mon-leau et al. [1988]; E: Galloni et al. [2001]; F: Borgovini [2003]. Lithostrati-graphic column modified after Conesa et al. [2007].FIG. 3. – Synthèse des principales études biostratigraphiques et de strati-graphie séquentielle, et comparaison avec cette étude. Log lithostratigra-phique modifié d’après Conesa et al. [2007].

microscope at 1600x magnification. The taxonomic identi-fication of the encountered calcareous nannoplankton taxafollows Perch-Nielsen [1985]. Smear-slides were prepareddirectly from the untreated samples, in order to retain theoriginal composition. For planktonic foraminiferal analy-ses, samples were wet sieved over 250 m, 149 m and 63 mscreens. When possible, a semiquantitative analysis wasmade on each sample and each size fraction in order to de-termine the First and Last Occurrences (FO and LO, respec-tively) of planktonic foraminiferal species. About 3700specimens were picked under a Wild Heerbrugg binocularmicroscope and identified. Taxonomic concepts and

nomenclature follow the atlas of Kennett and Srinivasan[1983].

Paleomagnetism

A geomagnetic polarity scale was recently established inOligocene – Miocene deposits in the Carry Le Rouet area[Demory et al., 2005]. In the present study, this magneto-stratigraphy was completed for the uppermost part ofthe Miocene deposits near Sausset-les-Pins. 41 cylindricalsamples and 6 cubic samples were added to the previousstudy for a total amount of 283 samples. Stepwise thermal



FIG. 4. – a) Panorama of the western flank of the Rouet beach showing the geometry of the Oligocene-Miocene syn-rift deposits. b) Geological cross-sec-tion illustrating the onlap geometry of syn-rift deposits onto the Urgonian basement and their thickening towards the South in the Carry-le-Rouet area.c) Panorama illustrating the tectonic contact between syn-rift and ante-rift deposits sealed by post-rift deposits (“upper prograding group”, middle Burdi-galian). d) Geological cross-section illustrating the onlap of post-rift deposits onto the Urgonian basement in the La Couronne Area. Location of panora-mas, cross-sections and boreholes on figure 2. Black stick: studied sections and borehole.FIG. 4. – a) Panorama de la bordure ouest de la plage du Rouet montrant la géométrie des dépôts oligo-miocènes syn-rift. b) Coupe géologique illustrantl’onlap des dépôts syn-rift sur le substratum urgonien ainsi que l’épaississement de ces dépôts vers le sud au niveau de Carry-le-Rouet. c) Panorama illus-trant le contact tectonique entre les dépôts syn-rift et ante-rift scellé par les dépôts post-rift du groupe progradant supérieur (Burdigalien moyen).d) Coupe géologique montrant l’onlap des dépôts post-rift sur le substratum urgonien. Localisation des panoramas, des coupes et des puits sur la figure 2.Trait noir épais : coupes étudiées et puits.


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demagnetisation was performed on 33 samples. Naturalremanent magnetisation (NRM) and remanent magnetis-ation at steps 100, 200, 300, 350, 400 and 450oC were mea-sured using the superconducting squid magnetometer2G760 (2G enterprises) of the CEREGE (Aix-en-Provence,France). The demagnetisation step 450oC was considered tobe sufficient to remove the viscous overprint and to accessto the primary paleomagnetic signal.

Stepwise alternative field demagnetisation was per-formed on 14 samples. Natural remanent magnetisation andremanent magnetisation were measured at steps 2.5, 5, 10,15, 20, 30, 45, 60, 80 and 100 mT. The data from thepaleomagnetic study are presented in figure 5. Four types ofresults were determined on the base of the quality of thedata, from high quality data (type 1) to the unexploitabledata (type 4) following the criteria defined in Demory et al.[2005]. For the exploitable data, Characteristic RemanentMagnetisation (ChRM) was calculated using PaleoMacsoftware [Cogné, 2003]. In order to highlight the geomag-netic polarity, the reversal angle (RA) [see Demory et al.,2005 for detailed explanation] was also calculated for types1, 2 and 3 data and is shown in figure 5. Values ofRA � 45o are assigned a normal polarity (black), and val-ues RA � 90o are assigned a reserved polarity (white). In-termediate directions of 45o � RA � 90o indicate noevident polarity.

Seismic data

In order to link the onshore geology with the industrial seis-mic data available five kilometers offshore, we conducted aseimic survey in the coastal offshore domain. It was ac-quired in 2006 (EOSYS Company, France). A 25 m longstreamer with a sparker seismic source shooting at 1500 jwere used. A total of 30.05 km of very high resolution seis-mic data has been acquired. Three of the new seismic linesintersect ancient industrial seismic lines (figs 1b and 2).This allows constraining the depth of the basement becauseit cannot be imaged in the coastal offshore survey. This alsoallows us to propose a new interpretation of the industrialseismic dataset, and to map syn-rift and post-rift depositson the continental shelf SW of Marseille. Moreover, weused four very high resolution seismic lines from theCALMAR cruise (IFREMER, 1999) allowing us to seek theoffshore prolongation of the Marseille basin towards theNerthe area. The interpretation of both the very high resolu-tion seismic reflection data and the industrial seismic datawas based on the seismic stratigraphic principles ofMitchum et al. [1977] and has been performed in theKingdomSuite software package.

RESULTS

Calcareous nannoplankton biostratigraphy

Calcareous nannofossils are scarce in sediments from theParareefal Unit to the Biodetrital Sausset-les-Pins Unit andfrequent in those from the Plan de Sausset Unit. Specimenswere moderately well preserved, except for samples Vq1,Vq2, Vq13 and NR06 in which preservation was good(plate I).

Reworked early Cretaceous and Eocene taxa werefound in all collected samples. Samples from the base of theParareefal Unit yielded Sphenolithus capricornutus andSphenolithus delphix (fig. 5). The LO of S. capricornutuscorresponds to the base of NN1 Biozone of Martini [1971],emended by Aubry and Villa [1996]. The FO and LO of S.delphix straddle the NP25/NN1 biozones limit of Martini[1971], emended by Aubry and Villa [1996], just before theChattian/Aquitanian boundary. Sphenolithus conicus, whoseLO is placed in the Aquitanian [Aubry and Villa, 1996], isalso present within the above-mentioned interval. Conse-quently, the base of the Parareefal Unit deposited during theend of the NP25 Biozone and the beginning of the NN1Biozone of Martini [1971] emended by Aubry and Villa[1996], indicating a late Chattian age.

In the middle upper part of the Bioclastic Carry Unitand in the lower part of the Biodetrital Sausset-les-PinsUnit, the concomitant occurrence of Sphenolithusdisbelemnos, Triquetrorhabdulus carinatus and the absenceof Helicosphaera ampliaperta indicate the lower part ofNN2 biozone of Martini [1971], NN2b subzone of Aubryand Villa [1996], or the MNN1d-MNN2a subzones ofFornaciari and Rio [1996], late Aquitanian in age.Fornaciari and Rio [1996] noted that the occurrence ofSphenolithus disbelemnos marks the base of MNN1dwhereas the cross over (CO) Helicosphaera carteri/Helicosphaera euphratis or the acme end of H. euphratismarks the base of MNN2a. In the studied samples we iden-tified the FO of S. belemnos, followed by the Helicosphaeracarteri/Helicosphaera euphratis CO, whereas H. carteri ismost abundant than H. euphratis. Notably, H. euphratis/H.carteri CO is calibrated at 20.92 Ma [Lourens et al., 2004]or at 20.89 Ma [Raffi et al., 2006], slightly preceeded bythe FO of the foraminiferal species Paragloboratalia incog-nita, situated at 20.93 Ma [Lourens et al., 2004]. A similarsuccession of bio-events was observed in the investigatedBioclastic Carry Unit and in the lower part of theBiodetrital Sausset-les-Pins Unit. The nannofossils H.ampliaperta and T. carinatus were found at the top of theBioclastic Carry Unit, indicating that the top of the Unit de-posited during the uppermost part of the NN2 Biozone ofMartini [1971] and, respectively in the MNN2b Biozone of



FIG. 5. – Chronostratigraphic results. Magnetostratigraphic data: declination, inclination, reversal angle and magnetic polarity scale (black: normal polari-ty, white: reverse polarity, grey: undetermined). Empty rhomb: very good quality data, full rhomb: good quality data, square: poor quality data. Biostrati-graphic determinations for nannoplankton and planktonic foraminifera. Open circle, index taxa (also bold characters); full circle, other taxa.FIG. 5. – Résultats chronostratigraphiques. Données magnétostratigraphiques : déclinaison, inclinaison, reversal angle et échelle de polarités magnéti-ques (noir : polarité normale, blanc : polarité inverse, gris : polarité indéterminée). Losange vide : donnée de très bonne qualité, losange plein : donnéede bonne qualité, carré : donnée de mauvaise qualité. Déterminations biostratigraphiques du nannoplancton et des foraminifères planctoniques. Cerclevide, taxons marqueurs (caractères en gras) ; cercle plein, autres taxa.


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PLATE. I. – Calcareous nannoplankton microphotographs (light microscope). 1-2 : Helicosphaera scissura MILLER, 1981; 1 : crossed nicols; 2 : transmit-ted light; Sample FB04. 3-4 : Helicosphaera carteri (WALLICH, 1877) KAMPTNER, 1954; 3 : crossed nicols; 4 : transmitted light; Sample FB02. 5 : Pon-tosphaera multipora (KAMPTNER, 1948) ROTH, 1970 emended BURNS, 1973; transmitted light; Sample FB05. 6-7 : Helicosphaera ampliapertaBRAMLETTE and WILCOXON, 1967; 6 : transmitted light; 7 : crossed nicols; Sample Vq13. 8 : Helicosphaera intermedia MARTINI, 1965; crossed nicols;Sample CpR6. 9 : Discoaster deflandrei BRAMLETTE and RIEDEL, 1954; transmitted light; Sample CpR6. 10, 16 : Spenolithus capricornutus BUKRY andPERCIVAL, 1971; crossed nicols, 0o; Sample NR06. 11 : Sphenolithus moriformis (BRÖNNIMANN and STRADNER, 1960) BRAMLETTE and WILCOXON,1967; crossed nicols, 0o; Sample FB05. 12-13 : Sphenolithus heteromorphus DEFLANDRE, 1953; crossed nicols; 12 : 0o; 13 : 45o; Sample FB05. 14-15 :Sphenolithus conicus BUKRY, 1971; crossed nicols; 14 : 0o; 15 : 45o; Sample CpR6. 17 : Calcidiscus leptoporus (MURRAY and BLACKMAN, 1898) LOE-BLICH and TAPPAN, 1978; crossed nicols; Sample NR06. 18-19 : Reticulofenestra minuta ROTH, 1970; 18 : transmitted light; 19 : crossed nicols; SampleCpR1. 20-21 : Sphenolithus belemnos BRAMLETTE and WILCOXON, 1967; crossed nicols; 18 : 0o; 19 : 45o; Sample FB01. 22 : Rhabdosphaera clavigerMURRAY and BLACKMAN, 1898; crossed nicols; Sample CpR6. 23-24 : Reticulofenestra pseudoumbilicus (GARTNER, 1967) GARTNER, 1969; 23 : crossednicols; 24 : transmitted light; Sample FB010. 25 : Coccolithus pelagicus (WALLICH, 1877) Schiller, 1930; crossed nicols; Sample FB09. 26-27 : Coccoli-thus miopelagicus BUKRY, 1971, emended WISE, 1973; 25 : transmitted light; 26 : crossed nicols; Sample FB09. 28 : Cyclicargolithus floridanus (HAY etal., 1967) BUKRY, 1971; crossed nicols; Sample NR06. 29 : Triquetrorhabdulus carinatus MARTINI, 1965; crossed nicols; Sample NR06. 30 : Helicosphae-ra ampliaperta BRAMLETTE and WILCOXON, 1967; crossed nicols; Sample Vq13. 31 : Braarudosphaera bigelowii (GRAN and BRAARUD, 1935)DEFLANDRE, 1947; crossed nicols; Sample FB06. 32 : Thoracosphaera sp., crossed nicols; Sample FB010.PL. I. – Micrographie du nannoplancton calcaire (microscope optique).

Fornaciari and Rio [1996] that corresponds to early-middleBurdigalian times.

Sediments from the Plan de Sausset Unit yielded diversi-fied assemblages of calcareous nannoplankton. The co-oc-currences of H. ampliaperta and Sphenolithus belemnos inthe lower part of the Unit indicate that deposition took placeduring the middle Burdigalian (NN3 Biozone of Martini[1971] or the MNN3a Biozone of Fornaciari and Rio[1996]). Finally, we found the FO of Sphenolithusheteromorphus that indicates the base of the NN4 Biozone ofMartini [1971] and respectively, the MNN4a Biozone ofFornaciari and Rio [1996]. Thus, the middle-uppermost partof the Plan de Sausset Unit is middle-late Burdigalian in age.

To conclude, in the studied sections, the calcareousnannofossils biozonation is comprised between the upper-most part of NP25 Biozone of Martini [1971], respectivelythe NP25c subzone of Aubry and Villa [1996], and thelower part of NN4 Biozone of Martini [1971]. In terms ofthe Mediterranean calcareous nannofossil zonation ofFornaciari and Rio [1996], the investigated deposits cov-ered the MNN1b-MNN4a zones (fig. 9).

Planktonic foraminifera biostratigraphy

Planktonic foraminifera are scarce in the Bioclastic CarryUnit and in the Biodetrital Sausset-les-Pins Unit. They areabundant in the Plan de Sausset Unit. Preservation is gener-ally moderate except in samples FTU1, FVA2, FBO1 andFBO4 where preservation is good (plate II). The FO ofGloboquadrina dehicens and FO of Paragloborotaliasemivera were evidenced in the Bioclastic Carry Unit, while

Paragloborotalia incognita was absent, then suggesting thatthis Unit encompasses the M1 and the base of M2 Zones ofBerggren et al. [1995] corresponding to the Aquitanian.

Within the base of the Biodetrital Sausset-les-Pins Unit,the co-occurrences of P. incognita and Catapsydraxdissimilis and the absence of Globigerinoides altiaperturusindicate the base of the M2 Biozone of Berggren et al.[1995] corresponding to the late Aquitanian – earlyBurdigalian. The uppermost part of the BiodetritalSausset-les-Pins Unit and the Plan de Sausset Unit aremarked by co-occurences of Gs. altiaperturus and C.dissimilis indicating the M2 and the M3 Biozones ofBerggren et al. [1995] corresponding to the early-middleBurdigalian.

Palaeomagnetism

Lithostratigraphic and biostratigraphic correlations betweensections (fig. 5) allow the construction of a composite mag-netic polarity sequence of the deposits of the southern flankof the Nerthe. The magnetic polarity sequence has been de-fined using only good-quality data (types 1 and 2). It in-cludes 7 normal and 7 reversed magnetozones. The polaritysequence is characterised by the presence of a long reverseinterval with two short normal magnetozones in the threefirst lithostratigraphic units (fig. 5). The Bioclastic CarryUnit shows a reverse polarity at its base, then a reversal. Inits upper part it shows mainly a normal polarity with a smallreversal. The two following lithostratigraphic units presenta dominance of reverse polarity including three normalpolarity intervals.



FIG. 6. – L1 VHR seismic profile (Eosys, 2006) and corresponding line drawing showing the geometry of the 5 overall prominent seismic reflectors (H1 toH5) and of the 4 defined seismic units (SU1 to SU4). Location of profile on figure 2.FIG. 6. – Profil sismique THR L1 (Eosys, 2006) et son interprétation montrant la géométrie des 5 principaux réflecteurs sismiques (H1 à H5) et des 4 uni-tés sismiques définies (SU1 à SU4). Localisation du profil sur la figure. 2.

Geometry of onshore deposits

In all, 107 dips were carefully measured during the fieldwork. They show that the direction of dip is rather constantin the whole succession (SSW orientation, mean of N195 ±33o). However, the angle of dip varies significantly alongthe succession. From the Continental units to the top of theR3 reef within the Bioclastic Carry Unit, 58 angles of dipare comprised between 3o and 15o with a mean of 9o ± 4o

and one 25o angle of dip was measured but located in the vi-cinity of a fault in the Petit Nid Bay. From the top of R3 tothe top of the series, angles of dip are significantly lower:48 measured angles of dip fall between 1o to 8o with a meanof 3o ± 2o. Thus the unconformity on top of R3 is a majorunconformity. No significant difference in angle of dipcould be evidenced on both sides of the unconformity be-tween the Biodetrital Sausset-les-Pins Unit and the Plan deSausset Unit. Moreover, we measured the lithostratigraphicunits of 27 cross-sections. The thickness of the deposits be-low R3 decreases by a half from east (near Carry-le-Rouet)to west (near Sausset-les-Pins). Moreover we evidencedthat these deposits thicken toward the south by 200% andthat they show an onlap geometry onto Urgonian basementbackstepping toward the north in the Carry-le-Rouet area(fig. 4b). The thickness of the deposits above the top of R3

does not vary significantly along an E-W trend. Howeverthey thicken from 2 m to 20 m toward the south and onlapthe Urgonian basement toward the north (fig. 4d). Thus,field investigations evidenced two parts in the Oligo-Mio-cene series. The unconformity on top of R3 may thus berelated to a major tectono-sedimentary event.

Geometry of offshore deposits

The seismic survey do not allow a precise interpretationfrom a sequence stratigraphic point of view because of therapid occurrence of the multiple. Four main seismic units(SU), namely SU1 to SU4 (fig. 6), have been distinguished.Their stratigraphic relationships are described on the seis-mic line L1 (North-South profile, fig. 6). From bottom totop the units are:

– SU1: the base of this unit is hidden by the first multi-ple but the upper boundary is an erosional truncation (fig. 6;Horizon 5). This unit is characterized by high amplitude aswell as continuous reflectors, and its stacking pattern andthickness cannot be correctly constrained because of the oc-currence of the first multiple. Near the top of the unit, be-tween horizons H6 and H5, dome-shaped reflectors show anaggradational pattern with onlaps and downlaps onto theconformable horizon H5. We interpret this sub-unit as a


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PLATE. II. – Scanning electron photomicrographs of planktonic foraminiferal species of biostratigraphic significance in the Tamaris composite section.1: umbilical a) and spiral b) views of Globigerinoides altiaperturus. 2: umbilical a) and spiral b) views of Globigerinoides trilobus s.l.. 3: umbilical a) andspiral b) views of Globoquadrina dehicens. 4: umbilical a), edge b) and spiral c) views of Paragloborotalia incognita. 5: umbilical a), edge b) and spiralc) views of Globoconella zealandica. Specimens 1, 2, 3, 4 from sample FBO4; specimen 5 from sample FBO10. Scale bar = 100 m.PL. II. – Photomicrographies en microscopie électronique à balayage des espèces significatives de foraminifères planctoniques de la coupe composite desTamaris. 1 : Vues ombilicale a) et spirale b) de Globigerinoides altiaperturus. 2 : Vues ombilicale a) et spirale b) de Globigerinoides trilobus s.l.. 3 : Vuesombilicale a) et spirale b) de Globoquadrina dehicens. 4 : ombilicale a), de profil b) et spirale c) de Paragloborotalia incognita. 5 : Vues ombilical a), deprofil b) et spirale c) de Globoconella zealandica. Les spécimens 1, 2, 3, 4 proviennent de l’échantillon FBO4 ; le spécimen 5 de l’échantillon FBO10.Echelle = 100 m.

reefal buildup. This seismic unit is observable only on theseismic line L1 and it plunges towards the west;

– SU2: it is an around 60 ms two way travel time (twtt)thick unit bounded below by an onlap surface and above byan almost conformable surface (fig. 6, horizons 3 and 2 re-spectively). The seismic facies is variable with moderate tohigh amplitude and semi-continuous to continuous reflec-tors. The internal configuration is sub-parallel. Near thebase of SU2, reflectors clearly exhibit a retrogradationalpattern toward the north (i.e. below Horizon 3). Thus, suc-cessively younger topset strata are deposited further land-ward forming a succession of backstepping stackingpatterns. SU2 is interpreted as a transgressive system tract,indicative of a relative sea level rise. Above Horizon 3, re-flectors are sub-parallel with an aggradational-progradational geometry toward the south. Their topsetscould not be imaged because they correspond to outcrop-ping strata. Thus, we cannot determine whether theycorrespond to a stillstand sea level or still to a transgressivesystem tract;

– SU3: The SU3 thickness is about 40 ms twtt and isbounded below by an almost conformable surface andabove by an erosional truncation (fig. 6; horizons 3 and 1respectively). SU3 exhibits two different seismic facies sep-arated by Horizon 2. Below Horizon 2 the seismic facies ischaracterized by semi-continuous, high amplitude sub-par-allel reflectors almost concordant at the bottom. Above Ho-rizon 2 the seismic facies shows chaotic reflections withweak amplitude. The internal geometry suggests anaggradational pattern indicative of a sea level rise;

– SU4: it is a sheet-shaped, 20 ms twtt thick unit,bounded below by a basal conformable surface (Horizon 1)and above by the sea-floor. The facies is characterized bysub-parallel continuous high amplitude reflectors. This unitunconformably overlies SU3 and SU2.

This very high resolution seismic dataset clearly showsthe occurrence of at the least 100 ms twtt thick Oligocene-Miocene deposits 4 km offshore. It allows us to propose that

Horizon 4, corresponding to a truncation surface and overly-ing a reefal buildup, may correspond to the unconformity ontop of R3. We consider it as a sequence boundary SB1. Hori-zon 3 may then correspond to the unconformity between theBiodetrital Sausset-les-Pins Unit and the Plan de SaussetUnit. We consider it as a sequence boundary SB2. Horizon 1may correspond to a Quaternary unconformity stacked withthe Messinian erosional surface, and is the sequence bound-ary SB3. SU1 has been identified only on line L1 and L2 andshows a progressive deepening westwards in accordancewith the general southwest plunge of the corresponding on-shore deposits. Last, we want to point out that neither tec-tonic structures nor syn-sedimentary tectonic activityfeatures have been evidenced, peculiarly within seismic unitsSU2 to SU4 that have been well imaged.

As seismic lines L2, L3 and L5 crosscut some commer-cial seismic lines (fig. 1b), we can also propose a new inter-pretation of the latter. In particular, the top of the Mesozoicbasement has been re-evaluated (fig. 7). The Oligocene torecent sedimentary succession in the northeasten sectorof the Golfe du Lion margin was divided in threemegasequences: a syn-rift megasequence limited at its topby an erosional surface named “break-up unconformity”, apost-rift megasequence limited at its top by the Messinianerosional surface and a Pliocene-Quaternary megasequence.This is in perfect accordance with previous works in otherparts of the Golfe du Lion margin [Gorini et al., 1993].In the studied area, neither terrigenous nor evaporiticMessinian deposits occur. Thus, we showed that theOligocene-Miocene syn-rift to post-rift deposits are presentin some parts of the northeasternmost continental shelf andcan reach a thickness of 0.6 s twtt s at maximum (figs 7 and8). In the southwestern most part of the studied area, withinthe Grand Faraman hemi-graben, such deposits are around1.5 s twtt thick (fig. 8). The syn-rift deposits are mainly re-stricted to the two major offshore NNE-SSW striking struc-tures, the Grand Faraman and Arlésiens extensional basins(fig. 8). They are also preserved into E-W trending lowsoffshore south of Nerthe relief in the prolongation of the



FIG. 7. – Seismic profile showing the occurrence of the two units of the post-rift deposits on the continental shelf of the northeastern sector of the Golfe duLion margin.FIG. 7. – Profil sismique illustrant la présence des deux unités des dépôts post-rifts sur le plateau continental du secteur NE de la marge du golfe du Lion.

Marseille basin and onshore in the Crau plain. Thanks tothe seismic lines from the CALMAR cruise, the offshoreprolongation of the Marseille basin can be now clearly iden-tified (fig. 8). The thickness of syn-rift deposits is low(600 ms twtt) in the whole studied area. As previously pro-posed [Gorini et al., 1993], the post-rift megasequence canbe divided in two seismic units: a lower unit with atransgressive retrogradational geometry (onlapping seismicreflectors) and an upper unit with a progradational geome-try (downlapping seismic reflectors) (fig. 7). The lowerunit, such as the syn-rift deposits, is restricted to pre-existing topographic lows whereas the upper unit largely

overflows these areas and reaches substratum highs (fig. 7).The second unit rests unconformably upon the first unit(figs 7 and 8). The whole post-rift deposits are lackingwithin the Marseille basin whereas they have been identi-fied within the Nerthe area.

DISCUSSION

Integrated stratigraphy

We propose to correlate the composite magnetostratigraphicscale to the Geomagnetic Time Scale of Cande and Kent[1995] calibrated by Gradstein et al. [2004] on the basis ofthe distinctive pattern of polarity reversals and on bio-chronological constraints (fig. 9). All calibrated ages ofbioevents (First Appearence Datum: FAD, Last AppearenceDatum: LAD, Cross-Over: CO) are thus from Lourens et al.[2004].

Assuming a reverse polarity and the presence of S.delphix (FAD 23.21 Ma and LAD 23.11 Ma), the uppermostpart of the first reversed magnetozone at the base of the sec-tion can be unambiguously correlated to Sub-ChronC6Cn.2r from the latest Oligocene (fig. 9). The lowermostpart of the section below the FO of S. delphix (i.e. Conti-nental Unit) could be correlated to C6Cr. The ChronC6Cn.3n could be missing because of the low sampling res-olution within this part of the section. The FO of Gq.dehiscens (FAD 22.38 Ma) within the Bioclastic Carry Unitallows correlating the observed normal magnetozone withthe sub-Chron C6Bn.2n. We can then correlate the two nor-mal magnetozones between the two previous calibratedones with the Chrons C6Cn.2n and C6Cn.1n. Thus, theChattian-Aquitanian boundary, corresponding to the base ofthe sub-Chron C6Cn.2n, is located within the R1 reef in theParareefal Cap de Nautes Unit. Consequently, it is nowdemonstrated that the first marine transgression is lateChattian in age in the Nerthe area.

The base of the Biodetrital Sausset-les-Pins Unit ismarked by the FO of the planktonic foraminifera P. incog-nita (FAD 20.93 Ma), the dominance of the nannofossilHelicosphaera carteri over the genus Helicosphaera (CO20.92 Ma) and the absence of the nannofossil H.ampliaperta (FAD 20.43 Ma). The time span constrained bythese two bioevents corresponds to the Chrons C6Ar andC6An.2n. Thus, the reversed magnetozone at the base ofthis unit can be correlated with the late AquitanianChron C6Ar. As the underlying normal magnetozone iscorrelated with the Chron C6Bn, the erosional surface at thetop of R3 reef (SB1) corresponds to a hiatus that lastedapproximatively 800 ka. The uppermost part of BiodetritalSausset-les-Pins Unit is marked by the FO of the planktonicforaminifera Gs. altiaperturus (FAD 20.06 Ma) and by theabsence of the nannofossil S. belemnos (FAD 19.03 Ma). Inthe studied section, the FO of Gs. altiaperturus was foundat the top of a normal magnetozone and in the overlying re-versed magnetozone. As the FAD of Gs. altiaperturus iscalibrated at the top of normal polarity Chron C6An.1n, wepropose to correlate the reversed magnetozone and the un-derlying normal magnetozone from the top of theBiodetrital Sausset-les-Pins Unit with the Chrons C6r andC6An.1n, respectively. The correlation of the magneto-stratigraphic scale of the Biodetrital Sausset-les-Pins Unit


602 OUDET J. et al.

FIG. 8. – Isopach maps (in meter) of syn-rift deposits (a) and post-rift depo-sits (b) of the northeastern sector of the Golfe du Lion margin. Interval ve-locities of 3000 m/s and 2600 m/s were used to calculate the thickness ofthe syn-rift and post-rift deposits, respectively.FIG. 8. – Carte des isopaques (en mètres) des dépôts syn-rift (a) et post-rift(b) du secteur NE de la marge du golfe du Lion. Des vitesses d’intervallede 3000 m/s et 2600 m/s ont été utilisées pour calculer l’épaisseur des dé-pôts syn-rift et post-rift respectivement.

is not straightforward because we could not evidence theChron C6An.2n. This may be related to hidden outcropsand to the low sampling resolution within this part of thesection (fig. 5, fig. 9). However, this unit encompasses theAquitanian/Burdigalian boundary.

The marls from the Plan de Sausset Unit are character-ized by the FO and then the LO of S. belemnos (FAD19.03 Ma and LAD 17.95 Ma), by the FO of S.heteromorphus (FAD 17.71 Ma) and by the absence of T.carinatus (LAD 18.28 Ma). Moreover, the planktonic



FIG. 9. – Correlation of the magneto-biostratigraphic scale of the Oligocene-Miocene deposits of the Nerthe area (a) with the Geomagnetic Polarity TimeScale (GPTS), pelagic zonations and selected datums of calcareous nannoplankton and foraminifera (with ages in Ma recalculated following Lourens et al.[2004]) (b). O and M Zones from Berggren et al. [1995]; NP and NN Zones from Martini [1971]; MNN Zone from Fornaciari and Rio [1996].FIG. 9. – Corrélation de l’échelle bio-magnétostratigraphique établie pour les dépôts oligo-miocènes de la région de la Nerthe (a) avec l’échelle des pola-rités géomagnétiques de référence, les zonations des organismes pélagiques et certains datums sélectionnés de nannoplancton calcaire et de foraminifèreplanctonique (avec des ages en Ma recalculés selon Lourens et al., 2004) (b). Zones O et M [Berggren et al., 1995]; zones NP et NN [Martini, 1971] ; zoneMNN [Fornaciari et Rio, 1996].

foraminifera Gl. zealandica is always present in these marls(LAD 17.26 Ma). Thus, the corresponding large reversedmagnetozone is correlated to the Chron C5Dr and the un-derlying normal magnetozone is then correlated to theChron C5En. The unconformity at the base of the Plan deSausset Unit (SB2) corresponds to a hiatus that lastedapproximatively 1.2 Ma. The uppermost part of the section,the La Couronne Limestones Unit, corresponds to a normalmagnetozone that could be correlated either to the ChronC5Dn or to a younger one. We cannot precise the durationof the hiatus corresponding to the basal erosional surface ofthis unit.

Integration of onshore and offshore geology andconsequences

Thanks to the very high resolution seismic dataset, we inte-grated onshore field data with the available offshore database.This integration was achieved by building a 3D geologicalmodel [Oudet, 2008].

Our data do not allow precising the onset of the rifting.Following the work of Hippolyte et al. [1993], we considerthat it started during late Chattian. In the northeastern sec-tor of the Golfe du Lion, rifting was strongly influenced byinherited structures such as intracontinental rift basinslinked to late Eocene-early Oligocene W-European rifting(Grand Faraman, Arlésiens and Marseille basins), but alsosuch as E-W structures formed during the Pyrenean com-pression (Nerthe and Crau basins) (fig. 8) [Mauffret andGorini, 1996]. In the Nerthe area, late Chattian to middleAquitanian deposits (i.e. below SB1) are characterized by aretrogradational geometry and a southward thickening andthus may be related to the infilling of a pre-existing topo-graphic low. However some minor extensional tectonic ac-tivity occurred during this period with metric-scalesouth-verging normal faults in the Nerthe area. The lateChattian to middle Aquitanian, continental to marine, de-posits correspond to the syn-rift sequence and the overlyingunconformity SB1 could correspond to the syn-rift/post-riftboundary (“break-up unconformity”). Indeed, SB1 is a ma-jor erosional, angular unconformity and it corresponds to ahiatus between 21.8 and 21 Ma (fig. 9). Depending on theauthors, the age of the “break-up unconformity” was con-sidered late Aquitanian to early Burdigalian [Gorini et al.,1993], or at the Aquitanian/Burdigalian transition[Benedicto, 1996], or at the Oligocene/Aquitanian transi-tion [Bache et al., 2010]. This proposal (late Aquitanian,21.8-21 Ma) is in perfect accordance with the estimated ageof 20.5-21.5 Ma for the end of the rifting, or the onset ofthe drifting, from palaeomagnetic studies [Edel et al., 2001;Gattacceca et al., 2007]. The age of the first marine inva-sion in the Nerthe area, now dated from late Chattian, dem-onstrates that open marine domains occurred, during earlyrifting, on the northern flanks of the rift shoulder and evenin the northeasternmost sector of the Golfe du Lion margin.Such a result is in accordance with ancient works showingthe occurrence of late Chattian marine sediments within theMarseille basin [Colomb et al., 1979]. More largely, a lateChattian transgression is known from several areas of theproto-LPB [Cravatte et al., 1974 ; St Tropez canyon:Groupe Estocade, 1978: central and western parts of theGolfe du Lion margin; Cherchi et al., 2008: southern Sar-dinia]. In the northeastern sector of the Golfe du Lion

margin, the syn-rift deposits are thin (0.6 s twtt) but notnegligible such as elsewhere in the Golfe du Lion margin[Bache et al., 2010; 1 s twtt]. Their thickness may reach700 m at maximum in the Marseille basin following the de-scription of the “Stéarinerie Fournier” well [Répelin, 1935].Our new seismic interpretation indicates that the GrandFaraman and Arlésiens structures may have also beensyn-rift depocenters (fig. 8) during early rifting, not majoraxes of erosion as proposed by Bache et al. [2010]. As thelate Chattian open marine deposits reached the Nerthe areaand the Marseille basin, we propose that the Grand Faramanand Arlésiens structures acted as marine passageways to thenortheast during early rifting. This is in accordance with alate Chattian marine ingression issued from the southwestthrough the southern Sardinia grabens and/or a Sardo-Iberiancorridor [Nury and Schreiber, 1997; Meulenkamp andSissingh, 2003].

Above SB1, marine sedimentation started again duringthe late Aquitanian with the last deposits of the BioclasticCarry Unit. These deposits may correspond to post-rift de-posits and comprise two units separated by SB2, asubparallel-bedded erosive aerial surface.

The oldest unit, below SB2, displays a retrogradationalgeometry offshore. Onshore, it rests unconformably uponsome tilted syn-rift deposits and corresponds to atransgressive system tract onlapping the Urgonian basement(fig. 2). These deposits result in a marine transgression aftera tilt of the previous deposits. They correspond to thepost-rift “Lower transgressive group” of Gorini et al.[1993] that can be now precisely dated from late Aquitanianto earliest Burdigalian. They appear to be very thin (0,3 stwtt) and restricted to the pre-existing lows of the relief cre-ated during the rifting, suggesting a low subsidence of themargin. It must be underlined that these deposits have suf-fered synsedimentary deformations and are topped by anunconformity (SB2). The latter reached directly the substra-tum only on some topographic highs, offshore, mainly atthe footwall of the Beauduc and Rhône horsts and, onshore,in the Nerthe area (near Sausset-les-Pins; figs 2 and 4c).Moreover, our results show that this unconformity (SB2)corresponds to a hiatus between 19.7 and 18.5 Ma (fig. 9).All these features may indicate that a low tectonic activitystill occurred during this period and consequently that theperiod between 21.8 and 18.5 Ma may correspond to a tran-sitional period between the syn-rift and the post-rift. Thusthe deposition of the “Lower transgressive group” (betweenSB1 and SB2) may have occurred in a transitional stage af-ter the onset of the oceanic accretion but before thecomplete cessation of tectonic activity on the Golfe du Lionmargin.

Above SB2, the youngest unit reveals a progradationalgeometry with a thickening towards the southwest. Itlargely overflows previous depositional areas and displaysan onlap geometry onto Mesozoic substatum highs in boththe offshore and onshore domains (fig. 4c). This unit corre-sponds to the post-rift “Upper prograding group” of Goriniet al. [1993] whose onset can now be precisely dated fromthe middle Burdigalian (within Chron C5En; fig. 9). Ournew interpretation of the seismic lines revealed theprogradational geometry of these deposits even on the innershelf, not conformable as previously proposed [Gorini etal., 1993]. These deposits may be considered as the post-riftdeposits sensu stricto. As the thickness of these deposits is


604 OUDET J. et al.

significant (up to 0.8 s twtt) even on topographic highs [e.g.Cravatte et al., 1974; 302 m in the Sirocco well on thefootwall of the Grand Faraman hemi-graben], the wholenortheastern sector of the Golfe du Lion margin was not asubstratum high subjected to aerial erosion after the end ofthe rifting, as proposed by Bache et al. [2010]. Post-rift de-posits above SB2 underline a second marine transgressionthat has also been recorded onshore in the opposite marginsof the LPB [Ferrandini et al., 2003: Corsica; Cherchi et al.,2008: Sardinia; Oudet, 2008; Oudet et al., 2010: Sardiniaand Golfe du Lion] and offshore in the central grabens andtopographic highs of the substratum [Cravatte et al., 1974;Sirocco well]. This transgression is marked there by the oc-currence of Globigerinoides bisphaericus whose FAD fallswithin the Chron C5En [Mancin et al., 2003]. This age isperfectly in accordance with our results from southern Pro-vence. This major, post-rift marine transgression occuredsynchronously with the end of the rapid ccw rotation ofCSb dated at around 18 Ma [Gattacceca et al., 2007]. Itcould thus correspond to the onset of an accelerated thermalsubsidence of the margins related to the slowdown of theoceanic accretion in the narrow LPB. This slowdown hasbeen correlated with the onset of the collision of CSb withApulia [Faccenna et al., 2002; Gattacecca et al., 2007].

CONCLUSION

The land and sea approach used in this study allows integrat-ing onshore and offshore data in spite of their different scalesand highlights the need for such studies in order to describemore precisely the tectono-sedimentary organization of pas-sive margins. From a spatial point of view, the integrationwas achieved through a very high resolution seismic cruiserealized in the gap between coastal onshore outcrops and off-shore commercial seismic lines. From a chronostratigraphicpoint of view, new bio-magnetostratigraphic investigationswere necessary onshore as a reappraisal of ancient works, inboth onshore and offshore domains. The main results are:

– the first marine transgression is now precisely datedlatest Chattian within the syn-rift deposits and appears to besynchronous with the first marine deposits in the offshorewells and others marginal basins. As in the whole Golfe duLion margin, syn-rift deposits are relatively thin and indica-tive of a low syn-rift subsidence. However in the northeast-ern sector of the Golfe du Lion margin, syn-rift depositswere deposited not only within extensionnal structures butalso within E-W topographic lows inherited from thePyrenean orogeny;

– the transition from syn-rift to post-rift appears grad-ual. Onshore and offshore, this transition corresponds totwo erosional unconformities (SB1 and SB2) and to the“Lower transgressive group”. This transition lasted 3.3 Maat maximum, between 21.8 and 18.5 Ma (late Aquitanian toearly Burdigalian). Each of the unconformities bears a hia-tus of around 1 Ma. The deposits are restricted to the preex-isting lows and are thin. The subsidence of the marginremained low during the transitional period;

– the post-rift started with a major marine transgressionthat is now dated from middle Burdigalian, at around18.5 Ma. This transgression was synchronous with the oneobserved elsewhere in the LPB. It largely overflowed thedomain of extensional basins and reached topographichighs previously emerged. The thickness of the post-riftunit (“Upper prograding group”), previously underesti-mated, is indicative of a high subsidence of the margin evenin the northeastern sector of the Golfe du Lion margin. Theonset of this high post-rift subsidence appears to be syn-chronous with the slowdown of the CSb motion.

Acknowledgements. – The funding of this study was supported by the GDRMarge program (INSU, French CNRS). The authors thank Eosys companyfor his contribution to this work. The authors thank Total for making theirseismic database available. We also thank S. Berné for providing seismiclines from the CALMAR cruise. We acknowledge L. Andréani for his edi-torial support. We acknowledge E. Fornaciari and M. Séranne for impro-ving the manuscript.

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