According to the most recent Paratethyan and Mediterranean stratigraphic scale correlations, the IPU erosional surface is consideredas the analogue to the Messinian erosional surface described down the slope offshore the Bosporus. In addition to recently

1. Introduction

In the early 70s, seismic reflection surveys followed

Marine Geology 244 (2007 Corresponding author. Present address: UMR 5805, EPOC,discovered inland erosional signature, the wide regional erosional surface we underline on the Western Black Sea margins validatesthe Black Sea Messinian desiccation hypothesis. We also demonstrate that evaporative draw-down of the Black Sea implied theinstallation in the basin of a negative hydrologic budget during the Messinian. The lack of a major Messinian Danube canyon on theRomanian shelf supports the hypothesis of a Messinian Danube trapped in the Dacic basin. However, the presence of a Messiniansuperficial incision network connecting the location of the modern Danube delta to deeply incised Messinian canyons on theouter shelf (IPU) makes the hypothesis of a Danube reaching the partially desiccated Black Sea still possible. 2007 Elsevier B.V. All rights reserved.

Keywords: Black Sea; Messinian erosional surface; seismic stratigraphy; Messinian salinity crisisHerv Gillet a,c,, Gilles Lericolais b, Jean-Pierre Rhault c

a Universit Bordeaux 1; CNRS; UMR 5805-EPOC, Talence, F-33405 Franceb IFREMER, Centre de Brest, DRO/GM, Technople Brest-Iroise, BP 70, 29280 Plouzan, France

c UMR 6538 - Domaines Ocaniques, UBO-CNRS, IUEM, Place Nicolas Copernic, 29280 Plouzan, France

Received 1 February 2006; received in revised form 12 June 2007; accepted 22 June 2007

Abstract

In 1975, sediment cores from leg DSDP 42b (sites 380A and 381) revealed a thin sediment layer in the Black Sea basin whichpoints to a shallow water environment at the Miocene-Pliocene boundary. With these facts and in the wake of hypothesis of theMessinian Salinity Crisis (MSC), it was proposed that the Black Sea, like the Mediterranean Sea, suffered a desiccation period at theend of the Messinian (Hs, K.J. and Giovanoli, F., 1979. Messinian event in the Black Sea. Palaeogeography, Palaeoclimatology,Palaeoecology, 29: 7593). Whereas the main topics of the MSC in the Mediterranean Sea is now widely accepted, the lack ofevidence for a Messinian erosional surface in the Black Sea left the debate about the Messinian desiccation of this basin open untiltoday. The analysis of high resolution multi-channel seismic data acquired during the BlaSON surveys brings important newelements for this scientific debate: (1) Down the slope offshore the Bosporus, we show a clear erosional surface correlated to the topof the Upper Miocene shallow water environment unit of site DSDP 381. The overlying Lower Zanclean unit inevitably dates thiserosional surface of the Messinian event. (2) Awide intra-Pontian erosional surface (IPU) is evidenced on the Romanian shelf. TheIPU is characterized by a sharp decrease in the incision rate from outer (deep canyons) to inner shelf (superficial incisions network).Messinian event in the black sea: Evidence of aMessinian erosional surfaceUniversit Bordeaux 1, Bat B18, Avenue des Facults, 33405 Talence,France. Tel.: +33 5 40 00 36 04; fax: +33 5 56 84 08 48.

E-mail address: h.gillet@epoc.u-bordeaux1.fr (H. Gillet).

0025-3227/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.margeo.2007.06.004) 142165www.elsevier.com/locate/margeoby DSDP drilling data revealed an unexpected thicklayer of Messinian evaporites within the sediment pile of

Geolothe deep Mediterranean basins (Ryan and Hs, 1973)which led to the famous hypothesis of the MessinianSalinity Crisis (Hs et al., 1973).

In the Black Sea, some years later, the cored boreholesof the DSDP Leg 42B recovered an unusual lithogicalunit at the Miocene-Pliocene boundary at the foot of theslope offshore the Bosporus (Ross and Neprochnov,1978). This unit, which includes coarse clastic pebblybreccia and stromatolitic dolomite, is considered asdeposited in a very shallow water environment (Stoffersand Mller, 1979) suggesting that the sea-level of theBlack Sea was very shallow at that time. In the wake ofthe previous Mediterranean discovery, Hs and Giova-noli (1979) have interpreted these data as evidence of adrop in level of the Black Sea of up to 1600 m below theglobal sea-level in response to the Messinian SalinityCrisis already described in the Mediterranean. Suchhypothesis was recently supported by the identificationof a Messinian Danube canyon filled by a Zanclean(Lower Pliocene) Gilbert Delta at the outlet of the IronGates (western Dacic basin) (Clauzon et al., 2005).

However, the hypothesis of a Messinian sea-leveldrop in the Black Sea suffers a major handicap. Whereasthe Messinian Salinity Crisis hypothesis is supported inthe Mediterranean Sea by the combined identification oftwo main erosional and sedimentary signatures (Clau-zon, 1973; Hs et al., 1973), the weak point of such ahypothesis in the Black Sea lies in the absence of suchan erosional signature on the basin margins.

We present here a high-resolution (HR) seismic stra-tigraphy study based on multichanel HR seismic datacollected during the BlaSON 1 and 2 IFREMER surveys.In order to verify the Black Sea Messinian event hypo-thesis, we concentrated our research on the identifica-tion of major erosional surfaces in the Black Sea. Thefield investigations of BlaSON surveys was mainlyconcentrated on the Romanian shelf, where seismic datawere correlated with abundant industrial drilling data,and on the Turkish slope offshore the Bosporus, wherethe two DSDP sites that prompted the Hs hypothesis(sites 380A and 381) are located.

2. Paleogeographic setting

The Black Sea is a land-locked basin, located betweenEurope and Asia Minor. It is generally considered as aresult of a back-arc extension associated with Mesozoicnorthward subduction of the Tethyan Ocean beneath theEurasian Continent (Letouzey et al., 1977; Zonenshainand Le Pichon, 1986; Finetti et al., 1988; Okay et al.,1994). At the end of the Eocene times, the paleogeo-

H. Gillet et al. / Marinegraphic reorganization stemmed from the closure of theTethys and the associated collision of continental blocsresulted in the individualization of two new sedimentaryrealms on both sides of the alpine orogenics belts: theMediterranean Sea to the south, and the Paratethys to thenorth.

The wide intracontinental Paratethyan Sea extendedthrough Central Europe; from the western alpineforedeeps towards the Aral Sea in Asia (Steiningerand Papp, 1979). Its present relics are the French Rhoneand Swiss Molasse basins (Western Paratethys), thePannonian basin (Central Paratethys), the Dacian, theEuxinic (i.e. Black Sea) (Fig. 1) and the Aralo-Caspianbasins (Eastern Paratethys).

The Upper Eocene to Middle Miocene palaeoen-vironmental and palaeogeographical evolution of theParatethys was characterized by a long term trend ofdecreasing marine influence and a correlative reductionin size of the sedimentation domains both resulting fromthe alpine orogenic activity (Rgl, 1999; Meulenkampand Sissingh, 2003). After the Middle Miocene times,Paratethyan conditions evolved into drastically restrict-ed marine environments. The palaeogeographicalreshaping rose to a paroxysm: the overall uplift aroundParatethys led to the progressive isolation, dislocationand, during Pliocene, to the final filling of most of theWestern Paratethyan basins (Meulenkamp and Sissingh,2003). This Neogene evolution was characterized byseveral successive closure episodes of the Paratethysmarine connections towards the Mediterranean Sea andIndian Ocean (Rgl, 1999; Meulenkamp and Sissingh,2003).

On one hand, severance of these connections resultedin the development of largely endemic faunas and floraswhich led to the establishment of specific Neogenestratigraphic scales for the Paratethyan sub-basins (Pappet al., 1974; Papaianopol and Marinescu, 1995; Rgl,1998; Chumakov, 2000).

On the other hand, the episodic closures of the openseaways led to potential eustatic responses within theisolated basins. Depending on the hydraulic budget of thebasin, its base level would evolve toward two maintendencies during isolation phases. Either the hydraulicbudget is positive and the level rises rapidly, or thehydraulic budget is negative and the level falls drasti-cally. Because the basins have relatively small superficies,these eustatic responses could reach large amplitudes in avery short time.

In the Late Miocene times, just before the MessinianSalinity Crisis in the Mediterranean Sea (Hs et al.,1973), the Eastern Paratethys, including Black Sea andDacic basin, was connected to the Mediterranean realm

143gy 244 (2007) 142165by a shallow sill north of the Aegean Sea (Rgl, 1999;

rtoni

GeoloFig. 1. Geographic setting in Central and Eastern Paratethys. A. Late To

144 H. Gillet et al. / MarineMeulenkamp and Sissingh, 2003) (Fig. 1). Presence ofsuch a connection is supported by the influx of Medi-terranean fauna (NN11) recorded in the Dacic basin(Marunteanu, 1992; Clauzon et al., 2005). With regard tothis palaeogeographical situation, it has been proposedthat the Mediterranean Messinian Salinity Crisis resultedin a complete isolation episode of the Eastern Paratethys.The question of the eustatic response of the Eastern Pa-ratethys to the Mediterranean Messinian Crisis thenappeared unavoidable.

3. Signs of a Messinian event in Eastern Paratethys

In the Mediterranean domain, the Messinian SalinityCrisis strongly impacted the sedimentation in two ways(Clauzon et al., 2005): (1) as an immediate consequence,the dramatic fall in sea-level induced the deposition ofevaporites in marginal and deep basins (Ryan and Hs,1973;Montadert et al., 1978), coupledwith the creation ofa widespread erosional surface on the shelves (Ryan andCita, 1978; Guennoc et al., 2000) submitted to subaerialerosion and retrogressive downcutting of deep canyons(Chumakov, 1973; Clauzon, 1973) and the edification ofdetritic cones at the mouth of the main rivers (Barber,1981; Savoye and Piper, 1991; Lofi et al., 2005,); (2) theeffect of the crisis persisted after its achievement withan (87 MA), after Meulenkamp and Sissingh (2003). B. Present time.

gy 244 (2007) 142165delayed responses such as the re-filling of the floodedMessinian canyon heads with Gilbert deltas (Gilbert,1885) during the Lower Pliocene (Clauzon et al., 1995)and the complete rebuilding of the shelf during the PlioQuaternary (Lofi et al., 2003).

In the Eastern Paratethys, similar signatures havebeen identified locally suggesting the advent of theMessinian event in this area. However, these signaturesconcern only a few scattered sites in the Black Sea(sedimentary signature) and the Dacic basin (erosionalsignature and delayed effects).

3.1. In the Black Sea (Euxinic basin)

In 1975, sediment cores from the DSDP Leg 42B inthe Black Sea revealed a thin layer of stromatoliticdolomite associated to clastic sediments at the MiocenePliocene boundary (Ross and Neprochnov, 1978). Thesedeposits were recovered downslope offshore the Bos-porus, at sites 380A and 381 drilled at 2107 and 1728 mwater depth respectively. These deposits correspond tothe lithological units IVd and 6 described in Figs. 2and 3. Detailed geophysical, lithological and biostrati-graphical analysis of these two drillings is available inthe initial report of DSDP Leg 42b (Ross and Nepro-chnov, 1978).

GeoloH. Gillet et al. / MarineUnit IVd, encountered at the 864884m depth intervalin Hole 380A (Fig. 2), consists of an association of pebblymudstone, stromatolitic dolomite, and cobble-clasts ofconglomerates (Fig. 3). The middle part of this unit showsallochtonous tilted dolomitic blocks overlying autochtho-nous horizontally laminated stromatolitic dolomite beds(Section 3 of core 58, Fig. 3). On both side of theseunusual stromatolitic dolomite deposits, the dominantlithology of unit IVd is a coarse clastic, generally referredto as pebbly breccia, composed of large-sized angular

Fig. 2. Stratigraphy and lithology of sites 380A and 381 of leg DSDP 42b, afand representative of a shallow water depositional environment. Index map145gy 244 (2007) 142165dolomitic clasts embedded in a mudstone matrix. From alithological point of view, the clasts of the upper part of theunit (Sections 1 and 2 of core 58, Fig. 3) are identical tothe underlying stromatolitic dolomite beds (Section 3 ofcore 58, Fig. 3), thus suggesting that they were rippedfrom underlying deposits and transported for a very shortdistance (Ross and Neprochnov, 1978).

Unit 6, recovered at the 352437 m depth interval inHole 381 (Fig. 2), consists of a mixture of pebbly mud-stone, breccia, shellhash, sand, drilling mud and common

ter Hs (1978). Unit IVd (380A) and Unit 6 (381) are Messinian in ageshows the seismic lines covering the study area (Blason Survey).

Geolo146 H. Gillet et al. / Marineangular dolomite fragments (Ross and Neprochnov,1978).

Thin section studies from the dolomitic rocks of bothunit IVd (site 380A) and 6 (site 381) revealed a great va-riety of criteria (e.g. intraclasts, algal mats, crusts, pellets,oolites) indicating a shallow depositional environmentwith occasional subaerial exposure and evaporitic andmeteoric diagenetic alterations (Stoffers and Mller,1979). Brecciated sediments of site 381 also showabundant evidence of diagenetic modifications (Stoffersand Mller, 1979). Shallow water depositional environ-ment is also evidenced at site 380A by the diatom as-semblages suggesting a water depth in the range of a fewmeters (Schrader, 1978). It has been proposed that theexclusive precipitation of dolomite (first carbonatemember of the evaporitic series) has resulted from the

Fig. 3. Detailed lithology of unit IVd (site 380gy 244 (2007) 142165predominant alkaline composition of the Black Sea waterat this time, thus suggesting that these units deposited infresh-water environments (Hs and Giovanoli, 1979;Kojumdgieva, 1983).

It has thus been proposed that Unit 6 from Hole 381and unit IVd from Hole 380 A are correlative. In addi-tion, at both sites, the sediment recovered below andabove these shallow water deposits represent deeperwater depositional environments (Hs and Giovanoli,1979), thus supporting the hypothesis of a temporarysea-level fall in the Black Sea during the UpperMiocene.Borehole analysis also revealed a change in salinity at theMiocene/Pliocene boundary. Indeed, the diatoms sam-pled in unit 7 at site 381 are thought to indicate a freshwater depositional environment, whereas the aragoniticmud in units 5 and IVc contains diatom assemblage

A), after Ross and Neprochnov (1978).

Geoloindicating an increased salinity (brackish-marine envi-ronment) (Schrader, 1978).

From a chronostratigraphic point of view, biostratigra-phic studies conducted on diatoms (Jous and Mukhina,1978; Schrader, 1978) and benthic foraminifera andcrustaceans (Gheorghian, 1978), as well as palynologicalstudies (Koreneva and Kartashova, 1978; Traverse,1978), indicate that units IVd and 6 are Late Miocene inage and that the top of these two unitsmarks the PlioceneMiocene boundary (Hs and Giovanoli, 1979; Stoffersand Mller, 1979). This dating is supported by a recenthigh-resolution pollen study at site 380A. According to aglobal climatostratigraphic approach (Popescu, 2006), thelowermost Zanclean age of the aragonitic mud (Unit IVc)overlying the pebbly breccia is demonstrated. In addition,this study confirms the proximal status of the upper-most laminated dolomites recovered in unit IVd.

Hs interpreted this detritico-evaporitic sedimentlayer as the sedimentary signature of a drastic loweringof the water-level within the Black Sea basin at the endof the Messinian (Hs and Giovanoli, 1979). As for theMediterranean Messinian Salinity Crisis model, he pro-posed that the Black Sea water level, due to evaporation,dropped very near to the level of the abyssal plain. Inthis model, the evaporative draw-down was related to are-organisation of the drainage patterns in centralEurope, involving the capture of the Danube water bythe retrogressive erosion affecting the exunded Medi-terranean margins. The author also interpreted theoverlying aragonitic unit as the signature of a marinetransgression corresponding to the Zanclean re-floodingof the Mediterranean which ended the Messinian eventin both the Mediterranean and the Black Seas. Such aZanclean re-flooding of the Black Sea is supported bythe influx of Mediterranean nannoplanctons (zoneNN12) recorded in the Kertch region (Semenenko andLyul'eva, 1978).

3.2. In the Dacic basin

The Dacic basin, which comprises the southernCarpathians foredeep, is today totally filled. It corres-ponds to the present Southern Romania and is delimitedto the north and west by the Carpathians, to the south bythe Balkan chain and to the east by the Black Sea.During the Late Miocene, this basin was incorporatedinto the realm of the Eastern Paratethys as the westernappendage of the Euxinian basin (Black Sea) (Meulen-kamp and Sissingh, 2003). The connections betweenthese two basins were moderately restricted due to thepresence of the Dobrogea horst (Rgl and Steininger,

H. Gillet et al. / Marine1983) (Fig. 1).Recent studies in the Eastern Paratethys led to thefollowing discoveries (Fig. 4):

(1) At the outlet of the Iron Gates, where the Danubeflows into the Dacic basin after cutting a passagethrough the Carpathians (Fig. 4), Clauzon et al.(2005) evidenced the presence of a very largeZanclean Gilbert-type fan delta lying above anerosional surface. This surface is visible on theGura Vii outcrop (Fig. 4), and is interpreted as aMessinian canyon, incising the Carpathians bedrock(Jurassic limestone). This canyon, attributed to a pastDanube tributary, joins the axial incision which isalmost parallel to the Danube's modern thalweg(Clauzon et al., 2005). The Zanclean age of theGilbert delta is confirmed by the presence in thebottomset beds of typical molluscs of the Bosphorianregional substage together with nannofossils repre-sentative of the zone NN12 (which span the MioPliocene boundary). Additionally, this assemblageimplies a Mediterranean influx in this part of theDacic basin and is correlated by the authors to theZanclean transgression.(2) Boreholes of the western and southern Carpathianforedeep (Fig. 4) reveal a hiatus between the LatePontian/Dacian deposits and the underlying series:middle Pontian (Ticleni borehole) (Drivaliari et al.,1997) or older layers such as Sarmatian, Oligocene orCretaceous (Clauzon et al., 2005). This sedimentaryhiatus, supports the Iron Gates discoveries as it mayillustrate an erosional gap resulting from the presencein this area of the MessinianDanube and tributaries(Clauzon et al., 2005). In contrast, outcrops locatedin the north-east part of the Dacic basin (Fig. 4),exhibit continuous sedimentation from the LateMiocene to the Early Pliocene (Marinescu et al.,1981). Subsurface data are consistent with fieldobservations, as many wells revealed a complete LateMiocene sedimentation in the north-eastern part ofthe Dacic basin (Clauzon et al., 2005). It has thusbeen proposed that in the southwestern Dacic basinthe Messinian erosional surface developed along theDanube and its main tributaries courses, whereas thenortheastern part of the basin remained as a perchedlake having recorded a continuous sedimentationduring the Messinian (Clauzon et al., 2005) (Fig. 4).The classical Mediterranean signatures of theMessinian Salinity Crisis (major erosion and Plio-cene Gilbert Delta edification in canyon heads) arerecovered at the outlet of the Iron Gates, thus asser-ting that the Dacic basin was severely affected by the

147gy 244 (2007) 142165Messinian Crisis (Clauzon et al., 2005).

Geolo148 H. Gillet et al. / MarineMoreover, the Bosphorian from the north-east part ofthe Dacic basin recorded two successive Mediterraneaninfluxes (zone NN11 and NN12) (Marunteanu and Pa-paianopol, 1998; Snel et al., 2006). These influxes implytwo successive connections at high sea-level between theMediterranean Sea and the Dacic basin before and rightafter the Messinian desiccation of the Mediterranean.

The above observations led to a new understandingof the Late Neogene relationships between theMediterranean Sea and the Dacic basin and providednew elements for the understanding of the modalities ofthe Messinian Crisis in these basins. Clauzon et al.(2005) thus recently reconsidered the chronostratigra-phy of the Lago Mare event at the scale of theMediterranean and Black Seas. Otherwise, these in-

Fig. 4. Modalities of the Messinian event in the Dacic basin and considered lsouthwestern part of the Dacic basin the Messinian erosional surface hasnortheastern part remained as a perched lake having recorded a continuous sgy 244 (2007) 142165fluxes led to a precise intercalibration of the Mediter-ranean standard and the regional Paratethyanchronostratigraphic scales (Snel et al., 2006) (seeSection 4.4).

3.3. The missing argument

The deep desiccated Black Sea model from Hs andGiovaneli (1979) matches with both the sedimentaryarguments and the subsidence constraints of the Euxinicbasin and with the Dacic basin recent discoveries.Nevertheless, this hypothesis suffers the lack of a mainargument: until now, the major regional erosionalsurface related to such a Messinian Black Sea desic-cation has not been clearly evidenced.

ocalities, after Clauzon et al. (2005). These authors propose that in thedeveloped along the Danube and its mains tributaries, whereas theedimentation.

GeoloH. Gillet et al. / MarineSome clues however suggest that such a surfaceexists, at least locally. Russian authors pointed out anerosional hiatus at the PontianKimmerian (MessinianZanclean) boundary in several sections of the Kerch andTaman peninsulas (northeastern Black Sea, Fig. 1)(Muratov, 1951; Semenenko, 1987; Zubakov, 2000;Chumakov, 2000). Semenenko (1987) and Chumakov(2000) proposed that these erosional signs could berelated to a Messinian event in the Black Sea. In

Fig. 5. Index map showing the study area, the location of the BlaSON surveyof the Petrom Company boreholes (black dots). Also are shown on the Turkishthe TPAO-Turkey Westates Petroleum exploration wells Karadeniz-1 and Infigures with additional information.149gy 244 (2007) 142165addition, on the basin margins, several authors locallyspotted on seismic data the presence of an erosionalsurface at the MioPliocene boundary (Nikolayevaet al., 1980; Finetti et al., 1988; Robinson et al., 1996;Can, 1996). However, such scarce and poorly detailederosional signs cannot suffice to support the Black Seadesiccation hypothesis. As for the MediterraneanMessinian Salinity Crisis, the definitive validation ofthe Messinian Black Sea desiccation hypothesis implies

s HR seismic lines (black lines) and the location on the Romanian shelfmargin the locations of the DSDP sites 380A and 381 (black stars), andeada-1 (black dots). Heavy gray lines and number in figure related to

description). In contrast to the Romanian wells, thestratigraphical analysis of the boreholes of the Euro-pean part of the Turkish margin (including the twoDSDP sites) is based on the standard Mediterraneanstage concept. This disparity leads us to consider twodistinct study areas: the Romanian shelf on the onehand and the European part of the Turkish margin onthe other hand.

4.3. Seismic/drilling correlation

In order to maintain coherence in the interpretation ofseismic data, we chose to work with the two-way traveltime sections. The stratigraphic calibration of theseismic data by the boreholes thus implied the con-version of the well depth information into travel timevalues. Because the in-situ measured velocities were notavailable for the Petrom and TPAO-Turkey WestatesPetroleum boreholes, we based their depth to timeconversion on our seismic stack velocities (converted inintervals velocities by mean of the Dix equation (Dix,1955)). The use of these stack velocities has beenvalidated using boreholes data. The stratigraphic limitdepth-time correlation (computed from the in-situ

Geology 244 (2007) 142165the recognition of a wide regional subaereal erosionalsurface at the scale of the basin.

4. Data and methods

4.1. Seismic data

The BlaSON 1 and 2 surveys (1998, 2002)conducted by the French Research Institute IFREMERin a joint research program between France andRomania, Bulgaria and Turkey, collected more than9000 km of High Resolution (HR) multichannelseismic profiles (Fig. 5). These lines were shot acrossthe Western Black Sea, using a mini-GI air-gun seismicsource (frequency range centered at 150 Hz) and a 24-channel streamer. Such a device displays a resolution ofabout 10 to 15 ms combined with a penetration of ap-proximately 1500 ms (two way time). We processed thedata using Landmark-Promax software. The conven-tional processing flow included the removal of noisetraces, CDP gather formation, velocity analysis, normalmoveout correction and stack, migration and seabedmute. The analysis of seismic data followed a classicalseismic stratigraphic procedure based on the analysis ofthe reflection ending (erosional truncation, onlap,downlap) and configuration (Mitchum and Vail,1977). It allowed the identification of the seismicunits and their boundaries.

4.2. Drilling data

The stratigraphic calibration of seismic data is basedon 26 exploratory and scientific boreholes (Fig. 5).They are located on the shelf, except for DSDP 380Aand 381, drilled downslope offshore the Bosporus.

On the Romanian shelf, we used data from 22 explo-ratory wells (Fig. 5) provided by the Romanian Petromcompany within a collaboration with the University ofBucharest. Due to confidentiality, these data were res-tricted to information on the depth (in meters) of thestratigraphic limits encountered. The stratigraphic anal-ysis of these Romanian boreholes is based on the regio-nal Paratethyan stage concept.

On the European part of the Turkish shelf, besidesDSDP boreholes, data from two TPAO-Turkey Wes-tates Petroleum exploratory boreholes (Karadeniz-1and Ineada-1) are available (Fig. 5). On these drillings(Can, 1996; Aks et al., 2002) information is alsolimited to the depth (in m) of the stratigraphic limits.On the base of the slope offshore the Bosporus, theseismic data were correlated with drillings 380A and

150 H. Gillet et al. / Marine381 of DSDP Leg 42b (Fig. 5) (see Section 3.1 formeasurements) was available for some of the Romanianwells. We observed only a slight average difference

Fig. 6. Late MiocenePliocene chronostratigraphic relationshipsbetween the standard Mediterranean and the Regional Paratethyan(Dacic basin) stratigraphic scales revised by Snel et al. (2006). The

Regional Pontian stage overlaps the MioPliocene boundary. Thisproposition of correlation is used in the present work.

Geolo(6%) between travel times computed from the in-situmeasurements and the travel-times calculated from thestack velocities.

Correlation of the two DSDP cores with the seismicdata is based on the velocities measured in the 380Aboreholes (Ross and Neprochnov, 1978). Extrapolationof these in-situ measured velocities from well 380A towell 381 is based on the lithological correlation pro-posed by Hs (1978).

4.4. Stratigraphic scales used

The direct biostratigraphic correlation of geologicalevents that occurred in the Mediterranean and EasternParatethys during the terminal Mioceneinitial Plioceneremained impossible for a long time, because the regionalstratigraphic scale for the latter region was based onevolutionary succession of strictly endemic malakofauna(Steininger and Papp, 1979; Chumakov, 2000). Thus,global chronostratigraphic considerations on the Roma-nian shelf depended on the quality of the correlation ofthe Paratethyan and Standard Mediterranean scales. Thediscovery of periodic Mediterranean nanoplancton influ-xes (NN11 and NN12) in the Dacic basin (Marunteanuand Papaianopol, 1998) and Kertch region (Semenenkoand Lyul'eva, 1978) led to the recent definition of theMioPliocene boundary in the Eastern Paratethys(Chumakov, 2000; Zubakov, 2000; Snel et al., 2006),which thus made possible the study over the Romaniansequences and their correlation at a global scale.

Although they are located in the Euxinic basin (BlackSea), the MioPliocene stratigraphic analysis of theRomanian boreholes (Petrom) was based on the Dacicbasin regional stages concept (Pontian, Dacian, Roma-nian). We thus refer for interpretation to the most recentcorrelation propositions between the Dacic basinregional and Mediterranean stratigraphic scales madeby Snel et al. (2006) (Fig. 6).

5. Evidences for a Messinian erosional surface inBlack Sea

5.1. The European part of the Turkish margin

5.1.1. Around the downslope DSDP sites, offshoreBosporus

The b2.050 HR seismic line shot downslope offshorethe Bosporus links the twoDSDPdrillings from site 380A(2107 m water depth) up to site 381 (1728 m water depth)(Fig. 7). Line b2.051, starts from site 381 and is strikeoriented (Fig. 7). It crosses several submarine canyons,

H. Gillet et al. / Marinecutting the slope. Because of a difficult stratigraphiccorrelation between each interfluve, the proposed inter-pretation of this line is restricted to the neighborhood ofsite 381. Correlation between the seismic lines and thedrillings led to the identification of twomajor stratigraphicunits: a PlioQuaternary unit unconformably overlying aMiocene unit. Correlation of well DSDP 381 with lineb2.050 shows that the top of the Late Miocene detritico-evaporitic unit (unit 6) correlates with a clear erosionalsurface (Figs. 7 and 8). This erosional discontinuitycorresponds to a strong amplitude reflector (M reflector)underlined by several erosional truncations. The mor-phology of this surface is marked by many secondaryorder incisions. This surface is sealed by the overlyinglowermost Zanclean aragonitic mud (unit 5) (Popescu,2006) (Fig. 2) thus inevitably dating the underlyingerosional surface of the Messinian Crisis time (Fig. 8).Down-slope of site DSDP 381 along line b2.050, theMessinian erosional surface extends seaward down to theBlack Sea basin (Fig. 7). Although the corresponding Mreflector becomes more and more discontinuous, theMessinian erosional surface can be easily identified downto 750 ms under the sea-floor (15 km up-slope of DSDPsite 380A), because this high impedance reflector stronglycontrasts with the surrounding low energy deep acousticfacies. Deeper, identification of the Messinian erosionalsurface is difficult (Fig. 7). At site 380A, the top of UnitIVd does not correlate with any meaningful reflector. Itseems that erosion does not reach the location of this deepdrilling site, or that it was tooweak to give a strong answeron the seismic data. However, prolongation of this surfacealong the same slope, down to site 380 suggests a goodcorrelation between the Messinian surface and the top ofthis shallow water environment unit.

5.1.2. The European part of the Turkish shelfTwo exploration wells provide the stratigraphy for this

southwestern part of the Black Sea shelf. Karadeniz-1 andIneada-1were drilled at 79 and 85mwater depth, and are2597 and 3118 m long, respectively (Can, 1996; Akset al., 2002) (Fig. 9). Karadeniz-1 recovered a very thinveneer of unconsolidated muds of Quaternary age,unconformably overlying a 107-m-thick package ofmudstone interbedded with shelly horizons of Plioceneage. The Pliocene deposits, in turn, unconformably over-lies 1386 m of prograded deltaic mudstones, sandstonesand conglomerates of late Oligoceneearly Miocene tomiddle Miocene age (Can, 1996). The Ineada-1 wellrecovered a 140-m-thick northward prograding Quater-nary succession, unconformably overlying a 284-m-thickPliocene shallow marine succession of mudstones withshelly horizons. The Pliocene deposits unconformably

151gy 244 (2007) 142165overlie 1380 m of prograding deltaic mudstones,

Fig. 7. Correlation between the DSDP sites 380A and 381 and the HR seismic line b2.050 (deep) and b2.051 (strike), down the slope offshore the Bosporus. The Messinian erosional surface correlateswith the top of the Unit 6 at site DSDP 381. Although the Messinian erosional surface is identified downslope of site 381 down to 3500 ms, it seems that erosion does not reach 380A. See Fig. 5 forlocation of the seismic lines.

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GeoloH. Gillet et al. / Marinesandstones and conglomerates of late OligoceneearlyMiocene to middle Miocene age (Can, 1996). In bothwells, the Pliocene systematically unconformably over-lies the middle Miocene series evidencing a correlativestratigraphic hiatus, which spans the whole Upper Mio-cene. Correlation between both hiatus is confirmed byseismic data analysis.

Correlation of both boreholes Karadeniz-1 (Fig. 11)and Ineada-1 (projected) (Fig. 10) with the BlaSONHRseismic lines allowing the identification of the Miocene,Pliocene and Quaternary stratigraphic units. These se-dimentary series show a strong basinward dip (Fig. 10)that may be related to the combination of the NeogenePontides-Balkanides uplift (Sinclair et al., 1997;Nikishin et al., 2003) and an acceleration of the basinsubsidence during the PlioQuaternary (Nikishin et al.,2003). As a result of this tectonic tilt, the Miocene strataare outcropping on the inner shelf (Fig. 10). On theseismic profiles, we evidence a major erosional uncon-

Fig. 8. Zoom on the correlation of the top of unit 6 (site 381) with the MessiniMessinian age breccia containing stromatolitic dolomite clasts. The erosionalLower Pliocene in age. See Fig. 7 for location.153gy 244 (2007) 142165formity at the top of the Miocene sequence. This seismicdiscontinuity is underlined by the strong amplitude Mreflector which tangentially cuts the underlying intra-Miocene reflectors generating numerous erosionaltruncations (Fig. 10). This erosional surface correlateswith the Upper Miocene hiatus shown in the Ineada-1well (Fig. 10).

The direct correlation of the Karadeniz-1 boreholewith the line b2.052, located on the middle shelf,supports the previous interpretation (Fig. 11). Here, theMiocenePliocene discontinuity evidenced in the bore-hole correlates with an erosional surface clearly visibleon the HR seismic line. This erosional discontinuitycorresponds to a discontinuous strong acoustic imped-ance reflector (M reflector), truncating the underlyingMiocene reflectors. This erosional surface is rugged anddisplays locally high relief. Westward of the drilling site,it defines a local incision that reaches 100 ms in depth inthis area.

an erosional surface (seismic lines b2.050 and 051). Unit 6 consists of asurface is sealed by unit 5 of site 381 which consists of aragonitic mud

Geolo154 H. Gillet et al. / MarineSeismic profile analysis thus clearly shows that thestratigraphic hiatus encountered at the MiocenePlioceneboundary in boreholes Karadeniz-1 and Ineada-1 has anerosional origin, and does not results from a lag deposit.Moreover, despite less accurate chronostratigraphiccalibration, this single MioPliocene boundary erosionalsurface can be chronostratigraphically correlated with theMessinian erosion observed downslope on site DSDP381 (Section 5.1.1).

5.2. The Romanian shelf

In contrast tomost of the shelves of theBlack Seawhichare relatively narrow, theRomanian shelf is wide. The post-rift sedimentation of this shelf consists of a stack of fivemajor stratigraphic units (Eocene, Oligocene, Badenian-Sarmatian, Pontian, Dacian-Romanian-Quaternary), which

Fig. 9. Stratigraphy and lithology of the Karadeniz-1 and Ineada-1 explora(1996) and Aks et al. (2002).gy 244 (2007) 142165geometries are strongly influenced by the morphology ofthe underlying rifting structures. The four oldest units arebounded below by erosional unconformities (Robinsonet al., 1996; Gillet et al., 2003) reaching a maximum depthalong the axis of the Histria Depression (Gillet et al., 2003;Gillet, 2004), an asymmetric half-graben opened on theWestern Black Sea basin (Dinu et al., 2005).

The most recent propositions for the correlationbetween the Regional Dacic and Mediterranean strati-graphic scales states that the Regional Pontian stageoverlaps the MioPliocene boundary (Snel et al., 2006)(Fig. 6). We thus concentrated our research on thePontian stratigraphic unit and evidenced the presence ofa major intra-Pontian seismic discontinuity, splitting thePontian into two distinct sub-units (P1 and P2). ThisIntra Pontian Unconformity (IPU) corresponds to amajor erosional surface with atypical characteristics.

tion boreholes on the European part of the Turkish margin, after Can

GeoloH. Gillet et al. / MarineFirstly, in contrast with usual high amplitude seismicsignature of an erosional surface, the IPU corresponds toan alignment of low-energy discontinuous reflectors(Fig. 12). It is underlined essentially by the numerouserosional truncations affecting the subparallel reflectorsof the underlying unit P1 which is characterized by highfrequency and strong amplitude subparallel reflectorswith relatively good lateral continuity. In addition, astrong acoustic facies contrast is visible between P1 andthe overlying unit P2 which consists of a low-energyacoustic facies characterized by discontinuous low am-plitude reflectors showing however some scatteredseaward prograding clinoforms (Fig. 12).

Secondly, the IPU erosional surface is characte-rized by a limited regional extension (approximately13,400 km2 over the study area) and by a sharp decreaseof incision rate from outer to inner shelf. On a wide halfpart of the outer-shelf, the IPU corresponds to a set ofdeep incisions (canyons) (Fig. 12). Their lateral exten-sion is limited to a 150 km wide corridor plumb with theHistria Depression suggesting a deep tectonic control onthe morphology of the IPU. On the distal seismic lines,

Fig. 10. Correlation between the Ineada-1 borehole and the HR seismic liboundary and is interpreted as theMessinian erosional surface (discussed in tex155gy 244 (2007) 142165this erosional surface slopes down below the penetra-tion depth of our HR seismic. Strike line BlaSON b010(Fig. 12) shows some IPU canyons reaching 550m depth(using a 2200 m/s interval velocity). Nevertheless, noneof the described canyons extend landward on the inner-shelf and accordingly none of these deep incisionsreached the modern coastline position. Indeed, canyonerosion stops abruptly some kilometres landward of lineBlaSON b010. On the inner-shelf, the set of deepincisions gives place to a superficial incisions network(Fig. 13). These superficial incisions (50 m deep at most)occur on a corridor spreading from the deep incisionszone to the location of the modern Danube delta(Fig. 14). The seaward prograding clinoforms of thesub-unit P2 illustrate the rapid clastic sediment fillingof the space made available by the IPU erosion phase.

The fact that the IPU is contained in the Pontiansequence allows us to narrow down the potential timewindow for its formation to 6.255.2 Ma. Because thistime interval contains the Miocene/Pliocene boundary,we propose that this erosional surface potentiallycorresponds to the Romanian shelf analogue of the

nes b2.051 and b2.056. M reflector correlates with the MioPliocenet, see section 6.1). This surface is overlain by a Pliocene prograding unit.

Geology156 H. Gillet et al. / MarineMessinian erosional surface evidenced offshore theBosporus on site DSDP 381 (Section 5.1.1).

6. Discussion

6.1. Validation of the Black Sea desiccation hypothesis

From reanalysis of the lithology of DSDP sites 380Aand 381, improved with high resolution seismic dataacquired during the BlaSON surveys, we arrive at thefollowing observations:

(1) On DSDP site 381, the Messinian erosionalsurface evidenced on the seismic lines correlates withthe top of the Late Miocene detritico-evaporitic unit(Unit 6) (Fig. 8) presenting shallow depositional

Fig. 11. Correlation between the Karadeniz-1 borehole and the HR seismic lin(discussed in text, see section 6.1) which is clearly erosional on this part of the mQuaternary, Pliocene and Miocene shading.244 (2007) 142165environment and evidences for occasional subaerialexposure and meteoric diagenetism (Stoffers andMller, 1979). These observations imply (at least inthis area) a subaerial origin for the Messinian ero-sional surface. In addition, the deeper water sedi-ments recovered below and above unit 6 suggest thatthis surface has been created during a temporarylowering in sea-level (Hs and Giovanoli, 1979).This is in accordance with the presence of Messinianautochtonous shallow water sediments (Unit IVd ofDSDP 380A) downslope of the Messinian erosionalsurface (Fig. 7). This observation confirms the sub-aerial origin of the erosional surface located moreupslope of site DSDP 380A. Detritic breccias,including stromatilitic dolomite clasts, recovered inUnit 6 (site 381) and IVd (site 380A) would than

e b2.052. M reflector underlines the Messinian erosional surfaceargin. See Fig. 5 for location of the seismic profile. See Fig. 10 for

Fig. 12. HR seismic line b.010 on the Romanian shelf and its interpretation. Lebada and Minerva boreholes (Petrom Company) allowed stratigraphic calibration of this line. A major erosional surface(IUP) splits the Pontian series into two distinct sub-units (P1 and P2) and corresponds here to a set of deep incisions (canyons). According to the most recent Paratethyan and Mediterraneanstratigraphic scales (Snel et al.,2006), we propose that the IPU surface corresponds to the Messinian erosional surface on the Romanian shelf (discussed in text, see section 6.1). See Fig. 5 for locationof the seismic profile.

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Figsubstraconseis

158 GeologyH. Gillet et al. / Marinecorrespond to the coarser clastic products issued fromthe subaerial erosion.(2) Using a subsidence model for the Black Seabasin, Nikishin et al. (2003) propose that the waterdepth was 2250 m in the Black Sea Western basin inLate Miocene time. Both arguments (subaerial originof the downslope erosional surface and pre-existingdeep basin) support the hypothesis of Hs andGiovanoli (1979), who proposed that these LateMiocene shallow water sediments (Unit 6 and IVd)were deposited in a pre-existing deep basin partiallydesiccated at the end of the Messinian.

. 13. HR seismic line b.080 on the inner Romanian shelf and its interpretation.-units (P1 and P2) and corresponds here to a superficial incision network.tigraphic calibration of the line. According to the most recent Paratethyan andsider the IPU surface as the Messinian erosional surface on the Romanian shemic profile.244 (2007) 142165(3) The presence of the Messinian erosional surface atthe top of the Unit 6 at site 381 (Figs. 7 and 8)demonstrates that these sediments have been exundedafter their deposition in a shallow water environment.However, at site 380A, the erosional surface is notvisible (Fig. 7), suggesting that this site was notexunded, but probably remained under shallow waterconditions as it is testify by the stromatolitic dolomiteof Unit IVd. This is in agreement with the work ofStoffers and Mller (1979) who have proposed that (a)Unit 6 (site 381) was deposited in shallow waterconditions before the maximum drop of the sea level,

The IPU erosional surface splits the Pontian series into two distinctLotus, Iris and Poseidon boreholes (Petrom Company) allowedMediterranean stratigraphic scales correlation (Snel et al.,2006), welf (discussed in text, see section 6.1). See Fig. 5 for location of the

FigdecFig

GeologyH. Gillet et al. / Marinewhen at the same time site 380A was in a deeperenvironment, (b) the deposition of Unit IVd at site380A in shallow water environment illustrates the

. 14. A. Isochron map of the IPU erosional surface on the Romanian shelf; B.rease of incision rate from outer (deep canyons) to inner shelf (superficial incis. 12 and 13. White dots refer to industrial drill holes used to calibrate these159244 (2007) 142165maximum drop of the sea level drop implyingexundation of site 381 and subaerial erosion at thetop of Unit 6.

Morphologic map showing that the IPU is characterized by a sharpsions network). White lines on A refer to seismic lines presented onseismic lines.

Figdur

160 GeologyH. Gillet et al. / Marine(4) The above interpretations are also supported bythe discoveries made on the Turkish and Romanianshelf respectively. The presence of present-daycanyons on the Turkish slope makes impossible thedirect correlation between the seismic lines acquiredon the Turkish shelf and the lithological logs of thedeep DSDP sites. However, the presence of a majorerosional surface at the MiocenePliocene boundaryin neighborhood of Karadeniz-1 and Ineada-1drillings (Figs. 10 and 11) support the concept ofthe creation of a generalized erosional surface at thescale of the Black Sea basin during the Messinian.We thus consider this erosional surface as theanalogue on the European part of the Turkish shelfof the Messinian erosional surface discovered on thebase of the slope between DSDP sites 380A and 381

. 15. Proposed working hypothesis for the lower course of the Messinian Daning basin desiccation; B. the Messinian Danube was reaching the partially244 (2007) 142165(Fig. 7). Similarly, the correlation by the way of HRseismic lines between the Romanian and Turkishmargins was impossible due to the wide occurrenceof shallow gas and related acoustic masking on theBulgarian shelf. So, the partial Messinian desiccationof the Black Sea demonstrated on the Turkish marginshould have an analogue surface of erosion along theentire Black Sea margin, including the Romanianone. According to the most recent Paratethyan andMediterranean stratigraphic scales correlation (Snelet al., 2006) (Fig. 6), we consider the IPU erosionalsurface pointed out on the Romanian shelf (Figs. 12,13 and 14) as the expected analogue of the Messinianerosional surface previously described on the Euro-pean part of the Turkish margin (shelf and slope).The tectonic stability of the Romanian shelf during

ube: A. the Messinian Danube remained trapped in the Dacic basindesiccated Black Sea.

GeoloTertiaryQuaternary times (Robinson et al. 1996),suggests a eustatic origin rather than tectonic origin.

To summarize, we evidence the existence of a wideregional Messinian erosional surface on the WesternBlack Sea margins supporting the previously incompleteargumentation for theMessinian draw-down in the BlackSea: the deep basin shallow water sediments describedby Stoffers and Mller (1979), the inland erosional anddelayed signature of Clauzon et al. (2005) (Fig. 4) andpre-existing deep basin interpreted by Nikishin et al.(2003) and Robinson et al. (1995). Our results confirmthe hypothesis of a drastic lowering of the water-levelwithin the Black Sea basin at the end of the Messinian(Hs and Giovanoli, 1979).

6.2. About the origin of the Black Sea Messinian event

Although the presence of a Messinian erosionalsurface is in accordance with the hypothesis of Hs andGiovanoli (1979), some details of this model do notmatch the recent discoveries made in the Dacic basin(Clauzon et al., 2005). Indeed, Hs andGiovanoli (1979)proposed that the evaporative Messinian draw-down ofthe Black Sea was related to the capture of the centralEurope Danube waters towards the desiccated Medi-terranean basin. Clauzon et al. (2005) neverthelessdemonstrate that the Danube has been captured duringthe Messinian towards the Eastern Paratethys.. Theyalso demonstrate that the proto-Danube did not flow intoEastern Paratethys (includingDacic andBlackSea basins)before the Messinian event.. The lower course of thisproto-Danube was restricted to the Pannonian Basin.Thus, a hypothetical deviation of the course of this proto-Danube cannot have had any consequences on thehydraulic budget of the eastern Paratethys and cannot bethe factor that resulted in the Messinian draw-down of theBlack Sea. According to this argument, this part of themodel of Hs and Giovanoli (1979) must be rejected.

The evaporative Messinian draw-down of the BlackSea implies: (1) the isolation of the basin and (2)persistence or installation of a negative hydraulic budgetin which evaporation exceeds basin water inputs.

TheMessinian isolation can be easily explained by theanalysis of the paleogeographic situation. In the LateMiocene time, the Eastern Paratethys isolated from otherparts of the world ocean, was connected to the Mediter-ranean realm by a shallow sill north of the Aegean Sea(Rgl, 1999; Meulenkamp and Sissingh, 2003). The re-cord of NN11 influx in the Eastern Paratethys (Seme-nenko and Lyul'eva, 1978; Marunteanu and Papaianopol,

H. Gillet et al. / Marine1998,) demonstrates that such connections persisted untilLate Messinian time (Clauzon et al., 2005). Then, at theend of the Messinian, the evaporative draw-down of theMediterranean Sea resulted in the complete isolationof the Eastern Paratethys (including Black Sea basin)behind the North-Aegean sill.

Persistence or installation of a negative hydraulicbudget in this basin must thus be discussed. Paleoenvir-onmental studies demonstrate that in the Late Miocenetime, just before theMessinian Salinity Crisis, the EasternParatethys was characterized by reduced salinity envi-ronment (Papp et al., 1974; Semenenko and Lyul'eva,1978; Steininger and Papp, 1979; Papaianopol andMarinescu, 1995; Rgl, 1999). Persistence of reducedsalinity in the Eastern Paratethys along with a context ofconnection with the Mediterranean realm implied apositive hydraulic budget restricting the Mediterraneanmarine water intakes. It also means that the Messinianevaporative draw-down of the Black Sea followingisolation of the basin implied a sudden change frompositive to negative hydraulic budget in the Late Mes-sinian time. We propose the following working hypoth-esis to explain this sudden change.

The installation of a negative hydraulic budget in thisbasin in Late Messinian time could be related to a drasticclimatic change. Such climate change could be the resultof the partial desiccation of the Mediterranean Sea, asthe disappearance of an epicontinental sea can influenceglobal atmospheric circulation. This is in accordancewith Steininger and Papp (1979) who proposed that themarked increase of open country type mammal fauna(e.g.: antelopes, hyanids and endemic African taxa)recorded on the entire Paratetyan realm resulted from asudden change from established Lower Pontian wet-temperate climate to a distinct Upper Pontian dry phase.However, this proposition appears inconsistent with arecent high-resolution pollen study showing the persis-tence of a warm-temperate climate (with moderatedamplitude variations) on the Eastern Paratethys duringthe Upper MioceneLower Pliocene period (Popescu,2001). Both arguments are not completely in disagree-ment as one concerns a wide regional record (faunamigration) and the other deals with a more accurateEastern Paratethys climatic record (Popescu study).

As another working hypothesis, we propose that theinstallation of a negative hydraulic budget in the basincould be related to a sudden Eastern Paratethys drainagepattern reorganization. This re-organization would resultfrom the eustatic tuning on both sides of the North-Aegean sill following the beginning of the Messiniandraw-down in the Mediterranean. As a result of theclosure of Mediterranean water inputs, sea-level in the

161gy 244 (2007) 142165Black Sea may have temporary dropped to the depth of

Geolothe sill. This may have resulted in a fragmentation of theEastern Paratethys into several sub-basins bounded byintern sills (such as Dobrogean sill). Such a fragmen-tation would cause the capture of part of the water inputsof the Eastern Paratethys into marginal perched basins(such as the northeastern part of the Dacic basin(Clauzon et al., 2005)) leading to the establishment ofa negative water budget in the isolated Black Sea.

Considering a climate component, a low-amplitudesea-level fall could lead to the creation of such lakes,thus bringing an increased contribution to the reductionof the water inputs to the Black Sea. Both hypotheses(climate and river pattern changes) are not incompatible.

6.3. About the lower course of the Messinian Danube

Clauzon et al. (2005) proposed that in the south-western Dacic basin the Messinian erosional surface hasdeveloped along the Danube and its mains tributariescourses, whereas the northeastern part remained as aperched lake having recorded a continuous sedimenta-tion (Fig. 4). The complete lack of information about theMessinian paleogeography of the Eastern part of thisbasin leads to crucial interrogation concerning the lowercourse and outlet of the Messinian Danube. Our resultson the Western Black Sea margin bring some importantarguments that unfortunately appear contradictory: (1)complete absence of a large Messinian Danube canyonincision on the Romanian shelf, (2) presence of aMessinian superficial incision network connecting themiddle shelf deep incisions zone to the location of themodern Danube delta (Fig. 14).

On the one hand, the absence in the Black Sea of aDanube analogue to the Messinian Rhne or Nilecanyons supports the hypothesis of a Danube's Messi-nian lower course restricted to theDacic basin (Fig. 15A).In this hypothesis, we propose that theMessinianDanubeflowed into the remnant Dacic perched lake described byClauzon et al. (2005) and we suggest that the deep IPUincisions evidenced on the middle Romanian shelfcorrespond to the retrogressive subaerial erosion of theMessinian paleo-shelf break related to the installationof a Messinian coastal drainage pattern (superficialincision network) on the exunded shelf.

On the other hand, the presence of a Messiniansuperficial incisions network on the inner part of theRomanian shelf is not incompatible with the hypothesisof a Messinian Danube flowing into the desiccatedBlack Sea basin (Fig. 15B). Because this networkconnects the location of the modern Danube delta to themiddle shelf Messinian canyons, it suggests that the

162 H. Gillet et al. / MarineMessinian Danube reached the Black Sea via a similarcourse to its modern one. According to this hypothesis,the superficial incisions network would correspond tothe Messinian anastomosed lower course of the Danubeon the inner shelf, reaching down stream the very lowlevel of the Messinian Black Sea through the describedIPU canyons. However, to support such a hypothesis amajor erosional canyon of the Danube on the shelfwould have been evidenced instead of the restrictedretrogressive subaerial erosion of the paleo-shelf break.

Analogue situation have been described for the paleo-Berre and paleo-Aude systems in the Gulf of Lions(Northwestern Mediterranean Sea) (Lofi et al., 2005).Despite the presence of two deep Aude and BerreMessinian valleys beneath the inner shelf, no relatedcanyons are visible onshore. Lofi et al. (2005) proposedthat karstic resurgencesmay have been at the origin of thedeep Messinian incisions noted on the middle shelf. Onthe Romanian shelf, the dominant lithology of the serieseroded by the IPU canyons (Pontian series) consists ofshales with rare thin sandstone beds (Robinson et al.,1996; Tambrea et al., 2002). Such karstic origin forthe IPU canyon must therefore be rejected.

7. Conclusion

High resolution multichannel seismic reflection dataacquired during the BlaSON surveys allow the recog-nition of a widespread Messinian erosional surface inthe Black Sea on both the European part of the Turkishmargin and on the Romanian shelf. In addition to formerarguments describing the presence of shallow waterenvironment sediments in the deep basin (Stoffers andMller, 1979), the existence of a Messinian Danubecanyon onland filled by Lower Pliocene Gilbert deltasystem (Clauzon et al., 2005) and the pre-existing deepstatus of the Miocene basin (Robinson et al., 1995;Nikishin et al., 2003), this new element bring the lastargument that validates the hypothesis of Hs andGiovanoli (1979). Indeed, we confirm that the Black Seawas severely affected by the Mediterranean MessinianSalinity Crisis and suffered a drastic lowering of itswater-level at the end of the Messinian.

Down the slope offshore the Bosporus, the correla-tion between our seismic reflection data and DSDPdrilling of site 381 led to the extremely accuratestratigraphic calibration of this Messinian erosionalsurface. Erosional truncation of the top of the Unit 6 atsite 381 suggests that this site was exunded duringmaximum draw-down of sea-level whereas absence ofsuch an erosional surface at the top of the Unit IVd atsite 380A suggests that this deeper site never exunded

gy 244 (2007) 142165and was covered by shallow water at the same time.

163H. Gillet et al. / Marine Geology 244 (2007) 142165Because the direct standardMediterranean stratigraph-ic calibration is difficult to apply to the interpreteddrillings on the Romanian shelf, the identification of theMessinian erosional surface was here based on (1) theanalogy between the local erosional surface (IPU) and thediscoverymade on the Turkishmargin and (2) on themostrecent proposition of correlation between the Paratetyanand Mediterranean stratigraphic scales (Snel et al., 2006).

We also demonstrate that evaporative draw-down ofthe Messinian isolated Black Sea implied the installationon the basin of a negative hydrologic budget. Theclimatic origin and the implication of drainage changeshave to be explored.

On the Romanian shelf, the Messinian erosionalsurface (IPU) is characterized by a sharp decrease ofincision rate from outer (deep canyons) to inner shelf(superficial incisions network). The absence of a majorMessinian Danube canyon on the Romanian shelfsupports the hypothesis of a Messinian Danube whichremained trapped in the Dacic basin. However, because ofthe presence of a superficial incision network connectingthe location of the modern Danube delta to the outer shelfMessinian canyons, the hypothesis of a Danube reachingthe partially desiccated Black Sea remains possible.

Acknowledgements

This PhD work was supported by a grant of the Frenchministry of research and prolonged by a European projectof the 5th framework program called ASSEMBLAGE(EVK3-CT-2002-00090). We thank Prof. Corneliu Dinu(University of Bucharest) for providing stratigraphicinformation for drillings of Romanian Petrom Company,Prof. Nicolas Panin (Geoecomar), whowas at the initiativeof theBlaSON research projects, C. Seyve and J.L. Rubino(Total), G. Clauzon (CEREGE) and J.P. Suc (Univ. Lyon1)for fruitful discussions. We also thank First Crew of the R/V Le Surot and then Herv Nouz and Estelle Threau forthe assistance in seismic processing, Alison Chalm,WalterRoest and Anna Pienkowski-Furze for having helped uswith the English and J.J. Corne, J. Lofi and the editors fortheir constructive reviews and comments. This is UMR5805 EPOC contribution no. 1666.

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165H. Gillet et al. / Marine Geology 244 (2007) 142165

Messinian event in the black sea: Evidence of a Messinian erosional surfaceIntroductionPaleogeographic settingSigns of a Messinian event in Eastern ParatethysIn the Black Sea (Euxinic basin)In the Dacic basinThe missing argument

Data and methodsSeismic dataDrilling dataSeismic/drilling correlationStratigraphic scales used

Evidences for a Messinian erosional surface in Black SeaThe European part of the Turkish marginAround the downslope DSDP sites, offshore BosporusThe European part of the Turkish shelf

The Romanian shelf

DiscussionValidation of the Black Sea desiccation hypothesisAbout the origin of the Black Sea Messinian eventAbout the lower course of the Messinian Danube

ConclusionAcknowledgementsReferences