ORIGINAL

Late Quaternary evolution of the Çanakkale Strait region(Dardanelles, NW Turkey): implications of a major erosionalevent for the postglacial Mediterranean-MarmaraSea connection

Erkan Gökaşan & Hüseyin Tur & Mustafa Ergin &

Tolga Görüm & Fatma Gül Batuk & Nurcan Sağcı &Timur Ustaömer & Ozan Emem & Hakan Alp

Received: 20 May 2009 /Accepted: 9 September 2009 /Published online: 25 September 2009# Springer-Verlag 2009

Abstract Seismic and bathymetric data from the Çanak-kale Strait and its extensions onto the shelves of theMarmara and Aegean seas indicate that the strait wasformed mainly by an erosional event. Four seismic units areobserved on seismic profiles. The lower two of these (units4 and 3) constitute the basement of a regionally widespreaderosional unconformity (ravinement), which developed

during marine isotope stage 2 (MIS 2). The two upperunits (units 2 and 1), which overlie the ravinement surface,form a higher-order sequence. Sequence stratigraphicanalysis indicates that units 2 and 1 deposited as lowstandand highstand systems tracts respectively, since the end ofMIS 2. The transgressive systems tract is represented by amajor erosional event which occurred throughout the

E. Gökaşan (*) : T. GörümNatural Sciences Research Center, Yıldız Technical University,Beşiktaş,34349 İstanbul, Turkeye-mail: egokasan@yildiz.edu.tr

H. Tur :H. AlpDepartment of Geophysical Engineering, İstanbul University,Avcılar,34850 İstanbul, Turkey

M. ErginDepartment of Geological Engineering, Ankara University,Tandoğan,06100 Ankara, Turkey

F. G. Batuk :O. EmemDepartment of Geodesy and Photogrammetric Engineering,Yıldız Technical University,Davutpaşa,34210 İstanbul, Turkey

N. SağcıDepartment of Geophysical Engineering, Institute of Science,İstanbul University,Beyazit,34452 İstanbul, Turkey

T. UstaömerDepartment of Geology, İstanbul University,Avcılar,34850 İstanbul, Turkey

Present Address:T. GörümDepartment of Earth Systems Analysis, International Institutefor Geo-information Science and Earth Observation (ITC),P.O. Box 6, 7500 AA Enschede, The Netherlands

Present Address:M. ErginGeological Research Center for Fluvial,Lacustrine and Marine Environments (AGDEJAM),Ankara University,06100 Ankara, Turkey

Geo-Mar Lett (2010) 30:113–131DOI 10.1007/s00367-009-0166-2

Çanakkale sill area when the Mediterranean-Marmara Seaconnection and, hence, the Çanakkale Strait was formed.The existence of the erosive Şarköy Canyon along the shelfedge of the southern Marmara Sea demonstrates that theflow direction causing the erosion was from south to north,thus proving that it was produced by Mediterranean waterflowing over the sill into the Marmara Sea basin.

Introduction

The Çanakkale Strait (CS) constitutes the shallow andnarrow south-western gateway to the Marmara Sea locatedin NW Turkey (Fig. 1). The Marmara Sea and its straits(İstanbul or Bosphorus, and Çanakkale or Dardanellesstraits), known as the Marmara waterway, separate Anatolia(Asia) from Thrace (Europe). As such, they constitute aunique water connection between the Mediterranean andBlack seas (Fig. 1). In the course of Quaternary sea-levelfluctuations, water exchanges between the Mediterraneanand the Black Sea probably produced this waterway due tothe peculiar morphology in the vicinity of the İstanbul andÇanakkale straits. These straits have therefore been impor-tant focal areas for late Quaternary oceanographic andpalaeoceanographic studies (Defant 1961; Sholten 1974;Stanley and Blanpied 1980; Sur 1988; Oğuz and Sur 1989;Ergin et al. 1991, 1997; Beşiktepe et al. 1994; Smith et al.1995; Lane-Serff et al. 1997). Especially the suggestion ofRyan et al. (1997) that the last transgression in the Black Seawas caused by an abrupt and rapid incursion of Mediterra-nean waters through the İstanbul Strait, and the subsequentproposition of Ryan and Pitman (1999) that this eventrepresented Noah’s Flood narrated in the Bible, precipitated adramatic increase in the number of studies focusing on thelatest connection between the Mediterranean and Black seas.

While some studies supported and expanded this newevolutionary model (e.g. Ballard et al. 2000; Major et al.2002; Ryan et al. 2003; Eriş et al. 2007), others questionedthe idea and instead postulated a persistent and strong outflowfrom the Black Sea during the last glacial-interglacialtransition period, based on multidisciplinary data from theMarmara Sea and the Black Sea exits of the İstanbul Strait(e.g. Aksu et al. 1999, 2002a, 2002b; Hiscott and Aksu 2002;Hiscott et al. 2002; Kaminski et al. 2002; Mudie et al. 2002).

Besides the two scientific groups supporting twodifferent models, other independent studies have beenperformed along the Marmara waterway, also with the aimto understand the evolution of the strait system and thelatest Mediterranean-Black Sea connection (e.g. Alavi et al.1989; Ergin et al. 1997, 2007; Gökaşan et al. 1997, 2005a,2005b, 2008; Demirbağ et al. 1999; Çağatay et al. 2000;Yaltırak et al. 2000; Algan et al. 2001, 2002; Görür et al.2001; Oktay et al. 2002; Kerey et al. 2004; Kırcı-Elmas et

al. 2008). Again, some of these studies support the abruptflooding theory of Ryan et al. (1997), whereas otherscontradict it. This has not only kept the debate about thelatest Mediterranean-Black Sea water connection alive, buthas triggered numerous new studies in the region to test thetwo theories (see Ryan 2007 for further information).

Independently of any controversy, the origin andgeological evolution of the Çanakkale Strait has been thefocus of research ever since Hochstätter (1870) andsubsequent researchers suggested that the strait evolved asa fault-controlled river valley or as a graben structure(Darkot 1938; Yalçınlar 1949; Erol 1968, 1992; Alpar et al.1996; Demirbağ et al. 1998; Yaltırak et al. 2000; Ergin et al.2007; Gökaşan et al. 2008). Thus, the presence of severalelevated marine terraces along the CS and the westerncoastline of the Marmara Sea proves the occurrence oftectonic uplift and river incision (e.g. Erol and Nuttal 1973;Yaltırak et al. 2002). Recent studies by Yaltırak et al. (2000)and Gökaşan et al. (2008) have shown that the CS has beencut into upper Miocene deposits widely outcropping alongthe coast of the strait. Gökaşan et al. (2008) also suggestedthat the present seafloor morphology of the CS has beencontrolled by a combination of factors ranging fromtectonics to current erosion and deposition. In addition,Ergin et al. (2007) argued that the Şarköy Canyon, which isincised into the outer shelf and slope of the southernMarmara Sea, and which can be regarded as a submarineextension of the CS (Fig. 2), was formed by a majorerosional event along a previously formed tectonic depres-sion during a sea-level lowstand.

Therefore, earlier work on the Çanakkale Strait and theŞarköy Canyon region indicates that tectonics and erosionhave been dominant factors in the evolution of thesechannels. However, in these studies it was assumed thatthe present channel of the CS developed progressively overa long period. Thus, previous authors suggested that beforethe last transgression in the area, the present channel of theCS had already existed, and the sill depth between theAegean and Marmara seas was accepted as similar to eitherthe present seafloor or the basement depth of the CS. Inaddition, effects of water exchange on the evolution of theCS during the last connection between the Aegean andMarmara seas have not been discussed to date.

In this study, a new model for the evolution of theÇanakkale Strait is presented, based on evidence for a close

Fig. 1 a Location map of the study area showing major tectonicfeatures (modified after Yılmaz et al. 2009). The inset shows thelocation of the Marmara Sea and major faults in Anatolia. CSÇanakkale Strait (Dardanelles), GP Gelibolu Peninsula; CS-NAFZcentral segment of the North Anatolian Fault Zone, NS-NAFZ northernsegment of the North Anatolian Fault Zone, SS-NAFZ southernsegment of the North Anatolian Fault Zone, SS southern shelf. bLocation map of the seismic profiles

b

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relationship between the origin of the strait and the latestMediterranean-Black Sea water connection. It is essentiallya reinterpretation of erosional features at the base of the CSand Şarköy Canyon previously reported by Ergin et al.(2007) and Gökaşan et al. (2008), supported by newseismic and multi-beam bathymetric data from the MarmaraSea exit of the CS. In addition, data previously publishedby Aksu et al. (1999), Hiscott and Aksu (2002), Hiscott etal. (2002), Yaltırak et al. (2002) and Çağatay et al. (2009)are evaluated to clarify the chronostratigraphy of the model.

Geological and geomorphological constraints

The onshore geology of the region surrounding theÇanakkale Strait consists of pre-Miocene to Miocene

basement and upper Miocene and younger (Pliocene,Quaternary) deposits (inset of Fig. 2). Upper Miocene andPliocene deposits cropping out along the coast on both sidesof the strait consist of siliciclastic and carbonate sedimentsdeposited in fluvio-lacustrine, beach and shallow-marineenvironments (inset of Fig. 2; Erol 1968, 1972, 1981, 1985,1992; Elmas and Meriç 1998; Çağatay et al. 1998; Tüysüzet al. 1998; Yaltırak et al. 1998, 2000, 2002). Quaternarymarine terraces and alluvial deposits of modern riversconstitute the recent sediments around the CS (inset ofFig. 2; Erol and Nuttal 1973; Yaltırak et al. 2002).

The study area has been strongly influenced by tectonicprocesses active in NW Turkey. The most remarkabletectonic feature is the Ganos Fault, which is the western-most extension of the northern segment of the NorthAnatolian Fault Zone (NS-NAFZ in Fig. 1a; e.g. Şengör

Fig. 2 Digital elevation model of the Çanakkale Strait and itssurrounding areas. I, II (Nara Passage) and III indicate segments ofthe Çanakkale Strait reported in Gökaşan et al. (2008), which havedifferent coastal and seafloor morphologies. BI Bozcaada Island, CSÇanakkale Strait, CV Çanakkale Valley, EI Ekinlik Island, KPKaraburun Peninsula, KaP Kapıdağ Peninsula, KR Karamenderes

River, MI Marmara Island, N Nara Promontory, PI Paşalimanı Island,SC Şarköy Canyon, SG Saros Gulf, TB Tekirdağ Basin, TI TürkeliIsland. The inset shows a simplified geological map of the ÇanakkaleStrait and surrounding area, including the faults along the strait(compiled from Erol and Nuttal 1973; Sümengen et al. 1987; Siyakoet al. 1989; Gökaşan et al. 2008; Ustaömer et al. 2008). Y Yelkenkaya

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1979; Barka and Kadinsky-Cade 1988; Şaroğlu et al. 1992;Okay et al. 1999; Yaltırak and Alpar 2002; Gökaşan et al.2003; Ustaömer et al. 2008; Yılmaz et al. 2009). This faultcuts the western part of the Marmara Sea, and enters theAegean Sea after passing through Thrace (Fig. 1a). Thecentral and southern segments of the NAFZ (CS-NAFZ andSS-NAFZ in Fig. 1a), on the other hand, cross the southernshelf of the Marmara Sea and adjacent terrestrial areas (e.g.Barka and Kadinsky-Cade 1988; Şaroğlu et al. 1992; Güreret al. 2003, 2006; Kurtuluş and Canbay 2007). Severalactive secondary faults have been identified between thesemajor faults, some of which have affected the morphology ofthe CS (e.g. the Nara Fault in Fig. 1a; Gökaşan et al. 2008).

Morphologically, the Çanakkale Strait, together with itssubmarine extensions, constitutes a shallow sill (ÇanakkaleSill) at approx. —70 m elevation between the deep(>1,000 m) Aegean and Marmara sea basins (Fig. 2). TheAegean side of the study area consists of a shelf plainframed by the Gelibolu and Biga peninsulas and theGökçeada and Bozcaada islands. Here, a N-S-aligned valleydefined by the 80 m depth contour, and extending from theAegean Sea exit of the CS to the shelf edge, is the mainmorphological feature (Fig. 2). This valley, here named theÇanakkale Valley, begins in the southernmost, E-W-alignedpart of the CS and bends sharply to the north upon enteringthe Aegean Sea. The CS was morphologically subdivided byGökaşan et al. (2008) into three segments: (1) a NE-SW-oriented wide southern segment at the Aegean Sea exit; (2) anarrow, zigzag-shaped central segment around the NaraPromontory, also known as the “Nara Passage”; and (3) arelatively straight, NE-SW-oriented northern segment at theMarmara Sea exit (Fig. 2; Gökaşan et al. 2008).

The morphology of the CS strongly affects the hydro-dynamics in the strait, which is governed by a two-layeredflow system, a surface layer flowing from the Black Sea tothe Aegean Sea, and a bottom layer comprising Mediterra-nean waters flowing towards the Black Sea (Sur 1988;Oğuz and Sur 1989; Beşiktepe et al. 1994). The channel ofthe CS extends along and parallel to the coast of the BigaPeninsula, but disappears off Değirmencik where, at anaverage water depth of 60 m, it merges with the broad andflat plain of the south-western Marmara Sea shelf (Fig. 2).The shelf itself is incised by the 160-m-deep, NE-SW-aligned Şarköy Canyon (Ergin et al. 2007). Downslopetowards the Marmara Sea basin, the Şarköy Canyon isreplaced by small gullies and a large submarine mud-flowdeposit (Gazioğlu et al. 2002; Fig. 2).

Materials and methods

In the course of numerous cruises between 1979 and 1996,a total of 1,532 km of high-resolution, single-channel

seismic data were collected by the Turkish Navy, Depart-ment of Navigation, Hydrography and Oceanography (TN-DNHO; Fig. 1b). During these surveys, an analogueUniboom seismic system consisting of an energy supplyunit (300 J), a transducer, a single-channel hydrophonestreamer and an analogue recorder was used. The shipspeed was about 4 knots, and the recorder was set at a 100-to 400-ms scan. Positioning was achieved by a Trispondersystem. The seismic data, which had previously beeninterpreted by Özturan (1995), Smith et al. (1995), Alparet al. (1996), Çağatay et al. (1998), Demirbağ et al. (1998),Yaltırak et al. (1998, 2000), Vardar (2006), Ergin et al.(2007), Gökaşan et al. (2008) and Ustaömer et al. (2008),were geologically reinterpreted for this study.

In addition, 413 km of digital seismic and approx.8,000 km2 of multi-beam bathymetric data were collectedbetween 2004 and 2007 by the research vessel TCGÇubuklu of the TN-DNHO within the framework of ascientific project supported by the Scientific and Techno-logical Research Council of Turkey (TUBITAK-CAYDAG-104Y024; Figs. 1b, 2). Of this, the seismic data from theÇanakkale Strait and the Şarköy Canyon were acquired in2005 and 2006 (Fig. 1b). The seismic system consisted of a300-J seismic energy source, a single-channel hydrophonestreamer (receiver) and a recorder set at a 200-ms scan. Theship speed was about 4 knots, positioning being achieved bya D-GPS system. Part of this seismic data was previouslypublished by Ergin et al. (2007) and Gökaşan et al. (2008).

Seismic units were distinguished on the basis of seismicreflection configurations, reflection terminations and un-conformity surfaces (cf. Posamentier and Allen 1999; Coeand Church 2003). To extend unit boundaries across thestudy area, the boundaries of the seismic units were correlatedalong and between profiles using crosscuts. In this way, bothisopach maps of individual units and palaeo-topographicmaps were produced. The depths to and thicknesses of theseismic units were calculated using velocity approximationsof 1,500 m/s for seawater, 1,700 m/s for unit 1 and 2,000 m/sfor units 2 and 3, based on a seismic refraction study in theİstanbul Strait by Uluğ et al. (1987). With due considerationof previous seismic stratigraphic investigations, sedimento-logical analyses, and 14C datings on core samples from theÇanakkale Strait, the Marmara Sea and the Aegean Sea bySmith et al. (1995), Çağatay et al. (1998), Aksu et al. (1999,2002a), Hiscott and Aksu (2002), Hiscott et al. (2002),Yaltırak et al. (2002), Ergin et al. (2007), Eriş et al. (2007),Gökaşan et al. (2008) and İşler et al. (2008), as well asstudies on global marine isotope stages (MISs) and sea-levelfluctuations by Waelbroeck et al. (2002) and Lisiecki andRaymo (2005), the seismic units were additionally analyzedwithin a sequence stratigraphic context.

The multi-beam bathymetric data were collected in 2004,2005 and 2007 by the research vessel TCG Çubuklu of the

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TN-DNHO (Fig. 2). The system used (an Elac BCC MarkO) operates with 56 beams at 50 kHz. It has a range of2,500 m and employs a fan of echo-sounders covering anangle of approx. 120° below the survey vessel. The horizontalcoverage of the beams corresponds to three times the waterdepth. A D-GPS was used for positioning, the vessel speedbeing held at 8–10 knots. Part of the data covering theÇanakkale Strait and Saros Gulf were previously published byGökaşan et al. (2008) and Ustaömer et al. (2008).

Results

The seismic profiles from the Çanakkale Strait and itsextensions along the Marmara and Aegean seas display fourunits (units 1, 2, 3 and 4). In place of the three seismic units1–3 formerly identified by Gökaşan et al. (2008), furtherevaluation of the seismic data led to the recognition of fourseismic units, which are described in detail below.

Unit 1 comprises the uppermost deposits in the studyarea (Figs. 3a, 4, 5, 6a, b, 7). It unconformably overlies unit2, but may also lie directly on units 3 and 4 where units 2and 3 do not occur. Unit 1 has flat-lying parallel reflectionsterminating with onlap onto older units indicative ofaggradational deposition in relatively low-energy condi-tions. In the study area, the uppermost layer of unit 1 formsthe seafloor, thus probably constituting the modern depositsin the CS. On the open shelf on both sides of the CS, unit 1consists of a thin drape mostly less than 10 m thick(Figs. 3a, 4b, c, 5, 8a). However, within channel-likedepressions such as the CS, the Çanakkale Valley and theŞarköy Canyon, unit 1 is almost absent or restricted toisolated sediment drifts reaching 30 m in thickness(Figs. 3b, 5b, 6a, c, 8a). Gökaşan et al. (2008) noted that,in some places in the CS, unit 1 consisted of two subunitsseparated by an unconformity (Fig. 6b), which they relatedto hydraulic variations along the strait. In the mouths ofrivers discharging into the CS, unit 1 does not overlie, butlaterally abuts against unit 2 (Fig. 6a).

On seismic profiles, unit 2 is observed with differentreflection configurations in different places of the CS andthe nearby shelf areas. Thus, unit 2 can be divided intothree subunits, 2a, 2b and 2c. On the shelves of the Aegeanand the Marmara seas, unit 2 is characterized by sigmoidal-oblique reflectors indicating prograding delta deposition(Figs. 3, 5), whereas at the Marmara Sea exit of the CS ittakes the form of incised channel-fill sequences (Fig. 4b).The upper boundary of unit 2 is erosional.

On the NE Aegean Sea shelf, a delta sequence prograd-ing from south to north is interpreted as unit 2a (insets ofFigs. 3a, b, 8b). The latest topset-foreset transition of unit2a is observed at a depth corresponding to approx. 150 ms(~112 m; insets of Fig. 3a). Towards the CS, the thickness

of unit 2a progressively decreases from 45 m to less than10 m, the contours abruptly turning to the east into thesouthern exit of the CS (Fig. 8b). On the other hand, unit 2bconsists of a channel-fill sequence along an incised valleyat the Marmara Sea entrance of the CS, and anotherprograding delta sequence on the SW Marmara Sea shelf(inset of Fig. 4b, Fig. 5). Here, the latest topset-foresettransition of unit 2b is observed at approx. 130 ms(~97.5 m, Fig. 5; Smith et al. 1995). The maximumthickness of unit 2b is observed to the west of MarmaraIsland at 60 m (Fig. 8b). Off the Karabiga Peninsula,however, unit 2b is absent (Fig. 8b). Along the CS, unit 2claterally abuts against unit 1 where local rivers enter thestrait, the deposits constituting modern deltas in front of theriver mouths (Fig. 6a; Gökaşan et al. 2008). Here, unit 2creaches maximum thicknesses of up to 80 m, whereas alongthe other parts of the strait it is either absent or occurs as athin (<5 m thick) drape (Fig. 8b; Gökaşan et al. 2008).

Unit 3 is observed mainly on the Aegean and Marmarashelves. In these areas, unit 3 consists of high-amplitude,continuous parallel reflectors terminating with onlap anddownlap onto the erosional upper surface of unit 4 (Figs. 3,4c, 5), indicating aggradational deposition. Unit 3 iswidespread on both shelves, thickening towards the shelfbreaks where it may exceed 200 m (Fig. 9a).

In the CS, parallel-bedded sediment packages, whichdirectly overlie unit 4 in the form of onlapping successions,appear locally along the incised valleys (Fig. 7, insets ofFig. 9a and b). The upper boundary of this unit is alsoerosional, being overlain by units 1 and 2c (inset of Fig. 7).Since in the CS unit 3 deposits are observed only offYelkenkaya where the largest marine terrace outcrops (insetof Fig. 2), Gökaşan et al. (2008) interpreted it to be thesubmarine extension of this marine terrace without assign-ing any specific unit number to it. Due to its stratigraphicposition and reflection configuration, this deposit is nowinterpreted to be part of unit 3.

Unit 4 is the lowermost unit observed on the seismicprofiles, the only exception being an area between theKaraburun Promontory and Marmara Island (Figs. 3, 4, 5a,6, 7) where parallel reflectors of unit 4 unconformablyoverlie pre-Miocene basement, which also crops out on theKaraburun Peninsula and Marmara Island (Ergin et al.2007). The upper surface of unit 4 is erosional. Yaltırak etal. (2000) and Gökaşan et al. (2008) interpreted unit 4(formerly unit 3) as the submarine extension of the upperMiocene deposits observed on the adjacent shores.

On the NEAegean Sea shelf, the surface of unit 4 is incisedby a N-S-oriented channel located between the Bozcaada-Gökçeada islands and the Gelibolu-Biga peninsulas (Figs. 3b,9b). It deepens and widens from south (−40 m) to north(−160 m), the deeper parts being infilled by deposits of unit3, both overlain by unit 2a. The surface morphology of unit

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4 reveals a relict valley of a palaeo-river flowing from southto north (Fig. 9b). The Çanakkale Valley was subsequentlyincised into this succession along the northern margin of thepalaeo-channel. In the CS, the surface morphology of unit 4is characterized mostly by a wide, horizontal erosion surfacelocated at depths of −80 to −90 m. Only in the Nara Passagedoes the erosional surface transform into a narrow “V”-typevalley reaching depths of −113 m (Fig. 9b; Gökaşan et al.2008). Along the Şarköy Canyon, the depth at which unit 4appears increases dramatically to values exceeding 500 m

(Fig. 9b), the morphology suggesting the former existence ofa NE-SW-oriented narrow basin.

Discussion

Sequence stratigraphic analysis of seismic units

Depositional environments on the south-western shelf ofthe Marmara Sea were initially determined by Smith et al.

Fig. 4 a–c Interpreted seismic profiles from the SW Marmara Sea shelf at the northern exit of the Çanakkale Strait (modified after Ergin et al.2007; see inset map and Fig. 1b for location). M Multiple

Fig. 3 a, b Interpreted seismic profiles from the NE Aegean Sea shelfoff the southern exit of the Çanakkale Strait (modified after Çağatay etal. 1998; see inset map and Fig. 1b for location). a N-S-aligned

seismic profile. b E-W section across the Çanakkale Valley located inthe northern part of a. M Multiple

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(1995). According to these authors, three depositional unitsexisted in the area, consisting of (from top to bottom) drapesediments (unit A), basinward-prograding deltaic deposits(unit B) and flat-lying parallel-bedded subaqueous sedi-ments (unit C; Fig. 10). On the basis of sediment cores andseismic data from the southern shelf of the Marmara Sea,Ergin et al. (1997) and Aksu et al. (1999) subsequentlyrecognized that the upper surface of the subaqueoussediments (unit C of Smith et al. 1995) represented aregional unconformity which must have developed duringthe last sea-level lowstand in the Marmara Sea. Thisunconformity was interpreted as a sequence boundary byHiscott and Aksu (2002) which, according to several 14Cdates from the Marmara Sea available to them, must havedeveloped between ~23 ka and 12 ka b.p. Subsequently,Dolu et al. (2007), Eriş et al. (2007) and Çağatay et al.(2009), using 14C and ESR dating methods on sedimentsfrom the northern shelf of the Marmara Sea and İzmit Gulf,dated the uppermost unit (unit A of Smith et al. 1995) ashaving been deposited in the Marmara Sea since ~12 ka b.

p. (Fig. 10). Ergin et al. (2007), in turn, showed that thecross-stratified unit (unit B of Smith et al. 1995)was depositedbetween ~25 ka and ~11.5 ka b.p. (based on 14C datings).On the other hand, the marine terraces along the ÇanakkaleStrait (inset of Fig. 2), which may possibly be westernextensions of unit C of Smith et al. (1995), have been datedby the U/Th method by Yaltırak et al. (2002) as having beenemplaced between 200 ka and 52.7 ka b.p. Similarstratigraphic successions were also observed on the northernshelf of the Saros Gulf by Çağatay et al. (1998) and on thenorth-eastern shelf of the Aegean Sea by İşler et al. (2008).

The stratigraphic background outlined above is used inthe present study for a sequence stratigraphic analysis of theseismic units. As a consequence of the stratigraphiccorrelations, the upper two units deposited on the uncon-formity surface (units 1 and 2) are interpreted as belongingto the last deglacial and the present interglacial periods,whereas the lower two units (units 3 and 4), which underliethe erosional surface, must have been deposited before thelast glacial maximum.

Fig. 5 a, b Interpreted seismic profiles from the SW Marmara Sea shelf at the northern exit of the Çanakkale Strait (modified after Smith et al.1995 and Ergin et al. 2007; see inset maps and Fig. 1b for location). M Multiple

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Throughout the study area, which comprises the Çan-akkale Strait and its extensions on the Marmara Sea andAegean Sea shelves, the lowermost seismic unit, i.e. unit 4(Fig. 11), was formerly, together with unit 3, interpreted asrepresenting the basement of the strait (Yaltırak et al. 2000;Gökaşan et al. 2008). Unit 3, which unconformably overliesthe upper surface of unit 4, is now interpreted as a separateunit forming the shallow counterpart of marine sedimentsdeposited on the distal parts of the shelves and in the deepbasins of the Marmara and Aegean seas during MIS 3 andolder periods of the Pliocene-Quaternary era (Çağatay et al.1998, 2009; Okay et al. 1999). The aggradational characterof these sediments on the shelves suggests that unit 3possibly represents a transgressive systems tract. Alongthe CS, unit 3 occurs as a valley fill off Yelkenkaya,having also been equated with the marine terrace atYelkenkaya. The parallel reflectors of these sedimentsindicate a low-energy environment such as an estuary,rather than a river. This unit is therefore interpreted as atransgressive valley fill deposited in incised valleys duringMISs 7 and 5.

The erosional surface, which truncates units 3 and 4 andwhich is overlain by units 1 and 2, is interpreted as part ofthe regional unconformity surface identified in the shelfsequences of the Marmara and Aegean seas. This erosionalsurface must have formed during the sea-level lowstand ofthe last glacial period (MIS 2). As such, it must obviouslyhave been reshaped by a ravinement surface whichdeveloped during the last transgression across the glacial-

interglacial boundary. Since the erosional unconformitysurface is interpreted as a sequence boundary, units 2 and 1,which overlie the erosional surface, constitute a higher-order depositional sequence.

Unit 2 comprises a variety of delta deposits which occur,on one hand, in deeper water on the shelves of the Aegean(unit 2a; Figs. 3, 11) and Marmara (unit 2b; Figs. 5, 11)seas and, on the other hand, in shallow water off rivermouths discharging into the Çanakkale Strait (unit 2c;Figs. 6a, c, 7, 11). These deltaic deposits are interpreted torepresent the lowstand and highstand systems tracts of thesequence respectively. Units 2a and 2b on the Aegean andthe Marmara shelves respectively represent progradationalsequences associated with the late-lowstand systems tractduring initiation of MIS 1. After retrogradation of thetransgressional systems tract during MIS 1 from the shelfedges of the Marmara and Aegean seas to the coast of theÇanakkale Strait, unit 2c was deposited in the strait bydeltaic sedimentation in front of the rivers entering the straitduring the present sea-level highstand. There are nodeposits between units 2a and 2b, and units 2b and 2c,indicating that, during transgression, erosion (rather thandeposition) was the dominant factor along the strait and itsextensions.

Within this sequence stratigraphic framework, unit 1represents the modern sediment dispersal system of thestudy area, being deposited simultaneously with unit 2c. Onthe adjacent shelves, unit 1 is observed in the form of thinmud drape deposits (e.g. Fig. 3). Their aggradational nature

Fig. 6 a – c Interpreted seismic profiles from the Çanakkale Strait (modified after Gökaşan et al. 2008; see inset maps and Fig. 1b for location). MMultiple

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suggests that they represent the postglacial highstandsystems tract. Along the CS, this unit consists of mostlyreworked sediments indicative of high-energy conditions(Gökaşan et al. 2008).

The sequence stratigraphic analysis of the seismic unitsindicates that the sediment deposits found in the study arearepresent a succession from lowstand to highstand systemstracts, which overlie an erosional unconformity and hencebelong to a higher-order sequence formation. However,during development of the sequence, the CS has beenaffected mostly by erosional conditions due to its uniquemorphological position pivoted between two deep basins,the Aegean and Marmara seas. After development oflowstand deposits on the shelves, the rising water leveleventually overstepped the Çanakkale Sill, enabling waterto flow through the CS, thereby causing a massive erosionalevent on the sill. The erosional conditions probablycontinued until the sea-level highstand was reached onboth sides and the levels of the Aegean and Marmara seasbecame balanced. Once sea level stabilized, units 2c and 1began to be deposited under the present-day two-layeredcurrent system.

Erosion along the Çanakkale Strait and implicationsfor the latest Mediterranean-Marmara Sea connection

On seismic profiles from the Çanakkale Strait, an erosionalsurface truncating units 3 and 4 is seen to form the base ofthe channel (Figs. 4, 5, 6, 7). This clearly indicates that thepresent shape of the strait has been formed by an erosionalevent. In addition, faults along the CS, which wereobserved by Gökaşan et al. (2008; inset of Fig. 2), areanother important factor controlling the morphology ofthe channel. Sequence stratigraphic analysis suggests thatthe CS was excavated very recently and rapidly during thelatest transgressional phase. Recent erosion also occurredalong the Şarköy Canyon (Fig. 12; Ergin et al. 2007), thecanyon having probably been cut by the same erosionalevent which excavated the CS. Figure 13a illustrates themorphology of the CS immediately after the erosional eventformed the channel, while Fig. 13b shows the thickness ofsediments, which consist of units 2b, 2c and 1, depositedsubsequently.

The flow direction associated with the erosional event isan important issue in the context of models dealing with the

Fig. 7 Interpreted seismicprofile from the ÇanakkaleStrait (see inset map and Fig. 1bfor location). M Multiple

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latest Mediterranean-Marmara Sea connection. On thesouth-western Marmara Sea shelf, Aksu et al. (1999)recorded westerly and south-westerly prograding deltadeposits indicative of flow towards the Çanakkale Strait atthe end of the last glacial period. This was interpreted asevidence for the outflow of Black Sea water into theAegean Sea. A number of subsequent studies from theMarmara Sea and the Black Sea supported this idea (e.g.

Hiscott and Aksu 2002; Aksu et al. 2002a, 2002b; Hiscottet al. 2002; Kaminski et al. 2002; Mudie et al. 2002). Thisinterpretation is potentially also supported by the existenceof a delta on the Aegean Sea shelf (unit 2a; Figs. 3, 8b).However, this delta may just as plausibly be associated withthe Karamenderes River (Fig. 2), which extended onto theshelf during the last glacial and deglacial periods. It shouldbe noted here that the Çanakkale Valley extending

Fig. 8 a, b Distribution andthickness (m) of units 1 and 2in the Çanakkale Strait and itsextensions to the Marmara andAegean seas (compiled fromGökaşan et al. 2008 and thisstudy). KP Karaburun Peninsula

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northwards across the Aegean shelf west of the GeliboluPeninsula does not actually end in a substantially incisedcanyon near the shelf break. There is thus little evidence onthe Aegean side of the CS for strong and sustained erosionwhich one would expect if Black Sea water had filled theMarmara Sea and then overflowed through the CS into theAegean Sea.

An opposing model for the latest Mediterranean-BlackSea connection was proposed by Ryan et al. (1997) whosuggested that the latest sea-level rise in the Marmara andthe Black seas was caused by overflow of Mediterranean

waters through the Çanakkale Strait and the İstanbul Straitrespectively. According to this theory, the flow responsiblefor erosion along the CS was from south (Mediterranean) tonorth (Black Sea). Indeed, the existence of the ŞarköyCanyon on the shelf margin of the south-western MarmaraSea strongly mitigates in favour of northerly flow from theAegean Sea to the Marmara Sea. As pointed out above,there is no evidence for erosion on the Aegean shelf whichcan match the deeply incised Şarköy Canyon on theMarmara Sea side (Fig. 2; İşler et al. 2008; Ustaömer etal. 2008). In fact, there is no plausible alternative

Fig. 9 a Distribution and thick-ness (m) of unit 3 in the Çan-akkale Strait and its extensionsto the Marmara and Aegean seas(compiled from Gökaşan et al.2008 and this study). b Palaeo-topography (m) of the uppersurface of unit 4 in the Çanak-kale Strait and its extensions tothe Marmara and Aegean seas(compiled from Gökaşan et al.2008 and this study). The reddashed line indicates the axisand flow direction of thepalaeo-river. KR KaramenderesRiver

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explanation for the existence of the large Şarköy Canyon.For this reason, the “Ryan” model is favoured here.According to this model, the latest erosional event, causedby overstepping of the Çanakkale Sill by Mediterraneanwaters, excavated and shaped the Çanakkale Strait and theŞarköy Canyon. The shelf valley between the incisedchannels of the CS and the Şarköy Canyon is interpretedas representing a bypass channel (Fig. 13a). Seismic andbathymetric data indicate that progradation directions ofunit 2b variably changed from SE to NW, NW to SE, and Sto N (Fig. 5, inset of Fig. 8b), indicating basinwarddevelopment as previously proposed by Smith et al.(1995). Thus, the delta formation (unit 2b) and the channel

fill at the Marmara Sea exit of the Çanakkale Straitprobably took place when the strait was excavated byMediterranean waters (Fig. 13b).

Evolutionary stages of the Çanakkale Strait

For the model favoured here, four conditions are required.These are (1) the CS has been formed mainly during the lasttransgression, and after overflow of Mediterranean waterfrom the Aegean Sea into the Marmara Sea; (2) the incisionof the Şarköy Canyon was caused by the overflowingwaters which, at the same time, excavated the CS; (3) unit2b, consisting of the channel-fill deposits at the Marmara

Fig. 11 Stratigraphic columns of the NE Aegean Sea shelf, the Çanakkale Strait and the SW Marmara Sea shelf, showing the relationshipbetween Upper Miocene and younger deposits based on available seismic data

Fig. 10 Comparison of stratigraphic subdivisions according to different authors, including that of the present study

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Sea exit of the CS and the delta on the Marmara Sea shelf,was emplaced during the formation of the CS by sedimentseroded along the strait; and (4) the delta on the Aegean Seashelf (unit 2a) was deposited by the Karamenderes Riverprior to the excavation of the CS. The delta is thus a fluvialfeature unrelated to the formation of the CS. In thisscenario, the erosion forming the CS truncated units 4, 3and 2a, which were subsequently overlain by units 2b, 2cand 1 (Fig. 11).

The evolutionary model of the Çanakkale Strait illus-trated in Fig. 14 spans MISs 7–1 in the late Pleistocene-Holocene period. During MISs 7 and 5, marine watersinvaded the Marmara Sea (e.g. Yaltırak et al. 2002; Dolu etal. 2007; Çağatay et al. 2009), thereby creating the marineterraces recorded along the strait. The occurrence of theterraces is restricted to the eastern section of the strait

between the Nara Promontory and the Marmara Sea exit(inset of Fig. 2). This would suggest that, during MISs 7and 5, marine incursions entered from the Marmara Sea anddid not penetrate beyond the Nara Promontory, controlledby the Nara Fault (Fig. 14a). Before the formation of theÇanakkale Strait, another Mediterranean-Marmara Seawater connection must therefore have existed. Yaltırak etal. (2002) actually suggested that such a gateway waslocated at the boundary between the Gelibolu Peninsula andThrace (Fig. 14a; Bolayır Channel of Yaltırak et al. 2002).At present, the two regions are connected by a narrow andlow land plug (Fig. 2). When the gateway was open, theÇanakkale Basin, which formed a southern appendix to theMarmara Sea, was a shallow embayment or even a lagoon,in which shallow-marine sediments were deposited(Fig. 14a). According to Çağatay et al. (2009), during

Fig. 12 Seismic evidence forerosion along the Şarköy Can-yon (see inset map for location;after Ergin et al. 2007)

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MIS 3 a water connection between the Marmara Sea andthe Mediterranean was not established, the Marmara Seahaving thus become a lake with its shelf areas subaeriallyexposed. This environmental situation also continuedduring the last glacial period (MIS 2; e.g. Ergin et al.1997; Aksu et al. 1999; McHugh et al. 2008). During MIS2, a similar lowstand environment existed in the AegeanSea (Çağatay et al. 1998; İşler et al. 2008), major riversextending to the shelf edges on both sides. Of these, the

Karamenderes River reached down to the southern shelfbreak of the Saros Gulf where, at the end of the lastdeglaciation period, progressive delta deposits formed alate-lowstand systems tract (unit 2a; Fig. 14b).

During the following transgression, the rivers retreatedto the coast of the Çanakkale Strait, thereby endingdeposition of unit 2a (Fig. 14c). The rising Mediterraneanwaters created a ravinement surface by transgressionalerosion on the Aegean shelf before overstepping the

Fig. 13 a Palaeo-topographicmap illustrating the base of theÇanakkale Strait immediatelyafter the erosion forming thechannel of the strait. b Thick-ness of sediments depositedsince the Çanakkale Strait chan-nel formed

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shallowest point of the Çanakkale Sill, mostly following theactive and recently active fault zones, to flow into the deepMarmara Sea basin (Fig. 14c). The Nara Fault, in particular,controlled the flow direction, being responsible for thezigzag shape along the Nara Passage (Fig. 14c). On thebasis of dated sediments from the Marmara Sea (e.g.Çağatay et al. 2000; McHugh et al. 2008), Mediterraneanwaters invaded the Marmara Sea at about 12 ka b.p. Asshown by McHugh et al. (2008), sea-level rise in theMarmara Sea was rapid, the fast flow of the marineincursion excavating the Çanakkale Strait and incising theŞarköy Canyon in a very short time period. In the process,both marine and terrestrial sediments occurring along theflow path, including basement deposits of the strait, were

eroded and carried into the deep Marmara Sea basin. Sincesevere erosion was centred around the flow path, parts ofunit 3 located in previously formed incised valleys andelevated marine terraces survived (Fig. 14c). In the confinedzigzagging Nara Passage, the flow was particularly strong,thus explaining the exceptionally deep incision in this part ofthe strait. At the same time, the Şarköy Canyon was incisedinto the shelf break and slope of the southern Marmara Sea(Fig. 14c). When the water level of the Marmara Sea hadrisen above the depth of the Şarköy Canyon head, erosionended (Fig. 14d), leaving behind a relict canyon. Sedimentscontinued to be eroded in the strait, and now began to bedeposited on the Marmara Sea shelf, the deltaic depositsforming unit 2b (Fig. 14d). When sea level reached the

Fig. 14 Schematic evolutionary stages of the Çanakkale Strait. aDuring the interglacial periods of MISs 7 and 5, marine watersinvaded the Marmara Sea and the northern part of the Çanakkale Straitthrough the Bolayir Channel, a waterway between Thrace and theGelibolu Peninsula. b At the end of the last glacial period, theKaramenderes River extended onto the NE Aegean shelf andemplaced unit 2a as a late-lowstand delta deposit. c In the course ofthe last transgression in the Aegean Sea, the rising Mediterranean

waters eventually overstepped the Çanakkale Sill and flowed into theMarmara Sea, thereby excavating the Çanakkale Strait and the ŞarköyCanyon. d When the water level in the Marmara Sea had risen abovethe depth of the Şarköy Canyon head, erosion along the canyon endedand unit 2b began to be deposited. After sea levels on both side of theÇanakkale Strait stabilized, the modern two-layered current systemestablished itself in the strait, and units 2c and 1 began to be deposited

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present level at approx. 6 ka b.p. (McHugh et al. 2008), thetwo-layered modern current system was established, andunits 2c and 1 were deposited along the Çanakkale Strait, asextensively illustrated by Gökaşan et al. (2008).

Conclusions

Seismic data indicate that recent sediments deposited alongthe Çanakkale Strait and its extensions on the shelves of theMarmara and Aegean seas constitute a high-order sequencedeveloped on a regional erosion surface, which was formedduring MIS 2. Late-lowstand and highstand systems tractsare represented by units 2a and 2b, and units 2c and 1respectively. During the subsequent transgression, the risingMediterranean waters overstepped the Çanakkale Sill andflowed into the Marmara Sea, thereby excavating theÇanakkale Strait and the Şarköy Canyon.

According to this model, the Çanakkale Strait wasformed by an erosional event associated with the latestMediterranean-Marmara Sea connection, the sill depth inthe strait at the time of overflow being unknown, althoughit must have been shallower than the present level. Manyprevious studies, which mostly assumed a sill depth moreor less identical to its present depth, ignored the fact that itsdepth must have been lowered during overflow due toexcavation by the strong turbulent flow. Similar to theÇanakkale Strait, the sill depth along the İstanbul Strait isalso unknown, as this one too must have been lowered byerosion, as documented by Gökaşan et al. (2005b).

The formation of the Çanakkale Strait must have occurredwithin a short period of time, as suggested by the rapid riseof the water level in the Marmara Sea (McHugh et al. 2008).The overflow of Mediterranean waters into the Marmara Seabasin, and later into the Black Sea basin, must have beenwitnessed by early man as major local catastrophes, as aconsequence of which the environmental conditions in andaround these basins changed dramatically.

Acknowledgements This study was supported by The Scientific andTechnological Research Council of Turkey, Project No. TUBİTAK-ÇAYDAG-104Y024. We thank the officers and crew, as well as thescientists and technicians onboard the TCG Çubuklu and TCG Çeşmeof the Turkish Navy, Department of Navigation, Hydrography, andOceanography, for their unfailing assistance. We are also grateful forthe constructive comments of two anonymous referees and the journaleditor B.W. Flemming.

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