ORIGINAL

Erkan Gokasan Æ Oya Algan Æ Huseyin Tur

Engin Meric Æ Ahmet Turker Æ Mehmet Simsek

Delta formation at the southern entrance of Istanbul Strait(Marmara sea, Turkey): a new interpretation basedon high-resolution seismic stratigraphy

Received: 13 April 2004 / Accepted: 20 May 2005 / Published online: 12 April 2005� Springer-Verlag 2005

Abstract A detailed stratigraphic investigation based onhigh-resolution seismic profiles revealed that the delta atthe southern entrance of the Istanbul Strait consists ofthree parasequence sets. The lowermost parasequenceshows a sea-level stillstand at the beginning of the low-stand systems tract, possibly at 11,000±1,100 a b.p.,whereas the upper two parasequences reflect depositionat lowstand and during the subsequent transgression. Amaximum flooding surface may be developing on thedelta at present. The delta is located on the eastern sideof the Istanbul Strait canyon, with east–west progradingparasequences. The development of the delta is clearlyassociated with the Kurbagalı Stream on the east coast,and not with the Black Sea outflow through the strait.The geometry of the delta indicates a radial architecturearranged from northeast to southwest.

Introduction

The Marmara Sea constitutes an oceanographic link be-tween two large, semi-enclosed basins, theMediterranean

and Black seas (Fig. 1a). It has been demonstrated thatthe sea level and water exchanges between the Black andMediterranean seas during glacial–interglacial periodswere controlled by the two narrow and shallow straits ofIstanbul and Canakkale in the Marmara Sea. However,there are controversial views about sea-level changes in,and water exchange between these basins in the periodfrom the Last Glacial Maximum to the Holocene.

Ryan et al. (1997) suggested that the last transgres-sion in the Black Sea was fast and abrupt, having beencaused by invading Mediterranean waters through theIstanbul Strait to the subaerially exposed shelf area ofthe Black Sea. In a follow-up study, Ryan and Pitman(1999) refer to this event as being the biblical Noah’sFlood. By contrast, a persistent and strong outflow ofthe Black Sea during the last glacial–Holocene periodwas suggested by Aksu et al. (2002a, b), Hiscott et al.(2002), and Kaminsky et al. (2002), using multidisci-plinary data from the Marmara Sea.

One of the main arguments for strong and persistentBlack Sea outflow into the Marmara Sea is the apparentexistence of a delta at the southern exit of the IstanbulStrait (Hiscott et al. 2002). According to Hiscott et al.(2002), the delta (unit 2 delta) was formed by strongBlack Sea outflow at about 9–10 ka b.p. However, al-ready Alavi et al. (1989) and later Oktay et al. (2002)recognized this delta, and interpreted it to be the productof the Kurbagalı Stream.

In this short contribution, we present new data,which throw new light on this controversy. Since thestudy focuses mainly on the delta formation itself, otherdepositional units and faults observed in the area arebeyond the scope of this paper.

Materials and methods

A total of 380 km of high-resolution, single channelseismic data was analysed (Fig. 1b, c). The data werecollected by the Turkish Navy, Department of Navi-gation, Hydrography and Oceanography (TN-DNHO)

E. Gokasan (&)Natural Sciences Research Center,Yıldız Technical University,34349 Besiktas, Istanbul, TurkeyE-mail: egokasan@yildiz.edu.tr

O. Algan Æ E. MericInstitute of Marine Sciences and Management,Istanbul University, 34470 Vefa, Istanbul, Turkey

H. TurDepartment of Geophysics, Istanbul University,Avcılar, Istanbul, Turkey

A. Turker Æ M. SimsekDepartment of Navigation, Hydrography and Oceanography,Turkish Navy, cubuklu, Istanbul, Turkey

Geo-Mar Lett (2005) 25: 370–377DOI 10.1007/s00367-005-0215-4

during 1992, 1993 and 1995 cruises, using an analogueUniboom seismic system composed of a seismic energyunit (300 J), a transducer, a single channel hydro-phone streamer and an analogue recorder. Ship speedwas about 4 knot, and the recorder was set at a

100-ms scan on the recording paper. Positioning wasachieved by a Trisponder system. The depths of theseismic units were calculated using velocities of1,500 m/s for seawater and 1,700 m/s as an averagefor the soft sediments.

Fig. 1 a Location map showingthe Black, Marmara andMediterranean seas. bBathymetric map of the studyarea (modified after Gunnersonand Ozturgut 1974). c Seismicreflection profiles in the studyarea. Thick continuous linesdenote the sections shown inFigs. 4–6, dashed lines theprofiles in Figs. 2–5

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Results

Seismic stratigraphic interpretation

Examination of the seismic profiles revealed that thedelta sequence consists of three parasequence sets. Thelowest one is underlain by a high-amplitude horizontalreflection (Fig. 2a, b), which crosses the shelf areathrough the submarine canyon of the Istanbul Strait.The three parasequence sets must have been depositedunder the control of changing accommodation spaceand/or amount of sediment supply.

Parasequence set 1 (PS 1)

This parasequence set is the lowermost member of thedelta and is located on top of the horizontal high-amplitude reflector. The stacked foreset packages of thisset are represented by oblique clinoforms, prograding ina seaward direction. The clinoforms overly each otherwith a toplap ending (Fig. 2a, b). The top of the PS 1unit shows evidence of slight truncation. The obliquelyprograding configuration indicates that no verticalaccommodation space was available. In fact, the uppersurface was evidently eroded. Such a depositional pat-tern is typical for a stillstand during relative sea-level

Fig. 2 a, b Seismic reflectionprofiles and their interpretationfor the parasequence sets PS 1,PS 2 and PS 3 (see Fig. 1c forlocations). a Dataset extractedfrom Oktay et al. (2002),b dataset extracted fromGokasan et al. (2002)

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rise, and is observed mainly at the beginning of thelowstand systems tract or the falling stage systems tract(Coe and Church 2003). The youngest topset–foresettransition of PS 1 is observed at 60 ms (�47 m waterdepth).

Parasequence set 2 (PS 2)

The internal configuration of PS 2 consists of parallelreflectors on the landward side and clinoforms withsigmoidal–oblique reflectors on the seaward side(Fig. 2a, b). The sigmoidal (aggradational) progradationindicates that accommodation space was available abovethe PS 1. This kind of sigmoidal progradation can de-velop in either highstand or lowstand system tracts.

Since the PS 1 parasequence set represents the stillstandphase, PS 2 should represent the lowstand systems tract.The topset–foreset transition of the youngest parase-quence of the PS 2 is at about 50 ms (�40 m waterdepth). This is in agreement with the previous findings ofHiscott et al. (2002) and Oktay et al. (2002).

Parasequence set 3 (PS 3)

PS 3 represents the uppermost set of the delta formation.It overlies PS 2 with backstepping deposition (Figs. 2a,b, 3). In the landward part, up to a water depth of 9 m,the depositional system displays oblique–sigmoidalprogradation. Although the shallow part of PS 3has locally been dredged for marine transportation

Fig. 3 Seismic reflection profileand its interpretation for the PS3 parasequence set (see Fig. 1cfor location)

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(Prof. Dr. M.C. Barla, personal communication; Fig-s. 2a, 3), the inner reflection patterns can be seen in thedata shown in Fig. 3. The top of this parasequence set,which also forms the present seafloor, has been trun-cated and possibly shows current-drifted sediments.Retrogradation of the PS 3 clearly indicates that theamount of accommodation space was at first dramati-cally increased. However, the progradational parase-quences in the upper part indicate that accommodationspace decreased again, suggesting that the relatively ra-pid rise in sea level had slowed down or had come to an

end. Hence, PS 3 represents a transgressive to highstandsystems tract.

Location and physical properties of the delta

The delta is located to the east of the submarine canyonmarking the entrance of the Istanbul Strait, its base lyingat a water depth of about 40 m (Figs. 1b, 4). The offlapsof PS 2 along the delta front form the eastern slope ofthe canyon (Figs. 2a, 4). Towards the north, at thesouthern exit of the Istanbul Strait where the canyonpresents a sharp profile, the delta disappears. In thesouth, the delta deposits are replaced by shallow marinesediments (Fig. 4).

A NNW–SSE-oriented seismic profile crossing thedelta indicates a channel fill and offlaps passing into non-deltaic parallel-bedded sediments both in the north andthe south (Fig. 5). These northerly and southerly offlapsas well as the channel incision indicate that the mouth ofthe delta is located towards the east. The sediment sourcefor this delta must therefore have been the KurbagalıStream. The progradation directions of PS 2 and PS 3 aremainly NW–ESE and NW–S, respectively (Figs. 6, 7).The shape of the delta body shows a fan-like geometryformed by radial progradations. This geometry is

Fig. 4 The distribution of the delta based on latitudinal projectionsof seismic profiles from south to north (see Fig. 1c for locations)

Fig. 5 Seismic reflection profileand its interpretation for aNNW–SSE cross section of thedelta (see Fig. 1c for location)

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controlled mainly by sediment supply and, to a lesserdegree, by the current dynamics of the area.

Discussion and conclusions

A detailed sequence stratigraphic investigation indicatesthat the delta at the southern entrance of the IstanbulStrait consists of three parasequences. This is at variancewith previous interpretations, which claimed the delta toconsist of a single parasequence set (Hiscott et al. 2002;Oktay et al. 2002). The reflection configurations of theseparasequence sets indicate that the development of thedelta started with a stillstand, and that it continued todevelop during the lowstand and transgressive phases.At present the delta is probably in the maximumflooding surface stage. This situation implies a sequencecycle formed in the course of sea-level rise.

During the Late Glacial sea-level lowstand and up to12 ka b.p., the Marmara Sea was an isolated lake(Stanley and Blanpied 1980; Cagatay et al. 2000). Theshelf areas were exposed to subaerial erosion and subjectto delta progradations (Ergin et al. 1997; Aksu et al.1999). A series of delta deposits and barrier islands onthe southern shelf of the Marmara Sea were deposited atthe maximum lowstand during the late Pleistocene,being subsequently submerged in the course of theHolocene transgression (Aksu et al. 1999). Gradualtransition from lacustrine to marine conditions occurredat about 12 ka b.p., with the inflow of Mediterraneanwaters through the Canakkale Strait. The developmentof the ‘‘Kurbagalı’’ delta from PS 1 to PS 3 musttherefore be related to the sea-level rise, which occurredduring the last 12,000 years.

The lowermost parasequences (PS 1) indicate a pausein sea-level rise at about –47 m. Hiscott et al. (2002)pointed out that the delta was active from 10 to 9 ka b.p.Their age determination corresponds to the topset–fo-reset transition of PS 2 in this study. Therefore, thestillstand period indicated by PS 1 must have occurredbetween 12 and 9 ka b.p. An onshore well drilled in thehinterland of the delta in the Kurbagalı Stream valley

(Fig. 1c) was interpreted by Meric et al. (1991) and byUnsal et al. (2002) who recognized three unconformitiesin the well log (Fig. 8). The uppermost unconformity at�18 m indicates that a relative sea-level fall occurred at11,000±1,100 a b.p. After this unconformity, sea-levelrose continuously until the present day, the invasion ofMediterranean life forms documenting the inflow ofMediterranean waters (Unsal et al. 2002). This strati-graphic interpretation of the well log is in agreementwith the stratigraphy derived from the seismic data. Itstarts with a stillstand phase (PS 1) and continues with alowstand, followed by a transgressive systems tract (PS 2and PS 3). Thus, the PS 1 may be the coastal equivalentof the uppermost unconformity in the Kurbagalı Streamprofile which developed at 11,000±1,100a b.p. Thistiming is also compatible with the duration of sea-levelrise during the Younger Dryas.

Sea-level initially began to rise from �47 to �40 m,as documented by the increasing accommodation spaceand subsequent deposition of PS 2. Later, when PS 3was deposited, the rate of sea-level rise increased

Fig. 6 The distribution of the delta based on longitudinal projec-tions of seismic profiles from west to east (see Fig. 1c for locations)

Fig. 7 Locations and progradation directions of PS 2 (above) andPS 3 (below)

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dramatically. Thus, if any rapid rise in sea-level occurredin the Marmara Sea, be it by Black Sea outflow orotherwise, it must have happened during the depositionof PS 3, rather than of PS 2, as suggested by Hiscottet al. (2002). The increase in sea-level rise may also havebeen caused by tectonic subsidence of the northernMarmara region, as suggested in previous studies (Alaviet al. 1989; Oktay and Sakınc 1991; Gokasan et al. 1997;Demirbag et al. 1999; Oktay et al. 2002; Gokasan et al.2002).

The delta was previously interpreted to be the prod-uct of strong Black Sea outflow (Hiscott et al. 2002)because apparently no riverine sediment source existedto explain the southward-climbing topset–foreset tran-sitions of the delta. However, the results of the presentstudy clearly do not support this view. The seismicstratigraphic investigation presented here is in agree-ment with the rising sea-level in the Marmara Sea duringthe latest Pleistocene–Holocene. The location of the

delta and the progradational directions of the PS 2parasequence set clearly indicate that the source was theKurbagalı Stream. The delta is situated on the easternside of the Istanbul Strait canyon and off the mouth ofthe Kurbagalı Stream (Fig. 7). It can therefore not havebeen produced by Black Sea outflow through theIstanbul Strait because, in that case, it should have beendeposited along the axis of the canyon. The NNW–SSE-oriented seismic sections (Figs. 5, 6) show that theparasequences of PS 2 prograde to the north as well as tothe south, due to the radial geometry of the delta whensea-level stood at �40 m (Fig. 9).

Acknowledgements This study was supported by the ResearchFoundation of Istanbul University with Grants 75/15052003. Wethank the Turkish Navy, Department of Navigation, Hydrographyand Oceanography, and the personnel of the R/V T.C.G. cubuklufor their enthusiasm and care during seismic data collection.

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