ORIGINAL

Quaternary evolution of the Gulf of İzmit (NW Turkey):a sedimentary basin under control of the North AnatolianFault Zone

Erdal Dolu & Erkan Gökaşan & Engin Meriç &

Mustafa Ergin & Tolga Görüm & Hüseyin Tur &

Berkan Ecevitoğlu & Niyazi Avşar & Muhittin Görmüş &

Fatmagül Batuk & Buğser Tok & Oktay Çetin

Received: 20 February 2006 /Accepted: 16 January 2007 / Published online: 27 February 2007# Springer-Verlag 2007

Abstract The Quaternary evolution of the Gulf of İzmit,situated on the tectonically active North Anatolian FaultZone (NAFZ), was investigated using seismic reflection,paleontologic, and sediment textural data. On the basis ofseismic stratigraphic and sedimentologic-paleontologicinterpretations, four depositional units were distinguished

within the Plio-Quaternary sequence of the Gulf of İzmit.According to these data, Plio-Quaternary deposits suppliedfrom the northern terrestrial area started to accumulateduring a progradational phase, in a south-facing half-graben. A coarse-grained sedimentary unit prograding intothe gulf from the south since 200 ka B.P. indicates a

Geo-Mar Lett (2007) 27:355–381DOI 10.1007/s00367-007-0057-3

E. DoluInstitute of Marine Science and Management, Istanbul University,Vefa,34470 Istanbul, Turkey

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

E. MeriçModa, Hüseyin Bey Sok. 15/4, Kadıköy,34710 Istanbul, Turkey

M. ErginFaculty of Engineering, Department of Geological Engineering/Geological Research Center for Fluvial, Lacustrine,and Marine Environments (AGDEJAM), Ankara University,Tandogan,06100 Ankara, Turkey

H. TurFaculty of Engineering, Department of Geophysical Engineering,Istanbul University,Avcılar,34850 Istanbul, Turkey

B. EcevitoğluDepartment of Geophysics Engineering, Ankara University,Tandoğan,06100 Ankara, Turkey

N. AvşarEngineering-Architecture Faculty,Geology Department,Çukurova University,Balcalı,01330 Adana, Turkey

M. GörmüşEngineering-Architecture Faculty,Department of Geology,Süleyman Demirel University,Çünür,32100 Isparta, Turkey

F. BatukFaculty of Civil Engineering,Department of Geodesy and Fotogrametric Engineering,Yıldız Technical University,Beşiktaş,34349 Istanbul, Turkey

B. TokDepartment of the Navigation,Hydrography and Oceanography, Turkish Navy,Çubuklu,81647 Istanbul, Turkey

O. ÇetinScience-Lettres Faculty, Department of Physics, Abant İzzetBaysal University,Gölköy,14280 Bolu, Turkey

dramatic variation in the evolution of the gulf, with theinitiation of a new strike-slip fault of the NAFZ and acorresponding uplift of the Armutlu Peninsula in the southof the gulf. During the evolution of this fault from a wideshear zone consisting of right-stepped strike-slip faults andpull-apart basins to a localized principal fault zone,sediments were deposited under the influence of northerlyprograding terrestrial and shallow-marine conditions due torelative sea-level fluctuations in the Marmara Sea. Duringthis period, the Gulf of İzmit was invaded mainly byMediterranean and partly by Black Sea waters. In the latestglacial period, shallow areas in the gulf became subaeriallyexposed, whereas the central and western sub-basins of thegulf turned into lakes. The present evolution of the Gulf ofİzmit is controlled by the after effects of the new rupture ofthe NAFZ and the estuarine nature of the gulf environment.

Introduction

The North Anatolian Fault Zone (NAFZ), lying roughlyparallel to the Black Sea coast of Anatolia from theKarlıova region in the east to the northern Aegean Sea inthe west, is one of the longest and most active strike-slipfaults in the world (e.g., Ketin 1948, 1968; Şengör 1979;Şengör et al. 1985; Barka and Kadinsky-Cade 1988; Barka1992; Şaroğlu et al. 1992; Barka and Rellinger 1997;Yılmaz et al. 1997; Gürbüz et al. 2000; Fig. 1a). At the eastof the Marmara Sea, it bifurcates into two segments, thenorthern one extending into the Marmara Sea through anE–W-oriented narrow basin, the Gulf of İzmit (GI). Fartherwestward, the NAFZ continues along an E–W-orienteddeep trough, the Marmara Trough, which consists of fourbasins, i.e., the Çınarcık, Central, Tekirdağ, and Silivribasins, and three ridges, i.e., the Eastern, Central, andWestern ridges, which separate the basins (Gazioğlu et al.2002; Fig. 1b,c). To explain the evolution of these relativelydeep basins from the activity of the present dextral NAFZ,various models have been suggested that assumed theNAFZ to consist of en-echelon strike-slip faults formingpull-apart basins (Barka and Kadinsky-Cade 1988), tocomprise a negative flower structure (Aksu et al. 2000),or to be part of a transform–transform–transform type oftriple junction (Okay et al. 2000) in NW Anatolia.However, some recent studies carried out after the greatMarmara Sea earthquake in 1999 argued that the normalfaults that deepened the basins along the GI and MarmaraSea were inactive or less active today, and that a newmaster strike-slip fault cutting the whole basin from the GIto the western end of the Marmara Sea has been active forthe past 200 ka (İmren et al. 2001; Gökaşan et al. 2001,2003; Le Pichon et al. 2001; Gazioğlu et al. 2002; Kuşçu etal. 2002; Rangin et al. 2004; Şengör et al. 2005). This

indicates that this fault, which has been called the “MainMarmara Fault” by İmren et al. (2001) and Le Pichon et al.(2001), or the “New Marmara Fault (NMF)” by Gökaşan etal. (2003; Fig. 1b), developed neither the Gulf of İzmit northe Marmara Trough.

The Gulf of İzmit is an elongated basin located in thenortheastern corner of the Marmara Sea along the path ofthe western portion of the NAFZ (Fig. 1b–d). The GIconsists of three sub-basins, the Eastern, Central, andWestern basins (Fig. 1d). With 35 m water depth, theEastern Basin is the smallest and shallowest, beingconnected to the larger and deeper Central Basin by anarrow gorge. The maximum water depth of the CentralBasin is approx. 200 m. A northward-prograding delta (theHersek Delta or Hersek Peninsula) separates the CentralBasin from the westernmost basin (the Western Basin;Fig. 1d). The Western Basin forms the eastern edge of theÇınarcık Basin, which is the easternmost, largest anddeepest basin of the Marmara Sea (Fig. 1b–d). A submarinecanyon connects the approx. 200-m-deep floor of theWestern Basin to the 1,250-m-deep abyssal plain of theÇınarcık Basin. Therefore, in general, the GI appears to bean elevated trough relative to the Marmara Sea.

In addition to active tectonics of the NAFZ (Barka andKadinsky-Cade 1988; Barka and Kuşçu 1996; Alpar 1999;Altınok et al. 1999; Barka et al. 2000; Lettis et al. 2000;Gökaşan et al. 2001; Alpar and Yaltırak 2002; Kuşçu et al.2002, 2005), the present environment of the GI is controlledmainly by estuarine conditions in the gulf (Ergin and Yörük1990; Algan et al. 1999), which include sediment erosion,transportation, and deposition on the seafloor. Especially thelast major earthquake of 17 August 1999 strongly affectedthe deepwater and coastal morphologies of the gulf (Barka1999; Öztürk et al. 2000; Kuşçu et al. 2005).

Previous studies suggested that the GI has developedsince the Pliocene, and that very thick basin sediments weredeposited in the gulf during its evolution (Ketin 1968;Barka and Kadinsky-Cade 1988; Sakınç and Bargu 1989;Bargu and Yüksel 1993; Meriç 1995; Seymen 1995; Edigerand Ergin 1995; Koral and Eryılmaz 1995; Barka andKuşçu 1996; Bargu 1997; Alpar 1999; Altınok et al. 1999;Gökaşan et al. 2001; Alpar and Yaltırak 2002, 2003; Kuşçuet al. 2002, 2005; Çağatay et al. 2003; Polonia et al. 2004).The last 817 ka of this evolution has been dated by the ESRmethod on sediment samples from boreholes between theHersek and Kaba promontories (Çetin et al. 1995). Thetectonic history of the Gulf of İzmit was postulated to becontrolled by the present active faults in the area. Hence,active faults in and around the gulf were considered to be ofnormal or transtensional character in explaining the originof the depression of the gulf and the present dextral activityof the NAFZ. After the earthquake on 17 August 1999,however, it was found that a single dextral fault exists along

356 Geo-Mar Lett (2007) 27:355–381

the Marmara Sea and the Gulf of İzmit, as mentionedabove. Thus, a modification of current views of theevolution of the Gulf of İzmit and the NAFZ along thegulf is necessary.

Within this context, in the present study offshore seismicdata have been reinterpreted following a seismic strati-

graphic approach, also including data from ESR dating,sedimentology, and paleontology of drill-hole samples.Thus, new data in the form of isopach and paleo-topography maps for each depositional unit in the gulfwere produced in order to better understand the Quaternarytectonics and sedimentary evolution of the GI. The

Fig. 1 a–d Maps showing the location of the study area and itssurroundings. a Main fault zones (white lines) superimposed on thedigital elevation model of Anatolia and surrounding areas. b Activefaults along the northern segment of the NAFZ (NMF New MarmaraFault). NS Northern shelf, TB Tekirdağ Basin, CB Central Basin, SBSilivri Basin, ÇıB Çınarcık Basin, GI Gulf of İzmit, B Bosphorus,GMS Ganos Mountain system, KP Kocaeli Peninsula, AP Armutlu

Peninsula, WR Western Ridge, CR Central Ridge, ER Eastern Ridge,SS southern shelf, GS Gulf of Saros (based on Gökaşan et al. 2003). cFault map of the terrestrial areas around the Gulf of İzmit (modifiedfrom Emre and Awata 2003). Inset map shows the locations and figurenumbers of the seismic profiles from the gulf. d Borehole locationsand digital elevation model (reprocessed from Gökaşan et al. 2001) ofthe Gulf of İzmit and surrounding areas

Geo-Mar Lett (2007) 27:355–381 357

stratigraphic interpretation presented here also throws newlight on the effects of NAFZ activity during the time periodin question.

Data sources

A total of 165 km of single-channel high-resolution seismicprofiles were recorded in the Gulf of İzmit from onboardthe R/V Arar of Istanbul University (inset of Fig. 1c). Theseismic data had previously been interpreted mainly interms of structural aspects (Alpar 1999; Altınok et al. 1999;Gökaşan et al. 2001; Alpar and Yaltırak 2002, 2003). In thisstudy, these data were reinterpreted from a seismicstratigraphic viewpoint based on borehole data, and for thispurpose, isopach and paleo-topography maps of each unitwere produced.

An additional 380 km of seismic data, collected from theeastern part of the southern shelf of the Marmara Sea (insetof Fig. 1b) aboard the R/V TCG Çubuklu operated by theTurkish Navy Department of Navigation, Hydrography andOceanography (TN-DNHO), were used for the first time inthis study to correlate sedimentary units and faults in the

Gulf of İzmit to the southern shelf plain of the Marmara Seaoff the Armutlu Peninsula. Multi-beam bathymetric datafrom the GI had also been collected onboard the R/V TCGÇubuklu of TN-DNHO. These data have been used inseveral other studies focusing on the Marmara Sea and theGI (Ecevitoğlu 2000; İmren et al. 2001; Gökaşan et al.2001, 2003; Gazioğlu et al. 2002; Fig. 1b). Part of thebathymetric data (from the GI) were reprocessed for thisstudy using ArcGIS 8.3 software (Fig. 1d).

Altogether, 164 sediment samples were taken from nineboreholes drilled across the Gulf of İzmit (Fig. 1d) north ofthe Hersek Delta. These were previously subjected to grain-size, paleontologic and ESR-dating analyses, the resultingdata having been presented in Ediger and Ergin (1995),Çetin et al. (1995), and Meriç et al. (1995). Results of theseinterpretations were reevaluated and correlated with seismicdata in the present study.

Materials and methods

The seismic profiles of Istanbul University (inset of Fig. 1c)were collected using a 1.25-kJ spark array system with a

Fig. 1 (continued)

358 Geo-Mar Lett (2007) 27:355–381

maximum recording length of 300 ms (two-way-travel time).Positioning was by means of an integrated GPS with anaccuracy of ±20 m. To produce the isopach and paleo-topography maps, depths of the seismic units were calculatedusing velocities of 1.500 m/s for seawater and 2.000 m/s asan average for the basin deposits, corresponding to theaverage velocity of the Plio-Quaternary channel deposits ofthe Bosphorus determined by Uluğ et al. (1987).

Additional seismic data of TN-DNHO (inset of Fig. 1b)were acquired using a spark array system comprising aseismic energy unit (300–600–900 J), a transducer, a single-channel hydrophone streamer, and an analogue recorder.Positioning in this case was achieved by a Trisponder system,with a recording length of up to 500 ms (two-way-traveltime).

Multi-beam bathymetric data were collected using theswath system Elac BCC Marko, consisting of 56 beamsoperated at 50 kHz and with a range of 2,500 m. Thesystem employs a series of echo sounders mounted such asto cover approx. 120° below the survey vessel. Thehorizontal coverage of the beams is three times the waterdepth. DGPS was used for positioning, the vessel speedbeing held between 8–10 kn.

Sediment samples were taken from nine boreholesdrilled across the Gulf of İzmit (Fig. 1d). Eight of theboreholes were located within the gulf (B1–B8), whereasthe southernmost one (B9) was drilled on the HersekPeninsula (Fig. 1d).

Electron spin resonance (ESR) was used for sedimentdating (Çetin et al. 1995). This method was first applied inpractice by Ikeya (1975). Since then, there have beensubstantial contributions based on carbonate materials,bones, and quartz covering an age range of up to 2 Ma(e.g., Schwarcz 1994; Çetin et al. 1995; Ulusoy 2004).

In order to assess the depositional environments of thesedimentary facies observed in the borehole data, use wasmade of the global sea-level curve of Skene et al. (1998;Fig. 2).

Results

Sedimentology and paleontology

On the basis of grain size and microfaunal distribution inESR-dated sediments from nine boreholes, at least fivesedimentary units (borehole units, BU) were distinguishedin the GI. Field observations and stratigraphic correlationson land (Seymen 1995) suggest that surrounding Paleozoicand Mesozoic rocks of the Kocaeli and Armutlu peninsulasform the pre-Miocene basement (Triassic limestone) in thegulf region. The basement underlying the five units wasobserved only in the two northernmost boreholes (Fig. 3).

Two of these units (BU4 and BU2) consist mainly ofrelatively coarse-grained sediments into which two mudlayers of units BU5 and BU3 are intercalated, and atopmost layer of unit 1 representing recent deposition in thegulf. From bottom to top, the five units are:

Lower muddy unit (BU5)

This unit is characterized by a predominance of fine-grained sediments defining a pro-delta/offshore mud focuswith local occurrences of gravel-bearing layers indicatinghigher energy/turbiditic events. The lower muddy unitunderlying the coarse-grained deposits indicates brackish-marine intercalations of Black Sea and Mediterranean Seaorigin, respectively (Table 1). Based on sedimentologic

Fig. 2 Relative sea-level model over the last 540 ka (Skene et al.1998)

Geo-Mar Lett (2007) 27:355–381 359

interpretation of the longest borehole, which was drilleddown to 118.45 m below the surface (mbs) in the HersekDelta, a mud layer and overlying silty-slightly gravely mudand mud intercalation containing Mediterranean, andsubsequently Black Sea and Mediterranean foraminiferalfaunas exist at the base of borehole B9. The ages of thesedeposits were estimated as latest Pliocene-early Pleistocenebased on paleontologic evidence (Toker and Şengüler 1995;Fig. 3, Table 1). At approx. 55 mbs in borehole B9, ESRdatings of the pelecypods Cardium sp. and Dreissenapolymorpha suggest that deposition of mud unit 5 com-menced at 817±105 ka B.P., and continued until 693±126 ka under Black Sea brackish water conditions. Mudlayers consisting of intercalations of mud-silty and gravelymud deposited up to 199±22 ka B.P. contain foraminiferalfaunas of both Mediterranean and Black Sea origin(Table 1). The thickness of BU5 increases toward thesouthern coast of the gulf (Fig. 3). There is evidence of verythin drape or an absence of unit 5 in the northernmostboreholes (B1 and B2). The thickness of this unit increasesstrongly between boreholes B2 and B3. The base of the unitwas not reached in boreholes B3–B8, and not even in thelongest borehole (B9) located in the Hersek Delta.

Comparison of BU5 with the global sea-level curve(Fig. 2) suggests that, at this site, mud was deposited fromapprox. 700 to 200 ka B.P. under both transgressive andregressive conditions. The paleo-sill depths (<−120 m) ofthe Çanakkale Strait system (Yaltırak et al. 2000) wouldhave enabled Mediterranean waters to enter the MarmaraSea over a wide time span. The eustatic sea-level curvewould support a transgression-induced Mediterranean in-cursion lasting from about 275 until 200 ka B.P. However,the available data are insufficient to assess whether or notalso a Black Sea water incursion could have taken placeeither through the Bosphorus in the west or via a paleo-river (“Sakarya”) channel in the east.

Coarser-grained sediments with mud intercalations (BU4)

Sediments of BU4 are rich in terrigenous sand- and gravel-sized material, including eroded and rounded rock debris,indicative of high-energy conditions (Ediger and Ergin1995). This unit extends from about 200 to 180 ka B.P., andon the basis of the eustatic sea-level curve, representsregressive conditions (marine-brackish and coastal environ-ments; Table 1). This is also supported by the decreasing

Fig. 3 Sedimentologic and pa-leontologic interpretation of theborehole data from the Gulf ofİzmit (for location, see Fig. 1d).Modified from Ediger andErgin (1995). The dashedsquare indicates the site of theseismic profile on Fig. 5

360 Geo-Mar Lett (2007) 27:355–381

Tab

le1

Sedim

entologicandpaleon

tologicinterpretatio

nof

thebo

reho

ledataa

Grain

size

Env

iron

ment

Electronspin

resonance

(ESR)datin

g(ka)

Fossils:Fforaminifer,O

ostracod

,Ppelecypo

d,G

gastropo

d,Nnano

plankton

Determination

Coarsesediments

with

mud

intercalation

Marine-Mediterranean

0.5±0.2,

5.0±0.9,

6.6±0.7,

11.6±2.8,

12.2±1.9

(Çetin

etal.19

95)

F:Sp

irop

lectinella

sagittu

la,Textularia

sagittu

la,T.

trun

cata,Adelosina

cliarensis,

A.mediterran

ensis,Sp

iroloculinaan

gulosa,S.

excavata,S.

orna

ta,S.

tenu

iseptata,

Siph

onap

erta

aspera,S

.irregularis,C

ycloforina

julean

a,Lachlan

ella

bicornis,L

.und

ulata,

Massilin

asecans,Quinq

ueloculin

aberthelotia

na,Q.laevigata,

Q.seminula,

Miliolinella

subrotun

da,Trilo

culin

amarioni,Sigm

oilin

itacostata,

Dentalin

alegu

minifo

rmis,

Amph

icorynascalaris,Favulinahexago

na,Brizalin

aspathu

lata,Cassidu

linacarina

ta,

Rectuvigerina

phlegeri,Buliminaelon

gata,Reussella

spinulosa,

Valvulineria

brad

yana

,Stom

atorbina

concentrica,

Rosalinabrad

yi,Discorbinella

bertheloti,Lob

atulaloba

tula,

Cibicides

florida

nus,Plano

rbulinamediterran

ensis,Sp

haerog

ypsina

glob

ula,

Asterigerinata

mam

illa,

Non

ionella

turgida,

Aub

ignyna

perlucida,

Ammon

iacompa

cta,

A.pa

rasovica,

A.p

arkinson

iana

,A.tepida,

Cribroelphidium

poeyan

um,E

lphidium

aculeatum,E

.advenum

,E.complan

atum

,E.crispu

m,E.macellum,E.po

nticum

Recentsediments

(unit1)

O:Callistocytheresp.,Leptocytheresp.,Cyprideissorbyana

,C.torosa,Pon

tocytheresp.,

Trachyleberissp.,Cythereisjonesi,C

arinocythereiscarina

ta,C

.qua

dridentata,C

osta

batei,

C.edwardsii,Bosqu

etinasp.,Aurila

sp.,Urocythereisbrita

nnica,

Loxocon

chasp.,

Paracytherideapa

rallia,

Semicytherura

sp.,Xestoleberissp.

P,G:Nucula(N.)nu

cleus,Nuculan

a(N.)pernula,

Thracia

papyracea,

Ana

dara

(A.)diluvii

pertransversa,

Ostreasp.,Jago

niareticulata,

Divaricella

(L.)divaricata,Cardium

edule,

Mon

odacna

caspia

taman

ica,

Spisula(S.)subcarinatatriang

ula,

Dosinia

sp.,Venu

s(V.)

verrucosa,

Corbu

la(V.)gibb

a,Hyalin

asp.,Bittium

reticulatum

,Thericium

sp.,Tu

rritella

(T.)terebra,

Hydrobiastag

nalis,Pseud

amnicola

sp.,Trun

catella

(T.)albida

Coarsesediments

Coastal

14.6±1.6,

15.4±3.7

(Çetin

etal.19

95)

F:Aub

ignyna

perlucida,

Ammon

iapa

rasovica,Elphidium

complan

atum

,E.crispu

mUpp

ercoarse

unit

(unit2)

O:Cyprideistorosa,Pon

tocythereelon

gata,Urocythereissp.

G:Bittium

reticulatum

,Hydrobiasp.,Pho

smolleri

Mud

Marine-Mediterranean

24.8±3.7,

31.7±5.4,

33.7±

7.2,

35.2±8.1(Çetin

etal.

1995)

F:Quinq

ueloculin

alaevigata,

Q.seminula,

Lag

enamollis,Brizalin

aalata,

B.spathu

lata,

Buliminaaculeata,Rosalinabrad

yi,Lob

atulaloba

tula,Asterigerinatamam

illa,

Ammon

iacompa

cta,

A.pa

rkinsonian

a,A.tepida

,Elphidium

crispu

m,E.macellum

Upp

ermud

dyun

it(unit3)

O:Callistocytheresp.,Cyprideistorosa,Pon

tocythereelon

gata,Carinocythereissp.,

Bosqu

etinasp.,Loxocon

chasp.,Can

dona

parallela

pann

onica

P,G:Sp

isulasp.,Tellina

(M.)do

nacina

,Corbu

la(V.)gibb

a,C.(L.)mediterran

ensis,

Bittium

sp.,Trun

catella

(T.)albida

,Rissoina(S.)bryerea,

Rissoasp.,Pseud

amnicola

sp.,

Cyathara(M

.)alteun

ata

Mud

-coarse

sediment

intercalations

Coastal-m

arine-

brackish-

Mediterranean–

Black

Sea

186.0±20

.0,19

5.0±20

.0,

198.0±23

.0(Çetin

etal.

1995)

F:Ammon

iacompa

cta,

A.pa

rkinsonian

a,Elphidium

complan

atum

,E.crispu

mCoarsematerial-mud

intercalations

(unit

4)

O:Callistocytheresp.,Leptocytheresp.,Pon

tocytheresp.,Cythereissp.,Bun

toniasp.,

Aurila

sp.,Loxocon

chasp.,Can

dona

parallela

pann

onica

P,G:Mod

iolussp.,Dreissena

polymorph

a,Tu

rritella

(T.)terebra,

Hydrobiasp.

Geo-Mar Lett (2007) 27:355–381 361

Tab

le1

(con

tinued)

Grain

size

Env

iron

ment

Electronspin

resonance

(ESR)datin

g(ka)

Fossils:Fforaminifer,O

ostracod

,Ppelecypo

d,G

gastropo

d,Nnano

plankton

Determination

Mud

Marine-Mediterranean

199.0±22

.0,25

4.0±39

.0,

306.0±39

.0,32

0.0±37

.0(Çetin

etal.19

95)

F:Sp

iroloculinaexcavata,S.

orna

ta,Quinq

ueloculin

alaevigata,

Brizalin

aspathu

lata,

Cassidu

linacarina

ta,Buliminaelon

gata,Uvigerina

peregrina,

Rosalinabrad

yi,Hyalin

eaba

lthica,

Lob

atulaloba

tula,Cibicides

florida

nus,Asterigerinatamam

illa,

Non

ion

depressulum,Non

ionella

turgida,

Aub

ignyna

perlucida,

Ammon

iacompa

cta,

A.

parkinsonian

a,A.tepida

,Cribroelphidium

poeyan

um,Elphidium

aculeatum,E.ad

venu

m,

E.crispu

m,E.depressulum,E.macellum

Low

ermud

dyun

it(unit5)

O:Callistocytheresp.,Leptocytheresp.,Cyprideistorosa,Pon

tocytheresp.,Carinocythereis

quad

ridentata,

Costa

batei,C.edwardsii,

Aurila

sp.,Urocythereisbrita

nnica,

Loxocon

cha

rhom

boidea.,Semicytherura

sp.,Xestoleberissp.,Can

dona

parallela

pann

onica

P,G:Mod

iolussp.,Corbu

la(V.)gibb

a,Gastrocha

enasp.,Tu

rritella

(T.)terebra,

Hydrobia

stag

nalis,Pseud

amnicola

sp.,Rissoasp.

Siltymud

Brackish–

Chaud

ian–

Black

Sea

664.0±94

.0(Çetin

etal.

1995)

O:Can

dona

(C.)bu

rdurensis

P:Dreissena

rostriform

ispo

ntocaspia

Mud

Marine-brackish-

Mediterranean–

Black

Sea

693.0±12

6.0(Çetin

etal.

1995)

F:Non

ionsp.,Cribroelphidium

sp.

O:Leptocytheresp.,Loo

xoconcha

sp.,Hirscha

man

niasp.,Macrocyprissp.

P,G:Mod

iolussp.,Hydrobiasp.

Siltyandslightly

gravelymud

layer

Brackish-Black

Sea

817.0±10

5.0(Çetin

etal.

1995)

O:Loxocon

charhom

boidea,Can

dona

(C.)bu

rdurensis

P:Cardium

sp.,Dreissena

polymorph

aMud

Marine-Mediterranean

Early

Pleistocene

(Tok

erandŞeng

üler

1995

)F:Sp

iroloculinaexcavata,Quinq

ueloculin

aseminula,

Brizalin

aspathu

lata,Cassidu

lina

carina

ta,Buliminaelon

gata,B.margina

ta,Va

lvulineria

brad

yana

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tula,

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(?)

LatestPlio

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erandŞeng

üler

1995

)Nofossil

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cene

(Tok

erandŞeng

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1995

)F:Brizalin

aalata,

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lata,Cassidu

linacarina

ta,Buliminaelon

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ta,

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brad

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atulaloba

tula,Non

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atlantica,

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mediterran

ensis,Elphidium

macellum

O:Carinocythereissp.,Costa

edwardsii,

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dona

(C.)bu

rdurensis

P:Mysella

bidentata

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spelagicus,Discoasterbrou

weri,Reticulofenestrapseudo

umbilica,

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oemilian

alacuno

sa

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ifiedfrom

Schorniko

v(196

9),Yanko

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(198

7),Schütt(198

8),Yanko

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90),Altınsaçlı(199

3),Federov

(199

3),Taner

(199

5),Tok

erandŞeng

üler

(199

5),Gülen

etal.

(199

5),andMeriç

etal.(199

5)

362 Geo-Mar Lett (2007) 27:355–381

Fig. 4 Original and interpretedseismic profile obtained fromthe northern shelf of the CentralBasin of the gulf (modified fromDolu 2002). For location, seeinset of Fig. 1c. Note the intensedeformation by the NMF, whichis the new rupture of the NAFZin the Marmara Sea (based onGökaşan et al. 2003, and thisstudy), and the southerly deltadevelopment (inset) of unit SU4.T Toplap, M multiple

Geo-Mar Lett (2007) 27:355–381 363

thickness toward the shore (Fig. 3). In addition, theintercalated mud layers suggest an increased terrigenousinput even at times of sea-level lowering.

Upper muddy unit (BU3)

This unit was observed in only three boreholes along thesouthern slope of the gulf (Fig. 3), and covers a periodbetween 35.2±8.1 and 24.8±3.7 ka B.P. (Table 1). Based onthe sea-level curve (Fig. 2), BU3, which is characterized bya Mediterranean fauna, indicates mud deposition in the gulfduring lowering of sea level, i.e., immediately before theLast Glacial Maximum. The presence of Mediterraneanwaters in the Marmara Sea, and thus in the gulf, between 35and 24 ka B.P. has also been previously reported (Meriç etal. 1995; Kaminski et al. 2002; Çağatay et al. 2003).

Upper coarser-grained unit (BU2)

This unit consists of gravel, sand, and silt layers depositedin relatively shallow and higher-energy waters, or evensubaerially exposed coastal environments of the HersekPromontory during the lowstands of sea level at the end ofthe Last Glacial Maximum (14–16 ka B.P.; Table 1),immediately before the onset of the Flandrian transgression.

Recent sediments (BU1)

BU1 constitutes the uppermost layer in the boreholes. Mudlayers intercalated with sandy and gravely deposits con-taining a Mediterranean fauna constitute this unit, dated at12.2±1.9 to 0.5±0.2 ka B.P. (Table 1). It is thereforesuggested that sediments of BU1 were deposited in the gulffrom the beginning of the last interglacial period in theMarmara Sea, i.e., from about 12 ka B.P. (Çağatay et al. 2000)when Mediterranean waters invaded the Marmara Sea.Similar fossils were also observed by Kerey et al. (2004) inthe eastern part of the gulf, indicating that the gulf extendedtoward the east already at the beginning of the Holocene.

Seismic stratigraphy

Sedimentologic and paleontologic data indicate that rela-tively thick Pliocene-Quaternary sedimentary sequenceshave been deposited in the basin since the opening of theGulf of İzmit. Seismic profiles from the gulf (cf. inset ofFig. 1c) reveal four seismic units (seismic units, SU)distinguished by pronounced unconformities that can becorrelated in the entire GI.

Fig. 5 Original and interpreted seismic profile obtained between the Hersek and Kaba promontories in the Gulf of İzmit (modified from Dolu 2002).For location, see inset of Fig. 1c. Note shifting of the delta prograding direction from south to north during deposition of unit SU3. M Multiple

364 Geo-Mar Lett (2007) 27:355–381

Unit SU4

This unit is the lowermost member of the deposits observedon the seismic profiles from the gulf. It appears only in theshallower areas of the gulf such as the shelves, the EasternBasin, and around the Hersek Promontory. Along theWestern and Central basins, the seismics did not penetratedown to the upper surface of this unit. Unit SU4 showsobliquely prograding reflections with toplap terminations,indicating that it may represent southward-progradingdeltaic deposits (Fig. 4). Toplap progradations indicate astill-stand period of relative sea level at −67 m. Comparisonof SU4 with the global sea-level curve (Fig. 2) indicatesthat this unit may have been deposited during lowstandperiods that have occurred since 600 ka B.P. The chaoticreflections in the Eastern Basin are possibly due to theeffect of gas charging in the sediments (Kuşçu et al. 2005).The upper surface of SU4 has mostly been eroded (Fig. 5). Inthe Western and Central basins, the regional paleo-morphol-ogy of this surface indicates the existence of deep troughs(deeper than 250 m) separated by a 100-m-deep ridge at theHersek Promontory (Fig. 6). In the Eastern Basin, however,the upper surface of SU4 forms a plain at −50 m.

Unit SU3

SU3 unconformably overlies unit SU4 (Figs. 4 and 5). Thisunit was observed on all the seismic profiles from the gulf. Itmainly shows parallel reflections with some chaotic inter-calations (Figs. 4, 5 and 7). This indicates that SU3 is adeposit formed under low-energy conditions, as would beexpected in a lake, lagoon, or gulf environment. In theCentral Basin, divergent reflections are interpreted to becaused by normal fault activity (Fig. 7). In addition, chaoticreflections may have formed by mass movements triggeredby slope instabilities. The seismic profile off the Hersek

Delta (Fig. 5) shows downlap reflection terminationsindicating delta wedges of the Hersek intercalated withparallel reflections of SU3. This suggests that the presentnortherly progradation sedimentation of the Hersek Deltabegan during the deposition of this unit. The upper surface ofSU3 is truncated along the shelf (Fig. 7), whereas in theremainder of the gulf (basins) it is concordant with theoverlying SU1 (insets A and B of Fig. 7). The thickness ofthis unit increases strongly in the troughs of the Central andWestern basins (Fig. 8a). In these basins, the thickness ofSU3 must be more than 100 m, since its base (upper surfaceof SU4) was not recorded. By contrast, this unit occurs as athin layer in the Eastern Basin and around the Hersek Delta,with an average thickness of 10 m (Fig. 8a). Furthermore, themorphology of the upper surface of SU3 indicates that thetroughs at the upper surface of SU4 have become narrower(Fig. 8b). This unit is overlain by both SU2 and SU1.

Unit SU2

Unit SU2 unconformably overlies SU3. The reflectors in thisunit change from chaotic to prograding (Figs. 9 and 10, cf.also Figs. 4 and 5). The prograding reflections occurringalong the corridor between the NW Central Basin and thecenter of the Western Basin indicate riverine deposition fromthe northern side of the gulf into the Western Basin, andpossibly up to the Çınarcık Basin of the Marmara Sea(Figs. 10 and 11a). The chaotic reflections in the EasternBasin may indicate the existence of alluvial fan deposits inthis part of the basin. SU2 occurs over a very limited area inthe gulf, e.g., in the Eastern Basin, along the eastern edge ofthe Central Basin, and along a corridor on the northwesternshelf of the Central Basin toward the Western Basin(Fig. 11a). SU2 laterally shifts with SU3 in the WesternBasin (Fig. 10). As a result, and relative to Fig. 8b, the depthchart to the surface of SU2 (Fig. 11b) shows no changes in

Fig. 6 Upper surface paleo-topography of unit SU4 (modified from Dolu 2002). Present depth below sea surface is in meters. Note the deepWestern and Central basins, the total depths of which are presently unknown

Geo-Mar Lett (2007) 27:355–381 365

366 Geo-Mar Lett (2007) 27:355–381

most of the Central Basin, minor changes in the EasternBasin, and more substantial changes only in the transitionalcorridor between the Central and Western basins.

Unit SU1

Unit SU1 represents the uppermost deposits in the Gulf ofİzmit. A good example is shown in Fig. 10 (cf. also Figs. 4,5, 7 and 9). This unit was observed over the entire area ofthe gulf (Fig. 12). It mainly shows parallel reflectorsindicative of generally low-energy conditions, althoughsome chaotic intercalations do occur. These may beassociated with occasional tectonic events. Downlappingreflectors occur offshore of the Hersek and Çatal deltas

(Fig. 13, cf. also Fig. 9). These indicate that SU1 is ofshallow-marine and deltaic origin controlled by modernenvironmental conditions. On the shelf, this unit overliesSU3 with an erosional unconformity, whereas the transitionbetween the two units appears to be conformable in theCentral Basin (Fig. 7). It also overlies SU2 unconformably.No truncation surface can be observed in the internalreflectors of the SU1 unit. This indicates that SU1sediments have been continuously deposited since it wasinitiated. In addition, the top of SU1 forms the modernseabed. The seismic evidence therefore suggests that theseupper sediments have been deposited since the end of thelast glacial period. The thickness of these deposits is morethan 20 m in the deepest parts of the Western and Çınarcıkbasins, and more than 30 m off the Hersek and Çatal deltas(Fig. 12). Sediment clouds off the deltas visible on satelliteimages demonstrate the modern depositional activity ofSU1 sediments in the gulf (inset of Fig. 12).

Interpretation of seismic profiles from the southernMarmara Sea shelf

Seismic profiles from the southern shelf of the MarmaraSea are interpreted in this study mainly to understand

Fig. 7 Original and interpreted seismic profile from the Central Basinof the Gulf of İzmit (modified from Gökaşan et al. 2001). Divergentreflectors of unit SU3 toward the fault in the center of the gulf in insetA show the present major fault is located in the center of the gulf. The1 in inset B shows the boundary between units 2b and 3 (boundary ofthe last glacial and interglacial periods) of [Alpar and Yaltırak (2002,2003)], which is interpreted in the present study as an internal baselevel controlled by a tectonic depression, rather than a relative sea-level change. The 2 in inset B shows the boundary between units SU1and SU2 (boundary of the last glacial and interglacial periods) ofGökaşan et al. (2001) and this study. For location, see inset of Fig. 1c.NMF New rupture of the NAFZ in the Marmara Sea, M multiple

�

Fig. 8 a Sediment thickness map of unit SU3 (modified from Dolu 2002). Thickness values are in meters. b Depth to the upper surface of unitSU3 (modified from Dolu 2002) relative to present sea level

Geo-Mar Lett (2007) 27:355–381 367

tectonic aspects of the eastern part of the southern shelfand the Armutlu Peninsula. Four seismic units can beidentified to the west of the Armutlu Peninsula, whichmay relate with the deposits in the Gulf of İzmit(Fig. 14a). These units can be compared with previousobservations by Aksu et al. (1999), who used own seismicdata.

According to Aksu et al. (1999), their units 1 and 2represent deposition during the last glacial and post-glacialperiods, which may be compared with SU1 and SU2 in thegulf. The underlying unit 3 on Fig. 14a, which has high-amplitude continuous reflections, may have been depositedduring the highstand environment established before theglacial period. Thus, unit 3 on the southern shelf (inset A of

Fig. 9 Original and interpretedseismic profile from the east ofthe Hersek Delta in the gulf(modified from Dolu 2002, andDolu and Gökaşan 2003). Forlocation, see inset of Fig. 1c.The new rupture of the NAFZwas named as New MarmaraFault (NMF) by Gökaşan et al.(2003). NMF New rupture of theNAFZ in the Marmara Sea,M multiple

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Fig. 10 Original and interpreted seismic profile from the WesternBasin of the Gulf of İzmit, illustrating the draping nature of unit SU1covering all the older deposits (modified from Dolu 2002, and Dolu

and Gökaşan 2003). For location, see inset of Fig. 1c. Note progradingclinoforms of unit SU2. M Multiple

Fig. 11 a Sediment thickness map of unit SU2 (modified from Dolu 2002). Thickness values are in meters. b Depth of the upper surface of unitSU2 (modified from Dolu 2002) relative to the present sea level

Geo-Mar Lett (2007) 27:355–381 369

Fig. 14a) may be the western extension of SU3 in the Gulfof İzmit.

Unit 4 represents the lowermost sediments on theseprofiles. It appears along the southern coastal zone of theMarmara Sea (Fig. 14a). It has chaotic reflectors, and layersof the covering units terminate with onlap onto the uppersurface of unit 4. This unit extends deeper than the seismicpenetration and disappears offshore (Fig. 14a). Thus, it maybe the pre-Tertiary basement described onland by Gürer etal. (2003). This unit may be correlated with the Triassicbasement in boreholes from the northern side of the gulf.

High-amplitude reflectors of unit 3 diverge from the coasttoward the shelf plain. Similar reflectors of unit 3 withdivergent configuration exist at the shelf edge but withopposite direction (i.e., from the shelf edge to the shelf plain).These reflectors indicate subsidence in the center of the shelfplain, and the occurrence of a local basin along the shelf plain(Fig. 14a). Units 1 and 2 disappear near the edge of thesouthern shelf, whereas a high-amplitude continuous reflec-tor of unit 3 is uplifted and crops out on the seafloor (inset Bof Fig. 14a). In this area, reflectors of unit 3 are cut anduplifted by approx. 20 ms (15–20 m) where some steps onthe shelf floor exist (inset B of Fig. 14a). To the north ofthese steps, the otherwise even and continuous strata of unit3 are folded, and their tops are truncated (inset C ofFig. 14a). This truncation surface, which forms the presentseafloor, probably formed during the Würm glacial lowstandperiod (Ergin et al. 1997). In other places, however, thepresent seafloor undulates with the folded strata of unit 3(inset C of Fig. 14a). This indicates that the compressioneffect is presently still active, and that the steps on the shelf

floor and the slope at the shelf edge can thus be interpretedas reverse fault scarps. Folded sediments along the shelf edgeare also clearly seen to the west of İmralı Island (Fig. 14b).

Discussion

Faults in the Gulf of Izmit

An E–W-oriented strike-slip fault of the NAFZ (NMF inthis study and in Gökaşan et al. 2003) unconformablycutting all the basins and ridges along the gulf wasobserved as a submarine extension of the rupture of theearthquake that occurred on 17th August 1999 (Gökaşan etal. 2001; Alpar and Yaltırak 2002, 2003; Kuşçu et al. 2002,2005; Çağatay et al. 2003; Polonia et al. 2004). Theinterpretation of the seismic data from the Central Basinsupports the idea that the present trough of this basin hasbeen tilted toward the fault at the center, and that thesediments of SU1 and SU3 become thicker toward this fault(inset A of Fig. 7). This indicates that the fault along thecentral axis of the gulf is the main rupture of the NAFZ inthe study area. The seismic stratigraphy also indicates thatonly the upper part of SU3 and subsequent units have beenaffected by syn-tectonic sediment deformation caused bythis fault (Fig. 7; Gökaşan et al. 2001). Thus, the faultfollowing the central axis of the gulf is evidently a veryrecent feature. Seismologic evaluations and field studiescarried out after the earthquake of 17 August 1999 supportthis interpretation. The seismologic data suggest that theepicenter of the earthquake occurred in the center of the

Fig. 12 Sediment thickness map of unit SU1 (modified from Dolu 2002). Thickness values are in meters. The depth map to the surface of unitSU1 corresponds to the modern bathymetry of the Gulf of İzmit. The inset indicates actual delta development of the Hersek and Çatal deltas

370 Geo-Mar Lett (2007) 27:355–381

gulf, fault plane solutions indicating a pure strike-slip motion(Örgülü and Aktar 2001; Pınar et al. 2003). In addition, fieldobservations in the eastern and southern hinterlands of thegulf showed that the landward extension of the rupture liesalong the plain, rather than along the slopes of the basin,thereby mostly forming a strike-slip morphology along a

very narrow fault zone, with some minor normal motions atthe end of individual fault segments (Emre and Awata2003; Awata et al. 2003; Fig. 15a,b; cf. also Fig. 1c). Rightsteps on the E–W-oriented fault ruptures of the earthquakewere also observed. These data clearly indicate that thefault at the center controls the latest phase of the evolution

Fig. 13 Original and reinter-preted seismic profile from theeast of the Çatal Delta (WesternBasin) of the Gulf of İzmit(modified from Alpar 1999). Forlocation, see inset of Fig. 1c.The new rupture of the NAFZwas named the New MarmaraFault (NMF) by Gökaşan et al.(2003). Note the reverseddownlap of the Çatal Delta

Geo-Mar Lett (2007) 27:355–381 371

Fig. 14 a, b Original andinterpreted seismic profiles afrom the west of the ArmutluPeninsula, and b from the westof İmralı Island. M Multiple

372 Geo-Mar Lett (2007) 27:355–381

of the gulf, cutting it into two parallel sections, rather thanforming it. This suggestion is consistent with the new ideaof the NAFZ in the Marmara Sea (İmren et al. 2001; LePichon et al. 2001; Gökaşan et al. 2003; Rangin et al. 2004;Şengör et al. 2005).

According to Gökaşan et al. (2001), this fault has bothnormal and reverse components, due to its geometry anddextral motion forming restraining and releasing bends.Multi-beam bathymetry indicates that the northern bound-ary of the modern trough of the Central Basin consists ofWNW–ESE-oriented concave and NE–SW-oriented linearslopes (Fig. 16, cf. also Fig. 1d). This morphology suggeststhat the linear parts of this slope are controlled by right-

stepping strike-slip faults, as observed on land (Emre andAwata 2003; Awata et al. 2003), and that the concave partsare accommodational normal faults. This implies thedevelopment of a small pull-apart basin by the fault in theCentral Basin. This slightly modifies the seismic interpre-tation of Gökaşan et al. (2001) for the Central Basin, from asingle negative flower structure to strike-slip and normalfaults of a pull-apart basin (Fig. 16, cf. also Fig. 7). Someactive folds to the north of the strike-slip fault (cf. Fig. 7)have probably developed due to the restraining bends onthe fault controlled by the NE–SW orientation and E–W-directed dextral motion of the sliding blocks. There is moreevidence for compressional structures than for extensional

Fig. 14 (continued)

Geo-Mar Lett (2007) 27:355–381 373

ones in the Western Basin. On a profile from the WesternBasin (cf. Fig. 13), submarine delta deposits of the ÇatalRiver cover the older units. The downlapping lapout marksthe northern boundary of this delta. However, as clearlyvisible on Fig. 13, the downlap termination is reversed bythe new rupture of the NAFZ. This evidently indicates theeffect of the compressional component of the NAFZ in thegulf. A transpressional effect of the NAFZ in the ÇınarcıkBasin was studied in detail by Gökaşan et al. (2003). Inaddition, along the southern shelf, the effect of thecompressional component can be observed on the seismicprofiles from the west of the Armutlu Peninsula (Fig. 14).Due to this effect, a mature erosional surface on theArmutlu Peninsula, which developed up to late Miocene–Pliocene times around the Marmara Sea, has been uplifted(Emre et al. 1998; Fig. 17).

The above discussion demonstrates that the new ruptureof the NAFZ in the gulf actively controls the tectonics inthe region. The trough of the Central Basin is formed by achain of successive pull-apart depressions, which alsodisplay some active compressional structures such as thefolds in the Central Basin, and the reverse faulting in theWestern Basin and the west of the Armutlu Peninsula.Although this new fault controls the present tectonics in thegulf, a much wider transtensional shear zone or normalfault generation along the basin boundaries are required toexplain the entire depression of the gulf. Very steepnorthern and southern scarps delimiting the thick basindeposits of the gulf indicate that some normal andtranstensional faults exist along the coastline, which havebeen the subject of numerous other studies (e.g., Ketin1968; Barka and Kadinsky-Cade 1988; Bargu and Sakınç1989; Barka 1992; Bargu and Yüksel 1993; Seymen 1995;Smith et al. 1995; Wong et al. 1995; Bargu 1997; Armijo etal. 1999, 2002; Aksu et al. 2000; Barka et al. 2000; Lettis etal. 2000; Öztürk et al. 2000; Alpar and Yaltırak 2002, 2003;Gürer et al. 2003). Some of these faults may have initiatedthe development of the gulf, and are today observed asinactive or less active faults along the coastline (Emre andAwata 2003; Awata et al. 2003; Figs. 1c and 15a). Similarinactive or less active normal faults have been recordedalong the boundaries of the Marmara Sea Basin (İmren etal. 2001; Le Pichon et al. 2001; Gazioğlu et al. 2002;Gökaşan et al. 2003; Rangin et al. 2004).

Correlation of the seismic stratigraphy, sedimentology, andpaleontology

One of the seismic profiles located between the Hersek andKaba promontories (Fig. 5) more or less lines up with theboreholes shown in Fig. 1d. However, no direct correlationbetween the boreholes B1, B2, B8, and B9 and the seismicdata is possible because the profile does not reach up to

these locations. Thus, the Triassic basement of the gulfobserved in boreholes B1 and B2, and older sediments(dated at 817±105 ka B.P. and before) in borehole B9(Figs. 3 and 5) cannot be directly correlated with the unitsobserved on the seismic data.

In this comparison, SU4 based on seismic interpreta-tion corresponds to BU5 based on borehole data (Figs. 3and 5). The depths to the top of the two units demonstratethis. In addition, deposits of BU5 become younger fromnorth to south between boreholes B3 (693±126 ka) andB6 (199±22 ka), as shown by the ESR dates. Thiscorrelates well with the southward-prograding reflectionsof SU4. Thus, between approx. 700 and 200 ka B.P., adelta developed from the northern coast of the gulf underboth transgressive and regressive sea-level fluctuations(Skene et al. 1998; Fig. 2). From this point of view, thetoplap termination of SU4 may indicate lowstand periods inthe sea-level curve either at 340 ka or at 250 ka B.P. The ESRdating of BU5 in borehole B7 (320±37 ka), which appears atthe same level as the younger part of BU5 (199±22 ka), isinterpreted to indicate that the older part of BU5 wastectonically uplifted by the fault observed on the seismicprofile (Fig. 5).

BU5 is overlain by BU4. Coarse-grained materialconsisting of a sand layer overlain by gravels marks theboundary between BU4 and BU5 (Figs. 3 and 5). Towardthe north of BU4, these sediments are laterally replaced bya muddy layer. This stratigraphy correlates well with SU3on the seismic profile, where coarse-grained material ofBU4 is correlated with northerly downloaded reflections ofSU3 interpreted as pro-delta sediments of the Hersek Delta.The muddy sediments of BU4 correspond to the parallelreflections in the northern part of SU3 (Figs. 3 and 5).

In the southern boreholes, BU4 is overlain by muddysediments of BU3. However, since the seismic data do notresolve the boundary between BU4 and BU3 in this area, weassume that SU3 also includes BU3 (Figs. 3 and 5). Accordingto the ESR datings of BU3 and BU4, SU3 began to bedeposited during the sea-level highstand at 200 ka B.P. (Skeneet al. 1998; Fig. 2), and this continued during the followingregressive and transgressive periods between 200 and 35 kaB.P. This interpretation indicates that delta development fromthe north ended, and a new deltaic succession began toprograde from south to north with the development of theHersek Delta at approx. 200 ka B.P. Paleontologic data indeedindicate that a Mediterranean fauna existed in the gulf until35 ka B.P. (Table 1). Ever since the conformable stratigraphybetween SU1 and SU3 in the Central Basin (Fig. 7), this sub-basin of the gulf has never drained completely during any ofthe regressive periods in this time interval. Coarse-grainedsediments of BU2, and the youngest sediments depositedsince 12 ka B.P. (BU1), correlate well with the chaoticreflections of SU2 and with the uppermost unit (SU1) based

374 Geo-Mar Lett (2007) 27:355–381

on seismic interpretation (Figs. 3 and 5). Thus, SU1 and SU2,which correlate with BU1 and BU2, are interpreted assediments deposited in the gulf during the Late GlacialMaximum and the subsequent post-glacial period.

This stratigraphic sequence can be partly correlated withthat found in other coastal and offshore areas of the gulf(Fig. 18; cf. Bargu and Yüksel 1993; Gökaşan et al. 2001;Alpar and Yaltırak 2002, 2003; Kuşçu et al. 2002; Çağatay

Fig. 15 a, b Surface ruptures ofthe 17th August 1999 earth-quake, a in the Çatal Delta, andb on the east coast of the gulf(modified from Emre and Awata2003, and Awata et al. 2003)

Fig. 16 Right-stepped faultsand pull-apart basins of thenew rupture of the NAFZ(NMF) in the gulf (reprocessedfrom Gökaşan et al. 2001)

Geo-Mar Lett (2007) 27:355–381 375

et al. 2003). Alpar and Yaltırak (2002, 2003) used the sameseismic data as that of the present study. Although theirseismic interpretation differs in some details from thatpresented here, e.g., in delineating the boundaries of theseismic units, which resulted in a different calibration withseismic and borehole data, we can nevertheless partlycorrelate our interpretations with their results. Since thefolded sediments on the seismic profiles were interpreted asunit 1 by Alpar and Yaltırak (2002), this unit may becorrelated with SU4 of our study (Fig. 18). In addition, thetiming of unit 2a of Alpar and Yaltırak (2002) is identicalwith that of SU3 and SU4. As a result, unit 2a alsocorrelates with SU3 (Fig. 18). Units 2b and 3 of Alpar andYaltırak (2002, 2003) correlate well with SU1 and SU2 ofour study, but the boundary between these units has beeninterpreted differently. They followed an unconformitysurface, which we place within SU1, assuming that thissurface was the boundary between their units 1 and 2a(Alpar and Yaltırak 2002, 2003). However, as we havepointed out above, this unconformity surface is the resultof an internal lowering of the base level caused bytectonic activity (inset B of Fig. 7). This new interpreta-tion is also supported by the fact that this unconformitysurface does not correlate with any other unconformity inthe gulf.

On the other hand, our interpretations correlate very wellwith the results of Çağatay et al. (2003). These authorsdistinguished five units in the southern coastal and offshoreareas of the gulf. Their lowermost unit is a sedimentarysequence occurring between two erosional unconformities. Itbegins with pebbly sand at the base, and is overlain by mud,pebbly sand and sand layers containing a Mediterraneanmollusk fauna (Çağatay et al. 2003). A 14C date from the topof this unit gives an age of 35,750 years B.P. They alsocorrelated this unit with some marine terraces identified inprevious studies, which were deposited between 130–260and 53–210 years B.P. along the Gulf of İzmit and theMarmara Sea coasts (Sakınç and Bargu 1989; Paluska et al.1989; Görür et al. 1997; Sakınç et al. 1999; Yaltırak et al.2002). This unit may actually correlate with SU3 of ourseismic interpretation. Thus, according to the study ofÇağatay et al. (2003) and our study, during the time periodfrom 200 ka B.P. (based on the ESR dating of Çetin et al.1995) to the last glacial period, the Marmara Sea and theGulf of İzmit were inundated by Mediterranean waters.Çağatay et al. (2003) observed that unit 2, which consists ofcoarse-grained material, was possibly deposited from 36 to10 ka B.P., which coincides with the later part of the lastglacial period. Unit SU2 corresponds well with this, eventhough deposition of SU3 is interpreted to end after 24.8±

Fig. 17 Digital elevation modelof the Gulf of İzmit and sur-rounding areas, viewed from thewest. The solid line illustratesthe new rupture of the NAFZ,and dashed lines show theboundary faults of the basin

Fig. 18 A comparison of stratigraphic units in the Gulf of İzmit as interpreted in this and in previous studies

376 Geo-Mar Lett (2007) 27:355–381

3.7 ka B.P. on the basis of ESR dating (Çetin et al. 1995).According to Çağatay et al. (2003), an organic-rich mudlayer marks the boundary between units 2 and 3. Two 14Cdates from this layer suggest that units 3–5 have beendeposited since the latest Mediterranean water inflow intothe Marmara Sea at 12 ka B.P. (Çağatay et al. 2000). The

three units of Çağatay et al. (2003) thus correlate with SU1of our study.

Gökaşan et al. (2001), who used the same seismic data asthose of the present study, and Kuşçu et al. (2002)distinguished two depositional units in the gulf. Both studiesare based on similar interpretation procedures, and both

Fig. 19 a–d Stages of the tec-tonic evolution of the Gulf ofİzmit, based on Figs. 1d, 6, 8b,and 11b

Geo-Mar Lett (2007) 27:355–381 377

conclude that the deposits of the gulf consist of an upper unitcomprising horizontal parallel reflectors and a lower unitconsisting of chaotic reflectors. They conclude that the upperunit has been deposited since the beginning of the Holocene,and that the lower one was thus Pleistocene in age (Gökaşanet al. 2001; Kuşçu et al. 2002). This upper unit perfectlymatches SU1, the remainder of our units corresponding withtheir lower unit (Fig. 18).

One of the first studies combining offshore seismic datawith coastal information from the gulf was that of Barguand Yüksel (1993). They distinguished four units in thegulf. They also attempted to draw a first isopach map oftheir uppermost unit using two seismic profiles from thegulf. The boundaries of the seismic units identified alongthe northern shelf of the Central Basin mostly fit ourinterpretation (Fig. 4), and their units 2 and 3 are wellcorrelated with SU1 and SU2 (Fig. 18).

Tectonic history of the basin and effect of the NAFZ

In the north (Kocaeli Peninsula) and the south (ArmutluPeninsula), the GI is delimited mainly by the Paleozoic andMesozoic basement. It contains a very thick succession ofbasin sediments. This situation basically indicates that thegulf evolved largely as a depressional area. Onland scarsfollowing the coastline of the gulf may be interpreted asnormal and transtensional faults of this depression.

There are no seismic and borehole data available that reachdown to the basement of the gulf. There is thus no directevidence to outline the basement morphology. However, theborehole data do give some indirect evidence of the basementmorphology of the GI. Boreholes B1 and B2, located in thenorth, reach the Triassic basement at −26 and −50 m,respectively, whereas the southernmost borehole B9 ends at−118 m without reaching the basement (Fig. 3). The knowndepth of the Triassic surface in the gulf suggests that themorphology of the Triassic basement of the gulf may be asouth-facing asymmetric half-graben (Fig. 3). Seismicprofiles from the Tekirdağ Basin reveal a similar half-grabenstructure for the western Marmara Sea (Okay et al. 1999;Gökaşan et al. 2003). Using these data for basin modeling,Gökaşan et al. (2003) concluded that the Marmara Basin wasinitiated as a south-facing half-graben. Since the GI formsthe eastern edge of this basin, it may also have been initiatedas a south-facing half-graben (Fig. 19a). However, the uppersurface of SU4 (Fig. 6) indicates that during deposition ofthis unit, a strike-slip fault consisting of right-stepped, NE–SW-oriented en-echelon faults developed in the gulf, thesebeing possibly related to the initiation of the NAFZ in theMarmara Sea (Fig. 19b). Previous studies suggested that inthe course of the last 200 ka, a new rupture of the NAFZformed in the GI and the Marmara Sea (İmren et al. 2001;Gökaşan et al. 2001, 2003; Le Pichon et al. 2001; Şengör et

al. 2005). This coincides with the end of the period in whichSU4 was deposited. In addition, the sediment sources of thegulf changed from a southerly prograding delta to thenortherly prograding Hersek Delta and shallow-marinesedimentation marking the begin of SU3 (Fig. 5). A changein depositional regime at approx. 200 ka B.P. has beenreported for the Gulf of Saros located on the path of theNAFZ (Çağatay et al. 1998; Fig. 1b). This indicates that withthe development of the new rupture of the NAFZ, theArmutlu Peninsula may have started to rise. In this period,the Eastern, Central, and Western basins were initiated aspull-apart basins, and the Hersek Promontory and the gorgebetween the Eastern and the Central basins remained at arelatively higher elevation, as they are situated along the pathof the faults (Fig. 19b). On the other hand, the upper surfaceof SU3 shows that the sub-basins of the gulf becamenarrower (Fig. 8b). This indicates that during deposition ofSU3, the shear zone of this new fault may have been locallyconfined, and the orientation of faults of the previous en-echelon system was closely aligned in an E–W direction(Fig. 19c).

As mentioned above, onland extensions of these NE–SW-oriented faults in the north and south of the area (Kocaeli andArmutlu peninsulas) were identified as inactive or less activefaults in field studies (Emre and Awata 2003; Fig. 1c). In thisperiod, the pull-apart basins mainly formed the present shapeof the GI. Multi-beam bathymetric data for the upper surfaceof SU1 indicate that the present E–W-oriented strike-slipfaults unconformably cut all previously formed basins andridges in the gulf, its western extension also unconformablycutting the basins and ridges in the Marmara Sea (Fig. 19d,cf. also Fig. 1c). During this stage, the principal slip zone ofthe fault appeared, and the evolution of the new rupture ofthe NAFZ in the older extensional basin of the gulf wascompleted. This course of events outlines the evolution of astrike-slip fault system in an extensional basin, whichresulted in the collapse of the basin during the generationof the strike-slip fault. The system thus evolved from a largershear zone in front of en-echelon faults to a localized, singlerupture representing the new rupture of the NAFZ (NMF).

With the evolution of the NMF in the Marmara Sea and thenorthern Aegean Sea, the present restraining bend with anactive NW–SE-directed compressional stress formed in theMarmara Sea and northeastern Aegean Sea (Emre et al. 1998;Kahle et al. 1998; Boztepe-Güney et al. 2001; Yaltırak et al.2002; Gökaşan et al. 2003; Ocakoğlu et al. 2004, 2005;Fig. 1b). This restraining bend has reversed the southern partof the NAFZ in the Marmara Sea, as observed on presentand previous seismic profiles (Okay et al. 1999, 2000; İmrenet al. 2001; Yaltırak 2002; Gökaşan et al. 2003). With thiscompressional effect, the southern portion of a matureerosional surface, which is evident over the entire terrestrialarea around the Marmara Sea, has been uplifted from the GI

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to the Ganos Mountain system (Emre et al. 1998; Yaltırak etal. 1998, 2000, 2002; Gökaşan et al. 2001, 2003; Fig. 17).Folds and reverse faults observed on the seismic profilesfrom the Gulf of İzmit (Figs. 7 and 13), and also on seismicprofiles from the west of the Armutlu Peninsula (Fig. 14a,b),are possibly also related to this compressional effect.

Conclusions

On the basis of seismic stratigraphy, sedimentologic, andmorphologic data interpretation, the following conclusionson the evolution of the Gulf of İzmit can be drawn.

The gulf began to develop in late Pliocene times as asouth-facing half-graben that subsequently filled withPliocene–Quaternary sediments. In the central section,sediment infill initially proceeded in form of a southward-prograding delta fed from the high-relief areas of theKocaeli Peninsula in the north.

Since about 200 ka B.P., sedimentation switched to asoutherly source and the Hersek Delta began to progradeinto the gulf as the Armutlu Peninsula was uplifted bycompressional effects associated with the renewed faulting.This switch in sediment source was induced by the newrupture of the NAFZ. The new fault evolved from a largershear zone consisting of en-echelon strike-slip faults andpull-apart basins into a localized principal fault zone,changing the initial half-graben geometry of the gulf intoits present complex morphology.

In the course of its evolution, the Gulf of İzmit wasvariably invaded by Mediterranean and Black Sea waters.Although inflow of Mediterranean waters dominated, theoccurrence of Black Sea foraminiferal faunas, and alsofreshwater and brackish water faunas, document occasionalBlack Sea influence. It is not clear whether the connectionwith the Black Sea occurred in the İzmit Channel followingthe path of the Sakarya River, or via the Bosphorus.

Acknowledgements This paper is based on the MSc thesis of ErdalDolu. We gratefully acknowledge the captains and crews as well as thescientists and technicians of the R/V Arar of Istanbul University, andthe R/V TCG Çubuklu of the Turkish Navy, Department ofNavigation, Hydrography and Oceanography, for their help duringthe bathymetric and seismic surveys. We thank Dr. Timur Ustaömerfor his very valuable criticism. We also acknowledge the helpfulcomments of two anonymous referees and the editor of the journal, Dr.Burg Flemming, to improve the paper.

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