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Marine and Petroleum Geology xxx (2011) 1e12

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Marine and Petroleum Geology

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Acoustic evidence of shallow gas accumulations and active pockmarks in the _IzmirGulf, Aegean sea

Derman Dondurur a,b,*, Günay Çifçi a, Mahmut Göktu�g Drahor b,c, Süleyman Coskun a

aDokuz Eylül University, Institute of Marine Sciences and Technology, Bakü Street, No. 100, 35340 _Inciraltı, _Izmir, TurkeybDokuz Eylül University, Center for Near Surface Geophysics and Archaeological Prospection (CNSGAP), Tınaztepe Campus, 35160 Buca, _Izmir, TurkeycDokuz Eylül University, Engineering Faculty, Department of Geophysics, Tınaztepe Campus, 35160 Buca, _Izmir, Turkey

a r t i c l e i n f o

Article history:Received 9 November 2010Received in revised form19 April 2011Accepted 5 May 2011Available online xxx

Keywords:Gas seepagesGas chimneysPockmarksMud diapirsProtracted seep modelFault-associated seepage

* Corresponding author. Dokuz Eylül University, InsTechnology, Bakü Street, No: 100, 35340 _Inciraltı, I2785565; fax: þ90 232 2785082.

E-mail address: derman.dondurur@deu.edu.tr (D.

0264-8172/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.marpetgeo.2011.05.001

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a b s t r a c t

Based on high-resolution Chirp seismic, multibeam bathymetry and side scan sonar data collected in the_Izmir Gulf, Aegean Sea in 2008 and 2010, gas-related structures have been identified, which can beclassified into three categories: (1) shallow gas accumulations and gas chimneys, (2) mud diapirs, and (3)active and inactive pockmarks. On the Chirp profiles, shallow gas accumulations were observed along thenorthern coastline of the outer _Izmir Gulf at 3e20 m below the seabed. They appear as acoustic turbidityzones and are interpreted as biogenic gas accumulations produced in organicerich highstand fan sedi-ments from the Gediz River. The diapiric structures are interpreted as shale or mud diapirs formed underlateral compression due to regional countereclockwise rotation of Anatolian microplate. Furthermore,the sedimentary structure at the flanks suggests a continuous upward movement of the diapirs. Severalpockmarks exist close to fault traces to the east of Hekim Island; most of them were associated withacoustic plumes indicating active degassing during the survey period in 2008. Another Chirp survey wascarried out just over these plumes in 2010 to demonstrate if the gas seeps were still active. The surveysindicate that the gas seep is an ongoing process in the gulf. Based on the Chirp data, we proposed that thepockmark formation in the area can be explained by protracted seep model, whereby sediment erosionand reedistribution along pockmark walls result from ongoing (or long lasting) seepage of fluids overlong periods of time. The existence of inactive pockmarks in the vicinity, however, implies that gasseepage may eventually cease or that it is periodic. Most of the active pockmarks are located over thefault planes, likely indicating that the gas seepage is controlled by active faulting.

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1. Introduction

Significant evidence for gassy sediments in shallow marine envi-ronment, and their distribution and effects on the sediments havebeen realized after the initiation of acoustic mapping systems(Hovland and Judd, 1988). Although geological sources cause theinput of several different gases into the atmosphere, the mostcommon gas in the marine environment is methane. Methane accu-mulation inmarinesediments and seepage into thewatercolumncanbe evidenced in numerous ways: higher methane concentrations inthe water column, acoustic plumes on echosounder and subbottomprofiler records, existence of chemosynthetic communities on theseafloor, authigenic carbonates and fluidecreate geological features

titute of Marine Sciences and_zmir, Turkey. Tel.: þ90 232

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, D., et al., Acoustic evidence o(2011), doi:10.1016/j.marpetg

such as pockmark depressions (Hovland and Judd, 1988; Baraza andErcilla, 1996; Ergün et al., 2002; Çifçi et al., 2003; Gay et al., 2006;Rogers et al., 2006; Iglesias and GarcíaeGil, 2007; Judd andHovland, 2007; Savini et al., 2009; Crutchley et al., 2010).

Since methane production and migration in the shallow sedi-ments result in different acoustic anomalies, the accumulation zonesand their different effects in the sedimentary strata can be observedbyacousticmapping techniques.Gasaccumulationzonescanoftenbedistinguished as “acoustic turbidity” or “wipeeout” zones. Acousticturbidity zones are represented on seismic records by significantdisruptions caused by scattering and higher attenuation of theseismic energy due to free gas bubbles. Columnareshapeddisruptionzones with suppressed seismic amplitude are termed “acousticchimneys” and generally interpreted to indicate fluid migrationpathways (Judd and Hovland, 2007).

Pockmarks are described as cratereshaped seabed depressionsthat are generally related to gas and/or fluid discharge in surficialsediments (King and MacLean, 1970). They are commonly found inareas of high sedimentation rates such asdeltas, often near petroleum

f shallow gas accumulations and active pockmarks in the _Izmir Gulf,eo.2011.05.001

D. Dondurur et al. / Marine and Petroleum Geology xxx (2011) 1e122

fields or tectonically active continental shelves and slopes (see Rogerset al., 2006 and references therein). Biogenic or thermogenic gas, orhydrocarbonmigration fromdeep reservoirs, gashydratedissociation,freshwater supply from aquifers are generally considered as potentialmechanisms required for pockmark generation (Hovland and Judd,1988; Çifçi et al., 2003; Ussler et al., 2003; Dondurur, 2005; Rogerset al., 2006; Chand et al., 2009). Widely accepted model for thepockmark formation is that thisfluiddischargeoccurs as a suddenandcatastrophic expulsion of overpressured pore fluids from deeperhorizons (e.g. Hovland and Judd, 1988; Yun et al., 1999; Cole et al.,2000; Hovland et al., 2002; Çifçi et al., 2003; Gay et al., 2006; Rogerset al., 2006; Cathles et al., 2010). Although most of the pockmarksreported worldwide appear to be inactive, they could be episodicallyactive, driven by climatic changes and other cyclic forces as tides andstorm waves (Hovland et al., 2002) as well as during changes intectonically-induced stress regimes producing vertically stackedpockmarks (Baraza and Ercilla,1996; Çifçi et al., 2003). The pockmarkformation is considered as a complicated process and needs furtherinvestigation to account for the other possible mechanisms.

In this study, high-resolution acoustic methods are used to mapthe shallow gas occurrence and gas-related seabed structures at the

Fig. 1. (a) General tectonic framework of the Anatolian microeplatedNAF North Anatolian Fa_Izmir Gulf and surrounding areadGF Gülbahçe Fault, UF Urla Fault, SF Seferihisar Fault, UAF UFault, GG Gediz Graben (adapted from Ocako�glu et al., 2005; Emre et al., 2005; Aktar et al., 22010.

Please cite this article in press as: Dondurur, D., et al., Acoustic evidence oAegean sea, Marine and Petroleum Geology (2011), doi:10.1016/j.marpet

Gediz Delta in the outer part of the _Izmir Gulf. To date, shallow gasaccumulations or pockmarks have never been reported for the area.We investigate the occurrence, distribution and formation mech-anism of shallow gas and active/inactive pockmarks in the area. Acomparison of the Chirp data collected in 2008 and 2010 over theactive pockmarks is provided. The relationship between gasseepage and active faulting in the gulf is also discussed.

2. Tectonic and geomorphological setting

The study area is located off the west coast of Turkey, in thecentral part of the Aegean Sea (Fig. 1). The general tectonic frame-work of Anatolia is controlled by WSW block rotation along twoactive strike slip fault systems, the dextral North Anatolian Fault andthe sinistral East Anatolian Fault (Fig. 1a). The slip rate of theAnatolian micro-plate movement is 18e22 mm/year in the centralpart towards the west, and it is approx. 40 mm/year in the westernpart as a countereclockwise rotation, in response to the collision ofthe Eurasian Plate with the Arabian Plate that was initiated about 40million years ago (Reilinger et al., 1997; Yılmaz et al., 2000). Thiscollision caused compression in the eastern Anatolia associated with

ult, EAF East Anatolian Fault, DSF Dead Sea Fault, HA Hellenic Arc, (b) active faults in thezunada Fault, BF Bornova Fault, TF Tuzla Fault, MF Menemen Fault Zone, SHF Sahilevleri007; Drahor et al., 2009) and (c) locations of the acoustic profiles collected in 2008 and

f shallow gas accumulations and active pockmarks in the _Izmir Gulf,geo.2011.05.001

D. Dondurur et al. / Marine and Petroleum Geology xxx (2011) 1e12 3

NeS extension in thewestern part, forming several EeWeextendinghorstegraben systems such as the Gediz, Büyük Menderes, KüçükMenderes and Alasehir grabens (McKenzie, 1978; Sengör et al.,1984). Yılmaz et al. (2000) suggested that these graben systemsstarted their formation after Late Miocene, possibly in the Pleisto-cene. Several different formationmodels have been proposed for thedriving mechanism of the extension, such as tectonic escape,backearc spreading or orogenic collapse, which are discussed byBozkurt (2001) and Aktar et al. (2007). The _Izmir Gulf is located atthe western tip of the Gediz Graben (Fig. 1b).

The main active faults at the _Izmir Gulf and surrounding areas arethe _Izmir, Gülbahçe, Tuzla, Seferihisar, Uzunada and Urla faults(Saro�glu et al., 1992; Ocako�glu et al., 2004, 2005; Emre et al., 2005;Aktar et al., 2007; Fig. 1b). They are strike-slip faults, with the excep-tionof theSahilevleri Fault,whichexhibitsnormal fault characteristicsand forms the southern coastal zone of the inner gulf (Drahor et al.,2009). Several different tectonic models were suggested for theformation of _Izmir Gulf (see Ocako�glu et al., 2004); however, thedetailed tectonic historyand sedimentary structure arepoorly known.Based on multiechannel marine seismic data, Ocako�glu et al. (2005)suggested that normal faults exist in the outer gulf and are cut byNeSeextending strike-slip faults. They also suggested two mainstratigraphical units, lower and upper units. They interpreted thelower unit as Miocene basement consisting of parallel or wavyreflectors, top of which was defined by high amplitude reflectionsindicating an unconformity. The upper unit was interpreted as Plio-ceneeQuaternary basin deposits that onlap the Miocene basementsurface. They also calculated the thickness of PlioceneeQuaternarysediments to the Miocene bedrock, which reached 900 m in thecentral part of the outer _Izmir Gulf (to the east of Uzun Island).

The _Izmir Gulf can be subdivided into two physicoegeographicprovinces, an inner gulf and an outer gulf separated by the YenikaleStrait (Fig. 1c), a 13 m deep sill. The maximum water depth in theinner gulf is 20 m, while it is approx. 65 m in the outer gulf. Theinner gulf extends in an EeW direction, which conforms to thegeneral trend of the graben structures, whereas the outer gulfextends NNWeSSE. The graben systems also constitute thedrainage system of western Anatolia towards the Aegean Sea. TheGediz River is the drainage system of the Gediz Graben via the outer_Izmir Gulf. The ancient Gediz River bed in the central part ofthe outer gulf was active until the end of 1800s (Fig.1c). In 1886, theriver bed was moved to the outside of the gulf because of theinfilling risk of the outer gulf by sediment load from the Gediz River.Although it does not directly flow into the central part of the outergulf today, the Gediz River has had a considerable effect on theformation of the present day morphology of the _Izmir Gulf as it hascontrolled the sediment accumulation in the gulf since the lastglacial highstand, over an area of about 20,000 km2.

3. Materials and methods

The acoustic data were collected during a survey aboard the R/VK. Piri Reis of Dokuz Eylül University, Institute of Marine Sciencesand Technology in March 2008 and June 2010. In 2008 survey,multibeam bathymetry, Chirp seismic and deep-tow side scansonar data were collected, while only Chirp seismic data werecollected in 2010. A global DGPS system was utilized during theentire survey with an integrated navigation system. Bathymetricdata were obtained using a poleemounted Elac SeaBeam 1185multibeam sonar operating at 180 kHz. This system utilizes 126beams with 1.5� resolution which provides a total swath coverageof 153�. The multibeam data were processed using the Caraibessoftware with the following processing steps: beam editing anddeespiking, correction of navigation errors, data interpolation,DTM construction and gridding with 5 m grid interval.

Please cite this article in press as: Dondurur, D., et al., Acoustic evidence oAegean sea, Marine and Petroleum Geology (2011), doi:10.1016/j.marpetg

The shallow sedimentary structure was investigated usinga poleemounted Chirp subbottom profiler utilizing a sweep signalbetween 2.75 and 6.75 kHz centred at 3.5 kHz. Gain correction,deechirping and amplitude envelope calculations were applied toChirpdata. Thehigheresolution seismicdatawere analyzedusing theKingdomSuite Software andprocessedusingProMax fromLandmarkGraphics. These acoustic systems allowed us to map the shallowsedimentary structure andgeomorphologyof theouter gulf aswell asgassy structures and pockmarks. A GeoAcoustics DT2000 deep-towcombined side scan sonar and GeoChirp II sub-bottom profilersystem was also deployed to map sediment backscatter images insome anomalous areas such as fault scarps and gas seeps. The sidescan sonar was operated at 110 kHz with a total coverage of 660 m,andthedeep-towChirpsub-bottomprofilerutilizeda2e7kHzsweepsignal. Timeevaried gain, speckle removal, alongetrack speedcorrections and slant range correction were applied to the side scansonar data before mosaic preparation. Deep-tow Chirp data wereprocessed using the same processing steps given above. A total ofabout 1125 km of sideemounted Chirp seismic and 122 km (approx.80 km2) of combined side scan sonar with deep-tow Chirp datawerecollected during 2008 survey. The primary purpose of the survey in2010 was to check if the active gas seeps in specific zones observedduring 2008 survey were still active or not. Approx. 45 km of Chirpdata was collected during 2010 survey. The entire survey area andsurvey lines in the outer gulf are shown in Figure 1c.

4. Observations

Based on the acoustic dataset for the outer part of _Izmir Gulf,three distinct structural categories plausibly related with gasaccumulation and migration were identified:

1. Shallow gas accumulations and gas chimneys,2. Diapirs, and3. Active and inactive pockmarks.

4.1. Shallow gas accumulations, diapiric structures and gaschimneys

Extensive shallow gas accumulations were identified in thehighstand sediments, especially along the northern coastline of theouter _Izmir Gulf approx. 3e20 m below the seafloor (see Fig. 2 forexamples). The gassy sediments mask deeper horizons resulting intransparent zones on the Chirp seismic data, plausibly due to a highattenuation of the acoustic signal in the gassy sediments producingacoustic blankingetype turbidity zones. The top of the acousticturbidity appears blurry (Fig. 2a), rather than as a distinct brightreflection, which may be due to the scattering of the acoustic signalat the wateresaturated to gassy sediment interface. There are verysharp vertical boundaries between gasebearing and gasefreesediments. In places, cloudy turbidityetype gas anomalies wereobserved in the water column at areas with underlying gas accu-mulation (Fig. 2a).

A number of diapiric structures exist in the outer gulf (e.g.immediately NE of Uzun Island; Fig. 2b) and to the northernmostpart of Gülbahçe Gulf (not shown here). None of these diapirsextends to the seabed. Their top is generally located less than 3 mbelow the seabed and their widths range from 70 to 100 m atcentral part. Internal structure of the diapirs is completely trans-parent without any significant reflections. Presumably because ofthe plastic deformation due to the upward movement of thematerial (possibly fine-grained), the sediments on both sides of thediapir flanks bend upwards. These features are somehow found tobe associated with gas.

f shallow gas accumulations and active pockmarks in the _Izmir Gulf,eo.2011.05.001

Fig. 2. Examples of sideemounted Chirp profiles showing (a) transparent acoustic turbidity zone with sharp vertical boundaries and cloudy turbidity in the water column, (b)a diapiric structure in the outer gulf and (c) gas chimneys. Locations of the profiles are shown in Figure 1c.

D. Dondurur et al. / Marine and Petroleum Geology xxx (2011) 1e124

In some areas, gas chimneys as narrow gas columns are alsoobserved on the Chirp data (Fig. 2c). Their width ranges from 40 to110 m and they are distinguished by very bright reflections fromtheir upper boundary generally located less than 50 m below theseabed. It was not possible to determine the bottom of gassysediments and chimneys, due to the high attenuation of theacoustic signal.

The depth to the top of gassy sediments as well as the distri-bution of gas chimneys in the prodelta area of Gediz River is shownin Figure 3a. In the outer gulf, it was not possible to determine thelandward boundary of the acoustic turbidity zone, since it extendsinto very shallow coastal waters which were not mapped by ouracoustic systems. The chimneys accumulate to the SE of HekimIsland whereas they show a random distribution southwards.

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4.2. Active and inactive pockmarks

Several pockmarks are observed in the outer gulf, especially tothe eastern and southeeastern part of Uzun Island along the Uzu-nada fault zone. Most of them are associated with active gas seepsdetected by our acoustic systems, which appear as narrowcolumnar acoustic anomalies in the water column. The distributionof active and inactive pockmarks on the multibeam bathymetricmap as well as the active fault zones are mapped using the linea-tions on the multibeam bathymetry and the fault cuts on the Chirpseismic lines (Fig. 3b).

Based on our acoustic data, both active and inactive pockmarkscould be classified into two types: fault-related and isolated pock-marks. The former are associated with fault planes, the latter not.

f shallow gas accumulations and active pockmarks in the _Izmir Gulf,geo.2011.05.001

Fig. 3. (a) The boundary of gassy sediments and depth to top of the acoustic turbidity zone together with the distribution of gas chimneys along the basin deduced from Chirpprofiles, superimposed on the proposed boundary of the Gediz delta sediments (shaded area). (b) Closeeup of the pockmark area with an active fault map of the outer _Izmir Gulf,based on the acoustic data.

D. Dondurur et al. / Marine and Petroleum Geology xxx (2011) 1e12 5

Although these soecalled isolated pockmarks are generally veryclose to the faults observed on the Chirp data, they are not locatedat the fault planes as such. In some cases, they occur in basinal areaswhere no faults are observed. A Chirp seismic line for both fault-related and isolated pockmarks is given in Figure 4a.

The gas seep creates hyperbolic anomalous distortions in theshallow sediments. These distortions could be used to identify theformer seep zones recognized as narrow columnar high reflectivityzones just below the seabed. This allows us to distinguish inactivepockmark zones using seismic data. If there is no seep over suchcolumnar disruption zones in the shallow sediments, it is inferred toan inactivepockmark. Theyare generally located inareas veryclose tothe active pockmarks to the east of Hekim Island, while they showa random distribution further east towards the basinal areas of thegulf. Figure 4b shows an example Chirp profile for inactive pock-marks. The highly reflective column-shaped disturbance zones in theshallowsediments can be traceddown to the erosional surface. Thesecolumnsare interpretedas remnantsof thephysicaldisruptions in thesediments during the active gas venting in the past. Active andinactive pockmarks can also be seen on the side scan sonarmosaic ashighbackscattering zonesof circularor semi-circular isolatedpatches(Fig. 4c) with the diameters no more than 20 m.

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The depressions of the pockmarks sometimes cannot beobserved clearly on the poleemounted Chirp data. However, thepockmark craters, their feeder channels and the shallow gasreservoir fuelling the pockmark can be seen more clearly on thedeep-tow Chirp data. This is due to the different spatial resolutionsof both systems arising from the survey speed difference during thedata collection. The sideemounted Chirp data was collected at5.5e6 knots, whereas deep-tow Chirp data was collected at 2.5e3knots survey speed. Therefore, the deep-tow data have muchhigher spatial sampling than the data collected by poleemountedChirp system, which indicates that the deep-tow Chirp data hashigher spatial resolution and hence the more accuracy to resolvethe lateral extends of the structures. Figure 5 shows an exampledeep-tow Chirp profile from northern pockmark zone with itscorresponding side scan sonar data and the line drawings. There aretwo distinct gas seeps from a large pockmark crater on the Chirpline, which also appear as two distinct gas plumes on the watercolumn zone of the side scan sonar data. Thewidth and depth of thecrater are approx. 180 and 7 m, respectively. The feeder channel ofthis active pockmark can be clearly traced down to an acousticturbidity zone which drives the seeps. The top of acoustic turbidityzone is distinguished by a bright horizontal reflection about 700 m

f shallow gas accumulations and active pockmarks in the _Izmir Gulf,eo.2011.05.001

Fig. 4. (a) An example Chirp profile illustrating active and inactive pockmarks (see text for details). (b) A poleemounted Chirp and (c) a section of a side scan sonar mosaic showinginactive pockmarks (arrows) as high backscattering patches. Locations of the profiles are shown in Figure 1c.

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wide, located about 22 m below the seabed (assuming a soundvelocity of 1500 m/s in the sediments).

5. Discussion

5.1. Gas accumulation, seep activity and sedimentary structure

The acoustic data indicate that the highstand sediments overliean erosional surface in the outer gulf, and the depth to the erosionalsurface (e.g. thickness of the postglacial sediments) close to the NEborder of the outer gulf is more than 80 m. Gassy sediments arerecognized by distinctive seismic anomalies, such as high seismicattenuation and scattering of the signal, creating acoustic turbidityzones and are generally located in the highstand sediments ofGediz River. Aksu et al. (1995) suggested that the Gediz River carries

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approx. 4.7 million tons of sediment per annum. Based on thethickness calculations for the PleistoceneeHolocene sedimentsafter the last highstand, Coskun (2009) suggests an apparentsedimentation rate of 2.5e4 m/ka for the foreset area of the Gedizdelta. It is suggested that the Gediz River transported extensiveamounts of organic material (mostly plant debris) rich sediments,at relatively high sedimentation rate to the outer gulf causinga rapid burial of the fine grained sediments in the Gediz prodeltaarea. This rapid burial prevented the oxidization of organic mate-rial. It also provided an efficient delivery of this organic material tomethanogenic archaea to produce biogenic methane in shallowsediments, which then produced shallow gas accumulation zoneswhen the methane concentrations exceeded the solubility. There-fore, shallow gas accumulations in the outer _Izmir Gulf are directlysourced in the highstand fan sediments of the Gediz River delta,

f shallow gas accumulations and active pockmarks in the _Izmir Gulf,geo.2011.05.001

Fig. 5. (a) Deep-tow Chirp profile (bottom), corresponding side scan sonar data (top) and (b) interpretative line drawing, showing two adjacent gas seeps. The associated gaschimneys connect the pockmark to an area of deeper gas accumulation, indicated by a bright reflection (EHR) and an acoustic turbidity zone. S Surface (ghost) reflection, G1, G2

multiple reflections. Location of the profile is shown in Figure 1c.

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where biogenic gas would be produced in organicerich terrigenousdeltaic sediments by bacterial activity. Active or inactive pockmarksare not observed in the acoustic turbidity area. Furthermore, thereis no active faulting in this part of the gulf. Therefore, it is proposedthat the absence of faulting in the gassy sediments area preventsthe gas from migrating to the seabed, and without suitableconduits, the gas accumulates in the pore spaces of fine-grainedsediments, resulting in acoustic turbidity zones.

The cloudy turbidity zones also exist in the water column oversome acoustic turbidity areas located at the mouth of ancient GedizRiver bed (Fig. 2a). It is inferred that this type of gas anomalies areformed by reesuspension of the seabed sediments by gas escapefrom underlying acoustic turbidity zone as reported in offshoreSpain by GarcíaeGil et al. (2002).

Most of the active pockmarks are located along the fault tracesto the east of Hekim and Uzun Islands (Fig. 3b). As discussed byvarious researchers (e.g. Bøe et al., 1998; Rise et al., 1999; Yun et al.,1999; Hübscher and Borowski, 2006), fault planes and glacialerosion are two mechanisms for gas seepage from a deeper gasreservoir. According to the acoustic data, it is inferred that the gasseeps in the outer _Izmir Gulf are related to faulting. In this case,fault planes act as suitable conduits for fluid escape, which allowfault-related pockmark generation on the seabed and gas seepageinto the water column.

Both active and inactive pockmarks can also be seen on thedeep-tow Chirp data in Figure 6, which crossecuts some seeps that

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are either exactly over or very close to active fault planes. The line inFigure 6 runs more or less parallel to an active fault to the SE ofHekim Island and, in places, crossecuts the same fault planerepeatedly. The sedimentary structure along this line is subdividedinto two packages, referred to as Unit 1 and Unit 2, both of whichare affected by the Uzunada Fault. Based on thickness variations ofsediments of Unit 1, it is inferred that the fault has a significantstrike slip component. While sediments of Unit 1 show parallel andless disturbed character, deformation is much more evident insediments of Unit 2. The seismic line shows several seep zonesindicated from S1 to S5 together with small-scale pockmark craters(e.g. at the location of S4) forming active pockmarks, as well asa number of inactive pockmarks indicated by IP (Fig. 6). The gasseeps associated with active pockmarks can be distinguished asnarrow strips in the water column ascending from seabed to seasurface. Similar gas seeps were also observed at NW Spain andtermed “acoustic smokes” by Iglesias and GarcíaeGil (2007). Thepockmarks of seeps S2 and S3 are classified as fault-related pock-marks since they are located directly over active fault planes, andthe seeps S1, S4 and S5 are interpreted as isolated ones.

Together with these observations, one can conclude that the gasis produced in (or below) Unit 2 which consists of organic materialrich (possibly terrigenous) sediments. Therefore, the interfacebetween Unit 1 and Unit 2 is an important surface which possiblyseparates organic-material rich sediments of Unit 2 and the lessdisturbed sediments of Unit 1. Any active fault surfaces that may

f shallow gas accumulations and active pockmarks in the _Izmir Gulf,eo.2011.05.001

Fig. 6. Deep-tow Chirp profile showing active and inactive pockmarks, gaseenhanced reflections (EHR) and active faulting (top), and interpretative line drawing of the profile(bottom; see text for details). Location of the profile is shown in Figure 1c. S1 to S5 seeps, IP inactive pockmarks.

D. Dondurur et al. / Marine and Petroleum Geology xxx (2011) 1e128

affect the whole sedimentary column through to the seabed, whereavailable, appear to constitute the only pathways for the gas tomigrate. If the fault surfaces do not penetrate to the seabed,then the gas migration would have to occur either along themicroecracks or other appropriate pathways, which may becreated by migrating gas itself along local disturbances. Unit 2 isalso distinguished by some enhanced reflections labelled EHR inFigure 6, which are interpreted as bright spots related to shallowgas. Therefore, it is also concluded that the source of the gas whichcurrently drives the active pockmarks today (and has driven theinactive pockmarks in the past) is located in and/or below Unit 2.

There is no evident connection between the shallow gas accu-mulation area along the NE coastline of the gulf and the pockmarkarea with respect to gas migration. This indicates that the gasmigration pathways are along the vertical fault traces in the outergulf rather than along horizontal paths. The deeper sedimentaryand structural system of the area is not known since there are nodetailed deep seismic or well data. Our higheresolution seismicdataset, however, clearly shows that the gas seepage is predomi-nantly controlled by active faulting in the outer gulf. This situationindicates the possibility that all of the active pockmarks may havea connection to a deeper reservoir. This theory needs further

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investigation, especially groundetruthing and gas chemistry, toevaluate the gas composition and hence its possible connection toa possible deeper thermogenic gas reservoir.

5.2. Seep models

Most researchers suggest that pockmarks are formed byepisodic, sudden and often catastrophic fluid (mostly gas) expul-sions possibly due to local overpressure conditions (e.g. Hovlandand Judd, 1988; Prior et al., 1989; Solheim and Elverhøi, 1993;Yun et al., 1999; Cole et al., 2000; Hovland et al., 2002, 2010; Çifçiet al., 2003; Gay et al., 2006; Cathles et al., 2010). These expul-sions are also considered to be the agent responsible for the erosionand sediment reesuspension inside the pockmark craters, since thegas expulsion through a pockmark is a significant force which canerode and redistribute the sediments along the pockmark walls. Inall cases, the pockmark walls consist of erosive surfaces and there isno sediment infill in the craters.

In widely accepted catastrophic expulsion model of pockmarkformation, the pockmarks do not necessarily show active gas seeps,but formed by expulsions occurring episodically, that is why mostof the pockmarks seem to be inactive on the acoustic data (Hovland

f shallow gas accumulations and active pockmarks in the _Izmir Gulf,geo.2011.05.001

D. Dondurur et al. / Marine and Petroleum Geology xxx (2011) 1e12 9

et al., 2002). As suggested by Hovland et al. (2010), the mechanismsand processes involved in pockmark maintenance can be estab-lished after a long-term monitoring has been performed, as inPatras Gulf, Greece (Marinaro et al., 2006). Therefore, we haveperformed an additional Chirp survey in 2010 to monitor the activeseeps mapped in 2008 in the outer gulf. Figure 7 illustrates anexample comparison of Chirp data collected in 2008 and 2010surveys over the same active pockmark. The data clearly show thatthe gas seep observed in 2008 is still active during 2010 survey.Based on both surveys, all of the pockmarks observed in the outergulf were degassing during the survey periods in 2008 and 2010.

Hovland (1989) indicated that pockmarks can develop bya continuous mechanism of gas seepage. Based on the availabledataset and the observations of active gas seeps in 2008 and 2010surveys, here we suggest that the pockmarks in the _Izmir Gulf areformed by “protracted seepmodel”. This model can be explained bythree stages outlined on the schematic illustration in Figure 8together with deep-tow Chirp seismic data examples obtainedfrom the outer gulf. According to the model, a gas seep originatesalong or close to an active fault surface in the first stage (Fig. 8a). Insome areas, bright reflections just below the seep are observed andthey are interpreted as gas fronts driving the seep through a gaschimney that extends from the seafloor to the deeper gas source. Atthis stage, no depressions or craters attributable to a pockmarkexist on the seabed. Afterwards, during this ongoing seepagethrough the seabed, a small scale pockmark originates on theseafloor as a small crater (Fig. 8b). It is inferred that this crater arisesdue to the erosional effect of ongoing gas seepage which can easilysuspend the muddy sediments. As the gas seepage continues, thiserosionwill continue to erode pockmarkwalls and re-distribute thesediments inside the newlyeformed pockmark crater (Fig. 8c). Asa consequence, a typical pockmark with erosive crater walls isproduced. However, it is not known howmuch time is required fora pockmark formation by the suggested model, because a long-term acoustic monitoring is necessary to evaluate the time spanfor a pockmark generation. The suggested protracted seep model

Fig. 7. (a) Chirp profile from 2008 survey over a fault-related active pockmark. (b) Chirp prFigure 1c. The data show that the gas seep is an ongoing process in the gulf.

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does not need overpressure conditions for the gas or pore fluids tocreate pockmarks immediately after a sudden eruption as in thecatastrophic expulsion model. Instead, the model requires localdisruption zones, such as active fault surfaces for the gas to migratefrom local gas source to seabed in long periods of time.

If the protracted seep model is the case for the pockmarkdevelopment in the outer _Izmir Gulf, this means that we get largerand larger pockmarks over the time provided that the gas seepagedoes not cease. By means of this, the structural form of pockmarksin the study area can be further subdivided into two groups, thesouthern and northern zones. The southern zone is locatedimmediately SE of Hekim Island, while the northern zone is locatedto the EeESE of Uzun Island (Fig. 3b). The craters are relativelysmaller and it is not possible to determine the source of the gas onChirp data in the southern zone. The seeps are, however, associatedwith larger pockmark craters, and furthermore, one can generallyobserve the gas source beneath the pockmarks in the southernzone. Therefore, it is suggested that the pockmarks are only in theirfirst or second growth stages in the southern zone. To the north, onthe other hand, larger pockmark craters exist (see Figs. 5 and 8c).Therefore, it is interpreted that the southern zone is defined asa younger pockmark basin in its initial development stage.According to our model, the pockmarks in the northern zone,however, must be older because they lasted over long periods oftime to create larger craters, assuming a long-lasting or protractedgas expulsion throughout the area.

In the central part of the outer basin, several inactive pockmarksexist, which are directly located over highly reflective columnardisturbances in the shallow sediments. Although there is no clearevidence for their past activity, it is suggested that they areremnants of pockmarks which were actively degassing in the past.In spite of this, one should consider that there still might beaqueous emissions of dissolved hydrocarbonerich pore water inthe sites inferred to inactive since we do not perform any geo-chemical analysis for the dissolved gas in the water column. Eventhough the inactive pockmarks show a random distribution over

ofile from 2010 survey over the same pockmark. Locations of the profiles are shown in

f shallow gas accumulations and active pockmarks in the _Izmir Gulf,eo.2011.05.001

Fig. 8. Schematic illustration (left column) of the protracted seep model suggested for pockmark formation in the study area, and examples of corresponding deep-tow Chirp data(right column). (a) Formation of gas seep with a gas chimney below, (b) initiation of a pockmark crater resulting from ongoing fluid seepage, and (c) formation of a pockmark craterresulting from erosion due to seepage (see text for details). Locations of the Chirp profiles are shown in Figure 1c.

D. Dondurur et al. / Marine and Petroleum Geology xxx (2011) 1e1210

the basin, it is argued that they possibly follow the local disturbancezones along the deeper fault traces which cannot bemapped by ourhigheresolution acoustic dataset.

A number of possibilities could be suggested for cessation of gasseepage in the _Izmir Gulf: (1) the entire gas has been releasedduring the active seeping, (2) seepage currently continues, but thegas concentration is not high enough for our acoustic systems todetect the ascending gas bubbles, and (3) vertical gas migrationpathways along the local zones with high permeability havechanged. Since inactive pockmarks in the study area are very closeto active ones, and both possibly follow deeper fault traces, wepropose that inactive pockmarks were fueled by the same gas

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reservoir as that for the active pockmarks today. Therefore, it issuggested that the most possible explanation for the occurrence ofinactive pockmarks in the area is the lateral change in gasmigrationpathways from inactive pockmark zones into currently activepockmark zones. The internal structure and the local permeabilityof the local disruption zones such as the microecracks in theshallower sediments could easily change under the active tectonicregime of the _Izmir Gulf. It is proposed that the most possible agentfor closing of the microecracks is tectonically induced local stressaccumulation originating from active faulting in the gulf. In addi-tion, it is also likely that fluid pressure changes associated withseepage might affect the gas migration pathways in such away that

f shallow gas accumulations and active pockmarks in the _Izmir Gulf,geo.2011.05.001

D. Dondurur et al. / Marine and Petroleum Geology xxx (2011) 1e12 11

the release of fluid pressure can likely cause fluid flow pathways toclose off.

5.3. Diapiric structures

Themud diapirs in the gulf do not reach the seabed to formmudflows. They are considered as the earlier state of the mud volcanoesas discussed by Dimitrov (2002), who suggested that many of thesubmarine mud volcanoes are developed at the crests of the muddiapirs. The mud diapirs observed in the gulf might be related togas and/or fluid escape to the seabed, consistent with the results ofLimonov et al. (1997) and Milkov (2000). Upward bending of thesediments at the flanks is larger for deeper sediments andChristmas treeetype structures at both sides are not observed.Stacked outflow lenses produce Christmas treeetype structuresand they indicate multiple pulses in the upward movement ofdiapir or mud volcano activity as the case in the Gulf of Cadiz(Somoza et al., 2003; Van Rensbergen et al., 2005). The absence ofstacked lenses and larger bends at deeper sediments at the flanksindicate a continuous upward movement of the diapirs.

Although the deeper structure, internal morphology and fluidcomposition of the diapiric structures are not known, it is possible todiscuss their formation and driving mechanisms. The formation ofamud diapir is linked to two key processes: (1) high sediment inputand 8e22 km thick sedimentary cover (in passive margins), and (2)lateral tectonic compression (in active margins), and the othermechanisms result from these two key processes (for more detail,see Milkov, 2000). If the formation of mud diapirs in our study areawas related to the first process, there would have to be extremelythick sedimentary cover in the area. However, there is no geologicalinformation of such a thick sedimentary cover in the areawhere thediapirs are observed. In addition, Ocako�glu et al. (2005) suggestamaximumof 300msediment thickness at the locationof thediapirin Figure 2b. Therefore, it is concluded that their formation could berelated to the tectonic structure of the area, which can be defined byNWeSEetrending normal faults with strike-slip components. Thecountereclockwise rotation of thewestern Anatolia including _IzmirGulf could create some locallyeinduced lateral compression whichresults in the upward movement of shale or muddy fluids along thelocal disruption zones. The effects of such lateral compression arequite evident in the outer GülbahçeGulf, where indications of thrustfaults are observed (Drahor et al., 2009). Therefore, it is suggestedthat faulting and lateral tectonic compression togetherwithpossibleabnormal pore pressures due to the abundant gas accumulationscause the formation of shale or mud diapirs along the local disrup-tion zones such as fault planes. Similar interpretations have alsobeen suggested by Hovland and Curzi (1989) for Adriatic Sea andHovland (1990) for the offshore Norway for the diapiric structures,where the diapirs are formed when plastic clay just below theseabed is charged with the gas migrating from deeper sediments.When the gas fills more and more of the clay pore volume, the claysurface deforms due to the static instability (Hovland, 1990).Anyway, deeper structures and their possible connection to a deeperoverpressured shale formation and their relation to the gas chargedsediments in the area should be carefully investigated.

6. Conclusions

It is concluded that high deposition rate of organicerich materialfrom the Gediz River drives biogenic gas generation in the muddysediments resulting in acoustic turbidity, cloudy turbidity and gaschimneys on the seismic data. Several active and inactive pockmarksexist close to fault traces to the eastofHekim Island. It is likely that gasseepage is controlled by active faulting. The Chirp data collected in2008 and 2010 surveys indicate that the gas seepage is an ongoing

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phenomenon in the gulf. Therefore, it is concluded that the formationof pockmarks in the area can be explained by protracted seepmodel,inwhich the sediment reedistribution inside the craters and erosionon the pockmark walls are formed by the erosive nature of the fluidsseeping over long periods of time. Existence of inactive pockmarkssuggests thatgas seepageat a specific areamaynotbecontinuousbut,rather, periodic. The most probable explanation for the inactivepockmark existence is the change in the gas migration pathways,which might be local disruption zones of high permeability.

We conclude that diapiric structures are formed under lateralcompression due to regional countereclockwise rotation ofAnatolian microplate along the local disruption zones. The absenceof a Christmas treeetype sedimentary structure and stacked lensesat the flanks and presence of larger upward bends at deeper sedi-ments suggest an uninterrupted upward movement of the diapirs.

Although we have shown evidence for shallow gas relatedformations in the outer _Izmir Gulf, some questions still remain. Wedo not have groundetruthing to determine the origin and source ofthe gas using independent pore water geochemical analysis toexplain a possible connection to a deeper thermogenic reservoir.We also do not know the composition of the diapirs as well as theirrelationship to fluid flow in the gulf. A long-term acoustic moni-toring is also necessary to evaluate the pockmark formation and gascessation in detail.

Acknowledgements

We would like to thank the officers, crew and scientificmembers aboard the K. Piri Reis research vessel for their valuableeffort during the cruise. George M. Osborne from INTECSEA, Dr.Jean-Paul Foucher, Dr. Gareth J. Crutchley and Dr. Jorge Iglesias arethanked for their fruitful comments on an earlier version of themanuscript. This research was supported by a grant from TheScientific and Technical Research Council of Turkey (TUBITAK,project code 104Y027) and by a Dokuz Eylül University ResearchFoundation Grant (project code 2005.KB.FEN.065). The acousticsystems and data processing facilities of the seismic laboratoryfounded with the financial support of the Turkish State PlanningOrganization (DPT, project code 2003K120360) were used for thiswork.

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