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Chapter 1

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Introduction: Tectonic Elementsof the Black Sea Region

Andrew G. RobinsonJKX Oil & Gas plc

Guildford, United Kingdom

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INTRODUCTION

In 1967 and 1969, two oceanographic cruises weremade in the Black Sea under the guidance of theWoods Hole Oceanographic Institute: The cruisesincluded scientists from many countries and disci-plines. Their aims were to determine the recent geo-logical and geochemical evolution of the Black Sea, tomap the shallow structure of the basin, and to studythe interaction between the oxidized surface watersand the anoxic waters beneath them. The results werepublished 23 years ago, as AAPG Memoir 20 (Rossand Degens, 1974). During the 1969 cruise, the vesselAtlantis II collected 40 piston cores, which formed thebasis of most of the subsequent geological studies thatwere restricted to very recent sedimentation. Specula-tions concerning the origin of the basin and the rela-tionship of the geology offshore to that exposedaround the margins of the Black Sea were rooted inpre-plate tectonic concepts of basin formation andwere in any case hampered by a lack of relevant data(Brinkmann, 1974).

In 1976, the Glomar Challenger visited the Black Seaon Leg 42B of the Deep Sea Drilling Project and drilledand cored three deep-water sites (379, 380, and 381).Well 381 north of the Bosporus encountered sedimentsas old as Miocene, including some apparentlydeposited in shallow water (Ross, 1978).

The next major volume in Western literature to dealwith the Black Sea was published a decade later, col-lecting papers presented two years earlier at a confer-ence in Yalta. In this volume, a number of seismicreflection lines—shot in 1975 by an Italian vessel—were published and, for the first time, the scientificcommunity had a picture of the deeper structure of theBlack Sea (Finetti et al., 1988). The interpretation of theseismic data, however, was still impeded by the smallnumber of wells drilled in the Black Sea to which thelines could be tied, by the ambiguity of even these fewties, and by the presence of numerous multiples. Bythis time, the Black Sea was nonetheless widelyregarded as a marginal basin developed to the north ofthe northward-subducting Tethyan oceanic plate,

apparently rifting during the mid-Cretaceous (Görür,1988; Manetti et al., 1988). There was also the hint thatthe Black Sea had originated not as a single basin, butas two—an eastern and a western Black Sea—that hadcoalesced during their postrift phases to form the sin-gle depocenter observed today.

The volumes referred to above did not reflect even asmall part of the total geological work carried out onthe Black Sea. Unfortunately, the vast volume of Sovietliterature was inaccessible and, although often of goodquality, was just as often difficult for a Western readerto understand. This was also largely true of the Turk-ish literature, although research—mainly on the Pon-tides—by the group at Istanbul Technical Universitywas widely disseminated. None of the countriesaround the Black Sea made information obtained frompetroleum exploration programs available either toeach other or to the scientific community. It was evenimpossible to obtain geological maps from the sur-rounding onshore areas (other than old 1:500,000 geo-logical maps of northern Turkey). As much as anysedimentary basin, the Black Sea suffered from thecold war.

During the first few years of the 1990s, however, theprofound political changes that took place in EasternEurope began to have an equally profound effect onthe quantity and quality of geological work carried outin the Black Sea region. Romania, Bulgaria, Ukraine,Georgia, and Russia became independent sovereignstates and, to a greater or lesser extent, opened theiroffshore areas to exploration by Western oil compa-nies (see Benton, this volume). In doing so, they madeavailable a large amount of well and seismic data thathad until that time been regarded as state secrets. Thenew petroleum exploration programs led to the acqui-sition of new high-multiplicity marine reflection seis-mic data and to the drilling of new exploration wells inBulgarian, Romanian, and Ukrainian offshore areas.At the same time, previously secret detailed geologicalmaps of excellent quality became available for all ofthe former Soviet and Eastern European states(although, for the most part, these remain difficult toobtain). It also became possible to visit these countries

Robinson, A.G., 1997, Introduction: tectonic elementsof the Black Sea Region, in A.G. Robinson, ed.,Regional and petroleum geology of the Black Seaand surrounding region: AAPG Memoir 68, p. 1–6.

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more or less independently, to carry out field studies,and to talk without hindrance to their scientists. Mean-while, in the southern part of the Black Sea, the early1990s saw the licensing of much of the Turkish off-shore sector to Western companies for petroleumexploration. No new wells have been drilled in thelicensed areas, but a large amount of reflection seismicdata has been acquired, interpreted, and in part pub-lished (Robinson et al., 1996). It is unfortunate thatdetailed geological maps of Turkey are still consideredsecret and remain unpublished.

As a result of this activity, a large amount of newhigh-quality geological work has been carried out onthe Black Sea and surrounding areas. Much of this ispublished in this volume for the first time. The contri-butions to the volume cover most of the major tectonicelements in and around the Black Sea. They do not doso evenly, and are biased principally toward thoseareas that have received the most attention by oil com-panies, and toward northern Turkey, where Turkishgeologists have been active for many years. The chap-ters not only cover different areas but also encompassvery different types of study, based on different kindsof data. Thus, papers on the Pontides are essentiallybased on field mapping; those on the offshore shelfareas are based on seismic and well data. Perhaps notsurprisingly, different workers using different data-bases sometimes come up with quite different andeven opposing and incompatible conclusions. In edit-ing this volume, I have made no attempt to resolvesuch differences or to sweep them under the carpet; Iwish to draw attention to them on the basis that theidentification of a problem is the first step to its solu-tion. The volume begins with a series of chapters thatdeal with general aspects of the development of theBlack Sea; subsequent chapters are organized in anessentially counterclockwise fashion, beginning withRomania and ending in the Gulf of Odessa (Ukrainianoffshore). There are notable gaps at Thrace (althoughthe Thrace Basin is well documented in literature else-where; see later), the Greater Caucasus, the Russianmargin of the Eastern Black Sea, the Azov Sea, andonshore Crimea.

To those not particularly familiar with the Black Seaarea, the complexity of the geology and the volume ofliterature available (although often inaccessible) canprove somewhat daunting. This chapter is thereforeintended to serve not just as a background and intro-duction to the Memoir and the chapters in it, but alsoto the main tectonic elements in the Black Sea region. Ihope that this will help the reader put the other chap-ters into a regional context quickly. The complexity ofthe area is such that it is impossible to write an uncon-troversial review, and this chapter inevitably reflectsmy prejudices. Where others have very different prej-udices, I have tried to point this out. Indeed, a secondaim of this chapter is to indicate where there are majordifferences of interpretation.

Following, I give a brief description of the main tec-tonic elements in the Black Sea region, explaining howthese are covered in this Memoir. For detailed referencelists, the reader should look in these chapters. I illustratethe descriptions with reference to a map (Enclosure 1).

This is a slightly odd map in that it tries to show a num-ber of attributes, principally basin origin—age andtype—but also the latest deformation event. In manyareas, the nature and exact positions of the boundariesof sedimentary basins are unclear; the origin of some isdisputed; and in some parts of the Black Sea, the map isbased on scanty information. It is therefore a somewhatunsatisfactory hybrid and will be open to criticism fromthose who have detailed knowledge of particular areas.So, after many dark hours with blunted colored pencilsduring which I many times nearly consigned the map tothe bin, I have ended up including it, having convincedmyself that it is better to have some map of this kindcovering the entire Black Sea region than it would be tohave nothing at all. I encourage those who know whereit is wrong or misleading to write and tell me.

GENERAL SETTING OF THE BLACK SEA BASINS

The present Black Sea morphology has been pro-duced by the coalescence of two basins during theirpostrift phases (Robinson et al., 1996). Both formed asextensional basins north of a northward-subductingoceanic plate—the Tethys Ocean. The suture zonewhere this ocean eventually closed is marked by anophiolite belt and subduction-accretion complex run-ning from Izmir on the Turkish Mediterranean coast,through Ankara and Erzincan, and into the Sevanregion of Armenia. The Western basin is widelyaccepted as having rifted in the Aptian-Albian. Thereis plenty of evidence of synrift deposits of this ageonshore in Turkey, and of Aptian-Albian synriftgeometries on offshore seismic data. The opening ofthe Eastern Black Sea is harder to date, partly becausesynrift deposits appear to be absent, but a consensusseems to be emerging that it is younger than the West-ern Black Sea, within end-Cretaceous to Paleocene.Spadini et al. (this volume) summarize the results oftheir work modeling the subsidence and sedimenta-tion history of the two basins. They show how West-ern and Eastern basins appear to have formed on quitedifferent lithospheres with quite different thermal andmechanical properties. They show how these differ-ences influenced the subsequent development of thebasins, and how they can explain the absence of synriftdeposits in the Eastern basin and the variable thick-nesses of the early postrift stratigraphies.

These two most recent Black Sea basins are verymuch the tip of a marginal-basin iceberg. Back- andintra-arc basins have been opening and closing nearthe southern margin of the East European continentsince at least the beginning of Mesozoic. Examplesinclude the Greater Caucasus basin (Early Jurassic rift)and the Küre-Tavric basin (Triassic), now splitbetween the Central Pontides and Crimea by thespreading of the Western Black Sea (indeed, the West-ern Black Sea rifted on the site of this older basin).Reconstructing the original settings and geometries ofthese older basins is not easy and is not helped by acomplete lack of plausible paleomagnetic data. Banks

and Robinson (this volume) attempt an explanationfor the early Mesozoic basins in terms of strike-slipmotion along major NW-SE faults such as the Pece-neaga-Camena fault in Romania. They envisage a set-ting behind a zone of oblique subduction, rather likethe present-day Andaman Sea.

As the Tethys Ocean finally began to close duringthe Paleogene, the elevation of the Pontides led to thedevelopment of a restricted basin system, partly iso-lated from the world’s oceans and known asParatethys. At times, Paratethys stretched from thePannonian basin in the west as far as the SouthCaspian. Due to widespread provinciality of faunasand flora, the biostratigraphy of Paratethys is a subjectin its own right. A digestible review of its stratigraphyis provided by Jones and Simmons (this volume), whoattempt their explanation in a global sequence strati-graphic framework.

WESTERN MARGIN OF THE WESTERNBLACK SEA: FROM THE SCYTHIAN

PLATFORM TO RHODOPE

The East European, Scythian, and Moesian plat-forms appear to have had rather similar geological his-tories and share a common general stratigraphy. TheMoesian Platform is described by Tari et al. (this vol-ume). They present a series of seismic lines andregional isopach maps that show that this area wasanything but platform-like during its pre-Late Jurassichistory. The area was affected by a Hercynian (pre-Permian) deformation and, during the Middle Triassic,E-W trending rift basins developed in northern Bul-garia and southern Romania. Tari et al. describe theseas failed arms of the Paleotethys Ocean. During theNorian-Rhaetian, the area was again subjected to com-pressional deformation. A foreland fold and thrust beltdeveloped in northern Bulgaria, with the effects of thecompression dying out to the north in Romania. Thisthrust belt was presumably the external (northern) partof the Strandzha orogenic belt (Banks, this volume).The disappearance of compressive structures of LateTriassic age to the north is interesting because north ofthe Peceneaga-Camena fault and the North Dobrogeanorogenic belt, on the southern edge of the ScythianPlatform, there is another belt of thin-skinned compres-sive deformation beneath undeformed Upper Jurassiclimestones that also appears to be of Triassic age(Odessmorgeologia, unpublished data; see map, Figure1). These two foldbelts—in north Bulgaria and south-ern Ukraine—could be juxtaposed by 200–300 km ofsinistral strike-slip movement along the Peceneaga-Camena fault during or prior to the Late Jurassic. Suchmovement is independently proposed by Banks andRobinson (this volume) to explain the development ofLate Triassic? extensional basins that closed prior to orduring the Middle Jurassic.

During the Late Jurassic and Neocomian (and pos-sibly earlier), a south-facing passive margin devel-oped, on which a carbonate ramp formed. The UpperJurassic carbonates are of interest from the point of

view of petroleum exploration, as they form at leastone oil reservoir—at the Tyulenovo oil field on the Bul-garian coast. The sedimentology of the carbonates isdescribed in detail by Harbury and Cohen (this vol-ume). They show a transition between shallow-marinecarbonates in the north and deep-marine facies in thesouth, with the transition trending approximately east-west and passing through Varna in northern Bulgaria.By the Hauterivian, a clastic wedge had developed onthe margin. Following a mid-Cretaceous hiatus, pre-sumably reflecting the location of the area at the west-ern margin of the Aptian Western Black Sea rift, UpperCretaceous facies are outer shelf to bathyal. At thistime, the southern margin of the Moesian Platform(where the Balkanides are now) would have beenlinked to the Western Black Sea. The subsidence andstructural history of the Balkanides are the subject ofthe chapter by Sinclair et al. (this volume). Using acombination of seismic, well, and outcrop data, theyshow how, at around the end of the Cretaceous, theJurassic extensional faults on the south-facing passivemargin were inverted as compression in the Balkanideregion began. Most of the shortening is, however, thin-skinned and of early to middle Eocene age. Compres-sion had ended by the beginning of the Miocene.

The Balkanides are also described by Banks (thisvolume) in a chapter that attempts to integrate thegeological histories of all of the major tectonic unitsalong a north-south transect from the Danube delta toThrace. Banks describes how the early Mesozoicstratigraphy of the Strandzha Zone (Istranca in Turk-ish) also restores to a south-facing passive margin, andhow the stratigraphy is similar to that exposed in theBalkanides. He suggests a Late Triassic age for theonset of compression (which is the same as thatdemonstrated by Tari et al. for the Moesian Platformfurther north), but believes the main deformation to bepost-Middle Jurassic. Strandzha has also been affectedby Tertiary compression. These relationships suggestthat there is little extension associated with theSrednogorie Zone, which contains more than 3 km ofUpper Cretaceous marine volcanics and volcaniclasticsediments. The chemistry of the volcanics suggeststhat the zone principally represents a subduction-related volcanic arc.

SOUTHERN MARGIN OF THE BLACKSEA BASINS: THE PONTIDES, ACHARA-

TRIALET BELT, AND ASSOCIATEDFORELAND BASINS

The Pontides are a complex belt of compressionaldeformation that runs from Thrace in the west toGeorgia in the east, forming the northern coast ofTurkey and extending offshore to affect the postriftfills of both Western and Eastern Black Sea basins. Thenorthern deformation front mainly runs a few tens ofkilometers offshore. The Pontides are bounded to thesouth by a broad subduction-accretion complex withophiolites that was assembled through the Mesozoic

Introduction: Tectonic Elements of the Black Sea Region 3

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and early Tertiary as the Tethys Ocean closed. ThePontides are in fact geologically quite diverse andinclude at least three major terranes (and probablyother smaller ones): the Western, Central, and EasternPontides. Each of these has its own characteristic his-tory of compressive, extensional, and strike-slip defor-mation, and each incorporates more than one basintype. An overview of the entire province is providedhere by Yılmaz et al. (this volume), who summarizeyears of field studies by Turkish workers. Althoughthis chapter is long and presupposes at least some geo-graphic familiarity with northern Turkey, it includes atremendous amount of detailed information that hasfor the most part been published only in Turkish.

The Western Pontides are characterized by anunmetamorphosed Cambrian–Ordovician to Carbonif-erous sedimentary sequence resting on Precambrianmetamorphic basement. On this is superimposed a Tri-assic rift to passive margin sequence followed by a LateJurassic carbonate ramp. The similarity to the MoesianPlatform is clear. Indeed, it is now widely accepted thatthe Western Pontides rifted from the Moesian Platformduring the Aptian–Albian and moved south to theirpresent position during the Late Cretaceous, leavingbehind the Western Black Sea. In north Turkey, thetransition from shallow to deep-marine limestones inthe Upper Jurassic takes place close to the position ofthe North Anatolian fault zone. Prior to the Aptian, thisarea would therefore restore to a position immediatelyeast of the present-day Balkanides. The onshore evi-dence for Aptian–Albian rifting and Late Cretaceouspostrift subsidence is summarized by Görür (this vol-ume). The relevant stratigraphy has been exposed byTertiary inversion of the mid-Cretaceous extensionalfaults. Görür and Tüysüz (this volume) provide a sum-mary of the petroleum potential of this part of northernTurkey. Despite many years of onshore, as well as someoffshore, exploration, there has been no petroleum dis-covery in the Pontides despite the presence of severaloil and gas seeps (Derman and Iztan, this volume). Myguess is that this is due to a timing problem: Sourcerocks in the Ordovician/Silurian and possibly theAptian were largely matured during or prior to rifting.Most of the traps are compressional and of principallyEocene age.

The Central Pontides are quite different from theWestern Pontides. They have an ophiolitic basementcovered by Triassic–Lower Jurassic turbidites andintruded by Mid-Jurassic postcollisional granites. Theevolution of this area is covered by Ustaömer andRobertson (this volume). They describe the presence ofa late Paleozoic oceanic arc and back-arc basin. Theback-arc basin closed before Late Jurassic. Whatappears to be the same Triassic to Late Jurassic back-arc basin fill can be found in Crimea and in the off-shore part of the Ukrainian Black Sea, the two areashaving been separated by Late Cretaceous spreadingin the Western Black Sea. Banks and Robinson (thisvolume) accordingly believe that the eastern trans-form margin of the Western Black Sea runs betweenthe Eastern and Central Pontides, along the westernedge of the Mid-Black Sea High, and that the bound-ary between the Western and Central Pontides can be

linked to the Peceneaga-Camena fault in Romania.Görür (this volume) links the southern and northernmargins of the Western Black Sea in a different way,joining the Western/Central Pontides boundary to apostulated West Crimean fault west of the CrimeanPeninsula (see also Okay et al., 1994).

The Eastern Pontides have been the subject of littleattention until recently. Their geological history isdescribed by Okay and Sahintürk (this volume). Thepre-Jurassic deposits have a typically Laurasian charac-ter, indicating that the boundary with Gondwana layfurther to the south. The usual south-facing carbonate-dominated passive margin can be recognized for theLate Jurassic to Barremian period and is located towardthe south of the Eastern Pontides. The Eastern Pontidesare dominated at outcrop by two more or less coincidentensialic volcanic arcs of Late Cretaceous and Eocene(Lutetian) age. Okay and Sahintürk date the onset ofsubduction here as Cenomanian–Turonian and suggestthat the Eastern Black Sea extensional basin opened inan intra-arc setting during the Maastrichtian, rather ear-lier than the Late Paleocene suggested by Robinson et al.(1996). There appears to be a consensus emerging thatthe Eastern Black Sea is not just a separate basin, but issignificantly younger than the Western Black Sea.

Further east, the deformation front jumps (or bends)to the north, and the Eastern Pontides merge into theAchara-Trialet Belt in Georgia. Banks et al. (this vol-ume) describe a study of this fold belt and the relatedRioni and Kartli foreland basins, incorporating inter-pretations of geological maps, fieldwork, and seismicand onshore well data. The Achara-Trialet Belt appearsto be a Paleogene extensional (back- or intra-arc) basinthat began to close during the late Eocene or Oligocene.The stratigraphy is dominated by a very thick deep-marine Eocene sequence, including volcanics and vol-canogenic turbidites in the Middle Eocene, theequivalents of the Eastern Pontides Lutetian arc. Thissequence—in particular the very thick Eocene—is verysimilar to the stratigraphy interpreted for the EasternBlack Sea by Spadini et al. (this volume). Indeed, Bankset al. consider that the Achara-Trialet Belt represents theclosed eastern end of the Eastern Black Sea. The Rioniand Kartli basins developed mainly during Miocene, byflexure of the northern margin of the Eastern BlackSea—Achara-Trialet Basin (the Shatsky Ridge) beneaththe compressional fold belts to the north (Greater Cau-casus) and the south. There has been oil explorationonshore in Georgia for around a century, mostly con-centrating on the frontal folds of the Achara-Trialet Beltand the Rioni and Kartli basins. The petroleum geologyof this area is described by Robinson et al. (this volume).They provide further evidence for the regional develop-ment of an Upper Eocene oil-prone source rock (see alsoRobinson et al., 1996).

NORTHERN MARGIN OF THE BLACKSEA BASINS: GREATER CAUCASUS,

CRIMEA, AND GULF OF ODESSA

Regretfully, the northern margins of the Black Seaare not well represented in this volume, or in any

accessible literature. The Russian coast is dominatedby the Greater Caucasus. This appears from geologicalmaps to represent an Early Jurassic extensional basinthat accumulated a great thickness of Jurassic and Cre-taceous deep-marine sediments before being closedfrom the Late Eocene onward (Robinson et al., 1996). Itcan perhaps thus be considered a forerunner of theEastern Black Sea. The main foreland basin develop-ment is to the north of the mountain belt (Indolo-Kuban Basin), but the Greater Caucasus had an effecton the Kartli Basin (in Georgia) (Robinson et al., thisvolume) and produced the small flexural TuapseTrough (offshore Russia).

Toward the northeast, the Greater Caucasus seemto die out. It is tempting to link them with theCrimean fold belt, but the stratigraphy and style ofdeformation in the Crimea are quite different. Robin-son and Kerusov (this volume) describe the seismicstratigraphy of the offshore Gulf of Odessa (knownalso locally as the North-West Shelf) and relate theoffshore to the exposed geology in Crimea. Theyinterpret offshore features and the Crimean Moun-tains to represent Aptian–Albian half-grabens (devel-oped during Western Black Sea rifting) that wereinverted from the middle Eocene onward. The degreeof inversion is minor offshore, but very substantial inCrimea. Gas-condensate fields offshore are found ininversion anticlines above Aptian–Albian extensionalfaults and may have been sourced from the Paleozoicof the Scythian Platform.

ACKNOWLEDGMENTS

I would like to thank JKX Oil & Gas plc for sup-porting the preparation of this Memoir, in particularMartin Miller. Many other people have been mosthelpful: I am grateful especially to Alexandru Maftei(Enterprise Oil) and to David Roberts (BP Explo-ration). Istanbul Technical University wins the awardfor preparing papers on time. Thanks to Petroconsul-tants for providing a Black Sea base map.

REFERENCES CITED

Banks, C.J., this volume, Basins and thrust belts of theBalkan Coast of the Black Sea, in A.G. Robinson, ed.,Regional and petroleum geology of the Black Seaand surrounding region: AAPG Memoir 68, p. 115-126

Banks, C.J., and A.G. Robinson, this volume, Mesozoicstrike-slip back-arc basins of the Western Black Searegion, in A.G. Robinson, ed., Regional andpetroleum geology of the Black Sea and surround-ing region: AAPG Memoir 68, p. 53–62

Banks, C.J., A.G. Robinson, and M.P. Williams, thisvolume, Structure and regional tectonics of theAchara-Trialet Fold Belt and the Adjacent Rioni andKartli Foreland Basins, Republic of Georgia, in A.G.Robinson, ed., Regional and petroleum geology ofthe Black Sea and surrounding region: AAPG Mem-oir 68, p. 331–346.

Benton, J., this volume, Exploration history of theBlack Sea province, in A.G. Robinson, ed., Regionaland petroleum geology of the Black Sea and sur-rounding region: AAPG Memoir 68, p. 7–18.

Brinkmann, R., 1974, Geologic relations between theBlack Sea and Anatolia, in D.A. Ross and E.T.Degens, eds., 1974, The Black Sea—geology, chem-istry, and biology: AAPG Memoir 20, p. 63–76.

Derman, A.S., and Y.H. Iztan, this volume, Results ofgeochemical analysis of seeps and potential sourcerocks from Northern Turkey and the Turkish BlackSea, in A.G. Robinson, ed., Regional and petroleumgeology of the Black Sea and surrounding region:AAPG Memoir 68, p. 313–330.

Finetti, I., G. Bricchi, A. Del Ben, M. Pipan, and Z.Xuan, 1988, Geophysical study of the Black Sea area:Bolletino di Geofisica Teorica ed Applicata, v. 30, p. 197–324.

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Görür, N., and O, Tüysüz, this volume, Petroleumgeology of the southern continental margin of theBlack Sea, in A.G. Robinson, ed., Regional andpetroleum geology of the Black Sea and surround-ing region: AAPG Memoir 68, p. 241–254.

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Manetti, P., M. Boccaletti, and A. Peccerillo, 1988, TheBlack Sea: remnant of a marginal basin behind theSrednogorie-Pontides island arc system during UpperCretaceous–Eocene time: Bolletino di Geofisica Teor-ica ed Applicata, v. 30, p. 39–51.

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