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An overview of lithological, palaeontological and geochronological evidence existing for the Palaeozoic formations from Dobrogea and Pre-Dobrogea has enabled a better understanding of the Palaeozoic history of these areas.
Turkish Journal of Earth Sciences (Turkish J Earth Sci.), Vol.A 21,SEGHEDI 2012, pp 669–721 Copyright ©TÜBİTAK doi:10.3906/yer-1101-20 First published online 11 December 2011 Palaeozoic Formations from Dobrogea and Pre-Dobrogea – An Overview ANTONETA SEGHEDI National Institute of Marine Geology and Geoecology, 23−25 Dimitrie Onciul Street, 024053 Bucharest, Romania (E-mail: seghedi@geoecomar.ro) Received 19 January 2011; revised typescript received 02 November 2011; accepted 11 December 2011 Abstract: An overview of lithological, palaeontological and geochronological evidence existing for the Palaeozoic formations from Dobrogea and Pre-Dobrogea has enabled a better understanding of the Palaeozoic history of these areas The Lower Palaeozoic of Pre-Dobrogea, in places in continuity with the pelitic-silty facies of the underlying Vendian (Ediacaran) deposits, was one of the peri-Tornquist basins of Baltica, suggesting that the Scythian Platform in the Pre-Dobrogea basement represents the rifted margin of the East European Craton In North Dobrogea two types of Palaeozoic succession have formed in different tectonic settings Deep marine Ordovician–Devonian deposits, including pelagic cherts and shales, associated with turbidites, and facing Devonian carbonate platform deposits of the East European Craton, form northward-younging tectonic units of an accretionary wedge, tectonically accreted above a south-dipping subduction zone South of the accretionary prism, the basinal to shallow marine Silurian–Devonian deposits of North Dobrogea, showing a similar lithology to the East Moesian successions, accumulated on top of lowgrade Cambrian clastics with Avalonian affinity indicated by detrital zircons Late Palaeozoic erosion was accompanied by deposition of continental alluvial, fluvial and volcano-sedimentary successions, overlying their basement above an imprecise Carboniferous gap The low-grade metamorphic Boclugea terrane, showing Avalonian affinity, and the associated Lower Palaeozoic deposits represent East Moesian successions, docked to Baltica by the Lower Devonian and subsequently involved in the Hercynian orogeny, being affected by Late Carboniferous–Early Permian regional metamorphism and granite intrusion The Late Carboniferous–Early Permian syn-tectonic sedimentation, regional metamorphism of Palaeozoic formations and development of a calc-alkaline volcano-plutonic arc indicate an active plate margin setting and an upper plate position of the Măcin-type successions during the Variscan collision, when the Orliga terrane, with Cadomian affinity, was accreted to Laurussia along a north-dipping subduction zone of the Rheic Ocean The East Moesian Lower Palaeozoic succession, overstepping its Ediacaran basement, represents an Avalonian terrane, docked to the Baltica margin in the Early Palaeozoic A narrow terrane detached from the Trans-European Suture Zone (TESZ) margin of the Baltica palaeocontinent forms a tectonic wedge within the East Moesian basement The Palaeozoic sedimentary record of East Moesia shows a quartzitic facies in the Ordovician, graptolite shales in Upper Ordovician–Wenlock, black argillites in the Ludlow-Pridoli and fine-grained clastics in the Lower Devonian Eifelian continental sandstones are followed by a carbonate platform from Givetian to Tournaisian times and coalbearing clastics in the Carboniferous, indicating a foredeep basin evolution By the Eifelian both East Moesia and Pre-Dobrogea were part of Laurussia, sharing the same old red sandstone facies The Permian is a time of rifting in Dobrogea and Pre-Dobrogea, although evidence for rifting in the East Moesian sedimentary record is very limited In the eastern basins of Pre-Dobrogea, Permian rifting was accompanied by alkaline bimodal volcanism of the basalttrachyte association, that affected also the northern margin of North Dobrogea Late Permian within-plate alkaline magmatic activity emplaced plutonic and hypabyssal complexes along the south-western margin of North Dobrogea The model proposed for the Palaeozoic history based on existing data for the north-western margin of the Black Sea records early Palaeozoic docking to Baltica of the Avalonian terrane of East Moesia, including the Boclugea terrane of North Dobrogea Late Carboniferous–Early Permian accretion of the Cadomian Orliga terrane from North Dobrogea, accompanied by Hercynian metamorphism and granite intrusion, correlates with the closure of the Rheic Ocean Subsequently, Avalonian and Cadomian terranes, together with a narrow terrane detached from the TESZ margin of Baltica palaeocontinent, were displaced southward along the strike-slip fault system of the TESZ Key Words: North Dobrogea Orogen, Moesian Platform, Scythian Platform, lithology, biostratigraphy Dobruca ve Ön-Dobruca’nın Paleozoyik Formasyonları Özet: Dobruca ve Ön-Dobruca’daki Paleozoyik formasyonlarının litolojik, paleontolojik ve jeokronolojik ửzelliklerinin gửzden geỗirilmesi bu bửlgelerin Paleozoyik tarihỗelerinin daha iyi anlaşılmasını sağlar Ön-Dobruca’nın Alt Paleozoyik 669 PALAEOZOIC FROM DOBROGEA AND PRE-DOBROGEA istifleri, Ưn-Dobruca’nın bazı bưlgelerinde daha altta yer alan Vediyen (Edikariyen) pelitik-siltli fasiyeslerle devamlılık gösterir, ve Baltika’nın peri-Tornquist havzalarndan birini oluturur Bu durum ện-Dobrucay da iỗine alan skit Platformu’nun Doğu Avrupa Kratonu’nundan riftleşme ile ayrıldığına işaret etmektedir Kuzey Dobruca’da farklı tektonik ortamlara işaret eden iki tip Paleozoyik istif bulunur Çưrt ve şeyl ve bunlarla ilişkili türbiditlerden oluşan derin denizel OrdovisyenDevoniyen ỗửkelleri, ve bu ỗửkellerin Dou Avrupa Kratonuna bakan kenarında gelişmiş Devoniyen karbonat platformu, güneye doğru dalan bir dalma-batma zonunda gelişmiş bir eklenir prizma oluşturur Eklenir prizmanın güneyinde, derinden s denize kadar deien Kuzey Dobrucann SiluriyenDevoniyen ỗửkelleri, Dou Moezya istflerine benzerlik gösterir, ve kırıntılı zirkonlarla Avalonya’ya bağlı olduğu saptanan dỹỹk dereceli Kambriyen krntllar ỹzerinde ỗửkelmitir Geỗ Paleozoyikte gelien erozyon ve bửlgenin karalamas sonucu karasal ỗửkeller ve volkanik kayalar, arada bir Karbonifer boşluğu olmak üzere bu temel üzerinde yer alır Düşük dereceli metamorfik kayalardan oluşan Bokluca mıntıkası Avalonya özellikleri gösterir, ve Bokluca’ya bağlı Alt Paleozoyik kayaları Doğu Moezya özellikleri taşır; bu birimler Erken Devoniyende Baltika ile ỗarpm ve daha sonra Geỗ KarboniferErken Permiyende rejyonal metamorfizma ve granitik sokulumlar ile tanmlanan Hersiniyen orojenezi geỗirmitir Bu ửzellikler ve Geỗ KarboniferErken Permiyen yal tektonizma ile eşyaşlı sedimentasyon, bölgenin bu dönemde aktif bir kıta kenarı konumunda olduğuna işaret eder Kadomiyen özellikler gösteren Orliga mıntıkası, Reik Okyansu’nun kuzeye doğru dalıp yok olması sonucu Lavrasya’ya eklenmiştir Doğu Moezya’nın Alt Paleozoyik istifleri, Erken Paleozoyik’te Baltika’ya yamanan Avalonya tipi bir mıntıkaya aittir Baltika’nın Trans-Avrupa Kenet Zonu (TESZ) kıta kenarnan ayrlm ince bir mntka Dou Moezya temeli iỗinde bir kıymık oluşturur Doğu Moezya’nın sedimenter istifi, Ordovisyen’de kuvarsitik fasiyesler, Üst OrdovisyenVenlokta graptolitli eyller, LudlovPridolide siyah ỗamur talar, Alt Devoniyende ince taneli kırıntılılardan yapılmıştır Efyeliyen yaşlı karasal kumtaşlarını takiben Givetiyen– Turnaziyen zaman aralığında platform karbonatları gelişmiş, ve daha sonra Karbonifer’de kömür iỗeren krntllar, bir ửn-ỹlke havzasnda ỗửkelmitir Eyfeliyende hem ện-Dobruca hem de Doğu Moezya, Lavrasya’nın yamanmıştır ve benzer kırmızı kumtaşı fasiyesleri gưsterirler Permiyen’de Dobruca ve Ưn-Dobruca’da riftleşme gưzlenir, buna karşın Doğu Moezyada rifitleme ile ilgili ỗửkel kaytlar ỗok ktdr ện-Dobrucann dou havzalarında Permiyen riftleşmesi ile beraber bazalt-trakit birlikteliğinden oluşan alkalin bimodal volkanizma gelişmiş, ve bu volkanizma Kuzey Dobruca’nın kuzey kenarını da etkilemitir Geỗ Permiyen levha-iỗi alkalin magmatizma sonucu Kuzey Dobrucann gỹneybat kenarı boyunca derinlik ve yarı-derinlik kayaları yerleşmiştir Burada sunulan model, Erken Paleozoyik’te Baltika’nın güney sınırı boyunca Doğu Moezya ve Kuzey Dobrucann Bokluca mntkasn iỗeren Avalonya kửkenli kta parỗaỗklarnn Baltikaya eklenmesini iỗerir Kuzey Dobrucann Kadomiyen kửkenli Orliga mntkas Geỗ Karbonifer Erken Permiyen’de kuzeye eklenmiş ve bu olay sonucu Reik Okyanusu kapanarak Hersiniyen metamorfizmas ve granit yerleimi gerỗeklemitir Bu olaylar takiben Avalonya ve Kadomiyen kökenli mıntıkalar, ve Baltika’nın TESZ kenarından kopan ince bir mıntıka, TESZ boyunca gelişen doğrultu-atımlı faylar boyunca güneye doğru ötelenmiştir Anahtar Sözcükler: Kuzey Dobruca orojeni, Moezya Platformu, İskit platformu, litoloji, biyostratigrafi Introduction The western margin of the East European Craton, a major terrane boundary along the contact between the stable Precambrian Fennoscandian-East European Craton and the younger structures of Western and Southern Europe, was defined as the Trans-European Suture Zone or TESZ (Pharaoh 1999) (Figure 1) Along the TESZ, peri-Gondwanan terranes of Far East Avalonia are found, accreted to the former Baltica palaeocontinent during the Lower Palaeozoic (Ziegler 1986, 1988; Pharaoh 1999; Winchester et al 2002, 2006), are mingled with proximal Baltican terranes All these terranes with Avalonian and Baltican affinity are variously displaced together along the strike-slip faults of the TESZ (Winchester et al 2002, 2006; Nawrocki & Poprawa 2006; Oczlon et al 2007) The southeastern segment of the TESZ 670 runs through the southwestern part of Ukraine and Moldavia, continuing to the Black Sea through south-eastern Romania The northwestern margin of the Black Sea includes three major structural units: the westernmost segment of the Scythian Platform, the North Dobrogea Orogen and the Moesian Platform All these units include Palaeozoic formations, concealed in the platforms and exposed in North Dobrogea In order to better understand the geological evolution of this area and improve palaeogeographic models, it is important to establish or update terrane affinities Due to limited reliable information and still poorly defined terrane affinities, most palaeocontinental reconstructions for Moesia and/or Dobrogea assume that each forms one single terrane (Mosar & Seghedi 1999; Stampfli 2000; Kalvoda et al 2002; Cocks & Torsvik 2005, A SEGHEDI Figure Location of Dobrogea on a simplified terrane map of Europe (modified after the TESZ map of EUROPROBE project) US– Ukrainian shield, VM– Voronezh massif, EC– East Carpathians, SC– South Carpathians, SP– Scythian Platform, MP– Moesian Platform, NDO– North Dobrogea orogen 2006; Nawrocki & Poprawa 2006; Winchester et al 2006) However, from detailed analysis of various existing data, terranes with both Baltican and Avalonian palaeogeographic affinities were inferred to make up the Moesian Platform and a model for their displacement along the TESZ, together with the North Dobrogea terrane, was proposed (Oczlon et al 2007) Detrital zircon data enabled separation of Avalonian and Cadomian terranes in the North Dobrogea metamorphic suites, brought together following the closure of the Rheic Ocean (Balintoni et al 2010) The goal of this paper is to provide an overview of the lithological, biostratigraphical and geochronological information from the Palaeozoic formations in the northwest Black Sea area and comment on the evidence for Palaeogeographic affinities The Palaeozoic record from Pre-Dobrogea presented here is the result of correlation of borehole data across state borders, based on the petrographic studies of thin sections provided by the former Oil Institute in Bucharest and the Geological Institute from Kishinev Core samples stored at the Geological Institute from Kishinev and the Geological Institute of Romania have also been examined For North Dobrogea, the review of geological data is based on papers published in local journals, abstracts and field guide books, as well as on unpublished reports and PhD theses The data on East Moesia are summarized according to the synthesis of the Moesian Palaeozoic from Romania (Seghedi et al 2005a, b), to serve as a basis for comparison with North Dobrogea and PreDobrogea 671 PALAEOZOIC FROM DOBROGEA AND PRE-DOBROGEA Geological and Tectonic Background The north-western margin of the Black Sea is a highland area referred to as Dobrogea, a geographical and historical province confined between the Black Sea shore, the Sfântu Gheorghe Distributary of the Danube Delta and the lower reaches of the Danube River Dobrogea is surrounded by the lowlands of the Pre-Dobrogea Depression in the north and the Romanian Plain in the west (Figure 2) The largest part of Dobrogea belongs to Romania, except for its southern margin that continues for some distance in Bulgaria North of the Dobrogea highlands, the flatlying Pre-Dobrogea Depression is shared by three countries: Romania, Moldavia and Ukraine The area consists of three main tectonic units, two Palaeozoic platforms (Moesian and Scythian) and the Cimmerian Orogen of North Dobrogea (Săndulescu 1984) (Figure 3) While the Scythian Platform is Figure The main geotectonic units of the western Black Sea margin The area with darker grey shading represents exposures of pre-Cenozoic rocks EM– East Moesia; WM– West Moesia; SGF– Sfantu Gheorghe Fault; PCF– Peceneaga-Camena Fault; COF– Capidava-Ovidiu Fault; PF– Palazu Fault; EF– Eforie Fault; IMF– IntraMoesian Fault 672 A SEGHEDI Post-tectonic cover (Babadağ Basin) Palaeozoic platform cover platform cover Figure Schematic map showing the main structural units of Dobrogea (modified from Seghedi et al 2005a) ND– North Dobrogea; CD– Central Dobrogea; SD– South Dobrogea; LCF– Luncaviţa-Consul Fault; other abbreviations as in Figure entirely concealed by Quaternary deposits, the eastern parts of the North Dobrogea orogen and of the Moesian Platform are exposed in North Dobrogea and Central and South Dobrogea, respectively was part of the northern rift shoulder of the Western Black Sea Basin, which sourced the kaolinite-rich Aptian syn-rift sediments preserved both in the PreDobrogea depression and South Dobrogea (Rădan 1989; Ion et al 2002) Separated from the Scythian Platform by the Sfântu Gheorghe Fault and bounded southward by the Peceneaga-Camena Fault, North Dobrogea represents the south-eastern part of the North Dobrogea Cimmerian Orogen, where the Hercynian basement and its Mesozoic cover are exposed (Figures & 3) The north-western part of the belt is covered by Cenozoic deposits of the Carpathian foredeep Surrounded by the Carpathians and the Balkans, the Moesian Platform is an area with a heterogeneous and complex Precambrian basement overlain by a thick Palaeozoic to Cenozoic cover West of the Black Sea, the eastern part of the platform (East Moesia) consists of two tectonic provinces separated by the Capidava-Ovidiu Fault (Figure 3) The stratigraphic gap between the Late Jurassic and Cenomanian sediments which seal both the Cimmerian structures of North Dobrogea, as well as the Peceneaga-Camena Fault, suggests that throughout the Early Cretaceous North Dobrogea Confined between the Peceneaga-Camena and Capidava-Ovidiu crustal faults, Central Dobrogea exposes the Neoproterozoic Moesian basement The flat-lying Palaeozoic cover, preserved only west of the Danube, above the subsided part of the 673 PALAEOZOIC FROM DOBROGEA AND PRE-DOBROGEA Neoproterozoic basement, has been completely removed from the uplifted block of Central Dobrogea during an unspecified period of pre-Bathonian erosion In the outcrop area, the Neoproterozoic basement is unconformably covered by Bathonian– Kimmeridgian carbonate platform successions above local remnants of a pre-Bathonian weathering crust (Rădan 1999) (Figure 3) Pre-Dobrogea Depression Other authors regard this area as the southern passive margin margin of the East European craton reworked by Late Proterozoic (Early Baikalian) and younger tectonism (Kruglov & Tsypko 1988; Gerasimov 1994; Drumea et al 1996; Milanovsky 1996; Pavliuk et al 1998; Poluchtovich et al 1998; Stephenson 2004; Stephenson et al 2004; Saintot et al 2006) South Dobrogea is a subsided Moesian block, along the Capidava-Ovidiu and Intramoesian faults This block exposes only the Mesozoic–Cenozoic Moesian cover with frequent discontinuities and gaps West and north-west of the Danube River, the corresponding parts of these main units of Dobrogea, concealed by Cenozoic deposits, lie at depths of over 600 m in the footwall of the Danube Fault (Gavăt et al 1967) (Figure 2) The basement of the Pre-Dobrogea depression is a highly tectonized area about 100 km wide, defined against the neighbouring units by the crustal faults Baimaklia-Artiz (or Leovo-Comrat-Dnestr) and Sfântu Gheorghe, known from geophysical and drilling data (Figure 4) The structure of the preMesozoic basement is defined by the intersection of two major fault systems A system of WNW– ESE-trending, parallel faults has controlled the development of intrabasinal longitudinal ridges This is intersected by a N–S-trending fault system, best developed east of the Prut River (Neaga & Moroz 1987; Drumea et al 1996; Ioane et al 1996; Visarion & Neaga 1997) The Pre-Dobrogea Depression The Pre-Dobrogea depression represents a Mesozoic– Tertiary depression superimposed on a pre-Triassic basement According to Săndulescu (1984), the PreDobrogea basement is the westernmost segment of the epi-Variscan Scythian Platform Running E–W along the south-western corner of the East European Craton, the Scythian Platform is buried westward beneath the Tertiary molasse foredeep of the East Carpathians Both the age of cratonization and Variscan history of this tectonic unit are quite controversial Located between the East European Craton and the Alpine-Cimmerian folded belts on its southern border, the Scythian Platform is classically defined as a wide Variscan belt referred to as the ‘Scythian orogen’ (Mouratov & Tseisler 1982; Milanovsky 1987; Zonenshain et al 1990; Nikishin et al 1996) With active orogenesis supposed to have occurred from Early Carboniferous to Permian times (Nikishin et al 1996, 2001; Stampfli & Borel 2002), it has usually been considered to represent the link between the Variscan orogen of western and central Europe and the Uralian belt at the eastern edge of the East European Platform In the Mesozoic, the ‘Scythian orogen’ showed a platform stage of development (Mouratov 1979), its basement being concealed by the superimposed Mesozoic–Tertiary deposits of the 674 Beneath the flat-lying Middle Jurassic–Tertiary cover, the area of the Pre-Dobrogea Depression is a Permian palaeorift (Neaga & Moroz 1987) A longitudinal high, the Bolgrad-Chilia (or Lower Prut) Horst (Neaga & Moroz 1987; Moroz et al 1997; Visarion & Neaga 1997), separates two depressions elongated E–W (Figures & 5) The northern depression includes the Sărata-Tuzla and Aluat basins, separated by the OrehovkaSuvorovo basement high The Sărata-Tuzla basin is complicated by a minor longitudinal intrabasinal ridge The Aluat basin continues into Romania in the WNW-elongated Bârlad depression; this basin shows a staircase geometry, its bottom being progressively downthrown westward towards the East Carpathian foredeep This is accommodated by a system of N–S faults, reactivated as result of nappe stacking in the East Carpathians The Sulina (or Lower Danube) basin, with its depocentre situated in the Danube Delta, developed south of the Bolgrad-Chilia high (Figure 4) The basement lies at depths of 1–1.5 km in areas of basement highs and at depths of 3–4 km in depressions (Mouratov & Tseisler1982) The basement of the Pre-Dobrogea consists of magmatic A SEGHEDI Figure Distribution of the main basins of the Scythian Platform in the Predobrogea depression, with location of the main boreholes (compiled after Neaga & Moroz 1987; Paraschiv 1986; Pană 1997) Shades of grey represent grabens and highs, respectively rocks (granites, diorites and gabbros), that yield Neoproterozoic K-Ar ages (790, 640–620 Ma) (Neaga & Moroz 1987) (Figure 5) In the central part of the Bolgrad-Chilia and Orehovka-Suvorovo highs, the magmatic basement is unconformably overlain by undeformed Vendian deposits These deposits were intersected by the Orehovka and Suvorovo boreholes for a thickness of over 2000 m Remnants of a palaeo-weathering crust were found on top of the magmatic basement rocks (Neaga & Moroz 1987) The Neoproterozoic Ediacaran (Vendian) age is derived from a phytocenosis with Vendotaenia antiqua Grujilov, identified in the Orehovka borehole (Visarion et al 1993) A K-Ar age of 600±20 Ma yielded by pelitic rocks is consistent with the palynological data The Vendian succession (Avdărma Series) is upward shallowing, including marine sediments (conglomerates, sandstones, volcanic sands, black 675 PALAEOZOIC FROM DOBROGEA AND PRE-DOBROGEA Figure Mesozoic subcrop map of the Scythian Platform, showing the distribution and structure of the Neoproterozoic and Palaeozoic formations (compiled after Visarion et al 1993; Ioane et al 1996 and Visarion & Neaga 1997) shales rich in phosphate nodules and grey siltstones, sandstones and mudstones) grading upward into continental deposits (conglomerates and purple 676 pebbly sandstones, with siltstone and mudstone interbeds) With these features, the Vendian deposits of the Pre-Dobrogea resemble the Avdărma Series A SEGHEDI from the East European Platform cover, exposed along the Dnestr River, the only difference being the lesser thickness of the latter (only 600–700 m) The platform cover of the Pre-Dobrogea comprises mainly Cretaceous to Quaternary sediments with a total thickness of 2500 to 3500 m (Permyakov & Maidanovich 1984) Due to the high mobility of the area, repeatedly subjected to oscillatory movements, the sedimentary cover shows stratigraphic gaps and unconformities, being characterized by the absence of the Lower Jurassic and thin Lower Cretaceous deposits (Ionesi 1994) According to the synthesis of Ion et al (2002), the Mesozoic cover includes Callovian–Oxfordian black shales in the Danube Delta and carbonates in the rest of the Pre-Dobrogea, overlain by Oxfordian–Kimmeridgian carbonate platform limestone Dolomites, clastics and evaporites develop in the Berriasian–Valanginian; dolomites, dolomites and clastics in the Middle–late Aptian, while clastics occupy the northern half of the Delta in the Sarmatian followed by Late Meotian shales, and the Quaternary is represented by sand and clay of marine, fluvial and lacustrine origin North Dobrogea The narrow, NW-trending Cimmerian fold and thrust belt of the North Dobrogea orogen (Figure 3) is a basement with a Hercynian history of magmatism and deformation This basement was subsequently involved in early Alpine (Cimmerian) events, experiencing extension during the Late Permian– Middle Triassic, and compression (transpression) in the Late Triassic–Middle Jurassic (Seghedi 2001) A major high-angle reverse fault with a NW–SE strike (Luncaviţa-Consul Fault, Savul 1935), represents the contact between the Măcin and Tulcea zones (Mutihac 1964) (Figure 6) These zones refer to areas with dominantly Palaeozoic and Triassic exposures respectively; Jurassic deposits occur in limited areas in both zones The Măcin zone (Măcin Nappe, Săndulescu 1984) is a Cimmerian tectonic unit exposing mainly Palaeozoic formations in the western part of North Dobrogea The Tulcea zone is the larger, eastern part of North Dobrogea, exposing mostly Triassic and Jurassic formations included in three Cimmerian thrusts (Săndulescu 1984) All four major Cimmerian thrust bounded units recognised in North Dobrogea have Hercynian deformed basement and Triassic or Triassic–Jurassic cover (Mirăuţă, in Patrulius et al 1973; Săndulescu 1984) (Figure 7) The Cimmerian tectonic units are interpreted either as low-angle nappes, based largely on geophysical data (Săndulescu 1984; Visarion et al 1993), or as high-angle thrusts, based on borehole information (Baltres 1993) The latter tectonic model is figured in the transect VII of the TRANSMED Atlas (Papanikolaou et al 2004) The Măcin Cimmerian tectonic unit (corresponding to the descriptive Măcin zone), exposes largely Palaeozoic formations, with only minor exposures of Triassic deposits (Figures & 6) In the Tulcea zone, exposing mainly Triassic deposits and a few Palaeozoic and Jurassic formations, are three Cimmerian tectonic units (Săndulescu 1984): the Consul, Niculiţel and Tulcea nappes The Triassic succession, unconformable on the Hercynian basement, starts with lower Scythian (Werfenian) continental fanglomerates, followed by sandstones and upper Scythian limestone turbidites (Baltres 1993) Rhyolites and basalts started to be emplaced in the late Scythian, with the basaltic volcanism continuing up to Middle Anisian (Baltres et al 1992) The basaltic volcanism is partly coeval with deposition of nodular and bioturbated limestones and cherty limestones The basinal succession terminates with Late Anisian– late Carnian Halobia marls (Baltres et al 1988) Turbiditic deposits accumulated, starting in the late Carnian and ranging up to the middle Jurassic, with an upward coarsening trend of coarse members (Baltres 1993; Grădinaru 1984) Shallow marine carbonate sedimentation took place along the basin margin from the Scythian to the Norian and in the Oxfordian–Kimmeridgian (Grădinaru 1981; Baltres 1993) Apart from numerous unpublished reports, a detailed presentation of the Triassic–Jurassic deposits of North Dobrogea is given in Grădinaru (1981, 1984, 1988) and Seghedi (2001) An early Cretaceous history is not preserved in the stratigraphic record, except for scarce remnants of a kaolinitic weathering crust, found in several locations on top of the the Hercynian basement or the Triassic deposits In one locality this crust 677 PALAEOZOIC FROM DOBROGEA AND PRE-DOBROGEA Figure Quaternary subcrop map showing the outcrop areas of Palaeozoic formations and the main Cimmerian faults in North Dobrogea (modified from Seghedi 1999) is preserved beneath transgressive Cenomanian calcarenites and was ascribed to the Aptian (Rădan 1989) The post-tectonic cover of the North Dobrogea orogen is represented by the shallow-marine Upper Cretaceous sediments of the ‘Babadag basin’ They seal the deeply truncated Hercynian and Cimmerian structures, overstepping the easternmost segment of the Peceneaga-Camena Fault and overlying the 678 Histria Formation (Figure 3) The Upper Cretaceous succession includes Albian bioclastic limestones, Cenomanian–Turonian detrital limestones with conglomerate interbeds in the eastern part, Coniacian detrital limestones with cherts, chalk and glauconite and Santonian–Campanian nodular limestones developed only along the eastern margin of North Dobrogea (Ion et al 2002 and references therein) A SEGHEDI formed either of recycled crust, or from material not affected by Avalonian-Cadomian continental margin magmatism The Cadomian terranes were supplied with detrital zircons derived from the African craton The configuration of palaeocontinents at the end of the Neoproterozoic is shown in Figure 19 Laurentia, which started to separate from West Gondwana opening the Iapetus Ocean, includes North America, Greenland and the British Isles north of the Iapetus Suture (Cocks & Torsvik 2005) Baltica includes most of Scandinavia and Russia eastward to the Urals Peri-Gondwanan terranes of West and East Avalonia include British Isles and NW Europe south of the Iapetus Suture, Eastern Newfoundland, most of the Maritime Provinces of Canada, parts of the eastern United States (Cocks & Torsvik 2005) Terranes forming the basement of Danubian Nappes in the South Carpathians, as well as the East Moesian basement and the Boclugea terrane are included here (Balintoni et al 2010) Cadomia includes Armorica, Iberia, Saxothuringia, Tepla-Barrandia, proto-Alps, Sakarya, Eastern Pontides, Menderes and Kırşehir (Linnemann et al 2008) to which the Carpathian and Orliga terranes were added (Balintoni et al 2010) The Scythian Platform – Considering the two main models proposed for its evolution, the Scythian Platform may show either Baltican or Avalonian/ Cadomian affinities Discrimination between the two models is not possible yet based on detrital zircon data, which are still lacking If the Scythian Platform represents the southern margin of the East European Craton, involved in Neoproterozoic deformation, it was part of the Baltica palaeocontinent Evidence in favor of this affinity comes from stratigraphical and geophysical data As with the Neoproterozoic–Lower Palaeozoic cover of the East European Platform, the undeformed platform cover of the Scythian Platform starts with the Neoproterozoic clastics and contains Silurian carbonates Geophysical evidence also indicates that the lithosphere and crust of the southern East European Platform and Scythian Platform are thicker than those of the Phanerozoic Western Europe terranes (Yegorova et al 2004; Yegorova & Starostenko 2002) According to the ‘Scythian orogen’ model, the southernmost part of the East European Platform formed a passive margin of Baltica from the Cambrian until the collision and docking of the Scythian Platform crust in the Late Palaeozoic, which in this case would include terranes with both Baltican and peri-Gondwanan affinity This model is based on the assumption that the Scythian Platform represents an eastward prolongation of the Variscan orogen (or Palaeozoic platform) of central and western Europe (Mouratov & Tseisler 1982; Milanovsky 1987; Zonenshain et al 1990; Visarion et al 1993; Nikishin et al 1996, 2001) However, there is no irrevocable evidence that the basement crust of the Scythian Platform includes a Late Palaeozoic (Variscan) accretionary orogenic belt (Seghedi et al 2003; Stephenson et al 2004) As observed in borehole cores, the Palaeozoic successions of the Scythian Platform not show a penetrative deformation related to folding (Seghedi et al 2003b), as seen in North Dobrogea More steeply inclined dips of bedding noticed in boreholes can be explained as result of block tilting and rotation connected to Permian rifting (Seghedi et al 2003b; Saintot et al 2006) North Dobrogea – Results from detrital zircons from samples of Boclugea lithologies indicate several age groups, corresponding to the late Neoproterozoic– early Cambrian, the Mesoproterozoic, the 1.7 and 2.2 Ga interval of the Palaeoproterozoic and the 2.55– 2.85 Ga interval of the Neoarchaean, with age gaps or minima in the intervals 0.75–0.85 Ga, 1.65–1.7 Ga and 2.2–2.5 Ga (Balintoni et al 2010) Considering the significant input from Mesoproterozoic and Palaeoproterozoic sources and only minor zircon contributions from Grenvillian sources, the authors conclude that an East Avalonian palaeogeographic affinity seems likely for the Boclugea terrane Detrital zircon ages for the Orliga terrane show clusters at 0.5–0.75 Ga, 1.75–2.1 Ga, 2.3–2.5 Ga and 2.55–2.85 Ga, with a gap covering the entire Mesoproterozoic and the Grenvillian (Balintoni et al 2010) Because such patterns characterize the Cadomian terranes that originated close to the West African craton (Samson et al 2005; Nance & Murphy 1994), the Orliga terrane is considered a true Cadomian terrane (Balintoni et al 2010) 707 PALAEOZOIC FROM DOBROGEA AND PRE-DOBROGEA Figure 19 Reconstruction of the Late Vendian (ca 550 Ma) (modified from Cocks & Torsvik 2005) Spreading centres are shown as black lines, subduction zones are shown as lines with ticks Transform faults are accompanied by arrows Black areas represent Grenville-Sveconorwegian mobile belts (about Ga) Dark grey shaded areas are regions that show evidence of Avalonian-Cadomian-Pan-African-Baikalian-Timanian deformation, terrane amalgamation and magmatism East Moesia (EM), Boclugea, Danubian, and other correlatives in the Balkans (DB) represent East Avalonian Peri-Gondwanan terranes (Balintoni et al 2010) Cadomia (figured after Linnemann et al 2008), includes Armorica (A), Iberia (Ib), Saxothuringia (ST), Tepla-Barrandia (TB), proto-Alps (PA), Sakarya, Eastern Pontides, Menderes and Kirshehir, with the Carpathian terranes (C) and Orliga (O) added by Balintoni et al (2010) P represents Palazu terrane Based on U-Pb detrital zircon ages, a correlation of Orliga with the Cadomian Sebeş-Lotru terrane and of Boclugea with the Avalonian Danubian terranes from the South Carpathians is possible (Balintoni et al 2010) This correlation suggests that the South Carpathians and North Dobrogea were connected during the Variscan orogeny The continuity was disrupted in the Early Cretaceous, by westward displacement of Moesia, related to the opening of the western Black Sea basin (Balintoni & Baier 1997) East Moesia Evidence for the palaeogeographic affinity of East Moesia is still controversial Based on a detailed analysis of tectonostratigraphic, palaeontological and geochronological data, four distinct terranes, 708 two with Baltican and two with Avalonian affinities have been separated in the entire Moesian Platform (Oczlon et al 2007) According to these authors, East Moesia consists of an Avalonian terrane comprising Central and a large part of South Dobrogea, separated by the narrow Palazu terrane with a basement of Baltican affinity The review of terranes along the southwestern margin of the East European Craton, between the North Sea and the Black Sea, suggests that dextral strike-slip dominated the southwestern Baltican margin during Late Ordovician–Early Devonian accretion of Far Eastern Avalonia (Oczlon et al 2007) Southward displacement of some Avalonian terranes, including East Moesia, occured consequently to the Variscan indentation of the Bohemian Massif, while current juxtaposition of the Moesian terranes is the result of sinistral strike-slip A SEGHEDI displacement during the opening of Mesozoic backarc basins Palynological evidence based on Chitinozoans show a North Gondwanan affinity of the East Moesian Lower Palaeozoic deposits (Vaida et al 2004; Vaida & Verniers 2005), in good agreement with the affinity of the Lochkovian chitinozoans from West Moesia (Northwest Bulgaria) (Lakova 1995; Yanev et al 2005) This however contrasts with the results from miospores (Steemans & Lakova 2004), which support a Baltican affinity for Moesia at that time Lithology and macrofauna of the Lower Palaeozoic in the 5082 Mangalia borehole were previously used to suggest an affinity with the macrofauna identified in the Bosfor area (Bithynia), in basins with continuous marine sedimentation across the Silurian/Devonian boundary (Iordan 1967) The macrofauna of trilobites and tentaculites is also comparable to that from the Armorican and Bohemian Massifs that are considered part of Cadomia (Linnemann et al 2008) accretion of peri-Gondwanan terranes of Far East Avalonia (Pharaoh 1999; Winchester et al 2002, 2006); accretion to Laurussia during the Hercynian orogeny; rifting and displacement along the TESZ, together with slivers of proximal Baltican terranes accommodated along strike-slip faults (Winchester et al 2006; Oczlon et al 2007) Palaeogeographic affinities proposed for the terranes presented here are based mainly on lithology and palaeontological evidence derived from the Palaeozoic record Detrital zircon ages are published only for the Central Dobrogea basement (Żelaźniewicz et al 2001, 2009; Balintoni et al 2011) and for the early Early Palaeozoic Boclugea and Orliga terranes later affected by Hercynian regional metamorphism (Balintoni et al 2010) (Figure 8) The existing evidence indicates that terranes with Baltican, Avalonian and Cadomian affinities occur in the Dobrogea and Pre-Dobrogea area Discussion Baltican affinities are shown by the Palazu terrane and the Pre-Dobrogea basement The Palazu terrane, isolated in the basement of South Dobrogea between the Capidava-Ovidiu and Palazu faults, is a proximal Baltican sliver (Oczlon et al 2007) It is made of Archaean and Palaeoproterozoic crust, similar in composition to the Ukrainian shield (Giuşcă et al 1967, 1976), or the Sarmatia segment of Baltica (as defined by Bogdanova et al 1997) Ediacaran riftrelated volcano-sedimentary successions (Cocoşu Group) are preserved on top of the metamorphic crust This geological association suggests an initial location along the TESZ margin near the VolhynOrsha aulacogen, about 500 km north of the present location of the Palazu terrane The Volhynian series from the Volhyn depression developed within the aulacogen includes lower Vendian flood basalts formed during continental rifting that accompanied Rodinia breakup at about 750 Ma (Kumpulainen & Nystuen 1985; Torsvik et al 1996; Nikishin et al 1996) Lithological, petrographic and geochemical features of the Volhynian basalts (Bakun-Czubarow et al 2002; Białowolska et al 2002) correlate well with those of the Cocoşu Group (Seghedi et al 2000) The geological evolution of the Palaeozoic terranes from Dobrogea and Pre-Dobrogea took place during several events occurring at the south-western margin of the East European Craton: Early Palaeozoic In a series of palaeogeographic maps for the East European Craton (Nikishin et al 1996), the Pre-Dobrogea area is considered part of the Baltica palaeocontinent Fine-grained Vendian lithologies Detrital zircon data exist so far for the Ediacaran basement of East Moesia Detrital zircons yielded U/Pb SHRIMP ages of 1497±8 Ma, 1050±1 Ma, 603±5 Ma and 579±7 Ma (Żelaźniewicz et al 2001), interpreted as Avalonian (Seghedi et al 2005a, b) or Far East Avalonian sources (Oczlon et al 2007) Detrital zircons published by Balintoni et al (2011) show the following age concentrations: the greatest age grouping of 2.65–3.0 Ga is Archean; Palaeoproterozoic ages show a very low frequency around 1.8 Ga and between 2.2–2.6 Ga; the Mesoproterozoic ages peak around 1.5 Ga; Palaeoproterozoic ages cluster around 1.7 Ga and between 1.95–2.2 Ga; late Neoproterozoic ages between 633–583 Ma., suggesting that the Brasiliano orgen (Nance et al 2009) was the most important Neoproterozoic detrital zircon source for the Histria Formation A detailed discussion of the zircon sources and their discrimination is found in Balintoni et al (2011) 709 PALAEOZOIC FROM DOBROGEA AND PRE-DOBROGEA from Pre-Dobrogea, with phosphate nodules and Vendotaenia antiqua, accumulated in the Dnestr basin of Baltica This is one of the very large shelf basins, established along the western slope of the East European Craton during the Ediacaran and referred to as the Peri-Tornquist basin (Nikishin et al 1996) During the Cambrian, the Dnestr basin developed into a passive continental margin, with shallowmarine clastics overstepping the Vendian deposits This evolution is confirmed in the Aluat and Bârlad basins, where Cambrian clastics have been identified The typical facies of Silurian carbonates from the Moldavian and Baltic basins (Sliaupa et al 2006) was not found in Pre-Dobrogea However, the Ordovician and Silurian deposits, identified by chitinozoans in the upper part of what had been considered a Vendian succession, show a siltstone-mudstone facies with sandstone and limestone interbeds (Vaida & Seghedi 1997) This suggests that the transition from the carbonate facies of the Peri-Tornquist Basin to the fine-grained clastic facies of the Peri-Caspian basin (Nikishin et al 1996) took place by lateral facies changes in the Pre-Dobrogea area The Avalonian affinity of East Moesian Lower Palaeozoic successions is suggested by the unconformities within the Silurian and at the base of the Silurian and Devonian, characteristic of Caledonian displaced terranes along the TESZ (Oczlon et al 2007) The peri-Gondwanan, Avalonian provenance of Central Dobrogea is based on the detrital zircon spectrum in the Ediacaran turbidites presented so far (Żelaźniewicz et al 2001, 2009; Balintoni et al 2011) and on cold water acritarchs from the East Moesian Palaeozoic cover (Oczlon et al 2007) In North Dobrogea, terranes with both Avalonian and Cadomian affinities are indicated by detrital zircon ages (Balintoni et al 2010) The location of the Palaeozoic shelf facies south of deep marine Palaeozoic sediments indicates a north-northeast facing continental margin of North Dobrogea, consistent with a North Gondwanan provenance (Oczlon et al 2007) The Măcin-type Palaeozoic successions overlie the Cambrian clastics of the Avalonian Boclugea terrane, so they might have travelled together across the Tornquist sea Initial relations, as well as the progressive metamorphism 710 from the low-grade facies Boclugea rocks to the very low-grade Silurian–lower Devonian deposits is obscured by subsequent thrusting, due to involvement in the Hercynian orogeny, following the accretion of the Cadomian Orliga terrane Indirect evidence for an Avalonian provenance of the Lower Palaeozoic terranes in North Dobrogea can be derived from a critical analysis of their lithostratigraphy and from comparison with the Palaeozoic successions of East Moesia In the Măcin zone of North Dobrogea, the basinal, anoxic Silurian lithofacies is shallowing upward to Lower Devonian shelf sandstones and carbonates (Figure 11) The biostratigraphy of these successions is ill-defined due to scarcity of fossil remains, the only exception being the Bujoare fossil site rich in Lower Devonian macrofauna The entire Middle– Upper Devonian section is missing here due to erosion This is indicated by the abundant limestone clasts reworked in the lowest fanglomerates of the Carapelit Formation exposed near the Lower Devonian outcrops As the Carapelit conglomerates are proximal deposits, provenance from more distal sources of conodont-bearing limestone clasts is precluded Clast petrography and their fossil content in the lower members of the Carapelit Formation represent evidence that the middle and late Devonian in the Măcin zone developed in limestone facies, similar to East Moesia and the Scythian Platform Considering all lithological and structural information about the Silurian–Devonian successions in the Măcin zone of North Dobrogea, lithological correlation with East Moesian coeval deposits leads to a more reasonable lithostratigraphy, even if palaeontological evidence is scarce The dark shales of the Cerna Formation might be ascribed to the ?Wenlock–Lower Ludlow, based on their black shale facies The overlying black argillites might represent the Upper Ludlow, as recorded in lithological logs of several boreholes from East Moesia, where the transition to upper Ludlow is marked by a change in lithology from dark shales to argillites (Iordan 1999; Seghedi et al 2005a) The overlying argillites with thin interbeds of sandstones and calcarenites could represent the Pridoli Discontinuous, thin layers of volcanic tuffs are interbedded in the Upper Ludlow– Pridoli succession in the Măcin zone, as with the Pridoli A SEGHEDI lithofacies from the Zăvoaia borehole in East Moesia Identification of the conodont Icriodus woschmidti in the same lithofacies marks the presence of the Lower Devonian, indicating the continuity of the argillite facies into the Lochkovian The Lochkovian, welldated on the brachiopod-dominated macrofauna from the Bujoare Hills, contains several beds of mica-rich quartzitic sandstones, capped by a thin orthoquartzite member, which might be lithologically correlated with the Eifelian lithofacies of East Moesia The shallow-marine limestones overlying the Eifelian quartzites, rich in crinoidal ossicles and silicified solitary corals, could be ascribed to the Givetian The rest of the Devonian shallow marine carbonate facies succession was probably removed by erosion during deposition of the Carapelit Formation Following this reasoning, it is possible to conclude that the Măcin-type Silurian–Devonian deposits of North Dobrogea represent East Moesian successions Based on a previous palaeotectonic model (Mosar & Seghedi 1999) (Figure 20) it is proposed that during the Cambrian–Silurian, the Avalonian part of North Dobrogea was part of the East Moesian terrane, docked to Baltica during the Silurian, and experiencing a common history with the Scythian margin of Baltica during the Devonian The terranes were further involved in the complex evolution of the Hercynian orogen, when the Avalonian North Dobrogea represented the upper plate where regional metamorphism, crustal melting and granitic plutonism and volcanism were taking place The Cadomian Orliga terrane became part of North Dobrogea in the Late Carboniferous, after the closure of the Rheic Ocean Synthesis and Conclusions As part of the Baltica palaeocontinent, the area of the Scythian Platform was a shelf basin from the Vendian until the Silurian Early Devonian rifting disrupted the shelf basins that evolved further as a carbonate ramp during the Middle Late Devonian– Lower Carboniferous Following the Middle–Late Carboniferous subduction in the areas of the West European Variscan and South European Scythian orogens (as defined by Ziegler 1989), siliciclastic deposits with coal measures prograded on the passive margin carbonates The sedimentary record suggests that during the Palaeozoic, the basement of the Pre-Dobrogea Depression was part of the Baltican foreland involved in the amalgamation of Avalonia/ Baltica with Laurentia, and further evolved as part of the Variscan foreland Palaeozoic deposits of East Moesia show a wellpreserved palaeontological record and not show any structural elements indicative of metamorphism The succession of Palaeozoic facies is well documented based on biostratigraphic zones: graptolites for the Ordovician and Silurian, conodonts and chitinozoans for the Lochkovian, tentaculites and trilobites for the Pragian–early Givetian and spiriferids, conodonts and foraminifera for the Givetian–Visean The Namurian–Westphalian is dated on placoderm and ostracoderm fishes, terrestrial plants, algae and foraminifera Gaps in the succession correspond to the Llandovery, Upper Wenlock and lower Upper Ludlow Based on these unconformities, a periGondwanan Avalonian affinity is inferred for East Moesia, considering also the Avalonian affinity of the East Moesian Ediacaran basement The Precambrian basement of the Moesian Platform between the Capidava-Ovidiu and Palazu faults is a terrane with proximal Baltican derivation This terrane, devoid of Palaeozoic deposits, was probably detached from the northwestern slope of the Ukrainian shield (where Volhyn flood basalts overlie the Palaeoproterozoic and Archaean Sarmatia margin), and was displaced along the TESZ Ordovician to Devonian deep marine sediments developed south of the Sfantu Gheorghe Fault (Figure 10) show a strong contrast in facies and structural style with the Lower Palaeozoic passive margin successions of the Scythian Platform The Tulceatype Palaeozoic sediments, dated on conodonts and palynological assemblages, form several northwardyounging, upward-coarsening terranes, consisting of radiolarian cherts, siliceous shales and distal turbidites Rocks show a southward increase in metamorphic grade, from subgreenschist to lower greenschist facies Trace fossil assemblages and facies association indicate that turbidites accumulated in deep basins below the CCD Lithological association and upward-coarsening facies architecture from siliceous pelagic rocks to turbidites suggests that these terranes represent oceanic and trench sediments, 711 PALAEOZOIC FROM DOBROGEA AND PRE-DOBROGEA Figure 20 Plate tectonic reconstruction showing a possible scenario for the evolution of North Dobrogea, East Moesia and PreDobrogea and surrounding oceanic realms in the Palaeozoic (modified from Mosar & Seghedi 1999) The model is based on the constitution of North Dobrogea Palaeozoic basement, that includes an Ordovician–Devonian accretionary wedge separated from a Late Palaeozoic magmatic arc and retro-arc foreland basin by a strike-slip fault The model shows accretion to Baltica of East Moesia (including the Boclugea terrane), following the closure of the Tornquist Sea and Variscan accretion of Orliga terrane upon closure of the Rheic Ocean North Dobrogea shows south-dipping subduction in the Lower Palaeozoic and finally rather gently docks with the SE edge of Baltica and develops a strong syn- to post-collisional strikeslip deformation along the Teyssiere zone This late Hercynian wrench tectonics is also related to the destruction of the Variscan Orogen and strongly affects the Dobrogea accretionary prism remnants, as well as the continental margin arc 712 A SEGHEDI which record the trenchward movement of an oceanic plate They are interpreted as underthrust units of an accretionary wedge, accreted to the upper plate during southward subduction of the Scythian margin of the East European Craton (Mosar & Seghedi 1998; Seghedi 1999) The Upper Palaeozoic Carapelit Formation, ascribed to the Upper Carboniferous–Early Permian, obviously represents a continental, volcanic molasse, intruded by calc-alkaline granites It contrasts in composition and style with the Carboniferous coal measures from the Scythian Platform of PreDobrogea The Carapelit Formation strongly contrasts in facies and explosive calc-alkaline volcanism with the Lower Permian continental red bed facies from the Pre-Dobrogea Depression, accompanied by effusive-explosive alkaline bimodal volcanism A model explaining the Upper Palaeozoic continental sedimentation and magmatism in North Dobrogea as a result of thrust propagation in a retro-arc foreland basin (Seghedi et al 1987; Oaie & Seghedi 1994; Seghedi & Oaie 1995) (Figure 20c) accounts for: the great thickness of fluvial and alluvial continental sediments of the Carapelit Formation, accumulated in active tectonic condition; mixed clast composition, suggesting sediment recycling; very-low-grade metamorphism (anchizone metamorphism) of the Palaeozoic sediments of the upper plate, connected to retro-arc thrusting and progressive increase in metamorphic grade of Palaeozoic sediments toward the thrusts footwalls Compared to East Moesia, the Palaeozoic of North Dobrogea is loosely dated, due to the scarcity of its fossil record This can be explained by the penetrative Hercynian deformation in amphibolite to greenschist and subgreenschist facies conditions and thermal metamorphism connected to granite intrusion Nevertheless, based on a vertical lithofacies succession, the shallow-marine Lower Palaeozoic of North Dobrogea correlates well with coeval East Moesian deposits On this basis, an Avalonian affinity can be inferred for the Măcin-type Lower Palaeozoic, consistent with the Avalonian affinity of its basement, the Cambrian Boclugea terrane, as suggested by detrital zircons Unlike East Moesia and Pre-Dobrogea, the continental Late Palaeozoic of North Dobrogea was involved in Hercynian collision, with metamorphism, volcano-plutonic development and thrusting This suggests an active margin setting and upper plate position The deep marine Palaeozoic pelagic cherts and distal turbidites, were part of the forearc wedge, tectonically accreted to the upper plate along a south-dipping subduction plane The south polarity of the subduction zone is indicated by the deep marine Palaeozoic of North Dobrogea facing the shallow-marine Devonian carbonate platform facies of the Scythian Platform A north-dipping subduction zone of Palaeotethys developed in the Late Palaeozoic as shown in Figure 20a, b The trace of the Rheic suture runs through North Dobrogea, where both Avalonian and Cadomian terranes are found, as in the Hercynian basement reworked in the Alpine nappes of the South Carpathians (Balintoni et al 2010) Permian rifting accompanied by bimodal alkaline volcanism occurred both in North Dobrogea and the Pre-Dobrogea In North Dobrogea, Late Permian rifting is explained as a consequence of extensional collapse of the Variscan orogen, overthickenned by thrusting and granite intrusion (Figure 20d) The transition from early Permian syn-collisional calc-alkaline magmatism to Late Permian alkaline magmatism reflects the changing stress regime from compression to transtension and was explained as result of slab roll-back (Figure 20d) The extensional (transtensional) regime propagated from the Hercynian belt into its foreland, where rifting was accompanied by accumulation of thick volcanosedimentary successions in the Sărata-Tuzla basin from the eastern part of the rift Active rifting of the basement of the Pre-Dobrogea is suggested by emplacement of the bimodal alkaline volcanism of the basalt-trachyte association This volcanism was fed by the suite of the bimodal basalt-trachyte dykes emplaced during the Permian along the northern margin of North Dobrogea, as well as by small bodies of syenites and ultrapotassic alkaline rocks emplaced in the basement highs of the rift basin In the meantime, alkaline volcanism developed from Late Permian to early Triassic times along the southern border of North Dobrogea, parallel to the PeceneagaCamena Fault Acknowledgements This paper was written as result of the participation in the second ISGB conference in held in Ankara in 713 PALAEOZOIC FROM DOBROGEA AND PRE-DOBROGEA 2009 and supported from project PROMED, contract 31-030/2007, financed by the Ministry of Education and Research of Romania The author is extremely grateful to Aral Okay for the strong support and encouragement for writing this paper, as well as for his infinite patience The comments and suggestions of Ayda Petek Üstaömer and an unknown reviewer are highly appreciated, considerably increasing the quality of this paper The English language of the paper was greatly 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Fanning, M., Seghedi, A & Zaba, J 2009 More evidence on Neoproterozoic terranes in southern Poland and southeast Romania Geological Quarterly 53, 93– 123 Żelaźniewicz, A., Seghedi, A., Jachowicz, M., Bobiñski, W., Buła, Z & Cwojdzinski, S 2001 U-Pb SHRIMP data confirm the presence of a Vendian Foreland Flysch Basin next to the East European Craton EUROPROBE Conference Ankara, Turkey, Abstracts, 98–101 721 ... Boclugea, Megina and Orliga metamorphic terranes and Silurian–Lower Devonian sediments; P– Permian alkaline complexes; C-P– Carboniferous– Permian granitoids; C2-P 1– Carboniferous–Early Permian Carapelit... and geochemical signature and even the same type of extensive 697 PALAEOZOIC FROM DOBROGEA AND PRE -DOBROGEA hydrothermal alteration as the Permian volcanic rocks from the Scythian Platform, and. .. North Dobrogea and PreDobrogea 671 PALAEOZOIC FROM DOBROGEA AND PRE -DOBROGEA Geological and Tectonic Background The north-western margin of the Black Sea is a highland area referred to as Dobrogea,