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Paleogeographic reconstruction of a segment of the North-Tethyan margin in Bulgaria from Barremian to Albian
Abstract
In this synthetic work are reconstructed seven paleogeographic maps corresponding to the main stages of the geodynamic evolution, from Barremian to Albian, along a complex segment of the North-Tethyan margin in Bulgaria. This reconstruction follows the recent publication of three paleogeographic maps related to the Berriasian - Hauterivian interval. The herein proposed facies maps are explained by palinspastic cross-sections, variously oriented, showing the geometric assemblage of the most classic formations used in the Bulgarian literature, constrained by a new biostratigraphy, particularly founded on ammonites, and our recent studies in terms of sequence stratigraphy. Between the emerged (or croded) areas of the Romanian Dobrogea to the north and the Rhodopes/Serbo-Macedonian massif to the south (itself fringing the Tethyan oceanic crust), the Bulgarian Balkanides and Moesia correspond during early/middle Cretaceous times to a wide east-west oriented arm of the sea, filled up during the Hauterivian by the terrigenous series of the "Axial Basin". Just before the Barremian, this basin was flanked, to the south, by a mobile boundary subdivided into fault-blocks (supplying the basin in siliciclastics) and, to the north/north-east, by a much stabler margin (Russe). From Barremian to lower Aptian, an incessant competition occurs between the thick terrigenous basinal sediments (generally external/distal) and the rudist/orbitolinid-bearing "Urgonian-type" carbonate platforms, overlying alignments of shoals (Russe, Lovech, Vratsa, Brestnitsa, Eleshnitsa, Simeonovo), either separated or coalescent. Locally, turbidites with olistolites of Urgonian limestones have been induced by extensional movements, generating sedimentary slopes. During middle/late Aptian, the carbonate platforms drown, then completely disappear and only the terrigenous sedimentation continues within a more and more reduced Axial Basin. During the Albian, the tectonic inversion (first compressions) of the Austrian phase induces the creation of a narrow, but anoxic, foreland basin probably supplied in glaucony by the erosion of hypothetic meridional laterites.
... There are no ophiolites farther to the south, but Upper Jurassic-Cretaceous outer shelf and slope sedimentation (turbidites, basinal marls) is preserved on the southern margin of the Moesian Platform and the northern margin of the Balkan terrane (cf. Tchoumatchenco and Sapunov, 1994;Minkovska et al., 2002). ...
... This is implicit from Paleogene-early Neogene oceanic subduction under the Inner Carpathian region (Seghedi et al., 2004a) and underlined by lack of any crustal thickening around the Moesian Platform during late Early Cretaceous time (cf. Savu, 1985, andMinkovska et al., 2002). The Ceahlȃu-Severin Ocean was subducted during Late Cretaceous time under the Balkan terrane of northern Bulgaria (Fig. 10) and the Banat area in the South Carpathians (BA in Fig. 5). ...
The Moesian Platform is a crustal block within southern Europe, located beyond the southwestern margin of the East European craton. Along this margin lie terranes that were accreted to Baltica as part of Far Eastern Avalonia during Late Ordovi- cian-Early Devonian time and terranes that already formed part of Cambrian Baltica, displaced as proximal terranes together with Far East Avalonian terranes. The tectonic history and crustal affinity of the Moesian Platform, however, remain poorly understood. A review of available tectonostratigraphic, paleontological, and geochro- nologic data suggests that the Moesian Platform comprises four distinct terranes, two with Baltican and two with Avalonian affinities. A fifth terrane, North Dobrogea, lies between the Moesian Platform and the East European craton and records Variscan (Carboniferous) accretion. This accretionary record leads to the paradox that the youngest accreted crust (North Dobrogea) lies closest to the craton, whereas the earlier accreted crust and crust derived from the craton itself are now located more externally. A review of terranes along the southwestern margin of the East European craton, between the North Sea and the Black Sea, suggests that a dextral strike-slip dominated the southwestern Baltican margin during Late Ordovician-Early Devonian accretion of Far Eastern Avalonia, much as is the case in western North America today. Variscan indentation of the Bohemian Massif led to escape-displacement of some Caledonian terranes, and strike-slip displacement during the Mesozoic opening of Mediterranean-style oceanic basins led to the current juxtaposition of Moesian ter- ranes, inverted with respect to their accretionary history.
... During the Barremian-Early Aptian (Early Cretaceous) several carbonate platforms existed on the northern Tethyan margin in Bulgaria (MINKOVSKA et al., 2002). Abundant and diversified corals (about 120 species) were described from marls and limestones of the Lovech Urgonian Group in the Central Fore-Balkan. ...
Euendolithic microorganisms (boring endoliths) syn-vivo associated with modern corals are commonly reported, but their fossil record is extremely rare. This paper reports the new finding recognized in the colonial scleractinian coral Clausastrea saltensis from the Upper Barremian of Bulgaria. Large microborings (up to 50 μm, most ca. 15-25 μm in diameter) filled with calcite cement are distributed medially along coral septa of some corallites. Borings were produced by microeuendoliths growing from the skeleton interior outward during the life of the coral host. They are compared to traces produced by the recent oligophotic filamentous chlorophyte Ostreobium, which is known to be the most common skeleton-dwelling alga in modern living corals and regarded as neutral or beneficial to the coral. In terms of general morphology, diameter and distribution pattern, the borings are similar to those recently recognized in the Early Cretaceous microsolenid coral.
... The Lower Cretaceous deposits in Bulgaria contain numerous corals, mostly colonial forms building bioconstructions both in carbonate as well as in siliciclastic deposits (for lithostratigraphy and paleogeography of Lower Cretaceous in Bulgaria see Minkovska et al., 2002 andNikolov et al., 2007). These coral assemblages represent two time spans. ...
... Representatives of Mathesia have been collected from the Emen Formation, the middle part of the Lovetch Urgonian Group in the Central Fore-Balkan domain of the Central Northern Bulgaria (Figs. 2 and 3). The Barremianelower Aptian Lovetch Urgonian Group consists of four terrigenous and four interbedded carbonate formations (Khrischev, 1966;Nikolov, 1969;Peybernès et al., 1979; for review see Minkovska et al., 2002;Nikolov et al., 2007) (Fig. 3). In the Emen Formation bioclastics and coral-rich beds are the dominant facies; rudist occurrences are uncommon or have a limited local extent (Minkovska, 1996). ...
Mathesia darderi, a slender cylindrical monopleurid genus, formerly documented from the late Aptian–Albian of Spain, France, Tunisia, Algeria, Egypt and Turkey, has been discovered in the upper Barremian and the lower Aptian of Bulgaria and Spain. Notwithstanding some morphological changes, Barremian–lower Aptian forms and those of the upper Aptian–Albian possess the same myocardinal organisation and the same microstructural attributes. The inner shell margin of the right valve displays scalloped, festooned, tubular and vermiform microstructures. The most prominent evolutionary trait of M. darderi is the increase in body size through time. A statistical analysis of size distributions show that populations of the late Barremian–early Aptian, and the late Aptian, and those of the early to middle Albian, are significantly different; a pattern which has a biostratigraphic potential. Ecological changes through time are expressed by a displacement of communities from the central/distal part, to the proximal part of carbonate platforms. M. darderi is present locally in the upper Barremian–lower Aptian, and has its major spreading over the European and Arabo–African margins of the Mediterranean Tethys during the Clansayesian–lower to middle Albian. The disappearance of the species at the Middle–Upper Albian boundary, correlates with a critical, spatial reduction of carbonate platforms.
... The basement of the Srednogorie zone experienced amphibolite-grade late Variscan and weak early Late Cretaceous metamorphism and associated ductile deformation between 105 and 99 Ma (Velichkova et al. 2004 ). The southward increasing , weak metamorphic overprint also affected the Late Palaeozoic-Triassic to Albian cover (Minkovska et al. 2002) and was associated with ductile top-W shear due to transport of the tectonic overburden. The metamorphic overprint reached very low-to low-grade conditions and is dated at 102 to 100 Ma by the 40 Ar/ 39 Ar method (Velichkova et al. 2004). ...
Amphibole and mica 40Ar/39Ar ages have been determined from Late Cretaceous plutonic and volcanic successions from the Panagyurishte region (Bulgaria), which is part of the calc-alkaline to alkaline Banatite belt spreading from the Apuseni Mountains to the Black Sea and hosting world-class Cu-Au-deposits. The post-collisional magmatism is younger than early Late Cretaceous orogenic events and contemporaneous with formation of the collapse-type Srednogorie sedimentary basin. Three volcanic intercalations within the oldest sedimentary succession of the Srednogorie zone exposed along the Tolponitsa river show 40Ar/39Ar ages between 93 and 89 Ma and at ca. 86 Ma. Biotites from the Elshitsa granodiorite give an age of 86 Ma. Muscovite from an alteration zone at Elshitsa yield an age of 88 Ma. Amphiboles from a dacite porphyry from Vlaykov Vruh yield an age at 80 Ma. Biotites from the Velichkovo granodiorite give ages at 78–80 Ma.
Amphibole ages in part date hydrothermal alteration as the ages are clearly similar to U-Pb zircon ages from the same plutons, i.e. subvolcanic intrusions. Together with previous 40Ar/39Ar amphibole and mica and numerous U-Pb zircon ages from mainly Late Cretaceous plutonic rocks of the Srednogorie area, the new ages display the following major trends: Magmatism lasted over a long period ranging from ca. 93 to 78 Ma. Both the plutonic and volcanic successions yield similar age groups, which are: 93–89 Ma, 86–84 Ma and 80–78 Ma. The age groups and their distribution show that magmatism was discontinuous and shifted southwards. Older subvolcanic rocks (93–89 Ma) developed in a regime of ca. N–S extension. They show that the southward prograding magmatism is older than dextral shearing, which developed between 86–78 Ma within ca. N–S compressional tectonic conditions. The geodynamic setting of the late Cretaceous magmatism in the Srednogorie zone is still controversial. The relationships within the Panagyurishte region, specifically the early stage of extension combined with calc-alkaline magmatism, could be explained by either retreat of a subduction zone or by slab break-off. The subsequent compressional phase is likely related to intra-orogenic shortening following collision.
... (b) Map of the Balkan, Rhodopian and Serbomacedonian units (modified after Haydoutov et al., 1997 ). to represent zones with a metamorphic, continental basement and cover successions comprising Late Carboniferous to Eocene strata (Fig. 2). The basement is locally termed as Balkanide type (Ivanov, 1989aIvanov, ,b, 2002) or Balkan terrane (Haydoutov and Yanev, 1997) and includes migmatitic orthogneisses, some paragneisses and amphibo- lites. ...
40Ar/39Ar dating of amphibole and biotite from volcanic and plutonic rocks from the Panagyurishte region of the Srednogorie Zone (Bulgaria) shows a clear trend in post-magmatic cooling ages from north to south: A monzonite dyke from the Elatsite deposit yields ages of 90.78 ± 0.44 Ma (amphibole), and 91.72 ± 0.70 Ma (biotite). Further south, an andesite from the Vosdol neck exposed in the valley NE of Chelopech records an age of 89.95 ± 0.45 Ma (biotite). In the Medet mine, a coarse-grained granodiorite yields an age of 85.70 ± 0.35 Ma (amphibole), amphibole from a similar granodiorite from Elshitsa 84.07 ± 0.54 Ma. An andesite lava breccia exposed in the Sv. Nikola hill south of Panagyurishte records an age of 80.21 ± 0.45 Ma (amphibole). Our new 40Ar/39Ar ages are in line with recently reported UPb zircon ages. Older subvolcanic rocks (ca. 9290 Ma) developed in a regime of ca. NS extension. They show that the southward prograding magmatism is older than dextral shear, which developed between ca. 8678 Ma within a ca. N-S compressional tectonic regime.
... (b) Map of the Balkan, Rhodopian and Serbomacedonian units (modified after Haydoutov et al., 1997 ). to represent zones with a metamorphic, continental basement and cover successions comprising Late Carboniferous to Eocene strata (Fig. 2). The basement is locally termed as Balkanide type (Ivanov, 1989aIvanov, ,b, 2002) or Balkan terrane (Haydoutov and Yanev, 1997) and includes migmatitic orthogneisses, some paragneisses and amphibo- lites. ...
The Panagyurishte region, east of Sofia in Bulgaria, is characterised by amphibolite facies metamorphic basement rocks, overlain by Late Carboniferous to Triassic and Late Cretaceous cover sequences, which together comprise the Central Part of the Srednogorie unit. This succession is intruded by subvolcanic Cretaceous calcalkaline magmatic bodies, which are part of the so-called Banatite Belt and host world-class CuAu-deposits. The goal of this study is to constrain the age of tectonometamorphic events recorded in the Srednogorie basement.The amphibolite facies metamorphic Srednogorie basement consists of two-mica gneisses, micaschists, orthoamphibolites, small serpentinite bodies, and anatexites of pre-Upper Carboniferous age. New 40Ar/39Ar ages of white mica and biotite from southern and northern sectors of the central part of the Srednogorie unit show well-defined Late Palaeozoic plateau ages of ca. 317305 Ma, which indicate a Variscan age for the postmetamorphic cooling after the last amphibolite facies metamorphic overprint. Subsequent post-Variscan intrusion and slow cooling of the crosscutting Poibrene diorite is indicated by Late Palaeozoic cooling ages of 302.9 ± 1.4 and 263.00 ± 0.69 Ma reported for hornblende and biotite, respectively.During the Cretaceous, southern sectors of the Srednogorie basement and overlying Late Carboniferous to Triassic cover successions were affected by greenschist facies metamorphic overprint. The latter event is limited to southern sectors near Alpine low-angle normal faults. Muscovite and biotite samples from several subhorizontal ductile shear zones yield consistent Cretaceous 40Ar/39 Ar plateau ages of ca. 99105 Ma. Greenschist facies retrogression affected the basement mainly along basement-cover (Stefanian-Permian) contacts, which are strongly sheared. Subsequently, the steep ductile dextral Maritsa shear zone formed along a belt of granitoid intrusions within rheologically weakened crust along southern margins of the Panagyurishte region, whereas the Central Srednogorie collapse-type basin seals structures formed during early Late Cretaceous.
The Southern Carpathians and the Balkan Mountains define a broad physiographic area, placed at the centre of current debates on the emergence of the earliest Upper Palaeolithic and the Aurignacian technocomplex, the migration and dispersal routes of Anatomically Modern Humans in Europe, and the pre-Neolithic and the neolithization of the Balkan area. In this archaeological context and state of research, the Upper Palaeolithic sites from the Lower Danube Valley represent a relevant piece in the jigsaw puzzle of past human land use and mobility patterns. The aim of this article is to investigate the similarity between intraclastic-bioclastic cherts from Giurgiu-Călăraşi area and “Kriva Reka” type of Ludogorie chert from NE Bulgaria, by focusing on their macroscopic and microscopic traits and their geological contexts. The distribution of eluvial and primary deposits of Ludogorie chert types from NE Bulgaria reflects the sedimentary facies belts of the Lower Cretaceous Sea. Also, the alluvial deposits reveal the role played by rivers in the erosion, transport, and redeposition further and further away of the Ludogorie cherts, thus generating an extended area abundant in such materials. The geological distribution of Kriva Reka type similar cherts in Romania was confirmed in alluvial deposits around Giurgiu (Frăteşti Formation, Lower Pleistocene, and Danube’s lower terrace deposits, Upper Pleistocene). The archaeological distribution was confirmed in the Upper Palaeolithic open-air sites from Giurgiu-Malu Roşu, Slobozia-Râpa Bulgarilor, and Nicolae Bălcescu-La Vii. Their use by Boian and Gumelniţa Neolithic communities from southern Romania suggests a long time exploitation of local available cherts.
All stages of the Lower Cretaceous series are developed in Bulgaria, from Berriasian to Albian, with no missing or undated intervals. Lower Cretaceous rocks are present in several areas in Bulgaria, mainly in the north and the southwest, in each of these regions characterized by specific features. The full spectrum of terrigenous, terrigenous-carbonate and carbonate rocks is represented. Their spatial distribution in different regions shows a general trend of progressive replacement of terrigenous and terrigenous-carbonate sediments to the south by carbonate sedimentation to the north. Marine deposits predominate, with restricted development of variegated continental deposits in the Black Sea area in Northeast Bulgaria. The Bulgarian Lower Cretaceous basin depositional environment was a back-arc developed along the southern periphery of the Moesian microplate. It was situated to the north of one large island arc extended from the northwest of the area of the Pannonian massif and the Apuseni Mt., passing through Bulgaria and reaching the Caucasus to the east. In general, the Lower Cretaceous basin was asymmetrical, with a steep southern edge and a broad northern edge, developed on the southern part of the Moesian platform. Two major depositional areas (southern and northern) are distinguished. The southern one, known as "Nish-Troyan geosynclinal flysch depression" or "Peri-Moesian marginal flysch basin", was very narrow and of nearly trench-like character. Rather mobile tectonic regime and intensive compensated subsidence were typical for this basin too. The northern area was a wide shelf with a less thick sedimentary sequence. A large carbonate platform is the most characteristic feature of this area.
Within the Emerici zone-Barremense zone biostratigraphic interval, the Barremian deposits of Central Fore-Balkan (Lovech-Veliko Tarnovo shelf) consist of a succession of several formations where alternate terrigenous argillaceous/sandy-dominated facies (Kormjansko Fm., Balgarene Fm.) and carbonate-dominated ('Urgonian') facies (Krushevo Fm., Emen Fm.). The qualitative and, particularly, quantitative facies analysis of the carbonate successions observed along 13 detailed cross-sections and in one drill hole show the stacking of about 40 fifth-order T-R cycles induced by numerous eustatic jerks contributing to the progressive settlement of this shelf. These high-frequency cycles of about 100 000 years must be regarded as valuable correlation tools for the subsurface hydrocarbon research. (C) 2004 Academie des sciences. Publie par Elsevier SAS. Tous droits reserves.
The Lower Cretaceous is of wide geographical distribution in Bulgaria, mainly in the northern and southwestern part of the country. Marine deposits predominate, with exception of the most northeastern part of the country, where paralic and continental deposits occur. The main part of the Lower Cretaceous sediments were deposited in shelf environments, and a small part of them - in moderately deep-water (epibathial) ones. The shelf sediments consist of conglomerates, gritstones, various sandstones, marls and different types of limestones. In the lower (Berriasian-Valanginian) part of the series, as well as in the Uppermost Barremian-Middle Aptian, flysch and flyschoid (flysch-like) clastic sediments were formed in deeper-water environments. Pelagic limestones alternating with marls (in the Western Fore-Balkan and partly in South-West Bulgaria) were developed, too, and typical catastrophic sediments (debrites, tempestites, sediments with hummocky-cross stratified bedding) - in the boundary Jurassic-Cretaceous beds. These sediments are also observed in several higher stratigraphic levels (Upper Valanginan, Upper Barremian-Aptian). At the beginning of the Late Barremian (up to the Early Aptian) a considerable build-up of large carbonate platforms of Urgonian type associated with terrigenous sediments in the Northern Fore-Balkan band commenced. The Lower Cretaceous sediments are grouped (united) into nine sediment associations, each of them integrating with the main lithostratigraphic successions and facial types.
A new formal lithostratigraphic unit - Damyanovo Formation, is defined in the Lower Cretaceous sediments of NW Bulgaria. Its lower and middle part is composed of rhythmic flysch alternation of sandstones, siltstones and silty marls, whereas the upper part is built up by rhythmic thin packets of sandy limestones and calcareous sandstones. Damyanovo Formation is underlain by Mramoren Formation and is covered by Sumer Formation. Laterally, it passes to Sumer and Botunya Formations, and interfingers with Roman Formation. The chronostratigraphic range of the formation is Late Barremian - Late Aptian, based on ammonite and calcareous nannofossil finds. The deep-water turbiditic sediments of Damyanovo Formation are deposited in a depression of deficient sedimentation with slightly asymmetrical basin profile. They mark the final turbidite impulse in the geological development of the Early Cretaceous basin on the territory of North Bulgaria.
Diversified and abundant corals of the suborder Pachythecaliina (order Hexanthiniaria) are described from Upper Barremian, biostromal reefs of the Emen Formation, Lovech Urgonian Group, north central Bulgaria. The corals are mostly of the phaceloid growth form and represent 14 species (six new), 12 genera (three new), belonging to five families. Pachythecaliines occur with the small, monopleurid cylindrical rudist Mathesia darderi. The rudists frequently are densely clustered, occur between coral branches or are in contact with them. Other corals, with the exception of the phaceloid Calamophylliopsis, and other rudists, are rare. Non-laminated microbialite crusts provided additional, structural support for bioconstruction development. Microbialites (automicrites) can be interpreted as a product of microbial activity, or alternatively, as a result of carbonate precipita-tion, brought about by non-living organic substrates (organomineralization s.s.). In addition to microbialites, meta zoans are encrusted by heterotrophic skeletal microorganisms, while photophilic and oligotrophic microencrusters, usually common in other coral-bearing limestones of the Emen Formation, are very rare. The section at the Rusalya Quarry (NW of Veliko Tarnovo), about 42 m thick, provides the sedimentary and environmental context for the reefal biostromes. The vertical biotic and sedimentary succession displays a general shallowing trend: from the outer carbonate platform with bioclastic limestones containing small boundstone patches (corals, but not pachythecaliines, Lithocodium aggregatum), to the inner platform with rudist biostromes. The pachythecaliinerich biostromes, 2.5 m thick, were developed in a low-energy environment, referred to the distal part of the rudist-dominated area of the platform. The development of microbialites was facilitated by a low sedimentation rate, and possibly by increased nutrient level. Only poorly diversified and non-phaceloid pachythecaliines occur in other coral-rich limestones and marls of the Urgonian complex in Bulgaria. The assemblage described is the most remarkable, Early Cretaceous coral community worldwide, with regard to pachythecaliines. Phaceloid pachythecaliines are only more common in the Upper Jurassic rocks, being particularly diversified in the Tithonian-Lower Berriasian Štramberk Limestone (Czech Republic) and its equivalent in the Polish Outer Carpathians. However, their sedimentary context differs from that described for the corals of the Emen Formation.
Conicopfenderina ? balkanican. sp. (larger benthic Foraminifera), new index-species of the Valanginian from Balkanides (North-Tethyan margin, Bulgaria). Description of a new species of Pfenderinidae (streptospiral, then rectilinear, larger benthic foraminifer with a keriothecal wall), Conicopfenderina ? balkanica n. sp.. This orbitolinid-like high-sized new taxon is observed in abundance within the internal/proximal shelf limestones of the Slivnitsa Formation (Western Balkan, Bulgaria). On the East-European part of the North-Tethyan margin, this foraminifer appears to constitute a good index of the Valdanchella miliani zone (rather Upper Valanginian in age). Until now, such a biostratigraphic subdivision was thought to be paleobiogeographically restricted to the « Catalan sub-province », linking, on the same margin but more to the west, Jura to Iberian Range. To cite this article: B. Peybernès, C. R. Palevol 3 (2003).
Dans l'intervalle biostratigraphique zone à Emerici–zone à Barremense, les dépôts barrémiens du Prébalkan central (plate-forme de Lovech–Veliko Tarnovo) s'articulent en plusieurs formations, où alternent faciès terrigènes argilo-gréseux dominants (Fm. Kormjansko, Fm. Balgarène) et faciès carbonatés « urgoniens » dominants (Fm. Krushevo, Fm. Emen). L'analyse qualitative et, surtout, quantitative des faciès, effectuée sur 13 coupes détaillées et un forage, révèle, dans les deux formations carbonatées de Krushevo et d'Emen, la succession d'une quarantaine de cycles T–R de haute fréquence (5e ordre) correspondant à autant d'à-coups d'origine eustatique, qui contribuent à l'édification progressive de cette plate-forme. Ces cycles d'environ 100 000 ans constituent un précieux outil de corrélation pour la recherche d'hydrocarbures en subsurface. Pour citer cet article : V. Minkovska et al., C. R. Geoscience 336 (2004).
Within the Emerici zone–Barremense zone biostratigraphic interval, the Barremian deposits of Central Fore-Balkan (Lovech–Veliko Tarnovo shelf) consist of a succession of several formations where alternate terrigenous argillaceous/sandy-dominated facies (Kormjansko Fm., Balgarene Fm.) and carbonate-dominated (‘Urgonian’) facies (Krushevo Fm., Emen Fm.). The qualitative and, particularly, quantitative facies analysis of the carbonate successions observed along 13 detailed cross-sections and in one drill hole show the stacking of about 40 fifth-order T–R cycles induced by numerous eustatic jerks contributing to the progressive settlement of this shelf. These high-frequency cycles of about 100000 years must be regarded as valuable correlation tools for the subsurface hydrocarbon research. To cite this article: V. Minkovska et al., C. R. Geoscience 336 (2004).
The present paper reports for the first time the presence of representatives of the ammonite subfamily Venezuellinae Kvantaliani in the Mediterranean area (in particular Bulgaria). Until now they were considered as endemic and were known only from Aptian sediments in South America (Venezuela). Species of g. g. Venezuella and Renziella were found in three sections on the territory of North Bulgaria. The lithology and the ammonite sequences of the sections are described. The representatives of g. Venezuella and Renziella associate with characteristic, including also zonal Aptian ammonites from A. nolani and H. jacobi zones. The concept of the Aptian Age of the ammonites of this group is advocated. The phyllogenetic concept for the origin and evolution of the subfamily is rejected. Five species of g. Venezuella and 1 species of g. Renziella are described and figured. Three of them – V. bulgarensis sp. n., V. sumerensis sp. n., V. gracilis sp. n., are new for the science.
We present an integrated geomagnetic polarity and stratigraphic time scale for the Triassic, Jurassic, and Cretaceous periods of the Mesozoic Era, with age estimates and uncertainty limits for stage boundaries. The time scale uses a suite of 324 radiometric dates, including high-resolution Ar-40/Ar-39 age estimates. This framework involves the observed ties between (1) radiometric dates, biozones, and stage boundaries, and (2) between biozones and magnetic reversals on the seafloor and in sediments. Interpolation techniques include maximum likelihood estimation, smoothing cubic spline fitting, and magnetochronology. The age estimates for the 31 stage boundaries (in mega-annum) with uncertainty (millions of years) to 2 standard deviations, and the duration of the preceding stages (in parentheses) are Maastrichtian/Danian (Cretaceous/-Cenozoic) is 65.0 +/- 0.1 Ma (6.3 m.y.), Campanian/Maastrichtian is 71.3 +/- 0.5 Ma (12.2 m.y.), Santonian/Campanian is 83.5 +/- 0.5 Ma (2.3 m.y.), Coniacian/Santonian is 85.8 +/- 0.5 Ma (3.2 m.y.), Turonian/Coniacian is 89.0 +/- 0.5 Ma (4.5 m.y.), Cenomanina/Turonian is 93.5 +/- 0.2 Ma (5.4 m.y.), Albian/Cenomanian is 98.9 +/- 0.6 Ma (13.3 m.y.), Aptian/Albian is 112.2 +/- 1.1 Ma (8.8 m.y.), Barremian/Aptian is 121.0 +/- 1.4 Ma (6.0 m.y.), Hauterivian/Barremian is 127.0 +/- 1.6 Ma (5.0 m.y.), Valanginian/Hauterivian is 132.0 +/- 1.9 Ma (5.0 m.y., Berriasian/Valanginian is 137.0 +/- 2.2 Ma (7.2 m.y.), Tithonian/Berriasian (Jurassic/Cretaceous) is 144.2 +/- 2.6 Ma (6.5 m.y.), Kimmeridgian/Tithonian is 150.7 +/- 3.0 Ma (3.4 m.y.), Oxfordian/Kimmeridgian is 154.1 +/- 3.2 Ma (5.3 m.y.), Callovian/Oxfordian is 159.4 +/- 3.6 Ma (5.0 m.y.), Bathonian/Callovian is 164.4 +/- 3.8 Ma (4.8 m.y.), Bajocian/Bathonian is 169.2 +/- 4.0 Ma (7.3 m.y.), Aalenian/Bajocian is 176.5 +/- 4.0 Ma (3.6 m.y.), Toarcian/Aalenian is 180.1 +/- 4.0 Ma (9.5 m.y.), Sinemurian/Pliensbachian is 195.3 +/- 3.9 Ma (6.6 m.y.), Hettangian/Sinemurian is 201.9 +/- 3.9 Ma (3.8 m.y.), Rhaetian/Hettangian (Triassic/Jurassic) is 205.7 +/- 4.0 Ma (3.9 m.y.), Norian/Rhaetian is 209.6 +/- 4.1 Ma (11.1 m.y.), Carnian/Norian is 220.7 +/- 4.4 Ma (6.7 m.y.), Ladinian/Carnian is 227.4 +/- 4.5 Ma (6.9 m.y.), Anisian/Ladinian is 234.3 +/- 4.6 Ma (7.4 m.y.), Olenekian/Anisian is 241.7 +/- 4.7 Ma (3.1 m.y.), Induan/Olenekian is 244.8 +/- 4.8 Ma (3.4 m.y.), Tatarian/Induan (Permian/Triassic) is 248.2 +/- 4.8 Ma. The uncertainty in the relative duration of each individual stage is much less than the uncertainties on the ages of the stage boundaries.
In Bulgaria, the palaeogeography of Balkanides and the Moesian ‘microplate’ (or Moesia), at the beginning of the Cretaceous Period, was marked by the presence of a deep NE–SW to E–W trough (Axial Basin, or Nish-Trojan Trough), inherited from Late Jurassic times and situated in the southern part of the future Alpine tectorogen (Fore-Balkan). This trough was filled with terrigenous, more or less turbiditic, basinal sediments, particularly during the Berriasian (Cherni Osam flysch), which prograded towards the axis of this palaeostructure. Laterally, the two passive margins of this trough are asymmetrical: (1) to the south, a mobile and narrow margin, restricted to some fault-blocks (more or less tilted) against the Rhodopes (‘Thracian Island’) and covered by coarse siliciclastics (Kostel Formation); (2) to the north (Moesia), a more stable and considerably wider shallow-water carbonate platform (Kaspichan Formation). Towards the north-west, the transition between the trough and the northern margin is characterized by a NW–SE framework of sub-basins (Glozhene and Salash formations) and small carbonate Bahamian-type ‘islands’ (Brestnitsa Formation, Slivnitsa Formation) whereas, to the south, the transition is sharper. The geodynamic evolution of this system is particularly marked by the centrifugal backstepping of the terrigenous facies on the two margins. The end of flysch-type/turbiditic sedimentation during the Valanginian coincided with the spectacular narrowing of the northern carbonate internal/proximal platform during the Hauterivian. In this synthesis, three successive palaeogeographic maps complemented by NE–SW-orientated cross-sectional (palinspastic) profiles are presented which demonstrate the evolution of this part of the Peritethyan area of Bulgaria, situated to the north of the ‘Vardar Ocean’, part of the Tethyan oceanic realm.
The sediments of the Loveč Urgonian Group are very rich in bivalves, gastropods, echinoderms, brachiopods, corals, chaetetids, sponges, algae, but quite rare in cephalopods. For that reason, the outcrops of some cephalopods in Magâra Tongue in the Bâlgarene Formation and the Debelcovo-Mladen Tongue of the Emen Formation at the village of Puševo, Veliko Târnovo District are interesting. They all date the Barremian Age.
In the Western Fore-Balkan, micropaleontologic assemblages (Calpionellids, benthic Foraminifera, Algae) observed within the lithostratotype ofthe Magura Formation (Magura cave - Rabisha hill, Northern limb of the Belogradchik anticline) evidence its Late Tithonian and Berriasian (Early to Middle) age. This formation can be regarded as a lateral tongue (more or less reefal) of the lower part of the Makresh Fonnation, described in the well R-l, situated more the North, close to the village of Makresh on the Moesian Platfonn. Magura Formation is also older than the "urgonian"-type massive limestones, Barremian in age, from the Oreshets-Rouzhintsi area (SE), previously included in the same formation, which must be rather correlated to the Simeonovo Formation, more the North, and the Banitsa tongue, first unit of the Vratsa Urgonian Group cropping out to the South-East.
Along a Danube-Gabrovo cross-section, from Central Fore Balkan to Moesian Platform, the Barremian-Aptian deposits are distributed within three distinctpaleogeographic domains: a central terrigenous basin (Pleven-Polski Trǎmbeš) and, laterally (N: Danube, S: Gabrovo) two carbonate-dominated platforms where "Urgonian" facies can be well-developed. This pattern is induced by synsedimentary extensional tectonics already active from the end of Jurassic times. The subdivision of the various series into eight 3 d order depositional sequences and the recent discovery of ammonites allow us to propose new and more reliable correlations between the traditional lithologic formations characterizing those three domains.
The Bulgarian Early Cretaceous basin developed within the northern periphery of the Tethyan Ocean adjacent to the European fragment of the Eurasian lithospheric plate. The basin was of a back-arc type, and formed over the Moesian microplate. It had an arc configuration convex to the North. An avolcanic island arc was situated South of it. Further South, a comparatively narrow interarc basin developed that was bounded to the South by a well-expressed volcanic island arc. The midocean zone of the Tethys Ocean passed through the Aegean.
The morphostructural main features of the Balkanides and the evolution of the ideas on this alpine chain are firstly recalled. Then the most important stages are described: the structural units due to the Austrian phase (during the Albian) are obliterated by the Illyrian movements (later Mid-Eocene), at the origin of the present tectonic zones, that are briefly described. Finally, the Neogene to Quaternary phases are marked by various activities of the lineaments systems. The Rhodope zone, which is especially interpreted, reveals a pile of thick, nappes. The Balkanides arc is considered as a subduction-collision belt, marked by the sinking of the Arabia-Africa plate beneath the Eurasia one. -from English summary
The Lower Cretaceous is developed all over the Northwestern Bulgaria and is usually represented by the Berriasian to the Albian stages. The lithostratigraphic units have different chronostratigraphic range, wide spectrum of lithological varieties and complicated lateral relations. The sedimentary evolution in Northwestern Bulgaria is represented by the change in lithofacies distribution during the various stages of the Early Cretaceous, variations in the sediment thicknesses and general regularities in basin's development. Two zones of facial transitions are established - the Kozlodui-Kenza and Yarlovica-Byalo Pole. -from Authors
In the western Fore-Balkan (northwest Bulgaria), the characterization of numerous depositional sequences within the Barremian–Albian interval allows us to reconstruct from southeast to northwest the successive palaeogeographies of this part of the north Tethyan margin: two carbonate (Urgonian) platforms flanking a central terrigenous basin during the Barremian; a single centrifugal terrigenous basin (Aptian) marked, along its axis, by an olistostrome and siliciclastic turbidites; a single Albian basin characterized by the stacking of glauconite-rich condensation sections and anoxic black marls.
Along a Danube-Gabrovo cross-section, from Central Fore Balkan to Moesian Platform, the Barremian-Aptian deposits are distributed within three distinct paleogeographic domains: a central terrigenous: basin (Pleven-Polski Trămbeš) and, laterally (N: Danube, S: Gabrovo) two carbonate-dominated platforms where “Urgonian” fades can be well-developed. This pattern is induced by synsedimentary extensional tectonics already active from the end of Jurassic times. The subdivision of the various series into eight 3d order depositional sequences and the recent discovery of ammonites allow us to propose new and more reliable correlations between the traditional lithologic formations characterizing those three domains.
The study, in terms of sequence stratigraphy (with ammonite zone control), of numerous cross-sections of Lower Cretaceous (Hauterivian to Aptian) series cropping out along two transects A-B (Fore Balkan and southern part of the Moesian Platform) and C-D (northeastern part of the Moesian Platform) shows the eastwards extension of the North-Tethyan palaeogeographic pattern platform-basin-platform, previously established in central Bulgaria. However, the two boundaryplatforms are strongly assymetrical, this assymetry being revealed by the importance, to the south, of the relative lowstand prograding siliciclastic bodies (supplied by continental erosions linked to extensional tectonics) during the synrift phase (not marked to the north) and by the age, the facies (oolitic to the north, Urgonian to the south) and the number of the calcareous tongues interpreted as the transgressive systems tracts of backstepping depositional sequences, well-marked at the shelf/basin transition.
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