NW Bulgaria Research Articles

Late Oligocene and Miocene climate and vegetation in the Eastern Paratethys area (northeast Bulgaria), based on pollen data

The results of palynological studies of the Oligo-Miocene in the western part of the Euxinian Basin (Eastern Paratethys) are based on the analysis of 57 samples from core C-136A drilled near the town of Balchik, Varna district. Some samples were barren or contained very little pollen and were not included in the analysis. Quantitative data are thus confined to 26 polleniferous samples. We present results for the Upper Oligocene as well as the Tarkhanian, Karaganian, Bessarabian, and Chersonian stages of the middle to upper Miocene. Mixed deciduous forests were widespread during the time period studied. Swamp forests are also recorded, with distribution maxima observed in the Oligocene and in the Bessarabian. The widest extent of herbaceous vegetation is recorded in the late Bessarabian and Chersonian, when it occupied open landscapes after a marine regressional phase. The climatic data reconstructed by the Coexistence Approach indicate mean annual temperatures (MATs) of ca. 15–17 °C and mean annual precipitation (MAPs) of 800–1300 mm for the Upper Oligocene. The narrowest coexistence intervals of the Tarkhanian and Karaganian display MATs of 14–17 °C and MAPs of 800–1300 mm. The most substantial data are for the Bessarabian, in which the lower part is characterized by MATs of 15.6–16.6 °C and MAPs from 800–1200 (1300) mm. The results for the upper part of the Bessarabian are more variable, especially with respect to precipitation that is generally lower (700–800 mm). The same is true for the Chersonian, where besides lower precipitation, lower annual temperature and temperature of coldest month are observed. These results correlate with previous vegetation and palaeoclimate data from the Forecarpathian Basin (NW Bulgaria).

Distribution of diagenetic alterations in fluvial and paralic deposits within sequence stratigraphic framework: Evidence from the Petrohan Terrigenous Group and the Svidol Formation, Lower Triassic, NW Bulgaria

Elucidation of diagenetic alterations in the Petrohan Terrigenous Group (fluvial; highstand systems tract HST) sandstones and Svidol Formation (tide-dominated deltaic and tidal flat, transgressive systems tract TST and highstand systems tract HST, respectively) sandstones and calcarenite, Lower Triassic, NW Bulgaria was constrained within a sequence stratigraphic framework. Eogenetic alterations in the fluvial HST sandstones include (i) formation of grain-coating infiltrated clays as a result of percolation of mud-rich surface waters into underlying coarse-grained and permeable channel-fills and crevasse splay sandstones; (ii) formation of pseudomatrix by mechanical compaction of mud intraclasts that were incorporated into the coarse-grained channel sandstones during their lateral avulsion; and (iii) cementation by calcite (δ 18O VPDB = − 6.5‰ to − 3‰; δ 13C VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ 18O VPDB = − 6.1‰ to − 0.3‰; δ 13C VPDB = − 7.2‰ to − 5.8‰) in the crevasse splay and floodplain sediments. Mesogenetic alterations that are encountered in the fluvial HST sandstones include (i) illitization of grain-coating clays, mud intraclasts, and mica, possibly because of simultaneous albitization of feldspars; (ii) cementation by calcite (δ 18O VPDB = − 14.5‰ to − 8.4‰; δ 13C VPDB = − 7.7‰ to + 0.6‰) and dolomite (δ 18O VPDB = − 15.8‰ to − 5‰; δ 13C VPDB = − 7.9‰ to + 1.5‰); and (iii) limited amounts of quartz overgrowths in the channel sandstones owing to occurrence of thick grain-coating clays. Conversely, the tide-dominated deltaic TST sandstones and the tidal flat HST calcarenite were pervasively cemented by calcite (δ 18O VPDB = − 6.6‰ to − 3.1‰; δ 13C VPDB = − 5.1‰ to + 0.6‰) and siderite (δ 18O VPDB = − 7.2‰ to − 5.7‰; δ 13C VPDB = + 0.3‰ to + 0.9‰) particularly below marine and maximum flooding surfaces, due to the presence of abundant bioclasts and prolonged residence time of the sediments under certain geochemical conditions along these surfaces. The remaining open pores were cemented during mesodiagenesis by calcite (δ 18O VPDB = − 6.6‰ to − 3.1‰ and δ 13C VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ 18O VPDB = − 6.6‰ to − 3.1‰ and δ 13C VPDB = − 5.1‰ to + 0.6‰). This study shows that constructing a conceptual model for the distribution of diagenetic alterations is possible by integration of diagenesis with sequence stratigraphy. The model shows that tide-dominated deltaic TST sandstones and tidal flat HST calcarenite were pervasively cemented by carbonates during near-surface eodiagenesis, owing to the presence of abundant bioclasts. Conversely, fluvial LST sandstones remained poorly cemented during near-surface eodiagenesis due to the lack of bioclasts, but were cemented by mesogenetic calcite, dolomite and quartz overgrowths instead.

Monomineralic carbonate ooid types in the Triassic sediments from Northwestern Bulgaria

Oolitic rocks occur in almost all lithostratigraphic units from the Iskur Carbonate Group (Lower – Upper Triassic) in NW Bulgaria. Primarily monomineralic ooids with homogenous cortices belong to several fabric types: micritic/microsparitic, brickwork, sparry relic, blocky, radial fibrous, radial sparry, shrunken and mottled. In addition, ooids with compound cortical fabrics were distinguished: blockymicritic, micritic-radial and radial-brickwork. Cracked, spalled, broken, abraded, distorted and pitted types were differentiated according to outer deformed shape. Apart from the single individuals polyooids were found in some rocks. The primary ooid fabrics were modified by various isochemical diagenetic processes including microbial micritization, compaction, aragonite transformation, recrystallization, dissolution and cementation. These proceeded in the marine-phreatic, meteoric-phreatic and deep-burial connate diagenetic environments. Only one specific ooid association shows relation with a particular depositional setting. At the very base of the Lower Triassic peritidal sequence (Mogila Fm) mottled and radialbrickwork ooids coexist with bimineralic individuals and broken shapes. The formation of such association is attributed to hypersaline conditions, as this interpretation is based on other sedimentological data and analogy with similar occurrences from the geological record. The original mineralogy of some ooid types was inferred mainly on the basis of petrographic and geochemical data, but actualistic evidence was considered as well. The precipitation of aragonite and high-Mg calcite ooids was proved. Their stratigraphic occurrence conforms well to the global distribution of carbonate ooids through the Triassic period. Thus aragonite ooid precipitation was predominant during Late Olenekian – Middle Anisian times. The main control was high carbonate saturation state of seawater promoting higher precipitation rates for aragonite due to kinetic reasons. In this particular case, the high degree of seawater carbonate saturation was probably further promoted by an increased salinity reflecting intense evaporation in an arid peritidal setting. The contrastingly large predominance of calcitic ooids in the Upper Anisian – Lower Carnian sediments corresponds to the secular increase of calcite abiotic precipititation during the Middle Triassic epoch. This change was related to the appearance of global highstand, more intense seafloor spreading, warmer climate, higher CO2 levels, and consequently, to low carbonate saturation state of seawater favouring calcite precipitation. In this particular case, the seawater reached normal salinity from the Middle Anisian onwards, thus probably further promoting calcite ooid formation. Locally controlled short-term shifts in the carbonate precipitation also took place in the epicontinental Triassic sea. This is inferred from the coexistence or interbedding of primary aragonitic and calcitic ooids, as well as presence of bimineralic ooid cortices. It is suggested that subtle temperature and/or salinity changes ultimately controlled the carbonate saturation state of seawater. Unlike the established trend of primary ooid mineralogy, marine phreatic cements in the studied Triassic rocks were predominantly composed of high-Mg calcite. This discrepancy is generally explained by the lower supply of carbonate ions into interparticle pores, which favoured calcite cementation, although poor preservation of marine aragonite cements is another possibility. Further evidence on the abiotic carbonate precipitation might be obtained from investigation of the limestone micritic matrix.

Magnetic Susceptibility Screening of Anthropogenic Impact on the Danube River Sediments in Northwestern Bulgaria - Preliminary Results

Several events of anthropogenic impacts on the floodplain river sediments of the Danube along a section near the town of Oryakhovo (NW Bulgaria) have been detected using magnetic technique. In the field, magnetic susceptibility was measured using three susceptibility meters – MS2D, KT-5 and SM30. The differences in the surface susceptibility values measured with the different instruments are ascribed to the various penetration depths which depend on several parameters like sensor diameter, frequency and field strength. This is supported by the detailed laboratory study on penetration depth and sensitivity of the new SM30 susceptibility meter and the comparison with the already existing data for the other two sensors. Boundaries between different flooding events are clearly linked with significant variability of the measured susceptibility values corresponding to one and the same level. Sediment cores, taken at different distances from the water level, show the history of depositional events and corresponding degree of anthropogenic pollution. Identification of the magnetic phases responsible for the signal was carried out by means of thermomagnetic κ(T) curves. The main carrier is magnetite with Tc of 580°C or oxidized magnetite with Tc of 600°C. A second kink at about 300°C is better expressed for samples with lower susceptibilities. It may correspond to several minerals, e.g. maghemite, pyrrothite, titanomagnetite, as well as different mineral transformations of paramagnetic minerals (decomposition, dehydroxilation, etc. for example siderite, lepidocrocite). The samples showing strong magnetic enhancement are characterized by the predominance of magnetite. Optical microscopy on magnetic extracts shows the presence of small spherical particles, typical for the anthropogenic magnetic phases from high-temperature technological processes. Well expressed susceptibility variations along the cores suggest the presence of several stages of different degree of pollution, covering an estimated period of about 30 years.

Palynological evidence for Miocene climate change in the Forecarpathian Basin (Central Paratethys, NW Bulgaria)

We reconstruct quantitatively the Middle to Upper Miocene climate evolution in the southern Forecarpathian Basin (Central Paratethys area, Northwest Bulgaria) by applying the coexistence approach to 101 well-dated palynofloras isolated from three cores. The climatic evolution is compared with changes in vegetation and palaeogeography. The Middle Miocene was a period of a subtropical/warm–temperate humid climate with mean annual temperature (MAT) between 16 and 18°C and mean annual precipitation (MAP) between 1100 and 1300 mm. Thereby, during the entire Middle Miocene a trend of slightly decreasing temperatures is observed and only small climate fluctuations occur which are presumably related to palaeogeographic reorganisations. The vegetation shows a corresponding trend with a decrease in abundance of palaeotropic and thermophilous elements. The Upper Miocene is characterised by more diverse climatic conditions, probably depending on palaeogeographic and global climatic transformations. The beginning of this period is marked by a slight cooling and a significant drying of the climate, with MAT 13.3–17°C and MAP 652–759 mm. After that, fluctuations of all palaeoclimate parameters occur displaying cycles of humid/dryer and warmer/cooler conditions, which are again well reflected in the vegetation. Our study provides a first quantitative model of the Middle–Upper Miocene palaeoclimate evolution in Southeastern Europe and is characterised by a relatively high precision and resolution with respect to the climate data and stratigraphy.

Earliest finds of crossbills (genus Loxia) (Aves: Fringillidae) from Varshets (NW Bulgaria)

A review of the fossil history of g. Loxia is presented. Bulgarian record of crossbills consists of several finds of 4 sites – 3 of Upper Pleistocene (Wurmian) age and one of Middle Villafranchian (ca. 2.2 mya). The last site (a ponor near the town of Varshets, NW Bulgaria) has provided the earliest record of the genus. These finds are described as Loxia patevi sp. n.

Paleobiogeographical affinities of Pridolian and Lochkovian chitinozoans from North Bulgaria

Chitinozoan assemblages of Pridolian and Lochkovian Age from the subsurface of North Bulgaria have been compared with coeval chitinozoan faunas from numerous localities in Laurassia and Gondwana. The Pridolian and Lochkovian chitinozoan faunas of NW Bulgaria seem related without any doubt to the North Gondwanian realm, whereas those from NE Bulgaria are not diverse enough to precisely estimate their biogeographic position within the Laurassia - Gondwana interface.

A geodynamic model of the Alpine magmatism in Bulgaria

The territory of Bulgaria covers part of the active continental margin of the Eurasian plate. Several first-order tectonic units may be distmguished: Moesian platform, paraautochthonous margin of the platform, zone of Mid-Mesozoic collage units, Late Cretaceous island-arc system and a system of Cenozoic collisional and post-collisional grabens. The Alpine magmatism is genetically related to the evolution of the Eurasian margin and the Tethyan ocean south of it. The earliest Alpine magmatic activity is represented by Triassic, basic and intermediate, strongly altered volcanics (NW Bulgaria, in boreholes). They are related to initial, embryonal rifting of the Moesian platform. The Triassic and Jurassic-Lower Cretaceous small bodies of basic volcamcs in the SE collage units are interpreted as ensimatic. During the Upper Cretaceous an ensialic island-arc system originated. The related intensive magmatism formed the Srednogoric volcano-intrusive zone (SVIZ). The magmatic rocks are products of complex differentiation processes. Their formation was accompanied by deep-water sedimentation. All magmatic groups, according to SiO2 content, occur. By the K2O/SiO2 ratio they belong to the TH, CA, HKCA, SH, HKTR (high-K transitional) and BG (bulgaritic) series. Longitudinal and transversal zonahties may be traced. In a global aspect SVIZ is one of the most ancient segments of the Alpine-Himalayan volcano-intrusive belt. The bulgaritic petrochemical trend is related to the earliest generation of K-cnriched magmas in the Mediterranean region. The products of the collisional magmatism are exposed in the Macedonian-Rhodope-North Aegean volcanic zone (MRNAVZ) located south of the Late Cretaceous island arc. It originated during the Eocene-Oligocene (37-25 Ma) as a result of the collision between Eurasia and the Apulian promontory of Africa. The magmatic rocks belong dominantly to the intermediate and acid groups and to the CA, HKCA and SH series. The distribution of intermediate and acid rocks in this zone is controlled by the thickness of the crust. In the Eastern and Central Rhodopes the K-content increases from south to north. The volcanic activity in the zone occurred in conditions of intensive block orogeny and was accompanied by terrigenous molasse sedimentation. During the Neogene a zone of transversal faulting developed in the collisional orogen (remainding of Himalayan-type orogen) and parts of the Moesian platform. This zone is marked by small bodies of basic and ultrabasic, Na-alkalinc and subalkalinc rocks. The K-content increases in southward direction, toward the collisional front.

A palaeomagnetic and petromagnetic study of Upper-Carboniferous, Permian and Triassic sediments, NW Bulgaria

In order to obtain basic palaeomagnetic data on Upper Carboniferous, Permian and Triassic sediments collected from the NW Bulgaria, laboratory stability tests were extended from A.C. and thermal treatments to studies of mineral phase changes and to investigations of changes of magnetic anisotropy during laboratory procedures. Laboratory criteria were found which permitted to distinguish samples suitable for palaeomagnetic analyses from those representing rocks totally or almost totally chemically reworked during their history. Palaeomagnetic directions and pole positions derived from Stephanian, Lower Permian and Triassic rocks from the southern margin of the Moesian Platform are compatible with the values obtained for the tectonically stable North-European Platform.

Hydracariens souterrains de Bulgarie. I. Mideopsis (Nudomideopsis) motasi n. sp.

A new species of Mideopsis (Nudomideopsis) motasi n. sp. coming from a well near Gorni-Ciflik, close to Belogradchik in NW Bulgaria, is described. For the structure of the palp it is distinguished from the other subterranean European species Mideopsis (Mideopsis (Nudomideopsis) longipalpis Szalay and Mideopsis (Nudomideopsis) fonticola Tanasachi et Orghidan and resembles some Japonese Mideopsis species found also in wells.