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Oxfordian to Valanginian palaeoenviron- mental evolution on the western Moesian Carbonate Platform: a case study from SW Bulgaria

Ivanova D., Kołodziej B., Koleva-Rekalova E., Roniewicz E.

Annales Societatis Geologorum Poloniae

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2008

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Vol. 78, No 2

65-90

EN

Three sections (Rebro, Lyalintsi and Velinovo) of the Upper Jurassic-Lower Cretaceous carbonate sequences from the Lyubash unit (Srednogorie, Balkanides, SW Bulgaria) have been studied for elucidation of biostratigraphy and palaeoenvironmental evolution. Palaeontological studies of foraminifera, supplemented by studies of calcareous dinoflagellate cysts and corals, enabled the determination of the Oxfordian-Valanginian age of the analysed sequences. They were deposited on the Dragoman Block (western part of the Moesian Platform), and during Mid-Late Cretaceous included to the Srednogorie. A possible Middle to Late Callovian age of the lowermost part (overlying the Bajocian-Lower Bathonian Polaten Formation) of the studied sections assumed till now has not been confirmed by the present studies. Eleven facies have been distinguished and attributed to depositional environments. Marine sedimentation on a homoclinal ramp started in the Oxfordian and till the Early Kimmeridgian - in all three sections - was dominated by fine-grained peloidal-bioclastic wackestones to grainstones. Since the Late Kimmeridgian, when a rimmed platform established, facies pattern underwent differentiation into (i) the inner platform (lagoon and tidal flat facies) - only in Velinovo, (ii) reef and peri-reef facies/bioclastic shoals - mainly in Lyalintsi, and (iii) platform slope - mainly in Rebro. Sedimentation generally displays a shallowing-upward trend. Two stages in evolution of the rimmed platform are postulated. The mobile stage lasting till the Tithonian/Berriasian boundary was followed by a more stable stage in the Berriasian to Valanginian time. Reefs are developed mainly as coral-microbial biostromes, lower coral bioherms or coral thickets, in the environment of moderate energy and sedimentation. They contain highly diversified corals (72 species). Micro- bialites contributed to the reef framework, but they never dominated. Locally, microencrusters and cement crusts formed important part of reefal framework. During the mobile stage of the platform evolution a relative sea-level rise interrupted reef development, as evidenced by intercalations of limestones with Saccocoma. During the second stage high carbonate production and/or regressive eustatic events, not balanced by subsidence, decreased accommodation space, limiting reef growth and enhancing carbonate export to distal parts of the platform.

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Carbon and oxygen isotope record of Lower-Middle Jurassic belemnites and rocks from the Western Balkan Mts. (Bulgaria)

Metodiev L., Koleva-Rekalova E.

Volumina Jurassica

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2006

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Vol. 4, no. 1

188-190

EN

Methods This work is based on the study of 76 belemnites and bulk samples, collected from section Dobravitsa-1, located at 60 km to the north of Sofia. Elemental measurements were performed with an ICP-OES instrument (Perkin Elmer, Optima 3000), following microwave digestion (MLS Ethos 1600). Stable isotope data from belemnites (ä13Ccarb and ä18Ocarb) and rocks (ä13Corg) were collected by Finnigan Mat 251/252 and Finnigan Mat Delta E mass spectrometers. TC and TOC measurements were realized with a CS-200 Carbon/Sulfur Analyzer. Overview of results (Fig. 1) We have documented a negative ä13Corg excursion from a background of c. -25.74‰ of the Upper Pliensbachian to -28.63‰ in the base of the Lower Toarcian Tenuicostatum Zone. The ä13Corg record displayed upwards a shift to -25.43‰ in the Falciferum Zone, a fall to -27.41‰ in the Dispansum Zone and rise to -25.83‰ in the Pseudoradiosa Zone. Two smaller rises of ä13Corg were detected in the Aalenian. The ä13Ccarb and ä18Ocarb signatures provided coeval positive C isotope excursions and negative O isotope shifts localized in the Lower Toarcian. The samples produced the following values: Upper Pliensbachian (ä18O from -0.47 to -2.16‰, ä13C from +0.74 to +2.73‰); Toarcian (ä18O from -1.01 to -3.94‰, ä13C from -0.03 to +3.21‰); Aalenian and Lower Bajocian (ä18O from -1.0 to -2.86‰, ä13C from +0.02 to +1.47‰). The extreme values came from the base of the Tenuicostatum Zone, as well as from the Falciferum and Bifrons zones. The TOC contents is low, mostly <0.5 wt%, with maximum of 1.25 wt% in the Pseudoradiosa Zone. The S values range from 0.05 to 2.24 wt%. Both TOC and S show fluctuations that correlate with the isotope curves. The Mn values compose a flat temporal trend (range from 224 to 568 ppm) with an apparent rise to 1263 ppm in the Bifrons Zone. The TC values (varying from 2.72 to 9.97 wt%) are inferring short-term depositional changes. Summary Stable carbon and oxygen isotope data (ä13Corg, ä13Ccarb and ä18Ocarb) are used to appraise the pattern of their stratigraphic variations in an Upper Pliensbachian – Lower Bajocian succession of the Western Balkan Mts. (Bulgaria). A section, which is composed of alternating offshore ferruginous marls, shales and limestones, and divided into 15 ammonite zones (from the Spinatum Zone to the Discites Zone), has been sampled. Several coeval excursions were found to be superimposed on the O and C isotope trends that are pursued by concomitant variations of the organic-carbon (TOC), total-carbon (TC), S and Mn contents. The observed variations seem to be of both global and local significance. Some of them show a synchronicity with other similar events widely recorded in coeval strata in Europe (Jenkyns et al. 2002).

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Callovian to Berriasian pelagic carbonates in the Western Fore-Balkan, Bulgaria: microfacies, ammonite and microfossil zonations

Ivanova D., Koleva-Rekalova E., Lakova I., Metodiev L.

Volumina Jurassica

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2006

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Vol. 4, no. 1

175-177

EN

The Callovian to Berriasian pelagic carbonates in the Western Fore-Balkan crop out as a part of the Middle Jurassic – Lower Cretaceous peri-platform marine sediments deposited on the northern Tethyan continental margin. This pelagic record consists of marl-limestone alternation (Bov Formation), grey micritic limestones with reddish nodular limestones (Javorets Formation), Ammonitico Rosso type red nodular limestones (Gintsi Formation) and Biancone-type grey micritic regularly bedded limestones (Glozhene Formation) (Sapunov 1976). The total thickness of this succession exceeds 400 m. Rich ammonite faunas recorded from the Bov, Javorets and Gintsi Formations enabled ammonite zonation and age assignment: Macrocephalites spp., Hecticoceras spp. and Kosmoceras spp. zones (Callovian), P. athletoides, C. renggeri, P. (D.) episcopallis, P. (D.) antecedens and G. riazi zones (Oxfordian), H. beckeri zone (Upper Kimmeridgian) and H. hybonotum, S. schwertschlageri and V. rothpletzi zones (Tithonian). The stratigraphic distribution and relative abundance of pelagic microplankton organisms (thin-shelled bivalves, planktonic foraminifers, radiolarians, calcareous dinocysts, pelagic echinoderms and calpionellids) have been used for biostratigraphy and/or recognition of microbiofacies. Within the Oxfordian- Berriasian interval the calcareous dinocyst zones: C. fibrata, C. borzai, C. tithonica, P. malmica, C. tenuis, C. fortis, St. proxima and St. wanneri are recorded. The Middle Tithonian to Berriasian interval is characterized by the successive calpionellid zones: Chitinoidella, Praetintinnopsella, Crassicollaria, Calpionella and Calpionellopsis (Lakova et al. 1999). Five microbiofacies within the pelagic carbonates are superposed: mudstone and wackestone with filaments of pelagic bivalves (Callovian), Globuligerina wackestone and radiolarian wackestone [Oxfordian-Kimmeridgian(?)], Saccocoma wackestones (Kimmeridgian – Lower Tithonian) Globochaete mudstone (Middle Tithonian) and calpionellid mudstone (Upper Tithonian and Berriasian) (Fig. 1). The estimated average rate of sedimentation within the Callovian-Berriasian pelagic succession in the Western Fore-Balkan varying from 9 to 26 mm/10 3 years is characteristic for the transition from relatively condensed to stratigraphically expanded sections in the Upper Jurassic of the Tethyan region. This rate is lower during the Callovian to Kimmeridgian and increased significantly in the Tithonian and Berriasian. Probable explanations are partial carbonate dissolution of the red nodular limestones in the Late Jurassic and the increased bioproductivity of nannoplankton in the Berriasian.

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The Triassic and Jurassic sediments in eastern Stara Planina Mts. (Bulgaria) - an example of classification of geosites in sedimentary rocks

Budurov K., Ivanova D., Koleva-Rekalova E., Petrunova L., Tchoumatchenco P., Yaneva M., Zagorcev I.

Polish Geological Institute Special Papers

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2004

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Vol. 13

199-208

EN

Two types (basin and shelf) of Triassic and Jurassic Tethyan sediments participate in the structure of eastern Stara Planina Mts. (eastern Bulgaria). A parautochtonous position is assumed for the basin type rocks. The shelf sediments are allochtonous and can be observed as olistolites included in the Lower Jurassic Sini Vir Formation and in the Middle Jurassic Kotel Formation. The parautochtonous sediments take part in the composition of the probable overthrust structures refolded in antiformal and synformal structures, and intensely eroded before Late Cretaceous times. From the geological heritage point of view, the Triassic and Jurassic sediments of the region are included in a large unit - geosites framework - composed of 19 geosites: 10 in the parautochtonous and 9 in the allochtonous sediments. They exhibit different geological (tectonical, stratigraphical, palaeontological, etc.) features.

PL

W strukturze wschodniej Starej Planiny (wschodnia Bułgaria) występują dwa typy triasowych i jurajskich osadów Tetydy: basenowe i szelfowe. Skały pochodzenia basenowego uważane są za parautochtoniczne. Osady szelfowe są allochtoniczne. Występują one w dolnojurajskiej formacji Sini Vir oraz w środkowojurajskiej formacji Kotel, w formie olistolitów. Osady parautochtoniczne uczestniczą w prawdopodobnej strukturze płaszczowinowej, przefałdowanej w struktury antyklinalne i synklinalne, intensywnie zerodowanej przed górną kredą. Z punktu widzenia dziedzictwa geologicznego, osady triasowe i jurajskie omawianego regionu tworzą dużą zbiorczą jednostkę geotopową, obejmującą 19 geotopów: 10 w osadach parautochtonicznych, 9 w osadach allochtonicznych. Posiadają one różnorodne cechy geologiczne (tektoniczne, stratygraficzne, paleontologiczne itp.).

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Laminated aragonite-clay sediments (varves) of Chersonian age from north Bulgaria

Koleva-Rekalova E.

Biuletyn Państwowego Instytutu Geologicznego

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1999

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nr 387

120--122