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1

Towards an ammonite subzonation of the Toarcian in Bulgaria

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Metodiev L.

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tom Vol. 4, no. 1

186-188

EN

A scheme of 9 ammonite zones and 16 subzones for the Toarcian of the Balkan Mts. is given. It is an amplification of the standard put forward by Sapunov (1968) for the Toarcian in Bulgaria. The subzones and some zones are newly proposed. They are still in working phase and need refinement. This zonal (subzonal) set is enhanced by range chart of the ammonite genera found in Bulgaria and juxtaposed to the scheme of Elmi et al. (1997) (Fig. 1). Tenuicostatum Zone (Crosbeyi and Semicelatum subzones). The zone corresponds to the range of Dactylioceras (Orthodactylites). Protogrammoceras and Tiltoniceras are also available. The subzones are defined by the ranges of D. (O.) clevelandicum and D. (O.) crosbeyi as well as D. (O.) tenuicostatum and D. (O.) semicelatum. Falciferum Zone (Serpentinum and Falciferum subzones). The zone embraces the ranges of Harpoceras and Hildaites. The former were recently found to come out upwards outside the zone. The ranges of H. serpentinum and Hildaites define the Serpentinum Subzone. The advent of the zonal index and the mass-incoming of Hildoceras fix the Falciferum Subzone. Bifrons Zone (Lusitanicum, Bifrons and Semipolitum subzones). The subzones accepted herein are based on the successive ranges of the species of Hildoceras. Associates of Dactylioceratidae are also present, though too sporadically. Haugia and Phymatoceras arise at the top of the zone, which ends at the last occurrence of Hildoceras. Variabilis Zone. The zone was formerly placed in the Lower Toarcian, while here it is laid in the Upper Toarcian. The zone comprises the latest Dactylioceratidae, Chartronia and Denckmannia. It is limited at the top by the advent of Pseudogrammoceras and Podagrosites. Haugia ammonites are rare and of less biostratigraphical value than in NW Europe. Two subzones could be used: Collina spp. and Chartronia- Denckmannia spp. Thouarsense Zone (Bingmanni, Thouarsense and Fascigerum subzones). The zone as here understood is narrower than in the older scheme. It is defined in terms of its subzones, being composed of species of Pseudogrammoceras, Grammoceras and Esericeras. The top is taken at the extinction of Grammoceras and Esericeras. Fallaciosum Zone. Owing to the particular abundance of its index, it is used as separate zone. The extinction of Pseudogrammoceras and Podagrosites and the advent of Phlyseogrammoceras and Pseudolillia trace the upper limit. Dispansum Zone. No authentic record caused it wasn’t be into the standard. It is now clearly discernible by species of Phlyseogrammoceras, Hammatoceras, Hudlestonia and Pseudolillia. Pseudoradiosa Zone (Levesquei and Pseudoradiosa subzones). It is instead of the older Levesquei and Moorei zones as being framed throughout by finely ribbed Dumortieria. These are ruling in the Pseudoradiosa Subzone. Coarsely ribbed species are dominant in the Levesquei Subzone. Aalensis Zone (Mactra and Aalensis subzones). A succession of finely ribbed and more evolute Pleydellia and P. (Cotteswoldia) (Mactra Subzone) followed by P. (Walkericeras) and finely ribbed but less evolute Pleydellia (Aalensis Subzone) defines the zone. First Pseudammatoceras, Bredyia and Czernyeiceras appear at the top.

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

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Metodiev L. , Koleva-Rekalova E.

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tom 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

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

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tom 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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Biostratigraphy and palaeoenvironmental development of the Callovian-Valanginian carbonate platform from the Lyubash unit (SW Bulgaria)

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

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tom Vol. 4, no. 1

174-175

EN

This study deals with the successions located in the Lyubash unit (SW Bulgaria), which represents a part of the Moesian Carbonate Platform in Middle Jurassic – Early Cretaceous time interval. According to the recent tectonic scheme of Bulgaria (Dabovski et al. 2002) the Lyubash unit belongs to Srednogorie tectonic zone of the Balkan orogenic system. Three sections have been sampled and studied – Rebro, Lyalintsi and Velinovo, which are built up by thick-bedded to massive light grey to whitish organogenic and micritic limestones of the Javorets, Gintsi and Slivnitsa formations. Detailed studies on foraminifera and calcareous dinocyst, provided new biostratigraphic data, and confirmed the Middle Callovian-Valanginian age of the examined sediments as assumed previously by Sapunov et al. (1985). Foraminifera are particularly abundant in the lagoon facies, and represented by genera: Globuligerina, Ophthalmidium, Cornuspira, Ammobaculites, Mesoendothyra, Labyrinthina, Kurnubia, Pseudocyclammina, Dobrogelina, Rumanoloculina, Hechtina, Meandrospira, Valvulina, Trocholina, and Neotrocholina. The sedimentological analysis allowed recognizing microfacies groups assigned to three facies zones: platform slope facies, reef and peri-reef facies and lagoon-tidal flat facies. The development of the platform is characterized by complex vertical and lateral alternation of the studied microfacies. Platform slope facies are prevalent at the section Rebro; the Lyalintsi section is dominated by reef and peri-reef facies, and the section Velinovo is mainly dominated by lagoon facies. Sea-level fluctuations are observed, however generally, a shallowing-upward trend in the platform evolution is observed in all three sections. Coral-microbial reefs are developed as biostromes, and contain numerous and highly differentiated scleractinian corals: 70 species (6 new) from 50 genera (4 new). Among them following corals are the most common: phacelloid Latomeandra, Stylosmilia, Thecosmilia, Cladophyllia, ramose Solenocoenia, Meandrophyllia, lameller Synastraea and Microsolena. Reef development has been disturbed by sea level rise, evidenced by limestones with Saccocoma, intercalated with coral biostromes.