Givetian carbonate microfacies from the Totleben-2 well ... · Givetian carbonate microfacies from the Totleben-2 well (Moesian Platform, central North Bulgaria) Polina Andreeva Geological

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GEOLOGICA BALCANICA, 45, Sofia, Dec. 2016, pp. 21–31.

Givetian carbonate microfacies from the Totleben-2 well (Moesian Platform, central North Bulgaria)

Polina AndreevaGeological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Block 24, 1113 Sofia, Bulgaria; e-mail: [email protected](Accepted in revised form: December 2016)

Abstract. Seven carbonate microfacies types (MFT 1–7) have been distinguished and described in the Givetian dolomite formation from the Totleben-2 well (Moesian Platform, central North Bulgaria). They have been grouped in three microfacies associations: 1) shallow subtidal microfacies with open circulation (MFT 1, coral-stromatoporoid floatstone/rudstone; and MFT 2, brachiopod floatstone/rudstone); 2) protected shallow subtidal microfacies (MFT 3, intraclastic-ooidal packstone/grainstone; MFT 4, palaeosiphonoclad wacke-stone/packstone; and MFT 5, bioclastic-peloidal packstone/grainstone); 3) intertidal-supratidal microfacies (MFT 6, fenestral microbial bindstone; and MFT 7, mudstone to packstone with Palaeomicrocodium). These microfacies have been interpreted as formed in dominantly shallow subtidal lagoon setting with open to mod-erate circulation and locally developed intertidal/supratidal environments. Most microfacies are comparable with Wilson’s (1975) Standard Microfacies Types (SMFT) and/or with Givetian microfacies described from the eastern part of the Moesian Platform, as well as with other shallow-marine Devonian successions from Europe.

Andreeva, P. 2016. Givetian carbonate microfacies from the Totleben-2 well (Moesian Platform, central North Bulgaria). Geologica Balcanica 45, 21–31.

Keywords: microfacies, depositional settings, Devonian, Moesian Platform, Bulgaria.

INTRODUCTION

Devonian deposits in northern Bulgaria were drilled in more than twenty wells located mainly in the east-ern parts of the Moesian Platform (Budurov, 1961; Spassov and Yanev, 1966; Spassov, 1971; Yanev, 1972). They have been studied by many authors in terms of stratigraphy and sedimentology (e.g., Ya-nev, 1974; 1995; Bokov and Ognyanov, 1978; Spas-sov, 1983; 1987; Lakova and Yanev, 1989; Lakova, 1993, 1995a, b; Boncheva, 1995; Yanev and Bonche-va, 1995; Boncheva et al., 2000; Belivanova, 2002; Andreeva, 2008, 2010, 2015). On the other hand, geological data about coeval deposits in the central part of the platform are very scarce. Therefore, the drilled Devonian succession in the Totleben-2 well represents a unique opportunity to study carbon-ate microfacies in this part of the Moesian Platform (Fig. 1). These deposits were described for the first time by Ovcharov et al. (1973), who suggested De-

vonian age of the rocks and assigned them to three informal lithostratigraphic units: shale-limestone formation (Lower Devonian), dolo stone formation (Middle Devonian) and dolostone-limestone forma-tion (Middle to Upper Devonian). The early Devon-ian age of the shale-limestone formation (depth interval 3209–3501 m) was proved on the basis of chitinozoans by Lakova (1986). Later, the Devon-ian strata in the Totleben-2 well were subdivided by Yanev (1995) and Yanev and Boncheva (1995) into several informal units, following the lithostrati-graphic scheme of Devonian deposits from wells in northeastern Bulgaria (Yanev, 1972; Fig. 2).

The present paper is focused on the carbonate rocks (limestones and dolomitic limestones) from the Givetian dolostone formation (Fig. 2) drilled in the Totleben-2 well (Moesian Platform, central North Bulgaria). This study aims to distinguish and describe the major microfacies types, as well as to interpret their respective depositional environments.

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GEOLOGICAL SETTING

According to the tectonic scheme of Dabovski and Zagorchev (2009), the studied Devonian sediments from the Totleben-2 well occur on the Iskar-Yantra step of the Moesian Platform (Fig. 1). There, the Low-er to Upper Devonian carbonates were subdivided by Yanev (1995) and Yanev and Boncheva (1995) into four informal lithostratigraphic units (Fig. 2). The banded limestone formation (Frasnian), the dolomite formation (Givetian), the calcareous-clayey package of the carbonate-sulphate formation (Eifelian), and the calcareous-siliciclastic-clayey formation (Lower De-vonian), were distinguished in the depth interval from 2707 m to 3501 m.

The Givetian sequence of the dolomite formation was drilled within the depth interval 2776–3213 m. The lower part of this unit (from 2872 m to 3213 m) is composed of dark grеy to black, predominantly massive, or locally brecciated, dolostones. The lat-ter commonly contain dolomitized bioclasts (most-ly stromatoporoids?) that are difficult to identify. The upper part (from 2776 m to 2872 m) consists of creamy grey, light grey, and dark grey, massive or laminated dolomitic limestones and limestones (Fig. 2). Some of these rocks contain various well-preserved bioclasts (e.g., brachiopods, stromatoporo-ids, corals, bivalves).

MATERIAL AND METHODS

Microfacies analysis is based on detailed microscopic study of 16 thin-sections (hosted by the Geological Institute, Bulgarian Academy of Sciences) observed

under standard petrographic microscope. The primary core-logging documentation of Slavcho Yanev (un-published results) was additionally used. Seven mi-crofacies types (MFT 1–7) were described, following Dunham’s (1962) textural classification and later am-plified by Embry and Klovan (1971). The comparison charts of Baccelle and Bosellini (1965) were applied to estimate the relative frequency of constituent par-ticles.

The diagnostic microfacies criteria of Flügel (2004) were used for defining the microfacies types. These criteria include: depositional texture, qualita-tive composition (fine-grained matrix, grain types, carbonate cements), quantitative composition (relative proportions of fine-grained matrix, grains, and inter-particle carbonate cements), and depositional fabrics (biofabrics, bedding and lamination, burrowing and bioturbation, fenestral fabrics).

Most of the identified microfacies types were com-pared with the classical microfacies scheme created by Wilson (1975), and expanded by Flügel (2004), and/or with Givetian microfacies described from the eastern part of the Moesian Platform, as well as with other shallow-marine Devonian successions from Europe.

MICROFACIES TYPES AND ASSOCIATIONS

Seven microfacies types (MFT 1–7) have been dis-tinguished and described (Table 1). They are ordered from the most distal to the most proximal and are grouped in three microfacies associations: 1) shallow subtidal microfacies with open circulation; 2) pro-tected shallow subtidal microfacies; and 3) intertidal-supratidal microfacies.

Fig. 1. Tectonic subdivision of the Moesian Platform in Bulgaria (modified from Dabovski and Zagorchev, 2009) and location of the Totleben-2 well plus drilled Devonian wells in the eastern part of the platform.

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Fig. 2. Lithostratigraphic subdivision of the Devonian deposits in northeastern Bulgaria (after Yanev, 1995, and Yanev and Bonche-va, 1995) and lithological log of the studied Givetian carbonates from the dolomite formation, with distinguished microfacies types.

Table 1Summary of the distinguished microfacies types, microfacies associations, and interpreted depositional environments

Microfacies type Microfacies association Depositional environment

MFT 1 Coral-stromatoporoid floatstone/rudstone Shallow subtidal with open circulation

Back reef setting or open lagoon

MFT 2 Brachiopod floatstone/rudstone

Shallow subtidal with open circulation Open lagoon

MFT 3 Intraclastic-ooidal packstone/grainstone Protected shallow subtidal Lagoon with moderate

circulation

MFT 4 Palaeosiphonoclad wackestone/packstone Protected shallow subtidal Lagoon with moderate circulation

MFT 5 Bioclastic-peloidal packstone/grainstone Protected shallow subtidal Lagoon with moderate circulation

MFT 6 Fenestral microbial bindstone Intertidal-supratidal Intertidal microbial mats

MFT 7 Mudstone to packstone with Palaeomicrocodium Intertidal-supratidal Pedogenic carbonates

(palaeocaliche)

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Shallow subtidal microfacies with open circulation

This microfacies association includes two microfa-cies types (MFT 1, 2) containing well-preserved, mainly open-marine fossil assemblages (e.g., corals, stromatoporoids, brachiopods, crinoids). They are in-terpreted as formed in shallow subtidal settings with normal water salinity and locally agitated hydrody-namic conditions (probably due to sporadic periods of stronger wave activity).

MFT 1 Coral-stromatoporoid floatstone/rudstone

Description. This MFT is composed of well-preserved fossil remains of tabulate corals (mainly Thamnop-ora) and encrusting and scarce branching stromato-poroids (Fig. 3a, b) within wackestone/packstone matrix. Brachiopods, crinoids, calcimicrobial crusts, and rare calcispheres also occur. The encrusting stro-matoporoids are locally developed on coral skeletons or some brachiopod shells. The partly dolomitized matrix is micritic/microsparitic and contains variable

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amount of peloids. MFT 1 is documented from the lower and middle parts of the Givetian succession (Fig. 2).

Interpretation. The open-marine fossil assemblage (e.g., corals, stromatoporoids, brachiopods, crinoids) indicates waters with normal salinity and open circula-tion. The high percentage of relatively well-preserved, reef-derived bioclasts suggests close similarity to or-ganic build-ups (coral-stromatoporoid biostromes?). The deposits of this MFT are interpreted as formed in a relatively calm shallow subtidal setting, locally interrupted by sporadic high-energy events (strong storms or currents). A similar microfacies has been described from Givetian carbonate rocks in Belgium and defined as inner-shelf deposits with open circula-tion (Pas et al., 2016). MFT 1 can be also compared with SMFT 5 “Allochthonous bioclastic grainstone, rudstone, packstone and floatstone or breccia”, when formed in back-reef setting and lagoon (Flügel, 2004).

MFT 2 Brachiopod floatstone/rudstone

Description. MFT 2 (Fig. 3c–e) is characterized by abundant, whole or disarticulated brachiopod shells, and scarce sections of Trypanopora, a sessile tabular fossil with uncertain affinity (Weedon, 1991). Other skeletal grains include branching stromatoporoids, crinoids, single calcispheres, ostracods, and palaeo-siphonoclad algae. Brachiopod shells are often con-centrated in separate irregular layers. Rounded mic-ritic intraclasts and peloids are also common. Some skeletal grains (brachiopods, Trypanopora) are broken due to mechanical compaction. The limestone ground-mass is micrite/microsparite matrix or sparite cement being locally replaced by dolomite. Some echinoderm and palaeosiphonoclad algae bioclasts display syntaxial calcite overgrowths. This microfacies occurs only in the middle part of the studied Givetian interval (Fig. 2).

Interpretation. The irregular layers of brachiopod shells associated with Trypanopora, stromatoporoids and crinoids, and the presence of micritic intraclasts characterize a relatively high-energy, well-oxygenat-ed, shallow-subtidal environment with normal water salinity. In particular, Trypanopora has been assigned

to the tabulate corals or serpulid worms and, more recently, to the tentaculitoids and the microconchids (Weedon, 1991). A similar Givetian microfacies con-taining mainly brachiopods and less-common Trypa-nopora was described by Galli (1985) from the Carnic Alps (Italy) and interpreted as deposited in a non-re-stricted, agitated lagoon environment. This MFT can be also correlated with SMFT Type 12 “Limestone with shell concentration” (Flügel, 2004), which is characterized by accumulation of brachiopod shells (SMFT 12-Brach).

Protected shallow subtidal microfacies

This association includes three microfacies types (MFT 3–5), which are interpreted as formed in pro-tected shallow-subtidal settings with moderate water circulation. They are characterized by fossil assem-blages containing mainly palaeosiphonoclad green algae, calcispheres, ostracods, and rare gastropods. Typical open-marine fossils (e.g., brachiopods, cri-noids, corals, and stromatoporoids) occur only locally.

MFT 3 Intraclastic-ooidal packstone/grainstone

Description. MFT 3 is represented by packstones/grainstones consisting of poorly to moderately sorted ooids (up to 50% of the rock volume). Most ooids are strongly micritized and rarely contain laminae hav-ing poorly preserved radial-fibrous fabric (Fig. 3f). The ooids are characterized by ellipsoidal or irregular shapes and size between 0.3 mm and 1.0 mm. Peloids and intraclasts (derived from microbial bindstones and palaeosiphonoclad wackestones) also occur. The intraclasts range in size from 0.2 mm to 7.0 mm and display oval or irregular shapes. Skeletal grains (brachiopod shells, palaeosiphonoclad algae, and os-tracods) are sparse. The groundmass is often partly dolomitized, being represented by micritic matrix and blocky cement. This MFT occurs in the lower and middle levels of the studied interval from the dolomite formation (Fig. 2).

Interpretation: The common occurrence of ooids indicates a warm-water, shallow subtidal environ-

Fig. 3. a) Large coral fragment from coral-stromatoporoid floatstone/rudstone (MFT 1). Dolomite formation, core interval 2864–2879 m, depth 2870.40 m; b) Branching stromatoporoid (white arrow) and brachiopod bioclast (yellow arrow), MFT 1. Dolo-mite formation, core interval 2864–2879 m, depth 2870.40 m; c–d) Different sections of the sessile tabular fossil Trypanopora, MFT 2. Dolomite formation, core interval 2830–2833 m, depth 2830 m; e) Brachiopod shells concentrated in separate layers, MFT 2. Dolomite formation, core interval 2830–2833 m, depth 2830 m; f) Intraclastic-ooidal packstone/grainstone (MFT 3) con-taining micritized ooids with rare, poorly preserved radial-fibrous laminae (white arrows). Dolomite formation, core interval 2830–2833 m, depth 2831.30 m. Note: All microphotographs in plane-polarized light.

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ment. The irregular micritic ooids and those with fine radial laminae are often associated with lagoon set-tings (Strasser, 1986). Most probably, the sediments of MFT 3 were deposited in a subtidal lagoon with moderate water energy. A similar MFT was described by Pas et al. (2016) in Givetian carbonates from Bel-gium and interpreted as formed in non-agitated to moderately agitated water conditions in a lagoon envi-ronment. This microfacies may likewise be compared with SMFT 15 “Oolite” (Flügel, 2004).

MFT 4 Palaeosiphonoclad wackestone/packstone

Description. MFT 4 is represented by wacke stones/packstones with commonly broken palaeosiphonoclad green algae bioclasts (Fig. 4a). They are associated with whole gastropod shells, calcispheres, ostracods, and rare brachiopods and crinoids. A variable amount of peloids and rounded micritic intraclasts is also pre-sent. The micritic/microsparitic matrix is partly dolo-mitized. Some bioclasts of palaeosiphonoclad algae

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are characterized by syntaxial calcite overgrowths. MFT 4 is found only in the middle part of the studied Givetian succession (Fig. 2).

Interpretation. Palaeosiphonoclad green algae are commonly present in Middle to Upper Devonian subtidal carbonates, deposited in relatively calm set-tings with moderate water circulation (Preat and Ma-met, 1989; Skompski and Szulczewski, 1994; da Silva and Boulvain, 2002; 2006; Pas et al., 2016). A similar microfacies has also been described in the Givetian–Frasnian carbonates from the northeastern part of the Moesian Platform in the Kardam-119, Nikola Koz-levo-24, Mihalich-2, Chereshovo-1, and Preslavtsi-2 wells (Fig. 1) and interpreted as formed in a subtidal environment with moderate hydrodynamic condi-tions (Andreeva, 2010). The palaeosiphonoclad algae wackestones/packstones of MFT 4 are regarded as de-posited in a similar subtidal setting.

MFT 5 Bioclastic-peloidal packstone/grainstone

Description. MFT 5 is characterized by packstones/grainstones composed of abundant peloids and vari-ous skeletal grains (Fig. 4b, c). The latter comprise mainly calcispheres, ostracods, and whole or disar-ticulated brachiopod shells (?Emanuella spp.) plus palaeosiphonoclad green algae, rare corals, crinoids, and stromatoporoids. Intraclasts having various sizes (from 0.02 mm to 1.00 mm) and oval or irregular shape also occur. Most of them are micritic or, rarely, de-rived from peloidal packstones and grainstones. Relic bladed cement locally forms isopachous rims around some allochems. Drusy calcite cement and dark brown micritic matrix also fill the intergranular porosity. This MFT occurs in the middle part of the studied interval (Fig. 2).

Interpretation. The abundant peloids associated with calcispheres and ostracods indicate a protected shallow subtidal environment with moderate water circulation. The brachiopod taxon Emanuella is also indicative of restricted calm-lagoon to peri-reefal settings (Raski, 1986; Skompski and Szulczewski, 1994). MFT 5 has previously been distinguished in

Fig. 4. a) Wackestone/packstone (MFT 4) containing palaeosiphonoclad green algae bioclasts (white arrows). Dolomite formation, core interval 2830–2833 m, depth 2833 m; b) Bioclastic-peloidal grainstone (MFT 5) containing peloids, intraclasts (yellow ar-row) and well-preserved brachiopod shells (?Emanuella spp., white arrow). Dolomite formation, core interval 2830–2833 m, depth 2832.10 m; c) Calcispheres (white arrow) in MFT 5. Dolomite formation, core interval 2830–2833 m, depth 2832.10 m. d) Fenes-tral microbial bindstone (MFT 6) composed of clotted micrite, small-sized microbial peloids and spar-filled irregular fenestrae (white arrows). Dolomite formation, core interval 2808–2810 m; e) Inhomogeneous limestone texture showing in situ brecciation, MFT 7. Dolomite formation, core interval 2794–2804 m; f) Palaeomicrocodium aggregates (MFT 7). Dolomite formation, core interval 2794–2804 m. Note: All microphotographs in plane-polarized light.

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the Givetian–Frasnian lagoon carbonates from the Ograzhden-120, Kardam-119, Vaklino-1, Nikola Koz-levo-24, Mihalich-2, Chereshovo-1, and Preslavtsi-2 wells (Fig. 1) from the eastern part of the Moesian Platform (Andreeva, 2010). Similar microfacies have been described in Devonian shallow-marine carbon-ates from Belgium, France, and Germany (Preat and Mamet, 1989; Garland et al., 1996; Garland 1997; Chamley et al., 1997; da Silva and Boulvain, 2002; Mabille and Boulvain, 2007) and also interpreted as formed in protected subtidal lagoons with moderate water circulation. MFT 5 can be correlated with SMFT 16 (non-laminated subtype) “Non-laminated peloidal packstones and grainstones”, which is typical for pro-tected shallow-marine environments with moderate water circulation (Flügel, 2004).

Intertidal-supratidal microfacies

This association includes two microfacies types (MFT 6 and 7) formed in intertidal and supratidal settings.

MFT 6 Fenestral microbial bindstone

Description. This MFT is represented by fenestral microbial carbonates (microbialites) composed of ho-mogeneous and clotted micrite framework (Fig. 4d). The fenestrae are spar-filled, predominantly irregular in shape, ranging in size from <0.4 mm to 1.2 mm and corresponding to the bird’s-eye type. Small-sized microbial peloids are also common. They are often cemented by isopachous fibrous marine-phreatic ce-ment. Poorly preserved calcimicrobes are only rarely observed. Trapped disarticulated ostracod valves and single calcispheres are in minor amount. MFT 6 is found in the middle and upper parts of the studied sec-tion (Fig. 2).

Interpretation. The microbial deposits of MFT 6 are interpreted as microbial mats developed in an in-tertidal environment. A similar microfacies has been distinguished and described in Ograzhden-120, Kar-dam-119, Vaklino-1, Nikola Kozlevo-24, Mihalich-2, Chereshovo-1, and Preslavtsi-2 wells in the northeast-

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ern part of the Moesian Platform (Fig. 1) and likewise defined as intertidal deposits (Andreeva, 2007, 2010). Similar fenestral microbial bindstones have also been documented in Middle to Upper Devonian peritidal carbonates from western and central Europe (Preat and Mamet, 1989; Skompski and Szulczewski, 1994; Gar-land et al., 1996; Garland, 1997; da Silva and Boul-vain, 2002, 2004, 2006; Boulvain et al., 2005; Casier and Preat, 2006; Preat et al., 2007; Mabille and Boul-vain, 2008; Casier et al., 2010; da Silva et al., 2010). MFT 6 can be correlated with SMFT 21 (fenestral sub-type) “Fenestral packstones and bindstones” formed in intertidal and supratidal settings (Flügel, 2004).

MFT 7 Mudstone to packstone with Palaeomicrocodium

Description. These deposits are characterized by inho-mogeneous mudstone to packstone texture and com-monly dispersed Palaeomicrocodium aggregations. The limestones consist of dense micrite, clotted mic-rite and small-sized peloids. Locally developed in situ brecciation also occurs (Fig. 4e). Palaeomicrocodium is observed as discrete spherical bodies or cluster-like and rosette-like aggregations having various sizes (from 0.1 mm to >2.0 mm; Fig. 3f). Disarticulated ostracod valves are scarce. MFT 7 occurs only in the upper part of the studied interval of the dolomite unit (Fig. 2).

Interpretation. The deposits of MFT 7 are inter-preted as possibly pedogenic carbonates (palaeocali-che) developed in high-intertidal/supratidal settings. This interpretation is supported by the presence of diagnostic criteria for palaeocaliche (Flügel, 2004): Palaeomicrocodium aggregations, inhomogeneous dense to clotted micritic microfabric, micritic peloids, and breccia-like texture. In particular, the occurrence of Palaeomicrocodium is often related to extremely shallow settings and subaerial exposure (Antoshkina, 2006, 2013). There are different assumptions about the origin of Palaeomicrocodium. The latter has been assigned to cyanobacteria, green algae, problematic organo-mineral bodies, or has been interpreted as a product of symbiosis between soil fungi and roots of higher plants (Antoshkina, 2006, 2013). More re-cently, on the basis of micropetrographic and geo-chemical studies, Antoshkina (2013) suggested that Palaeomicro codium structures belong to ancient li-chens or actinolichens indicative of the early stage of weathering crust formation and palaeosol formation.

DISCUSSION AND CONCLUSIONS

Depositional model

The studied Givetian carbonates from the dolomite formation are interpreted as formed in various shal-low-marine subtidal to intertidal/supratidal environ-

ments (Fig. 5). The subtidal microfacies (MFT 1–5) were deposited in a lagoon setting with open to mod-erate water circulation. MFT 1 (coral-stromatoporoid floatstone/rudstone) and MFT 2 (brachiopod float-stone/rudstone) contain open-marine fossil assem-blages (mainly corals, crinoids, brachiopods, and stromatoporoids) that indicate well-oxygenated shal-low-marine environment with normal salinity. The ooid packstones/grainstones with micritized ooids (MFT 3), palaeosiphonoclad wackestones/packstones (MFT 4), and bioclastic-peloidal packstones/grain-stones of MFT 5 are interpeted as formed in a protect-ed lagoon setting. They are characterized by low-di-versity fossil associations (e.g., Paleosiphonocladales, ostracods, calcispheres) that suggest shallow-marine settings with moderate water circulation. The fenestral microbial bindstones of MFT 6 are interpreted as mi-crobial mats developed in the intertidal zone. The in-homogeneous mudstones to packstones with Palaeo-microcodium aggregations (MFT 7) are considered as possible pedogenic carbonates (palaeocaliche) formed in a high-intertidal/supratidal environment.

A general shallowing-upward trend is distin-guished within the studied Givetian succession from subtidal lagoon sediments (MFT 1–5) to intertidal (MFT 6) and supratidal (MFT 7) deposits (Fig. 2).

Comparison with other Devonian shallow-marine carbonates

Some of the described MFTs from the Totleben-2 well resemble previously studied Givetian peritidal microfa-cies from northeastern Bulgaria (Andreeva, 2010). For example, MFT 4 (palaeosiphonoclad wackestone/pack-stone) and MFT 5 (bioclastic-peloidal packstone/grain-stone) have been distinguished in the Ograzhden-120, Kardam-119, Vaklino-1, Nikola Kozlevo-24, Miha-lich-2, Chereshovo-1, and Preslavtsi-2 wells and simi-larly interpreted as products of shallow subtidal lagoon with moderate water circulation (Andreeva, 2010). Fenestral microbial bindstones of MFT 6 commonly occur in the intertidal units of the same Devonian wells.

Other microfacies (e.g., MFT 1, coral-stromato-poroid floatstone/rudstone; MFT 2, brachiopod float-stone/rudstone; MFT 3, intraclastic-ooidal packstone/grainstone; and MFT 7, mudstone to packstone with Palaeomicrocodium) have not been reported from De-vonian wells drilled in the Moesian Platform. Most of them reflect deposition in a shallow-marine lagoon with open (MFT 1 and MFT 2) or moderate (MFT 3) water circulation and locally agitated hydrodynamic conditions. On the other hand, the mudstones to pack-stones with Palaeomicrocodium (MFT 7) were formed in a high-intertidal/supratidal setting.

The obtained microfacies data suggest that, dur-ing the Givetian, sediments from the central part of the Moesian Platform were deposited in dominantly shallow subtidal lagoon setting with open to moder-ate water circulation and locally developed intertidal/

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supratidal environments. In comparison, the coeval deposits from the eastern part of the platform were formed in arid peritidal carbonate/evaporite environ-ments (from shallow subtidal zones, adjacent to the tidal flat, to supratidal sabkha) with more restricted water circulation.

Similar microfacies types have been described from various shallow-marine Devonian successions in the Polish Holy Cross Mts (Skompski and Szulc-zewski, 1994), the Ardennes in Belgium and France (Preat and Mamet, 1989; Chamley et al., 1997; Gar-land, 1997; da Silva and Boulvain, 2002, 2004, 2006; Casier and Preat, 2006; Preat et al., 2007; Mabille and Boulvain, 2007, 2008; Casier et al., 2010; Pas et al.,

2016), and the Rhenisches Schiefergebirge area in Germany (Garland, 1997).

Acknowledgements

I am thankful to Slavcho Yanev (Geological Institute, Bulgarian Academy of Sciences) for allowing me to use his thin-section collection and primary documen-tation. Athanas Chatalov (Sofia University “St. Kli-ment Ohridski”) is thanked for his critical review of early version of the manuscript. Thanks to Georgi Granchovski and Lubomir Metodiev (Geological In-stitute, Bulgarian Academy of Sciences) for their tech-nical and linguistic help.

Fig. 5. Idealized depositional model of the Givetian carbonates (dolomite unit) from the Totleben-2 well (Moesian Platform, central North Bulgaria), with distribution of the main allochems.

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