MICROFACIES ANALYSIS OF MIDDLE DEVONIAN (EIFELIAN) CARBONATE ROCKS FROM DEEP WELLS IN NORTH-EASTERN BULGARIA. PRELIMINARY RESULTS

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1 Доклади на Българската академия на науките Comptes rendus de l Académie bulgare des Sciences Tome 61, No 10, 2008 GEOLOGIE Sédimentologie MICROFACIES ANALYSIS OF MIDDLE DEVONIAN (EIFELIAN) CARBONATE ROCKS FROM DEEP WELLS IN NORTH-EASTERN BULGARIA. PRELIMINARY RESULTS Polina Andreeva (Submitted by Corresponding Member I. Zagorchev on August 1, 2008) Abstract Ten microfacies types (MFT 1 10) are distinguished and described in the Eifelian clayey-carbonate package (carbonate-sulphate formation) from the deep wells R-1 Vaklino, R-119 Kardam and OP-2 Mihalich (NE Bulgaria). They are respectively interpreted as inner- and mid-ramp carbonates developed in a shallowing-upward sequence. Thus, a gradual transition from open-marine (MFT 1 4) through shallow subtidal (MFT 5 6) to more restricted subtidalintertidal deposition (MFT 7 10) is established. Most of these microfacies are comparable with Wilson s Standard Microfacies Types and/or with other microfacies described from Middle Devonian successions from Western Europe. Key words: microfacies, carbonate ramp, shallowing-upward sequence, Devonian, NE Bulgaria Introduction. The Devonian rocks in North-eastern Bulgaria are established only in subsurface sections. The Eifelian (Middle Devonian) sequence in this area is composed of dark grey to black variously clayey limestones, locally interbedded with scarce black shales. This succession is referred to an informal lithostratigraphic unit (clayey-carbonate package), which builds up the lowermost parts of the carbonate-sulphate formation [ 1 ]. These sediments have been described from a general palaeoenvironment viewpoint by Yanev [ 2 ], who interpreted them as deposited in a shallow marine setting with normal salinity. This study was undertaken at the Geological Institute of the Bulgarian Academy of Sciences and is a contribution to Project NZ-1501, which is financed by the National Science Fund of the Bulgarian Ministry of Education and Science. 1309

2 The present paper is based on preliminary results obtained from the microfacies analysis of carbonate rocks in the clayey-carbonate package, presented in three well sections: R-1 Vaklino, R-119 Kardam and OP-2 Mihalich. This study aims to distinguish and describe the major microfacies types as well as to interpret their respective depositional environments. Microfacies description. Ten microfacies types (MFT 1 10) were identified following the textural classification proposed by Dunham [ 3 ] and later supplemented by Embry and Klovan [ 4 ]. Most of them were compared with the classical microfacies scheme created by Wilson [ 5 ], and expanded by Flügel [ 6 ], and/or with other microfacies types of Middle Devonian successions known from Western Europe. MFT 1 consists of strongly bioturbated bioclastic wackestones (Fig. 1, A) and packstones. The latter are built up predominantly of skeletal detritus of brachiopod shells, crinoids, ostracods, gastropods, trilobites and sporadic tentaculites and bryozoans which occur within clay-rich micritic/microsparitic matrix. This microfacies is established mostly in R-1 Vaklino and only rarely in R-119 Kardam (Fig. 2). It might be compared with Standard Microfacies Type (SMF) 9 ( strongly burrowed bioclastic wackestone ), as well as with microfacies types defined in Middle Devonian carbonates from Belgium and France [ 7 13 ]. MFT 2 is represented by bioclastic floatstones. Rich fossil association, including mainly large well-preserved skeletons of brachiopods and tabulate corals plus subordinate bryozoans, trilobites, tentaculites, gastropods and crinoids, characterizes these rocks. Most brachiopod shells occur in convex-up position, forming spar-filled shelter porosity below them (Fig. 1, B). The micritic/microsparitic matrix is bioturbated and commonly has dark brown appearance due to the higher clay content. This microfacies commonly occurs in all investigated well sections (Fig. 2). It corresponds to SMF 8 ( wackestones and floatstones with whole fossils and well preserved infauna and epifauna ) and is similar to other microfacies types described in Eifelian and Givetian sequences in Western Europe [ 7, 12, 14 ]. MFT 3 is composed of bioclastic grainstones alternating with clay-rich mudstone/wackstone laminae (Fig. 1, C). The skeletal grains include abundant moderately sorted well-rounded and abraded crinoid ossicles as well as rare brachiopod Fig. 1. (A) MFT 1 bioturbated bioclastic wackestone; (B) MFT 2 brachiopod floatstone with spar-filled shelter porosity below shells; (C) MFT 3 bioclastic grainstone with clay-rich mudstone/wackestone laminae; (D) MFT 4 bioclastic rudstone with ferriferous ooids (polished slab surface); (E) Skeletal grains associating with ferriferous ooids (MFT 4); (F ) MFT 5 bioclastic grainstone with variously micritized skeletal grains; (G) Fully micritized bioclast of calcareous green algae (MFT 5); (H) MFT 6 bioclastic wackestone with gymnocodiacean green algae; (I) MFT 7 - peloidal packstone/grainstone; (J) MFT 8 bioclastic wackestone with skeletal grains represented by palaeosiphonoclad algae (arrow) and ostracods; (K) MFT 9 fenestral Porostromata microbial bindstone; (L) MFT 10 laminated peloidal microbial bindstone. Note: All microphotographs in plane-polarized light 1310 P. Andreeva

3 А C B E D F 0.25 mm 1 cm 0.25 mm J I H G 2.5 mm K L Fig. 1

4 OP-2 Mihalich MFT R-1 Vaklino MFT Lower Devonian Middle Devonian Eifelian Givetian R-119 Kardam Limy-terrigenous-clayey formation shales sandy limestones sandstones MFT Clayey-carbonate package ( Carbonate-sulphate formation ) limestones shales core interval Fig. 2 Carbonate-sulphate formation dolostones anhydrites unconformable boundary

5 shells, ostracods, tabulate corals, tentaculites, trilobites and gastropods. Sporadic clastic quartz grains are also locally observed. This microfacies type is represented only in the lowermost parts of the clayey-carbonate package in R-1 Vaklino (Fig. 2). MFT 4 consists of bioclastic rudstones containing variable amount of ferriferous ooids (Fig. 1, D and E). The fossil remains (crinoids, tabulate corals, brachiopods, ostracods, trilobites, etc.) commonly exceed 2 mm by size and occur within partly recrystallized and/or dolomitized clayey-calcareous matrix. This microfacies is established in well R-119 Kardam (Fig. 2). MFT 5 is characterized by bioclastic grainstones with partly or completely micritized skeletal grains (Fig. 1, F ). The latter comprise abundant calcareous green algae (Fig. 1, G), subordinate crinoids, and scarce gastropods, ostracods, calcispheres, tabulate corals, trilobites and brachiopod shells. Various amounts of peloids occur locally being scattered within the sparitic cement. This microfacies type is represented only in the uppermost parts of the clayey-carbonate package in OP-2 Mihalich (Fig. 2). It could be compared with SMF 11 ( coated bioclastic grainstone with sparry cement ) and other microfacies types defined in Middle Devonian sections from Belgium [ 11 ]. MFT 6 includes wackestones and packstones, containing common gymnocodiacean (Fig. 1, H) and other calcareous green algae. They are observed in association with crinoids, gastropods, sparse fragmented tabulate corals, trilobites, brachiopods and peloids. The matrix is micritic, or is neomorphically altered to microspar. This microfacies type is established only in R-119 Kardam (Fig. 2). It is similar to SMF 18 ( bioclastic grainstones and packstones with abundant benthic foraminifera or calcareous green algae ). MFT 7 is distinguished for the presence of packstones and grainstones with abundant peloids (Fig. 1, I). Skeletal grains (mostly ostracods, calcispheres and gastropods as well as subordinate crinoids, brachiopod shells, palaeosiphonoclad algae and single tabulate corals) and micritic rounded intraclasts are also represented. The groundmass is sparitic or mixed micritic/sparitic. Some bioclasts (predominantly crinoids and brachiopods) are variously micritized. This microfacies occurs in the upper parts of clayey-carbonate package in R-119 Kardam (Fig. 2) and could be compared to SMF 16 Non-laminated ( non-laminated peloidal grainstone and packstone ). Similar microfacies are established also in Middle Devonian sequences in Western Europe [ 7, 9, 10 ]. MFT 8 consists of bioclastic mudstones and wackestones with poor fossil association of predominantly whole skeletons of well preserved palaeosiphonoclad green algae and/or ostracods plus rare crinoid ossicles (Fig. 1, J). The mi- Fig. 2. Generalized lithological logs of the clayey-carbonate package and microfacies distribution in the studied well sections Compt. rend. Acad. bulg. Sci., 61, No 10,

6 critic/microsparitic matrix commonly contains fine skeletal debris. Peloids are locally observed, too. This microfacies type is represented in the sections of R-1 Vaklino and R-119 Kardam (Fig. 2). Similar Middle Devonian microfacies have been described in Belgium and France by Preat and Mamet[ 7 ], Chamley et al. [ 10 ] and Preat et al. [ 13 ], being also characterized by monotonous fossil assemblages. MFT 9 is represented of fenestral Porostromate microbial baindstone (Fig. 1, K). It is composed of clotted or homogeneous micrite, calcified cyanobacterial sheaths (calcimicrobes) and sporadic vermiform gastropods. Variable in size fenestrae with irregular shape are very typical. This microfacies is established only in R-119 Kardam (Fig. 2) and could be compared with SMF 21 ( fenestral bindstone with porostromate microfossils ), or with other microfacies types from Middle Devonian carbonate successions in Belgium and France [ 7, 12 ]. MFT 10 consists of laminated peloidal microbial bindstones (Fig. 1, L). These rocks are built up of alternating peloidal laminae with wackestone or packstone/grainstone fabrics. Scarce trapped thin-shelled ostracods locally occur as well. Microfacies type 10 is established in well R-1 Vaklino (Fig. 2) and is similar to SMF 16-Laminated ( laminated peloidal bindstone ) as well as to microfacies described from Givetian carbonates in Belgium [ 12 ]. Microfacies interpretation. Microfacies types MFT 1 4 are interpreted as deposited in an open marine subtidal environment below normal wave base. The various fossil associations (crinoids, brachiopods, trilobites, tentaculites, bryozoans, tabulate corals, etc.) characterize waters with normal salinity and good circulation. The micritic/microsparitic clay rich and bioturbated matrix of MFT 1 and MFT 2 testifies to precipitation in weakly agitated setting. On the other hand, grainstone and rudstone textures with predominantly abraded and rounded bioclasts in MFT 3 and MFT 4, as well as aligned in hydraulically stable position brachiopod shells in MFT 2 indicate periods of higher-energy events. The latter were presumably related to occasional stronger storm activity, interrupting the background low-energy sedimentation. Microfacies types MFT 5 10 are assumed as deposited in shallow marine environment above normal wave base. The fossil assemblages with common calcareous green algae are typical of MFT 5 and MFT 6 indicating precipitation in shallow subtidal zones with normal salinity and good oxygenation of the seawater. Winnowed textures of MFT 5 suggest deposition under constant high-energy conditions (probably due to wave action), whereas micritic matrix of MFT 6 is characteristic for a low- to modarate-energy setting. Microfacies MFT 7 10 contain poor and low-diversity flora and fauna (mostly palaeosiphonoclad algae, ostracods and calcispheres) that characterize more restricted water circulation. The abundant peloids and packstone/grainstone textures of MFT 7 are indicative of a shallow subtidal setting with moderate water agitation [ 15 ]. The well preserved skeletal grains and non-winnowed fabrics of MFT 8 testify to subtidal conditions with weak water circulation. Finally, MFT P. Andreeva

7 and MFT 10 represent the shallowest recognized deposits formed as intertidal microbial mats. The available microfacies data, obtained only from short core intervals and the small study area do not allow a more precise reconstruction of the ancient Middle Devonian carbonate platform. However, it could be assumed that the described sediments were most probably formed on a carbonate ramp, whereupon MFT 1 4 were deposited in the storm-dominated mid-ramp zone, and MFT 5 10 represent inner-ramp (MFT 5 6) and back-ramp lagoonal and peritidal deposits (MFT 7 10) (cf. Burchette and Wright [ 16 ]). A shallowing-upward depositional trend is recognized in the range of clayeycarbonate package. Thus, a gradual transition from open-marine (mid-ramp) (MFT 1 4) through shallow subtidal (inner-ramp) setting (MFT 5 6) to more restricted (back-ramp) sedimentation (MFT 7 10) is established (Fig. 2). Another solid evidence for the presence of such shallowing-upward sequence is the temporal transition from lime-dominated to sulphate- and/or dolomite-dominated lagoonal and peritidal deposition, which is observed in the framework of the whole carbonate-sulphate formation [ 2, 17 ]. Conclusions. The reported Eifelian microfacies from subsurface sections in NE Bulgaria are interpreted as possible mid- and inner-ramp deposits. They are similar to microfacies types of Middle Devonian successions described from Western Europe [ 7 14 ], and/or are comparable with Wilson s Standard Microfacies Types, which are also represented in inner- and mid-ramp settings (cf. Flügel [ 6 ]). A future microfacies investigation of other Devonian well sections from the same study area will provide new data for discussions and more precise correlations and interpretations. REFERENCES [ 1 ] Yanev S. Bull. Geol. Inst., 21, 1972, (in Bulgarian). [ 2 ] Yanev S. Bull. Geol. Inst., 23, 1974, (in Bulgarian). [ 3 ] Dunham R. J. In: Classification of carbonate rocks. A symposium (ed. E. W. Ham), Amer. Assoc. Petrol. Geol. Mem., 1, 1962, [ 4 ] Embry A. F., J. E. Klovan. Bull. Can. Petrol. Geol., 19, 1971, No 4, [ 5 ] Wilson J. L. Carbonate Facies in Geologic History, Berlin, Springer-Verlag, 1975, 473 pp. [ 6 ] Flügel E. Microfacies of Carbonate Rocks, Berlin, Springer, 2004, 976 pp. [ 7 ] Preat A., B. Mamet. Bull. Centres Rech. Explor.-Prod. Elf-Aquitaine, 13, 1989, No 1, [ 8 ] Preat A., R. Kasimi. Bull. Centres Rech. Explor.-Prod. Elf-Aquitaine, 19, 1995, No 2, [ 9 ] Garland J. PhD Thesis, University of Durham, 1997, 282 pp. Compt. rend. Acad. bulg. Sci., 61, No 10,

8 [ 10 ] Chamley H., J.-N. Proust, J.-L. Mansy, F. Boulvain. Palaeogeogr., Palaeoclim., Palaeoecol., 129, 1997, No 3 4, [ 11 ] Mamet B., A. Preat. Geologica Belg., 8, 2005, No 3, [ 12 ] Casier J.-G., A. Preat. Bull. Inst. r. Sci. nat. Belg. Sci. Terre, 76, 2006, [ 13 ] Preat A., S. Blockmans, L. Capette, V. Dumoulin, B. Mamet. Geologica Belg., 10, 2007, No 1 2, [ 14 ] Hubmann B. Jb. Geol. Bundesanst., 136, 1993, No 2, [ 15 ] Tucker M. E., V. P. Wright. Carbonate sedimentology, Oxford, Blackwell, 482 pp. [ 16 ] Burchette T. P., V. P. Wright. Sed. Geol., 79, 1992, No 1 4, [ 17 ] Andreeva P. Compt. rend. Acad. bulg. Sci., 60, 2007, No 2, Geological Institute Bulgarian Academy of Sciences Acad. G. Bonchev Str., Bl Sofia, Bulgaria polina a@geology.bas.bg 1314 P. Andreeva