Доклади на Българската академия на наукитеComptes rendus de l’Academie bulgare des Sciences

Tome 61, No 10, 2008

GEOLOGIE

Sedimentologie

MICROFACIES ANALYSIS OF MIDDLE DEVONIAN(EIFELIAN) CARBONATE ROCKS FROM DEEP WELLS INNORTH-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 inthe Eifelian clayey-carbonate package (carbonate-sulphate formation) from thedeep wells R-1 Vaklino, R-119 Kardam and OP-2 Mihalich (NE Bulgaria). Theyare respectively interpreted as inner- and mid-ramp carbonates developed ina shallowing-upward sequence. Thus, a gradual transition from open-marine(MFT 1–4) through shallow subtidal (MFT 5–6) to more restricted subtidal-intertidal deposition (MFT 7–10) is established. Most of these microfacies arecomparable with Wilson’s Standard Microfacies Types and/or with other mi-crofacies 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 establishedonly in subsurface sections. The Eifelian (Middle Devonian) sequence in this areais composed of dark grey to black variously clayey limestones, locally interbeddedwith scarce black shales. This succession is referred to an informal lithostrati-graphic unit (clayey-carbonate package), which builds up the lowermost parts ofthe carbonate-sulphate formation [1]. These sediments have been described froma general palaeoenvironment viewpoint by Yanev [2], who interpreted them asdeposited in a shallow marine setting with normal salinity.

This study was undertaken at the Geological Institute of the Bulgarian Academy of Sciencesand is a contribution to Project NZ-1501, which is financed by the National Science Fund of theBulgarian Ministry of Education and Science.

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The present paper is based on preliminary results obtained from the micro-facies analysis of carbonate rocks in the clayey-carbonate package, presented inthree well sections: R-1 Vaklino, R-119 Kardam and OP-2 Mihalich. This studyaims to distinguish and describe the major microfacies types as well as to interprettheir respective depositional environments.

Microfacies description. Ten microfacies types (MFT 1–10) were identi-fied following the textural classification proposed by Dunham [3] and later sup-plemented by Embry and Klovan [4]. Most of them were compared with theclassical microfacies scheme created by Wilson [5], and expanded by Flugel [6],and/or with other microfacies types of Middle Devonian successions known fromWestern Europe.

MFT 1 consists of strongly bioturbated bioclastic wackestones (Fig. 1, A)and packstones. The latter are built up predominantly of skeletal detritus ofbrachiopod shells, crinoids, ostracods, gastropods, trilobites and sporadic tentac-ulites and bryozoans which occur within clay-rich micritic/microsparitic matrix.This microfacies is established mostly in R-1 Vaklino and only rarely in R-119Kardam (Fig. 2). It might be compared with Standard Microfacies Type (SMF)9 (“strongly burrowed bioclastic wackestone”), as well as with microfacies typesdefined in Middle Devonian carbonates from Belgium and France [7–13].

MFT 2 is represented by bioclastic floatstones. Rich fossil association, includ-ing mainly large well-preserved skeletons of brachiopods and tabulate corals plussubordinate bryozoans, trilobites, tentaculites, gastropods and crinoids, charac-terizes these rocks. Most brachiopod shells occur in convex-up position, formingspar-filled shelter porosity below them (Fig. 1, B). The micritic/microsparitic ma-trix is bioturbated and commonly has dark brown appearance due to the higherclay content. This microfacies commonly occurs in all investigated well sections(Fig. 2). It corresponds to SMF 8 (“wackestones and floatstones with whole fos-sils and well preserved infauna and epifauna”) and is similar to other microfaciestypes described in Eifelian and Givetian sequences in Western Europe [7,12,14].

MFT 3 is composed of bioclastic grainstones alternating with clay-rich mud-stone/wackstone laminae (Fig. 1, C). The skeletal grains include abundant mod-erately sorted well-rounded and abraded crinoid ossicles as well as rare brachiopod

Fig. 1. (A) MFT 1 – bioturbated bioclastic wackestone; (B) MFT 2 – brachiopod float-stone with spar-filled shelter porosity below shells; (C) MFT 3 – bioclastic grainstone withclay-rich mudstone/wackestone laminae; (D) MFT 4 – bioclastic rudstone with ferriferousooids (polished slab surface); (E) Skeletal grains associating with ferriferous ooids (MFT 4);(F ) MFT 5 – bioclastic grainstone with variously micritized skeletal grains; (G) Fully mi-critized bioclast of calcareous green algae (MFT 5); (H) MFT 6 – bioclastic wackestonewith 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

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shells, ostracods, tabulate corals, tentaculites, trilobites and gastropods. Sporadicclastic quartz grains are also locally observed. This microfacies type is repre-sented 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 fer-riferous ooids (Fig. 1, D and E). The fossil remains (crinoids, tabulate corals,brachiopods, ostracods, trilobites, etc.) commonly exceed 2 mm by size and occurwithin partly recrystallized and/or dolomitized clayey-calcareous matrix. Thismicrofacies is established in well R-119 Kardam (Fig. 2).

MFT 5 is characterized by bioclastic grainstones with partly or completelymicritized skeletal grains (Fig. 1, F ). The latter comprise abundant calcareousgreen algae (Fig. 1, G), subordinate crinoids, and scarce gastropods, ostracods,calcispheres, tabulate corals, trilobites and brachiopod shells. Various amounts ofpeloids occur locally being scattered within the sparitic cement. This microfaciestype is represented only in the uppermost parts of the clayey-carbonate packagein OP-2 Mihalich (Fig. 2). It could be compared with SMF 11 (“coated bioclasticgrainstone with sparry cement”) and other microfacies types defined in MiddleDevonian sections from Belgium [11].

MFT 6 includes wackestones and packstones, containing common gymnoco-diacean (Fig. 1, H) and other calcareous green algae. They are observed in as-sociation with crinoids, gastropods, sparse fragmented tabulate corals, trilobites,brachiopods and peloids. The matrix is micritic, or is neomorphically altered tomicrospar. This microfacies type is established only in R-119 Kardam (Fig. 2).It is similar to SMF 18 (“bioclastic grainstones and packstones with abundantbenthic foraminifera or calcareous green algae”).

MFT 7 is distinguished for the presence of packstones and grainstones withabundant peloids (Fig. 1, I). Skeletal grains (mostly ostracods, calcispheres andgastropods as well as subordinate crinoids, brachiopod shells, palaeosiphonocladalgae and single tabulate corals) and micritic rounded intraclasts are also rep-resented. The groundmass is sparitic or mixed micritic/sparitic. Some bioclasts(predominantly crinoids and brachiopods) are variously micritized. This micro-facies 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-laminatedpeloidal grainstone and packstone”). Similar microfacies are established also inMiddle Devonian sequences in Western Europe [7,9, 10].

MFT 8 consists of bioclastic mudstones and wackestones with poor fossilassociation of predominantly whole skeletons of well preserved palaeosiphono-clad 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 distri-bution in the studied well sections

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critic/microsparitic matrix commonly contains fine skeletal debris. Peloids arelocally observed, too. This microfacies type is represented in the sections of R-1Vaklino and R-119 Kardam (Fig. 2). Similar Middle Devonian microfacies havebeen described in Belgium and France by Preat and Mamet[7], Chamley etal. [10] and Preat et al. [13], being also characterized by monotonous fossil as-semblages.

MFT 9 is represented of fenestral Porostromate microbial baindstone (Fig. 1,K). It is composed of clotted or homogeneous micrite, calcified cyanobacterialsheaths (calcimicrobes) and sporadic vermiform gastropods. Variable in size fen-estrae with irregular shape are very typical. This microfacies is established only inR-119 Kardam (Fig. 2) and could be compared with SMF 21 (“fenestral bindstonewith porostromate microfossils”), or with other microfacies types from MiddleDevonian 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 pack-stone/grainstone fabrics. Scarce trapped thin-shelled ostracods locally occur aswell. Microfacies type 10 is established in well R-1 Vaklino (Fig. 2) and is similarto SMF 16-Laminated (“laminated peloidal bindstone”) as well as to microfaciesdescribed from Givetian carbonates in Belgium [12].

Microfacies interpretation. Microfacies types MFT 1–4 are interpretedas deposited in an open marine subtidal environment below normal wave base.The various fossil associations (crinoids, brachiopods, trilobites, tentaculites, bry-ozoans, tabulate corals, etc.) characterize waters with normal salinity and goodcirculation. The micritic/microsparitic clay rich and bioturbated matrix of MFT 1and MFT 2 testifies to precipitation in weakly agitated setting. On the other hand,grainstone and rudstone textures with predominantly abraded and rounded bio-clasts in MFT 3 and MFT 4, as well as aligned in hydraulically stable positionbrachiopod shells in MFT 2 indicate periods of higher-energy events. The latterwere presumably related to occasional stronger storm activity, interrupting thebackground low-energy sedimentation. Microfacies types MFT 5–10 are assumedas deposited in shallow marine environment above normal wave base. The fossilassemblages with common calcareous green algae are typical of MFT 5 and MFT 6indicating precipitation in shallow subtidal zones with normal salinity and goodoxygenation of the seawater. Winnowed textures of MFT 5 suggest depositionunder constant high-energy conditions (probably due to wave action), whereasmicritic 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 (mostlypalaeosiphonoclad algae, ostracods and calcispheres) that characterize more re-stricted water circulation. The abundant peloids and packstone/grainstone tex-tures of MFT 7 are indicative of a shallow subtidal setting with moderate wateragitation [15]. The well preserved skeletal grains and non-winnowed fabrics ofMFT 8 testify to subtidal conditions with weak water circulation. Finally, MFT 9

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and MFT 10 represent the shallowest recognized deposits formed as intertidalmicrobial mats.

The available microfacies data, obtained only from short core intervals andthe small study area do not allow a more precise reconstruction of the ancientMiddle Devonian carbonate platform. However, it could be assumed that thedescribed sediments were most probably formed on a carbonate ramp, whereuponMFT 1–4 were deposited in the storm-dominated mid-ramp zone, and MFT 5–10represent 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 clayey-carbonate package. Thus, a gradual transition from open-marine (mid-ramp)(MFT 1–4) through shallow subtidal (inner-ramp) setting (MFT 5–6) to morerestricted (back-ramp) sedimentation (MFT 7–10) is established (Fig. 2). An-other solid evidence for the presence of such shallowing-upward sequence is thetemporal transition from lime-dominated to sulphate- and/or dolomite-dominatedlagoonal and peritidal deposition, which is observed in the framework of the wholecarbonate-sulphate formation [2,17].

Conclusions. The reported Eifelian microfacies from subsurface sectionsin NE Bulgaria are interpreted as possible mid- and inner-ramp deposits. Theyare similar to microfacies types of Middle Devonian successions described fromWestern Europe [7–14], and/or are comparable with Wilson’s Standard MicrofaciesTypes, which are also represented in inner- and mid-ramp settings (cf. Flugel [6]).A future microfacies investigation of other Devonian well sections from the samestudy area will provide new data for discussions and more precise correlationsand interpretations.

REFERENCES

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Amer. Assoc. Petrol. Geol. Mem., 1, 1962, 108–171.[4] Embry A. F., J. E. Klovan. Bull. Can. Petrol. Geol., 19, 1971, No 4, 730–781.[5] Wilson J. L. Carbonate Facies in Geologic History, Berlin, Springer-Verlag, 1975,

473 pp.[6] Flugel 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, 47–86.[8] Preat A., R. Kasimi. Bull. Centres Rech. Explor.-Prod. Elf-Aquitaine, 19, 1995,

No 2, 329–375.[9] Garland J. PhD Thesis, University of Durham, 1997, 282 pp.

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[10] Chamley H., J.-N. Proust, J.-L. Mansy, F. Boulvain. Palaeogeogr., Palaeo-clim., Palaeoecol., 129, 1997, No 3–4, 369–385.

[11] Mamet B., A. Preat. Geologica Belg., 8, 2005, No 3, 85–111.[12] Casier J.-G., A. Preat. Bull. Inst. r. Sci. nat. Belg. Sci. Terre, 76, 2006, 5–29.[13] Preat A., S. Blockmans, L. Capette, V. Dumoulin, B. Mamet. Geologica

Belg., 10, 2007, No 1–2, 3–25.[14] Hubmann B. Jb. Geol. Bundesanst., 136, 1993, No 2, 393–461.[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, 3–57.[17] Andreeva P. Compt. rend. Acad. bulg. Sci., 60, 2007, No 2, 79–182.

Geological InstituteBulgarian Academy of SciencesAcad. G. Bonchev Str., Bl. 24

1113 Sofia, Bulgariae-mail: polina a@geology.bas.bg

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