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

Tome 60, No 2, 2007GEOLOGIESedimentologie

GIVETIAN MICROBIAL CARBONATES FROM THECARBONATE-SULPHATE SUITE IN WELL R-119 KARDAM

(NORTH-EASTERN BULGARIA)

P. Andreeva

(Submitted by Academician T. Nikolov on January 9, 2007)

Abstract

Microbial carbonates (limestones and dolostones) are established in the MiddleDevonian carbonate-sulphate suite in well R-119 Kardam (North-eastern Bulgaria).They are determined as stromatolites on the basis of the observed macrofabric (hor-izontal or crinkled laminae) and microfabric features (fine-grained and agglutinatedlaminae characterized by clotted fabric or composed of trapped grains). The availablelithofacies association (carbonates and evaporates) testifies to deposition in an aridperitidal (sabkha) environment, whereupon the microbialites represent former inter-tidal microbial mats. Calcified cyanobacterial sheaths are the only preserved skeletalparts from microbes, but the observed clotted microfabric and at least part of thehomogeneous micrite may also have microbial origin. A major problem still to be re-solved is which particular microbes were involved in the formation of the non-skeletalparts of the initial mats. Most probably the latter were produced by cyanobacterialcalcification, although heterotrophic bacteria and non-living organic macromoleculesmay have also favoured CaCO3 precipitation. A bibliography review confirms thatpre-Mesozoic carbonate microbialites have not been investigated in Bulgaria so far.

Key words: microbialite, stromatolite, microbial mat, sabkha, Devonian, NEBulgaria

Introduction. Microbially produced carbonates (microbialites) are known in thegeological record from Archaean to Recent time. They occur in a wide variety of depo-sitional settings such as reefs, tidal flats, mud mounds, lakes, springs, caves and soils [1].Major processes creating microbialites are calcium carbonate precipitation and locallyimportant grain trapping. Microbes participating in their formation encompass bacte-ria (particularly cyanobacteria), fungi, small algae and protozoans.

A review of the Bulgarian sedimentological literature shows that microbial car-bonates (laminites, stromatolites, oncoids and trombolites) have been described fromMesozoic [2–4] and Tertiary rocks [5–8], while pre-Mesozoic microbialites have not beeninvestigated so far.

The present study is focused on Middle Devonian (Givetian) microbial carbonatesestablished in a deep well R-119 Kardam from North-eastern Bulgaria. The main goalsare to explain the origin of the described macro- and microfabrics as well as to interpretthe microbialite formation in the background of the general depositional environment.

Geological notes. Paleozoic rocks in the Moesian platform are known only fromdeep wells drilled for oil and gas prospecting (Fig. 1) and their sedimentology was

This study was undertaken at the Geological Institute (Bulgarian Academy of Sciences) and is acontribution to Project NZ-1501 financed by the National Science Fund.

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treated for the first time by Спасов and Янев [9]. According to Янев [10], duringa part of the Middle Devonian epoch shallow marine carbonate and lagoonal sulphatesediments were deposited in the most north-eastern part of the present territory ofBulgaria. They comprise the bulk of the carbonate-sulphate suite and are furthersubdivided into first, second and third sulphate-bearing packages.

All these packages have Givetian age and are represented in well R-119 Kardam(Fig. 2). Predominating bedded anhydrites and fine-crystalline dolostones as well assubordinate limestones build up the first and second sulphate-bearing packages. Litho-clastic breccias (rudstones and floatstones), coarse crystalline dolostones and limestonesare the leading lithotypes in the upper parts of the carbonate-sulphate unit. Microbiallimestones and dolostones occur irregularly in the three sulphate-bearing packages ascentimetre- to decimetre-scale layers.

Macrofabric and microscopical description of the microbialites. The ob-served microbialites are determined as stromatolites on the basis of their general macro-and microfabric features and by following the definition of Riding [11], e.g. “laminatedbenthic microbial deposits”. They are characterized by alternation of brown to darkgrey subparallel laminae with variable thickness (from < 1mm to 5 mm). The latterare horizontal or crinkled and may be locally discontinuous. Some small low-relief (upto 5 cm in height) dome shapes are also observed (Fig. 3). White to greyish spar-filledvoids (fenestrae) with oval, spherical or irregular morphology and maximum size of5 mm are common.

Under the microscope, it is observed that the microbialites are composed predomi-nantly of fine-grained (micrite/microsparite or dolomicrite/dolomicrosparite) laminaewith clotted microfabric (Fig. 4a,b). The latter is distinguished by sponge-like globularand irregular clots that grade into homogeneous micrite or individual peloids. Somelaminae appear dark brown possibly due to the presence of more organics and/or non-carbonate admixtures (clay, ferric oxides). Poorly preserved tube-like structures occuronly scarcely (Fig. 4c). Trapped ostracods shells, torn out rounded and sorted peloids,intraclasts with clotted microfabric as well as single calcisphers are also observed andlocally built up irregular agglutinated laminae (Fig. 4d). Some of the bioclasts have beenreplaced by anhydrite, but elsewhere carbonate pseudomorphs after anhydrite crystalsor aggregates are present. Scarce fenestrae filled with calcispar, dolospar or evaporateminerals occur (Fig. 4a) and microscale vertical desiccation cracks locally crosscut thelamination (Fig. 4e).

General depositional environment and microbialite formation. The pre-sence of evaporates in the sulphate-bearing suites (Fig. 2) indicates arid climatic con-ditions [10]. The available lithofacies association testifies respectively to deposition in aperitidal (sabkha) environment. This conclusion is supported by the well distinguishedsupratidal, intertidal and subtidal subfacies units [12]. Thus, the anhydrites representsupratidal (e.g. subaerial) deposits and further evidence for such interpretation (e.g.precluding alternative subaqueous origin) comprises their relatively small bed thickness,replacive/displacive nature as well as the presence of nodular (e.g. chicken-wire) structure(cf. Warren and Kendall [13]). Also of supratidal origin are the rudstones/floatstoneswhich consist of clasts derived through desiccation and disintegration of intra/supratidalcarbonates (cf. Shinn [14]; Chatalov [4]). The presence of abundant fine crystallinenon-fossiliferous dolostones likewise supports the hypothesis for sabkha environment asFig. 1. Location map of the major wells drilled through Devonian sediments in North-easternBulgariaFig. 2. Generalized lithological log of the carbonate-sulphate suite in well R-119 KardamFig. 3. Polished surface of microbial carbonate (dolostone) showing laminated macrofabric andsmall low-relief domes (arrows)

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they resulted from penecontemporaneous replacement of primary CaCO3 (aragonite)sediments in the intertidal to lower supratidal zone. The associated limestones, whichare represented by bioclastic mudstones, wackestones and packstones, contain wellpreserved thin-shelled bivalves, moravamminids, ostracods and gastropods plus sporadiccrinoid fragments and echinoid spines. The initial sediments are interpreted as productsof shallow subtidal deposition with the non-winnowed textures and relatively low biotadiversity indicating restricted circulation and elevated salinity conditions.

The observed microbialites emerged through lithification of former microbial matsdeveloped in the intertidal zone. Stromatolites composed of fine-grained and aggluti-nated laminae were produced which is also typical of modern sabkhas in the PersianGulf [14]. The lamination resulted from episodic accretion related to seasonal microbialgrowth, periodical trapping of sediment material or both. The fine-grained laminaeprobably represent parts of former mats with relatively smooth morphology whichtrapped and bound only little amount of very fine grains. On the contrary, the occurrenceof agglutinated laminae indicate locally irregular surface mat topography [1], facilitatingthe trapping and binding of coarser sedimentary material derived from disintegration ofthe microbial mats (peloids and intraclasts) or supplied from the shallow subtidal zone(ostracod shells) during stronger storm activity. Lamination of some mats was disruptedby small desiccation cracks produced during periods of their subaerial exposure. In turn,fenestral voids were formed by gas bubble release during organic decay and/or matshrinkage during desiccation. The scarce evaporate crystal growth is probably indicativeof the transition interval between the lower supratidal and the upper intertidal zone.However, the observed relicts of laminated microbialites in some anhydrites (Fig. 4f)rather testify to seaward progradation of the sabkha and are another solid evidence forexistence of a peritidal setting.

The observed tubular structures represent calcified cyanobacterial sheaths and arethe only preserved skeletal remains from former microbes. However, at least part of themicritic matrix in the stromatolitic fine-grained laminae is assumed to have microbialorigin such as calcified bacterial cells or from whiting precipitation (cf. Riding [1]). Inthis context, the relative amount of trapped and bound carbonate mud is impossibleto determine. In turn, the clotted microfabric may have resulted from calcificationof microbially produced EPS (extracellular polymeric substances). The peloids withinsome laminae with clotted fabric are also interpreted as microbially induced in situprecipitates. Such microbial peloids are considered by Chafetz [15] as calcified bacterialaggregates rimmed by euhedral calcite crystals. According to Dupraz et al. [16] themicrite nucleation is initiated within a polymer biofilm embedding microbial commu-nities and undergoes discontinuous calcification generating micropeloidal structure.Meanwhile, the observed clotted fabric gradations from homogeneous micrite to in-dividual peloidal grains may have resulted from different intensity of the microbialmat calcification. According to Kazmierczak et al. [17] mats growing in conditions ofhigh calcium carbonate supersaturation produce almost homogeneous micrite, whereasin less intensively calcified mats micritic peloid-like bodies are formed. On the otherhand, the relatively better roundness and sorting of peloids from agglutinated laminaeindicate slight transport and redeposition processes. These particles are interpreted aspossible individual calcified cell aggregates that have been teared out from the mat dueto hydrodynamic events (bottom currents, storms) and redeposited as peloid grains andintraclasts (cf. Kazmierczak et al. [17]).

A major problem of this study still to be resolved is which microbes were involved

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Fig. 4. (A) Clotted microfabric and fenestrae (f) in fine-grained laminae; (B) Fine-grained laminaewith clotted fabric, demonstrating gradation from homogenous micrite to individual peloids; (C)Tube-like structures (arrow) representing calcified microbe sheaths; (D) Trapped ostracod shells(arrow) and torn out peloids in agglutinated laminae; (E) Microscale dessication crack (arrow)crosscutting the lamination; (F) Relic microbial laminae (m) in bedded anhydrite (a)

Compt. rend. Acad. bulg. Sci., 60, No 2, 2007 181

in the formation of the non-skeletal parts of the initial mats. Most probably thesewere cyanobacteria whose calcification appears to result from creation of alkalinitygradients in mucilaginous sheaths associated with photosynthetic uptake of CO2 and/orHCO−3 uptake that raises alkalinity [18]. However, a wide range of other bacterialprocesses (ammonification, denitrification, sulphate reduction and anaerobic sulphideoxidation) can also lead to raised alkalinity favouring CaCO3 precipitation [19]. Theyresult in the formation of fine-grained carbonate and are localized within decayingmats as heterotrophic bacteria degrade organic matter. Finally, an assumption shouldbe made that precipitation of “automicrite” in association with non-living organicmacromolecules (so-called organomineralization by Trichet and Defarge [20]) mayhave also played a significant role.

Conclusions. The reported occurrence of Paleozoic stromatolites helps to moreprecisely clarify the general depositional environment for part of the Middle Devoniansequence from North-eastern Bulgaria. Thus, their presence and some characteristicsof the associated carbonate and evaporitic rocks testify to the existence of an ancientsabkha whereupon the microbial mats were formed in the intertidal subenvironment.The described Givetian microbialites were deposited during a time interval which wasclose to one of the major cyanobacterial calcification events of the Phanerozoic – theLate Devonian epoch [1].

Acknowledgements. The author is grateful to Atanas Chatalov for the usefulcomments.

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Geological Institute, Bulgarian Academy of SciencesAcad. G. Bonchev Str., Bl. 24, 1113 Sofia, Bulgaria, e-mail: polina_a@geology.bas.bg

182 P. Andreeva