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Microbial sediments from the Upper Triassic Main Dolomite Formation of Žumberak Mts. (Northwestern Croatia)

Dolomite has been considered as one of the biggest geological problems. It is very rare in modern sediments and has the inability to synthesize under low-temperature conditions in the laboratory, so the origin of dolomite has remained an enigma in sedimentology, often called the ‘Dolomite Problem’ (MCKENZIE & VASCONCELOS, 2009). Many dolomitisation theories has been established but have difficulties to explain vast (both in thickness and regional extent) Main Dolomite Formation (Hauptdolomite, Dolomia Principale). Main Dolomite has thickness up to 2000 m, and is spread all through wider Alpine area, consist entirely of dolomite, with limestone intercalations only in the uppermost part near the transition to the Dachstein Limestone. Only discovery of VASCONCELOS & MCKENZIE (1997, and later papers) has provided new understanding of the mechanisms that is involved in the primary dolomite precipitation under Earth surface conditions, in contrary to other theories that included secondary replacement of meta-stable calcium carbonates. Authors develop the “Microbial dolomite model” based on a study of a modern dolomite-forming hypersaline coastal lagoon, Lagoa Vermelha, Brazil, where sulphate-reducing bacteria induce dolomite precipitation. That conclusion was confirmed by culture experiments. Bacteria are not only crucial for production of dolomite in Lagoa Vermelha case, but also are part of microbial mat communities that form stromatolites (SPADAFORA et al., 2009). It is very interesting that stromatolite taken from Lagoa Vermelha continues to grow in aquarium, where no trapping and binding of sediment is possible. MASTANDREA et al. (2006) found in stromatolitic facies of the Main Dolomite in Southern Italy small round structures that they interpret as fossilized bacteria, supposedly similar to the living ones from Lagoa Vermelha. The other ‘problem’ with the Main Dolomite is the origin of sediments, although this topic is not much discussed in literature. The origin of sediments that formed limestone is well known, thanks to investigations of modern carbonate platform environments. The Main Dolomite has almost no fossil remnants, and its structure, when preserved, consists mainly of micritic particles. ZANKL & MERZ (1994) investigated cyanobacteria from Everglades swamp (USA) and found out that quantity of carbonate sediments that cyanobacteria precipitate corresponds to the sedimentation rates calculated for the Main Dolomite. Presented solution of these two ‘geological problems’ enables us to consider the Main Dolomite as a paragon of a microbial rock (or microbialite). The Main Dolomite (Glavni dolomit) is present also in Žumberak Mts in northwest Croatia. There it lies between dolomites of Carnian Slapnica Formation and dolomites (with rare limestone intercalations) of Rhaetian Posinak Formation (GRGASOVIĆ, 1997 ; GRGASOVIĆ 2007 ; GRGASOVIĆ et al. 2007). In the uppermost part of the Main Dolomite, an interval of intraformational tempestite dolomite breccia of Kalje Member is separated. The geological age of the Main Dolomite in this area is Norian-Rhaetian, based on foraminifera and calcareous algae biozonation (GRGASOVIĆ, 1997). The Main Dolomite is characterised by the frequent alternation of three lithofacies: dolomicrites, fenestral dolomicrites and dolostromatolites, all presumably of microbial origin. The dolomicrite lithofacies predominates over the others. It contains dense mass of micritic particles or minute peloids and aggregated grains, with rare intraclasts. Most often has been diagenetically changed to dolomicrosparite. Sedimentary environment is interpreted as subtidal. The origin of mud particles is probably from cyanobacteria, as suggested by ZANKL & MERZ (1994). The fenestral dolomicrite lithofacies has typical micobialite characteristics, and it is stromatolite actually, but to distinguish it from the next member it was named ‘fenestral dolomicrite’, since it is characterized by lamellar and irregular fenestrae, with geopetal filling due to vadose intertidal environment. It consists of a matrix of minute peloids and trombolites, originated possibly by trapping and binding of subtidal carbonate mud to intertidal microbial mats. It would, according to RIDING (1991) correspond to ‘agglutinated stromatolites’. Dolostromatolite lithofacies are the most prominent characteristic of the Main Dolomite Formation. They are very finely wavy laminated, so they have been colloquially called ‘microwave’ stromatolite. They have a much darker colour than other structural types. These stromatolites are built of very small sediment particles that were probably precipitated by microbes in microbial mats (as suggested by SPADAFORA et al. (2009)), and of thin micritic tubes that represent cyanobacterial skeletons as well. It would, according to RIDING (1991), correspond to ‘skeletal stromatolite’. Between the laminae built of micrite particles and cyanobacterial skeletons there are thin lamellar fenestrae which suggest an intertidal to supratidal sedimentary environment. Both ‘fenestral dolomicrite’ and ‘dolostromatolite’ lithofacies has completely preservative structure, meaning that all components of microfacies, from the thinnest mud particle to coarse-grained cement, is fully preserved, and in thin section cannot be distinguished from the limestone microfacies, so we use Alizarin colouring to prove 100% dolomite mineralogy. Also, rare fossils are preserved, that enabled their detailed investigation (GRGASOVIĆ, 1997 ; SOKAČ & RGASOVIĆ, 1998). Such preservation suggests a model of very fast and early dolomitisation, as “Microbial dolomite model” of VASCONCELOS & MCKENZIE (1997). Dolomicrite lithofacies has been fully dolomitised somewhat later during diagenesis, as suggested by larger crystals, although the initial dolomitisation can be also explained by above mentioned model.

stromatolites, microbial sediments, Upper Triassic, main Dolomite, Žumberak Mts.

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