Origin of fabric-selective dolomitization recognizable in the field: two case studies from Anisian carbonate rocks in the western Balkanides

Article in Press, Corrected Proof
Pages: 
pp. 43-60
Sofia University “St Kliment Ohridski”, 15 Tsar Osvoboditel Blvd, 1504 Sofia, Bulgaria
Abstract: 

The origin of dolomitized burrows (Cruziana ichnofacies) in limestones and dolomitic layers in limestone-dolostone ribbon rocks was studied on the basis of petrography, X-ray diffractometry, geochemical data and isotope signatures of the dolomites. Selective dolomitization of the burrows with a local source of Mg occurred in a near-surface setting from non-evaporitic solutions with low Mg/Ca ratio. The low-temperature precipitation of non-stoichiometric (with Ca excess) dolomite was microbially mediated and controlled by bacterial sulphate reduction. The carbon for dolomite formation was largely derived from seawater and/or dissolution of precursor carbonate sediments, and partly derived from the decomposition of organic matter. Selective dolomitization of primarily argillaceous carbonate layers in the ribbon rocks was mainly associated with an autochthonous source of Mg. Clay mineral transformations (i.e., illitization of smectite) at intermediate burial depths supplied Mg ions for the formation of Ca-rich (locally ferroan) dolomite. However, differential diagenesis of the originally clay-rich and clay-poor layers may also have favoured early dolomite precipitation, whereupon contemporaneous seawater was the primary Mg source during shallow burial. The obtained results show that various factors control the formation of fine-grained, non-stoichiometric dolomite with macroscopically recognizable fabric-selective character of replacement.

Keywords: 

dolomitization, fabric-selective, burrows, ribbon rocks, Middle Triassic, western Balkanides

VOLUME 47 (1)/May 2018

Faculty of Geology and Geography, Department of Mineralogy, Petrology and Economic Geology, Sofia University “St Kliment Ohridski”, 15 Tsar Osvoboditel Blvd, 1504 Sofia

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University of Mining and Geology “St Ivan Rilski”, Department of Geology and Geoinformatics, 1 Prof. Boyan Kamenov Str., Studentski Grad, 1700 Sofia, Bulgaria
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Sofia University “St Kliment Ohridski”, Department of Geology, Palaeontology and Fossil Fuels, 15 Tsar Osvoboditel Blvd, 1504 Sofia, Bulgaria

Regional Museum of History Kardzhali, 4 Republikanska Str., 6600 Kardzhali, Bulgaria
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Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 24, 1113 Sofia, Bulgaria

Department of Sedimentary Basins and Petroleum, Shahid Beheshti University, 1983969411 Tehran, Iran

Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Block 24, 1113 Sofia, Bulgaria