Stromatolite-like cherts in the Barrandian Upper Proterozoic: A review


Authors: Pouba Z, Kříbek B, Pudilová M

Published in: Bulletin of Geosciences, volume 75, issue 3; pages: 285 - 296;

Keywords: Upper Proterozoic, Neoproterozoic, Barrandian, stromatolite, chert, stiriolite, geysirite microfossils, sabkha facies, geochemistry, stable isotopes, organic matter,

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Microfossiliferous stromatolitic cherts morphologically similar to stromatolites of the Collenia type can be traced over a discontinuous horizon in the southeastern part of the Barrandian Upper Proterozoic (Neoproterozoic). Stromatolitic cherts occur in the form of lenticular bodies associated with volcanics, volcaniclastics, black cherts and with sediments of sabkha or black shale facies. The amount of organic carbon ranges between 0.3 and 0.4 % in the dark laminae and is lower than 0.002 % in light-coloured laminae. The isotope composition of organic carbon ( delta;13C = -28 to -32 ‰, PDB) falls within the range of carbon isotope composition of Precambrian stromatolites. Trace amounts of isoprenoid hydrocarbons, phytane and pristane identified in the rock extract point to the phototrophic character of microorganisms in stromatolitic buildups. Isotope composition of oxygen of quartz in light ( delta;18O = 20.1 to 20.8 ‰, SMOW) and dark ( delta;18O = 20.1 to 20.9 ‰, SMOW) siliceous laminae is practically identical and indicates no significant temperature differences during their deposition. Organic carbon-rich laminae are enriched in K, P, V and U, organic matterpoor laminae have an increased content of Cr, Sb and other trace metals. The REE pattern in stromatolites resembles that of associated volcanics of OIT- and OIA-type and does not differ substantially from the REE pattern in hydrothermal, Cr-rich cherts, which commonly cement stromatolite fragments. It is suggested that stromatolites originated in an inter- or supratidal marine environment and were closely associated with active submarine or subaerial hydrothermal springs. The increased temperature of water and probably of CO2 resulted in increased activity of algae and in periodical growth of algae and/or bacterial mats on the surface of precipitated material deposited around or close to hydrothermal vents. At the present stage of knowledge, it is difficult to assess whether stromatolitic cherts originated from siliceous material or represent a product of complete silicification of originally calcareous precipitates, which seems to be more probable.