Neoproterozoic sedimentary carbonates and their silicified equivalents: Barrandian, Czech Republic


Authors: Skoček V, Pouba Z

Published in: Bulletin of Geosciences, volume 75, issue 3; pages: 241 - 260;

Keywords: Neoproterozoic, limestones, dolomites, siliceous rocks, sedimentary environment, ooids, pisoids, microfossils,

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The Neoproterozoic sequence incorporates sedimentary limestones and dolomites in thin layers and lenses. Oolitic, pisolitic, stromatolitic and micritic limestones with an admixture of various allochemical particles were distinguished in addition to breccias. Coated grains occur as two distinctly different types. Microoids (diam. below 0.3 mm) and pisoids (diam. 3-6 mm) are considered a product of repeated growth in agitated water. Volcanic accretionary lapilli are mostly small (diam. below 2 mm) carbonatized bodies with poorly developed concentric texture. Limestones with unusual, characteristic crystalline texture are the result of the replacement of chemically unstable layers of evaporites (sulfates). Arenites and greywackes with carbonate cement and shales with carbonate lenses are the products of diagenetic carbonate redistribution. The sedimentary carbonates differ from carbonatized volcanogenic rocks by relict textures, elevated amounts of Sr and typical negative 18O/16O ratios. The similarity of structures and textures, between sedimentary carbonates and some siliceous rocks evokes the suspicion that some cherts in the sequence are of replacement origin. This is evidenced by quartzose pseudomorphs of dolomite rhombs and by the presence of silicified sulfate aggregates in them. The small size of stromatolitic build-ups indicates a limited extent of favourable circumstances for the growth of algal mats. The areal association of carbonates with shales, silciciclastics, volcanic and volcaniclastic rocks shows that the majority of detrital carbonates were deposited from gravity currents initiated at the edges of volcanic islands.