Distribution and Origin of Reduction Spheroids in the Upper Paleozoic Sequences, Czech Republic


Authors: Skoček V

Published in: Bulletin of Geosciences, volume 74, issue 2; pages: 119 - 126;

Keywords: Upper Paleozoic, reduction spheroids, red beds, geochemistry, diagenesis,

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Green spots or spheroids frequently occur in fluvio-lacustrine redbeds of the Stephanian C and Permian age whereas they are extremely rare in red sediments of the Stephanian A and totally lacking in red sediments of the Westphalian C and D. Cores are more common at certain levels whereas at other levels are totally absent due to processes leading to their erasing. In centres of spheroids, vanadium and uranium oxides, present as earthy porous matter or pigment of calcite crystals or concretions, often occur.
The study of distribution has shown that spots originated in an early diagenetic stage, not deep below the sediment/atmosphere interface, probably within the zone of microbial activity. It is presumed that a fluctuating moistening was one of the prerequisites of discrete sites origin which were centres of Fe3+ reduction. Such sites represented inhomogenities with pH and Eh gradient compared to the hosting sediment. Organic matter and vanadium and uranium compounds played the decisive role in the process. The mobility of aforementioned elements was enhanced due to elevated temperature and fluctuating mineralization of pore solutions. The precipitation and following dissolution of V and U was accompanied by an irreversible Fe3+ removal.
The uneven distribution of spheroids evidences their relation to digenesis of porous sediments under specific facial and climatic circumstances. The classical explanation of spots as simple reduction features induced by fragments of plants, grains of mafic minerals or pyrite clusters is in contradiction to actual observations.