Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Identification of algal-rich beds and indicia of clay minerals in lacustrine deposits of Cypris Formation, Sokolov Basin, using optical spectroscopy in visible range (VIS)


Karel Martínek, Juraj Franců, Ondřej Bábek, Petr Rojík, Veronika Kopačková

Geoscience Research Reports 48, 2015 (GRR for 2014), pages 95–98
Map sheets: Sokolov (11-23)

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Published online: 29 October 2015

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Lacustrine claystones of the Miocene Cypris Formation, Sokolov Basin, were examined for a paleoclimatic study. A > 70 m thick succession recovered in borehole Dp-333-09 was sampled every 10 cm giving the total number of 718 samples that were analysed with a visible light reflectance spectroscopy (VIS). VIS data are promising proxies in paleoclimatic research partly because the measurements are very fast, cheap and easy. Here we tested the relationships of VIS, organic matter content and clay mineral composition. First results suggested that algal organic matter might have distinct spectral signatures in VIS (see Fig. 1). To validate the hypothesis, 25 analyses of organic matter (TOC - total organic carbon, TIC - total inorganic carbon, HI - hydrogen index, OI - oxygen index) and 19 samples for X-Ray diffraction of clay mineral fraction were used. The results show that orange/violet ratio has a high correllation (R = 0.88) with hydrogen index (HI; a proxy for algal organic matter contents; Fig. 2B), brightness correlates (R = 0.77) with kaolinite content (Fig. 2D), and green highely correlates with illite content (R = 0.79; Fig. 2F). Oxygen index (OI), which serves as a proxy for terrestrial organic matter contents, has a good negative correlation with orange/violet ratio (R = 0.47; Fig. 2C); hence, this color ratio can be considered as a complementary proxy for terrestrial organic matter. “Redness” of sediment expressed either by a red/green ratio or raw reflectance in the red spectral region correlates well with total organic matter (TOC). However, it also correlates with smectite content, which makes this parameter an ambiguous proxy; other supporting parameters are needed to differentiate TOC and smectite in sediment “redness”. The results presented here cannot be generalized for other deposits with different mineralogy and organic matter maturity. The main result important for further paleoclimatic research is that using a small number of analytical results for validation, we can interpret a large number of VIS measurements in terms of mineralogy and organic geochemistry.


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