8th International Nannoplankton Association Conference
Jörg Mutterlose, Kai Kessels:
The Jurassic/Cretaceous boundary interval of the Russian Platform is characterised by the widespread occurrence of black shales. In order to understand the controlling factors which caused the black shale sedimentation, calcareous nannofossils were analysed from two sections of the Volga Basin (SE Russia). Calcareous nannofossils are ideal proxies for deciphering fluctuations of nutrients, temperature and salinity.
A total of 110 samples were examined for calcareous nannofossils. Material from the Gorodische section (stratotype of the Volgian) and from the Kashpir section was studied. The lithology of both sections consists of calcareous mudstones and intercalated grey, organic-rich shales. The presence of Ethmorhabdus gallicus throughout both successions allows an assignment to the E. gallicus Nannofossil Biozone (NJ17b), which corresponds to the D. panderi Ammonite Biozone of the Middle Volgian. The calcareous nannofossils have boreal affinities. The calcareous mudrock beds at Kashpir yield a well-preserved rich and diverse nannoflora, dominated by Watznaueriaceae (Watznaueria barnesiae, Watznaueria communis, Watznaueria britannica, Watznaueria fossacincta), Biscutum constans and Zeugrhabdotus erectus (in order of abundance). All six black shale beds are essentially barren of calcareous nannofossils, although occasional heavily-etched specimens of Watznaueria barnesiae occur. Only the uppermost part of the black shale succession yielded an assemblage of extreme low diversity and abundance. These observations suggest a diagenetic overprint in the black shale horizons. The composition of the assemblages shifts from Watznaueria-dominated assemblages to Biscutum constans-dominated assemblages in the uppermost part. B. constans makes upto 50% of the overall abundance. These horizons also show an increase of Crucibiscutum salebrosum. B. constans and Zeugrhabdotus spp. are considered to be taxa indicative of a high nutrient supply, while C. salebrosum is a cold-water species.
The increase of black shale horizons in the upper part of the section suggests a trend towards more-stable anoxic conditions of the bottom-waters. This correlates well with an increase of nannofossil species which are considered to indicate higher productivity of surface-waters. We think that this shift from low to higher surface-water productivity was caused by a cooler fresh-water cap. This cool, stratified water would be best developed in a cool climatic phase (confirmed by stable isotopes) with low circulation and high fresh-water runoff. These interpretations fit well with the occurrence of C. salebrosum in the uppermost part of the section.
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