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Calcareous nannoplankton in the Upper Jurassic marine deposits of the Bohemian Massif: new data concerning the Boreal–Tethyan communication corridor

100%

Holcová K. , Holcová M.

tom Vol. 60, No. 3

624--636

Calcareous nannoplankton assemblages from the Jurassic relict deposits in the northern part of the Bohemian Massif are described here for the first time. They are generally of low diversity and dominated by watznaueriaceans. Some of them are diagenetically affected, probably due to dolomitisation. Calcareous nannoplankton enables the stratigraphical range of the Northern Bohemia Jurassic succession to be extended to the Tithonian by reference to the stratigraphical range of Jurassic platform sequences in Central Poland and the eastern part of the Bohemian Massif. The Oxfordian–Kimmeridgian nannofossil assemblages indicate a generally oligotrophic condition of the restricted sea with episodic fluvial input containing terrestrial nutrients. The character of the upper part of the water column was generally uniform and did not reflect variability at the sea-floor expressed by lithofacies diversity. The palaeoenvironment interpreted for the famous former palaeontological locality “Sternberk Quarry” was characterized by a higher nutrient content and more stable environment. The Tithonian nannofossil assemblages contain warm-water Tethyan taxa which suggest south-north migration of nannoplankton due to warming during the Jurassic–Cretaceous boundary interval.

The earliest Badenian Planostegina bloom deposit : reflection of an unusual environment in the westernmost Carpathian Foredeep (Czech Republic)

61%

Kopecká J. , Holcová K. , Nehyba S. , Hladilová Š. , Brzobohatý R. , Bitner M. A.

tom Vol. 62, No. 1

18--37

The Miocene sedimentation history of the Brus denudation relict (the western part of the Carpathian Foredeep, Czech Republic) has been inferred from 20 m of silt/siltstones, sand/sandstones and limestones penetrated by the Brus-1 borehole. Detailed multiproxy lithofacies and biofacies analyses have allowed facies and palaeoenvironment interpretations. The presence of molluscs, brachiopods and fish fauna, as well as large benthic and epiphytic foraminifera indicates a generally shallow, subtropical, marine environment. Despite the scarcity of biostratigraphical markers, the section can be correlated with the lowermost Badenian (~15-16 Ma). In the siltstones in the lower part of the borehole, there are abundant low-salinity foraminifera, which may indicate increased rainfall. Linked to this is the nutrient enrichment of the sea bottom water inferred from the presence of the high-nutrient taxa accompanied by abundant calcareous nannoplankton eutrophic taxa such as Coccolithus pelagicus and eutrophic planktonic foraminifera of the Globigerina bulloides/praebulloides group. The main Planostegina bloom was recorded in a sandstone in the interval 12.5-7 m, and was not influenced by a shift from high-organic content Valvulineria assemblage of small foraminifera to an epiphytic one. The boundary between siliciclastic and carbonate sedimentation (-7.5 m) shows a drop in K and Th concentrations, and in the Th/U ratio, but a rise in the Th/K ratio. This change in radioactive element content may generally indicate a significant decrease in terrestrial sediment input which is further supported by the onset of limestone deposition. Reversal of the deposition regime and a decrease of clastic input into the basin may be related to the changing of the precipitation regime, probably triggered by orbitally-forced cyclicity.