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

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

Geological Quarterly

2018

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.

Reconstruction of the unusual Middle Miocene (Badenian) palaeoenvironment of the Carpathian Foredeep (Lomnice/Tišnov denudational relict, Czech Republic)

Holcova K., Brzobohatý R., Kopeckáa J., Nehyba S.

Geological Quarterly

2015

Vol. 59, No. 4

654--678

Multiproxy sedimentological, gamma-spectrometric, foraminiferal, calcareous nannoplankton, and otolith data were used for the reconstruction of the Badenian (Middle Miocene) history of the Moravian part of the Carpathian Foredeep. The study material originated from the new borehole LOM-1, which drilled >20 metres of monotonous clayey siltstones with exceptionally rich and well-preserved microfossil assemblages. Distal parts of the Carpathian Foredeep (a forebulge depozone) are exposed in this succession. Generally, a quiet environment of outer shelf to upper bathyal of monotonous clayey silts was interpreted, which is typical for the proximal parts of a peripheral foreland basin. The section studied can be subdivided into six intervals, confirming the cyclical character of Middle Miocene sedimentation in the Central Paratethys. Interannual oscillations of nutrient content, temperature and/or salinity are interpreted based on the oscillations of geochemical as well as palaeobiological data. The turnover connected with the initiation of the Middle Miocene Climatic Transition is recorded above the LO (last occurrence) of Helicosphaera waltrans in agreement with previous observations in the Carpathian Foredeep. The changes include cooling, a decrease in nutrients, a probable increase of the salinity of surface water, and increase of seasonality. Seasonality was manifested by an alternation of mixed and stratified water columns with a seasonal input of nutrients. Concerning nutrients, sources of seasonal riverine input or seasonal upwelling are both possible.