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

• Main Authors: Kopecká, Jitka, Holcová, Katarína, Nehyba, Slavomír, Hladilová, Šárka, Brzobohatý, Rostislav, Bitner, Maria Aleksandra
• Format: Article in Journal/Newspaper
• Language: English
• Published: Polish Geological Institute-National Research Institute 2018
• Subjects: Middle Miocene; Central Paratethys; siliciclastic sedimentation; carbonate sedimentation; large benthic foraminifera; palaeoecology; Planktonic foraminifera
• Online Access: https://gq.pgi.gov.pl/article/view/25744; https://doi.org/10.7306/gq.1398

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.