--> Palaeoenvironmental reconstructions based on Middle Miocene foraminifera assemblages collected from several exploration boreholes located in the south-eastern Transylvanian Basin

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Palaeoenvironmental reconstructions based on Middle Miocene foraminifera assemblages collected from several exploration boreholes located in the south-eastern Transylvanian Basin

Abstract

Foraminifera assemblages were studied from 26 exploration wells drilled in SE Transylvanian Basin by Romgaz SA in order to establish their statigraphic distribution and paleoenvironmental significance.

Four different assemblages were identified, named according the dominant taxa: Anomalinoides dividens, Bogdanowiczia pocutica, Velapertina, and Globigerina.

The assemblage with Anomalinoides dividens is dominated by this index taxon. Sometimes, rare miliolids, small planktonic foraminifera (Globigerina prabulloides, G. tarchanensis, G. subcretacea), tubular benthic agglutinants, pyritized pteropods and fish skeletal remnants also appear. This assemblage is characteristic for the base of the Sarmatian. Anomalinoides dividens signals a major change in the environment, with a surface brackish layer in stratified waters, in a transgressive setting (Filipescu, 2004b).

The Bogdanowiczia pocutica assemblage consists of tubular benthic agglutinated foraminifera and planktonic foraminifera, which are characteristic to the late Badenian. This species suggests a re-oxygenation of the bottom waters following the transgressive interval (Filipescu, 2004a).

The Velapertina and Globigerina assembalges are alternating and are present in the samples collected at deeper depths. Beside Velapertina species, such as: V. luczkowskae (Popescu), V. indigena (Luczkowska), some globigerinids and a few pyritized pteropods have also been noticed. The Globigerina assembalge is diverse and includes:

G. bulloides, G. praebulloides, G. falconensis, G. ottnangiensis, G. diplostoma etc. In some samples rare tubular agglutinants, trochospiral, planispiral calcareous benthic foraminifera and a few fish skeletal remnants have also been noticed. These assemblages are characteristic for the late Badenian (Krezsek & Filipescu, 2005) and suggest normal salinity deep marine environments. The spreading of large planktonics (e.g. Velapertina) probably was stimulated by the transgressive interval, characterized by optimum climatic conditions and oligotrophic environments (Schmidt et al., 2008). Smaller planktonics (e.g. Globigerina) may point out unstable habitats with high turbulence (Schmidt et al., 2008) and high organic input (eutrophic conditions).

The assemblages give a good image on facies migrations associated to the sea-level changes and occur repeatedly in the stratigraphic record (e.g. Globigerina/Velapertina assemblages). Further work could improve the biostratigraphic zonation of the late Badenian by giving more importance to the correlation of the planktonic assemblages.

Acknowledgements

This paper is part of the doctoral research made possible by the financial support of the Sectorial Operational Programme for Human Resources' Development for 2007-2013, co-financed by the European Social Fund, under the project POSDRU/159/1.5/S/133391 - “Doctoral and postdoctoral excellence programs for training highly qualified human resources for research in the fields of Life Sciences, Environment and Earth”.