Coiling direction in Middle Miocene globorotaliids (Foraminiferida) – a case study in the Paratethys (Upper Silesia Basin, Poland)

Abstract

The paper deals with sub-evaporite Badenian deposits (Middle Miocene, Paratethys) that display a sudden appearance of globorotaliid taxa showing preferences in the taxonomic set and coiling direction. An interval of sinistrally coiled Jenkinsella mayeri with proportionally coiled Globoconella bykovae is at the bottom of the studied sequence. It is followed by an interval of proportionally coiled Jenkinsella transsylvanica. Proportionally coiled Globoconella bykovae with a small number of proportionally coiled Jenkinsella transsylvanica are at the top of the Moravian (CPN 7) substage (Orbulina suturalis alias Lagenidae Zone). The sub-evaporite part of the Wielician (CPN 8) substage (Uvigerina costai alias Spiroplectammina Zone) began with a globorotaliid-barren interval (IIC biozone). It is followed by biozone IID displaying three globorotallid peaks amongst globorotallid-barren deposits. Only globoconellas are present in all these three events. Proportionally coiled Globoconella bykovae appeared first. It is followed by proportionally coiled Globoconella minoritesta and sinistrally coiled Globoconella bykovae at the end of the biozone IID. Coiling preferences of globorotaliids have been compared to changes in the hydrographic regime unfolding during the CPN 7 and CPN 8 in the studied area of the Paratethys. The study discovers that the coiling of globorotaliids responded selectively to the hydrographic changes. Globoconella bykovae is highly opportunistic and can tolerate considerable temperature changes. Specimens of this taxon persist to coil proportionally during the sub-evaporite Badenian, except for the shift to sinistral coiling just prior to the Wielician evaporites. Sinistrally coiled Jenkinsella mayeri is the most sensitive to hydrographic changes. This taxon retreated at the onset of the temperature drop during the mid-Moravian cryptic cooling. This cooling is marked by the new jenkinsellid occurrence – proportionally coiled Jenkinsella transsylvanica. Proportionally coiled Globoconella minoritesta is present in the middle of the IID globorotaliid peaks. The globorotaliid coiling data, presented in this paper, has been calibrated to the global stratigraphy. The integrated data (foraminifera, nannofossils, oxygen and carbon stable isotopes, radiometric measurements) supports the view that the CPN 7/CPN 8 boundary is coeval to the Langhian/Serravalian one. In the Polish part of the Carpathian Foredeep, the boundary runs within the Skawina Formation.