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Jernej Pavsic, Bogoljub Anicic:
Lower Miocene Braarudosphaera and Micrantholithus rich layers of the eastern Slovenia
(Poster)

During the stratigraphic examination of Oligocene and Lower Miocene beds at Plohov Breg, E Slovenia, in the lower part of the profile a concentration of braarudospheres and microantholiths was detected, which were otherwise very rare throughout the profile (Pavsic & Anicic, 1998). This interval, which is an indication of a short-term change in living conditions in the ancient environment, was determined in detail. The enrichment of beds with pentaliths is most probably the consequence of cooling and a salinity decrease in the sea-water. In these beds, the following species of calcareous nannoplankton were determined: Braarudosphaera bigelowii, Coccolithus pelagicus, Cyclicargolithus abisectus, Discoaster druggi*, Discoaster barbadiensis*, Discoaster deflandrei, Discoaster distinctus*, Dictyococcites bisectus, Eiffellithus turriseiffelii*, Helicosphaera euphratis, Helicosphaera recta, Micrantholithus vesper, Prediscosphaera cretacea*, Reticulofenestra lockeri, Sphenolithus moriformis, Sphenolithus radians*, Triquetrorhabdulus carinatus (*reworked species).

Based on an increase of two species of nannoplankton in the otherwise monotonous profile of the Lower Miocene beds at Plohov Breg in E Slovenia, a 2.5m horizon could be delimited in which, according to our opinion, the salinity was drastically decreased. The reason for this decrease of salinity remains, for the moment, without a plausible explanation. We believe that mixing of larger volumes of water, owing to upwelling of sea-water as proposed by Siesser et al. (1992) for the Atlantic, cannot be applied to Paratethys. Other hydrological mechanisms of sea-water in that sedimentation region cannot be applied because of our deficient knowledge of the palaeogeography of the region. We consider that the decreased salinity might have been caused by incursion of river-water or, more probably, by drastically increased rainfall (Peleo-Alampay et al., 1999).

We will try to find evidence for the decreased salinity also with isotopic analyses. The interdependence of numbers of subjects of the two species is indicative of the limitation of the carrying capacity of the environment, when considering the ecological parameters that are important for the existence of two species with very similar, or even equal, requirements.

Figure 1: Relative distribution of Braarudosphaera bigelowii and Micrantholithus vesper in the interval of beds enriched with pentaliths. Frequency of the other species do not exceed ten specimens. Numbers (1, 2, 3) indicate the number of specimens. x100

References

Pavsic, J. & AniCiC, B. 1998. Nannoplankton stratigraphy of Oligocene and Miocene strata on Plohov Breg, near Podcetrtek (Eastern Slovenia). Razprave 4, razreda SAZU, 34(2): 55-79.

Peleo-Alampay, A.M., Mead, G.A. & Wei, W. 1999. Unusual Oligocene Braarudosphaera-rich layers of the South Atlantic and their palaeoceanographic implications. J. Nannoplankton Res., 21(1): 17-26.

Siesser, W.G., Bralower, T.J. & de Carlo, E.H. 1992. Mid-Tertiary Braarudosphaera-rich sediments on the Exmouth Plateau. Proc. ODP, Sci. Res., 122: 653-663.

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