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Lilian Švábenická:
Helicosphaera waltrans in the Carpathian foredeep, central Paratethys and its biostratigraphic and palaeogeographic significance
(Poster

The nannofossil species, Helicosphaera waltrans, was found in the Lower/Middle Miocene boundary sediments of the western part of the Carpathian Foredeep (western part of central Paratethys), in the area of Lower Austria and Moravia, Czech Republic. Helicosphaera waltrans shows the following features: morphologically distinct species; short stratigraphical range; first occurrence (FO) probably corresponds to last occurrence of Helicosphaera ampliaperta; lower part of its range: characterised by significant presence of other helicoliths; upper part of its range associated with species Helicosphaera walbersdorfensis, Discoaster exilis, D. variabilis, Umbilicosphaera rotula, Calcidiscus premacintyrei, and others; occurrence partially coincides with paracme/low-abundance interval of Sphenolithus heteromorphus which is correlated with MNN4b (Mediterranean Neogene Nannoplankton Biozone sensu Fornaciari et al. (1996)), the lowermost part of Middle Miocene.

In the Carpathian Foredeep, nannofossil assemblages with H. waltrans form a significant horizon. This is limited to a short stratigraphical interval which approximately corresponds with the stratigraphic range of the foraminiferal species, Globigerinoides bisphericus, and includes the FO of Orbulina suturalis (Svábenická & Ctyroká, 1999). This interval can be correlated with the upper part of M4 and with M5-M6 ((sub)tropical zones sensu Berggren et al. (1995)). The M4/M5 boundary was postulated as the Lower/Middle Miocene boundary.

H. waltrans has been mentioned rarely and exclusively from the Miocene deposits of the Mediterranean area till present (Theodoridis, 1984; Fornaciari et al., 1996). Fornaciari et al. (1996) compare its short range within the middle part of MNN5 (approximately middle part of the Langhian), above the Sphenolithus heteromorphus paracme/low-abundance interval. The occurrence of this species in the intercontinental central Paratethys basin is therefore a surprising find, and an interesting phenomenon from the biostratigraphical and palaeogeographical point of view.

According to the above-mentioned data, the FO of H. waltrans in the Carpathian Foredeep was recorded lower than that described by Fornaciari et al. (1996) and Young (1998) from the Mediterranean, already in the lowermost part of the Middle Miocene. It is possible to assume that the original area of distribution of H. waltrans was the western part of central Paratethys and that this species migrated through the 'Trans-Tethyan-Trench Corridor' to the Mediterranean. In accordance with Rögl (1998), this seaway supplied the Pannonian, Styrian and Vienna Basins and a part of the Carpathian Foredeep. Intermittent seaways and regional closure of basins with endemic development are supposed here in the period from the Karpatian (Late Burdigalian) to Early-Middle Badenian (Langhian).

References

Berggren, W.A., Kent, D.V., Swisher, C.C. & Aubry, M.P. 1995. A revised Cenozoic geochronology and chronostratigraphy. Geochronology Time Scales and Global Stratigraphic Correlation, SEPM Special Publication, 54: 129-212.

Fornaciari, E., Di Stefano, A., Rio, D. & Negri, A. 1996. Middle Miocene quantitative calcareous nannofossil biostratigraphy in the Mediterranean region. Micropaleontology, 42(1): 37-63.

Rögl, F. 1998. Paleogeographic Considerations for Mediterranean and Paratethys Seaways (Oligocene to Miocene). Ann. Naturhist. Mus. Wien, 99A: 279-310.

Svábenická, L. & Ctyroká, J. 1999. Biostratigraphic correlation (Foraminifers and Nannofossils) of the Karpatian and Lower Badenian sediments in the Alpine-Carpathian Foredeep (Moravia and Lower Austria). Geologica Carpathica, 55 (special issue): 79-80.

Theodoridis, S.A. 1984. Calcareous nannofossil biozonation of the Miocene and revision of the helicoliths and discoasters. Utrecht Micropaleontological Bulletins, 32: 1-271.

Young, J.R. 1998. Neogene. In: P.R. Bown (Ed.). Calcareous Nannofossil Biostratigraphy. Cambridge University Press: 225-265.

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