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Eva Halásová:
Report on the Hauterivian/Barremian boundary in the NW part of the Slovak western Carpathians based on calcareous nannoplankton
(Poster)

Calcareous nannoplankton assemblages have been observed in the Hauterivian/Barremian boundary in a quarry at Polomec Hill, near Lietavská Lúçka (part of Zilina, NW Slovakia). The exposed Lower Cretaceous carbonate sequence belongs to the Zliechov Unit of the Krízna Nappe, Central Western Carpathians. The biostratigraphy of the section was based on ammonites and microplankton (Vasíçek et al., 1995). A magnetostratigraphical study is in progress.

This section has been suggested as the national reference section for the Hauterivian/Barremian boundary. In accordance with results of the Copenhagen (1983) and Mula (1993) workshops (Hoedemaeker et al., 1993), this boundary was situated between the angulicostata and hugii Ammonite Biozones.

Calcareous nannofossils were studied from: (1) the Ptychoceras Beds - violet-grey, marly biomicritic limestones with microplankton represented by Tintinopsella carpathica, Cadosina fusca fusca, Colomisphaera heliosphaera. Throughout the sequence (except for its base), fragments and more-complete shells of Euptychoceras meyrati, E. borzai, Haploceras and Barremites occur, accompanied by rarer Ptychoceras puzosianum and true crioceratitid ammonites. Aptychi (Lamellaptychus angulocostatus) occur mostly as fragments (Vasíçek & Michalík, 1995). This part of the sequence was attributed to the Late Hauterivian meyrati Biozone. Associated nannofloras are typified by rare Lithraphidites bollii, the first appearance datum (FAD) of Rucinolithus terebrodentarius, Calcicalathina oblongata, Assipetra infracretacea. Nannofossil assemblages enriched in the species Micrantholithus hoschulzii, Nannoconus steinmannii, N. globulus, Watznaueria barnesiae and Zeugrhabdotus embergeri were observed in this part of the section. Nannoconus bermudezii is common in the upper part. Coccoliths such as Zeugrhabdotus erectus, Glaukolithus diplogrammus, Cretarhabdus crenulatus, Vekshinella stradneri, Diazomatolithus lehmanii, Percivalia fenestrata, Cretarhabdus conicus and Rhagodiscus sp. are also present in the assemblages; and (2) the yellowish-brown, reddish-grey or greenish-grey biomicrites of the Pseudothurmania Beds with a marly admixture, which are remarkable for the presence of synsedimentary slumping and brecciated beds. Tintinopsella no longer occurs, hedbergellid foraminifers are fairly common. Representatives of Pseudothurmania and Crioceratites dominate over other ammonite genera. Aptychi (Lamellaptychus angulocostatus angulocostatus), belemnites (Duvalia dilatata), brachiopods (Terebratulina, Pygites) are locally abundant. The Pseudothurmania Beds are regarded as basal Barremian. Accompanying calcareous nannofloras are of a similar nature to those mentioned above, although Braarudosphaera bigelowii starts to be abundant, and the last occurrence datum (LAD) of Lithraphidites bollii was observed. An horizon with common (Figure 1a) Micrantholithus sp. (new species?) and (Figure 1b) M. speetonensis was recognised. Pseudothurmania and Crioceratites disappear at the base of the sequence of well-bedded limestones with Barremites difficilis, Hamulina lorioli, Hamuli sp., Karsteniceras sp., Holcodiscus sp. and other ammonites. Hamulina lorioli, Veveysiceras escheri and Spitidiscus ex. gr. hugii occurring more sporadically, indicating the hugii Biozone (Vasíçek & Michalík, 1988). The composition of the microfauna is similar to the assemblage mentioned above. The nannoplankton assemblages are characterised again by enrichment in the species M. hoschulzii, Nannoconus steinmannii, Nannoconus globulus, and an horizon with abundant Nannoconus bermudezii was found. This part of the sequence is of Early Barremian age.

The interval studied correlates with Nannoplankton Biozone Lithraphidites bollii (CC5). The LAD of Lithraphidites bollii and FAD of Rucinolithus terebrodentarius are recognised here. Calcicalathina oblongata occurs throughout the section (discontinuously). The nannofossil assemblages are correlatable with those found in the Hauterivian/Barremian transition in the Butkov Quarry (Manín Unit, Krízna Nappe, Central Western Carpathians).

Figure 1a

Figure 1b

References

Hoedemaeker, P. & Company, M. (Eds) 1993. Ammonite zonation for the Lower Cretaceous of the Mediterranean region: basis for the stratigraphic correlations within IGCP Project 262. Revista Esp. Paleont., 8: 117-120.

Vasíçek, Z. & Michalík, J. 1988. Some heteromorphic ammonites from Polomec (Hauterivian/Barremian, Central Western Carpathians, Czechoslovakia). Geol. Zbor. Geol. Carpath., 39: 655-674.

Vasíçek, Z., Michalík, J. & Reháková, D. 1995. Hauterivian/Barremian boundary in the Western Carpathians. In: A.V. Dhondt (Ed.). Cretaceous stage boundaries. Abstracts volume, Second Int. Symposium, Brussels: 122.

Vasíçek, Z. & Michalík, J. 1995. The last lamellaptychi in the Hauterivian sequence of the Krízna Nappe, Central Western Carpathians. Geol. Carpathica, 46: 303-310.

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