8th International Nannoplankton Association Conference


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András Nagymarosy:
Lower Oligocene nannoplankton in anoxic deposits of the central Paratethys

A widespread anoxic event occurred in the whole Paratethys during the Early Oligocene (Nannoplankton Biozones NP22-23). Thick deposits of anoxic black shale indicate this event in the western and central regions of the Paratethys basin-system, including the area of the N Alpine Molasse, most of the Outer Carpathian flysch troughs, the Buda-type Palaeogene basin in Hungary, Slovenia and Croatia, and the Transylvanian Basin.

The main cause of the anoxia was a stratification of denser, normal-salinity and less-dense brackish water layers as a consequence of the increasing isolation of the Paratethys from the World Ocean. The whole process can be very well observed in the profiles of the Tard Clay Formation, in Hungary. The decrease of salinity is well reflected in the change of the nannoplankton associations as well as by the presence or extinction of planktonic foraminifers.

The underlying Buda Marl Formation (NP19 - lower NP22) contains full-marine nannoplankton assemblages. These diverse assemblages are characterised by moderately warm to subtropical, pelagic forms, with different species of the genera Sphenolithus, Discoaster, Lithostromation, Triquetrorhabdulus, Pseudotriquetrorhabdulus, Ilselithina and Peritrachelina. In the top part of the Buda Marl, these assemblages have been slightly influenced by the immigration of pelagic/cold-water forms of Reticulofenestra hesslandii, R. callida, Chiasmolithus oamaruensis and Coccolithus pelagicus, as well as by blooms of the cold-water species, R. bisecta.

In the non-laminated lowermost part of the Tard Clay Formation (base NP22), another group became dominant with nearshore, full-marine affinities: holococcoliths (Lanternithus minutus, Zygrhablithus bijugatus, Isthmolithus recurvus); helicosphaerids; and some pontosphaerids, such as P. enormis, P. desueta, P. multipora, P. rothii, Transversopontis pulchra, T. pulcheroides, T. pygmaea. Very similar assemblages have been found in the Schönecker Fischschiefer (Vorlandmolasse), the Psekhian Horizon (Caucasus), and the Mittlere Pechelbronner Schichten (Rheingraben) from the same time-interval. Impoverished planktonic foraminifera associations are still present in these layers.

In the upper, slightly-laminated part of the Tard Clay Lower Member (NP22/23 transition) the normal salinity forms become impoverished and a new group of nannoplankton appears, indicating nearshore conditions with fluctuating salinity: Reticulofenestra lockeri, Coccolithus crassipons, Pontosphaera latelliptica, P. latoculata, Transversopontis obliquipons, T. pulchra ('dwarf' variety), Orthozygus aureus, Micrantholithus attenuatus. O. aureus, P. latoculata, P. multipora, Transversopontis obliquipons, T. pulchra, Braarudosphaera bigelowii and R. lockeri produce several mass-blooms at this level. Planktonic foraminifera disappear in this horizon.

The almost-complete isolation of the C Paratethys (middle part of NP23) resulted in the evolution of Paratethys-endemic nannoplankton species, such as Reticulofenestra ornata, Transversopontis latus and Transversopontis fibula. These low-salinity, low-diversity assemblages contain also Reticulofenestra cf. R. tokodensis and B. bigelowii, accompanied by elements of the brackish Cardium lipoldi mollusc assemblage (Tard Clay middle part in Hungary, Bizusa Formation in Transylvania, Dynów Marl in the Outer Carpathians, Heller Mergelkalk in the Vorlandmolasse, Polbinian/Solenovian Horizon in the Caucasus). The latter three horizons are pure nannochalks of R. ornata. Although the brackish conditions persisted, the bottom anoxia paused for a short time.

The peak of anoxia took place in upper NP23 (upper, strongly laminated member of the Tard Clay, Menilite and Dysodile Beds in the Outer Carpathians, Ileanda Beds in Transylvania, Bändermergel in the Vorlandmolasse). Both planktonic and benthic assemblages are absent in these layers, except rare blooms of low-salinity nannoplankton, such as R. ornata, R. cf. R. tokodensis, B. bigelowii. The mono- or duo-specific assemblages appear in thin, white calcareous laminae, embedded into black, non-calcareous pelite. These blooms formed widespread nannochalk horizons in the Outer Carpathians (Tylawa Limestone).

Near to the NP23/24 boundary, the connection with the world ocean had been restored. The slowly increasing salinity resulted first in blooms of R. lockeri and B. bigelowii (boundary of the Tard Clay and the overlying Kiscell Clay Formation), or in monospecific blooms of Cyclicargolithus floridanus (Jaslo Limestone, Outer Carpathians). From NP24 on, the water stratification and, thus, the anoxia ceased. Full-marine planktonic and benthic assemblages invaded the Paratethys from the North Sea and from the Mediterranean region. The rich, high-diversity pelagic nannoplankton of the Kiscell Clay (NP24) is dominated by several species of the genera Sphenolithus, Reticulofenestra, Coccolithus and Helicosphaera.


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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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