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Catalogue of Tithonian–Berriasian radiolarians
Luis O'Dogherty, Špela Goričan, Peter O. Baumgartner, Atsushi Matsuoka, Marco Chiari

Building: Muséum d'Histoire Naturelle de Genève
Room: Lobby /Hall/
Date: 2018-12-05 12:15 PM – 12:18 PM
Last modified: 2018-11-30

Abstract


Radiolarians have a great potential for defining the Jurassic–Cretaceous boundary, but they are still underexplored. The number of publications dealing with taxonomy or stratigraphy of Tithonian–Berriasian radiolarians is significantly lower than for other Mesozoic stages. A great effort was made at the end of the century by the Mesozoic Working Group of InterRad (International Association of Radiolarists), however, the resolution across the Jurassic–Cretaceous boundary was unsatisfactory due to the low number of samples investigated at that time.

In this meeting we present a new taxonomic atlas of radiolarians for low- and mid-latitude areas. This atlas consists only of species whose occurrence has been reported in Tithonian and/or Berriasian strata. It includes 210 revised species, 40 of them were not previously considered in the Catalogue of Baumgartner et al. (1995). Synonymies and illustrations of all of them should facilitate their recognition to build the most complete lists of taxa in samples across the J/K boundary.

The catalogue of taxa has been elaborated based on selected radiolarian-rich samples from well-constrained stratigraphic levels through the Tithonian–Berriasian stages. The taxa reported in these samples have been very-well studied and illustrated in order to positively confirm their occurrence in every sample. These rich and well-preserved samples record up to 115 already described taxa (see Dumitrica & Zügel, 2003; Jud, 1994; Matsuoka, 1998; Matsuoka et al., 2005). The well-constrained stratigraphic position of samples allows us to create an artificial succession of levels and hence to create an empirical stratigraphic chart which is exhibited during this symposium. Actually, this stratigraphic atlas is the starting point of the project underway by the Mesozoic Working Group of InterRad in order to complement the definition of the J/K boundary with radiolarians. This would be of great interest because radiolarians, conversely to calpionellids, are common in the Neotethys, assuring the traceability of the J/K boundary beyond the Central Atlantic–Western Tethysian areas.

Analysis of taxa on Italian sections (Bosso and Valdorbia, Umbria-Marche Apennines) reveals that a major turnover encompassing the first appearance of up to 25 species took place just before the J/K boundary (inside the lower part of the Maiolica facies). These preliminary results should not to be regarded as an artifact related to a major change due to preservation imposed by the change from chert- to carbonate-dominated lithologies in the Alpine–Mediterranean region because the occurrence of these new species are recorded rapidly through around the first 20m of the Maiolica facies and concomitantly with the occurrence of first true calpionellids in the uppermost Tithonian. Another valuable argument supporting this hypothesis is that the major facies from the underlying lime-poor to lime-free siliceous deposits to the Maiolica-type limestone is not marked by an extinction event. The extinctions through the Tithonian are gradual and 97 species survive the J/K boundary.

The best candidate to define the J/K boundary in the Alpine–Mediterranean region are those located in the Umbria-Marche Apennines in which magnetostratigraphy, calpionellids and calcareous nannofossils have been also investigated (Bosso and Valdorbia sections). Counterpart areas in the Northern Calcareous Alps encompass also a great potential to redefine our radiolarian scales; they belong to the Oberalm and Schrambach formations at Trattberg-Gartenau and Kaltenhausen sections (Steiger, 1992).