Bio- (calcareous nannofossil and calpionellid) magnetostratigraphy across the Jurassic/Cretaceous boundary - An integrated approach to approximate the Jurassic/Cretaceous (J/K) boundary at Torre de’ Busi section, Southern Alps (Italy)

Integrated magneto- (calcareous nannofossil and calpionellid) biostratigraphy have been investigated at high resolution (one sample every 5-20 cm) across the Jurassic/Cretaceous (J/K) boundary at Torre de’ Busi section (Southern Alps, Northern Italy). The J/K boundary interval according to The Berriasian Working Group is the interval from CM19n to M18r. The Tithonian-Berriasian time interval is characterized by a major calcareous nannofossil speciation episode and by the appearance of the calpionellid group: several genera and species first appear and rapidly evolve, showing an increase in diversity, abundance and, particularly, in calcification degree. Biostratigraphic investigations have been performed directly on un-heated magneto-core end pieces as well as on samples collected specifically from the Jurassic/Cretaceous boundary interval. Calcareous nannofossil investigations have been performed on simple smear slides and ultra-thin sections (7-8 µm thick), while calpionellid has been investigated on the same thin sections used for nannofossil. The J/K boundary interval at Torre de’ Busi is represented by light brownish to whitish limestone belonging to the lowermost part of the Maiolica formation, representing a lithified nanno-ooze also rich in calpionellids. Calcareous nannofossil M.chiastius Zone (Bralower et al., 1989); Crassicollaria and Calpionella Zones (Remane, 1986; Pop, 1994b; Reháková & Michalík, 1997) and CM19 and CM18 polarity chrons have been recognized across the J/K boundary. Four reliable and well-marked calcareous nannofossil datums have been identified: the FO of N.wintereri and C.cuvillieri both correlating with the middle part of CM19n; the FO of N.steinmannii minor at the top of CM19n and the FO of C.octofenestratus at the base of CM18r. The ‘explosive’ appearance of a monospecific association of small, globular Calpionella alpina (C.alpina “acme”) has been identified in the uppermost part of CM19n. It is here emphasized that integrated stratigraphy based on calcareous nannofossil and capionellid events and magnetostratigraphy is a powerful tool for characterizing the J/K boundary interval at enhanced resolution, and is crucial for defining the J/K boundary. Bralower, T.J., Monechi, S. and Thierstein, H.R., 1989. Calcareous nannofossil Zonation of the Jurassic-Cretaceous Boundary Interval and Correlation with the Geomagnetic Polarity Timescale. Marine Micropaleontology, 14: 153-235. Pop, G., 1994b. Calpionellid evolutive events and their use in biostratigraphy. Rom. J. Stratigraphy, 76: 7-24. Reháková, D. and Michalík, J., 1997: Evolution and distribution of calpionellids- the most characteristic constituents of Lower Cretaceous Tethyan microplankton. Cretaceous Research, 18: 493-504 Remane, J., 1986: Calpionellids and the Jurassic-Cretaceous boundary. Acta Geologica Hungarica, 29: 15-26