New bio-magnetostratigraphy for the Upper Jurassic and lowermost Cretaceous of the Southern Alps (Italy)

Calcareous nannofossil and magnetic stratigraphies of several sections spanning the Middle-Upper Jurassic to Lowermost Cretaceous have been studied in the Lombardian Basin (Torre di Busi, Colle di Sogno) and Trento Plateau (Colma di Vignole; Foza; Frisoni; Sciapala; Bombatierle; Passo Branchetto) in order to integrate calcareous nannofossil events with the polarity chron sequence and, where available, with calpionellid biostratigraphy. Calcareous nannofossil biostratigraphy has been carried out on smear slides prepared from un-heated end-pieces of the same samples used for magnetostratigraphy. Similar variations in nannofloral abundance and composition have been documented in all sections. In the Jurassic/Cretaceous boundary time interval, all previously known calcareous nannofossil Zones and corresponding Subzones (Bralower et al., 1989) have been clearly recognized. In the Callovian-Kimmeridgian interval, a tentative integration of biostratigraphy (De Kaenel et al., 1996; Bown, 1998; Cobianchi, 2002) with polarity chrons is proposed. The magnetic stratigraphies span the CM16R (late Berriasian) to the CM25 (Oxfordian) interval. The identification of polarity zones is based on polarity-zone pattern fit, and, where available, on the previously established correlations of polarity chrons to nannofossil events/zones (Channell & Grandesso, 1987; Channell et al., 1987; Weissert & Channell, 1989; Bralower et al., 1989). Bown, P.R. and Cooper, M.K.E. 1998. Jurassic. In: P.R. Bown (EDS.). Calcareous nannofossil stratigraphy. British Micropalaeontological Society Publications Series. Kluver Academic Publishers, Dordrecht, Boston, London, pp. 34-85. 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. Channell, J.E.T., and Grandesso, P. 1987. A revised correlation of Mesozoic polarity chrons and calpionellids zones. Earth Planet. Sci.Lett., 85: 222-240. Channell, J.E.T., Bralower, T.J. and Grandesso, P., 1987. Biostratigraphic correlation of M-sequence chrons at Capriolo and Xausa (S. Alps, Italy). Earth Planet. Sci.Lett., 85: 203-221. Channell, J.E.T., Erba, E., Nakanishi, M. and Tamaki, K. 1995. Late Jurassic-Early Cretaceous time scales and oceanic magnetic anomaly block models. Geochronology Time scale and global stratigraphic correlation 54: 51-63. Cobianchi, M. 2002. Calcareous nannofossils from the Middle and Upper Jurassic of the Belluno Basin (Southern Calcareous Alps). Att.Tic.Sci.Terrra, 43: 3-24. De Kaenel, E., Bergen, J.A. and Perch-Nielsen, K. 1996. Jurassic calcareous nannofossil biostratigraphy of Western Europe. Compilation of recent studies and calibration of bioevents. Bull. Soc. geol. France, 167(1): 15-28. Weissert, H. and Channell, J.E.T. 1989. Tethyan carbonate carbon isotope stratigraphy across the Jurassic/Cretaceous boundary: an indicator of decelerated global carbon cycling? Paleoceanography, 4(4): 483-494.