Combined magnetostratigraphic, palaeomagnetic and calpionellid investigations across Jurassic/Cretaceous boundary strata in the Bosso Valley, Umbria, central Italy

The principal aim of a detailed magnetostratigraphic and micropalaeontological investigation of the Jurassic/Cretaceous (J/K) boundary limestones in the basal portion (39 m) of the Bosso Valley section in Umbria, central Italy, was to determine precisely the boundaries of magnetozones and narrow reverse subzones, and to find a correlation between magnetostratigraphic data (reflecting global events) and calpionellid zonation. Two reverse subzones were detected in magnetozones M20n and M19n. These are in the same positions relative to the magnetozones above and below as the reverse subzones at the recently studied locality of Brodno, near Žilina, West Slovakia. Both the Brodno and Bosso sections so far represent the only magnetostratigraphic profiles across J/K boundary strata in continent-based outcrops in the Tethyan realm displaying both reverse subzones, which correlate well with analogous subzones in the M-sequence of marine magnetic anomalies. The samples of the upper Tithonian and lower Berriasian limestones studied are characterized by a three- or even four-component remanence, with the carrier of palaeomagnetic directions being the C-component, separated by multi-component analysis after progressive thermal demagnetization in the interval of ca. 400°C to the magnetite unblocking temperature (around 550°C). Other aspects of the use of magnetization changes in limestones for possible correlation are also discussed. In addition, the existence of a prominent post-tectonic component of remanent magnetization is indicated. This imprint, recorded in the Bosso Valley and elsewhere in the Tethyan realm, dates most probably to the Neogene and is worthy of further investigation.