Tithonian–Berriasian calpionellid and calcareous dinocyst biostratigraphy, and microfacies in the Torre de’ Busi section (Lombardian Basin, northern Italy)

Tithonian and lower Berriasian pelagic carbonate deposits of the Torre de’ Busi section (Lombardian Basin, northern Italy) were studied in detail in terms of their microfacies, calpionellid and calcareous dinocyst biostratigraphy, and integrated with calcareous nannofossils and magnetic stratigraphy. Calpionellid zonation was revised, modifying the archival, unpublished data. Three calpionellid zones (Chitinoidella, Crassicollaria and Calpionella) and six subzones (Longicollaria dobeni, Chitinoidella boneti, Tintinnopsella remanei, Crassicollaria intermedia, Calpionella alpina and Remaniella ferasini) were identified, spanning from the topmost lower Tithonian to the lower Berriasian. Correlations between the calpionellid zones and magnetozones combined with calcareous nannofossil zones, are consistent with most Tethyan sections. The base of the C. alpina Subzone is quantitatively illustrated and based on high-resolution frequency analysis of calpionellid taxa. Calcareous cysts were studied in the Torre de’ Busi section for the first time. Five calcareous dinoflagellate zones were distinguished. The lower Tithonian contains the Committosphaera pulla–Carpistomiosphaera tithonica, Parastomiosphaera malmica and the lowest part of Colomisphaera tenuis zones. In the upper Tithonian the Colomisphaera tenuis, Colomisphaera fortis and the lower part of Stomiosphaerina proxima zones were documented. The upper part of the St. proxima Zone is present in the lower Berriasian. The calcareous dinocyst zones and their correlation with magnetozones align well with available Tethyan records. The Torre de’ Busi section represents a fully pelagic environment with a relatively large amount of radiolarians and sponge spicules. A major microfacies change, from radiolarian to calpionellid-dominated microfacies, occurs in the upper Tithonian (base of the Cr. intermedia Subzone, base M19r), close to the base of the Maiolica Formation. This level is characterized by a decrease in calcareous dinocyst frequencies, and an increase in calpionellids, which correlates with a major speciation phase of nannoconids.