Integrated stratigraphy (radiolarians and calcareous nannofossils) of the Middle to Upper Jurassic Alpine radiolarites (Lombardian Basin, Italy): Constraints to their genetic interpretation

New biostratigraphic data obtained by integrating radiolarians and calcareous nannofossils document a depositional history for the Middle to Upper Jurassic radiolarites of the Italian Southern Alps characterized by strong fluctuations in the sedimentation rates, associated to shifts from gravity-driven to hemipelagic sedimentary process. We studied the radiolarites in four sections, located in different settings of the Lombardian Basin, a rifted basin of the northern Adriatic plate. The sections studied were described and sampled in detail to obtain data on their lithology (using facies and microfacies analysis and calcimetry) and their nanno- and microfossils content (especially radiolarians, and calpionellids). An almost complete record of radiolarians was detected throughout the Selcifero Lombardo Formation, so that the radiolarian zonation (UAZs) for the Middle to Upper Jurassic interval, partially integrated with calcareous nannofossil and calpionellid biostratigraphic data, was successfully applied. Accordingly, the lower member (Radiolariti) correlates with the lower Bathonian (UAZ 5) to the Kimmeridgian (UAZ 11) interval, and the upper member (Rosso ad Aptici) displays an age spanning from the Kimmeridgian to the base of Upper Thitonian (from UAZs 10–11 to nannofossil NJK zone). We interpret the radiolarites as pelagic sediment relatively enriched in radiolarian during a long time span of basin starvation, due to the near absence of periplatform exportation. If we except Bathonian, a period of relatively higher sedimentation rates, the radiolarites are characterized by very low sedimentation rates, implying equally low radiolarian productivity. Therefore, it is not necessary to claim for high Tethyan water fertility during Middle to Late Jurassic, even if this latter condition likely played a role in the production/exportation of the carbonate platforms.