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

Chiari M., Cobianchi M., Picotti V.

Volumina Jurassica

2006

Vol. 4, no. 1

152-153

New biostratigraphic data obtained by integrating radiolarian and calcareous nannofossil stratigraphy document a depositional history for the Middle to Upper Jurassic radiolarites of the Southern Alps characterized by strong fluctuations in the sedimentation rates, associated with shifts from turbiditic to hemipelagic sedimentary processes. The radiolarites were studied in four sections located in different settings of the Lombardian Basin, a rifted basin of the northern Adriatic Plate, where this interval is known as Selcifero Lombardo Formation. The latter consists of two members, the Radiolarite and the Rosso ad Aptici, respectively, and passes, gradually, upwards to the Maiolica Formation. The sections studied were sampled in detail to obtain data on their lithology (using facies and microfacies analyses 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 (UAZones) for the Middle to Upper Jurassic interval, partially integrated with calcareous nannofossils and calpionellid biostratigraphic data, was successfully applied. Accordingly, the Radiolarite unit correlates with the Lower Bathonian (UAZ. 5) to the Kimmeridgian (UAZ. 11) interval, and the Rosso ad Aptici member displays an age spanning from Kimmeridgian to the base of Upper Tithonian (from the UAZ. 10-11 to nannofossil NJK zone). The base of the Maiolica Formation is assigned to Upper Tithonian, in agreement with the previous authors. We interpret the radiolarites as pelagic sediments relatively enriched in radiolarian during a long time span of basin starvation, due to the near absence of periplatform exportation. Because the very low sedimentation rates equally imply low radiolarian productivity, it is not necessary to claim for high water fertility during Middle to Late Jurassic, even if this latter condition likely played a role in development of the carbonate platforms.