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2 Volumina Jurassica

2 2006

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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

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Chiari M. , Cobianchi M. , Picotti V.

tom 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.

The Jurassic biosiliceous sediments from the Sicanian basin in Western Sicily: new data

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Chiari M. , Di Stefano P. , Parisi G.

tom Vol. 4, no. 1

153-154

Detailed stratigraphic analyses on Jurassic radiolarites from centralwestern Sicily provide new data on the onset of the biosiliceous sedimentation in the Sicanian Basin. Studies were performed in two structural units of the Maghrebian chain that crop out south of Corleone, i.e. the Monte Barraců and the Campofiorito units. The radiolaritic unit at Monte Barraců consists of radiolarian-bearing cherty calcilutites alternating with cherts and levels of siliceous marls. The lower zone is green-grey coloured, while the upper one is red. This unit covers uppermost Triassic – Lower Jurassic cherty calcilutites through a large-scale stepped unconformity of submarine origin. The upper boundary (affected, in places, by synsedimentary deformations) is transitional to a thin unit of red wackestones with Saccocoma grading upwards to pink to whitish calpionellid cherty wackestones. The radiolarians from samples collected in the radiolaritic unit (lower zone) indicate a Late Bajocian to latest Bajocian – Early Bathonian age (UAZ. 4-5) for the presence of Ares cylindricus flexuosus (Takemura) and Parahsuum(?) magnum Takemura. The transitional zone between green and red radiolarites contains a radiolarian assemblage which indicates a Middle-Late Oxfordian age (UAZ. 9) for the presence of Emiluvia pentaporata Steiger & Steiger and Saitoum levium De Wever. Samples collected in the uppermost zone yielded Podocapsa amphitreptera Foreman and Hexasaturnalis minor (Baumgartner) that indicate a Middle-Late Oxfordian to Late Kimmeridgian – Early Tithonian age (UAZ. 9-11). In the Campofiorito section the radiolaritic unit covers a 20 m thick unit of basaltic pillow lavas and hyaloclastites, resting in turn on Lower Jurassic cherty calcilutites and marls. The lithologies of the radiolaritic unit are well comparable to the Monte Barraců ones. In this section samples from the basal zone could be dated as Early-Middle Bajocian to Middle Bathonian (UAZ. 3-6) for the coexistence of Paronaella kotura Baumgartner with Unuma echinatus Ichikawa and Yao. In the upper part of the section a poor preservation of radiolarians in the studied samples prevents further biostratigraphic evaluations. The new data fit well the biostratigraphic evaluations already available from the adjacent structural units of Monte Genuardo and Giuliana. They indicate that the onset of the biosiliceous sedimentation, in a large sector of the Sicanian Basin, can be “bracketed” between Late Bajocian and Early-Mid Bathonian times.