The analysis of calpionellid associations from jebels Amar and Jédidi sections in North-Atlasic Tunisia provides, for the first time, a precise biozonation of the Jurassic-Cretaceous transition succession. In the light of the new data obtained and considering recently published results, the age of Upper Jurassic formations is clarified, allowing correlations with the Tunisian 'Dorsale' and the North-South Axis successions. Within the Maghrebides' range, sections from the external zones correlated to the Tunisian successions are quite distinctive from their equivalent in the internal zones. Both have evolved in different palaeogeographic domains related to the early structuration of the northwestern and southwestern Tethys margins.

International audienceSince the 1970s, radiolarians have been used extensively to study and characterize basin successions from the peri-Mediterranean regions. However, no data was ever produced in Tunisia in spite of the occurrence of Jurassic siliceous series previously described by many authors. Our study presents the biostratigraphical results obtained on sections from the Jédidi Formation, which characterizes the basinal series of the Tunisian Trough. We provide new direct age determinations from the base, the middle part and the top of the formation, which ranges from the latest Bajocian–middle Bathonian to the Oxfordian (Tethyan biozones UAZ 5-6 to 8-9). The onset of these radiolarianbearing series of the Tunisian Trough, adjacent to partly coeval Ammonitico Rosso successions, is interpreted as resulting from regional palaeotopographical gradient and palaeoceanographical conditions. Inaccurately interpreted as “true” radiolarites, theses series are different from coeval radiolarites of the Maghrebian “Dorsale calcaire” and from more recent homologous deposits of the Flysch domain. They are instead correlative with series exposed in the Babors and the west-Numidian ranges of northern Algeria that belonged to the same Jurassic North African margin of western Tethys. During Alpine convergence s.l., this external domain was overthrust by nappe complexes of the Maghrebide inner zones and the Flysch domain units

Integrated biostratigraphy and palaeogeographical interpretations of Upper Jurassic-Lower Cretaceous (Callovian-Berriasian) successions are synthesised on the basis of published data and our recent investigations of key sections from the "Tunisian Dorasale" (TD) and "Tunisian Trough" (TT) domains of northern Tunisia. After a revised biostratigraphy had been proposed for the "rosso ammonitico" series, bed-by-bed sampling led to a radiolarian-based first direct dating of the partly coeval biosiliceous series in the TT. Upward within the Jurassic column, an integrated biostratigraphy (ammonites, calpionellids and associated biomicrofacies) allowed precise biozonation for the Kimmeridgian-middle Berriasian interval. The heterogeneity of the Late Jurassic facies in northern Tunisia is the result of two main geodynamical and palaeogeographical events. The first corresponds to the fragmentation of the initial Early Jurassic platform linked to Tethyan rifting. The second event, coeval to the radiolarian-bearing series of the TT, is expressed by a significant deepening in north-west Tunisian palaeoenvironments. During Kimmeridgian -Tithonian times, previous troughs evolved back into carbonate platforms. Within the Maghrebian Belt, Upper Jurassic-lowermost Cretaceous series comprising three major facies groups are easily correlated; minor differences in age are interpreted as due to local tectonic control. Regional correlations confirm, on the one hand, a common geological history for the external segments of the African margin and, on the other hand, a distinct palaeogeographical evolution of the inner domain including the "Dorsales calcaires" and ancient massifs, both being separated by intermediate flysch zone segments.

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International audienceOur study presents preliminary biostratigraphic results from the Jurassic siliceous series of northwestern Tunisia. For the first time, radiolarians are extracted from the Jédidi formation and provide a direct age determination. They are the first radiolarian fauna documented from Tunisia. Two age assignments are comprised within the following intervals: (1) Late Bathonian–Early Callovian, (2) Late Bathonian–Early Oxfordian. These ages are compatible with recent stratigraphic synthesis proposed for the Jurassic series of Tunisia. The data suggest the correlation of the Jédidi formation with siliceous series of Middle–Late Jurassic age from the external zone of the Maghrebides belt rather than with true oceanic units from the Maghrebian flyschs or the internal zones of western Tethys

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