1 Introduction

Depuis leur découverte dans les années 60, les séries siliceuses à radiolaires du Nord-Ouest tunisien ne bénéficiaient pas jusqu'à présent de contrôle stratigraphique direct, celui-ci étant basé sur des corrélations lithologiques ou par encadrement. Nous présentons ici les premiers résultats biostratigraphiques obtenus sur ces séries, en documentant les faunes de radiolaires extraites de la coupe-type de la formation Jédidi (Figs. 1 et 2). Grâce à elles, nous proposons de préciser les corrélations stratigraphiques ainsi que le contexte paléogéographique associé à ces séries.

2 Cadre stratigraphique et études antérieures

3 Localisation et description de la coupe-type de la formation Jédidi

La coupe-type de la formation Jédidi est située à 10 km à l'ouest de la ville de Mejez el Bab, dans la partie orientale du jebel Sidi Jédidi (Fig. 1). Au-dessus des calcaires noduleux gris rougeâtre (formation Kef el Orma, reconnue par [23] dans le secteur d'étude), la coupe peut être subdivisée en trois unités (Fig. 3) :

4 Assemblages de radiolaires et âges

Des tentatives précédentes de datation de la formation Jédidi n'ont pas conduit à l'extraction de radiolaires identifiables (R. Enay, comm. pers., 2004). Nous avons intégré dans notre approche une sélection d'échantillons sur le terrain [7], puis un traitement chimique HF de faible concentration (5 %). Jusqu'à présent, deux niveaux ont livré des assemblages diagnostiques. MB4 (Figs. 3 et 4) contient : Cinguloturris carpatica Dumitrica, Palinandromeda podbielensis (Ozvoldova), Parahsuum sp., Parvicingula sp. aff. blackhornsensis Pessagno et Whalen, Protunuma sp., ?Sethocapsa sp., Stichocapsa robusta Matsuoka, Transhsuum brevicostatum (Ozvoldova), Transhsuum maxwelli (Pessagno), Tricolocapsa plicarum Yao, ?Tricolocapsa sp. Ces morphotypes sont présents dans la zonation de Baumgartner et al., basée sur la méthode des associations unitaires [2]. Ils sont corrélés à l'UAZ 7, d'âge Bathonien supérieur ou Callovien inférieur. Le second échantillon productif, MB18, comprend : Protunuma sp., Sethocapsa sp., Transhsuum maxwelli (Pessagno), Williriedellum sp. A sensu Matsuoka, et Wrangellium sp. ; cet assemblage peut être attribué aux UAZ 4–8. La présence d'UAZ 7 dans l'échantillon MB4 permet de réduire l'attribution de MB18 aux UAZ 7–8, correspondant à un âge compris dans l'intervalle Bathonien supérieur–Oxfordien inférieur. Ce sont les premières datations directes obtenues sur des séries siliceuses tunisiennes. Ce sont aussi les premiers radiolaires documentés de Tunisie. Les âges obtenus sont, pour l'instant, compatibles avec les évaluations antérieures effectuées par encadrement à partir des formations sous-jacente Kef el Orma et sus-jacente Beni Kleb, d'âge Bajocien supérieur à Oxfordien supérieur respectivement [12,22,23]. Il sera cependant nécessaire de dater plus précisément la base et le sommet de la formation Jédidi.

5 Discussion

Since their discovery in the 1960s, radiolarian-bearing siliceous units of northwestern Tunisia underwent several stratigraphic correlations. These series had no direct age attribution so far, these being based on lithological correlations and/or on biostratigraphic data obtained from underlying and overlying units (see below). We present here the first direct age controls on these siliceous units by documenting radiolarian fauna extracted from the type-section of the Jédidi formation (Mejez el Bab region) (Figs. 1 and 2). We propose to discuss previous correlations, as well as place these biosiliceous successions in their regional, west-Tethyan palaeogeographic setting.

For a complete overview of Jurassic stratigraphy from the Tunisian Atlas, see [22–26]. In the Jurassic series, the facies heterogeneity observed in the Early Jurassic is accentuated during the Middle and Late Jurassic (Fig. 2). The Tunisian ‘Dorsale’ documents platform (Zaghouan) and slope-type deposits (Hammam-Lif), whereas North-Atlasic Tunisia comprises basinal successions (Tunisian trough), where the Jédidi formation is represented (H, Fig. 2). Originally, the siliceous successions from Tunisia were broadly attributed to the Lower Cretaceous [5,6,21]. From jebels Jédidi, Chaâbane and Tamrhoura, exposures of siliceous series were discovered by Pini and Salaj [17], who identified a thick succession (70 m) of reddish limestones yielding chloritized radiolarians attributed to the Callovian–Oxfordian interval by comparison with known successions of northern Tunisia. A lateral equivalent to these biosiliceous deposits was discovered by Glaçon and Rouvier [11] in Thuburnic area (region of Ghardimaou, northwestern Tunisia). Later on, Rouvier [19] assigned the ‘green radiolarian jaspers’ to the Kimmeridgian–Tithonian interval. The first attribution to ‘true’ radiolarian chert for these siliceous successions is from Alouani et al. [1] who inferred a Late Oxfordian–Lower Kimmeridgian age range, by comparison with the stratigraphy of Ammonitico-Rosso unit (Zaress formation) from the Tunisian ‘Dorsale’, as well as the occurrence of Kimmeridgian ammonites within overlying limestones [10]. Then Peybernès et al. [16] described at Jebel Ammar (region of Sidi Thabet, northern Tunisia) alternations of red to green pelites and limestones-marls that could laterally grade into red/green radiolarian-bearing pelites and radiolarites called ‘Jédidi facies’, previously described [1]. Peybernès et al. proposed a Late Kimmeridgian–Late Tithonian age [16, (fig. 2, p. 158)]. Taking into account their new attribution of the underlying nodular limestones to the Kef el Orma Fm (rather than to the Zaress Fm as proposed in correlations by [16]) and on the basis of new ammonite fauna from adjacent series gathered at J. Ammar and Thuburnic, Soussi et al. [23] revised previous correlations and assessed a Late Bajocian–Late Oxfordian age for the siliceous series of northwestern Tunisia. Soussi [22] defined a new formation at the type-locality of Jebel Jédidi-Chaâbane, including exposures from jebels Tamrhoura, Oued Maâden (or Thuburnic) and Ammar. All these biochronological attributions were based on indirect evidence.

The studied section is located 10 km to the west of the town of Mejez el Bab, near highway No. 6 linking Tunis to Ghardimaou (Fig. 1). Between Mejez el Bab and Oued Zarga, a gravel road heads south to Sidi Ahmed Jédidi, then to the southeastern flank of jebels Sidi Jédidi and Chaâbane, where the section is exposed (Fig. 1B). Within the range, Jurassic units are bounded to the south by the Téboursouk fault (Fig. 1A), cropping out along a NE–SW belt parallel to large exposures of Triassic salt formations bordering the fault. Lower Jurassic limestones, presently worked in a gravel quarry, bound the Jurassic southernmost exposures. Jurassic units extend to the north as the base of a wide syncline where are identified subvertical to slightly reversed Middle and Late Jurassic strata. These strata grade into Cretaceous (vast plain crossed by Ghardimaou highway) and Eocene successions (Toukabeur–Chaouache area).

The Jédidi formation type-section can be subdivided into three lithological units (Fig. 3). The base of the formation overlies a layer of pink–grey nodular limestone containing belemnites (Formation Kef el Orma), as recognized by [23] in the study area and maintained in this paper:

• – unit U1, dominantly composed of thick red pelites (locally not exposed); this unit comprises two beds of thinly laminated siliceous limestone, dark brown to brick red in colour with tints of yellow and green. The siliceous content varies along beds. Locally, these beds display potentially radiolarian-rich layers;
• – unit U2, which differs from the underlying unit by a lesser amount of pelites and siliceous facies. Calcareous beds are dominant and are harder, thicker and darker than those of U1;
• – unit U3, resembling U1. Limestone beds are relatively thin, red or green in colour, along with decreasing siliceous content towards the top of the formation, grading into marl-limestone alternations of the Beni Kleb formation [15].

Although the Jédidi formation comprises several layers of primary radiolarian-bearing biogenic chert, we consider it as improperly described in the literature as ‘radiolarites’ [1] or ‘pseudoradiolarites’ [22]. True radiolarites (also classically named ‘radiolarian chert’, ‘ribbon chert’, or ‘bedded chert’) are typically composed of alternations of chert-shale couplets devoid of limestone layers, whereas the Jédidi formation comprises several intercalations of limestone beds and marls in which the siliceous component is present (as shown by occurrence of radiolarians) but obviously not dominant. Although some beds are composed of biosiliceous jasper, other apparent ‘radiolarite’ beds are actually silicified limestone layers (Fig. 3). Occurrence of calcium carbonate is observed within the entire succession (units 1 to 3). The presence of radiolarians is significant, but cannot be considered the dominant lithological component, as is the case in a true primary biosiliceous succession. There is, to our knowledge, no true radiolarite formation in Tunisia. In summary, the so-called ‘radiolarites’ of the Jédidi formation should instead be described as ‘siliceous series’. In terms of depositional environments, the occurrence of carbonates within these siliceous successions suggests that related depocentres, although basinal and potentially deep, were located above the CCD and probably linked with a high siliceous productivity. Such successions are usually found in bathyal environments.

Previous attempts for extracting radiolarians from the Jédidi formation were undertaken without success (R. Enay, pers. comm., 2004). Our approach included field selection of appropriate samples [7], then processed in the HF laboratory of the University Claude-Bernard–Lyon-1, France. As is usually applied to biosiliceous samples, low concentration HF etching procedures were undertaken. So far, two layers released diagnostic radiolarians. Sample MB4 (Fig. 3) from the base of U2 contains the following association (Figs. 3 and 4): Cinguloturris carpatica Dumitrica, Palinandromeda podbielensis (Ozvoldova), Parahsuum sp., Parvicingula sp. aff. blackhornsensis Pessagno and Whalen, Protunuma sp., ?Sethocapsa sp., Stichocapsa robusta Matsuoka, Transhsuum brevicostatum (Ozvoldova), Transhsuum maxwelli (Pessagno), Tricolocapsa plicarum Yao, ?Tricolocapsa sp. These identified morphotypes are present within the Unitary Association Zonation of Baumgartner et al. [2], showing a common occurrence within the UAZ 7 of Late Bathonian or Early Callovian age [2]. Although we do not use the following taxon in our correlation due to slight morphological differences, Parvicingula blackhornsensis Pessagno and Whalen, reported and described from the Snowshoe formation, Oregon [14] is of Late Bathonian–Callovian age pro parte, an age consistent with our age establishment.

The second productive sample MB18, collected from the upper part of U2, contains (Figs. 3 and 4): Protunuma sp., Sethocapsa sp., Transhsuum maxwelli (Pessagno), Williriedellum sp. A sensu Matsuoka and Wrangellium sp. Zonation [2] shows that this association is correlative with UAZ 4–8. The occurrence of UAZ 7 in MB4 establishes that MB18 can be attributed to UA7–8, corresponding to an age placed within the interval Late Bathonian–Early Oxfordian.

These age data provide, for the first time, a direct age control on the Jurassic radiolarian-bearing siliceous strata from Tunisia. As indicated in the stratigraphic framework above, the age of the Jédidi formation was previously assessed indirectly from the underlying Kef el Orma and the overlying Beni Kleb formations, providing a Late Bajocian to Late Oxfordian age range [12,22,23]. Our data are compatible with this assessment. More diagnostic layers need to be found in the future, for instance radiolarian-bearing beds near the base and the top of the Jédidi formation.

Within the Tunisian ‘Dorsale’, the transition between Middle and Late Jurassic successions is characterized by the onset of ‘Ammonitico Rosso’ facies, usually of Middle Callovian (Anceps zone) up to Late Oxfordian (Bimammatum and/or Planula zones) age range. At Bou Kornine du Pont de Fahs, Raouas-Beni Kleb, Aziz, Oust and Ammar jebels, these series are instead represented by a thick succession of greenish or dark-red limestones (‘faciès ocellés’ [3] or Fahs formation sensu Soussi [22]). The siliceous series of the Jédidi formation probably represent for their upper part a lateral equivalent, within the Tunisian trough, of these carbonate successions. The lower part of the formation can also be correlated to part of, or the whole of Bent Saïdane Fm (Fig. 2).

In their discussion, Alouani et al. [1] have interpreted the siliceous Jédidi formation and associated Jurassic basinal units as the first pelagic deposition following a general collapse of the Tunisian trough area. The direct, although partial, age assignment of the siliceous deposits of the Jédidi formation provides an argument for integrating the pelagic deposits of the Tunisian trough into the biosiliceous series identified in the external Maghrebide Range (e.g., Kabylias located to the northwest of Constantine, Algeria) to which the same Middle-?Late Jurassic age was assigned [9]. Although the conditions of radiolarian palaeoecology are still a matter of debate (see [18] and references therein), this period witnesses a combination of high productivity siliceous depocentres with a general deepening of segments of west Circumtethyan passive margins. The absence of true Jurassic radiolarites is to be paralleled with the absence, in Tunisia, of true ophiolites, suggesting a lack of true oceanization found instead within the Maghrebian flyschs unit [13] and in the internal zones of western Tethys [8].

Appropriate radiolarian detection and extraction techniques show their potential for directly dating Jurassic siliceous series of Tunisia. Although more work is probably needed, the type-section of the Jédidi formation released so far two assemblages of Late Bathonian–Early Callovian and Late Bathonian–Early Oxfordian ages respectively. These results show that the Jédidi formation is partly coeval to radiolarite successions from the Mediterranean region. However, they suggest that the Jurassic siliceous series of the Tunisian trough are to be correlated with units from the external zone of the Maghrebides belt rather than true oceanic units from western Tethys.