Due to the poor preservation and the calcitization of many radiolarian shells, identification of radiolarian taxa was difficult. In order to improve our results, specimens were extracted after repeated chemical treatments. The softer samples have been washed using standard techniques (soaked in hydrogen peroxide for few hours), then sieved (63 μm – 160 μm – 500 μm). Assemblages have been photographed with a Scanning Electronic Microscope (SEM) at the University of Claude-Bernard Lyon I, France. Abundance of radiolarian was assessed by counting the entire radiolarian content in each washed residue, as well as in thin sections.

Three samples were selected, based on the good preservation of the faunal assemblages. Among 35 radiolarian morphotypes, only 23 species were figured. Biostratigraphic analysis of the fossil record and planktic foraminifer's zones (Fig. 3, and Plate 1 and Plate 2) correlation allow us to distinguish the following three radiolarian assemblages:

1. The sample 37 has provided a diversified radiolarian fauna with the co-occurrence of Dictyomitra montisserei (SQUINABOL), Obeliscoites perspicuus (SQUINABOL), Tubilustrium transmontanum O’DOGHERTY, Dictyomitra gracilis (SQUINABOL), Holocryptocanium barbui DUMITRICA, Stichomitra aff. navalis O’DOGHERTY, Cryptamphorella conara (FOREMAN), Torculum dengoi (SCHMIDT-EFFING), Stichomitra communis SQUINABOL, Torculum coronatum (SQUINABOL), Distylocapsa micropora (SQUINABOL), Patellula verteroensis (PESSAGNO), Godia concava (LI & WU).

Bak (Arthur et al., 1990) established a radiolarian zonation (H. barbui – H. geysersensis) for the Albian–Cenomanian, based on the co-occurrence of Holocryptocanium barbui DUMITRICA, Holocryptocanium geysersensis PESSAGNO, Novixitus weyli SCHMIDT-EFFING, Squinabollum fossile (SQUINABOL), Crymptamphorella macropora DUMITRICA, Hemicryptocapsa tuberosa DUMITRICA. O’Dogherty (O’Dogherty, 1994) proposed a radiolarian zonation for the Albian based on Unitary Associations. He described for the Upper Albian to the base of the Cenomanian Spoletoensis zone divided into three radiolarian subzones: the Romanus, Missilis and Anisa subzones.

The first appearance of B. breggiensis, recorded within Upper Albian basal intervals, coincides with first appearance of radiolarian species Tubilustrium transmontanum O’DOGHERTY, which is confined with the upper part of the Romanus subzone (O’Dogherty, 1994). An assemblage containing Stichomitra navalis and Torculum coronatum was recorded also within this subzone. Babazadeh and de Wever (Babazadeh and de Wever, 2004) described a radiolarian assemblage yielding the co-occurrence of Dictyomitra gracilis, Holocryptocanium barbui and Dictyomitra montisserei and assigned it to Middle–Late Albian age. Nevertheless, the presence of T. dengoi, whose first appearance coincides with the Missilis – Anisa subzones boundary (O’Dogherty, 1994), allows rejuvenating the assemblage age.

2. Radiolarian assemblage recovered from sample 62 is highly diversified at the top of Mouelha blackshales. Likewise, it records an acme of species belonging to Hagiastridae and Cavaspongiidae taxa. This interval shows the co-occurrence of Dispongotripus acutispinus SQUINABOL, Dactyliosphaera maxima (PESSAGNO), Pessagnobrachia sp., Cavaspongia euganea (SQUINABOL), Cryptamphorella conara (FOREMAN), Pessagnobrachia rara (SQUINABOL), Dorypyle communis (SQUINABOL), Pseudodictyomitra paronai (ALIEV), Pseudodictymitra sp., Torculum coronatum (SQUINABOL), Holocryptocanium tuberculatum DUMITRICA, Distylocapsa micropora (SQUINABOL), Obeliscoites perspicuuus (SQUINABOL), Dactyliosphaera acutispina SQUINABOL, Dictyomitra gracilis (SQUINABOL), Thanarla spoletoensis O’DOGHERTY, Dactyliosphaera lepta (FOREMAN), Patellula verteroensis (PESSAGNO), Savaryella novalensis (SQUINABOL), Savaryella quadra (FOREMAN), Pessagnobrachia fabianii (SQUINABOL), Stichomitra communis SQUINABOL, Holocryptocanium barbui DUMITRICA, Xitus aff. spicularius (ALIEV), Torculum coronatum (SQUINABOL), Crolanium aff. spineum (PESSAGNO),

Samples recovered from the succession overlying the Mouelha blackshales show an assemblage composed of Cryptamphorellla conara Dumitrica, Pessagnobrachia sp., and Thanarla spoletoensis O’DOGHERTY, which correspond to the lower part of the Appenninica zone and the middle part of the Anisa subzone of O’Dogherty, 1994. Although the coexistence of D. lepta, Stichomitra communis and Patellula verteroensis is assigned to early Late Cenomanian age (Erbacher, 1998), this assemblage possibly characterizes the Late Albian taking into account the presence of D. maxima whose last occurrence is coeval with the base of Anisa subzone (O’Dogherty, 1994).

3. Sample 68 is characterized by the abundance of cryptocephalic nassellaria (Holocryptocanium). Moreover, we notice the first occurrence and bloom of Mallanites triquetrus. This interval shows the co-occurrence of Xitus mclaughlini (PESSAGNO), Hexapyramis pantanelli SQUINABOL, Mallanites triquetrus (SQUINABOL), Thanarla spoletoensis O’DOGHERTY, Dictyomitra montisserei (SQUINABOL), Godia concava (LI & WU), Cryptamphorella conara (FOREMAN), Torculum coronatum (SQUINABOL), Cavaspongia euganea (SQUINABOL), Distylocapsa micropora (SQUINABOL), Dactyliosphaera maxima (PESSAGNO), Holocryptocanium barbui DUMITRICA, Dactyliodiscus longispinus (SQUINABOL), Dispongotripus acutispinus SQUINABOL.

The radiolarian assemblages are characterized by high diversity rate thus making it possible to bring additional details about the J.Srassif pelagic successions age. It is worth noting in these assemblages, the abundance of cryptothoracic nassellaria Holocryptocanium barbui, which seems to be ubiquistic specie in these successions.

The correlation of these assemblages with biozonations based on Unitary Association established by O’Dogherty (1994) allows assigning these levels to the interval comprised between UAZ 13 and UAZ14 of Late Albian age.

Radiolarian-bearing carbonate pelagic successions investigated in the Srassif section might not be considered as true radiolarite successions in the absence of true chert layers. The presence of interstratified limestone beds in the Jurassic radiolarian-bearing Jedidi Formation has been mentioned by Cordey et al., 2005 who emphasized the absence of true ‘radiolarites’ in Tunisia. They considered that such an occurrence of carbonates is indicative of a depocenter, although basinal, located above the CCD.

The peculiar co-occurrence of organic rich deposits (blackshales) and radiolarian blooms discovered in the studied section are to be outlined. Radiolarian-bearing deposition associated with blackshales has previously been observed within cherty limestones and/or bituminous deposits (Bogdanov and Vishnevskaya, 1992, Danelian et al., 2002, De Wever and Baudin, 1996, Erbacher et al., 1996, Musavu-Moussavou and Danelian, 2006, Robaszynski et al., 1993 and Salvini and Marcucci Pesserini, 1998).

During the Mid-Cretaceous, organic rich-sediments were widely spread in Mediterranean Tethys basins as a result of Oceanic Anoxic Event (OAE) (Arthur et al., 1990 and Jenkyns, 1980). Geochemical and biostratigraphic data recovered from Albian pelagic sequences of northwestern Tunisia have yielded the records of OAE1b, OAE1c and OAE1d, which can be correlated with time-equivalent of northern Tethyan margins (Ben Fadhel et al., 2010). Basins configuration inherited from Early Jurassic rifting phase affecting the Tunisian platform (Boughdiri et al., 2007) were enhanced during the Mid-Cretaceous period by halokinetic dynamics, which have probably preserved basins geometry during post-rift period (Karakitsios, 1995). The resulting basins were filled then by organic-rich deposits, which were coeval with a sea level rise and hypersilicious period characterizing the Tethys Realm during the Albian–Cenomanian (Racki and Cordey, 2000).

During the early Late Albian, the Srassif basin (Fig. 4) was the site of upwelling currents, an elevated radiolarian productivity triggered by silica enrichment, provided by halokinetic-induced hydrothermalism. This fact is suggested by depleted O¹⁸ values obtained from authigenic carbonate nodules enclosed within black shale levels (unpublished data), which are indicative of carbonate precipitation derived from hot and meteoric hydrothermal waters (Hemadi, 1987 and Layeb et al., in press). A high nutrient supply was provided by the continental runoff of the emerged area of Kairouan Island (M’rabet, 1981).

The decrease of S/N ratio (Fig. 2) can be interpreted as a result of a semi-enclosed basin structured with tilted blocs, leading to the widespread of the oxygen minimum zone (OMZ). Thus, assemblages dominated by nassellaria are indicative of deep-dwelling taxa, eutrophic conditions and vigorous upwelling regime (Premoli Silva et al., 1999). The Late Albian period is characterized by a more oxygenated and oligotrophic environment, following changes in basin configuration and the sudden collapse of the stratified water column, may be responsible for radiolarian diversification and the increase of S/N ratio.

Particular attention was paid to the interstratifications of radiolarian-rich layers into rhythmic limestone marls couplet, possibly reflecting a control of radiolarian productivity by Milankovitch cycle parameters variation. According to Ogg et al., 1992, such a trend is responsible for an abrupt change in tropical upwellings and periodically enhanced radiolarian production.

Radiolarians from Mid Cretaceous (Upper Albian) pelagic successions of the Tunisian trough are documented for the first time and used to propose age correlations. Further investigations are needed to enhance resolution of the biochronology as well as paleoecological interpretations, as faunal abundances may be related to taphonomic artefacts. However, it seems that the Srassif paleobasin recorded an anoxic environment and an associated biotic response, controlled by sea level rise and extensive tectonics, which prevailed during the Albian. Nutrient availability and radiolarian high productivity were possibly triggered by the continental runoff of the Kairouan Island, the upwelling currents and Si enrichment of the seawater probably related to halokinetic-induced hydrothermalism. Halokinetic dynamics were responsible for the preservation of basin configuration inherited from rifting phase. This could have favored the widespread of anoxic conditions and the deposition of organic-rich and radiolarian-bearing sediments.