Radiolarian Age Constraints of Mid-Cretaceous Black Shales in Northern Tunisia

Mid-Cretaceous pelagic deposits outcropping in Northern Tunisia include organic-rich beds locally associated with high abundance of radiolarian microfauna, which are interpreted as the signature of the two global oceanic anoxic events OAE1 and OAE2 (Talbi, 1991; Saidi & Belayouni; 1994; Caron et al., 1999; Amedro et al., 2005, Heldt et al., 2008; Khazri et al., 2009; Soua et al., 2009; Robascynski et al., 2010; Ben Fadhel et al., 2011). Several studies have stated the close association between organic-rich sediments and radiolarian in the Atlantic and Tethyan realms (Marcucci-Passerini et al, 1991; O’Dogherty, 1994; Erbacher & Thurow, 1998; Danelian et al., 2004, 2007). In North African margins, the radiolarian biostratigraphy have focused upon radiolarianbearing Jedidi Formation which has been thoroughly discussed by Cordey et al, (2005) and Boughdiri et al, (2007). The first attempts at dating radiolarian series in Northern Tunisia show that radiolarian associated with carbonate-siliceous beds, have yielded useful diagnostic radiolarian assemblages (Cordey et al., 2005; Soua et al., 2006; Ben Fadhel et al., 2010). Albian and Cenomanian-Turonian black shales of Northern Tunisia were considered to have good generative oil source rock (Layeb, 1990; Saidi & Belayouni, 1994; Bechtel et al., 1998; Ben Fadhel et al., 2011). In this overall context, the restudy and high-resolution biostratigraphy of Albian black shale beds of Lower Fahdene Formation and C/T cherty beds of organic-rich Bahloul Formation outcropping in Northern Tunisia domain have yielded well-preserved and age-diagnostic radiolarians species. The aim of this paper is to: 1) give new illustrations of radiolarian taxa recovered from albian pelagic deposits of north african margins 2) establish a direct age of black shales using radiolarian assemblages 3) compare the radiolarian assemblages with time equivalent investigated in tethyan and east Pacific domains.


Introduction
Mid-Cretaceous pelagic deposits outcropping in Northern Tunisia include organic-rich beds locally associated with high abundance of radiolarian microfauna, which are interpreted as the signature of the two global oceanic anoxic events OAE1 and OAE2 (Talbi, 1991;Saïdi & Belayouni;1994;Caron et al., 1999;Amédro et al., 2005, Heldt et al., 2008Khazri et al., 2009;Soua et al., 2009;Robascynski et al., 2010;Ben Fadhel et al., 2011). Several studies have stated the close association between organic-rich sediments and radiolarian in the Atlantic and Tethyan realms (Marcucci-Passerini et al, 1991;O'Dogherty, 1994;Erbacher & Thurow, 1998;Danelian et al., 2004Danelian et al., , 2007. In North African margins, the radiolarian biostratigraphy have focused upon radiolarianbearing Jedidi Formation which has been thoroughly discussed by Cordey et al, (2005) and Boughdiri et al, (2007). The first attempts at dating radiolarian series in Northern Tunisia show that radiolarian associated with carbonate-siliceous beds, have yielded useful diagnostic radiolarian assemblages (Cordey et al., 2005;Soua et al., 2006;Ben Fadhel et al., 2010). Albian and Cenomanian-Turonian black shales of Northern Tunisia were considered to have good generative oil source rock (Layeb, 1990;Saidi & Belayouni, 1994;Bechtel et al., 1998;Ben Fadhel et al., 2011). In this overall context, the restudy and high-resolution biostratigraphy of Albian black shale beds of Lower Fahdene Formation and C/T cherty beds of organic-rich Bahloul Formation outcropping in Northern Tunisia domain have yielded well-preserved and age-diagnostic radiolarians species. The aim of this paper is to: 1) give new illustrations of radiolarian taxa recovered from albian pelagic deposits of north african margins 2) establish a direct age of black shales using radiolarian assemblages 3) compare the radiolarian assemblages with time equivalent investigated in tethyan and east Pacific domains.

Geological setting
The area of investigation is located in Northern Tunisia (Fig. 1). Three sections are selected in this study on the basis of occurrence of organic and radiolarian-rich layers: Aptian -Albian unconformity in outcrops (Ouahchi et al., 1998). (4) During the Albian, the geodynamic evolution is marked by the sealing of lower Cretaceous structures during an extensional phase that persisted to form graben systems promoting organicrich and siliceous strata deposition throughout upper Cenomanian to Lower Turonian times (Soua et al., 2009). The major faults in this area are represented by N140° and N70° trend features. The Bahloul thickness is significantly variable in this area. It may varies from 10m to 40m in thickness (Layeb & Belayouni, 1989;Soua & Tribovillard, 2007). Uniquely, in this area, the top of the Bahloul represents many cenomanian olistolith levels (Soua et al., 2006) marking syndepositional tectonic activities (Turki, 1985).  Chikhaoui et al., 1991;Meddeb, 1986)

Jebel Garci section
The condensed section of Jebel Garci (Fig. 2) begins with orbitolinids-rich green to gray clay alternating with discontinuous sandy limestone beds which are attributed to the Hameima Formation. The clay intervals have also provided fragments of rudist and bryozoans (GA1). The upper part contains olistolites deposits that gradually pass to a reefal limestone which is outlined at the top by burrowed hardground. The next successions (GA9 -GA23) which correspond to the "Allam" Member consist of centimeter-thick grey to dark laminated limestone bed and organic-rich black marl intervals.

Jebel Srassif section
The base of Jebel Srassif section (Fig. 3) which constitutes the "Marnes Moyennes" Member, consists of 130 meter-thick alternations of grey marl and limestone, which become dark and laminated at the top. A cyclic marl/limestone bundles (10m) can be distinguished having an organic and radiolarian-rich mudstone texture. It is capped by a thick organic-rich limestone bed (20 m) characterized by bituminous odor and yellowish color in patina. This level corresponds to the Mouelha member (Burollet, 1956). The 40 meters of the top consist of an alternation of grey limestones and dark grey-ochre marls yielding septarian nodules characterizing the Defla member. They are overlain by a succession of lenticular limestone beds and grey marl of Azreg member (50 m). 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 Plates 1 and 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 (

Oued Kharroub section
The outcrop (Fig. 4) is composed mainly by dark clayey limestone and organic-rich black shales with abundant planktic foraminifera. These organic-rich deposits include siliceous beds with abundant radiolarians, an equivalent to "Livello Bonarelli" bed marker (Marcucci Passerini et al, 1991;Salvini and Marcucci Passerini, 1998;Premoli-Silva et al, 1999;Scopelliti et al, 2004 A total of twenty five of radiolarian species are recognized belonging to nassellarians and spumellarians with maximum of eighteen (18) species in sample OKS 11. Their differential stratigraphical range and relative abundance allow to distinct two successive assemblages (R I and R II ) through the C-T transition.
Although, the studied radiolarian species do not exhibit a good potential for biostratigraphic dating, the section is calibrated either by foraminifers and ammonites. The R II assemblage spans the upper part of the OAE-2 interval and the organic-poor deposits overlying this interval. It is characterized by a decrease trend of the nassellarian relative abundances (from 87% to 42%). Therefore, maybe dissolution or bad preservation conducted to the absence of this group close to the base of the upper half of the section, across the OKS40-OKS45 samples interval. Many species show rapid and gradual disappearing following a stepwise-like pattern (e.g. Guttacapsa sp., Spongostichomitra elatica, Novixitus sp., Stichomitra stocki, Mita gracilis, Pseudodictyomitra pseudomacrocephala, Thanarla pacifica D. montisserei). About the associated spumellarians, several species from the R I assemblage persisted more or less long time (e.g. Archaeocenosphera aff. vitalis, Crucella messinae, Praeconocaryomma lipmanae, Rhopalosyringium hispidum Pyramispongia glascockensis Pessagno., Cavaspongia euganea (Squinabol),, C. Californiaensis Campbell and Nishimura, Pseudoeucyrtis spinosa (Squinabol), Archaeocenosphaera ? mellifera O'Dogherty,). Nevertheless, very few species of nassellarians first occurred across the upper half part of the studied section. All these species are represented by dwarf and poorly preserved specimens.

Discussion
Detailed analysis of radiolarian assemblages allows us to attribute a biostratigraphic framework for the organic-rich beds. In the following section, we used zonal scheme proposed by O'Dogherty (1994) (1994). Although the coexistence of D. lepta, Stichomitra communis and Patellula verteroensis is assigned to early Late Cenomanian age (Erbacher, 1998), this assemblage possibly characterize 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). Samples recovered from basal beds (GA2-GA6) show high abundance of Pseudodictyomitra lodogaensis and contain some early Cretaceous taxa from Turbocapsula Z o n e s u c h a s A.portmanni and Th. pacifica (O' Dogherty, 1994;Erbacher and Thurow, 1998;Danelian et al., 2007;Michalik et al., 2008). Thus, a late Aptian age of these beds could not hitherto be ruled out. According to Erbacher & Thurow (1998), the first occurrence of Pseudodictyomitra lodogaensis coincides with the upper part of G. algerianus Zone. Its last occurrence coincides with the Aptian-Albian boundary and the first occurrence of Mita gracilis (= Dictyomitra gracilis). This taxon is also reported from the Albian to Cenomanian deposits of the Atlantic domain, California and Pacific realms (Thurow, 1988;Karminia, 2006;Palechek et al., 2010). It is possible that black shale unit of Jebel Garci could underlines the Aptian-Albian boundary. In fact, Danelian (2008) have reported the presence of Thanarla praeveneta from the Upper Aptian -Lower Albian bed which occurs in GA7 beds underlying the black shale successions.

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In that score, an assemblage recovered from Mid Cretaceous outcrops of Northern Tethys margins was described by Danelian et al., (2007), shows the co-occurrence of P. lodogaensis, Dictyomitra gracilis, Thanarla brouweri, Archaeodictyomitra aff.vulgaris assigning it to the early Albian UA10-11 biochronozone. Danelian et al (2004) consider that an early Albian age of Dercourt Member cannot be ruled out despite the presence of Angulobracchia portmanni and pseudoeucyrtis hanni characteristic of U.A.9. These species are observed hitherto within assemblage from GA15, associated with Dictyomitra montisserei. Kurilov & Vishnevskaya (2011) described an assemblage extracted from Early Cretaceous outcrops of Pacific domain that does not differ from GA21. It contains Thanarla brouweri, Pseudodictyomitra paronai, Pseudodictyomitra lodogaensis, Holocryptocanium barbui, Dictyomitra cf. montisserei, Dictyomitra communis, and Dictyomitra gracilis indicating an early Albian age. The sample GA26 has provided an assemblage characterized by high abundance of Hiscocapsa asseni, co-occurring with D.gracilis and D.montisserei. It lies with the UA10 biochronozone of Romanus zone (O'Dogherty, 1994;Danelian et al., 2004). We suggest that lower part of black shale intervals could be assigned to the upper part of Costata z o n e ( G A 5 -G A 1 4 ) b a s e d o n t h e p r e s e n c e o f A p t i a n t a x a ( i , e . Angulobracchia portmanni, Pseudoeucyrtis hanni). The lower part of this zone coincide with the first occurrence of Microhedbergella praeplanispira planktic foraminifera. Whereas the top coincide with the last occurrence of Angulobracchia portmanni and Pseudoeucyrtis hanni associated with a relative increase in abudance of Archaedictyomitrae and Williriedellidae families. The Romanus zone (GA14 -GA27) show the dominance of high diversified nassellarian species. The assemblage recovered from GA17 is composed of Thanarla brouweri, Archaeodictyomitra montisserei, Thanarla conica which is attributed to the middle Albian Mallanites romanus subzone (U.A. 10 -11 biochronozone) (O'Dogherty, 1994;Danelian et al, 2004). However, the first occurrence of Ticinella primula planktic foraminifera is recorded 24 m above GA17 bed. Thus, we suggest that lower part of Romanus zone may be attributed to the Early Albian. Studies on Cenomanian -Turonian boundary interval show that deposition of radiolarian, organic-rich sediment and large positive carbon isotopic excursion are coeval with extreme fertility conditions and correspond to a large-scale proxy that indicate a hypersiliceous period (Premoli Silva et al, 1999;Racki & Cordey, 2000) The Bonarelli equivalent in Tunisia is commonly known by the Bahloul Formation (Burollet, 1956). In the Bargou area, the Bahloul Formation shows organic-rich intervals interbedding cherty and radiolarian limestone layers (Layeb andBelayouni 1999, Soua andTribovillard, 2007) Although the C/T boundary interval outcropping in the Tunisian realm was extensively studied by planktic foraminifera and ammonite biostratigraphy (Maamouri et al, 1994;Nederbragt and Fiorentino, 1999;Abdallah et al., 2000;Amédro et al, 2005), radiolaria assemblages have provided a useful tool for age calibration and subdivision of C/T organicrich beds in this study. Two black shale levels were identified in Oued Kharroub section: 1. The first lies with the lower part of Withinella archaeocretacea planktic foraminifera zone, above the highest occurrence of Rotalipora cushmani 2. The second coincides with the middle part of Heterohelix moremani zone The calibration of these levels is based on age-diagnostic radiolarian recovered from biosiliceous limestone beds (Fig. 4).
The OSK 24 yields an assemblage composed of Rhopalosyringium radiosum O'Dogherty, Praeconocaryomma lipmanae Pessagno, Acaeniotyle vitalis O'Dogherty Rhopalosyringium hispidum O'Dogherty. The three first taxa have been described by Bak et al (2005) and attributed them to the late Cenomanian -early Turonian. Erbacher (1998) attribute Rhopalosyringium radiosum to the early Turonian, but later Musavu-Moussavou and Danelian (2006) expand its range to late Cenomanian. The assemblage contains Xitus picenus Salvini and Marcucci -Passerini which its range do not exceed the Silviae Zone of Bonarelli (O'Dogherty, 1994;Salvini & Marcucci-Passerini, 1998 (Bandini et al., 2006;Smreckova, 2011). In East Pacific domain, this taxon, associated with C. californaensis and Pyramispongia glascockensis PESSAGNO, is recorded within the Silviae Zone of late Cenomanian age (Bragina, 2009). Salvini & Marcucci-Passerini (1998) stated that C. californiaensis occurs only in the base of upper assemblage C of Bonarelli Level which lies with the base Superbum Zone defined by O'Dogherty (1994). In the Atlantic domain, the last occurrence of C. californiaensis is recorded in the late Cenomanian just beneath the organic-rich beds related to the OAE2 (Musavu-Moussavou and Danelian, 2006). Taking into account the paleogeographic similarities between northern and southern Mediterranean Tethys margins, the radiolarian assemblage recovered from OKS44 level could be correlated with upper assemblage (Superbum Zone) of Bonarelli level in Central Italy. Thus, the second black shale lie with the upper part of Biacuta s u b z o n e o f l a t e C e n o m a n i a n a g e , i f w e t a k e i n t o consideration the position of turonian Watinoceras spp. ammonite (Amédro et al., 2005)

Conclusion
Biostratigraphic investigations of Albian and C/T boundary intervals in Northern Tunisia show that organic-rich beds are generally associated with high abundance of radiolarian fauna. Age constraint of organic-rich sediments is established and correlated with biochronozones of O'Dogherty (1994). In the light of these results, we deduce that: 1. Black shale interval of Jebel Garci which is embedded within the "Allam" Member is assigned to the early Albian U.A.10 biochronozone. However, the latest Aptian could not be excluded for the lower part. 2. Late Albian organic-rich beds of Jebel Srassif including cyclic limestone/marl beds of "Marnes Moyennes" and Mouelha Members lie with the boundary interval between U.A. 13 and U.A. 14 biochronozones. 3. Two black shale levels embedded within Bahloul Formation are probably of late Cenomanian age and confined with the U.A.18 biochronozone. The first occurrence of turonian Watinoceras spp. ammonite is recorded 70 cm above the second black shale bed (OSK40) It seems that distribution of radiolarian assemblages of albian and cenomanian-turonian boundary intervals shows some difference from those of Atlantic and east Pacific domains. Preservation index and range discrepancies of some radiolarian species could affect the subdivision resolution. Further studies on radiolarian distribution assemblages and relationships with environmental changes during Mid-Cretaceous time are needed to establish paleogeographic reconstructions of southern tethyan margins. www.intechopen.com