Albian oceanic anoxic events in northern Tunisia: Biostratigraphic and geochemical insights

Cretaceous Research 32 (2011) 685e699

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Albian oceanic anoxic events in northern Tunisia: Biostratigraphic and geochemical insights Moez Ben Fadhel a, b, *, Mohsen Layeb c, Afif Hedfi b, Mohamed Ben Youssef a a

Département de Géoressources, CERTE e Technopole de Borj Cedria, 8020 Soliman, Tunisia Département de Géologie, Faculté des Sciences de Bizerte, Université de Carthage, 7000 Bizerte, Tunisia c Institut Supérieur des Arts et Métiers de Siliana, Université de Jendouba, 6100 Siliana, Tunisia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 July 2010 Accepted in revised form 18 April 2011 Available online 23 April 2011

Albian pelagic successions of the Nebeur area in northwestern Tunisia consist of radiolarian-bearing and organic-rich black shale beds, which represent the lower part of the Fahdene Formation. The carbonate content of the organic-rich beds ranges between 40 and 48%. Total organic carbon (TOC) analyses via Rock Eval pyrolysis yielded values ranging between 0.7 and 2.8% and a mixed marine/terrestrial origin. Tmax values vary between 424 and 450
C, indicative of submature to mature organic matter. High resolution planktic foraminiferal and radiolarian biostratigraphy suggest that the black shales beds span the mid- to late Albian, confined to the middle part of the Ticinella primula zone, upper Biticinella breggiensis zone and lower appeninica þ buxtorfi zone. Episodes of organic-rich deposition in the “Tunisian Trough” are interpreted as being the sedimentary record of the global oceanic anoxic events OAE1b, c, and d respectively. Age-diagnostic radiolarian assemblages recovered from late Albian organicrich black shales lie within the UA13eUA14 boundary biochronozones. The abundance of radiolarian and calcispheres (i.e. pithonella) within the black shales suggests high productivity periods and eutrophic conditions probably triggered by upwelling currents. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Albian Northern Tunisia Planktic foraminifera Radiolarians Black shales Oceanic anoxic events

1. Introduction The Albian time was a period characterized by global sea level rise and geodynamic activity expressed by high production of oceanic crust leading to a large degassing rate of CO2 (Caldeira and Rampino, 1991; Larson (b), 1991). Increased atmospheric CO2 contents are thus thought to have induced a global “greenhouse” climate (Barron and Washington, 1985; Bice and Norris, 2002). A direct consequence of the above events was the enhanced preservation of organic-rich sediments throughout Western Mediterranean basins of the Tethys (Bréhéret, 1997; Karakitsios et al., 2004; Luciani et al., 2004; Coccioni et al., 2006), recording discrete Oceanic Anoxic Events (Schlanger and Jenkyns, 1976). Oceanic Anoxic events (OAEs) are widely thought to represent short-lived periods (<1 my) of increased organic carbon burial (Leckie et al., 2002), accompanied by positive carbon isotope excursions in bulk carbonate and organic matter. In some instances, such as with the Toarcian and lower Aptian OAEs, these positive isotopic

* Corresponding author. Département de Géoressources, CERTE e Technopole de Borj Cedria, Route Touristique, 8020 Soliman, Tunisia. E-mail address: [email protected] (M. Ben Fadhel). 0195-6671/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2011.04.004

excursions are preceded by rapid negative excursions (Jenkyns,1980; Bralower et al., 1993; Strasser et al., 2001; Leckie et al., 2002; Gröcke et al., 2006; Robaszynski et al., 2010). It is also speculated that the Albian was specifically a “hyper-siliceous” period, characterized by the deposition of both organic-rich and biogenic silica-rich sediments (De Wever and Baudin, 1996; Racki and Cordey, 2000). The widespread occurrence of marine anoxia during OAEs is believed to have been caused largely by massive igneous activity (Turgeon and Creaser, 2008) and increased phytoplankton productivity (Jarvis et al., 2002). Several theories and models have been proposed to explain black shale accumulation scenarios. Erbacher et al. (1996) have distinguished between P-OAE and D-OAE, reflecting increased oceanic productivity and sedimentation of terrestrial organic matter respectively. Galeotti et al. (2003) have postulated two main models for OAE development, namely 1) the preservation model, involving decreased ventilation on the seafloor and low rate of remineralization of organic matter; and 2) the productivity model, which is based on elevated primary productivity causing dysoxic/anoxic environment on the sea floor. The Albian organic-rich black shale successions outcropping in the northwestern Tethyan margin have been intensively studied using planktic foraminiferal, calcareous nannofossil and radiolarian biostratigraphy, in order to constrain the timing of the OAEs 1b, c and d,

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associated marine biota extinction and turnover events (Erbacher and Thurow, 1997; Premoli silva et al., 1999; O’Dogherty and Guex, 2002; Luciani et al., 2004; Coccioni et al., 2006; Danelian et al., 2007). However, this is not the case for sections of the North African margin, where radiolarian and planktic foraminiferal agecalibration of the Albian OAEs has received less attention. Ammonite and planktonic foraminifera biostratigraphy in conjunction with geochemistry, have been focused primarily on the study of Albian pelagic successions of Northern Tunisia (Burollet, 1956; Massin and Salaj, 1970; Salaj, 1980; Dali-Ressot, 1987; Talbi, 1991; Saïdi and Belayouni, 1994; Memmi, 1999; Ben Haj Ali et al., 2002; Robaszynski et al., 2007; Ben Haj Ali and Ben Haj Ali, 2008; Chihaoui et al., 2010). Although radiolarian-bearing and organic-rich black shales of Albian age have been identified in the same region (Ben Haj Ali and Ben Haj Ali, 1996; Tandia, 2001), no previous studies have hitherto provided a precise age-calibration of organic-rich black shale horizons recording oceanic anoxic events. The objectives of this paper are therefore: 1) to provide an integrated biostratigraphic and geochemical framework of such organic-rich sections based on planktic foraminifera and radiolarian assemblages; 2) to correlate identified OAE intervals with time-equivalent sections from the northwestern Tethyan margin; and 3) to interpret these records in the context of depositional and palaeo-environmental evolution in the Tethyan realm during the Albian. 2. Geological setting The Nebeur area is located in the northwestern extremity of the ‘Domes Belt’ of Northern Tunisia (Fig. 1A) and it is palaeogeographically included within a subsidence basin known as the ‘Tunisian Trough” (‘Sillon Tunisien’ of Burollet, 1956). The Domes Belt is a complex structural domain characterized by strike slip faults and Triassic extrusions. The latter have been interpreted as either diapiric (Perthuisot, 1978) and/or interstratified with Albian and Turonian pelagic sediments (Vila et al., 1996; Ghanmi et al., 1999). The tectonic complexity of the Domes Belt is the result of the succession of two main deformation phases. The first phase is of an early Cretaceous age and is the consequence of the AfricaeEurasia

relative plate motion that led to the opening of the Ligurian Tethys Sea (Chikhaoui et al., 1998) and, subsequently, to the disintegration of the Tunisian margin (Martinez et al., 1991). This tectonic event resulted in tilted block topography and triggered halokinetic dynamics. Such tectonic architecture is responsible for instability in the sedimentation patterns, characterized by hiatuses at the horst top as well as in subsiding basins (Chikhaoui et al., 1991, 1998; Memmi, 1999). The second deformation phase is linked to orogenic processes during the late Miocene. It is characterized by compressive tectonics which affected post-Neogene structures and reactivated pre-existing faults. The resulting inter-fingering faults have created tectonic ‘corners’ and multidirectional folds trending NEeSW, NeS and EeW (Chikhaoui and Turki, 1995). The associated depositional systems are characterized by mixed argillaceous and siliclastic facies (Valanginian to Aptian) (Bolze, 1954; Chikhaoui et al., 1998), overlain by an Albian to late Cenomanian pelagic facies of monotonous alternations of marl and limestone couplets, termed the Fahdene Formation (Burollet, 1956). The late Cretaceous (Cenomanian to Late Campanian) succession of the Mellegue paleograben comprises an up to 5000 m-thick sequence of largely pelagic sediments. The study area can be divided into three major structure components, namely the Mellegue paleograben, and the Koumine and southern Nebeur horsts (Chikhaoui et al., 1991) (Fig. 1B). Based on outcrop exposure and the interpreted geodynamic context, two sections were chosen for this study (Figs. 2,3): the Koudiat Berkouchia section, which borders the Triassic extrusion of South-Nebeur horst, and the Srassif section which is located in the Mellegue paleograben. 3. Materials and methods A total of 140 samples of marl and limestone were collected for biostratigraphic and geochemical examinations. Organic-rich levels were sampled at relatively higher resolution (approximately every 50 cm). Soft samples were soaked in hydrogen peroxide solution and then washed using standard techniques. Samples were sieved sequentially using meshes of 500 mme250 mm e 125 mme63 mm.

Fig. 1. Location of the study area (A) e Structural map of Nebeur area (B).

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Fig. 2. Planktic foraminiferal biostratigraphy of the J. Srassif section.

Each sieve was stained with methyl blue (10 g/l) to avoid contamination from previous samples. Specimens with good preservation were determined and picked under a Wild Herbrugg light binocular microscope. Abundances of planktic foraminifera and radiolaria are expressed semi-quantitatively by counting the

total number of preserved specimens (per 3 g of sample). A total of twenty-nine least-weathered samples were powdered and analyzed at the ETAP research center (Tunis) via Rock Eval pyrolysis, in order to obtain total organic carbon (TOC) contents (Espitalié et al., 1986). Results (in wt%) are presented in Table 1.

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Fig. 3. Planktic foraminiferal biostratigraphy of the Koudiat Berkouchia section.

4. Results 4.1. Lithostratigraphy 4.1.1. The Albian successions of Jebel Srassif section The Albian successions of Jebel Srassif (Fig. 2) are in vertical contact with chaotic Triassic evaporites (gypsum, mineralized dolomite) and they contain at their base, 1 m-thick layer of glauconite-rich clay containing barite and iron oxide veins. The

immediately overlying unit is a w10 m-thick succession of sandstone beds and green marl alternations, which contain ostracods, phosphate coprolites and abundant benthic microfauna. These are overlain by a blue to gray metric packstone bed (4 m) showing slump structures and reworked intraclastic material (SRF 7). Stratigraphically higher is a gray-colored marly interval (90 m) containing limestone beds and reworked Triassic lithoclasts (SRF10). This interval is capped by a bioturbated and glauconite-rich limestone bed (SRF12). This is overlain by alternations of gray-colored

M. Ben Fadhel et al. / Cretaceous Research 32 (2011) 685e699 Table 1 Rock Eval data from TOC-rich intervals from Koudiat Berkouchia.

Upper Albian

Mid. Albian

Sample

TOC

S1

S2

Tmax

S3

HI

OI

86 84 80 79 76 63 53 33 29 23 17 6 3

0.78 1.37 1.25 1.3 1.81 1.27 2.44 2.39 0.85 1.77 1.19 0.17 0.36

0.14 0.36 1.23 0.37 0.57 0.71 1.66 0.88 0.15 0.84 0.4 0.09 0.09

0.82 2.41 3.84 2.59 4.23 3.6 7.56 7.17 0.55 5.16 2.73 0.02 0.03

440 440 450 443 443 444 446 444 436 448 444 334 424

0.73 0.56 0.18 0.43 0.38 0.21 0.32 0.36 0.59 0.26 0.35 0.41 0.59

105 176 307 199 234 283 310 300 065 292 229 12 8

94 41 14 33 21 17 13 15 69 15 29 241 164

limestone and fissile black marl, which grades into an organic-rich, thin-laminated mudstone at its top (SRF15). The base of this unit comprises intraformational conglomerates (SRF 14) and marly intervals, which contain abundant detrital quartz. The top unit is attributed to the Allam Member (Burollet, 1956), has scarce faunal content and contains some spherical radiolaria and belemnites rostrums. Toward the top, the Allam unit is overlain by greyish nodular limestones and marls containing carbonate concretions (“calcaire à miches”). They gradually change to cyclic laminated limestone/marl bundles (SRF37 to SRF 44). The gray and marly intervals (4e6 m thickness) occasionally evolve into millimeter-thick, black-colored, organic and radiolarian-rich marl/limestone beds (Black shales). These beds are organized in individual couplets and are overlain by a nodular, biostromal limestone bed (2 m) (SRF 50). The topmost of biostromal limestone bed is capped by a yellowish-colored wackstone bed (12 m) characterized by a bituminous odor (SRF 53). This level is assigned to the Mouelha Member, which is considered as a regional stratigraphic marker (Burollet, 1956). The uppermost section consists of a lower (w40 mthick) monotonous alternation of gray limestone beds and ochrecolored marls yielding septarian nodules (SRF76) and bioherm concretions. The final 50 m up-section consists of gray-colored marl intervals and limestone beds. 4.1.2. The Albian succession of Koudiat Berkouchia section This succession (Fig. 3) is in vertical contact with chaotic Triassic evaporites (gypsum and pseudostratified dolomite beds), and begins with greenish, glauconite-rich clay (w3 m). The latter is

Table 2 Rock Eval data from TOC-rich intervals from J. Srassif.

Upper Albian

Mid. Albian

Lower Albian Upper Aptian

Sample

TOC

S1

S2

Tmax

S3

OI

HI

68 57 48 36a 36 25 23 18 14 13 11 09 07 05 02 01

0.79 1.7 1.3 0.79 1.34 0.76 0.7 0.67 1.49 1.04 0.84 0.68 0.07 0.1 0.28 0.21

0.15 0.16 0.13 0.34 0.38 0.2 0.23 0.24 0.75 0.53 0.33 0.41 0.06 0.06 0.06 0.07

0.54 2.19 1.77 1.44 2.29 1.07 1.16 0.82 4.7 1.83 1.4 1.03 0.06 0.04 0.05 0.05

442 441 441 441 442 446 443 442 444 437 443 449 447 454 450 452

0.72 0.93 0.73 0.4 0.54 0.87 0.42 0.54 0.19 0.53 0.35 0.21 0.12 0.5 0.1 0.11

91 55 56 51 040 114 60 81 13 51 42 31 171 50 36 52

68 129 136 182 171 141 166 122 315 176 167 151 86 40 24 24

689

overlain by an alternation of black marls and nodular, reddishcolored limestone beds. The limestone contains dentritic manganese oxide encrustations and ammonite impressions (BRK 8), and constitutes the lower part of the Allam Member (15 m). This interval is separated by an erosive surface, from an alternation of w20 m in total of radiolarian-rich, gray and wackstone beds (BRK 22) and marls. The alternation is capped by a massive gray limestone bed (4 m) (BRK 30). Stratigraphically higher is approximately 27 m of rhythmic laminated bundles (BRK 32eBRK 58) (Plate 3, Fig. 2) which are composed of couplets of centimeter-thick dark marl, and greyish laminated limestone beds containing interbedded black shale layers. The rhythmic bundles are overlain by 20 m of bioturbated, dark laminated limestone and gray to dark marl, which contain veins filled with calcite and galena. The succession continues with 10 m of recrystallized, pseudo-laminated limestone beds (BRK 56eBRK 59). Stratigraphically above is an interval consisting of alternations of gray limestone and marl, which are overlain by an organic-rich and yellowish limestone bed corresponding to the Mouelha Member. The succession continues with gray marl and laminar limestone beds with ammonite impressions (40 m) (BRK 89). The upper part consists of an alternation of yellow-colored limestone beds and greenish marls containing septarian nodules (SRF 91). 4.2. Biostratigraphy 4.2.1. Planktic foraminifera biotic events The planktic foraminiferal assemblages within the lower part levels of Hedbergella planispira zone are remarkably poor compared to the high abundance of benthic foraminifera within the lower Albian successions. A decrease in number of planktic foraminifera compared to radiolarian is also observed within black shales mainly of late Albian age (Figs. 2 and 3). An increase in the abundance of benthic and planktic foraminifera is recorded across the AlbianeCenomanian boundary. At the Jebel Srassif section, the AptianeAlbian boundary lies at the last appearance datum (LAD) of Ticinella bejaouaensis SIGAL. Microfacies recovered from limestone beds of this interval display rich benthic foraminifera assemblages (i.e. Textularia sp., Plate 3, Fig. 3; Gavelinella sp., Plate 3, Fig. 7 Fursenkoina viscida, Plate 3, Fig. 6). On the contrary, planktic foraminifera are poorly to very poorly-preserved throughout this interval. Moreover, we notice an upward decreasing content of siliciclastic material. Shaly and glauconitic limestone beds (SRF12) underlying the Allam Member, provide very poorly-preserved planktic foraminifera fauna. The marly interval (SRF 13) overlying the glauconitic limestone beds and underlying limestone beds with intraformational conglomerates (SRF14), has yielded Ticinella primula LUTERBACHER, Favusella washitensis (CARSEY) (Plate 2, Fig. 7) and tiny forms of Hedbergella. The thin black limestone and marl beds alternations (SRF15) provided poorly-preserved planktic foraminifera, whereas radiolaria are abundant, particularly within limestone beds (Plate 3, Fig. 8). The first appearance datum (FAD) of Biticinella breggiensis (GANDOLFI) (SRF23) in J Srassif is recorded 25 m above the topmost portion of the Allam Member. However, B. breggiensis (GANDOLFI) (Plate 2, Fig. 3) appears 15 m above the Triassic evaporites (BRK 6), indicating a hiatus of H. planispira and the lower part of T. primula zone in Koudiat Berkouchia. Finely laminated limestone beds (BRK 21, Plate 3, Fig. 10) and gray marl alternations yielded T. primula LUTERBACHER (Plate 2, Fig. 5), B. breggiensis (GANDOLFI), Hedbergella delrioensis (CARSEY) and Ticinella praeticinensis (SIGAL). By contrast, in the Jebel Srassif section the planktic foraminifera assemblages recovered from the equivalent marly intervals are scarce and poorly-preserved.

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Plate 1. Radiolarians. 1 e Dactyliosphaera maxima (PESSAGNO), scale bar: 100 mm, sample 62. 2 e Holocryptocanium barbui DUMITRICA, scale bar: 50 mm, sample 37. 3 e Torculum dengoi (SCHMIDT-EFFING), scale bar: 50 mm, sample: 37. 4 e Cryptamphorella conara (FOREMAN), scale bar: 50 mm, sample 37. 5 e Dictyomitra gracilis (SQUINABOL), scale bar: 50 mm, sample 62. 6 e Dictyomitra montisserei (SQUINABOL), scale bar: 50 mm, sample 68. 7 e Tubilustrium transmontanum O’DOGHERTY, scale bar: 50 mm, sample 37. 8 e Mallanites triquetrus (SQUINABOL), scale bar: 100 mm, sample 68.

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Plate 2. Planktic foraminifera (scale bar: 100 mm). 1 & 2 e Thalmanninella appenninica (RENZ), sample 62. 3 e Biticinella breggiensis (GANDOLFI), sample 37. 4 & 6 e Planomalina buxtorfi (GANDOLFI), sample 68. 5 e Ticinella primula LUTERBACHER, sample 22. 7 e Favusella washitensis (CARSEY), sample 15a. 8 e Pseudothalmanninella ticinensis (GANDOLFI), sample 62. 9 e Pseudothalmanninella subticinensis (GANDOLFI).

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Fig. 4. Total organic carbon and CaCO3 content diagrams of the Koudiat Berkouchia and J. Srassif sections.

The first occurrence of Pseudothalmanninella subticinensis (GANDOLFI) (Plate 2, Fig. 9), in Koudiat Berkouchia section is recorded below the massive and dark limestone bed (BRK30), whereas in Jebel Srassif section, it precedes the first bundle of rhythmic marl/limestone beds. Although benthic foraminiferal assemblages are scarce and less diversified, these successions have provided a rich planktic foraminifera assemblage composed of T. primula LUTERBACHER, B. breggiensis (GANDOLFI), P. subticinensis (GANDOLFI) and Thalmanninella balernaensis (GANDOLFI). The planktic foraminifera recovered from black shale beds are interbedded within rhythmic marl/limestone alternations outcropping in both sections, are less abundant and they occur in tiny forms. The gray-colored and marly interval underlying these alternations (BRK 32) has yielded planktic foraminifera assemblages composed of T. primula LUTERBACHER, B. breggiensis (GANDOLFI), P. subticinensis (GANDOLFI), T. balernaensis (GANDOLFI) and T. praeticinensis (SIGAL). Benthic forms are poorly-represented and planktic foraminifera keeled forms are almost absent from the black shale intervals whereas radiolaria are abundant (Plate 3, Fig. 11). A short bloom period of keeled forms associated with radiolarian diversification is recorded in the BRK61 bed. Stratigraphically upward, the first occurrence of Pseudothalmanninella ticinensis (GANDOLFI) (Plate 2, Fig. 8) is recorded at the topmost limestone bed of the rhythmic successions (BRK64). In Jebel Srassif section, its first occurrence lies with the last rhythmic bundle. The successions overlying the limestone/marl rhythmic bundles of Koudiat Berkouchia are impoverished in benthic and planktic foraminifera. Nevertheless, a sample of BRK74 has recorded the FAD

of Thalmanninella appenninica (RENZ) (Plate 2, Figs. 1 and 2) and the LAD of T. primula LUTERBACHER, just below the dark yellowish limestone bed of Mouelha Member. The uppermost successions of both sections record an environmental event characterized by the bloom of keeled forms (i, e; Thalmanninella and Planomalina genus) particularly within the sequences containing the AlbianeCenomanian boundary. The bloom is associated with a high diversification rate of radiolaria as well as a high abundance of benthic foraminifera (i.e. BRK 83; SRF 62). 4.2.2. Radiolarian assemblages The Upper Albian marl/limestone alternations of Jebel Srassif section have provided well-preserved, diagnostic radiolarian assemblages (Plate 1) (Ben Fadhel et al., 2010). By analyzing the abundance curves Ben Fadhel et al. (2010) reported that:  the radiolarians are confined mainly within light-colored levels underlying or overlying organic-rich black shales;  abundance peaks of radiolarians are within the breggiensis zone and the lower part of appeninica þ buxtorfi zone; and,  abundance fluctuations are confined to subticinensis and ticinensis zones. The assemblages are dominated by the cryptothoracic species (Holocryptocanium, cryptamphorella). High rates of radiolarian diversification are recorded mainly within the rhythmic successions of ticinensis planktic foraminifera zone and in the lower part of the appenninica zone. The assemblages are dominated by spumellarians

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Sample SRF 37 has yielded an assemblage containing Cryptamphorella conara (FOREMAN), Tubilustrium transmontanum O’DOGHERTY, Holocryptocanium barbui DUMITRICA, Torculum dengoi (SCHMIDT-EFFING) and Dictyomitra gracilis (SQUINABOL). Sample SRF 62 recovered from gray-colored marl overlying organic-rich black shale of the Mouelha Member, has yielded an assemblage containing H. barbui Dumitrica, Mallanites triquetrus (SQUINABOL, )Dictyomitra montisserei (SQUINABOL) and Dactyliosphaera maxima (PESSAGNO). The assemblage described previously contains species of middle to late Albian age; however, the presence of T. dengoi (SCHIMDT-EFFING) and Dactyliosphaera maximum (PESSAGNO) suggests a younger age, probably Late Albian sensu stricto. It also lies within the UA13eUA14 boundary biochronozones according to the radiolarian taxonomy proposed by O’Dogherty (1994). 4.3. Organic geochemistry

Fig. 5. Tmax versus hydrogen index and hydrogen index versus oxygen index plot diagrams for Albian black shales of this study.

with discoidal forms (i.e. Godia) and multi-segment nassellarians with elongated test (Dictyomitra). Other spumellarians species, belonging to the cavaspongidae family, are recognized within the topmost of ticinensis zone (mainly cavaspongia).

The total organic carbon values on both sections range between 0.2 and 2.8 wt% (Tables 1 and 2), with a maximum value recorded within marl and limestone rhythmic successions (Fig. 4). Albian organic-rich sediments are mainly dominated by kerogen type II with an input of terrigenous organic source material (Figs. 5,6). High values of TOC are confined to the upper part of Allam Member (middle Albian), the medium part of cyclic marl/limestone couplets, and the Mouelha Member. Tmax values carried out by Rock Eval pyrolysis vary between 436
C and 452
C, which indicates submature to mature organic matter. Marked peaks of TOC values are observed within successions of breggiensis zone of the Koudiat Berkouchia section and are higher than in the time equivalents of the J. Srassif section. This trend indicates an enhanced preservation of organic matter and higher rates of marine productivity at least during the early late Albian. The carbonate content in both sections (Fig. 4) varies between 15% and 80% CaCO3 with a maximum peak recorded across AlbianeCenomanian boundary successions of the Koudiat Berkouchia section. The lower part of the late Albian section is characterized by low CaCO3 content, whereas radiolarian-bearing levels attributed to the breggiensis zone show values ranging between 42 and 50% CaCO3.

Fig. 6. Tmax versus hydrogen index and hydrogen index versus oxygen index plot diagrams for Albian black shales of this study.

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Fig. 7. Bioevent correlation of black shale beds of the Koudiat Berkouchia and Jebel Srassif sections with time equivalents in the western Mediterranean Tethyan realm.

5. Discussion 5.1. Biostratigraphic calibration By correlating time-equivalent sections in Northwestern Mediterranean Tethys margins and using planktic foraminiferal bioevents (Fig. 7), we will attempt to constrain the records of Albian oceanic anoxic events in the “Tunisian Trough”. The organic-rich horizons observed in the J. Srassif section span the middle part of primula zone just 25 m below the first occurrence of B. breggiensis. The TOC contents here vary between 0.89 and 1.49% (Fig. 4). In Koudiat Berkouchia, a sedimentary hiatus of this unit spans the lower part of primula zone. Tsikos et al. (2004), and Karakitsios et al. (2007) identified several dm-thick organic-rich black shales within the Vigla Shale Member (TOC content: 1e28%) in the Ionian zone, just below the first occurrence of B. breggiensis. Radiolarian-rich material recovered from the black shales level of Sopoti section (Ionian Zone) have permitted Danelian et al. (2007) to attribute this level to late Aptianeearly Albian age (UA10e11) biochronozones of O’Dogherty (1994). The organic materials are type III according to Rock Eval analysis of Danelian et al. (2007), similar to those of our study (Fig. 5). It has not been possible to identify radiolarian biochronozones in our studied sections because of poor preservation of index species. However, the uppermost Allam black shale unit possibly corresponds to the oceanic anoxic event OAE1b “Paquier event”, which is widespread within the middle Albian outcrops of Northern Tethys margins (Bréhéret, 1985; Erbacher et al., 1996; Tsikos et al., 2004; Danelian et al., 2007). Black shale layers interbedded within cyclic marl/limestone alternations bearing moderately abundant radiolarian, are confined

to the upper part of breggiensis zone in Jebel Srassif. In the Koudiat Berkouchia section, they span the entire uppermost portion of the breggiensis zone. Arthur et al. (1990) and Bralower et al. (1993) noticed that the OAE1c straddles the entire breggiensis zone. They also report values of TOC ranging from 1.34 to 2.44% and relatively lower CaCO3 content. Similar ranges of values were reported by Galeotti et al. (2003) from the orbitally-controlled black shales of the ‘Amadeus Segment’ unit, which lies within the upper part of praeticinensis, zone (Luciani et al., 2007). The Mouelha black shale is a calcispherae-rich, dark yellowish bed (4e15 m thick, Plate 3, Fig. 12) yielding rich and diversified radiolarian microfauna. It lies within the lower part of the Rotalipora appenninica zone. It precedes the first appearance datum (FAD) of Planomalina buxtorfi (GANDOLFI) (Plate 2, Figs. 4 and 6) in Koudiat Berkouchia section whereas in J. Srassif, it precedes the FAD of R. appenninica and above the FAD datum of P. buxtorfi. A black shale event has been identified by Robinson et al. (2008) within the base of the appenninica zone in the Pacific realm of the USA, showing TOC values ranging between 0.25 and 2.0%. Petrizzo and Huber (2006) stated that Upper Albian organic-rich black shales level “fall in the upper part of the planktonic foraminiferal Rotalipora appenninica Zone, and is totally comprised within the stratigraphic interval identified by the total range of the distinctive biomarker Planomalina buxtorfi”. Taking into consideration the position of P. buxtorfi first appearance datum, the Mouelha black shales can be correlated with the Breistroffer level (OAE1d) identified in the Vocontian Basin (Erbacher et al., 1996; Bréherét, 1997; Giraud et al., 2003; Bornemann et al., 2005) and with the ‘Pialli’ level in the Italian Apennines domain (Coccioni, 2001)

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Fig. 8. Correlation of Albian pelagic successions in central-north Tunisia with the sections investigated in this paper.

5.2. Depositional environment 5.2.1. The geodynamic and paleogeographic framework The Albian paleogeography and the distribution of depositional systems in the Tunisian realm are the reflection of a late Jurassic rifting phase that preceded the opening of the Atlantic and Ligurian domains (Bouaziz et al., 2002). The rifting phase lasted until at least the Early Aptian, and then fragmented the Tunisian margin in tilted blocks and half grabens (Martinez et al., 1991). During the late Aptianeearly to late Albian time span, the tectonic regime varied from transpressional to extensional deformation (Bouaziz et al., 2002; Zouaghi et al., 2009; Chihaoui et al., 2010) and was accompanied by active salt extrusions (Vila et al., 1996). The early Albian paleogeography was characterized by the individualization of four main domains (Fig. 9A): 1) an emerged area, which has provided siliciclastic sediments to adjacent basins; 2) areas of low sedimentation rate in which early Albian hiatus are recorded (Tandia, 2001); 3) a stable domain characterized by the infill of pre-existing grabens; and, 4) an area of high subsidence rate, which has received pelagic siliciclastic sediments of Hameima and/or Bir M’Cherga Formation. During the mid to late Albian, Northern Tunisia and emerged areas of Central Tunisia were overwhelmed by the global eustatic sea level rise documented worldwide (Hardenbol et al., 1998). From the south to the north, the paleogeography is subdivided in four main domains (Fig. 9B): 1) The emerged Sahara shield as an area of nondeposition, 2) The carbonate reefal platform of the Zebbag

Formation which covered a large part of south and central Tunisia; 3) The transgressiveeregressive facies of the ‘Selloum Sequence’ constituting the transition area between the Fahdene Basin and Zebbag carbonate platform, which was under the control of the Kasserine and Sbiba extensional fault systems (Bismuth et al., 1982; Zouaghi et al., 2009); and, 4) The Fahdene subsidence basin, which filled with deep-water sediments of the Fahdene Formation (Burollet, 1956). The lower Albian to mid-Albian sedimentation in the Nebeur Basin was characterized by a high detrital quartz input (Plate 3, Figs. 3e4) that represents the Bir M’Cherga Formation (Tandia, 2001). Siliciclastic input was supplied by the runoff of emerged areas in Central Tunisia and subsequent to the global eustatic sea level rise (Hardenbol et al., 1998) .The deposition of marls and shaly limestones is coeval with a high subsidence rate, a deepening phase following the sea level rise, a seafloor instability and probably with an active salt extrusion, the latter two are evidenced by slumps, intraformational conglomerates and reworked Triassic clasts respectively (Plate 3, Fig. 5). The global transgression which started at the Early Albian continued through the Mid to Late Albian. The extensional tectonics seems to have triggered halokinetic dynamics, tilted block architecture and restricted basin configuration (Karakitsios, 1995). The configuration is responsible for the variations in thickness and for the hiatuses of Early to Mid Albian that are recorded at the top of tilted blocks (i.e. Koudiat Berkouchia) in Northern Tunisia, as well as in other areas of north Central Tunisia (Fig. 8). Restricted basins

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(i.e. “cul-de-sac” basin configuration of Danelian et al., 2004) could have developed upwelling currents thus enhancing eutrophic conditions, primary productivity rate of marine biota and later the preservation of organic matter. The biotic events and large-scale correlation with North Tethyan margins (Fig. 7) show that the timing of OAEs must have been diachronous, probably due to differences in sedimentation rates, local geodynamic settings and preservation of biostratigraphic markers. Another cause of diachroneity can be speculated when considering OAEs as coeval with periods of high productivity of marine biota: diachronous black shales could then be interpreted as the result of the position of sites with respect to centers of upwelling at the time of deposition (White et al., 1992; De Wever et al., 1996). 5.2.2. Paleoenvironmental implications of radiolaria- and organicrich deposits The three episodes of black shale deposition recorded in Nebeur Basin are coeval with a high abundance of radiolarian, whereas planktic foraminifera are less abundant and dominated by small Hedbergella species. This trend is inverted near the AlbianeCenomanian boundary where planktic foraminifera are more abundant and keeled forms are well-represented. Radiolarian abundance peaks are generally associated with light to gray-colored beds, which overlie or underlie organic-rich black shales. A similar association between radiolaria and planktic foraminifera within black shales was described by Gebhardt et al. (2010) who emphasized that unlike the calcareous plankton community, radiolaria skeletons may have resisted extreme acidity in the water column. Several works have pointed that the occurrence of radiolaria within black shales (Plate 3 Figs. 9, 11, 12) indicates eutrophic and highly productive environments (Premoli Silva et al., 1999; Danelian et al., 2007). A hydrothermal event which occurred during the AptianeAlbian time span (Mattoussi-Kort et al., 2009) and mainly an input of fresh meteoric water at least during the late Albian (Hemadi, 1987) could be speculated at this stage, creating upwelling currents and consequently radiolarian blooms. Radiolarian-bearing limestone beds described in Koudiat Berkouchia and Jebel Srassif sections do not show any radiolarites or chert beds. Danelian et al. (2007) have mentioned that the Fourcade level, an early Aptian radiolarian-rich equivalent outcropping in the Ionian zone was not associated with radiolarites. Following Danelian et al. (2002), the radiolarian assemblages recovered from the Fourcade level are associated with carbonate-free and organicrich levels overlying carbonate-poor beds. The CaCO3 percentage measured in our study (Fig. 4) shows high abundance of radiolaria and decreased values (from 56 to w42%) from the base to the middle part of the black shale unit (breggensis and subticinensis zones), indicating increased dissolution and consequently shoaling of the lysocline (Bralower et al., 2002). Despite a possible rise of the CCD and related increased dissolution, it seems that absence of radiolarites in the Nebeur Basin is interpreted as basinal deposition located above the CCD (Cordey et al., 2005), coeval with a highly fertile environment (Danelian et al., 2002) Plate 3. (a) Fig. 1 e Limestone bed affected by slump figures and reworkings (AptianeAlbian boundary, Jebel Srassif section). Fig. 2 e Marl and limestone rhythmic bundles (Koudiat Berkouchia section e Upper Albian). Fig. 3 e Micritized fecal pellets packstone with Textularia sp. benthic foraminifera (arrow). Pyrite crystals (dark spots) scattered within quartz-rich matrix. 10, sample SRF7a (Aptian e Albian boundary). Fig. 4 - Quartz-rich packstone with glauconite grains (arrow). 10, sample SRF7a. (b) Fig. 5 e Wackestone with disseminated pyrite (dark spots) containing subangular Triassic clasts (arrow) sample SRF 10, 10 (Lower Albian). Fig. 6 e wackestoneepackstones with Fursenkoina viscida benthic foraminifera (arrow) and dessiminated pyrite, SRF 10, 10 (Lower Albian). Fig. 7 e Spicula-rich wackstone with scattered glauconite grains and Gavinella sp. benthic foraminifera (arrow), sample SRF 10, 10 (Lower Albian). Fig. 8e Organic-rich mudstone with radiolarians (arrow),

sample SRF 15, 10 (Middle Albian, uppermost Allam Member). (c) Fig. 9 e Compacted organic and radiolarians-rich packstone with glauconite grains, BRK21a, 10. (Upper Albian). Fig. 10 e Organic-rich wackstone enclosing Ticinella primula planktic foraminifera (white arrow) and spheric radiolarians. Calcispheres (pithonella) are also present (black arrow), sample BRK 22, 20. (Upper Albian). Fig. 11 e Radiolarian-rich packstone from cyclic bundles. Note the laminated texture, which are constituted by thin dark layer devoid of microfauna and radiolarians-rich light layers, sample BRK 34, 2.5 (Upper Albian). Fig. 12 e Calcified radiolarians -rich packstone (uppermost Mouelha Member) associated with Planomalina buxtorfi planktic foraminifera (arrow). Calcispheres are abundant within organic-rich matrix containing pyrite crystals (dark spots). 10, sample SRF 62. (Upper Albian).

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Fig. 9. Paleogeographic sketch maps of Tunisia during the Albian (after Marie et al., 1984; Zghal and Arnaud, 2005; modified). (A) Early to mid-Albian. (B) Late Albian. Star: localization of sections used for regional correlation. SM: J. Semmama; Bk: Koudiat Berkouchia; Sr: Jebel Srassif; OS: Oued Siliana; As: Ain Slim.

5.2.3. The marl/limestone rhythmicity Rhythmic alternations of marl and limestone beds, interpreted as orbitally-forced (Herbert and Fischer, 1986; Sageman et al., 1998; Fiet et al., 2001; Galeotti et al., 2003; Grippo et al., 2004; Tyszka, 2009), are common in the study area and exhibit prominent topographic features particularly in the Koudiat Berkouchia section (Plate 3, Fig. 2). They are grouped in bundles separated by 1.5e6 m of gray marl intervals; the latter sometimes grade into black shale beds. Each bundle contains 10 to 20 couplets of millimeter-thick, laminated, gray to black marl and light to black-colored limestone beds, which lie within the breggiensis planktic foraminifera zone. Strasser et al. (2001) stated that variations of orbital cycles were responsible for this hierarchy; they assigned a single bundle to a 400 ka orbital eccentricity cycle and a limestone/marl couplet to 20 ka precession cycle. At microscopic scale, the limestone beds exhibit cyclic alternation and laminated textures. They are constituted by the superposition of white-colored, thin, planktic foraminifera-rich and/or radiolarian-rich layers and dark organic-rich layers devoid of microfauna (Plate 3 Fig. 11) indicative of changing dysoxic and anoxic conditions at the sea floor (Leckie et al., 2002). Dysoxiceanoxic oscillations in mid-Cretaceous pelagic rhythmic beds have been interpreted as orbitally-controlled (Herbert and Fisher, 1986; Sageman et al., 1998; Grippo et al., 2004), and mid-Cretaceous OAEs have been considered the extreme response of orbitallycontrolled oceanic anoxic cycles (Mitchell et al., 2008). We deduce that upwellings and related blooms of marine biota could be linked to monsoonally-forced high productivity triggered by high eccentricity periods (Herrle et al., 2003; Grippo et al., 2004; Fisher et al., 2009), accounting for the planktic foraminifera or radiolarian -rich layers. By

contrast, periods of high precession at eccentricity maxima, which caused water column stratification, nutrient depletion and decreased productivity (Wendler et al., 2002), are expressed by organic-rich layers devoid of microfauna. In the light of these results, it becomes plausible that late Albian climate in low latitude domains (i.e. northern Tunisia) was characterized by monsoonality that favored seasonal upwellings and increased biological productivity. We assume that runoff following late Albian rainfall periods affecting the emerged Kairouan Island (Fig. 9) has provided terrigenous material and high nutrient input to the Nebeur Basin, thus promoting the deposition of organic and radiolarian -rich deposits. It is worth noting that halokinetic dynamics were responsible for the formation of salt-withdrawal basins (Zouaghi et al., 2005) (i.e. Koudiat Berkouchia), which were suitable environments of high productivity and enhanced preservation of the organic matter. 6. Conclusions Integrated radiolarian and planktic foraminifera biostratigraphy and bulk organic geochemical analysis allow us to identify and constrain the formation of black shale beds in response to Albian oceanic anoxic events OAE1b, c, and d in the “Tunisian Trough” realm. The Middle to Upper Albian black shale beds outcropping in the Nebeur area have also been shown to be a mature and potential oil-source rock. Diachnroneity in the records of OAEs across the Albian could be assigned to the preservation of marker species or to paleoenvironmental conditions linked to a local geodynamic context. It seems

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that salt ascent in Koudiat Berkouchia has induced an early to midAlbian hiatus at the top of tilted blocks; during the late Albian, this was followed by the development of a restricted, salt-withdrawal basin configuration which favored upwelling and led to high biological productivity and water column stratification. Deposition of organic- and radiolarian-rich Albian sediments in the Nebeur area may have been triggered by orbitally-forced seasonality that controlled the supply of CaCO3, the position of CCD and radiolarian productivity. Enhanced dissolution of carbonate would have accounted for the selective abundance of radiolarians; this, in turn, indicates eutrophic conditions at least during the late Albian, an event that has so far been identified in northwestern Mediterranean Tethyan margins. Acknowledgments We thank Mr Aïdi Ben Said, senior geologist in Office National des Mines of Tunis for his assistance during the field component of this study. We are also indebted to Drs Moncef Saidi, Fabrice Cordey, Mourad Bedir, Taher Zouaghi and Mrs Neila Chine, for their invaluable support during this project. Our special thanks go to K. Lasta and Dr H. Tsikos for helping with linguistic improvements to our manuscript. We are also grateful to Professor Hassan Abdallah for his help during the preparation of the manuscript. Dr M. B. Hart and two anonymous reviewers are warmly acknowledged for their constructive reviews and feedback. References Arthur, M.A., Jenkyns, H.C., Brumsack, H.J., Schlanger, S.O., 1990. Stratigraphy, geochemistry and paleoceanography of organic carbon-rich Cretaceous sequences. In: Ginsburg, R.N., Beaudoin, B. (Eds.), Cretaceous Resources, Events and Rhythms. Kluwer, Dordrecht, pp. 75e119. Barron, E.J., Washington, W.M., 1985. Warm Cretaceous climates: high atmospheric CO2 as a plausible mechanism. In: Sundquist, E.T., Broeker, W.S. (Eds.), The Carbon Cycle and Atmospheric CO2: Natural Variations Achaean to Present, vol. 32. American Geophysical Union Monograph, Washington, DC, pp. 546e553. Ben Fadhel, M., Layeb, M., Ben Youssef, M., 2010. Upper Albian planktic foraminifera and radiolarian biostratigraphy (Nebeurenorthern Tunisia). Comptes Rendus Palevol 9, 73e81. Ben Haj Ali, N., Ben Haj Ali, M., 1996. Caractéristiques lithologiques et biostratigraphiques du Crétacé inférieur de la région du Krib (Tunisie septentrionale). Géologie de l’Afrique et de l’Atlantique Sud: Actes Colloque. Angers, France, 585e597. Ben Haj Ali, N., Tandia, I., Razgallah, S., 2002. Les événements biologiques et sédimentologiques de l’Aptien-Albien du Nord-Ouest de la Tunisie. Documents des Laboratoires de Géologie de la Faculté de Sciences de Lyon 156, 38. Ben Haj Ali, N., Ben Haj Ali, M. (2008). Paleogeographic evolution and lower Cretaceous biochronostratigraphic subdivision of a southern Tethyan margin: Example of Tunisia. International Geological Congress. Abstract. August 6 e 14, Oslo, Norway. Bice, K.L., Norris, R.D., 2002. Possible atmospheric CO2 extremes of the middle Cretaceous (Late AlbianeTuronian). Paleoceanography 17, 1e17. Bismuth, H., Boltenhagen, C., Donze, P., Le Févre, J., Saint-Marc, P., 1982. Etude sédimentologique et biostratigraphique du Crétacé Moyen et Supérieur du Djebel Semmama (Tunisie du Centre Nord). Cretaceous Research 3, 171e185. Bolze, J., 1954. Ascension et percée des diapirs au Crétacé moyen dans les monts de Téboursouk (Tunisie septentrionale). Compte-rendu sommaire et bulletin de la Société Géologique de France 8, 139e141. Bornemann, A., Pross, J., Reichelt, K., Herrle, J.O., Hemleben, C., Mutterlose, J., 2005. Reconstruction of short-term palaeoceanographic changes during the formation of the ‘Niveau Breistroffer’ (OAE 1d, SE France). Journal of the Geological Society of London 162, 623e639. Bouaziz, S., Barrier, E., Soussi, M., Turki, M.M., Zouari, H., 2002. Tectonic evolution of the northern African margin in Tunisia from paleostress data and sedimentary record. Tectonophysics 357, 227e253. Bralower, T.J., Sliter, W.V., Arthur, M.A., Leckie, R.M., Allard, D.J., Schlanger, S.O., 1993. Dysoxic/anoxic episodes in the AptianeAlbian. In: Pringle, M.S., Sager, W.W., Sliter, W.V., Stein, S. (Eds.), The Mesozoic Pacific: Geology, Tectonics, and Volcanism. American Geophysical Union Monograph, vol. 77, pp. 5e37. Bralower, T.J., Premoli Silva, I., Malone, M.J., 2002. New evidence for abrupt climate change in the Cretaceous and Paleogene: an Ocean Drilling Program expedition to Shatsky rise, northwest Pacific. GSA Today (Geological Society of America) 11, 4e10. Bréhéret, J.G., 1985. Indice d’un évènement anoxique étendu à la Téthys alpine, à l’Albien inférieur (évènement Paquier). Comptes Rendus de l’Académie des Sciences, Paris, t. 300 8 (série II), 355e358.

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