M Ben Fadhel, T Zouaghi, A Amri, M Ben Youssef. Radiolarian and Planktic Foraminifera Biostratigraphy of the Early Albian Organic Rich Beds of Fahdene Formation, Northern Tunisia. Journal of Earth Science, 2014, 25(1): 45-63. doi: 10.1007/s12583-014-0399-5
Citation:
M Ben Fadhel, T Zouaghi, A Amri, M Ben Youssef. Radiolarian and Planktic Foraminifera Biostratigraphy of the Early Albian Organic Rich Beds of Fahdene Formation, Northern Tunisia. Journal of Earth Science, 2014, 25(1): 45-63. doi: 10.1007/s12583-014-0399-5
M Ben Fadhel, T Zouaghi, A Amri, M Ben Youssef. Radiolarian and Planktic Foraminifera Biostratigraphy of the Early Albian Organic Rich Beds of Fahdene Formation, Northern Tunisia. Journal of Earth Science, 2014, 25(1): 45-63. doi: 10.1007/s12583-014-0399-5
Citation:
M Ben Fadhel, T Zouaghi, A Amri, M Ben Youssef. Radiolarian and Planktic Foraminifera Biostratigraphy of the Early Albian Organic Rich Beds of Fahdene Formation, Northern Tunisia. Journal of Earth Science, 2014, 25(1): 45-63. doi: 10.1007/s12583-014-0399-5
The lower part of Fahdene Formation outcropping in northeastern Tunisia is represented by pelagic sequences and characterized by an important siliciclastic fraction that includes radiolarian and organic-rich beds of Allam Member. Litho-biostratigraphic analysis based on planktic foraminifera and radiolaria shows that deposition of organic-rich beds of Allam Member is confined to the Microhedbergella rischi Zone through the lower part of Ticinella primula Zone. Age diagnostic radiolaria recovered from these beds has been identified to constrain a direct age of black shale deposition. The assemblages can be correlated with the radiolarian biochronozone U.A.10–11 indicating an Early Albian to early Middle Albian in age. Radiolarian assemblages composed of species characteristic of U.A.10 biochronozone (A. montisserei, D. gracilis) are associated with radiolarian taxa belonging to Costata Zone (U.A.9 Pseudoeucyrtis hanni and Thanarla pseudodecora), which can be correlated with Dercourt Member in the Ionian Zone and Paquier level of the Vocontian Basin (southeast of France). Biostratigraphic and abundance curves analysis has demonstrated significant diversification of cryptocephalic Nassellaria and Archaeodictyomitrae, probably adapted to eutrophic conditions that characterized the Aptian-Albian transition. Ecological conditions may have governed the stratigraphic range of many cosmopolitan taxa (i.e., Pseudodictyomitrae lodogaensis) compared with stratigraphic distributions schemes reported from other domains. The timing of black shales deposition was diachronic due to local geodynamic conditions and upwelling currents distribution. The Allam black shales are correlative with the oceanic anoxic event OAE1b which is characterized by the widespread of supraregionnally organic-rich beds in the Mediterranean Tethys basins across the Aptian-Albian transition.
The Early Albian pelagic sequences of western Tethyan margins include discrete organic-rich beds yielding diagnostic radiolarian microfauna (Danelian et al., 2007; Erbacher and Thurow, 1997). Previous biostratigraphic works on AptianAlbian transition of north Central Tunisia focused on ostracods (Zghal et al., 1997; Khayati-Ammar, 1996; Bismuth et al., 1981), ammonites (Chihaoui et al., 2010; Lehmann et al., 2009; Memmi, 1999) and planktic foraminifera (Ben Haj Ali, 2005). These authors have discussed the deposition age of Late Aptian Serj and/or Hameima Formation and Early Albian Lower Fahdene Formation, and concluded that Lower to Middle Albian pelagic successions are diachronic and affected by hiatuses. In addition, some authors stated that Lower to Midlle Albian pelagic successions of northeastern Tunisia can't be differentiated due to poor preservation of planktic foraminifera (Jauzein, 1967).
So far, there are only few studies on Early Albian radiolaria. The first dating attempts in the Tunisian realm using radiolarian assemblages were focused on Jurassic "radiolarites" of the Tunisian Ridge (Dorsale "Tunisienne") (Boughdiri et al., 2007; Cordey et al., 2005). More recently, Ben Fadhel et al. (2010) and Soua et al. (2006) discussed the paleoenvironmental significance respectively of Late Albian and Cenomanian-Turonian radiolarian-rich beds of northern Tunisia.
Here we present integrated biostratigraphic analysis of radiolarian and planktic foraminiferal assemblages from Allam Member in northern Tunisia outcropping in Jebel Garci, Ain Slim and Jebel Ragoubet Lahnèche sections.
The aim of this article is also to present: (1) new documentation of radiolarian species extracted from organic-rich beds which is the first attempt of Early Albian successions dating in northeastern Tunisia; (2) an integrated biostratigraphy using planktic foraminifera and radiolarian of organic-rich beds of Allam Member; (3) constraining the timing of black shales deposition and correlation with equivalents black shales recorded in the Mediterranean Tethys basins.
2.
GEOLOGICAL SETTING AND PREVIOUS WORKS
The studied sections are located in northeastern Tunisia, and belong to two different structural domains. The section of Ain Slim-Zebbas (Fig. 1a) is bordering the "salt-glacier" extrusion of Bir Lafou (Ben Chelbi et al., 2006) located in Bir M'Cherga area which belongs to the northeastern part of the Dome belt (Vila et al., 1996; Perthuisot, 1978). The area is characterized by NNE-SSW-trending anticlines dominated by the Triassic structure of Jebel Aouinet. It is bordered to the southeast by the Zaghouan thrust and to the north by the prolongation of the "Té- boursouk suture" (Fig. 1).
The Triassic of Jebel Aouinet anticline consists of "Germanic" facies (anhydrites, dolomites and cargneules) and shows in its eastern slope, evaporites lenses, interstratified within Aptian to Early Albian shales (Ben Chelbi et al., 2006).
Early Cretaceous outcrops in Jebel Zebbas located northeast of Jebel Aouinet, consist of Barremian to Late Aptian pelagic successions of gray clay and quartzite beds which yield Puzosia getulina, Ptuchoceras laeve, Phylloceras winckleri ammonites and belemnite fragments (Jauzein, 1967). The Albian sequences consist of laminated limestone beds and organic-rich black marls including pyrite nodules (Talbi, 1991)
The Fadeloun-Garci-Mdeker structure (Fig. 1b) is composed of three anticlines, trending north-south and is considered as the northern prolongation of the N-S axis (Saadi, 1990). The anticlines are separated from the Atlas domain by the Zaghouan thrust, which its northeastern part becomes south-verging, commonly defined by the Chérichira-Kondar thrust (Khomsi et al., 2004).
The Cretaceous sedimentation was, under the control of syn-sedimentary faults trending 140ºN–160ºN reflected by chaotic and gravitational deposits (Saadi, 1991).
Early Cretaceous successions show northward, reduced thickness and affected by hiatus and extreme condensations in Hammam Zriba (Saadi, 1990). The motion of a corridor trending north-south by 140ºN–160ºN faults has led to the fragmentation of the seafloor in the lozenge-shaped basin (Saadi and Duee, 1991). During the Valanginian–Barremian time span, these basins were supplied by siliciclastic deposits while condensed sedimentation occupied uplifted horsts (Saadi et al., 1994; Biely et al., 1973).
The Ragoubet Lahneche anticline (Fig. 1c) is located westward of Ain Slim Section, 20 km far from the Teboursouk Village. The structure is a part of the Jebel Goraa structure, an NE-SW roosted syncline. The NW flank is cross cut by a Triassic lens separating the Neogene from Cretaceous successions. Southward, Aptian and Albian deposits outcropping in western border of Ragoubet Lahneche structure are pierced by the Triassic salt extrusion. The faults bordering the western flank of Jebel Goraa structure are responsible for a normal downthrow of Cretaceous successions which were overlain by quaternary alluvium of Oued Tessa and Arkou rivers (Hammami, 1999).
The first stratigraphic nomenclature of Fahdene Formation was proposed by Burollet (1956). That author subdivided the Fahdene Formation into five members: (1) The Lower Shale Member, composed of dark shale and marls, including an ammonite-rich horizon which Burollet et al. (1983) attributed it to the Latest Aptian; (2) the Allam Limestone Member, composed of black massive limestone and dark shale attributed to the Middle Albian; (3) the Middle Shale Member, which is composed of black shale and marl, attributed to the Late Albian; (4) the Mouelha Limestone Member, composed of black, laminated and bituminous limestone bed of Late Albian age; (5) the Upper Shale Member, composed of black shales embedding some marly intervals of Vraconian age (Uppermost Albian).
Then Bismuth (1973) established a biostratigraphic framework for the Aptian-Albian transition based on ostracod fauna. Saadi (1991) discussed the biostratigraphic framework of Jebel Garci Section based on planktic and benthic foraminifera as well as faunal content of reefal limestones of Serj Formation. She assigned the black shales of Lower Fahdène Formation to Early– Midlle Albian age and the hardground surface outlining the reefal limestone bed as an unconformity within the Aptian-Albian boundary.
Later, Robaszynski et al.(2008, 1994) proposed a detailed biostratigraphic and sedimentological analysis of the Fahdène Formation based on planktic foraminifera and ammonites and described the microfossil distribution within the AlbianCenomanian boundary.
More recently, biostratigraphic studies were focused mainly on the lower part of Fahdene Formation which outlined the Aptian-Albian transition, based mainly on ammonite (Chihaoui et al., 2010; Memmi, 1999) or using planktic foraminifera for age calibration of the "interstatified salt glaciers" outcropping in Ragoubet Lahnéche and Fej Lahdoum areas (Ghanmi et al., 2001; Vila et al., 1996, 1994).
Studies on structural and geodynamic of Triassic salt bodies outcropping in Bir M'Cherga area and cross cutting the Cretaceous succession of Jebel Zebbas show that salt wall has an Aptian age based on the presence of Planomalina cheniourensis
(Ben Chelbi et al., 2006). The Late Aptian to Early Albian age of salt roof (80 m thickness) is estimated based on the presence of an association of Hedbergella planispira Tappan, Vaginella recta Reuss, Frondicularia filocuicta Reuss, Lenticulina turgidula Reuss, L. pulchella Reuss, Tristix concavata Reuss (Ben Chelbi et al., 2006).
Ben Chelbi et al. (2006) stated that normal microfaults, cross cutting Clansayesian limestone beds and sealing Albian marls of Lower Shale Member, indicate an extensive tectonic phase which is responsible for the tilting of the sedimentary blocks. The time equivalent outcrops in Jebel Garci show olistostromes, burrowed surfaces, and condensed deposits as indicators of tectonic activity associated with marine sea-level fall (Saadi et al., 1994; Saadi, 1991).
3.
MATERIALS AND METHODS
A total of 48 samples of marl and limestone were collected for biostratigraphic analysis. Organic-rich levels were sampled at relatively higher resolution (approximately every 50 cm). Soft samples were crushed and soaked in hydrogen peroxide solution and then washed using standard techniques. Samples were sieved sequentially using meshes of 500 mm-250 mm-125 mm-63 mm. Each sieve was stained with methyl blue (10 g/L) to avoid contamination from previous samples. Specimens with good preservation determined and picked under Wild Herbrugg light binocular microscope. Abundance of radiolaria is expressed semiquantitatively by counting the total number of preserved specimens (per 1 g of sample). Planktic foraminifers and radiolarians were mounted on slide, using double-faced adhesive tape, coated with silver and photographed using JEOL JSM 5400 scanning electron microscope of the ETAP Research Center, Tunis.
4.
RESULTS
4.1
Jebel Garci Section
The condensed section of Jebel Garci (Fig. 2) begins with orbitolinids-rich green to gray clay and discontinuous sandy limestone beds alternations which are attributed to the Hameima Formation. The clay intervals have also provided fragments of rudist and bryozoans (GA1). The top is outlined by olistolites deposits that gradually pass to a reefal limestone. The top is capped by an oxidized hardground which is overlain by detrital quartz rich-marly interval that provided Hedbergella trocoidea
(Gandolfi), Microhedbergella pseudodelrioensis Huber and Leckie, and Microhedbergella praeplanispira Huber and Leckie (GA6).
Figure
2.
Radiolarian and planktic foraminifer biostratigraphy of Jebel Garci Section. 1. Thin bedded limestone; 2. thin laminated limestone; 3. reefal limestone; 4. sandy limestone; 5. black shales; 6. marl (gc. light gray; gf. dark to gray); 7. green to gray shale.
Upwards, the black gray marl interval (GA9–10) did not provide well-preserved planktic foraminifera. Some small-sized planktic foraminifer species are identified associated with benthic foraminiferal assemblage composed by rare epistominids and Conorboides lamplughi (Sherlock) (GA10).
The next successions (GA13–GA20) consist of centimeterthick grey to dark laminated limestone bed and organic-rich black marls intervals. Upwards, the succession is rhythmic and the marly intervals increase in thickness contrary to limestone beds. The microfauna content yields poor planktic foraminiferal assemblages and radiolarian rich microfauna.
Planktic foraminifera reappear on GA20 bed that yields Hedbergella gorbachikae Longoria Microhedbergella praeplanispira Huber and Leckie and Microhedbergella pseudodelrioensis Huber and Leckie. The benthic foraminiferal assemblages consist of Osangularia schloenbachi (Reuss) and Valvulineria infracretacea Crespin.
Fractures related to a strike slip fault outlining the black hales unit are onlapped by a marly interval and gray limestone beds alternation (GA24–GA27). This unit is characterized by highly abundant of benthic GA25 sample has provided benthic foraminiferal assemblages composed of Fursenkoina viscida
(Khan), Spirillina minima Schacko, Lenticulina sp., Nodosaria sp., Ramulina aculaeta (D'Orbigny), Valvulinoides sp., Gavelinella sp.. Planktic foraminifera are represented by M. praeplanispira, (Gandolfi) and Microhedbergella pseudodelrioensis Huber and Leckie.
The organic-rich beds comprised between GA9 and GA23 have released a moderately to well-preserved and agediagnostic radiolarian species.
Radiolarians appear with few discrete taxa within GA6 level. It provides an assemblage composed of Holocryptocanium barbui Dumitrica, Spongostichomitra elatica (Aliev), Pseudoeucyrtis hanni (Tan), Archeodictyomitra vulgaris Pessagno, Thanarla brouweri (Tan), Stichomitra simplex (Smirnova and Aliev), Angulobracchia portmanni Baumgartner, Thanarla pacifica Nakaseko and Nishimura. They become diversified and abundant within GA7. This latter provided an association of Dictyomitra aff.
gracilis (Squinabol), Dictyomitracommunis Squinabol, Archaeodictyomitra montisserei (Squinabol), Pseudodictyomitralodogaensis Pessagno, Thanarla praeveneta Pessagno, Archaeodictyomitra aff. A. vulgaris Pessagno, Hiscocapsa sp., Thanarla aff. pulchra (Squinabol), Spongostichomitra elatica (Aliev),
Thanarla brouweri (Tan), Thanarla pseudodecora (Tan), Angulobracchia portmanni Baumgartner, Stichomitra simplex (Smirnova and Aliev), Stichomitracommunis Squinabol.
The upper part of black shales interval (G17–23), composed by rhythmic bundles of limestone and marl beds, is characterized by a decrease of radiolarian abundance. The sample GA20 has released a radiolarian assemblages composed of Thanarla brouweri
(Tan), Spongostichomitra phalanga O'Dogherty, Pseudodictyomitraparonai (Aliev), Dictyomitracommunis Squinabol, Holocryptocanium barbui Dumitrica, Pseudodictyomitralodogaensis Pessagno, Dictyomitragracilis (Squinabol), Archaeodictyomitra montisserei (Squinabol), Spongostichomitra elatica (Aliev),
Pseudodictyomitraparonai (Aliev).
Marly interval of the top GA27 has released an assemblage of Holocryptocanium barbui Dumitrica, Stichomitrasimplex
(Smirnova and Aliev), Pseudodictyomitraparonai (Aliev), and Dactyliosphaera maxima (Pessagno).
4.2
Ain Slim Section
The Ain Slim Section, in vertically contact with Triassic evaporites (Fig. 3), begins with detrital and bipyramidal quartz-rich silty marls (AS1–6). They released diversified planktic foraminiferal assemblages composed of Globigerinelloides ferreoloensis (Moullade), Globigerinelloides aptiensis Longoria, Hedbergella trocoidea (Gandolfi), Planomalina cheniourensis Sigal,
Clavihedbergella (Lilliputianella) bizonae (Chevalier). The benthic microfauna is composed of Lenticulina sp., Valvulineria sp.,
Gavelinella berthelini (Ten Dam). Upward, barite nodules-rich marly interval has yielded Paraticinella eubejaouaensis (Randrianasolo and Anglada) and some tiny hedbergellids (AS7–AS10).
Figure
3.
Planktic foraminifera biostratigraphy of Ain Slim Section. 1. Triassic; 2. thin bedded limestone; 3. green shale; 4. green to gray shale; 5. black shales; 6. shaly limestone; 7. barite concretions; 8. marls (gc. light gray; gf. dark to gray; gn. dark).
We also identified sporadic occurrence of Saracenaria sp. and some spherical radiolarian fauna (AS10).
The next intervals (AS11–AS14) consist of dark pyrite nodules-rich marls and limestone beds that include stratiform barite mineralizations. Planktic foraminifera are less diversified in opposition to benthic foraminifera. They are poorly preserved within dark detrital quartz-rich marls (AS11–12–13) and are represented only by Microhedbergella pseudodelrioensis Huber and Leckie, Hedbergella trocoidea (Gandolfi) and Microhedbergella praeplanispira Huber and Leckie.
AS11 sample provided an association of Globulina prisca Reuss, Gavelinella baltica Brotzen, Gyroïdinoides sp., Lingulogavelinella albiensis Malapris, Berthelina intermedia
(Berthelin).
The basal part of shaly limestones and pyritic nodules-rich gray alternations (AS13) provided Lenticulina sp., Gavelinella intermedia (Berthelin), Ramulina aculatea Wright, Valvulineria infracretacea Crespin, Gyroidinoïdes niditus (Reuss), Gavelinella ammonoïdes (Reuss) and Ticinella madecassiana Sigal.
AS16 beds underlying organic-rich laminated marl and limestone beds show the first occurrence of Ticinella roberti (Gandolfi). Well-preserved species of benthic foraminifera were identified such as Lenticulina munsteri (Roemer), Marginulina inaequalis Reuss and Gavelinella sp. (AS17).
Stratigraphically upward, the abundance of benthic foraminifera decreases whereas planktic foraminifera become abundant (AS20). The first occurrence of Ticinella primula Luterbacher coincides with shaly limestone bed (AS18). The marly interval (AS21), rich in detrital materials, provided Ticinella roberti (Gandolfi), Ticinella primula Luterbacher and a benthic association composed of lenticulines, Spiroplectinata annectens (Parker and Jones), Gavelinella intermedia (Berthelin),
Laevidentalinacommunis (D'Orbigny).
4.3
Ragoubet Lahneche Section
The section (Fig. 4) begins with chaotic Triassic evaporites and conglomerates which are overlain by dark greyish, slumped marl interval (BLH0–BLH2, thickness: 2.50 m) showing centimeter-thick secant fractures and filled with calcite mineralization (BLH0–BLH2). The marl interval provided pyrite cubes, tiny hedbergellids, mineralized skeletons of spherical radiolarian and some specimens of Microhedbergella praeplanispira Huber and Leckie. The next successions (BLF2, BLF4–6) consist of 40 m of grey to dark marl, passing upward into bituminous dark, splintery and organic rich limestone beds of the Allam Member (BLF3, BLF4 to 6). They yield relatively abundant planktic foraminiferal microfauna with partially mineralized skeletons. Upward, the microfauna become rare, particularly within organic-rich beds.
Figure
4.
Radiolarian and planktic foraminifer biostratigraphy of Ragoubet Lahneche Section. Legend see Figs. 2 and 3.
The interval comprised between BLH2 and BLH3 has provided Microhedbergella praeplanispira Huber and Leckie, Microhedbergella pseudodelrioensis Huber and Leckie, Ticinella roberti (Gandolfi), Globigerinelloides maridalensis Bolli, Globigerinelloides eaglefordensis (Moreman), Ticinella aff. yezoana (Takayanagi and Iwamoto). The first occurrence of Ticinella primula Luterbacher is recorded within BLH3 bed, associated with scarce ostracods. It also yields a moderately-preserved radiolarian microfauna composed of Crolanium sp., Dactyliospharea maxima
(Pessagno), Obeliscoites perspicuous (Squinabol), Cavaspongia sphaerica O'Dogherty, Stichomitracommunis Squinabol. Sample from BLH8 has yield diversified microfauna of radiolarian and planktic foraminifera dominated by species of the genus Globigerinelloide. The assemblage is composed by Globigerinelloides bentonensis Morrow, Hedbergella sp. and Ticinella madecassiana Sigal.
Radiolarians are very abundant within organic-rich bed of Allam Member. BLH8 sample has provided an assemblage composed of Pessagnobrachia fabianii (Squinabol), Godia concava
(Li & Wu), Savaryella novalensis (Squinabol), Dactyliosphaera acutispina Squinabol, Cavaspongia sphaerica O'Dogherty, Holocryptocanium barbui Dumitrica, Stichomitracommunis Squinabol, Savaryella quadra (Foreman), and Torculum coronatum
(Squinabol).
The interval comprised between BLH9 and BLH11 (thickness: 11 m) correspond to grey marl and grey coarse-massive limestone bed alternations. The radiolaria microfauna is rare. The BLH11 has provided some specimens of Microhedbergella praeplanispira Huber and Leckie Globigerinelloides bentonensis Morrow.
5.
DISCUSSION
5.1
Biological Events and Biostratigraphic Attribution
The turbidic reefal and sandy limestone and green shales alternations did not yield diagnostic planktic foraminifera microfauna. However, this interval could lithologically be correlated with the Hameima Formation in northeastern Tunisia of Upper Gargasian to Clansayesian in age (Chihaoui, 2009).
The next successions overlying the hardground surface have yielded abundant Microhedbergella praeplanispira Huber and Leckie that indicates an Early Albian age (Moullade et al., 2002). In addition, co-occurrence of Early Albian taxa such as Osangularia schloenbachi (Reuss), Fursenkoina viscida (Khan) (Holbourn et al., 2001; Erbacher et al., 1998) respectively in GA20 and GA25 beds, assign at least the upper part of the black shales to the Early Albian (GA17 to GA23).
In the following section, we used radiolarian calibration in order to improve the resolution of biostratigraphic record. The assemblages recovered from black shale beds are discussed herein.
Samples recovered from basal beds (GA2–GA6) show high abundance of Pseudodictyomitralodogaensis Pessagno and contain some Early Cretaceous taxa from Turbocapsula Zone such as Thanarla pseudodecora (Tan) and Pseudoeucyrtis hanni (Tan)
(Zyabrev, 2011; Michalik et al., 2008; Danelian et al., 2007; Erbacher and Thurow, 1998; O'Dogherty, 1994). Thus, a Late Aptian age of these beds could not hitherto be ruled out.
According to Erbacher and Thurow (1998), the first occurrence of Pseudodictyomitralodogaensis Pessagno coincides with the upper part of Globigerinelloides algerianus Zone. Its last occurrence coincides with the Aptian-Albian boundary and the first occurrence of Mitagracilis (=Dictyomitra gracilis). On the other side, this taxon is also recorded within Albian to Cenomanian deposits in the Atlantic domain, California and Pacific realms (Palechek et al., 2010; Kariminia, 2006; Thurow, 1988).
Slaczka et al. (2009) described an assemblage containing Angulobracchia portmanni Baumgartner, Dictyomitra communis (Squinabol), Hiscocapsa asseni (Tan), Pseudodictyomitralodogaensis Pessagno, Pseudoeucyrtis hanni (Tan), almost similar to GA7 taxa. These authors attributed the assemblage to Costata Zone that is confined to U.A.6–9 biochronozones of Midlle to Late Aptian age (O'Dogherty, 1994).
It is noteworthy to point to the coexistence of Albian species in all samples such as Dictyomitramontisserei Squinabol and Dictyomitragracilis Squinabol with Aptian taxa particularly in GA7, GA15 and GA26. In this overall context, an assemblage recovered from Mid Cretaceous outcrops of northern Tethys margins was described by Danelian et al. (2007), shows the co-occurrence of Pseudodictyomitralodogaensis Pessagno, Dictyomitragracilis Squinabol Archaeodictyomitra aff. vulgaris Pessagno assigning it to the Early Albian U.A.10–11 biochronozone. Danelian et al. (2004) consider that an Early Albian age of Dercourt Member cannot be ruled out despite the presence of pseudoeucyrtis cf. hanni (Tan) characteristic of U.A.9. These species are hitherto observed within an assemblage from GA15, associated with Archaeodictyomitra montisserei (Squinabol).
Kurilov and Vishnevskaya (2011) described an assemblage extracted from Early Cretaceous outcrops of Pacific domain that does not differ from GA21. It contains Pseudodictyomitralodogaensis Pessagno, Holocryptocanium barbui Dumitrica, Dictyomitra cf. montisserei Squinabol, Dictyomitracommunis
(Squinabol) and Dictyomitragracilis Squinabol indicating an Early Albian age.
The samples GA26 have provided an assemblage characterized by high abundance of Hiscocapsa asseni (Tan) co-occurring with Dictyomitragracilis Squinabol and Dictyomitramontisserei Squinabol. It lies with the U.A.10 biochronozone of Romanus Zone (Danelian et al., 2004; O'Dogherty, 1994).
Basal beds of Ain Slim Section, overlying Triassic evaporites (AS1–AS6), have provided planktic foraminifera-rich assemblages mainly composed of Globigerinelloides species (G. ferreolensis, Globigerinelloides aptiensis (AS1), Clavihedbergella (Lilliputianella) bizonae (Chevalier), Globigerinelloides aptiensis (Longoria) and Planomalina cheniourensis Sigal (AS3). These occurrences are confined to the upper part of algerianus Zone, based on the lowest occurrence of Planomalina cheniourensis, which indicates a Middle Aptian age (Huber and Leckie, 2011).
The sample AS6 shows an assemblage composed of Hedbergela trocoidea, Hedbergella excelsa, Hebergella gorbachikae and G. ferreolensis (AS6) which is confined to the Upper Aptian. Upward, the marly interval has yielded depauperate planktic foraminifera fauna. The samples AS9 and AS10 contain small well-preserved specimens of Paraticinella eubejaouaensis which is considered by many authors as a marker specie to identify the Aptian-Albian boundary (Bellier and Moullade, 2002; Silva and Sliter, 1999; Sliter, 1989).
The next upper beds yield strongly impoverished planktic faunas in opposition to a high abundance of benthic foraminifera (GA11), an event which was regarded as a timeline outlining the Aptian-Albian boundary (Ben Haj Ali, 2005)
The next dark marly interval (AS11–AS15) contains small nodules of pyrite, and yields Mic. praeplanispira, Micr. praedelrioensis and benthic-rich assemblages composed of Globulina prisca Reuss, a cosmopolitan specie of Lower Cretaceous age associated with Osangularia schloenbachi (Reuss), Berthelina intermedia (Berthelin) which indicates an Early Albian age (Tyszka, 2006; Holbourn et al., 2001; Erbacher et al., 1998; Holbourn and Moullade, 1998).
Shaly limestones of Allam Member (AS14–16) which overlain by organic-rich thin laminated limestone beds (OX2–OX8) show an assemblage composed typically by Ticinella madecassiana and Micr. praeplanispira, associated with and benthic microfauna indicating an Early Albian age.
The occurrence of Ticinella primula (AS18) is recorded 17 m above the black shale beds (OX2–OX8). Upward, the series are composed of fractured limestone beds and gray marl intervals (AS19–31) which are confined to the acme zone of T. roberti and T. raynaudi and characterizing the upper part of primula Zone (Sliter, 1989).
The Ragoubet Lahneche anticline was the subject of an integrated study using biostratigraphy and seismic reflection data in order to explain the salt tectonic evolution in northern Tunisia (Vila et al., 2002). These authors have postulated that conglomerate bed overlying the Triassic evaporites have a Middle Albian age based on the presence of Hedbergella planispira and Favusella washitensis. Then, Ben Haj Ali (2005) stated that marlous bed in contact with Triassic evaporites has a Late Albian age based on the presence of Hedbergella planispira, Biticinella breggiensis and Ticinella primula together with abundant radiolarian microfauna.
Slumped marly interval (BLH0-2) overlying Triassic evaporites (Fig. 4) provided an abundant tiny Hedbergellids such as Microhedbergella praeplanispira Huber and Leckie. The first occurrence of Ticinella primula Luterbacher is recorded within BLH3 bed thus the organic-rich beds of Allam Member may outline the late Early Albian.
The assemblage provided by the interval BLH3–BLH11 is composed of Microhedbergella praeplanispira Huber and Leckie,
Ticinella roberti (Gandolfi), Ticinella yezoana Takayanagi and Iwamoto Microhedbergella rischi Huber and Leckie, Microhedbergella pseudodelrioensis Huber and Leckie, Ticinella primula Luterbacher, an association that is inferred to the Middle Albian.
The BLH8 has provided an assemblage containing strictly Albian species together with Dactyliospharea acutispina
(Squinabol). Following (Erba et al., 1999) the first occurrence of Dactyliosphaera acutispina (Squinabol) species is recorded within the Selli Livello black shales of Early Albian age. Bak (1999) stated that first occurrence of Dactyliospharea acutispina
(Squinabol) is recorded within the upper part of Rotalipora ticinensis-Planomalina praebuxtorfi foraminiferal zone through R. appenninica foraminiferal zone. Then, Kemkin (2009) described an assemblage from the Far East domain of Russia containing Dactyliosphaera acutispina (Squinabol) together with Dactyliosphaera maxima (Pessagno). He attributed the assemblage to the Middle–Late Albian. An assemblage containing this species is also described within Late Albian black shales of northeastern Tunisia (Ben Fadhel et al., 2010). The taxonomic and stratigraphic range discrepancies of Dactyliospharea acutispina
(Squinabol) has been debated in Kemkin and Filippov (2010) who stated that range of the species is the Middle Albian–Early Cenomanian and not the Barremian. Taking into account the microfauna content, we suppose that the organic-rich beds of Allam Member lies with the upper part of U.A.10–U.A.11 biochronozones of Early to Middle Albian in age (O'Dogherty, 1994).
We think that variation of stratigraphic ranges between Mediterranean Tethys and East Pacific domains may be influenced by ecological and paleoceanographic conditions including water temperature and connections between domains during the Albian.
Studies on radiolarian biostratigraphy show that Early Albian zonal schemes still not well established (Danelian et al., 2004; O'Dogherty, 1994). For example, O'Dogherty (1994) has assigned the Early–Middle Albian interval to the Mallanites romanus Zone. Then, he modified the subdivision and assigned the upper part of the Costata subzone to the Early Albian which the top coincides with the upper part of Ticinella primula planktic foraminiferal zone (O'Dogherty and Guex, 2002).
Following the zonal schemes of (Erbacher and Thurow, 1998, 1997), the first occurrence of Mita gracilis (=Dictyomitra gracilis) is recorded within the Hedbergella planispira Zone (=Microhedbergella rischi Zone of Huber and Leckie, 2011) and considered as a marker species of Aptian-Albian boundary. Furthermore, these authors have assigned the base of Pseudodictyomitralodogaensis Zone to the upper half G. algerianus planktic foraminifera zone of Late Aptian age which the top coincide the FO (first occurrence) of Mita gracilis. However, it was stated that Mita gracilis occurs much earlier in Hauterivian–Barremian successions of Umbria Marche Basin in Central Italy (Danelian et al., 2007; Jud, 1994).
In their study about Mid Cretaceous black shales of Ionian domain, Danelian et al. (2007), Danelian (2008) stated an abundant diversified archaeodictyomitrae taxa within Lowermost Albian successions. They postulated that last occurrence of Thanarla brouweri (Tan) is recorded within Middle Albian U.A.11 biochronozone, equivalent to the planktic foraminifera T. primula Zone. Nevertheless, it was demonstrated that stratigraphic range of Thanarla brouweri could reach the CenomanianTuronian boundary, particularly within the lower part of Bonarelli Horizon (Musavu-Moussavu et al., 2007).
Considering biotic changes across the Aptian-Albian boundary (Danelian, 2008) we think that constraining the age of black shale beds based solely on radiolarian assemblages, may reveals discrepancies because of similarities between archaeodictyomitrae morphospecies and mixing with strictly Aptian taxa.
We suggest that lower part of black shale intervals could be assigned to the upper part of Costata Zone based on the presence of Aptian taxa (i.e., Pseudoeucyrtis hanni, Thanarla pseudodecora). 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 Pseudoeucyrtis hanni associated with a relative increase in abundance increase of Archaedictyomitrae and Williriedellidae families.
The Romanus Zone described in Jebel Garci show the dominance of high diversified nassellarian species. The assemblage composed of Thanarla brouweri, Archaeodictyomitra montisserei, Thanarla conica may be attributed to the Middle Albian Mallanites romanus Subzone (U.A.10–11 biochronozone)
(Danelian et al., 2004; O'Dogherty, 1994). However, the first occurrence of Ticinella primula planktic foraminifera is recorded 24 m above GA17 bed. Thus, an Early Albian age for the lower part of Romanus Zone cannot be ruled out.
In Ragoubet Lahneche Section, the top of Costata Zone which is outlined by the first set of Allam black shales shows the first occurrence of Ticinella primula planktic foraminifera. The lower part of Romanus Zone which lies with the upper part of Allam black shales is characterized by appearance of an abundant Hagiastridae assemblage (i.e., Savaryella quadra and Savaryella novalensis).
5.2
Relationships between Radiolarian Abundance and Black Shales Deposition
The abundance curves of Allam black shales show gradual increase of radiolarian abundance (Fig. 5) which could indicates a rise of productivity rate. The continuous input of nutrients has created eutrophic environment associated with high rate of planktic productivity. As consequences, significant quantity of organic matter produced by the phytoplankton was degraded by the bacterial activity, thus minimizing the amount of oxygen and enhancing the widespread of anoxic conditions.
Figure
5.
(a) Radiolarian abundance within Allam Member black shales of Jebel Garci Section; (b) Spumellaria vs. Nassellaria ratio (S/N) recorded from the Allam black shales in the Jebel Garci Section.
The decrease in Spumellaria abundance, subsequent to the abundance peak at GA10 bed (Fig. 5), indicates an unstable environment with low rate of productivity. By consequence, opportunistic species (mainly cryptocephalic Nassellaria, and some Archaeodictyomitrae) adapted to low oxygen conditions increased in abundance.
Silva and Sliter (1999) suggest that low S/N (Spumellaria vs. Nassellaria) ratio indicates a high productivity due to an important input of nutrients in the surface sea-water and coupled with intensified upwelling currents. However, the rise of S/N ratio could reflect in certain cases a low-oxygen environment (Gorican et al., 2003; Kuhnt et al., 1986).
Such a tendency was observed GA10 and GA17 where S/N ratio shows linear increase (Fig. 5). It can be explained by the widespread of strictly anoxic conditions following the exhaustion of nutrient supply. If eutrophic conditions become increasingly accentuated, the environment becomes hostile to bloom of Spumellaria (Silva and Sliter, 1999). Other factors, such as the complexity of Spumellaria test structure, made their mobility difficult, which explains the dominance of Nassellaria in S/N ratio (O'Dogherty and Guex, 2002).
Studies on radiolarian paleoecology have demonstrated the existence of relationship between radiolarian abundance and organic matter content (de Wever and Baudin, 1996). Generally speaking, the organic-rich sedimentation was coeval with transgressive system tracts (TST) (Amèdro, 2008; O'Dogherty and Guex, 2002; Arthur and Sageman 1994; Schlanger and Jenkyns, 1976). Moreover, the vertical flux of nutrient-rich waters depends on oceanic circulation and intense upwelling currents which occur during the transition from regressive to transgression episode (O'Dogherty and Guex, 2002). It was also shown that radiolarian high abundance recorded from mid Cretaceous organic-rich and biosiliceous beds indicates an environment characterized by high rate of productivity (Danelian, 2008; Silva and Sliter, 1999).
5.3
Age Correlation of Black Shale Beds
Although Lower Albian didn't provide well preserved planktic foraminifera, radiolaria are thought to be a reliable biostratigraphic tool of large-scale correlation (Boughdiri et al., 2007). The main target of this correlation (Fig. 6) is to compare the biological events as well as the Albian sedimentary record in the Mediterranean Tethys basins.
Figure
6.
Correlation of Albian bio-events and sedimentary records in northeastern Tunisia with equivalent outcrops in northern Tethys margins.
The Jebel Garci Section shows the association of Aptian radiolarian assemblages (i.e., Pseudoeucyrtis hanni, Hiscocapsa sp., Thanarla pseudodecora) with Albian species that is mentioned by Danelian et al. (2004) who suggested that an Early Albian age attributed to Dercourt Member is not excluded. However, we cannot with certainty assign the absolute age of these black shales, since the Lower Albian radiolarian zonation was not identified in the nomenclature of O'Dogherty (1994).
These black shales can hitherto be attributed to the biochronozone U.A.10 based on the co-occurrence of Dictyomitragracilis, Archaeodictyomitra montisserei and Hiscocapsa sp.. Toward the north-east, the age of Allam black shale is rather diachronic. It outlines the Early–Middle Albian successions overlaying the Triassic evaporites. A late Middle Albian age cannot be excluded on the basis of abundant Hagiastridae species (Savaryella quadra and Savaryella novalensis).
Studies on radiolarian biostratigraphy in Atlantic domains (Erbacher and Thurow, 1998) show that organic-rich beds occur within Aptian-Albian transition which lies with the boundary between M. gracilis and P. lodogaensis zones. These two species are abundant within the middle part of black shale set. It is noteworthy that Pseudodictyomitrae lodogaensis records it last occurrence in the Costata Zone. This species has't been found neither in Romanus Zone nor in Late Albian successions (Ben Fadhel et al., 2010). Thus, we attribute an Early Albian to early Middle Albian to the Allam black shales. They can be correlated with the "Upper Shaly Siliceous" interval (Fig. 6) of Sopoti Section in southern Albania (Danelian et al., 2007).
Chihaoui (2009) has postulated through carbon isotope signature, the presence of an equivalent of Paquier level (Herrle et al., 2003; Bréhéret, 1988) between SA3 and SA4 sequence in Tajerouine area (northwestern Tunisia). The author attributed it to the Early Albian Hypacanthoplites buloti ammonite zone. Recently, Ben Fadhel et al. (2011) have proposed a biostratigraphic framework for Albian pelagic successions in Nebeur area. They ascribed the Upper Allam organic-rich beds to T. primula planktic foraminiferal zone.
In this study, biostratigraphic results from northeastern Tunisia sections show that organic-rich beds precede the lowest occurrence of Ticinella primula (AS18) and 88 m above the highest occurrence of Paraticinella eubejaouaensis in Ain Slim Section. This could confirm the assessment of Huber and Leckie (2011) who stated that disappearance of Paraticinella eubejaouaensis is recorded 52 m below the Paquier level in the Vocontian Basin.
Large scale correlations show that timing of black shales deposition is diachronic between northern Mediterranean Tethys margins and Tunisian domains.
The diachronic beginning of Allam black shales deposition was controlled mostly by local geodynamic conditions and upwelling currents distribution in southwest and northeast basins on the one hand Tunisian and northern Tethys areas on the other (Ben Fadhel et al., 2011). In fact, basin configuration of northern Tunisia was the result of SW-NE extension tectonic regime and halokinetic dynamics recorded across the Aptian-Albian transition (Rigane et al., 2010; Guiraud and Maurin, 1992; Boltenhagen, 1985). This tectonic phase and the resulted restricted basins were coeval with the widespread of supraregionnally black shales of Niveau Paquier which is considered as the record of the oceanic anoxic event OAE1b (Danelian, 2008; Tsikos et al., 2004; Herrle et al., 2003; Bréhéret, 1997; Bralower et al., 1993; Bréhéret et al., 1986).
Total organic carbon and Tmax vs. hydrogen index plots analysis carried on Early Albian black shales of Allam Member has demonstrated a good to moderate potential source rock and the organic matter is mainly terrigenous Type III (Ben Fadhel et al., 2011). Southward, the equivalent of Allam black shales (Lower Fahdene Formation) is represented by the Late Aptian– Early Albian reefal limestones of Serj Formation which is considered as a potential reservoir rock of hydrocarbons (Burollet, 1988).
6.
CONCLUSION
Radiolarian and planktic foraminifer biostratigraphic study of Lower Fahdene Formation has provided an age constraint of black shales embedded within the Allam Member. Radiolarian assemblages analysis together with planktic foraminifera biological events show that deposition of Allam Member black shales lies with U.A.10–U.A.11 biochronozone and Microhedbergella rischi through the lowest part of Ticinella primula planktic foraminiferal zone. However, an Earliest Albian age could not be excluded on the basis of typical Aptian taxa.
The deposition of organic-rich beds in northern Tunisia is characterized by radiolarian bloom which reflects high productivity rates and eutrophic environment. As consequences, numerous Aptian taxa (i.e., pseudoeucyrtis hanni, Thanarla pseudodecora) disappeared and many taxa adapted to these conditions (i.e., Archaeodictyomitrae) are represented by various morphotypes. It is noteworthy that ecological conditions may have governed the stratigraphic range of many cosmopolitan taxa (i.e., Pseudodictyomitraelodogaensis) compared with stratigraphic distributions schemes reported from other domains.
The large-scale correlation the black shales of Allam Member with sections reported from North Mediterranean Tethys margins show that organic rich beds are correlable with Niveau Paquier of the Vocontian Basin and equivalent to the supraregionnally distributed oceanic anoxic event OAE1b.
ACKNOWLEDGMENTS:
This work was supported by the «le Ministère de L'Enseignement Supérieur et de la Recherche Scientifique» and «Centre des recherches des technologies des eaux–CERTE». Drs. Moncef Saidi and Hedia Bessaies from ETAP Research Center gave all facilities needed for SEM photographs. We thank Prof. Imen Khemiri for improving the English text. Authors are grateful to Miroslava Smrečková from Comenius University, Bratislava, Slovakia for providing us with documentations and citations about Cretaceous radiolarians in Pacific domain. Comments provided by an anonymous reviewer and the editor are gratefully acknowledged.
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Figure 2. Radiolarian and planktic foraminifer biostratigraphy of Jebel Garci Section. 1. Thin bedded limestone; 2. thin laminated limestone; 3. reefal limestone; 4. sandy limestone; 5. black shales; 6. marl (gc. light gray; gf. dark to gray); 7. green to gray shale.
Figure 3. Planktic foraminifera biostratigraphy of Ain Slim Section. 1. Triassic; 2. thin bedded limestone; 3. green shale; 4. green to gray shale; 5. black shales; 6. shaly limestone; 7. barite concretions; 8. marls (gc. light gray; gf. dark to gray; gn. dark).
Figure 4. Radiolarian and planktic foraminifer biostratigraphy of Ragoubet Lahneche Section. Legend see Figs. 2 and 3.
Figure 5. (a) Radiolarian abundance within Allam Member black shales of Jebel Garci Section; (b) Spumellaria vs. Nassellaria ratio (S/N) recorded from the Allam black shales in the Jebel Garci Section.
Figure 6. Correlation of Albian bio-events and sedimentary records in northeastern Tunisia with equivalent outcrops in northern Tethys margins.
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