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The “Aptian Crisis” of the South-Tethyan margin: New tectonic data in Tunisia
Journal of African Earth Sciences 57 (2010) 360–366
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The ‘‘Aptian Crisis” of the South-Tethyan margin: New tectonic data in Tunisia Adel Rigane a,*, Moncef Feki a, Claude Gourmelen b, Mabrouk Montacer a a b
Département des Sciences de la Terre, Faculté des Sciences de Sfax, Université de Sfax, Unité de recherche, GEOGLOB, BP. 1171, Sfax 3000, Tunisia Laboratoire de Géodynamique des Rifts et des Marges Passives, Faculté des Sciences, Université du Maine, 72085 Le Mans cedex, France
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Article history: Received 27 May 2009 Received in revised form 11 November 2009 Accepted 16 November 2009 Available online 20 November 2009 Keywords: South-Tethyan margin Aptian/Albian Aptian Crisis Normal fault and drape fold Diapirism Tunisia
a b s t r a c t The distribution of Cretaceous sedimentary basins in the central Tunisian Atlas is controlled by major faults. The reactivation of these faults and the diapiric activity of the Triassic evaporites has resulted in a complex tectonic history. Most studies show that the Saharan platform in Tunisia, located on the South-Tethyan margin, was compartmentalized by an extensive tectonic phase towards the end of the Aptian, during an episode known as the ‘‘Aptian Crisis”. In the study area located in Central-Western Tunisia, the Aptian series of Jebel El Hamra and Jebel El Ajered form the core of an anticlinal fold. The Aptian russet-coloured dolomites (Serj formation) are rimmed by alternating marls and limestones of end-Aptian age and the Fahdene formation composed of Albo-Cenomanian marls. The structural and tectono-sedimentary study of Jebels El Hamra and El Ajered highlights the prominent influence of paleostructures acquired during the evolution of the South-Tethyan basin and their control on the existing geometry of the fold belt. Indeed, the present geometry of folds and faults results directly from the block tectonics developed during the Aptian. This Aptian tectonic style is characterized by extensive and/or transtensive deformation leading to multiscal fracturing in structural blocks. This deformation is characterized by a normal or strike-slip normal fault and drape fold system. The age and the extension direction (SW–NE) are fully consistent with the regional extensional phase probably disturbed by diapirism. This ‘‘Aptian Crisis” can be related to the opening of the equatorial Atlantic and Tethys oceans. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In the Tunisian Central Atlas, the distribution of the Cretaceous sedimentary basins is controlled by major structural trends. The reactivation of major faults associated with these structural trends and the Triassic diapirism has resulted in a complex tectonic history. Most studies (Bismuth et al., 1982; Boltenhagen, 1985a,b; Soyer and Tricart, 1987; Martinez et al., 1991; Bouaziz et al., 2002; Feki et al., 2005a,b; Rigane et al., 2005) propose that the Saharan platform in Tunisia was compartmentalized by a distensive tectonic phase towards the end of the Aptian. The studied area is located in Central-Western Tunisia and corresponds to Aptian series of Jebel El Hamra, which is bounded in the East by the Foussana graben (Fig. 1). This morphological unit exhibits three principal culminations separated by three pass: Khanguet Sloughi, Fej El Fakkat and Khanguet Zitoun. Globally, this structure corresponds to a faulted anticline with a dolomitic core (Serj formation, Aptian age) and marly-limestone limbs (Hamaïma and Fahdène formations, of Albian age) cropping out around the rim. In Tunisia, these formations are of indisputable
* Corresponding author. Tel.: +216 74 276 400; fax: +216 74 274 437. E-mail address: [email protected] (A. Rigane). 1464-343X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2009.11.005
economic interest which make up oil (Bishop, 1975) and mining targets represented by reservoir rocks (Serj), source rocks (Fahdène) and ore deposits (galena and barite) in the roof of the Serj formation. By studying the tectono-sedimentary events recorded in the geological formations around Jebel El Hamra, we can confirm the existence of a tectonic crisis at the end of the Aptian, while specifying the nature and style of the corresponding deformation and providing further information on Tunisian basin geodynamics during this episode. 2. Apto-Albian paleogeography in central Tunisia The facies and thickness distribution of the sediments is controlled by tectonic structures associated with halokinetic movements. All authors agree that the Early Cretaceous sedimentary succession of central Tunisia corresponds to an unstable platform with clastic deposits in the South and open marine facies in the North (Marie et al., 1984; Dlala, 1999). At the end of the Early Cretaceous, positive vertical movements are followed by erosion, in particular along the North–South Axis (NSA) where the Upper Albian in many places lies unconformably on the underlying succession (Burollet, 1956; Biely et al., 1973). During the Early
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Fig. 1. Structural sketch map of Tunisia and location of study area.
Fig. 2. Lithostratigraphic correlation of Cretaceous units in Tunisia (after Ben Ferjani et al. (1990).
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Cretaceous, elevated areas are commonly developed to the East of the NSA and become denuded of their sediments (M’Rabet, 1987). Finally, a distensive tectonic phase towards the end of Upper Aptian, known as the ‘‘Aptian Crisis” (Soyer and Tricart, 1987), appears to have led to the breakup of the northern edge of the Saharan platform into distinct compartments (Bismuth et al., 1982; Martinez et al., 1991). The periodic recurrence of these blocks in tilted blocks has a major influence in determining the paleogeography of central Tunisia during the ‘‘Middle Cretaceous”. 3. Stratigraphic framework 3.1. The Cretaceous in Tunisia The Cretaceous crops out from the North to the extreme South of the country. Generally, a regular progression of facies is observed from South to North, from continental, then lagoonal or neritic and finally marine (Fig. 2). Very often, the transition in central Tunisia takes place via platform carbonate facies. However, the very irregular subsidence leads locally – and sometimes abruptly – to large variations in thickness and facies (Marie et al., 1984; Rigane et al., 2005). Lastly, in Eastern Tunisia, both on land and offshore, the Cretaceous rocks are well-known in drill-holes, which encounter many volcanic intercalations: dykes, sills, tuffs, ashes and rare lava flows (Burollet, 1956; Bajanik, 1971; Ellouz, 1984; Patriat et al., 2003). 3.2. Apto-Albian sedimentary sequences in central Tunisia At this epoch, the whole of central Tunisia corresponds to a vast carbonate platform which follows on from the sandy sedimentary facies of the Barremian (M’Rabet, 1987). The Aptian is represented by the Serj formation (or its lateral equivalent, the Orbata formation), which overlies Barremian sands (Boudinar Formation). The Albian corresponds to the Fahdène formation (or its lateral equivalent, the Zebbag Formation), being characterized primarily by thick clay units, marls and some limestone intercalations or argillaceous limestone. The transition between these two formations (Serj–Fahdène) is never sharp, but corresponds in most cases to a sequence made up of green clays alternating with thin dolomite beds or bioclastic limestones defining the Hamaïma formation (Burollet, 1956) of Late Aptian age. However, more recent chronostratigraphic studies reveal that the boundaries of the Hamaïma formation are clearly diachronous, according to the different sectors and authors (El Euchi, 1993; Zghal, 1994).
4. Structure of Jebel El Hamra 4.1. Cartographic data The geological and structural map of Jebel El Hamra and Jebel El Ajered (Fig. 4) shows a stratigraphic succession extending from the Aptian to the present-day with, however, a significant gap from the Campanian to the Middle Miocene. The dolomitic Aptian makes up the core of the morphological unit. It is rimmed by the marlycalcareous deposits of the Hamaima Formation (Intermediate Sequence), and then by the marly deposits of the Fahdène formation. The more recent members of the succession appear discontinuous and occur in a much more external position. The overall geometry is simple and displays two types of megastructures (Fig. 4): a major fold and a series of faulted blocks: – An anticlinal fold, slightly sigmoidal and with an axial trend striking close to N030, forms the framework of Jebel El Hamra. The anticlinal fold axis plunges towards the SSW, which causes the pitching of the thin carbonate Serj formation under the Hamaïma and Fahdène formations towards the South. Towards the North, on the contrary, the oldest beds of the Aptian appear at outcrop. The El Jebel Hamra fold is often non symmetrical, with a gently sloping western flank and a slightly more steeply dipping eastern flank, indicating a general vergence (or facing) towards the SE. – The folded structure is crosscut by three cartographic fault generations. The first (F1) strikes NW–SE and subdivides the fold into four independent anticlines each having its own geometry (Fig. 4). The second (F2) fault system is N–S striking, bounding
3.3. Aptian–Albian transition in Jebel El Hamra Around Jebel El Hamra in the Kasserine area, the Serj formation has been worked by several mining operations due to the fact that its top is made up of karstified carbonates rich in minerals containing Zn, Ba and Pb. For this reason, the Aptian–Albian transition has been thoroughly studied (El Euchi, 1993), with all authors agreeing that it corresponds to an argillaceous limestone sequence intercalated between the Serj and Fahdène formations (Fig. 3). Hence, these transition beds are frequently referred to as the ‘‘Intermediate sequence” (Pervinquière, 1903; El Euchi, 1993). Although, in detail, the age and position of this sequence are a matter of debate, its contacts always coincide with two significant discontinuities D and D0 , named D3 and D4 by M’Rabet (1987), as shown in recent studies (Touir et al., 2005). We rely on this definition to draw up a simple and clear picture of the Aptian–Albian transition in Jebel El Hamra (Fig. 3).
Fig. 3. Synthetic chronostratigraphic and lithostratigraphic section (not to scale) of Aptian–Albian transition in study area. D: emergence with karsts features, galena, barite, quartz and phosphate, D0 : long period of emergence with karsts and pedogenetic features.
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the Serj dolomites in the East, while the third (F3) strikes NE– SW and is especially well developed towards the NW and SE (Fig. 5). These three major faults system (F1, F2 and F3) segment the Serj formation dolomites into four large hectometric-scale blocks (BI, BII and BIII) within which other minor faults can be observed. The size of the fault blocks decreases regularly from the SE towards the NW, and the boundary faults are systematically sealed by the marly-calcareous beds of the Hamaïma formation (Fig. 5). Blocks and faults boundaries are the result from the Aptian Crisis and the F0 reverse faults on contrary (Figs. 3 and 5) correspond to tertiary compression phases. 4.2. Late Aptian faults The F1 (NW–SE) system is represented by four major kilometerscale faults that cut right across the mountain (Fig. 6). From SW towards the NE, this set comprises the faults of Khanguet Zitoun, Fej El Fakkat, 1112 Signal and Khanguet Slougui. These faults are stee-
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ply dipping towards the North, each with a downthrown block on its NE side, thus delineating a series of half-grabens succeeding each other in this direction (Figs. 4 and 7). The F2 system (N–S trend) is especially well developed on the eastern edge of the northerly blocks. Although the kinematics of this system is still normal, the faults exhibit a strong sinistral strike-slip component. The F3 system (NE–SW) also comprises a normal strike-slip fault on the western limb of the fold. While the F3 fault system strikes on the SE limb is also NE–SW, the faulting is clearly reverse and thrusting post-dates the structural inversion associated with the Miocene compression. 5. Tectonics of the Aptian Crisis The persistent instability of the sedimentary basin floor during this stage of the Cretaceous is frequently highlighted and pre-
Fig. 4. (a) Simplified geological and structural map of El Hamra-El Ajered area and stereographic plots of faults (equal-area, lower hemisphere). Slickenside lineations are shown by small arrows. Black arrows: direction of extension.
Anticlinal axis;
synclinal axis;
fault system T: Trias; Ap: Aptian; f-Ap:
Uppermost Aptian; Cce: Cenomanian; C: Cenozoïc; Q: Quaternary; CT: Turonian. (b) Interpretation of the study area: two negative lozenge-shaped depocentre bounded by a N–S shear zone.
Fig. 5. Transverse geological section (X–X0 ) in block B. T: Trias; Ap: Aptian; f-Ap: Uppermost Aptian; Al–C–T: Albian–Cenomanian–Turonian, F2, F3: end-Aptian strike-slip normal fault with drape fault in Hama formation, F0 : Reverse fault in relation with Miocene folding (probably inverted normal fault).
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Fig. 6. Transverse (F1) and N–S-trending (F3) fault system.
sented as characteristic of the Aptian Crisis (Soyer and Tricart, 1987). In the present study, we provide more detailed information on this crisis by investigating the tectono-sedimentary history of Jebel El Hamra. The specific character of this crisis is expressed in the geometry of the deposits, their deformation style and the dominant role of inherited structural features. 5.1. Basin geometry The geometry is characterized by a lozenge-shaped depocentre associated with a transtensive regime in which the pre-existing structural framework is expressed by F2 and F3 faults, and the newly-formed structures by the F1 faults. The extension direction is consistent with all the preceding data (Fig. 4). During the Miocene, the changeover from extensional to transpressive deformation results in the present-day pattern of independent faulted and folded blocks, whose geometry is controlled by the Aptian structural heritage (Fig. 8). One of the direct consequences of this ‘‘inherited” structure is the false sinistral mega-virgation of the fold axis.
erosion and perhaps emersion. Its origin is purely eustatic. The discontinuity D0 which has a tectonic origin is also marked by a hard ground. Indeed, the Serj formation is conformably overlain by the Hamaïma formation, which marks a clear change of depositional environment. These marly-calcareous deposits are characterized by rhythmic sedimentation on an unstable and subsiding floor. At the top of this formation, we find major faults (F1, F2 and F3) appear in a transtensive regime with a brittle style in Serj carbonates and ductile style in marly-limestone alternation of Hamaïma formation which presents drap folds formed at the level of these major faults (Figs. 5 and 8), giving rise to a faulting system related to extensional tectonics (Reddy et al., 1982; Larroque, 1987; Ameen, 1988; Rigane et al., 1994; Gourmelen et al., 2000; Khalil and McClay, 2002). The end of the Aptian Crisis in Tunisia corresponds to a renewed period of emersion, associated with erosion and karstification, which is picked out by discontinuity D0 (Figs. 3 and 8). Indeed, the return to a calmer tectonic regime is reflected by the deposition of Albian black marls (Fahdène formation) unconformably on top of the Aptian (Fig. 8).
5.2. Deformation style 5.3. Role of inherited features The pre-rift series correspond to platform carbonates of the Serj formation and to marly-calcareous alternations of the Hamaïma formation. The post-rift sediments mainly composed by marl, correspond to the Fahdene formation (Fig. 3). The discontinuity D corresponds to a hard ground marking the end of the basin filling with
Structural analysis of the Aptian and Late Aptian faults (F1, F2 and F3) reveals two extensional phases trending NE–SW and NW–SE. The former phase is consistent with the original extension direction already demonstrated in other studies (Boltenhagen,
Fig. 7. Longitudinal geological section (Y–Y0 ) with Triassic diapiric dome and tilted blocks (BI, BII, BIII and BIV). T: Trias; p-Ap: pre-Aptian formations; Ap: Aptian; f-Ap: Final Aptian; Al–C–T: Albian–Cenomanian–Turonian, KZ: Khanguet Zitoun Fault, FK: Fej El Fakkat Fault, 1112: 1112 Signal Fault, KS: Khanguet Sloughi Fault.
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References
Fig. 8. Drape fold in a layered ductile cover (Hma formation) above brittle basement (Serj formation) with normal fault, in an extensional setting.
1985a,b). We attribute the second phase to the diapiric activity of Triassic evaporites that are currently at outcrop to the North of Jebel El Hamra and south of Jebel El Ajered (Fig. 7). Indeed, the cartographic absence, here, of all formations before Aptian and the existence of Cenomanian deposits lying stratigraphically on top of the Trias at the NE extremity of Jebel El Hamra, implies the presence of a positive zone associated with diapirism that controls the Aptian sedimentation. This phase of diapirism is probably responsible for the local development of a NW–SE extension direction. 6. Conclusions The structural and tectono-sedimentary study of Jebel El Hamra to the West of Kasserine (central Tunisia) underlines the predominant role of paleostructures acquired during the evolution of the South-Tethyan basin, and their influence of the present-day geometry of the fold belt. Indeed, the existing pattern of folding and faulting results directly from tectonic blocks formed during the Aptian. At that time, Tunisia can be regarded as an intracontinental basin at the northern margin of the African craton, characterized by depocentres directly related to the activity (reactivation) of pre-existing faults. In this case, mobility is concentrated within an N–S trending tectonic bundle in a transtensive regime. Hence, the depocentres appear as two pull-apart basins bounded by a lower-shaped structure (Fig. 4b). Lastly, the existence of tectonic nodes supports the hypothesis that diapirism can induce local tectonic extension. The tilted blocks of Jebel El Hamra can be, consequently, the final result of this transtensive tectonics, locally associated to halocinetic vertical movements of Triassic salt. At a large scale, in the African geodynamic context of this period (Guiraud and Maurin, 1991; Guiraud and Maurin, 1992; Guiraud et al., 2005; Basile et al., 2005), this late Aptian phase, very exactly dated in the studied area to the end of Clansaysian age, is integrated in the rifting phase shown by Guiraud and Maurin (1991) in all the intracontinental basins of the Northern part of the African craton, in particular in Libya (gulf of Syrte). In addition, as noticed by these authors (Guiraud and Maurin, 1991, 1992), we conclude also that the opening was done on a mobile zone of the crust in Tunisia, more precisely on a submeridian fault, inherited from the African craton. Finally, the SW–NE detected extension originates from the rifting and oceanic activity of the Eastern Tethyan Margin, i.e. of Mesogea (Dercourt et al., 1985; Ricou, 1994). Acknowledgements The authors would like to thank Mr. J.-J. Tiercelin for his constructive review and would also like to acknowledge anonymous reviewer for his useful comments. Mr H. Bejaoui (English Unit Faculty of Sciences of Sfax), is thanked for the English improvement of the manuscript.
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The ‘‘Aptian Crisis” of the South-Tethyan margin: New tectonic data in Tunisia Adel Rigane a,*, Moncef Feki a, Claude Gourmelen b, Mabrouk Montacer a a b
Département des Sciences de la Terre, Faculté des Sciences de Sfax, Université de Sfax, Unité de recherche, GEOGLOB, BP. 1171, Sfax 3000, Tunisia Laboratoire de Géodynamique des Rifts et des Marges Passives, Faculté des Sciences, Université du Maine, 72085 Le Mans cedex, France
a r t i c l e
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Article history: Received 27 May 2009 Received in revised form 11 November 2009 Accepted 16 November 2009 Available online 20 November 2009 Keywords: South-Tethyan margin Aptian/Albian Aptian Crisis Normal fault and drape fold Diapirism Tunisia
a b s t r a c t The distribution of Cretaceous sedimentary basins in the central Tunisian Atlas is controlled by major faults. The reactivation of these faults and the diapiric activity of the Triassic evaporites has resulted in a complex tectonic history. Most studies show that the Saharan platform in Tunisia, located on the South-Tethyan margin, was compartmentalized by an extensive tectonic phase towards the end of the Aptian, during an episode known as the ‘‘Aptian Crisis”. In the study area located in Central-Western Tunisia, the Aptian series of Jebel El Hamra and Jebel El Ajered form the core of an anticlinal fold. The Aptian russet-coloured dolomites (Serj formation) are rimmed by alternating marls and limestones of end-Aptian age and the Fahdene formation composed of Albo-Cenomanian marls. The structural and tectono-sedimentary study of Jebels El Hamra and El Ajered highlights the prominent influence of paleostructures acquired during the evolution of the South-Tethyan basin and their control on the existing geometry of the fold belt. Indeed, the present geometry of folds and faults results directly from the block tectonics developed during the Aptian. This Aptian tectonic style is characterized by extensive and/or transtensive deformation leading to multiscal fracturing in structural blocks. This deformation is characterized by a normal or strike-slip normal fault and drape fold system. The age and the extension direction (SW–NE) are fully consistent with the regional extensional phase probably disturbed by diapirism. This ‘‘Aptian Crisis” can be related to the opening of the equatorial Atlantic and Tethys oceans. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In the Tunisian Central Atlas, the distribution of the Cretaceous sedimentary basins is controlled by major structural trends. The reactivation of major faults associated with these structural trends and the Triassic diapirism has resulted in a complex tectonic history. Most studies (Bismuth et al., 1982; Boltenhagen, 1985a,b; Soyer and Tricart, 1987; Martinez et al., 1991; Bouaziz et al., 2002; Feki et al., 2005a,b; Rigane et al., 2005) propose that the Saharan platform in Tunisia was compartmentalized by a distensive tectonic phase towards the end of the Aptian. The studied area is located in Central-Western Tunisia and corresponds to Aptian series of Jebel El Hamra, which is bounded in the East by the Foussana graben (Fig. 1). This morphological unit exhibits three principal culminations separated by three pass: Khanguet Sloughi, Fej El Fakkat and Khanguet Zitoun. Globally, this structure corresponds to a faulted anticline with a dolomitic core (Serj formation, Aptian age) and marly-limestone limbs (Hamaïma and Fahdène formations, of Albian age) cropping out around the rim. In Tunisia, these formations are of indisputable
* Corresponding author. Tel.: +216 74 276 400; fax: +216 74 274 437. E-mail address: [email protected] (A. Rigane). 1464-343X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2009.11.005
economic interest which make up oil (Bishop, 1975) and mining targets represented by reservoir rocks (Serj), source rocks (Fahdène) and ore deposits (galena and barite) in the roof of the Serj formation. By studying the tectono-sedimentary events recorded in the geological formations around Jebel El Hamra, we can confirm the existence of a tectonic crisis at the end of the Aptian, while specifying the nature and style of the corresponding deformation and providing further information on Tunisian basin geodynamics during this episode. 2. Apto-Albian paleogeography in central Tunisia The facies and thickness distribution of the sediments is controlled by tectonic structures associated with halokinetic movements. All authors agree that the Early Cretaceous sedimentary succession of central Tunisia corresponds to an unstable platform with clastic deposits in the South and open marine facies in the North (Marie et al., 1984; Dlala, 1999). At the end of the Early Cretaceous, positive vertical movements are followed by erosion, in particular along the North–South Axis (NSA) where the Upper Albian in many places lies unconformably on the underlying succession (Burollet, 1956; Biely et al., 1973). During the Early
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Fig. 1. Structural sketch map of Tunisia and location of study area.
Fig. 2. Lithostratigraphic correlation of Cretaceous units in Tunisia (after Ben Ferjani et al. (1990).
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Cretaceous, elevated areas are commonly developed to the East of the NSA and become denuded of their sediments (M’Rabet, 1987). Finally, a distensive tectonic phase towards the end of Upper Aptian, known as the ‘‘Aptian Crisis” (Soyer and Tricart, 1987), appears to have led to the breakup of the northern edge of the Saharan platform into distinct compartments (Bismuth et al., 1982; Martinez et al., 1991). The periodic recurrence of these blocks in tilted blocks has a major influence in determining the paleogeography of central Tunisia during the ‘‘Middle Cretaceous”. 3. Stratigraphic framework 3.1. The Cretaceous in Tunisia The Cretaceous crops out from the North to the extreme South of the country. Generally, a regular progression of facies is observed from South to North, from continental, then lagoonal or neritic and finally marine (Fig. 2). Very often, the transition in central Tunisia takes place via platform carbonate facies. However, the very irregular subsidence leads locally – and sometimes abruptly – to large variations in thickness and facies (Marie et al., 1984; Rigane et al., 2005). Lastly, in Eastern Tunisia, both on land and offshore, the Cretaceous rocks are well-known in drill-holes, which encounter many volcanic intercalations: dykes, sills, tuffs, ashes and rare lava flows (Burollet, 1956; Bajanik, 1971; Ellouz, 1984; Patriat et al., 2003). 3.2. Apto-Albian sedimentary sequences in central Tunisia At this epoch, the whole of central Tunisia corresponds to a vast carbonate platform which follows on from the sandy sedimentary facies of the Barremian (M’Rabet, 1987). The Aptian is represented by the Serj formation (or its lateral equivalent, the Orbata formation), which overlies Barremian sands (Boudinar Formation). The Albian corresponds to the Fahdène formation (or its lateral equivalent, the Zebbag Formation), being characterized primarily by thick clay units, marls and some limestone intercalations or argillaceous limestone. The transition between these two formations (Serj–Fahdène) is never sharp, but corresponds in most cases to a sequence made up of green clays alternating with thin dolomite beds or bioclastic limestones defining the Hamaïma formation (Burollet, 1956) of Late Aptian age. However, more recent chronostratigraphic studies reveal that the boundaries of the Hamaïma formation are clearly diachronous, according to the different sectors and authors (El Euchi, 1993; Zghal, 1994).
4. Structure of Jebel El Hamra 4.1. Cartographic data The geological and structural map of Jebel El Hamra and Jebel El Ajered (Fig. 4) shows a stratigraphic succession extending from the Aptian to the present-day with, however, a significant gap from the Campanian to the Middle Miocene. The dolomitic Aptian makes up the core of the morphological unit. It is rimmed by the marlycalcareous deposits of the Hamaima Formation (Intermediate Sequence), and then by the marly deposits of the Fahdène formation. The more recent members of the succession appear discontinuous and occur in a much more external position. The overall geometry is simple and displays two types of megastructures (Fig. 4): a major fold and a series of faulted blocks: – An anticlinal fold, slightly sigmoidal and with an axial trend striking close to N030, forms the framework of Jebel El Hamra. The anticlinal fold axis plunges towards the SSW, which causes the pitching of the thin carbonate Serj formation under the Hamaïma and Fahdène formations towards the South. Towards the North, on the contrary, the oldest beds of the Aptian appear at outcrop. The El Jebel Hamra fold is often non symmetrical, with a gently sloping western flank and a slightly more steeply dipping eastern flank, indicating a general vergence (or facing) towards the SE. – The folded structure is crosscut by three cartographic fault generations. The first (F1) strikes NW–SE and subdivides the fold into four independent anticlines each having its own geometry (Fig. 4). The second (F2) fault system is N–S striking, bounding
3.3. Aptian–Albian transition in Jebel El Hamra Around Jebel El Hamra in the Kasserine area, the Serj formation has been worked by several mining operations due to the fact that its top is made up of karstified carbonates rich in minerals containing Zn, Ba and Pb. For this reason, the Aptian–Albian transition has been thoroughly studied (El Euchi, 1993), with all authors agreeing that it corresponds to an argillaceous limestone sequence intercalated between the Serj and Fahdène formations (Fig. 3). Hence, these transition beds are frequently referred to as the ‘‘Intermediate sequence” (Pervinquière, 1903; El Euchi, 1993). Although, in detail, the age and position of this sequence are a matter of debate, its contacts always coincide with two significant discontinuities D and D0 , named D3 and D4 by M’Rabet (1987), as shown in recent studies (Touir et al., 2005). We rely on this definition to draw up a simple and clear picture of the Aptian–Albian transition in Jebel El Hamra (Fig. 3).
Fig. 3. Synthetic chronostratigraphic and lithostratigraphic section (not to scale) of Aptian–Albian transition in study area. D: emergence with karsts features, galena, barite, quartz and phosphate, D0 : long period of emergence with karsts and pedogenetic features.
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the Serj dolomites in the East, while the third (F3) strikes NE– SW and is especially well developed towards the NW and SE (Fig. 5). These three major faults system (F1, F2 and F3) segment the Serj formation dolomites into four large hectometric-scale blocks (BI, BII and BIII) within which other minor faults can be observed. The size of the fault blocks decreases regularly from the SE towards the NW, and the boundary faults are systematically sealed by the marly-calcareous beds of the Hamaïma formation (Fig. 5). Blocks and faults boundaries are the result from the Aptian Crisis and the F0 reverse faults on contrary (Figs. 3 and 5) correspond to tertiary compression phases. 4.2. Late Aptian faults The F1 (NW–SE) system is represented by four major kilometerscale faults that cut right across the mountain (Fig. 6). From SW towards the NE, this set comprises the faults of Khanguet Zitoun, Fej El Fakkat, 1112 Signal and Khanguet Slougui. These faults are stee-
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ply dipping towards the North, each with a downthrown block on its NE side, thus delineating a series of half-grabens succeeding each other in this direction (Figs. 4 and 7). The F2 system (N–S trend) is especially well developed on the eastern edge of the northerly blocks. Although the kinematics of this system is still normal, the faults exhibit a strong sinistral strike-slip component. The F3 system (NE–SW) also comprises a normal strike-slip fault on the western limb of the fold. While the F3 fault system strikes on the SE limb is also NE–SW, the faulting is clearly reverse and thrusting post-dates the structural inversion associated with the Miocene compression. 5. Tectonics of the Aptian Crisis The persistent instability of the sedimentary basin floor during this stage of the Cretaceous is frequently highlighted and pre-
Fig. 4. (a) Simplified geological and structural map of El Hamra-El Ajered area and stereographic plots of faults (equal-area, lower hemisphere). Slickenside lineations are shown by small arrows. Black arrows: direction of extension.
Anticlinal axis;
synclinal axis;
fault system T: Trias; Ap: Aptian; f-Ap:
Uppermost Aptian; Cce: Cenomanian; C: Cenozoïc; Q: Quaternary; CT: Turonian. (b) Interpretation of the study area: two negative lozenge-shaped depocentre bounded by a N–S shear zone.
Fig. 5. Transverse geological section (X–X0 ) in block B. T: Trias; Ap: Aptian; f-Ap: Uppermost Aptian; Al–C–T: Albian–Cenomanian–Turonian, F2, F3: end-Aptian strike-slip normal fault with drape fault in Hama formation, F0 : Reverse fault in relation with Miocene folding (probably inverted normal fault).
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Fig. 6. Transverse (F1) and N–S-trending (F3) fault system.
sented as characteristic of the Aptian Crisis (Soyer and Tricart, 1987). In the present study, we provide more detailed information on this crisis by investigating the tectono-sedimentary history of Jebel El Hamra. The specific character of this crisis is expressed in the geometry of the deposits, their deformation style and the dominant role of inherited structural features. 5.1. Basin geometry The geometry is characterized by a lozenge-shaped depocentre associated with a transtensive regime in which the pre-existing structural framework is expressed by F2 and F3 faults, and the newly-formed structures by the F1 faults. The extension direction is consistent with all the preceding data (Fig. 4). During the Miocene, the changeover from extensional to transpressive deformation results in the present-day pattern of independent faulted and folded blocks, whose geometry is controlled by the Aptian structural heritage (Fig. 8). One of the direct consequences of this ‘‘inherited” structure is the false sinistral mega-virgation of the fold axis.
erosion and perhaps emersion. Its origin is purely eustatic. The discontinuity D0 which has a tectonic origin is also marked by a hard ground. Indeed, the Serj formation is conformably overlain by the Hamaïma formation, which marks a clear change of depositional environment. These marly-calcareous deposits are characterized by rhythmic sedimentation on an unstable and subsiding floor. At the top of this formation, we find major faults (F1, F2 and F3) appear in a transtensive regime with a brittle style in Serj carbonates and ductile style in marly-limestone alternation of Hamaïma formation which presents drap folds formed at the level of these major faults (Figs. 5 and 8), giving rise to a faulting system related to extensional tectonics (Reddy et al., 1982; Larroque, 1987; Ameen, 1988; Rigane et al., 1994; Gourmelen et al., 2000; Khalil and McClay, 2002). The end of the Aptian Crisis in Tunisia corresponds to a renewed period of emersion, associated with erosion and karstification, which is picked out by discontinuity D0 (Figs. 3 and 8). Indeed, the return to a calmer tectonic regime is reflected by the deposition of Albian black marls (Fahdène formation) unconformably on top of the Aptian (Fig. 8).
5.2. Deformation style 5.3. Role of inherited features The pre-rift series correspond to platform carbonates of the Serj formation and to marly-calcareous alternations of the Hamaïma formation. The post-rift sediments mainly composed by marl, correspond to the Fahdene formation (Fig. 3). The discontinuity D corresponds to a hard ground marking the end of the basin filling with
Structural analysis of the Aptian and Late Aptian faults (F1, F2 and F3) reveals two extensional phases trending NE–SW and NW–SE. The former phase is consistent with the original extension direction already demonstrated in other studies (Boltenhagen,
Fig. 7. Longitudinal geological section (Y–Y0 ) with Triassic diapiric dome and tilted blocks (BI, BII, BIII and BIV). T: Trias; p-Ap: pre-Aptian formations; Ap: Aptian; f-Ap: Final Aptian; Al–C–T: Albian–Cenomanian–Turonian, KZ: Khanguet Zitoun Fault, FK: Fej El Fakkat Fault, 1112: 1112 Signal Fault, KS: Khanguet Sloughi Fault.
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References
Fig. 8. Drape fold in a layered ductile cover (Hma formation) above brittle basement (Serj formation) with normal fault, in an extensional setting.
1985a,b). We attribute the second phase to the diapiric activity of Triassic evaporites that are currently at outcrop to the North of Jebel El Hamra and south of Jebel El Ajered (Fig. 7). Indeed, the cartographic absence, here, of all formations before Aptian and the existence of Cenomanian deposits lying stratigraphically on top of the Trias at the NE extremity of Jebel El Hamra, implies the presence of a positive zone associated with diapirism that controls the Aptian sedimentation. This phase of diapirism is probably responsible for the local development of a NW–SE extension direction. 6. Conclusions The structural and tectono-sedimentary study of Jebel El Hamra to the West of Kasserine (central Tunisia) underlines the predominant role of paleostructures acquired during the evolution of the South-Tethyan basin, and their influence of the present-day geometry of the fold belt. Indeed, the existing pattern of folding and faulting results directly from tectonic blocks formed during the Aptian. At that time, Tunisia can be regarded as an intracontinental basin at the northern margin of the African craton, characterized by depocentres directly related to the activity (reactivation) of pre-existing faults. In this case, mobility is concentrated within an N–S trending tectonic bundle in a transtensive regime. Hence, the depocentres appear as two pull-apart basins bounded by a lower-shaped structure (Fig. 4b). Lastly, the existence of tectonic nodes supports the hypothesis that diapirism can induce local tectonic extension. The tilted blocks of Jebel El Hamra can be, consequently, the final result of this transtensive tectonics, locally associated to halocinetic vertical movements of Triassic salt. At a large scale, in the African geodynamic context of this period (Guiraud and Maurin, 1991; Guiraud and Maurin, 1992; Guiraud et al., 2005; Basile et al., 2005), this late Aptian phase, very exactly dated in the studied area to the end of Clansaysian age, is integrated in the rifting phase shown by Guiraud and Maurin (1991) in all the intracontinental basins of the Northern part of the African craton, in particular in Libya (gulf of Syrte). In addition, as noticed by these authors (Guiraud and Maurin, 1991, 1992), we conclude also that the opening was done on a mobile zone of the crust in Tunisia, more precisely on a submeridian fault, inherited from the African craton. Finally, the SW–NE detected extension originates from the rifting and oceanic activity of the Eastern Tethyan Margin, i.e. of Mesogea (Dercourt et al., 1985; Ricou, 1994). Acknowledgements The authors would like to thank Mr. J.-J. Tiercelin for his constructive review and would also like to acknowledge anonymous reviewer for his useful comments. Mr H. Bejaoui (English Unit Faculty of Sciences of Sfax), is thanked for the English improvement of the manuscript.
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