Geometry and structural evolution of Lorbeus diapir, northwestern Tunisia: polyphase diapirism of...

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Int J Earth Sci (Geol Rundsch) (2014) 103:881–900DOI 10.1007/s00531-013-0992-3

ORIGInal PaPER

Geometry and structural evolution of Lorbeus diapir, northwestern Tunisia: polyphase diapirism of the North African inverted passive margin

Amara Masrouhi · Olivier Bellier · Hemin Koyi

Received: 15 May 2013 / accepted: 11 December 2013 / Published online: 9 January 2014 © Springer-Verlag Berlin Heidelberg 2014

Introduction

Inverted salt-involved passive margins are commonly char-acterized by polyphase salt structure evolution. Polyphase salt tectonics is one of several features usually characteris-tic of inverted salt-involved passive margins. The superpo-sition of extensional and contractional deformation deter-mines diapir emplacement and evolution. The evolution of inherited faults controls and influences the piercement effi-ciency and the style and geometry of the diapir. Thematic and/or regional studies were focused on deformations of salt in passive margins (e.g., Wu et al. 1990; Vendeville and Jackson 1992a, b; Talbot 1993; Gaullier et al. 1993; Fletcher et al. 1995; Diegel et al. 1995; Guglielmo et al. 1997; Mc Bride et al. 1998; Maillard et al. 2003; Jack-son 1995; Vendeville 2005; Gaulier and Vendeville 2005; Hudec and Jackson 2006, 2007). Studies focused on defor-mation of polyphase salt diapirism are less frequent com-pared with studies of deformation of salt in passive mar-gins (e.g., Talbot and Jackson 1987; Vially et al. 1994; letouzey et al. 1995; nilsen et al. 1995; Koyi 1998; Gug-lielmo et al. 2000; Rowan et al. 2003; Fort et al. 2004; Rowan and Vendeville 2006; Callot et al. 2007, Trocmé et al. 2011; Callot et al. 2012), and most of them focus on evolution of structures that were reactivated by a moder-ate shortening. Studies paying attention to the evolution of diapirs created in extensional phase and reactivated by important compression (Rowan and Vendeville 2006; Roca et al. 2006; Callot et al. 2007, 2012; Trocmé et al. 2011) are even less frequent than the last mentioned. In addition to the difficulty in interpreting the geometry and the evo-lution of diapirs in such a tectonics context, the faulting associated with diapirism is generally deformed as com-plex structures during successive tectonics phases. Fur-thermore, the difficulty in understanding such evolution

Abstract Detailed geologic mapping, structural analy-sis, field cross-sections, new dating based on planktonic foraminifera, in addition to gravity signature of lorbeus diapir, are used to characterize polyphase salt diapirism. This study highlights the role of inherited faulting, which controls and influences the piercement efficiency and the style and geometry of the diapir; and also the localization of evaporite early ascent displaying diapiric growth during extension. Salt was extruded along the graben axis devel-oped within extensional regional early Cretaceous tectonic associated with the north african passive margin evolution. Geologic data highlight reactive diapirism during albian time (most extreme extension period) and passive diapirism during the late Cretaceous post-rift stage. northeastern Maghreb salt province gives evidences that contractional deformations are not associated with significant diapirism. During shortening, the initial major graben deforms as complex anticlines where diapirs are squeezed and pinched from their feeding.

Keywords Inverted passive margin · atlas system · northeastern Maghreb · Polyphase salt diapirism · Diapir

a. Masrouhi (*) Geo-Resources laboratory, Centre de Recherches et des Technologies des Eaux de Borj Cedria, B.P. 273, 8020 Soliman, Tunisiae-mail: [email protected]

a. Masrouhi · O. Bellier CnRS, CEREGE UM 34, aix-Marseille Université, 13545 aix en Provence, France

H. Koyi Hans Ramberg Tectonic laboratory, Department of Earth Sciences, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden

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can be also complicated if we do not pay attention to the use of the specific terminology: it is important to use terms such as diapir, dome, passive diapir, active diapir, reactive diapir… accurately and consistently. During the last cen-tury, geoscientists used the terms diapir and dome inter-changeably. It was not until the 1990s that the geoscience community discussed the problem of terminology and new definitions were proposed. In 1992 (Vendeville and Jack-son 1992a, b) and Jackson et al. (1994) presented system-atic works defining a set of terms: diapir, active diapir, passive diapir and reactive diapir. The use of such terms is currently a broader definition. Therefore, we use in this paper this aforementioned terminology.

The atlas system is one of few salt provinces which expose evaporite rocks affected by multiple tectonic phases, such as the Zagros Mountains, alps, Pyrenees, axel Heiberg in the Sverdrup basin, the Flinders Ranges in australia and the Sivas basin in Turkey (Roca et al. 2006; Jackson and Harrison 2006; Rowan and Vendeville 2006; Callot et al. 2012). The atlas system corresponds to a fold-thrust-belt related to the convergence between africa and Eurasia. It corresponds to one of the world’s salt provinces that expose salt structures with significant exposure qual-ity and important numbers of structures. In this paper, we use field data from exposed lorbeus diapir in the northern Tunisia, to outline some of the main characteristic features of diapiric growth in inverted passive margin. This study give emphasis to the aptian–albian inherited rifting faults in the structuring and the development of the diapir and the following evolution during late Cretaceous post-rift extension, upper Eocene compression, Oligocene–lower Miocene second extension and middle Miocene second compression. The lorbeus diapir shows greatly deformed strata in both limbs. This deformation could be related to the diapirism and/or the tectonics compressions. In order to distinguish diapirism from tectonics compressions, we present separately the stratigraphic results and the tectonic observations. Both the nW- and SE-limbs are composed of Cretaceous and Tertiary formations.

North Tunisia geological and diapirism setting

The atlas system of north africa is an alpine fold-thrust-belt formed during Cenozoic compressions as a result of the convergence between africa and Europe that began during the late Cretaceous. This system is bordered to the south by the african Saharan platform that is considered stable and not affected by the continuous Tertiary tectonic compressions. From north to south, this system is com-posed, in Tunisian–algerian onshore territory, by three major domains: (1) the internal zone corresponds in algeria to greater and lesser Kabylies and to the Galite archipelago

in Tunisia; (2) the Tellian domain corresponds to a thrust sheet that moved southeastward mainly during middle Miocene time; (3) an atlassic foreland domain that corre-sponds to the complex fold belt of northeastern algeria and northern Tunisia. This latter represents the Central Saharan atlas in algeria and Tunisian atlas in Tunisia. northeast-ern algeria and northern Tunisia geodynamic evolution is usually divided into two major periods. The first one, a rift-ing stage followed by passive margin context, began during late Permian–Middle Triassic and continued until late Cre-taceous time (Bouaziz et al. 2002; Soussi 2002; Guiraud et al. 2005; Gharbi et al. 2013). The second period began during late Cretaceous until present time and corresponds to shortening tectonic period with compressional tectonic episodes intercalated with periods of relative tectonic qui-escence (Guiraud and Bosworth 1997; Bracene et al. 1998; Bracene and Frizon de lamotte 2002; Marmi and Guiraud 2006; Herkat and Guiraud 2006; Masrouhi et al. 2007, 2008a; Frizon de lamotte et al. 2009, 2011).

The northeastern part of the algerian Saharan atlas and northern part of the Tunisian atlas, pertaining to the east-ern part of the Maghrebides chain, is called the northeast-ern Maghreb domain. This domain exposed numerous salt structures that vary greatly in size and style and has also been called the northeastern Maghreb salt province (Vila et al. 2002; Masrouhi and Koyi 2012; Masrouhi et al. 2013). The northeastern Maghreb domain was regarded in recent publications as a domain subjected to thin-skin tectonic, characterized by frequent S–SE-verging thrust-ing with a detachment in Triassic salt rocks (Outtani et al. 1995; Bracene et al. 1998; ahmadi et al. 2006). Recently, during the last decade, more emphasis has been put on the reactivation of inherited Tethyan major faults as the main mechanism of thrusting and folding of this fold-thrust-belt (Souquet et al. 1997; Morgan et al. 1998; Masrouhi et al. 2007; Dhahri and Boukadi 2010; Gharbi et al. 2013).

The Meso–Cenozoic geodynamic evolution and related depositional environment can be summarized as follows. along the northern margin of africa, rifting began during the late Permian–Middle Triassic period and culminated at the transition between the Triassic and Jurassic. The Early Mesozoic rifting created a Triassic basin, dominated by salt deposits showing thick layers of evaporites (mainly halite and gypsum), clay and dolomites of an epicontinental envi-ronment (adil 1993; Kamoun et al. 2001). This region of the southwestern part of the Tethys Ocean was character-ized by extensional deformation with major normal fault-ing and crustal thinning and subsidence. This episode was followed by passive margin basin style tectonics, and Tri-assic salt series are capped throughout Tunisia and algeria by Jurassic carbonate strata which were formed during the Tethyan extension (Soussi 2002; Boughdiri et al. 2007). During the Jurassic and Early Cretaceous, an extensional

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tectonic regime predominated, related to the opening of the Central atlantic. This period shows a dislocation of the con-tinental platform and development of major synsedimentary normal fault systems produced tilted blocks basin geometry (Guiraud et al. 2005; Gharbi et al. 2013). Early Cretaceous extension is accompanied in this margin by volcanic activ-ity testifying to the passive margin style (laridhi Ouazaa and Bédir 2004). at the end of the Early Cretaceous, the aptian–albian ages correspond to the most extreme exten-sion period of the south Tethyan margin in northeastern algeria and north Tunisia (Masrouhi et al. 2013). During this period, the continued extensional tilting produced a paleogeographic differentiation between platforms in cen-tral and southern regions (Gharbi et al. 2013) and a deep trough in the northern region, i.e., Tunisian through in northern Tunisia (Burollet 1956) and the Selaoua through in northeastern algeria (Herkat and Guiraud 2006, Marmi and Guiraud 2006). The late Cretaceous is an extensional period of post-rift stage showing pelagic depositional envi-ronments controlled essentially by the isostatic response of different major fault systems and/or depozones (Guiraud et al. 2005). Since the late Cretaceous, a positive tectonic inversion has occurred at the transition between lower Seno-nian and upper Senonian (Guiraud and Bosworth 1997; Bosworth et al. 1999; Masrouhi et al. 2008a). The Cenozoic corresponds to a period of shortening related to convergence between the africa and Eurasia plates. The Cenozoic tec-tonics of this margin created complex structures from the superposition of three main Cenozoic events, listed as fol-lows (Frizon de lamotte et al. 2000; Bracène et al. 2002; Piqué et al. 2002; Masrouhi et al. 2008a; Frizon de lamotte et al. 2009): (1) an upper Eocene contractional event with a WnW–ESE main regional stress (σ1). This phase led to moderate regional shortening that generated nnE-trending gently folded structures. On the Maghreb scale, even this event was the subject of debate; the majority of recent pub-lications highlight this tectonic phase (Vially et al. 1994; Beauchamp et al. 1999; Frizon de lamotte et al. 2000, 2009; Bosworth et al. 2008; Masrouhi et al. 2008a). This tectonic shortening event is named, in the atlas system, the atlassic tectonic event. (2) after this first shortening event, an Oligocene–lower Miocene extension was accompanied by a general subsidence phase, recorded firstly by continen-tal Oligocene deposits and then by marine lower Miocene strata. The Oligocene is generally an azoic stratum. On the contrary, lower Miocene strata were dated, in northern Tuni-sia, as Burdigalian–Serrevalian (Masrouhi et al. 2007). This extensional tectonics is related to the Mediterranean dynam-ics in the Tertiary period, with the opening of the algerian trough. Following this event, a second paroxysmal and gen-eralized compressional tectonic event with a nW–SE main regional stress (σ1) is responsible for the relief building of the atlas belt. The alpine event (Masrouhi et al. 2007,

2008a) occurred during the middle Miocene (Turki et al. 1988; Chihi 1992). Recent publications highlighted this event in northern Tunisia and northeastern algeria as part of the north african orogen (Sébrier et al. 2006; Frizon de lamotte et al. 2009; Roure et al. 2012). authors emphasize that the north african fold-thrust-belt is related to this event and is still active up to the present (Jolivet and Faccenna 2000). During this later phase, the main regional stress (σ1) direction rotated, from the Miocene to the present, from nW–SE to n–S (Mejri et al. 2010).

The northeastern Maghreb salt province is one of the world’s salt provinces (Hudec and Jackson 2007). This province exposes a Triassic salt (200–250 Ma). Two main salt tectonics models have been proposed to explain salt emplacement and kinematics: (1) a diapir or dome model (e.g., Bolze 1950; Perthuisot 1978, 1981; Perthuisot et al. 1998; Jallouli et al. 2005; Benassi et al. 2006) describing a scenario where the salt actively pierced the overburden and tectonically reduced the thickness of the overburden sequences in the limbs of the structures. This model, which was accepted by the geoscientists who worked on salt tectonics in north africa, was presented as a generalized scheme for all northeastern Maghreb salt province struc-tures (Perthuisot et al. 1998; Jallouli et al. 2005). (2) a sub-marine salt glacier model proposes a scenario where salt flows at the sediment–water interface or below a thin layer of marine sediments (Vila et al. 1996, 2002; Ghanmi et al. 2001; Masrouhi and Koyi 2012). The original salt position, forming a sheet of allochthonous salt concordant with the underlying sediments, can be complicated later by Tertiary compressions. In addition, improvements in understand-ing other margins, using field and/or 2D/3D seismic data as well as analog and/or numerical models, emphasize that the extreme variability in salt structures can be recognized within a single margin (e.g., Curnelle and Marco 1983; Wu et al. 1990; Jackson 1995; Diegel et al. 1995; Fletcher et al. 1995; Mc Bride 1998; Mc Bride et al. 1998; Hafid 2000; Hudec and Jackson 2004, 2006; Rowan et al. 2003; Fort et al. 2004; Canerot et al. 2005; Gaulier and Vendeville 2005; Vendeville 2005; Roca et al. 2006; Hudec and Jack-son 2007; Mohr et al. 2007; Yin and Groshong 2007; Talbot and Pohjola 2009). Recently, authors have documented the northeastern Maghreb salt province as a province showing variable salt tectonics evolution and related different salt structures (salt glacier, Masrouhi and Koyi 2012; salt can-opy, Masrouhi et al. 2013; salt diapir, this paper). Further-more, in the northeastern Maghreb salt province, the major salt structures were deformed later during Eocene and Mio-cene compressional events. This multiphase tectonic his-tory makes the interpretation of the original salt emplace-ment ambiguous. Thus, the present configuration of the salt structure from this margin is the result of a long and com-plex geological history with successive tectonic events.


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The lorbeus diapir is located in northwestern Tunisia. This region is characterized by numerous nW-trending faults through that cut nE-trending atlassic folds (Fig. 1). This study, based on new, detailed geologic mapping, field cross-sections, sedimentologic revision, new sedimentary formations dating by micropaleontological associations, detailed gravity map and tectonics analysis, has the follow-ing goals: (1) to illustrate the sedimentary record of dia-pirism in this margin; (2) to show the geometric and kin-ematics characteristic of an area where diapirs grow firstly in extensional phase and later affected by polyphase defor-mations (first compression, second extension, second com-pression); (3) to deduce a model of passive and/or active diapirism from the north african margin.

Tectono‑sedimentary data

The lorbeus diapir is a asymmetric anticline structure with a curved axis that changes from a nE strike to nnE strike from west to east. This anticline structure is cored by Triassic salt intruded through Cretaceous and Tertiary overburden series (Fig. 2). This structure, about 10 km long and 1.5–2.5 km wide, is limited to the north by the Wadi lorbeus Syncline. To the south, the lorbeus diapir is limited to the southwest by the Wadi Tessa Syncline and to the southeast by the Sers subsiding fault trough (Fig. 1). This structure was preliminary studied by Perthuisot (1975,

1976, 1981) who interpreted it as a diapir structure, using the term “atectonic diapir” to mean diapir evolution with-out associated faults (Fig. 3). In this study, we propose a reassessment of the tectonic evolution of this structure as one controlled by faulting associated with diapirism. Sub-sequently, we illustrate a different kinematics evolution. We present new tectono-sedimentary data in order to illustrate the position and relations of the Triassic salt relative to the surrounding Mesozoic and Cenozoic series. locations are given in this work by lambert Coordinates of north Tuni-sia (origin Carthago Tunisia 1922) longitudes (X) and lati-tudes (Y).

Triassic salt rocks

The Triassic salt rocks outcropping in the Jebel lorbeus are famous as the first Triassic diapiric rocks to be dated, with Myophoria cf. goldfussi var. alberti, Posidonomy, Ostrea cf. montis caprilis (Pervinquere 1903). In this structure, the salt rocks in the diapir correspond in some localities to intensely folded gypsum strata. Triassic rocks are mainly formed by mass of gypsum, anhydrite, sand-stone and carbonate. If the halite is missing at the surface, several springs and wadis (rivers) characterized by saline water indicate the presence of the halite at depth. This fact was described by Perthuisot (1981) and confirmed in several domains in Tunisia by mining and/or petroleum

Fig. 1 a Map showing the location of the north african domain; b simplified regional structural map of northern Tunisia and northeast algeria showing the distribution of outcropping Triassic salt in this province, some important structural features; and c loca-tion of lorbeus diapir with its neighboring structures: black. Salt, 1 Cretaceous, 2 Paleogene, 3 neogene


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boreholes. In northern Tunisia, the multifaceted salt struc-ture and outcropping conditions make it difficult to estab-lish the succession of the Triassic rocks. nevertheless,

authors who worked on this problem (Perthuisot 1978; adil 1993; Masrouhi 2006) proposed that it is probably more than 1,000 m thick.

Fig. 2 a lithostratigraphical type section of the Triassic series crop-ping out in northern Tunisia, redrawing after adil (1993), Masrouhi (2006) (1 Evaporites, 2 Basaltic rocks, 3 Dolomites, 4 limestone, 5

Marls, 6 Grainstones). b Composite lithostratigraphic subdivision, biostratigraphic keys (fauna), bulk density of outcropping series in the study area and regional tectonic events

Fig. 3 Previous structural evo-lution model (redrawing after Perthuisot 1981) interpreted the lorbeus as a diapir structure with use of the term “atectonic diapir” to mean diapir evolution without associated faults


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Stratigraphy

Our study presents a structural revision based on detailed new micropaleontological data of Cretaceous formations, in order to date precisely the successive events. It also attempts to determine the position of the Triassic salt rela-tive to surrounding Mesozoic and Cenozoic series. We pre-sent below detailed stratigraphic description of key sectors.

The Koudiat Tabet Sardouk (Fig. 4; X = 413–414, Y = 309) region constitutes the northern part of the lor-beus diapir. This area, identified by XX′ section in Fig. 5, demonstrates diapirism as old as albian. The Koudiat Sardouk region, just south of the present strike-slip fault, exposes a Cretaceous series showing a “forceful” contact with the mass of the diapir. The contact is characterized by strongly fractured and veined strata, suggesting pow-erful frictional force acting on the Cretaceous beds. Frac-ture, veins and friction are probably the result of Tertiary

compressions. The Cretaceous series begins with limestone and marls containing Rotalipora appenninica, Rotalipora gr. ticinensis–subticinensis, ticinella sp. that allow attribu-tion to the Middle–Upper albian age. albian series con-tain abundant black shale levels, testifying for deep sea environment deposition. This first level overlies marly series with rare calcareous bed intercalations. This level is dated as lower–Middle albian based on a micropaleon-tological assemblage of Hedbergella sp. Ticinella sp. and Radiolarae. These strata are about 70°–80° SE-dipping toward the diapir. Under this series, as described above, a Cenomano-Turonian sequences crop out that begins with marly series, well dated as Cenomanian age by Pithonnella ovalis, Pithonnella sphaerica, Calcisphaerula innominata, Botetocardiella conoida, Ticinella sp., R. appenninica, Rotalipora gr. ticinensis. Under the Cenomanian marls, a platy limestone bar is found, securely identified as Turo-nian by its facies and Helvetoglobotruncana helvetica

Fig. 4 Detailed geologic map of the lorbeus diapir; num-ber: lambert Coordinates; lithostratigraphic units: see figure


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foraminifera of Turonian stage. Under the Cenomanian and Turonian sequences, the Coniacian–Santonian crops out. From the top to the base, these series correspond to Conia-cian marls and limestone, well dated with Marginotrun-cana sigali, Marginotruncana coronata, Marginotruncana pseudolineana, Dicarinella imbricata. The base of this series corresponds to Santonian marls with P. ovalis, P. sphaerica, C. innominata, D. imbricata, Dicarinella con-cavata, M. gr. sigali. The series described above overlie the Campanian white limestone, corresponding to the lower member of the abiod Formation (Burollet 1956), and eas-ily recognized in the field. This member was dated previ-ously (Perthuisot 1981) by Globotruncana stuartiformis, Globotruncana eleveta, Globotruncana fornicate, and we confirm the same micropaleontological assemblage. The Campanian limestone bar is repeated through a subverti-cal fault. nW-ward the Campanian white limestones are adjacent to the Maastrichtian limestone that corresponds to the second member of the abiod formation. This latter, vertical at surface, is followed by Paleocene shale under a thin Quaternary soil. Moreover, this arrangement is fol-lowed by Eocene limestone gently dipping nW-ward. This latter is dated as Ypresian (Blondeau in Perthuisot 1978), containing especially Nummulites irregularis and Nummu-lites rollandi. Stratigraphic study based on abundant micro-paleontological assemblage demonstrates overturned strata adjacent to the diapir.

The Koudiat El Kechrid locality, 2 km southward (Fig. 4, X = 408, Y = 306–307), shows a similar geomet-ric arrangement (Fig. 5, YY′). adjacent to the main diapiric salt, SE-dipping Cenomanian–Turonian beds crop out. Strata are also well dated in this locality and correspond to overturned sequences. Mesozoic outcropping series start, from SE to nW, with marly and platy limestone formations. The first formation is dated as Cenomanian–Turonian age,

characterized at its top by P. ovalis, Pithonnella sphearica, small Heterohelix sp., R. appenninica, Rotalipora reicheli, Rotalipora greenhouensis—microfauna characteristic of Middle Cenomanian. Under the Cenomanian marl, the Turonian limestone, containing usually H. helvetica, char-acterizes Turonian fauna. Under this, the Coniacian–Santo-nian crops out and is also well characterized with M. sigali, M. coronata, M. pseudolineana, D. imbricata dating the Conician age. The base of this series corresponds to San-tonian marls, with P. ovalis, P. sphaerica, C. innominata, D. imbricata, D. concavata, M. gr. sigali. Similarly to the Koudiat Tabet Sardouk, the Campanian limestone crops out (Fig. 5) and is repeated through a subvertical fault. as is the case with the Koudiat Tabet Sardouk, the stratigraphic study of the Koudiat El Kechrid, based on an abundant micropaleontological assemblage, demonstrates overturned strata adjacent to the diapir.

Southwestward of the diapir, the region of Sidi Msid (Fig. 4, X = 407, Y = 306) exposes the following series. The nW-limb is composed of vertical to overturned Cam-panian and Maastrichtian limestone. The Sidi Msid River offers better conditions for observing geometry than the adjacent plains (Fig. 5, ZZ′). The dip of strata gradually decreases from south to north, i.e., the Campanian–Maas-trichtian limestones are vertical to overturned and followed by Paleocene and Eocene strata dipping 35°–40°nW. Cre-taceous and Paleogene strata are covered unconformably by the Oligocene–lower Miocene strata, which are them-selves folded (ca. 30°–35° of dip). This unconformity dates a first regional compression. It corresponds to a well-recog-nized late Eocene tectonic event.

The southeastern flank of the lorbeus structure corre-sponds to the Koudiat El louatia, Koudiat Safia and Kou-diat Hanich outcrop series. The Koudiat El louatia shows a Cretaceous series beginning with the Coniacian–Santonian

Fig. 5 Geologic cross-sections prepared from surface data (see Fig. 4 for location) showing the structural configuration of the Triassic salt sheet of the lorbeus diapir


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aleg Formation. Strata just adjacent to the contact with the diapir correspond to the Coniacian marls and limestones dated with, from the base to the top, M. sigali, M. coronata, M. pseudolineana, D. imbricata. This assemblage dates to the Coniacian age. The top of this series corresponds to Santonian marls with P. ovalis, P. sphaerica, C. innomi-nata, D. imbricata, D. concavata, M. gr. sigali. The aleg Formation is overlain by the abiod Formation showing by two characteristic white limestone bars intercalated by a marl level characterized by Globotruncana calcarta fauna. In the Koudiat Safia and the Koudiat Hanich, the same lithostratigraphic units crop out in the aleg Formation and the abiod Formation. Upper Cretaceous and Paleocene–Eocene series are covered unconformably by Oligocene–lower Miocene strata. These series begin with azoic Oli-gocene, which is conglomeratic sand and shale sequences, and followed by Miocene marine strata. The Oligocene base consists of deltaic siliceous sandstones containing abundant reworked Triassic insoluble elements. This obser-vation makes the base of the Oligocene different from the Paleocene and Eocene series. Reworking abundant Triassic elements, the Oligocene deposits give evidence of a second piercement of Triassic salt body. all the above-mentioned series are covered by Mio-Pliocene continental strata with an angular unconformity. This unconformity is of Mio-Pli-ocene continental strata dating the middle Miocene parox-ysmal tectonic event.

Tectono‑sedimentary data and structural configuration

The geologic map (Fig. 4) and the cross-sections (Fig. 5) clearly show a Triassic evaporites–cored anticline structure with strongly deformed limbs. a concentric zone of folded strata is present around the diapirs (Figs. 5, 6, 7, 8, 9, 10). Remarkably, the nW-limb exposes usually overturned strata. The SW limb is also formed of strongly deformed strata but usually with normal stratigraphic polarity. The oldest exposed series in contact with the diapir corre-spond to the albian sequences. These series, described for the first time, outcrop in the south of the Koudiat Tabet Sardouk and in the Oued El Yhoudi region (Fig. 4). The albian age was recognized to be the most extreme exten-sion of the south Tethyan margin in Tunisia. above the albian strata, the Cenomanian marly deposits show abun-dant carbonate clasts re-sedimented (Fig. 6) in Cenoma-nian time. These carbonate clasts are a unique sedimen-tary feature in Tunisia (we are not aware of an equivalent described in the Tunisian geologic literature). These fea-tures have been described in other salt provinces (Garrison and Mc Millan 1999; Giles and lawton 2002; Rowan et al. 2003; Jahani et al. 2009). In the Jebel lorbeus, they cor-respond to angular carbonate clasts likely entrained in the

diapir by a debris flow dominant movement. Clasts were likely cemented in place (Fig. 6) by salt (regularly gyp-sum). This observation testifies for two essential facts: (1) the angular form of clasts and the cement reprecipitated in place indicate a proximal facies adjacent to the diapir. (2) Cementation of the albian clasts by salt indicates that this salt existed at the surface and pierced the overburden, to be reprecipitated in place. Thus, data fit with an irregu-lar bathymetry with a high zone on the diapir apex. The Cenomanian series correspond to deep marine environ-ment. an anoxic environment probably preserved the salt that cemented the clasts, proceeding from diapir apex to peripheral zones. The major inherited faults are reactivated by reverse and strike-slip faulting. Even if this is the case for most of the faults, observed small-scale extensional deformation has been preserved. Indeed, the Cenomanian–Turonian series expose numerous sealed normal faults (Fig. 7). This latter testifying for the first extensional syn-sedimentary faulting that controls structure during the Mesozoic era (Beauchamp et al. 1999; Vially et al. 1994; Herkat and Guiraud 2006; Masrouhi et al. 2008a; Frizon de lamotte et al. 2009. The Coniacian–Santonian series exhibits great thickness variations, from 70 to 120 m in the diapir limbs to several hundred meters in the adja-cent plains of Sers and lorbeus River (Zied Tbaibi per-sonal communication). Thickness variations highlight that accommodation space was not uniform within this region, reflecting either incipient structural growth or inherited tectonic controls. This fact is testified by the bioturbated sequences (Fig. 6) indicating that the lorbeus area corre-sponds to a high zone within the deep bathymetric region-ally recognized for this stage. The Coniacian–Santonian series thin toward the diapir, near to which this sequence is truncated by a Campanian unconformity (Fig. 8). This configuration should be related to the diapir growth. Fur-thermore, laterally, the angular unconformity grades to conformities away from the diapir. In addition, the base of the Campanian strata is usually conglomeratic. Cam-panian base level corresponds to polygenic conglomerate reworking different elements from Early Cretaceous series (Fig. 8).

These observations clearly display that the Cretaceous sedimentation was influenced by pierced evaporite body. Their sedimentologic characteristic identifies them as dif-ferent from those of usual facies recognized within this margin. Thus, it is possible to characterize them as salt-related growth strata. Depositional patterns influenced by the diapir growth are known as “halokinetic sequences” (Giles and lawton 2002).

at the lorbeus diapir, halokinetic sequences can be divided into four sequences, i.e., albian, Cenomanian–Turonian, Coniacian–Santonian and Campanian. (1) The albian, marly with rare limestone intercalations, deposits


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are thickest on the northwest and southeast margin of the diapir. This series has a regional thickness of about a few hundred meters in the Kef (north) and Sers (south) regions. This series has a thickness <80 m in the lorbeus diapir area. (2) The Cenomanian–Turonian series comprise a well outcropping, carbonates clasts recycled albian clasts derived from the diapir apex. (3) The thickness variations of these series and their facies are also influenced by diapir proximity. Facies show progressives sequences in a diapir-proximal position with a more carbonated facies and gray

to black marls in the deep zone away from the diapir. The Campanian limestone bar corresponds undoubtedly to a halokinetic sequence like those described in the la Popa basin, northeastern Mexico where Giles and lawton (2002) described these halokinetic features. These series contain conglomeratic level associated with an angular unconform-ity at the base of the sequences.

The geologic map (Fig. 4) and field cross-sections (Fig. 5) display evidence that the lorbeus structure appears as a nE-trending, folded zone bonded on the

Fig. 6 Photographic plate illus-trating halokinetic sequences recognized in the lorbeus dia-pir. a, b, c, d albian carbonate clasts re-sedimented in Cenom-anian deposits entrained in the diapir by debris flow move-ments, black arrows indicate the clast cemented (red arrows) by salt (gypsum) testifying a pierced salt diapir during albian time (e, f), and Coniacian–San-tonian aleg formation showing a Coniacian bioturbated series (e, f) and abundant nodules in Santonian marls (g, h) testifying to a synsedimentary submarine slope


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northern and southern sides by several n110°–n130°E faults zones. These correspond to inherited normal faults reactivated as reverse to dextral or sinistral strike-slip faulting. Detailed cross-sections established from field work show that the strike-slip component does not corre-spond to important movement. They are usually limited to some meters of displacement (Fig. 9). Previous stud-ies (Gharbi et al. 2013) demonstrated that across-fault sediment thickness variations and stratigraphic onlap relationships show that these n°110–130°E faults were active during the Early Cretaceous extensional period, with nnE–nE-trending minimum stress (σ3). This period was responsible for the development of nW-trending gra-bens and half-grabens. The Sers Fault trough (Fig. 1) is an example that begun at least in Cretaceous time. The

nW-limb corresponds usually to an overturned package. Detailed observations demonstrate that during Tertiary shortening, the nW-limb was deformed preferentially by flexural slip; no obvious fault is observed (Figs. 9, 10). Even if the faulting could have been active in deforma-tion of the nW-limb, most of them are localized within the formation boundary. Thus, it is sometimes not easy to define them as true faulting or as flexural slip faulting/movement (Fig. 9). The SE-limb usually exposes normal-polarity stratigraphic layers. This limb shows a strongly deformed refolded strata outcropping in the Koudiat El louatia, Koudiat Safia and Koudiat El Hanich (Figs. 4, 5). The strong deformation is located on the SE-lorbeus limb between the lorbeus anticline and the adjacent Oued Tessa syncline. In addition, the strongly refolded strata

Fig. 7 Cenomanian series of lorbeus area was corresponding to a deep marine environment; b, c small-scale preserved extensive deformation showing a sealed normal fault


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would explain that shortening does not preferentially use fault planes. Geologic cross-sections show a reverse slip of fault in the aïn El Melah region and a normal slip in the Koudiat Safia region. This latter frequently exposes fault planes showing reverse and strike-slip slickenside. Reverse slip, from the Middle Miocene to the present day, likely occurred along pre-existing normal faults. This fact is clearly displayed by several fault systems where inversion does not compensate the previous normal fault-ing throw. The geologic map (Fig. 4) and cross-section (Fig. 5) clearly show that both the nW- and SE-limbs currently have a “forceful” contact with the salt body of the diapir. The nW contact of the diapir dips usually SE-ward, in contrast to the SE contact that dips usually nW-ward. Field data prove that the first one is located (Fig. 9a, b) on Cretaceous overturned strata deformed preferentially by flexural slip. The second shows nW-dip direction on vertical to steeply dipping normal-polarity stratigraphic layers (Fig. 9e). The contact convergence allows envisaging that diapir geometry can be squeezed at a depth of 1–1.5 km. This simple limit-geometric pro-jection can highlight a detached salt body from its source layers at a depth of at least 1.5 km.

Gravity data

Gravity analysis is used here to confirm geological field data, understanding salt position and the comparison between models. The Bouguer anomaly map (Fig. 11) used is produced from the analysis of surface measure-ments of at least one per square km (and, in some locali-ties from denser networks, arfaoui 2000, national office of Mine, Tunisia). In order to determine whether the Triassic

salt material is rooted or not, stripping or modeling is not needed, since this material crops out (nely 1980).

The resulting complete Bouguer gravity anomaly map provides evidence for a general nE-trend in the lorbeus area and the Wadi lorbeus syncline in the north–west. Southeastward, the complete Bouguer gravity anomaly map also provides evidence for a general nW-trend in the Sers fault trough. In the northeastern part, gravity values range from −31.73 to −25.90 mGal in the Wadi lorbeus syn-cline. In the lorbeus area, values increase and range from −20.62 to −17.71 mGal. In the Sers Fault trough, values increase again and range from −17.71 to −15.04 mGal.

Two geologic pieces of evidence are highlighted by the Bouguer gravity map: (1) the Bouguer anomaly curves are not influenced by the Triassic body that shows simple limits. The gravity values fit with the geologic data of this study (Fig. 12), i.e., in the northwestern part, low-density values represent the syncline of the Wadi lorbeus, with the development of a thick Miocene series characterized by low-density values (Figs. 2, 4). In the southeastern part of this region, values increase and reflect the Sers fault trough, corresponding to thick Mesozoic series. This latter is formed by thick lower and Upper Cretaceous series that consist of marls and limestones characterized by high den-sity values (Fig. 2). (2) The lorbeus structure’s gravity map lines reflect a regional pattern of an elongate nE-trending structure. These results are confirmed by geologic data pro-vided by this study. This trend of gravity data appears to reflect the overall fold and fault pattern and highlight an atlasic (nE–SW) direction confirmed by our field work (Figs. 2, 4, 5). Gravity data confirms the presumption that the salt body detached from its source layers at a depth of at least 1.5 km, according to gravity surface measurements of at least one per square km.

Fig. 8 Photographic plate looking to south illustrating the Koudiat El Guenoua structural style. a Detailed map. b, c angular unconformity of Cam-panian limestone and marls on the aleg formation. d Conglom-eratic base of the Campanian series


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Discussion, thematic implications and conclusions

Field work, consisting of detailed geologic mapping and field cross-sections based on micropaleontological study of planktonic foraminifera, supported by gravity data, shows that all major faults of lorbeus area are mainly WnW–E-trending. These faults correspond to inherited Cretaceous normal faults with nE-trending extension. In addition, Cretaceous faulting is associated with numer-ous sedimentary features, such as calcareous nodules and slumps, usually reflecting sedimentation over a submarine slope for which faults are likely responsible. Structural data (Masrouhi 2006; Dhahri and Boukadi 2010; Gharbi

et al. 2013) confirm in Tunisia an extensional Mesozoic period related to the south Tethyan margin well character-ized at in the atlas system scale (Beauchamp et al. 1999; Guiraud and Bosworth 1997; Piqué et al. 2002; Herkat and Guiraud 2006; Frizon de lamotte et al. 2009). according to the terminology of Giles and lawton (2002), it is pos-sible to characterize the albian to Campanian strata as salt-related growth strata known as “halokinetic sequences”. Few horizons can be interpreted to correspond to debris flows sequences into the deeper part of the Cretaceous basin. Maastrichtians to upper Eocene series appear to be not influenced by the diapir growth. This period coincides with the tectonic inversion. Detailed investigations in the

Fig. 9 Photographic plate illustrating some important structural evi-dences of the lorbeus diapir. a Photographic plate looking to the south showing the aleg Formation SE-dip toward the diapir. b Dex-tral strike-slip faulting corresponding to the inversion of the inher-ited cretaceous normal faulting; note that the strike displacement is

limited to some meters (11–12 m). c, d Developed flexural slip used as preferential mechanisms during the folding of lorbeus anticline n-limb. e Photographic plate looking to the north showing the aleg formation vertical at surface overlain by diapir salt mass showing a nW-dip contact


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field and analysis of thin sections provide evidence that this series does not rework Triassic element. In the northwest of lorbeus area outcrops an Upper Cretaceous series which is unconformably covered by nW-dipping Oligocene–lower Miocene marine deposits (Figs. 4, 10). This unconformity give evidences of the Pyrenean tectonic event deformations known as the atlassic event in north africa (Masrouhi et al. 2007, 2008a; Riley et al. 2011). atlassic deforma-tion resulted in open folding (15°–20° dipping limbs) and inversion of major Mesozoic normal faults. This event, which occurred during the middle to late Eocene, is well recognized in north africa (Guiraud and Bosworth 1997; Frizon de lamotte et al. 2009, 2011; Bracene and Frizon de lamotte 2002; Masrouhi et al. 2007, 2008a). The presence

in the base of the Oligocene of abundant reworked Trias-sic sediments provides evidence for a second piercement of Triassic salt body. In addition, the unconformity angle of the Oligocene is about 80°–85° in the Sidi Msid and Wadi el Yhoudi region (Figs. 4, 5, 10) which we interpret as related to the diapir growth. However, this unconform-ity when it is related to the Eocene tectonic event it does not exceed 15°–20° in northern Tunisia as reported in sev-eral studies (Masrouhi et al. 2007, 2008b; Ben Slama et al. 2009; Melki et al. 2010, 2011). This structural framework provides evidence that the Oligocene unconformity was generated by salt growth, as demonstrated by the abun-dant reworked elements and by the high-angle unconform-ity. The results allow us to define the Oligocene series as a

Fig. 10 Photographic plate illustrating the Oligocene unconformity in the Oued El Msid and the Oued El Yhoudi region. a Photographic plate looking northwestward showing vertical Campanian–Maastrich-tian limestone overlain unconformably by Oligocene. This 75°–80°

angle unconformity (b–d) gives evidence for strata influenced by dia-pir growth, defining the Oligocene beds as halokinetic sequences. e, f Base of the Oligocene reworking abundant insoluble Triassic ele-ments demonstrating a second period of salt piercing


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second “halokinetic sequences.” The Oligocene thus repre-sents a second diapirism growth period. after that a second compression occurred during the middle Miocene to pre-sent day. The second unconformity concerns the Miocene–Pliocene contact. It corresponds to the alpine intra-Torto-nian major tectonic phase (Guiraud and Bosworth 1997; Masrouhi et al. 2008a).

Tectonic analysis shows that the lorbeus diapir is bounded on the northwest by a SE-dipping faults and on the southeast by nW-dipping faults. This geometry confirms that lorbeus zone was a graben structure. The lorbeus diapir emplacement was guided by normal faults of both nW- and SE-fault hanging walls. Taking into

account the general framework of this margin, the Meso-zoic extensional faulting probably was affecting both the cover and the basement. Even there is no evidence for a basement faulting component in the evolution of the lor-beus structure, the structural style appears very similar to those demonstrated as controlled by basement faults. Basement fault motion is one of the trigger mechanisms of diapirism in other salt provinces similar to the lorbeus diapir, such as the Eastern Betics (Rubinat et al. 2012). During the Miocene major compression, regional short-ening led to inversion of major faults that bounded the pre-existing lorbeus graben. lorbeus major faults appear as not strongly inverted as thrust system during alpine

Fig. 11 Bouguer gravity map of lorbeus area, superimposed on the outcropping Triassic salt, the contour interval is 1 mGal; a, b location of Fig. 12 section

Fig. 12 East–west section (see Fig. 11 for location) showing the Bouguer gravity profile model highlighting a posi-tive gravity anomaly over the lorbeus anticline and nega-tive gravity anomaly over the lorbeus syncline and the Sers fault trough


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compression. This fact can be explained by the high-angle pre-existing normal faults (equal or more than 70° dip-ping) that are mechanically weaker regarding the graben geometry. In this area of north Tunisia, there are major thrust systems, the Teboursouk and Zaghouan thrust sys-tems, located, respectively, north and south of the lor-beus area (Fig. 1). These thrust systems correspond to inverted major inherited normal faults (Masrouhi et al. 2013). By contrast, deformation of the overburden and the viscous salt layers was mainly accommodated along the lorbeus graben for which present geometry corre-sponds to the present-day complex anticline cored by Tri-assic salt. Geologic data, presented above and supported by gravity data, indicate a salt mass detached from its source layers at a depth of at least 1.5 km. The Cenozoic compressional tectonics was necked and/or squeezed the pre-existing diapir. The data set permits to establish the present-day geometry of a diapir bulb isolated from its source layers (Fig. 13).

Taking into account recent improvement in salt tectonics within inverted tectonic style context (letouzey et al. 1995; Sans and Koyi 2001; Scheck et al. 2003; Rowan et al. 2003; Roca et al. 2006; Callot et al. 2007, 2012; Trocmé et al. 2011; Rubinat et al. 2012) and key tectonic event in Tuni-sia (Bouaziz et al. 2002), we propose a simplified model of an inverted diapir with the following steps in its kinematic evolution (Fig. 14).

(a) During the Jurassic–Early Cretaceous period exten-sional faulting develops related to the south Tethyan margin evolution. This period corresponds to thick

depositional basin sediments. In north Tunisia, the basin evolution is also recognized to be controlled by nW-dipping basement faults.

(b) The aptian–albian period is recognized as an extreme extensional tectonic period in south Tethyan margin (Dercourt et al. 1993; Guiraud and Bosworth 1997; Guiraud et al. 2005; Masrouhi et al. 2008a). This is confirmed by the presence of nodule reworking, slumps and conglomerate lenses. The major faults rep-resent the source of ascending hot mineralizing fluids generating Celesto-baritine mineralizations (amouri 1989).

(c) With further extension, a system of horst and grabens has been emplaced, where undeformed blocks were bounded by high-angle normal faults. This period, rec-ognized as an extreme extensional phase, likely corre-sponds to a low ratio of aggradation/extension (rapid extension/slow sedimentation). These parameters pro-mote salt diapirism. The viscous salt material advances. This evolution corresponds to a definition of a reactive diapirism (Vendeville and Jackson 1992a, b) which extruded along the axis of the lorbeus graben. active diapirism, if it exist, corresponds to a brief albian epi-sode (Vendeville and Jackson 1992a; Schultz-Ela et al. 1993) unrecognized in Cretaceous strata and/or the dia-pir phenomenon.

(d) During the Cenomanian–Coniacian and the base the Campanian, the salt mobility is testified by a thick Coniacian–Santonian depocenter located around the diapir and thinner sequences on the lorbeus diapir. Diapirism ceases during Campanian period when salt

Fig. 13 Simplified cross-section of the present-day lorbeus diapir arrangement showing the structural style of the diapir demonstrated in this study. Basement architecture and depth are based on previous gravity and structural studies (e.g., Jallouli and Mickus 2000)


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growth cannot keep up with sedimentation. Campa-nian deposits seal the diapir with a significant angular unconformity.

(e) a tectonic inversion that occurred during the late Cre-taceous (Senonian) was followed by deep marine Pale-ocene and shallow Eocene deposits.

(f) a first compressional episode occurred during the Middle Eocene as demonstrated in the Maghreb mar-gin (Masrouhi et al. 2008a, b; Frizon de lamotte et al. 2009; Khomsi et al. 2009). Parts of the resulting struc-ture have been gently folded (limbs dip of c. 20°–30°). Palaeogene deposits and some of the Upper Cretaceous

Fig. 14 Simplified seven stage reconstruction (not to scale) of tecono-sedimentary evolution of the lorbeus structure. note that these schematic sections are not intended to show the mode of triggering mechanism of the initial salt extrusion, for more detail see text


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series were eroded during uplifting by the folding. This episode corresponds to moderate shortening that slightly inverted pre-existing normal faults but does not significantly deforms the diapir geometry.

(g) The Oligocene–lower Miocene corresponds to a second extensional period. This latter promoted a second phase of reactive diapirism which extruded along the axis of the lorbeus graben. Taking into account the moder-ate amount of Eocene compression, the diapir was not squeezed and the graben not inverted. Thus, the second salt extrusion undoubtedly is located in the central lor-beus graben similarly than during Cretaceous time.

(h) a middle Miocene paroxysmal shortening strongly deformed the overburden. The diapir was squeezed at a depth of ≈1–1.5 km. With increasing shortening, the diapir bulb was isolated from its source layers and a large complex fold formed. The geometry and the distri-bution of folds were strongly controlled by the pre-exist-ing structures. like those described in others provinces (Roca et al. 2006; Trocmé et al. 2011; Callot et al. 2012, 2013) synclines nucleated in the areas of pre-existing horsts. anticlines structures are formed at the pre-exist-ing graben axis; areas where the overburden was thinner.

Detailed analysis of geologic surface data from the lor-beus diapir completed by its gravity signature and compari-son with the Djebissa algerian structure allow us to suggest the following conclusions:

1. Diapirism only grows during extension. Salt extruded along the axis of the lorbeus graben, developed within extensional regional early Cretaceous tectonic in north-ern Tunisia. at this stage, salt supply was greater than the rate of sedimentation and resulted in the formation of salt diapir extruded reactively during albian time and passively during the late Cretaceous post-rift stage.

2. Contractional deformation does not promote diapirism. The geometry and the distribution of folds are strongly controlled by pre-existing structures. During shorten-ing, the initial major graben deforms as complex anti-clines and diapirs are squeezed.

Acknowledgments We are indebted to Pr. Mohamed Ben Youssef for dating lots of Cretaceous samples. amara Masrouhi thanks the aix-Marseille University, through the both programs of visiting Pro-fessor (2011–2013) and OT-Med Program (2012–2013) for found-ing its last visits in Cerege. amara Masrouhi thanks as well the IFC (Institut Français de cooperation) through SSHn program (2011 and 2013). We are indebted to François Roure and Jean-Paul Callot for their thoughtful reviews of the manuscript. We are grateful to Wolf-Christian Dullo, Editor in Chief of International Journal of Earth Sci-ences. This paper is a contribution to the labex OT-Med (no. anR-11-laBX-0061) funded by the «Investissements d’avenir» program of the French national Research agency through the a*MIDEX pro-ject (no. anR-11-IDEX-0001-02).

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