Introduction

This work aimed to show the role of salt tectonics in the nature and organization of the sedimentary deposits near Triassic salt bodies in the North-West of Tunisia (Fig. 1). The studied outcrops, studied in the ElKef-Tajerouine area evidence the occurrence of halokinetic sequences (HS) Rowan and Giles (2021), as primarily defined at the El Papo diapir in Mexico (Giles and Lawton 2002; Giles and Rowan 2012, ). The halokinetic sequences were formed in response to variations in sediment-accumulation rate versus diapir-rise rate and roof thickness. These sequences around the salt bodies indicate passive diapirism (Jackson and Talbot 1994).

Fig. 1
figure 1

A) The Maghrebian chain in North Africa. B) Structural domains of Tunisia and localization of areas and sections studied in this work. C) Geological map of the El Kef-Tajerouine area, (coordinate system Carthage UTM, N32°). The Triassic of J. Debadib and Ben Gasser is included in a NE-SW late Cretaceous anticline. Towards the south, the Guern Halfaya Triassic is linked to that of Debadib. D) Geological section along the ElKef -Tajerouine area. The SW-NE elongated structure with Triassic core of the fold axis results from the superposition of several deformation episodes. The ElKEF morphology builds during the Neogene phases of compression. These deformations reactivate an earlier halokinetic consecutive to the Jurassic-Cretaceous distensions, accompanied by a period of dome stages and piercing of the Triassic

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Halokinetic sequences are defined in the salt-sediment interface and described in the literature (Fig. 2). These constitute conformable successions of growth strata genetically influenced by near-surface or extrusive salt movement. The sequences are bounded by angular unconformities at their top and bottom, and become unconformable to conformable when the series moves away from the diapir (Giles and Lawton 2002; Giles and Rowan 2012). Halokinetic sequences (HS) form angular unconformity-bounded growth strata that were directly deposited adjacent to the margins of topographically raised salt domes, canopies, and sheets. The presence of diapir-derived clasts in the gravity-driven deposits at the base of the halokinetic sequences shows that the diapirs periodically forced their way out of the diapir roof when the rate of accumulation of sedimentation was lower than the eustatic change (Giles and Lawton 2002; Giles and Rowan 2012; Hearon et al. 2014). Two HS types (hook and wedge) are defined (Giles and Lawton 2002), depending on the geometry close to the diapir. A stack of wedge halokinetic sequences forms a tapered composite halokinetic sequence (Tapered-CHS). Tapered-CHS exhibits convergent base and top boundaries, a broad zone of thinning toward the diapir, and the axial trace of the drape fold monocline is inclined away from the diapir margin. A stack of hook halokinetic sequence exhibits a tabular geometry named tabular CHSs. These have narrow zones of stratal upturn (50-200 m), whereas tapered CHSs have a broad area of the stratal upturn and thinning (300-1000 m). The tapered CHSs are formed when the long-term sediment-accumulation rate outpaces the long-term diapir-rise rate (Giles and Rowan 2012; Jackson and Hudec 2017) while the tabular CHSs appear in the opposite case. Recently, Roca et al. 2020 has proven that in the case of deep deposits, the halokinetic sequences geometry is defined by the roof edges thickness, the depth of the water above the roof of the diapir and the dip of the salt sedimentation interface. The edges of the diapir roof are controlled by the ratio between the salt rise and the sediment aggradation. If slopes are high, gravity-driven deposits also develop in the wedge sequences.

Fig. 2
figure 2

Characteristics of halokinetics sequences described in the world

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This work is based on field studies, a biostratigraphic study and facies analysis performed on the collected samples. A sequential stratigraphic analysis by the Embry (2009) led to the definition of the third-order sequences within which fourth-order sequences controlled by the halokinesis were recognized (halokinetic sequences of Giles and Lawton 2002; Giles and Rowan 2012) and deposited around Triassic bodies or their equivalent condensed series. Deposits associated with salt halokinesis (salt tectonics) and evaporite diapirism are known in Northern Tunisia and the adjacent Algerian region. The present study therefore dealt with:

  1. a)

    A record of the Lower Cretaceous sequences in the ElKef-Tajerouine area showing a series of transitions from shelf to basin.

  2. b)

    The recognition of halokinetic sequences (sense Giles and Lawton 2002) around Triassic bodies by facies and morphostructural analysis and the study of relationships between the outcropping Triassic and the sections.

  3. c)

    Improving the paleogeographical map to show the role of Triassic diapirism in the NW saline province of Tunisia.

  4. d)

    A proposition of an evolutionary model of the studied region.

The objective of this study is to primarily provide evidence regarding the control of sedimentation by halokinesis.

The remainder of this research work was organized as follows: the “Geological and stratigraphic setting” and the “Material and methods” sections present the geological and stratigraphic setting of the area under study, whereas the “Structural and sedimentary study” section provides the structural and sedimentary study of the region. The interpretation and discussion of the results as well as the paleographical study are detailed, respectively, in the “Interpretation and discussion” and “Paleogeography” sections.

Geological and stratigraphic setting

The stratigraphic synthesis of (Ben Ferjani et al. 1990) as well as the geophysical studies of (Bobier et al. 1991) interpreted Northern Tunisia as rift basins formed during Triassic to Jurassic times (Bouaziz et al. 2002; Boughdiri et al. 2007; Bobier et al. 1991; Martinez et al. 1991; Bédir et al. 2000; Ben Youssef 1999; Chikhaoui et al. 1998; Hajji et al. 2013) and evolved after a break-up to a passive continental margin during the Cretaceous. The earliest Mesozoïc sedimentation consists of Triassic supratidal to continental deposits accumulated during Tethyan rifting. These are overlain by a Jurassic shelf to open-marine carbonates that are formed on tilted blocks (Bouaziz et al. 2002; Kamoun 1989; Boughdiri et al. 2007). The Tunisian Atlas Mountains are the eastern part of the Maghreban Chain (Fig. 1A). They extend from the internal Tellian (NNW of Tunisia, Fig. 1B) to the salt province “zone of diapirs.” The North-Tunisian Atlas is a fold-thrust belt driven by basement fault reactivation (Fig. 1D) and halokinetic activity involving the Triassic salt, (Rouvier et al. 1998; Snoke et al. 1988; Ben Ayed 1986; Piqué and Tricart 2002; Ben Ferjani et al. 1990; Khomsi et al. 2009). The Atlas structures (Fig 1B), correspond to a series of elongate outcrops of Triassic shaly evaporites offset by south verging reverse faults and thrusts in the Tunisian Atlas, (Boccaletti et al. 1990; Morgan et al. 1998; Anderson 1996; Rigo et al. 1996; Brusset 1999; Khomsi et al. 2004).

The NW salt province or “diapir zone”

The structural evolution of northern Tunisia is subdivided into five successive stages: Mesozoic rifting, Upper Cretaceous to Eocene compression (Atlasic stage), Middle Miocene compression (Alpine stage), Late Miocene extension and Pliocene compression. The Kef-Tajerouine area, belonging to the so-called “diapirs zone” situated in the NW part of the Tunisian Trough (Fig. 1B), is characterized by large reliable flat synclines, cored by a thick succession of Cretaceous to Paleogene with dominantly carbonates series. The NE-SW narrow anticlines have cores that mostly consist of complexly-deformed Triassic rocks. The anticlines have a thrust component along the south and south-east flank.

The NE-SW trending Triassic bodies of the “diapir zone” have been interpreted as diapiric structures due to their gypsum nature, chaotic internal composition, and their frequent association with faults (Burollet et al. 1956; Snoke et al. 1988; Bouhlel 1993; Perthuisot et al. 1999; Hatira et al. 2000; Chikhaoui et al. 2002; Boukadi and Bédir 1996). The role of diapirism has always been a hot subject of debate. However, the most adopted interpretation is that the Albian diapirism was superimposed by the late Cretaceous-Paleogene compression and then by the Tertiary stage forming the present-day structures (Hicheri et al. 2018).

Morphologies of mushroom diapirs explain the thinning and inversion of sedimentary successions in their flanks (Snoke et al. 1988; Martinez et al. 1991; Smati 1986; Rouvier et al. 1998; Hatira et al. 2000). Bolze (1954) reported extrusive diapirs within the Aptian in this area. Vila et al. (1996, 1998) proposed the salt glaciers model for the salt-bearing bodies or the Tajerouine -ElKef area. Vila et al. (1996), Ghanmi et al. (2006), Masrouhi and Koyi (2012), Masrouhi et al. (2013) argued that the Triassic is rather allochthonous and folded later during the Cenozoic contraction. More recently, Jaillard et al. (2017) suggested that the Tunisian margin is an Atlantic type margin submitted to salt tectonics.

The Lower Cretaceous in the ElKef area

The lower Cretaceous of the Oued Mellegue area (OM—Fig. 1C) outcrops North of ElKef–Tajerouine area. The series consist of thick (500 m) Valanginian siliciclastic turbidites deposits. Following a hiatus of the Hauterivian (Fig. 3), the Barremian- Aptian (400 m) interval marks a change to shallower deposits (marly sediments of a distal shelf) and well-developed limestone shelf. In the ElKef-Tajerouine area, the Aptian and the early Albian are characterized by shallow mixed siliciclastic and carbonate sediments and some developed shallow carbonate deposits (Serj Formation at J.Slata, J. Hamaima, and J.Harraba). The Albian (Fahdene Formation, Burollet 1956) is characterized by thick deepwater deposits (200 to 400 m) consisting of black-shales and hemipelagic marly-limestones (Jaillard et al. 2021).

Fig. 3
figure 3

General stratigraphic section of the Lower Cretaceous of the Oued Mellègue dam situated North of Elkef area. The stages subdivisions are based on the stratigraphy of Ben Haj Ali (2005)

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The Aptian–Albian cycle in the northern-central part of Tunisia

The Aptian succession studied at the north of Tunisia (Oued Mellègue, OM and J. Oust section, Fig. 1) is storm-dominated siliciclastic and carbonate shelf deposits (Fig. 4, modified from Souquet et al. 1997). These evolve into carbonates which deepen upward to organic-rich sediments (black-shales of the Albian Fahdene Formation, Fig. 4). The depositional sequences of the Aptian - early Albian, defined in the north-east of Tunisia (Saadi et al. 1994; Souquet et al. 1997), are condensed series (Enfidha area, Fig. 4). The subsiding northern domains (Oued Mellègue section, Fig. 3), distal shelf limestones correspond to the transgressive system tract (TST). The shallow-marine marly sandstones-limestones (orbitolinas-rich, with lamellibranchs and echinoderms) represent the high stand system tracts (Hamaima Formation Fig. 4). The Aptian-Albian transition cycle was defined (Souquet et al. 1997) according to a N-S transect explaining the hiatus of a part of the Aptian-Albian by truncated second-order sequences due to synsedimentary tectonics and the beveling of the series in the area of platform-basin transition (J. Oust-Fig. 4). In this study, we would like to highlight the role that the halokinetic movement played at this time span.

Fig. 4
figure 4

A) Correlation between the Aptian-Albian of Oued Mellègue and J.oust section. B) Depositional model and depositional sequences of the Tunisian margin during the Aptian to the early Albian, (from Souquet et al. 1997, modified). The Aptian to late Albian is subdivided into 4 depositional sequences (3rd order of Vail et al. 1991). When the sequences are complete the TSTs correspond to limestones bar (topped by hard grounds and ammonites) and are followed by shallows sandstones-marlstones and carbonates bioclastics sediments forming the HSTs. The sequences boundaries are characterized by abrupt facies change and regional unconformities

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The Aptian-Albian transition in Tunisia

The latest Aptian - Albian unconformity is described in numerous areas of the northern- central part of Tunisia (Bismuth 1973; Bismuth et al. 1981; Mrabet 1984; Ben Youssef 1999). This stage is now debated (Hichi et al. 2020) and rejuvenated to the middle Albian (Latil 2011; Chihaoui et al. 2010; Jaillard et al. 2013; Chaabani and Razgallah 2006; Trabelsi et al. 2020). Trabelsi et al. (2020) evidenced a prominent eustatic sea-level fall of latest Aptian to earliest Albian age that enhanced the emersion of the Central Tunisian Atlas domain. This was followed by the Albo-Cenomanian transgression known at the global scale (Ben Youssef et al. 1985; Abdallah 1989; Chaabani and Razgallah 2006; Chihaoui et al. 2010; Ben Chaabane et al. 2019). This interval is marked by two successive unconformities: (1) the first unconformity is due to a eustatic sea level fall at the Aptian-Albian boundary, and (2) the second unconformity is related to a tectonic event that culminates in Middle Albian times (Jaillard et al. 2013). The Albian tectonics is related to a strike-slip rejuvenation of ancient master basement faults in relation to the opening of central African grabens and the South Atlantic Ocean (Zouari et al. 1999; Bouaziz et al. 2002; Zouaghi et al. 2016).

In the South-Tethyan domain (Tunisia, Algeria and Morocco), this period corresponds to the Austrian stage of African-Eurasian convergence (Brusset 1999; Chikhaoui et al. 2002; Bodin et al. 2010; Chekhma and Ben Ayed 2013; Soua 2016). In contrast, several authors consider that the Albian transition is a stage of regional extensional tectonic activity (Snoke et al. 1988; Chihi et al. 1984; Martinez et al. 1991; Vila et al. 1996; Gharbi et al. 2013; Masrouhi and Koyi 2012; Jaillard et al. 2013).

The tectonics at the Albian was attributed to the rotation of the Iberian-Apulian Plates (Guiraud and Maurin 1992; Guiraud 1998; Janssen et al. 1995). In the southern Mediterranean domain, the continental collision began in the late Cretaceous (Cherchi and Trémolières 1984; Letouzey and Trémolières 1980; Dercourt et al. 1986; Frizon de Lamotte et al. 2009); however, the Maghrebian Atlas has undergone tectonic subsidence since the Early Cretaceous (Guiraud 1998; Souquet et al. 1997; Moragas et al. 2016).

Material and methods

We logged the stratigraphy of the latest Aptian to Albian succession in outcrops (located in Fig. 1B) diapiric structures located in the Kef-Tajerouine area, (Fig. 1C). A detailed sedimentological study based on field examination, biostratigraphy and facies analysis of the series allows us to characterize the facies and sedimentary environments. The four studied sections are located at the contact with Triassic bodies in the El Kef -Tajerouine area (named KNA, KA, KS, OK, KD, SM Fig. 1C).

Sequence stratigraphic analysis led to the recognition of the depositional sequences and their systems tracts. The comparison with the sequences of J. Oust and those of the Enfidha areas leads to the recognition of unconformities and then to constraint the role of Triassic during the Aptian-Albian transition.

We used published seismic line in the north of Tunisia (Rigo et al. 1996) and gravity model proposed by numerous authors (Chikhaoui et al. 2002, Jallouli et al. 2005; Talbot 2005; Vila et al. 1998; Ayed-Khaled et al., 2012; Gharbi et al. 2013, Hicheri et al. 2018) to further define the morphology of the Triassic bodies in the studied area. The results were compared with published analog model of salt tectonics (Koyi 1988; Jackson and Talbot 1994; Jackson and Hudec 2017; Costa and Vendeville, 2002; Berastegui et al. 1998).

Structural and sedimentary study

El Kef–Tajerouine area (Atlas domain)

The Debadib and Ben Gasser Triassic body

The geological map (Fig. 1C) shows two main NE-SW Triassic salt bodies (Ben Gasser and Debadib), surrounded by upper Cretaceous carbonate rocks. The J. Debadib structure forms a box fold anticline with Triassic evaporites filling the core. The existence of inverted layers in the North of J. Debadib indicates a Triassic flow towards the northeast (Snoke et al. 1988). The Triassic-Cretaceous interface shows locally inverted faults testifying the subsequent regional deformation (Perthuisot et al. 1999, Hatira et al. 2000; Chikhaoui 2002).

The contact between the Triassic and the latest Aptian-Albian overburden corresponds to an unconformity (clearly visible at Koudiat Ennab El Azreg, KNA—Fig. 5), complicated by subsequent tectonics. The Koudiat Ennab El Azreg (KNA, Figs. 1 and 5) structure belongs to the overburden of the Triassic of J. Debadib. The KNA section is the reduced series of the latest Aptian–Albian units compared to the lateral outcrop of the Mellègue dam section (Fig. 3). The KNA syncline shows a steep flank in contact with the Triassic salt body (Ph1, Fig. 6). Towards the core of the syncline, the dips weaken as the layers crests become thinner (Fig. 5B, KNA). Breccias and mineralization underline the base of the section forming an erosional unconformity. The series is subdivided into four (Fig. 6) lenses units bounded by abrupt facies changes and erosional surfaces.

Fig. 5
figure 5

A) Geological map of Debadib at the West of ELKef city with the values of dips, drawn from Burollet et al. (1956), Snoke et al. (1988), Vila et al. (1996). B) Geological section established across the Triassic of J. Debadib, showing a drape- fold structure at the North flank of the Debadib correlated with the small syncline of KNA. C) Correlation between the Oued El Kohol and KNA sections located over (KNA) and close to (OK) the Triassic of Debadib. The thinning of Albian series from KNA to OK section resulted from the Triassic rise

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Fig. 6
figure 6

A, Stratigraphic log of Koudiat Enab el Azreg section (KNA), characterized by the latest Aptian? - Lower middle Albian halokinetic sequences. The series is subdivided in one hook halokinetic sequence (Units 1-2) breccias level capped by glauconitic limestone, followed by reefal lenticular limestones. (Units 3 – 4) are middle-late Albian wedge sequence. Ph.1-2) Relationship between lenses of mineralized dolomite (lateral cap rock), and reefal limestones of unit 2. Ph.3) Panoramic view of KNA showing a straightened layer by Triassic rise. Ph.4) Debris flows breccias of Unit1, the arrow points to small Triassic clasts (dm) into a yellow dolomitic matrix

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The polymictic breccia of Unit 1 involves subangular dolomicrite extraclasts varying in size from centimeter to decimeter. They originate in the Triassic and pre-albian deposits (oolithic dolostones clasts of Aptien age, Ph. 3-Fig. 7). The dolomitic grainstone matrix (Ph.1-2-3, Fig.7) shows sericite plages. A discontinuous and glauconitic dolo-limestones level (50 cm) with echinoderms, brachiopods and glauconite caps unit 1.

Fig. 7
figure 7

Photomicrograph of thin sections of the KNA stratigraphic log. Ph.1) Thin section of polymictic breccia (Unit1) reworking triassic extraclasts (T), and centimeter-sized caprock clasts (dolomicrite). Ph.2) Photomicrograph of the polymictic breccias, the cement corresponds to a dolosparite with quartz idiomorphic crystals (Q). Ph.3) clasts of oolithic grainstones (CO,) reworked in red- algal peloidal-grainstones. Ph.4) Magnification of Ph.3 showing red algal and quartz idiomorphic crystals. Ph.5) Photomicrograph of the reefal limestones, the patch reef characterized by stomatopores (S) encrusted by red algae (AR). Notice the geopetal structures (arrow) indicating an emersive stage at the top of the limestones. Ph.6) Photomicrograph at the top of unit 2 (rudstones with coral) notice the iron-oxide (hard ground) and bioturbations (yellow arrow). Ph.7) Wackestones with glauconite (Gl) rich in planktonic foraminiferas (Fp), some of the grains were altered (AGL), grains of phosphate (phosp). Ph.8) Photomicrographs of the laminated late Albian series, Radiolarians(R) mudstones with sponges spicules (Sp)

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The following unit (Unit2) belongs probably to the early Albian which contains red alga (Archeolithotamnium rude) and some rarely Foraminifera cf. Favusella washitensis. Theses reefal limestones (5 to 10 m) consist of red algal-peloidal-echinodermal grainstones-bindstones with stromatoporoid and rudist bioclasts (Ph. 5-4, Fig. 7). This unit is capped by a hard ground (iron oxide, geopetal features, Ph.5-Fig. 7) that corresponds to a sequence boundary. Units 1 and 2 contain quartz idiomorphic crystals (Ph. 4-Fig. 7) with relicts of microcrystals of anhydrite as described by Hatira (1988) in the cap-rock of Sakiat diapir located at the NW of the Ben Gasser structure.

Unit 3 is attributed to the early Albian and is more continuous than the previous ones. This reduced deposit (1m thick) corresponds to the Allam member of the Fahdene Formation (Burollet 1956). These deposits are glauconitic packstones with radiolarian, planktonic foraminiferas and echinoderms, of the distal ramp (Ph. 7-Fig. 7). In the overlying series (unit 4), limestone are dark and thinly laminated. Microfacies show radiolarian wackestones/packstones with rarely sponge spicules and planktonic foraminiferas (Ph. 8-Fig. 7). The association: Biticinella breggiensis, Rotalipora aff. ticinensis; Ticinella roberti, indicates Breggiensis zone corresponding to the base of Late Albian (Jaillard et al. 2021).

The Albian series of KNA (unit 1, unit 2, and 3, and unit4 Fig. 6) is subdivided into two depositional sequences (TST–RST of Embry’s model). The halokinetic sequences (HSs-Fig. 6) corresponds to the stage of diapirs growth. The breccias level evidence of piercing and erosion of the diapir roof, while the reef limestones develop when the diapir was more stable. This preserved roof forms a small syncline whose deposit was synchronous with a diapir ascent phase.

Origin of lenticular dolomite at the contact of overburden with Triassic

The breccias of unit 1 (KNA-Fig. 6) reworks yellow centimetre-sized clasts (Ph. 2-Fig. 7) coming from lime-dolomite lenses located between the reef limestones and the breccias (cp, Fig. 6). These lenses are here interpreted as lateral cap rock as defined for the Neoproterozoic Patawarta salt sheet in the Flinders Ranges, South Australia (Giles et al. 2012) and allochthonous salts (Kernen et al. 2019). Cap rock (a cap of carbonate–anhydrite), is generally believed to develop from a salt solution from the top of the salt core. This leaves a residue of insoluble anhydrite altered to gypsum, calcite, and sulfur. Cap rocks are massive fabrics with homogeneous mineralogy and lack any internal structure. The lack of sedimentary structures or fossils in these lenses supports this observation. The geometric relationship with the overlying layers (B-Fig. 6) led to consider these lenses as cap rock relict that were formed on the roof of the diapir, then rotated to a flanking position by (KNA-Fig. 6) the process of halokinetic drape-folding (Giles et al. 2012; Kernen et al. 2019). This indicates two stages of stabilization of the diapir: (1) first stabilization and formation of the cap rock, and (2) second stabilization deposition of the shallow limestones of Unit 2 (B-Fig. 6).

In the NW flank of the J. Debadib diapir, the late Albian black-shales of Oued El Kohol (OK section-Fig. 5) are overturned and Triassic evaporites rest on their top. However, the equivalent of units 1,2,3, of KNA, is absent by lateral thinning (C-Fig. 5). The section consists of alternating laminated black shales.

North-East of Tajerouine area (J.Sif-Guern Halfaya)

Toward the south of the Debadib-Ben Gasser Triassic wall (crop out at 15 km) and separated by late Cretaceous deformed thick succession (Fig. 1) appears a second smaller diapir structure (J. Sif, Guern Halfaya, Fig. 1C, D and Fig. 8A). The Triassic body forms the prolongation of J. Debadib. This Triassic evaporites core NE-SW anticline supports a Miocene perched syncline (J. Sif). The Triassic rocks are flanked to the South of J. Sif, by steep layers of Albian sediments.

Fig. 8
figure 8

A) J.Sif-Guern Halfaya geological map (drawn from Burollet et al. 1956; Vila et al. 1998). The structure corresponds to a small NE–SW trending anticline that was part of the Albian to late Cretaceous succession straightened by the salt extrusion (GA- KA - KS section). B) Geological section at J. Sif - Guern Halfaya. The pinch out of the Albian series around the Triassic indicates the rising Triassic at that time. C) Geological section at Koudiat edelaa (KD) displaying inverted late Albian series on the north flank of the Triassic evaporites (Ben Gasser)

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The Albian forms the cover of the southern flank of J. Sif (Koudiat Hamra, KA-Fig. 8A). The series is subdivided into three units (Fig. 9). Unit 1 rests directly on a deformed narrow Triassic body and consists of grey thin-bedded lithographic biomicritic limestones (5 m thick), belonging to the Allam Member of Late-Early Albian age, Chihaoui et al. 2010). The overlying unit (2) is dated as Late Albian by the association: Globigerinelloïdes casei, Globigerinelloïdes benthonensis, and Hedbergellas sp. The bottom is 5 m thick amalgamated distal storms deposits (brown sandy-limestones). These consist of sandy glauconitic packstones rich in planktonic foraminifera, Ph. 1-Fig. 9) overlain by marly-limestones rich in trace of ammonites, glauconitic and planktonic foraminiferas (Ph. 2-Fig. 9). The series evolves to radiolarian-rich brown hemipelagic marly-limestones of Unit 3 (Fig. 9). The latter unit is characterized by Rotalipora apenninica, Thalmanninella balernaensis, Globigerinelloïdes simplex, Globigerinelloïdes simplissima, and some Planomalina praebuxtorfi, indicating the upper part of the late Albian (Mouelha Mb of the Fahdène Formation). The lateral thickness changes in the Albian storm deposits (Unit 2, Fig. 9 and B, Fig. 10) related to NNE-SSW trending faults in the Koudiat Hamra area (KA-Fig. 10A), suggesting that these faults were synsedimentary (ElGaraa, Fig. 8A). Thickness trends and stratal pinch out, from NE to SW, away from the Triassic outcrop (B-Fig. 10) constrain the timing of the Triassic inflation. Units 2 and 3 are identified as halokinetic sequences bounded by unconformity and facies changes.

Fig. 9
figure 9

Koudiat Hamra stratigraphic log, ((KA-Fig. 8, Tajerouine area). The stratigraphic section was performed across the SW part of the J. Sif structure. The series is subdivided into three middle-late Albian units. Unit 2 consists of amalgamated distal storm deposits. At least two halokinetic sequences are recognized, by microfacies analysis. Ph.1) Planktonic foraminiferas rich silteous wackestones (FP), subangular quartz grains are covered by iron oxides. Ph2) Microfacies of the unit2, planktonic foraminiferas and glauconites rich wackestones. We identify a wedge halokinetic sequence

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Fig. 10
figure 10

A) Satellites imagery of the Tajerouine area located South J.Debadib-BenGasser (from Google Earth). Notice the beveling of the layers of the Albian (El Garaa) toward the Triassic outcrop. B) Correlation between Albian sections south of J. Sif, close to the Triassic. The thinning toward the NW, indicates sedimentary control by Triassic salt rise. Ph.1-2) Lenses of breccias with Triassic clasts, and cap rock blocks “yellow dolomite”

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The second studied outcrop is situated 500 m southwest of KA section, south of the Guern Halfaya structure and labelled KS (Fig. 8A, Fig. 10). The series begins with basal lens-shaped breccia layers rich in Triassic dolomite and cap rock clasts (Ph. 1and Ph. 2, Fig. 10) and is overlain by an alternating laminated marly–wackstones rich in pithonellas and radiolarians. According to radiolarians such as Squinabollum fossilis, Torculum coronatum and the planktic foraminifera Biticinella breggiensis, this section belongs to the Albian and reaches the latest Albian with the appearance of Planomalina Buxtorfi, (Mouelha Mb of the Fahdene Formation).

This sequence is characterized by lenses of breccias (diapir - derived clasts) with rapid lateral facies variations (tens of meters in size). Although complicated by a later set of faults, the outcrop shows layers having undergone a torsion linked to the salt rise during the formation of a hook halokinetic sequence type.

Southwest of Tajerouine area (J. Slata)

Jebel Slata (JS-Fig. 1C) forms an Aptian–Albian mushroom-shaped anticline trending NE with a Triassic core detached to the Southwest. In the SE part of the Jebel Slata, the Fahdene Fm overlies the Hameima Fm, of earlier Albian age, by an angular unconformity (Fig. 11, A and B). This major unconformity occurs between Aptian reefal facies (late Aptian) overlying by shelf limestones- sandstones of the Hameima Formation (lower Albian) deposits (Smati 1986; Perthuisot et al. 1988; Chikhaoui et al. 2002; Chihaoui et al. 2010; Jaillard et al. 2017), and by late Albian deep marly – limestones of the Fahdene Formation (Ph. 1-Fig. 11). The contact is underlined by brown to red developed breccias rich in Aptian and Triassic clasts (Smati 1986), corresponding to a salt tongue evolving probably to salt weld (A, Fig. 11) and is onlapped by the late Albian sequences. A “discontinuous weld is a surface or thin zone marking a vanished salt body. The weld results from complete or nearly complete loss of salt by creep or dissolution. As such, a weld is a negative salt structure. Welds can separate concordant strata but are easiest to recognize where either or both contact strata are discordant to the weld surface.” (Jackson 2017). This major unconformity at the vicinity of rotated Aptian and early Albian layers (A, Fig. 11) is related to salt movement.

Fig. 11
figure 11

A) Geological map of the J.Slata from Smati (1986), showing a mushroom structure affecting the Aptian- Early Albians layers. B) Geological section established at the southern part of the J.Slata. Ph.1, Panoramic view of the unconformity between the Early Albian and the Late Abian. Ph2

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The base of the succession, in the SW part of Jebel Slata (see location: JS, Fig. 1C) is dated by Biticinella breggiensis, indicating the base of the late Albian. The upper part contains Rotalipora praeticinensis and Rotalipora ticinensis of the Late Abian age. The alternating grey-black marls and hemipelagic radiolarians, belemnites, cherts are rich in bioturbated limestones. It is subdivided into five Units. Each unit starts with a slump level, to form an HS unconformity. Over these unconformities occur breccias and debris flow layers (with Triassic extraclasts and yellow metric lenses of Triassic material mixed with centimetre-sized clasts derived from early carbonate deposits). The clasts’ size and frequency increase towards the south of the J.Slata structure. The slumped glauconitic layers (Ph1-Fig. 12) evolve upward to laminated calcarenites. Hemipelagic mudstones characterize the upper part of each unit. These lenses disappear laterally in the thickest Albian series notably (at El Garaa, Fig. 8) and north and east of J. Slata.

Fig. 12
figure 12

J. Slata section corresponds to at least four HSs of a slope-carbonate succession, with glauconitic debris flows containing both shelf and pelagic (Radiolarian) bioclasts. Ph.1) slumped nodular limestones rich in Triassic clasts, glauconite and bioclasts originated from shallow deposits. Ph.2) slight variations in the stratal dips of the Albian series, Ph.3) Photomicrograph of the brecciated level at the bottom of the series, Packstone rich in bivalves, glauconite (Gl) (some of them altered), Triassic clasts and Planktonic Foraminiferas. Ph.4) cherty packstones rich in Radiolarias and Glauconites. The cherts developed from sponges spicules (SP)

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The development of nodular facies with Triassic clasts and debris deriving from a near platform indicates a slope created by the diapiric extrusion. The sediment derived from the shelf within the debris-flows facies (lens-shaped deposit with lateral variations in thickness) and the slight variations in the layers dip (Ph. 2-Fig. 12) reflect the growth stages of the diapir. The laminated mudstones and black shales characterize the relative calm of the diapir. The evolution towards the summit with more mature sediments, and the sequences of the type couplet debrites - turbidites of “Einsele sequence” (Einsele 1991), indicates a slope setting. Units (1, 2, 3, and 4) correspond to halokinetic sequences with a basal sequence boundary marked by slumps indicating the slope form created by the rising roof of the diapir. Debris flows adjacent to the unconformity contain Triassic detritus and shelf bioclasts (ph.2, 3-Fig. 12) and are abundant in the two first sequences. The set shows a vertical evolution to glauconite-rich sediments (altered and non-altered grains, Ph. 1-2-3-4, Fig. 12) and cherts (Ph. 4-Fig. 12). This indicates a transgressive system tract of late Albian age (T. Breggiensis zone).

At Koudiat Edalaa (KD- Fig. 1, C-Fig. 8), north of Ben Gasser, and at the interface with the Triassic, inverted late Albian black shales were observed (Rouvier et al. 1998, D-Fig. 8). We correlated KD section with the Oued El Kohol section and the upper part (unit 4) of the KNA series (Figs. 5–6). The inverted dip of the Albian layers associated with foliation occurrence is related to a pinch of the NW side of the diapir during the Triassic evaporites flows towards the north-west.

Sidi Mbarka structure (South West of Ben Gasser Triassic body)

The studied structure is located close to the Algerian border, on the SSE extension of the Debadib-Ben Gasser salt wall (J. Harraba- SM, Fig. 13A). Sidi Mbarka (SM) corresponds to an NW-SE anticline cored with Aptian core sediments (Fig. 13A–B). The series was dated as Barremian-Aptian by Dubourdieu (1956) and then revised by Peybernès et al. (1996), Latil (2011), Chikhaoui et al. (2002), Jaillard et al. (2013) to latest Aptian-late Albian. The series is characterized by Aptian shallow mixed carbonate-silicoclastic deposits evolving to deep Albian sediments (Peybernès et al. 1996).

Fig. 13
figure 13

A) Geological map of J. Harraba, drawn from Dubourdieu (1956). B) Morphostructural analyses of a satellite imagery (from google Earth) of Sidi Mbarka structure (S-E of J. Harraba), showing an Aptian - Albian series characterized by the occurrence of composites of HS sequence in contact with the south-west termination of the Triassic of Ben Gasser. C) Correlation between SS1 and SS2 mining boreholes of Sidi Sahbi (Hatira et al. 2000). We recognize at least two successive Tabular-Tapered CHSs from the early Albian to the late Albian, in a relationship with the Triassic extrusion. A minor cusp formed where the unconformities (septaria level) intersect the diapir

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The early Albian is topped by an ammonite level (Dubourdieu 1956, Burollet et al. 1956). The level was attributed firstly to the late Aptian then revised to the lower Albian (Peybernès et al. 1996; Latil 2011; Jaillard et al. 2013, Fig. 13C). This ammonite-rich level is known throughout the region. Black-shales overlain by septaria layer follow the sequence which intersects an unconformity at cusp (C-Fig. 13) delineating the late Albian tapered halokinetics sequences. This level evolves laterally to glauconitic breccias lenses reworking Triassic clasts (Peybernès et al. 1996).

The morphostructural analysis based on satellites imagery (Fig. 13C) and a review of the mining surveys of Sidi Sahbi (SS1 and SS2, Fig. 13D, Hatira et al. 2000; Chikhaoui et al., 2000) prove the occurrence of composite halokinetic sequences (CHS) during the Albian. The satellite imagery shows a convergence of the Albian layers toward the Triassic outcrop, where the strata become thinner close to the evaporitic material. The summit of the Albian layers straightens up and turns back under the salt material that forms an overhang spread laterally to the west (Fig. 13C). This setting is caused by the emerging salt, which on its way to the surface causes pinching and beveling of the overburden.

In this case, the tapered composites HS (Hameima Formation) corresponds to the high stand systems tract as defined by Giles and Lawton (2002). The composites HS created by the rising salt of Ben Gasser area was tapered in its shape in the case of the HST and tabular in the case of TST (reefal limestones of the Latest Aptian-Early Albian localized at the NW part of Sidi Mbarka structure, (Fig. 13C).

Interpretation and discussion

The facies and thickness variations observed in the Albian successions in the ElKef–Tajerouine area indicate lateral changes in accommodation due to passive diapirism, (Jackson and Talbot 1994; Rowan and Giles 2021) into the Triassic bodies in this area. The local appearance of high-energy facies on top and at the flank of salt structures, which contrasts with the distal shelf deposits, is here emphasized. It illustrates the influence of uplift of Triassic evaporites on the paleo-bathymetry and sedimentation. Halokinetic sequences in the KNA section, KH-KS section, and the Late Albian of Jebel Slata illustrate different stages of halokinetic movements. The morphostructural analysis of the KNA section above the Triassic salt body of the Debadib corresponds to a deformed roof with drape-fold geometry (elongated syncline of KNA, Fig. 5B–C). The reduction of strata thickness toward the north flank, stratigraphic pinch-outs, and unconformities were caused by salt rise. The defined Hook and Wedge sequences were deposited above and around salt uplifts (Giles and Rowan 2012; Rowan et al. 2003). Their position above the Triassic evaporites of J. Debadib reveals salt rise movements within the Triassic body to a pre-Late Albian and Late Albian age. The replacement of breccias by reefal facies in the KNA section marks an evolution of the depositional setting from slope created by diapir rise then to a thin shallow platform by quiescence of the diapir in a halokinetic sequence (Giles and Lawton 2002; Giles and Rowan 2012). The breccias then reefal limestones form the basal part of a hook sequence (Giles and Lawton 2002) trapped at the top of the rising salt body of the J. Debadib. The uplift of the Triassic salt of J. Debadib is at the origin of the slope. The sediment settled above this slope contains clasts resulting from the erosion of the diapir and its roof. The correlation between the KNA and Oued el Kohol sections (Fig. 5C) indicates the rise of Triassic salt of the J. Debadib during the Albian (pinch out of the HS sequences observed at the KNA syncline towards the north flank). The geometry and sedimentary content of the KNA sequence correspond to the HCS defined by Giles and Lawton (2002) and Giles and Rowan (2012) when they studied the region of the El Papalote diapir in Mexico. In the same case, the vertical evolution of debris-flow facies towards reef lenses was also observed around the Salif diapir in Yemen (Davison et al. 1996).

Albian gravity-driven deposit consisting of breccias and glauconitic debris-flows (south-east of J. Slata, Fig. 11) underline the effects of eustacy triggering the deepening of the basin. Debris-flows in the Late Albian sediments with clasts originated in Triassic extrusions were formed over deep ramp setting generated by the erosion of the top of the diapir. The angular unconformity and upturned growth strata represent the development of halokinetic sequences adjacent to a salt weld or a prolongation of a salt tongue (south of J. Slata, Fig. 11A). This allochthonous salt was then buried and remobilized by the thick Albian sediments (Fig. 12).

These HSs are included in a tapered composite halokinetic sequence and correspond to a transgressive systems tract of the T. Breggiensis zone. Jaillard et al. (2017) suggest the extrusion of the diapir during the Middle Albian by a hiatus recognized in several zones of the Tajerouine area. The Albian sequences of J. Slata are very similar to the carbonate series described around the Bakio diapir. They were formed in a slope, over an angular unconformity (between early and late Albian). The Albian transgression is recognized both in Tunisia and Spain. The Albian series around the Bakio diapir in Spain (Rowan et al. 2012; Poprawski et al. 2016) is richer in sediments in the breccia levels suggesting that the formed roof over Bakio diapir was thicker than the one of J. Slata. However, the evolution of the series from proximal towards distal facies was observed in both cases.

Composites of the halokinetic sequences (CHS) correspond to systems tracts related to passive diapirism (Giles and Lawton 2002; Giles and Rowan 2012; Hearon et al. 2015). The CHS type is defined by the interplay between salt-rise and sediment-accumulation rates. However CHS geometries around the diapir within the same interval suggested that the ultimate control is the roof thickness (Hearon et al. 2014). Rowan et al. (2012), Poprawski et al. (2016) and Roca et al. (2020) described tapered composites of halokinetic sequences in the case of carbonates deep-water sedimentation, around the Bakio diapir in Spain. Tabular CHSs are linked to carbonate development over the roof of Bakio diapir, during high stand sea level, while tapered CHS are related to transgressive system tracts. These characteristics contrast with the established model, in which wedge HSs/tapered CHSs are constituted by a relative lack of debris (Giles and Rowan 2012). Roca et al. (2020) support and refine these results by invoking the role of water depth and the importance of the slope created by halokinesis in the building sequences. In our case, the Albian deep basin with predominant carbonate sediment shows that CHS-bounding unconformities correlate to periods of transgression (Hearon et al. 2014).

At Sidi Mbarka, South of J. Harraba, the middle-late Albian HCSs unconformity is in contact with a maximum flooding surface (Zone of Clansayes and Septarian level, Fig. 13C). This unconformity is at the base of tapered HCSs which corresponds to an HST. The tapered CHSs of the late Albian TST confirm that the sediment rate accumulation exceeded the rising diapir velocity (Giles and Rowan 2012) due to the Late Albian transgression becoming more pronounced. The rise in sea level affects the type of sequences that form on and near the diapir, the salt rise and the roof thickness (Roca et al. 2020; Rowan and Giles 2021). Sedimentary descriptions reminiscent of the HS sequences have been described around the diapir of Jebel Lorbeus located southeast of the study area during the Upper Cretaceous (Masrouhi et al. 2014). At the scale of the Maghreb chain, these sequences have also been described in the Jurassic of the Tazoult basin in Morocco, based on seismic and structural studies (Martin-Martin et al., 2017; Saura et al. 2014) (Fig. 14).

Fig. 14
figure 14

A) Paleogeography of Tunisia during the Latest Aptian (compiled from Mrabet 1984, Marie et al. 1984, Mass et al., 1984; Bouhlel 1993; Saadi and Duée 1991;Ben Youssef 1999; Zghal 1994 ;Ben Chaabane et al. 2019, Trabelsi et al. 2020). B, Paleogeography of Tunisia during the early Albian (from Marie et al. 1984)

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The HS around the Triassic salt bodies of the salt province of the Algerian-Tunisian border suggest an initiation of passive diapirism controlled by sediment aggradation rates (Giles and Rowan 2012; Hearon et al. 2014, 2015; Jackson and Hudec 2017; Roca et al. 2020; Rowan and Giles 2021) during the Albian. Passive diapirism (Jackson and Talbot 1994) is expressed to the salt crest remaining at or near the seafloor throughout its evolution.

Paleogeography

Paleogeographic maps of Tunisia (Fig. 15A and B) show a zone of uplift structures on the salt province underlining Triassic salt extrusion at late Aptian to early Albian. The Aptian-Albian boundary characterized by the development of mixed shelf deposits of the Hameima Fm, then by drowning of the domain by deeper sediments of the Fahdène formation. This geodynamic change resulted from the transtensive stage during Early Albian (Haq 2014) following the previously rifted margin (Jurassic and Lower Cretaceous). The margin triggered gravity-gliding over the Triassic with thrusts and folds in the northern (Tellian) domain and extensional faults in the central-south of Tunisia, as already proposed locally by Masrouhi et al. (2014), and more generally by Jaillard et al. (2017).

Fig. 15
figure 15

A) Location of the following sections in Tunisia. B) NW-SE Geological section developed from an interpreted seismic line in the NE of Tunisia (drawn from Rigo et al. 1996). A broad Triassic body of Lansarine linked to Cretaceous compartments affected by thrusts and duplex structures. C) Regional NW-SE cross-section established by correlation between drills at the NE part of Tunisia (offshore), (from Soua and Smaoui 2008). The Triassic form a large deep rooted body that was preserved from the Tertiary shortening. Relationship beetwen the series crossed by the surveys indicate a structure similar to the diapIr lateral model of Berastegui et al. 1998). D) Analog model of diapiric lateral emplacement explaining how diapirs can be integrated into orogenic wedges (in Spain) during successive compressive stages (after Berastegui et al. 1998) where the salt lobes form the core of the thrusts

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The proposed model takes into account gravity and structural studies and seismic profiles in other areas of northern Tunisia involving the Triassic bodies of equal importance. Published seismic lines (Rigo et al. 1996, Fig. 15A) and gravity models have been proposed and discussed by numerous authors (Vila et al. 1998; Perthuisot et al. 1999; Chikhaoui et al. 2002; Jallouli et al. 2005; Talbot 2005) to further define the morphology of the Triassic salt bodies in the studied area. The Triassic body of Lansarine, located 100 km NE of the ElKef area, is an allochthonous salt (salt canopies from Masrouhi and Koyi 2012). However, gravity analyzes of the Triassic evaporites of Debadib reveal large and rather deep-rooted Triassic bodies (Chikhaoui et al. 2002; Jallouli et al. 2005; Talbot 2005) like the structure of the Gulf of Tunis (Fig. 15C). In addition, this shows subsequent shortening events affecting the north of the Tunisian Atlas. Low-angle thrusts and duplexes moved southward using the Triassic evaporites as a detachment level. The Debadib Triassic evaporites of the Kef area forms a diapir lateral structure (Fig. 15B–C) as modelled in the Betic fold-and-thrust belt at the southern margin of Guadalquivir Basin in Spain (Berastegui et al. 1998). The salt wall is squeezed toward the external zones by the weight of internal zones that advanced during contraction (Fig. 15D). The model can be applied to the Triassic body in the El Kef-Tajerouine area as other Triassic salt outcrops in northern Tunisia. The final structure is shown by diapirs of Triassic evaporites with the flow of salt during the Albian-Cenomanian (Snoke et al. 1988; Talbot 2005). The results are similar to the analog model of salt tectonics (Koyi 1988; Jackson and Talbot 1994; Jackson and Vendeville 1995; Jackson and Hudec 2017).

We proposed an evolutionary model in 4 stages (Fig. 16); the first stage corresponds to the activity of Tethyan extensional rift faults. The second stage pillow developed from the Jurassic onward by mobilization of the Triassic salt due to sedimentary loading/or extensional tectonics (Chihi et al. 1984; Bédir et al. 2000; Talbot 2005; Hajji et al. 2013). The third period is reefs and oolitic banks on the top and the flanks of the domes (Serj Formation). The development of halokinetic sequences on the edges of the Triassic axes due to passive diapirism correspond to Albian.

Fig. 16
figure 16

Paleostructure proposed for Kef–Tajerouine area during the Albian. The morpho-structural study suggests that the Triassic salt is relatively deep-rooted in the Debadib-Ben Gasser structure. We revealed four stages of the halokinesis at the Kef–Tajerouine area from the Jurassic to the Albian

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Conclusion

The facies and thickness variations described in the stratigraphic sections of the Albian successions in the ElKef-Tajerouine area indicate relative lateral changes in accommodation due to passive-diapirism of structures into the Triassic salt bodies in this area. The main results of this study can be summarized as follows:

  • *This phenomenon occurred during the Early to Late Albian, especially before and during the T. breggiensis zone of early-late Albian age.

  • *The local appearance on the top and near the salt structures of high-energy facies instead of distal shelf deposits is remarkable. This illustrates the influence of vertical movements of diapirs on paleobathymetry and sedimentation. This has been already described by Masse and Thieuloy (1979), Masse and Chikhi-Ouimer (1982), Mrabet (1984), Camoin et al. (1990) and by Jaillard et al. (2013) in neighbouring areas. The morphostructural analysis of the KNA section allows us to identify a drape-fold halokinetic sequence (hook sequence) located above the Triassic salt body of Debadib-BenGasser.

  • *The thinning of this series towards the Oued El Kohol section (North flank of the Debadib) is related to the rise of salt diapir during the Albian.

  • *The late Albian gravity-driven deposits of J. Slata are similar (facies and halokinetic sequences) to the one described around the Bakio diapir (Poprawski et al., 2016). Both were formed in deep carbonate sediment over a regional angular unconformity. This deposit is no longer uniquely reserved for the hook-like sequence as evidenced around Bakio's diapir in Spain (Popwarski et al., 2016, Roca et al. 2020).

  • *The HS sequences highlighted around the Triassic salt bodies of the salt province, of the Algerian-Tunisian border reveal a stage of passive diapirism during the Albian. The Triassic salt bodies of the NW salt province formed stocks and walls, which survived to a regional thrust tectonic stage during late Cretaceous-Tertiary N-S shortening. These structures in the NW Tunisia account for the interest of petroleum explorations in this region.