Introduction

Located between the Saharan platform and the Atlasic domain, and separated by the Jerid trough, a thick upper Cretaceous sedimentary succession developed before the emergence of the Saharan platform at the end of Maastrichtian. The upper Cretaceous strata in this area were strongly affected by two important tectonic phases and are expressed by two major discontinuities: Apto-albian and Turonian.

Indeed, the lower Cretaceous deposits of South Tunisia have been the major topic of several works (Busson 1967, 1987; Barale et al. 1998; Barale and Ouaja 2001, 2002; Ouaja et al. 2004; Fanti et al. 2012). However, the upper Cretaceous sediments in this region have been less studied (Boltenhagen 1981; Ghanmi et al. 1993; Abbes et al. 1994; Zouari et al. 1999 Khila et al. 2016a, b, 2017).

The present paper is the first contribution of a detailed sedimentological and sequential analysis of the Coniacian conglomerate recorded in the Chotts region (southern Tunisia).

The aims of this work are as follows: (1) identification of the depositional processes and environments of the carbonate deposits, (2) documentation of the geometry of the conglomerate bodies, and (3) evaluation of the likelihood of tectonic or eustatic controls on the emplacement of these conglomerates.

Material and methods

The conglomerate study is based on detailed several measured-stratigraphic sections at Zemlet el Beidha area (34° 04′ 25″ N and 9° 52′ 17″ E). This study is based on stratigraphic and sedimentological analysis, in addition to sampling in the most representative sections. The mapping carte of the region is represented in Fig. 2. Besides, E-W correlation of different units was based on litho-chronostratigraphic information. Finally, facies and facies associations are described; then, for illustration, field pictures of lithofacies are presented.

Background-study area

The study area is located to the southeast of Tunisia in the Zemlet el Beida area which is bordered to the west by the northern Chotts ranges, to the east by the Mediterranean Sea, and to the south by the Saharan platform (Fig. 1). Sedimentary strata in this area are mainly of Cretaceous to Miocene age.

Fig. 1
figure 1

Study area location, 1: Aptian, 2: Albian, 3: Cenomanian–Turonian, 4: Coniacian–Campanian, yellow: Pleistocene deposits

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The Chotts chains form an anticlinal megastructure, where the lower Cretaceous strata occupy a central axial position. This megastructure represents the transition between the central Tunisian Atlas to the North and the Saharan platform to the South. The folds are comparable with those of the central Tunisian Atlas (Zargouni 1985; Ben Ayed 1993). They trend to the WNW–ESE except in the proximity of the great Gafsa fault where they are directed WNW–ESE, parallel to the Gafsa fault.

During the late Cretaceous, accommodation might be controlled by fault systems. The main faulting directions are N 100–110 E (Bouaziz et al. 1996, 2002).

The Gattar member

The Gattar member corresponds to the upper unit of the Zebbag Formation (upper Albian-Lower Turonian) (Fournié 1978). It is about a large massive brown dolomite bar which caps the Cretaceous cliff. The type locality is the jebel Ben Younes (Gafsa area). It has a large extension and variable thickness ranging from 20 to 50 m.

The Haidoudi formation

It is about a thick argillaceous carbonate succession is preserved: the Haidoudi Formation. It has been subdivided into a lower unit of conglomeratic sandstone, a middle unit of dolomitic limestone, and an upper unit of argillaceous coquina limestone. The type locality is Haidoudi Mountain in El Hamma area (Abbes et al. 1994). This Formation has a Coniacian-Lower Campanian age. This Formation is transgressive above various series ranging from Barremian (Sidi Aïch Formation), Aptian (Orbata Formation), and Lower Turonian (Gattar member).It suggests that these conglomerates are uncomformably cutting reworking the strata below. This regional unconformity is upper Aptian–Turonian in age. Thus, it is related to the reactivation of NW trending faults that have delimited the Zemlet el Beida horst (Gharbi et al. 2013).

In terms of paleogeographical setting, in contrast to the Lower to Mid-Cretaceous evaporates to siliciclastics, the upper Cretaceous interval is characterized by an extensive carbonate platform. However, during the upper Cretaceous, Zemlet el Beida area was comprised by several small basins separated by structural highs, and accommodation in these basins was controlled by active tectonics (Louhaïchi and Tlig 1993; Chaabani and Razgallah 2006; Bodin et al. 2010; Gharbi et al. 2013).

Facies and depositional environment

Four facies associations have been distinguished (AF1–AF4) from base to top of Haidoudi Formation. The facies associations are as follows.

Facies association AF1: conglomerates

Conglomerate bed is overall tabular and laterally continuous and has thickness ranging from 25 cm to 5 m (Fig. 6). Generally, these conglomerates are coarser grained to the northeast. However, there is no systematic regional variation in either roundness or sphericity. According to their composition, three facies are identified.

Facies type F1a, structureless polymictic conglomerate

Description

This clast-supported, muddy-sand matrix conglomerate consists mainly of angular to well-rounded, pebble- to cobble-sized clasts and has a structureless internal structure. Carbonate clasts range in long axis length from a few centimeters to 50 cm; they have a grainstone texture. Clasts are clast-supported in a poorly sorted sandy mud matrix.

Interpretation

The poorly sorted matrix, angular to subrounded shape and the thickness-maximum clast size relationship, indicates a debris flow origin generated through the reworking of calcareous deposits outcropping in the area. This type of conglomerate, because of its coarseness, seems to be the most proximal of all alluvial fan deposits.

F1b, subaqueous debris flow

Description

This conglomerate type has a variable grain size from pebble to very coarse sandstone with only rare cobbles. Conglomerates are poorly sorted with clast-supported or matrix-supported textures. All blocks consist of dolosparite containing dissolved allochems. Conglomerate beds are commonly inverse-to-normal graded.

Interpretation

This conglomerate has debris flows origin because of (1) the well-bedded character of the conglomeratic succession; (2) the inverse-to-normal graded character of beds, which also strongly suggests a subaqueous character; and (3) the general lack of lamination within beds.

F1c, coarse pebbly sandstone

Description

These medium to course-grained pebbly sandstone beds are poorly to well-sorted, with a domination of the sand matrix. Sand grains are generally spherical in shape. Grading is locally well developed in individual beds (partly normal graded and some inversely graded).

Interpretation

This pebbly sandstone facies is interpreted as turbidites on submarine depositional lobes, generally found in the steep-faced slope of modern fan deltas.

F1d, conglomerates with muddy and sandy pebbles

Description

These conglomerates are moderately to well-sorted, dominated by fine to coarse-grained sand. Generally, they are normal to inversely graded with pebble-sized clasts in a sandy matrix. The pebbles are widely dispersed and have a lenticular shape.

Interpretation

This conglomerate is interpreted as a turbidite bed with isolated floating clasts in sand matrix, emplaced in the steep-faced slopes that pass laterally to submarine lobes.

Facies association AF2: bioconstruction

This facies association is represented by bioturbated dolomite and dolomite with rudists, corals, and algae which contains up to half-meter thick. These are constructive bodies formed by colonial organizations (rudists, corals, and algae; Fig. 6). On the top of the conglomerate bodies, these patch reefs are lenticular in shape. The presence of rudists indicates an agitated, shallow, and oxygenated environment.

Facies association AF3: dolomitic limestone rich with Inoceramus

This facies association contains two facies (Fig. 6):

  1. 1.

    F3a: Corresponds to limestone beds with Inoceramus. These wackstone to packstone limestones are mainly present in the Chotts and the Saharan platform uncompressing the Haidoudi Formation. Units are formed by a succession of half-meter to meter-thick beds. Inoceramus has variable size ranging from few centimeters to one meter.

  2. 2.

    F3b: It is about skolithos facies corresponds to bioturbated limestone with Inoceramus laid out in metric benches. This facies is well observed on the top of the carbonatic Haidoudi Formation with a packstone to grainstone texture.

    The packstone to grainstone texture with the presence of Inoceramus is an indication of a warm and shallowing environment with high energy given the extremely thin and large, entirely preserved shells.

Facies association AF4: fossiliferous marl

This association has a variable thickness ranging from 40 to 100 m (Fig. 6). It is primarily made up by fossiliferous marl (F4a) intercalated with gremulus bioturbated carbonate (F4b) and coquina beds (F4c).

  1. 1.

    F4a corresponds to a fossiliferous marl alternating with decimetric limestone beds. This marl has a rich fauna comprised of Echinoderms, gastropods, and mollusks with benthic foraminifera (name) and Ostracods.

  2. 2.

    F4b is a fossiliferous bioturbated marly limestone, in beds up to 10 m thick, rich in Echinodermata, gastropods, and bryozoan debris.

  3. 3.

    F4c consists of lumachellic marl made of Ostrea shells.This facies is present in thick benches within the fossiliferous marl of the argillaceous Haidoudi member.

This marl settled out not within an open marine environment but rather in a lagoon environment indicated by the presence of benthic foraminifera associated with Echinodermata, gastropods, and mollusks. This lagoon was dominantly low energy but was affected sporadically by wave action.

Geometry of the conglomerate body

The mapping study of the Zemlet el Beida area shows the occurrence of six units (Fig. 2) which are the Bouhedma Formation, the Sidi Aïch Formation, and the Haidoudi Formation which is subdivided into the Conglomerate body, the building up level, the Coniacian bar, and the Haidoudi successions.

Fig. 2
figure 2

The mapping of the Zemlet el Beida area; 1: Bouhedma Formation, 2: the brown bar at the top of the Bouhedma Formation, 3: Sidi Aïch Formation, 4: Orbata Formation, 5: conglomerates, 6: fossiliferous level, 7: Coniacian bar, 8: Aleg Formation, 9: Miocene sandstone

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The Sidi Aïch Formation maintains the same thickness toward the study area and is overlain in places by the Orbata Formation or by conglomeratic bodies. The Orbata Formation has a uniform thickness but it is absent in the middle part of the study area where the conglomerate is directly deposited on Sidi Aïch sand. The Haidoudi Formation is unconformable above the rest (Fig. 3). The conglomerate (AF1), about 3 m in thickness, lenticular in shape, pinches out laterally toward the east. The rudist level (AF2) with coral and algae observed generally on the top of these conglomerates; pass into dolomitic conglomerate containing variously sized pebble (Fig. 6). Laterally, toward the south, the thickness of the Haidoudi Formation has been progressively reduced. On the top, the argillaceous carbonate alternance of the Haidoudi Formation is overlain unconformably by coarse sand attributed to the Miocene.

Fig. 3
figure 3

E-W correlation of the upper Cretaceous series of the Zemlet el Beida anticlinal

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Consequently, the conglomerates in the Zemlet el Beida area range from zero to approximately 5 m in thickness and extend about 3 km in lateral extent (Fig. 3). The deposits show no systematic variation in thickness, but there is an uneven basal surface and a flat top, as well as a general tendency to disappear toward the east. The thickness is variable, and some changes are observed over relatively short distances (50 to 100 m), indicating moderate relief of the conglomerate depositional surface. An erosional escarpment limits the conglomerate outcrop on the northeast. This conglomerate can be observed also laterally toward the southwest, maintaining more or less the same thickness. Further north, it is observed in Central Tunisia in jebel Kbar where thickness reaches 20 m.

Sedimentologic and stratigraphic interpretation

The thick conglomerates with poorly sorted pebbles and rare blocks strongly suggest a sedimentary response to local tectonism. The study conglomerates are likely to be a syntectonic (Fig. 4) sedimentation response to normal faulting during an extensional period in the region (Chaabani and Razgallah 2006; Gharbi et al. 2013). This extensional tectonic period is well known in the south of Tunisia during the upper Turonian–Coniacian (Bouaziz et al. 2002; Chaabani and Razgallah 2006; Bodin et al. 2010; Gharbi et al. 2013). Indeed, the region is characterized by a tilted block system of horsts and grabens. In fact, this structural frame is responsible for the distribution of the upper Cretaceous sediments in the Zemlet el Beida area (Bouaziz et al. 1996; Chaabani and Razgallah 2006).

Fig. 4
figure 4

Cross section of the upper Cretaceous series of the Zemlet el Beida anticlinal

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The northern Chotts range was dominated by WNW and NW normal faults which are mainly inherited from the Jurassic and reactivated during the Early Cretaceous–Albian times (Gharbi et al. 2013).

The upper Cretaceous sequence is marked at its base by a major unconformity SB which generated the incision of many valleys (Figs. 5 and 6). This sequence begins with conglomerates that fill incised valleys and constitute the first transgressive deposits. At the top, the sedimentary facies are mainly made up of bioclastic limestone with inoceramids. The maximum flooding surface is situated at the base of a limestone level with mollusks coquina. These limestones are overlain by the top of the Haidoudi formation represented by bioclastic limestone, which materialize the regressive interval.

Fig. 5
figure 5

Depositional and sequence stratigraphic interpretation of the upper Cretaceous series of the Zemlet el Beida anticlinal

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Fig. 6
figure 6

a The unconformity between the Sidi Aïch Formation and the Coniacian conglomerates. b The structureless conglomerates. c, d Dolomite with rudists. e The irregular limit between the Haidoudi and Orbata Formation. f Limestone beds with Inoceramus. g Coquina beds. h Gremulus bioturbated carbonate

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On the other side of the basin, the sequence is represented only by the transgressive and regressive intervals without the incised valley fills. This means that the conglomerates were not deposited there and this area is an uplifted zone.

Discussion

The Zemlet el Beida area has attracted the attention of many geologists for over a century, and many fundamental aspects of the structure of the region have been studied. However, the Coniacian conglomerates have been neglected and their nature and geometry have never been described or discussed.

Many interpretations have been proposed to explain the upper Aptian–Lower Albian regional unconformity. Several authors refer this unconformity to the Austrian movements related to a western progradation of the rift that developed eastward along the whole northern African margin (Ben Ayed 1993; Bouaziz et al. 2002; Zouaghi et al. 2005; Bodin et al. 2010 and Gharbi et al. 2013). Other authors relate this unconformity to an extensional event (normal faulting regime) (Guiraud 1998; Zouari et al. 1999; Guiraud et al. 2005; Bodin et al. 2010; Rigane et al. 2010). Bodin et al. (2010) suggest that this unconformity is the result of a northeastward movement of the Arabian–Nubian block during the Aptian–Albian transition. The conglomerate thickness variations and the main tectonic elements support the interpretation that the Aptian sedimentation was controlled by north-south, NW-SW, and east-west faults (Zargouni 1985). Chaabani and Razgallah (2006) proposed that the sedimentary break corresponding to a rapid sea-level fall followed and the hardground presents a marked stratigraphic gap extending from the Aptian to the Middle Albian. A recent study claimed that the N100–110_E trending normal fault was associated with upper Aptian syn-depositional conglomerates (Gharbi et al. 2013).

Conglomerate sediment within the paleovalley would seem to be in part autochthonous and this is according to the clast analysis which corresponds to the erosion of the carbonate series of the Zemlet el Beida area especially Orbata and Zebbag Formations. The second origin is allochthonous or parautochthonous and this is because the varying degrees of particle abrasion and rounding.

The general sequence started with an erosional sequence boundary that could be corresponded to a major unconformity related to the Austrian event (Bouaziz et al. 2002; Chaabani and Razgallah 2006; Bodin et al. 2010; Gharbi et al. 2013). This boundary is qualified as a limit of a second-order sequence. The sequence started with conglomeratic sediments which correspond to valley fill sediments when the sea level rises. The transgressive interval corresponds to the fossiliferous limestone with Inoceramus and the regressive interval is formed by lumachellic limestone.

Conclusion

Various measured section along the Zemlet el Beida area in southern Tunisia provide lithostratigraphic and sedimentological details of upper Cretaceous (Coniacian–Maastrichtian) stratigraphy in this region of North Africa. This study reveals eight facies within four facies associations, corresponding to four different depositional environments ranging from alluvial plain to the open marine environment. These series could be subdivided into a standard transgressive/regressive sequence which started by incised valley fills representing by the conglomerate deposits. This sequence is interpreted as a shallowing upward successions on a carbonate platform. The organization of these series is related to the reactivation of the sedimentary floor accidents indicated by the spectacular variation of facies and thickness.