Introduction

This present study is the result of an investigation undertaken over a 1-year period, which focused mainly on the characterization of the paleoenvironment of mid-Cretaceous deposits, located in the area of Tébessa, in north-eastern Algeria.

The selected site is located in the wilaya (province) of Tébessa, about 20 km from the main town of the wilaya and a few kilometers away from the town of Morsott (Fig. 1), with the Tunisian border situated a few kilometers to the east. The 2.5 km sampled section, of SW-NE orientation, lies between two mountains, namely the Djebel Hameimat North which is situated at the base (altitude of 900 m), and Djebel Chemla, which is situated at the top and considered to be a more impressive mountain culminating at an altitude of 1200 m (Fig. 2). These mountains dominate the Morsott Basin that constitutes the geographical continuity of the southern Tébessa Basin.

Fig. 1
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Geographical location of the site

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Fig. 2
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Detail of the Chemla cross-section

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From the study of the fossiliferous contents of these deposits (Foraminifera and Ostracods), our aim is to characterize the paleoenvironment of this area during the mid-Cretaceous.

Geological setting

The series studied are located in the oriental part of the Saharan Atlas of Algeria that extends into Tunisia as the Tunisian Atlas. The area of Tébessa presents as outcrop only sedimentary sequences which range in age from the early Cretaceous (Albian) to Miocene, the whole largely covered with quaternary deposits. Triassic diapirs often pierce the more recent series, and frequently cause an upheaval of the wall-rocks, with reductions in thickness and even an inversion of the dip of the concerned layers. In the geological map of Morsott (scale 1/50,000, sheet 178), there are no less than three diapirs listed, one being located at the base of our section, in the Djebel Hameimat North area (Fig. 3). During Cenomanian to Turonian, the Tébessa area would have been in a deep domain, at the limit between slope and basin (Herkat 2003; Herkat and Kechid 2006). Cenomanian is described as largely transgressive, which allowed the deposit of important quantities of sediment (Burollet 1956).

Fig. 3
figure 3

Geological map of the Morsott area with location of the studied section

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Materials and methods

The deposits studied, approximately 700 m thick, are composed mainly of marls, in particular in the lower part of the section, and marly limestones or limestones, especially in the upper part.

Sampling was carried out on average every 5 m; a denser sampling was not possible considering the high density of the deposits. Furthermore, in the lower part of the section, the absence of outcrops sometimes prevented an appropriate sampling, as oppose to the top of the section where more samples were collected because of a faster variation of the facies.

The oysters levels were also sampled, allowing us to discover a vast number of fossils.

Two different methods were used, according to the type of deposits concerned:

  • The soft deposits (marls and silty marls) were initially soaked in a mixture of water and hydrogen peroxide for several hours, then washed through two sieves of 0.063 and 2 mm mesh. The collected microfossils were then sorted. For each sample, at least 200 specimens were selected, in order to carry out statistical studies.

  • The hardened deposits (marly limestones and limestones) were cut in order to produce thin sections. The study of the thin sections was not aimed at counting the microfossils.

In addition, two other parameters were measured, namely the amount of calcium carbonate and the amount of total organic carbon (TOC). This latter parameter could be measured only on a small number of samples, selected on the one hand in the levels of transition between Vraconnian and Cenomanian, at the base (three samples), and on the other hand in the transition levels between Cenomanian and Turonian, towards the summit (seven samples). Measurements of TOC were made using an apparatus of type LECO IR212 at the Tectonics laboratory of Paris VI University.

The counting carried out made it possible to determine the various populations of microfossils, foraminifera (F), benthic (B) or planktic (P), and ostracods (O). The ratios of abundance between these populations are usually employed in paleoenvironmental analysis (Groshény and Malartre 1997). We were therefore able to determine the relationship between planktic foraminifera and the total foraminifera (P/P + B), or either the relationship between the ostracods and the total of the studied microfossils (F/F + O). This latter ratio, in particular, is referred to by Rey (1983) and Andreu (1991).

Results

Lithostratigraphy and study of the microfauna

The following description is subdivided into three parts, and references to the three formations described by Burollet, in 1956, in central Tunisia. These three formations are (a) the Fahdene formation, corresponding to the Albian and Cenomanian, (b) the Bahloul formation, of Late Cenomanian to Early Turonian age, and finally (c) the Annaba member, lower term of the Aleg formation (which extends from Turonian to Campanian). The first two formations were initially described on the level of the Wadi Bahloul, an area that has since been extensively studied as a type-locality for the study of the Cenomanian/Turonian limit on the southern margin of the Tethys (Accarie et al. 2000; Burollet and Robaszynski 1991; Robaszynski et al. 1990, 1993a, 1993b; Caron et al. 1999, 2006, Zagrarni et al. 2008). Our study area is located only a little over 10 km to the west.

  • Fahdene formation

Lithology

The first levels encountered was characterized by finely laminated black limestone levels, which alternate with black marls, together with an approximate thickness estimated at 30 m. These levels correspond a priori to the Vraconnian stage, generally not very thick in the area. The limestone levels frequently contain prints of ammonites, often pyritic, but the identification of which is difficult to determine.

Following these levels, there is a thick succession of dark marly levels (grey or brown), often rich in gypsum, pyrite and ferruginous nodules (of microscopic size). The intercalated limestones beds are rare (generally consisting of marly limestone levels). Only two samples contained glauconite (15 and 16). It is also important to note that towards the base, these levels are frequently covered with quaternary deposits, which sometimes made sampling impossible. The description made by Burollet (1956) seems to perfectly correspond to our own observations: the Fahdene formation can indeed be described as a “very powerful series of grey or black clays and marls, which presents a certain number of beds of limestone or marly limestone”.

It was observed that some horizons are fossil-rich, with frequent bivalves (generally Ostreidae), Echinoderms (the omnipresent species is Hemiaster batnensis (COQUAND)) and rarer ammonites. The only ammonites discovered appear as prints, overall rather badly preserved.

Geochemistry

Except for the first limestone levels, the carbonate content is rather low (Fig. 4).

Fig. 4
figure 4

Main results: 1 proportion of CaCO3 (%), 2 proportion of benthic foraminifera B (%), 3 proportion of planktic foraminifera P and heterohelicidae H (%), 4 proportion of ostracods O (%), 5 ratio benthic/(benthic + planktic), 6 ratio foraminifera/(foraminifera + ostracods)

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The TOC measured on three samples, is also relatively low, with the measured values that oscillate around 0.4%.

Study of the microfauna

The proportion between benthic and planktic foraminifera is variable, but overall the planktic forms dominate, comprising almost only foraminifera with a globulous morphology (Favusella washitensis, Globigerinelloides sp., Heterohelix sp., very small Hedbergella sp.). Hedbergella delrioensis, Hedbergella planispira, and rarer Hedbergella simplex have been noticed.

Together with benthic, the calcareous and agglutinated foraminifera are collectively present, but their relative abundance varies according to the samples. The most frequent agglutinated forms are Ammomarginula sp., Trochamminoides sp., Textularia sp., while the calcareous forms are greatly diversified, they are however always small forms of the type Gavelinella sp., Gyroidinoides sp., Lagena sp., Nodosaria sp., Lenticulina sp., etc. Miliolidae are generally very rare, except for one sample (N°36) in which these organisms represent almost completely the forms observed (the main species is Spiroloculina sp.), in partnership with many ostracods. The ostracods were also rare in the other samples (Fig. 4). They form very slightly diversified assemblages, comprising of only some species with smooth carapace, and in particular the genus Cytherella sp. and Paracypris sp.

Specific diversity is variable, rather low in the first levels (only two to three different species observed, of planktic type), it greatly increases in the middle part of the interval (with more than ten different species), then decreases again in the final levels that contain only slightly diversified benthic forms).

  • Bahloul Formation

Lithology

Advancing in the section, the limestone layers are increasingly numerous, as are the oyster levels. The latter contain numerous bivalves, always mainly Ostreidae, as well as gastropods, with rare Echinoderms (Hemiaster sp.). The Cephalopods, however, are poorly represented: ammonites (always rare and in a poor state of conservation), nautiloidae (Nautilus sp.), some belemnite rostra. These levels form a transition between the typical Fahdene formation, seen below, and the true Bahloul formation. Therefore, we will thus regard this as the “Pre-Bahloul” zone, in line with the distinction already suggested by Accarie et al. (1996), always on sections located in central Tunisia, not far from our study area.

In the true Bahloul formation, the limestone beds clearly dominate, and the marls are much rarer. The oyster levels are absent, which explains why no fossils were discovered in this zone. The transition between Cenomanian and Turonian is marked by the presence of finely laminated black limestone, called “in plates”, corresponding completely to the description made by Burollet in 1956 on the wadi Bahloul levels. These levels are at most only about ten meters in thickness.

As in the Fahdene formation, the gypsum, the pyrite and the ferruginous microscopic nodules are frequent, while glauconite is always rare (sample 103).

Geochemistry

The carbonate content is overall higher than previously, although very variable. The majority of the limestone deposits are actually composed of marly limestone, with a carbonate content not exceeding 80%.

The measured TOC is relatively low before the Bahloul levels (<0.4%), then clearly increases (from 0.8% to 2% for samples 100 to 106) and decreases strongly thereafter (0.1% only for sample 108). Such values are in accordance with those obtained in the area for some comparable levels (Naili et al. 1995), but are notable lower than the values measured in central Tunisia by Caron et al. (2006): certain levels indeed have a TOC reaching 8%.

Study of the microfauna

  • Pre-Bahloul zone: as in the subjacent levels, the benthic/planktic ratio is highly variable, but, overall, planktic foraminifera dominate the whole of the microfauna (44.5%), practically in equality with the benthic forms (43%), while the ostracods remain always rather rare (12.5%) (Fig. 4). The foraminifera/ostracods ratio is almost constant, near to 1.

Among planktic foraminifera, the globulous forms are still definitely dominant (99%), and the species encountered are the same ones as previously. Heterohelicidae are appreciably more abundant, and account for on average 32% of the planktic forms (against approximately 17% for the Fahdene formation). Specific diversity is overall lower than previously.

Benthic foraminifera are mainly comprised of agglutinated forms (almost 75% of the benthic total), primarily represented by Lituolidae (Trochamminoides sp., Ammobaculites sp., Ammomarginulina sp.) and Textularidae (Textularia sp.). The species Thomasinella punica (Schlumberger), very characteristic because of its large size, was only found in two samples (52, 54).

The ostracods comprise, in addition to the forms already observed in the Fahdene levels (especially Cytherella sp. and Paracypris sp.), some very different forms, with ornamented carapace, which are sometimes rather abundant. These new forms can represent up to 44% of all of the ostracods for certain samples, with an average of 6%. The most common ornamented form is Amphicytherura (Sondagella) distincta (Gerry and Rosenfeld), already observed in the Tébessa area by Vivière (1985) and also described in Morocco by Andreu (1991). The species Phlyctocythere citreum (Viviere), described for the first time by Vivière 1985 at Tébessa, was also observed (sample 56).

  • Bahloul formation (sensu stricto): In these levels, we note a clear evolution of the microfauna. Planktic foraminifera are definitely dominant here (74% of the total) and the benthic microfauna are poorly represented.

Planktic foraminifera are only represented by globulous forms, the keeled forms are completely absent (whereas rare species were until now observed frequently). Species of the genus Hedbergella, Whiteinella, and Heterohelix are common. Heterohelicidae are very numerous, and account for on average 67% of all of the planktic microfauna, and up to 92% for certain samples (101, 108). Samples for which thin sections were produced were not included in the numerical data. These levels are also characterized by a very large size microfauna, strongly contrasting with our observations concerning the subjacent levels. The microfossils are sometimes very abundant (levels 101 to 106 observed in thin sections).

On the contrary, benthic foraminifera are much rarer, and poorly diversified. Both calcareous and agglutinated forms are observed, sometimes in partnership with ostracods. These are often dominant within the benthic microfauna where they account for on average 16% of the microfauna, against only 9% for benthic foraminifera.

Finally, some filaments are observed in certain thin sections (103, 104, 106), but their number remains limited. These forms correspond to juvenile bivalves, whose two valves are in an open position, giving a linear appearance. Such filaments were already detected in the area, in particular by Nederbragt and Fiorentino on the Tunisian side of the Wadi Mellègue (1999), and in the Kalaat Es Senam area (Accarie et al. 2000, Caron et al. 2006). In addition, in eastern Algeria, their presence is already attested by Naili et al. (1995).

  • Annaba member

Lithology

The levels of the Annaba member, whose appearance strongly contrasts with the Bahloul levels, are composed primarily of thick limestone beds of clear color, intercalated with very few marl levels. These levels are easily identifiable in the landscape, because they give rise to an important relief.

Very few fossils were discovered, comprising some regular Echinoderms (Heterodiadema libycum (Desor)) and undetermined gastropods.

Geochemistry

The carbonate rate is generally high (nearly 100% for the limestones beds). The TOC was not measured. It is 0.1% for sample 108, located completely at the base of this formation.

Study of the microfauna

The marly levels are almost completely absent; consequently no counting could be carried out in this zone. The study of the thin sections however gave certain useful indications. The microfacies are completely different from those of the Bahloul formation. Texture here is always micritic, but the very clear color seems to indicate the complete absence of organic matter. The microfauna observed is very rare, and some thin sections did not contain any microfossil. The only forms observed are rare and consist of planktic foraminifera (of globulous type) and ostracods.

  • Main characteristics of the microfauna

The main results concerning the Fahdene formation (a), the pre-Bahloul zone (b) and the Bahloul formation s.s (c) are summarized in Fig. 5. This figure presents the principal data obtained from the sorting of the microfauna:

  • Slight predominance of planktic foraminifera (48.7%), before benthic foraminifera (38.4%) and ostracods (12.9%). The planktic/benthic ratio has a mean value of 0.54 and the foraminifera/ostracod ratio of 0.87.

  • Clear predominance of the globulous forms among planktic foraminifera (99%),

  • Predominance of the agglutinated (60%) among benthic foraminifera.

  • Predominance of the ostracods with smooths carapace (96%) among the ostracods.

Fig. 5
figure 5

Counting summary: main results about microfauna (%)

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Biostratigraphy

The biostratigraphic framework of the section is mainly based on planktic foraminifera. The biozonation of Robaszynski and Caron (1979, 1995), Caron (1985) has been applied. In our case, the ammonites could not be used to give more precise details, because they were very rare, and generally in a poor state of preservation. The Bahloul levels in particular are completely sterile from this point of view. This scarcity of the ammonites contrasts with the abundance in the levels of the Wadi Bahloul section, where a rich ammonite fauna was recently described (Caron et al. 2006).

This present study, which is therefore based on planktic foraminifera, has made it possible to distinguish four biozones between the final Albian (corresponding to Vraconnian) and the beginning the early Turonian (Fig. 6). The limit between Vraconnian and Cenomanian could not be clearly recognized, but lithostratigraphic contrast is clearly apparent, with on one side the laminated limestone levels of a darkish color, and on other side the dark marls, almost completely decarbonated, typical of the Fahdene formation, and which mark the beginning of the Cenomanian stage. Vraconnian is regarded here as the upper part of the Albian (in agreement with Robaszynski et al. 2007). This stage has generally a much reduced thickness in the area, and is sometimes completely absent.

Fig. 6
figure 6

Biostratigraphic framework. We have only listed the useful species for the biostratigraphic study. The number of the samples here does not correspond to the whole part of the samples studied

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Before specifying the definition of the biozones, it was advisable to reconsider the scarcity of planktic keeled foraminifera, whose utility in terms of biostratigraphy is essential for this time period. Indeed, 99% of planktic foraminifera are of globulous type, and the majority of the sorted samples do not contain any keeled foraminifera. This is why in Fig. 6, we choose to identify only the samples containing one or more identifiable keeled specimens. This small number of samples does not reflect all of the studied samples.

The four biozones identified are as follows:

  • Rotalipora appenninica (pro-parte) biozone is included in late Vraconnian.

  • Rotalipora globotruncanoides biozone (this species is regarded as equivalent to Rotalipora brotzeni, according to Robaszynski and Caron 1995): it comprises species Hebergella sp., Globigerinelloides sp., Heterohelix moremani, F. washitensis, R. appenninica, and R. brotzeni.

  • R. cushmani biozone: the index species was not identified with certainty, but the entry in this zone is assumed to be, due to the absence of the preceding species (F. washitensis, R. appenninica, R. brotzeni). The species Hedbergella sp., Globigerinelloides sp. and Heterohelix sp. remain very common. The last specimen of Rotalipora genus is observed at level 86.

  • Whiteinella archaeocretacea biozone (pro-parte): it is marked by the absence of Rotalipora and the appearance of Whiteinella (W. baltica, W. paradubia, and W. archaeocretacea). The genus Hedbergella and Heterohelix are always very abundant, with the appearance of the species Heterohelix globulosa.

Discussion

Paleoenvironment

The paleoenvironment's determination is possible thanks to the study of many indices, such as micropaleontology, geochemistry or mineralogy. The counting carried out in the marly levels can thus give valuable indications for the determination of the paleo-depths.

Indeed, the planktic/benthic ratio increases with depth, just as the foraminifera/ostracod ratio (Rey 1983). It is also very interesting to study the relationship between the various populations of microfossils (calcareous and agglutinated for benthic foraminifera, globulous and keeled for the planktic ones). Various authors tried to characterize the depositional environment from these indices: for example, for Vivière (1985) and Masse (1988), the planktic/benthic ratio would be around 30% to 40% on the external platform, and would reach 50% to 70% in a deeper environment (slope).

Furthermore, the environment of the planktic foraminifera would have been very different according to their morphology. The globulous forms (Hedbergella, Heterohelix, Whiteinella) prefer surface waters, with a maximum depth estimated between 100 and 200 m, while the keeled forms, much more complex (with the example of Rotalipora), need deep water (Caron and Homewood 1982; Hart and Bailey 1979; Hart 1980; Groshény and Malartre 1997; Groshény et al. 2006). Consequently, an increase in the proportion of keeled forms will generally be interpreted as an index of the deepening of the environment.

Foraminifera, just like other organisms, are adapted to their environment, and evolve when this changes, which is the case particularly during crisis periods (Caron 1983). These periods are generally characterized by rarefaction, even disappearance, of the most advanced forms of life, necessarily more demanding (in our case keeled forms) whereas at the same time the simplest morphotypes are favored and multiply in great number. It is possible to make the distinction between periods called “r-strategy” period, characterized by a predominance of the least demanding forms of life, and other periods called “K-strategy” (MacArthur and Wilson 1967).

The results obtained here concerning the study of the Chemla section argue in favor of a calm and relatively deep depositional environment, but not enough to have allowed the development of keeled planktic forms in a great number. The maximum supposed depth would be approximately of 200 m, which corresponds to the depth of the external platform or the upper part of the slope. This assumption is confirmed by the mean value of the planktic/benthic ratio (54%), the absence of coarse detrital elements in all of the levels and the global scarcity in ostracods (which represent only 13% of the microfossils). A very similar interpretation has already been made by Benkherouf (1988) in the Djebel Dyr area (see Fig. 3 for the geographical localization).

In addition, in the Aurès area, located to the west, the paleogeographic model of a monoclinal ramp is generally preferred with that of a carbonated platform (Herkat 2005; Herkat and Kechid 2006). This model is almost most probably valid for the area studied here.

This paleoenvironment such as it is defined seems moreover characterized by a poor oxygenation of the water, even with a tendency to anoxia, a tendency that is found throughout the Cenomanian and up to the beginning of the Turonian. The elements that indicate this anoxic tendency are several. First of all, some minerals need an anoxic environment for their formation, as is the particular case of pyrite (Baudin et al. 2008), and also of the microscopic ferruginous nodules observed in many levels. It is also necessary to consider the gypsum abundance, disseminated, on the entire section. This element could probably be regarded as a secondary element, resulting from the transformation of pyrite; indeed this phenomenon being usually observed currently in surface deposits (Cormier 2000), where it results in many problems in the field of construction. Furthermore, it is necessary to insist on the fact that the gypsum, although abundant in the studied series, never forms layers, but is on the contrary closely mixed within the levels. Some poorly oxygenated water also results in the presence of specific organisms, such as benthic agglutinated foraminifera, which prevail among the benthic ones, and that are known to be resistant to anoxic conditions (Schlanger et al. 1987).

The Cenomanian/Turonian transition

The interest in this limit is due to the fact that it is characterized by the occurrence of a major biological crisis, caused by an anoxia of the bottom waters. This crisis, named Cenomanian/Turonian Boundary Event or Oceanic Anoxic Crisis 2 was studied by many authors, and initially by Schlanger and Jenkyns 1976. Since, it was recognized worldwide, as well as in Asia (Wan et al. 2003), as in Europe (Hart et al. 2005; Groshény and Malartre 1997; Tsikos et al. 2004; Keller et al. 2001, etc.) and in the USA, in the Western Interior Basin (Caron et al. 2006; Desmares et al. 2007). In North Africa, its repercussions were also widely studied, in particular in Tunisia (Accarie et al. 2000; Burollet and Robaszynski 1991; Robaszynski et al. 1990, 1993a, 1993b; Caron et al. 1999, 2006; Zagrarni et al. 2008), and to a lesser extent in Morocco (Tsikos et al. 2004; Ettachfini and Andreu 2004; Ettachfini et al. 2005) and in Algeria (Naili et al. 1995; Groshény et al. 2007). According to these various authors, the C/T crisis, which involved a major upheaval of the environment, is at the origin of the deposit of important quantity of organic carbon at the bottom of the oceans (Schlanger et al. 1987; Arthur et al. 1988; Jenkyns et al. 1994), which appears in the form of ‘black shales' layers. It caused also the disappearance of the Rotalipora genus, a complex keeled foraminifera that occupied hitherto the deep water (Hart and Bailey 1979). These organisms, were typically demanding forms of life, having experienced an important development during the K-strategy periods (Groshény and Malartre 1997), and which could not survive at the time of the crisis (r-selection period, during which the opportunist species are developed in great numbers).

The Djebel Chemla series also carry the print of the C/T crisis. Characteristic levels are encountered, of black shales type, the famous Bahloul levels described at many places in the area. These levels of laminated appearance and black color are rich in organic carbon (values of TOC up to 2%). Other important characteristics are also observed there: the disappearance of Rotalipora (last occurrence observed on level 86), very low specific diversity, predominance of planktic forms of life, and in particular of globulous foraminifera of Heterohelix type, presence of filaments.

The presence of these filaments, already evoked previously, is a characteristic feature, so much in North Africa (Nederbragt and Fiorentino 1999; Accarie et al. 2000; Caron et al. 2006; Naili et al. 1995) and elsewhere in the world (see for example Desmares et al. 2007, in the USA).

The great development of Heterohelicidae was also noted by several authors, first of all by Leckie (1985) which called this event “Heterohelix shift”. These morphotypes are rather primitive forms of life, which live in surface water and have an opportunist behavior, i.e., they adapt very well to rapid changes of their environment, contrary to the more complex forms of life. The' Heterohelix shift' occurs after the disappearance of Rotalipora. It was already described in central Tunisia (Robaszynski et al. 1990; Nederbragt and Fiorentino 1999; Caron et al. 1999, 2006) and in the USA for example (Caron et al. 2006; Desmares et al. 2007).

Lastly, the evolution of specific diversity is also very characteristic. Indeed, C/T crisis, like all the crisis, is marked by a very low specific diversity. The tolerant species are rare and are composed mainly of globulous planktic foraminifera (Hedbergella, Whiteinella, and especially Heterohelix), of rare benthic foraminifera, and some anoxia-resistant ostracods. Specific diversity, particularly low in the levels of C/T transition, is on the contrary definitely stronger in the average part of the section (medium part of the Fahdene formation), which agree with the observations already made by Hart and Ball (1986).

Summary

The study undertaken on the Djebel Chemla series has allowed us to characterize paleoenvironment (approximate depth, paleo-oxygenation) and to identify the print of the C/T universally recognized crisis.

However, the study remains incomplete in certain aspects. On the one hand, it does not enable us to identify with certainty the beginning of Turonian stage and the end of the anoxic crisis. The studied levels (Annaba member) present a very different facies but have a very poor micropaleontological content. No keeled foraminifera could be observed, the presence of specimen of the Helvetoglobotruncana genus would have in fact made it possible to verify that these levels belong to the Turonian.

In addition, we also have to point out the heterogeneity of our results. Indeed, although the dominant marly series seem homogeneous, it reveals a very variable micropaleontological content, sometimes dominated by planktic forms, sometimes by benthic ones (Fig. 4). The environmental conditions should have changed, which is hardly astonishing considering the studied time interval (more than 4 My). Perhaps the evolution observed reflects eustatic variations, but it appears premature to attempt a sequential interpretation on the basis of the study of only one section.

Conclusion

The micropaleontological study undertaken on the Djebel Chemla series allowed, through the systematic countings carried out in particular in the marly levels, the biostratigraphic and paleoenvironmental context to be specified.

There are indeed four biozones that could be defined, for some of them only partially. This lack of precision is partly due to the low population of ammonites and keeled foraminifera. The depositional environment considered, in view of our results, is a calm and relatively deep environment of external platform type or upper part of the slope, of which the depth did not exceed 200 m and whose water was often poor in oxygen.

The Cenomanian/Turonian crisis was also highlighted in our deposits, in which several distinctive criteria of this event were located, such as the presence of Bahloul facies levels, typical in the area as in central Tunisia.

These results should not however make one forget the important variations of the micropaleontological content, where the planktic and benthic predominant phases alternate, which doubtless reveals a variation of the depositional environment, for example of eustatic origin. Interpretation of a sequential type could be possible following the study of several sections, which would also make it possible to specify the paleoenvironmental framework of this poorly known area of Algeria.