Geological investigations carried out on the Dahra Massif have revealed sedimentary changes and bioevents characterizing the post-gypsum detrital sediments (from Messinian to Piacenzian), which are followed by the Trubi equivalent Pliocene marls or white marly limestones.

Structured into two superimposed steps, the late Messinian deposits yielded two successive ostracod assemblages. They indicate a brackish environment for the lower and a fairly open shallow brackish environment for the second. Based on their ostracod content, assemblage 1 (Cyprideis, Loxoconcha muelleri) corresponds to the Lago Mare biofacies 1 of the Apennine foredeep, which is correlated with the Lago Mare 1 episode dated between 5.64 and 5.60 Ma. Assemblage 2 (Loxocorniculina djafarovi) is referred to the Lago Mare biofacies 2 described in the same region. It is correlated with the Lago Mare 3 episode, dated between 5.46 and 5.33 Ma.

Moreover, the stratigraphic succession is marked by a major discontinuity indicated by a hardground, separating step 1 from step 2 and corresponding to the ostracod assemblages 1 and 2, respectively. This discontinuity is considered here to be equivalent to the Messinian Erosional Surface, already evidenced in the region and widely known around the Mediterranean Basin.

These late Messinian deposits and their ostracod assemblage 2, notably the detrital sedimentation with Ceratolithus acutus, Globorotalia margaritae, Reticulofenestra cisnerosii document a marine incursion into the Lower Chelif Basin, corresponding to the latest Messinian marine reflooding of the Mediterranean Basin, that happened before the earliest Zanclean R. cisnerosii occurrence. Finally, the bioevents evidenced in the Dahra Massif, reinforce the evidence of the late Messinian Lago Mare 3 episode, and support the ante-Zanclean age of the marine reflooding of the Mediterranean.

The overlying deposits are marked by coral constructions (cf. Cladocora cf. caespitosa, Dendrophyllia sp) never described before and covering the entire early Zanclean, testifying the existence, at that time, of warm enough conditions, which may correspond to the marine isotopic stage TG5.

Les études géologiques menées sur le massif du Dahra ont révélé des changements sédimentaires et des événements paléobiologiques caractérisant les sédiments détritiques post-gypse (du Messinien au Plaisancien), suivis par des marnes ou des calcaires marneux blancs pliocènes équivalents au faciès Trubi.

Structurés en deux étapes superposées, les dépôts du Messinien terminal ont livré deux assemblages d’ostracodes. Le premier indique un environnement saumâtre et le second un environnement saumâtre peu profond assez ouvert. D’après leur contenu en ostracodes, l’assemblage 1 (Cyprideis, Loxoconcha muelleri) correspond au Lago Mare biofaciès 1 de l’avant fosse apenninique, qui est corrélé avec l’épisode Lago Mare 1 daté entre 5,64 et 5,60 Ma. L’assemblage 2 (Loxocorniculinadjafarovi) est rapporté au Lago Mare biofaciès 2 mis en évidence dans la même région. Il est corrélé avec l’épisode Lago Mare 3, daté entre 5,46 et 5,33 Ma.

De plus, la succession sédimentaire est affectée par une discontinuité majeure matérialisée par une surface rubéfiée (hardground), séparant l’étape 1 de l’étape 2, correspondant respectivement aux assemblages d’ostracodes 1 et 2. Cette discontinuité est considérée comme l’équivalent de la Surface d’Erosion Messinienne, déjà reconnue dans la région et abondamment décrite tout autour du bassin méditerranéen.

Ces dépôts du Messinien terminal et leur assemblage d’ostracodes 2, notamment la sédimentation détritique avec Ceratolithusacutus, Globorotaliamargaritae, Reticulofenestracisnerosii documentent une incursion marine dans le bassin du Bas Chélif, correspondant à la remise en eau marine du bassin méditerranéen au Messinien terminal, avant l’apparition de Reticulofenestracisnerosii indiquant la base du Zancléen.

Ainsi, les bioévénements mis en évidence dans le massif du Dahra permettent-ils de renforcer l’existence du Lago Mare 3 d’âge Messinien terminal et de la remise en eau marine de la Méditerranée antérieurement au début du Pliocène.

Les dépôts sus-jacents sont caractérisés par des constructions coralliennes (cf. Cladocora cf. caespitosa, Dendrophyllia sp) jamais décrites auparavant et couvrant l’ensemble du Zancléen inférieur. Elles témoignent de l’existence, à cette époque, de conditions suffisamment chaudes, pouvant correspondre au stade isotopique TG5.

Miocene-Pliocene sedimentation in the Lower Chelif Basin is highly diversified according to its platform to basin facies. The Messinian (7.25–5.33 Ma) is well-known for its pre-reef deposits (Saint Martin, 1990): blue marls, diatomites bearing fish fauna (Arambourg, 1927, Gaudant et al., 1997). These deposits evolve vertically into bioclastic sandstones to coralline calcareous algae, leading up to coral bioconstructions (Djebel Murdjadjo: Cornée et al., 1994; Saint Martin et al., 1995).

The subtropical marine environments characterized by coral reefs (Porites, Tarbellastraea and Siderastraea) as well as impoverished Avicennia mangrove (Saint Martin, 1990; Chikhi, 1992) are succeeded by a post-reefal sedimentation including stromatolites, oolitic accumulations and gypsum (Rouchy, 1982a; Saint Martin, 1990; Cornée et al., 1994). These sedimentary deposits indicate a degradation of marine conditions and are often associated to the so-called Terminal Carbonate Complex (Esteban, 1979; Cunningham et al., 1997; Cunningham and Collins, 2002; Cornée et al., 2004; Roveri et al., 2009, Roveri et al 2020; Clauzon et al., 2015).

Pre-evaporitic sediments including diatomites characterize the sedimentary succession seaward of the platform (Rouchy, 1982b). This evolution leads to the formation of gypsum deposits with varying thickness, from a few meters south of the Lower Chelif Basin (Beni Chougrane: Sahaouria) to several hundred meters northward (Tazgaït) and south (Ouled Maallah) of the Dahra Massif. The post-evaporitic facies are diverse (Anderson, 1936; Perrodon, 1957; Welter et al., 1959 ; Rouchy, 1982a, b), indicating the onset of a desalination process where environments became palustrine to lacustrine (Rouchy, 1982b; Rouchy and Saint Martin, 1992; Orszag-Sperber et al., 2000; Orszag-Sperber, 2006; Rouchy and Caruso, 2006; Rouchy et al., 2007).

The Upper Miocene sedimentary succession comprises the Lago Mare biofacies (Anderson, 1936; Perrodon, 1957; Rouchy, 1982b). The chronological context and causative mechanism of such biofacies are still the subject of debate (Gautier et al., 1994; DeCelles and Cavazza, 1995; Clauzon et al., 1996, 2015; Riding et al., 1998; Butler et al., 1999; Krijgsman et al., 1999; Rouchy and Caruso, 2006; Bassetti et al., 2006; Manzi et al., 2013; Roveri et al., 2014b, 2016; Pellen et al., 2017).

The deposition of evaporites occurred in two steps. The first step (5.97–5.60 Ma) involved the deposits of gypsum (sulfates) in the peripheral basins, while the second step (5.60–5.46 Ma) involved the deposit of evaporite giant in the central basins (chlorides: K, Na, and Mg). This second step is believed to correspond to a drop in the Mediterranean Sea level and strong subaerial erosion of its margins (Clauzon et al., 1996, 2015; CIESM, 2008; Bache et al., 2012; Andreetto et al., 2021).

In light of these considerations, Rouchy et al. (2007) described several localities corresponding to these facies in the Lower Chelif Basin, including Beni Chougrane (Sig, Sahaouria, El Ghomri) and the Dahra Massif (Djebel Meni-Abreuvoir, Oued El Aicha). Additionally, Osman et al. (2021) described similar facies in the Dahra region at Azaizia and Ain Yakoub. These studies instead of authors identified the presence of a Lago Mare (Rouchy et al., 2007), also referred to as Lago Mare 1 (Osman et al., 2021), during the Messinian Salinity Crisis (MSC). This Lago Mare that has been interpreted as a result of the flow of freshwater coming from the Paratethys into the Mediterranean Sea represents a high sea-level exchange (Clauzon et al., 2005; Snel et al., 2006; Popescu et al., 2009, 2015; Manzi et al., 2009; Suc et al., 2011; Do Couto et al., 2014).

Several times, particularly during two distinct events, Lago Mares (LM1 and LM3) seem to have characterize this water exchange (Clauzon et al., 2005; Popescu et al., 2015). LM1, estimated from 5.64 to 5.60 Ma, overlying peripheral evaporites is affected by the Messinian Erosional Surface (MES) (Gautier et al., 1994; Clauzon et al., 2005; Popescu et al., 2009; Manzi et al., 2009; Clauzon et al., 2015). LM3, following the marine reflooding of the Mediterranean Basin is dated from 5,46 to 5,33 Ma (Krijgsman et al., 2001; Clauzon et al., 2005; Popescu et al., 2007, 2009, 2015; Bache et al., 2012; Do Couto et al., 2014). In addition, a LM2, reported from deep central basins (ca. 5.50–5.46 Ma), is considered as a Paratethys discharge after erosion of the Hellenic Arc or overflow over it (Popescu et al., 2015). Manzi et al. (2013) and Roveri et al. (2014a, b, 2016) consider that LM1 and LM3 represent in fact a single phase, located between 5.42 and 5.33 Ma, corresponding also to LM2 in the central basins.

The return to normal marine conditions into the Lower Chelif Basin is usually characterized by the widespread occurrence of Zanclean blue marls or "Trubi facies"; whitish in color at the surface. These blue marls are rich in microfauna (Perrodon, 1957; Mazzola, 1971; Belkebir and Anglada, 1985; Thomas, 1985; Belkebir et al., 1996). This transgression appears to have resulted in the inundation of certain morphological structures inherited from the MSC (Dahra: Osman et al., 2021; Oued Rhiou Boukadir: Moulana et al., 2021, 2022). The overlying grey marls are still of Zanclean age and are in turn overlain by Piacenzian alternating marls and sandstones. They are marked by bivalve shell concentrations (Rouchy et al., 2007; Belhadji et al., 2008; Mansouri et al., 2008; Atif et al., 2008; Satour et al., 2013, 2020; Bendella et al., 2021; Satour, 2021; Osman et al., 2021; Benyoucef et al., 2021; Mansouri, 2021).

The present study focuses on the post-gypsum sedimentation (late Messinian − Piacenzian) in the Dahra Massif (Fig. 1), with a particular emphasis on three newly studied sections: Djebel El Abiod, Hgaf Tamda, and Sidi Brahim Telegraph. The results of these sections are supplemented by data from other sections, including the basal part of the Oued Tarhia section. Previous studies have only inventoried deposits with Globorotaliapuncticulata, more than 200 m above the gypsum (Osman et al., 2021). In this study, the Oued Tarhia section is described in detail with a focus on its lower part (brown to variegated marls, sandy marls and sandstones), overlying the gypsum and capped by the Pliocene marls.

Structured like an ENE-WSW oriented depression, the intra-mountain (Tellian) basin of the Lower Chelif (Fig. 1) has undergone significant sedimentation during the Neogene and the Quaternary periods. with a thickness that can reach over 4800 m (Brive, 1897; Anderson, 1936; Perrodon, 1957; Mazzola, 1971; Delfaud et al., 1973; Thomas, 1985; Meghraoui et al., 1988; Neurdin-Trescartes, 1992; Arab et al., 2015). This region shows evidence of Alpine tectonics, still active today (Guardia, 1975; Meghraoui, 1982; Meghraoui et al., 1986; Meghraoui et al., 1988; Derder et al., 2013; Leprêtre et al., 2018; Abbouda et al., 2018). The structure of the Dahra Massif, in titled blocks, dates back to the end of the Cretaceous (Brive, 1897; Anderson, 1936; Leprêtre et al., 2018). In relation to this, numerous authors have contributed to the understanding of the Cenozoic stratigraphy of this basin and the surrounding Tell massifs (Pomel, 1892; Brive, 1897; Anderson, 1936; Perrodon, 1957).

With regard to the Miocene marine sedimentation of this region, two major sequences are distinguished (Delfaud et al., 1973; Thomas, 1985; Neurdin Trescartes, 1992); they generally correspond to the first and second post-nappe cycles (Perrodon, 1957; Meghraoui, 1982; Meghraoui et al., 1988; Fig. 2). Their ages are estimated from late Burdigalian to Serravallian (Belkebir and Anglada, 1985; Belkebir et al., 1996; Bessedik et al., 2002; Belkebir et al., 2008) concerning the first sequence and from Tortonian to Messinian concerning the second one (Mazzola, 1971; Neurdin-Trescartes, 1992, 1995; Belkebir et al., 2008; Belhadji et al., 2008). Continental sedimentation is also widespread on the southern and northern margins of the Lower Chelif Basin, in addition to the marine sedimentation areas (Guardia, 1975, 1976; Ouda and Ameur, 1978; Ameur-Chehbeur, 1992; Bessedik et al., 1997; Bessedik et al., 2002; Belkebir et al., 1996; Mahboubi et al., 2015).

The Late Miocene sedimentation is characterized by an unconformity covering earlier marine and continental deposits; it shows a transgressive to a regressive trend (Fig. 2). It consists of marls evolving to diatomite and diatomitic marl alternation, then evaporites and finally post-evaporitic lagunal sediments (Anderson, 1936; Perrodon, 1957; Rouchy, 1982a, b; Thomas, 1985; Saint Martin, 1990; Neurdin-Trescartes, 1992, 1995; Mansour et al., 1999, Rouchy et al., 2007).

The gypsum, which marks the start of the MSC in the Mediterranean peripheral basins, can reach thicknesses of up to 4 meters in some places (with 2 beds) or even more than 250 to 300 meters in the Lower Chelif Basin (massive gypsum), particularly on the southern and northern slopes of the Dahra Massif (Ouled Maallah, Tazgaït: Fig. 2). In the central part of the Basin, post-evaporitic Messinian sedimentation evolved into palustrine to lacustrine deposits. The Pliocene is characterized by “Trubi facies” and marine blue to whitish marls, well represented in the Sidi Brahim Telegraph section with thicknesses up to 750–800 m (Brive, 1897; Anderson, 1936; Perrodon, 1957; Mazzola, 1971; Fenet and Irr, 1973; Belkebir and Anglada, 1985; Thomas, 1985; Neurdin-Trescartes, 1992; Rouchy, 1982a; Rouchy et al., 2007; Atif et al., 2008; Abbouda et al., 2018). In fact, many authors describe compressive tectonics that affected the Pliocene cycle, with some faults still being active (Perrodon, 1957; Thomas, 1985; Meghraoui, 1982; Meghraoui et al., 1988; Derder et al., 2013; Arab et al., 2015; Abbouda et al., 2018).

The dating of the Messinian and Pliocene deposits specified, particularly with regard to the northern and southern margins of this basin. Since several works have highlight the diversity of facies and identified several Messinian and Pliocene bioevents in the planktonic foraminifera and calcareous nannoplankton (Saint Martin, 1990; Rouchy et al., 2007; Atif et al., 2008; Osman et al., 2021). These bioevents are calibrated on radiometric and/or astronomic ages (Channell et al., 1988, 1992; Sprovieri, 1993; Lourens et al., 2004, 2005; Sprovieri et al., 2006; Raffi et al., 2006; Di Stefano and Sturiale, 2010; Backman et al., 2012; Gradstein et al., 2012; Lirer et al., 2019) (Fig. 3).

This study addresses the post-gypsum deposits located near Hassi Ben Mekki quarry (central Dahra) (RN90) (Djebel El Abiod and Hgaf Tamda sections). Other sections are partially detailed (bottom of the Oued Tarhia section) for their biostratigraphic interest (Messinian-Pliocene boundary). The Sidi Brahim Telegraph section is explored upwards (Pliocene) and enable correlations with the Azaïzia section (a). The boundaries between lithostratigraphic (sub)units have been sought and carefully described.

The extraction of planktonic foraminifera and calcareous nannofossils was performed on the same samples. Due to their significant thickness, more than 50% of the sampling concerned the Djebel El Abiod and Sidi Brahim Telegraph sections. A large number of samples were taken (over 250), but only 190 were selected for their microfossil content. The extraction of foraminifera involves a phase of deflocculation of 200 to 300 grams of sediment, achieved by soaking in lukewarm water. Washing is carried out under a trickle of water through a sieve with a mesh size of 80 and 100 µm. Foraminifera, ostracods, charophytes are identified using a binocular microscope, with magnification ranging from x250 to 500. Extraction of calcareous nannofossils (smear slides) consists in placing a fragment of sediment on a slide before dilution with a drop of distilled water. The slide is stored on hot plate for drying for a few seconds and finally covered with a coverslip, glued using Eukitt resin. The slide is analyzed using a polarizing optical microscope (magnification ×500). The analysis is done by systematically scanning the slide. After targeting the organisms, the determination was carried out with a magnification of ×500 to ×1000 µm.

Our objective is to first specify a reliable stratigraphic framework based on all available data on foraminifera and calcareous nannoplankton in the Miocene and Pliocene marine deposits from the Lower Chelif Basin (Magné in: Perrodon, 1957; Bizon in: Thomas, 1985; Belkebir and Anglada, 1985; Rouchy, 1982a, b; Saint Martin, 1990; Bizon in: Neurdin-Trescartes, 1992; Osman et al., 2021). The planktonic foraminifera bioevents recorded in the Lower Chelif Basin constitute an important background for the reconstruction of the local biostratigraphy, which is correlated with those established in the Mediterranean (Bizon and Bizon, 1972; Zachariasse, 1975; Cita, 1975; Thunell, 1979; Langereis and Hilgen, 1991; Hilgen et al., 2012; Iaccarino et al., 2007) as reported on the standard Blow scale (Blow, 1969). Our biostratigraphy is also based on calcareous nannofossils correlates with the scale proposed by Martini (1971) and Backman et al., (2012). Some microfossils constitute, for the base of the Pliocene, important landmarks in relation to the top of the Sphaeroidinellopsis subdehiscens acme, reported at 5.21 Ma and 5.30 for its base (Lourens et al., 2005; Lirer et al., 2019). Globorotalia. margaritae and Ceratolithus acutus can also date together the latest Messinian layers.

Four geological sections carried out on the southern edge of the Dahra Massif are described in distinct lithostratigraphic units, showing their detailed sedimentological and paleontological contents.

Chronologically, they cover the Upper Miocene and the Pliocene periods. The Djebel El Abiod section is composed of six units (Fig. 4; S1), which respectively belong to Messinian (Units I-III and IV), early Zanclean (Unit V; coral facies), late Zanclean (p.p.) and early Piacenzian (Unit VI). The Hgaf Tamda section (Fig. 5; S2) crops out in a syncline with NE-SW oriented axis with five lithostratigraphic units: units I-III are attributed to Messinian, units IV (coral facies) and V belong to Pliocene. The Oued Tarhia section (Fig. 6; S3) is composed of five lithological units: units I-IV and V p.p. belong to Messinian; upper part of unit V is integrated within Zanclean. The Sidi Brahim Telegraph section (Fig. 7; S4) is subdivided into four units dated from latest Messinian (UI-II) to Zanclean (UIII) and Piacenzian (Unit IV).

Post-gypsum deposits or Lago Mare

The post-gypsum sediments, well represented in the Djebel El Abiod section (S1), are reduced or even absent in the Hgaf Tamda (S2) and Oued Tarhia (S3) sections. Their extension reveals, from East to West, a discontinuity in their facies and an irregularity of their topographic background. Two steps characterize this sedimentary succession, which unconformably overlies the selenite gypsum (Fig. 8).

Step 1

It is represented by variegated clays corresponding to a filling sedimentary sequence (clay-sandy marls-sandstone-conglomerates), witness to intense erosion (Fig. 4). Ostracofauna is abundant (Pl. 1): Cyprideis (assemblage 1) associated with Loxoconcha muelleri, L. sp.1 and L. sp.2 indicates a brackish, shallow environment attesting some episodic fluvio-lacustrine contributions (Chara cf. hispida, Pseudocatillus sp., and quartz). This ostracofauna (S1) is of late Messinian age (Gliozzi, pers. comm.), comparable to that described by Rouchy et al. (2007) in the Beni Chougrane (Djebel Touakas), and in the Dahra Massif (Oued El Aïcha).

Step 2

Grey ruby clays are marked by a late Messinian ostracod assemblage 2 (Gliozzi, pers. comm.) over green marine marls with dwarf planktonic foraminifera (Djebel El Abiod): Loxocorniculina djafarovi, Euxinocytherepraebaquana, Amnicythere cf. accicularia, A. sp., Cytherura pyrama, Camptocypria sp., Zalanyiella venusta (Pl. 1). Above, variegated clays have yielded another assemblage with Cyprideis (abundant), Tyrrhenocythere cf. ruggierii, Amnicythere sp., Zalanyiella venusta (see S1). This assemblage 2 corresponds to open shallow to brackish marine conditions (L. djafarovi), which became brackish to slightly lacustrine (hypo-mesohaline, 5-15‰) at the top (Cyprideis abundant). This deposit is unconformably overlain by marine sandy marls (SM1, SM2) containing planktonic foraminifera.

The ostracod association of Djebel El Abiod (Fig. 4) is slightly different from that revealed in the Oued Tarhia (Fig. 6). The T16 sample assemblage (Loxocorniculina djafarovi, Euxinocythere praebaquana, Amnicythere sp., Cytherura pyrama, Loxoconcha muelleri, Tyrrhenocythere cf. ruggierii), is different from that from the T17 sample (Cyprideis, Loxoconcha sp.1, L. sp.2, Tyrrhenocythere pontica, Amnicythere sp., Amnicythere propinqua, showing a red gangue on some reworked individuals of Loxoconcha muelleri). In addition, the L. djafarovi assemblage (Djebel El Abiod) evolves at the top into assemblage with Cyprideis (abundant), comparable to that, in the same position, of the Oued Tarhia section (Cyprideis, Loxoconcha sp .1, L. sp.2, Tyrrhenocythere pontica, Amnicythere propinqua, A. sp.).

The Loxocorniculina djafarovi assemblage described in the Sidi Belattar and Sidi Brahim Telegraph sections by Atif et al. (2008) seems to present some reworking (Loxoconcha muelleri). Not far from this locality, the ostracod assemblages, collected in the Oued Tarhia section (Fig. 6, S3), give rise to similar remarks, in particular the presence of several individuals (shells in situ) belonging to Loxoconcha muelleri (sample T16), having provided the L. djafarovi association. The sample T17 shows individuals of L. muelleri with carapaces within a red gangue that suggests their reworking. These observations firstly concern the presence of L. muelleri (in situ) within the L. djafarovi assemblage, mainly in the western localities (Sidi Belattar, Sidi Brahim Telegraph: Atif et al., 2008; Oued Tarhia: this work). Secondly, the assemblage with L. djafarovi occurs without L. muelleri in the sections of Djebel Meni −Abreuvoir (Rouchy et al., 2007) and Djebel El Abiod.

The assemblage of Cyprideis associated with L. muelleri (assemblage 1) followed by that of L. djafarovi (assemblage 2) constitutes a chronological landmark succession in the Lower Chelif Basin and the Dahra Massif.

The grey ruby and variegated clays of Djebel El Abiod (step 2), marked by a brackish character, are interrupted by an unconformity and overlain by marine sandy marls (SM1, SM2). These latter have yielded foraminifera and calcareous nannofossils from Miocene to Pliocene with a strong planktonic representation (Globigerinoides, Globigerina, Globorotalia, Coccolithus pelagicus, Helicosphaera carteri, Discoaster variabilis and some Sphenolithus), before the emplacement of Pliocene deposits. This marine episode seems to correspond to the grey sandy marls with Globorotalia gp, G. margaritae, Globigerinoides gp, Globigerina gp, and Reti-culofenestra pseudoumbilicus, belonging to the Sidi Brahim Telegraph section (Pl. 1).

The lithologic and paleontological successions (step 1, step 2), characterized by brackish or brackish to slightly lacustrine environments are correlated with the gypsum and post-gypsum sedimentation attributed to Lago Mare (Rouchy et al., 2007) even partly to the Lago Mare 1 (Osman et al., 2021) from late Messinian.

Three pre-Zanclean discontinuities are recorded (latest Messinian) in the neighbouring localities of Djebel El Abiod and Hgaf Tamda (Figs. 4 and 5). They locate between: (i) the lower variegated clays and the alternating sandy marl and sandstone (VCI/SMS), (ii) the top of the hardground with distorted structure, belonging to the sandy marls and sandstones and the green marine marls and, (iii) the variegated clays and the marine sandy marls (VCII/SM1).

The most important discontinuity (ii) is recorded in the Djebel El Abiod section (Fig. 4). It is materialized by the surface of the hardground with its subsequent deformation that resulted in varying dip from 25 to 30° towards the NNW, prior to the overlying deposits; we consider it as a major discontinuity equivalent to the Messinian Erosional Surface (MES). It is also underlined by a paleontological change, separating the pre-hardground assemblage 1 from the post-hardground assemblage 2; this is clearly visible in the Hgaf Tamda, Oued Tarhia and Sidi Brahim Telegraph sections (Fig. 8).

SM1, SM2, Conglomerate and age of Djebel El Abiod

Two sandy marl levels belonging to the Unit IV (SM1, SM2: Fig. 4; S1) rest unconformably on the post-hardground variegated clays (Fig. 4: e: VCII/SM1; e1: SM2, M/Z boundary), which are estimated as latest Messinian in age based on its own ostracod assemblage 2 (Gliozzi, pers. com.). Their microfossils show an extensive representation of planktonic foraminifera and calcareous nannofossils (Globigerinoides, Globigerina, Globorotalia, Coccolithus pelagicus, Helicosphaera carteri, Discoaster variabilis and some Sphenolithus), witnesses of a marine incursion whose age may be Messinian to Zanclean based on the calcareous nannoflora. These deposits (SM1, SM2) are crowned by an undulating surface (unconformity) overlain by a thin conglomeratic level (Fig. 4: d1, d2 and e1: intermediate conglomerate), the top of which is also interrupted by another undulating surface (called dcs = double corrugated surface, see Fig. 4: a). The overlying deposit is a coralliferous white marly limestone (CWML), which yielded Globorotalia margaritae, Reticulofenestra cisnerosii of earliest Zanclean age. The latter are correlated with the lower part of the white marls of the Sidi Brahim Telegraph section whose extreme base reveals the presence of Ceratolithusacutus (TSB9), followed (Fig. 7, S4) by the presence of Reticulofenestra cisnerosii (TSB11).

The age of the conglomerate could be estimated between the late Messinian and the earliest Zanclean. It must locally express the gap of the Sphaeroidinellopsis subdehiscens biozone, commonly recognized in the Lower Chelif Basin (Mazzola, 1971; Belhadji et al., 2008), possibly incomplete in some places (Rouchy et al., 2007; Osman et al., 2021). This attribution can be supported by the succession of C. acutus (samples T18, T19, T20) and Globorotalia margaritae (T19, T20), observed in the Oued Tarhia (S3; Fig. 6). These deposits evidence that the marine reflooding happened prior to the R. cisnerosii occurrence (T20). Thus, the data outlined above corroborate dating of the ostracod assemblage 2 highlighted in the Dahra Massif sections and confirm, by correlation, its ascription to the late Messinian.

Consequently, the unconformity and the conglomeratic level observed in the Djebel El Abiod and Hgaf Tamda sections separating the coralliferous white marly limestones from the underlying marine sandy marls (i.e., SM1 and SM2) could be Miocene to Pliocene in age. In the absence of biomarker, the marine sandy marls (SM1 and SM2) are attributed to the late Messinian, which must correspond again or partly to the marine reflooding in the Lower Chelif Basin, coeval with the Lago-Mare assemblage 2 with Loxocorniculina djafarovi (latest Messinian) described in the Djebel El Abiod section.

Coral bioconstructions, associated fauna, age and environment of white marly limestones

The restoration of marine conditions in the Dahra Massif began in the latest Messinian with significant sedimentation of grey sandy marls, generally detrital at the base and recorded in quite deep areas. Other shallow areas are marked by the development of carbonate platforms materialized by lenticular white limestone with Neopycnodonte cochlear or algae (Lithothamnium) (Brive, 1897; Perrodon, 1957). These deposits include also scaphopods, echinoderms, ostreids, etc.

The extensive exploration of Miocene and Pliocene outcrops in the Dahra Massif allows obtaining new paleontological and stratigraphic data for these white limestones, particularly in the Djebel El Abiod and Hgaf Tamda sections (Figs. 4 and 5). These sediments, unconformable on the Messinian deposits, present alternations of coralliferous white limestone beds and marly limestone (CWML) with abundant specimens of Neopycnodonte cochlear, Megerlia truncata and some lenticular clusters of scleractinians evolving to coral bioconstructions with Dendrophyllia sp. and cf. Cladocora cf. caespitosa. These CWML and coral bioconstructions are dated from the early Zanclean, according to the record of Globorotalia margaritae (biozone N18 p.p.: Mazzola 1971; Belkebir and Anglada, 1985) and Reticulofenestra cisnerosii (biozone NN12: Mansouri, 2021), and to the upper Zanclean based on Globorotalia puncticulata and Discoaster asymmetricus (Osman et al., 2021).

Biostratigraphy suggests a basal Zanclean age for the lowermost Dahra white marly limestones (i.e.: FO of G. margaritae at 5.08 Ma and LO of R. cisnerosii at 5.119 Ma, i.e., the upper NN12 biozone, knowing that the appearance of G. margaritae is reported before 5.08 Ma: Fig. 3). On their western extension, these white marly limestones yielded, at their base, Globorotalia puncticulata associated with Discoaster asymmetricus (Osman et al., 2021) corresponding respectively to N19 and NN14/15 biozones. These biochronostratigraphic results imply that the CWML of Ouled Slama are older than those registered in the Ouled Maallah, suggesting a chronological relay of coral bioconstructions.

Consequently, these bioconstructions would have started before 5.119 Ma and disappeared a little before 3.60 Ma. Furthermore, the presence of corals like Ceratotrochus (Edwardsotrochus) pentaradiatus in the whitish marls (Sidi Brahim Telegraph section: Fig. 7, S4), collected between the G. puncticulata biozone and until before the G. crotonensis presence (Mazzola, 1971), suggests a degradation of this type of coral environment during the upper Zanclean as observed in the Ouled Maallah and Ouled Slama localities (Figs. 4, 5 and 6).

The abundance of Megerlia truncata (brachiopod) may indicate depths down to 100 m or more (Emig, 1988); its alternating abundance between the coral banks means some variations of bathymetry during the Lower Pliocene, oscillations which would probably be linked to readjustment of the margin in relation with coastal reliefs. Similarly, the species Neopycnodonte cochlear can affectionate this depth (up to about 50 m) in temperate waters with low turbidity (Ben Moussa, 1994). Like their modern Mediterranean representatives, these coral biobuilders seem to require, during the Lower Pliocene, a warm environment for their development, the conditions of which began to deteriorate since the disappearance of Globorotalia puncticulata in relation with the onset of a shallow environment. Indeed, TSB levels 32 to 49 yielded a diversified ostracofauna with a rich representation of Aurila, Loxoconcha, Cytheropteron, Cytherella, among others. This implies the presence of a coastal euhaline environment where frequency of planktonic foraminifera is low (levels TSB 30 to 37) to the benefit of benthic species (Ammonia sp, Bulimina sp, Elphidium sp.).

Dating of the Messinian-Pliocene succession in the Dahra Massif

The Sidi Brahim Telegraph section (S4; Fig. 7) is the reference section for the Lower and Upper Pliocene in the Dahra Massif. The analysis reveals the presence of several paleontological features and/or bioevents; Globorotalia margaritae species collected in the grey marls (TSB03 and TSB04), followed by Ceratolithus acutus (TSB09, associated with G. margaritae) in the white blue marls and Reticulofenestra cisnerosii (TSB11, associated with G. margaritae and C. acutus). This succession is also listed in the Oued Tarhia section (grey marine marls: T20. Fig. 6). The underlying grey marls are characterized there by an abundant ostracofauna dominated by L. djafarovi, (assemblage 2), attributed to the Lago Mare (Rouchy et al., 2007), which belongs to the late Messinian (Gliozzi, pers. com.). The absence of the upper part of the sandy marl and sandstone alternation is observed, the conglomerates and the hardground overlying the sediments with the ostracod assemblage 1; that demonstrates the importance of the stratigraphic gap in some localities (Oued Tarhia section), a phenomenon linked to erosion.

G. margaritae indicates the N18 biozone (Mazzola, 1971; Belkebir and Anglada, 1985; Belhadji et al., 2008), of Zanclean age (samples TSB3 and TSB4). The succession of G. margaritae and Ceratolithus acutus, before the appearance of Reticulofenestra cisnerosii, suggests a latest Messinian age for these sediments (Popescu et al., 2017; Mansouri, 2021).

The sample TSB11 (blue whitish marls) reveals the presence of G. margaritae, C. acutus and R. cisnerosii, indicating the base of Zanclean (N18 biozone of Blow, 1969/biozone NN12 of Martini, 1971). The Messinian-Zanclean boundary, dated at 5.33 Ma, corresponds to the FAD of R. cisnerosii (Mazzola, 1971; Belkebir and Anglada, 1985; Backman et al., 2012; Popescu et al., 2017; Osman et al., 2021; Mansouri, 2021). It thus dates the marine reflooding before the early Zanclean (R. cisnerosii occurrence) (Mansouri, 2021; Cavazza and De Celles, 1998; Londeix et al., 2007; Carnevale et al., 2008; Bache et al., 2012; Do Couto et al., 2014; Clauzon et al., 2015; Suc et al., 2015; Popescu et al., 2021; Van Dijk et al., 2023).

The appearance of C. acutus followed by C. rugosus both associated with G. margaritae, evidences the lower Zanclean (level TSB 17), that corresponds to biozones N18 (Mazzola, 1971; Belkebir and Anglada, 1985) and NN13 (Martini, 1971; Backman et al., 2012; Tchouar, 2013; Osman et al., 2021; Mansouri, 2021). The occurrence of Globorotalia puncticulata associated with Discoaster asymmetricus, collected in the white marls of the Sidi Brahim Telegraph section (TSB23; Fig. 7), indicates the upper Zanclean (Fig. 9).

Occurrence of G. puncticulata associated with G. puncticulata cf. padana, Discoaster asymmetricus, D. tamalis and Aurila cf. convexa emathiae in the TSB29 sample allows attributing this part of the whitish marls to the upper Zanclean (biozones N19-NN14/NN15). The species Aurila cf. convexa emathiae (ostracod) constitutes, according to Carbonnel and Ballesio (1982), a biozone equivalent to the G. puncticulata biozone (Uliczny, 1969; Sissingh, 1972, 1976).

This attribution is also valid for levels TSB30 to TSB34, some of which (TSB32-34) are marked by the presence of G. puncticulata cf. padana. In particular, the species D. asymmetricus and D. tamalis persisted there (TSB41-49). The presence of the species Globorotalia crotonensis (Mazzola, 1971) in the sandy marls of the Sidi Brahim Telegraph section (equivalent to level TSB51) marks the Piacenzian Stage. However, this last species may be absent in some localities of the Lower Chelif Basin; it is replaced by G. crassaformis (Belkebir, 1986; Belkebir and Anglada, 1985) or G. cf. crassaformis (Osman et al., 2021) or even G. hirsuta aemiliana (Belhadji et al., 2008).

The presence of Discoaster tamalis and the disappearance of Sphenoliths (Sphenolithus abies, among others) in the TSB66 level confirm the Piacenzian age of the Sidi Brahim Telegraph section (Mansouri, 2021). The latter section thus corresponds to a stratigraphic extension going from the late Messinian to the Piacenzian (Figs. 2 and 8) (biozones N18-N19-N20 of Blow, 1969 corresponding to biozones NN12-NN13-NN14/NN15-NN16 of Martini, 1971).

The CWML (Hgaf Tamda section) revealed the presence of five successive species. The Hg10 sample yields G. margaritae associated with R. cisnerosii. This succession indicates the N18 (Blow, 1969)/NN12 (Martini, 1971; Backman et al., 2012) biozones. The latter can be attributed to the basal Zanclean (Fig. 2). The Hg14 and Hg15 levels recorded G. margaritae associated with G. puncticulata indicating the N19 biozone of Blow (1969), attributed to the middle to upper Zanclean (Mazzola, 1971; Belkebir and Anglada, 1985). The species G. puncticulata associated with G. bononiensis and G. cf. crotonensis are recorded in the samples Hg16 and Hg17, indicating the N20 biozone of Blow (1969) of Piacenzian age (Fig. 10).

G. margaritae occurs in the Djebel El Abiod section; in the CWML (Ab30, Ab31), this species is associated with R. cisnerosii (Ab32, Ab33, Ab34, Ab35), then with G. puncticulata (Ab36). These bioevents assign to the CWML a Lower Pliocene age (N18 biozone of Blow, 1969 and NN12 biozone of Martini, 1971) (Fig. 10). The lower part of the overlying grey marls is attributed to the middle to upper Zanclean (biozone N19 from Blow, 1969) based on the presence of G. margaritae associated with G. puncticulata (Fig. 3). The upper part of the grey marls is assigned to the Piacenzian (biozone N20 of Blow, 1969), which yielded G. crotonensis over the first sandstone bar in the Djebel El Abiod section (Fig. 4: sample 49).

According to biostratigraphy, the first marine deposits of the Sidi Brahim Telegraph (i.e., grey marls with G. margaritae and the lower part of the blue whitish marls with C. acutus associated with G. margaritae) point out the latest Messinian marine reflooding (Figs. 7 and 9). This interpretation is also valid for the grey marls of the Oued Tarhia section (Figs. 6 and 10). These data seem to confirm the observations on the bioturbated deposits from Djebel Meni-Abreuvoir and Oued El Aïcha considered as being witnesses of the restoration of marine conditions although they were ascribed to the basal Pliocene without paleontological argument (Rouchy et al., 2007). These data confirm the uppermost Messinian age of the Mediterranean reflooding (Cavazza and De Celles, 1998; Londeix et al., 2007; Carnevale et al., 2008; Bache et al., 2012; Do Couto et al., 2014; Clauzon et al., 2015; Suc et al., 2015; Popescu et al., 2021; Van Dijk et al., 2023).

In addition, the Piacenzian deposits of the localities of Hgaf Tamda and Djebel El Abiod (Figs. 4, 5 and 10) underwent a significant deformation of Pliocene to Pleistocene age, having generated folds whose structural axes (syncline and anticline) are NE-SW oriented, and bounded by faults trending NS, NNE/SSW. This (transpressional) deformation is consistent with that described in the Dahra Massif (Perrodon, 1957; Arab et al., 2015).

Ouled Slama (Djebel El Abiod, Hgaf Tamda, Oued Tarhia), and Sidi Brahim Telegraph sections correlated with the Azaïzia one display a Miocene to Pliocene detrital sedimentation (Fig. 11), limited at the base by two beds of selenite gypsum, well known in the Lower Chelif Basin (Brive, 1897; Anderson, 1936; Perrodon 1957; Delfaud et al., 1973; Rouchy, 1982a, b; Neurdin-Trescartes, 1992). This post-evaporitic sedimentation is generally ascribed to the Messinian, being of brackish or even lacustrine character or “Lago Mare” (Rouchy 1982a, b; Saint Martin, 1990; Saint Martin and Rouchy, 1990; Rouchy et al., 1992, Rouchy and Caruso, 2006; Rouchy et al., 2007; Atif et al., 2008; Caruso et al., 2020). According to a recent study at Ouled Maallah (Dahra), its stratigraphic position was correlated with other reef platforms in the Western Mediterranean (Melilla, Sorbas, etc.). The post-evaporitic deposits from Ouled Maallah are attributed to “Lago Mare 1” (Osman et al., 2021).

In light of our new stratigraphic and paleontological results, we need to discuss this post-evaporitic sedimentary succession.

Assemblages 1, 2 and Lago Mare episodes in the Mediterranean region

The late Messinian (post-evaporitic) sedimentation deposits were characterized by two successive steps (step 1 aboved by step 2), having respectively provided two successive ostracod contents called assemblage 1 and assemblage 2 (Fig. 11).

The late Messinian ostracod assemblage 1 is of Parathethyian origin (Cyprideis, Loxoconcha muelleri, L. sp.1 and L. sp.2) (Carbonnel and Ballesio, 1982; Gliozzi, pers. comm.). Cyprideis agrigentina, C. anlavauxensis associated with Loxoconcha muelleri and L. cf. eichwaldi (Rouchy et al., 2007) complete this assemblage indicating a brackish and shallow with some episodic fluvio-lacustrine environment. Ostracod assemblage 2 refers to the same origin and is also attributed to late Messinian (Carbonnel and Ballesio, 1982; Gliozzi and Grossi, 2008; Gliozzi, pers. comm.). It is dominated by Loxocorniculina djafarovi with E. praebaquana, Amnicythere cf. accicularia, A. sp., Cytherura pyrama, Camptocypria sp., Zalanyiella venusta (Pl. 1). It is followed by another assemblage of abundant Cyprideis with Tyrrhenocythere cf. ruggierii, Amnicythere sp., Zalanyiella venusta (see S2) of open shallow marine to brackish (L. djafarovi) or to slightly lacustrine (Cyprideis abundant) conditions.

These assemblages (1 and 2:Fig. 11) are comparable to those known in the late Messinian in the Mediterranean region. They indicate the “Lago Mare” biofacies (Bonaduce and Sgarrella, 1999; Iaccarino and Bossio, 1999; Gliozzi, 1999, Gliozzi et al., 2006, 2007; Rouchy and Caruso, 2006; Gennari et al., 2008; Guerra-Merchán et al., 2010; Grossi et al., 2015; Stoica et al., 2016; Mas and Fornós, 2020). The assemblage 1 of Cyprideis associated with L. muelleri is comparable to the biofacies 1 of Bonaduce and Sgarrella (1999) where Cyprideis is associated with Tyrrhenocythere ruggierii, Loxoconcha kochi, Loxoconcha muelleri and Caspiocypris alta (Iaccarino and Bossio, 1999). Assemblage 2 would correspond to biofacies 2 of Bonaduce and Sgarrella (1999), characterized by L. djafarovi associated with Amnicythere, Loxoconcha, Loxocauda, Cytheromorpha, Cyprinotus and Tyrrhenhocythere. Moreover, this shows that assemblages 1 and 2 of the Dahra Massif are approximately consistent with the Apennine geological formations p-ev1 and p-ev2 where the species L. djafarovi appears, according to Roveri et al., (2008), near the p-ev1/p-ev2 boundary.

The assemblage 1 of Cyprideis associated with L. muelleri and followed by that (assemblage 2) of L. djafarovi constitutes, like those well known in the Mediterranean, a chronological landmark sequence in the Lower Chelif Basin and the Dahra Massif. These assemblages are respectively correlated to Lago Mare biofacies 1 and Lago Mare biofacies 2 whose age is estimated between 5.59 and 5.40 Ma for the first and between 5.40 and 5.33 Ma for the second (Roveri et al., 2008; Grossi et al., 2008; Andreetto et al., 2022). The Lago Mare 1 of Clauzon et al. (2005) and Bache et al. (2012) is estimated between 5.64 and 5.60 Ma. It is almost coeval with Lago Mare biofacies 1 (see above). The LM3 corresponds to the post MSC marine reflooding according to Bache et al. (2012, 2015) and Popescu et al. (2017), estimated between 5.460 and 5.332 Ma (Clauzon et al., 2005; Popescu et al., 2007, 2009, 2015; Do Couto et al., 2014; Bache et al., 2012: Fig. 15). Finally, its age is slightly comparable to that recognized for Lago Mare biofacies 2 (5.40 and 5.332 Ma: Roveri et al., 2014b; Gliozzi and Grossi, 2008; Grossi et al., 2011, 2015; Gliozzi et al., 2007, Gliozzi et al., 2012).

Messinian Erosional Surface

A major discontinuity is materialized in the Dahra Massif by the hardground. This surface is underlined by a paleontological change, separating the assemblage 1 from the assemblage 2. As a consequence, we interpret this discontinuity as the Messinian Erosional Surface (MES: Fig. 11) that was already evidenced in the Lower Chelif Basin thanks to a geometrical approach (Osman et al., 2021), so much highlighted in many other Mediterranean basins (e.g., Clauzon et al., 1996, 2005; 2015; El Euch-El Koundi et al., 2009; Rubino et al., 2010).

Age and status of the conglomerate underlying the CWML

Because of its post-hardground position, the conglomerate level of Djebel El Abiod, which is overlain by the Pliocene Coralliferous White Marly Limestone (CWML with Reticulofenestra cisnerosii), is estimated to be late Messinian − early Pliocene in age. This conglomeratic deposit corresponds to a sedimentation phase slightly prior to the CWML, which itself is correlated with the whitish blue marls (equivalent to the Trubi facies) of the Sidi Brahim Telegraph section. This situation is comparable to that described in the Kalamaki section (Greece) where Pierre et al. (2006) note the presence of deformed microconglomerates below Zanclean Trubi limestones. In the case of Djebel El Abiod, this deposit may have a status corresponding to the missing of the Sphaeroidinellopsis subdehiscens biozone, commonly recognized in the Lower Chelif Basin (Mazzola, 1971; Belhadji et al., 2008) or possibly incomplete elsewhere (Rouchy et al., 2007; Osman et al., 2021). This interpretation is supported by the succession of Ceratolithus acutus and Globorotalia margaritae observed in the grey marls of the Oued Tarhia section, the deposition of which predates the occurrence of R. cisnerosii.

From these data, the stratigraphic position of the conglomerate is limited at the base by the ostracod assemblage 2 highlighted in the Dahra massif and correlated with the late Messinian age reserved to the Lago Mare biofacies 2 (Gliozzi et al., 2012; Grossi et al., 2011; Roveri et al., 2008, 2014b; Andreetto et al., 2022), between 5.40 and 5.346 Ma. Therefore, the age of conglomerate will be more later to that of the Lago Mare 3, estimated between 5,460 and 5,332 Ma (Clauzon et al., 2005; Bache et al., 2012, Bache et al., 2015; Do Couto et al., 2014, Popescu et al., 2015, 2017, 2021).

Coral constructions, associated fauna, age and environment of white limestones

The post-MSC restoration of normal marine conditions in the Dahra Massif (Fig. 11) began with significant sedimentation of whitish grey marls, recorded in fairly deep areas (Sidi Brahim Telepgraph). At the same time, the shallow lateral zones are marked by development of carbonate platforms (Djebel El Abiod and Hgaf Tamda) materialized by lenticular white limestones. They are dated from the early Zanclean, according to Globorotalia margaritae (Mazzola 1971; Belkebir and Anglada, 1985) and Reticulofenestra cisnerosii (Mansouri, 2021), and to the upper Zanclean based on the occurrence of Globorotalia puncticulata and Discoaster asymmetricus (Osman et al., 2021).

Biostratigraphy suggests for the CWML an age going from the earliest to the late lower Zanclean. This dating suggests a chronological relay of coral bioconstructions from eastern to western Dahra. The interval between 4.04 Ma (FO of D. asymmetricus) and 3.60 Ma (LO of G. puncticulata without G. crassaformis or G. crotonensis) provides the age to the topmost CWML.

Furthermore, the presence of corals like Ceratotrochus (Edwardsotrochus) pentaradiatus in the whitish marls (Sidi Brahim Telegraph) collected between G. puncticulata biozone and until before the G. crotonensis occurrence (Mazzola, 1971) suggests a degradation of this type of coral environment during the late Zanclean in these localities.

The builder cf. Cladocora cf. caespitosa and Dendrophyllia sp. are generally colonial species (Laborel and Laborel-Deguen, 1978; Zibrowius, 1980; Jiménez et al., 2016; Altuna and Poliseno, 2019). Their modern representatives live in the Mediterranean (e.g., Cladocora caespitosa) at shallow depth (Kersting and Linares, 2009, Kersting and Linares, 2012;Kružić and Požar-Domac, 2003; Laborel, 1961; Peirano et al., 1998; Kružić et al., 2012; Özalp and Alparslan, 2011) in a warm environment. Some others are solitary and shallow organisms (Desmophyllum: Altuna and Poliseno, 2019). The associated Neopycnodonte cochlear and Megerlia truncata indicate depths down to 50 m or more (Ben Moussa, 1994; Emig, 1988). Their alternating abundance (Megerlia truncata) between the coral banks means some bathymetrical variations during the Lower Pliocene, oscillations which would probably be linked to the readjustments of the margin in relation with the coastal reliefs.

Like their modern Mediterranean representatives, these coral builders seem to live, during the Lower Pliocene, under warm conditions, which began to deteriorate since the disappearance of G. puncticulata in relation with the onset of a shallow environment. The corals evidenced in the Dahra Massif are well known in the Mediterranean (Dendrophyllia sp., cf. Cladocora cf. caespitosa, cf. Desmophyllum sp., D. cf. cristagalli, Ceratotrochus sp., Ceratotrochus (Edwardsotrochus) pentaradiatus), from the Lower Miocene up to Present (Vertino et al., 2014). Their presence here during the earliest Zanclean is new, compared to what is known elsewhere in the Mediterranean: late (to latest) Zanclean from Spain and Italy (Aguirre and Jiménez, 1998; Vertino et al., 2014; Spadini, 2019).

The bioconstructions (cf. Cladocora cf. caespitosa, Dendrophyllia sp.), highlighted in the white marly limestones with Neopycnodonte cochlear, attest the presence of marine conditions during the lower Zanclean, warm enough for their development. Such a context is also evidenced by the presence of Proboscidea remains in this locality (Osman et al., 2021). This may correspond to the warm isotopic stage TG5 (Vidal et al., 2002).

The sedimentary record of the Dahra Massif provides valuable insights into the geologic history of the Lower Chelif Basin from the Messinian to the Pliocene. The post-gypsum Messinian detrital deposits of the Ouled Slama series reflect intense erosion on the continent and are followed by the Trubi equivalent Pliocene marls or coralliferous marly limestones and sandstones, including coral constructions never described before. The sections of the Dahra Massif offer a comprehensive view of the detrital laminated deposits located between the Messinian gypsum and those of the Pliocene.

Lago Mare

Two successive steps, separated by a major discontinuity (Fig. 11), characterize this deposition. They correspond to two superimposed ostracod assemblages 1 and 2, respectively. The first assemblage includes Cyprideis and Loxoconcha mulleri, indicative of a brackish environment affected by episodic fluvio-lacustrine inputs. The second assemblage, characterized by Loxocorniculina djafarovi, suggests a fairly open shallow brackish environment becoming more brackish at the top where Cyprideis was abundant (Fig. 11). The assemblage 1 corresponds to the Lago Mare biofacies 1 (Grossi et al., 2015; Roveri et al., 2008), that we correlate with the LM1 (Clauzon et al., 2005; Popescu et al., 2015; Bache et al., 2012). The assemblage 2 is referred to the Lago Mare biofacies 2 (Grossi et al., 2011; Roveri et al., 2008), that we correlate with the LM3 (Clauzon et al., 2005; Bache et al., 2012; Do Couto et al., 2014; Popescu et al., 2015).

Messinian Erosional Surface and post-crisis marine reflooding

A hardground allows to subdivide the late Messinian post-gypsum sediments into a lower part including the ostracod assemblage 1 and an upper part showing the ostracod assemblage 2. This hardground marks of a major discontinuity, interpreted here as corresponding to the Messinian Erosional Surface (Fig. 11), previously evidenced in the Lower Chelif Basin (Osman et al., 2021) and so many times widely identified around the Mediterranean Basin (e.g., Clauzon et al., 1996, 2005, 2015; El Euch-El Koundi et al., 2009; Rubino et al., 2010).

The late Messinian deposits belonging to the ostracod assemblage 2, notably the detrital sedimentation with the successive occurrence of planktonic microorganisms (Ceratolithus acutus, Globorotalia margaritae, Reticulofenestra cisnerosii) document a marine incursion into the Lower Chelif Basin (Fig. 11). Accordingly, these deposits represent the marine reflooding of the Mediterranean Basin, which occurrend in the latest Messinian (e.g., Popescu et al., 2021).

Bioevents subsequent to the Lago Mare 3

Several bioevents were successively evidenced in the Dahra Massif, sealing the late Messinian Lago Mare 3 (Fig. 11). The biostratigraphic succession is comparable to that of the Sorbas Basin and other localities in the Mediterranean region where the marine reflooding has been robustly identified (Clauzon et al., 2015). Globorotalia margaritae, Ceratolithus acutus and C. rugosus indicating an early Zanclean age are followed by Globorotalia puncticulata with Discoaster asymmetricus (late Zanclean). Therefore, Globorotalia crotonensis, G. crassaformis or G. aemiliana and D. tamalis bioevents complete the Piacenzian Stage in the Lower Chelif Basin.

Coral constructions

Scleractinian bioconstructions (cf. Cladocora cf. caespitosa, Dendrophyllia sp.) are reported for the first time in the Dahra Massif in white marly limestones dated from the entire lower Zanclean. These bioconstructions testify to the existence, at that time, of warm enough conditions, therefore also favorable for Proboscidea (Osman et al., 2021). This relatively warm phase may correspond to the TG5 marine isotopic stage.

This study was performed within the framework of the doctoral training of 3rd Cycle “Geology of Marine and Continental Environments: Integrated Stratigraphy, Chronology and Dynamics of Paleoenvironments”. This work is carried out thanks to the support of the DGRSDT. It fits into the PRFU (E04N01UN310320200001) projects of the Ministry of Higher Education and Scientific Research. Professor E. Gliozzi is thanked for her help to authenticate our ostracod determinations and for making us aware of the ongoing revision of the Paratethyan ostracoda in progress. We acknowledge the anonymous reviewer and Dr. M.C. Melinte-Dobrinescu for their critics and suggestions, which allowed to significantly improving the mansucript. The local authorities of Mazouna and El Guettar were of great help to us in our numerous field trips. The determination of some brachiopod taxa was possible thanks to the assistance of Professors A. Ouali Mehadji and P. Moissette. The authors thank Dr. F.-Z. Bessedik for valuable assistance in the English language.

Cite this article as: Atik A, Mansouri MEH, Bessedik M, Osman MK, Belkebir L, Saint Martin J-P, Chaix C, Belkhir A, Gorini C, Belhadji A, Satour L.. 2024. New insights on the latest Messinian-to-Piacenzian stratigraphic series from the Dahra Massif (Lower Chelif Basin, Algeria): Lago Mare, reflooding and bio-events, BSGF - Earth Sciences Bulletin 195: 2.

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