New insights in the latest Messinian to Piacenzian stratigraphic series from the Dahra Massif (Lower Chelif Basin, Algeria): Lago Mare, reflooding and bio-events

Recent geological studies conducted in the Dahra Massif located in the Lower Chelif Basin (specifically, the sections of Djebel El Abiod, Hgaf Tamda, Oued Tarhia and Sidi Brahim Telegraph) revealed the bio-sedimentary events that characterize the post-gypsum deposits (Messinian to Piacenzian periods). At Djebel El Abiod, for instance, variegated clays were deposited above the gypsum layer, which were later followed by alternating layers of sandy marls and sandstones (Chara cf. hispida), evolving towards (filling sequence) conglomerates with Pseudocatillus sp, inclined and fractured, whose upper surface is affected by a hardground. Above, this are marine green marls, ruby grey clays evolving to variegated clays, a sequence from which the detrital base is absent (erosion). This event (deformation, erosion) located between these sedimentary sequences, fits well with the Messinian Erosional Surface. This interpretation is supported by the palaeontological content of these deposits. The first sequence, prior to the MES, is marked by an assemblage of Cyprideis ostracodes associated with Loxoconcha muelleri, which can be linked to Lago Mare biofacies 1 ( Lago Mare 1 event, LM1); the second sequence, characterized by the assemblage with Loxocorniculina djafarovi, is linked to Lago Mare biofacies 2 (Lago Mare 3 event, LM3). The unconformable subsequent marine sedimentation (SM1, SM2), marks the reflooding ascribed to the latest Messinian; these sandy marls are sealed by an unconformity overlain by a conglomerate. In this locality, the Pliocene sedimentation begins with coralliferous white marly limestones (CWML). Unconformable marine marls follow the latter, with intercalations of sandstone bars. Simultaneously, white marls (Trubi facies) followed by sandy marls and sandstones alternation develop during the Pliocene (Sidi Brahim Telegraph: SBS). Ages have been established through the identification of several bioevents: Globorotalia margaritae and Reticulofenestra cisnerosii collected from the CWML with Neopycnodonte cochlear, indicating a lower Zanclean age. The succession of G. puncticulata and G. cf. crotonensis in the grey marls corresponds to the Zanclean and Piacenzian (Hgaf Tamda, Djebel El Abiod). Therefore, the deposits located between the gypsum (included) and the CWML are attributed to the latest Messinian. The underlying sandy marls (SM1, SM2) are related to the latest Messinian marine reflooding, preceding the Pliocene marine deposits. This bioevent succession is evident in the SBS section, which is of Messinian and Pliocene age defined by the calcareous nannoplankton. The CWML includes a coral construction (Cladocora cf. caespitosa and Dendrophyllia sp), dated between the presence of R. cisnerosii associated with Globorotalia margaritae at the base and Discoaster asymmetricus associated with G. puncticulata at the top. It indicates a relatively warm event (lower Pliocene: 3 Ma) that correlates to the TG5 stage of the oxygen isotope curves.