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

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 etudes geologiques menees sur le massif du Dahra ont revele des changements sedimentaires et des evenements paleobiologiques caracterisant les sediments detritiques post-gypse (du Messinien au Plaisancien), suivis par des marnes ou des calcaires marneux blancs pliocenes equivalents au facies Trubi.Structures en deux etapes superposees, les depots du Messinien terminal ont livre deux assemblages d'ostracodes. Le premier indique un environnement saumatre et le second un environnement saumatre peu profond assez ouvert. D'apres leur contenu en ostracodes, l'assemblage 1 (Cyprideis, Loxoconcha muelleri) correspond au Lago Mare biofacies 1 de l'avant fosse apenninique, qui est correle avec l'episode Lago Mare 1 date entre 5,64 et 5,60 Ma. L'assemblage 2 (Loxocorniculina djafarovi) est rapporte au Lago Mare biofacies 2 mis en evidence dans la meme region. Il est correle avec l'episode Lago Mare 3, date entre 5,46 et 5,33 Ma.De plus, la succession sedimentaire est affectee par une discontinuite majeure materialisee par une surface rubefiee (hardground), separant l'etape 1 de l'etape 2, correspondant respectivement aux assemblages d'ostracodes 1 et 2. Cette discontinuite est consideree comme l'equivalent de la Surface d'Erosion Messinienne, deja reconnue dans la region et abondamment decrite tout autour du bassin mediterraneen.Ces depots du Messinien terminal et leur assemblage d'ostracodes 2, notamment la sedimentation detritique avec Ceratolithus acutus, Globorotalia margaritae, Reticulofenestra cisnerosii documentent une incursion marine dans le bassin du Bas Chelif, correspondant a la remise en eau marine du bassin mediterraneen au Messinien terminal, avant l'apparition de Reticulofenestra cisnerosii indiquant la base du Zancleen.Ainsi, les bioevenements mis en evidence dans le massif du Dahra permettent-ils de renforcer l'existence du Lago Mare 3 d'age Messinien terminal et de la remise en eau marine de la Mediterranee anterieurement au debut du Pliocene.Les depots sus-jacents sont caracterises par des constructions coralliennes (cf. Cladocora cf. caespitosa, Dendrophyllia sp) jamais decrites auparavant et couvrant l'ensemble du Zancleen inferieur. Elles temoignent de l'existence, a cette epoque, de conditions suffisamment chaudes, pouvant correspondre au stade isotopique TG5.