Similar to the Miocene, the Pliocene of the outcrops of Bas Chelif Basin delivered abundant and much diversified fossiliferous levels of malacofauna (bivalvia). The paleontological study of two typical Pliocene series, in the center (Sidi Brahim) and at the northwestern margin of the Bas Chelif Basin (Sassel Beach), permit the identification of 4 orders, 8 families, 15 genera, and 20 species among the bivalvia. The simultaneous study of autoecological and taphonomic features of the fauna contained in the Sidi Brahim geological section allowed distinguishing at once four fauna associations, in which each one characterizes a particular ecozone which is attributed to an environment: (1) the muddy deep environment association (Cristatopecten cristatum , Pelecyroa brochii , and Tellina donacina ); (2) the muddy instable and frankly deep environment association (Anadara diluvii, C. cristatum, P. brochii, and T. donacina); (3) the association of the infralittoral environments, rich in sandy supplies; and (4) the association of the upper infralittoral sandy environment and three shell assemblages-the "3D census assemblage," "3D multihabitat time-averaged assemblage," and "3D allochthon assemblage"-where their spatiotemporal enchainment allowed making obvious four phases of transgressive-regressive oscillation in the general transgressive tendency of Sidi Brahim section, during the lower and average Pliocene. With a reduced thickness, the Sassel Beach section is characterized, on one hand, by the setup of two fauna associations of the infralittoral sandy environments (Pecten benedictus and Ostrea lamellosa) and infralittoral muddy environments (Aequipecten opercularis , Aequipecten seniensis, and P. benedictus), and of other parts by the piling up of three types of assemblages (3D allochthonous assemblage, 3D census assemblage, and 3D multihabitat time-averaged assemblage), defining three phases, comparatively less pronounced of the transgressive cycle of the Pliocene.
