Etude paléontologique et dynamique d'un intervalle condensé à gastropodes dans les calcaires à grands bivalves du Pliensbachien (Ouarsenis, Algérie)

The large bivalve limestones of the Grand Pic de l'Ouarsenis have yielded a gastropod fauna attributed to the Middle Carixian (Lower Pliensbachian) by Metaderoceras sp. and to the Cl biozone of the benthic foraminifer chart. Its palaeontological study allows to recognize and to describe eight species belonging to seven different superfamilies, despite their poor state of preservation. Some taxa are known on both sides of the Tethys (northwestern and maghrebian margins). The environmental setting of the Ouarsenis fauna is mainly controlled by an insular palaeotopography and by autocyciclic processes (tectonic and hydrodynamism). The deposit of the gastropod fauna has occurred during the transgressive phase of a 3rd order sequence (condensed interval sensu Vail et al., 1991). These events, occurring during the Late Carixian times, are well documented along the south-tellian border. The eustatic deepening is complicated by the local tilted-blocks tectonics. In these conditions, the gastropods level has been deposited in a weak sedimentation rate setting and its environment is located to the upper part of the photic zone and subjected to low hydrodynamic conditions. The marine waters were warm, calm, with a normal salinity.