Age and dynamic context of the multicostate zeilleriids limestones of the Grand Pic de l'Ouarsenis (Algiers-Oran tell, western Algeria).

The vertical range of the brachiopods in the Ouarsenis depends closely on t he sedimentary: evolution and on the local geodynamic controls. During the Late Sinemurian and the Early Carixian. several Zeilleriid coquinas have ac cumulated on a shallow carbonate platform. These brachiopods are well level led in the biochronologic scale and their age is confirmed by the discovery of an ammonite (Gemmellarocerns sp.). Aborted reefs were built laterally f ollowing tectonic movements. The sedimentary evolution has happened during a relative sea-level rise. The brachiopods appeared at the beginning and di sappeared when the filling progressed. A forced regression occurred at the end of the Early Carixian. It is marked by incisions, emersions and karstif ications. Large bivalve limestones (Cochlearites, Opisoma) have been deposi ted later. Their local chronological position ranges from the Middle Carixi an to the Late Carixian (Demonense and Dilectum Zones). (C) 2000 Academic d es sciences / Editions scientifiques et medicales Elsevier SAS.