Characteristics and significance of brachiopod major shell beds, an event in the Devonian evolution of the Saoua (northwestern Sahara, Algeria)

In the Saoura, the brachiopod shell beds, so-called niveau coralligene, correspond to a major shell deposit dated to the Late Emsian. Brachiopods and crinoids dominate the benthic assemblage that contains also corals, bryozoans, trilobites, goniatites, and orthocones. This major level has a large geographic distribution and it is characterized by a wide brachiopod diversity due to time-averaging, taphonomic feedback and alternate bottom conditions changing from soft to shelly and firm. This kind of brachiopod association is linked to a transgressive onlap system. At regional extent, we can correlate this major shell bed to similar shell deposits from the Ahnet-Mouydir, Tindouf, and Zemmour areas. It indicates an important transgressive event underlined by change in the sedimentation from detritic deposits to carbonate sediments. (C) 2004 Academie des sciences. Publie par Elsevier SAS. Tons droits reserves.