Cenomanian-Turonian transition in Ksour Mountains (West Saharan Atlas, Algeria): Biostratigraphy, Geochemistry and deposit environments.

In the Ksour Mountains (Western Saharan Atlas), the Cenomanian-Turonian transition has been studied stratigraphically and geochemically from two cross-sections: Djebel M'daouer and Chebket Tamednaia. In these two sections, as on most of the shallow carbonate platforms of the South-Tethyan domain, the anoxic facies of the Cenomanian-Turonian boundary materialized by "black-shales" deposits are absent. In addition, the missing biostratigraphic data makes it difficult to establish a detailed biozonation for these two studied sections. The only two levels with rare ammonites of the Ghoundjaia Formation at the M'daouer section allow to characterize the Vibrayeanus, Gamai and Cauvini zones. So, for both sections, it seems useful to apply an isotopic study which showed a positive excursion zone of the delta C-13 lying along the Lower Member and the lower part of the Middle Member Ghoundjaia Formation. This anomaly zone is marked by three well-defined and characteristics peaks of the Cenomanian-Turonian transition and the boundary between these two stages stands between peaks 2 and 3 coincident with the transition from Gamai to Cauvini zones. The recording of these three peaks allowed the local correlation with the Tamednaia section where fauna of biostratigraphic value is missing.