Le passage Cénomanien–Turonien dans le Nord-Ouest algérien (coupe de Douar Menkouchi, Ouarsenis) : biostratigraphie des radiolaires

Abstract

The Ouarsenis Massif belongs to the Algerian Tell domain which is considered as the eastern part of the Maghrebian Tethys former margin. The Cenomanian-Turonian Douar Menkouchi section outcropping in the eastern side of the Ouarsenis massif is composed of marls and laminated limestone alternations, including massive limestone and a biosiliceous bed toward the upper part of the succession. It reveals the presence of well-preserved and diversified radiolarians. In this study, we have identified for the first time in the Algerian domain, age-diagnostic radiolarian assemblages characterizing the Cenomanian-Turonian transition (C/T). The organic-rich laminated beds (black shales) confined to Silvae Zone, Cenomanian in age and underlying a biosiliceous interval, have provided radiolarian assemblages dominated by Pseudodictyomitrae. The successions overlying the biosiliceous interval have provided diversified radiolarian assemblages including Hagiastridae (i,e. Crucella cachensis, Alievium cf. superbum) and abundant Dictymitrae taxa (i,e. Dictyomitra cf. napaensis and Dictyomitra undata) indicating early Turonian age. These marl-limestone alternations are assigned to the Alievium superbum Zone. Considering taphonomic and diagenetic implications on assemblages content, we conclude that organic-rich beds and biosiliceous interval outline the latest Cenomanian. The radiolarian assemblage composition exhibits similarities to microfauna recovered from Bonarelli Horizon in Central Italy. Morphological similarities are also deduced from assemblages containing Pseudodictyomitrae tiara morphotypes (synonym of P. nakasekoi) which resemble those recovered from ophiolitic intervals in Cyprus and Oman areas.