An exceptional facies in the Atlas domain: the Toarcian Ammonitico-rosso of Oued Zraa (Central Middle Atlas, Morocco)

The atlasic troughs, the places of Jurassic deposits of the Middle Atlas and central and eastern High Atlas, are South Tethyan diverticulums. In Oued Zraa area, located at the junction between the Middle Atlas trough and the... [ view full abstract ]

The atlasic troughs, the places of Jurassic deposits of the Middle Atlas and central and eastern High Atlas, are South Tethyan diverticulums. In Oued Zraa area, located at the junction between the Middle Atlas trough and the northwest platform, meets a rare deposit in the atlasic domain: an ammonitico-rosso dated of Toarcian.

Sedimentological analysis and detailed stratigraphic studies (El Hammichi et al., submitted) revealed the presence of a condensed series extended from lower Toarcian (Polymorphum zone) to middle Toarcian with the diverses horizons (Sublevisoni, Lusitanicum, Bifrons and Semipolitum) of the Bifrons zone. The fauna is abundant and quite unbalanced, dominated by platycones (Hildoceras). They are mainly represented by small ammonites (4 to 5 cm) generally poorly preserved: crushed, abraded, fragmentary and distorted that indicate an unstable depositional environment associated with the isolation of ammonoids communities.

A thin calcareous ammonitico-rosso passes laterally towards the E to a marly ammonitico-rosso slightly dilated and affected by different synsedimentary deformations that reflect a slope in the opposite direction of the marine basin. All Ammonitico-rosso are covered by pelagic limestone called "Calcaires à filaments" associated laterally to yellow marls which constitute a transition facies with black marls of marine subsiding basin.

The Ammonitico-rosso and the attendant facies are dependent of an original tectono-sedimentary compartmentation with double structuring (see figure): directional (North Middle Atlasic Fault, N40 °E) and transverse (Oued Zraa Fault, N100-120 ° E). This original palaeogeographical framework corresponding to an apex of block raised and intensely faulted, explains that the Oued Zraa ammonitico-rosso could be the site of an ecological and geographic isolation on the edge of the outer platform, while undergoing the influences of the surrounding marine basin.

The Oued Zraa site and other type of ammonitico rosso deposits located to the NE mark out the palaeogeographic and structural boundary between the stable platform and the subsiding Middle Atlasic trough. The Oued Zraa ammonitico-rosso is related to the real Toarcian ammonitico-rosso marly-calcareous and argillaceous that are well developed in the eastern Rif foreland (Béni Snassen) and western Algeria (Traras, Tiaret ) in neighbouring palaeogeographic contexts.

Reference: El HAMMICHI F., BENSHILI Kh., El ARABI H., TABYAOUI H. & CHARRIERE A. (submitted) L’ammonitico-rosso toarcien de l’Oued Zraa (bordure NW du Moyen-Atlas plissé, Maroc) : études biostratigraphiques, sédimentologiques et cadre paléogéographique. Rev. Paléobiogéographie Genève.