Biostratigraphic and environmental data on the history of Jebel Bou Dahar (eastern High Atlas, Morocco) palaeo-shoal during the early and middle Jurassic

During Middle Liassic times (= Pliensbachian), the Jebel Bou Dahar (near Be ni Tajjit, eastern High Atlas) was a carbonate platform where the " Calcair es du Bou Dahar " Formation has been accumulated. These limestones have bee n previously attributed to the Upper Pliensbachian (Domerian) on the suppos ed age of the large bivalves assemblages (mainly Opisoma, Cochlearites, Lit hioperna). We have collected a Tropidoceras sp. in the bedded limestones co vering the bioaccumulations with Cochlearites sp. and rare Opisoma menchiko ffi. These new data indicate that these bivalve beds have been deposited fr om the Demonense Zone upward. The lack of ammonites prevents a more precise dating of the top of this formation. On the Bou Dahar plateau (top of the palaeo-shoal), it is overlain by condensed beds yielding Upper Toarcian and Aalenian ammonites. This change in the environment resulted from the break ing and of the sinking of the platform and it is illustrated by large fissu res developed inside the shallow Pliensbachian carbonates which are filled up by Toarcian-Aalenian ammonites-bearing pink micrites. This palaeostructu re is surrounded and sealed by marls and marry limestones. The Sebbab Sghri r profile indicates a succession of five foraminifera assemblages from the late Carixian to the early Bajocian. In the uppermost "Calcaires du Bou Dah ar", we have recognized the equivalent of the C2 and D biozones of Septfont aine [1984, 1985]. During the early Bajocian, the succession of two microfa unas, 4 and 5, is displayed. Their boundary coincides with the limit betwee n the Concavum and Discites ammonite zones. The protoglobigerinids appearan ce (FAD; 5b) is linked to the structural homogeneization of the region. The Bou Dahar shoal has been previously differentiated around a Palaeozoic rel ief. The structural pattern consisted of E-W faults crossed by transverse N W-SE or SSW-NNE faults as it is documented by the eastern and western rapid change from the shallow platform to the open sea domain. The Toarcian and Aalenian marls onlap the shoal to the west, indicating that the palaeostruc ture was dissymetric. It sank progressively during a deepening episode whic h accelerated during the early Bajocian.