Abstract

During Middle Liassic times (= Pliensbachian), the Jebel Bou Dahar (near Beni Tajjit, eastern High Atlas) was a carbonate platform where the "Calcaires du Bou Dahar" Formation has been accumulated. These limestones have been previously attributed to the Upper Pliensbachian (Domerian) on the supposed age of the large bivalves assemblages (mainly Opisoma, Cochlearites, Lithioperna). We have collected a Tropidoceras sp. in the bedded limestones covering the bioaccumulations with Cochlearites sp. and rare Opisoma menchikoffi. These new data indicate that these bivalve beds have been deposited from 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 breaking and of the sinking of the platform and it is illustrated by large fissures developed inside the shallow Pliensbachian carbonates which are filled up by Toarcian-Aalenian ammonites-bearing pink micrites. This palaeostructure is surrounded and sealed by marls and marly limestones. The Sebbab Sghrir profile indicates a succession of five foraminifera assemblages from the late Carixian to the early Bajocian. In the uppermost "Calcaires du Bou Dahar", we have recognized the equivalent of the C2 and D biozones of Septfontaine [1984, 1985]. During the early Bajocian, the succession of two microfaunas, 4 and 5, is displayed. Their boundary coincides with the limit between the Concavum and Discites ammonite zones. The protoglobigerinids appearance (FAD; 5b) is linked to the structural homogeneization of the region. The Bou Dahar shoal has been previously differentiated around a Palaeozoic relief. The structural pattern consisted of E-W faults crossed by transverse NW-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 palaeostructure was dissymetric. It sank progressively during a deepening episode which accelerated during the early Bajocian.

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