Abstract

The lithostratigraphic analysis of Toarcian and Aalenian deposits from central and northern Middle-Atlas shows a diversity in the facies and important variations of thickness which allow us to separate the depositional area into three palaeogeographic units: the marine basin and two boundary zones. The subsiding basin corresponds to a trough where have been accumulated-ammonite bearing marls which include, to the NE, a turbiditic intercalation. On the instable boundary to the NW, of this trough, are developed nodular facies assigned to "ammonitico-rosso" facies. The other boundary, to the SW, is characterized by bioclastic limestones and numerous stratigraphic gaps interrupting sedimentation. Detailed biostratigraphic and sedimentologic studies within sectors where series are thick and rich in ammonites allow to recognize several 3 d order depositional sequences interpreted as elements of a simple filling model of the Toarcian basin. Other complementary data on the Toarcian-Aalenian series forming the two boundary zones are integrated with the basin ones in order to reconstruct, in several phases, the geodynamic evolution of this segment of the South-Tethyan margin: (i) the Upper Domerian-Lower Toarcian interval is marked by geographically restricted deposits; (ii) the beginning of the Middle Toarcian corresponds to the maximum flooding of the deposits, particularly on the NW boundary; (iii) the upper part of Middle Toarcian, Upper Toarcian and Aalenian are the filling-period of the trough towards NE. Vertical and horizontal distribution of Toarcian and Aalenian facies, particularly induced by the regional tectonic framework (N040 degrees northern Middle Atlasic Fault and N110 degrees -130 degrees transverse faults), are nevertheless globally integrated within the corresponding 2 d order regressive-transgressive cycle.

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