Toarcian and Aalenian series from the folded northern part of Middle Atlas(Morocco): diversity of the depositional processes and reconstruction of the palaeogeographic setting

The lithostratigraphic analysis of Toarcian and Aalenian deposits from cent ral and northern Middle-Atlas shows a diversity in the facies and important variations of thickness which allow us to separate the depositional area i nto three palaeogeographic units: the marine basin and two boundary zones. The subsiding basin corresponds to a trough where have been accumulated-amm onite bearing marls which include, to the NE, a turbiditic intercalation. O n the instable boundary to the NW of this trough, are developed nodular fac ies 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 s eries are thick and rich in ammonites allow to recognize several 3(d) order depositional sequences interpreted as elements of a simple filling model o f the Toarcian basin. Other complementary data on the Toarcian-Aalenian ser ies forming the two boundary zones are integrated with the basin ones in or der to reconstruct, in several phases, the geodynamic evolution of this seg ment of the South-Tethyan margin : (i) the Upper Domerian-Lower Toarcian in terval is marked by geographically restricted deposits; (ii) the beginning of the Middle Toarcian corresponds to the maximum flooding of the deposits, particulary on the NW boundary; (iii) the upper part of Middle Toarcian, U pper Toarcian and Aalenian are the filling-period of the trough towards NE. Vertical and horizontal distribution of Toarcian and Aalenian facies, part icularly induced by the regional tectonic framework (N040 degrees northern Middle Atlasic Fault and N110 degrees 130 degrees transvers faults), are ne vertheless globally integrated within the corresponding 2(d) order regressi ve-transgressive cycle.