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The Early Toarcian of the Moroccan Atlasic Domain (Middle Atlas and High Atlas central in particular): important period of geodynamic and sedimentary changes

Sadki D., Akasbi A.

Volumina Jurassica

2006

Vol. 4, no. 1

64-64

In the western Tethys, the Early Toarcian constitues one of the Jurassic stages marked by major palaeo-environmental and palaeo-oceanographic events. The changes could be linked to one of the most important and active phases of rift of Atlantic and Mesogee. In the Moroccan Atlasic chains (Middle Atlas and High Atlas in particular), these changes are responsible for important geodynamic, sedimentary and palaeogeographic events. From a geodynamic point of view: The Early Toarcian represents in Moroccan Atlasic chains, an intense distensive episode which corresponds to the peak of the break-up of the Liassic platform. This tectonic phase, which can be qualified as a "sudden rifting" is located in the Levisoni Zone that marked the formation of individual Atlasic basins, framed by residual platforms. The indices of this synsedimentary tectonic events are: lateral variations of thickness and facies, stratigraphic wedging, progressive unconformities, slumps, slides and resedimented blocks, synsedimentary normal faults, neptunians dykes. From a sedimentary point of view: This period corresponds to a change in sedimentary conditions: Sedimentation - becomes essentially marly, ending the carbonate episode of the Early and Middle Lias. These changes of depositional conditions are accompanied by sedimentary perturbations which are recorded as phases of erosion and non deposition in high zones (internal ridges and basin borders) or by allodapic deposits in depocenters as sandy turbidites ("Niveau brun" of Middle Atlas and turbiditic member of the Tagoudite Formation of the High Atlas). This turbiditic event of the Levisoni Zone is well known at a regional scale (Portugal) expressing the peak of the formation of individual basins within the western Tethys.