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The Lower Toarcian of the High-Atlas, Morocco (Todrha-Dades area):stratigraphy and sedimentation

Ettaki M., Chellai E.

Volumina Jurassica

2006

Vol. 4, no. 1

160-160

The Toarcian represents an important stage in the evolution of the Atlasic domain develped during the rifting on the West Tethyan margin during the Early Jurassic. In the Todrha-Dades area, the ammonites are apparently absent in deposits of the Tagoudite Formation, but a rich fauna of foraminifera and ostracods is meet in the marls and marly sandstones particularly in the lower member of the formation. The micropaleontological study indicates the Early Toarcian age (Polymorphum-Levisoni zones) for the Tagoudite Formation. In the Todrha-Dades area, the Lower Toarcian deposits increase in thickness from SW to NE. Towards SW, the sedimentation is characterized by large terrigenous sediment supply. In the NE direction the marly-sandstone succession of the Tagoudite Formation was deposited in a deep basin (umbilicus) created by a block tilting unfavourable for development of the microfauna. The impact of the tectonics during the Early Toarcian (Polymorphum Zone) is recorded by normal faults and plastic distortions.Tectonic structure analysis shows that the sedimentation took place in a transtensional tectonic regime. In general, the Todrha-Dades area is characterized during the Early Toarcian by the lateral passage between the different domains of sedimentation. The change in the sedimentation conditions took place after the Synpolymorphum Crisis. This change, accentuated by distensive tectonic, begins in the Early Toarcian and it is shown by sedimentary hiatus occurring in the platform and the seamounts ridges (individualized already during Pliensbachian). The neptunian dykes in Jbel Akenzoud and Tarhia n’Dades are associated with this level. Afterwards the detritical sedimentation occurred before the Middle Toarcian. Sedimentologic and paleontologic analysis prove that the Tagoudite Formation has been deposited in a relatively deep marine area. It marks a radical sedimentary change in the whole of the central High-Atlas domain after the Polymorphum Crisis during the Early Toarcian. The distensive tectonic was responsible for isolation of the basin and led to the development of anoxic conditions that controled the replacements of the fauna.