Sedimentary dynamics and palaeotectonic structuring during the Toarcian–Aalenian in the southwestern Middle Atlas (Morocco)

• A lot of data about Liassic and Dogger series of the SW Middle Atlas, Morocco.
• A stratigraphic, palaeontological, sedimentological and tectonic observations.
• An original geometry and evolution of the corresponding marine basins.
• The relationship between the Tethyan and Atlantic influences.

The southwestern Middle Atlas (Morocco), which has long been regarded as a southern extension of the Tabular Middle Atlas (Ain Leuh Causse), actually consists of a broad syncline and narrow anticlinal ridges that were folded between the Liassic and Dogger epochs. Using lithostratigraphic, biostratigraphic and structural analysis, we draw the geometry of structures during the Late Liassic–Early Dogger epoch and reconstruct the various stages of their evolution. This area, which has been considered a coastal border of the tethyan atlasic trough, corresponds to a distinct basin composed of a mosaic of sub-basins tilting to the west (i.e., toward the Atlantic). These sub-basins are attributable to a double-structuring: (1) SW–NE superimposed on existing anticlinal ridges and (2) submeridian which original direction SSW–NNE seems to indicate an influence of the Atlantic coast. Tectonic segmentation preceded the Early Toarcian anoxic crisis, and tectono-eustatic interactions continued during the Late Liassic and Early Dogger epochs. Sub-basins subsequently functioned alternately as depressions in the early Late Toarcian and early- and -middle Aalenian epochs or as depocentres in the Late Toarcian and Late Aalenian epochs. Tectonics and eustasy caused a palaeobiological subdivision during the Early and Middle Aalenian epochs, as indicated by significant SE–NW segregation between ammonite faunas of Tethyan and northwestern European origin. This study indicates that the original palaeogeographic position of the southwestern Middle Atlas was at the crossroads of the Tethyan and Atlantic margins.