Triassic sedimentation on the Eastern Atlantic margin: two examples from Moroccan Meseta and Portugal

The continental deposits of the Triassic basins developed along the eastern margin of the Central and North Atlantic show a similar sedimentological evolution, as those of the western margin resulting from the interaction of various processes.

The examples chosen in this work are those of the Mohammedia-Benslimane-ElGara-Berrechid basin MBEB in the Moroccan meseta that we studied in detail in the field, and that we tried to compare with Portugal which is on the same East Atlantic margin.

At the begininig of the Mesozoic, the northwestern part of the African continent was affected by an initial fracturing associated with the early stages of the opening of the Central Atlantic (Atlantic rift) during which several Moroccan Triassic basins are open.

The Mohammedia-Benslimane-ElGara-Berrechid basin is part of the Moroccan western Triassic province, which corresponds to all the basins of the Moroccan Atlantic margin in direct relation with the Atlantic rift. In this basin, an asymmetric rift is set up on the old Hercynian structures during the Carnien-Norien, the paroxysm is reached at the Trias-Lias passage with the installation of basalts (CAMP: Central Atlantique Magmatic Province).

During rifting (syn-rift stage in the Upper Triassic), the MBEB basin experienced three major phases of sediment filling. The first phase is purely continental, the first deposits to arrive in the opening basin are of proximal fluvial origin. Subsequently, the decrease of the paleopente and the rise of the base level generated paleoenvironmental changes in the basin (2nd phase), and the deposition system evolved towards distal environments. During the third phase, the syn-rift sedimentary series recorded a marine incursion in the late Triassic with saliferous sedimentation. This marine intervention is deduced from the presence of a thick saliferous series with a large lateral extension and whose isotopic ratios of sulfur and bromine contents indicate their marine origin. These marine waters are probably of Tethysian origin and are also linked to the opening of the Proto-Atlantic.

In Portugal, the Upper Triassic is represented by two formations in the north of the Lusitanian basin (Palain, 1976): Silves Fm which is fluvial sandstone and Dagorda Fm which includes first dolomites and then evaporites. In this Portuguese basin, the proximal-distal fluvial transition took place at the Norien-Rhétien limit. This also rift-type basin was filled with continental fluvial and alluvial clastic rocks of the Silves Formation, largely derived from the adjacent Iberian highlands of the Meseta. Locally, black shales are present at the top of the Silves and may represent the first marine incursion into the basin.

The comparison between the two basins shows that they followed a similar evolution at the base and in the middle of the series but at the top the MBEB basin presented thick layers of evaporites while that of Portugal presented mainly dolomites attributed to paralic facies.

Palain, C., 1976. Une série détritique terrigene; 'les grès de silves'; Trias et Lias inférieur du Portugal. Mem. Serv. Geol. Portugal, p. 25 (377 pp.)