Terrigenous successions of the late Triassic-early Jurassic in the High Moulouya (Morocco): stratigraphy and geodynamic context

The Triassic-Jurassic basins of the High Moulouya area record the evolution of the Atlasic rifting in the SW-NE direction, showing a syntectonic continental sedimentation and an active volcanism. Such mobility has led to the differentiation of asymmetric half basins, whose gradual subsidence took place in successive stages. These basins, particularly during the deposition of the terrigenous successions of the base of the Jurassic (early Liassic age), show a typical syntectonic continental infill. It consists of a mixed sedimentation, detrital and volcanoclastic, generally coarse-grained towards the base. Coarser deposits, fed by volcanic clasts, are structured as sequences of spreading river cones, alternating with irregular clay facies of flood plain. They reflect a rejuvenation of the relief, following a reactivation of the faults. The sedimentary infill was provided by intermittent flows with considerable discharge, forming deposits along fault escarpments. The top of the terrigenous series shows a dominance of thin detritic sediments and a lack of volcanoclastics, reflecting the peneplanation of the relief and the creation of a flood plain. It records the widespread homogenization of the prior relief, causing the disappearance of the pre-existing structures during a period of tectonic quiescence. The volcanoclastics, witnessing an explosive volcanism and a concomitant sedimentation, are clustered in the tectonically active regions, where they are thicker and diversified. This late volcanism occurred during the final phases of rifting, owing to the replay of major Hercynian and tardi-hercynian faults. The Triassic-Jurassic period is thus marked by an extensional dynamics controlled by the influence of the first stages of the opening of the central Atlantic and the western Tethys.