The Middle Jurassic - Lower Cretaceous paleodrainage evolution of the Central High Atlas (Morocco): implications for the development of the Moroccan Atlasic System.

The High Atlas is an orogenic system resulting from the Cenozoic-Recent tectonic inversion of Triassic-Jurassic rift systems. Its present setting derives from a long and complex tectono-sedimentary evolution related to the Early Mesozoic opening of the Atlantic Ocean (pre-orogenic period) and, then, to the Cenozoic convergence between the African and European plates which led to a full tectonic inversion. The WSW-ENE trending chain is bounded by the North Atlas Fault to the north and the South Atlas Fault to the south, that represented the master faults of the rifted basins during the pre-orogenic period, then reactivated in inversion during the orogenic period. Early Jurassic syn-rift carbonate platforms, related to a marine ingression, were replaced in the Middle Jurassic-Late Cretaceous by post-rift fluvial and lacustrine environments. The related continental successions, regionally known as Couches Rouges, are not unanimously interpreted in the frame of the tectono-sedimentary evolution of the High Atlas. According to some authors they record localized early compressive-transpressive stages of deformation, others refer them to a period of tectonic quiescence. This study illustrates a revised stratigraphy, facies analyses and paleodrainage reconstruction of the Guettioua (Bathonian) and Jbel Sidal Formations (Barremian), that represents the fluvial units of the Couches Rouges, outcropping at the core of several syncline basins throughout the Central High Atlas. The aim is to understand if there was a tectonic forcing on the development of the Middle Jurassic – Lower Cretaceous fluvial systems. The sedimentological characters observed in the two fluvial formations suggest that they have been deposited by wide ephemeral fluvial systems, characterized by a high variable discharge. The reconstructed paleogeography, based on paleodrainage analysis, shows the fragmentation of the fluvial systems in several drainage basins, separated by local thresholds, uplifting during the sedimentation. Our reconstruction suggests that the post-rift fluvial systems were controlled by the presence of topographic highs in the area of the High Atlas of Marrakech, in the axial part of the chain and at its southern front, next to the South Atlas Fault. This conclusion, together with structural data and tectonic observations collected in some of the study areas, supports the idea that, from the Middle Jurassic, the Central High Atlas were affected by early compressive-transpressive stages of deformation.