Tectono-sedimentary events and geodynamic evolution of the Northern Middle Atlas Miocene basins (Morocco): impact on the NW-SE Atlantic transgression

Abstract

This paper aims to identify and map the Miocene series in the Northern Middle Atlas. This is to enhance our knowledge about its spatial-temporal variation. Field investigations mainly based on microtectonic measurements combined with structural analyses, lithostratigraphic studies and stress tensor inversion have allowed to reconstruct the genesis and geodynamic evolution of the Northern Middle Atlas Miocene basins. The lithostratigraphic analysis of the Neogene series unconformably deposed on the Jurassic formations shows a transgressive depositional evolution. Several stratigraphic levels have been identified; successively: (i) a conglomerate capped by Vallesian volcanic-sedimentary, lacustrine and palustrine continental series, (ii) Upper Tortonian sandstones, fluvio-deltaic conglomerates and sandy marls with channeled sandstone, (iii) Upper Tortonian-Messinian molasse, pink calcarenite, reef limestone, ochre sandy silt and blue marl. These formations are unconformably overlain by the Skoura puddingstones assigned to the Lower-Middle Pliocene. These deposits are synchronous with a major brittle tectonic phase divided into three episodes: (i) N120 to N140 Vallesian compression (N30 to N50 horizontal σ3 axis), (ii) Upper Tortonian – Messinian N45 extension (σ1 vertical). The history of the Middle Atlas during the Upper Miocene shows an opening megasequence which begins with continental sedimentation during the Upper Vallesian-Tortonian period evolving towards marine sedimentation associated to the NW-SE Atlantic transgression during the Upper Tortonian.