Triassic stratigraphy and structural evolution of a rift basin: the Eç Çour basin, High atlas of Marrakech, Morocco

Abstract

Stratigraphic and structural analysis of the Triassic sedimentary basins of the High Atlas of Marrakech, in particular of the Eç Çour basin, indicates that the Proterozoic–Paleozoic block (“Massif Ancien”) formed a paleogeographic high during this period, with the basement block acting as an east–west trending horst, separating two large half-grabens to the north and to the south. The marginal half-grabens, which were filled by red beds essentially of continental origin, were most likely linked via transverse pathways. In the Eç Çour basin, the Triassic sedimentary deposits are subdivided into three formations of which the first two are continental and the third shows marine influences. Structurally, this area is characterized by two main (ENE–WSW and NNE–SSW) fault directions. The ENE–WSW trending faults, which include the major South Atlas fault, have an extended history of displacement during pre-Triassic Hercynian to Late Hercynian and during Triassic times. In contrast, the NNE–SSW-striking faults appear to have only a syndepositional Triassic history with, rarely, a late Hercynian heritage. This structural pattern is the result of a pure extensional regime during which synsedimentary normal faulting caused the assymmetry of the basin. This was reflected by high subsidence rates in the NE that progressively decrease to the NW towards the rising Proterozoic–Paleozoic massif of the High Atlas. During the tertiary Alpine–Atlasic orogeny, the ENE–WSW-striking normal faults were inverted.