Liassic geodynamic evolution of the transect Midelt-Errachidia (Central High Atlas, Morocco)

This work presents the data gathered through a stratigraphic, sedimentologic and geodynamic study of the Liassic formations cropping out along the Midelt-Errachidia transect. This area belongs to the eastern part of the Moroccan Central High Atlas. This work was inspired to the following steps: - to recognize the lithostratigraphic succession from the analysis of the different selected field sections; - to define a local biostratigraphy by using ammonites, foraminifera and algae to establish the age of the sedimentary units within each section; - to characterize through the facies analysis, the sedimentary environments during the Liassic interval of time; The purpose of this work was to propose correlation diagrams for the corresponding formations and draw, from Early Sinemurian to Early Toarcian, palaeogeographic maps showing the Liassic historical evolution of this Atlasic area. During the Sinemurian, the regional geodynamic processes involved the growth of microbialitic sponge-bearing micritic mounds. These build-ups that will be here detailed represent a key-step within the development of the studied atlasic basin. The collected data and their interpretation lead us to propose a geodynamic evolution diagram involving main steps that will be illustrate and their correlation with a second order transgression-regression cycles. The end of the different regressive periods corresponds to a general structuring phase with a related geometrical change in the shape of the basin. In this frame, two main structuring steps have been distinguished: - the first step, corresponding to the unconformity D2 at the transition between Early and Upper Sinemurian, is the responsible of the breakdown of the Early Liassic platform, and leads to the creation of a central subsiding trough. The Sinemurian carbonates mounds are closely linked to this fracturing event. - the second step corresponding to the unconformity D11 at the base of the Domerian Spinatum zone, occurs on the edges of the trough, and indicates a centrifugal migration of the fracturing wave mainly towards the South. This step induces the creation of the asymmetrical southward collapsed basin. The three transgressive periods are indicative of subsiding and maximum opening conditions of the sedimentary environments. They are capped by the intra Sinemurian D1 unconformity, the Middle Carixian D10 unconformity and the D16 unconformity, at the base of the Toarcian Bifrons zone. The deposits related to the last maximum transgression suggest that the beginning of the sedimentary homogenization process within the whole basin (central trough and lateral platforms) moving to the following Aalenian-Bajocian complete infilling.