Jurassic rifting to post‐rift subsidence analysis in the Central High Atlas and its relation to salt diapirism

Abstract

The subsidence evolution of the Tethyan Moroccan Atlas Basin, presently inverted as the Central High Atlas, is characterized by an Early Jurassic rifting episode, synchronous with salt diapirism of the Triassic evaporite-bearing rocks. Two contrasting regions of the rift basinwith and without salt diapirismare examined to assess the effect of salt tectonics in the evolution of subsidence patterns and stratigraphy. The Djebel Bou Dahar platform to basin system, located in the southern margin of the Atlas Basin, shows a Lower Jurassic record of normal faulting and lacks any evidence of salt diapirism. In contrast, the Tazoult ridge and adjacent Amezra€ ı basin, located in the centre of the Atlas Basin, reveals spectacular Early Jurassic diapirism. In addition, we analyse alternative Central High Atlas post-Middle Jurassic geohistories based on new thermal and burial models (GEN-EX â 4.0.3 software), constrained by new vitrinite reflectance data from the Amezra€ ı basin. The comparison of the new subsidence curves from the studied areas with published subsidence curves from the Moroccan Atlas, the Saharan Atlas (Algeria) and Tunisian Atlas show that fast subsidence peaks were diachronous along the strike, being younger towards the east from Early-Middle Jurassic to Late Cretaceous. This analysis also evidences a close relationship between these high subsidence rate episodes and salt diapirism.

Diapiric growth within an Early Jurassic rift basin: The Tazoult salt wall (central High Atlas, Morocco)

“…Therefore, the syndepositional deformation and facies change observed within the Pliensbachian to Bajocian flanking sediments are attributed to the rise of the evaporite‐bearing Upper Triassic rocks (Figures and ). The case of the Tazoult Ridge constitutes a well‐exposed example of the widespread synrift diapirism that took place in the central High Atlas during the Lower Jurassic [ Saura et al, ; Moragas et al, ].…”

“…Diapirism of the Upper Triassic evaporite‐bearing deposits is thought to have been triggered by basement normal faulting related to the extensional‐transtensional tectonic regime dominating the central High Atlas rift basin between Late Triassic and Early Jurassic times [ Bouchouata, ; McClay et al, ; Saura et al, ; Moragas et al, ] (Figure ). A similar association between extension and diapirism has been extensively reported in the literature based on outcrop data, seismics, and analog modeling [e.g., Mart and Ross, ; Koyi, ; Vendeville and Jackson, ; Koyi et al, ; Dooley et al, ; Hudec and Jackson, ; Burliga et al, ].…”

“…RSM and field mapping have been supported by the mapping of stratal relationships onto photopanoramas and field maps. Field work has included the collection of samples for facies, biostratigrahy, radiometric dating (Sr‐isotopy), and vitrinite reflectance work and basin modeling [i.e., Moragas et al, ].…”

“…The western High Atlas was part of the Atlantic Jurassic passive margin and is separated from the central and eastern High Atlas by the Paleozoic Massif, an area of pre‐Mesozoic basement exposures located south of Marrakesh [ Brechbühler et al, ] (Figure a). The central and eastern High Atlas basin developed as a multiphase rift system opened toward the east by two major rift episodes occurring during Middle‐Late Triassic and Early Jurassic (late Sinemurian and Pliensbachian) [ Ellouz et al, ; Frizon de Lamotte et al, ; Wilmsen and Neuweiler, ; Lachkar et al, ; Moragas et al, ] (Figure ). At present, the central High Atlas is characterized by a folded domain that is dominated by NE‐SW striking structural highs or ridges separating elongated and wide synclines filled by Early and Middle Jurassic sediments [ Poisson et al, ; Teixell et al, ; Frizon de Lamotte et al, ; Michard et al, ].…”

“…Chart showing the age of the sedimentary record in the Tazoult diapir wall (red line) and most important geodynamic processes occurring in the central High Atlas [Based on Saura et al, ]. Data compiled from (1) Marzoli et al [], (2) Ellouz et al [], (3) Michard et al [], (4) Baudon et al [] and Domènech et al [], (5) Moragas et al [], (6) Charrière et al [], and (7) Saura et al []. Absolute ages following Gradstein et al [].…”