Moroccan Central Atlantic Margin: Paleoenvironment reconstruction of a late syn-rift series (Berrechid sub-basin)

The Berrechid sub-basin contains records of the opening history of the Central Atlantic Margin (CAM) during the late Triassic-Early Jurassic. This syn-rift sub-basin encompasses (i) a Lower Salt-Mudstone Formation (LSM Fm), (ii) tholeiitic basalt flows related to the Central Atlantic Magmatic Province (CAMP), and (iii) an Upper Salt-Mudstone Formation (USM Fm). Significant tectonic, sedimentary, and climatic episodes have determined the depositional environment of the (USM Fm) which remains a matter of debate. We thoroughly investigate the sedimentological and mineralogical features of core materials, mine, and field outcrops covering the Hettangian evaporites, dated recently using palynological assemblage, and red beds of the Lower and Upper Members that constitute the (USM Fm). The following interpretations were based on the identified lithology, mineralogy, sedimentary structures, and textures. Particular consideration was also given to the lithostratigraphic variation along the sub-basin.

The Lower Member comprises a repetitive sequence of alternating primary bedded halite and syn-depositional displacive halite, whereas the Upper Member consists of bedded anhydrite/gypsum and siliciclastic mudstone. The bedded halite displays chevron and cumulate crystals, implying precipitation in shallow saline brines. The displacive halite encloses cubic crystals, randomly oriented in mudstone, suggesting the deposition in a wet saline mudflat. The siliciclastic mudstone associated with the bedded anhydrite/gypsum has various sedimentary aspects, characteristic of a subaerial dry mudflat environment. The distinct diagenetic features recognizable throughout the (USM Fm) include grey reduction spots and dissolution pipes filled with blocky clear halite cement. All these lithologies have registered periods of flooding, evapoconcentration, and desiccation, suggesting deposition in an arid continental setting. The absence of distinctive marine lithofacies and the lack of carbonates are additional evidence for our inference.

Both Lower and Upper Members are affected by a network of NNE-SSW to NE-SW normal faults. They show a varying thickness along the cores and outcrops, indicating the syn-sedimentary tectonic character of the studied Formation during the Early Jurassic time. The lateral migration of the paleoenvironments mentioned above is hence mainly controlled by the sub-basin’s architecture as half-graben jointly with the ongoing subsidence and sediments supply.

These interpretations of the USM Fm’s paleoenvironment highlight the continental context of the series during the Early Jurassic time. These results provide new insights on the paleogeography of the late syn-rift phase of the Moroccan Central Atlantic Margin.