Microbialites and sponges are the most significant components of the Sinemurian mounds of the Foum Tillicht Range in the central High Atlas of Morocco. Although these spectacularly well-exposed mounds have been reported in literature since the sixties, their genesis and their palaeoenvironmental settings are poorly understood. Recent studies have demonstrated that the characterization of the micritic component (allochthonous vs autochthonous micrite) and its distribution in the mound architecture represent a fundamental tool in reconstructing mound geometries and palaeoenvironmental evolution during their deposition. In this study, three main depositional intervals of the Lower/Upper Sinemurian mounds of the Foum Tillicht locality are recognized and a depositional model is suggested based on the characterization of their micritic component. The proposed depositional model includes a variation of the palaeoenvironmental conditions during deposition of the mounds. The allochthonous micrite and bioclast-dominated small mounds (ca. 1 m high and 0.5 m wide), observed within the lower depositional interval, suggest a marine palaeoenvironment with well oxygenated marine shallow water and high hydrodynamic conditions as well as, cyclically alternating suboxic/anoxic phases. These small mounds developed during short phases of relatively deeper water conditions. The mound evolution toward larger skeletal-rich (sponge) microbialite mounds indicates an evolution from a relatively fluctuating sea level to a more stable deeper water depositional environment, thus allowing time for the formation of a massive microbialite mound frame. The abundance of peloidal and aphanitic micrite associated with sponge tissue suggests an organic (microbial) origin through heterotrophic activities within a stressed suboxic environment. The decrease of the allochthonous micrite and the intermound deposit abundance involves a reduction of sedimentation rate and water energy. All these findings suggest that the sponge microbialite mounds were deposited within a stressed suboxic to anoxic setting characterized by calm and deep water conditions.

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