Lower Jurassic automicrite-sponge mounds and coral-sponge reefs in the Djebel Bou Dahar ramp to high-relief platform (High Atlas, Morocco)

The Djebel Bou Dahar (DBD) carbonate platform (High Atlas, Morocco) developed in an intracratonic rift basin during the early Jurassic. From Hettangian?-Sinemurian to Pliensbachian, the DBD evolved from an extensive low-relief depositional system into a high-relief platform as a result of progressive extensional tectonics. Platform growth was terminated by drowning in the earliest Toarcian. Six major stages of DBD platform evolution have been distinguished on the basis of geometry, character of depositional facies and internal architecture. This study focuses on the automicrite-rich bioherms accumulated during the Upper Sinemurian stage III and the Pliensbachian stages IV-VI and documents variations in composition, morphology and depositional setting of the sponge-automicrite boundstone. During the late Sinemurian stage III, siliceous sponge-automicrite mounds, up to 15 m-thick and tens of metres in width developed surrounded by skeletal coated grain packstone. These beds were deposited in a middle-ramp to gentle dipping slope setting, below wave base and include crinoids, echinoid spines, brachiopods, bryozoans, bivalves, siliceous demosponges and hexactinellids, foraminifers (Involutina liassica, Lenticulina, lagenids and nodosarids) and the problematic Radiomura cautica. The mounds consist of automicrite displaying homogeneous, clotted peloidal and laminated fabrics, gravity defying structures and isolating stromatactis-like cavities filled by radial fibrous cement. The automicrite is generally associated with sponge spicules, demosponges and hexactinellid sponges. The complex fabrics and the rapidly lithification at the seafloor suggest biologically-induced precipitation for the origin of the micrite. The Pliensbachian Stages IV to VI were characterized by steep slopes (up to 450-600 m relief and dip angles up to 30°) formed as a result of fault-controlled adjustment of the originally low-relief topography. Sponge-automicrite boundstone accreted on the upper slope down to a depth of 140 m below the platform break, while coral-sponge-automicrite boundstone occurred from the platform-break and outer platform down to depths of 70-100 m. Boundstone alternated with redeposited grainstone, packstone and rudstone beds. The deeper sponge-automicrite boundstone accumulations were devoid of corals and dominated by homogeneous to clotted peloidal micrite embedding sponge spicules of demosponges and hexactinellids. Irregular mm to cm wide stromatactis-like cavities were filled by radial fibrous cement. Sponges can be attributed to lithistid demosponges and lyssakine hexactinellids encrusted by Radiomura cautica, Terebella and serpulids. The margin and uppermost slope were characterized by the growth of metre-scale colonies of phaceloid corals surrounded by a patchy distribution of sponge-automicrite boundstone associated to stromatoporoids, chaetetids, Bacinella ordinata, Baccanella floriformis and Tubiphytes. The DBD platform developed in a setting suitable for the biologically induced precipitation of microcrystalline carbonate in association with the presence of sponges. These favourable conditions might have been facilitated by the availability of substrates following the end-Triassic extinction event and the specific chemico-physical properties of the adjacent basin seawater. The narrow rift basin configuration and arid climate setting may have promoted highly saturated and possibly mesotrophic waters that appear necessary for the biologically induced precipitation of calcium carbonates during degradation of biofilms and sponge-derived organic substrates. In addition, the character, composition and geometry of the sponge-automicrite boundstone were affected by the evolution of the geometry of the depositional system driven by extensional tectonics and eustatic sea level.