Controls on the growth and demise of a Lower Jurassic carbonate platform (Djebel Bou Dahar, High Atlas, Morocco)

The Djebel Bou Dahar (DBD) carbonate platform (High Atlas, Morocco) is an exceptionally well-exposed lower Jurassic carbonate platform developed in an intracratonic rift basin from Hettangian to Pliensbachian and drowned in the earliest Toarcian. It measures 35 by 10 km and reached a relief of nearly 450-600 m. Traditional geologic field mapping assisted by the use of RTK GPS (real time kinematic global positioning system), and interpretation of satellite imagery were used to spatially capture lithofacies patterns, stratal architectures, and structural features. The spatially constrained data set was used to build a digital outcrop model (DOM), which, in turn, allowed the generation of a (interpreted) spatially constrained stratigraphic framework. Six genetic stratigraphic units (stages I-VI), each with a distinct stratal anatomy, lithofacies composition and spatial arrangement, have been identified in the DBD. The primary goal of this study was to evaluate the relative contribution of factors controlling the evolution the DBD carbonate system, including tectonics, eustatic sea level fluctuations, and trends in carbonate-producing biota. The thick aggradational to retrogradational strata packages with limited development of progradational regressive strata packages that characterize the DBD stages are inferred to be the result of the combined effect of: 1) in-situ carbonate production; 2) high rates of creation of accommodation space (driven by the rapid tectonic subsidence and the overall eustatic transgressive trend characterizing the Early Jurassic; and inferred low amplitude eustatic base-level falls. Stages I and II (Hettangian?-Early Sinemurian) are dominated by shallow marine intertidal and supratidal depositional environments representing a broad low-relief and low-angle carbonate system that developed following a late Triassic rifting phase. Renewal of extensional tectonics at the Early-Late Sinemurian transition (Stages II-III boundary) resulted in compartmentalization through km-spaced block faulting. The low relief stages of the isolated carbonate system (Stage III and early Stage IV) were characterized by a grainy carbonate factory dominated by benthic heterotrophic communities in a probable nutrient-rich sea-water. During the Pliensbachian (Stages IV-VI) colonial scleractinian corals along with siliceous and calcareous sponges and stromatoporoids formed reefs on steep slopes (25-30°), leading to the development of a steep-fronted carbonate platform. This was coincident with the reestablishment of the shallow-water photozoan factory with a peritidal platform interior in oligotrophic conditions. Subaerial exposure during the Pliensbachian-Toarcian boundary eustatic sea-level fall, environmental changes associated with the Toarcian oceanic anoxic event, and the synchronous mass extinction critically affected the carbonate factory favouring platform drowning and termination. The observation that synsedimentary extensional tectonics played a primary, if not dominant, role in controlling accommodation space variations and in influencing the depostional facies architecture and geometries is of critical importance in modeling and predicting stratigraphic architecture and facies patterns in similar environments.