Sequence development in an isolated carbonate platform (Lower Jurassic, Djebel Bou Dahar, High Atlas, Morocco): influence of tectonics, eustacy and carbonate production

摘要

Six sequences, each with a distinct shape, stratal anatomy, bounding surfaces, lithofacies composition and stacking patterns, were identified in the Djebel Bou Dahar isolated carbonate platform which developed in a half-graben in the High Atlas marine rift basin during the Early Jurassic. Sequences are believed to have a complex, multi-process origin as the result of tectonic processes (regional subsidence and fault-block rotation) overprinting the eustatic signal. In addition, changing carbonate producers with shifting centres of deposition and relative accumulation potential influenced the filling and anatomy of the accommodation space that had been created by the hybrid tectonic and eustatic processes. Sequences I and II are tabular in shape and interpreted to primarily reflect the combined processes of uniform tectonic subsidence and eustacy which resulted in a ramp to shelf carbonate system with lagoon and tidal flat lithofacies. Onset of a rifting episode around the base of the Lotharingian and fault block rotation were responsible for the development of the angular unconformity with the underlying Sequence II, and the progressive development of a shallow-water isolated platform and the wedge-shaped depositional Sequences III to VI during the Lotharingian and Pliensbachian. Concurrent variations in palaeoceanographic conditions and in the carbonate factory caused a shift towards a flat-topped platform geometry with highenergy coated grains and sub-photic sponge-microbial deposition around the margin. The aggradational to retrogradational character of Sequences III, IV and VI is inferred to be a result of the combined effect of in situ carbonate production coupled with high rates of creation of accommodation space in the hanging wall. Environmental effects on carbonate production were the most probable cause for drowning of the Djebel Bou Dahar platform during the Toarcian but also influenced the filling and geometry of Sequences IV to VI. This study suggests that the commonly accepted model of pulse-driven sequence-development in half-grabens may not be fully applicable for the Djebel Bou Dahar, where a more continuous rotation at sequence scale, associated with subsidence pulses of the Djebel Bou Dahar block, seems to have interacted with eustacy and carbonate production to generate sequences. Continuous rotation is suggested by the presence of regular patterns of diverging strata and numerous minor exposure surfaces in lagoonal deposits of Sequences V and VI. In addition, this study demonstrates that eustacy and variations in the carbonate factory might be as significant as tectonics in sequence development and platform drowning in carbonate platforms within rift basins. This observation could imply that the effects of eustacy and the carbonate factory might not have been correctly recognized in other similar carbonate platforms due to a tectonic-biased interpretation.