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Subsidence Modeling of the Central High Atlas Jurassic Rift Basin (Morocco): Implications for Carbonate Platform Development


In syn- and post-rift settings, carbonate platform development and demise can be closely related to subsidence history and/or clastic sediment supply, a situation that can be modified in basins with associated diapirism. The lower Jurassic of the Central High Atlas exposes classic carbonate platform strata developed on the margins and block shoulders of the rift system, such as the Djbel Bou Dahar platform, with platform and basinal deposits developed along the rift axis. In contrast to the rift margin, the rift axis contains a mobile substrate of Triassic strata that gives rise to numerous elongate diapiric structures that bound mini-basins. Structural analysis reveals that many of the diapiric structures developed over buried normal faults that were active from middle Hettangian to late Pliensbachian times. In this setting we integrate detailed sedimentological and structural work with subsidence analysis between the rift margin and rift axis in order to gain a better understanding of the controls on carbonate platform initiation and demise in a syn-post rift setting with and without active diapirism. For the subsidence analysis we use thermal and 1D burial modelling (GENEX software, BEICIP-FRANLAB), constrained by new vitrinite reflectance data. Stratigraphic thicknesses, paleowater depths and age of different stratigraphic units were selected from 2 basin margin localities and from 2 basinal localities. For comparison, we also calculated subsidence history for the Djel Bou Dahar Platform on the rift shoulder, where there is no diapirism. Results indicate the occurrence of two periods of rapid subsidence. The first has a duration of c. 22 My and extended from Late Triassic to early Pliensbachian, and is attributed to the syn-rift stage. The second period, of c. 15 My is characterized by 2.5 times greater subsidence rate (Pliensbachian-Toarcian boundary to Bajocian-Bathonian limit). A significant increase of siliciclastic supply along the entire rift basin resulted in widespread carbonate platform demise, and is coincident with higher subsidence rates in basinal provinces. This was associated with active diapirism and salt withdrawal beneath the minibasins. The intricate relationships between tectonics, salt withdrawal, clastic sediment input, and paleowater depth have a direct impact on the development of Jurassic carbonate platforms in the Central High Atlas, and provide insights with valuable subsurface application.