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Abstract

With the initiation of rifting in the Late Permian (Kazanian; 259.3 Ma) and the onset of drifting in the Early Jurassic (Pliensbachian/Toarcian 189.6 Ma), the Tarfaya-Laâyoune Basins are part of the oldest passive continental margin segments in the Atlantic domain. A basin analysis of the Tarfaya-Laâyoune Basins has been performed based outcrop data, well data, and a multi 2D-seismic survey dataset. Present organo-geochemical, heavy mineral, and thermochronological studies have been considered. The workflow includes: i) post-processing /filtering of seismic data, ii) outcrop and well data screening, iii) seismic interpretation, iv) sequence stratigraphy, v) time/depth conversion, and vi) numerical reverse modeling. The Meso-/Cenozoic basin fill is bound by two superordinated unconformities. The Initial Rift Unconformity (IRU, 259.3 Ma) at the base and the seafloor at the top represents an incomplete 1st order sequence. This sequence can be subdivided into two phases, a transgressive phase and a regressive phase. The change between transgression and regression takes place during the Late Cretaceous, which marks the onset of basin inversion. Six 2nd order tectonic sequences are bound at the top by major unconformities: i) the rift stage by the Initial Drift Unconformity (IDU, 189.6 Ma), ii) the early drift stage by the Mature Drift Unconformity (MDU, 144.2 Ma), iii) the mature drift stage by the Peak Spreading Unconformity (PSU, 112.2 Ma), iv) the mature drift stage with initial Atlasian convergence by the Initial Atlasian Unconformity (IAU, 65 Ma), v) the mature drift stage with initial Atlasian compression and uplift by the Peak Atlasian Unconformity (PAU, 28.5 Ma), and vi) the mature drift stage with peak Atlasian uplift and erosion is bounded on top by the seafloor (present day). All 2nd order sequences are characterized by a specific sedimentary environment: the first by an alluvial, fluvial and restricted marine setting, the second by carbonate ramps and platforms, the third by deltaic environments, the fourth by open marine clastic and carbonate deposits, the fifth by the development of a clastic shelf margin wedge and the sixth by terrestrial runoff via deltas and widespread bypass sedimentation. In total, 31 3rd order sequences and 113 4th order sequences have been identified on three key transects of the dataset. The northern transect is located in the Tan Tan Delta region, the central transect is crossing the Cap Juby oilfield and the southern transect is located at the transition between the Tarfaya and the Laâyoune Basin. Based on the developed high resolution sequence stratigraphy in combination with well data screening, 13 potential source rock formations have been identified, their distribution circumscribed by using chronostratigraphic plots (Wheeler diagrams). Twelve of these potential source rocks have been deposited during the transgressive phase of the 1st order sequence, one, the source rock of the PETM event, after the onset of basin inversion. Reservoirs have been identified in the Triassic rift, the Jurassic carbonate, the Early Cretaceous deltaic and the Cenozoic deep marine successions. With the end of Late Permian to Early Jurassic rifting, subsidence and sediment flux was controlled by far-field intra-plate stresses. Persistent ridgepush during the Early to Late Jurassic with sea floor spreading (SFS) half-rates of 19 mm/y resulted in peak subsidence rates of about 150 m/m.y in outer shelf areas. Sediment supply from the hinterland was inhibited, initially resulting in unfilled accommodation space, subsequently filled by carbonate platforms. During the Tithonian to Berriasian, the tendency of divergence of the Central Atlantic margins has been reduced by the remaining Gondwana continent. Even though rifting in the South Atlantic started in Late Tithonian/ Early Berriasian, SFS halfrates decreased stepwise to 7 mm/y. Thermo-tectonic uplift in the South Atlantic rift area and decreased ridge push-forces resulted in an uplift of the Central Atlantic passive continental margins. The Jurassic shelf was subaerially exposed during the Berriasian and early Valanginian and major erosion started in the Anti-Atlas and the Sahara Craton. Supplied sediment was transported over the Jurassic shelf and formed the Tan Tan and Boujdour delta complexes on the outer continental shelf to lower slope. Combined flexural and compaction-induced subsidence added up to 120 m/m.y. in outer shelf areas. The delta development continued until the Albian. Its end correlates with initial drifting in the South Atlantic and strongly increasing SFS half-rates up to 24 mm/y in the Central Atlantic. Although ridge-push forces were smaller than in Jurassic times, water depths on the shelf top increased to several hundred meters. Sediment supply from the continent was starved. Cenozoic subsidence and sediment flux history was controlled by Alpine convergence and Atlasian uplift.