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Raft-Related Structures of the Albian Madiela Formation, Offshore South Gabon


We investigate the raft-related turtle structures of the post-salt, mixed carbonate-siliciclastic Albian Madiela formation, located offshore south Gabon. Recent 3d broadband RTM seismic data shows the Madiela structures comprised of a Lower Madiela pre-rafting section, with overlying Mid-Upper Madiela and Cenomanian Cap Lopez formation syn-rafting growth stratigraphy. The lower Madiela section directly overlies the salt-related detachment and is isopachous, with foot-wall and folded hanging-wall cut-offs observed at the edges of the raft blocks. Depo-centers predominantly stacked vertically to create symmetric structures as rafting continued during Mid-Upper Madiela deposition, although a few asymmetric structures are observed. The structures have an elongate map pattern that varies in orientation across the study area. Seismic facies vary across individual structures from high amplitude crests to low amplitude flanks. Inversion and turtle structure development caused crestal faulting, especially within the Cap Lopez section. Welding of the salt beneath the turtle structures is more extensive up-slope than down-slope in the study area. Map and section-view structural restorations indicate the maximum down-dip translation of the Lower Madiela rafts from a proximal setting varies from 25 km in the up-dip portion of the study area to 50 km down-dip. Structural restoration, adjacent well control, seismic facies transitions, and seismic attribute observations indicate inner neritic water depths during Albian deposition, deepening to outer neritic in the Cenomanian. Rafting initiated in the early Albian due to the effects of thermal subsidence and ongoing seafloor spreading on the margin. The variance in curvature of the raft trends near the Congo border mimics, and may be partially inherited from, the trend of the up-dip breakaway fault zone. The raft structures reach their present day positions in the study area during the Turonian. Inversion of the turtle flanks and basal welding initiated in the Turonian, with down-building of the inter-raft salt diapirs continuing to the Paleogene-to-present across the area. Local submarine erosion of the Cap Lopez formation is due to local inter-raft salt diapir inflation associated with steepening of the margin during the late Cretaceous. The structures of southern Gabon vary from similar Albian structures in northern Gabon and Congo in burial history, thickness, and proximity to siliciclastic depo-centers.