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Intraslope Basin Stratigraphy Documents the Evolution of Salt Wall Growth in the Lower Congo Basin, Offshore Angola


Post salt stratigraphy in the Lower Congo Basin provides a detailed insight into the evolution of elongate salt walls from the Late Cretaceous to the present. A large number of horizons were generated from 3D reflection seismic surveys using a commercial, semi-automatic, geo-model based interpretation technique. Visualizing these horizons and associated isochron, variance and amplitude maps allows us to analyze structural, thickness and depositional trends in the intra-slope basins and their implications for the evolution of the salt walls through time. The elongate salt walls in the study area are oriented NE-SW and occur to the SW and along strike of a number of major normal faults and isolated diapirs. They can be over 60 km long and up to 2200 ms TWT in height above the mother salt unit. The width of the intraslope basins between the salt walls is fairly consistent (10 – 15 km). Significant relief of the present day sea floor (up to 700 ms TWT) and local mass transport deposits in the shallowest subsurface bear witness to recent salt movement. Salt movement was initiated along closely spaced normal faults (spacing < 1 km) parallel to the regional strike (NE-SW). These faults, which have offsets of up to 200 ms TWT, result in pronounced local thickening of the early post salt stratigraphy underlain in places by residual pockets of salt in areas where the salt layer has otherwise welded. After this initial phase, salt mobilization becomes progressively focused towards the present day location of the salt walls although some local pockets of salt remain active into the Oligocene, evidenced by faulted turtle back structures with associated rim synclines (up to 700 ms TWT in height). By the Miocene, the turtle back anticlines have become inactive, and from then on, salt movement mostly occurs in the walls. Variations in post salt stratigraphic thickness and deflection of polygonal faults in the Pliocene are evidence for either segmented development of, or differential movement of salt within, the salt walls. The increasing structural significance of the salt walls is recorded by a switch in the general direction of sediment transport. Throughout the Late Cretaceous and Oligocene transport is mostly controlled by the regional dip of the basin and perpendicular to the salt walls (i.e. towards NW). However, from the Miocene onward sediment transport was towards SSW, confined within the intraslope basins and strongly controlled by the presence of the salt walls.