Tectono-Stratigraphic Evolution of the Sierra Leone Continental Margin

Jose L. Huedo, Ricardo Gerster, Hector Gonzalez, Hernán Verzi, Joan F. Flinch, and Abdul Mansaray
West Africa Exploration, Repsol-YPF, Madrid, Spain

The analysis of 2D and 3D seismic, well and regional data has led to a reconstruction of the tectono-stratigraphic history and evolution of the continental transform margin in Sierra Leone.

The Sierra Leone continental margin is strongly influenced by the tectonic activity associated to adjacent transform zones. These transform zones were originally generated during the rifting stage, and they continued its development during the sea-floor spreading in the post-rift stage. Therefore, transform zones extend into the continental crust and affect the structure of the continental margin. Differential subsidence across the Sierra Leona Transform Fault generates a low subsidence area to the North (Sulima Plateau) and an Upper Cretaceous-Cenozoic sedimentary depocenter to the South.

The evolution of the continental margin to the South of the Sierra Leone Transform Fault is characterized by progressive subsidence and deepening. The sedimentary filling of the basin evolved from continental to shallow marine deposits in the syn-rift stage (Lower Cretaceous), and from a ramp to a shelf-slope-deep basin in the post-rift stage (Upper Cretaceous and Cenozoic). Several turbidite complexes have been deposited in the slope and deep basin during the post-rift stage. The structural evolution of the margin forced the Cretaceous turbidites to be deposited over a gentler slope and therefore are less confined and exhibit a more meandering pattern than those in the Tertiary that are more confined to master canyons.

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