Shallow Structure and Stratigraphy of Liberian Continental Margin

Abstract

The rifting of Africa from North and South America has affected the structural framework off Liberia in two episodes. As shown by bathymetry, seismic-reflection profiles, magnetic data, and stratigraphy, the southeastern third of the margin is cut by west-southwest-trending fracture zones which we interpret as the extension of the St. Paul's fracture zone. These fracture zones intersect the continental margin off Cape Palmas to give rise to a blockfaulted and slump topography, similar to that in the area where the Romanche fracture zone intersects the African continent off Cape Three Points, Ghana. The fracture zones are covered by a prograded wedge of presumed Tertiary and Cretaceous sedimentary rock off central Liberia. Adjacent to the northwestern third of the Liberian margin, northwest-trending basins filled mainly with Lower Cretaceous paralic sediments are under the continental shelf; they extend to the upper slope, where they are downdropped along a northwest-striking fault zone that separates the shelf deposits from a thick prism of sediment beneath the continental rise. The southeastern third of the margin appears to have formed during the separation of Africa and South America in the Late Jurassic-Early Cretaceous. The rest of the margin seems, more strongly influenced by the tensional forces created during the rifting of Africa and North America; volcanic rocks are Late Triassic to Early Jurassic in age, and shelf sedimentation occurred mainly after the continents broke apart.