Structure of continental margin off Sierra Leone, West Africa

Abstract

Seismic-refraction data from the ocean are combined with geological data from the shore to determine the structure of the continental margin and the continent-ocean transition off West Africa. More than 5 km of Cretaceous-Quaternary sediments accumulated on the continental shelf and slope as the surface of the underlying continental crust subsided. On the shelf the pre-Mesozoic rocks consist of a 2- to 3-km thickness of Cambrian-Devonian sedimentary rocks over a crystalline Precambrian continental crust. These Paleozoic sedimentary rocks can be interpreted as a pinching out under the upper continental slope off Sierra Leone and may not extend any great distance into the South Atlantic. Layer 2 (oceanic basement) with a velocity range similar to that of the Paleozoic sedimentary rocks appears to extend all the way from the mid-Atlantic ridge to the lower continental slope. This layer may consist entirely of post-Paleozoic volcanies. The Cretaceous-Quaternary embayment of the Senegal basin is separated from the less well developed embayment of Sierra Leone by a west-southwest-trending arch in the pre-Mesozoic surface under the continental shelf. Mesozoic or Tertiary faulting produced the present configuration of the arch, but this faulting probably followed the zones of Paleozoic folding and faulting along the northwest limb. An anomalously high-velocity crust, 7.0–7.3 km/sec, occurs near the Sierra Leone rise. This may be similar to the crust that occurs beneath the Bermuda rise, a crust formed by metamorphism due to the igneous activity of the many seamounts comprising the rise. This anomalous crust does not extend to the ridge flank, suggesting that the Sierra Leone rise is an independent center of volcanism.