THE RELATIONSHIP BETWEEN SUBMARINE-CANYON FILL AND SEA-LEVEL CHANGE -AN EXAMPLE FROM MIDDLE MIOCENE OFFSHORE GABON, WEST-AFRICA

A series of Middle Miocene submarine canyons has been mapped in offsho re Gabon, West Africa using a combination of seismic facies analysis a nd well information. Three distinct seismic facies are recognized in t he canyon fill; the evolution of these facies is inferred to be relate d to sea-level change. During sea-level lowstand, deposition was chara cterized by lateral accretion on the southern canyon wall whilst progr essive erosion occurred on the northern wall. This depositional patter n was controlled by a local coast-parallel current, that flowed northw estwards along West Africa during the Middle Miocene. As relative sea level rose, the erosion on the northern wall ceased and the canyons we re filled by sediments showing an aggradational stratal pattern. Final ly, during the early part of the transgression, the canyons were fille d with horizontal or subhorizontal sedimentary strata. The sedimentary fill is thought to consist of alternating sands and mud within its lo wer part, but as relative sea level rose, clayey sediments gradually d ominated the canyon fill.