STRUCTURAL EVOLUTION AND SEQUENCE FORMATION OFFSHORE SOUTH GABON DURING THE TERTIARY

The present investigation of the Tertiary succession offshore South Ga bon is based on seismic surveys of 1985-1989 vintage and wells coverin g the southern part of Gabonese waters. In general, the Tertiary basin off South Gabon evolved during two structural stages. During the Pale ocene and Early-Middle Eocene, a trough-like basin subsided slowly tow ards the west. During this period there was a cyclic development of th e succession which consists of up to four sequences mostly bounded by Type 2 sequence boundaries. The sequential changes were induced by eus tatic sea-level changes. This pattern changed during the Neogene when the evolution was characterized by continental crustal flexuring in Ea rly-Middle Miocene times. This resulted in strong subsidence of the we stern part of the basin and uplift of the eastern part. Consequently, the eastern part was exposed to strong erosion and a thick Middle Mioc ene succession was deposited west of the shelf (approximately the pres ent-day shelf), especially on the slope. The clinoformal and lens-shap ed Middle-Late Miocene succession which consists of four sequences bou nded by Type 1 sequence boundaries has been controlled by eustatic cha nges in sea level. In addition to the overall subsidence pattern, salt tectonics has modified the external and internal architecture of the sequences on a local scale. Canyon formation in connection with slumpi ng results in the establishment of well developed fan systems on the l ower part of the slope and on the basin floor area.