Depositional evolution and sequence stratigraphy of the shelf and slope area off South Gabon, West Africa

Geological Survey of Denmark and Greenland, Copenhagen, Denmark

The Tertiary succession off South Gabon is divided into stratigraphic sequences. The study includes interpretation of seismic data and data from seven wells within the study area, covering 26,000 km ² . Four Paleogene sequences (P1-P4) are recognized within the Danian to earliest early Ypresian time interval. There was a major hiatus from the middle Eocene to the early Miocene in the study area. Six sequences (N1-N6) are identified in the Neogene, N1-N4 lower and middle Miocene and N5 and N6 probably upper Miocene to recent. The Danian and Thanetian sequences (P1-P3) are separated by Type 2 sequence boundaries, and the most widespread flooding of the area occurred at the Danian- Thanetian transition within sequence P2. The boundary between P3 and P4, close to the Thanetian-Ypresian transition, is probably of Type 1. The Paleocene sequences consist of transgressive systems tracts and highstand systems tracts. The lower Miocene sequence (N1) was formed mainly by progradation of the shelf as a result of high sediment supply and had a tectonic origin. The middle Miocene sequence boundaries are characterized by deep erosion, e.g., canyon formation into the substratum, and thus are Type 1 sequence boundaries. The middle Miocene sequences (N2-N4) consist of lowstand systems tracts, transgressive systems tracts (early part), and late highstand systems tracts. The upper Miocene to Holocene interval (N5-N6) could not be subdivided because the deposits are relatively thin. The Tertiary mainly siliciclastic sediments are mainly fine-grained sediments, though sandstones are identified at sequence boundaries of Danian to Ypresian ages. Sandstones are also inferred to have been deposited during the early fill of canyons and basinward in the middle Miocene. The processes involved in the formation of these sequences is thought to be a combination of basin type (tectonics), sediment supply, and sea-level changes. Curves of relative sea level and eustatic sea level were constructed for the studied interval, and these curves are compared with earlier published sea-level curves for the Tertiary.

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