| Popis: |
Seismic and acoustic reflection profiles demonstrate the presence of 4 seismic and 2 acoustic units. From the inner shelf outwards they are successively: Unit 1, interpreted as Upper Cretaceous starting from the Cenomanian, and a Palaeocene and Eocene Unit 2. Both units are folded, the latter more gently. The Oligocene regression induced the discordant boundary with two monoclinal units 3 and 4 representing the Miocene, and separated by a Middle to Late Miocene regressional unconformity. Late Cretaceous tensional tectonics, accompanying the early opening of the South Atlantic, was followed by at least 2 phases of compressional tectonics during Palaeocene to Eocene times. The orientation of the axes, 90° to 120°, is different from the known directions in western Africa. Fractures belonging to a Miocene tensional regime are proposed to have caused offsetting of the mapped strata. The thickness distributions of 2 acoustic units were mapped. The Lower Unit, less than 5 m thick, consists of Middle Weichselian marine clays. Their relicts are concentrated at the outer shelf, due to the fall of the sea level (100 to 115 m) around 18 000 yBP. The Upper Unit forms an at maximum 15 m thick Holocene lobe of clays originating from the Congo River. Both units show strips oriented ∼110°, controlled by the direction of the Cretaceous and Tertiary subcrops. |
