GEOMETRY AND STRUCTURAL EVOLUTION OF EASTERN NIGER DELTA TOE-THRUST BELT, EQUATORIAL GUINEA

Southeast-vergent mid-Miocene gravity-driven thrusting began along the eastern margin of the Niger Delta where the delta front begins to climb onto the bathymetrically high Cameroon Volcanic Line (CVL). Bioko Island and other volcanic features form a bathymetric high not present around the remainder of the delta margin. Filling of the deep marine basin between the Niger Delta and the CVL by sediment from the Cross River drainage system progressively choked off further development of the toe thrust belt, and forced the collapse of the delta front and consequent thrusting to migrate southwestward from Miocene through Recent time. There are relatively few imbricate thrust sheets in the easterly area, which is bordered on the hinterland side (northwest) by a zone of shale diapirism. Thrust sheets containing basin floor fans of the Upper Oligocene-Miocene Isongo Group are generally lobate in map view and detach within a thick shale zone rather than along a discrete stratigraphic horizon. Submarine canyon systems were incised into the growing thrust belt, and debris flows from the thrust sheets and shelf-sourced turbidites covered the adjacent basin floor. This influx of sediment both contributed to the loss of surface slope essential to thrusting, and buttressed the rising thrust sheets, thereby inhibiting further thrust progradation. The eroded thrust front was later buried by the Plio-Pleistocene Qua Iboe Group (QIG), in which canyon fill turbidites host the major hydrocarbon accumulations of Zafiro Field. The QIG cover thins to the southwest. Where the QIG is thin the leading edges of thrust sheets are not truncated by erosion, and the thrust belt has continued to grow until the present day. This segment of the toe thrust belt is characterized by more numerous and more tightly compressed thrust sheets. Significantly greater involvement of mobile shale is indicated by subdued tilted box-fold geometries. The buttressing effect of the sediment fill has forced gravitational collapse of the delta margin southwestward around the end of the partially filled Niger Delta – CVL basin. This has resulted in development of a zone of lateral offset, apparent in bathymetric data, which separates the leading southeastern edge of the thrust belt from the main frontal belts in Nigerian waters.