Post-tectonic subsidence of the Cote D'Ivoire-Ghana Marginal Ridge: Insights from FMS data

Abstract

During Ocean Drilling Program (ODP) Leg 159, Sites 959 and 960 were drilled on the northern slope and near the top of the Cote d'Ivoire-Ghana Marginal Ridge, at the intersection of the divergent Ivorian Basin and a transform margin. On seismic lines, this ridge appears as an acoustic basement tilted northward. It comprises Albian siliciclastic synrift sediments, unconformably overlain by Turonian to Santonian carbonate debris flows (postrift but syntransform sediments). These sediments are overlapped along the northern ridge slope by Upper Cretaceous-Paleocene claystones and Paleocene-Eocene porcellanite. The Formation MicroScanner (FMS) logging tool has recorded images of the borehole wall between 546 meters below sea-floor (mbsf) (porcellanite) and 924 mbsf (claystones) in Hole 959D, and between 331 and 351 mbsf (grainstones) in Hole 960C. From these images, strata and fault utitudes have been systematically measured (1771 beds and 694 faults measured in Hole 959D, and 257 beds and 13 faults in Hole 960C) to study the progressive tilting of the ridge and coeval deformation. In Hole 959D, the average dip of the bedding increases with depth, from 7°at 550 mbsf to 16°at 920 mbsf. The mean azimuth remains constant, around 320°N. These changes indicate a progressive northwestward tilt of the marginal ridge from the Upper Cretaceous to the Eocene. However, both dip and azimuth measurements exhibit important variations that cannot be ascribed to measurement resolution. In Hole 960C, the attitudes of bedding exhibit the same variations, with an average azimuth around 20°. Changes in dip and azimuth values can be correlated at a decimeter-to-meter scale and indicate progressive rotations about horizontal axes. At both Sites 959 and 960, direct observations on cores and strong correlation between rotation axis and fault strikes indicate that many rotations are related to tilting along low-dipping and gravity-driven normal faults. Sedimentary structures such as cross lamination or progradation can also explain rotations of successive beddings, especially at Site 960 where grainflows were observed. Averaging the dip and azimuth over a large interval (20 m) emphasizes the occurrence of several wavelengths in the depth domain for the progressive tilts. Finally, the northern slope of the Marginal Ridge results from a combination of tilting around several horizontal rotation axes. The rotations are mainly controlled by the contemporaneous thermal subsidence of the transform and divergent margins. Moreover, some faults and rotation axis directions can be associated with paleo-stresses, especially in the syntransform limestones of Hole 960C.