Sumito Morita2 and Asahiko Taira2


Anisotropy of magnetic susceptibility analysis was performed to determine the type of magnetic fabric, variation with lithostratigraphy, structural geology, and physical properties of the fine-grained sediments cored at the Côte d’Ivoire-Ghana Transform Margin during Ocean Drilling Program Leg 159. Results show three types of magnetic fabric: Type 1 is a spheric ellipsoid, which has a low degree of magnetic anisotropy; Type 2 is an oblate ellipsoid, which has a positive T value with a high degree of anisotropy; and Type 3 is a prolate ellipsoid, which has a negative T value with a high degree of anisotropy. An unconformity that occurred between the upper Albian and lower Turonian separates Type 1 from Type 2. Samples exhibiting Type 1 fabric retain the original sedimentary fabric, and Types 2 and 3 reflect modification resulting from syntransform deformation from the initiation of the Atlantic during the Albian to early Turonian. Type 3 sediments have a vertical magnetic lineation, which is controlled by secondary mineralized and vertically aligned siderite grains. The secondary mineralization of siderite may suggest that a horizontal compressional contraction occurred in the latest Albian to early Turonian during the time of uplift of the Côte d’Ivoire-Ghana Marginal Ridge at the end of the Syntransform Basin Stage. The disordered stratigraphic occurrence of magnetic fabric types may indicate tectonic slicing of the sequence with varying degrees of deformation.

1Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998. Proc. ODP, Sci. Results, 159: College Station, TX (Ocean Drilling Program).
2Ocean Research Institute, University of Tokyo, 1-15-1, Minami-dai, Nakano-ku, Tokyo 164, Japan. moritas@ori.u-tokyo.ac.jp