Kari Strand2


During Ocean Drilling Program Leg 159 four sites were drilled along the Côte d’Ivoire-Ghana Transform Margin (Sites 959-962), all located on a marginal ridge that defines the continental margin along a transform boundary. The Cretaceous rift to transform margin deposits document the early evolution of an active transform margin and its transition to a passive margin, thus defining a second-order, tectono-eustatic cycle of 10-50 m.y. duration. Emphasis in this paper is on sedimentary facies, sediment composition changes, and tectono-eustatic cycles in a rift/transform margin transition In the late Albian, restricted intracontinental sedimentation was characterized by laminated lacustrine and mixed-source deltaic deposits. This was followed by a progressive marine transgression with the deposition of horizontally laminated to cross-laminated, storm-influenced, shallow-shelf sandstones and claystones that terminate in a Turonian unconformity, thus defining one total third-order (1-10 Ma) tectono-eustatic cycle. The overlying periplatformal carbonate deposits of Turonian to Coniacian age were derived from shallow reefs by gravity-induced processes and from suspension, and indicate the beginning of a marginal ridge uplift. A significant siliciclastic component with lithic fragments, accompanied by carbonate sedimentation, indicates mixed, reworked sediment sources due to supposed further uplift of the marginal ridge. This uplift reduced sediment accommodation space and caused termination of this next third-order cycle during the late Coniacian–Santonian. Origin of the uplift is thought to be related to the thermal effect of the passage of a spreading ridge south of the margin. The next cycle, overlying the sandy limestones, is recorded by a condensed section of claystones, containing phosphatic debris and hardgrounds deposited over a time period of more than 10 m.y. In a deeper setting, this condensed interval is overlain by a sequence of organic-rich black claystones, whose deposition was promoted by diminished accommodation space and partial damming of the basin by the marginal ridge during late Coniacian to early Eocene. The dominant tectonic setting shifted closer to a passive margin at each of the four drill sites after the late Paleocene.

1Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998. Proc. ODP, Sci. Results, 159: College Station, TX (Ocean Drilling Program).
2Department of Geology, Institute of Geosciences, University of Oulu, FIN-90570 Oulu, Finland. kari.strand@oulu.fi