Leg 159 drilled a series of four sites (Sites 959-962) within continental
crust adjacent to the continent-ocean transition along the transform
passive margin of Côte d'Ivoire-Ghana (CIG). This leg represents the
first application of deep-sea drilling to the tectonics of transform-
margin development. A series of tectonized Albian sediments documents
an early phase of intra-continental transform motion. The deep-water
lacustrine sequence passed into a progressively more marine sequence as
breakup continued, probably as part of a pull-apart basin system. The
coexistence of compressional and extensional features, as well as rarer
strike-slip faults, shows the intense deformation that affected a broad
zone of the continental margin at that time. Subsequent inversion of the
pull-apart basin into the Marginal Ridge occurred during the Cenomanian-
Turonian. The period of maximum uplift of the ridge is shown by the
development of shallow-water reefal carbonates and associated high-
energy coarse, clastic sediments of Turonian-Santonian age. Tectonic
models of transform margins indicate that, following continental
breakup, an active ocean-continent transform phase is ended by the
migration of an active oceanic spreading center along the margin. Transfer
of heat from the spreading ridge is predicted to cause major uplift and
may correspond to this phase of shallow-water sedimentation. Subsequent
cooling of the continental lithosphere would produce subsidence. Post-
Santonian sedimentation at Site 959, situated on the flanks of the Deep
Ivorian Basin, was marked by a deeper water, organic-rich, black-shale
facies that continued until the late Paleocene. In contrast, Sites 960, 961,
and 962, which are closer to the crest of the CIG Marginal Ridge, show
hiatus and condensed claystone facies with glauconitic hardgrounds for
this time. This ridge may have acted as a sill to the Deep Ivorian Basin,
where free circulation with the open Atlantic was restricted. A major
unconformity seen at all four sites in the upper Paleocene may mark the
end of rapid thermal subsidence of the margin, but may also reflect
changes in current activity at that time. Much of the Eocene and Oligocene
is represented by pelagic siliceous sedimentation, with slumping off the
uplifted ridge crest seen at Site 959. Neogene sedimentation is dominated
by a hemipelagic clayey nannofossil sedimentation, except at Site 962,
which accumulated clay alone because of its position below the carbonate
compensation depth (CCD). Study of the Neogene sediments at Sites 959
and 960 is expected to yield a rare, high-resolution (1,000-10,000 yr)
record of the intermediate-water history in the Eastern Equatorial
Atlantic, with implications for paleoceanographic changes during
glacial/interglacial cycles.
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