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Abstract: Cretaceous to Late Oligocene Development of Sedimentary and Organic Facies at ODP Leg 159: Paleoenvironmental Implications for the Eastern Equatorial Atlantic


Late Albian to early Oligocene sections recovered during ODP Leg 159 (Cote d'Ivoire-Ghana Transform Margin) were studied using detailed organic geochemical, clay mineralogical and micropaleontological methods. Changes in lithology from late Albian organic poor siliciclastics and carbonates to Turonian organic rich black shales, bearing kerogen type II material, record the transition from oxic to anoxic depositional conditions related to the early opening of the Equatorial Atlantic gateway.

Clay assemblages showing smectite and chlorite in Turonian sections indicate intense terrestrial weathering from mixed source areas. Repetitive fluctuations in the position or establishment of oxygen deficient conditions off Ivory Coast/Ghana are inferred from variations in content and composition of sedimentary organic carbon during the Coniacian and Santonian-early Campanian. Clay data do not evidence a pronounced physical weathering on the African hinterland. Subsequent progressive deepening of the eastern Equatorial Atlantic during the middle Campanian to late Paleogene is deduced from generally organic poor, monotonous black, carbonaceous claystones. Simultaneously, an aridification of African climate is suggested by the deposition of eolian derived clay-sized quartz. A trend to higher productive surface water conditions throughout the Eocene to earliest Oligocene is indicated by the deposition of porcellanites. Organic facies of these sediments is characterized by intermediate organic carbon contents of mixed kerogen type II/III.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky