Abstract:
We studied the lower Turonian to Eocene section of Hole 959D (Leg 159, Cote d'Ivoire-Ghana Transform Margin, eastern Equatorial Atlantic), using sedimentologic, mineralogic, organic geochemical and micropalaeontologic techniques. We focussed on a 230m interval of Santonian to Upper Paleocene black, carbonaceous claystones. Because these are barren of calcareous planktic microfossils, their biostratigraphy is based on benthic foraminifers. Albian siliciclastics are unconformably overlain by some 40 m of Turonian to Coniacian calcareous sandstones characterized by a mixture of clay minerals including smectite, illite, kaolinite mixed-layers and minor chlorite. Around the Coniacian/Santonian boundary, facies changes to the black, carbonaceous claystones. Carbonate carbon content in the claystones is close to zero, whereas organic carbon attains more than 15% at the base of the claystones, where hardgrounds and accumulations of phosphatic debris occur. Smectite is the dominant, sometimes the only clay mineral in the claystones. Kaolinite, illite-smectite mixed-layer and illite amounts fluctuate, but are generally low. At the K/T-boundary, kaolinite increases markedly and declines to zero 20m above, where smectite remains the only clay mineral until the onset of Early Eocene siliceous deposition. We interpret the variations in type and quantity of clay minerals and organic matter, and of the composition of benthic foraminiferal assemblages of this apparently monotonous sequence mainly in terms of marine productivity and terrestrial input. Some of the variations correlate with super-regional sea-level and climatic events, whereas others appear to be related to the peculiar tectonic situation along the transform margin.