30. TURONIAN–SANTONIAN BENTHIC FORAMINIFER ASSEMBLAGES FROM SITE 959D (CÔTE D’IVOIRE-GHANA TRANSFORM MARGIN, EQUATORIAL ATLANTIC): INDICATION OF A LATE CRETACEOUS OXYGEN MINIMUM ZONE1Ann E.L. Holbourn2,3 and Wolfgang Kuhnt2 |
ABSTRACTTuronian–Santonian organic-rich fissile black claystones with laminated intervals from Hole 959D on the Côte d’Ivoire-Ghana Transform Margin, drilled during Ocean Drilling Program Leg 159, contain benthic foraminifer assemblages dominated by buliminid associations. The lower Turonian assemblage from Core 159-959D-68R is strongly dominated by Bolivina anambra, Praebulimina sp. 1, Praebulimina sp. 2, Praebulimina sp. 3, and Gavelinella spp. The upper Turonian to lower Coniacian assemblage from Core 159-959D-67R displays low abundance and low diversity and consists mostly of organically cemented agglutinated taxa and/or some corroded tests of Lenticulina, Bolivina, Gyroidinoides ex gr. nitidus. The middle Coniacian to lower Santonian assemblage from Core 159-959D-66R and from the base of Core 159-959D-65R contains high numbers of Praebulimina robusta, Praebulimina fang, Neobulimina subregularis, and Buliminella cf. gabonica, but shows marked fluctuations in abundance and diversity, which appear to be related to changes in total organic carbon. The distinct composition of the two buliminid associations in Core 159-959D-68R and Core 159-959D-66R suggests that endemism was stronger during the early Turonian, when circulation was probably more restricted and connections between equatorial Atlantic basins were limited. We interpret the late Coniacian–early Santonian depositional environment to be an oxygen minimum zone in a more open marine outer shelf or upper slope setting. A sudden increase in the proportion of deep-water agglutinated foraminifers in Sample 159-959D-65R-5, 30-34 cm, points to rapid deepening during the early Santonian. The higher diversity and the abundance of cosmopolitan taxa in samples from Cores 159-959D-66R and 159-959D-65R indicate that deep circulation between the North and South Atlantic was well established by the middle Coniacian to early Santonian. |
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