34. PLANKTONIC FORAMINIFER BIOSTRATIGRAPHY: EASTERN EQUATORIAL ATLANTIC1Richard D. Norris2 |
ABSTRACTFour sites were drilled during Ocean Drilling Program Leg 159 on the continental slope off the Côte d’Ivoire and Ghana in the eastern equatorial Atlantic. Samples rich in planktonic foraminifers range in age from the Holocene to early Miocene, whereas Cenozoic samples older than this contained either poorly preserved foraminifer assemblages or were completely barren. Preservation of foraminifers is best at the two shallowest sites (Site 959, 2090 m water depth, and Site 960, 2048 m water depth) and degrades with increasing water depth. Foraminifers are almost completely absent at Site 962 at 4648 m. Even in the shallow water sites, preservation is moderate in the Pleistocene, the earliest Pliocene, and the early to middle Miocene where there is extensive fragmentation of the <150-µm fraction and often dissolution of the interiors of large, robust specimens. Preservation is good through much of the upper Pliocene and the upper Miocene. Planktonic foraminifer assemblages are typical of the tropical oceans, although temperate species such as Neogloboquadrina pachyderma (sinistral), Globorotalia panda, and Globorotalia miozea are found sporadically. Globorotaloides hexagona is absent from Leg 159 sites throughout upper Pliocene Zones Pl4 and Pl5 and may prove to have a pattern of disappearance from upper Pliocene Atlantic sediments similar to Globorotalia tumida and species of Pulleniatina. The recovered section at Site 959 is almost biostratigraphically complete between lower Miocene Zone N7 and the Holocene, although there is probably a hiatus in the middle Miocene that has cut out, or greatly thinned, Zone N9. The other Leg 159 sites are less complete. The middle Miocene section is missing at both Sites 960 and 961, and Site 961 also has a hiatus across the Miocene/Pliocene boundary. |
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