Calcareous Nannofossils from the JOIDES Blake Plateau Cores, and Revision of Paleogene Nannofossil Zonation
AbstractThe calcareous nannofossil zonation for the Paleocene-Eocene interval has been refined by study of samples previously dated with planktonic foraminifers and of samples from continuous sections. In the middle Paleocene one additional distinctive zone, the Cyclococcolithina robusta Zone, is recognizable. The middle and upper Eocene interval, formerly divided into two zones, the Discoaster tani nodifer Zone (=Corannulus germanicus Zone) and the Isthmolithus recurvus Zone, can be divided into nine zones:1. the Discoaster tani s.I.-Sphenolithus radians Zone;2. the Reticulofenestra umbilica-Sphenolithus furcatolithoides Zone;3. the Pemma papillatum Zone;4. the Bramletteius serraculoides Zone;5. the Helicopontosphaera compacta-Chiasmolithus grandis Zone;6. the Hayella situliformis Zone;7. the Isthmolithus recurvus Zone (emended);8. the Sphenolithus predistentus-Discoaster barbadiensis Zone;9. the Helicopontosphaera reticulata Zone.The lower Tertiary sediments penetrated in JOIDES Blake Plateau cores J- 3, J-4, and J-6B can be zoned effectively with calcareous nannofossils. J-3 contains lower, middle and upper Eocene sediments, but the section is discontinuous and contains numerous gaps. J-4 contains an upper Paleocene section from the Discoaster gemmeus Zone through the Discoaster multiradiatus Zone. J-6B contains a discontinuous record of lower and middle Eocene sediments and a continuous upper Eocene section, from the Hayella situliformis Zone on up. The Eocene-Oligocene boundary is contained in cores J-3 and J-6B, but because of the inadequacy of the definition of this boundary in terms of biostratigraphic criteria, its exact level remains undetermined.