Calcareous nannofossil biostratigraphy of ODP Hole 108-667A and DSDP Site 85-574

In Hole 667A in the equatorial Atlantic Ocean, Zones CP19 through CN5 were recognized, whereas at Site 574 in the equatorial Pacific Ocean, only the CN4/CN5 boundary could be determined. Boundaries were identified by sharp rises and declines in abundance at the beginnings and ends, respectively, of index fossil ranges.

The sharp rise in abundance at the beginning of the range of Triquetrorhabdulus rugosus provided a good datum level in both regions; the same is true for the sharp decline in abundance at the end of the range of Cyclicargolithus floridanus. The last occurrence of Helicosphaera ampliaperta was used to mark the CN3/CN4 boundary in Hole 667A, while at Site 574, H. ampliaperta was absent. The abundance pattern of Triquetrorhabdulus carinatus obtained from Hole 667A makes it impossible to observe a distinct disappearance level.