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A relatively expanded and largely complete upper Paleocene to lower Oligocene sequence was recovered from the pelagic cap overlying Allison Guyot, Mid-Pacific Mountains. The sequence consists of calcareous ooze with a high planktonic foraminifer content. Two separate holes (865B and 865C) were drilled with the advanced piston coring system. Samples from these holes have been the target of intensive calcareous nannofossil biostratigraphic investigations. Calcareous nannofossils are moderately well preserved and diverse throughout the sequence recovered, which extends from nannofossil Zone CP3 to CPI 6. Our data show that unconformities occur in the uppermost lower Eocene and at the Eocene/Oligocene boundary, correlating to part of Zones CPU and CP 12 and Zones CP 15 and CP 16, respectively. Most traditional zonal markers are present; however, the rarity of several of them, particularly discoasters, and the overgrowth of others, including species of Tribrachiatus, in the uppermost Paleocene and lower Eocene makes zonal subdivision of part of this sequence difficult. For this reason, more attention has been paid to establishing the precise ranges of nonzonal taxa. We were able to determine 142 zonal and nonzonal events in the Paleogene section by intensively sampling both holes (1-5 samples in each core section). Sample density increased toward the Paleocene/Eocene boundary. Although the events are spread fairly evenly throughout the section, some of the most dramatic turnover occurs in the boundary and early Eocene interval. Currently unobserved gradational forms in nannofossil lineages indicate that parts of this sequence are more expanded than any other yet recovered. Plate reconstructions show that Site 865 was close to the equator at the time Paleogene sediments were deposited. The near completeness, expansion, paleolocation, and shallow burial depth of this sequence render it an ideal low-latitude Paleogene reference section.

Calcareous Nannofossil Biostratigraphy of Site 865, Allison Guyot, Central Pacific Ocean: A Tropical Paleogene Reference Section