Abstract
We examined calcareous nannofossil assemblages to determine the geologic age of the Kiyosumi Formation of the Awa Group, central Boso Peninsula, Japan. Prior to analyzing the Kiyosumi Formation, we examined size variations of Reticulofenestra specimens in lower Pliocene sediments in Hole 806B, Western Equatorial Pacific, revealing a number of distinct bioevents defined by changes in the sizes of Reticulofenestra specimens.
As a result, we identified a nannofossil datum (top of Amaurolithus spp.) in the Kiyosumi Formation, and the boundary between Zones CN10 and CN11 was identified in the uppermost part of the formation. Moreover, the Kiyosumi Formation records a similar stratigraphic size variation of Reticulofenestra specimens and bioevents to those seen in Hole 806B. Geological ages estimated from the datum and bioevents appear to coincide with previous interpretations based on magnetic reversals recorded in the formation. The results suggest that changes in the size of Reticulofenestra are useful in terms of biostratigraphy.
