Calcareous nannofossil biostratigraphy and geologic age of the Kiyosumi Formation of the Awa Group, Boso Peninsula, central Japan: Age determination based on size variations of <i>Reticulofenestra</i> specimens

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  • 安房層群清澄層の石灰質ナンノ化石層序と地質時代－<i>Reticulofenestra</i>属の大きさの変化に基づく年代の決定－
  • 安房層群清澄層の石灰質ナンノ化石層序と地質時代--Reticulofenestra属の大きさの変化に基づく年代の決定
  • アワソウグン キヨスミソウ ノ セッカイシツ ナンノ カセキソウ ジョ ト チシツ ジダイ Reticulofenestraゾク ノ オオキサ ノ ヘンカ ニ モトズク ネンダイ ノ ケッテイ
  • Calcareous nannofossil biostratigraphy and geologic age of the Kiyosumi Formation of the Awa Group, Boso Peninsula, central Jaman: age determination based on size variations of Reticulafwnwstra specimens
  • Calcareous nannofossil biostratigraphy and geologic age of the Kiyosumi Formation of the Awa Group, Boso Peninsula, central Japan: Age determination based on size variations of Reticulofenestra specimens (in Japanese with English abstract)

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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.<br>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.

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