National Institute of Advanced Industrial Science and Technology (AIST) This page is a page of the former research institute. We stopped updating on March 31.2001.
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National Institute of Advanced Industrial Science and Technology (AIST) This page is a page of the former research institute. We stopped updating on March 31.2001.
E-mail to webmaster (Japanese) E-mail to webmaster (English)

Integrated Stratigraphy

Thick marine sequences of some Miocene sedimentary basins in Japan interbeds volcaniclastic layers frequently, some of which are suitable for radiometric dating. Combining the biostratigraphy with geochronology, we can refine the integrated time scale, which enable us to know the age of strata precisely. We investigate the integrated study on Middle Miocene Boso sequence and Karasuyama sequence in central Japan, because both calcareous and siliceous microfossils are yielded from these sequences and dateable volcaniclastic rocks are interbedded.
The Middle to Late Miocene Arakawa Group is well exposed along the Arakawa River, Tochigi Prefecture, central Japan. More than 700m marine sequence, composed mainly of fossiliferous siltstone, interbedded more than 100 felsic volcaniclastic rocks. We found 20 tuff layers contain fresh biotite and/or hornblende, which are suitable for radiometric dating. We measured K-Ar ages of some key tuff beds as followed.
Kb-1
　is the lowest tuff in this sequence, of which 14.28+/-0.23 Ma K-Ar biotite age was obtained. The CN4/CN5a calcareous nannofossil boundary is recognized (Tanaka & Takahashi, 1998) approximately 20m above the Kb-1. The previous estimated age for CN4/CN5b boundary is 13.6 Ma (Berggren et al., 1995).
Kb-23
　is locate about 23m lower than the CN5a/CN5b boundary, which yielded 12.39+/-0.20 Ma K-Ar biotite age.
Kb-29<BR> 　locates only 5m lower than the CN5a/CN5b boundary. The K-Ar biotite age of 12.08+/-0.20 Ma indicates an older limit for the CN5a/CN5b boundary.
Og-1
　is one of the most important volcaniclastic layers in this sequence, because the Og-1 locates only 1m upper than the CN5a/CN5b boundary. The K-Arbiotite age of 11.76+/-0.19 Ma represents a boundary age directly, however K-Ar hornblende age shows somewhat younger (11.40+/-0.2 Ma).
Og-25
　contains fresh biotite, whose K-Ar age is 11.48+/-0.18 Ma. The first occurrence (FO) of G. nepenthes, indicates the N.13/N.14 foraminiferal boundary, is expected to locate between the Og-1 and Og-25.
Og-61
　locates the upper most part of the Ogane Formation, which gave K-Ar biotite age of 10.17+/-0.17 Ma, while no biostratigraphic control around this tuff is obtained.

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