Chronostratigraphy of the middle Miocene marine sequence in the Karasuyama area, central Japan. With special reference to the CN 5a/CN 5b boundary.

Abstract

Four volcanic tuff layers (Kb23, Kb29, Og1 and Og25) of the Karasuyama area, Tochigi Prefecture, central Japan, were selected from the middle Miocene marine sequence of the Arakawa Group and dated by K-Ar method for biotite and hornblende minerals. The CN 5a/CN 5b boundary, defined by the LO of Cyclicargolithus floridanus, is located five meters above the Kb29 and only one meter below the Og1 Tuffs. The K-Ar biotite ages of the Kb23 Tuff (12.39±0.20 Ma; 1σ error), Kb29 Tuff (12.08±0.20 Ma), Og1 Tuff (11.76±0.19 Ma) and Og25 Tuff (11.48±0.18Ma) represent a stratigraphic consistency, but the K-Ar hornblende age of Og1 Tuff (11.40±0.20Ma) is somewhat younger. We adopted the eruption age of the Og 1 as determined on biotite (11.76±0.19 Ma) which represents the age of the CN 5a/CN 5b boundary, directly. The previous estimation of the age for the CN 5a/CN 5b boundary (11.8Ma by Berggren et al., 1995) is secured by the precisely determined K-Ar ages.