Journal of The School of Marine Science and Technology,Vol.2 No.3
The Geological Structure of the Tanzawa Mountains from Consideration of the
Benthonic Large Foraminifera Fossils
Masahito KADOTA, Tetsuro SUEKANE and Yoshifumi MISAWA
Abstract
The Tanzawa group of the Miocene age is distributed in the Tanzawa mountains. There is plutonic mass on a
large scale at the center of geological tectonic unit and the mass exhibits a typical dome structure.It is partly decided
on the basis of larger foraminifera’s fossil and calcareous microfossil that the age of the Oyama Subgroup in the
center of the Tanzawa Group, which consist of pyroclastics, is about 13〜15Ma. However, under the influence of
noticeable change of the rock facies and their metamorphism caused by a great quantity of magma intrusion, the
stratigraphical correlation of the Miocene rocks in a whole area of the Tanzawa mountains is not completed yet.
Matsumaru (1971)determined the fossil age of larger foraminifera fossil Nephrolepidina observed in the eastern
and the southern part of the Tanzawa Group. In this report we have assigned the fossil age of Nephrolepidina seen
at Morotosawa in the southwest of the Tanzawa. In addition, we add the data for the fossil age of calcareous
microfossil (Aoike et. al., 1997) outcroped at Hayatogawa in the east Tanzawa and consider the stratigraphical
correlation for the whole area of the Tanzawa.
We present a detailed report on the following two themes.
Firstly, the green tuff of the Oyama Subgroup in Tanzawa Group is distributed at Morotozawa in Yamakitamachi,
which is located in the south of the Tanzawa mountains. We present a detailed report on the fossil age of
Nephrolepidina japonica (Yabe) sampled from limestone, in which the density of Miocene larger
foraminifera’s fossil can be seen information from N8zone of Blow (1969).
Secondly, on the basis of the first theme, we consider a stratigraphical correlation for the whole area of the
Tanzawa, which is not established yet.