In Ôita City, there are distributed the Sekinan and the Ôita groups which deposited during the late Tertiary and the early Quaternary.
By pollen analysis of these deposits, the following results are obtained.
1) The floral changes of that time in this region are similar to those of the Ôsaka group and its equivalents of the other regions in Japan, and summarized as follows.
a) At the middle part of the Higashiwasada formation (the middle part of the Sekinan group), among the Tertiary floras
Nyssa becomes extinct, but
Metasequoia is still found in this stage.
b) The first appearance of
Cryptomeria in this region is shown at the middle part of the Handa formation (the upper part of the Sekinan group).
c) In the upper part of the Takio formation (the lower part of the Ôita group)
Metasequoia has already disappeared, while
Fagus becomes predominant.
2) The first cooling of the climate may be indicated by the appearance of deciduous
Quercus at the middle part of the Higashiwasada formation. Therefore, it is presumed that this horizon indicates the Plio-Pleistocene boundary in this region.
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