The Fundamental Stratigraphy of the Late Tertiary Kiyosumi and the Anno Formations along the Inokawa River in the Central Boso Peninsula
1980 Volume 89 Issue 2 Pages 113-123
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1980 Volume 89 Issue 2 Pages 113-123
The Amatsu Formation and the Toyooka Subgroup consisting of the Kiyosumi and the Anno Formations are well exposed along the Inokawa River, the uppermost tributary of the Obitsu River which runs northwestward from the northern foot of the Kiyosumi mountain (315m) in the Boso peninsula.
The valley of the Inokawa River is one of the areas well-known for the distribution of the Toyooka Subgroup and the basal member of the Kazusa Group. The writers observed on the stratigraphy of the Kiyosumi and the Anno Formations along the Inokawa River paying the special attention to the rhythmic aspects of sediment, the development of conglomerate, and the existences of pyroclastic layers, such as white or pink or so-called gomashio tuffs, pumice and scoria. Some results of the present work are as follows:
1) The Kiyosumi Formation, consisting mainly of sandstone with thin bedded siltstone and conglomerate, is subdivided into three members, namely the lower conglomeratic sandstone (about 45m thick), the middle sandstone (about 140 m thick) and the upper sandstone (about 80 m thick) by the lithologic facies and the sedimentation of the strata.
2) The Anno Formation which is characterized by the alternation of tuffaceous sandstone and siltstone with many kinds of pyroclastic sediments, is subdivided also into three members, i. e., the lower siltstone (about 95m thick), the middle alternation of sandstone and siltstone (about 85 m thick) and the upper sandstone (about 120m thick).
It is remarkable that the middle alternation contains abundantly so-called gomashio tuff in several horizons, but that the upper sandstone is interbedded frequently with rather thin scoria which is black or dark in colour.
3) In this surveyed area the thickness of the Anno Formation is more or less larger than that of the Kiyosumi Formation, compared with the case in other areas where two Formations are distributed.
4) The thickness of the Kiyosumi Formation in the Inokawa River area is almost same as that of the Formation in the Ubara area along the Pacific Ocean, but the lithologic facies is somewhat different from each other between two areas, namely the conglomerate facies is developed considerably in the Inokawa River area, but it is very rare in the Ubara area.
5) In the Inokawa River area the carbonaceous layers which contain fossil pollen grains and spores are abundant, but in the Ubara area they are rather few.
6) The Kiyosumi and the Anno Formations in this area yield microfossils such as foraminifers and calcareous nannoplanktons, but macrofossils are very rare.
