Abstract
Because the Tertiary coal fields located in northwestern Kyushu are very imporant in Japan, respective geology has for a long time been investigated by many researchers.In spite of a large number of these previous works, however, some incomplete problems dealing with biostratigraphy, stratigraphic boundary between the Cretaceous and the Teritary formations, geological history, etc. still remain.
In this paper, general remarks on these coal fields are commented, discussing these problems in detail, based on a great deal of new data obtained recently by many peoples. Essential problems discussed and inferential results are as follows:
(1) Boundary problem between the Cretaceous and the Tertiary formations.
From paleontological evidence, it is proved that the lower part of t he Koyagi formation, the lowermost formation of the paleogene in the Takashima coal field, is of the cretaceous, and is named the Mitsuse formation. Since the rest (upper) part of the Koyagi formation, however, has nearly the same rock facies to those of the Mitsuse, it is impossible to draw the Cretaceous-Tertiary boundary between these two formations. As the unconformable relation between the Koyagi and the overlying formation is observed at other several localities, the possibility has newly raised that the equivalents having the same rock facies in the other coal fields like the Miike, are also of the cretaceous.
(2) Biostratigraphical Problem.
Employing the metho d that takes gross faunal continuity or discontinuity as a basis of correlation (after Shuto), the Paleogene/Neogene boundary should be settled between the Mazean and Nishisonogian, and the lower/middle Miocene between the Nishisonogian and Ainouran in Northwest Kyushu, but in the Sakamizu formation in North Kyushu. Because the discontinuity of molluscan fauna is not marked between the Kase and the Fukui formation but the former and the overlying Ohya formation, the boundary between the Sasebo and Nojima Group (= the upper and the middle Miocene) should be settled between these two formations.
(3) Extension of the coal field.
For future coal exploitat i o n, the northern continued area of the Chikuho coal field (= Hibiki-Nada), the western part of the Ariake sea, and the off shore area between the Ike-shima and the Ioh-jima should be put forward.
(4) Formation and development of the coal fields from the geotectonic point of view.
Resulting from geotectonic movements of the Ryukyu Island Arc, not onlyelo ngated basins being parallel to the Arc (like the Okinawa trough) but also many depressive basin perpendicular to the arc might be also formed.
Because geological and geographical circumstances in the latter basins were very suitable for deposition of plant materials, these basins become coal fields. Conclusively said, the coal fields in northwestern Kyushu might be rift valleys formed in the back deep, which had occured through the Ryukyu tectonic movement.
