Reconsideration on the Possibility of a New Coal Field in Kumamoto Prefecture, Central Kyushu

Abstract

In 1960, H. URATA and the writer reported on the Paleogene formation in the vicinity of Hokonoko, Kumamoto Prefecture and discussed the possibility of finding a new coal field under the ground of this district.
Meanwhile, it has become known that on the mountain-side in the northern part of Kamoto Machi, a conglomerate formation is distributed narrowly but continuously. The writer names this conglomerate the Fudogan formation, and correlates it with the Akasaki group, because of the close similarity in rock facies, being non-marine, non-fossiliferous, and purple in colour.
The Fudogan formation, about 120m thick, dips southward, and in the plain to the south it goes under the Aso volcanic products but still bears the characters of a basal conglomerate.
During October 1964, seismic prospecting by refractive method was carried out in the plain to the south of the hill composed of the Fudogan formation. The measure line was extended 4.8 km in a NNE direction.
In the above prospecting the following three layers were discriminated:
Seismic wave velocity (m/sec) Thickness (m)
1st layer 1, 645-1, 700 150-300
2nd layer 3, 620-3, 800 400-650
3rd layer 5, 670
Comparing these data with those of the geological and seismic investigations previously obtained, it may be safely said that the 1st layer corresponds to the Quaternary formation and the 3rd layer to metamorphic rocks or granite. After careful examination, the writer comes to a conclusion that the 2nd layer is probably Palaeogene formation ranging from the Amakusa stage to the Ariake or Nogata stage.
If this presumption is correct, the paleo-Ariake Basin proposed by H. MATSUSHITA might have been branched off, extending to this area.
The writer here presumes a possible stretch of a new coal field underlying the Quarternary deposits in this area.