Sedimentary petrographical experiments were performed using core samples from a bore hole for petroleum drilled at the Jikiba district, Kumamoto Prefecture. The main purpose of the present study is to decide a Cretaceous-Tertiary boundary from a standpoint of petro-graphical characters of sandstones.
In the major constituents, samples collected at the depth of 550 meters and 990. 20 meters are characterized by the prominence of quartz, and those below 1550 meters are by that of rock fragments. The heavy mineral compositions of the samples collected at the points above the level of 1, 100 meters in depth represent a dominance of zircon and a inferiority of garnet, and those of the samples below it show a predominance of garnet and epidote, although some excep-tions are met with.
As compared these results with previous ones (OHARA, 1960; SATO, 1961, 1969; MIKI, 1972; MIKI and UEMATSU, 1973), the writer may conclude that, in general, the samples above 1, 100 meters are of Paleogene and those below 1, 155. 50 meters are of Cretaceous.
According to the macroscopic and microscopic observations and also to the heavy mineral analysis, two reserve faults may be presumed; the one may be at the depth nearly of 500 meters, and the other may lay between 1, 200 and 1, 300 meters, if the exceptional assemblage of heavy minerals of this column is taken into account.