1973 Volume 23 Issue 119 Pages 227-236
This paper describes basic stratigraphical data, geological structure, petrographical characters of sandstones, and relationship between Paleogene formations and their Upper Cretaceous basement rocks in the southern part of Shimoshima, Amakusa Coal-field, Kumamoto Prefecture. Unconformable features between the Paleogene Fukuregi Formation and the H-Formation, which includes the so-called Fukami Formation having been interpreted as of Tertiary, are recognized at some localities in the studied area. The time-gap is not clear because of scarcity of fossils to indicate definite ages. The formations below and above the unconformity, however, show considerable dissimilarities in geological structure. For instance, the Upper Cretaceous formations are controlled by the tectonic movement, to which the Asami dome structure might be attributed; while the Paleogene strata are only influenced by the movement to form the so-called Ittyoda synclinal structure. These facts indicate a considerable time-gap between two formations. Sandstones of the Cretaceous and Paleogene formations are dissimilar in the lithological characters. The formers are characterized by abundant rock fragments, particularly those of igneous rocks. The latters predominate in quartz (Fig.5).The Cretaceous sandstones contain garnet and epidote in greater quantities than the Paleogene ones. The results of the petrographical study are consistent with the geotectonic interpretation mentioned above.
