The Stratigraphic Relation Between the Ouchi and the Kishima Groups in the Karatsu Coal Field, Kyushu, as Deduced From the Varying Thicknesses of Strata

Abstract

It has been previously discussed among some geologists that the Kishima group was deposited on the pre-Kishima erosional surface of the underlying Ouchi group. The discussion has been based mainly on the remarkably varying thickness of the uppermost part of the Yoshinotani formation which occupies. the upper part of the Ouchi group.
According to the writer's observation of the boundary between the two groups, however, the vertical changes in lithofacies from the Yoshinotani formation to the Kishima formation are very gradual and any evidence to imply unconformable relation is not found at most of the outcrops in the coal field.
In order to re-examine the thickness variation of the strata, the writer drew isopach maps based on some marker horizons as illustrated in figure 2, for the Yoshinotani formation and the lower units of the Kishima group, such as the Kishima formation, the Sari Sandstone and the Honeishi Zone. Comparison of the patterns of the isopach maps reveals that the pattern of the isopach map of the uppermost. Yoshinotani formation (figure 3) is very similar to those of the units of the Yoshinotani formation and the Kishima formation (figures 4 and 5), but is different from those of the Sari Sandstone and the Honeishi Zone (figures 6 and 7).
The result of the pattern comparison suggests that the remarkably variable thickness of the uppermost Yoshinotani formation is attributable to local variations in subsidence of the depositional basin than to erosion of the pre-Kishima age, and that the late Oligocene Ashiya marine transgression may have extended mainly over this area during the deposition of the Sari Sandstone.