Problems on the Prospecting of the Coal Seams of Taira District

Part II. Theories of Sedimentation of the Coal-bearing Complex, and Causes of Deterioration of Coal or Coal Seams

Abstract

The Iwaki Coal-bearing Beds of the Taira-Yoshima District contain the following six coal seams or groups of seams, named in ascending order : the second Kaso group, the first Kaso group, Honso group, Nakaso, Joso and Funtanso. This coal bearing formation repeats the cycles of sedimentation in a very regular order, beginning with the deposition of sandstone and conglomerate soon after subsidence of the coal area and finishing with the deposition of coal and shale after the silting up of the area. The cycles may be classified into tow types, i.e., the Kaso-type and. the Joso-type.
Coal seams above the Honso group belong to the Joso-type. This type of cycle (or cyclothem) begins with marine sedimentation, represented by sandstone and conglomerate or conglomeratic sandstone accompanying marine shell fossils and sometimes sand pipes, which was caused by a temporary marine invasion after sudden subsidence of the coal-forming field.
Coal seams below the Honso group consist of three groups, each group containing a few coal seams intercalating sandstone and shale, and between each group occurs a dominant conglomerate bed. The coal seams of each group have the tendency to converge into one seam in the western, shallower portion, and, on the contrary, split into several seams in the eastern, deeper region. These belong to the Kaso-type, and the roof of each coal seam is generally composed of shale or inudstone, or, rarely, of sandstone.
The sedimentation bf the Kaso-type must signify that of a lake basin. The transition to the Joso-type above the Honso group may imply the silting up of the basin and the direct connection of the coal-forming field with marine water. The cycles of the Joso-type may represent a sort of deltaic cyclothein, and also possibly a piedmont cyclothem, because of the narrowness of coal field along the coast line. The area of sedimentation in the Joso-type coal seams should have become narrower in each successive seam finally giving way to a purely marine facies without coal.
The roof of the Joso-type of coal seam such as the Honso and the Joso is composed mostly of coarse grained sandstone and conglomerate, and the coal itself has been washed out locally by the action of marine waves and currents, frequently forming a sort of intraformational conglomerate directly above the coal seam. This conglomerate has been named by the writer as "coaly simultaneous conglomerate".
The deterioration of the quality of coal of the Joso-type is caused, on one hand, by the thinning of the coal from above by erosion, or sometimes by the overlapping of the coal directly on the basement rock. On the other hand, the deterioration of quality by an increase in the ash content occurs in the eastern region. Among the Kaso-type of coal seams, the second Kaso group is thick and of good quality where it is deposited in the valley bottoms between hidden hills, which represent the true hills and valleys of coal forming age.
Away from the hidden hill region, however, the coal seam of the second Kaso shows a transition to the mere plant fossil bed with latifoliate tree leaves, and moreover was eroded away, in some places, probably by streams.