The present study is an attempt to work out the geology and geological structure, especially the stratigraphy and historical geology of the Tertiary of the marine region situated between the Sakito-Matsushima and Takashima Coalfields located in the sea west of the Nishisonogi Peninsula, Nagasaki Prefecture.
No geological information on this area has been available up to the present, but geophysical prospecting and drilling investigations (Figs. 3 and 4) have shown the existence of a coal einbeded Paleogene Systems having sediments correlated to the Tertiary in the Sakito-Matsushima and Takashima Coalfields (Figs. 1 and 2) considered to be submarine coalfields. Accordingly, all these coalfields have been collectively designated as the Nishisonogi Coalfield by the author. The study on the history of its formation and development are presented in summary as follows:
(1) Based on the main features of basements such as Nomo Peninsula, Barriers of Mitsuse, Miezaki and Terashimaoki, the sedimentary basin of this area may be divided into a number of depressions where the formation of the Nishisonogi Coalfield began in the late Mesozoic Era by local sedimentation action of the Mitsuse Formation and reddish purple beds and coaly shale. (Figs. 6, 7-1 and 7-2). This was followed by the Paleogene sedimentation period during which repeated unification and separation of the depression took place accompanied by movement of each barrier. However, it is concluded that the sedimentation gradually extended from the south to the north.
(2) During the Terashima Age (Fig. 7-3) in the Paleogene sedimentation period, the central coalfield was an uplift belt, and the south side area of the coalfield was environment preferable to the formation of coal seam. In the succeeding Matsushima Age (Fig. 7-4), the uplift belt caved in and the south side of the coalfield became a marine environment and the conditions became favorable for extensive coal sedimentation on the north side between the Sakito-Matsushima and Ikeshima terrains.
(3) In the Nishisonogi Age (Fig. 7-5), the area of the sedimentary basin of this coalfield markedly increased, and extended to the sea region. Not only did the formation of the coalfield cease on the north of the uplift belt in the central coalfield, but also brine water intruded to some extent into the Nishisonogi Peninsula where the Nishisonogi Group had deposited. The northern boundary of this Nishisonogi Sea extended to the Sasebo Coalfield.
(4) The northeast boundary of the Nishisonogi Coalfield is cut by a basement fault (Yobuko-no-Seto Fault) whose continuous activity before and after the Tertiary sedimentation has played an important role in the coalfield formation. The normal and reverse faults, and folding in the Tertiary period also aided the basement in forming a buried hill, thus causing the barriers to become extinct. This greatly influenced the formation of the coalfield.
(5) This coalfield is of moderate scale compared with other coalfields in North Kyushu. Nevertheless, it has existed over a long period of time from the Mitsuse Age in the late Mesozoic to the sedimentation of Nishisonogi Group, whose formation history itself may be considered to be a representation of the historical geology of the coalfields in North Kyushu.
