北部九州の第三紀堆積盆地のテクトニクスと堆積作用  [in Japanese] Tectonics and sedimentation of the Tertiary sedimentary basins in the northem Kyushu  [in Japanese]

This paper deals with changes of depositional environments and tectonic movements of Paleogene sedimentary basins in North and West Kyushu and discusses on the origin of depositional and tectonics ettingin several stages of basin development. A sedimentary basin where the Taishu Group in Tsushima and Katsumoto Formation in Iki Islands and coalfield formations in North Kyushu were deposited was intra-arc basin in between the volcanic chain that was jointed to the Paleogene volcanic front in Chugoku district and old non-volcanic arc in Middle Kyushu. Inland sea of the intra-arc basin is newly named the Paleo-Tsushima Bay. The Taishu Group was deposited in shelf to deltaic environments in the central part of the Paleo-Tsushima Bay, on the other hand the coalfield formations were deposited along its marginal nearshore and onshore area. Paleogene volcanoes along a volcanic front estimated in between the Tsushima-Goto Islands and Korea Peninsula supplied a huge amount of pyroclastics to sedimentary basins of the Taishu Group and coalfields in mainland of Kyushu. Newly compiled time table of the Paleogene sequence based on ages obtained from microfossils and fission track dating of major tuff beds indicates that well known transgression and regression recorded in coalfield sequence occurred corresponding to global sea level changes. The Chikuho-type structure which is characterized by syn-sedimentary NW-SE〜NNW-SSE trending normal faults running along the eastern boundary of each coalfield basin was born in middle Eocene, in correspond to a change of plate motion of the Pacific plate. Thick pile of fanglomerate and braided river sediments of the Ideyama Formation and its correlative formations were likely to have been deposited when the normal faulting was most actively going on. The faulting became in active in around 30 Ma, because the Shikoku Basin initiated its opening, and lateral movement prevailed along the plate boundary. In accordance with weakening of fault activity, the coal field basins diminished. During 21 to 18 Ma, tectonic setting of the sedimentary basins and character of volcanic activity in North and Northwest Kyushu were totally changed. Volcanic activity occurred nearby the mainland of Kyushu and its character changed from felsic to andestic. Stress field of the northwestern margin of Kyushu turned to be compressional, and thrust movement represented by the Sasagawa Thrust started its activity and fold structures in the Tsushima and Amakusa Islands were formed. Opening of the Japan Sea brought about these changes.