関門地域のいわゆる非海成第三紀層 : その1 北九州市地区の出山層について

Tertiary sediments are distributed widely in the Kwanmon area (so-called Kokura Coal-field) covering both sides of the Kwanmon Straight dividing Kyushu from Honshu. The lowermost division of the sediments cropping out in northern central hilly areas of Kitakyushu City are stated to be non-marine and has been correlated with the Ideyama Formation in the Chikuho Coal-field. Owing to indigence of paleontological evidence, and also to scarcity of outcrops at the surface, however, our knowledge of this formation is still incomplete. Based on newly found exposures and a lot of drilling data, this paper gives descriptions on the details of the distribution of the formation with its concealed extension beneath the Quaternary in the lowland city area, the stratigraphy, lithology and sedimentary features. Some comments on the sedimentary history of the formation are also presented. The Ideyama Formation in this district is characterized by a remarkably coarsegrained sediments consisting predominantly of conglomerate and conglomeratic sandstone with intercalated siltstone and shale and by the cyclic sedimentation by which the formation is divided into seven members. The lower six members of the formation settled by lWAHASHI and OHARA (1959) in the Itozu-Hiagari hills area are continued into the lowland city area beneath the Quaternary with a similar thickness and lithologic sequence. The uppermost member, about 250m, which formerly called the "undivided member" and herein named the Kurobaru sandstone member, is also found in the concealed area and forms a distinct unit consisting mostly of conglomerate and conglomeratic sandstone with a fewer and thinner bands of shale than the lower members. This member is overlain conformably by the coal-bearing Onga Formation. Heavy mineral analysis of sandstones of the Ideyama Formation shows that zircon and tourmaline are dominant, pyroxene, hornblende and monazite are little in many specimens, and epidote is found only in upper members. Garnet is scarcely found in a few specimens. The heavy mineral contents of this formation are divided into two groups, and those of the upper four members contain much fewer zircon and more abundant tourmaline than the lower three members. These heavy mineral assemblages, with a few local exceptions, bear no similarlity to that of the rocks of the basement Cretaceous Shimonoseki Subgroup cropping out widely in the southern and western areas of this district. Lithological analysis of gravels of several conglomerate beds and the paleocurrent system estimated from the diagonal bedding suggest that the main provenance of elastics of the Ideyama Formation was probably a mountain area composed of the Palezoic and Mesozoic systems developed formerly to the north. It may be concluded that the Tertiary sedimentary basin o~ this district was originated in a remarkably undulated land and had been sinking with a rhythmic and cyclic oscilation throughout the period of sedimentation of the Ideyama Formation