On the Hachinokubo Gravel Bed in the Sasebo Coal Field and its surrounding area, northwestern Kyushu, Japan

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  • 北松地域およびその周辺にみられる八ノ久保砂礫層（新称） について : ''佐世保炭田''の研究（その2)
  • "佐世保炭田"の研究-2-
  • サセボ タンデン ノ ケンキュウ 2

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Abstract

The Hachinokubo Gravel Bed occurs in the Sasebo coal fieid and extends over the northern part of the Nishisonogi and Higashimatsuura Peninsulas. It unconformably overlies on the basement rocks which consist of Paleozoic metamorphic complex, coalbearing and non-coal-bearing Tertiary groups, and older basalts. They are overlain unconformably by the Matsuura Basalts. In several coal pits, the Tertiary shale and sandstone over the coal, and the coal seam itself, gradually become thinner where the Gravel Bed lies directly above them. Consequently, the coal seam becomes thinner and thinner until it disappears completely due to erosion. In the western part of the area, gravels of the Gravel Bed are predominantly round to subangular in shape but are somewhat flattish in directions along the bedding planes. They are of pebble to cobble size and the matrix contists of relatively well sorted loose coarse sand. The Gravel Bed is presumed to be of fluvial origin. In the eastern part, however, the Gravel Bed is formed of fine-grained materials, such as sand and clay, in which a few plant remains are found. Gravels are not as well developed here as in other areas. They may be of lacustrine origin in the eastern region. Gravels of the Gravel Bed are composed of white quartzite, quartz schists, brownish purple to dark greenish gray chert, hard black siltstone, gray sandstone, gray conglomerate, granites, granodiorite, aplite, granophyre, rhyolites, basalts, and andesites. Among these various rock types, fossil radiolaria are frequently found in the gravels of chert, black siltstone, and the same kind of chert fragments contained in the conglomerate. Sandstone and conglomerates are fairly hard and indurated. They might have been derived from the Mesozoic or Paleozoic formations far removed from this area. The maximum thickness of the Gravel Bed is about 30 meters, but it is not uniform in thickness. In some places it vanishes completely. With the maximum dip of less than 4°, the Gravel Bed is inclined northward at an angle of about 1°. Several faults that extend in the directions northwest by west to southeast by east and east to west intersect both the Gravel Bed and the Matsuura Basalts. These conditions lead to the conclusion that the faults might have been active before the Gravel Bed was deposited. They show different amounts of vertical separation between the Gravel Bed and the underlying Tertiary formations.

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