Formation Process of Coal Basin, with Special Reference to that of the Ishikari Coalfield, Japan

Abstract

The Ishikari Coalfield in the central part of Hokkaido is the most important, productive coalfield in Japan with about 2500 squarekilometers of total area. It is built of early Eocene-middle Oligocene deposits, known as the Ishikari Group, including several formations, with thickness up to almost 3000 meters. These formations are mostly of non-marine origin with numerous workable, high quality coal seams and a few are of marine facies. (Fig. 1)
The Ishikari Groop is underlain by the 6, 000 meters thick Cretaleous marine sediments named Ezo-and Hakobuchi group and is overlain by the upper Oligocene marine Poronai Groop, 1, 600 meters in thickness, being bounded both ends by slight ungular unconformity. The Ishikari coal basin is observed to lie in between of the Cretaceoos Yezo-geosynclinal basin on the east and the Oligocene Poronai embayment on the west. This migration of basins since Mesozoic time down to later Tertiary date shows the westward propagation of crustal movement which is a part of the Alpine orogenic chain (Fig. 2). These coalbearing formations are the product of very calm period in the middle Paleogene Tertiary, just after the extensive Cretaceous transgression and before the marine invasion of late Paleogene period. This tendency, namely, coal basins are usually provided between or, just after crustal movements, is universally acceptable at various parts of the world throuogh ages.
The coal basin was formed, paleogeographicaly, by gentle, broad downwarping of wide, very flat area like peneplain which developed between Cretaceous and Tertiary interval. Local depressions, embayments and barriers were existed at places. (Fig. 3) Sinking of basins at various stages of the Ishikari epoch were took place very gently, mostly not so deep, and intermittently, changing center of subsidence, so as to yield cyclothems and also suitable to accumulate plant materials which formed coalseam in later stage.
From paleogeological point of view, during the period of Ishikari basin, exposed rocks on the eastern backland were of Cretaceous and Pre-Cretaceous sediments, part of which metamor phosed into schist and migmatitic gneiss, being accompanied by granitic rocks and basic to ultrabasic Intrusive rocks such as serpentinite. This kind of rocks are evidenced by examining heavy minerals in sandstones aud also pebbles in conglomerates in the Ishikari group. It also suggests us the rising up and exposition of Hidaka and Yubari mountains at that time. On the contrary, exposures of Pre-Cretaceous beds with granitic intrusive mass, with effusive rocks such as rhyolite, andesite and basalt are proved to exist on the western land area. (Table 1)
Vegetations prevailed at the Ishikari period, as recognised by fossils of plant leaf and pollen, were mainly of mixture of Arcto-Tertiary Geoflora and Paleo-Tropical Geoflora with genus of temperate zone with occasional intermixing of warm-temperate ones. This reminds us the climate of that time as that of north Formosa or Okinawa Islands. Only a slight changes of flora were observed throughout the Ishikari period, being a little warmer in the middle and got a little cooler climate at the close of that age. Almost all plant species were of lowland habit and it is also noticeable that a very few species were of waterflooding area.
Coal seams were formed by accumulation of plant materials when the watered area were shallow enough to grow vegetation, repeating growingup, falling down and also by partial transportation. Plant seams were laid down not so high and not so low from water level, during pause of subsidence long enogh to fill up basin by sedimens, after a gentle sinking. (Fig. 4) Intermittency of subsidence and deposition and its repetition were shown by existence of cyclothems in coalbearing foromations of the Ishikari group.
Three facies of plant deposition as seams are able to classify from mode of occur rence of coal seams and from coal petrographical point of view.