THE COAL-BEARING FORMATION OF CARBONATITE TYPE IN RESTRICTED PLATFORM-SOME SEDIMETARY CHARACTERISTICS OF HESHAN FORMATION IN THE MATAN REGION OF CENTRAL GUANGXI
Zhang Pengfei Liu Huanjie Zhuo Yue Jia Yuru Chen Changronh Ho Chuyu Yin Zongchang ( China Mining Institute )
The Upper Permian Heshan formation, the most important coal-bearing strata in central Guangxi, is a main source of industrial coal. The Heshan formation is a coal-bearing formation of carbonatite type of restricted platform. It belongs to a special type.To study it in detail is of great significance to sedimentology, the theory of coal-formation and to the industrial production. The chief characteristics of the coal-bearing formation of this type are as follows: 1. More than 85% of the total formation is composed of limestones, predominantly organic limestone, bioclastic limestone and reef limestone. 2. The dominant facies are mainly restricted subtidal, intertidal and reef facies. 3. Biological fossils and bioclasts of various genera and species are exceedingly rich, chiefly Chlorophyta, Fusulina and Spongia. 4. Organic reefs ( especially the Sponge reefs ) develop well. They occur chiefly on the top and ia the middle-lower parts of the section. 5. The whole formation consists of marine facies ( including coal seams ) . There are no continental sediments. It can also be proved by the results of trace element, DTA, X-ray diffraction and electron microscope analyses of the coal seams and their top, bottom and slooms. The coal seams formed in the "peat-flat" environment, which is different from the common coal-forming swamp. The differences are as follows: 1. It is situated mainl yat the intertidal zone, but not in the subaeral envi-ironment. 2. It is situated on the flat behind the reef in the calm, shallow environment. The organic reef serves as a barrier. 3. The original coal-forming substance is perhaps some halophytes, quite similar to mangroves growing in the recent tropical and subtropical littoral zone, but it is not a fresh water terrigenous plant. 4. The coal seams formed in the "peat-flat" are characterized by extremely high ash and sulphur. The "peat-flat" is a special environment for the formation of coal. Sometimes it developed from the tidal channel distributed on the intertidal flat. For example, the bottom of the upper layer of the 4th coal seam workable throughout the Heshan coal field, in some places, is the tidal channel originally composed of calcirudite. The organic reef (probably belonging to the biostrome accordingto their morphology ) is one of the characteristics of coal-bearing formation of carbonatite type of restricted platform. The formation and distribution of coal seams are influenced and controlled directly by their presence. The calm, shallow water environment of back-reef is favourable for the formation of coal seam. In general, the longer the interval of reef-forming stage and the better the reef-flat develops, the more favourable for the formation of coal seams.