DEPOSITIONAL ENVIRONMENTS AND COAL-FORMING CHARACTERISTICS OF COAL BED OF SHANXI FORMATION IN SHANMIAN COALFIELD, WESTERN HENAN

Shanraian coalfield, which is located in the western part of Henan Province, is one of the major coalfields in southern zone of the Late Palaeozoic coal-forming basin of North China.The coal-bearing strata consist of Taiyuan Formation of Late Carboniferous, Shanxi Formation of early Early Permian and Shihezi Group of Late Early Permian to early Late Permian. Coal II₁, located in the lower part of Shanxi Formation, is the main minable coal bed in the study area.The deposits underlying coa II₁bed are mainly formed on the shoreline and in the shallow sea. They can be divided, from bottom to top, into four menbers: bottom bauxitic mudstone member, lower clastic rock member, middle limestone member and upper clastic rock membes.The first three members are part of Taiyuan Formation, the last member belongs to top of Taiyuan Formation and lower part of Shanxi Formation.Chiefly descussed in this paper are the sedimentary fades, depositional environments and coal-forming characteristics of Upper Clastic Member and middle upper part of Shanxi Formation in the study area.The clastic rocks of upper member are mainly deposited on microtidal coast with extensive clastic tidal channels (medium to fine sandstone, fining upward sequence, sandbody shows branching from south to north (land diretion) in plan view and lenticular shape in trend section and lagoon-flats (grey silt, mudstone with abundant pyritc nodules, interbedcd bedding, lenticular and flaser bedding and biturbate structure) ， and barrier islands (medium to fine quartz sandstone, lower angle planar cross-bedding) in the study area. The middle-upper part of Shanxi Formation (which overlies the coal II₁)is regarded as fluvial-dominated, tide-influenced delta system filling in a shallow bay.It is composed of extensive developed distributary channels (with abundant mica in bed plan, sandbody shows branching and thinning from north to south in plan view and lenticular shape in trend section* palaeocurrent is mainly single direction, some with reverse direction) ， interdistributary bay -tidal flats (interbedcd bedding, smalt ripple cross bedding, horizontal and incline biologic hole) and less developed subaquatic delta plain (distributary mouth bar, distal bar and pro delta) . On the whole, the deposits overlying coal II phow a regressive sequence from the deposits of subaqueous and lower delta plain (middle upper part of Shanxi Formation) towards those of upper delta plain (Shihezi Group). The thickness and quality of coal bed are affected directly by the precedent and contempora-neous depositional environments. Cloal bed (coal II₁) fomed on banierlLagoonltidal flats of microtidal coast tends to be thick (0-14.10, mean 3.66m), laterally continuous and high in sulphur (2.5% 14.0%, mean 3.91%) . Conversely, delta plain coals are thin (0-3.22m,mean 0.52m), laterally discontinuous and low in sulphur (0.60%-0.73%, mean 0.66%). As far as a single coal bed, such as coal II₁, is concerned, the thickness, laterally extent and quality of coal bed are mainly dependent Op sedimentary environments, where the tidal channels developed, the overlying coals tend to be thin (0-3m), low in sulphur (0-2.5%,but high in ash (higher than 40%). Where the lagoon-tidal flats developed, the overlying coals tend to be thick (thicker than 3m) , high in sulphur(higher than 2.5% ), but low in ash (10%-25%). The postdepositional environments of peat bed, such as delta distributary channels erode slightly the underlying peat deposits, fonning local thinning zone. The postedpositional tectonics is minor facters Influencing the thickness of coal scams.