DEPOSITIONAL ENVIRONMENTS AND COAL- FORMING SEDIMENTARY CHARACTERISTICS OF TAIYUAN FORMATION OF LATE CARBONIFEROUS IN PINGDINGSHAN COAL-FIELD, HENAN PROVINCE

Pingdingshan coal-field, which is located in the middle part of Henan province, is one of the major coal basins in China. The coal-bearing area is 330 km². The coal-bearing strata are the Taiyuan Formation of Late Carboniferous, Shanxi Formation of early Early Permian and Shihezi Group of late Early to early Late Premian. Mainly discussed in this paper are the depositional environments and coal- forming sedimentary characteristics of Taiyuan Formation. The deposits of Taiyuan Formation are formed, in the mixed carbonate shallow sea and terrigenous clastic seashore. Taiyuan Formation can be divided, from bottom to top, into three members: Lower Limestone Member, Middle Clastic Member and Upper Limestone Member and Upper Limestone Member. The Lower and Upper Limestone Members Also recongnized in Limestone Members are mainly formed on the shore (intertidal zone) grey micrite interbedded with biogenic debris limestone, vertical burrows) and in the shallow sea (dark grey biogenic debris limestone, horizontal burrows). Also recognized in Limestone Members is the tempestite terbidite which interbedded with normal shallow marine carbonate. The vertical sedimentary sequence of Limestone Members is as follows: in the lowre part is the shallow marine limestone and in the upper part are the fine elastics and coal beds (or carbonaceous shale). Often found in the upper part of this sequence is the tidal channel deposits which are more developed in the Upper Limestone Member. The Middle Clastic Member is interpreted as the deposits of barrier island-lagoon-tidal flat system. Extensively developed in the study area are the barrier island facies (quartz sandstone, coarsening upward in grain size, sandbody displaying the belt strike of east to west), tidal channel facies (detritus quartz sandstone, large scale planar crossbedding, erosive base, finning upward both in grain size and sedimentary structure scale, sandbody showing splitting and thinning northwards), lagoon-tidal flat facies (interbedded, lenticular, flaser and bioturbation bedding and vertical burrows). In the lighUof petrographic properties, sedimentary structures etc., the Middle Clastic Member can be recognized as three types of vertical sedimentary sequences, i. e., the vertical sequences which are respectively dominated by barrier island facies, tidal channel facies and lagoon-tidal flat facies. The barrier island facies can be further diviede into regressive sequence (underlain by the shallow marine limestone facies) and transgressive sequence (underlain by the lagoon-tidal flat facies). Generally, there are about 5 to 6 coal seame beds (from 0 to more than 10 beds) in Taiyuan Formation. The average total thickness of coals is 3.36m (from 0.33m to 10.73m, based on the data of 130 wells). In the Lower and Upper Limestone Members, the deposits below the coal beds are often the clastic deposits and rootlet beds, but the roofs of coal beds are dominately by shore and shallow marine limestone, which means that coals are formed on the platform evoluted by the shore and shallow sea when filled by the clastic deposits. In the Middle Clastic Member, the undeveloped, discontinuous and thin flaser coals overlying the barrier island are resulted from the quick regession (because dominated overlying barrier island are the lagoon-tidal flat facies ) or the frequently shifting of barrier island landwards or seawards (refer to the section of vertical sequence). The average group maceral composition of the coals in Taiyuan Formation are vitrinite (70%-90%), inertinite (1.0%-29.8%) andexinite (0-1.0%), so the type of coal is vitrinertite. The inorganic composition of coals is mainly clay types (3.6%-11.8%). The average ash and sulphur content of coals are respectively 10% (from 9.9% to 20.4%) and 4% (from 3.87% to 5.48%). Thus the coals of Taiyuan Formation belong to the low ash (10%-15%) (middle ash content in part) and high sulphur content (higher than 4%) coals. To sum up, it is suggested that the coals of Taiyuan Formation are formed in the brackish to saline water mars