Sedimentological characteristics of the coal seams of Yacheng Formation in Qiongdongnan Basin, China

Yacheng Formation has been divided into one cycle sequence of ultra long-period, three cylce sequences of long-period and 12 cycle sequences of intermediate period based on data from comprehensive drilling cores, logging, the seismic survey and former studies. Every sequence and its interface have the corresponding response in lithology, facies, logging, and earthquake data. The development and transformation of the base level cycle of intermediate period in Yacheng Formation were controlled by the tectonic subsidence and the global sea level eustacy. Because the rising rate of the tectonic subsidence was obviously higher than that of the global sea-level. The dominating action of the tectonic subsidence was more obviously on the base-level cycle. The coal formation of Yacheng Formation was mainly caused by gelatination. The coal seams were mainly developed in the peat swamp of the delta plain of braided river and the upper intertidal zone and the supralittoral zone of tidal plat. In the early period of Yacheng Formation, the main coal-forming systems was mainly the braided river-braided river delta system, while in the late period, it was mainly the tidal plat-lagoon system.