Analysis of paleoclimatic cycles in the ultra thick coal seam of Yan'an Formation in the southern Ordos basin

Coal is a climate-sensitive sediment that contains abundant paleoclimate information. In order to find out the paleoclimate information contained in the coal seam and the evolution of its controlling factors, the No.4 thick coal seam in the first member of Yan’an Formation in the south of Ordos basin was studied. Based on the intensive sampling of the thick coal seam, the information of organic macerals, major and trace elements, and stable carbon isotopes of organic matter were systematically tested. By using the macerals, characteristic elements and carbon isotopes of coal and rocks, four cycles of paleoclimate with alternation of warm-wet and dry-heat have been identified in the thick coal seam. The results of this study are similar to those of the previous studies in the same seam in the adjacent area, and the paleoclimatic cycle identified by the method of paleovegetation content coefficient was very consistent. Based on the comparative analysis of the Milankovitch cycle information which controlled the climate evolution in the thick coal seam, it is considered that the development and evolution of the paleoclimate cycle in the thick coal seam were mainly controlled by the long period of eccentricity in the orbit parameters of the celestial body. The results confirm the reliability of the method and elucidate the genetic mechanism of paleoclimate cycloaddition.