Ordos Basin superposed evolution and structural controls of coal forming activities

The Ordos Basin is a tectonic basin formed by the superposition and transformation of multi-stage coal-accumulating basins. While the sedimentary evolution processes of Permo-Carboniferous, Triassic and Jurassic were different from one another, the time-space laws of coal resource formation in each period were very similar. The coal-accumulating period was controlled by the conversion process of tectonic structure. Permo-Carboniferous coal was formed in the conversion process of Caledonian-Hercynian tectonic movement, whereas Triassic and Jurassic coal was formed in the conversion process of Indochina-Yanshan tectonic movement. The coal-accumulating area was controlled by the conversion area of sedimentation. In offshore-type basin, the coal-accumulating area was found in the landside conversion area of river-lake sedimentation. In inland-lake-type basin, the coal-accumulating area had a ring-shaped distribution of conversion areas of river-lake sedimentation around the basin center. The deformation mode and occurrence state of coal resource were controlled by tectonic movement in the Yanshan period and later. The remarkable characteristics of the later transformation in the Ordos Basin were the thrust nappe structure in the west, the uplift in the east, and the concurrence of uplift and fault depression structures in the south and north. The thrust nappe developed in the west side of the Ordos Basin, not only did it cause large area erosion of Triassic and Jurassic coal but also repetition, fold and fracture of Permo-Carboniferous coal. The large area uplift developed in the east side of the Ordos Basin caused large area erosions of Permo-Carboniferous and Jurassic. The northwestward slope and extended northern syncline formed in this basin saved the coal resource of Late Paleozoic, Triassic and Jurassic. Uplift followed by fault depression in the south and north side of the Ordos Basin formed two fault depressions named Yinchuan-Hetao and Fenwei, which became the southern and northern boundaries of the Ordos Basin. The tectonic structure and conversion of sedimentation were important geological conditions in forming coal resources. Therefore, finding conversion areas amongst overlying rocks at the interface of tectonic movement is an effective strategy in coal finding explorations.