Interpretation

Thick sandstone with ORC in the sequence is interpreted as the deposits of the tidal channel (Figure 6L), mostly distributed in the upper Benxi and Taiyuan Formations. The formation of ORC, whose precursors are mainly organic-rich deposits in marsh or lagoon, is controlled by tidal processes, including basal erosion, bank erosion, and slump. With the changes in the positional relationship

between the sun, the moon and the earth, the energy of tide has the characteristic of periodic variations. The channel-bottom deposits are flood dominated, with irregular, poorly sorted ORC mixed with gravel or coarse sand. Some large particles transported by tidal turbulence probably trapped and deposited in singles within the massive sandstone, usually resulting in heavy bending and deforming (Figure 6B,K). When the channel is ebb dominated, tidal energy gradually weakens, and the small granule ORC transported by buoyancy tends to deposit as laminations in wavy bedding sandstone (Figure 6J).

#### *4.4. Facies Association IV*
