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A large area of deep-water sandstone was formed under the combined action of delta and gravity flow in the central Ordos Basin during the depositional period of the Ch6 and Ch7 oil-bearing formations of the Yanchang Formation. According to U-Pb dating of zircons by the method of LA-ICP-MS, the ages of the tuff layers at the base and upper parts of the Ch7 oil-bearing formation are 228.2±6.0 Ma and 222.2±2.0 Ma, respectively. Based on the ages of the tuff layers, it is suggested that the gravity flow complexes at the middle-upper Ch7 and lower-middle Ch6 oil-bearing formations were formed 215 to 224 Ma. The timing of deposition of the tuff and gravity flow is roughly the same as the period when tectonics are most active in the Qinling area, i.e. middle Indosinian period. Therefore, the even, widespread tuff at the bottom of the Ch7 oil-bearing formation and the thick deep-water sandstone of the Ch6 oil-bearing formation deposited during an early lake regression period, are the direct outcomes of the tectonic event. Tuff layers are deposited under the influence of the middle Indosinian tectonic events, and the clastic composition of the top and base tuff layers vary greatly. The events control the formation of large-area deep-water depressions, change of depositional system, migration of the depo-center, and occurrence of gravity flows.
Yang et al. (Tue,) studied this question.