| Abstract: | Using high-precision zircon U-Pb ID-TIMS geochronology, tuffs from the Chang 9 shale and the Chang 7 shale were dated. The tuff in the Chang 9 shale is 241.47 ± 0.17 Ma, which falls between the top tuff age of 241.06 ± 0.12 Ma and the bottom tuff age of 241.558 ± 0.093 Ma in the Chang 7 shale. These reveal that the Chang 9 and Chang 7 shales are contemporaneous, belonging to the Ladinian stage of the Middle Triassic. This insight expands the region of the main source rock of Chang 7 to the northeast and will inform the search for the deep Chang 9 shale petroleum system, increasing the scope for exploring the Chang 7 shale system in northern Shaanxi. The research results clarify the relationship between the two sets of shale in the Yanchang Formation and redefine the distribution range of the Chang 7 shale in the Ordos Basin. At the same time, it shows that there is a cross-layer problem in the stratigraphic division of the Yanchang Formation in different regions, the high-precision U-Pb dating technology providing a reference for the fine stratigraphic correlation of other continental basins in the world. |
