Zircon SHRIMP U-Pb age and geological implications of tuff at the bottom of Chang-7 Member of Yanchang Formation in the Ordos Basin

A table tuff interlayer at the bottom of Chang-7 Member of Yanchang Formation, the Ordos Basin, may serve as an indicator for stratigraphic division and correlation. In this study, zircon SHRIMP U-Pb dating was performed on the tuff at the bottom of Chang-7 Member using samples from wells Luo-36 and Zhuang-211 in the southwest of the basin, which yielded weighted average 206 Pb/ 238 U ages of 241.3±2.4 and 239.7±1.7 Ma, respectively. The cathodoluminescence images and the U/Th element ratio of tuff indicate that the zircons are magmatogenic and their ages represent that of the sedimentation age of the tuff at the bottom of Chang-7, which is 239.7–241.3 Ma in age. This finding confirms presence of the Middle Triassic strata at the bottom of Yanchang Formation. Taking previous findings into account, we suggest restricing the Late Triassic Yanchang Formation sensu stricto to the interval from Chang-7 to Chang-1 and assigning the interval from Chang-10 to Chang-8 as the Tongchuan Formation. The tuff at the bottom of the Chang-7 Member of Yanchang Formation represents the sedimentary response to the Qinling orogenic event in the early Indosinian. This tectonic event resulted in major changes in paleogeomorphology and the sedimentary environment of the lake basin during deposition of the Yanchang Formation. During this period, extremely favourable conditions were present for subsequent hydrocarbon generation and pooling of lake basin.