SHRIMP U-Pb Zircon Age and Its Stratigraphic Significance of Tuff at the Bottom of the Upper Triassic Nanying'er Group, Mount Baoji Area, Jingyuan County, Gansu Province

Previous studies suggest that the western margin of the Ordos basin in Late Triassic, extended westward across the Mount Liupan basin and connected with the Hexi Corridor and the associated strata in these two area can be corresponded each other; but the research was lack of supporting of high-precision, high confidence isotopic age data. In this paper, we conduct the SHRIMP U-Pb ages of tuff at the bottom of the Nanying’er Group, Mount Baoji area, Jingyuan County, Gansu Province. And acquire two sets of weighted average age of 439.3±4.1Ma and 244.1±3.8Ma. The former is inherited zircon crystallization age, while the latter is the deposition age of the tuff layers, which is quite consistent in the range of error with the depositional age of the tuff at the bottom of the 7th Member, Yanchang Formation, in the Ordos Basin. In other words, it is safe to say that the tuff at Mount Baoji area, Jingyuan County, and the tuff layer at the bottom of the 7th Member, Yanchang Formation, in the Ordos Basin can be used as a marker bed of the regional stratigraphic correlation. The results of this study provide a reliable basis for precise stratigraphic division and correlation for the Upper Triassic in the Ordos basin and the Hexi Corridor area.